An ALMA Survey of faint disks in the Chamaeleon I star-forming region: Why are some Class II disks so faint?

ALMA surveys of nearby star-forming regions have shown that the dust mass in the disk is correlated with the stellar mass, but with a large scatter. This scatter could indicate either different evolutionary paths of disks or different initial conditions within a single cluster. We present ALMA Cycle 3 follow-up observations for 14 Class II disks that were low S/N detections or non-detections in our Cycle 2 survey of the $\sim 2$ Myr-old Chamaeleon I star-forming region. With 5 times better sensitivity, we detect millimeter dust continuum emission from six more sources and increase the detection rate to 94\% (51/54) for Chamaeleon I disks around stars earlier than M3. The stellar-disk mass scaling relation reported in \citet{pascucci2016} is confirmed with these updated measurements. Faint outliers in the $F_{mm}$--$M_*$ plane include three non-detections (CHXR71, CHXR30A, and T54) with dust mass upper limits of 0.2 M$_\oplus$ and three very faint disks (CHXR20, ISO91, and T51) with dust masses $\sim 0.5$ M$_\oplus$. By investigating the SED morphology, accretion property and stellar multiplicity, we suggest for the three millimeter non-detections that tidal interaction by a close companion ($<$100 AU) and internal photoevaporation may play a role in hastening the overall disk evolution. The presence of a disk around only the secondary star in a binary system may explain the observed stellar SEDs and low disk masses for some systems.Comment: ApJ accepte

Hints for Small Disks around Very Low Mass Stars and Brown Dwarfs

The properties of disks around brown dwarfs and very low mass stars (hereafter VLMOs) provide important boundary conditions on the process of planet formation and inform us about the numbers and masses of planets than can form in this regime. We use the Herschel Space Observatory PACS spectrometer to measure the continuum and [O I] 63 μm line emission toward 11 VLMOs with known disks in the Taurus and Chamaeleon I star-forming regions. We fit radiative transfer models to the spectral energy distributions of these sources. Additionally, we carry out a grid of radiative transfer models run in a regime that connects the luminosity of our sources with brighter T Tauri stars. We find that VLMO disks with sizes 1.3-78 au, smaller than typical T Tauri disks, fit well the spectral energy distributions assuming that disk geometry and dust properties are stellar mass independent. Reducing the disk size increases the disk temperature, and we show that VLMOs do not follow previously derived disk temperature-stellar luminosity relationships if the disk outer radius scales with stellar mass. Only 2 out of 11 sources are detected in [O I] despite a better sensitivity than was achieved for T Tauri stars, suggesting that VLMO disks are underluminous. Using thermochemical models, we show that smaller disks can lead to the unexpected [O I] 63 μm nondetections in our sample. The disk outer radius is an important factor in determining the gas and dust observables. Hence, spatially resolved observations with ALMA—to establish if and how disk radii scale with stellar mass—should be pursued further. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA

Evolution of Protoplanetary Dust-Disk Sizes

The planets of our Solar System formed within a disk of dust and gas 4.5 billion years ago. While the Solar System’s primordial protoplanetary disk is now gone, we can begin to understand its possible properties by looking out to the disks within nearby star-forming regions. To understand planet formation, dust observations are used to constrain the mechanics and evolution of planet formation. The properties of disks around brown dwarfs and very-low mass stars (hereafter VLMOs) provide important boundary conditions on the process of planet formation and inform us about the numbers and masses of planets than can form in this regime. Radiative transfer models are fit to the spectral energy distributions of 11 VLMOs with known disks and find that these VLMOs do not follow previously derived disk temperature- stellar luminosity relationships if the disk outer radius scales with stellar mass. The 3 mm continuum observation of the highly inclined transition disk around the star T Cha reveals multiple dust structures: an inner disk, a spatially resolved dust gap, and an outer ring. When compared with previously published 1.6µm VLT/SPHERE imagery, it is found that the location of the outer ring is wavelength dependent. More specifically, the peak emission of the 3 mm ring is at a larger radial distance than that of the 1.6 µm ring, suggesting that millimeter-sized grains in the outer disk are located further away from the central star than micron-sized grains. The most likely origin of the dust gap is from an embedded planet or planets. The dust disk size of 152 protoplanetary disks is estimated from archival ALMA observations. This sample is combined with 47 disks from Tau/Aur and Oph whose dust disk radii were estimated, as here, through fitting radial profile models to visibility data. These 199 homogeneously derived disk sizes are used to identify empirical disk-disk and disk- host property relations as well as to search for evolutionary trends. In agreement with previous studies, we find that dust disk sizes and millimeter luminosities are correlated, but show for the first time that the relationship is not universal between regions. We find that disks in the 2-3 Myr-old are not smaller than disks in other regions of similar age, and confirm the Barenfeld et al. (2017) finding that the 5- 10 Myr USco disks are smaller than disks belonging to younger regions. Finally, we find that the outer edge of the Solar System, as defined by the Kuiper Belt, is consistent with a population of dust disk sizes which have not experienced significant truncation

Herschel spectra of 11 very low mass stars

VizieR online Data Catalogue associated with article published in journal Astronomical Journal (AAS) with title 'Hints for small disks around very low mass stars and brown dwarfs.' (bibcode: 2017ApJ...841..116H