2002A&A...387..624K

A unique and novel set of observations has been undertaken to probe the circumstellar envelope (CSE) of the nearby (130pc) 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 V=16 G-type 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, ∼2x10²¹cm^–2, and is large compared to that derived from the small interstellar extinction estimated in the zone of IRC +10° 216 at b=+43°, E_B–V<0.03mag. The CSE is certainly detected in the K I resonance lines, which are centred at the heliocentric velocity of IRC +10° 216 and have FWHM∼30km/s, 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 (6284Å) 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.