UV Interstellar Absorption Towards Gamma Velorum

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Session 14 -- Interstellar Dust and Extinction
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[14.10] UV Interstellar Absorption Towards Gamma Velorum

Edward L. Fitzpatrick and Lyman Spitzer, Jr. (Princeton University Observatory)

An analysis of high-resolution interstellar absorption line data toward HD 68273 ($\gamma^2$ Vel; O9 I + WC8), obtained with the GHRS on HST, reveals the presence of seven narrow absorption components, with LSR velocities between --23 and +10 km/s. Four of the narrow components arise in H I gas. Accurate depletions are obtained for Fe, Si, and Mg in the strongest of these components, at v(LSR) = 4 km/s, and are suggestive of ``warm'' neutral gas. The observed ratio of Fe and Mg atoms to Si atoms in the dust grains of this component is $2.04\pm0.10$, consistent with an olivine grain composition; the Fe/Mg ratio is $1.5\pm0.2$. The other three narrow features are identified with H II gas. The electron density in the two strongest H II components, as determined from the Si II* feature, is about 1 cm$^{-3}$. From the S column density in the H II components, we estimate the total pathlength through ionized gas to be 40 pc. With an assumed radius of 60 pc for a wind-blown bubble around $\gamma^2$ Vel, the effective Str\"{o}mgren radius is thus about 100 pc, requiring that $T \simeq 50000$ K for the WC8 component of the $\gamma^2$ Vel binary. Since the (perhaps) nearby star $\zeta$ Puppis is a comparable source of ionizing radiation, this temperature is an upper limit. The GHRS data also show broad absorption components of Si$^{+3}$, C$^{+3}$, and N$^{+4}$ which we identify with hot gas. The C$^{+3}$ and N$^{+4}$ features have widths corresponding to $T \simeq 4-8 \times 10^5$ K, consistent with the broad O$^{+5}$ line seen in Copernicus data. Despite some observational uncertainties, the ratios of column densities in the broad features agree (to $\pm$0.1 dex) with theoretical values for warm gas heated by thermal conduction and evaporating into an adjacent hot region. Outward evaporation from an isolated cloud in a hot ambient gas cannot be distinguished, on the basis of these data, from inward evaporation of a warm shell, compressed by an expanding, hot stellar-wind bubble. For halo stars, such as HD 93521, these ratios seem to have quite different values, consistent with cooling of infalling hot gas instead of conductive heating and evaporation.

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