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We study the IR-through-UV interstellar extinction curves towards 328 Galactic B and late-O stars. We use a new technique which employs stellar atmosphere models in lieu of unreddened "standard" stars. This technique is capable of virtually eliminating spectral mismatch errors in the curves. It also allows a quantitative assessment of the errors and enables a rigorous testing of the significance of relationships between various curve parameters, regardless of whether their uncertainties are correlated. Analysis of the curves gives the following results: (1) In accord with our previous findings, the central position of the 2175 A extinction bump is mildly variable, its width is highly variable, and the two variations are unrelated. (2) Strong correlations are found among some extinction properties within the UV region, and within the IR region. (3) With the exception of a few curves with extreme (i.e., large) values of R(V), the UV and IR portions of Galactic extinction curves are not correlated with each other. (4) The large sightline-to-sightline variation seen in our sample implies that any average Galactic extinction curve will always reflect the biases of its parent sample. (5) The use of an average curve to deredden a spectral energy distribution (SED) will result in significant errors, and a realistic error budget for the dereddened SED must include the observed variance of Galactic curves. While the observed large sightline-to-sightline variations, and the lack of correlation among the various features of the curves, make it difficult to meaningfully characterize average extinction properties, they demonstrate that extinction curves respond sensitively to local conditions. Thus, each curve contains potentially unique information about the grains along its sightline., Comment: To appear in the Astrophysical Journal, Part 1, July 1, 2007. Figures and Tables which will appear only in the electronic version of the Journal can be obtained via anonymous ftp from ftp://ftp.astronomy.villanova.edu . After logging in, change directories to "fitz/FMV_EXTINCTION". A README file describes the various files present in the directory

We present a new calibration of optical (UBV, Stromgren, uvby-beta, and Geneva) and near IR (Johnson RIJHK and 2MASS) photometry for B and early A stars derived from Kurucz (1991) Atlas model atmospheres. Our sample of stars consists of 45 normal, nearby B and early A stars which have high quality, low resolution IUE spectra and accurate Hipparcos parallaxes. The calibration is unique because it relies only on the UV spectral energy distributions, the absolute flux calibration of the V filter and the Hipparcos distances to determine the appropriate model atmospheres for the program stars. These models are then used to calibrate the synthetic photometry. We compare our results with previous, well accepted results and provide a thorough discussion of the random errors and systematic effects affecting the calibration. In particular, we demonstrate the influence of vsini on surface gravities derived from fitting model atmospheres. Finally, we discuss some of our intended applications of this new calibration., Comment: 56 pages, 11 Figures, electronic tables not included. To appear in AJ, March 2005

We have determined the fundamental properties and distance of a fourth eclipsing binary system (EB) in the Large Magellanic Cloud, HV5936 (B0.5 V + B2 III). As in our previous studies, we combine "classical" EB light curve and radial velocity curve analyses with modeling of the UV-through-optical spectral energy distribution of HV5936 to produce a detailed characterization of the system. In this paper, we also include an analysis of the high-resolution optical absorption line spectra of the binary components. We find HV5936 to be an Algol-class system, in which the masses of the primary and secondary stars have evolved via mass transfer to their current values of 11.6 Mo and 4.7 Mo, respectively. The properties of the primary star are indistinguishable from those of a "normal" star of the same current mass. The secondary is found to be overluminous for its current mass and exhibits a factor-of-2 enhancement in its surface He abundance. These results are compatible with "Case A" mass exchange occurring during the core hydrogen burning phase of the current secondary. The distance derived to the system, 43.2 +/- 1.8 kpc, implies a distance of ~44.3 kpc to the optical center of the LMC. This is several kpc closer than found in our analyses of other systems and we suggest that HV5936 lies "above" the LMC disk. This is supported by the very low interstellar H I column density and low E(B-V) found for the system - both of which are consistent with expected Milky Way foreground material - and may be associated with HV5936's location near the LMC supergiant shell LMC4., Comment: 44 pages, including 8 pages of tables and 11 pages of figures. Accepted for publication in the April 20, 2003 issue of the Astrophysical Journal

We have determined the distance to a second eclipsing binary system (EB) in the Large Magellanic Cloud, HV982 (~B1 IV-V + ~B1 IV-V). The measurement of the distance -- among other properties of the system -- is based on optical photometry and spectroscopy and space-based UV/optical spectrophotometry. The analysis combines the ``classical'' EB study of light and radial velocity curves, which yields the stellar masses and radii, with a new analysis of the observed energy distribution, which yields the effective temperature, metallicity, and reddening of the system plus the distance ``attenuation factor'', essentially (radius/distance)^2. Combining the results gives the distance to HV982, which is 50.2 +/- 1.2 kpc. This distance determination consists of a detailed study of well-understood objects (B stars) in a well-understood evolutionary phase (core H burning), and is free of the biases and uncertainties that plague various other techniques. After correcting for the location of HV982, we find an implied distance to the optical center of the LMC's bar of d(LMC) = 50.7 +/- 1.2 kpc. This result differs by nearly 5 kpc from our earlier result for the EB HV2274, which implies a bar distance of 45.9 kpc. These results may reflect either marginally compatible measures of a unique LMC distance or, alternatively, suggest a significant depth to the stellar distribution in the LMC. Some evidence for this latter hypothesis is discussed., Comment: To appear in the Astrophysical Journal (January 2002) 10 pages of text, plus 5 tables and 9 figures (ApJ-emulation style). This version incorporates new observations which double the number of ground-based radial velocity measurements and extend the spectrophotometric coverage to 1150--7500 A. The extended wavelength coverage eliminates a systematic error in the determination of E(B-V), which originally resulted from the absense of data longward of 4800 A

The distance to the Large Magellanic Cloud (LMC) is crucial for the calibration of the Cosmic Distance Scale. We derive a distance to the LMC based on an analysis of ground-based photometry and HST-based spectroscopy and spectrophotometry of the LMC eclipsing binary system HV2274. Analysis of the optical light curve and HST/GHRS radial velocity curve provides the masses and radii of the binary components. Analysis of the HST/FOS UV/optical spectrophotometry provides the temperatures of the component stars and the interstellar extinction of the system. When combined, these data yield a distance to the binary system. After correcting for the location of HV2274 with respect to the center of the LMC, we find d(LMC) = 45.7 +/- 1.6 kpc or DM(LMC) = 18.30 +/- 0.07 mag. This result, which is immune to the metallicity-induced zero point uncertainties that have plagued other techniques, lends strong support to the ``short'' LMC distance scale as derived from a number of independent methods., Comment: 6 pages, including 2 pages of figures. Newly available optical (B and V) photometry has revealed -- and allowed the elimination of -- a systematic error in the previously reported determination of E(B-V) for HV2274. The new result is E(B-V) = 0.12 mag (as compared to the value of 0.083 reported in the original submission) and produces a DECREASE in the distance modulus of HV2274 by 0.12 mag. ApJ Letters, in press

The star HD72089 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 to 1397.2 Angstroms at a resolving power lambda/Delta lambda = 110,000 and signal-to-noise ratio of 32 by STIS on the Hubble Space Telescope. We have identified 7 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 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., Comment: 11 pages, 2 figures, Latex. Submitted for the special HST ERO issue of the Astrophysical Journal Letters

Near-IR photometry for the B-type main-sequence star HD 62542 is presented and employed to investigate the interstellar extinction curve in the wavelength range 0.44-3.5 microns. The ratio of total to selective extinction is deduced to be R(V) = 3.24 +/- 0.05, very similar to the average value for diffuse clouds. From the resulting visual extinction (A(V) = 1.20 +/- 0.08) and an assessment of the literature on the spectral type and luminosity of the star, a distance of 405 +/- 35 pc is derived. The normality of the extinction law in the visible and near-IR is in marked contrast to the anomalous UV extinction. These results support the hypothesis that the complex formed as an accumulation of diffuse cloud material swept up by stellar winds and radiation.

IUE data are used to investigate the properties of the 2175 A interstellar extinction bump toward 45 reddened Milky Way stars. Using analytic fitting functions, parameters are derived which describe the central position, FWHM, and strength of the bump. The bump position is very stable, with an extreme deviation which is still significantly larger than the measurement uncertainty, indicating true variability. The bump FWHM has a large range of intrinsic values, 0.77/micron to 1.25/micron. The normalized bump strength varies by more than a factor of three. There are no convincing correlations among the three bump parameters, but the width of the bump is strongly correlated with the dust grain environments. Dense quiescent regions yield broad bumps, while diffuse interstellar medium and regions of recent star formation yield narrower bumps. The absence of any correlation between bump width and central position argues strongly against graphite grains in the size range prescribed by the Mathis, Rumpl, and Nordsieck (1977) model as carriers of the bump.

The properties of ultraviolet extinction in the LMC are reexamined using some of the existing LMC data from the International Ultraviolet Explorer (IUE) satellite and a large number of new observations obtained during the IUE Sixth Episode. The main goal is to determine if the LMC extinction properties are regionally modulated and, if so, to examine the type and range of the variations. Among the new IUE observations presented is a large grid of unreddened LMC flux standards, which will eliminate spectral mismatch effects as complications in the extinction analysis. Eight new reddened stars were observed. The properties of the program stars are summarized; the method for producing the extinction curves (including the criteria for pairing reddened and unreddened stars) is discussed; and the curves themselves are presented and discussed.

The UV extinction curves of 33 main-sequence B stars in five open clusters are presented and analyzed. An analytic fitting function is introduced which represents the observed curves and codifies the structure of the 2175 A extinction bump. A set of extinction parameters are also defined which describe the shape of the far-UV extinction. These data are first used to determine whether the extinction toward any of the clusters is uniform, i.e., whether it can be represented by a single curve for the cluster. This is accomplished by comparing the observed scatter of the far-UV extinction measurements toward a given cluster with that predicted by an error model. The extinction toward four of the five clusters is found to be uniform. The extinction data for those clusters with uniform extinction are then used to determine the observational uncertainties in other extinction parameters for which a priori error estimates cannot be made. With one exception, all of the parameters which describe the shapes of UV extinction curves differ significantly from cluster to cluster. The exception is the central position of the UV extinction bump, which has the same value for all of the cluster extinction curves. Applications and interpretation of the results are given elsewhere.

Weak absorption lines of C I, O I, Mg I, Mg II, Si I, Si II, P I, Cl I, Cr II, Mn II, Fe I, Ni II, Zn II, CO and C2 are detected in neutral gas in front of the 30 Doradus H II region by IUE spectra of R 136. The Large Magellanic Cloud abundances from the absorption lines are a factor of 2 or 3 below those of the Milky Way, in agreement with emission line study results. Neutral gas density and temperature are estimated from the observed excitation and ionization to be about 300/cu cm and 100 K, respectively; this implies a gas pressure of 30,000/cu cm K.

The structural and kinematic properties of the Small Magellanic Cloud (SMC) were studied by combining IUE satellite observations of individual bright stars (Sk 159 and HD 5980) with existing optical and radio data. It is shown that the SMC has a complex structure due to gravitational interaction with the Milky Way and the Large Magellanic Cloud (LMC). Stars observed in the southwest regions of the SMC were associated with the 134 km/s H I complex. H II regions in the SMC were distributed according to a rotational velocity of 167 km/s. All IUE high-dispersion targets exhibit ultraviolet absorption near 100 km/s and 200 km/s. It is shown that the absorption arises in the widespread H I complexes at velocities similar to the H II regions. Some possible explanations for the origin of H II and H I regions in the SMC are discussed. The locations of the nine high-dispersion IUE targets in the SMC are given.

High-dispersion International Ultraviolet Explorer satellite spectra of seven stars in the Small Magellanic Cloud (SMC) are examined to study the properties of interstellar C IV and Si IV absorption in the SMC. Absorption by C IV or Si IV or both is found near 160 km/s for all the stars. The velocity and the relative C IV and Si IV strengths suggest UV-photoionized nebular gas as the origin of this absorption. In addition, the stars show absorption by C IV and, sometimes, Si IV in the velocity range 100-130 km/s. This velocity is 30-60 km/s more negative than that expected for normal nebular gas, and the relative C IV and Si IV strengths indicate an ionization source other than stellar UV photoionization by normal Population I stars. Possible global origins are considered for this absorption, including a hot phase of the SMC interstellar medium and a circum-SMC distribution of highly ionized gas. The only detection of interstellar N V toward a SMC star is for HD 5980. The line is broad, possibly complex, and spans the velocity range of the nebular absorption and the 100-130 km/s absorption.

On the basis of a study of high-dispersion spectra of stars in the Large and Small Magellanic Clouds (LMC and SMC) obtained with the International Ultraviolet Explorer (IUE) satellite, Savage and de Boer (1979, 1981) have detected the Milky Way halo gas. The existence of gaseous coronae around both the LMC and SMC was also suggested. The evidence for this included the presence of strong interstellar absorption lines from high-ionization stages (e.g., C IV, Si IV, and Al III). Fitzpatrick and Savage (1983) concluded that the strongest C IV and Si IV features in the spectrum of the SMC star HD 5980 are probably nebular in origin, produced by stellar photoionization in NGC 346. However, other evidence indicated that at least one additional region of high-ionization stage gas is present in the HD 5980 line of sight. The present investigation is concerned with a reobservation of the star SK 159. It is found that the Milky Way disk and halo absorption features toward Sk 159, except for Al III are very similar to those seen toward HD 5980, also in the SMC.

A study of ultraviolet interstellar extinction in and near the core of the 30 Doradus nebula is presented. The pair method is used to determine the shape of the ultraviolet extinction curve, and reddened stars from within 80 pc of the core and unreddened stars from a variety of locations in the Large Magellanic Cloud (LMC) are considered. All stars near the core appear to be reddened by E(B-V) = 0.09-0.16 with an extinction law similar to previous LMC extinction laws. Some stars, including R136a, R145, and R147, are additionally reddened by E(B-V) = 0.18 with a 'nebular-type' extinction law. A model consisting of a layer of 'LMC foreground dust' which affects all of the stars and a deeper layer of 'nebular dust' which affects some of the stars is used for an explanation of the extinction properties. The extinction curves are then applied to the ultraviolet energy distribution of R136a for a determination of its intrinsic continuum shape. Evidence showing that the brightest LMC OB supergiants have intrinsic (B-V) colors and UV-to-visual continuum slopes which are redder than expected is presented.

UV data show R136a to be an extremely luminous object having spectral characteristics similar to that of O3 stars. It is suggested that the bright component of R136a is either a single, supermassive hydrogen-burning star, or a compact multiple system of supermassive stars. In the former case, the observational data imply a mass exceeding that of the most massive stars known by a factor of 20 and raise important theoretical problems concerning stellar formation and stability. Formation may occur through ordinary collapse in a region with peculiar dust properties, or through coalescence in the core of a compact cluster.

Individual high-dispersion International Ultraviolet Explorer (IUE) spectra of extragalactic stars are noisy because of the long exposure times required and the inherent limitations of the IUE detectors. A program has, therefore, been undertaken to obtain multiple ultraviolet spectra of the brightest Magellanic Cloud stars in order to produce composite spectra with higher signal-to-noise ratios than in individual spectra. The present investigation is concerned with results for HD 5980. The target star, HD 5980, is one of the brightest extragalactic far-ultraviolet sources in the sky. HD 5980 is located in the northeast quadrant of NGC 346, the largest H II region and OB star cluster in the SMC. The analysis of the interstellar line spectrum of HD 5980 provides a number of conclusions about the complex absorption characteristics of the interstellar gas toward the SMC.

In connection with investigations of the physical characteristics of interstellar dust, a study of objects with peculiar extinction might provide new insights about the composition, size distribution, formation, and destruction of the particulate matter. Meyer and Savage (1981) have found that the dust toward many stars exhibits peculiar UV extinction. The present investigation is concerned with International Ultraviolet Explorer (IUE) spectrophotometric measurements for a sample of stars judged by Meyer and Savage to have highly anomalous UV extinction as inferred from the broad-band Astronomical Netherlands Satellite (ANS) data. The reported IUE measurements provide the opportunity to determine if the ANS photometry measurements of Meyer and Savage are anomalous because of peculiar extinction.

Narrow, unshifted UV absorption lines, principally of Fe v, have been detected in the spectra of two Wolf-Rayet stars, HD 93162 (WN7) and HD 193793 (WC7). Both stars are known to show upper Balmer-line absorption but exhibit no periodic Doppler shifts that might indicate the presence of a companion. If intrinsic to the Wolf-Rayet stars, these lines presumably form near the base of the envelope in the photosphere. The potential significance of absorption-line studies of Wolf-Rayet stars is discussed.