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

Abstract

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.

Constraints from Dust Mass and Mass Accretion Rate Measurements on Angular Momentum Transport in Protoplanetary Disks

“…Disk Size.The characteristic temperature at which the disk emits depends on the spatial distribution of dust, in particular disk size (e.g., Hendler et al 2017). Spatially resolved millimeter observations show that protoplanetary disks vary in size by an order of magnitude (e.g., Andrews et al 2010, s » 0.4 dex).…”

On the diversity and statistical properties of protostellar discs

“…Testi et al (2016) found evidence that two discs of VLM objects in Ophiuchus may have sharp outer disc radii of ≈ 25 au, with three other discs having radii between 50 and 150 au, depending on model parameters. From modelling spectral energy distributions, Hendler et al (2017) find that out of 11 young stars with masses M * ∼ < 0.2 M , 7 likely have disc radii smaller than 10 au, with the remaining four objects having radii from 10-80 au.…”

“…The exact dependence, however, is model dependent, for example, whether or not disc temperature is scaled with stellar luminosity, and the assumptions made about the disc size; see, for example, Pascucci et al (2016);Hendler et al (2017). Andrews et al (2013) found that the millimetre flux scales as Fmm ∝ M 1.5−2.0 * for Class II discs in the Taurus region and they argue that, accounting for dust temperature scaling, this supports a roughly linear scaling of disc mass with stellar mass (i.e.…”

“…Recent observations have found evidence that the discs of brown dwarfs may typically be smaller than the discs of more massive T Tauri stars Hendler et al 2017). The first resolved observations of discs around very low mass (VLM) objects found disc sizes may range from 30 − 70 au to larger than 200 au .…”

ALMA Observations of the Young Substellar Binary System 2M1207

“…We adopted the opacity law from Beckwith et al (1990) and the dust temperature using the prescription from Andrews et al (2013), which calculates a characteristic dust temperature in the outer regions of a disk surrounding a star of a given luminosity. This approach gives the most conservative upper limit for the dust mass (a higher T dust , as might be expected for a disk smaller than the 200 au assumed in Andrews et al, would result in a lower dust mass upper limit, see Hendler et al 2017). We do not attempt a more precise calculation of the dust temperature for each disk, e.g., via radiative transfer calculations, because the uncertainty on the dust mass estimate is dominated by the uncertainty on the dust opacity coefficient κ ν .…”