FUSEDetermination of a Low Deuterium Abundance along an Extended Sight Line in the Galactic Disk

Abstract

We present a study of the deuterium abundance along the extended sight line toward HD 90087 with the Far Ultraviolet Spectroscopic Explorer (FUSE ). HD 90087 is a O9.5 III star located in the Galactic disk at a distance of $2.7 kpc away from the Sun. Both in terms of distance and column densities, HD 90087 has the longest and densest sight line observed in the Galactic disk for which a deuterium abundance has been measured from ultraviolet absorption lines so far. Because many interstellar clouds are probed along this sight line, possible variations in the properties of individual clouds should be averaged out. This would yield a deuterium abundance that is characteristic of the interstellar medium on scales larger than the Local Bubble. The FUSE spectra of HD 90087 show numerous blended interstellar and stellar features. We have measured interstellar column densities of neutral atoms, ions, and molecules by simultaneously fitting the interstellar absorption lines detected in the different FUSE channels. As far as possible, saturated lines were excluded from the fits in order to minimize possible systematic errors. IUE (International Ultraviolet Explorer) archival data are also used to measure neutral hydrogen. We report D/O ¼ (1:7 AE 0:7) ; 10 À2 and D/H ¼ (9:8 AE 3:8) ; 10 À6 (2 ). Our new results confirm that the gas-phase deuterium abundance in the distant interstellar medium is significantly lower than the one measured within the Local Bubble. We supplement our study with a revision of the oxygen abundance toward Feige 110, a moderately distant ($200 pc) sdOB star, located $150 pc below the Galactic plane. Excluding saturated lines from the fits of the FUSE spectra is critical; this led us to derive an O i column density about 2 times larger than the one previously reported for Feige 110. The corresponding updated D/O ratio on this sight line is D/O ¼ (2:6 AE 1:0) ; 10 À2 (2 ), which is lower than the one measured within the Local Bubble. The data set available now outside the Local Bubble, which is based primarily on FUSE measurements, shows a contrast between the constancy of D/O and the variability of D/ H. As oxygen is considered to be a good proxy for hydrogen within the interstellar medium, this discrepancy is puzzling.

Variations in D/H and D/O from NewFar Ultraviolet Spectroscopic ExplorerObservations

“…These measurements strengthen the argument that these high ratios cannot be due to some systematic problem associated with the different analyses (see also discussion below) or that they are peculiar cases. In particular, it has been argued that the high D/H ratios found in the ISM could be due to systematic effects that affect the N(H i) measurements (Hébrard et al 2005). Because the D/O ratios derived for these two sight lines are also high and independent of N(H i), the high D/ H ratios are likely due to a high abundance of D and not to a systematic problem with the N(H i) determination (see also discussion in x 7.1).…”

“…References.-(1) Hébrard et al 1999;(2) Hébrard & Moos 2003;(3) Dring et al 1997;(4) Linsky et al 1995;(5) Wood et al 1996;(6) Wood et al 2000; (7) Wood et al 2002b;(8) Piskunov et al 1997;(9) Wood et al 2002a;(10) Hébrard et al 2002;(11) Kruk et al 2002;(12) Lemoine et al 2002;(13) Vennes et al 2000;(14) Oliveira et al 2005;(15) Lehner et al 2002;(16) Oliveira et al 2003;(17) York & Rogerson 1976;(18) York & Kinahan 1979;(19) Sonneborn et al 2002;(20) Allen et al 1992;(21) Gry et al 1985;(22) This work; (23) Friedman et al 2002;(24) Hébrard et al 2005;(25) Ferlet et al 1980;(26) Meyer et al 1998;(27) York 1983;(28) Sonneborn et al 2000;(29) Jenkins et al 1999;(30) Bluhm et al 1999;(31)...…”

“…As mentioned above, the apparent lack of scatter in the D/O ratios has been used as an argument against the existence of sight lines with high D/H ratios (Hébrard et al 2005). However, one has to be careful in analyzing these ratios, as they depend Meyer et al [1998] value for O/ H).…”

Galactic evolution of D, 3He and 4He

“…The dispersion (by a factor of 3) which has been found in the measurements (Linsky et al 2006) makes it hard to interpret the data in the context of standard GCE models. It has been suggested (Hébrard et al 2005, Linsky et al 2006 that either the lowest (see also text by G. Hébrard, this volume) or the highest (see also text by J. Linsky, this volume) observed abundances are indicative of the actual value of the deuterium abundance in the local ISM (LISM). However, neither of these values can be reproduced by GCE models in agreement with all the major observational constraints for the solar neighbourhood (Romano et al 2006).…”

“…(1) Cyburt et al (2008); (2) Pettini et al (2008); (3) Geiss & Gloeckler (1998); (4) Linsky et al (2006); (5) Hébrard et al (2005); (6) Prodanović et al (2009); (7) Bania et al (2002); (8) Gloeckler & Geiss (1996); (9) Peimbert et al (2007); (10) Steigman (2007); (11) Asplund et al (2009). can not be reproduced by the models, independently of how many low-mass stars burn their 3 He on the RGB.…”