1 research outputs found
Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. II. Observations toward Gamma2 Velorum and Zeta Puppis by the Interstellar Medium Absorption Profile Spectrograph
To measure interstellar atomic deuterium abundances, we used the Interstellar
Medium Absorption Profile Spectrograph (IMAPS) to obtain spectra of gamma2 Vel
and zeta Pup over the wavelength interval 930-1150 A at a resolving power of
80,000. The interstellar D I features are resolved and cleanly separated from
interstellar H I in the Ly-delta and Ly-epsilon profiles of both sight lines,
and also in the Ly-gamma profile of zeta Pup. The D I profiles were modeled
using a velocity template derived from several N I lines in the IMAPS spectra
recorded at higher S/N. To find the best D I column density, we minimized
chi-squares for model D I profiles that included not only the N(D I) as a free
parameter, but also the effects of several potential sources of systematic
error which could also be varied. For both stars, H I column densities were
measured by analyzing Ly-alpha absorption profiles in a large number of IUE
high dispersion spectra. Ultimately we found that D/H = 2.18(+0.36,-0.31)e-5
for gamma2 Vel and 1.42(+0.25,-0.23)e-5 for zeta Pup, values that contrast
markedly with D/H derived in Paper I for delta Ori (the stated errors are 90%
confidence limits). Evidently, the atomic D/H ratio in the ISM, averaged over
path lengths of 250 to 500 pc, exhibits significant spatial variability.
Furthermore, variations in D/H do not appear to be anticorrelated with N/H.
Within the framework of standard Big Bang Nucleosynthesis, the large value of
D/H found toward gamma2 Vel is equivalent to a cosmic baryon density of Omega_B
h^2 = 0.023 (+-0.002), which we regard as an upper limit since there is no
correction for the destruction of deuterium in stars.Comment: 37 pages, 10 figures, to appear in the Astrophysical Journa