Interstellar CO towards X Persei (HD 24534) - II. Two-component model

The observations made by the Goddard High Resolution Spectrograph (GHRS) ab oard the Hubble Space Telescope (HST) of molecular CO in absorbing gas towa rds X Persei are reported. The two-component statistical equilibrium model incorporating radiative excitation of CO by line emission at the same veloc ity that originates in nearby molecular clouds has been used to reproduce h igh-resolution GHRS spectra. Earlier analysis indicates that the cloud has a complex structure and at least a two-component model should be used to ob tain accurate results. The spectra obtained from the International Ultravio let Explorer (IUE) were used to complement GHRS data and constrain the spac e of possible solutions. The new oscillator strengths recommended by Eidels berg et al. for A-X bands have been used. The results show that one of the components may be attributed to the Perseus OB2 molecular cloud, and the ot her component to an extension of the Taurus dark cloud. The total CO column density N(CO)=(1.0 +/- 0.2)x10(16) cm(-2) has been determined. According t o the results about 85 per cent of the observed CO belongs to an extension of the Taurus dark cloud. The CO radiation that originates in nearby molecu lar clouds may be the dominant excitation mechanism of the observed CO. The early results of (CO)-C-13 line analysis indicate a (CO)-C-13/(CO)-C-12 ra tio of about 40.