885 research outputs found
Draco -- A Failure of the Tidal Model
We test whether the structural properties of the nearby dwarf spheroidal
(dSph) galaxy Draco, a well-studied Milky Way companion, can be reconciled with
the suggestion that dSphs are unbound tidal remnants with a large depth extent
along the line of sight. In order to apply the observational test of this
hypothesis suggested by Klessen & Zhao, we use public photometric data from the
Sloan Digital Sky Survey (SDSS) to explore the width of Draco's blue horizontal
branch over a range of areas covering 0.06 square degrees to 6.25 square
degrees centered on Draco. The SDSS database is the only currently existing
database with sufficient depth and area coverage to permit a stringent test of
the tidal models. We show that blue horizontal branch stars extend beyond the
previously inferred limiting radii of Draco, consistent with the observed
absence of a truncated stellar surface density profile of this dSph. We
calculate new models for a galaxy without dark matter, using Draco's
morphological properties as constraints. The resulting models are unable to
reproduce the narrow observed horizontal branch width of Draco, which stays
roughly constant regardless of the field of view. We conclude that Draco cannot
be the remnant of a tidally disrupted satellite, but is probably strongly
dark-matter dominated. (ABSTRACT ABBREVIATED)Comment: 26 pages, 9 figures included, accepted for publication in ApJ,
high-resolution version available at
http://www.aip.de./~ralf/Publications/p22.abstract.htm
SP_Ace: a new code to derive stellar parameters and elemental abundances
Aims: We developed a new method of estimating the stellar parameters Teff,
log g, [M/H], and elemental abundances. This method was implemented in a new
code, SP_Ace (Stellar Parameters And Chemical abundances Estimator). This is a
highly automated code suitable for analyzing the spectra of large spectroscopic
surveys with low or medium spectral resolution (R=2,000-20,000). Methods: After
the astrophysical calibration of the oscillator strengths of 4643 absorption
lines covering the wavelength ranges 5212-6860\AA\ and 8400-8924\AA, we
constructed a library that contains the equivalent widths (EW) of these lines
for a grid of stellar parameters. The EWs of each line are fit by a polynomial
function that describes the EW of the line as a function of the stellar
parameters. The coefficients of these polynomial functions are stored in a
library called the " library". SP_Ace, a code written in FORTRAN95, uses
the GCOG library to compute the EWs of the lines, constructs models of spectra
as a function of the stellar parameters and abundances, and searches for the
model that minimizes the deviation when compared to the observed
spectrum. The code has been tested on synthetic and real spectra for a wide
range of signal-to-noise and spectral resolutions. Results: SP_Ace derives
stellar parameters such as Teff, log g, [M/H], and chemical abundances of up to
ten elements for low to medium resolution spectra of FGK-type stars with
precision comparable to the one usually obtained with spectra of higher
resolution. Systematic errors in stellar parameters and chemical abundances are
presented and identified with tests on synthetic and real spectra. Stochastic
errors are automatically estimated by the code for all the parameters. A simple
Web front end of SP_Ace can be found at http://dc.g-vo.org/SP_ACE, while the
source code will be published soon.Comment: 36 pages, 34 figures, A&A accepte
R-process and alpha-elements in the Galactic disk: Kinematic correlations
Recent studies of elemental abundances in the Galactic halo and in the
Galactic disk have underscored the possibility to kinematically separate
different Galactic subcomponents. Correlations between the galactocentric
rotation velocity and various element ratios were found, providing an important
means to link different tracers of star formation and metal enrichment to the
Galactic components of different origin (collapse vs. accretion). In the
present work we determine stellar kinematics for a sample of 124 disk stars,
which we derive from their orbits based on radial velocities and proper motions
from the the literature. Our stars form a subsample of the Edvardsson et al.
(1993) sample and we concentrate on three main tracers: (i) Europium as an
r-process element is predominantly produced in Supernovae of type II. (ii)
Likewise, alpha-elements, such as Ca, Si, Mg, are synthesised in SNe II,
contrary to iron, which is being produced preferentially in SNe Ia. (iii) The
s-process element Barium is a measure of the relative contribution of AGB stars
to the Galaxy's enrichment history and has been shown to be an indicator for
distinguishing between thin and thick disk stars. All such studies reveal,
basically, that stars with low galactocentric rotational velocity tend to have
high abundances of alpha-elements and Eu, but lower abundances of, e.g., Ba.Comment: 5 pages, 2 figures, Poster contribution to appear in "Planets To
Cosmology: Essential Science In Hubble's Final Years", proceedings of the May
2004 STScI Symposium, M. Livio (ed.), (Cambridge University Press
Hubble Space Telescope Photometry of Hodge 301: An "Old" Star Cluster in 30 Doradus
We present Hubble Space Telescope Planetary Camera UVI data for the
little-studied cluster Hodge 301 3' northwest of 30 Doradus' central ionizing
cluster R136. The average reddening of Hodge 301 is found to be =
(0.28+-0.05) mag from published infrared and ultraviolet photometry. Using two
different sets of evolutionary models, we derive an age of ~ 20-25 Myr for
Hodge 301, which makes it roughly 10 times as old as R136. Hodge 301 is the
most prominent representative of the oldest population in the 30 Dor starburst
region; a region that has undergone multiple star formation events. This range
of ages is an important consideration for the modelling of starburst regions.
Hodge 301 shows a widened upper main sequence largely caused by Be stars. We
present a list of Be star candidates. The slope of the initial mass function
for intermediate-mass main sequence stars ranging from 10 to 1.3 solar masses
is found to be -1.4+-0.1 in good agreement with a Salpeter law. There is no
indication for a truncation or change of slope of the IMF within this mass
range. In accordance with the age of Hodge 301 no obvious pre-main-sequence
stars are seen down to 1 solar mass. We estimate that up to 41+-7 stars with
more than 12 solar masses may have turned into supernovae since the formation
of the cluster. Multiple supernova explosions are the most likely origin of the
extremely violent gas motions and the diffuse X-ray emission observed in the
cluster surroundings.Comment: To appear in the Astronomical Journal (Feb 2000 issue). 16 pages in
two-column style. 9 separate figures, in part in significantly reduced
resolution for space reasons (bitmapped postscript or jpg
New calibrations for abundance determinations in HII regions
Simple relations for deriving the oxygen abundance in HII regions with
intensities of the three strong emission lines R_2, R_3, and N_2 (R
calibration) or S_2, R_3, and N_2 (S calibration) in their spectra are
suggested. A sample of 313 reference HII regions of the counterpart method is
used as calibrating data points. Relations for the determination of nitrogen
abundances, the R calibration, are also constructed. We find that the oxygen
and nitrogen abundances in high-metallicity HII regions can be estimated using
the intensities of the two strong lines R_2 and N_2 (or S_2 and N_2 for oxygen)
only. The corresponding two-dimensional relations are provided. There are
considerable advantages of the suggested calibration relations as compared to
the existing ones. First, the oxygen and nitrogen abundances estimated through
the suggested calibrations agree with the Te-based abundances within ~0.1 dex
over the whole metallicity range, i.e., the relative accuracy of the
calibration-based abundances is 0.1 dex. Although we constructed distinct
relations for high- and low-metallicity objects, the separation between these
two can be simply obtained from the intensity of the N_2 line. Moreover, the
applicability ranges of the high- and low-metallicity relations overlap for
adjacent metallicities, i.e., the transition zone disappears. Second, the
oxygen abundances produced by the two suggested calibrations are in remarkable
agreement with each other. In fact, the R-based and S-based oxygen abundances
agree within ~0.05 dex in the majority of cases for more than three thousand
HII region spectra.Comment: 16 pages, 9 figues, accepted to MNRA
Impact of Reionization on the Stellar Populations of Nearby Dwarf Galaxies
Cold dark matter models for galaxy formation predict that low-mass systems
will be the first sites of star formation. As these objects have shallow
gravitational potential wells, the subsequent growth of their stellar
populations may be halted by heating and gas loss due to reionization. This
effect has been suggested to have profoundly influenced properties of
present-day dwarf galaxies, including their stellar populations and even
survival as visible galaxies. In this Letter we draw on results from
quantitative studies of Local Group dwarf galaxy star formation histories,
especially for Milky Way satellites, to show that no clear signature exists for
a widespread evolutionary impact from reionization. All nearby dwarf galaxies
studied in sufficient detail contain ancient populations indistinguishable in
age from the oldest Galactic globular clusters. Ancient star formation activity
proceeded over several Gyr, and some dwarf spheroidal galaxies even experienced
fairly continuous star formation until just a few Gyr ago. Despite their
uniformly low masses, their star formation histories differ considerably. The
evolutionary histories of nearby dwarf galaxies appear to reflect influences
from a variety of local processes rather than a dominant effect from
reionization.Comment: Accepted by The Astrophysical Journal Letters. 5 pages, one figur
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