科研之友
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摘要
We present medium-resolution ultraviolet interstellar absorption measurements recorded with the HST-COS and FUSE spectrographs toward the post-AGB star Barnard 29 in the M13 globular cluster. Inspection of the UV spectra has revealed absorption at V-LSR similar to -60 km s(-1) in the absorption profiles of the C I, C II, C II*, C IV, N I, N II, N V, O I, ALII, Si II, Si IV, S II and Fe II ions. This component can be associated with an intermediate-velocity cloud (IVC), previously identified by both radio and visible observations. More importantly, these data have newly revealed high-velocity absorption at V-LSR = -121 +/- 3 km s(-1) in the UV ions of C II, C III, C IV, N II, Si II, and Si IV. This absorption is thought to arise in a highly ionized, multiphase high-velocity cloud (HVC) located a distance of 0.46 < z < 4.7 kpc and has no measurable counterpart in the 21 cm neutral H I emission profile or the normal tracers of neutral gas in the ISM such as Na I, N I or O I. This makes it one of the four nearest highly ionized HVCs yet discovered. The observed column density ratios of the H V components to the C IV and Si IV lines are consistent with values predicted for a turbulent mixing layer or shock ionization of a gas cloud. We have derived estimates of the abundance ratios of C and Si with respect to N relative to their corresponding solar values. Our results indicate values of [Si/N] > +0.40 +/- 0.21 and [C/N] +0.10 +/- 0.25 for the HVC gas component. These values indicate that the H V gas has higher than solar relative abundances, consistent with gas of a supersolar metallicity. Several other highly ionized HVCs with distances of less than 7.5 kpc have recently been shown to possess similarly high-metallicity values that are consistent with an origin in condensing returning gas from a galactic fountain.
- 出版日期2011-8
