The carbon star IRAS 15194-5115: circumstellar CO radio and FIR rotational lines

Abstract

We have modelled radio and far-infrared rotational lines of ¹²CO and ¹³CO obtained from the circumstellar envelope of the infrared-bright carbon star IRAS 15194-5115. Eleven rotational lines between J=1-0 and 21-20 and nine rotational lines between J=1-0 and 22-21 in the ground vibrational states of ¹²CO and ¹³CO, respectively, provide the observational constraints. A model of the circumstellar envelope with a constant mass-loss rate (1x10^-5 M_sun yr^-1) and ¹²CO/¹³CO-ratio (5.5) is consistent with our observed FIR and radio data, suggesting that the wind characteristics have not changed significantly over the past few thousand years. Thus, IRAS 15194-5115 appears to be a highly evolved AGB-star, but the carbon star properties combined with the inferred low ¹²C/¹³C -ratio make the evolutionary status of this star uncertain. It may have been a J-star for which the ¹²C/¹³C -ratio has remained low, or it may be a star of 5 to 8 solar masses, which has recently become a carbon star due to quenching of hot bottom burning. The dust properties or the relative amount of dust in the envelope appear to be different from those in the envelope of the well known carbon star IRC+10 216. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.