Empirical Temperature Measurement in Protoplanetary Disks

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.Comment: 24 pages, 16 figures, ApJ in pres

On the Radio Detectability of Circumplanetary Discs

Discs around young planets, so-called circumplanetary discs (CPDs), are essential for planet growth, satellite formation, and planet detection. We study the millimetre and centimetre emission from accreting CPDs by using the simple α disc model. We find that it is easier to detect CPDs at shorter radio wavelengths (e.g. λ ≲ 1 mm). For example, if the system is 140 pc away from us, deep observations (e.g. 5 h) at ALMA Band 7 (0.87 mm) are sensitive to as small as 0.03 lunar mass of dust in CPDs. If the CPD is around a Jupiter mass planet 20 au away from the host star and has a viscosity parameter α ≲ 0.001, ALMA can detect this disc when it accretes faster than 10−10M⊙yr−110−10M⊙yr−1 . ALMA can also detect the \u27minimum mass sub-nebulae\u27 disc if such a disc exists around a young planet in young stellar objects. However, to distinguish the embedded compact CPD from the circumstellar disc material, we should observe circumstellar discs with large gaps/cavities using the highest resolution possible. We also calculate the CPD fluxes at VLA bands, and discuss the possibility of detecting radio emission from jets/winds launched in CPDs. Finally we argue that, if the radial drift of dust particles is considered, the drifting time-scale for millimetre dust in CPDs can be extremely short. It only takes 102–103 yr for CPDs to lose millimetre dust. Thus, for CPDs to be detectable at radio wavelengths, mm-sized dust in CPDs needs to be replenished continuously, or the disc has a significant fraction of micron-sized dust or a high gas surface density so that the particle drifting time-scale is long, or the radial drift is prevented by other means (e.g. pressure traps)

An ALMA Continuum Survey of Circumstellar Disks in the Upper Scorpius OB Association

We present ALMA 880 micron continuum observations of 20 K and M-type stars in the Upper Scorpius OB association that are surrounded by protoplanetary disks. These data are used to measure the dust content in disks around low mass stars (0.1-1.6 Msun) at a stellar age of 5-11 Myr. Thirteen sources were detected in the 880 micron dust continuum at >3 sigma with inferred dust masses between 0.3 and 52 Mearth. The dust masses tend to be higher around the more massive stars, but the significance is marginal in that the probability of no correlation is p ~ 0.03. The evolution in the dust content in disks was assessed by comparing the Upper Sco observations with published continuum measurements of disks around ~ 1-2 Myr stars in the Class II stage in the Taurus molecular cloud. While the dust masses in the Upper Sco disks are on average lower than in Taurus, any difference in the dust mass distributions is significant at less than 3sigma. For stellar masses between 0.49 Msun and 1.6 Msun, the mean dust mass in disks is lower in Upper Sco relative to Taurus by Delta log Mdust = 0.44 +/-0.26.Comment: Accepted by Ap

Identifying Anticyclonic Vortex Features Produced by the Rossby Wave Instability in Protoplanetary Disks

Several nearby protoplanetary disks have been observed to display large scale crescents in the (sub)millimeter dust continuum emission. One interpretation is that these structures correspond to anticyclonic vortices generated by the Rossby wave instability within the gaseous disk. Such vortices have local gas over-densities and are expected to concentrate dust particles with Stokes number around unity. This process might catalyze the formation of planetesimals. Whereas recent observations showed that dust crescent are indeed regions where millimeter-size particles have abnormally high concentration relative to the gas and smaller grains, no observations have yet shown that the gas within the crescent region counter-rotates with respect to the protoplanetary disk. Here we investigate the detectability of anticyclonic features through measurement of the line-of-sight component of the gas velocity obtained with ALMA. We carry out 2D hydrodynamic simulations and 3D radiative transfer calculation of a protoplanetary disk characterized by a vortex created by the tidal interaction with a massive planet. As a case study, the disk parameters are chosen to mimic the IRS 48 system, which has the most prominent crescent observed to date. We generate synthetic ALMA observations of both the dust continuum and 12CO emission around the frequency of 345 GHz. We find that the anticyclonic features of vortex are weak but can be detected if both the source and the observational setup are properly chosen. We provide a recipe for maximizing the probability to detect such vortex features and present an analysis procedure to infer their kinematic properties.Comment: 14 pages, 8 figures, Accepted for publication in Astrophysical Journa

ALMA Observations of Circumstellar Disks in the Upper Scorpius OB Association

We present ALMA observations of 106 G-, K-, and M-type stars in the Upper Scorpius OB Association hosting circumstellar disks. With these data, we measure the 0.88 mm continuum and $^{12}$CO $J$ = 3$-$2 line fluxes of disks around low-mass ($0.14-1.66$ $M_{\odot}$) stars at an age of 5-11 Myr. Of the 75 primordial disks in the sample, 53 are detected in the dust continuum and 26 in CO. Of the 31 disks classified as debris/evolved transitional disks, 5 are detected in the continuum and none in CO. The lack of CO emission in approximately half of the disks with detected continuum emission can be explained if CO is optically thick but has a compact emitting area ($\lesssim 40$ au), or if the CO is heavily depleted by a factor of at least $\sim1000$ relative to interstellar medium abundances and is optically thin. The continuum measurements are used to estimate the dust mass of the disks. We find a correlation between disk dust mass and stellar host mass consistent with a power-law relation of $M_{\mathrm{dust}}\propto M_*^{1.67\pm0.37}$. Disk dust masses in Upper Sco are compared to those measured in the younger Taurus star-forming region to constrain the evolution of disk dust mass. We find that the difference in the mean of $\log(M_{\mathrm{dust}}/M_*)$ between Taurus and Upper Sco is $0.64\pm0.09$, such that $M_{\mathrm{dust}}/M_*$ is lower in Upper Sco by a factor of $\sim4.5$.Comment: 18 pages of text, 11 Figures, 5 Tables. Published in Ap

Millimeter Imaging of MWC 758: Probing the Disk Structure and Kinematics

We investigate the structure and kinematics of the circumstellar disk around the Herbig Ae star MWC 758 using high-resolution observations of the ^(12)CO (3-2) and dust continuum emission at the wavelengths of 0.87 and 3.3 mm. We find that the dust emission peaks at an orbital radius of about 100 AU, while the CO intensity has a central peak coincident with the position of the star. The CO emission is in agreement with a disk in Keplerian rotation around a 2.0 M_⊙ star, confirming that MWC 758 is indeed an intermediate-mass star. By comparing the observation with theoretical disk models, we derive that the disk surface density Σ(r) steeply increases from 40 to 100 AU and decreases exponentially outward. Within 40 AU, the disk has to be optically thin in the continuum emission at millimeter wavelengths to explain the observed dust morphology, though our observations lack the angular resolution and sensitivity required to constrain the surface density on these spatial scales. The surface density distribution in MWC 758 disk is similar to that of "transition" disks, though no disk clearing has been previously inferred from the analysis of the spectral energy distribution (SED). Moreover, the asymmetries observed in the dust and CO emission suggest that the disk may be gravitationally perturbed by a low-mass companion orbiting within a radius of 30 AU. Our results emphasize that SEDs alone do not provide a complete picture of disk structure and that high-resolution millimeter-wave images are essential to reveal the structure of the cool disk mid-plane

Recent Results from CARMA

The Combined Array for Research in Millimeter-wave Astronomy (CARMA) is a heterogeneous array of 23 telescopes designed to operate in the 1 cm, 3 mm, and 1 mm atmospheric windows. The array is a merger of the eight 3.5 m antennas from the Sunyaev-Zel’dovich Array, the nine 6.1 m antennas from the Berkeley-Illinois-Maryland array, and the six 10.4 m antennas from the Owens Valley Radio Observatory (OVRO). As a signature of Tom Phillips’ legacy, three of the CARMA antennas are from the original 3-element OVRO interferometer built under Tom’s leadership in the early 1980’s. Recent CARMA results are presented on the structure of circumstellar disks and the molecular gas distribution in M 51

Large-Scale Asymmetries in the Transitional Disks of SAO 206462 and SR 21

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations in the dust continuum (690 GHz, 0.45 mm) and 12CO J=6-5 spectral line emission, of the transitional disks surrounding the stars SAO 206462 and SR 21. These ALMA observations resolve the dust-depleted disk cavities and extended gaseous disks, revealing large-scale asymmetries in the dust emission of both disks. We modeled these disks structures with a ring and an azimuthal gaussian, where the azimuthal gaussian is motivated by the steady-state vortex solution from Lyra & Lin (2013). Compared to recent observations of HD 142527, Oph IRS 48, and LkHa 330, these are low-contrast (< 2) asymmetries. Nevertheless, a ring alone is not a good fit, and the addition of a vortex prescription describes these data much better. The asymmetric component encompasses 15% and 28% of the total disk emission in SAO 206462 and SR 21 respectively, which corresponds to a lower limit of 2 MJup of material within the asymmetry for both disks. Although the contrast in the dust asymmetry is low, we find that the turbulent velocity inside it must be large (~20% of the sound speed) in order to drive these azimuthally wide and radially narrow vortex-like structures. We obtain residuals from the ring and vortex fitting that are still significant, tracing non-axisymmetric emission in both disks. We compared these submillimeter observations with recently published H-band scattered light observations. For SR 21 the scattered light emission is distributed quite differently from the submillimeter continuum emission, while for SAO 206462 the submillimeter residuals are suggestive of spiral-like structure similar to the near-IR emission.Comment: ApJL, in press. 6 pages, 3 figures, 2 table