# Hyperion: The origin of the stars
A far-UV space telescope for high-resolution spectroscopy over wide fields

Erika T. Hamden  David Schiminovich  Department of Astronomy, Columbia
University, 550 W 120th St, New York, NY 10025, USA  Shouleh Nikzad  Jet
Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109,
USA  Neal J. Turner  Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, CA 91109, USA  Blakesley Burkhart  Department of Physics
and Astronomy, Rutgers University, 136 Frelinghuysen Rd, Piscataway, NJ 08854,
USA  Center for Computational Astrophysics, Flatiron Institute, 162 Fifth
Avenue, New York, NY 10010, USA  Thomas J. Haworth  Astronomy Unit, School of
Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
Keri Hoadley  Department of Physics & Astronomy, University of Iowa, 203 Van
Allen Hall, Iowa City, IA, 52242, USA  Jinyoung Serena Kim  Steward
Observatory, Department of Astronomy, University of Arizona, 933 N Cherry Ave,
Tucson AZ 85719  Shmuel Bialy  University of Maryland College Park, College
Park, MD, US  Geoff Bryden  Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, CA 91109, USA  Haeun Chung  Steward Observatory,
Department of Astronomy, University of Arizona, 933 N Cherry Ave, Tucson AZ
85719  Nia Imara  University of California, Santa Cruz, 1156 High Street,
Santa Cruz, CA 95064  Rob Kennicutt  Steward Observatory, Department of
Astronomy, University of Arizona, 933 N Cherry Ave, Tucson AZ 85719  Jorge
Pineda  Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA 91109, USA  Shuo Kong  Steward Observatory, Department of
Astronomy, University of Arizona, 933 N Cherry Ave, Tucson AZ 85719  Yasuhiro
Hasegawa  Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA 91109, USA  Ilaria Pascucci  Lunar and Planetary Laboratory,
University of Arizona, 933 N Cherry Ave, Tucson AZ 85719  Benjamin Godard
Paris Observatory, 77, Avenue Denfert-Rochereau, 75014 Paris, France  Mark
Krumholz  Australian National University, Canberra ACT 2600 Australia  Min-
Young Lee  Korea Astronomical and Space Sciences Institute, 776 Daedeok-daero,
Yuseong-gu, Daejeon 34055, Republic of Korea  Daniel Seifried  I. Physical
Institute, University of Cologne, Zülpicher Str. 77, 50937 Cologne, Germany
Amiel Sternberg  Tel Aviv University, P.O. Box 39040, Tel Aviv 6997801, Israel
Stefanie Walch  I. Physical Institute, University of Cologne, Zülpicher Str.
77, 50937 Cologne, Germany  Miles Smith  Jet Propulsion Laboratory, California
Institute of Technology, Pasadena, CA 91109, USA  Stephen C. Unwin  Jet
Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109,
USA  Elizabeth Luthman  Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, CA 91109, USA  Alina Kiessling  Jet Propulsion
Laboratory, California Institute of Technology, Pasadena, CA 91109, USA  James
P. McGuire  Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA 91109, USA  Mina Rais-Zadeh  Jet Propulsion Laboratory,
California Institute of Technology, Pasadena, CA 91109, USA  Michael Hoenk
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA
91109, USA  Thomas Pavlak  Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, CA 91109, USA  Carlos Vargas  Steward Observatory,
Department of Astronomy, University of Arizona, 933 N Cherry Ave, Tucson AZ
85719  Daewook Kim  Steward Observatory, Department of Astronomy, University
of Arizona, 933 N Cherry Ave, Tucson AZ 85719  Wyant College of Optical
Sciences, University of Arizona, Tucson AZ 85721

###### Abstract

We present Hyperion, a mission concept recently proposed to the December 2021
NASA Medium Explorer announcement of opportunity. Hyperion explores the
formation and destruction of molecular clouds and planet-forming disks in
nearby star-forming regions of the Milky Way. It does this using long-slit,
high-resolution spectroscopy of emission from fluorescing molecular hydrogen,
which is a powerful far-ultraviolet (FUV) diagnostic. Molecular hydrogen (H2 )
is the most abundant molecule in the universe and a key ingredient for star
and planet formation, but is typically not observed directly because its
symmetric atomic structure and lack of a dipole moment mean there are no
spectral lines at visible wavelengths and few in the infrared. Hyperion uses
molecular hydrogen’s wealth of FUV emission lines to achieve three science
objectives: (1) determining how star formation is related to molecular
hydrogen formation and destruction at the boundaries of molecular clouds; (2)
determining how quickly and by what process massive star feedback disperses
molecular clouds; and (3) determining the mechanism driving the evolution of
planet-forming disks around young solar-analog stars. Hyperion conducts this
science using a straightforward, highly-efficient, single-channel instrument
design. Hyperion’s instrument consists of a 48 cm primary mirror, with an f/5
focal ratio. The spectrometer has two modes, both covering 138.5-161.5 nm
bandpasses. A low resolution mode has a spectral resolution of R$\geq$10,000
with a slit length of 65 arcmin, while the high resolution mode has a spectral
resolution of R$\geq$50,000 over a slit length of 5 armin. Hyperion occupies a
2 week long, high-earth, Lunar resonance TESS-like orbit, and conducts 2 weeks
of planned observations per orbit, with time for downlinks and calibrations.
Hyperion was reviewed as Category I, which is the highest rating possible, but
was not selected.

###### keywords:

Ultraviolet astronomy, Spectroscopy, Telescopes, Ultraviolet Spectroscopy

*Erika T. Hamden<EMAIL_ADDRESS>
##  1 Introduction

Hyperion is a FUV spectroscopic mission which was recently proposed to the
December 2021 NASA Medium Explorer (MIDEX) opportunity. Hyperion seeks to
answer three fundamental questions about our galaxy and how stars and planets
form within it. First, what is the relationship between the formation and
destruction of molecular hydrogen in molecular clouds and the star formation
rate of those clouds? Second, how quickly and by which mechanisms do massive
stars disperse molecular clouds? Third, what is the driving mechanism for disk
dispersal around young, solar-analog stars? Our current understanding of star
formation, the impact of feedback on molecular clouds and galaxies, and the
structure and lifetimes of protoplanetary disks are all limited by our
inability to directly sense and measure the most dominant component of clouds
and disks, molecular hydrogen (H2 ).

H2 is the most abundant molecule in the universe and is a critical component
in the life-cycle of galaxies, stars, and planets. Despite its prevalence,
direct observations of H2 are not the standard method of observing molecular
clouds or gas rich disks. H2 , a symmetric molecule, has no dipole moment, and
no lines accessible in the rest-frame visible. It does not radiate efficiently
when cold because of the lack of dipole. H2 does have significant sets of
lines that can be excited via fluorescence. These lines are visible in both
the infrared (IR) and UV. IR ro-vibrational lines are typically weak, making
them challenging (though not impossible) to observe from the ground. UV-pumped
H2 , which is the primary observational tracer for Hyperion’s mission, have
fluxes directly proportional to the incident exciting UV flux
($F(H_{2})_{UV}\sim B_{mn}F(\lambda_{UV})$, where $B_{mn}$ is the branching
ratio of the H2 UV transition from the excited electronic band energy $m$ and
its decayed energy level in the ground electronic band, $n$), whereas
subsequent IR transitions will branch out further, leading to additional
losses in line flux over the IR for a given UV emission line
($F(H_{2})_{IR}\sim zF({\rm H_{2}})_{UV}$[1], where $z$ is a constant
($\lesssim$0.35) and in the absence of additional contributions to the IR
emission line, like collisional effects).

In addition, IR ro-vibrational lines often require special conditions to
arise, either in the way that H2 is heated to represent higher thermal states
(e.g., shocks) or how hh emits IR photons (e.g., collisional decay). In most
situations where IR-H2 lines are observed, the role of UV-pumping is just one
of several possible pumping mechanisms, which makes interpretation of the
initial conditions which initiate the H2 fluorescence process challenging[2,
3]. Indeed, Ref. 3 shows that, even in star-forming regions where IR-H2 level
populations show the least deviation from pure UV excitation, they always
measure a notable deviation from a purely UV excitation population of H2 (most
likely due to collisions).

Combined, IR ro-vibration emission requires special boundary and excitation
conditions, whereas UV emission probes nominal conditions along PDR and
molecular cloud boundaries subject to UV irradiation from nearby, newly formed
(OB) stars. A great example comes from the recent image of NGC 3324 from
_JWST_. Ref. 4 dive into the NIRCAM observations of this famous star-forming
region, revealing structures illuminated by IR H2 ro-vibrational emission. Of
note, however, is that IR H2 emission is observed exclusively in outflows and
shocks littering the region. The PDR itself, and of note the molecular cloud
bound exhibiting streaming gas from the molecular cloud boundary in the form
of hydrogen Pa-$\alpha$ emission, is devoid of IR H2 emission. This type of
boundary represents a typical cloud boundary where Hyperion is uniquely able
to detect coincident UV H2 features, alongside the other evaporating gas at
the molecular cloud boundary, that IR H2 does not appear.

We note that, while the advent of JWST now offers exquisite sensitivity and
provides the first near- to mid-IR observatory with the power to uncover IR H2
in ways previously unobtainable, the spectral resolution of JWST’s instrument
suite makes it inadequate to perform the necessary observations to derive
critical physical conditions about the gas that make up Hyperion’s suite of
science goals. Previous PDR studies using IR H2 have required high spectral
resolution instruments[3], just as Hyperion’s UV H2 measurements require high
spectral resolution.

The UV lines span both the extreme UV (EUV) and the FUV, but have been
observed by a range of space telescopes for limited sightlines. For large area
surveys, astronomers have typically used other, less abundant, molecular
tracers that are easily accessible and bright in the near IR and millimeter
(NIR; i.e. CO, HCN, dust). CO and other tracer emission is then converted to
H2 mass using a scale factor (e.g., XCO, [5, 6]). These conversion factors can
vary but are not necessarily universal on all scales, yielding large
uncertainties in mass [7].

Astronomers have known about the importance of H2 for nearly 100 years. H2 in
the inter-stellar medium (ISM) was first postulated in 1934 [8], but not
detected [9] until the first UV sounding rockets in the late 1960s [10, 11]
which yielded the first absorption-line measurements in dense interstellar
regions [12]. Since then, many other observations have confirmed the presence
of H2 along sightlines throughout the galaxy, both in absorption [13, 14, 15,
16, 17] and emission. Models predicting UV emission from fluorescing H2 [18,
19, 20] were first tested in the laboratory [21] before being observed from an
astrophysical object. H2 was then detected in emission towards the reflection
nebulae IC63 by IUE [22] and in the diffuse ISM with the Berkeley UVX Shuttle
Spectrometer [23], followed by subsequent Shuttle- and rocket-based studies.
Additional studies with FUSE [24] and others [25, 26, 27] have detected H2 in
a range of targets. Most recently, FIMS/SPEAR revealed galaxy-wide H2
fluorescence [28]. While these data provide confidence that H2 fluorescence is
prevalent throughout most nearby molecular clouds, the existing data sets are
not sufficient in spectral or spatial resolution nor in area coverage to
complete Hyperion’s objectives.

One of the technical challenges of observing H2 in the UV is that most UV
missions have instruments which are count rate limited, and have a maximum
brightness cutoff above which the detector will saturate. The effect of this
limit is that observations of regions nearby to the UV bright stars that
stimulate the brightest fluorescence are not possible. Hyperion is well-suited
to covering large areas, for which the observing time metric is inversely
proportional to the product of the efficiency, aperture area, and the slit
area. By comparison, the only UV spectrographs operating today (HST/STIS and
HST/COS), which were optimized for observing point sources and individual
sightlines, show that Hyperion has a comparable efficiency, smaller primary
aperture area, but significantly larger slit. This means that Hyperion’s
planned large area survey of the brightest star forming regions is not
possible with existing telescopes. A recently flight-tested, high-performance
UV solid state detector technology, delta doping [29], applied to CCDs with
low noise and large full well capacity creates a high efficiency, high dynamic
range solution that eliminates the problem of count rate limits of bright
objects. Future UV mission such as Hyperion can now observe the brightest
parts of nearby star forming regions to capture H2 fluorescence, benefiting
from the large detector dynamic range.

Hyperion conducts its observations with a simple single channel instrument: a
high-resolution FUV spectrograph. Hyperion has a 48-cm two-mirror Cassegrain
telescope, with two additional optics to provide field correction at the edges
of the field. Hyperion has a single, very long slit (70 arcmin), with a
stepped geometry configuration. The slit center is 2 arcseconds wide, giving a
resolution R$>$50,000 over 5 arcmin, while the rest of the slit is 10
arcseconds wide, with a resolution of R$>$10,000 over a 65 arcmin field. The
current best estimate (CBE) of angular resolution along the slit is $<$4
arcseconds. The spectrograph is an Offner relay design, and includes a folding
mirror just before the detector to reduce the impact of radiation on the
detector.

Hyperion’s overall percentage throughput is similar to HST STIS, but
Hyperion’s large field-of-view lets it cover hundreds of square degrees of
molecular clouds in an 18 month survey mission. Achieving similar coverage
with HST would take $>$40,000 years (STIS high-resolution aperture: 0.034′′ ×
28′′ = 0.952 sq′′ vs. Hyperion narrow aperture: 2′′ × 300′′ = 600 sq′′;
Hyperion wide aperture: 10′′ × 3900′′= 39000 sq′′).

Moreover, the sensitivity of Hyperion itself is two orders of magnitude higher
than HST STIS. The emission line sensitivity of Hyperion with wide aperture
over a 10′′$\times$ 300 ′′area in 2,600 sec is 1$\times$10-17 erg s-1 cm-2
arcsec-2 (MEV). In contrast, the emission line sensitivity of HST STIS over
0.034′′ × 28′′ area in 2,600 sec is 1.77$\times$10-15 erg s-1 cm-2 arcsec-2.
These differences are primarily due to the small aperture of HST STIS. In
addition, HST has strict count rate limits that eliminate observations of the
UV bright stars and dynamic molecular regions that Hyperion can observe using
the high dynamic range detector.

The Hyperion mission was proposed in 2019 as a Small Explorer (SMEX; Choi et
al., 2021 30). The current version of Hyperion was re-formulated for the 2021
Medium Explorer call. The changes include an updated optical design, using
only a single grating instead of an additional cross disperser, a larger FPA
comprising a 4 detector linear mosaic vs. a single detector, a slightly larger
aperture (40 vs. 48 cm), and a different orbit (TESS-like for MIDEX, LEO for
SMEX). The MIDEX selections occurred in mid-August, 2022 and while Hyperion
was reviewed as a Category I mission, the highest rating possible, it was not
selected.

In this paper, we provide a brief overview of the physics of H2 fluorescence
(Section 1.2.1) and how it can be used to measure physical rates and incident
radiation, describe Hyperion’s Science Objectives in detail (SOs, Section 2),
and the Hyperion instrument implementation (Section 3). Section 5 describes
the Hyperion survey strategy over its 18 month mission while Section 7
describes how the Hyperion instrument can be used for science beyond its
specific objectives.

###  1.1 Critical science questions that can only be answered in the UV

Hyperion addresses several foundational science questions that currently can’t
be answered with existing data. Hyperion is specifically designed to fill both
the holes in our current knowledge of molecular clouds and disks and address a
missing observational capability.

Hyperion will determine how star formation relates to the flow of gas into and
out of molecular clouds; how massive stars disperse such clouds to end star
formation; and how the gas is removed from the planet-forming disks around
young stars like our Sun. Hyperion does this by focusing on the atomic-to-
molecular transition, which acts as a boundary layer or envelope for a
molecular cloud or planet forming disk. This transition layer is the interface
between the nursery-like environment within the H2 region and the harsh,
radiation-filled outside of a cloud or disk. By observing this layer, we
determine the evolutionary states of star-forming molecular clouds, the
interior of bubbles blown within molecular clouds by young stars, and in the
winds of gas rich planet-forming disks.

Hyperion first addresses the life cycle of H2 molecules and contrasts that
with the life cycle of molecular clouds. Is the formation of H2 a critical and
necessary step in the path to star formation, or is it just a side effect of
the actual evolutionary path? This is discussed in Sec. 2.1. Hyperion also
explores the end of a molecular clouds life, as it is dispersed via various
feedback mechanisms, in Sec. 2.2. Finally, Hyperion explores the molecular
content of planet forming disks in Sec. 2.3.

###  1.2 The diagnostic power of H2 fluorescence

The key innovation to answering Hyperion’s science questions is to use the FUV
fluorescence of H2 , which provides diagnostic details on the state of the
gas, formation and destruction rates, energy source, and other key pieces of
information.

H2 consists of two hydrogen atoms held together by a covalent bond. In the
ISM, H2 formation occurs on dust grains, a process first proposed by Ref. 31
and calculated by Ref. 32. Formation occurs from catalytic reactions on the
dust surface, and dust and H2 are commonly found together in the ISM. H2
itself is non-polar, a homonuclear and symmetric molecule with no dipole
moment [33]. The lack of a permanent diople means that ro-vibrational
transitions are forbidden and only weak electric-quadrupole transitions are
allowed [34]. This causes the “unfavorable location of its spectrum”, from the
perspective of Ref. 35 and many astronomers since. Despite this negative
attitude, H2 in fact has a rich spectrum of emission lines in the UV and IR
that are excited by fluorescence, typically from an external UV source,
exciting the Lyman and Werner bands of H2 . Some emission in the IR can also
be excited by collision excitation, which makes analysis of IR H2 emission
fairly complex. This confusion is not present in the UV because these
collisions don’t have enough energy to stimulate the UV bands.

####  1.2.1 Physics of H2 photodissociation and H2 fluorescence

Figure 1: Energy diagram for H2 molecule, with an example excited band

Fluorescence occurs when an H2 molecule is excited from its ground electronic
band (X${}^{1}\Sigma_{g}^{+}$) into an excited band (for example, Lyman, the
first excited band, B${}^{1}\Sigma_{u}^{+}$, up arrow in Figure 1). When this
happens, the molecule will decay back down to the ground level via one of many
pathways (blue down arrows), producing a dense line spectrum whose exact
wavelengths depend on which sub- states within the electronic bands are
involved; each de-excitation carries a 15% chance of dissociating the H2
molecule entirely (dissociation continuum, purple down arrow). Subsequent
cascades in the ground band release IR photons (red arrows). Excitation into
the Werner band, (C${}^{1}\Pi_{\mu}$) creates a different set of emission
lines following the same process [36, 37].

The exact lines that are excited by H2 fluorescence in the ultraviolet depend
strongly on the energy source [36]. This means H2 fluorescence also provides
information on the excitation source itself and can be distinguished by
different excited progressions. In addition, the overall level of fluorescent
emission is related to the incident FUV radiation field at the H2 surface. In
the Solar neighborhood, the FUV (integrated over 6-13.6 eV) interstellar
radiation field has a flux of $F_{0}=2.7\times 10^{-3}$ erg cm-2 s-1 [38, 39,
40]. As we will be exploring clouds of various properties, we use the
normalized FUV intensity, $I_{\rm UV}\equiv F/F_{0}$111In these units, Ref.
41’s estimate of the interstellar radiation field is equivalent to 0.6, and
Ref. 38’s field has $I_{\rm UV}=1$ to characterize the field intensity that
illuminates the cloud.

Because both processes of the FUV line emission and the H2 photodissociation
are initiated by the same process, namely, the H2 photo-pumping, the H2
photodissociation rate and the summed intensity of all H2 emission lines are
proportional to each other (e.g., see Eq. 10 in Ref. 1; see also Ref. 42 for
an analogous derivation for the case of cosmic-ray excitation). However,
because the emission lines are affected by dust extinction, the
proportionality between the total line emission power and the
photodissociation rate is only exact in optically thin regions, whereas in
optically thick gas, one must correct for extinction by dust. Thus, given an
estimate of the dust optical depth, a measurement of the H2 FUV spectrum may
be used to estimate the H2 photodissociation rate. An elaborated discussion
and numerical tests are presented in Bialy et al. 2022 (in prep.), and in
Burkhart et al. 2022 (in prep.).

####  1.2.2 Formation rates via ortho-to-para lines

A key signature found in the H2 UV-fluorescence emission lines is the ortho-
to-para ratio (OPR). In the ground state, ortho molecules have spins in the
same direction (total nuclear spin=1, only odd rotational quantum numbers j),
while para molecules have spins in opposite directions (zero total nuclear
spin, only even j states). These different states have subtly different
allowed transitions for fluorescence.

The imprint of H2 formation in the cloud, and how far out of equilibrium H2
formation may have occurred, is traced by the ratio of the H2 fluorescent
ortho-para lines, which can be used to infer the OPR in the illuminated gas.
The OPR measurement works as a chemical clock because H2 forms on dust grains
with an OPR near 3, but this value is far out of thermal equilibrium. Over
millions of years, slow collisional processes lead this elevated OPR to decay
down to the thermal equilibrium value of 0.001 for low temperature gas [43,
44]; measuring the OPR therefore reveals how long ago a particular population
of molecules formed. The exact decay rate depends on ionization, which is well
understood for cosmic rays [45] and modeled for other sources.

For equilibrium conditions in a photodissociation region (PDR), the
equilibrium OPR in the ground vibrational state is set by the competition
between the reactive collisions with protons, the H2 formation process, and
selective photodissociation via optically thick (“self-shielded”) absorption
lines (46, see Eqs. 4-7). A low OPR provides no ambiguity in the state of the
gas; measuring a low OPR is a sure sign of older, colder H2 gas. A high OPR
can be the result of two different processes. One is a warm PDR that has
already reached chemical equilibrium, while the other is cold, young H2 gas
that has not yet reached chemical equilibrium and has a high OPR due to the
formation process on dust grains. A way to distinguish between the two is via
a temperature measurement, which can be self-consistently estimated using the
H2 fluorescent emission. The temperature (in and out of equilibrium) will be
modeled using the Meudon PDR code [47]. Hyperion will incidentally collect
absorption line measurements in the process of mapping target clouds. This can
provide measurements of cold H2 through parts of the cloud, for which the
chemical equilibration time is unambiguously long and therefore the OPR
measurement can be straightforwardly interpreted as chemical age.

##  2 Science Motivation of the Hyperion Mission Concept

###  2.1 Formation: Understanding molecular cloud formation and evolution

Most interstellar gas is in the form of atomic hydrogen (HI) however
ultimately it is the molecular gas, predominately in the form of H2 , that
fuels the formation of stars. For example, the seminal results of Ref. 48 have
shown that HI is largely not observed to correlate with star formation and the
strong correlation of the star formation rate surface density with gas surface
density only begins as column densities greater than the atomic to molecular
transition [1, 49, 50, 51, 52, 53, 54, 55]. This may imply that the transition
between the atomic to molecular phase is important for the formation of stars
or it may simply be that the formation of molecular gas a natural step on the
pathway for gas to become dense and cold, a pathway which is also necessary
for star formation [56, 57, 58, 59, 60]. Ultimately, understanding how the
lifecycle of molecular gas connects to star formation necessitates
observations that capture the HI to H2 transition process in action [61].

With present telescopes and coverage of the electromagnetic spectrum, studying
the HI-H2 boundary layer can only be done on individual sight-lines or
indirectly using tracers other than H2 itself, e.g. CO [62] and dust [63].
Hyperion provides exactly this measurement, using the unique diagnostic of H2
fluorescence which, in an analogous manner to H$\alpha$ emission in
photoionized HII regions, provides a direct tracer of the process of H2
dissociation at the atomic to molecular boundary and phase transition mass
flux which determines if the boundary is advancing or receding.

Tracking the formation and destruction of H2 allows us to determine both how
clouds are assembled and how they are dispersed, tracing out key steps in the
cloud-to-star life cycle [64, 65, 66]. This opens up the possibility to answer
long-standing questions in galaxy and molecular cloud formation that, until
Hyperion, have largely been the purview of numerical experiments: How does gas
in galaxies flow from diffuse-atomic to dense-molecular gas? What sets the
size and distribution of molecular clouds in galaxies? Is the dominant process
for molecular cloud formation from large-scale disk gravitational instability
or via phase transitions driven by small scale local compression and/or via
cloud collisions into spiral arms? How does this cycle relate to larger scale
galactic and intergalactic environments?

Furthermore, while the formation of H2 represents the first step to star
formation, it is unclear how the two processes are linked. Do molecular clouds
evolve in distinct phases, first accumulating molecules, then forming stars,
and then dissociating because of stellar feedback, such that the lifetime of
an individual H2 molecule is comparable to that of a MC? Or do molecular
clouds spend most of their 20–30 Myr lives in a steady state, with H2
formation balancing mass loss by star formation and H2 destruction with
individual H2 molecules living much less time than molecular clouds (e.g.
undergo many cycles of formation and dissociation during the cloud life)? Or
does MC formation and destruction involve mechanical agglomeration and
dispersal of molecular mass, but with little or no chemical/phase change, so
that individual H2 molecules live much longer than molecular clouds?

By providing a direct measurement of H2 formation and destruction rates,
Hyperion will answer these critical questions for the first time.

####  2.1.1 Hyperion’s Measurements of H2 Dissociation and Formation

####  2.1.2 H2 Dissociation with Hyperion

We can use measured H2 line emission to estimate the H2 dissociation rate. The
idea is that the total surface brightness summed over the fluorescent line
emission is proportional to the H2 dissociation rate, integrated along the
LOS. This is because the H2 lines are excited by FUV LW radiation, which is
also responsible for H2 destruction [36]. H2 dissociation measured by Hyperion
will occur on the surface of the molecular cloud, where UV photons are easily
able to penetrate. While Hyperion cannot probe deeper within the cloud, our
primary science is concerned with the destruction of H2 at the cloud boundary.
We take care to estimate how much dust attenuation is occurring even at this
surface layer.

The complication is that the observed brightness of the emitted lines is
reduced compared to the real true excitation rate as the lines suffer from
dust absorption as they propagate from the cloud interior to the observer.
Through measurements of dust extinction, we can estimate the effective dust
attenuation and thus how much of the cloud volume do the H2 line observations
probe. Typically, $\tau\approx 10^{-21}N$ cm2. Thus, for cloud columns
$N<10^{21}$ cm-2 or lower, $\tau<1$, dust absorption is not significant, i.e.,
this is the optically thin limit. For regions in the cloud where $N\gg
10^{21}$ cm-2, dust absorption is significant and then the H2 lines do not
probe the H2 dissociation along the entire LOS and thus underestimate the true
integrated dissociation rate.

####  2.1.3 H2 Formation with Hyperion

Measuring H2 formation directly with the H2 florescent spectrum requires a
detailed understanding of how the OPR depends on the temperature and
ionization state of the region in question. The imprint of H2 formation in the
cloud is traced by the OPR, which can be used to measure how far out of
equilibrium the H2 is at present.

The OPR method is effective in measuring H2 formation time scales at low
observed OPR values, where there is no ambiguity as to the state of the gas.
Searches for low OPRs indicative of old molecular gas are best conducted in
the weak PDRs near cloud edges where the FUV field is dominated by the
background interstellar radiation field, rather than toward bright, local
sources of dissociating radiation surrounded by intense PDRs. In such weak PDR
regions the OPR we measure should be close to that of the bulk chemical state
in the cloud interior, since most of the H2 molecules we observe will be pre-
existing ones, rather than ones newly re-formed immediately following
dissociation. An added benefit is that, in such regions, dust absorption is a
minimal concern, since the volume and column densities are likely to be
moderate. Once we have measured the chemical age of the H2, we can deduce the
approximate H2 formation rate simply by dividing the H2 mass by the chemical
age.

As noted above, if we measure high OPR values in our target regions, this
could mean that the bulk of the molecular gas is truly young, but it could
also mean that we have not successfully probed the cloud interior, and we are
instead mostly measuring young, surface H2 that is chemically different from
the equilibrium gas deeper in the cloud. To distinguish between these two
possibilities, and provide an alternative estimate of the H2 formation rate if
our measurements find high OPRs, we will employ another method for deducing
the H2 formation rate, based on 21cm observations which trace the HI column
density. In a forthcoming series of papers (Bialy et al., in prep; Burkhart et
al., in prep.) we present the details of this latter method as well as the use
of H2 line emissions to trace the H2 dissociation rate, and test these methods
against synthetic observations and compare the observed rates to the global
star formation rate in the cloud.

####  2.1.4 Connecting H2 formation rates and H2 dissociation rates to star
formation rates

Hyperion’s measurements of H2 formation and dissociation rates can be combined
with measured cloud star formation rates and efficiencies to determine what,
if any, relationship exists between the formation of molecules and the global
properties of star formation.

A cartoon example of the comparison of the cloud star formation to H2
formation and dissociation is shown in Figure 2, which shows the ratios of H2
formation to star formation versus H2 destruction to star formation. A plot
like this is not possible with existing telescopes and is only possible with
Hyperion. Such a diagram demonstrates our ability to disentangle different
cloud evolution scenarios discussed above.

In a scenario where clouds have distinct phases, they should be born in the
upper left “cloud growing” region (purple oval), rapidly pass through the 1:1
“steadily cycling gas” region (yellow oval), and then rapidly move into the
“cloud dissociating” region (pink oval). In the extended steady-state
scenario, where H2 molecule lifetimes are short, we should see the exact
opposite distribution: most clouds on the 1:1 line, and concentrated in the
upper right “extended steady state” where H2 formation and destruction rates
greatly exceed star formation rates [67, 68]. If MC formation and dispersal is
mainly mechanical and not chemical, and molecules live much longer than
clouds, molecular clouds should concentrate in the lower left “rapid star
formation” where star formation rates exceed both H2 formation and
destruction. Within Galactic molecular clouds, different parts of the cloud
may appear in different regions on Figure 2, although we expect a high degree
of correlation on small scales ($\sim$0.1 pc) [69, 70] between formation and
destruction rates due to filamentary structures in molecular clouds.

Figure 2: Left panel: Cartoon of the molecular cloud boundary where Hyperion
observes. Hyperion is able to determine the rates at which molecular hydrogen
molecules are forming and dissociating at the cloud surface, determining if
the cloud is growing or shrinking over time. Right Panel: Hyperion compares
ratios of H2 formation and dissociation to star formation rate to pinpoint the
cloud’s evolutionary state. Four different evolutionary possibilities are
shown in different colored area. We show a comparison of simulated points
using the Simulating the Life-Cycle of molecular Clouds (SILCC) simulations.
Purple and blue points are early evolutionary states (i.e. t $<1.5$ Myrs)
while green and yellow points show later evolution but before the cloud is
dispersed by supernova $1.5<t<3$ Myrs. Hyperion will empirically sample all
evolutionary paths in this plot.

###  2.2 Feedback: Determining how molecular clouds are destroyed

Massive stars emit copious amounts of energy into their surroundings in the
form of ionizing and dissociating radiation, stellar winds, radiation pressure
and eventually supernovae [62, 71]. These feedback mechanisms impact the star
formation process and the broader interstellar medium in various ways.

Locally, feedback makes star cluster formation a self-regulating process by
setting the characteristic mass of the stellar initial mass function [72]. At
the scale of the star forming region, feedback acts to disperse the molecular
cloud that produces the young stars, limiting further star formation [73, 71,
74]. However, feedback can potentially trigger new star formation in the same
cloud and even neighbouring clouds [75, 76, 77, 78, 79, 80, 81, 82, 83, 84,
85]. At smaller scales, feedback can heat and disperse circumstellar disks
around young stellar objects, potentially impacting the planet formation
process (discussed further in Sec 2.3).

At much larger scales, feedback impacts the interstellar medium through
supernova explosions. Supernovae are capable of injecting energy out to much
larger distances which is important for regulating the gas distribution in the
interstellar medium and hence properties such as the galactic scale height and
the mean star formation rate [86, 87]. Indeed calibrated “feedback” is a key
sub-grid process for models of the formation and evolution of galaxies, which
are some of the worlds biggest astrophysics simulations [88, 89, 90]. However
as a sub-grid process the nature of the feedback itself is not really
understood in these models, with the prescription tuned to give results in
agreement with observations. Recent work has shown that the impact of
supernova feedback depends on preprocessing work by radiation and winds. A
larger volume of the interstellar medium is influenced by supernova feedback
if winds and photoionization sculpt low density channels in the star forming
cloud before the supernova goes off, [91, 92]. Therefore, a better
understanding of the preprocessing role of feedback, which is one of
Hyperion’s goals, is crucial to connecting the scale of star-forming regions
and that of the wider Galaxy, though recent theoretical efforts such as the
SILCC project [93, 94, 66, 95, 96] and other groups [97, 98, 99] are working
to address that.

For some time, photoionization has been expected to be the dominant feedback
mechanism prior to supernovae [100, 101]. However, recent observations in [C
II] as part of the SOFIA FEEDBACK survey have found new evidence for fast
flows driven by winds, which they suggest implies a more dominant role of
winds [102, 103, 104]. Whether these fast flows truly represent a widespread
wind that is a key driver of the cloud dispersal or they only trace a handful
of low density channels through which the wind efficiently escapes remains an
alive debate in literature [101, 105].

Given the importance of feedback discussed above, it is necessary to:

i) Observationally determine the relative impact of different feedback
mechanisms in dispersing molecular gas over time.

ii) Observationally determine whether supernovae in a region will be confined
and deposit their energy locally, or whether they will be able to inject
energy into larger scales.

But why has this been difficult to achieve to date, and how will Hyperion
succeed?

Determining the feedback mechanism responsible for dispersal requires an
estimate of the energy output from massive stars and the kinematics of the
surrounding gas. The issue with the kinematics is that widely used tracers
such as CO are downstream of the feedback in the neutral/outer PDR part of the
flow and tracers in the H ii region generally don’t probe the rate of
expansion. This is why the recent [C II] SOFIA observations in the FEEDBACK
survey have been so impactful and surprising. They have enabled us to probe
the kinematics of components of the feedback driven material that we
previously had no access to. What we ideally need is a tracer at the driving
interface, in the vicinity of the ionization front and H-H2 transition, to
properly trace the expansion kinematics. UV fluorescence emanating close to
the H-H2 transition offers this probe. Due to the high optical depth of the
ISM to the UV, Hyperion will primarily trace receding accelerated surfaces
through the large holes in molecular gas associated with HII regions. Hyperion
will use this to measure the expansion of feedback driven surfaces in Hii
regions and determine whether they are consistent with thermal expansion,
winds or supernova driving based on the velocity of the expansion for size of
the Hii region. Due to the high optical depth of the ISM to the UV, Hyperion
will primarily trace receding accelerated surfaces through the large holes in
molecular gas associated with HII regions

A valuable characteristic of the H2 fluorescent spectrum is that the intensity
of certain lines is a function of the exciting FUV radiation field strength
[36, 106]. The combination of the FUV field at the expanding front and the
expansion velocity can be used to distinguish the driving feedback mechanisms,
as illustrated in Figure 3. This figure was constructed using semi-analytic
models of expanding H ii regions [107, 108] and and wind blown bubbles[109].

In addition, the long slit of Hyperion can be used to map the radial profile
of the FUV radiation field by measuring line intensities as a function of the
distance to the FUV source near the bubble center. A line of sight through
which supernova energy can escape to larger distances will also be associated
with a radial FUV profile without an abrupt decrease associated with a dense
molecular wall. By sampling the angular profile of the FUV radiation field we
can determine the fraction of angles through which radiation is escaping to
larger distances and hence determine if the region is predominantly radiation
bounded (in which case the supernova energy deposition will be local) or
otherwise. Achieving this requires being able to map the radial profile of the
exciting UV out to sufficiently large distances from the UV source. Given that
the H2 UV fluorescent emission intensity is an increasing function of the
strength of the exciting UV radiation, the high spectral resolution high
sensitivity design of Hyperion is necessary to achieve this.

Hyperion maps the projected escape of FUV radiation, but given that the holes
in Hii regions are typically large (rather than a large number of small holes
like a sieve) the projected fraction of angles on the sky (i.e. fraction of
$2\pi$ in projection) associated with low density channels correlates with the
fraction of $4\,\pi$ steradians through which supernova energy can efficiently
escape to larger distances. Hyperion will map the FUV profiles of star forming
regions, which following the above allows us to quantitatively constrain the
dense cloud coverage and consequences for supernova feedback.

Figure 3: Left Panel: Cartoon of a bright UV source blowing a bubble in a
molecular cloud. Hyperion observes the edges of the bubble, at the HI-H2
transition, to determine the expansion speed of the front and the driving
mechanism of the bubble. Right Panel: Hyperion distinguishes mechanisms behind
feedback-driven bubbles by measuring their expansion speeds and FUV radiation
fields. Colored dots are semi-analytic models. Points with 1-$\sigma$ errors
are synthetic Hyperion observations of models.

The above applied to multiple massive star forming regions will combine to
tell us what is driving the pre-supernova feedback responsible for displacing
the bulk of the molecular gas, and the impact this has in enabling supernova
feedback to escape into the wider ISM. This will provide a vital link between
that of the star forming region and galaxy evolution.

###  2.3 Disks Dispersal: Evaluating the mechanisms for gas disk dispersal

It is now observationally well established that exoplanets are remarkably
diverse in their properties [110]. Determining the origin of this requires
understanding the formation and evolution of planetary systems, which in turn
requires understanding the evolution and dispersal of planet-forming disks.

Disk dispersal may proceed in a number of different ways:

i) The transport of material through the disk, resulting in accretion onto the
star [111, 112, 113]. Until relatively recently, this transport was thought to
be facilitated by a turbulent viscosity [114], though recent observations do
not provide firm evidence for the required degree of turbulence. There is
growing evidence for the required transport of angular momentum being
facilitated by magnetohydrodynamically driven winds (MHD winds; see point iii
below).

ii) The growth and radial drift of dust grains through the disk and onto the
host star [115]. Without pressure bumps in the disk to trap dust [116, 117],
this can lead to the very rapid depletion of the dust budget on short
timescales.

iii) The extraction of material through MHD driven winds. MHD winds can be
launched as a result of the magneto-rotational instability [118, 119, 120]
(MRI) or magnetocentrifugally [121, 122, 123, 124]. The aforementioned work
showed that, in addition to extracting material, MHD driven winds can also
efficiently redistribute or remove angular momentum, facilitating accretion.

iv) The extraction of material in internal photoevaporative winds. X-ray, EUV
and FUV radiation from the host star can heat material in the inner disk above
the escape velocity, driving a photoevaporative wind [125, 126, 127, 128, 129,
130, 131, 132, 133].

v) The extraction of material in external photoevaporative winds. Material in
the outer disk is less strongly bound and so can be driven in a wind by more
modest heating than the inner disk. The FUV radiation in a wide range of star
forming environments can be sufficient to drive material from the outer disk
[134, 135, 136, 137, 138, 139, 140, 141, 142, 143]. This is expected to reduce
the disk mass, radius and lifetime [144, 145, 146, 147] and could also
influence the migration and mass of massive planets in the outer disk (Winter
et al. in preparation). It is currently thought that the majority of young
stars form in an environment that would lead to significant external
photoevaporation [148, 149], however the nearest star forming regions and
hence sites of our best studied discs are low mass such as Taurus/Lupus with
an absence of strong UV sources [117, 150].

Figure 4: A cartoon illustrating the Hyperion measurements of protoplanetary
disks dispersing by different pathways and the diagnostics that will be used
to identify each regime.  Figure 5: Hyperion will determine under what
conditions planet-forming disks undergo mass accretion and dispersal through
winds driven by several mechanisms (colored regions and points). Points
indicate simulated observations by Hyperion.

Each of the above mechanisms have been and continue to be studied in detail.
However, to date, their full interplay is too complicated to study
theoretically, while observations can only probe a very limited subset of
processes at any given time. For example, there are separate tracers of the
gas and dust components of disks [117, 150], accretion [111, 112, 113], inner
winds [151, 152, 153, 154, 155, 156] and outer winds [134, 157, 158, 136,
159]. Semi-analytic theoretical models suggest that the interplay between
accretion and internal and external winds provides the key controlling
mechanism for disk evolution [160, 161]; however, no single instrument can
probe all of these. Hyperion offers the ability to do so for the first time.

Hyperion is capable of simultaneously measuring both internal and external
winds from disks through UV-H2 fluorescence and the relative level of
accretion using the C IV 1549Å emission doublet, as shown in Figure 4. The
internal and external winds are not spatially resolved by Hyperion. Rather, it
distinguishes them using the fact that specific emission lines arise in the
fluorescent spectrum, depending on the source of H2 excitation. In addition,
the intensity of H2 emission is a function of the incident FUV field strength,
and the outer wind is generally driven by a weaker FUV radiation field than
the inner component, leading to fainter emission lines in the outer wind.
Furthermore, the flow speed is estimated by velocity centroiding the H2 lines,
which provides further distinction between the slow ($\sim$4 km s-1) outer
winds and faster (tens of km s-1) inner winds.

Hyperion’s measurements will enable it to compare the rate of mass loss
through winds with the accretion rate, which will allow it to determine
whether the internal winds are sufficient to drive accretion and discover
whether additional mass loss due through external photoevaporation plays a
significant role in protoplanetary disk dispersion, and if so, in what kinds
of external UV radiation environments [160]. A cartoon of how to distinguish
between these is shown in Figure 5.

##  3 The Hyperion instrument design

To achieve the science objectives outlined in this paper, we have designed a
single instrument that provides a balance between spectral and spatial
resolution, design complexity, and cost. Briefly, Hyperion consists of a 48 cm
Cassegrain telescope, focusing light onto a 70 arcmin long slit, which feeds
an Offner style spectrometer. The spectra are captured by a linear mosaic of 4
high efficiency delta-doped CCDs, optimized to the Hyperion bandpass. All
optical surfaces are reflective, with a high heritage aluminum coating
protected with MgF2. At the Hyperion bandpass, this provides reflectance above
90%, even assuming typical contamination at end of life of 3% (based on HST
performance over time, [162]. The overall Hyperion instrument design
parameters are listed in Figure 6.

Figure 6: Overview of Hyperion Instrument Parameters. EOL=End of Life, CBE=
Current Best Estimate. Figure 7: Zemax raytrace of the Hyperion telescope
optics, including locations of the pupil stop and field stop.

The Hyperion telescope optical layout is shown in Figure 7. The telescope
design is an on-axis 48-cm Cassegrain with a two-element corrector with tilted
and decentered free form surfaces. Performance readily supports Hyperion
spatial resolution, which is SNR driven (it does not need to be diffraction
limited).

The long length of the slit (70 arcmin total length) is driven by needing to
cover as large an angular area at moderate resolution as possible in order to
map molecular hydrogen fluorescence over the large fields of view occupied by
nearby Galactic molecular clouds. Achieving good performance over this length
requires the addition of a 2-mirror relay (with above 90% reflectance) between
the Cassegrain telescope and the slit to keep the FWHM of the PSF to between
4′′-7′′ over the 70′ field. The scientific observation throughput gain from
the wide field of view surpasses the moderate loss from the two extra
reflections. The relay also provides an intermediate image plane and Lyot stop
(pupil plane) to suppress straylight.

Hyperion’s stepped slit design provides two different spectral resolutions,
enabling two observing modes without any moving parts. The majority of the
slit length is 10 arcsec wide, providing a resolution of R$\sim$10,000 over 65
arcmin. The central 5’ portion of the slit is 2 arcsec wide, giving a high
spectral resolution of 50,000 or greater. This geometrically shaped slit
design reduces complexity (no moving parts) while still providing flexibility
to achieve all of Hyperion’s science objectives.

The Hyperion spectrometer consists of a collimating mirror, a grating, a
camera mirror, and a fold mirror before the focal plane. We choose to use the
grating in the third order to simplify fabrication. This increases the period
to a manufacturable 330 nm. The efficiency fall-off with wavelength is
manageable because of the narrow Hyperion band pass. Zeiss has made
holographic gratings of comparable sizes and periods and also has experience
with convex substrates. The efficiency in the system model were based on
profiles achieved by Zeiss on previous gratings.

The Hyperion focal plane consists of 4$\times$1 mosaic of delta-doped UV
optimized CCDs, which are passively cooled by a radiator to $<$ 168K, and is
then actively controlled with a closed-loop heater circuit at 168 K. Each
detector is a Teledyne e2v quad readout CCD272-84, which is derived from the
CCD273 used on the European Space Agency’s (ESA) Euclid mission. The CCD has a
4k $\times$ 4k format with 12 $\mu$m pixels. The detectors are UV optimized
via delta-doping [29, 163, 164, 165] and a detector-integrated, directly-
deposited metal-dielectric filter (MDF) [166, 167] which provides out of band
rejection. The UV camera system meets all Hyperion performance and
environmental requirements, and has been demonstrated to be TRL 6 by means of
a combination of analysis and measurements on high fidelity prototypes.
Historically, UV missions with microchannel plate detectors (MCPs) have not
been able to observe in the UV bright regions that Hyperion investigates
because of their count rate limits. As a brief example, Hyperion’s observation
plan of the Orion region includes the bright star iota Orionis. This star has
a measured flux at 1565 of 8.483 $\times$ 109 erg cm-2 s-1 angstrom-1. Using
Hyperion’s bandpass and effective area, we find an expected count rate from
this star alone to be around 6M counts/s. In contrast, the Europa-UVS mission
uses an MCP with a global count rate limit of 1.2 M counts/s [168]. Hyperion’s
targets exceed these limits.

Hyperion’s ability to detect faint sources requires long exposures to meet the
sensitivity requirements at suitable SNR. The Hyperion instrument is read
noise dominated preventing the use of arbitrarily many shorter exposures. Long
exposures bring challenges with pixels being knocked out by cosmic rays. For
each objective, there is an optimization of exposure number, duration, and on-
chip binning to achieve the required SNR and ensure only 0.3-1.4% of bins are
lost to cosmic ray hits. Exposure durations range between 930 and 5000 s (16
to 66 mins) depending on target brightness and observing mode.

##  4 Hyperion spacecraft and payload overview

The Hyperion spacecraft is based on the BCPSmall product line from Ball
Aerospace. Hyperion benefits from the successful on-orbit NASA GPIM and WISE
along with USAF STPSat-2 and STPSat-3 missions. The IXPE, SPHEREx, and SWFO-L1
missions are currently under development for NASA using the BCP-Small S/C,
with SPHEREx providing the most similar mechanical configuration to Hyperion.

The passively-cooled payload is protected from the sun with a Kepler-style
wrap-around sunshield enabling the science-required $\pm 30^{\circ}$ boresight
roll (to ensure any position angle on a target over the course of a year).
Fixed solar arrays are mounted to the sunshield. The average science-mode sun
incidence is a $9^{\circ}$ roll and $36^{\circ}$ tilt resulting in a 31% EOL
power margin. The worst-case sun angle is $80^{\circ}$ tilt with no roll
leading to use of the batteries. Depth of discharge never exceeds 40% during
observing (batteries are sized for infrequent but long duration eclipses: 5
hours 3 times a year).

Achieving a lunar-resonant orbit requires a lunar flyby, mission orbit
insertion burn and a number of deterministic and statistical trajectory
correction maneuvers. These vary in number and duration depending on the day
within the 21-day launch period. The maximum required $\Delta$V is 193 m/s
including maneuver contingency, operational desaturation, and uncertainties
and residuals. No stationkeeping is required and the Hyperion science orbit
exhibits excellent long-term stability, maintaining a perigee altitude well
above the geosynchronous regime for at least 100 years. At end-of-mission, the
S/C will be passivated but no maneuvering will be required to ensure planetary
protection compliance. The tank will be loaded with 113kg of propellant (340
m/s total $\Delta$V for a 554 kg observatory dry mass), a 60% propellant
margin which can support either maneuver uncertainties or an extended mission.

The observatory has comfortable margins in launch mass, power, link budget,
data storage, slew speed and pointing accuracy (requirement: 95% probability
that a point source will be found $\pm$5′′ perpendicular to the slit). The
principal challenge for this mission is pointing stability sufficient to
ensure a 95% probability that a point source remains within the 2′′ slit
throughout an exposure of up to 5000 s. This can be achieved (with 30%
contingency and 25% margin) with the use of three optical head star trackers
mounted on the anti-sun side of the spacecraft, and benefiting from the stable
thermal environment of an orbit so far from Earth.

##  5 Hyperion’s orbit, observing plan, and survey strategy

Hyperion has a single instrument with a single observing mode – point-and-
stare, long-slit spectroscopy. Each science objective has its own strategy for
utilizing the instrument via optimal slit placement. To efficiently conduct
long exposure observations on faint sources, and to reduce the impact of
radiation from the Van Allen belts, Hyperion resides in a TESS-like Lunar-
resonant orbit. Low Earth orbit (LEO), in contrast, can accommodate exposures
of only up to $\sim$30 minutes in most directions. In a TESS-like orbit,
objects are continuously visible for nearly two weeks and the maximum exposure
time is only limited by the cosmic ray rates. In addition, most LEO’s have
regular periods of eclipse by the Earth’s shadow, yielding a challenging
thermal environment. Other orbits that were studied are an intermediate,
highly-elliptical orbit, which would result in passes through the inner Van
Allen belt twice per period, and an L2 orbit, which offers scant additional
benefits for observing, but has higher hardware costs. The TESS-like two-week
2:1 Lunar resonant orbit provides a stable orbit with apogee of 400,000 km, no
station-keeping, high-thermal stability, and an L2-like radiation environment.
Orbit right ascension of the ascending node and inclination are set by the
constraint to ensure Orion is visible for at least 5 months of the year.

The overall observing program consists of an 18 month science phase, split
between the three science objectives. All surveys for each objective are
completed within the first year, while the final 6 months provide 50%
observing time margin. The observing schedule is constrained mainly by the
solar exclusion angle (95∘), with additional requirements set by the Moon and
Earth exclusion angles. The strongest science constraint on the observing plan
is coverage of the Orion complex, the nearest high-mass-star-forming cloud,
whose intense UV environments provide valuable targets for all three of the
mission’s science objectives. The Orion complex is visible over 5 months of
the year from our planned orbit. To be robust to launch dates and adjustments
in observing requirements, the Hyperion survey plan has 113% science margin
and completes the baseline mission in a single year, while allocating the
remaining 6 months for revisits or follow-up studies. The observing schedule
for the targets in Figure 9 is constructed accounting for the Sun, Earth, and
Moon pointing constraints and using exposure durations that reach the required
SNRs for the expected source brightnesses with the instrument operating at its
expected performance. One-tenth of the time on orbit is reserved for
engineering activities including telecoms, slewing, calibration, and
decontamination. On this schedule, the baseline observations are completed
within 12 months. Figure 8, showing how targets are covered for each science
objective. For science objective 1, slits are placed across the targeted
molecular clouds to provide uniform $\geq$ 3% area coverage. This coverage
yields a 5-10% error on the true underlying dissociation rates, based on
analysis of the simulations. For science objective 2, the 2′′-wide portion of
the slits is placed at spoke angles $<$10∘ apart to cover 360∘ of the bubble
wall. For science objective 3, each compact disk is caught by five dithered
pointings, offset by 2′′ to cover the point spread function (PSF).

Figure 8: Figure showing the observing scenario for each science objective.

During the science mission, Hyperion only requires propellant for momentum
dumping (station-keeping and disposal maneuvers are unnecessary). If orbit
entry is achieved within the margined propellant budget, Hyperion carries
consumables for more than 30 years on orbit. In addition, the expected
hardware lifetime is longer than the 18 month science mission. Possible
science programs in an extended mission phase are described below.

Figure 9: Target list by science objective

##  6 Data Sufficiency

Hyperion collects spectra over wavelengths 138.5–161.5 nm to capture enough of
the H2 fluorescent lines to reach all three SOs. The spectra from the
10′′-wide portion of the slit have R=10,000 to separate the stronger lines,
including those of the molecule’s ortho and para spin states, while the
spectra from the slit’s narrower, 2′′-wide central portion have R=50,000 to
measure the lines’ Doppler shifts and widths in bubbles and disks. Each
molecular cloud target covers many degrees in area, but we are also interested
in the expected small scale features within each giant cloud. The angular
resolution along the slit of 7′′ is sufficient to resolve features in bubbles
and clouds within a few hundred parsecs and to distinguish target stars from
their neighbors.

The 10′′-wide portion of the slit is 65′ long, enough to survey 3% of the
target clouds’ area in the mission lifetime, while the 2′′-wide portion is 5′
long, enough to survey the bubbles at the higher spectral resolution.

The spectra exceed SNR=3 at the threshold surface brightness of 1E-17 erg s-1
cm-2 arcsec-2 for both the lower and higher spectral resolutions, so the
fluorescence is measured down to the level excited by the mean interstellar UV
radiation field. The corresponding sensitivity threshold for the compact
sources of SO3 is 1E-14 erg s-1 cm-2. Reaching these sensitivity thresholds
requires exposures longer than feasible in low Earth orbit. These are enabled
by Hyperion’s lunar-resonant orbit.

Hyperion surveys the fluorescence in eight clouds forming stars at rates 3 to
715 M⊙ Myr-1 (Figure 9), spanning the range found in our Galactic
neighborhood. The baseline mission maps more than twice the minimum fraction
of the eight target clouds’ area to determine their main evolutionary pathway
on Figure 2.

Hyperion also surveys nine bubbles with ages 0.5 to 8 Myr (Figure 9), enabling
testing the full spectrum of potential dispersal mechanisms (Figure 3). The
baseline mission returns exposures at slit angles spaced around the bubbles
twice as finely as the 10∘ spacing below which the leakiness can be gauged.
The spacecraft is capable of placing the slit at any orientation on a given
target over a 12-month period, so that all bubble features can be sampled.

Finally, Hyperion determines wind mass-loss rates from disks in each of eight
star-forming regions lit by the full local range of UV environments, from the
average interstellar field to the intensity of the Orion Nebula Cluster
(Figure 9). Fluxes are measured to a precision of 10%, so the disk winds’
masses and flow rates can be determined as SO3 requires. The baseline mission
measures 24 disks in each star-forming region, twice the minimum to
distinguish driving mechanisms (Figure 5). Hyperion completes its baseline
mission in 18 months.

##  7 Other critical science Hyperion can conduct

Hyperion’s observational capability can fuel an active guest investigator (GI)
Program after the primary mission ends. This includes observing interstellar
gas and star forming regions in low metallicity targets such as the SMC and I
Zw 18. Local metal-poor galaxies are ideal analogues of primordial galaxies
with a barely enriched ISM. Searches for molecular gas in metal-poor galaxies
[169, 170, 171, 172] are actively ongoing, as the impact of low metal
abundance on star formation and H2 formation remains unknown and of great
theoretical interest [173].

Additional Galactic science includes studying emission lines from shocked and
photoionized warm-hot gas in supernova remnants, planetary nebulae, and the
ISM. The brightest rest-frame FUV nebular diagnostic lines in the Hyperion
window are: CIV 1447, 1551, HeII 1640, NIV] 1483, 1486, NeIV 1600. Possible
Galactic investigations include the study of emission lines from shocked and
photoionized warm-hot gas in supernova remnants, planetary nebulae, star
forming regions, and the ISM. In particular, Hyperion’s feedback objective
simultaneously provide exquisite data on the exciting FUV field strength,
attenuation and $H_{2}$ distribution of PDRs. Beyond the Milky Way, Hyperion
can study stellar populations and emission line regions in nearby galaxies
near sites of intense star formation or Active Galactic Nucleus (AGN).

Beyond the Milky Way, Hyperion can study stellar populations and emission line
regions in nearby galaxies near sites of intense star formation or AGN.
Hyperion can also probe the circumgalactic medium (CGM) of distant galaxies,
observing redshifted SiIV1400, OIV]1404 (z$>$0.0035), CII1335 (z$>$0.05) and
H-Ly$\alpha$1216 (z$<$0.15), all important FUV diagnostic signatures. Hyperion
can study the impact of AGN and supernova feedback in the z$\sim$0.1
Ly$\alpha$ forest [174, 175] and will be the only observatory in the near-term
future after HST COS that can study the properties of the z$\sim$0.1
Ly$\alpha$ forest. Hyperion fills an important gap in UV spectroscopic
coverage.

##  8 Conclusion

Hyperion uses the UV fluorescence of molecular hydrogen to trace key phases in
the formations and lifetimes of molecular clouds, stars, and planets. Hyperion
speaks to key Astro2020 Decadal Survey [176] science priorities: Understand
how galaxies evolve through gas condensing to form stars.; and Trace stars and
planets from the disks out of which they form to the vast array of extra-
solar planetary systems. Hyperion does this by achieving 3 science objectives
which explore how MCs and disks lose or gain mass as a function of their
surroundings and internal actions.

Hyperion also provides critical UV capability during the period between the
end of life of the Hubble Space Telescope and the start of science for the
Astro2020-recommended large IR/visible/UV flagship space telescope in the
mid-2040s (Astro2020 Figure S.1). Hyperion’s high-resolution, wide-field
spectrograph partly closes this gap in UV access, enabling groundbreaking
discoveries well beyond the mission’s three science objectives.

## Acknowledgments

The Hyperion mission is a collaboration between the University of Arizona,
NASA’s Jet Propulsion Laboratory California Institute of Technology, and Ball
Aerospace. Each organization provided seed funding for mission development,
both in the 2019 SMEX version and the 2021 MIDEX version described here. The
Hyperion team thanks each organization for their support in developing this
mission concept. Science team members at other institutions contributed their
time to the mission concept, and the Hyperion team is grateful for their work.
Graphics support was provided by Eva Grall, Katie Peek, and Mark Seibert. The
Hyperion team thanks them for their contributions.

This research was carried out at the Jet Propulsion Laboratory, California
Institute of Technology, under a contract with the National Aeronautics and
Space Administration (80NM0018D0004)

TJH is funded by a Royal Society Dorothy Hodgkin fellowship.

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###### List of Figures

  1. 1 Energy diagram for H2 molecule, with an example excited band
  2. 2 Left panel: Cartoon of the molecular cloud boundary where Hyperion observes. Hyperion is able to determine the rates at which molecular hydrogen molecules are forming and dissociating at the cloud surface, determining if the cloud is growing or shrinking over time. Right Panel: Hyperion compares ratios of H2 formation and dissociation to star formation rate to pinpoint the cloud’s evolutionary state. Four different evolutionary possibilities are shown in different colored area. We show a comparison of simulated points using the Simulating the Life-Cycle of molecular Clouds (SILCC) simulations. Purple and blue points are early evolutionary states (i.e. t $<1.5$ Myrs) while green and yellow points show later evolution but before the cloud is dispersed by supernova $1.5<t<3$ Myrs. Hyperion will empirically sample all evolutionary paths in this plot.
  3. 3 Left Panel: Cartoon of a bright UV source blowing a bubble in a molecular cloud. Hyperion observes the edges of the bubble, at the HI-H2 transition, to determine the expansion speed of the front and the driving mechanism of the bubble. Right Panel: Hyperion distinguishes mechanisms behind feedback-driven bubbles by measuring their expansion speeds and FUV radiation fields. Colored dots are semi-analytic models. Points with 1-$\sigma$ errors are synthetic Hyperion observations of models.
  4. 4 A cartoon illustrating the Hyperion measurements of protoplanetary disks dispersing by different pathways and the diagnostics that will be used to identify each regime.
  5. 5 Hyperion will determine under what conditions planet-forming disks undergo mass accretion and dispersal through winds driven by several mechanisms (colored regions and points). Points indicate simulated observations by Hyperion.
  6. 6 Overview of Hyperion Instrument Parameters. EOL=End of Life, CBE= Current Best Estimate.
  7. 7 Zemax raytrace of the Hyperion telescope optics, including locations of the pupil stop and field stop.
  8. 8 Figure showing the observing scenario for each science objective.
  9. 9 Target list by science objective