Abstract
The thermal structure of protoplanetary disks is a fundamental characteristic of the system that has wide-reaching effects on disk evolution and planet formation. In this study, we constrain ...the 2D thermal structure of the protoplanetary disk TW Hya structure utilizing images of seven CO lines. This includes new ALMA observations of
12
CO
J
= 2–1 and C
18
O
J
= 2–1 as well as archival ALMA observations of
12
CO
J
= 3–2,
13
CO
J
= 3–2 and 6–5, and C
18
O
J
= 3–2 and 6–5. Additionally, we reproduce a Herschel observation of the HD
J
= 1–0 line flux and the spectral energy distribution and utilize a recent quantification of CO radial depletion in TW Hya. These observations were modeled using the thermochemical code RAC2D, and our best-fit model reproduces all spatially resolved CO surface brightness profiles. The resulting thermal profile finds a disk mass of 0.025
M
⊙
and a thin upper layer of gas depleted of small dust with a thickness of ∼1.2% of the corresponding radius. Using our final thermal structure, we find that CO alone is not a viable mass tracer, as its abundance is degenerate with the total H
2
surface density. Different mass models can readily match the spatially resolved CO line profiles with disparate abundance assumptions. Mass determination requires additional knowledge, and, in this work, HD provides the additional constraint to derive the gas mass and support the inference of CO depletion in the TW Hya disk. Our final thermal structure confirms the use of HD as a powerful probe of protoplanetary disk mass. Additionally, the method laid out in this paper is an employable strategy for extraction of disk temperatures and masses in the future.
Abstract Young stellar objects are thought to commonly undergo sudden accretion events that result in a rise in bolometric luminosity. These outbursts likely coincide with the onset of planet ...formation and could impact the formation of planets. The reason behind this dramatic enhancement of accretion is an active area of research, and the mass of the system is a critical parameter. Using the Northern Extended Millimeter Array, we survey five outbursting sources (three FU Ori, one EX Or, and one “peculiar” source) with the primary goal of determining the system’s mass using an optically thin line of CO. We estimate the mass of a central region for each object that using both continuum emission and C 17 O J = 2-1. The C 17 O emission likely includes both disk and inner envelope material, thus acts as an upper limit on the disk mass, ranging from 0.33 to 3.4 M ⊙ for our sources. These derived masses suggest that the inner ∼1000 au contains enough mass along the line of sight for these sources to be gravitationally unstable.
Abstract
Here we present high-resolution (15–24 au) observations of CO isotopologue lines from the Molecules with ALMA on Planet-forming Scales (MAPS) ALMA Large Program. Our analysis employs ...observations of the (
J
= 2–1) and (1–0) lines of
13
CO and C
18
O and the (
J
= 1–0) line of C
17
O for five protoplanetary disks. We retrieve CO gas density distributions, using three independent methods: (1) a thermochemical modeling framework based on the CO data, the broadband spectral energy distribution, and the millimeter continuum emission; (2) an empirical temperature distribution based on optically thick CO lines; and (3) a direct fit to the C
17
O hyperfine lines. Results from these methods generally show excellent agreement. The CO gas column density profiles of the five disks show significant variations in the absolute value and the radial shape. Assuming a gas-to-dust mass ratio of 100, all five disks have a global CO-to-H
2
abundance 10–100 times lower than the interstellar medium ratio. The CO gas distributions between 150 and 400 au match well with models of viscous disks, supporting the long-standing theory. CO gas gaps appear to be correlated with continuum gap locations, but some deep continuum gaps do not have corresponding CO gaps. The relative depths of CO and dust gaps are generally consistent with predictions of planet–disk interactions, but some CO gaps are 5–10 times shallower than predictions based on dust gaps. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
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 (MAPS) Large Program provides a detailed, high-resolution (∼10–20 au) view of molecular line emission in five protoplanetary disks at spatial ...scales relevant for planet formation. Here we present a systematic analysis of chemical substructures in 18 molecular lines toward the MAPS sources: IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. We identify more than 200 chemical substructures, which are found at nearly all radii where line emission is detected. A wide diversity of radial morphologies—including rings, gaps, and plateaus—is observed both within each disk and across the MAPS sample. This diversity in line emission profiles is also present in the innermost 50 au. Overall, this suggests that planets form in varied chemical environments both across disks and at different radii within the same disk. Interior to 150 au, the majority of chemical substructures across the MAPS disks are spatially coincident with substructures in the millimeter continuum, indicative of physical and chemical links between the disk midplane and warm, elevated molecular emission layers. Some chemical substructures in the inner disk and most chemical substructures exterior to 150 au cannot be directly linked to dust substructure, however, which indicates that there are also other causes of chemical substructures, such as snowlines, gradients in UV photon fluxes, ionization, and radially varying elemental ratios. This implies that chemical substructures could be developed into powerful probes of different disk characteristics, in addition to influencing the environments within which planets assemble. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
Carbon dioxide is an important tracer of the chemistry and physics in the terrestrial planet-forming zone. Using a thermochemical model that has been tested against the mid-infrared water ...emission, we reinterpret the CO
2
emission as observed with Spitzer. We find that both water UV-shielding and extra chemical heating significantly reduce the total CO
2
column in the emitting layer. Water UV-shielding is the more efficient effect, reducing the CO
2
column by ∼2 orders of magnitude. These lower CO
2
abundances lead to CO
2
-to-H
2
O flux ratios that are closer to the observed values, but CO
2
emission is still too bright, especially in relative terms. Invoking the depletion of elemental oxygen outside of the water midplane ice line more strongly impacts the CO
2
emission than it does the H
2
O emission, bringing the CO
2
-to-H
2
O emission in line with the observed values. We conclude that the CO
2
emission observed with Spitzer-IRS is coming from a thin layer in the photosphere of the disk, similar to the strong water lines. Below this layer, we expect CO
2
not to be present except when replenished by a physical process. This would be visible in the
13
CO
2
spectrum as well as certain
12
CO
2
features that can be observed by JWST-MIRI.
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.
Abstract
The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program provides a unique opportunity to study the vertical distribution of gas, chemistry, and temperature in the ...protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. By using the asymmetry of molecular line emission relative to the disk major axis, we infer the emission height (
z
) above the midplane as a function of radius (
r
). Using this method, we measure emitting surfaces for a suite of CO isotopologues, HCN, and C
2
H. We find that
12
CO emission traces the most elevated regions with
z
/
r
>
0.3
, while emission from the less abundant
13
CO and C
18
O probes deeper into the disk at altitudes of
z
/
r
≲
0.2
. C
2
H and HCN have lower opacities and signal-to-noise ratios, making surface fitting more difficult, and could only be reliably constrained in AS 209, HD 163296, and MWC 480, with
z
/
r
≲
0.1
, i.e., relatively close to the planet-forming midplanes. We determine peak brightness temperatures of the optically thick CO isotopologues and use these to trace 2D disk temperature structures. Several CO temperature profiles and emission surfaces show dips in temperature or vertical height, some of which are associated with gaps and rings in line and/or continuum emission. These substructures may be due to local changes in CO column density, gas surface density, or gas temperatures, and detailed thermochemical models are necessary to better constrain their origins and relate the chemical compositions of elevated disk layers with those of planet-forming material in disk midplanes. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
An understanding of abundance and distribution of water vapor in the innermost region of protoplanetary disks is key to understanding the origin of habitable worlds and planetary systems. ...Past observations have shown H
2
O to be abundant and a major carrier of elemental oxygen in disk surface layers that lie within the inner few astronomical units of the disk. The combination of high abundance and strong radiative transitions leads to emission lines that are optically thick across the infrared spectral range. Its rarer isotopologue
H
2
18
O
traces deeper into this layer and will trace the full content of the planet-forming zone. In this work, we explore the relative distribution of
H
2
16
O
and
H
2
18
O
within a model that includes water self-shielding from the destructive effects of ultraviolet radiation. In this Letter we show that there is an enhancement in the relative
H
2
18
O
abundance high up in the warm molecular layer within 0.1–10 au due to self-shielding of CO, C
18
O, and H
2
O. Most transitions of
H
2
18
O
that can be observed with JWST will partially emit from this layer, making it essential to take into account how H
2
O self-shielding may effect the H
2
O to
H
2
18
O
ratio. Additionally, this reservoir of
H
2
18
O
-enriched gas in combination with the vertical “cold finger” effect might provide a natural mechanism to account for oxygen isotopic anomalies found in meteoritic material in the solar system.
Formaldehyde (H2CO) is an important precursor to organics like methanol (CH3OH). It is important to understand the conditions that produce H2CO and prebiotic molecules during star and planet ...formation. H2CO possesses both gas-phase and solid-state formation pathways, involving either UV-produced radical precursors or CO ice and cold ( 20 K) dust grains. To understand which pathway dominates, gaseous H2CO's ortho-to-para ratio (OPR) has been used as a probe, with a value of 3 indicating "warm" conditions and <3 linked to cold formation in the solid state. We present spatially resolved Atacama Large Millimeter/submillimeter Array observations of multiple ortho- and para-H2CO transitions in the TW Hya protoplanetary disk to test H2CO formation theories during planet formation. We find disk-averaged rotational temperatures and column densities of 33 2 K, (1.1 0.1) × 1012 cm−2 and 25 2 K, (4.4 0.3) × 1011 cm−2 for ortho- and para-H2CO, respectively, and an OPR of 2.49 0.23. A radially resolved analysis shows that the observed H2CO emits mostly at rotational temperatures of 30-40 K, corresponding to a layer with z/R ≥ 0.25. The OPR is consistent with 3 within 60 au, the extent of the pebble disk, and decreases beyond 60 au to 2.0 0.5. The latter corresponds to a spin temperature of 12 K, well below the rotational temperature. The combination of relatively uniform emitting conditions, a radial gradient in the OPR, and recent laboratory experiments and theory on OPR ratios after sublimation, led us to speculate that gas-phase formation is responsible for the observed H2CO across the TW Hya disk.
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