HIGH-VELOCITY GAS IN GHRS SPECTRA OF 3 OB STARS

In Hubble Space Telescope (HST) GHRS spectra of the B1 V star 23 Ori, we observed interstellar absorption lines of C II, C II, Mg II, Al II , Al III, Si II, Si III, and S II at a heliocentric radial velocity of about -100 km s(-1). Lines of N I, O I, and Fe II that normally are s trongly saturated in low-velocity interstellar gas were not detected a t all in the high-velocity gas, apparently owing to collisional ioniza tion of these species. Echelle exposures of the Mg II lambda 2800 doub let resolved those high-velocity features into four blended line compo nents. The widths of the partially resolved components require upper l imits T-max less than or similar to 12,000 K, while the ionization bal ance of the gas is crudely characteristic of collisional ionizational equilibrium near T = 25,000 K. With a total column density N(H) approx imate to 6 x 10(17) cm(-2), these high-velocity clouds appear to consi st of warm, ionized gas, which is now cooling after having been previo usly shocked; the gas is over-ionized with respect to its temperature and has not yet reached a steady-state equilibrium. Comparable spectra of the O9 Ib star tau CMa revealed high-velocity interstellar clouds with properties generally similar to those of the 23 Ori clouds, excep t for a much smaller total column density of N(H) approximate to 1 x 1 0(17) cm(-2). Similar spectra of the B7 III star eta Tau showed absorp tion lines at heliocentric radial velocities of -149 and -42 km s(-1). Each absorbing region produces detectable, narrow absorption lines of C II, C II, Mg II, Al II, and Si III, although not of N I, O I, S II , and Fe II. The lambda 1335 C II lines from the excited J = 3/2 leve l are much stronger than the lambda 1334 C II lines from the ground le vel, even when uncertainties in the interpolated stellar continuum are taken into account. The suprathermal C II excitation seen in both hi gh-velocity regions arises in the spectrum of a relatively nearby star , which also is a close binary. These circumstances suggest that the a bsorption lines observed at high velocity are formed in matter flowing within or near the binary system, and that optical pumping by eta Tau may produce the high-C II excitation found in this circumstellar gas.