Context. Gas giants accrete their envelopes from the gas and dust of proto-planetary disks, and therefore it is important to determine the composition of the inner few astronomical units, where most ...giant planets are expected to form. Aims. We aim to constrain the elemental carbon and oxygen abundance in the inner disk (R < 2.3 AU) of TW Hya and compare with the outer disk (R > 2.3 AU) where carbon and oxygen appear underabundant by a factor of approximately 50. Methods. Archival Spitzer-IRS and VLT-CRIRES observations of TW Hya were compared with a detailed thermo-chemical model, DALI. The inner disk gas mass and elemental C and O abundances were varied to fit the mid-infrared H2 and H2O line fluxes as well as the near-infrared CO line flux. Results. Best-fitting models have an inner disk that has a gas mass of 2 × 10−4 M⊙ with C/H ≈ 3 × 10−6 and O/H ≈ 6 × 10−6. The elemental oxygen and carbon abundances of the inner disk are about 50 times lower than in the interstellar medium and are consistent with those found in the outer disk. Conclusions. The uniformly low volatile abundances imply that the inner disk is not enriched by ices on drifting bodies that evaporate. This indicates that drifting grains are stopped in a dust trap outside the water ice line. Such a dust trap would also form a cavity as seen in high-resolution submillimeter continuum observations. If CO is the major carbon carrier in the ices, dust needs to be trapped efficiently outside the CO ice line of ∼20 AU. This would imply that the shallow submillimeter rings in the TW Hya disk outside of 20 AU correspond to very efficient dust traps. The most likely scenario is that more than 98% of the CO has been converted into less volatile species, for example CO2 and CH3OH. A giant planet forming in the inner disk would be accreting gas with low carbon and oxygen abundances as well as very little icy dust, potentially leading to a planet atmosphere with strongly substellar C/H and O/H ratios.
The gas-phase CO abundance (relative to hydrogen) in protoplanetary disks decreases by up to two orders of magnitude from its interstellar medium value of ∼10−4, even after accounting for freeze-out ...and photodissociation. Previous studies have shown that while local chemical processing of CO and the sequestration of CO ice on solids in the midplane can both contribute, neither of these processes appears capable of consistently reaching the observed depletion factors on the relevant timescale of 1-3 Myr. In this study, we model these processes simultaneously by including a compact chemical network (centered on carbon and oxygen) to 2D (r + z) simulations of the outer (r > 20 au) disk regions that include turbulent diffusion, pebble formation, and pebble dynamics. In general, we find that the CO/H2 abundance is a complex function of time and location. Focusing on CO in the warm molecular layer, we find that only the most complete model (with chemistry and pebble evolution included) can reach depletion factors consistent with observations. In the absence of pressure traps, highly efficient planetesimal formation, or high cosmic-ray ionization rates, this model also predicts a resurgence of CO vapor interior to the CO ice-line. We show the impact of physical and chemical processes on the elemental (C/O) and (C/H) ratios (in the gas and ice phases), discuss the use of CO as a disk mass tracer, and, finally, connect our predicted pebble ice compositions to those of pristine planetesimals as found in the Cold Classical Kuiper Belt and debris disks.
Context. The total gas mass is one of the most fundamental properties of disks around young stars, because it controls their evolution and their potential to form planets. To measure disk gas masses, ...CO has long been thought to be the best tracer as it is readily detected at (sub)mm wavelengths in many disks. However, inferred gas masses from CO in recent ALMA observations of large samples of disks in the 1–5 Myr age range seem inconsistent with their inferred dust masses. The derived gas-to-dust mass ratios from CO are between one and two orders of magnitude lower than the ISM value of ~100 even if photodissociation and freeze-out are included. In contrast, Herschel measurements of hydrogen deuteride line emission of a few disks imply gas masses in line with gas-to-dust mass ratios of 100. This suggests that at least one additional mechanism is removing CO from the gas phase. Aims. Here we test the suggestion that the bulk of the CO is chemically processed and that the carbon is sequestered into less volatile species such as CO2, CH3OH, and CH4 in the dense, shielded midplane regions of the disk. This study therefore also addresses the carbon reservoir of the material which ultimately becomes incorporated into planetesimals. Methods. Using our gas-grain chemical code, we performed a parameter exploration and follow the CO abundance evolution over a range of conditions representative of shielded disk midplanes. Results. Consistent with previous studies, we find that no chemical processing of CO takes place on 1–3 Myr timescales for low cosmic-ray ionisation rates, <5 × 10−18 s−1. Assuming an ionisation rate of 10−17 s−1, more than 90% of the CO is converted into other species, but only in the cold parts of the disk below 30 K. This order of magnitude destruction of CO is robust against the choice of grain-surface reaction rate parameters, such as the tunnelling efficiency and diffusion barrier height, for temperatures between 20 and 30 K. Below 20 K there is a strong dependence on the assumed efficiency of H tunnelling. Conclusions. The low temperatures needed for CO chemical processing indicate that the exact disk temperature structure is important, with warm disks around luminous Herbig stars expected to have little to no CO conversion. In contrast, for cold disks around sun-like T Tauri stars, a large fraction of the emitting CO layer is affected unless the disks are young (<1 Myr). This can lead to inferred gas masses that are up to two orders of magnitude lower. Moreover, unless CO is locked up early in large grains, the volatile carbon composition of the icy pebbles and planetesimals forming in the midplane and drifting to the inner disk will be dominated by CH3OH, CO2 and/or hydrocarbons.
The atmospheric composition of giant planets carries the information of their formation history. Superstellar C/H ratios are seen in atmospheres of Jupiter, Saturn, and various giant exoplanets. ...Also, giant exoplanets show a wide range of C/O ratio. To explain these ratios, one hypothesis is that protoplanets accrete carbon-enriched gas when a large number of icy pebbles drift across the CO snowline. Here we report the first direct evidence of an elevated C/H ratio in disk gas. We use two thermo-chemical codes to model the 13C18O, C17O, and C18O (2−1) line spectra of the HD 163296 disk. We show that the gas inside the CO snowline (∼70 au) has a C/H ratio that is 1-2 times higher than the stellar value. This ratio exceeds the expected value substantially, as only 25%-60% of the carbon should be in gas at these radii. Although we cannot rule out the case of a normal C/H ratio inside 70 au, the most probable solution is an elevated C/H ratio that is 2-8 times higher than the expectation. Our model also shows that the gas outside 70 au has a C/H ratio that is 0.1× the stellar value. This picture of enriched C/H gas at the inner region and depleted gas at the outer region is consistent with numerical simulations of icy pebble growth and drift in protoplanetary disks. Our results demonstrate that the large-scale drift of icy pebble can occur in disks and may significantly change the disk gas composition for planet formation.
Abstract
Here we aim to explore the origin of the strong C
2
H lines to reimagine the chemistry of protoplanetary disks. There are a few key aspects that drive our analysis. First, C
2
H is detected ...in young and old systems, hinting at a long-lived chemistry. Second, as a radical, C
2
H is rapidly destroyed, within <1000 yr. These two statements hint that the chemistry responsible for C
2
H emission must be predominantly in the gas phase and must be in equilibrium. Combining new and published chemical models, we find that elevating the total volatile (gas and ice) C/O ratio is the only natural way to create a long-lived, high C
2
H abundance. Most of the C
2
H resides in gas with an
F
UV
/
n
gas
∼ 10
−7
G
0
cm
3
. To elevate the volatile C/O ratio, additional carbon has to be released into the gas to enable equilibrium chemistry under oxygen-poor conditions. Photoablation of carbon-rich grains seems the most straightforward way to elevate the C/O ratio above 1.5, powering a long-lived equilibrium cycle. The regions at which the conditions are optimal for the presence of high C/O ratio and elevated C
2
H abundances in the gas disk set by the
F
UV
/
n
gas
condition lie just outside the pebble disk as well as possibly in disk gaps. This process can thus also explain the (hints of) structure seen in C
2
H observations.
We present a synergic study of protoplanetary disks to investigate links between inner-disk gas molecules and the large-scale migration of solid pebbles. The sample includes 63 disks where two types ...of measurements are available: (1) spatially resolved disk images revealing the radial distribution of disk pebbles (millimeter to centimeter dust grains), from millimeter observations with the Atacama Large Millimeter/Submillimeter Array or the Submillimeter Array, and (2) infrared molecular emission spectra as observed with Spitzer. The line flux ratios of H2O with HCN, C2H2, and CO2 all anticorrelate with the dust disk radius Rdust, expanding previous results found by Najita et al. for HCN/H2O and the dust disk mass. By normalization with the dependence on accretion luminosity common to all molecules, only the H2O luminosity maintains a detectable anticorrelation with disk radius, suggesting that the strongest underlying relation is between H2O and Rdust. If Rdust is set by large-scale pebble drift, and if molecular luminosities trace the elemental budgets of inner-disk warm gas, these results can be naturally explained with scenarios where the inner disk chemistry is fed by sublimation of oxygen-rich icy pebbles migrating inward from the outer disk. Anticorrelations are also detected between all molecular luminosities and the infrared index n13-30, which is sensitive to the presence and size of an inner-disk dust cavity. Overall, these relations suggest a physical interconnection between dust and gas evolution, both locally and across disk scales. We discuss fundamental predictions to test this interpretation and study the interplay between pebble drift, inner disk depletion, and the chemistry of planet-forming material.
Recent theoretical, numerical, and observational works have suggested that when a growing planet opens a gap in its disk the flow of gas into the gap is dominated by gas falling vertically from a ...height of at least one gas scale height. Our primary objective is to include, for the first time, the chemical impact that accreting gas above the midplane will have on the resulting carbon-to-oxygen ratio (C/O). We compute the accretion of gas onto planetary cores beginning at different disk radii and track the chemical composition of the gas and small icy grains to predict the resulting C/O in their atmospheres. In our model, all of the planets which began their evolution inward of 60 AU open a gap in the gas disk, and hence are chemically affected by the vertically accreting gas. Two important conclusions follow from this vertical flow: (1) more oxygen-rich icy dust grains become available for accretion onto the planetary atmosphere; (2) the chemical composition of the gas dominates the final C/O of planets in the inner (<20 AU) part of the disk. This implies that with the launch of the
James Webb
Space Telescope we can trace the disk material that sets the chemical composition of exoplanetary atmospheres.
Context. The infrared ro-vibrational emission lines from organic molecules in the inner regions of protoplanetary disks are unique probes of the physical and chemical structure of planet-forming ...regions and the processes that shape them. These observed lines are mostly interpreted with local thermal equilibrium (LTE) slab models at a single temperature. Aims. We aim to study the non-LTE excitation effects of carbon dioxide (CO2) in a full disk model to evaluate: (i) what the emitting regions of the different CO2 ro-vibrational bands are; (ii) how the CO2 abundance can be best traced using CO2 ro-vibrational lines using future JWST data and; (iii) what the excitation and abundances tell us about the inner disk physics and chemistry. CO2 is a major ice component and its abundance can potentially test models with migrating icy pebbles across the iceline. Methods. A full non-LTE CO2 excitation model has been built starting from experimental and theoretical molecular data. The characteristics of the model are tested using non-LTE slab models. Subsequently the CO2 line formation was modelled using a two-dimensional disk model representative of T Tauri disks where CO2 is detected in the mid-infrared by the Spitzer Space Telescope. Results. The CO2 gas that emits in the 15 μm and 4.5 μm regions of the spectrum is not in LTE and arises in the upper layers of disks, pumped by infrared radiation. The v2 15 μm feature is dominated by optically thick emission for most of the models that fit the observations and increases linearly with source luminosity. Its narrowness compared with that of other molecules stems from a combination of the low rotational excitation temperature (~ 250 K) and the inherently narrower feature for CO2. The inferred CO2 abundances derived for observed disks range from 3 × 10-9 to 1 × 10-7 with respect to total gas density for typical gas/dust ratios of 1000, similar to earlier LTE disk estimates. Line-to-continuum ratios are low, in the order of a few percent, stressing the need for high signal-to-noise (S/N > 300) observations for individual line detections. Conclusions. The inferred CO2 abundances are much lower than those found in interstellar ices (~ 10-5), indicating a reset of the chemistry by high temperature reactions in the inner disk. JWST-MIRI with its higher spectral resolving power will allow a much more accurate retrieval of abundances from individual P- and R-branch lines, together with the 13CO2Q-branch at 15 μm. The 13CO2Q-branch is particularly sensitive to possible enhancements of CO2 due to sublimation of migrating icy pebbles at the iceline(s). Prospects for JWST-NIRSpec are discussed as well.
Context.
Water is a key molecule in star- and planet-forming regions. Recent water line observations toward several low-mass protostars suggest low water gas fractional abundances (<10
−6
with ...respect to total hydrogen density) in the inner warm envelopes (
r
< 10
2
au). Water destruction by X-rays is thought to influence the water abundances in these regions, but the detailed chemistry, including the nature of alternative oxygen carriers, is not yet understood.
Aims.
Our aim is to understand the impact of X-rays on the composition of low-mass protostellar envelopes, focusing specifically on water and related oxygen-bearing species.
Methods.
We computed the chemical composition of two proto-typical low-mass protostellar envelopes using a 1D gas-grain chemical reaction network. We varied the X-ray luminosities of the central protostars, and thus the X-ray ionization rates in the protostellar envelopes.
Results.
The protostellar X-ray luminosity has a strong effect on the water gas abundances, both within and outside the H
2
O snowline (
T
gas
~ 10
2
K,
r
~ 10
2
au). Outside, the water gas abundance increases with
L
X
, from ~10
−10
for low
L
X
to ~10
−8
–10
−7
at
L
X
> 10
30
erg s
−1
. Inside, water maintains a high abundance of ~10
−4
for
L
X
≲ 10
29
–10
30
erg s
−1
, with water and CO being the dominant oxygen carriers. For
L
X
≳ 10
30
–10
31
erg s
−1
, the water gas abundances significantly decrease just inside the water snowline (down to ~10
−8
–10
−7
) and in the innermost regions with
T
gas
≳ 250 K (~10
−6
). For these cases, the fractional abundances of O
2
and O gas reach ~10
−4
within the water snowline, and they become the dominant oxygen carriers. In addition, the fractional abundances of HCO
+
and CH
3
OH, which have been used as tracers of the water snowline, significantly increase and decrease, respectively, within the water snowline as the X-ray fluxes become larger. The fractional abundances of some other dominant molecules, such as CO
2
, OH, CH
4
, HCN, and NH
3
, are also affected by strong X-ray fields, especially within their own snowlines. These X-ray effects are larger in lower-density envelope models.
Conclusions.
X-ray-induced chemistry strongly affects the abundances of water and related molecules including O, O
2
, HCO
+
, and CH
3
OH, and can explain the observed low water gas abundances in the inner protostellar envelopes. In the presence of strong X-ray fields, gas-phase water molecules within the water snowline are mainly destroyed with ion-molecule reactions and X-ray-induced photodissociation. Future observations of water and related molecules (using, e.g., ALMA and ngVLA) will access the regions around protostars where such X-ray-induced chemistry is effective.
Abstract
Observations of protoplanetary disks have revealed them to be complex and dynamic, with vertical and radial transport of gas and dust occurring simultaneously with chemistry and planet ...formation. Previous models of protoplanetary disks focused primarily on chemical evolution of gas and dust in a static disk, or dynamical evolution of solids in a chemically passive disk. In this paper, we present a new 1D method for modeling pebble growth and chemistry simultaneously. Gas and small dust particles are allowed to diffuse vertically, connecting chemistry at all elevations of the disk. Pebbles are assumed to form from the dust present around the midplane, inheriting the composition of ices at this location. We present the results of this model after 1 Myr of disk evolution around a 1
M
⊙
star at various locations both inside and outside the CO snowline. We find that for a turbulent disk (
α
= 10
−3
), CO is depleted from the surface layers of the disk by roughly 1–2 orders of magnitude, consistent with observations of protoplanetary disks. This is achieved by a combination of ice sequestration and decreasing UV opacity, both driven by pebble growth. Further, we find the selective removal of ice species via pebble growth and sequestration can increase gas phase C/O ratios to values of approximately unity. However, our model is unable to produce C/O values of ∼1.5–2.0 inferred from protoplanetary disk observations, implying selective sequestration of ice is not sufficient to explain C/O ratios >1.
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