Jayant Murthy

A COMPILATION OF INTERSTELLAR COLUMN DENSITIES

We have collated absorption line data toward 3008 stars in order to create a unified database of interstellar column densities. These data have been taken from a number of different published sources and include many different species and ionizations. The preliminary results from our analysis show a tight relation [N(H)/E(B - V) = 6.12 Multiplication-Sign 10{sup 21}] between N(H) and E(B - V). Similar plots have been obtained with many different species, and their correlations along with the correlation coefficients are presented.

Probing the Role of Carbon in Ultraviolet Extinction along Galactic Sight Lines

We report previously undetermined interstellar gas and dust-phase carbon abundances along 15 Galactic sight lines based on archival data of the strong 1334.5323 A transition observed with the Space Telescope Imaging Spectrograph. These are combined with previously reported carbon measurements along six sight lines to produce a complete sample of interstellar C II measurements determined with the 1334 A transition. Our data set includes a variety of Galactic disk environments characterized by different extinctions and samples paths ranging over three orders of magnitude in average density of hydrogen (n(H)). Our data support the idea that dust, specifically carbon-based grains, are processed in the neutral interstellar medium. We, however, do not find that the abundance of carbon in dust or the grain-size distribution is related to the strength of the 2175 A bump. This is surprising, given that many current models have polycyclic aromatic hydrocarbons as the bump-producing dust.

Dust Properties in the Far-Ultraviolet in Ophiuchus

We have derived the albedo (a) and phase function asymmetry factor (g) of interstellar dust grains at 1100 A using archival Voyager observations of diffuse radiation in Ophiuchus. We have found that the grains are highly forward-scattering, with g = 0.55 ± 0.25 and a = 0.40 ± 0.10. Even though most of the gas in this direction is in the Ophiuchus molecular cloud, the diffuse FUV radiation is almost entirely due to scattering in a relatively thin foreground cloud. This suggests that one cannot assume that the UV background is directly correlated with the total amount of gas in any direction.

Mapping the Diffuse Ultraviolet Sky with the Galaxy Evolution Explorer

We present a map of the diffuse ultraviolet cosmic background in two wavelength bands (FUV: 1530 {\AA}; NUV: 2310 {\AA}) over almost 75% of the sky using archival data from the GALEX mission. Most of the diffuse flux is due to dust-scattered starlight and follows a cosecant law with slopes of 545 photons cm-2 s-1 sr-1 {\AA}-1 and 433 photons cm-2 s-1 sr-1 {\AA}-1 in the FUV and NUV bands, respectively. There is a strong correlation with the 100 {\mu}m IRAS flux with an average UV/IR ratio of 300 photons cm-2 s-1 sr-1 {\AA}-1 (MJy sr-1)-1 in the FUV band and 220 photons cm-2 s-1 sr-1 {\AA}-1 (MJy sr-1)-1 in the NUV but with significant variations over the sky. In addition to the large scale distribution of the diffuse light, we note a number of individual features including bright spots around the hot stars Spica and Achernar.

Voyager 2 observations of NGC 7023 - Dust scattering shortward of 1600 A

We report the first observational determination of the nebular-to-stellar flux ratio out to wavelengths as short as 1000 A for the reflection nebula NGC 7023, using a combination of spectroscopic data from Voyager 2 and from the Hopkins Ultraviolet Telescope (HUT) on the Astro 1 mission. After accounting for contributions by H2 fluorescence to the nebular flux, the wavelength variation of the ratio of the residual dust scattered flux to that observable from the embedded star is consistent with a reduction of the dust albedo by 25 percent over the wavelength interval 1300-1000 A. This is most easily interpreted, if the particles responsible for the FUV rise in the extinction curve are low-albedo, not necessarily zero-albedo, grains. The direct proportionality between the nebular surface brightness, measured at 22 arcsec offset by HUT, and the nebular flux suggests that the phase function of scattering does not vary in the 2000-1000 A range.

IUE observations of hydrogen and deuterium in the local interstellar medium

High-resolution Ly-alpha spectra of the late-type stars Epsilon Eri, Procyon, Altair, Capella, and HR 1099 taken with the short-wavelength camera on IUE are presented. The density, velocity dispersion, and bulk velocity of the interstellar H I toward each of the stars is derived from the spectra. Lower limits on the deuterium-to-hydrogen ratio toward these stars are obtained. 40 references.

OBSERVATIONS OF THE DIFFUSE FAR-ULTRAVIOLET BACKGROUND WITH THE FAR ULTRAVIOLET SPECTROSCOPIC EXPLORER

We have used observations taken under the Far Ultraviolet Spectroscopic Explorer (FUSE) S405/505 channel realignment program to explore the diffuse far-ultraviolet (FUV; 1000-1200 A) radiation field. Of the 71 independent locations in that program, we have observed a diffuse signal in 32, ranging in brightness from 1600 to a maximum of 2.9 × 105 photons cm-2 sr-1 s-1 A-1 in Orion. The FUSE data confirm that the diffuse FUV sky is patchy with regions of intense emission, usually near bright stars, but also with dark regions, even at low Galactic latitudes. We find a weak correlation between the FUV flux and the 100 μm ratio but with wide variations, perhaps due to differences in the local radiation field.We have used observations taken under the FUSE S405/S505 channel realignment program to explore the diffuse FUV (1000 1200 Å) radiation field. Of the 71 independent locations in that program, we have observed a diffuse signal in 32, ranging in brightness from 1600 photons cm sr s Å to a maximum of 2.9× 10 photons cm sr s Å in Orion. The FUSE data confirm that the diffuse FUV sky is patchy with regions of intense emission, usually near bright stars, but also with dark regions, even at low Galactic latitudes. We find a weak correlation between the FUV flux and the 100 μm ratio but with wide variations, perhaps due to differences in the local radiation field. Subject headings: ultraviolet: ISM, ISM: dust Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985.

GALEX OBSERVATIONS OF DIFFUSE UV RADIATION AT HIGH SPATIAL RESOLUTION FROM THE SANDAGE NEBULOSITY

We have observed a region of nebulosity first identified as starlight scattered by interstellar dust by Sandage using the Galaxy Evolution Explorer (GALEX) ultraviolet imaging telescope. Apart from airglow and zodiacal emission, we have found a diffuse UV background of between 500 and 800 photons cm–2 sr–1 s–1 A–1 in both the GALEX far-ultraviolet (FUV) (1350-1750 A) and NUV (1750-2850 A) bands. Of this emission, up to 250 photons cm–2 sr–1 s–1 A–1 is due to H2 fluorescent emission in the FUV band. The remainder is consistent with scattering from interstellar dust with forward scattering grains of albedo about 0.4. These are the highest spatial resolution observations of the diffuse UV background to date and show an intrinsic scatter beyond that expected from instrumental noise alone. Further modeling is required to understand the nature of this scatter and its implications for the ISM.

Constraints on the optical properties of interstellar dust in the far-ultraviolet - Voyager observations of the diffuse sky background

Four regions of the sky have been observed at various Galactic latitudes and H I column densities, with the ultraviolet spectrometer aboard Voyager 2 finding no evidence for the presence of a diffuse radiation field between 912 and 1100 A at a level of 100-200 photons per sq cm s sr A. As starlight scattered by dust grains may be expected to be a major contributor to the diffuse background, especially at low Galactic latitudes, it is possible to place a very stringent upper limit of less than 0.1 on the reflectivity of the dust in this spectral range. This albedo is much lower than the canonical value of about 0.5 for standard graphite-silicate dust models.

Measurement of dust optical properties in the coalsack nebula

We have used FUSE and Voyager observations of dust scattered starlight in the neighborhood of the Coalsack Nebula to derive the optical constants of the dust grains. The albedo is consistent with a value of 0.28 ± 0.04 and the phase function asymmetry factor with a value of 0.61 ± 0.07 throughout the spectral range from 900 – 1200 u in agreement with previous determinations as well as theoretical predictions. We have now observed two regions (Ophiuchus and Coalsack) with intense diffuse background radiation and in both cases have found that the emission is due to light from nearby hot stars scattered by a relatively thin foreground cloud, with negligible contribution from the background molecular cloud.