High-Velocity Gas in GHRS Spectra of Three OB Stars

Details High-Velocity Gas in GHRS Spectra of Three OB Stars High-Velocity Gas in GHRS Spectra of Three OB Stars

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...468..290t&link_type=abstract

Astrophysical Journal v.468, p.290

Astronomy and Astrophysics

Astronomy

26

Ism: Abundances, Ism: Kinematics And Dynamics, Stars: Early-Type, Ultraviolet: Stars

Scientific paper

In Hubble Space Telescope (HST) GHRS spectra of the B1 V star 23 Ori, we observed interstellar absorption lines of C II, C 11*, 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 strongly saturated in low-velocity interstellar gas were not detected at all in the high-velocity gas, apparently owing to collisional ionization of these species. Echelle exposures of the Mg II λ2800 doublet resolved those high-velocity features into four blended line components. The widths of the partially resolved components require upper limits Tmax ≤ 12,000 K, while the ionization balance of the gas is crudely characteristic of collisional ionizational equilibrium near Tmax = 25,000 K. With a total column density N(H) ≍6 x 1017 cm-2, these high-velocity clouds appear to consist of warm, ionized gas, which is now cooling after having been previously 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 τ CMa revealed high-velocity interstellar clouds with properties generally similar to those of the 23 Ori clouds, except for a much smaller total column density of N(H) ≍ 1 x 1017 cm-2.
Similar spectra of the B7 III star η Tau showed absorption 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 HI, although not of N I, O I, S II, and Fe II. The λ1335 C 11* lines from the excited J = 3/2 level are much stronger than the λ1334 C II lines from the ground level, even when uncertainties in the interpolated stellar continuum are taken into account. The suprathermal C II* excitation seen in both high-velocity regions arises in the spectrum of a relatively nearby star, which also is a close binary. These circumstances suggest that the absorption lines observed at high velocity are formed in matter flowing within or near the binary system, and that optical pumping by η Tau may produce the high-C II excitation found in this circumstellar gas.

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