Deuterium and the Local Interstellar Medium Properties for the Procyon and Capella Lines of Sight

Abstract

We present Goddard High-Resolution Spectrograph observations of the interstellar H I and D I Lyα lines and the Mg II and Fe II resonance lines formed along the lines of sight toward the nearby stars Procyon (3.5 pc, l = 214°, b = 13°) and Capella (12.5 pc, l = 163°, b = 5°). New observations of Capella were obtained at orbital phase 0.80, when the radial velocities of the intrinsic Lyα emission lines of each star were nearly reversed from those of the previous observations at phase 0.26 (analyzed by Linsky et al.). Since the intrinsic Lyα line of the Capella system the "continuum" against which the interstellar absorption is measured has different shapes at phases 0.26 and 0.80, we can derive both the intrinsic stellar profiles and the interstellar absorption lines more precisely by jointly analyzing the two data sets. We derive interstellar parameters from the simultaneous analysis of the two data sets as follows: (D/H)LISM = (1.60±0.09 [+0.05, - 0.10 systematic error]) × 10-5, temperature T = 7000±500 [±400 systematic error] K, and microturbulence ξ = 1.6±0.4 [and ±0.2 systematic error] km s-1. (All random errors determined in this paper are ±2 σ.) For the analysis of the Procyon line of sight, we first assumed that the intrinsic Lyα line profile is a broadened solar profile, but this assumption does not lead to a good fit to the observed D I line profile for any value of D/H. We then assumed that (D/H)LISM = 1.6 × 10-5, the same value as for the Capella line of sight, and we modified the broadened solar profile to achieve agreement between the simulated and observed line profiles. The resulting asymmetric intrinsic stellar line profile is consistent with the shapes of the scaled Mg II line profiles. We believe therefore that the Procyon data are consistent with (D/H)LISM = 1.6 × 10-5, but the uncertainty in the intrinsic Lyα emission-line profile does not permit us to conclude that the D/H ratio is constant in the local interstellar medium (LISM). The temperature and turbulence in the Procyon line of sight are T = 6900±80 (±300 systematic error) K and ξ = 1.21±0.27 km s-1. These properties are similar to those of Capella, except that the gas toward Procyon is divided into two velocity components separated by 2.6 km s-1 and the Procyon line of sight has a mean neutral hydrogen density that is a factor of 2.4 larger than that of the Capella line of sight. This suggests that the first 5.3 pc along the Capella line of sight lies within the local cloud and the remaining 7.2 pc lies in the hot gas surrounding the local cloud. We propose that nHI = 0.1065±0.0028 cm-2 be adopted for the neutral hydrogen density within the local cloud and that ξ = 1.21±0.27 km s-1 be adopted for the nonthermal motions. The existence of different second velocity components toward the nearby stars Procyon and Sirius provides the first glimpse of a turbulent cloudlet boundary layer between the local cloud and the surrounding hot interstellar gas. We speculate that what is often called "turbulence" may instead be velocity shear within the local cloud that is not a rigid comoving structure. We also derive gas phase abundances of iron and magnesium in the Procyon line of sight and the abundance of oxygen in the Capella line of sight. Within the context of standard big bang nucleosynthesis, our observed value of (D/H)LISM leads to 0.042 ≤ ΩBh250 ≤ 0.09, depending on the assumed model for Galactic chemical evolution of deuterium. Our lower limit (D/H)LISM > 1.41 × 10-5 provides a hard lower limit to the primordial D abundance and thus a hard upper limit on ΩBh250 ≤ 0.125. These limits are independent of Galactic chemical evolution models and only assume that D is destroyed with time.