Herschel/PACS view of disks around low-mass stars and brown dwarfs in the TW Hydrae association

Abstract

We conducted Herschel/PACS observations of five very low-mass stars or brown dwarfs located in the TW Hya association with the goal of characterizing the properties of disks in the low stellar mass regime. We detected all five targets at 70 μm and 100 μm and three targets at 160 μm. Our observations, combined with previous photometry from 2MASS, WISE, and SCUBA-2, enabled us to construct spectral energy distributions (SEDs) with extended wavelength coverage. Using sophisticated radiative transfer models, we analyzed the observed SEDs of the five detected objects with a hybrid fitting strategy that combines the model grids and the simulated annealing algorithm and evaluated the constraints on the disk properties via the Bayesian inference method. The modeling suggests that disks around low-mass stars and brown dwarfs are generally flatter than their higher mass counterparts, but the range of disk mass extends to well below the value found in T Tauri stars, and the disk scale heights are comparable in both groups. The inferred disk properties (i.e., disk mass, flaring, and scale height) in the low stellar mass regime are consistent with previous findings from large samples of brown dwarfs and very low-mass stars. We discuss the dependence of disk properties on their host stellar parameters and find a significant correlation between the Herschel far-IR fluxes and the stellar effective temperatures, probably indicating that the scaling between the stellar and disk masses (i.e., M disk ∝ M ) observed mainly in low-mass stars may extend down to the brown dwarf regime.

A homogeneous analysis of disks around brown dwarfs

“…The lower disk mass around cooler stars as revealed by far-IR measurements is consistent with the strong correlation between the stellar and disk masses given by Andrews et al (2013) from a millimeter survey of Class II sources in the Taurus molecular cloud. Several recent works obtained similar results, although the sample size and assumptions used to analyze the Herschel data differ (e.g., Alves de Oliveira et al 2013;Spezzi et al 2013;Olofsson et al 2013;Liu et al 2015). Note that the above disk comparisons were mostly conducted between two stellar mass bins, i.e., the sun-like stars and the BDs including very low-mass stars.…”

“…We observed a preferential values of β in the range 1.05−1.20, while the majority of the target disks are best modeled with H 100 of the order of 5−20 AU. As shown by Harvey et al (2012a) and Liu et al (2015), most parts (except for the inner region) of typical BD disks produce optically thin emission at far-IR wavelengths, demonstrating the applicability of PACS photometry as a diagnosis for the disk mass of BDs. Objects with detections at 70 and/or 160 μm enable reasonable determination of their disk masses, as demonstrated by clear peaks in the Bayesian probability distributions of M disk in these cases.…”

“…Strong grain growth up to millimeter sizes as detected in some BD disks (e.g., Ricci et al 2012Ricci et al , 2013Ricci et al , 2014Broekhoven-Fiene et al 2014) would remain undetected in our data and could affect the disk mass. Our prescription for the dust properties is identical to those used in Liu et al (2015).…”

M Stars in the Tw Hya Association: Stellar X-Rays and Disk Dissipation

“…The WISE point source catalog -W W 1 4colors of these three stars are 0.25, 0.20, and 0.08, respectively (where the TWA 15AB binary is unresolved by WISE), all of which lie well within the locus of TWA star colors for stars that lack IR excesses (Schneider et al 2012). The mid-IR excess stars TWA 3A, 7, 30A, 30B, and 31 also display relatively strong far-IR fluxes (Riviere-Marichalar et al 2013;Liu et al 2015). Reanalysis of archival HST imaging has yielded a direct detection of the TWA 7 dust disk via scattered starlight; TWA 25 also displays a compact, nearly edge-on dust disk in HST imaging despite its lack of detectable mid-IR excess (Choquet et al 2015).…”

“…Only three of these stars-TWA 30A, 30B, and 31-have been the subject of sensitive millimeter-and submillimeter-wave continuum and CO observations (with the Atacama Large Millimeter Array; Rodriguez et al 2015). All three display mid-to far-IR excesses indicative of warm dust (Schneider et al 2012;Liu et al 2015). However, only TWA 30B was detected as a submillimeter continuum source, and none of the three were detected as CO sources, in the Rodriguez et al (2015) ALMA survey of ultralow-mass members and candidate members of the TWA.…”