Abstract Observations of disks with the Atacama Large Millimeter/submillimeter Array (ALMA) allow us to map the chemical makeup of nearby protoplanetary disks with unprecedented spatial resolution ...and sensitivity. The typical outer Class II disk observed with ALMA is one with an elevated C/O ratio and a lack of oxygen-bearing complex organic molecules, but there are now some interesting exceptions: three transition disks around Herbig Ae stars all show oxygen-rich gas traced via the unique detections of the molecules SO and CH 3 OH. We present the first results of an ALMA line survey at ≈337–357 GHz of such disks and focus this paper on the first Herbig Ae disk to exhibit this chemical signature—HD 100546. In these data, we detect 19 different molecules including NO, SO 2 , and CH 3 OCHO (methyl formate). We also make the first tentative detections of H 2 13 CO and 34 SO in protoplanetary disks. Multiple molecular species are detected in rings, which are, surprisingly, all peaking just beyond the underlying millimeter continuum ring at ≈200 au. This result demonstrates a clear connection between the large dust distribution and the chemistry in this flat disk. We discuss the physical and/or chemical origin of these substructures in relation to ongoing planet formation in the HD 100546 disk. We also investigate how similar and/or different this molecular makeup of this disk is to other chemically well-characterized Herbig Ae disks. The line-rich data we present motivate the need for more ALMA line surveys to probe the observable chemistry in Herbig Ae systems, which offer unique insight into the composition of disks ices, including complex organic molecules.
Abstract The Atacama Large Millimeter/submillimeter Array (ALMA) can probe the molecular content of planet-forming disks with unprecedented sensitivity. These observations allow us to build up an ...inventory of the volatiles available for forming planets and comets. Herbig Ae transition disks are fruitful targets due to the thermal sublimation of complex organic molecules (COMs) and likely H 2 O-rich ices in these disks. The IRS 48 disk shows a particularly rich chemistry that can be directly linked to its asymmetric dust trap. Here, we present ALMA observations of the IRS 48 disk where we detect 16 different molecules and make the first robust detections of H 2 13 CO , 34 SO, 33 SO, and c-H 2 COCH 2 (ethylene oxide) in a protoplanetary disk. All of the molecular emissions, aside from CO, are co-located with the dust trap, and this includes newly detected simple molecules such as HCO + , HCN , and CS. Interestingly, there are spatial offsets between different molecular families, including between the COMs and sulfur-bearing species, with the latter being more azimuthally extended and radially located further from the star. The abundances of the newly detected COMs relative to CH 3 OH are higher than the expected protostellar ratios, which implies some degree of chemical processing of the inherited ices during the disk lifetime. These data highlight IRS 48 as a unique astrochemical laboratory to unravel the full volatile reservoir at the epoch of planet and comet formation and the role of the disk in (re)setting chemical complexity.
Observations of different molecular lines in protoplanetary disks provide valuable information on the gas kinematics, as well as constraints on the radial density and temperature structure of the ...gas. With ALMA we have detected H13CO+ (J = 4–3) and HC15N (J = 4–3) in the HD 97048 protoplanetary disk for the first time. We compare these new detections to the ringed continuum mm-dust emission and the spatially resolved CO (J = 3–2) and HCO+ (J = 4–3) emission. The radial distributions of the H13CO+ and HC15N emission show hints of ringed sub-structure whereas, the optically thick tracers, CO and HCO+, do not. We calculate the HCO+/H13CO+ intensity ratio across the disk and find that it is radially constant (within our uncertainties). We use a physio-chemical parametric disk structure of the HD 97048 disk with an analytical prescription for the HCO+ abundance distribution to generate synthetic observations of the HCO+ and H13CO+ disk emission assuming LTE. The best by-eye fit models require radial variations in the HCO+/H13CO+ abundance ratio and an overall enhancement in H13CO+ relative to HCO+. This highlights the need to consider isotope selective chemistry and in particular low temperature carbon isotope exchange reactions. This also points to the presence of a reservoir of cold molecular gas in the outer disk (T ≲ 10 K, R ≳ 200 au). Chemical models are required to confirm that isotope-selective chemistry alone can explain the observations presented here. With these data, we cannot rule out that the known dust substructure in the HD 97048 disk is responsible for the observed trends in molecular line emission, and higher spatial resolution observations are required to fully explore the potential of optically thin tracers to probe planet-carved dust gaps. We also report non-detections of H13CO+ and HC15N in the HD 100546 protoplanetary disk.
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
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 elemental composition of the gas and dust in a protoplanetary disk influences the compositions of the planets that form in it. We use the Molecules with ALMA at Planet-forming Scales ...(MAPS) data to constrain the elemental composition of the gas at the locations of potentially forming planets. The elemental abundances are inferred by comparing source-specific gas-grain thermochemical models with variable C/O ratios and small-grain abundances from the DALI code with CO and C
2
H column densities derived from the high-resolution observations of the disks of AS 209, HD 163296, and MWC 480. Elevated C/O ratios (∼2.0), even within the CO ice line, are necessary to match the inferred C
2
H column densities over most of the pebble disk. Combined with constraints on the CO abundances in these systems, this implies that both the O/H and C/H ratios in the gas are substellar by a factor of 4–10, with the O/H depleted by a factor of 20–50, resulting in the high C/O ratios. This necessitates that even within the CO ice line, most of the volatile carbon and oxygen is still trapped on grains in the midplane. Planets accreting gas in the gaps of the AS 209, HD 163296, and MWC 480 disks will thus acquire very little carbon and oxygen after reaching the pebble isolation mass. In the absence of atmosphere-enriching events, these planets would thus have a strongly substellar O/H and C/H and superstellar C/O atmospheric composition. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
Sulfur-bearing molecules play an important role in prebiotic chemistry and planet habitability. They are also proposed probes of chemical ages, elemental C/O ratio, and grain chemistry ...processing. Commonly detected in diverse astrophysical objects, including the solar system, their distribution and chemistry remain, however, largely unknown in planet-forming disks. We present CS (2 − 1) observations at ∼0.″3 resolution performed within the ALMA MAPS Large Program toward the five disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. CS is detected in all five disks, displaying a variety of radial intensity profiles and spatial distributions across the sample, including intriguing apparent azimuthal asymmetries. Transitions of C
2
S and SO were also serendipitously covered, but only upper limits are found. For MWC 480, we present complementary ALMA observations at ∼ 0.″5 of CS,
13
CS, C
34
S, H
2
CS, OCS, and SO
2
. We find a column density ratio
N
(H
2
CS)/
N
(CS) ∼ 2/3, suggesting that a substantial part of the sulfur reservoir in disks is in organic form (i.e., C
x
H
y
S
z
). Using astrochemical disk modeling tuned to MWC 480, we demonstrate that
N
(CS)/
N
(SO) is a promising probe for the elemental C/O ratio. The comparison with the observations provides a supersolar C/O. We also find a depleted gas-phase S/H ratio, suggesting either that part of the sulfur reservoir is locked in solid phase or that it remains in an unidentified gas-phase reservoir. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Abstract
We report the discovery of a circumplanetary disk (CPD) candidate embedded in the circumstellar disk of the T Tauri star AS 209 at a radial distance of about 200 au (on-sky separation of ...1.″4 from the star at a position angle of 161°), isolated via
13
CO
J
= 2−1 emission. This is the first instance of CPD detection via gaseous emission capable of tracing the overall CPD mass. The CPD is spatially unresolved with a 117 × 82 mas beam and manifests as a point source in
13
CO, indicating that its diameter is ≲14 au. The CPD is embedded within an annular gap in the circumstellar disk previously identified using
12
CO and near-infrared scattered-light observations and is associated with localized velocity perturbations in
12
CO. The coincidence of these features suggests that they have a common origin: an embedded giant planet. We use the
13
CO intensity to constrain the CPD gas temperature and mass. We find that the CPD temperature is ≳35 K, higher than the circumstellar disk temperature at the radial location of the CPD, 22 K, suggesting that heating sources localized to the CPD must be present. The CPD gas mass is ≳0.095
M
Jup
≃ 30
M
⊕
adopting a standard
13
CO abundance. From the nondetection of millimeter continuum emission at the location of the CPD (3
σ
flux density ≲26.4
μ
Jy), we infer that the CPD dust mass is ≲0.027
M
⊕
≃ 2.2 lunar masses, indicating a low dust-to-gas mass ratio of ≲9 × 10
−4
. We discuss the formation mechanism of the CPD-hosting giant planet on a wide orbit in the framework of gravitational instability and pebble accretion.
Abstract We present MIRI Medium-resolution Spectrograph observations of the large, multi-gapped protoplanetary disk around the T Tauri star AS 209. The observations reveal hundreds of water vapor ...lines from 4.9–25.5 μ m toward the inner ∼1 au in the disk, including the first detection of rovibrational water emission in this disk. The spectrum is dominated by hot (∼800 K) water vapor and OH gas, with only marginal detections of CO 2 , HCN, and a possible colder water vapor component. Using slab models with a detailed treatment of opacities and line overlap, we retrieve the column density, emitting area, and excitation temperature of water vapor and OH, and provide upper limits for the observable mass of other molecules. Compared to MIRI spectra of other T Tauri disks, the inner disk of AS 209 does not appear to be atypically depleted in CO 2 nor HCN. Based on Spitzer Infrared Spectrograph observations, we further find evidence for molecular emission variability over a 10 yr baseline. Water, OH, and CO 2 line luminosities have decreased by factors of 2–4 in the new MIRI epoch, yet there are minimal continuum emission variations. The origin of this variability is yet to be understood.
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.
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