A UBVR CCD Survey of the Magellanic Clouds

We present photometry and a preliminary interpretation of a UBVR survey of the Large and Small Magellanic Clouds. We determine improved values for the relative number of blue and red supergiants. We also compare the relative number of Red Supergiants (RSGs) and Wolf-Rayet stars, demonstrating a strong, tight correlation with metallicity, and reinvestigate the initial mass function slope of massive stars found in the field.Comment: complete postscript (including embedded figures) can be found at: ftp://ftp.lowell.edu/pub/massey/mcatlas.ps.gz Accepted for publication in the ApJ

An Atlas of FUSE Sight Lines Toward the Magellanic Clouds

We present an atlas of 57 Large Magellanic Cloud (LMC) and 37 Small Magellanic Cloud (SMC) observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The atlas highlights twelve interstellar absorption line transitions at a resolution of ~15 km/s. These transitions cover a broad range of temperatures, ionization states, and abundances. The species included are OVI, which probes hot (T~3x10^5 K) ionized gas; CIII and FeIII, which probe warm (T~10^4 K) ionized gas; SiII, PII, CII, FeII, and OI, warm neutral gas; and six different molecular hydrogen transitions, which trace cold (T<=500 K) gas. We include Schmidt Halpha CCD images of the region surrounding each sight line showing the morphology of warm ionized gas in the vicinity, along with continuum images near each FUSE aperture position. Finally, we present several initial scientific results derived from this dataset on the interstellar medium of the Magellanic Clouds and Galactic halo.Comment: 29 pages, 6 figures. Complete Atlas of 94 additional images (~800kB each) is available at http://fuse.pha.jhu.edu/~danforth/atlas Accepted to the ApJS March 200

Constraints on the Ionization Balance of Hot-Star Winds from FUSE Observations of O Stars in the Large Magellanic Cloud

We use a Sobolev with Exact Integration model to analyze the winds lines of 25 LMC O stars. The data include FUSE profiles of C III, N III, S IV, P V, S VI, and O VI and IUE or HST data for Si IV, C IV, and N V. Several of the FUSE lines are unsaturated, so meaningful optical depths (equivalently, mass loss rate times ionization fractions), as a function of wind velocity can be determined. Ratios of these quantities give the relative ionization as a function of velocity and demonstrate that, except for O VI in all stars and S VI in the later stars, the wind ionization shifts toward lower stages at higher velocity. Because O VI and S VI do not behave like the other ions, they must be produced by a different mechanism. Using mass-loss rates determined from the Vink et al. relationships, we derive mean ionization fractions. Because these are all less than one, the derived mass loss rates cannot be too small. However, the ion fractions for P V (expected to be dominant in some winds), never exceed 0.20. This implies that either the calculated mass loss rates or the assumed P abundances are too large, or the winds are strongly clumped. We examine correlations between the mean ion fractions and stellar parameters, and find two significant relationships. First, as expected, the mean ionization fraction of lower ions decreases with increasing temperature. Second, the mean ionization fraction of S VI in the latest stars and O VI in all stars increases with terminal velocity, re-affirming Cassinelli and Olson's conjecture that O VI is produced non-radiatively. Finally, we discuss peculiar aspects of three stars, BI 272, BI 208, and Sk-67 166.Comment: Accepted for publication in ApJ, 67 pages 8 figures -- update of Eq 1 and Table

ROSAT PSPC view of the hot interstellar medium of the Magellanic Clouds

Diffuse X-ray emission from the Magellanic Clouds (MCs) is studied by using all the archival data of pointed ROSAT Position Sensitive Proportional Counter (PSPC) observations. For this purpose, contributions from the point and point-like sources in the ROSAT High Resolution Imager (HRI) and PSPC source catalogues are eliminated and periods of high solar activity are excluded. The spectral analysis yielded characteristic temperatures of 10^6 - 10^7 K for the hot thin plasma of the ISM which extends over the whole Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The total unabsorbed luminosity in the 0.1 - 2.4 keV band within the observed area amounts to 3.2 x 10^38 erg s^-1 in the LMC and 1.1 x 10^37 erg s^-1 in the SMC, each with an uncertainty of -40%, +100%. The X-ray luminosity of the LMC is comparable to that of other nearby galaxies with pronounced star formation. In the LMC, hot regions were found especially around the supergiant shell (SGS) LMC 4 and in the field covering SGS LMC 2 and LMC 3. Highest temperatures for the SMC were derived in the southwestern part of the galaxy. The diffuse X-ray emission is most likely a superposition of the emission from the hot gas in the interior of shells and supershells as well as from the halo of these galaxies.Comment: accepted by A&

The effects of work experience during higher education on labour market entry: learning by doing or an entry ticket?

Graduates from higher education often enter the labour market with a considerable amount of work experience. Using German data, we address the question of whether early work experience pays off upon labour market entry. We compare the labour market benefits of different types of work experience. This comparison allows us to more generally test hypotheses about different explanations of why education pays off. Results indicate that tertiary graduates do not profit from work experience that is unrelated to the field of study or was a mandatory part of the study programme. Even though field-related and voluntary work experience helps graduates to realize a fast integration into the labour market, it is not linked to higher chances for entering a favourable class position or to higher wages in the long run. These results provide evidence for the signalling explanation of educational benefits in the labour market rather than the human capital explanation

The Galactic WN stars: Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation

CONTEXT: Very massive stars pass through the Wolf-Rayet (WR) stage before they finally explode. Details of their evolution have not yet been safely established, and their physics are not well understood. Their spectral analysis requires adequate model atmospheres, which have been developed step by step during the past decades and account in their recent version for line blanketing by the millions of lines from iron and iron-group elements. However, only very few WN stars have been re-analyzed by means of line-blanketed models yet. AIMS: The quantitative spectral analysis of a large sample of Galactic WN stars with the most advanced generation of model atmospheres should provide an empirical basis for various studies about the origin, evolution, and physics of the Wolf-Rayet stars and their powerful winds. METHODS: We analyze a large sample of Galactic WN stars by means of the Potsdam Wolf-Rayet (PoWR) model atmospheres, which account for iron line blanketing and clumping. The results are compared with a synthetic population, generated from the Geneva tracks for massive star evolution. RESULTS: We obtain a homogeneous set of stellar and atmospheric parameters for the Galactic WN stars, partly revising earlier results. CONCLUSIONS: Comparing the results of our spectral analyses of the Galactic WN stars with the predictions of the Geneva evolutionary calculations, we conclude that there is rough qualitative agreement. However, the quantitative discrepancies are still severe, and there is no preference for the tracks that account for the effects of rotation. It seems that the evolution of massive stars is still not satisfactorily understood.Comment: 19 pages, 11 figures, A&A, in press, additional Online-material on http://www.astro.physik.uni-potsdam.de/abstracts/galwn.htm

Fundamental Properties and Distances of LMC Eclipsing Binaries: III. EROS 1044

We present results from a detailed analysis of a third eclipsing binary (EB) system in the Large Magellanic Cloud, EROS 1044 (~B2 IV-V + ~B2 III-IV). Our study combines the "classical" EB study of light and radial velocity curves with detailed modeling of the observed spectral energy distribution, and yields an essentially complete picture of the stellar properties of the system and a determination of its distance. The observational data exploited include optical photometry, space-based UV spectroscopy, and UV/optical spectrophotometry. The advantages of our technique include numerous consistency checks and, in the case of the distance determinations, the absence of zero point uncertainties and adjustable parameters. We find the EROS 1044 system to consist of a pair of normal, mildly-evolved ~21000 K stars, whose derived properties are consistent with stellar evolution calculations. The distance to the system is 47.5+/-1.8 kpc. We discuss the implications of our results for three EB systems (HV 2274, HV 982, and EROS 1044) on the general distance to the Large Magellanic Cloud.Comment: 18 pages, 8 figures, accepted for publication in Ap

The Metallicity Distribution Function of Red Giants in the LMC

We report new metallicity determinations for 39 red giants in a 220 square arcminute region, 1.8 degrees southwest of the bar of the Large Magellanic Cloud. These abundance measurements are based on spectroscopy of the Ca II infrared triplet. The metallicity distribution function (MDF) shows a strong peak at [Fe/H] = -0.57 +/- 0.04. Half the red giants in our field fall within the range -0.83 < [Fe/H] < -0.41. We find a striking contrast in the shape of the MDF below [Fe/H] < -1 between our inner disk field and the distant outer field studied by Olszewski (1993). Our field-star MDF seems similar to that of the intermediate-age (1-3 Gyr) star clusters. We have also obtained abundance estimates using Stromgren photometry for ~1000 red giants in the same field. The Stromgren measurements, which are sensitive to a combination of cyanogen and iron lines, correlate well with the Ca II measurements, but a metallicity-dependent offset is found (abridged).Comment: 55 pages, 16 figures, AASTeX v5.0, extended data table not included, to appear in the October, 2000 issue of The Astronomical Journa