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

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( = 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( = 1), show pedestal emissions at radii 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 ±10% or more in different directions.