11institutetext: Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna,
Tenerife, Spain
11email<EMAIL_ADDRESS>22institutetext: Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain 33institutetext: Departamento de Matemática y Física Aplicadas, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile 44institutetext: Univ. Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France 55institutetext: Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA 66institutetext: Department of Astronomy, University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 77institutetext: Vanderbilt University, Department of Physics & Astronomy, 6301 Stevenson Center Ln., Nashville, TN 37235, USA 88institutetext: Observatoire de l’Université de Genève, Chemin des Maillettes 51, 1290 Versoix, Switzerland 99institutetext: NASA Ames Research Center, Moffett Field, CA, 94035, USA 1010institutetext: Caltech/IPAC, 1200 E. California Blvd. Pasadena, CA 91125, USA 1111institutetext: Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA 1212institutetext: U.S. Naval Observatory, Washington, DC 20392, USA 1313institutetext: NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA 1414institutetext: Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA 1515institutetext: George Mason University, 4400 University Drive, Fairfax, VA, 22030 USA 1616institutetext: Department of Physics & Astronomy, Swarthmore College, Swarthmore PA 19081, USA 1717institutetext: El Sauce Observatory, Coquimbo Province, Chile 1818institutetext: Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 19C Allèe du 6 Août, 4000 Liège, Belgium 1919institutetext: Astrobiology Research Unit, Université de Liège, 19C Allèe du 6 Août, 4000 Liège, Belgium 2020institutetext: Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco 2121institutetext: International Center for Advanced Studies (ICAS) and ICIFI(CONICET), ECyT-UNSAM, Campus Miguelete, 25 de Mayo y Francia (1650), Buenos Aires, Argentina 2222institutetext: Université de Montréal, Département de Physique & Institut de Recherche sur les Exoplanètes, Montréal, QC H3C 3J7, Canada 2323institutetext: European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile 2424institutetext: Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal 2525institutetext: Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal 2626institutetext: Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 2727institutetext: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 2828institutetext: Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA 2929institutetext: Department of Astrophysical Sciences, Princeton University, NJ 08544, USA 3030institutetext: SETI Institute, 189 Bernardo Ave, Suite 200, Mountain View, CA 94043, USA 3131institutetext: Space Telescope Science Institute, Baltimore, MD 21218, USA 3232institutetext: Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA 3333institutetext: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 3434institutetext: Department of Physics and Astronomy, University of New Mexico, 210 Yale Blvd NE, Albuquerque, NM 87106, USA 3535institutetext: Department of Space, Earth and Environment, Astronomy and Plasma Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden

# TOI-674b: an oasis in the desert of exo-Neptunes transiting a nearby M dwarf
††thanks: Based on observations made with the HARPS instrument on the ESO
3.6-m telescope at La Silla Observatory under programme ID 1102.C-0339

F. Murgas 1122 N. Astudillo-Defru 33 X. Bonfils 44 Ian Crossfield 55 J. M.
Almenara 44 John Livingston 66 Keivan G. Stassun 77 Judith Korth 3535 Jaume
Orell-Miquel 1122 G. Morello 1122 Jason D. Eastman 1414 Jack J. Lissauer 99
Stephen R. Kane 3232 Farisa Y. Morales 3333 Michael W. Werner 3333 Varoujan
Gorjian 3333 Björn Benneke 2222 Diana Dragomir 3434 Elisabeth C. Matthews 88
Steve B. Howell 99 David Ciardi 1010 Erica Gonzales 1111 Rachel Matson 1212
Charles Beichman 1010 Joshua Schlieder 1313 Karen A. Collins 1414 Kevin I.
Collins 1515 Eric L. N. Jensen 1616 Phil Evans 1717 Francisco J. Pozuelos
18181919 Michaël Gillon 1919 Emmanuël Jehin 1818 Khalid Barkaoui 19192020 E.
Artigau 2222 F. Bouchy 88 D. Charbonneau 1414 X. Delfosse 44 R. F. Díaz 2121
R. Doyon 2222 P. Figueira 23232424 T. Forveille 44 C. Lovis 88 C. Melo 2323 G.
Gaisné 44 F. Pepe 88 N. C. Santos 24242525 D. Ségransan 88 S. Udry 88 Robert
F. Goeke 2828 Alan M. Levine 2626 Elisa V. Quintana 1313 Natalia M. Guerrero
2626 Ismael Mireles 2626 Douglas A. Caldwell 303099 Peter Tenenbaum 303099 C.
E. Brasseur 3131 G. Ricker 2626 R. Vanderspek 2626 David W. Latham 1414 S.
Seager 262627272828 J. Winn 2929 Jon M. Jenkins 99

(Received 2021; accepted 2021)

###### Abstract

Context. The NASA mission TESS is currently doing an all-sky survey from space
to detect transiting planets around bright stars. As part of the validation
process, the most promising planet candidates need to be confirmed and
characterised using follow-up observations.

Aims. In this article we aim to confirm the planetary nature of the transiting
planet candidate TOI-674b using spectroscopic and photometric observations.

Methods. We use TESS, Spitzer, ground-based light curves and HARPS
spectrograph radial velocity measurements to establish the physical properties
of the transiting exoplanet candidate TOI-674b. We perform a joint fit of the
light curves and radial velocity time series to measure the mass, radius, and
orbital parameters of the candidate.

Results. We confirm and characterize TOI-674b, a low-density super-Neptune
transiting a nearby M dwarf. The host star (TIC 158588995, $V=14.2$ mag,
$J=10.3$ mag) is characterized by its M2V spectral type with
$\mathrm{M}_{\star}=0.420\pm 0.010$ M⊙, $\mathrm{R}_{\star}=0.420\pm 0.013$
R⊙, and $\mathrm{T}_{\mathrm{eff}}=3514\pm 57$ K, and is located at a distance
$d=46.16\pm 0.03$ pc. Combining the available transit light curves plus radial
velocity measurements and jointly fitting a circular orbit model, we find an
orbital period of $1.977143\pm 3\times 10^{-6}$ days, a planetary radius of
$5.25\pm 0.17$ $\mathrm{R}_{\oplus}$, and a mass of $23.6\pm 3.3$
$\mathrm{M}_{\oplus}$ implying a mean density of $\rho_{\mathrm{p}}=0.91\pm
0.15$ [g cm-3]. A non-circular orbit model fit delivers similar planetary mass
and radius values within the uncertainties. Given the measured planetary
radius and mass, TOI-674b is one of the largest and most massive super-Neptune
class planets discovered around an M type star to date. It is also a resident
of the so-called Neptunian desert and a promising candidate for atmospheric
characterisation using the James Webb Space Telescope.

###### Key Words.:

stars: individual: TOI-674 – planetary systems – techniques: photometric –
techniques: radial velocities

##  1 Introduction

M dwarf stars are the most common type of stars in the Milky Way and in the
solar neighborhood (e.g., Reid et al., 1995, Chabrier & Baraffe, 2000, Henry
et al., 2006, Bochanski et al., 2010, Winters et al., 2015). Due to their
relatively small sizes and low masses, they are good targets for the detection
of small planets using transit searches and radial velocity measurements,
respectively. More importantly, transiting planets orbiting bright M dwarfs
offer great opportunities for atmospheric characterization using, for example,
low and high resolution transmission spectroscopy (e.g., Kreidberg et al.,
2014, Knutson et al., 2014, Ehrenreich et al., 2014, Southworth et al., 2017).
Several Earth-sized and super-Earth planets have been found orbiting around M
dwarfs (e.g., GJ 876d Rivera et al., 2005, TRAPPIST-1 system Gillon et al.,
2017, LHS 1140b Dittmann et al., 2017, LHS 1140c Ment et al., 2019, GJ 357b
Luque et al., 2019), but also planets with masses and radii between those of
Neptune and Jupiter have been found (e.g., GJ 876c Marcy et al., 2001, GJ 436b
Gillon et al., 2007; HATS-71b Bakos et al., 2020; GJ 3512b Morales et al.,
2019; TOI-1728b Kanodia et al., 2020). Relatively few gas planets orbiting
around M dwarfs have been found in this mass and size range, in agreement with
the prediction of a paucity of gas giants orbiting around M dwarfs for core
accretion models (Laughlin et al., 2004). Specifying their occurrence rate
with better statistics can give new insights on planetary formation (e.g.,
core accretion versus disk instability: Boss, 2006) and orbital migration
processes (e.g., Correia et al., 2020).

Planetary population studies made for different types of stars have noticed a
lack of Neptune-sized planets with short orbital periods (i.e., highly
irradiated), this is the so-called ‘Neptunian desert’ (e.g., Szabó & Kiss,
2011, Mazeh et al., 2016, Fulton & Petigura, 2018). The low number of Neptune-
sized planets with orbital periods shorter than 4 days could possibly indicate
different formation mechanisms for close-in super-Earths and Jovian planets
(Mazeh et al., 2016), similar to what is observed for low-mass brown dwarfs in
short orbits around Sun-like stars (e.g., Grether & Lineweaver, 2006). Another
proposed mechanism for the origin of this gap in the radius distribution is
photo-evaporation of planetary atmospheres in response to high energy
radiation (ultraviolet, X-ray) from the star (e.g., Lopez & Fortney, 2013,
Chen & Rogers, 2016).

In recent years, more planets in the Neptunian desert have been identified.
Some examples of planets with masses close to Neptune and in short orbital
periods found recently are NGTS-4b (West et al., 2019), TOI-132b (Díaz et al.,
2020), LTT 9779b (Jenkins et al., 2020b), TOI-1728b (Kanodia et al., 2020),
TOI-442b (Dreizler et al., 2020), TOI-849b (Armstrong et al., 2020). The
addition of new objects in this mass, radius, and orbital period range can
help to shed some light on the physical mechanisms behind the Neptunian
desert.

The Transiting Exoplanet Survey Satellite (TESS; Ricker et al., 2014) is a
NASA-sponsored space telescope launched in April 2018. Its main mission is to
monitor the full sky in search of transiting planets orbiting around bright
stars ($5<I_{C}<13$ mag). The observing strategy of TESS is to observe with
its 4 cameras a $24^{\circ}\times 96^{\circ}$ area of the sky for 27 days. The
exposure time for each camera is of 2 seconds and during the Primary Mission
the images were stacked in two timing sampling modes: 2 minute and 30 minute
cadence. The planned time for the main mission was 2 years, but on July 2020
TESS entered its extended mission which is approved to continue until the end
of September 2022. For the first TESS extended mission the 30-minute cadence
for Full Frame Images (FFIs) has been reduced to 10 minutes.

Here, we report the discovery of TOI-674b, a super-Neptune transiting around a
nearby M dwarf, discovered using TESS data. The mass, radius, and orbital
period of this planet indicate that this is a new member of the Neptunian
desert, and it is a good candidate target for follow-up observations. This
paper is organized as follows: Section 2 presents the observations used in
this work, Section 3 presents the methods use in the data analysis, Section 4
presents the parameter estimates of the planet as well as a discussion in the
context of known planets. Finally, Section 5 presents the conclusions of this
work.

##  2 Observations

###  2.1 Space-based photometry

####  2.1.1 TESS photometry

The star TIC 158588995 (TOI-674) was observed by TESS during Sector 9 and
Sector 10 for 27 days each. The Sector 9 campaign started on 28 February 2019
and ended on 25 March 2019; for this campaign the target was positioned on
TESS CCD 3 Camera 2. The Sector 10 campaign started on 26 March 2019 and ended
on 22 April 2019, and for this TESS run the target was placed on CCD 4 Camera
2. For its prime mission, TESS images were stacked in two timing sampling
modes: 2 minute and 30 minute cadence; TOI-674 was selected to be observed
using the 2 minute short-cadence mode in both sectors (Stassun et al., 2018b).

The raw image data taken by TESS was processed by the Science Processing
Operations Center (SPOC) at NASA Ames Research Center. The SPOC pipeline
(Jenkins et al., 2016) calibrates the image data, performs quality control
(e.g., identifies and flags bad data), extracts photometry for each target
star in the TESS field of view, and searches the resulting light curves for
exoplanet transit signatures. The initial TESS light curve was produced using
simple aperture photometry (SAP, Morris et al., 2020) and then the
instrumental systematic effects were removed using the Presearch Data
Conditioning (PDC) pipeline module (Smith et al., 2012, Stumpe et al., 2014).
The light curves are searched with an adaptive, wavelet-based matched filter
(Jenkins, 2002, Jenkins et al., 2020a) , and then fitted to limb-darkened
transit models (Li et al., 2019) , and subjected to diagnostic tests to make
or break the planetary hypothesis (Twicken et al., 2018). Subsequently, the
TESS science office reviewed the transit signature identified in the SPOC
processing of Sector 9, promoting it to a planetary candidate as TESS Object
of Interest (TOI) TOI-674.01, and alerted the community in May 2019.

In this work we made used of the TESS PDC- SAP light curves that have been
corrected for instrumental systematics. These data sets (TESS sector 9 and 10)
are available at the Barbara A. Mikulski Archive for Space Telescopes
(MAST111https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html). The
TESS images around the position of TOI-674 in Sector 9 and 10 are shown in
Figure 1. The red squares indicate the aperture used to produce the PDC-SAP
light curves.

Figure 1: TESS target pixel file image of TOI 674 observed in Sector 9 (left)
and Sector 10 (right), made with tpfplotter (Aller et al., 2020). The pixels
highlighted in red show the aperture used as the target region in the
photometry. The position and sizes of the red circles represent the position
and TESS magnitudes of nearby stars respectively. Table 1: TOI-674
identifiers, coordinates, stellar parameters, and magnitudes. The adopted
stellar mass and radius used in this work are the ones presented in the middle
column (i.e., BT-Settl models SED fit).

Identifiers |  |  |  |  | Ref.
---|---|---|---|---|---
TIC |  |  |  | 158588995 |
2MASS |  |  |  | J10582099-3651292 |
Gaia EDR3 |  |  |  | 5400949450924312576 |
Equatorial coordinates |  |  |  |  |
RA (J2016) |  |  |  | $10^{\mathrm{h}}\;58^{\mathrm{m}}\;20^{\mathrm{s}}.77$ | 1
DEC (J2016) |  |  |  | $-36^{\circ}\;51^{\prime}\;29\aas@@fstack{\prime\prime}19$ | 1
$\mu_{\alpha}$ [mas/year] |  |  |  | $-163.255\pm 0.015$ | 1
$\mu_{\delta}$ [mas/year] |  |  |  | $-3.672\pm 0.015$ | 1
Parallax [mas] |  |  |  | $21.623\pm 0.016$ | 1
Parameters from SED fit |  | BT-Settl SED fit |  | BT-Settl + Stellar evol. models SED fit |
Effective Temperature [K]aa$a$As an independent check, we also derived Teff and [Fe/H] using the Machine Learning approach implemented in ODUSSEAS (Antoniadis-Karnavas et al., 2020). The obtained Teff (3220$\pm$70 K) is lower than the one obtained with SpecMatch-Emp (Yee et al., 2017), while the [Fe/H] (+0.05$\pm$0.11) is compatible within the 1-$\sigma$ uncertainties. | $\mathrm{T}_{\mathrm{eff}}$ | $3514\pm 57$ |  | $3505^{+28}_{-32}$ | 5
Stellar Luminosity [$\mathrm{L}_{\odot}$] | $\mathrm{L}$ | $0.0243^{+0.0012}_{-0.0009}$ |  | $0.0240^{+0.0013}_{-0.0008}$ | 5
Surface gravity [cm/s2]bb$b$Stellar surface gravity $\log(g)$ derived from SED fit. | $\log(g)$ | $5.28^{+0.51}_{-0.66}$ |  | $4.832^{+0.006}_{-0.009}$ | 5
Metallicity [dex]aa$a$As an independent check, we also derived Teff and [Fe/H] using the Machine Learning approach implemented in ODUSSEAS (Antoniadis-Karnavas et al., 2020). The obtained Teff (3220$\pm$70 K) is lower than the one obtained with SpecMatch-Emp (Yee et al., 2017), while the [Fe/H] (+0.05$\pm$0.11) is compatible within the 1-$\sigma$ uncertainties. | $[\mathrm{Fe}/\mathrm{H}]$ | $0.17\pm 0.12$ |  | $0.114\pm 0.074$ | 5
Stellar Age [Gyr] |  |  |  | $5.5^{+2.9}_{-1.9}$ | 5
Mass [M⊙] | $\mathrm{M}_{\star}$ |  $0.420\pm 0.010$cc$c$Mass derived using the empirical relations of Mann et al. (2019). |  | $0.442\pm 0.007$ | 5
Radius [R⊙] | $\mathrm{R}_{\star}$ | $0.420\pm 0.013$ |  | $0.421\pm 0.007$ | 5
Stellar density [g cm-3] | $\rho_{\star}$ | $7.99\pm 0.76$ |  | $8.36^{+0.22}_{-0.31}$ | 5
Surface gravity [cm/s2] | $\log(g)$ |  $4.81\pm 0.03$dd$d$Stellar surface gravity $\log(g)$ computed using the derived stellar mass and radius. |  | $4.832^{+0.006}_{-0.009}$ | 5
Distance [pc] | $\mathrm{d}_{\star}$ | $46.16\pm 0.03$ |  | $46.16\pm 0.03$ | 5
Other parameters |  |  |  |  |
Stellar activity index | $\log_{R_{HK}}$ |  | $-5.14\pm 0.01$ |  | 5
Stellar rotation period [days]ee$e$From empirical relations of Astudillo-Defru et al. (2017a). | $P_{\mathrm{rot}}$ |  | $52\pm 5$ |  | 5
Stellar rotation velocity [km/s]ff$f$From stellar radius and rotation period. |  |  | $0.4\pm 0.04$ |  | 5
Apparent magnitudes |  |  |  |  |
Gaia G [mag] |  |  |  | $13.073\pm 0.002$ | 1
B [mag] |  |  |  | $15.742\pm 0.024$ | 2
V [mag] |  |  |  | $14.203\pm 0.041$ | 2
Sloan g [mag] |  |  |  | $14.938\pm 0.022$ | 2
Sloan r [mag] |  |  |  | $13.626\pm 0.047$ | 2
Sloan i [mag] |  |  |  | $12.299\pm 0.068$ | 2
J [mag] |  |  |  | $10.359\pm 0.023$ | 3
H [mag] |  |  |  | $9.737\pm 0.023$ | 3
K [mag] |  |  |  | $9.469\pm 0.019$ | 3
WISE W1 [mag] |  |  |  | $9.345\pm 0.022$ | 4
WISE W2 [mag] |  |  |  | $9.237\pm 0.019$ | 4
WISE W3 [mag] |  |  |  | $9.163\pm 0.038$ | 4
WISE W4 [mag] |  |  |  | $8.332\pm 0.455$ | 4

(1) Gaia Collaboration et al. (2021); (2) Henden et al. (2015); (3) Cutri et
al. (2003); (4) Cutri & et al. (2014); (5) This work.

222

####  2.1.2 Spitzer photometry

A primary transit of TOI-674b was observed using the Spitzer Space Telescope
as part of a program dedicated to TESS target follow-up (GO-14084, Crossfield
et al., 2018). The data were taken with the instrument InfraRed Array Camera
(IRAC), which is a four-channel camera with a field of view of
$5.2^{\prime}\times 5.2^{\prime}$ and can take images at 3.6, 4.5, 5.8, and 8
$\mu$m. Each channel has $256\times 256$ pixels and a pixel scale of $\sim
1.2$ ″ pixel-1. The target was observed on 29 September 2019 using Spitzer’s
4.5 $\mu$m channel, the exposure time was set to 2 seconds and the total
observation time was $\sim 5$ hours. To compute the time series we used a
$3\times 3$ pixel subarray centered on the position of TOI-674.

###  2.2 Ground-based photometry

After the discovery of the transits made by the TESS team, several follow-up
ground-based observations were scheduled to confirm that the transits occur on
TOI-674 and rule-out some false positive scenarios (e.g., eclipsing binaries
unresolved by TESS). The data were acquired under the TESS Follow-up Observing
Program (TFOP) and uploaded to the Exoplanet Follow-up Observing Program for
TESS (ExoFOP-TESS333https://exofop.ipac.caltech.edu/tess/).

####  2.2.1 El Sauce Observatory

We observed a full transit of TOI-674b in the Rc band on 12 May 2019 from El
Sauce Observatory in Coquimbo Province, Chile. The 0.36 m telescope is
equipped with a $1536\times 1024$ SBIG STT-1603-3 camera. The camera was
operated using a $2\times 2$ bin mode, the image scale is 1.47 ″ pixel-1
resulting in a $18.8\arcmin\times 12.5\arcmin$ field of view. The photometric
data were extracted using AstroImageJ (Collins et al. 2017) with a 5 pixel
aperture.

####  2.2.2 LCOGT

Las Cumbres Observatory Global Telescope Network (LCOGT, Brown et al., 2013)
is a set of robotic telescopes operating in both hemispheres and with sites
distributed across several countries. A primary transit of TOI-674b was
observed on 16 May 2019 with the LCOGT 1 m telescope at Cerro Tololo
International Observatory (CTIO) in Chile. The LCOGT 1 m telescopes are
equipped with SINISTRO CCDs with a field of view of $26^{\prime}\times
26^{\prime}$ , and feature a pixel scale of 0.389 arcsec/pixel and read out
cadence of 28 seconds. The transit was observed in the Sloan g band, the
telescope was slightly defocused (0.1 mm from nominal value), and the exposure
time was set to 150 seconds. The photometry was extracted using AstroImageJ
software with an aperture of 10 pixels and the Point Spread Function (PSF)
diameter was 1.95 arcsec.

####  2.2.3 TRAPPIST-South

TRAPPIST-South, located at ESO-La Silla Observatory in Chile, is a 60 cm
Ritchey–Chretien telescope equipped with a thermoelectrically cooled $2K\times
2K$ FLI Proline CCD camera (Jehin et al., 2011; Gillon et al., 2013). It
features a field- of-view of $22^{\prime}\times 22^{\prime}$ and a pixel-scale
of 0.65 ″/pixel. We acquired 494 images during a full-transit observation on
2019-05-13 with the $I+z$ filter and an exposure time of 15 s. The optimum
photometric aperture was 6 pixels (3.9 arcsec) and the PSF diameter 2.2
arcsec. A second full-transit observation was carried out on 2019-05-21 using
the Sloan z filter with an exposure time of 15 s, and yielded a total of 412
images. The optimum aperture was 8 pixels (5.2 arcsec) and the PSF diameter
was 2.5 arcsec. These sets of observations confirmed the presence of a transit
event on the target star on time, and eliminated the possibility that the
events are due to an eclipsing binary outside of the PSF centered on the
target star. For both dates we made use of the TESS Transit Finder tool, which
is a customized version of the Tapir software package (Jensen, 2013), to
schedule the photometric time-series and we used AstroImageJ to perform
aperture photometry.

###  2.3 Spectroscopic observations

TOI-674 was observed by the High Accuracy Radial velocity Planet Searcher
(HARPS, Mayor et al., 2003) at the ESO La Silla 3.6m telescope. The
observations were carried out as part of the program 1102.C-0339 dedicated to
searching for planets orbiting around M dwarfs. From 24 May 2019 to 18 July
2019, we collected 17 spectra with a resolving power of $R\approx 115000$. The
image of the target star was placed in the aperture of the scientific fiber
while the calibration fiber was used to monitor the sky, the exposure time was
set to 30 minutes and the CCD was read using the slow read-out mode, resulting
in a signal-to-noise ratio between 5 and 13 (median of 9).

The spectra were calibrated and extracted using the HARPS online pipeline
(Lovis & Pepe, 2007; for improvements in the pipeline check Mayor et al.,
2009a, b and references therein). With the spectra and preliminary radial
velocity measurements from the HARPS online pipeline we produced a template
spectrum following Astudillo-Defru et al. (2017b). The final radial velocities
were obtained from likelihood functions constructed by comparing the stellar
template shifted by different velocities with each individual spectrum. The
velocity that produced the maximum likelihood for each data point was the
derived HARPS radial velocity. The RV time series present a dispersion of 17.4
m/s and have a median uncertainty of 7.3 m/s.

We also obtain from the spectra several activity indices:
$\mathrm{H}_{\alpha}$, $\mathrm{H}_{\beta}$, $\mathrm{H}_{\gamma}$, Na, and
calcium S index. The radial velocities and activity indices are given in Table
3.

###  2.4 High-resolution imaging

TESS has a pixel scale of 21 arcsec/pixel, hence it is possible that TOI-674
can present some level of flux contamination produced by nearby faint stars
not detected by seeing limited photometric observations. Contamination in the
flux of the transit host star can led to a wrong estimation of transit depth,
thus leading to an incorrect absolute planet radius. TESS Data Validation
Report (Twicken et al., 2018) performs a difference image centroiding
analysis, in the case of TOI-674 the report determined that the location of
the source of the transit signature was within 1 arcsec of the target star.
Nevertheless, we searched for previously undetected faint nearby companion
stars using adaptive optics and speckle imaging using Gemini Telescope.

####  2.4.1 Gemini/NIRI

We searched for visual companions with the Gemini/NIRI adaptive optics imager
(Hodapp et al., 2003). Such companions can dilute the light curve, thereby
biasing the measured radius, or even be the source of a false positive if the
visual companion is an eclipsing binary (e.g., Ciardi et al., 2015). We
collected 9 images in the Br$\gamma$ filter, each with exposure time 5.7 s,
and dithered the telescope by $\sim$2” between each exposure. This dither
pattern allows for a sky background frame to be constructed from the science
images themselves. We processed the data using a custom code which performs
bad pixel and flat corrections, subtracts the sky background, aligns the star
between images and coadds the frames. The final image can be seen in Figure 2.
We inspected the final image visually, and did not identify companions
anywhere in the field of view, which is 26.8”$\times$26.8”. To estimate the
sensitivity of these images to the presence of visual companions, we injected
scaled copies of the stellar PSF at several radial separations and position
angles, and scaled their brightness until each could be detected at
5-$\sigma$. Sensitivity was then averaged over position angle, and Figure 2
shows the sensitivity to visual companions as a function of radius. We are
sensitive to companions 5 mag fainter than the host beyond 270 mas, and reach
a contrast of 7.3 mag in the background limited regime, beyond $\sim$1.05”.

Figure 2: Gemini/NIRI high resolution image of TOI-674. Our observations rule
out candidates to 5 mag fainter than the host beyond 270 mas, and 7.3 mag
beyond $\sim$1.05″. Inset: thumbnail image of TOI-674, centered on the star.
The PSF is circular to the limit of our resolution, and no visual companions
are identified anywhere in the field of view, which extends at least 13″ from
the target in all directions.

####  2.4.2 Gemini/Zorro

We observed TOI-674 on 14 January 2020 using the Zorro instrument mounted on
the 8-m Gemini South telescope, located on Cerro Pachón in Chile. Zorro
simultaneously observes diffraction-limited images at 562 nm (0.017”) and 832
nm (0.028”). Our data set consisted of five 1000 X 60 ms exposures
simultaneously obtained in both band-passes, followed by a single $1000\times
60$ ms image, also in both band-passes, of a PSF standard star. Following the
procedures outlined in Howell et al. (2011), we combined all images and
subjected them to Fourier analysis, and produce re-constructed imagery from
which 5-$\sigma$ contrast curves are derived in each passband (Figure 3). Our
data reveal TOI-674 to be a single star to contrast limits of 5 to 7
magnitudes within the spatial limits of 0.8/1.3 AU (562/832 nm respectively)
out to 55 AU ($\mathrm{d}=46$ pc). A second Gemini/Zorro observation made in
25 February 2021 confirmed the previous result and no companion stars to
TOI-674 were detected within the angular and contrast limits explored.

Figure 3: Gemini/Zorro high resolution image of TOI-674 taken on 14 January
2020. TOI-674 is a single star to contrast limits of 5 to 7 mag within 0.8/1.3
AU (562/832 nm respectively) out to 55 AU ($\mathrm{d}=46$ pc). Inset:
1.2″x1.2″ thumbnail image of TOI-674, centered on the star.

##  3 Analysis

###  3.1 Stellar parameters

We computed an initial estimate of TOI-674 stellar parameters using the
weighted average of the HARPS spectra and analysed it with SpecMatch-Emp (Yee
et al., 2017). We thereby obtained $\mathrm{T_{eff}}=3470\pm 70$ K,
$\mathrm{R_{\star}}=0.413\pm 0.100$ $\mathrm{R_{\odot}}$, and
$[\mathrm{Fe}/\mathrm{H}]=0.18\pm 0.09$ dex. The parameters obtained with
SpecMatch-Emp were used as a prior to fit the Spectral Energy Distribution
(SED) of TOI-674. To estimate the stellar parameters we followed Díaz et al.
(2014). For the SED fit we adopted the distance and the apparent magnitudes
from Gaia (Gaia Collaboration et al., 2021), 2MASS (Cutri et al., 2003), and
WISE (Cutri & et al., 2014). We used as priors the distance to the star (based
on Gaia EDR3 parallax) and the effective temperature and value of
$[\mathrm{Fe}/\mathrm{H}]$ from SpecMatch-Emp; the uncertainty in
$[\mathrm{Fe}/\mathrm{H}]$ was changed from $0.09$ dex to $0.12$ dex according
to Table 3 of Yee et al. (2017). The fit was done by comparing the SED with
the synthetic spectra models from the BT-Settl library (Allard et al., 2011)
alone and the BT-Settl library plus stellar evolution models from Dotter et
al. (2008).

The stellar parameters for TOI-674 are presented in Table 1. The stellar mass
value for the SED fit without stellar evolution models was computed following
Mann et al. (2019) that uses the distance, K-magnitude, and metallicity to
obtain a mass estimate. For the SED fit using BT-Settl stellar models we find
a stellar effective temperature of $3514\pm 57$ K, indicating that the
spectral type must be close to M2V. To be conservative in our uncertainties,
we adopted as final stellar parameters the values found by the SED fit with
stellar atmosphere models without including the stellar evolution models,
i.e., middle column of Table 1. Hence, to derive absolute planet parameters we
use in this work a stellar mass and radius of $\mathrm{M}_{\star}=0.420\pm
0.010$ M⊙ and $\mathrm{R}_{\star}=0.420\pm 0.013$ R⊙ respectively.

To check our results and as an independent determination of the basic stellar
parameters, we performed an analysis of the broadband spectral energy
distribution (SED) of the star together with the Gaia EDR3 parallax (with no
systematic correction Stassun & Torres, 2021), in order to determine an
empirical measurement of the stellar radius, following the procedures
described in Stassun & Torres (2016); Stassun et al. (2017, 2018a). We pulled
the $JHK_{S}$ magnitudes from 2MASS, the W1–W4 magnitudes from WISE, the
$GG_{\rm BP}G_{\rm RP}$ magnitudes from Gaia, and the NUV magnitude from
GALEX. Together, the available photometry spans the full stellar SED over the
wavelength range 0.2–22 $\mu$m (see Figure 12).

We performed a fit using NextGen stellar atmosphere models (Hauschildt et al.,
1999), with the effective temperature ($T_{\rm eff}$), surface gravity ($\log
g$), and metallicity ([Fe/H]) adopted from the spectroscopic analysis. The
remaining free parameter is the extinction, $A_{V}$. The resulting fit (Figure
12) has a reduced $\chi^{2}$ of 3.1 (excluding the UV measurement which
appears to be in excess, see below), with best fit $A_{V}=0.02\pm 0.02$.
Integrating the (unreddened) model SED gives the bolometric flux at Earth,
$F_{\rm bol}=3.50\pm 0.12\times 10^{-10}$ erg s-1 cm-2. Taking the $F_{\rm
bol}$ and $T_{\rm eff}$ together with the Gaia parallax, gives the stellar
radius, $R_{\star}=0.413\pm 0.015$ R⊙; a value consistent with our adopted
stellar radius presented in Table 1.

Finally, we can use the star’s activity to estimate an age via empirical
rotation-activity-age relations. The mean chromospheric activity from the
time-series spectroscopy is $\log R^{\prime}_{\rm HK}=-5.14\pm 0.01$, which
implies a stellar rotation period of $P_{\rm rot}=52\pm 5$ days via the
empirical relations of Astudillo-Defru et al. (2017a). This estimated
$\mathrm{P}_{\rm rot}$ implies an age of $\tau_{\star}=3.2\pm 1.2$ Gyr via the
empirical relations of Engle & Guinan (2018), which is consistent with the age
derived from the SED fit plus stellar evolution models presented in Table 1.

###  3.2 Frequency analysis of the HARPS data

We searched for the planetary signal in the HARPS data using a generalized
Lomb-Scargle periodogram (Zechmeister & Kürster, 2009). With the radial
velocity measures alone a signal with a period of $\sim 2.0$ days is detected
with a False Alarm Probability (FAP) below 1% ( $\mathrm{FAP}(peak)=0.15\%$,
see Figure 4). This period is consistent with the orbital period originally
reported by TESS team as part of their alerts ($P=1.977$ days).

Figure 4: TOI-674b GLS periodogram for HARPS radial velocities measurements.
The peak with the maximum power corresponds to a period of $\sim 2$ days, the
dashed line shows the False Alarm Probability (FAP) level of 1%.

We find no signals in the RV or activity indices periodograms that points to
the rotational period of the star (see Fig. 13). M dwarfs can have rotational
periods ranging from a few hours to over one hundred days (e.g., Newton et
al., 2016) and our total baseline of HARPS observations is of $\sim 54$ days
(with 17 individual spectra). In section 3.1 we derived a rotational period
for the host star of $P_{\rm rot}=52\pm 5$ days from empirical relations,
hence it is possible that we barely observed a single full rotational period
of the star during the spectroscopic follow-up of TOI-674.

###  3.3 Transit light curve and radial velocity model

The light curves were modeled with the help of the transit modeling package
PyTransit444https://github.com/hpparvi/PyTransit (Parviainen, 2015) version
2.0. We used the PyTransit model implementation of the Mandel & Agol (2002)
analytic transit model with a quadratic limb darkening law.

Due to the large number of data points, the Spitzer and ground based light
curves were detrended before performing the joint fit to speed up the fitting
process. For the Spitzer time series we performed Pixel Level Decorrelation
(PLD) following Deming et al. (2015) to remove the instrumental noise known to
affect Spitzer data. We computed the normalized pixel intensities for each of
the $3\times 3$ Spitzer pixels

$\hat{P}_{i}^{t}=\frac{P_{i}^{t}}{\sum_{i=1}^{N}P_{i}^{t}}$ (1)

where $P_{i}^{t}$ is the $i$-th pixel inside the aperture at a time $t$ of the
time series. We modeled the total flux inside an aperture as

$S^{t}=\sum_{i=1}^{N}c_{i}\hat{P}_{i}^{t}+T(t)+ft+gt^{2}$ (2)

where $c_{i}$ are weighting coefficients for the normalized pixel intensities
$\hat{P}_{i}^{t}$, $T(t)$ is the transit model, and $ft+gt^{2}$ is a quadratic
function in time with constants $f$ and $g$. We fitted the Spitzer time series
using equation 2 and compute the $\chi^{2}$ for different aperture sizes and
binning the data in time with several bin sizes. The combination of aperture
and bin sizes that delivered the lowest $\chi^{2}$ value was used for the
global fit.

The ground based light curves were fitted simultaneously using a common
transit model and the systematic effects of each data set were accounted for
using a linear model with two free parameters dependent on the star position
in the detector (X and Y-axis), a term dependent on the Full Width at Half
Maximum (FWHM) of the PSF as a proxy for seeing variations, and a time-
dependent term to model any time-dependent slope present in the time series.

For the joint fit, the global free parameters for the transit modeling
included the planet-to-star radius ratio
$\mathrm{R}_{\mathrm{p}}/\mathrm{R}_{\star}$, the central time of the transit
$\mathrm{T}_{\mathrm{c}}$, the stellar density $\rho_{\star}$, and the transit
impact parameter $\mathrm{b}$. The quadratic limb darkening (LD) coefficients
$\mathrm{u}_{1}$ and $\mathrm{u}_{2}$ were set free, but during the fit these
values were compared to the predicted coefficients computed for each bandpass
using ldtk555https://github.com/hpparvi/ldtk (Parviainen & Aigrain, 2015).
Ldtk uses the Husser et al. (2013) spectral library to compute custom stellar
limb darkening profiles. For each light curve the predictd quadratic LD
coefficients were computed using the stellar parameters derived in section 3.1
and the fitted coefficients were weighted against these predicted values using
a likelihood function. During the fitting process we converted the LD
coefficients $(\mathrm{u}_{1},\mathrm{u}_{2})$ to the parameterization
proposed by Kipping (2013) $(\mathrm{q}_{1},\mathrm{q}_{2})$ with
$q_{i}\in[0,1]$.

For the TESS time series, besides using an analytical model for the transit we
fitted the stellar variability present in the time series using Gaussian
Processes (GPs; e.g., Rasmussen & Williams, 2010, Gibson et al., 2012,
Ambikasaran et al., 2015). The TESS GPs were computed using the python package
Celerite (Foreman-Mackey et al., 2017); we chose a Matern $3/2$ kernel:

$k_{ij\;\mathrm{TESS}}=c^{2}_{1}\left(1+\frac{\sqrt{3}|t_{i}-t_{j}|}{\tau_{1}}\right)\exp\left(-\frac{\sqrt{3}|t_{i}-t_{j}|}{\tau_{1}}\right)$
(3)

where $|t_{i}-t_{j}|$ is the time between points in the series, $c_{1}$ is the
amplitude of the variability, and $\tau_{1}$ is a characteristic time-scale.
The constants $c_{1}$ and $\tau_{1}$ were set as free parameters.

The radial velocity data was modeled using
RadVel666https://github.com/California-Planet-Search/radvel (Fulton et al.,
2018). The free parameters in the radial velocity fit were the planet induced
radial velocity semi-amplitude ($K_{\mathrm{RV}}$), the host star systemic
velocity ($\gamma_{0}$), the instrumental radial velocity jitter
($\sigma_{\mathrm{RV\;jitter}}$), while the orbital period and central transit
time were also set free but taken to be global parameters in common with the
fits of the light curves. To account for systematic noise present in the
radial velocity time series we use GPs with an exponential squared kernel
(i.e., a Gaussian kernel)

$k_{ij\;\mathrm{RV}}=c^{2}_{2}\exp\left(-\frac{(t_{i}-t_{j})^{2}}{\tau^{2}_{2}}\right)$
(4)

where $t_{i}-t_{j}$ is the time between points in the series, $c_{2}$ is the
amplitude of the exponential squared kernel, and $\tau_{2}$ is a
characteristic time-scale. The constants $c_{2}$ and $\tau_{2}$ were set as
free parameters.

In order to estimate the fitted parameter values we employed a Bayesian
approach. We started the fitting procedure by doing an uninformative transit
search in the TESS time series (from Sector 9 and 10) using Transit Least
Squares (TLS, Hippke & Heller, 2019). From TLS we obtained an estimate of the
period and epoch of the central time of the transit with their respective
uncertainties, we used these values as priors for the joint fit. Then, we
implemented a Markov chain Monte Carlo (MCMC) procedure using emcee (Foreman-
Mackey et al., 2013) to evaluate a likelihood plus a prior function. The
likelihood function was the sum of the log likelihood for each transit time
series and the log likelihood of the radial velocity observations. A total of
38 free parameters were sampled in the joint fit.

The fitting procedure started with a global maximization of the posterior
function using PyDE777https://github.com/hpparvi/PyDE. Once the minimization
converged we launched a burn-in MCMC with 125 chains and 2000 iterations.
After this burn-in stage was finished, the main MCMC ran for 5000 iterations
with the same number of chains as the burn-in stage. The final parameter
values and 1$-\sigma$ uncertainties were determined from the posterior
distributions of the fitted variables: we computed the percentiles of the
distribution corresponding to the median and lower and upper 1-$\sigma$ limits
(from the median) of the distribution for each variable.

A planet in such a short orbital period ($P\sim 1.9$ days) is likely to have
had its orbit circularized over time. However, this is not necessarily true.
There is evidence that Neptune-mass planets with orbital periods of the order
of a few days present non-zero orbital eccentricity (e.g., Kane et al., 2012,
Correia et al., 2020). Thus, we performed two global fits: one assuming a
circular orbit and the other allowed the orbit to have non-zero eccentricity.
For the case of the eccentric orbit we set as global free parameters the
square root of the eccentricity multiplied by the sine of the argument of the
periastron (i.e., $\sqrt{e}\sin(\omega)$ with parameter limits $[-1,1]$) and
the square root of the eccentricity multiplied by the cosine of the argument
of the periastron (i.e., $\sqrt{e}\cos(\omega)$ with parameter limits
$[-1,1]$). Using this parametrization we are sampling values of $e\in[0,1]$
and $\omega\in[0,2\pi]$ (after imposing $e<1$).

Table 4 presents the prior functions and limits used in both global fits. For
the circular orbit case we used an uninformative prior (uniform) for the
stellar density, with $\rho_{\star}\in[5,15]$ g cm-3. For the fit allowing a
non-zero eccentricity, we imposed a more restrictive range of values for the
stellar density, using a normal prior with the density and 2-$\sigma$
uncertainty found by our procedure used to obtain the stellar parameters (see
Section 3.1).

##  4 Results and discussion

The global fit parameter values and 1-$\sigma$ uncertainties are presented in
Table 2. Figures 14 and 15 present the correlation plots of the orbital fitted
parameters excluding the limb darkening coefficients and parameters related to
the red noise. Figures 5 and 6 show all the light curves included in this
study and the radial velocity measurements made by HARPS; the best circular
orbit model fit from our joint modeling is shown in red.

Figure 5: TOI-674b space and ground-based transit light curves after removing
systematic noise affecting the data. The best circular orbit model fit is
represented by the red line and below each panel the residuals of the fit are
shown. Figure 6: TOI-674b HARPS radial velocity measurements and best fitting
circular orbit model including Gaussian processes to model systematic noise
sources (red line). Left: HARPS RV time series measurements. Right: HARPS RV
measurements phase-folded to the orbital period of the planet.The shaded gray
area corresponds to $\pm 1$-$\sigma$ level from the mean of 5000 randomly
selected posterior samples. The RMS of the residuals is $8.25$ m/s. Table 2:
TOI-674b global fit parameters and 1-$\sigma$ uncertainties for the circular
and non-zero eccentricity fits. The prior functions and parameter limits used
in the joint fit are presented in Table 4. The adopted stellar mass and radius
used to derive the absolute planet parameters are the ones presented in the
middle column of Table 1 (i.e., BT-Settl models SED fit).

Fitted orbital parameter | Circular orbit ($e=0$) | Non circular orbit ($e\neq 0)$
---|---|---
$R_{p}/R_{\star}$ | $0.1140\pm 0.0009$ | $0.1143\pm 0.0009$
$T_{\mathrm{c\;BJD}}$ [days] | $2458641.404552\pm 0.000102$ | $2458641.404604\pm 0.000108$
$P$ [days] | $1.977143\pm 0.000003$ | $1.977143\pm 0.000003$
$\rho_{\star}$ [g cm-3] | $10.14\pm 1.04$ | $7.97\pm 0.59$
$b=(a/R_{\star})\cos(i)\left(\frac{1-e^{2}}{1+e\sin(\omega)}\right)$ | $0.624\pm 0.035$ | $0.635\pm 0.030$
$\sqrt{e}\cos(\omega)$ | —- | $0.217\pm 0.153$
$\sqrt{e}\sin(\omega)$ | —- | $0.211\pm 0.089$
$\gamma_{0}$ [m/s] | $13485.51\pm 4.78$ | $13486.59\pm 3.81$
$K$ [m/s] | $21.44\pm 2.94$ | $23.55\pm 3.70$
$\sigma_{\mathrm{RV}}$ [m/s] | $3.40\pm 2.95$ | $3.58\pm 2.85$
Derived orbital parameters |  |
$e$ | —- | $0.10\pm 0.05$
$\omega$ [deg] | —- | $45.4\pm 30.8$
$a/R_{\star}$ | $12.80\pm 0.42$ | $11.81\pm 0.30$
$i$ [deg] | $87.21\pm 0.24$ | $86.68\pm 0.20$
Fitted LD coefficients |  |
$\mathrm{u}_{\mathrm{1\;TESS}}$ | $0.25\pm 0.13$ | $0.24\pm 0.13$
$\mathrm{u}_{\mathrm{2\;TESS}}$ | $0.36\pm 0.19$ | $0.36\pm 0.17$
$\mathrm{u}_{\mathrm{1\;4.5\mu m}}$ | $0.071\pm 0.036$ | $0.073\pm 0.043$
$\mathrm{u}_{\mathrm{2\;4.5\mu m}}$ | $0.15\pm 0.07$ | $0.15\pm 0.07$
$\mathrm{u}_{\mathrm{1\;R_{c}}}$ | $0.29\pm 0.18$ | $0.30\pm 0.17$
$\mathrm{u}_{\mathrm{2\;R_{c}}}$ | $0.42\pm 0.26$ | $0.41\pm 0.24$
$\mathrm{u}_{\mathrm{1\;I+z}}$ | $0.24\pm 0.13$ | $0.23\pm 0.14$
$\mathrm{u}_{\mathrm{2\;I+z}}$ | $0.36\pm 0.19$ | $0.36\pm 0.19$
$\mathrm{u}_{\mathrm{1\;g}}$ | $0.48\pm 0.26$ | $0.48\pm 0.24$
$\mathrm{u}_{\mathrm{2\;g}}$ | $0.38\pm 0.30$ | $0.38\pm 0.29$
$\mathrm{u}_{\mathrm{1\;z}}$ | $0.38\pm 0.13$ | $0.39\pm 0.15$
$\mathrm{u}_{\mathrm{2\;z}}$ | $-0.01\pm 0.19$ | $-0.04\pm 0.21$
TESS GP parameters |  |
$\mathrm{c}_{1}$ | $(2.1\pm 0.4)\times 10^{-4}$ | $(2.1\pm 0.4)\times 10^{-4}$
$\tau_{1}$ [days] | $0.74\pm 0.30$ | $0.75\pm 0.35$
HARPS GP parameters |  |
$\mathrm{c}_{2}$ [m/s] | $4.3^{+9.9}_{-3.2}$ | $3.3^{+5.5}_{-2.4}$
$\tau_{2}$ [days] | $49\pm 31$ | $45\pm 30$
Derived planet parameters |  |
$\mathrm{R}_{\mathrm{p}}$ [$\mathrm{R}_{\oplus}$] | $5.25\pm 0.17$ | $5.26\pm 0.17$
$\mathrm{M}_{\mathrm{p}}$ [$\mathrm{M}_{\oplus}$] | $23.6\pm 3.3$ | $23.5\pm 3.3$
$\rho_{\mathrm{p}}$ [g cm-3] | $0.91\pm 0.15$ | $0.90\pm 0.15$
$\mathrm{g}_{\mathrm{p}}$ [m s-2] | $8.5\pm 1.3$ | $8.4\pm 1.3$
$\mathrm{a}$ [AU] | $0.0250\pm 0.0008$ | $0.0231\pm 0.0007$
$\mathrm{T}_{\mathrm{eq}}$ [K]11footnotemark: $1$ | $635\pm 15$ | $661\pm 14$
$\langle\mathrm{F}_{\mathrm{p}}\rangle$ [105 W/m2] | $0.523\pm 0.039$ | $0.612\pm 0.045$
$\mathrm{S}_{\mathrm{p}}$ [$\mathrm{S}_{\odot}$] | $38.73\pm 2.88$ | $45.48\pm 3.39$
888$\mathrm{T}_{\mathrm{eq}}$ computed assuming an albedo of 0.3.

###  4.1 The orbital eccentricity of TOI-674b

By using a normal prior on the stellar density with the values taken from the
stellar parameters for the host star and setting the eccentricity and argument
of the periastron free, we find that TOI-674b may have a slightly eccentric
orbit with $\mathrm{e}=0.10\pm 0.05$, although this eccentricity value could
be an spurious signal. According to Lucy & Sweeney (1971), an eccentricity
should not be considered significant if it is less than 2.45$\sigma$ from
zero, which our eccentric fit does not achieve. Nonetheless, many Neptune-mass
planets with short orbital periods posses non-circular orbits like HAT-P-11b
(Bakos et al., 2010, Yee et al., 2018), GJ 436b (Gillon et al., 2007, Lanotte
et al., 2014), and TOI-1728b (Kanodia et al., 2020) to name some examples.
Correia et al. (2020) proposed that these non-circular orbits could be
produced by any of several processes alone or in combination like a tidal
torque created by photo-driven evaporation of the planet’s atmosphere and/or
gravitational interaction between the short period planet and another
planetary object in a longer period orbit in the system.

We compared the Bayesian Information Criterion (BIC) statistics of the
circular and eccentric orbit model fits to the data. The BIC takes into
account the number of observed data points $n$, the number of fitted
parameters $k$, and the maximized likelihood $\mathcal{L}_{\mathrm{max}}$. The
BIC is defined by

$\mathrm{BIC}=k\ln{n}-2\ln{\mathcal{L}_{\mathrm{max}}}\;.$ (5)

The model with the lowest BIC value is the one that better fits the data. In
our case
$\Delta\mathrm{BIC}=\mathrm{BIC}_{\mathrm{Ecc}}-\mathrm{BIC}_{\mathrm{Circ}}=44$,
meaning that the circular model fit is preferred using this criterion.

It is worth pointing out that the stellar density $\rho_{\star}$, transit
impact parameter $\mathrm{b}$, eccentricity $\mathrm{e}$, and argument of the
periastron $\omega$ are correlated. In our non-zero eccentricity model fit we
used a normal prior to the stellar density which could have skewed our results
to eccentricity values higher than zero. Another point to consider is that our
radial velocity measurements were made with a rather nonuniform distribution
in orbital phase, in part due to the orbital period being almost an integer
number of days ($P\sim 1.9$ days) making a uniform coverage difficult from a
single observing site. The non-uniform phase coverage contributes to the
difficulty of establishing whether the orbit is slightly eccentric. Additional
RV measurements in different orbital phases and/or observations with higher
precision, for example using VLT/ESPRESSO (Pepe et al., 2010, 2021), could
help to put stronger constraints on the orbital eccentricity of TOI-674b.

On the other hand, the stellar density found by our circular orbit fit using a
uniform prior for this parameter is $\rho_{\star}=10.14\pm 1.04$ g cm-3, a
value that differs from the derived stellar density computed using our adopted
stellar mass and radius ($\rho_{\star}=7.99\pm 0.76$ g cm-3) by $\sim
2\sigma$. This discrepancy could be caused by an underestimation of the
uncertainties of the stellar parameters or an unknown systematic affecting our
fit. The different stellar density values found in the circular and non-
circular orbit fit affected the values of the semi-major axis over stellar
radius ($a/R_{\star}$) and orbital inclination ($i$) (see Table 2) since these
values were derived using the stellar density, impact parameter, eccentricity
and argument of the periastron.

Despite the different values of stellar density from our circular and non-zero
eccentricity results, both cases are in agreement within the 1-$\sigma$
uncertainties (see Table 2). Given the BIC results, for the rest of this work
we will adopt the planetary parameters derived from the circular orbit model
fit.

###  4.2 Transit timing variations

The presence of additional planets in the system could perturb the orbit of
TOI-674b and produce an orbit with non-zero eccentricity. The interactions
between TOI-674b and these hypothetical additional planets in the system could
also affect the central times of the observed transits. We searched for
transit timing variations (TTVs) of TOI-674b by fitting the TESS photometry
from Sector 9 and Sector 10, the Spitzer transit, and the 4 ground-based
transits jointly with PyTTV (Korth, 2020). This translated into 27 individual
transit fits spanning a time baseline of close to $\sim 200$ days. The fitted
model included individual transit centers as free parameters and shared the
remaining geometric and orbital parameters such as impact parameter, radius
ratio, and stellar density. We estimated the model parameter posteriors using
MCMC-sampling (emcee; Foreman-Mackey et al., 2013). The transit centers do not
show significant variations from the linear ephemeris. We show the TTVs in
Fig. 7.

Figure 7: TOI-674b transit timing variations from TESS data (blue) and follow-
up observations (green). The different shades of color of the error bars
represent the 1-$\sigma$ (dark) and 3-$\sigma$ (light) levels of uncertainty
of the measurements. No significant TTVs are detected in a $\sim 200$ days of
baseline covered by the observations.

The photometric measurements do not cover a long enough period ($\sim 200$
days) to allow us to put further constraints on hypothetical additional
planets in the system which could be responsible for an eccentric orbit of
TOI-674b via gravitational interactions. Further monitoring (spectroscopic and
photometric) is therefore desirable.

###  4.3 Mass, radius and composition

We find that TOI-674b has a radius of $5.25\pm 0.17$ $\mathrm{R}_{\oplus}$ and
a mass of $23.6\pm 3.3$ $\mathrm{M}_{\oplus}$. Combining these measurements we
find that TOI-674b has a mean density of $\rho_{\mathrm{p}}=0.91\pm 0.15$ g
cm-3.

We compared the mass and radius of TOI-674b with known exoplanets taken from
the TEPcat database (Southworth, 2011) and the composition models of Zeng et
al. (2019). The planet models consider an isothermal atmosphere and are
truncated at 1 milli-bar pressure level (defining the radius of the planet).
Figure 8 shows the mass-radius diagram for known transiting exoplanets with
masses measured with a precision better than 30% and the rocky planet models
of Zeng et al. (2019) with an equilibrium temperature of 700 K, i.e., the
equilibrium temperature closest to the likely temperature of TOI-674b. The
planet models shown in Fig. 8 are planets with pure iron cores (100% Fe),
earth-like rocky compositions (32.5% Fe plus 67.5% MgSiO3), a 100% water
compositions, and earth-like composition planet cores with 5% H2 gaseous
envelopes. We can see that TOI-674b is located far above all of these model
predictions, suggesting that this planet must possess a very large H/He
envelope.

Figure 8: Mass-radius diagram for planets with mass determinations with a
precision better than 30%. The red star is TOI-674b, the orange points are
planets orbiting M type stars, and the gray points are planets around other
types of stars. The dashed lines represent the mass-radius models of Zeng et
al. (2019): planets with pure iron cores (100% Fe, brown line), earth-like
rocky compositions (32.5% Fe plus 67.5% MgSiO3, black line), 100% water
compositions (blue line), and earth-like composition planet cores with 5% H2
gaseous envelopes (green line).

Comparing the mass and radius of TOI-674b with other planets discovered
orbiting around M type stars, we find that TOI-674b is one of the largest and
most massive super-Neptune class planets discovered around an M-dwarf to date.
The planets orbiting M-dwarfs closest to TOI-674b in the M-R diagram are
TOI-1728b (Kanodia et al., 2020) and TOI-442b (Dreizler et al., 2020).
TOI-1728b orbits an M0V star ($\mathrm{T}_{\mathrm{eff}}=3980$ K) and is a
super-Neptune with $\mathrm{R}_{\mathrm{p}}=5.05\pm 0.17$
$\mathrm{R}_{\oplus}$, $\mathrm{M}_{\mathrm{p}}=26.78\pm 5.43$
$\mathrm{M}_{\oplus}$ and an orbital period of $\sim 3.5$ days
($\mathrm{T}_{\mathrm{eq}}=767$ K). TOI-442b is another Neptune-like planet
orbiting an M0V star with $\mathrm{R}_{\mathrm{p}}=4.7\pm 0.3$
$\mathrm{R}_{\oplus}$, $\mathrm{M}_{\mathrm{p}}=30.8\pm 1.5$
$\mathrm{M}_{\oplus}$, and an orbital period of $\sim 4$ days. TOI-674b,
TOI-1728b and TOI-442b have very similar masses and radii; neither planet has
a very eccentric orbit. All host stars are main-sequence M dwarfs, but the
star TOI-674 is smaller and less luminous than TOI-1728 and TOI-442, therefore
even though its planet has a closer-in orbit, it has a slightly smaller
equilibrium temperature (for the same albedo). The similarity between these
three recent TESS discoveries means that most of the qualitative information
in the Discussion and Summary sections of Kanodia et al. (2020) and Dreizler
et al. (2020) also applies to TOI-674b.

The voluminous envelope of TOI-674b suggests that it has more H2 than could be
obtained from accretion and degassing of solid material (Rogers et al., 2011);
therefore it must have accreted gas directly from its protoplanetary disk.
Accretion of large gaseous envelopes so close to a star requires a very
massive solid core, so this planet very likely formed much farther from its
star and then moved inwards, either via gradual disk migration early in the
system’s history when the protoplanetary disk was still massive or later via
high-eccentricity migration and tidal circularization. Early arrival at its
current orbit would have subjected TOI-674b to a long epoch of substantial
photoionizing radiation, stripping the outer part of the atmosphere; tidal
heating would also have induced substantial mass loss. Thus, TOI-674b probably
formed with a substantially more massive envelope that was partially stripped
away, mostly in the first tens of millions of years after reaching its current
orbit (e.g., check Fulton et al., 2017, Fulton & Petigura, 2018 and references
therein).

###  4.4 TOI-674b and the Neptunian desert

Based on our derived mass and orbital period, TOI-674b is a new resident of
the so-called Neptunian Desert. In Figure 9 we plotted the $\log\mathrm{P}$ vs
$\log\mathrm{M_{p}}$ relationship for known exoplanets; the black lines are
the limits of the Neptunian Desert by Mazeh et al. (2016). TOI-674b is well
within the limits of the desert and in a region sparsely populated in the
orbital period versus radius space.

Figure 9: Mass versus orbital period diagram for planets with mass
measurements. Planets found by the transit method are shown in light cyan
circles, planets found by radial velocity measurements are shown in blue
triangles, and planets found by other methods are shown in gray squares. The
filled black symbols represent planets orbiting around M stars (i.e, stars
with $\mathrm{T}_{\mathrm{eff}}$ in the range 2500-4000 K), each symbol
representing the discovery method as explained before. The black dashed lines
represent the limits of the Neptunian Desert defined by Mazeh et al. (2016).
TOI-674b position in this diagram is marked by the red star.

There are multiple scenarios that explain the origin of the Neptunian desert.
One proposed explanation of the lack of Neptune-sized planets in short orbits
is photo-evaporation of the planet’s H/He envelope due to high energy
radiation coming from the star once the planet arrives to a position close to
its central star after formation (e.g., Owen & Wu, 2013, Lopez & Fortney,
2013, Chen & Rogers, 2016). The lower boundary of the desert in Mazeh et al.
(2016) (i.e., planets with low mass/small radius) is consistent with this
proposed photo-evaporation mechanism. Matsakos & Königl (2016) proposed that
tidal disruption of planets in highly-eccentric orbits (followed by
circularisation of the orbit due to tidal interactions with the host star) can
explain both boundaries of the desert. Owen & Lai (2018) proposed that both
mechanisms are able to explain the Neptunian desert, with photo-evaporation
creating the low mass/small radius and high mass/large radius boundary being
created by the tidal disruption limit for gas giants experiencing high-
eccentric migration. Although TOI-674b is inside the proposed boundary by
Mazeh et al. (2016), it is closer to the low mass/small radius limit
indicating that this planet may have experienced some photo-evaporation of its
upper atmospheric layers due to the high energy radiation of its host star.

It is also worth noting that there are few large planets known to orbit M
stars. Bonfils et al. (2013) estimated the occurrence of planets around M
dwarfs using HARPS data and showed that planets with large masses were not
common around these types of stars. Bonfils et al. (2013) found that the
occurrence for planets with $\mathrm{M}_{\mathrm{p}}\sin(i)$ in the range of
[10, 100] $\mathrm{M}_{\oplus}$ and with orbital periods shorter than 10 days
is $0.03^{+0.04}_{-0.01}$. A similar result was obtained by Dressing &
Charbonneau (2015) using Kepler data to estimate the rate of planets around M
stars. For the largest planet size range studied by Dressing & Charbonneau
(2015), i.e., $\mathrm{R}_{\mathrm{p}}\in[3.5,4.0]$ $\mathrm{R}_{\oplus}$ and
orbital periods shorter than 50 days, Dressing & Charbonneau (2015) find an
occurrence rate of $0.016^{+0.018}_{-0.070}$ planets per M dwarf.

###  4.5 Potential for atmospheric characterisation

Due to the large scale height of its atmosphere and relative brightness of its
host star in near infrared bands, TOI-674b is a promising target for
atmospheric studies. Kempton et al. (2018) proposed a metric for the expected
S/N of transmission spectroscopy with the James Webb Space Telescope (JWST,
Gardner et al., 2006) instrument Near Infrared Imager and Slitless
Spectrograph (NIRISS) for 10 hours of observation time. Following Kempton et
al. (2018), we find a Transmission Spectroscopy Metric (TSM) of $\sim 230$ for
TOI-674b. We compared this TSM value with those of previously discovered
exoplanets that are in the Neptunian Desert (see Figure 10). TOI-674b is one
of the planets belonging to the Neptunian Desert with the highest TSM factor
discovered to date, making it an interesting candidate for atmospheric
characterisation for JWST.

Figure 10: Apparent magnitude in J band versus the Transmission Spectroscopy
Metric (TSM) by Kempton et al. (2018) for TOI-674b and known transiting super-
Earths and Neptune-sized planets inside the Neptunian Desert limits by Mazeh
et al. (2016). The size and color of each circle represents the planet radius
in Earth units.

We computed a synthetic transmission spectrum of TOI-674b using petitRADTRANS
(Mollière et al., 2019). The model spectrum assume solar elemental abundances
and a cloud-free atmosphere with an isothermal temperature profile at a
temperature of 700 K. Figure 11 presents model optical and infrared spectra
potentially observable with JWST for an assumed atmospheric pressure level of
0.01 bar for the continuum. The atmosphere models include molecules such as
H2O, CO2, CO, CH4, and Na and K; Rayleigh scattering from H and He is also
included.

Figure 11 also shows simulated observations made with JWST of the model
spectrum. The simulations were made for the JWST instruments Near Infrared
Imager and Slitless Spectrograph (NIRISS) using Single Object Slitless
Spectroscopy (SOSS) mode (spectral resolution $\mathrm{R}\sim 700$), Near
InfraRed Spectrograph (NIRSpec) using the medium resolution grating G395M
($\mathrm{R}\sim 1000$), and Mid-Infrared Instrument (MIRI) using Low
Resolution Spectroscopy (LRS) mode ($\mathrm{R}\sim 100$). The simulated
observations cover a wavelength range of 0.8-10 $\mu$m.

For the JWST simulated data we used ExoTETHyS.BOATS999https://github.com/ucl-
exoplanets/ExoTETHyS (Morello et al., 2021) to select wavelength bins with
similar number of counts, in order to have comparable error bars from each
point of the spectrum. Using a constant resolution, or a fixed bin width
($\Delta\lambda$), would increase the error bars and the scatter in some
spectral regions by a factor of several units, especially in the MIRI longer
wavelengths. Although the total integrated S/N is independent on the choice of
bins, a higher scatter per point may hinder the detection significance of
broad molecular features (e.g., Tsiaras et al., 2018). Then, we recomputed the
spectra with PandExo (Batalha et al., 2017), which fully accounts for the
specific JWST instrument modes. We noted that the spectra simulated with
ExoTETHyS.BOATS and PandExo are consistent, including their error bars. The
main differences appear to be related with the adopted stellar templates.
Here, we show only the results obtained with PandExo. It is remarkable that
all the various H2O and CH4 features can be well sampled by ¿10 points with
order-of-magnitude smaller error bars than the amplitude of the features (for
a cloud-free atmosphere).

Figure 11: Synthetic transmission spectrum model of TOI-674b (gray line) in
the JWST spectral range computed with petitRADTRANS (Mollière et al., 2019).
The model assumes solar abundance and a cloud-free atmosphere with an
isothermal temperature of 700 K. PandExo (Batalha et al.) simulations of
TOI-674b transmission spectrum observations made with JWST instruments NIRISS
(blue), NIRSpec (orange), and MIRI (red) are also shown.

It should be noted that the cloud-free transmission spectrum presented here is
the most optimistic scenario. Transmission spectroscopy studies done in mostly
hot-Jupiters have shown that many planets posses molecular features with a
lower amplitude than what the cloud-free models predict (e.g., Sing et al.,
2011, Deming et al., 2013, Kreidberg et al., 2014). This lack of molecular
traits is often explained by the presence of clouds and hazes dampening the
strength of the expected features like, for example, the case of water bands
in the near-infrared for hot-Jupiters (Sing et al., 2016). Thus, it would be
preferable to establish if TOI-674b atmosphere is cloud-free with exploratory
transmission spectroscopy observations before JWST is operational. Optical low
resolution observations could detect evidence for Rayleigh scattering and the
broad wings of Na and K lines (e.g., Nikolov et al., 2018), although the
detection of the latter lines would be challenging due to the relative
faintness (for these type of studies) of the planet host star ($V=14.2$ mag).
In the near-infrared it would be possible to use Hubble Space Telescope (HST)
Wide Field Camera 3 (WFC3) to detect the water bands between 1.0-1.7 $\mu$m as
shown by several studies (e.g., Wakeford et al., 2013, Madhusudhan et al.,
2014, Sing et al., 2016 , Tsiaras et al., 2018).

Another interesting prospect for atmosphere characterisation is to search for
evidence of atmospheric escape using individual lines of hydrogen
(Lyman-$\alpha$, 121.5 nm) and helium (He I triplet at 1083 nm). These lines
have been detected on short-period warm Neptunes orbiting around M dwarfs like
for example GJ 436b (Lyman-$\alpha$, Ehrenreich et al., 2015) and GJ 3470b (He
I, Palle et al., 2020).

##  5 Conclusions

We report the discovery and confirm the planetary status of TOI-674b, a super-
Neptune transiting exoplanet orbiting around a nearby M dwarf. NASA’s TESS
observations led to the initial detection of the transits. Follow-up
photometric observations made with several facilities and radial velocity
measurements taken with the HARPS spectrograph made possible confirmation of
the planetary nature of the transiting object and establishment of its mass
and radius.

From a HARPS mean stellar spectrum we estimate that the spectral type of the
host star is M2V and that the star has an effective an effective temperature
of $\mathrm{T}_{\mathrm{eff}}=3514\pm 57$ K, and a stellar mass and radius of
$\mathrm{M}_{\star}=0.420\pm 0.010$ $\mathrm{M}_{\odot}$ and
$\mathrm{R}_{\star}=0.420\pm 0.013$ $\mathrm{R}_{\odot}$ respectively.

We analysed the data from Sector 9 and 10 two minute cadence time series
observations from TESS (PDCSAP curves) plus single transit follow-up
observations from Spitzer Space Telescope ($4.5\;\mu\mathrm{m}$ band), and
ground-based facilities: El Sauce ($\mathrm{R}_{\mathrm{c}}$ filter), LCOGT
(Sloan-g filter), and TRAPPIST-South telescopes ($I+z$, Sloan-z bands). The
single transit observations were detrended before performing a global joint
fit of the data in order to reduce the number of free parameters and speed up
the fitting procedure. The transit observations (TESS time series plus single
transit follow-up observations) and radial velocity measurements from HARPS
were fitted simultaneously using a MCMC procedure that included Gaussian
processes to model the systematic effects present in TESS and RV measurements.
For the joint fit we considered two cases to determine the orbital parameters
of the planet: a circular orbit and a non-zero eccentricity model.

For both fitted models (circular and eccentric model), we find that the
derived planetary mass and radius agree within uncertainties. The non-zero
eccentric solution presents an eccentricity value of $e=0.10\pm 0.05$, hence
this planet could potentially join the previous known short period Neptune-
sized planets with significant eccentricities. Since most of our radial
velocity measurements were made during quadrature, more follow-up observations
can help to further constraint the eccentricity value for this planet.

Using our circular orbit model fit we find that TOI-674b has a radius of
$\mathrm{R}_{\mathrm{p}}=5.25\pm 0.17$ $\mathrm{R}_{\oplus}$ and a mass of
$\mathrm{M}_{\mathrm{p}}=23.6\pm 3.3$ $\mathrm{M}_{\oplus}$, and it orbits its
star with a period of $1.977143\pm 3\times 10^{-6}$ days. We derived a mean
bulk density of $\rho_{\mathrm{p}}=0.91\pm 0.15$ g cm-3, indicating a large
atmosphere/envelope. Comparing the mass and radius of TOI-674b with literature
values for planets discovered orbiting around M type stars, we find that
TOI-674b is one of the largest and most massive super-Neptune class planet
discovered around an M-dwarf to date.

This planet is a new addition to the so-called Neptunian Desert. The
Transmission Spectroscopy Metric (TSM) of TOI-674b is $\sim 240$, one of the
highest TSM for Neptunian Desert planets. Thus, TOI-674b is a promising
candidate for atmospheric characterisation using JWST.

We searched for evidence of another planets in the system by measuring the
transit timing variations using the data from TESS, Spitzer, and ground-based
follow-up observations. We find no significant deviations from a linear
ephemeris for the central time of the transits in a time baseline of $\sim
200$ days. TOI-674 is expected to be observed again by TESS during its
extended campaign. The observations will be done from 7 March 2021 until 2
April 2021 (corresponding to Sector 36); these new TESS observations will help
to refine TOI-674b’s orbital parameters and in the search for additional
planets in the system.

###### Acknowledgements.

N.A.-D. acknowledges the support of FONDECYT 3180063. D. D. acknowledges
support from the TESS Guest Investigator Program grant 80NSSC19K1727 and NASA
Exoplanet Research Program grant 18-2XRP18_2-0136. X. D., T. F., and G. G.
acknowledge funding in the framework of the Investissements d’Avenir program
(ANR-15-IDEX-02), through the funding of the “Origin of Life” project of the
Univ. Grenoble-Alpes. G. M. has received funding from the European Union’s
Horizon 2020 research and innovation programme under the Marie Skłodowska-
Curie grant agreement No. 895525. This paper includes data collected by the
TESS mission, which are publicly available from the Mikulski Archive for Space
Telescopes (MAST). Funding for the TESS mission is provided by NASA’s Science
Mission directorate. Resources supporting this work were provided by the NASA
High-End Computing (HEC) Program through the NASA Advanced Supercomputing
(NAS) Division at Ames Research Center for the production of the SPOC data
products. This research has made use of the Exoplanet Follow-up Observation
Program website, which is operated by the California Institute of Technology,
under contract with the National Aeronautics and Space Administration under
the Exoplanet Exploration Program. The research leading to these results has
received funding from the ARC grant for Concerted Research Actions, financed
by the Wallonia-Brussels Federation. TRAPPIST is funded by the Belgian Fund
for Scientific Research (Fond National de la Recherche Scientifique, FNRS)
under the grant FRFC 2.5.594.09.F, with the participation of the Swiss
National Science Fundation (SNF). MG and EJ are F.R.S.-FNRS Senior Research
Associate. Some of the Observations in the paper made use of the High-
Resolution Imaging instrument Zorro. Zorro was funded by the NASA Exoplanet
Exploration Program and built at the NASA Ames Research Center by Steve B.
Howell, Nic Scott, Elliott P. Horch, and Emmett Quigley. Zorro was mounted on
the Gemini South telescope of the international Gemini Observatory, a program
of NSF’s OIR Lab, which is managed by the Association of Universities for
Research in Astronomy (AURA) under a cooperative agreement with the National
Science Foundation. on behalf of the Gemini partnership: the National Science
Foundation (United States), National Research Council (Canada), Agencia
Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia,
Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia,
Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science
Institute (Republic of Korea). This work was supported by FCT - Fundação para
a Ciência e a Tecnologia through national funds and by FEDER through
COMPETE2020 - Programa Operacional Competitividade e Internacionalização by
these grants: UID/FIS/04434/2019; UIDB/04434/2020; UIDP/04434/2020; PTDC/FIS-
AST/32113/2017 & POCI-01-0145-FEDER-032113; PTDC/FISAST/28953/2017 &
POCI-01-0145-FEDER-028953. This work made use of tpfplotter by J. Lillo-Box
(publicly available in www.github.com/jlillo/tpfplotter), which also made use
of the python packages astropy, lightkurve, matplotlib and numpy. This work
makes use of observations from the Las Cumbres Observatory Global Telescope
Network. This work made use of corner.py by Daniel Foreman-Mackey (Foreman-
Mackey, 2016).

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##  Appendix A Spectral energy distribution of TOI-674

Figure 12: Spectral energy distribution of TOI-674. Red symbols represent the
observed photometric measurements, where the horizontal bars represent the
effective width of the passband. Blue symbols are the model fluxes from the
best-fit NextGen atmosphere model (black).

##  Appendix B Spectroscopic data measurements

Table 3: HARPS radial velocity time series and spectroscopic activity indicators for TOI-674. BJD [-245800] | RV [ms-1] |  $\sigma_{\mathrm{RV}}$ [ms-1] | $\mathrm{H}_{\alpha}$ | $\sigma_{\mathrm{H_{\alpha}}}$ | $\mathrm{H}_{\beta}$ | $\sigma_{\mathrm{H_{\beta}}}$ | $\mathrm{H}_{\gamma}$ | $\sigma_{\mathrm{H_{\gamma}}}$ | NaD | $\sigma_{\mathrm{NaD}}$ | S | $\sigma_{\mathrm{S}}$
---|---|---|---|---|---|---|---|---|---|---|---|---
8627.630457 | 13481.91 | 10.85 | 0.0692 | 0.0014 | 0.0521 | 0.0037 | 0.1076 | 0.0130 | 0.0059 | 0.0021 | 0.4891 | 0.6417
8636.597318 | 13493.11 | 04.75 | 0.0675 | 0.0006 | 0.0495 | 0.0017 | 0.1085 | 0.0064 | 0.0076 | 0.0008 | 1.1554 | 0.3996
8637.628554 | 13470.93 | 06.80 | 0.0670 | 0.0008 | 0.0550 | 0.0028 | 0.1026 | 0.0106 | 0.0075 | 0.0012 | 0.7150 | 0.5353
8638.635832 | 13491.23 | 14.92 | 0.0668 | 0.0018 | 0.0503 | 0.0063 | 0.1001 | 0.0204 | 0.0069 | 0.0030 | 1.3946 | 0.8684
8639.622186 | 13476.06 | 11.65 | 0.0663 | 0.0014 | 0.0486 | 0.0050 | 0.1009 | 0.0161 | 0.0087 | 0.0022 | 0.4529 | 0.7547
8640.626096 | 13490.04 | 07.86 | 0.0679 | 0.0010 | 0.0452 | 0.0031 | 0.0981 | 0.0129 | 0.0081 | 0.0014 | 0.8246 | 0.6512
8641.611199 | 13479.48 | 10.45 | 0.0664 | 0.0013 | 0.0477 | 0.0044 | 0.1143 | 0.0192 | 0.0061 | 0.0020 | 1.1750 | 1.1082
8642.605629 | 13497.19 | 06.66 | 0.0677 | 0.0008 | 0.0517 | 0.0030 | 0.1134 | 0.0137 | 0.0083 | 0.0012 | 0.9768 | 0.6242
8643.595060 | 13483.99 | 06.48 | 0.0677 | 0.0008 | 0.0507 | 0.0026 | 0.1185 | 0.0103 | 0.0082 | 0.0011 | 0.5704 | 0.5792
8644.544065 | 13493.96 | 04.58 | 0.0667 | 0.0006 | 0.0524 | 0.0017 | 0.1264 | 0.0066 | 0.0088 | 0.0007 | 1.0437 | 0.4021
8645.519317 | 13462.41 | 08.60 | 0.0696 | 0.0011 | 0.0547 | 0.0030 | 0.1080 | 0.0099 | 0.0098 | 0.0016 | 1.0369 | 0.4539
8657.546337 | 13469.92 | 05.71 | 0.0690 | 0.0007 | 0.0602 | 0.0023 | 0.1071 | 0.0077 | 0.0092 | 0.0010 | 0.9712 | 0.4522
8660.523300 | 13490.75 | 09.88 | 0.0675 | 0.0012 | 0.0539 | 0.0037 | 0.0863 | 0.0125 | 0.0075 | 0.0019 | 1.0510 | 0.7247
8670.508395 | 13518.13 | 06.02 | 0.0692 | 0.0008 | 0.0500 | 0.0023 | 0.1152 | 0.0084 | 0.0072 | 0.0010 | 0.6041 | 0.3814
8676.515917 | 13523.37 | 09.43 | 0.0674 | 0.0012 | 0.0455 | 0.0034 | 0.1219 | 0.0121 | 0.0081 | 0.0018 | 1.0161 | 0.4858
8679.482254 | 13456.72 | 07.26 | 0.0680 | 0.0008 | 0.0623 | 0.0035 | 0.0900 | 0.0115 | 0.0095 | 0.0013 | 0.7379 | 0.4551
8682.501971 | 13504.79 | 04.83 | 0.0670 | 0.0006 | 0.0533 | 0.0018 | 0.1120 | 0.0065 | 0.0090 | 0.0008 | 0.8569 | 0.3134

Figure 13: GLS periodograms for the activity indices obtained from 17 HARPS
spectra. The period of the planet candidate is shown by the vertical green
line.

##  Appendix C Light curve and radial velocity joint fit

Table 4: Global fit parameters prior functions and limits; $\mathcal{U}$,
$\mathcal{N}$, and $\mathcal{J}$ represent Uniform, Normal, and Jeffreys prior
functions respectively.

Fitted parameter | Ccircular orbit ($e=0$) | Non-circular orbit ($e\neq 0$)
---|---|---
$R_{\mathrm{p}}/R_{\star}$ | $\mathcal{U}(0.05,0.35)$ | $\mathcal{U}(0.05,0.35)$
$T_{\mathrm{c\;BJD}}$ [days] | $\mathcal{U}(2458640.9,2458641.9)$ | $\mathcal{U}(2458640.9,2458641.9)$
$P$ [days] | $\mathcal{U}(1.7,2.1)$ | $\mathcal{U}(1.7,2.1)$
$\rho_{\star}$ [g cm-3] | $\mathcal{U}(5,15)$ | $\mathcal{N}(7.99,1.5)$
$b=(a/R_{\star})\cos(i)\left(\frac{1-e^{2}}{1+e\sin(\omega)}\right)$ | $\mathcal{U}(0,1)$ | $\mathcal{U}(0,1)$
$\sqrt{e}\cos(\omega)$ |  | $\mathcal{U}(-1,1)$
$\sqrt{e}\sin(\omega)$ |  | $\mathcal{U}(-1,1)$
$\gamma_{0}$ [m/s] | $\mathcal{U}(-10^{5},10^{5})$ | $\mathcal{U}(-10^{5},10^{5})$
$K_{\mathrm{RV}}$ [m/s] | $\mathcal{U}(0,110)$ | $\mathcal{U}(0,110)$
$\sigma_{\mathrm{RV\;jitter}}$ [m/s] | $\mathcal{U}(0,10)$ | $\mathcal{U}(0,10)$
LD coefficients |  |
$q_{1}=(u_{1}+u_{2})^{2}$ | $\mathcal{U}(0,1)$ | $\mathcal{U}(0,1)$
$q_{2}=0.5u_{1}/(u_{1}+u_{2})$ | $\mathcal{U}(0,1)$ | $\mathcal{U}(0,1)$
TESS GP parameters |  |
$\mathrm{c}_{1}$ | $\mathcal{J}(10^{-4},100)$ | $\mathcal{J}(10^{-4},100)$
$\tau_{1}$ [days] | $\mathcal{J}(10^{-4},200)$ | $\mathcal{J}(10^{-4},200)$
HARPS GP parameters |  |
$\mathrm{c}_{2}$ [m/s] | $\mathcal{U}(0,100)$ | $\mathcal{U}(0,100)$
$\tau_{2}$ [days] | $\mathcal{U}(10^{-3},80)$ | $\mathcal{U}(10^{-3},80)$
101010We also imposed $e<1$ for the non-circular orbit fit.

Figure 14: Correlation plot for the orbital parameters fitted using a circular
orbit model. Limb darkening coefficients and systematic effect parameters were
left out intentionally for easy viewing. The blue lines mark the median values
of the parameters. Figure 15: Correlation plot for the orbital parameters
fitted using a non-zero eccentricity orbit model. Limb darkening coefficients
and systematic effect parameters were left out intentionally for easy viewing.
The blue lines mark the median values of the parameters. Figure 16: TESS
Sector 9 and Sector 10 two minute cadence light curves of TOI-674. Top panel:
TESS PDCSAP flux time series and best circular orbit model from the joint fit.
Middle panel: TESS flux and transit model after removing stellar
variability.Bottom panel: residuals of the fit.