37 research outputs found
Small-scale hero: massive-star enrichment in the Hercules dwarf spheroidal
Dwarf spheroidal galaxies are often conjectured to be the sites of the first
stars. The best current contenders for finding the chemical imprints from the
enrichment by those massive objects are the "ultrafaint dwarfs" (UFDs). Here we
present evidence for remarkably low heavy element abundances in the metal poor
Hercules UFD. Combined with other peculiar abundance patterns this indicates
that Hercules was likely only influenced by very few, massive explosive events
- thus bearing the traces of an early, localized chemical enrichment with only
very little other contributions from other sources at later times.Comment: 4 pages, 3 figures, to appear in the proceedings of `First Stars IV -
from Hayashi to the future', M. Umemura, K. Omukai (eds.
Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy
We present an assessment of the barium abundance ratios for red giant member
stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are
drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II
6141.71 AA absorption line at low signal-to-noise ratios. For three brighter
stars we were able to gain estimates from direct equivalent-width measurements,
while for the remaining eight stars only upper limits could be obtained. These
results are investigated in a statistical manner and indicate very low Ba
abundances of log epsilon (Ba) < 0.7 dex (3 sigma). We discuss various possible
systematic biasses, first and foremost, a blend with the Fe I 6141.73 AA-line,
but most of those would only lead to even lower abundances. A better match with
metal-poor halo and dSph stars can only be reached by including a large
uncertainty in the continuum placement. This contrasts with the high
dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the
latter spreads are typical of the very low luminosity, dark-matter dominated
dSphs, a high level of depletion in heavy elements suggests that chemical
enrichment in Hercules was governed by very massive stars, coupled with a very
low star formation efficiency. While very low abundances of some heavy elements
are also found in individual stars of other dwarf galaxies, this is the first
time that a very low Ba abundance is found within an entire dSph over a broad
metallicity range.Comment: 10 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Clinical Efficacy of A Nutraceutical Approach for the Management of Dyslipidemia in Metabolic Disorders: A One-Year Treatment With Armolipid Plus
n/
Hot Cores : Probes of High-Redshift Galaxies
The very high rates of second generation star formation detected and inferred
in high redshift objects should be accompanied by intense millimetre-wave
emission from hot core molecules. We calculate the molecular abundances likely
to arise in hot cores associated with massive star formation at high redshift,
using several independent models of metallicity in the early Universe. If the
number of hot cores exceeds that in the Milky Way Galaxy by a factor of at
least one thousand, then a wide range of molecules in high redshift hot cores
should have detectable emission. It should be possible to distinguish between
independent models for the production of metals and hence hot core molecules
should be useful probes of star formation at high redshift.Comment: Updated to correspond to version accepted by MNRA
A new comprehensive set of elemental abundances in DLAs III. Star formation histories
We obtained comprehensive sets of elemental abundances for eleven damped
Ly-alpha systems (DLAs) at z_DLA=1.7-2.5. In Paper I of this series, we showed
for three DLA galaxies that we can derive their star formation histories and
ages from a detailed comparison of their intrinsic abundance patterns with
chemical evolution models. We determine in this paper the star formation
properties of six additional DLA galaxies. The derived results confirm that no
single star formation history explains the diverse sets of abundance patterns
in DLAs. We demonstrate that the various star formation histories reproducing
the DLA abundance patterns are typical of local irregular, dwarf starburst and
quiescent spiral galaxies. Independent of the star formation history, the DLAs
have a common characteristic of being weak star forming galaxies; models with
high star formation efficiencies are ruled out. All the derived DLA star
formation rates per unit area are moderate or low, with values between -3.2 <
log SFR < -1.1 M_sol yr^{-1} kpc^{-2}. The DLA abundance patterns require a
large spread in ages ranging from 20 Myr up to 3 Gyr. The oldest DLA in our
sample is observed at z_DLA=1.864 with an age estimated to more than 3 Gyr; it
nicely indicates that galaxies were already forming at z_f>10. But, most of the
DLAs show ages much younger than that of the Universe at the epoch of
observation. Young galaxies thus seem to populate the high redshift Universe at
z>2, suggesting relatively low redshifts of formation (z~3) for most
high-redshift galaxies. The DLA star formation properties are compared with
those of other high-redshift galaxies identified in deep imaging surveys with
the aim of obtaining a global picture of high-redshift objects.Comment: 19 pages, 11 figures, Accepted for publication in A&
A new comprehensive set of elemental abundances in DLAs - II. Data analysis and chemical variation studies
We present new elemental abundance studies of seven damped Lyman-alpha
systems (DLAs). Together with the four DLAs analyzed in Dessauges-Zavadsky et
al. (2004), we have a sample of eleven DLA galaxies with uniquely comprehensive
and homogeneous abundance measurements. These observations allow one to study
the abundance patterns of 22 elements and the chemical variations in the
interstellar medium of galaxies outside the Local Group. Comparing the
gas-phase abundance ratios of these high redshift galaxies, we found that they
show low RMS dispersions, reaching only up 2-3 times the statistical errors for
the majority of elements. This uniformity is remarkable given that the quasar
sightlines cross gaseous regions with HI column densities spanning over one
order of magnitude and metallicities ranging from 1/55 to 1/5 solar. The
gas-phase abundance patterns of interstellar medium clouds within the DLA
galaxies detected along the velocity profiles show, on the other hand, a high
dispersion in several abundance ratios, indicating that chemical variations
seem to be more confined to individual clouds within the DLA galaxies than to
integrated profiles. The analysis of the cloud-to-cloud chemical variations
within seven individual DLAs reveals that five of them show statistically
significant variations, higher than 0.2 dex at more than 3 sigma. The sources
of these variations are both the differential dust depletion and/or ionization
effects; however, no evidence for variations due to different star formation
histories could be highlighted. These observations place large constraints on
the mixing timescales of protogalaxies and on scenarios of galaxy formation
within the CDM hierarchical theory. Finally, we provide an astrophysical
determination of the oscillator strength of the NiII 1317 transition.Comment: 45 pages, 33 figures (high-resolution figures available on request
from the authors or in the A&A journal). Accepted for publication in A&
HII Region Metallicity Distribution in the Milky Way Disk
The distribution of metals in the Galaxy provides important information about
galaxy formation and evolution. HII regions are the most luminous objects in
the Milky Way at mid-infrared to radio wavelengths and can be seen across the
entire Galactic disk. We used the NRAO Green Bank Telescope (GBT) to measure
radio recombination line and continuum emission in 81 Galactic HII regions. We
calculated LTE electron temperatures using these data. In thermal equilibrium
metal abundances are expected to set the nebular electron temperature with high
abundances producing low temperatures. Our HII region distribution covers a
large range of Galactocentric radius (5 to 22 kpc) and samples the Galactic
azimuth range 330 degree to 60 degree. Using our highest quality data (72
objects) we derived an O/H Galactocentric radial gradient of -0.0383 +/- 0.0074
dex/kpc. Combining these data with a similar survey made with the NRAO 140 Foot
telescope we get a radial gradient of -0.0446 +/- 0.0049 dex/kpc for this
larger sample of 133 nebulae. The data are well fit by a linear model and no
discontinuities are detected. Dividing our sample into three Galactic azimuth
regions produced significantly different radial gradients that range from -0.03
to -0.07 dex/kpc. These inhomogeneities suggest that metals are not well mixed
at a given radius. We stress the importance of homogeneous samples to reduce
the confusion of comparing data sets with different systematics. Galactic
chemical evolution models typically derive chemical evolution along only the
radial dimension with time. Future models should consider azimuthal evolution
as well.Comment: Accepted for publication in Ap
A simple model for the evolution of disc galaxies: The Milky Way
A simple model for the evolution of disc galaxies is presented. We adopt
three numbers from observations of the Milky Way disc, the local surface mass
density, the stellar scale length (of the assumedly exponential disc) and the
amplitude of the (assumedly flat) rotation curve, and physically, the (local)
dynamical Kennicutt star formation prescription, standard chemical evolution
equations assuming and a model for spectral evolution of stellar populations.
We can determine the detailed evolution of the model with only the addition of
standard cosmological scalings with time of the dimensional parameters. A
surprising wealth of detailed specifications follows from this prescription
including the gaseous infall rate as a function of radius and time, the
distribution of stellar ages and metallicities with time and radius, surface
brightness profiles at different wavelengths, colours etc. At the solar
neighbourhood stars start to form ago at an increasing rate
peaking 4 billion years ago and then slowly declining in good agreement with
observations. The mean age of long lived stars at the solar neighbourhood is
about . The local surface density of the stars and gas are 35 and , respectively. The metallicity distribution of the stars at
the solar radius is narrow with a peak at .Both a
Salpeter IMF and a Chabrier IMF are consistent with observations. Comparisons
with the current and local fossil evidence provides support for the model which
can then be used to assess other local disc galaxies, the evolution of disc
galaxies in deep optical surveys and also for theoretical investigations such
as simulations of merging disc galaxies (abbreviated).Comment: acceppted for publication in MNRA
Extragalactic chemical abundances: do HII regions and young stars tell the same story? The case of the spiral galaxy NGC 300
(Abridged) We have obtained new spectrophotometric data for 28 HII regions in
the spiral galaxy NGC 300, a member of the nearby Sculptor Group. The detection
of auroral lines, including [OIII]4363, [SIII]6312 and [NII]5755, has allowed
us to measure electron temperatures and direct chemical abundances for the
whole sample. We determine for the first time in this galaxy a radial gas-phase
oxygen abundance gradient based solely on auroral lines, and obtain the
following least-square solution: 12+log(O/H)=8.57-0.41 R/R25, where the
galactocentric distance is expressed in terms of the isophotal radius R25. The
gradient corresponds to -0.077 dex/kpc, and agrees very well with the
galactocentric trend in metallicity obtained for 29 B and A supergiants in the
same galaxy. The intercept of the regression for the nebular data virtually
coincides with the intercept obtained from the stellar data. This allows little
room for depletion of nebular oxygen onto dust grains, although in this kind of
comparison we are somewhat limited by systematic uncertainties, such as those
related to the atomic parameters used to derive the chemical compositions.
We discuss the implications of our result with regard to strong-line
abundance indicators commonly used to estimate the chemical compositions of
star-forming galaxies, such as R23. By applying a few popular calibrations of
these indices based on grids of photoionization models on the NGC 300 HII
region fluxes we find metallicities that are higher by 0.3 dex (a factor of
two) or more relative to our nebular (Te-based) and stellar ones.
We confirm a metallicity dependence of the `softness' parameter
eta=(O+/O++)/(S+/S++), in the sense that softer stellar continua are found at
high metallicity.Comment: Accepted for publication in The Astrophysical Journa
Evolution of interstellar dust and stardust in the solar neighbourhood
The abundance evolution of interstellar dust species originating from stellar
sources and from condensation in molecular clouds in the local interstellar
medium of the Milky Way is studied and the input of dust material to the Solar
System is determined. A one-zone chemical evolution model of the Milky Way for
the elemental composition of the disk combined with an evolution model for its
interstellar dust component similar to that of Dwek (1998) is developed. The
dust model considers dust-mass return from AGB stars as calculated from
synthetic AGB models combined with models for dust condensation in stellar
outflows. Supernova dust formation is included in a simple parameterized form
which is gauged by observed abundances of presolar dust grains with supernova
origin. For dust growth in the ISM a simple method is developed for coupling
this with disk and dust evolution models. The time evolution of the abundance
of the following dust species is followed in the model: silicate, carbon,
silicon carbide, and iron dust from AGB stars and from SNe as well as silicate,
carbon, and iron dust grown in molecular clouds. It is shown that the
interstellar dust population is dominated by dust accreted in molecular clouds;
most of the dust material entering the Solar System at its formation does not
show isotopic abundance anomalies of the refractory elements, i.e.,
inconspicuous isotopic abundances do not point to a Solar System origin of dust
grains. The observed abundance ratios of presolar dust grains formed in SN
ejecta and in AGB star outflows requires that for the ejecta from SNe the
fraction of refractory elements condensed into dust is 0.15 for carbon dust and
is quite small () for other dust species.Comment: 29 pages, 19 figure