120 research outputs found
Evolution of the Volatile Inventory During Planet Formation
Today, with the wealth of data provided by the Atacama Large Millimeter/ submillimeter Array (ALMA), we are beginning to characterizing the chemistry associated with the early stages of planet formation. Planets are born within disks of gas, primarily in molecular form, and dust. ALMA enables us to, for the first time, resolve these disks down to the radii of giant planet formation, and in some instances even into the zone where Earth-like planets are born. In this dissertation I explore one of the major results from ALMA regarding the disposition of the primary carriers of carbon and nitrogen within protoplanetary disks. The state of carbon and nitrogen has important implications for the composition of planets. Knowing the abundance of gas phase species in the disk provides the starting composition for the atmospheres of gaseous giant planets while the composition of ices influence the composition of solid bodies, such as terrestrial planets.
Using both models and observations, this dissertation explores the evolution of volatile molecules in protoplanetary disks. Using chemical models, I have shown that volatile nitrogen in protoplanetary disks is likely found mainly in the form of molecular nitrogen, a molecule which remains in the gas phase throughout much of the disk (Chapter 2).
The rest of this dissertation focuses on the chemistry of carbon, as the main carbon carriers are more readily accessible to observational characterization. My analysis of CO isotopologue emission in the protoplanetary disk TW Hydrae, in conjunction with emission from the molecular hydrogen isotopologue HD, reveals that CO gas, the primary carrier of volatile carbon, is under-abundant relative to the total gas mass throughout the disk (Chapter 3). I thus demonstrate that it is CO, and not the total gas, which is missing in this one system. To explore the potential cause of this depletion I then ran a large grid of chemical models for disks with a wide range of physical conditions in order to analyze how effective chemical reactions are at removing volatile molecules from the gas. I found that in both the upper layers of the disk (Chapter 4) and in the midplane (Chapter 5), an ISM level cosmic ray ionization rate, one unattenuated by disk winds, is needed to reduce the CO gas abundance by greater than an order of magnitude during the typical disk lifetime. In the absence of cosmic rays, chemical processes involving ultraviolet or X-ray photons can also reprocess CO on timescales of several million years, though not to the extent seen in the high cosmic ray rate models. I conclude that chemistry is unlikely to be the only cause of volatile depletion, given that many young, 1-3 million year old, protoplanetary disks have measured CO abundances one to two orders of magnitude below expectations. Other processes, such as vertical mixing of the gas and grain growth, must also contribute.
The results of my chemical modeling suggest that, under certain circumstances, gas giants which form after a million years of chemical evolution may accrete envelopes under-abundant in volatile elements such as carbon, nitrogen, and oxygen. To conclude, a summary of the findings and future directions are discussed in Chapter 6.PHDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/146057/1/kamberrs_1.pd
Line Ratios Reveal N2H+ Emission Originates Above the Midplane in TW Hydrae
Line ratios for different transitions of the same molecule have long been
used as a probe of gas temperature. Here we use ALMA observations of the N2H+
J~=~1-0 and J~=~4-3 lines in the protoplanetary disk around TW Hya to derive
the temperature at which these lines emit. We find an averaged temperature of
39~K with a one sigma uncertainty of 2~K for the radial range 0.8-2'',
significantly warmer than the expected midplane temperature beyond 0.5'' in
this disk. We conclude that the N2H+ emission in TW Hya is not emitting from
near the midplane, but rather from higher in the disk, in a region likely
bounded by processes such as photodissociation or chemical reprocessing of CO
and N2 rather than freeze out.Comment: Accepted for publication in ApJ Letters, 5 pages, 1 figur
Mass inventory of the giant-planet formation zone in a solar nebula analog
The initial mass distribution in the solar nebula is a critical input to
planet formation models that seek to reproduce today's Solar System.
Traditionally, constraints on the gas mass distribution are derived from
observations of the dust emission from disks, but this approach suffers from
large uncertainties in grain growth and gas-to-dust ratio. On the other hand,
previous observations of gas tracers only probe surface layers above the bulk
mass reservoir. Here we present the first partially spatially resolved
observations of the CO J=3-2 line emission in the closest
protoplanetary disk, TW Hya, a gas tracer that probes the bulk mass
distribution. Combining it with the CO J=3-2 emission and the previously
detected HD J=1-0 flux, we directly constrain the mid-plane temperature and
optical depths of gas and dust emission. We report a gas mass distribution of
13(R/20.5AU) g cm in the
expected formation zone of gas and ice giants (5-21AU). We find the total
gas/millimeter-sized dust mass ratio is 140 in this region, suggesting that at
least 2.4M_earth of dust aggregates have grown to >centimeter sizes (and
perhaps much larger). The radial distribution of gas mass is consistent with a
self-similar viscous disk profile but much flatter than the posterior
extrapolation of mass distribution in our own and extrasolar planetary systems.Comment: Definitive version of the manuscript is published in Nature
Astronomy, 10.1038/s41550-017-0130. This is the authors' versio
Rapid Evolution of Volatile CO from the Protostellar Disk Stage to the Protoplanetary Disk Stage
Recent observations show that the CO gas abundance, relative to H, in
many 1-10 Myr old protoplanetary disks may be heavily depleted, by a factor of
10-100 compared to the canonical interstellar medium value of 10. When
and how this depletion happens can significantly affect compositions of
planetesimals and atmospheres of giant planets. It is therefore important to
constrain if the depletion occurs already at the earliest protostellar disk
stage. Here we present spatially resolved observations of CO, CO,
and CO =2-1 lines in three protostellar disks. We show that
the CO line emits from both the disk and the inner envelope, while
CO and CO lines are consistent with a disk origin. The
line ratios indicate that both CO and CO lines are optically
thick in the disk region, and only CO line is optically thin. The
line profiles of the CO emissions are best reproduced by
Keplerian gaseous disks at similar sizes as their mm-continuum emissions,
suggesting small radial separations between the gas and mm-sized grains in
these disks, in contrast to the large separation commonly seen in
protoplanetary disks. Assuming a gas-to-dust ratio of 100, we find that the CO
gas abundances in these protostellar disks are consistent with the ISM
abundance within a factor of 2, nearly one order of magnitude higher than the
average value of 1-10 Myr old disks. These results suggest that there is a
fast, 1 Myr, evolution of the abundance of CO gas from the protostellar
disk stage to the protoplanetary disk stage.Comment: 10 pages, 3 figures, 2 tables. Accepted for publication in ApJ
Unlocking CO Depletion in Protoplanetary Disks II. Primordial C/H Predictions Inside the CO Snowline
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.Comment: Accepted for publication in ApJ, 15 pages, 10 figures, 3 table
Systematic Variations of CO Gas Abundance with Radius in Gas-rich Protoplanetary Disks
CO is the most widely used gas tracer of protoplanetary disks. Its abundance
is usually assumed to be an interstellar ratio throughout the warm molecular
layer of the disk. But recent observations of low CO gas abundance in many
protoplanetary disks challenge our understanding of physical and chemical
evolutions in disks. Here we investigate the CO abundance structures in four
well-studied disks and compare their structures with predictions of chemical
processing of CO and transport of CO ice-coated dust grains in disks. We use
spatially resolved CO isotopologue line observations and detailed
thermo-chemical models to derive CO abundance structures. We find that the CO
abundance varies with radius by an order of magnitude in these disks. We show
that although chemical processes can efficiently reduce the total column of CO
gas within 1 Myr under an ISM level of cosmic-ray ionization rate, the
depletion mostly occurs at the deep region of a disk. Without sufficient
vertical mixing, the surface layer is not depleted enough to reproduce weak CO
emissions observed. The radial profiles of CO depletion in three disks are
qualitatively consistent with predictions of pebble formation, settling, and
drifting in disks. But the dust evolution alone cannot fully explain the high
depletion observed in some disks. These results suggest that dust evolution may
play a significant role in transporting volatile materials and a coupled
chemical-dynamical study is necessary to understand what raw materials are
available for planet formation at different distances from the central star.Comment: 17 pages, 8 figures, accepted for publication in the Ap
CO Depletion in Protoplanetary Disks: A Unified Picture Combining Physical Sequestration and Chemical Processing
The gas-phase CO abundance (relative to hydrogen) in protoplanetary disks
decreases by up to 2 orders of magnitude from its ISM value ,
even after accounting for freeze-out and photo-dissociation. 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 . In this study, we
model these processes simultaneously by including a compact chemical network
(centered on carbon and oxygen) to 2D () simulations of the outer
() disk regions that include turbulent diffusion, pebble
formation, and pebble dynamics. In general, we find that the CO/H 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 snowline. 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.Comment: Accepted for publication in The Astrophysical Journa
On the Commonality of 10-30AU Sized Axisymmetric Dust Structures in Protoplanetary Disks
An unsolved problem in step-wise core-accretion planet formation is that rapid radial drift in gas-rich protoplanetary disks should drive millimeter-/meter-sized particles inward to the central star before large bodies can form. One promising solution is to confine solids within small-scale structures. Here, we investigate dust structures in the (sub)millimeter continuum emission of four disks (TW Hya, HL Tau, HD 163296, and DM Tau), a sample of disks with the highest spatial resolution Atacama Large Millimeter/submillimeter Array observations to date. We retrieve the surface brightness distributions using synthesized images and fitting visibilities with analytical functions. We find that the continuum emission of the four disks is ~axisymmetric but rich in 10–30 AU-sized radial structures, possibly due to physical gaps, surface density enhancements, or localized dust opacity variations within the disks. These results suggest that small-scale axisymmetric dust structures are likely to be common, as a result of ubiquitous processes in disk evolution and planet formation. Compared with recent spatially resolved observations of CO snow lines in these same disks, all four systems show enhanced continuum emission from regions just beyond the CO condensation fronts, potentially suggesting a causal relationship between dust growth/trapping and snow lines
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