The Star Formation Newsletter

s of recently accepted papers Sub-Millimeter Continuum Observations of ρ OPH A: The Candidate Protostar VLA 1623 and Pre-Stellar Clumps Philippe André, Derek Ward-Thompson and Mary Barsony 1 Service d’Astrophysique, Centre d’Etudes de Saclay, F–91191 Gif-sur-Yvette Cedex, France 2 Mullard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, England 3 Harvard-Smithsonian Center for Astrophysics, MS–78, 60 Garden Street, Cambridge, MA 02138, USA We have mapped the sub-millimeter continuum emission from the rich star-forming core, ρ Oph A, at 350, 450, 800 and 1300μm using the JCMT and IRAM 30-m telescopes. In addition to the diffuse emission from the cloud core itself, these observations reveal four well-defined clumps, which, surprisingly, are barely visible in the single-dish molecular maps obtained so far in this region. The gas may have partially frozen out onto the dust grains, due to low temperature and/or high density in the clumps. We discuss the possibility that these clumps are pre-stellar or extremely young, low-mass protostars. One of them coincides with the (cm) radio source VLA 1623 which drives one of the youngest and best collimated CO bipolar flows known to date. Supplementing our JCMT measurements with stringent upper limits obtained at all four IRAS bands by Maximum Entropy Deconvolution of the raw IRAS data, we show that VLA 1623 is a highly obscured (AV > ∼ 1000) young stellar object (YSO) with a remarkably low bolometric luminosity (Lbol < ∼ 1 L ) and dust temperature (Td < ∼ 20 K). Its spectral energy distribution is well fitted by a single modified blackbody from dust at 15− 20 K with an opacity index β = 1.5. This indicates a mass of circumstellar material MC ∼ 0.6M and a bolometric luminosity Lbol ∼ 1 L . Our sub-mm observations resolve a nearly spherical dust envelope around VLA 1623, of FWHM size ≈ 2000 AU roughly independent of wavelength. This suggests that the bulk of the circumstellar mass of VLA 1623 is distributed in an “apple-like” structure rather than a disk. Detailed, spherical modelling of this dust structure, using the radiative transfer code of Wolfire & Cassinelli, shows the spectral energy distribution and sub-mm intensity profiles of VLA 1623 can be fitted simultaneously by a 1 L internal source surrounded by a circumstellar envelope with a surprisingly shallow density distribution (ρ(r) ∝ r−0.5). The remarkably cold temperature, high internal obscuration, and relatively massive circumstellar structure of VLA 1623 all point to an extremely young object, perhaps a true protostar. We compare VLA 1623 with a few other very young embedded sources similarly characterized by very low values of the ratio Lbol/Lsubmm, and suggest that these objects define an entirely new class of YSOs, which we call “Class 0” to indicate their extreme youth. Although these strong sub-mm YSOs are invisible in the nearand mid-IR, they are already powering highly collimated outflows (opening angles < 30◦) with great efficiency (Lflow/Lbol up to ∼ 50%). This may indicate that the outflow phenomenon starts earlier than previously thought in (proto)stellar evolution, i.e., prior to the near-IR Class I phase as in the scheme devised by Lada. Accepted by Astrophys. J. VLBI Survey of ρ Ophiuchi: A Population of Magnetized, Diskless Young Stellar Objects Philippe André, Bryan D. Deeney, Robert B. Phillips and Jean-François Lestrade 1 Service d’Astrophysique, Centre d’Etudes de Saclay, F–91191 Gif-sur-Yvette Cedex, France 2 Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085, USA 3 Massachusetts Institute of Technology, Haystack Observatory, Westford, MA 01886, USA 4 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91106, USA In the past years, extensive radio-continuum surveys conducted with the Very Large Array (VLA) have shown that only ∼ 10 of the ∼ 100 young stellar objects (YSOs) embedded in the central region of the ρ Ophiuchi molecular cloud have “strong” radio emission above S6cm = 1 mJy. Two of these ρ Oph radio stars, S1 and DoAr 21, have recently been detected and resolved by Very Long Baseline Interferometry (VLBI), implying that their radio emission is “peristellar”, i.e., originates in magnetic structures up to ∼ 25 R∗ in diameter. We report here VLBI observations of nine other ρ Oph YSOs, yielding positive detections in five cases (VSSG 14, WL 5, ROC 16, ROX 39, and VSSG 11), with flux densities ranging from 1.2 to 9 mJy at 3.6 cm. In contrast, while simultaneously detected by the VLA at