At a time when ALMA produces spectacular high resolution images of gas and dust in circumstellar disks, the next observational frontier in our understanding of planet formation and the chemistry of planet-forming material may be found in the mid- to far-infrared wavelength range. A large, actively cooled far-infrared telescope in space will offer enormous spectroscopic sensitivity improvements of 3-4 orders of magnitude, making it possible to uniquely survey certain fundamental properties of planet formation. Specifically, the Origins Space Telescope (OST), a NASA flagship concept to be submitted to the 2020 decadal survey, will provide a platform that allows complete surveys of warm and cold water around young stars of all masses and across all evolutionary stages, and to measure their total planet-forming gas mass using the ground-state line of HD. While this white paper is formulated in the context of the NASA Origins Space Telescope concept, it can be applied in general to inform any future space-based, cold far-infrared observatory

Banzatti, Andrea

Bergin, Edwin A.

Blake, Geoffrey A.

Bradford, Matt

de Val-Borro, Miguel

Fortney, Jonathan J.

Fraine, Jonathan

Kataria, Tiffany

Leisawitz, David T.

Meixner, Margaret

Melnick, Gary

Meshkat, Tiffany

Pontoppidan, Klaus M.

Salyk, Colette

Staguhn, Johannes G.

Stevenson, Kevin

Zhang, Ke

Caltech Authors - Main

Draft version August 20, 2018
Typeset using LATEX modern style in AASTeX61
THE NEED FOR A FAR-INFRARED COLD SPACE TELESCOPE TO UNDERSTAND
THE CHEMISTRY OF PLANET FORMATION
Klaus M. Pontoppidan,1 Edwin A. Bergin,2 Gary Melnick,3 Matt Bradford,4
Johannes G. Staghun,5, 6 David T. Leisawitz,6 Margaret Meixner,7
Jonathan J. Fortney,8 Colette Salyk,9 Geoffrey A. Blake,4 Ke Zhang,2
Andrea Banzatti,10 Tiffany Kataria,11 Tiffany Meshkat,12 Miguel de Val-Borro,6
Kevin Stevenson,7 and Jonathan Fraine7
1Space Telescope Science Institute,
2University of Michigan
3Harvard-Smithsonian Center for Astrophysics
4California Institute of Technology
5Johns Hopkins University
6NASA Goddard Space Flight Center
7Space Telescope Science Institute
8University of California, Santa Cruz
9Vassar College
10Lunar and Planetary Laboratory, University of Arizona
11Jet Propulsion Laboratory, California Institute of Technology
12IPAC, California Institute of Technology
ABSTRACT
At a time when ALMA produces spectacular high resolution images of gas and dust in cir-
cumstellar disks, the next observational frontier in our understanding of planet formation and
the chemistry of planet-forming material may be found in the mid- to far-infrared wavelength
range. A large, actively cooled far-infrared telescope in space will offer enormous spectro-
scopic sensitivity improvements of 3-4 orders of magnitude, making it possible to uniquely
survey certain fundamental properties of planet formation. Specifically, the Origins Space
Telescope (OST), a NASA flagship concept to be submitted to the 2020 decadal survey, will
provide a platform that allows complete surveys of warm and cold water around young stars
of all masses and across all evolutionary stages, and to measure their total planet-forming
gas mass using the ground-state line of HD. While this white paper is formulated in the
context of the NASA Origins Space Telescope concept, it can be applied in general to inform
any future space-based, cold far-infrared observatory.
Corresponding author: Klaus Pontoppidan
pontoppi@stsci.edu, (410) 338 4744
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1. INTRODUCTION
Giant planets, planetesimals, as well as comets and Kuiper belt objects form in a rich
chemical environment where primordial water and other volatiles mix with new complex
chemistry to create a vast diversity of planetary systems. While the formation and chemical
evolution of planets is complex, the initial chemical conditions of the carriers of oxygen,
carbon and nitrogen of the planet-forming region at 0.1-10 AU will play a vital role in
determining the final makeup of planets (Cleeves et al. 2014; Bergin et al. 2015). Indeed, it
is thought that many chemical signatures seen in ancient Solar System material originate in
the gas-rich protoplanetary disk phase (Busemann et al. 2006; Mumma & Charnley 2011).
We do not know whether protoplanetary disks are universally able to seed their planets
with water and other volatile species critical for the origin of life, as we know it. In order to
measure the abundance and distribution of water in gas and dust actively being incorporated
into planetesimals and planets, access to far-infrared lines of water vapor is required. The
5−600µm spectra of typical protoplanetary disks are rich in transitions from some of the most
abundant solids and molecular gas species, which are not accessible at other wavelengths.
This white paper is based on a quantitative yield study of a simulated far-infrared spectro-
scopic survey with the Origins Space Telescope (OST) of the Orion cluster (Pontoppidan et
al. 2018, in prep.). In that work, we argue that a cold, far-infrared telescope in space, with
its enormous improvement in sensitivity, will be able to open a new, and previously nearly
completely unexplored, window on planet formation and associated chemistry. We iden-
tify water and hydrogen deuteride (HD) as some of the most important examples of unique
far-infrared tracers of planet formation, but there are many others, including NH3, HDO, hy-
drides, OI and water ice. HD is a powerful direct tracer of the total planet-forming gas mass,
while water is a key tracer of volatiles needed to seed the atmospheres and hydrospheres of
potentially habitable planets and satellites.
The connection between water and life is self-evident, and a far-infrared telescope, such as
OST, is a critical need for understanding the origin of habitable worlds. A main connection
between planet formation and extra-solar planets is the C/O ratio and its potential link
between the composition of the disk at the birth location and the atmosphere of gas giants
(O¨berg et al. 2011). The C/O ratio links the core accretion theory of giant planet formation
with the disk chemical composition, which is driven by the condensation/sublimation fronts
of major volatile carriers of C (CO, CO2) and O (water). While CO can be accessed from
the ground, cold (T < 500 K) water can only be reliably detected from space. OST, with its
large spectral grasp, has the capability to constrain the location of the water snowline line
hundreds of disks. OST, through a combination of observations of multiple lines of water
vapor and HD, has the capability to observe and constrain the water vapor abundance at,
and beyond, the snowline in hundreds of disks, and thereby provide critical missing pieces
for measuring C/O ratios as well as absolute abundances (C/H and O/H). This type of
information will be transformative as we enter the age of giant planet characterization.
Thus, to understand the full diversity of planet-forming disks and their connection to
exoplanetary composition and the availability of water to potential biospheres, it is critical
Chemistry of planet formation with OST 3
to obtain a sensitive far-infrared spectroscopic census of large samples of protoplanetary disks
around stars of all masses, and for a wide variety of environments. We estimate that sensitive
far-infrared spectra of 1000 disks are needed, reflecting two dimensions (disk mass and stellar
age), 10 bins along each axis, and 10 disks in each bin. Such a survey must be conducted at
high spectral resolution (R = 45 000−200 000) in order to detect the lines against the bright
infrared continuum, and to spectrally resolve them to measure their spatial distribution by
means of fitting Keplerian line profiles. The minimum resolving power is defined as the
FWHM line width of the HD 1-0 line at 112µm and the 179.5µm ground-state line of water
in a typical protoplanetary disk, viewed at an inclination of 45◦ (see Figure 1).
2. PLANET-FORMATION AND OST
2.1. Measuring the mass of planet-forming matter in the Galaxy
The most fundamental quantity that determines whether planets can form, and on what
time scale, is the protoplanetary disk mass. Estimates of disk masses are complicated by
the fact that the molecular properties of the dominant constituent, molecular hydrogen, lead
it to be unemissive at temperatures of 10-30 K, which characterizes much of the disk mass.
To counter this difficulty, the thermal continuum emission of the dust grains (Andrews &
Williams 2007) or rotational lines of CO (Williams & Best 2014) are often used as a proxy
for total mass, under the assumption that they can be calibrated to trace the total gas mass.
However, sensitive observations have demonstrated that grains have undergone substantial
growth, making the determination via dust inherently uncertain (Ricci et al. 2010). Further,
recent observations indicate that the CO abundance in disks may be orders of magnitude
lower than in molecular clouds due to a combination of sequestration below the CO snow
line and chemistry (Favre et al. 2013; Kama et al. 2016). These uncertainties are well known,
with broad implications on our understanding of the time scale where gas is available to form
giant planets, the primary mode of giant planet formation, the dynamical evolution of the
seeds of terrestrial worlds, and the resulting chemical composition of pre-planetary embryos.
The fundamental rotation transition of HD at 112µm has been proposed as a more robust
measure of protoplanetary disk mass. HD is a tracer of H2, with an abundance closely
matching that of elemental deuterium. The D abundance, in turn, is known from UV
absorption spectroscopy of the local interstellar bubble (Linsky 1998). Using the Herschel
Space Telescope, (Bergin et al. 2013) measured the gas mass of the TW Hya disk by taking
advantage of the fact that the lowest rotational transition of HD is ∼ 106 times more emissive
than the lowest transition of H2 for a given gas mass at 20 K. The subsequent conversion
of HD emission to H2 gas mass is well calibrated. Due to Herschel’s limited lifetime, the
only other deep HD observations obtained were toward six disks, with the result being two
additional detections (McClure et al. 2016). In Pontoppidan et al. (2018), we find that,
with a 5σ line sensitivity of 10−21 W cm−2, we can measure the minimum-mass solar nebula
around solar-type stars out to a distance of Orion (410 pc) in one hour. Such sensitivity
is possible with a 4-5 K far-infrared telescope, with a collecting area of 25 m−2, and can be
achieved by the OST concept.
4 Pontoppidan et al.
2.2. Understanding the role of water in planet formation
To understand the full diversity of planet-forming disks and their connection to exoplane-
tary composition and the availability of water to potential biospheres, it is critical to obtain
an infrared spectroscopic census of the water content in large samples of protoplanetary
disks around stars of all masses, and for a wide variety of environments. This can be accom-
plished by observations of water lines. Based on discoveries by Spitzer, we now know that
the mid- to far-infrared spectrum of protoplanetary disks is often dominated by emission
from thousands of strong water vapor lines, of which a few hundred are currently detected
(see Figure 1). The corresponding transitions span a very wide range of excitation, or upper
level, energies, from less than 100 to 1000s of Kelvin (Carr & Najita 2008; Salyk et al. 2008;
Pontoppidan et al. 2010; Banzatti et al. 2017). Further, Spitzer observations suggest that
there are strong chemical differences in disks around stars of different masses, with water
being abundant around solar-mass stars, but much less pronounced around both low-mass
and intermediate-mass (> 2M) stars (Pascucci et al. 2013).
HD	1-0
H2O
H2O
H2O	bending	vibration
HCN+C2H2
H2O	“179.5	micron”
H2O	“557	GHz”
CO	fundamental Silicate
Figure 1. A disk model spectrum from Pontoppidan et al. (2018), rendered at 4-660µm, and at a
uniform 3 km s−1 spectral resolving power. Major spectral features are indicated. The insets show
examples of diagnostic molecular lines, including the HD 1-0 line.
This preponderance of water emission lines covering all relevant gas temperatures allows
for mapping the distribution of water vapor in protoplanetary disks. Essentially, each line
will trace gas in a relatively restricted temperature range, and the combination of many
water lines with an appropriate model can map the abundance of water vapor as a function
of radius in the disk. Up until now, this has been done for a few disks using unresolved
Chemistry of planet formation with OST 5
line spectroscopy with Spitzer and Herschel (Zhang et al. 2013; Blevins et al. 2016). At
longer wavelengths, Herschel-PACS was able to detect cool water in a few disks at low reso-
lution (Riviere-Marichalar et al. 2012; Blevins et al. 2016). The analysis of such unresolved
spectroscopy will always be model-dependent. However, one powerful way to break model
degeneracies is to spectrally resolve the lines and use Keplerian motion of the disk to obtain
an independent measure of the spatial distribution of the emission for each line.
Indeed, beautifully velocity-resolved Herschel-HIFI observations of the ground-state para
and ortho lines at 1113 and 557 GHz have been used to measure the cold water abundance
in the outer, coldest portions of two disks (corresponding to the Sun’s Kuiper Belt, 10-30 K),
TW Hya (Hogerheijde et al. 2011) and DG Tau (Podio et al. 2013). At the end of the Herschel
mission, we essentially lost the ability to observe water vapor from gas with temperatures in
the 50-500 K range, and therefore our ability to trace the planet- and comet-forming regions
at 1-50 AU, including the location of the snow line. See Figure 2 for a comparison of Herschel
and OST yields of water and HD.
1.00 10.00
Stellar mass [solar masses]
0
20
40
60
80
100
120
140
Nu
m
be
r o
f t
ar
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ts
Origins MRSS
Herschel capability
Figure 2. Distribution of Orion disks with detectable far-infrared water and HD lines in a notional
∼1500 hour program, compared to the distribution of disks that would have been observable with
Herschel PACS (at a much lower resolving power of ∼ 2000) in the same time, demonstrating the
improvement possible with OST.
3. WATER AND HD IN DISKS IN THE NEXT TWO DECADES
In the 2025-2030 time frame, we expect to have exquisite data sets revealing the physical
structure of dust and gas in perhaps ∼ 100 nearby protoplanetary disks, as observed with
ALMA. We will have a census of the amount of water vapor in the surfaces of a similar
number of inner disks at 1 AU from lines tracing hot gas as observed with JWST. JWST
will also provide measurements of the composition of a range of exoplanetary atmospheres,
and their complement of water and other volatile species.
Warmer water can be observed from the ground and will be observed by JWST at wave-
lengths below ∼ 28µm, although JWST is unable to observe any water line with upper level
energies less than 800 K, nor will it spectrally resolve the lines. Thus, JWST observations
will trace hot water inside of ∼ 1 AU and are complementary to OST. Additionally, a small
6 Pontoppidan et al.
number of warm-water lines may be observed by ALMA, under very favorable circumstances
and for a few bright disks. In the short term, SOFIA-HIRMES will be able to observe water
ice via the strong 43µm ice band in many disks, as well as HD 1-0 and warm water in a
handful of bright disks.
4. SUMMARY AND CHALLENGES
A broad, observational census of water colder than 500-1000 K in large samples of disks
requires a space-based, highly sensitive observatory operating at wavelengths of, at least, 30-
200µm, with spectral resolving powers of R & 45 000 for optimal detection, and R & 200 000
to fully resolve the line profiles of cold water and HD 1-0. High spectral resolving power is
needed to use emission line profiles to trace the location of the emitting gas in a Keplerian
disk. 5σ line sensitivities of 10−21 W cm−2 are needed to detect HD in ∼1000 disks around
stars with masses as low as 0.3M in one hour (water lines are 5-10 times brighter). This
requires a 4-5 K telescope with a collecting area of ∼ 25 m2 (equivalent to that of JWST).
A necessary component needed to fully take advantage of the low backgrounds of a cold
telescope operating at high spectral resolution are low-noise, multiplexed detectors operating
at 30-200µm, and technology for achieving sensitive high-resolution spectroscopy across the
far-infrared (30-200µm).
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The need for a far-infrared cold space telescope to understand the chemistry of planet formation

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The need for a far-infrared cold space telescope to understand the chemistry of planet formation

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