VLT/UVES and WHT/UES absorption spectroscopy of the circumstellar envelope of IRC + 10° 216 using background stars

First results and a search for DIBs

¹ Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
² Groupe d'Astrophysique, CNRS and Université de Montpellier, Case 072, Place Bataillon, 34095 Montpellier Cedex 05, France e-mail: mauron@graal.univ-montp2.fr
³ School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK e-mail: June.McCombie@nottingham.ac.uk; Peter.Sarre@nottingham.ac.uk (Based on observations made with the ESO VLT 8 m telescope with UVES (program 66.C-0220), WHT/UES (La Palma) and the 1.2 m telescope of the Haute–Provence Observatory, France. )

Corresponding author: T. R. Kendall, tkendall@oal.ul.pt

Received: 7 January 2002
Accepted: 7 March 2002

Abstract

A unique and novel set of observations has been undertaken to probe the circumstellar envelope (CSE) of the nearby (130 pc) carbon star IRC +10° 216 using optical absorption spectroscopy towards background stars lying beyond the envelope. The primary aim of the observations is to search for diffuse band (DIB) carriers in the CSE, for which the mass-losing envelopes of carbon stars are a likely place of origin. Our principal target is a star located 37´´ from IRC +10° 216 and was observed with VLT/UVES. A detailed model atmosphere and abundance analysis shows that it is somewhat metal-poor and has confirmed that it lies far beyond IRC +10° 216. The circumstellar H+2H₂ column density expected along the line of sight towards this target is relatively high, ~ cm^-2, and is large compared to that derived from the small interstellar extinction estimated in the zone of IRC +10° 216 at ,  mag. The CSE is certainly detected in the K i resonance lines, which are centred at the heliocentric velocity of IRC +10° 216 and have  km s^-1, consistent with twice the terminal expansion velocity of the circumstellar gas. The data show also that circumstellar Na i is very probably detected, as seen towards two background stars. The strongest DIB (6284 Å) present in the UVES wavelength coverage is detected but very probably arises in the foreground ISM. No DIB is detected at 6614 Å, or elsewhere. Overall, the data suggest that the DIB carriers, if present in the CSE, have a low abundance relative to H in the C-rich envelope of IRC +10° 216, in comparison with this ratio in the ISM.