Accurate measurement of temperature in protoplanetary disks is critical to understanding many key features of disk evolution and planet formation, from disk chemistry and dynamics, to planetesimal ...formation. This paper explores the techniques available to determine temperatures from observations of single, optically thick molecular emission lines. Specific attention is given to issues such as inclusion of optically thin emission, problems resulting from continuum subtraction, and complications of real observations. Effort is also made to detail the exact nature and morphology of the region emitting a given line. To properly study and quantify these effects, this paper considers a range of disk models, from simple pedagogical models, to very detailed models including full radiative transfer. Finally, we show how use of the wrong methods can lead to potentially severe misinterpretations of data, leading to incorrect measurements of disk temperature profiles. We show that the best way to estimate the temperature of emitting gas is to analyze the line peak emission map without subtracting continuum emission. Continuum subtraction, which is commonly applied to observations of line emission, systematically leads to underestimation of the gas temperature. We further show that once observational effects such as beam dilution and noise are accounted for, the line brightness temperature derived from the peak emission is reliably within 10-15% of the physical temperature of the emitting region, assuming optically thick emission. The methodology described in this paper will be applied in future works to constrain the temperature, and related physical quantities, in protoplanetary disks observed with ALMA.
We present ALMA observations of the \(^{12}\)CO, \(^{13}\)CO, C\(^{18}\)O J=2-1 transitions and the 1.3\,mm continuum emission for the circumbinary disc around HD 142527, at an angular resolution of ...\(\approx\)\,0\farcs3. We observe multiple spiral structures in intensity, velocity and velocity dispersion for the \(^{12}\)CO and \(^{13}\)CO gas tracers. A newly detected \(^{12}\)CO spiral originates from the dust horseshoe, and is rotating at super-Keplerian velocity or vertically ascending, whilst the inter-spiral gas is rotating at sub-Keplerian velocities. This new spiral possibly connects to a previously identified spiral, thus spanning > 360\(^\circ\). A spatial offset of ~30 au is observed between the \(^{12}\)CO and \(^{13}\)CO spirals, to which we hypothesize that the gas layers are propagating at different speeds (``surfing'') due to a non-zero vertical temperature gradient. Leveraging the varying optical depths between the CO isotopologues, we reconstruct temperature and column density maps of the outer disc. Gas surface density peaks at r\,\(\approx\)\,180\,au, coincident with the peak of continuum emission. Here the dust grains have a Stokes number of \(\approx\)\,1, confirming radial and azimuthal trapping in the horseshoe. We measure a cavity radius at half-maximum surface density of \(\approx\)\,100\,au, and a cavity eccentricity between 0.3 and 0.45.
Turbulent motions are believed to regulate angular momentum transport and influence dust evolution in protoplanetary disks. Measuring the strength of turbulence is challenging through gas line ...observations because of the requirement for high spatial and spectral resolution data, and an exquisite determination of the temperature. In this work, taking the well-known HD 163296 disk as an example, we investigated the contrast of gaps identified in high angular resolution continuum images as a probe for the level of turbulence. With self-consistent radiative transfer models, we simultaneously analyzed the radial brightness profiles along the disk major and minor axes, and the azimuthal brightness profiles of the B67 and B100 rings. By fitting all the gap contrasts measured from these profiles, we constrained the gas-to-dust scale height ratio \(\Lambda\) to be \(3.0_{-0.8}^{+0.3}\), \(1.2_{-0.1}^{+0.1}\) and \({\ge}\,6.5\) for the D48, B67 and B100 regions, respectively. The varying gas-to-dust scale height ratios indicate that the degree of dust settling changes with radius. The inferred values for \(\Lambda\) translate into a turbulence level of \(\alpha_{\rm turb}\,{<}\,3\times10^{-3}\) in the D48 and B100 regions, which is consistent with previous upper limits set by gas line observations. However, turbulent motions in the B67 ring are strong with \(\alpha_{\rm turb}\,{\sim}1.2\,{\times}\,10^{-2}\). Due to the degeneracy between \(\Lambda\) and the depth of dust surface density drops, the turbulence strength in the D86 gap region is not constrained.
While protoplanetary disks are often treated as isolated systems in planet formation models, observations increasingly suggest that vigorous interactions between Class II disks and their environments ...are not rare. DO Tau is a T Tauri star that has previously been hypothesized to have undergone a close encounter with the HV Tau system. As part of the DESTINYS ESO Large Programme, we present new VLT/SPHERE polarimetric observations of DO Tau and combine them with archival HST scattered light images and ALMA observations of CO isotopologues and CS to map a network of complex structures. The SPHERE and ALMA observations show that the circumstellar disk is connected to arms extending out to several hundred au. HST and ALMA also reveal stream-like structures northeast of DO Tau, some of which are at least several thousand au long. These streams appear not to be gravitationally bound to DO Tau, and comparisons with previous Herschel far-IR observations suggest that the streams are part of a bridge-like structure connecting DO Tau and HV Tau. We also detect a fainter redshifted counterpart to a previously known blueshifted CO outflow. While some of DO Tau's complex structures could be attributed to a recent disk-disk encounter, they might be explained alternatively by interactions with remnant material from the star formation process. These panchromatic observations of DO Tau highlight the need to contextualize the evolution of Class II disks by examining processes occurring over a wide range of size scales.
The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program provides a detailed, high resolution (\({\sim}\)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.
We explore the dynamical structure of the protoplanetary disks surrounding HD 163296 and MWC 480 as part of the Molecules with ALMA at Planet Forming Scales (MAPS) large program. Using the \(J = ...2-1\) transitions of \(^{12}\)CO, \(^{13}\)CO and C\(^{18}\)O imaged at spatial resolutions of \(\sim 0.^{\prime \prime}15\) and with a channel spacing of \(200\) \({\rm m\,s^{-1}}\), we find perturbations from Keplerian rotation in the projected velocity fields of both disks (\(\lesssim\!5\%\) of the local Keplerian velocity), suggestive of large-scale (10s of au in size), coherent flows. By accounting for the azimuthal dependence on the projection of the velocity field, the velocity fields were decomposed into azimuthally averaged orthogonal components, \(v_{\phi}\), \(v_r\) and \(v_z\). Using the optically thick \(^{12}\)CO emission as a probe of the gas temperature, local variations of \(\approx\! 3\) K (\(\approx\! 5 \%\) relative changes) were observed and found to be associated with the kinematic substructures. The MWC 480 disk hosts a suite of tightly wound spiral arms. The spirals arms, in conjunction with the highly localized perturbations in the gas velocity structure (kinematic planetary signatures), indicate a giant planet, \(\sim\! 1\) \(M_{\rm Jup}\), at a radius of \(\approx 245\) au. In the disk of HD 163296, the kinematic substructures were consistent with previous studies of Pinte et al. (2018a) and Teague et al. (2018a) advocating for multiple \(\sim\! 1\) \(M_{\rm Jup}\) planets embedded in the disk. These results demonstrate that molecular line observations that characterize the dynamical structure of disks can be used to search for the signatures of embedded planets. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
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 4 spectral setups, which together cover ~50 lines from over 20 different species. This paper introduces the ApJS 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 sub-structures, 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.
We present a high-resolution (\(\sim0.''12\), \(\sim16\) au, mean sensitivity of \(50~\mu\)Jy~beam\(^{-1}\) at 225 GHz) snapshot survey of 32 protoplanetary disks around young stars with spectral ...type earlier than M3 in the Taurus star-forming region using Atacama Large Millimeter Array (ALMA). This sample includes most mid-infrared excess members that were not previously imaged at high spatial resolution, excluding close binaries and highly extincted objects, thereby providing a more representative look at disk properties at 1--2 Myr. Our 1.3 mm continuum maps reveal 12 disks with prominent dust gaps and rings, 2 of which are around primary stars in wide binaries, and 20 disks with no resolved features at the observed resolution (hereafter smooth disks), 8 of which are around the primary star in wide binaries. The smooth disks were classified based on their lack of resolved substructures, but their most prominent property is that they are all compact with small effective emission radii (\(R_{\rm eff,95\%} \lesssim 50\) au). In contrast, all disks with \(R_{\rm eff,95\%}\) of at least 55 au in our sample show detectable substructures. Nevertheless, their inner emission cores (inside the resolved gaps) have similar peak brightness, power law profiles, and transition radii to the compact smooth disks, so the primary difference between these two categories is the lack of outer substructures in the latter. These compact disks may lose their outer disk through fast radial drift without dust trapping, or they might be born with small sizes. The compact dust disks, as well as the inner disk cores of extended ring disks, that look smooth at the current resolution will likely show small-scale or low-contrast substructures at higher resolution. The correlation between disk size and disk luminosity correlation demonstrates that some of the compact disks are optically thick at millimeter wavelengths.
Gap-like structures in protoplanetary disks are likely related to planet formation processes. In this paper, we present and analyze high resolution (0.17*0.11 arcsec) 1.3 mm ALMA continuum ...observations of the protoplanetary disk around the Herbig Ae star MWC 480. Our observations for the first time show a gap centered at ~74au with a width of ~23au, surrounded by a bright ring centered at ~98au from the central star. Detailed radiative transfer modeling of both the ALMA image and the broadband spectral energy distribution is used to constrain the surface density profile and structural parameters of the disk. If the width of the gap corresponds to 4~8 times the Hill radius of a single forming planet, then the putative planet would have a mass of 0.4~3 M_Jup. We test this prediction by performing global three-dimensional smoothed particle hydrodynamic gas/dust simulations of disks hosting a migrating and accreting planet. We find that the dust emission across the disk is consistent with the presence of an embedded planet with a mass of ~2.3 M_Jup at an orbital radius of ~78au. Given the surface density of the best-fit radiative transfer model, the amount of depleted mass in the gap is higher than the mass of the putative planet, which satisfies the basic condition for the formation of such a planet.
Rings are the most frequently revealed substructure in ALMA dust observations
of protoplanetary disks, but their origin is still hotly debated. In this
paper, we identify dust substructures in 12 ...disks and measure their properties
to investigate how they form. This subsample of disks is selected from a
high-resolution ($\sim0.12''$) ALMA 1.33 mm survey of 32 disks in the Taurus
star-forming region, which was designed to cover a wide range of sub-mm
brightness and to be unbiased to previously known substructures. While
axisymmetric rings and gaps are common within our sample, spiral patterns and
high contrast azimuthal asymmetries are not detected. Fits of disk models to
the visibilities lead to estimates of the location and shape of gaps and rings,
the flux in each disk component, and the size of the disk. The dust
substructures occur across a wide range of stellar mass and disk brightness.
Disks with multiple rings tend to be more massive and more extended. The
correlation between gap locations and widths, the intensity contrast between
rings and gaps, and the separations of rings and gaps could all be explained if
most gaps are opened by low-mass planets (super-Earths and Neptunes) in the
condition of low disk turbulence ($\alpha=10^{-4}$). The gap locations are not
well correlated with the expected locations of CO and N$_2$ ice lines, so
condensation fronts are unlikely to be a universal mechanism to create gaps and
rings, though they may play a role in some cases.
