The Experts below are selected from a list of 1920 Experts worldwide ranked by ideXlab platform

Edwin A Bergin - One of the best experts on this subject based on the ideXlab platform.

  • excess c h in protoplanetary disk gas from icy pebble drift across the co Snowline
    arXiv: Earth and Planetary Astrophysics, 2020
    Co-Authors: Ke Zhang, Arthur D Bosman, Edwin A Bergin
    Abstract:

    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 $^{13}$C$^{18}$O, C$^{17}$O, and C$^{18}$O (2-1) line spectra of the HD 163296 disk. We show that the gas inside the CO Snowline ($\sim$70 au) has a C/H ratio of 1-2 times higher than the stellar value. This ratio exceeds the expected value substantially, as only 25-60% of the carbon should be in gas at these radii. Although we cannot rule out the case of a normal C/H ratio inside 70 au, the most probable solution is an elevated C/H ratio of 2-8 times higher than the expectation. Our model also shows that the gas outside 70 au has a C/H ratio of 0.1$\times$ 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.

  • unlocking co depletion in protoplanetary disks ii primordial c h predictions inside the co Snowline
    The Astrophysical Journal, 2019
    Co-Authors: Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Kamber R Schwarz, Ke Zhang, Dana E Anderson
    Abstract:

    NSF [AST-1514670]; NASA [NNX16AB48G, NAS5-26555]; NASA through Hubble Fellowship Program - Space Telescope Science Institute [HST-HF2-51419.001, HST-HF2-51401.001]

  • unlocking co depletion in protoplanetary disks ii primordial c h predictions inside the co Snowline
    arXiv: Earth and Planetary Astrophysics, 2019
    Co-Authors: Kamber R Schwarz, Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Ke Zhang, Dana E Anderson
    Abstract:

    CO is thought to be the main reservoir of volatile carbon in protoplanetary disks, and thus the primary initial source of carbon in the atmospheres of forming giant planets. However, recent observations of protoplanetary disks point towards low volatile carbon abundances in many systems, including at radii interior to the CO Snowline. One potential explanation is that gas phase carbon is chemically reprocessed into less volatile species, which are frozen on dust grain surfaces as ice. This mechanism has the potential to change the primordial C/H ratio in the gas. However, current observations primarily probe the upper layers of the disk. It is not clear if the low volatile carbon abundances extend to the midplane, where planets form. We have run a grid of 198 chemical models, exploring how the chemical reprocessing of CO depends on disk mass, dust grain size distribution, temperature, cosmic ray and X-ray ionization rate, and initial water abundance. Building on our previous work focusing on the warm molecular layer, here we analyze the results for our grid of models in the disk midplane at 12 au. We find that either an ISM level cosmic ray ionization rate or the presence of UV photons due to a low dust surface density are needed to chemically reduce the midplane CO gas abundance by at least an order of magnitude within 1 Myr. In the majority of our models CO does not undergo substantial reprocessing by in situ chemistry and there is little change in the gas phase C/H and C/O ratios over the lifetime of the typical disk. However, in the small sub-set of disks where the disk midplane is subject to a source of ionization or photolysis, the gas phase C/O ratio increases by up to nearly 9 orders of magnitude due to conversion of CO into volatile hydrocarbons.

  • transport of co in protoplanetary disks consequences of pebble formation settling and radial drift
    arXiv: Earth and Planetary Astrophysics, 2018
    Co-Authors: Sebastiaan Krijt, Edwin A Bergin, Kamber R Schwarz, Fred J Ciesla
    Abstract:

    Current models of (exo)planet formation often rely on a large influx of so-called `pebbles' from the outer disk into the planet formation region. In this paper, we investigate how the formation of pebbles in the cold outer regions of protoplanetary disks and their subsequent migration to the inner disk can alter the gas-phase CO distribution both interior and exterior to the midplane CO Snowline. By simulating the resulting CO abundances in the midplane as well as the warm surface layer, we identify observable signatures of large-scale pebble formation and migration that can be used as `smoking guns' for these important processes. Specifically, we find that after $1\mathrm{~Myr}$, the formation and settling of icy pebbles results in the removal of up to $80\%$ of the CO vapor in the warm ($T>22\mathrm{~K}$) disk layers outside the CO Snowline, while the radial migration of pebbles results in the generation of a plume of CO vapor interior the Snowline, increasing the CO abundance by a factor ${\sim}2{-}6$ depending on the strength of the turbulence and the sizes of the individual pebbles. The absence of this plume of CO vapor in young nearby disks could indicate efficient conversion of CO into a more refractory species, or a reduction in the radial mass flux of pebbles by, for example, disk inhomogeneities or early planetesimal formation.

  • unlocking co depletion in protoplanetary disks i the warm molecular layer
    The Astrophysical Journal, 2018
    Co-Authors: Kamber R Schwarz, Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Ke Zhang, Dana E Anderson
    Abstract:

    CO is commonly used as a tracer of the total gas mass in both the interstellar medium and in protoplanetary disks. Recently, there has been much debate about the utility of CO as a mass tracer in disks. Observations of CO in protoplanetary disks reveal a range of CO abundances, with measurements of low CO to dust mass ratios in numerous systems. One possibility is that carbon is removed from CO via chemistry. However, the full range of physical conditions conducive to this chemical reprocessing is not well understood. We perform a systematic survey of the time dependent chemistry in protoplanetary disks for 198 models with a range of physical conditions. We vary dust grain size distribution, temperature, comic-ray and X-ray ionization rates, disk mass, and initial water abundance, detailing what physical conditions are necessary to activate the various CO depletion mechanisms in the warm molecular layer. We focus our analysis on the warm molecular layer in two regions: the outer disk (100 au) well outside the CO Snowline and the inner disk (19 au) just inside the midplane CO Snowline. After 1 Myr, we find that the majority of models have a CO abundance relative to H_2 less than 10^(−4) in the outer disk, while an abundance less than 10^(−5) requires the presence of cosmic-rays. Inside the CO Snowline, significant depletion of CO only occurs in models with a high cosmic-ray rate. If cosmic-rays are not present in young disks, it is difficult to chemically remove carbon from CO. Additionally, removing water prior to CO depletion impedes the chemical processing of CO. Chemical processing alone cannot explain current observations of low CO abundances. Other mechanisms must also be involved.

Karin I Oberg - One of the best experts on this subject based on the ideXlab platform.

  • Jupiter's composition suggests its core assembled exterior to the N2 Snowline
    The Astronomical Journal, 2019
    Co-Authors: Karin I Oberg, Robin Wordsworth
    Abstract:

    Jupiter's atmosphere is enriched in C, N, S, P, Ar, Kr and Xe with respect to solar abundances by a factor of ~3. Gas Giant envelopes are mainly enriched through the dissolution of solids in the atmosphere, and this constant enrichment factor is puzzling since several of the above elements are not expected to have been in the solid phase in Jupiter's feeding zone; most seriously, Ar and the main carrier of N, N2, only condense at the very low temperatures, 21-26 K, associated with the outer solar nebula. We propose that a plausible solution to the enigma of Jupiter's uniform enrichment pattern is that Jupiter's core formed exterior to the N2 and Ar Snowlines, beyond 30 au, resulting in a Solar composition core in all volatiles heavier than Ne. During envelope accretion and planetesimal bombardment, some of the core mixed in with the envelope causing the observed enrichment pattern. We show that this scenario naturally produces the observed atmosphere composition, even with substantial pollution from N-poor pebble and planetesimal accretion in Jupiter's final feeding zone. We note that giant core formation at large nebular radii is consistent with recent models of gas giant core formation through pebble accretion, which requires the core to form exterior to Jupiter's current location to counter rapid inward migration during the core and envelope formation process. If this scenario is common, gas giant core formation may account for many of the gaps observed in protoplanetary disks between 10s and 100 au.

  • unlocking co depletion in protoplanetary disks ii primordial c h predictions inside the co Snowline
    The Astrophysical Journal, 2019
    Co-Authors: Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Kamber R Schwarz, Ke Zhang, Dana E Anderson
    Abstract:

    NSF [AST-1514670]; NASA [NNX16AB48G, NAS5-26555]; NASA through Hubble Fellowship Program - Space Telescope Science Institute [HST-HF2-51419.001, HST-HF2-51401.001]

  • unlocking co depletion in protoplanetary disks ii primordial c h predictions inside the co Snowline
    arXiv: Earth and Planetary Astrophysics, 2019
    Co-Authors: Kamber R Schwarz, Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Ke Zhang, Dana E Anderson
    Abstract:

    CO is thought to be the main reservoir of volatile carbon in protoplanetary disks, and thus the primary initial source of carbon in the atmospheres of forming giant planets. However, recent observations of protoplanetary disks point towards low volatile carbon abundances in many systems, including at radii interior to the CO Snowline. One potential explanation is that gas phase carbon is chemically reprocessed into less volatile species, which are frozen on dust grain surfaces as ice. This mechanism has the potential to change the primordial C/H ratio in the gas. However, current observations primarily probe the upper layers of the disk. It is not clear if the low volatile carbon abundances extend to the midplane, where planets form. We have run a grid of 198 chemical models, exploring how the chemical reprocessing of CO depends on disk mass, dust grain size distribution, temperature, cosmic ray and X-ray ionization rate, and initial water abundance. Building on our previous work focusing on the warm molecular layer, here we analyze the results for our grid of models in the disk midplane at 12 au. We find that either an ISM level cosmic ray ionization rate or the presence of UV photons due to a low dust surface density are needed to chemically reduce the midplane CO gas abundance by at least an order of magnitude within 1 Myr. In the majority of our models CO does not undergo substantial reprocessing by in situ chemistry and there is little change in the gas phase C/H and C/O ratios over the lifetime of the typical disk. However, in the small sub-set of disks where the disk midplane is subject to a source of ionization or photolysis, the gas phase C/O ratio increases by up to nearly 9 orders of magnitude due to conversion of CO into volatile hydrocarbons.

  • unlocking co depletion in protoplanetary disks i the warm molecular layer
    The Astrophysical Journal, 2018
    Co-Authors: Kamber R Schwarz, Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Ke Zhang, Dana E Anderson
    Abstract:

    CO is commonly used as a tracer of the total gas mass in both the interstellar medium and in protoplanetary disks. Recently, there has been much debate about the utility of CO as a mass tracer in disks. Observations of CO in protoplanetary disks reveal a range of CO abundances, with measurements of low CO to dust mass ratios in numerous systems. One possibility is that carbon is removed from CO via chemistry. However, the full range of physical conditions conducive to this chemical reprocessing is not well understood. We perform a systematic survey of the time dependent chemistry in protoplanetary disks for 198 models with a range of physical conditions. We vary dust grain size distribution, temperature, comic-ray and X-ray ionization rates, disk mass, and initial water abundance, detailing what physical conditions are necessary to activate the various CO depletion mechanisms in the warm molecular layer. We focus our analysis on the warm molecular layer in two regions: the outer disk (100 au) well outside the CO Snowline and the inner disk (19 au) just inside the midplane CO Snowline. After 1 Myr, we find that the majority of models have a CO abundance relative to H_2 less than 10^(−4) in the outer disk, while an abundance less than 10^(−5) requires the presence of cosmic-rays. Inside the CO Snowline, significant depletion of CO only occurs in models with a high cosmic-ray rate. If cosmic-rays are not present in young disks, it is difficult to chemically remove carbon from CO. Additionally, removing water prior to CO depletion impedes the chemical processing of CO. Chemical processing alone cannot explain current observations of low CO abundances. Other mechanisms must also be involved.

  • excess c o and c h in outer protoplanetary disk gas
    The Astrophysical Journal, 2016
    Co-Authors: Karin I Oberg, Edwin A Bergin
    Abstract:

    The compositions of nascent planets depend on the compositions of their birth disks. In particular, the elemental compositions of gas giant gaseous envelopes depend on the elemental compositions of the disk gas from which the envelopes are accreted. Previous models have demonstrated that sequential freeze-out of O- and C-bearing volatiles in disks will result in supersolar C/O ratios and subsolar C/H ratios in the gas between water and CO Snowlines. However, this result does not take into account the expected grain growth and radial drift of pebbles in disks, and the accompanying redistribution of volatiles from the outer to the inner disk. Using a toy model we demonstrate that when drift is considered, CO is enhanced between the water and CO Snowline, resulting in both supersolar C/O and C/H ratios in the disk gas in the gas giant formation zone. This result appears to be robust for the disk model as long as there is substantial pebble drift across the CO Snowline, and the efficiency of CO vapor diffusion is limited. Gas giants that accrete their gaseous envelopes exterior to the water Snowline and do not experience substantial core-envelope mixing may thus feature both superstellar C/O and C/H ratios in their atmospheres. Pebble drift will also affect the nitrogen and noble gas abundances in the planet-forming zones, which may explain some of Jupiter's peculiar abundance patterns.

T J Millar - One of the best experts on this subject based on the ideXlab platform.

  • dust continuum emission and the upper limit fluxes of submillimeter water lines of the protoplanetary disk around hd 163296 observed by alma
    The Astrophysical Journal, 2019
    Co-Authors: Shota Notsu, Eiji Akiyama, Alice S Booth, Hideko Nomura, Catherine Walsh, Tomoya Hirota, Mitsuhiko Honda, Takashi Tsukagoshi, T J Millar
    Abstract:

    In this paper, we analyze the upper limit fluxes of submillimeter ortho-H2-16O 321 GHz, para-H2-18O 322 GHz, and HDO 335 GHz lines from the protoplanetary disk around the Herbig Ae star HD 163296, using the Atacama Large Millimeter/Submillimeter Array. These water lines are considered to be the best candidate submillimeter lines to locate the position of the H2O Snowline, on the basis of our previous model calculations. We compare the upper limit fluxes with the values calculated by our models with dust emission included, and we constrain the line-emitting region and the dust opacity from the observations. We conclude that, if the outer edge of the region with a high water abundance and the position of the water Snowline are both beyond 8 au, then themillimeter dust opacity κ mm will have a value larger than 2.0 cm2 g−1. In addition, the position of the water Snowline must lie inside 20 au if the millimeter dust opacity κ mm is 2.0 cm2 g−1. Future observations of the dust continuum emission at higher angular resolution and submillimeter water lines with a longer observation time are required to clarify the detailed structures and the position of the H2O Snowline in the disk midplane.

  • dust continuum emission and the upper limit fluxes of sub millimeter water lines of the protoplanetary disk around hd 163296 observed by alma
    arXiv: Earth and Planetary Astrophysics, 2019
    Co-Authors: Shota Notsu, Eiji Akiyama, Alice S Booth, Hideko Nomura, Catherine Walsh, Tomoya Hirota, Mitsuhiko Honda, Takashi Tsukagoshi, T J Millar
    Abstract:

    In this paper, we analyse the upper limit fluxes of sub-millimeter ortho-H$_{2}$$^{16}$O 321 GHz, para-H$_{2}$$^{18}$O 322 GHz, and HDO 335 GHz lines from the protoplanetary disk around the Herbig Ae star HD 163296, using the Atacama Large Millimeter/Submillimeter Array (ALMA). These water lines are considered to be the best candidate sub-millimeter lines to locate the position of the H$_{2}$O Snowline, on the basis of our previous model calculations. We compare the upper limit fluxes with the values calculated by our models with dust emission included, and we constrain the line emitting region and the dust opacity from the observations. We conclude that, if the outer edge of the region with high water vapor abundance and if the position of the water Snowline are beyond 8 au also, the mm dust opacity will have a value larger than 2.0 cm$^{2}$ g$^{-1}$. In addition, the position of the water Snowline will be inside 20 au, if the mm dust opacity is 2.0 cm$^{2}$ g$^{-1}$. Future observations of the dust continuum emission at higher angular resolution and sub-millimeter water lines with longer observation time are required to clarify the detailed structures and the position of the H$_{2}$O Snowline in the disk midplane.

  • Candidate Water Vapor Lines to Locate the H2O Snowline through High-dispersion Spectroscopic Observations. III. Submillimeter H2 16O and H2 18O Lines
    'American Astronomical Society', 2018
    Co-Authors: Notsu S, Nomura H, Walsh C, Honda M, Hirota T, Akiyama E, T J Millar
    Abstract:

    In this paper, we extend the results presented in our former papers on using ortho-H216O line profiles to constrain the location of the H2O Snowline in T Tauri and Herbig Ae disks, to include submillimeter para-H216O and ortho- and para-H218O lines. Since the number densities of the ortho- and para-H218O molecules are about 560 times smaller than their 16O analogs, they trace deeper into the disk than the ortho-H216O lines (down to z = 0, i.e., the midplane). Thus these H218O lines are potentially better probes of the position of the H2O Snowline at the disk midplane, depending on the dust optical depth. The values of the Einstein A coefficients of submillimeter candidate water lines tend to be lower (typically

Kamber R Schwarz - One of the best experts on this subject based on the ideXlab platform.

  • unlocking co depletion in protoplanetary disks ii primordial c h predictions inside the co Snowline
    The Astrophysical Journal, 2019
    Co-Authors: Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Kamber R Schwarz, Ke Zhang, Dana E Anderson
    Abstract:

    NSF [AST-1514670]; NASA [NNX16AB48G, NAS5-26555]; NASA through Hubble Fellowship Program - Space Telescope Science Institute [HST-HF2-51419.001, HST-HF2-51401.001]

  • unlocking co depletion in protoplanetary disks ii primordial c h predictions inside the co Snowline
    arXiv: Earth and Planetary Astrophysics, 2019
    Co-Authors: Kamber R Schwarz, Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Ke Zhang, Dana E Anderson
    Abstract:

    CO is thought to be the main reservoir of volatile carbon in protoplanetary disks, and thus the primary initial source of carbon in the atmospheres of forming giant planets. However, recent observations of protoplanetary disks point towards low volatile carbon abundances in many systems, including at radii interior to the CO Snowline. One potential explanation is that gas phase carbon is chemically reprocessed into less volatile species, which are frozen on dust grain surfaces as ice. This mechanism has the potential to change the primordial C/H ratio in the gas. However, current observations primarily probe the upper layers of the disk. It is not clear if the low volatile carbon abundances extend to the midplane, where planets form. We have run a grid of 198 chemical models, exploring how the chemical reprocessing of CO depends on disk mass, dust grain size distribution, temperature, cosmic ray and X-ray ionization rate, and initial water abundance. Building on our previous work focusing on the warm molecular layer, here we analyze the results for our grid of models in the disk midplane at 12 au. We find that either an ISM level cosmic ray ionization rate or the presence of UV photons due to a low dust surface density are needed to chemically reduce the midplane CO gas abundance by at least an order of magnitude within 1 Myr. In the majority of our models CO does not undergo substantial reprocessing by in situ chemistry and there is little change in the gas phase C/H and C/O ratios over the lifetime of the typical disk. However, in the small sub-set of disks where the disk midplane is subject to a source of ionization or photolysis, the gas phase C/O ratio increases by up to nearly 9 orders of magnitude due to conversion of CO into volatile hydrocarbons.

  • transport of co in protoplanetary disks consequences of pebble formation settling and radial drift
    arXiv: Earth and Planetary Astrophysics, 2018
    Co-Authors: Sebastiaan Krijt, Edwin A Bergin, Kamber R Schwarz, Fred J Ciesla
    Abstract:

    Current models of (exo)planet formation often rely on a large influx of so-called `pebbles' from the outer disk into the planet formation region. In this paper, we investigate how the formation of pebbles in the cold outer regions of protoplanetary disks and their subsequent migration to the inner disk can alter the gas-phase CO distribution both interior and exterior to the midplane CO Snowline. By simulating the resulting CO abundances in the midplane as well as the warm surface layer, we identify observable signatures of large-scale pebble formation and migration that can be used as `smoking guns' for these important processes. Specifically, we find that after $1\mathrm{~Myr}$, the formation and settling of icy pebbles results in the removal of up to $80\%$ of the CO vapor in the warm ($T>22\mathrm{~K}$) disk layers outside the CO Snowline, while the radial migration of pebbles results in the generation of a plume of CO vapor interior the Snowline, increasing the CO abundance by a factor ${\sim}2{-}6$ depending on the strength of the turbulence and the sizes of the individual pebbles. The absence of this plume of CO vapor in young nearby disks could indicate efficient conversion of CO into a more refractory species, or a reduction in the radial mass flux of pebbles by, for example, disk inhomogeneities or early planetesimal formation.

  • unlocking co depletion in protoplanetary disks i the warm molecular layer
    The Astrophysical Journal, 2018
    Co-Authors: Kamber R Schwarz, Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Ke Zhang, Dana E Anderson
    Abstract:

    CO is commonly used as a tracer of the total gas mass in both the interstellar medium and in protoplanetary disks. Recently, there has been much debate about the utility of CO as a mass tracer in disks. Observations of CO in protoplanetary disks reveal a range of CO abundances, with measurements of low CO to dust mass ratios in numerous systems. One possibility is that carbon is removed from CO via chemistry. However, the full range of physical conditions conducive to this chemical reprocessing is not well understood. We perform a systematic survey of the time dependent chemistry in protoplanetary disks for 198 models with a range of physical conditions. We vary dust grain size distribution, temperature, comic-ray and X-ray ionization rates, disk mass, and initial water abundance, detailing what physical conditions are necessary to activate the various CO depletion mechanisms in the warm molecular layer. We focus our analysis on the warm molecular layer in two regions: the outer disk (100 au) well outside the CO Snowline and the inner disk (19 au) just inside the midplane CO Snowline. After 1 Myr, we find that the majority of models have a CO abundance relative to H_2 less than 10^(−4) in the outer disk, while an abundance less than 10^(−5) requires the presence of cosmic-rays. Inside the CO Snowline, significant depletion of CO only occurs in models with a high cosmic-ray rate. If cosmic-rays are not present in young disks, it is difficult to chemically remove carbon from CO. Additionally, removing water prior to CO depletion impedes the chemical processing of CO. Chemical processing alone cannot explain current observations of low CO abundances. Other mechanisms must also be involved.

  • the radial distribution of h2 and co in tw hya as revealed by resolved alma observations of co isotopologues
    The Astrophysical Journal, 2016
    Co-Authors: Kamber R Schwarz, Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Ke Zhang, Ewine F. Van Dishoeck
    Abstract:

    CO is widely used as a tracer of molecular gas. However, there is now mounting evidence that gas phase carbon is depleted in the disk around TW Hya. Previous efforts to quantify this depletion have been hampered by uncertainties regarding the radial thermal structure in the disk. Here we present resolved ALMA observations of ^(13)CO 3-2, C^(18)O 3-2, ^(13)CO 6-5, and C^(18)O 6-5 emission in TW Hya, which allow us to derive radial gas temperature and gas surface density profiles, as well as map the CO abundance as a function of radius. These observations provide a measurement of the surface CO Snowline at ~30 AU and show evidence for an outer ring of CO emission centered at 53 AU, a feature previously seen only in less abundant species. Further, the derived CO gas temperature profile constrains the freeze out temperature of CO in the warm molecular layer to <21K. Combined with the previous detection of HD 1-0, these data constrain the surface density of the warm H_2 gas in the inner ~30 AU such that Σwarm gas = 4.7^(+3.0)_(-2.9) g cm^(-2)(R/10 au)^(-1/2). We find that CO is depleted by two orders of magnitude from R = 10-60 AU, with the small amount of CO returning to the gas phase inside the surface CO Snowline insufficient to explain the overall depletion. Finally, this new data is used in conjunction with previous modeling of the TW Hya disk to constrain the midplane CO Snowline to 17–23 AU.

Ke Zhang - One of the best experts on this subject based on the ideXlab platform.

  • excess c h in protoplanetary disk gas from icy pebble drift across the co Snowline
    arXiv: Earth and Planetary Astrophysics, 2020
    Co-Authors: Ke Zhang, Arthur D Bosman, Edwin A Bergin
    Abstract:

    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 $^{13}$C$^{18}$O, C$^{17}$O, and C$^{18}$O (2-1) line spectra of the HD 163296 disk. We show that the gas inside the CO Snowline ($\sim$70 au) has a C/H ratio of 1-2 times higher than the stellar value. This ratio exceeds the expected value substantially, as only 25-60% of the carbon should be in gas at these radii. Although we cannot rule out the case of a normal C/H ratio inside 70 au, the most probable solution is an elevated C/H ratio of 2-8 times higher than the expectation. Our model also shows that the gas outside 70 au has a C/H ratio of 0.1$\times$ 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.

  • Excess C/H in Protoplanetary Disk Gas from Icy Pebble Drift across the CO Snowline
    'American Astronomical Society', 2020
    Co-Authors: Ke Zhang, Bosman, Arthur D., Bergin, Edwin A.
    Abstract:

    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 $^{13}$C$^{18}$O, C$^{17}$O, and C$^{18}$O (2-1) line spectra of the HD 163296 disk. We show that the gas inside the CO Snowline ($\sim$70 au) has a C/H ratio of 1-2 times higher than the stellar value. This ratio exceeds the expected value substantially, as only 25-60% of the carbon should be in gas at these radii. Although we cannot rule out the case of a normal C/H ratio inside 70 au, the most probable solution is an elevated C/H ratio of 2-8 times higher than the expectation. Our model also shows that the gas outside 70 au has a C/H ratio of 0.1$\times$ 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.Comment: 9 pages, 4 figures, and 1 table. Accepted for publication in ApJ

  • unlocking co depletion in protoplanetary disks ii primordial c h predictions inside the co Snowline
    The Astrophysical Journal, 2019
    Co-Authors: Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Kamber R Schwarz, Ke Zhang, Dana E Anderson
    Abstract:

    NSF [AST-1514670]; NASA [NNX16AB48G, NAS5-26555]; NASA through Hubble Fellowship Program - Space Telescope Science Institute [HST-HF2-51419.001, HST-HF2-51401.001]

  • unlocking co depletion in protoplanetary disks ii primordial c h predictions inside the co Snowline
    arXiv: Earth and Planetary Astrophysics, 2019
    Co-Authors: Kamber R Schwarz, Karin I Oberg, Edwin A Bergin, Geoffrey A Blake, Ilsedore L Cleeves, Ke Zhang, Dana E Anderson
    Abstract:

    CO is thought to be the main reservoir of volatile carbon in protoplanetary disks, and thus the primary initial source of carbon in the atmospheres of forming giant planets. However, recent observations of protoplanetary disks point towards low volatile carbon abundances in many systems, including at radii interior to the CO Snowline. One potential explanation is that gas phase carbon is chemically reprocessed into less volatile species, which are frozen on dust grain surfaces as ice. This mechanism has the potential to change the primordial C/H ratio in the gas. However, current observations primarily probe the upper layers of the disk. It is not clear if the low volatile carbon abundances extend to the midplane, where planets form. We have run a grid of 198 chemical models, exploring how the chemical reprocessing of CO depends on disk mass, dust grain size distribution, temperature, cosmic ray and X-ray ionization rate, and initial water abundance. Building on our previous work focusing on the warm molecular layer, here we analyze the results for our grid of models in the disk midplane at 12 au. We find that either an ISM level cosmic ray ionization rate or the presence of UV photons due to a low dust surface density are needed to chemically reduce the midplane CO gas abundance by at least an order of magnitude within 1 Myr. In the majority of our models CO does not undergo substantial reprocessing by in situ chemistry and there is little change in the gas phase C/H and C/O ratios over the lifetime of the typical disk. However, in the small sub-set of disks where the disk midplane is subject to a source of ionization or photolysis, the gas phase C/O ratio increases by up to nearly 9 orders of magnitude due to conversion of CO into volatile hydrocarbons.

  • Unlocking CO Depletion in Protoplanetary Disks. II. Primordial C/H Predictions inside the CO Snowline
    'American Astronomical Society', 2019
    Co-Authors: Schwarz, Kamber R., Ke Zhang, Bergin, Edwin A., Öberg Karin, Blake, Geoffrey A., Cleeves L. Ilsedore, Anderson, Dana E.
    Abstract:

    CO is thought to be the main reservoir of volatile carbon in protoplanetary disks, and thus the primary initial source of carbon in the atmospheres of forming giant planets. However, recent observations of protoplanetary disks point toward low volatile carbon abundances in many systems, including at radii interior to the CO Snowline. One potential explanation is that gas phase carbon is chemically reprocessed into less volatile species, which are frozen on dust grain surfaces as ice. This mechanism has the potential to change the primordial C/H ratio in the gas. However, current observations primarily probe the upper layers of the disk. It is not clear if the low volatile carbon abundances extend to the midplane, where planets form. We have run a grid of 198 chemical models, exploring how the chemical reprocessing of CO depends on disk mass, dust grain size distribution, temperature, cosmic-ray and X-ray ionization rate, and initial water abundance. Building on our previous work focusing on the warm molecular layer, here we analyze the results for our grid of models in the disk midplane at 12 au. We find that either an ISM level cosmic-ray ionization rate or the presence of UV photons due to a low dust surface density are needed to chemically reduce the midplane CO gas abundance by at least an order of magnitude within 1 Myr. In the majority of our models CO does not undergo substantial reprocessing by in situ chemistry and there is little change in the gas phase C/H and C/O ratios over the lifetime of the typical disk. However, in the small subset of disks where the disk midplane is subject to a source of ionization or photolysis, the gas phase C/O ratio increases by up to nearly 9 orders of magnitude due to conversion of CO into volatile hydrocarbons.NSF [AST-1514670]; NASA [NNX16AB48G, NAS5-26555]; NASA through Hubble Fellowship Program - Space Telescope Science Institute [HST-HF2-51419.001, HST-HF2-51401.001]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu