First Sub-millimetre Detection Associated with Brown Dwarfs

Introduction

This is a short report on the first detection of sub-millimetre emission associated with Brown Dwarfs (BDs). It was achieved with SCUBA at the JCMT and MAMBO at the IRAM 30m telescope. The sub-millimetre emission is emitted by circum-"stellar" dust. Near and mid-infrared observations already indicated the presence of circumstellar dust around BDs by excess emission. While those observations allow conclusions on the geometry of the circumstellar matter causing the excess (e.g. Apai et al. 2002, ApJ, 573, L115) sub-millimetre observations enable us to assess the amount of the circumstellar matter.

The detections were achieved during observations between March 2002 and January 2003 targeting different populations of BDs. Our selection of BDs encompasses very young BDs (ages ~1 Myr) in star-forming regions like Taurus upto very close field BDs (ages >100 Myr). In this report we concentrate on the detections of circumstellar matter around two young BDs. Full account on the observing campaign and the results is given by Klein et al. (2003, ApJ, 593, L57).

Detected Dust Masses

Sub-millimetre emission associated with the BDs CFHT-BD-Tau 4 and IC 384 613 were detected consistently with SCUBA and MAMBO. The measured flux densities are listed in the table. Upper limits were derived for the other sources of the observing campaign of the order of a few mJy. The observations were executed in photometry mode using the central bolometer of SCUBA and MAMBO, respectively. The background (telluric and astronomical) subtraction was achieved by chopping with the secondary mirror and nodding the telescope. Its level has been estimated from the inner ring consisting of six bolometer pixels.

The measured flux densities can be used to derive the mass of the emitting dust, since the emission is mostly optically thin. However, one needs to assume the distance to the BD, the average dust temperature, and κ_ν, the mass absorption coefficient. Among the above quantities, the distances are well established. The distance to Taurus is 140 pc and it is 260 pc to the cluster IC 348. For dust around the young BDs, we choose a mass absorption coefficient of κ_ν=2 cm²g^-1 at 1.3 mm. The same value of κ_ν and a gas-to-dust ratio of 100 was applied by Beckwith et al. (1990, AJ, 99, 924) to derive disk masses for T Tauri disks. For the measurements at 850µm, we assume a wavelength dependence of κ_ν ~ λ^-β with β=1, also in accordance with Beckwith et al. The plausible range of dust temperatures is relatively small. We assume an average temperature of 10 to 20 K for the dust. This is the range of the mass-averaged dust temperature in the models for the disk around CFHT-BD-Tau 4 discussed by Pascucci et al. (2003, ApJ, 590, L111). The dust properties for the older sample of BDs have to be different (Klein et al.), but these are not needed here.

Applying the dust properties to the millimetre continuum measurements, we obtain the dust masses compiled in the table. The dust is certainly distributed in the form of disks as a detailed analyses of CFHT-BD-Tau 4 by Pascucci et al. show. The disk masses for the two BDs are 0.4…2.4 M_J (Jupiter mass) and 1.7…5.7 M_J, if we extrapolate the dust masses to disk masses assuming a gas-to-dust ratio of 100.

Summary

The observing campaign targeting BDs of several populations resulted in detections of millimetre emission associated with two young BDs, on which we concentrated here. For the other targets, upper limits on the circumstellar dust mass of a few Earth masses and lower were derived from the measured upper limits on the millimetre continuum flux densities. To estimate the dust masses, two sets of dust properties had to be applied: "Young" dust properties to BDs younger than 10 Myr and "debris" dust properties to BDs older than 100 Myr. The young dust properties were presente d here.

The detection of a few Jupiter masses of circumstellar matter around young BDs is an important result. To ensure this mass estimate, the dust properties have to be constrained further. However, a refinement of the dust properties will hardly change the fact that there are substantial amounts of circumstellar material around the two BDs CFHT-BD-Tau 4 and IC 348 613. Thus, the detections make BDs to places of possible planet formation. This fact opens a new set of targets for extrasolar planet searches, especially for direct imaging because of the low contrast between the central object and an prospective planet.

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