Ultraviolet properties of IRAS-selected Be stars

New IUE observations were obtained of 35 Be stars from a list of stars which show excess infrared fluxes in IRAS data. The IRAS-selected Be stars show larger C IV and Si IV equivalent widths than other Be stars. Excess C IV and Si IV absorption seems to be independent of spectral type for IRAS-selected Be stars later than spectral type B4. This is interpreted as evidence for a possible second mechanism acting in conjunction with radiation pressure for producing the winds in Be stars. No clear correlation of IR excess of v sin i with C IV or Si IV equivalent widths is seen, although a threshold for the occurrence of excess C IV and Si IV absorption appears at a v sin i of 150 km/sec

Diffuse band profiles in the Rho Ophiuchi cloud

High-resolution, high signal-to-noise ratio line profiles are presented for the 5780 and 5797 A diffuse interstellar bands toward six stars in the Rho Oph dark cloud. Target stars were chosen to exhibit a wide range of interstellar grain properties, as measured by grain polarization and far-UV extinction. The extreme case of the heavily reddened star HD 147889 is included; this star has one of the highest known lambdamax values, indicative of unusually large grains. Despite the differences in the grain properties, the line profiles and central wavelengths for the 5780 A band were found to be essentially identical for all lines of sight. This finding is in contradiction to the results of the embedded cavity grain model for diffuse bands, which predicts changes in both profile and central wavelength with grain size and impurity concentration. Results therefore support a molecular origin for the diffuse bands

On the size distribution of newly formed grains in red supergiant atmospheres

Theoretical ultraviolet extinction curves have been calculated for comparison with observed curves for circumstellar dust in M supergiants. The theoretical curves assumed a silicate grain composition, because silicate grains are expected in the oxygen-rich environments that are observed. Calculations were performed with and without the inclusion of scattering into the beam, with largely similar results. A comparison of the computed curves with the observed ultraviolet extinction curve for circumstellar dust in Scorpii indicates that the size distribution of the circumstellar grains must cut off near 800 Å that is, there are few or no grains smaller than this. Our conclusion is that smaller interstellar silicate grains, where they exist, must come from other sources such as grain fragmentation in shocks

Diffuse band profiles in the Rho Ophiuchi cloud

High-resolution, high signal-to-noise ratio line profiles are presented for the 5780 and 5797 A diffuse interstellar bands toward six stars in the Rho Oph dark cloud. Target stars were chosen to exhibit a wide range of interstellar grain properties, as measured by grain polarization and far-UV extinction. The extreme case of the heavily reddened star HD 147889 is included; this star has one of the highest known lambdamax values, indicative of unusually large grains. Despite the differences in the grain properties, the line profiles and central wavelengths for the 5780 A band were found to be essentially identical for all lines of sight. This finding is in contradiction to the results of the embedded cavity grain model for diffuse bands, which predicts changes in both profile and central wavelength with grain size and impurity concentration. Results therefore support a molecular origin for the diffuse bands

Diffuse band profiles in the spectrum of HD 29647: Evidence for a molecular origin?

High signal-to-noise ratio spectra have been obtained of the diffuse interstellar bands at 5780 and 5797 Å in the spectrum of HD 29647, a heavily reddened star within or behind a portion of the Taurus dark cloud complex. The observations were made using the coudé spectrograph on the Canada-France-Hawaii Telescope. The Reticon detector combined with the coudé spectrograph and excellent observing conditions allowed S/N ratios as high as 200 for this star, which was V=8.37 and E(B–V)=1.03. In two separate exposures both bands were found to be narrower and weaker than normal values for stars of similar reddening, and the profiles appear to deviate from those normally seen as well. Theories of band formation due to absorption centers in solid grains require bandwidths much greater than observed in HD 29647 and predict profile variations with grain size that are quite different from what is seen. Therefore we suggest that these observations argue for a molecular origin for the diffuse bands. The observed profiles may be explained as due to unusual rotational excitation in molecules

New Insights on Interstellar Gas-Phase Iron

In this paper, we report on the gas-phase abundance of singly-ionized iron (Fe II) for 51 lines of sight, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE). Fe II column densities are derived by measuring the equivalent widths of several ultraviolet absorption lines and subsequently fitting those to a curve of growth. Our derivation of Fe II column densities and abundances creates the largest sample of iron abundances in moderately- to highly-reddened lines of sight explored with FUSE, lines of sight that are on average more reddened than lines of sight in previous Copernicus studies. We present three major results. First, we observe the well-established correlation between iron depletion and and also find trends between iron depletion and other line of sight parameters (e.g. f(H_2), E_(B-V), and A_V), and examine the significance of these trends. Of note, a few of our lines of sight probe larger densities than previously explored and we do not see significantly enhanced depletion effects. Second, we present two detections of an extremely weak Fe II line at 1901.773 A in the archival STIS spectra of two lines of sight (HD 24534 and HD 93222). We compare these detections to the column densities derived through FUSE spectra and comment on the line's f-value and utility for future studies of Fe II. Lastly, we present strong anecdotal evidence that the Fe II f-values derived empirically through FUSE data are more accurate than previous values that have been theoretically calculated, with the probable exception of f_1112.Comment: Accepted for publication in ApJ, 669, 378; see ApJ version for small updates. 53 total pages (preprint format), 7 tables, 11 figure

Is There Enhanced Depletion of Gas-Phase Nitrogen in Moderately Reddened Lines of Sight?

We report on the abundance of interstellar neutral nitrogen (NI) for 30 sightlines, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Hubble Space Telescope (HST). NI column densities are derived by measuring the equivalent widths of several ultraviolet absorption lines and subsequently fitting those to a curve of growth. We find a mean interstellar N/H of 51+/-4 ppm. This is below the mean found by Meyer et al. of 62(+4,-3) ppm (adjusted for a difference in f-values). Our mean N/H is similar, however, to the (f-value adjusted) mean of 51+/-3 ppm found by Knauth et al. for a larger sample of sightlines with larger hydrogen column densities comparable to those in this study. We discuss the question of whether or not nitrogen shows increased gas-phase depletion in lines of sight with column densities log(H_tot) >~ 21, as claimed by Knauth et al. The nitrogen abundance in the line of sight toward HD 152236 is particularly interesting. We derive very small N/H and N/O ratios for this line of sight that may support a previous suggestion that members of the Sco OB1 association formed from an N-deficient region.Comment: Accepted in The Astrophysical Journal, 9/2006 (expected pub. date: 1/2007) 38 pages, 5 figures (4 color