ECHELLE SPECTROSCOPY OF THE NA-I-D ABSORPTION-LINE SYSTEMS OF MARKARIAN-231

We have obtained echelle spectra of the nucleus of Markarian 231 in 19 91 and 1994 and modeled the profiles of the sodium absorption lines th at are bluesfiifted up to 7800 km s(-1) less than the systemic recessi on velocity of this active galactic nucleus (AGN). We find that the br oadest absorption line (system I) can best be fitted with a blend of m ultiple, optically thin components rather than a single, or a blend of many, saturated lines. The total column density of neutral sodium in this broad line is N-Na (I) 10(14) cm(-2), a factor of 10 less than pr evious estimates. The line profile fit of the narrow Na I D absorption lines (systems II and III) yields a column density of N-Na (I) simila r or equal to 10(12) cm(-2). The variability of the absorption lines o f Mrk 231 is seen again in the disappearance of an Na I D absorption l ine at 6015 Angstrom sometime between 1991 and 1994 and a change in th e profile of the broad absorption line. The weak absorption lines away from the broad system have similar column densities and Doppler veloc ity widths to those seen in absorption lines due to extended gas from galaxies illuminated by background QSOs. We find no evidence in the sp ectra of regions 5'' east and west of the nucleus for Na I D absorptio n or the blend of H alpha + [N II] emission at the velocity of system I that had been previously observed. We consider the He I absorption s een in optical spectra and in the Hubble Space Telescope (HST) Faint O bject Spectrograph (FOS) spectrum of Smith et al., and we conclude tha t He I absorption arises from the metastable 2(3)S level in statistica l equilibrium due to irradiation by a dilute, steep spectrum radiation field; a similar mechanism to that proposed for Na I D and He I absor ption seen in shell stars.