Reddening law and interstellar dust properties along Magellanic sight-lines

This study establishes that SMC, LMC and Milky Way extinction curves obey the same extinction law which depends on the 2200A bump size and one parameter, and generalizes the Cardelli, Clayton and Mathis (1989) relationship. This suggests that extinction in all three galaxies is of the same nature. The role of linear reddening laws over all the visible/UV wavelength range, particularly important in the SMC but also present in the LMC and in the Milky Way, is also highlighted and discussed.Comment: accepted for publication in Astrophysics and Space Science. 16 pages, 12 figures. Some figures are colour plot

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

The Onfp Class in the Magellanic Clouds

The Onfp class of rotationally broadened, hot spectra was defined some time ago in the Galaxy, where its membership to date numbers only eight. The principal defining characteristic is a broad, centrally reversed He II $\lambda$4686 emission profile; other emission and absorption lines are also rotationally broadened. Recent surveys in the Magellanic Clouds (MCs) have brought the class membership there, including some related spectra, to 28. We present a survey of the spectral morphology and rotational velocities, as a first step toward elucidating the nature of this class. Evolved, rapidly rotating hot stars are not expected theoretically, because the stellar winds should brake the rotation. Luminosity classification of these spectra is not possible, because the principal criterion (He II $\lambda$4686) is peculiar; however, the MCs provide reliable absolute magnitudes, which show that they span the entire range from dwarfs to supergiants. The Onfp line-broadening distribution is distinct and shifted toward larger values from those of normal O dwarfs and supergiants with >99.99% confidence. All cases with multiple observations show line-profile variations, which even remove some objects from the class temporarily. Some of them are spectroscopic binaries; it is possible that the peculiar profiles may have multiple causes among different objects. The origin and future of these stars are intriguing; for instance, they could be stellar mergers and/or gamma-ray-burst progenitors.Comment: 27 pages, 8 figures, 2 tables; AJ accepte

Deep HST Imaging of Sextans A I. The Spatially Resolved Recent Star Formation History

We have measured stellar photometry from deep Cycle 7 Hubble Space Telescope/WFPC2 imaging of the dwarf irregular galaxy Sextans A. The imaging was taken in three filters: F555W ($V$; 8 orbits), F814W ($I$; 16 orbits), and F656N (H$\alpha$; 1 orbit). Combining these data with Cycle 5 WFPC2 observations provides nearly complete coverage of the optically visible portion of the galaxy. The Cycle 7 observations are nearly 2 magnitudes more sensitive than the Cycle 5 observations, which provides unambiguous separation of the faint blue helium burning stars (BHeB stars) from contaminant populations. The depth of the photometry allows us to compare recent star formation histories recovered from both the main sequence (MS) stars and the BHeB stars for the last 300 Myr. The excellent agreement between these independent star formation rate (SFR) calculations is a resounding confirmation for the legitimacy of using the BHeB stars to calculate the recent SFR. Using the BHeB stars we have calculated the global star formation history over the past 700 Myr. The history calculated from the Cycle 7 data is remarkably identical to that calculated from the Cycle 5 data, implying that both halves of the galaxy formed stars in concert. We have also calculated the spatially resolved star formation history, combining the fields from the Cycle 5 and Cycle 7 data. Our interpretation of the pattern of star formation is that it is an orderly stochastic process.Comment: 27 pages, 14 figures, 2 mpeg movies, accepted in the Astronomical Journa

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

Baryon number fluctuations in the QCD phase diagram from Dyson-Schwinger equations

We present results for fluctuations of the baryon number for QCD at nonzero temperature and chemical potential. These are extracted from solutions to a coupled set of truncated Dyson-Schwinger equations for the quark and gluon propagators of Landau-gauge QCD with Nf=2+1 quark flavors, that has been studied previously. We discuss the changes of fluctuations and ratios thereof up to fourth order for several temperatures and baryon chemical potential up to and beyond the critical endpoint. In the context of preliminary STAR data for the skewness and kurtosis ratios, the results are compatible with the scenario of a critical endpoint at large chemical potential and slightly offset from the freeze-out line. We also discuss the caveats involved in this comparison

The Relationship Between Stellar Light Distributions of Galaxies and their Formation Histories

A major problem in extragalactic astronomy is the inability to distinguish in a robust, physical, and model independent way how galaxy populations are related to each other and to their formation histories. A similar, but distinct, and also long standing question is whether the structural appearances of galaxies, as seen through their stellar light distributions, contain enough physical information to offer this classification. We argue through the use of 240 images of nearby galaxies that three model independent parameters measured on a single galaxy image reveal its major ongoing and past formation modes, and can be used as a robust classification system. These parameters quantitatively measure: the concentration (C), asymmetry (A) and clumpiness (S) of a galaxy's stellar light distribution. When combined into a three dimensional `CAS' volume all major classes of galaxies in various phases of evolution are cleanly distinguished. We argue that these three parameters correlate with important modes of galaxy evolution: star formation and major merging activity. This is argued through the strong correlation of Halpha equivalent width and broad band colors with the clumpiness parameter, the uniquely large asymmetries of 66 galaxies undergoing mergers, and the correlation of bulge to total light ratios, and stellar masses, with the concentration index. As an obvious goal is to use this system at high redshifts to trace evolution, we demonstrate that these parameters can be measured, within a reasonable and quantifiable uncertainty, with available data out to z ~ 3 using the Hubble Space Telescope GOODS ACS and Hubble Deep Field images.Comment: ApJS, in press, 30 pages, Figures 15 and 16 are in color. For a full resolution version, please go to http://www.astro.caltech.edu/~cc/cas.p