SMA observations of young disks: separating envelope, disk, and stellar masses in class I YSOs

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: dave@strw.leidenuniv.nl
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Observatorio Astronómico Nacional (IGN), Alfonso XII, 3, 28014 Madrid, Spain

Received: 26 March 2007
Accepted: 14 January 2008

Abstract

Context.Young stars are born with envelopes, which in the early stages obscure the central (proto)star and circumstellar disk. In the Class I stage, the disks are still young, but the envelopes are largely dispersed. This makes the Class I sources ideal targets for studies of the early stages of disks.

Aims.We aim to determine the masses of the envelopes, disks, and central stars of young stellar objects (YSOs) in the Class I stage.

Methods.We observed the embedded Class I objects IRS 63 and Elias 29 in the ρ  Ophiuchi star-forming region with the Submillimeter Array (SMA) at 1.1 mm.

Results.IRS 63 and Elias 29 are both clearly detected in the continuum, with peak fluxes of 459 and 47 mJy/beam, respectively. The continuum emission toward Elias 29 is clearly resolved, whereas IRS 63 is consistent with a point source down to a scale of 3″ (400 AU). The SMA data are combined with single-dish data, and both disk masses of 0.055 and ≤0.007  and envelope masses of 0.058 and ≤0.058  are empirically determined for IRS 63 and Elias 29, respectively. The disk+envelope systems are modelled with the axisymmetric radiative-transfer code RADMC, yielding disk and envelope masses that differ from the empirical results by factors of a few. HCO⁺ –2 is detected toward both sources, HCN –2 is not. The HCO⁺ position-velocity diagrams are indicative of Keplerian rotation and allow an estimate of the mass of the central stars. For a fiducial inclination of 30°, we find stellar masses of for IRS 63 and for Elias 29.

Conclusions.The sensitivity and spatial resolution of the SMA at 1.1 mm allow a good separation of the disks around Class I YSOs from their circumstellar envelopes and environments, and the spectral resolution makes it possible to resolve their dynamical structure and estimate the masses of the central stars. The ratios of the envelope and disk masses are found to be 0.2 for IRS 63 and 6 for Elias 29. This is lower than the values for Class 0 sources, which have , suggesting that this ratio is a tracer of the evolutionary stage of a YSO.