Energetic radiation and the sulfur chemistry of protostellar envelopes: submillimeter interferometry of AFGL 2591

Free access article

Issue		A&A Volume 475, Number 2, November IV 2007


Page(s)		549 - 558
Section		Interstellar and circumstellar matter
DOI		http://dx.doi.org/10.1051/0004-6361:20078032

A&A 475, 549-558 (2007)
DOI: 10.1051/0004-6361:20078032

Energetic radiation and the sulfur chemistry of protostellar envelopes: submillimeter interferometry of AFGL 2591

A. O. Benz¹, P. Stäuber¹, T. L. Bourke², F. F. S. van der Tak^{3, 4}, E. F. van Dishoeck⁵, and J. K. Jørgensen²

¹ Institute of Astronomy, ETH, 8092 Zurich, Switzerland
e-mail: benz@astro.phys.ethz.ch
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Max-Planck Institut für Radioastronomie, auf dem Hügel 69, 53121 Bonn, Germany
⁴ Netherlands Institute for Space Research (SRON), Landleven 12, 9747 AD Groningen, The Netherlands
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

(Received 7 June 2007 / Accepted 31 August 2007)

Abstract
Context.The chemistry in the inner few thousand AU of accreting envelopes around young stellar objects is predicted to vary greatly with far-UV and X-ray irradiation by the central star.
Aims.We search for molecular tracers of high-energy irradiation by the protostar in the hot inner envelope.
Methods.The Submillimeter Array (SMA) has observed the high-mass star forming region AFGL 2591 in lines of CS, SO, HCN, HCN( $\nu_2$ = 1), and HC¹⁵N with 0.6'' resolution at 350 GHz probing radial scales of 600-3500 AU for an assumed distance of 1 kpc. The SMA observations are compared with the predictions of a chemical model fitted to previous single-dish observations.
Results.The CS and SO main peaks are extended in space at the FWHM level, as predicted in the model assuming protostellar X-rays. However, the main peak sizes are found smaller than modeled by nearly a factor of 2. On the other hand, the lines of CS, HCN, and HC¹⁵N, but not SO and HCN( $\nu_2$ = 1), show pedestal emissions at radii $\la$ 3500 AU that are not predicted. All lines except SO show a secondary peak within the approaching outflow cone. A dip or null in the visibilities caused by a sharp decrease in abundance with increasing radius is not observed in CS and only tentatively in SO.
Conclusions.The emission of protostellar X-rays is supported by the good fit of the modeled SO and CS amplitude visibilities including an extended main peak in CS. The broad pedestals can be interpreted by far-UV irradiation in a spherically non-symmetric geometry, possibly comprising outflow walls on scales of 3500-7000 AU. The extended CS and SO main peaks suggest sulfur evaporation near the 100 K temperature radius. The effects of the corresponding abundance jumps may be reduced in visibility plots by smoothing due to inhomogeneity at the evaporation radius, varying by $\pm$ 10% or more in different directions.

Key words: accretion, accretion disks -- stars: formation -- submillimeter -- astrochemistry -- ISM: molecules -- X-rays: ISM

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