59 research outputs found
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
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 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 (; 8 orbits), F814W (; 16 orbits), and
F656N (H; 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
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