Abstract
Young stars are highly variable in the X-ray regime. In particular, bright X-ray flares can substantially enhance ionization in the surrounding protoplanetary disk. Since disk chemical ...evolution is impacted by ionization, X-ray flares have the potential to fundamentally alter the chemistry of planet-forming regions. We present two-dimensional disk chemical models that incorporate a stochastic X-ray flaring module, named XGEN, and examine the flares’ overall chemical impact compared to models that assume a constant X-ray flux. We examine the impact of 500 yr of flaring events and find global chemical changes on both short timescales (days) in response to discrete flaring events and long timescales (centuries) in response to the cumulative impact of many flares. Individual X-ray flares most strongly affect small gas-phase cations, where a single flare can temporarily enhance the abundance of species such as
H
3
+
, HCO
+
, CH
3
+
, and C
+
. We find that flares can also drive chemistry out of “steady state” over longer time periods, where the disk-integrated abundance of some species, such as O and O
2
, changes by a few percent over the 500 yr model. We also explore whether the specific history of X-ray flaring events (randomly drawn but from the same energy distribution) impacts the chemical evolution and find that it does not. Finally, we examine the impact of X-ray flares on the electron fraction. While most molecules modeled are not highly sensitive to flares, certain species, including observable molecules, are very reactive to the dynamic environment of a young star.
Young stars emit strong flares of X-ray radiation that penetrate the surface layers of their associated protoplanetary disks. It is still an open question as to whether flares create significant ...changes in disk chemical composition. We present models of the time-evolving chemistry of gas-phase H2O during X-ray flaring events. The chemistry is modeled at point locations in the disk between 1 and 50 au at vertical heights ranging from the midplane to the surface. We find that strong, rare flares, i.e., those that increase the unattenuated X-ray ionization rate by a factor of 100 every few years, can temporarily increase the gas-phase H2O abundance relative to H by more than a factor of ∼3-5 along the disk surface (Z/R ≥ 0.3). We report that a "typical" flare, i.e., those that increase the unattenuated X-ray ionization rate by a factor of a few every few weeks, will not lead to significant, observable changes. Dissociative recombination of H3O+, H2O adsorption and desorption onto dust grains, and ultraviolet photolysis of H2O and related species are found to be the three dominant processes regulating the gas-phase H2O abundance. While the changes are found to be significant, we find that the effect on gas-phase water abundances throughout the disk is short-lived (days). Even though we do not see a substantial increase in long-term water (gas and ice) production, the flares' large effects may be detectable as time-varying inner disk water "bursts" at radii between 5 and 30 au with future far-infrared observations.
Abstract
Planets form and obtain their compositions in dust- and gas-rich disks around young stars, and the outcome of this process is intimately linked to the disk chemical properties. The ...distributions of molecules across disks regulate the elemental compositions of planets, including C/N/O/S ratios and metallicity (O/H and C/H), as well as access to water and prebiotically relevant organics. Emission from molecules also encodes information on disk ionization levels, temperature structures, kinematics, and gas surface densities, which are all key ingredients of disk evolution and planet formation models. The Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program was designed to expand our understanding of the chemistry of planet formation by exploring disk chemical structures down to 10 au scales. The MAPS program focuses on five disks—around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480—in which dust substructures are detected and planet formation appears to be ongoing. We observed these disks in four spectral setups, which together cover ∼50 lines from over 20 different species. This paper introduces the Astrophysical Journal Supplement’s MAPS Special Issue by presenting an overview of the program motivation, disk sample, observational details, and calibration strategy. We also highlight key results, including discoveries of links between dust, gas, and chemical substructures, large reservoirs of nitriles and other organics in the inner disk regions, and elevated C/O ratios across most disks. We discuss how this collection of results is reshaping our view of the chemistry of planet formation.
Abstract
The Molecules with ALMA at Planet-forming Scales Large Program (MAPS LP) surveyed the chemical structures of five protoplanetary disks across more than 40 different spectral lines at high ...angular resolution (0.″15 and 0.″30 beams for Bands 6 and 3, respectively) and sensitivity (spanning 0.3–1.3 mJy beam
−1
and 0.4–1.9 mJy beam
−1
for Bands 6 and 3, respectively). In this article, we describe the multistage workflow—built around the CASA
tclean
image deconvolution procedure—that we used to generate the core data product of the MAPS LP: the position–position–velocity image cubes for each spectral line. Owing to the expansive nature of the survey, we encountered a range of imaging challenges: some are familiar to the submillimeter protoplanetary disk community, like the need to use an accurate CLEAN mask, and others are less well known, like the incorrect default flux scaling of the CLEAN residual map first described by Jorsater & van Moorsel (the “JvM effect”). We distill lessons learned into recommended workflows for synthesizing image cubes of molecular emission. In particular, we describe how to produce image cubes with accurate fluxes via “JvM correction,” a procedure that is generally applicable to any image synthesized via CLEAN deconvolution but is especially critical for low signal-to-noise ratio (S/N) emission. We further explain how we used visibility tapering to promote a common, fiducial beam size and contextualize the interpretation of S/N when detecting molecular emission from protoplanetary disks. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
Theoretical models and observations suggest that the abundances of molecular ions in protoplanetary disks should be highly sensitive to the variable ionization conditions set by the young ...central star. We present a search for temporal flux variability of HCO
+
J
= 1–0, which was observed as a part of the Molecules with Atacama Large Millimeter/submillimeter Array (ALMA) at Planet-forming Scales ALMA Large Program. We split out and imaged the line and continuum data for each individual day the five sources were observed (HD 163296, AS 209, GM Aur, MWC 480, and IM Lup, with between three and six unique visits per source). Significant enhancement (>3
σ
) was not observed, but we find variations in the spectral profiles in all five disks. Variations in AS 209, GM Aur, and HD 163296 are tentatively attributed to variations in HCO
+
flux, while variations in IM Lup and MWC 480 are most likely introduced by differences in the
uv
coverage, which impact the amount of recovered flux during imaging. The tentative detections and low degree of variability are consistent with expectations of X-ray flare-driven HCO
+
variability, which requires relatively large flares to enhance the HCO
+
rotational emission at significant (>20%) levels. These findings also demonstrate the need for dedicated monitoring campaigns with high signal-to-noise ratios to fully characterize X-ray flare-driven chemistry.
The regioselectivity of hydroxyl radical reactions with alkylarenes was investigated using a nuclear magnetic resonance (NMR)‐based methodology capable of trapping and quantifying addition and ...hydrogen ion products of the initial elementary step of the oxidation process. ion products are relatively minor components of the product mixtures (15–30 mol%), depending on the magnitude of the overall rate coefficient and the number of available hydrogens. The relative reactivity of addition at a given position on the ring depends on its relation to the methyl substituents on the hydrocarbons under study. The reactivity enhancements for disubstituted and trisubstituted rings are approximately additive under the conditions of this study.
An NMR‐based analytical method is described for investigating the regioselectivity of hydroxyl radical reactions with arenes in organic solvents. Product distributions derived from simple methylated arenes indicate that radical addition reactions dominate, with hydrogen ion from benzylic positions a minor, but significant, contributor.
The effect of ring substitution on the kinetics of reaction of arenes, heterocycles, and alkenes with hydroxyl radical is investigated in terms of reactivity and selectivity, using laser flash ...photolysis (LFP) in acetonitrile solution. The LFP data indicate that charge-transfer contributions in the transition state play an important role in dictating reactivity, and there is a correlation between the experimental and calculated ionization potentials of the arenes and alkenes and their respective reactivities. The reactivity observed for arenes in acetonitrile exhibits a much greater sensitivity toward substitution on the ring than in water, and therefore aqueous data cannot be used to predict reactivity in nonaqueous environments. Nonaqueous solution data may be predictable from gas phase data, and vice versa.
Cover Image Waggoner, Abygail R.; Abdulrahman, Yahya; Iverson, Alexis J. ...
Journal of physical organic chemistry,
December 2021, Letnik:
34, Številka:
12
Journal Article
Recenzirano
The cover image is based on the Research Article Reaction of hydroxyl radical with arenes in solution—On the importance of benzylic hydrogen ion by Abygail R. Waggoner, Yahya Abdulrahman, Alexis J. ...Iverson, Ethan P. Gibson, Mark A. Buckles and James S. Poole, https://doi.org/10.1002/poc.4278.
Young stars are highly variable in the X-ray regime. In particular, bright X-ray flares can substantially enhance ionization in the surrounding protoplanetary disk. Since disk chemical evolution is ...impacted by ionization, X-ray flares have the potential to fundamentally alter the chemistry of planet forming regions. We present 2D disk chemical models that incorporate a stochastic X-ray flaring module, named \xgen, and examine the flares' overall chemical impact compared to models that assume a constant X-ray flux. We examine the impact of 500 years of flaring events and find global chemical changes on both short time scales (days) in response to discrete flaring events and long time-scales (centuries) in response to the cumulative impact of many flares. Individual X-ray flares most strongly affect small gas-phase cations, where a single flare can temporarily enhance the abundance of species such as H\(_3^+\), HCO\(^+\), CH\(_3^+\), and C\(^+\). We find that flares can also drive chemistry out of "steady state" over longer time periods, where the disk-integrated abundance of some species, such as O and O\(_2\), changes by a few percent over the 500 year model. We also explore whether the specific history of X-ray flaring events (randomly drawn but from the same energy distribution) impacts the chemical evolution and find that it does not. Finally, we examine the impact of X-ray flares on the electron fraction. While most molecules modeled are not highly sensitive to flares, certain species, including observable molecules, are very reactive to the dynamic environment of a young star.
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