Observations Of Interstellar Na Research Articles

Ultra-high-resolution observations of interstellar Na i and K i towards the Scorpius OB1 association

I present ultra-high-resolution (R 9 × 10 5 ) observations of interstellar NaI and K I absorption lines towards three members of the Scorpius OB1 association (HD 152235, 152236 and 152249). These observations have, for the first time, resolved the intrinsic linewidths of most of the discrete absorption components present along these complex sightlines. The aims of the project were twofold: (i) to constrain the physical conditions prevailing in the highly blueshifted Sco OB 1 shell components, and especially to search for evidence of active shocks within them; and (ii) to further constrain the low-velocity structure, where the Na I spectra are fully saturated but the unsaturated K I lines allow a clear resolution of the individual velocity components. The results of these analyses are discussed. Perhaps the most surprising result is the lack of any obvious correlation between the velocity dispersion of a velocity component (b-value) and its velocity. Specifically, the high-velocity shell components are generally found to be no broader than the low-velocity components attributed to foreground (often molecular) clouds, and cannot therefore be any hotter or more turbulent. Thus, with the possible exception of the most blueshifted component towards HD 152236, there is no evidence for active shocks in the shell components at present. However, consideration of the relative time-scales for post-shock cooling and grain surface adsorption indicates that shock processing in the past may still account for the low Na I/Ca II ratios of these components found in previous work.

Kappa Velorum: another variable interstellar sightline?

We present ultra-high-resolution (R = 900 000) observations of interstellar Na i and K i absorption lines towards κ Vel (HD 81188) which show clear evidence for temporal variation between 1994 and 2000. Specifically, the column densities of K0 and Na0 in the main velocity component have increased by 40 and 16 per cent, respectively, over this period. Earlier work had suggested that this component actually consists of two unresolved sub-components; this result is confirmed here, and the overall line profile is found to be consistent with only one of these sub-components having increased in strength since 1994. We argue that this variation is consistent with the line of sight gradually probing a cold, dense interstellar filament of the kind recently proposed by Heiles to explain other observations of small-scale structure in the interstellar medium.

A study of interstellar Na I D absorption lines towards the Lupus molecular clouds

Intermediate-resolution (60 000 ≤ R ≤ 120 000) observations of interstellar Na I lines towards 29 stars in the general direction of the Lupus molecular clouds (330° ≲ l ≲ 350°; 0° ≲ b ≲ 25°) are presented. Previously published spectra towards an additional seven stars are also included. Based on the Hipparcos distances to these stars, and the minimum distance at which strong interstellar Na I lines appear in the spectra, I obtain a distance of ∼150±10 pc to the Lupus molecular complex. While in agreement with a number of other independent estimates, this result is at odds with the value of 100 pc recently obtained by Knude & Hog from a Hipparcos-based study of interstellar extinction. A possible explanation for this discrepancy is discussed, and it is concluded that the value of 150±10 pc obtained here is to be preferred. In addition, these observations have some other implications for the structure of the interstellar medium in this direction, and these are briefly considered.

Ultra–High‐Resolution Observations of Interstellar Naiand CaiiK toward the High Galactic Latitude Star HD 28497

We present very high resolution (0.32 km s-1) spectra of interstellar Na I D1, D2, and Ca II K absorption toward HD 28497 obtained with the Ultra-High-Resolution Facility at the 3.9 m Anglo-Australian Telescope. The star is located in projection in a highly disturbed interstellar region close to a number of identified features including the high galactic latitude molecular cloud MBM 20, the large Orion-Eridanus shell, seen in Hα and H I 21 cm maps, and a filamentary loop structure between vLSR = -12 and -4 km s-1 in the Berkeley H I 21 cm survey and visible on the IRAS 100 μm map. Toward HD 28497 we detect 13 absorption components in the Na I spectra, to a column density limit of 2 × 1010 cm-2, and 10 in Ca II K over a velocity range of ~70 km s-1. Four absorption components in the Na I spectra show s-resolved hyperfine structure with b-values from 0.31 to 0.40 km s-1 and column densities from 4.0 to 14 × 1010 cm-2. If we assume the clouds represented by these components have no internal turbulent velocities, their temperatures would range between 134 and 227 K. One of these hyperfine split (hfs) components, at vLSR = -11.1 km s-1, shows significant temporal variation in equivalent width compared to earlier (1977) observations, making this the first interstellar sight line outside the Vela supernova remnant to show a time-varying component. The feature may be associated with the filamentary loop structure seen in this region. There is poor correspondence between the Na I and Ca II profiles: we do not detect narrow Ca II profiles to the four hfs Na I components, and only three of the well-resolved components have the same Ca II and Na I radial velocities and consistent b-values. One of these components, at vLSR = -30.0 km s-1, has a low Na I/Ca II ratio and arises in a region where turbulent motions dominate—properties consistent with the hypothesis that the cloud lies close to HD 28497. In general, however, the Na I and Ca II occupy different gaseous phases in the ISM. We have compared our data with 21 cm emission profiles obtained from the recent Leiden/Dwingeloo H I survey. Based on agreement in the velocities, the Na I/Ca II ratio, and the kinetic temperatures, we conclude that the component at vLSR = -7.5 km s-1 is associated with the front side of the large, expanding Orion-Eridanus shell. Unexpectedly, the molecular cloud MBM 20 is not detected either in our absorption spectra or in the H I profiles, indicating that HD 28497 lies away from the core of MBM 20. Apart from the two features at -11 and -7.5 km s-1 there is almost no agreement between the H I profiles and the optical spectra. Although we cannot rule out the possibility that most of the H I lies behind the star, this explanation seems unlikely because many of the H I features have previously been attributed to foreground phenomena. The beam sizes of the H I and the optical studies are quite different and this suggests a different explanation, namely that the physical sizes of the interstellar structures we detect in Na I and Ca II are not extensive enough to be detected in H I. If so, this raises questions about the usefulness in general of combining results obtained from H I 21 cm studies with results obtained from optical (or ultraviolet) studies of the interstellar gas.

Corrigendum: Observations of interstellar Na I and Ca II towards five southern stars and some comments on the location of the denser diffuse clouds in the southern Milky Way

Corrigendum: Observations of interstellar Na I and Ca II towards five southern stars and some comments on the location of the denser diffuse clouds in the southern Milky Way

Observations of interstellar Na I and Ca II towards five southern stars and some comments on the location of the denser diffuse clouds in the southern Milky Way

High-resolution observations (3.6 km s–1 FWHM) of interstellar Na I and Ca II lines towards five southern early-type stars are presented. All five stars have strong absorption lines with low LSR radial velocities (|υlsr|≲5kms−1) which are interpreted as arising in material associated with the ridge of local molecular clouds identified by Dame et al. It is argued that the strong, low-velocity, atomic and molecular lines observed towards other southern stars close to the galactic plane also arise in this material, and special reference is made to the foreground interstellar spectrum of the Sco OB1 association. Four of the observed stars also have absorption components consistent with an origin in the Sagittarius–Carina spiral arm, and two (HD 111904 and 164402) have highly negative velocity components (υlsr≲−40kms−1) which are interpreted as arising in expanding shells centered on the Cen OB1 and Sgr OB1 associations, respectively.

Observations of interstellar NA I lines at 3302 A

Observations have been made of the equivalent widths of the interstellar Na i lambda3302 doublet toward 21 stars which have strongly saturated D-lines. Column densities of Na i have been derived, and the depletion of Na in the interstellar medium is discussed. Toward most stars Na is depleted by about a factor of 6. The distribution of line widths is found to be strongly peaked near 1 km s$sup -1$, in agreement with the isothermal sound velocity at 100 K. (AIP)

A Discussion on astronomy in the ultraviolet - The interstellar ionization balance due to ultraviolet radiation

A new estimate of the interstellar ultraviolet radiation field is used to analyse some well established observations of interstellar Na i, Ca n, Mg i, n in £, e and 8 Ori. Four models of the interstellar medium are used and chemical abundances derived relative to that of hydrogen, obtained from Lyman a data. The scatter of the results is discussed together with the apparent anomalous abundance of calcium.