One of the primary mechanisms for inferring the dynamical history of planets
in our Solar System and in exoplanetary systems is through observation of
elemental ratios (i.e. C/O). The ability to effectively use these observations
relies critically on a robust understanding of the chemistry and evolutionary
history of the observed abundances. Significant efforts have been devoted to
this area from within astrochemistry circles, and these efforts should be
supported going forward by the larger exoplanetary science community. In
addition, the construction of a next-generation radio interferometer will be
required to test many of these predictive models in situ, while simultaneously
providing the resolution necessary to pinpoint the location of planets in
formation.Comment: To be published in the ASP Monograph Series, "Science with a
  Next-Generation VLA", ed. E. J. Murphy (ASP, San Francisco, CA

Bergin, Edwin

Blake, Geoffrey A.

Burkhardt, Andrew M.

Cleeves, L. Ilsedore

Loomis, Ryan A.

McGuire, Brett A.

Remijan, Anthony J.

Shingledecker, Christopher N.

Willis, Eric R.

English

arXiv

One of the primary mechanisms for inferring the dynamical history of planets in our Solar System and in exoplanetary systems is through observation of elemental ratios (i.e. C/O). The ability to effectively use these observations relies critically on a robust understanding of the chemistry and evolutionary history of the observed abundances. Significant efforts have been devoted to this area from within astrochemistry circles, and these efforts should be supported going forward by the larger exoplanetary science community. In addition, the construction of a next-generation radio interferometer will be required to test many of these predictive models in situ, while simultaneously providing the resolution necessary to pinpoint the location of planets in formation

Caltech Authors - Main

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Observing the Effects of Chemistry on Exoplanets and Planet
Formation
Brett A. McGuire1, Edwin Bergin2, Geoffrey A. Blake3,4, Andrew M.
Burkhardt5, L. Ilsedore Cleeves6, Ryan A. Loomis1, Anthony J. Remijan1,
Christopher N. Shingledecker7,8 , and Eric R. Willis9
1National Radio Astronomy Observatory, Charlottesville, VA, 22903;
bmcguire@nrao.edu
2Department of Astronomy, University of Michigan, Ann Arbor, MI 48109
3Division of Geological and Planetary Sciences, California Institute of
Technology, Pasadena, CA 91125
4Division of Chemistry and Chemical Engineering, California Institute of
Technology, Pasadena, CA 91125
5Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138
6Department of Astronomy, University of Virginia, Charlottesville, VA 22904
7Max Planck Institute for Extraterrestrial Physics, Garching, Germany
8Institute for Theoretical Chemistry, University of Stuttgart, Germany
9Department of Chemistry, University of Virginia, Charlottesville, VA 22904
1. Background
The location of condensation fronts (snow lines) in protoplanetary disks (PPDs) can
have substantial effects on the physical and chemical evolution of planets in the system.
Indeed, many models predict that these condensation fronts are the locations where
planetesimal formation begins (Ros & Johansen 2013; Schoonenberg & Ormel 2017;
Drazkowska & Alibert 2017). The location of these condensation fronts is determined
both by the incident radiation from the protostar, but also, in large part, by the chemical
composition of the gas and ice. The desorption temperature of the bulk ice varies
significantly with the chemical composition and mixing ratio (Collings et al. 2004),
even of minor constituents. This can be further complicated by a variance in the sticking
efficiency of the icy grains with composition (Wang et al. 2005). It is clear that the
underlying chemical composition in these ices, even at low mixing ratios, can have a
substantial effect on the efficiency of planet formation.
Further, the relative gas vs solid-phase inventories at snow lines govern the com-
position of the planets and their atmospheres. These compositions (e.g., the C/O ratio
in planetary atmospheres; Öberg et al. 2011) may therefore be useful probes of the his-
tory of exoplanets. Effectively using observed exoplanet elemental ratios to test those
models therefore relies heavily on our ability to accurately interpret chemical abun-
1
2 Brett A. McGuire et al.
dances measured in disks to infer the inventory of undetectable species, and how these
measurements affect observed elemental ratios.
Constraining the chemical compositions in PPDs is challenging. The most com-
plex gas-phase species yet detected in PPDs are methanol (CH3OH; Walsh et al. 2016)
and methyl cyanide (CH3CN; Öberg et al. 2015), and more complex molecules are un-
likley to be observed (Walsh et al. 2014). Further, in cold or shielded regions within
disks, ices may be the major repository of C and O (Bergin et al. 2016). Direct obser-
vation of ices typically require space-based infrared facilities and a target source with
a sufficiently bright background continuum and favorable viewing geometry. Even in
massive young stellar objects with substantial ice columns, the list of firmly-detected
ice species numbers less than a dozen (Boogert et al. 2015). Our knowledge of gas
and ice compositions in disks must therefore rely on astrochemical models, using the
detectable simple gas species as the primary constraint.
These models are not yet robust, despite substantial efforts (Garrod 2013; Herbst
2017; Wakelam et al. 2015). To reliably infer chemical compositions in PPDs, in
which chemistry is closely coupled to physical conditions (temperature, density, UV
flux, evolving dust populations, etc.) that vary with radius and scale height, neces-
sitates that these models be well-constrained by observations in earlier stages of star
formation, where more robust inventories can be observed. Here, we briefly discuss the
state-of-the-art in progressively earlier stages of chemical evolution, and outline future
directions and the technology and community support needed to enable them.
2. Chemical evolution in PPDs ...
A number of larger scale studies of disk physical properties and chemistry have been
enabled by the sensitivity and resolution of ALMA. Efforts to measure fundamental
properties such as bulk mass (Ansdell et al. 2016), and larger compositional studies
examining nitrogen (Guzmán et al. 2017) and deuterium (Huang et al. 2017) ratios,
are now possible for larger samples sizes. With robust chemical modeling, ALMA ob-
servations of simple hydrocarbons (e.g., C2H) can perhaps be used to infer C/O ratios
indirectly (Bergin et al. 2016). As well, the JWST and, if commissioned, Origins Space
Telescope, have significant potential to observe major ice components (e.g. H2O, CO,
CO2, CH3OH) toward a wide variety of targets. Yet, these infrared facilities will still
largely be blind to the complex chemistry occurring within the ices, and to the com-
bined ice/gas coupling in warmer gas. That insight is only achievable through radio
observations of gas-phase species, that are sensitive to populations orders of magnitude
lower in abundance, coupled with chemical modeling.
3. ... and in complex, star-forming regions
Sgr B2(N), Orion-KL, and NGC 6334I are the most chemically and physically complex
stages of star formation observed and modeled (Garrod et al. 2008; Crockett et al. 2014;
McGuire et al. 2017). The extreme physical environments, interplay between the gas
and ice, and complex structures often result in models which can reproduce observed
abundances in, at best, a qualitative fashion (Laas et al. 2011; Garrod et al. 2008; Gar-
rod 2013). These types of physical and chemical interactions are the same that are at
Chemistry of Exoplanets and Planet Formation 3
play in PPDs, albeit on a much larger scale. It is therefore these environments that
provide a unique test environment for the similar PDR style models applied to PPDs.
Improvements in these models are most likely to be achieved by testing them against a
wide range of environments from quiescent clouds to massive star forming regions.
Existing facilities in the (sub-)millimeter regime, particularly ALMA, are capable
of reaching the line-confusion limit in broadband line surveys of complex, star forming
regions in a matter of hours. That is, the density of molecular lines in the resulting
spectra is such that there is at least one spectral line every FWHM; there are no line-
free channels. When line-confusion is reached, no additional information can be gained
from longer integration times at the same spatial resolution. We are rapidly approaching
the point at which deep drill observations with ALMA and the GBT will no longer
produce new spectral lines. The end result of this limit is that no further detections of
new species can be made, and thus the refinement of chemical networks will be limited
to those species detectable at the confusion limit.
In the cm-wave regime, however, the most sensitive survey, the Prebiotic Inter-
stellar Molecular Survey (PRIMOS; Neill et al. 2012) with the most sensitive facility
(the GBT) toward the most line-rich source (Sgr B2) has lines in only ∼10-20% of its
channels at cm-wavelengths. Diminishing returns on sensitivity for this survey have
already set in, and to increase the sensitivity substantially would require hundreds of
hours of dedicated observations for every ∼2 GHz window. Further, ALMA observa-
tions at higher frequencies are revealing that the region displays widely variable physi-
cal conditions within a relatively small environment, variation that is lost to single-dish
GBT observations (Belloche et al. 2016). This offers a valuable opportunity to probe
environment-induced chemical differentiation, but only with spatially-resolved obser-
vations which are not possible with the GBT.
While the brightest lines in Sgr B2 are readily detectable with extant facilities such
as the VLA and ATCA (Corby et al. 2015), the complex species are not efficiently ob-
servable, especially at high angular resolution. The only viable solution to this problem
is the construction of a next-generation interferometer like the next-generation Very
Large Array (ngVLA). Under the current design concept (Selina & Murphy 2017), the
ngVLA is likely to not only be capable of spatially resolving chemistry in Sgr B2(N),
but to reach the line-confusion limit even at cm-wavelengths, providing substantial
new, complex molecular detections and their correlation with the physical conditions at
work.
4. Applications to PPDs
Once refined, these models can be used to accurately interpret observations from ALMA
targeting the outermost condensation fronts within disks, primarily CO snow lines (Qi
et al. 2015). Application in the terrestrial planet forming region, near the H2O snow
line, will require AU-scale resolution with high sensitivity. A high-frequency ngVLA
is the only facility currently in design with the required specifications. Further, while
these models can then interpret results for C and O in disks, observations of the major
N-bearing species, NH3 will still be required at similar scales. An ngVLA operating at
least through the 24 GHz NH3 transition is therefore necessary.
4 Brett A. McGuire et al.
An ngVLA design concept is needed that supports broadband, high surface-brightness
sensitivity capabilities at the small to modest (0.1–3 ′′) scales required to probe the ef-
fects of the widely variable physical conditions on the chemical evolution in sources
like Sgr B2 and unlock chemical inventories obscured by line confusion suffered at ex-
tant facilities like ALMA. The same instrument should be capable of observing NH3
and imaging the planet forming zone inside the water snow line in disks, requiring AU-
scale (mas) resolution capabilities.
5. Chemical evolution in protostellar environments
The protostellar stage of star formation spans a wide range of sources, from ‘hot cori-
nos’ such as IRAS 16293 (Jørgensen et al. 2016) to chemically rich outflow sources
like L1157 (Mendoza et al. 2014). The molecular outflows from active young proto-
stars like L1157 provide a unique environment to test the immediate impact of rapid
changes in temperature and density on ‘pristine’ molecular clouds. In these sources,
distance from the protostar along the outflow corresponds directly to time, allowing
modeling of temporal evolution. Hot corinos are the next stage after L1157 and before
hot cores like Sgr B2(N).
The best method for constraining the chemistry in these sources is broadband spec-
tral line surveys. These sources are relatively compact, are beam-diluted in single-dish
observations, and their linewidths are typically such that high-sensitivity data are often
at or approaching line confusion limits, necessitating interferometric observation (Jør-
gensen et al. 2016). Because they are typically moderately warm (50–150 K; Jørgensen
et al. 2016; McGuire et al. 2015), complex molecules display bright features in the 1–
3 mm window well-covered by ALMA, NOEMA, and the SMA.
The SOLIS project at NOEMA and IRAM, which examines a sample of solar-type
protostellar sources (Ceccarelli et al. 2017), and the PILS project at ALMA, which has
focused particularly on an exquisitely detailed examination of the IRAS 16293 system
(Jørgensen et al. 2016), are the primary active dedicated investigations into this class of
objects. Extant observational facilities are likely sufficient to provide the required in-
sight. Ongoing and future efforts on ALMA, NOEMA, and the SMA examining a large
sample of protostellar environments are critical to constraining the effects of physical
environment on chemical evolution and incorporating these effects into models.
6. Chemical evolution in quiescent environments
Cold (Tex ∼10 K), dark, starless clouds are an ideal environment in which to test the
time evolution of chemical reaction networks under ‘constant’ physical conditions. The
typical sources for exploring this stage of star and planet formation are highly extended
(θs > 30′′). At these excitation temperatures, most molecules, especially those more
complex than CH3OH, will display bright emission at cm wavelengths. The best strat-
egy for obtaining the required abundance and excitation information is therefore deep,
broadband spectral line surveys at cm-wavelengths using single-dish facilities com-
bined with chemical modeling (Cordiner et al. 2017; Kaifu et al. 2004; McGuire et al.
2018). The best extant instrument for this work is the Green Bank Telescope (GBT).
Chemistry of Exoplanets and Planet Formation 5
Even in a source as extended as TMC-1, there exists evidence for large chemical
variation over small spatial scales (0.02–0.03 pc; Peng et al. 1998; Dickens et al. 2001),
and observations of such would probe the effects of the very earliest stages of collapse
on the chemistry. These observations will require high spatial resolution, high surface-
brightness sensitivity, and broad spectral bandwidth to be efficient. Because of the cold
temperatures, this further requires a cm-wavelength facility. While the VLA satisfies
this need, it lacks the combination of surface brightness sensitivity and bandwidth nec-
essary to conduct a sensitive line survey with any efficiency. The proposed ngVLA, in
its current conceptual design, would prove more than optimal for this task.
The chemical evolution of all ranges of environments from planetary atmospheres
(Arsenovic et al. 2016) to disk midplanes (Cleeves et al. 2013) to molecular clouds
(Hudson et al. 2008) is impacted by radiation. The last few years have seen signifi-
cant advances in modeling efforts to accurately include these effects (Shingledecker &
Herbst 2018), but these efforts are young, and require substantial training on ‘simple’
environments like quiescent clouds where the radiation effects can be easily decou-
pled from dynamical physical events such as outflows and shocks. In short, efforts at
the GBT and other large single-dish facilities are needed to survey quiescent chemical
clouds. These efforts would be greatly bolstered by the development of broadband ca-
pabilities on the ngVLA to enable investigations of the chemical evolution upon initial
gravitational collapse.
Summary
A combination of observational and theoretical efforts are needed that are focused
on placing rigorous constraints on models of chemical evolution at each stage of the
star and planet formation process. The observational requirements of these investiga-
tions necessitate a commitment by the community to support extant facilities and the
development of technological advancements for the same, as well as the development
of a next-generation VLA with high sensitivity and exquisite spatial resolution. The
ultimate goal should be producing models that can be used to better calibrate our esti-
mates of gas and ice compositions in PPDs, given the observations of simple gas-phase
species achievable in these sources, and the construction of a radio facility capable of
observationally exploiting these models in situ during planet formation.
References
Ansdell, M., Williams, J. P., van der Marel, N., et al. 2016, The Astrophysical Journal, 828, 1
Arsenovic, P., Rozanov, E., Stenke, A., et al. 2016, Journal of Atmospheric and Solar-Terrestrial
Physics, 149, 180
Belloche, A., Müller, H. S. P., Garrod, R. T., & Menten, K. M. 2016, Astronomy & Astro-
physics, 587, A91
Bergin, E. A., Du, F., Cleeves, L. I., et al. 2016, The Astrophysical Journal, 831, 1
Boogert, A. C. A., Gerakines, P. A., & Whittet, D. C. B. 2015, Annual Review of Astronomy
and Astrophysics, 53, 541
Ceccarelli, C., Caselli, P., Fontani, F., et al. 2017, The Astrophysical Journal, 850, 176
Cleeves, L. I., Adams, F. C., Bergin, E. A., & Visser, R. 2013, The Astrophysical Journal, 777,
28
Collings, M. P., Anderson, M. A., Chen, R., et al. 2004, Monthly Notices of the Royal Astro-
nomical Society, 354, 1133
6 Brett A. McGuire et al.
Corby, J. F., Jones, P. A., Cunningham, M. R., et al. 2015, Monthly Notices of the Royal
Astronomical Society, 452, 3969
Cordiner, M. A., Charnley, S. B., Kisiel, Z., McGuire, B. A., & Kuan, Y.-J. 2017, The Astro-
physical Journal, 850, 187
Crockett, N. R., Bergin, E. A., Neill, J. L., et al. 2014, The Astrophysical Journal, 787, 112
Dickens, J. E., Langer, W. D., & Velusamy, T. 2001, The Astrophysical Journal, 558, 693
Drazkowska, J., & Alibert, Y. 2017, Astronomy & Astrophysics, 608, A92
Garrod, R. T. 2013, The Astrophysical Journal, 765, 60
Garrod, R. T., Weaver, S. L. W., & Herbst, E. 2008, Astrophys. J., 682, 283
Guzmán, V. V., Öberg, K. I., Huang, J., Loomis, R., & Qi, C. 2017, The Astrophysical Journal,
836, 1
Herbst, E. 2017, International Reviews in Physical Chemistry, 36, 287
Huang, J., Öberg, K. I., Qi, C., et al. 2017, The Astrophysical Journal, 835, 1
Hudson, R. L., Moore, M. H., Dworkin, J. P., Martin, M. P., & Pozun, Z. D. 2008, Astrobiology,
8, 771
Jørgensen, J. K., van der Wiel, M., Coutens, A., et al. 2016, Astronomy & Astrophysics, 595,
A117
Kaifu, N., Ohishi, M., Kawaguchi, K., et al. 2004, Publications of the Astronomical Society of
Japan, 56, 69
Laas, J. C., Garrod, R. T., Herbst, E., &WidicusWeaver, S. L. 2011, The Astrophysical Journal,
728, 71
McGuire, B. A., Burkhardt, A. M., Kalenskii, S. V., et al. 2018, Science, 359, 202
McGuire, B. A., Carroll, P. B., Dollhopf, N. M., et al. 2015, The Astrophysical Journal, 812, 1
McGuire, B. A., Shingledecker, C. N., Willis, E. R., et al. 2017, The Astrophysical Journal
Letters, 851, L46
Mendoza, E., Lefloch, B., Lopez-Sepulcre, A., et al. 2014, Monthly Notices of the Royal As-
tronomical Society, 445, 151
Neill, J. L., Muckle, M. T., Zaleski, D. P., et al. 2012, The Astrophysical Journal, 755, 153
Öberg, K. I., Guzmán, V. V., Furuya, K., et al. 2015, Nature, 520, 198
Öberg, K. I., Murray-Clay, R., & Bergin, E. A. 2011, The Astrophysical Journal, 743, L16
Peng, R., Langer, W. D., Velusamy, T., Kuiper, T. B. H., & Levin, S. 1998, The Astrophysical
Journal, 497, 842
Qi, C., Öberg, K. I., Andrews, S. M., et al. 2015, The Astrophysical Journal, 813, 1
Ros, K., & Johansen, A. 2013, Astronomy & Astrophysics, 552, A137
Schoonenberg, D., & Ormel, C. W. 2017, Astronomy & Astrophysics, 602, A21
Selina, R., & Murphy, E. J. 2017, in ngVLA Memo Series, 17
Shingledecker, C. N., & Herbst, E. 2018, Physical Chemistry Chemical Physics, 20, 5359
Wakelam, V., Loison, J. C., Herbst, E., et al. 2015, The Astrophysical Journal Supplement
Series, 217, 1
Walsh, C., Millar, T. J., Nomura, H., et al. 2014, Astronomy & Astrophysics, 563, A33
Walsh, C., Loomis, R. A., Öberg, K. I., et al. 2016, The Astrophysical Journal Letters, 823, L10
Wang, H., Bell, R. C., Iedema, M. J., Tsekouras, A. A., & Cowin, J. P. 2005, The Astrophysical
Journal, 620, 1027


Science with an ngVLA: Observing the Effects of Chemistry on Exoplanets and Planet Formation

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Science with an ngVLA: Observing the Effects of Chemistry on Exoplanets and Planet Formation

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