Edward L. Fitzpatrick

Composition of interstellar clouds in the disk and halo. 3: HD 149881

In this paper we continue our investigation of diffuse clouds in the interstellar medium with an analysis of the line of sight toward the star HD 215733, located in the Galactic halo some 1700 pc below the plane. As in our previous papers, we utilize the component-fitting technique to determine velocities, velocity widths, and column densities for a variety of ions in each of the absorbing regions detected. Our data include a large number of ultraviolet absorption lines observed with the Goddard High Resolution Spectrograph and ground-based observations of Ca II K absorption and H I 21 cm emission. We detect 23 components (absorbing regions) in the low-ionization species toward HD 215733 and seven components in the highly ionized species (i.e., Si3+, C3+, and N4+). The low-ion components arise in H I gas. Gas-phase abundances measured for these components follow the pattern seen for halo stars in our previous studies. These have been interpreted as indicating either that a nearly indestructible population of interstellar grains is present or that the intrinsic abundances of a number of elements in the ISM are significantly subsolar. Kinetic temperatures are estimated for 16 low-ion components. Of these, four (with the highest values of |vLSR|) are warm, with T > 1000 K; six are cold, with T < 300 K. Extensive diagnostic information includes data on the excited atoms C+ and C0, showing that in the cold clouds n(H0) 25 cm-3, with n(H0)T 2500 K cm-3. The ionization equilibrium of C0, Mg0, S0, and Ca+ gives log ne values differing systematically by up to 1 dex between these different species. Correction for these differences, together with a somewhat uncertain overall calibration with excited C+, gives values of log ne in the range from -2.1 to -2.7 for the cold clouds in the four best determined cases, and from -1.2 to -1.8 for the 3 warm clouds with measured ne. The cold cloud values yield ne/n(H0) 2 × 10-4, which suggests ionization of the heavier elements only (with all H neutral), but values higher by half a dex would also be consistent with the data. Strong absorption features of Si3+ and C3+ both appear in three components. For two of these, the ratio of b values between these two species equals the square root of the mass ratio, suggesting thermal broadening at temperatures of 6 × 105 K and 5 × 104 K. The column density ratios would require a temperature of ~8 × 104 K in collisional equilibrium.

An analysis of the shapes of ultraviolet extinction curves. II - The far-UV extinction

In this paper the properties of interstellar extinction in the far-ultraviolet region are examined utilizing IUE extinction curves for a primary data sample of 45 reddened Milky Way OB stars. These results are combined with those derived for the 2175 A bump in a previous study. It is found that IUE extinction curves can be represented by linear combinations of a Lorentzian-like 2175 A bump profile, a well-determined FUV curvature term, and an underlying linear component. The parameters of the linear component are strongly correlated and therefore only five free parameters are required to fit all of the curves in our sample. Three parameters describe the 2175 A bump, one parameter describes the linear background, and one parameter describes the strength of the FUV curvature term. The shape of the FUV curvature is found to be identical, to within the observational errors, for all the curves in the sample - which spans virtually the entire range of extinction curve morphologies observed in the Milky Way. 23 references.

The H-R diagram of the Large Magellanic Cloud and implications for stellar evolution

An H-R diagram for the Large Magellanic Cloud is produced using a sample of 1375 normal spectral class O through G supergiants. The definition of normal includes all O stars that fit into the standard two-dimensional spectral classification framework and all later type stars that do not exhibit optical emission lines. The upper luminosity limit observed for this sample agrees with previous estimates and indicates that only stars initially less massive than about 40 solar masses evolve to the red supergiant region. Between luminosities of M(bol) = -8.l5 and M(bol) = -6.0, a discontinuity is found in the density distribution of stars in the region of the blue supergiants. On the hotter side of a line, the density of stars is over five times greater than on the cooler side. This feature is identified with an evolutionary time scale change occurring during the core helium-burning phase of the evolution of intermediate mass stars. It is suggested that this feature is best reproduced by stellar evolution models which contain blue loops. 64 refs.

The Composition of the Diffuse Interstellar Medium

Recent Goddard High Resolution Spectrograph measurements of Si, S, Cr, Mn, Fe, and Zn in interstellar clouds along lines of sight in the Galactic disk and into the lower halo are discussed. The gas-phase abundance of S relative to H in the interstellar clouds appears to be indistinguishable from the solar value. For the other elements, we find well-defined upper limits in the gas-phase abundances at significantly subsolar values. For Fe, Mn, and Cr (and probably Ti), there are no convincing cases in which the relative gas-phase abundances exceed roughly -0.5 dex, i.e., these elements are not seen in interstellar gas with an abundance greater than about one-third solar. For Si, the limit is roughly -0.15 dex, and for Zn a constant abundance of -0.13 dex is found from seven clouds along one halo sight line. These subsolar maximum abundances have two possible interpretations: (1) they indicate the presence of an essentially indestructible component of interstellar dust, which contains about two-thirds of the Ti, Mn, Cr, and Fe and about one-third of the Si (based on solar composition), or (2) they indicate that the true total abundances of these elements are substantially less than in the Sun.

Composition of interstellar clouds in the disk and halo. 2: Gamma(sup 2) Velorum

High-resolution observations of gamma(sup 2) Vel with the Goddard High-Resolution Spectrograph (GHRS) echelle on the Hubble Space Telescope reveal the presence of seven narrow absorption components, with LSR velocities between -23 and +9 km s(exp -1). Three of these show column density ratios N(S(++))/N(S(+)) and N(P(++))/N(P+)) of about 1 or more, and can be identified as H II regions, while the other four are H I regions, consistent with the O I profile and with the overall H(sup 0) column density of 5.9 x 10(exp 19) cm(exp -2), given the usual assumptions that S is undepleted while O has a depletion D(O) = -0.3 dex. The depletions of Fe, Si, and Mn, which could be measure accurately for two of the four H I regions (components 6 and 7), differ somewhat from the values of D(sub ws) found for slowly moving warm clouds in HD 93521; in particular, for the component at 4.0 km s(exp -1) (No. 6), abosolute of D exceeds absolute of D(sub ws) by 0.1-0.4 dex, while for that at 9.3 km s(exp -1) (No. 7), absolute of D equals absolute of D(sub ws) on the average. The observed ratio of Fe + Mg atoms to Si atoms in the grains of component 6 is 2.04 +/-0.10, consistent with an olivine grain composition; the Fe/Mg ratio is 1.5 +/- 0.2. The electron density in component 6, determined from the C II(sup *) feature, is 0.075 +/- 0.013 cm (exp -3), about two-thirds of that found for clouds of this velocity in HD 93521. In the two conspicuous H II regions, components 3 and 4, n(sub e), determined from the Si II(sup *) feature, is about 1 cm(exp -3). From the column density of S(+) + S(++) in these two components, the total H II path length is about 40 pc. With the radius of a wind-blown bubble around gamma(sup 2) Vel set equal to 60 pc, the effective Stromgren radius is about 100 pc, requiring that T approx. equal to 50,000 K for the Wolf-Rayet component of the gamma(sup 2) Vel binary. Since zeta Pup is a comparable source of ionizing radiation, this temperature is an upper limit. The profiles of the strongest H2 absorption features, from Copernicus archives, indicate that the absorbing molecules have a mean velocity identical with that of the strongest H II component (No. 4). We have no explanation for the possible presence of these H2 molecules in a region of ionized H. Alternatively, the H2 profiles can be explained by molecules in the two adjacent (in velocity) H I regions, components 2 and 5, provided their H I gas has densities and temperatures typical of normal cold clouds. The GHRS data show absorption by highly ionized atoms Si(3+) and C(3+), N(4+) in broad features, in addition to the narrow-line absorption by Si(3+) and C(3+) observed in the dominant H II components, Nos. 3 and 4. The broad C(3+) and N(4+) features have widths corresponding to T in the range (4-8) x 10(exp 5) K, consistent with the broad O(5+) line shown in Copernicus data. Despite some observational uncertainties, the ratios of column densities in the broad C(3+), N(4+), and O(5+) features agree to +/- 0.1 dex with theoretical values for warm gas, heating and evaporating by thermal conduction from an adjacent hot region. Outward evaporation from an isolated cloud in a hot ambient gas cannot be distinguished, on the basis of these data, from inward evaporation of a warm shell, compressed by an expanding, hot stellar-wind bubble. For several halo stars, the C IV/O VI ratio has a quite different average value, perhaps consistent with cooling of infalling hot gas instead of conductive heating and evaporation.

The relationship between the Eddington limit, the observed upper luminosity limit for massive stars, and the luminous blue variables

The observed upper luminosity limits in the Galaxy and the LMC are compared with the Eddington limit as estimated for plane-parallel LTE model atmospheres which include the full effects of metal line opacities in the ultraviolet. It is shown that the Humphreys-Davidson (HD) limit corresponds to the locus of extremely low effective gravities. This result suggests that stars approaching the HD limit will suffer high mass-loss rates because of the reduction of the effective gravity due to radiation pressure. These high mass-loss rates ultimtely lead to the core mass fraction reaching its critical value and the reversal of the stellar evolution tracks. It is shown that radiation pressure, as an agent for producing enhanced mass loss near the HD limit, can in a natural way explain the kink in the HD limit near T(eff) roughly 10,000 K and the upper luminosity limit for yellow and red supergiants. The high mass-loss rates of the luminous blue variables, their location in the HR diagram, and their evolutionary stage are also discussed. 27 references.

The Abundance of Mg in the Interstellar Medium

An empirical determination of the f-values of the far-UV Mg II ??1239, 1240 lines is reported. The strong near-UV Mg II ??2796, 2803 lines are generally highly saturated along most interstellar sight lines outside the local interstellar medium (ISM) and usually yield extremely uncertain estimates of Mg+ column densities in interstellar gas. Since Mg+ is the dominant form of Mg in the neutral ISM, and since Mg is expected to be a significant constituent of interstellar dust grains, the far-UV lines are critical for assessing the role of this important element in the ISM. This study consists of complete component analyses of the absorption along the lines of sight toward HD 93521 in the Galactic halo and ? Persei and ? Ophiuchi in the Galactic disk, including all four UV Mg+ lines and numerous other transitions. The three analyses yield consistent determinations of the ??1239, 1240 f-values, with weighted means of (6.4 ? 0.4) ? 10-4 and (3.2 ? 0.2) ? 10-4, respectively. These results are a factor of ~2.4 larger than a commonly used theoretical estimate, and a factor of ~2 smaller than a recently suggested empirical revision. The effects of this result on gas- and dust-phase abundance measurements of Mg are discussed.

Ultraviolet Absorption Lines from High-Velocity Gas in the Vela Supernova Remnant: New Insights from Space Telescope Imaging Spectrograph Echelle Observations of HD 72089*

The star HD 72089 is located behind the Vela supernova remnant and shows a complex array of high- and low-velocity interstellar absorption features arising from shocked clouds. A spectrum of this star was recorded over the wavelength range 1196.4-1397.2 ? at a resolving power of ?/?? = 110,000 and a signal-to-noise ratio of 32 by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We have identified seven narrow components of C I and have measured their relative populations in excited fine-structure levels. Broader features at heliocentric velocities ranging from -70 to 130 km s-1 are seen in C II, N I, O I, Si II, S II, and Ni II. In the high-velocity components, the unusually low abundances of N I and O I, relative to S II and Si II, suggest that these elements may be preferentially ionized to higher stages by radiation from hot gas immediately behind the shock fronts.

Highly ionized atoms toward HD 93521

Results are reported from the HST High Resolution Spectrograph observations of absorption features of C IV and Si IV in the spectrum of the high-latitude O star HD 93521 (l = 183 deg; b = 62 deg). A comparison of Si IV and C IV profiles showed that the FWHM of both features is about 50 km/sec, in contrast to the 7 km/sec found for one of the several S II features. The line centers for C IV and Si IV are at v = -67 km/sec and -60 km/sec, respectively. As the interval velocity decreases from 90 to 50 km/sec, the Si IV/C IV ratio of the column density per unit velocity interval increases from about 0.2 to 0.4. The result is qualitatively consistent with a fountain model of Shapiro and Benjamin (1991) if the slower gas has cooled and recombined more than the faster gas.

Interstellar Extinction in External Galaxies

In this paper I review the existing measurements of the wavelength dependence of interstellar extinction arising from dust situated outside the Milky Way galaxy. This paper emphasizes studies of extinction in the Large and Small Magellanic Clouds. I discuss the UV extinction properties, as well as the optical and near-IR properties as determined from ground-based photometry.