Brown dwarf disks in Ophiuchus

Brown dwarf disks with ALMA: evidence for truncated dust disks in Ophiuchus

Authors:

Testi et al

Abstract:

The study of the properties of disks around young brown dwarfs can provide important clues on the formation of these very low-mass objects and on the possibility of forming planetary systems around them. The presence of warm dusty disks around brown dwarfs is well known, based on near- and mid-infrared studies. High angular resolution observations of the cold outer disk are limited; we used ALMA to attempt a first survey of young brown dwarfs in the ρ-Oph star-forming region. All 17 young brown dwarfs in our sample were observed at 890 μm in the continuum at ∼0.′′5 angular resolution. The sensitivity of our observations was chosen to detect ∼0.5 M⊕ of dust. We detect continuum emission in 11 disks (∼65\%\ of the total), and the estimated mass of dust in the detected disks ranges from ∼0.5 to ∼6 M⊕. These disk masses imply that planet formation around brown dwarfs may be relatively rare and that the supra-Jupiter mass companions found around some brown dwarfs are probably the result of a binary system formation. We find evidence that the two brightest disks in ρ-Oph have sharp outer edges at R<~25 AU, in contrast to disks around Taurus brown dwarfs. This difference may suggest that the different environment in ρ-Oph may lead to significant differences in disk properties. A comparison of the Mdisk/M∗ ratio for brown dwarf and solar-mass systems also shows a possible deficit of mass in brown dwarfs, which could support the evidence for dynamical truncation of disks in the substellar regime. These findings are still tentative and need to be put on firmer grounds by studying the gaseous disks around brown dwarfs and by performing a more systematic and unbiased survey of the disk population around the more massive stars.

Dippers and Dusty Disks Edges: A Unified Model

Dippers and Dusty Disks Edges: A Unified Model

Authors:

Bodman et al

Abstract:

A search for dips in observed stellar flux in the Upper Scorpius and ρ Ophiuchus star formation regions with the Kepler mission by Ansdell et al. primarily identified young, low mass stars (dippers) with low accretion rates and hosting moderately evolved dusty circumstellar disks. These young stars likely exhibit rotating star spots that cause quasi-periodic photometric variations. However, a separate period associated with the dips is not evident in spectrograms constructed from the light curves. The material causing the dips in most of these light curves must be approximately corotating with the star. We find that disk temperatures computed at the disk corotation radius are cool enough that dust should not sublimate. Dippers are preferentially associated with young, low mass stars as they have low enough luminosities to allow dust to survive within a few stellar radii. Crude estimates for stellar magnetic field strengths and accretion rates are consistent with magnetospheric truncation near the corotation radius. Magnetospheric truncation models can explain why dusty material is lifted out of the midplane to obscure the star causing the light curve dips and why so many young low mass stars are dippers. We propose that variations in disk orientation angle, stellar magnetic field dipole tilt axis, and disk accretion rate are underlying parameters accounting for differences in the dipper light curves.

The Dust Masses Around 8 Brown Dwarfs and Very Low Mass Stars

Dust masses of disks around 8 Brown Dwarfs and Very Low-Mass Stars in Upper Sco OB1 and Ophiuchus

Authors:

van der Plas et al

Abstract:

We present the results of ALMA band 7 observations of dust and CO gas in the disks around 7 objects with spectral types ranging between M5.5 and M7.5 in Upper Scorpius OB1, and one M3 star in Ophiuchus. We detect unresolved continuum emission in all but one source, and the 12CO J=3-2 line in two sources. We constrain the dust and gas content of these systems using a grid of models calculated with the radiative transfer code MCFOST, and find disk dust masses between 0.1 and 1 M⊕, suggesting that the stellar mass / disk mass correlation can be extrapolated for brown dwarfs with masses as low as 0.05 M⊙. The one disk in Upper Sco in which we detect CO emission, 2MASS J15555600, is also the disk with warmest inner disk as traced by its H - [4.5] photometric color. Using our radiative transfer grid, we extend the correlation between stellar luminosity and mass-averaged disk dust temperature originally derived for stellar mass objects to the brown dwarf regime to ⟨Tdust⟩≈22(L∗/L⊙)0.16K, applicable to spectral types of M5 and later. This is slightly shallower than the relation for earlier spectral type objects and yields warmer low-mass disks. The two prescriptions cross at 0.27 L⊙, corresponding to masses between 0.1 and 0.2 M⊙ depending on age.

Infrared Observation of Transitional Disks in Ophiuchus

Infrared study of transitional disks in Ophiuchus with Herschel

Authors:

Rebollido et al

Abstract:

Context.

Observations of nearby star-forming regions with the Herschel Space Observatory complement our view of the protoplanetary disks in Ophiuchus with information about the outer disks.

Aims.

The main goal of this project is to provide new far-infrared fluxes for the known disks in the core region of Ophiuchus and to identify potential transitional disks using data from Herschel.

Methods.

We obtained PACS and SPIRE photometry of previously spectroscopically confirmed young stellar objects (YSO) in the region and analysed their spectral energy distributions.

Results.

From an initial sample of 261 objects with spectral types in Ophiuchus, we detect 49 disks in at least one Herschel band. We provide new far-infrared fluxes for these objects. One of them is clearly a new transitional disk candidate.

Conclusions.

The data from Herschel Space Observatory provides fluxes that complement previous infrared data and that we use to identify a new transitional disk candidate.

Demographics Circumstellar Disks in Ophiuchus and Taurus

Demographics of Transition Discs in Ophiuchus and Taurus

Authors:

Najita et al

Abstract:

Transition disc systems are young stars that appear to be on the verge of dispersing their protoplanetary discs. We explore the nature of these systems by comparing the stellar accretion rates and disc masses of transition discs and normal T Tauri stars in Taurus and Ophiuchus. After controlling for the known dependencies of stellar accretion rate and disc mass and on age, stellar accretion rate on stellar mass, and disc mass on the presence of stellar or sub-stellar companions, we find that the normal T Tauri stars show a trend of stellar accretion rate increasing with disc mass. The transition discs tend to have higher average disc masses than normal T Tauri stars as well as lower accretion rates than normal T Tauri stars of the same disc mass. These results are most consistent with the interpretation that the transition discs have formed objects massive enough to alter the accretion flow, i.e., single or multiple giant planets. Several Ophiuchus T Tauri stars that are not known transition disc systems also have very low accretion rates for their disc masses. We speculate on the possible nature of these sources.