Abstract
UV photochemistry in the surface layers of protoplanetary disks dramatically alters their composition relative to previous stages of star formation. The abundance ratio CN/HCN has long been ...proposed to trace the UV field in various astrophysical objects; however, to date the relationship between CN, HCN, and the UV field in disks remains ambiguous. As part of the ALMA Large Program MAPS (Molecules with ALMA at Planet-forming Scales), we present observations of CN
N
= 1–0 transitions at 0.″3 resolution toward five disk systems. All disks show bright CN emission within ∼50–150 au, along with a diffuse emission shelf extending up to 600 au. In all sources we find that the CN/HCN column density ratio increases with disk radius from about unity to 100, likely tracing increased UV penetration that enhances selective HCN photodissociation in the outer disk. Additionally, multiple millimeter dust gaps and rings coincide with peaks and troughs, respectively, in the CN/HCN ratio, implying that some millimeter substructures are accompanied by changes to the UV penetration in more elevated disk layers. That the CN/HCN ratio is generally high (>1) points to a robust photochemistry shaping disk chemical compositions and also means that CN is the dominant carrier of the prebiotically interesting nitrile group at most disk radii. We also find that the local column densities of CN and HCN are positively correlated despite emitting from vertically stratified disk regions, indicating that different disk layers are chemically linked. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
Gas mass remains one of the most difficult protoplanetary disk properties to constrain. With much of the protoplanetary disk too cold for the main gas constituent, H
2
, to emit, alternative ...tracers such as dust, CO, or the H
2
isotopologue HD are used. However, relying on disk mass measurements from any single tracer requires assumptions about the tracer’s abundance relative to H
2
and the disk temperature structure. Using new Atacama Large Millimeter/submillimeter Array (ALMA) observations from the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program as well as archival ALMA observations, we construct a disk physical/chemical model of the protoplanetary disk GM Aur. Our model is in good agreement with the spatially resolved CO isotopologue emission from 11 rotational transitions with spatial resolution ranging from 0.″15 to 0.″46 (24–73 au at 159 pc) and the spatially unresolved HD
J
= 1–0 detection from Herschel. Our best-fit model favors a cold protoplanetary disk with a total gas mass of approximately 0.2
M
⊙
, a factor of 10 reduction in CO gas inside roughly 100 au and a factor of 100 reduction outside of 100 au. Despite its large mass, the disk appears to be on the whole gravitationally stable based on the derived Toomre
Q
parameter. However, the region between 70 and 100 au, corresponding to one of the millimeter dust rings, is close to being unstable based on the calculated Toomre
Q
of <1.7. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
Emission substructures in gas and dust are common in protoplanetary disks. Such substructures can be linked to planet formation or planets themselves. We explore the observed gas ...substructures in AS 209 using thermochemical modeling with
RAC2D
and high-spatial-resolution data from the Molecules with ALMA at Planet-forming Scales (MAPS) program. The observations of C
18
O
J
= 2–1 emission exhibit a strong depression at 88 au overlapping with the positions of multiple gaps in millimeter dust continuum emission. We find that the observed CO column density is consistent with either gas surface-density perturbations or chemical processing, while C
2
H column density traces changes in the C/O ratio rather than the H
2
gas surface density. However, the presence of a massive planet (>0.2
M
Jup
) would be required to account for this level of gas depression, which conflicts with constraints set by the dust emission and the pressure profile measured by gas kinematics. Based on our models, we infer that a local decrease of CO abundance is required to explain the observed structure in CO, dominating over a possible gap-carving planet present and its effect on the H
2
surface density. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
Understanding the temperature structure of protoplanetary disks is key to interpreting observations, predicting the physical and chemical evolution of the disk, and modeling planet formation ...processes. In this study, we constrain the two-dimensional thermal structure of the disk around the Herbig Ae star HD 163296. Using the thermochemical code RAC2D, we derive a thermal structure that reproduces spatially resolved Atacama Large Millimeter/submillimeter Array observations (∼0.″12 (13 au)–0.″25 (26 au)) of
12
CO
J
= 2 − 1,
13
CO
J
= 1 − 0, 2 − 1, C
18
O
J
= 1 − 0, 2 − 1, and C
17
O
J
= 1 − 0, the HD
J
= 1 − 0 flux upper limit, the spectral energy distribution (SED), and continuum morphology. The final model incorporates both a radial depletion of CO motivated by a timescale shorter than typical CO gas-phase chemistry (0.01 Myr) and an enhanced temperature near the surface layer of the the inner disk (
z
/
r
≥ 0.21). This model agrees with the majority of the empirically derived temperatures and observed emitting surfaces derived from the
J
= 2 − 1 CO observations. We find an upper limit for the disk mass of 0.35
M
⊙
, using the upper limit of the HD
J
= 1 − 0 and
J
= 2 − 1 flux. With our final thermal structure, we explore the impact that gaps have on the temperature structure constrained by observations of the resolved gaps. Adding a large gap in the gas and small dust additionally increases gas temperature in the gap by only 5%–10%. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
Constraining the distribution of gas and dust in the inner 20 au of protoplanetary disks is difficult. At the same time, this region is thought to be responsible for most planet formation, ...especially around the water ice line at 3–10 au. Under the assumption that the gas is in a Keplerian disk, we use the exquisite sensitivity of the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA large program to construct radial surface brightness profiles with a ∼3 au effective resolution for the CO isotopologue
J
= 2–1 lines using the line velocity profile. IM Lup reveals a central depression in
13
CO and C
18
O that is ascribed to a pileup of ∼500
M
⊕
of dust in the inner 20 au, leading to a gas-to-dust ratio of around <10. This pileup is consistent with an efficient drift of grains (≳100
M
⊕
Myr
−1
) and a local gas-to-dust ratio that suggests that the streaming instability could be active. The CO isotopologue emission in the GM Aur disk is consistent with a small (∼15 au), strongly depleted gas cavity within the ∼40 au dust cavity. The radial surface brightness profiles for both the AS 209 and HD 163296 disks show a local minimum and maximum in the C
18
O emission at the location of a known dust ring (∼14 au) and gap (∼10 au), respectively. This indicates that the dust ring has a low gas-to-dust ratio (>10) and that the dust gap is gas-rich enough to have optically thick C
18
O. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
During the main phase of evolution of a protoplanetary disk, accretion regulates the inner-disk properties, such as the temperature and mass distribution, and in turn, the physical ...conditions associated with planet formation. The driving mechanism behind accretion remains uncertain; however, one promising mechanism is the removal of a fraction of angular momentum via a magnetohydrodynamic (MHD) disk wind launched from the inner tens of astronomical units of the disk. This paper utilizes CO isotopologue emission to study the unique molecular outflow originating from the HD 163296 protoplanetary disk obtained with the Atacama Large Millimeter/submillimeter Array. HD 163296 is one of the most well-studied Class II disks and is proposed to host multiple gas-giant planets. We robustly detect the large-scale rotating outflow in the
12
CO
J
= 2 − 1 and the
13
CO
J
= 2 − 1 and
J
= 1 − 0 transitions. We constrain the kinematics, the excitation temperature of the molecular gas, and the mass-loss rate. The high ratio of the rates of ejection to accretion (5–50), together with the rotation signatures of the flow, provides solid evidence for an MHD disk wind. We find that the angular momentum removal by the wind is sufficient to drive accretion though the inner region of the disk; therefore, accretion driven by turbulent viscosity is not required to explain HD 163296's accretion. The low temperature of the molecular wind and its overall kinematics suggest that the MHD disk wind could be perturbed and shocked by the previously observed high-velocity atomic jet. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
We observed HCO
+
J
= 1 − 0 and H
13
CO
+
J
= 1 − 0 emission toward the five protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480 as part of the MAPS project. HCO
+
is ...detected and mapped at 0.″3 resolution in all five disks, while H
13
CO
+
is detected (S/N > 6
σ
) toward GM Aur and HD 163296 and tentatively detected (S/N > 3
σ
) toward the other disks by a matched filter analysis. Inside a radius of
R
∼ 100 au, the HCO
+
column density is flat or shows a central dip. At outer radii (≳100 au), the HCO
+
column density decreases outward, while the column density ratio of HCO
+
/CO is mostly in the range of ∼10
−5
–10
−4
. We derived the HCO
+
abundance in the warm CO-rich layer, where HCO
+
is expected to be the dominant molecular ion. At
R
≳ 100 au, the HCO
+
abundance is ∼3 × 10
−11
− 3 × 10
−10
, which is consistent with a template disk model with X-ray ionization. At the smaller radii, the abundance decreases inward, which indicates that the ionization degree is lower in denser gas, especially inside the CO snow line, where the CO-rich layer is in the midplane. Comparison of template disk models with the column densities of HCO
+
, N
2
H
+
, and N
2
D
+
indicates that the midplane ionization rate is ≳10
−18
s
−1
for the disks around IM Lup, AS 209, and HD 163296. We also find hints of an increased HCO
+
abundance around the location of dust continuum gaps in AS 209, HD 163296, and MWC 480. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
The complex organic molecules (COMs) detected in star-forming regions are the precursors of the prebiotic molecules that can lead to the emergence of life. By studying COMs in more evolved ...protoplanetary disks we can gain a better understanding of how they are incorporated into planets. This paper presents ALMA band 7 observations of the dust and ice trap in the protoplanetary disk around Oph IRS 48. We report the first detection of dimethyl ether (CH3OCH3) in a planet-forming disk and a tentative detection of methyl formate (CH3OCHO). We determined column densities for the detected molecules and upper limits on non-detected species using the CASSIS spectral analysis tool. The inferred column densities of CH3OCH3 and CH3OCHO with respect to methanol (CH3OH) are of order unity, indicating unusually high abundances of these species compared to other environments. Alternatively, the 12CH3OH emission is optically thick and beam diluted, implying a higher CH3OH column density and a smaller emitting area than originally thought. The presence of these complex molecules can be explained by thermal ice sublimation, where the dust cavity edge is heated by irradiation and the full volatile ice content is observable in the gas phase. This work confirms the presence of oxygen-bearing molecules more complex than CH3OH in protoplanetary disks for the first time. It also shows that it is indeed possible to trace the full interstellar journey of COMs across the different evolutionary stages of star, disk, and planet formation.
Abstract
Deuterium fractionation is dependent on various physical and chemical parameters. Thus, the formation location and thermal history of material in the solar system is often studied by ...measuring its D/H ratio. This requires knowledge about the deuteration processes operating during the planet formation era. We aim to study these processes by radially resolving the DCN/HCN (at 0.″3 resolution) and N
2
D
+
/N
2
H
+
(∼0.″3–0.″9) column density ratios toward the five protoplanetary disks observed by the Molecules with ALMA at Planet-forming scales (MAPS) Large Program. DCN is detected in all five sources, with one newly reported detection. N
2
D
+
is detected in four sources, two of which are newly reported detections. We derive column density profiles that allow us to study the spatial variation of the DCN/HCN and N
2
D
+
/N
2
H
+
ratios at high resolution. DCN/HCN varies considerably for different parts of the disks, ranging from 10
−3
to 10
−1
. In particular, the inner-disk regions generally show significantly lower HCN deuteration compared with the outer disk. In addition, our analysis confirms that two deuterium fractionation channels are active, which can alter the D/H ratio within the pool of organic molecules. N
2
D
+
is found in the cold outer regions beyond ∼50 au, with N
2
D
+
/N
2
H
+
ranging between 10
−2
and 1 across the disk sample. This is consistent with the theoretical expectation that N
2
H
+
deuteration proceeds via the low-temperature channel only. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
We present ALMA observations of four different molecular species in showcasing their potential as tracers of physical and chemical conditions in planet forming Herbig Ae disks.
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