41 research outputs found
Abundance Patterns in Stars in the Bulge and Galactic Center
We discuss oxygen and iron abundance patterns in K and M red-giant members of
the Galactic bulge and in the young and massive M-type stars inhabiting the
very center of the Milky Way. The abundance results from the different bulge
studies in the literature, both in the optical and the infrared, indicate that
the [O/Fe]-[Fe/H] relation in the bulge does not follow the disk relation, with
[O/Fe] values falling above those of the disk. Based on these elevated values
of [O/Fe] extending to large Fe abundances, it is suggested that the bulge
underwent a rapid chemical enrichment with perhaps a top-heavy initial mass
function. The Galactic Center stars reveal a nearly uniform and slightly
elevated (relative to solar) iron abundance for a studied sample which is
composed of 10 red giants and supergiants. Perhaps of more significance is the
fact that the young Galactic Center M-type stars show abundance patterns that
are reminiscent of those observed for the bulge population and contain enhanced
abundance ratios of alpha-elements relative to either the Sun or Milky Way disk
at near-solar metallicities.Comment: requires iaus.cls; to appear in Formation and Evolution of Galaxy
Bulges, Proceedings IAU Symposium No. 245, 2007, M. Bureau et al. eds., in
pres
Testing the universal stellar IMF on the metallicity distribution in the bulges of the Milky Way and M31
We test whether the universal initial mass function (UIMF) or the integrated
galaxial IMF (IGIMF) can be employed to explain the metallicity distribution
(MD) of giants in the Galactic bulge. We make use of a single-zone chemical
evolution model developed for the Milky Way bulge in the context of an
inside-out model for the formation of the Galaxy. We checked whether it is
possible to constrain the yields above 80 M_{\sun} by forcing the UIMF and
required that the resulting MD matches the observed ones. We also extended the
analysis to the bulge of M31 to investigate a possible variation of the IMF
among galactic bulges. Several parameters that have an impact on stellar
evolution (star-formation efficiency, gas infall timescale) are varied. We show
that it is not possible to satisfactorily reproduce the observed metallicity
distribution in the two galactic bulges unless assuming a flatter IMF () than the universal one. We conlude that it is necessary to assume a
variation in the IMF among the various environments.Comment: 9 pages, 4 figures, accepted for publication in A&
Evolution of chemical abundances in Seyfert galaxies
We computed the chemical evolution of spiral bulges hosting Seyfert nuclei,
based on updated chemical and spectro-photometrical evolution models for the
bulge of our Galaxy, made predictions about other quantities measured in
Seyferts, and modeled the photometry of local bulges. The chemical evolution
model contains detailed calculations of the Galactic potential and of the
feedback from the central supermassive black hole, and the spectro-photometric
model covers a wide range of stellar ages and metallicities. We followed the
evolution of bulges in the mass range 10^9 - 10^{11} Msun by scaling the star
formation efficiency and the bulge scalelength as in the inverse-wind scenario
for elliptical galaxies, and considering an Eddington limited accretion onto
the central supermassive black hole. We successfully reproduced the observed
black hole-host bulge mass relation. The observed nuclear bolometric luminosity
is reproduced only at high redshift or for the most massive bulges; in the
other cases, at z = 0 a rejuvenation mechanism is necessary. The black hole
feedback is in most cases not significant in triggering the galactic wind. The
observed high star formation rates and metal overabundances are easily
achieved, as well as the constancy of chemical abundances with redshift and the
bulge present-day colours. Those results are not affected if we vary the index
of the stellar IMF from x=0.95 to x=1.35; a steeper IMF is instead required in
order to reproduce the colour-magnitude relation and the present K-band
luminosity of the bulge.Comment: 17 pages, 15 figures, 3 tables, accepted for publication in A&
Chemical similarities between Galactic bulge and local thick disk red giant stars
The evolution of the Milky Way bulge and its relationship with the other
Galactic populations is still poorly understood. The bulge has been suggested
to be either a merger-driven classical bulge or the product of a dynamical
instability of the inner disk. To probe the star formation history, the initial
mass function and stellar nucleosynthesis of the bulge, we performed an
elemental abundance analysis of bulge red giant stars. We also completed an
identical study of local thin disk, thick disk and halo giants to establish the
chemical differences and similarities between the various populations.
High-resolution infrared spectra of 19 bulge giants and 49 comparison giants in
the solar neighborhood were acquired with Gemini/Phoenix. All stars have
similar stellar parameters but cover a broad range in metallicity. A standard
1D local thermodynamic equilibrium analysis yielded the abundances of C, N, O
and Fe. A homogeneous and differential analysis of the bulge, halo, thin disk
and thick disk stars ensured that systematic errors were minimized. We confirm
the well-established differences for [O/Fe] (at a given metallicity) between
the local thin and thick disks. For the elements investigated, we find no
chemical distinction between the bulge and the local thick disk, which is in
contrast to previous studies relying on literature values for disk dwarf stars
in the solar neighborhood. Our findings suggest that the bulge and local thick
disk experienced similar, but not necessarily shared, chemical evolution
histories. We argue that their formation timescales, star formation rates and
initial mass functions were similar.Comment: Accepted for publication in A&A, 5 page
Formation & evolution of the Galactic bulge: constraints from stellar abundances
We compute the chemical evolution of the Galactic bulge in the context of an
inside-out model for the formation of the Milky Way. The model contains updated
stellar yields from massive stars. The main purpose of the paper is to compare
the predictions of this model with new observations of chemical abundance
ratios and metallicity distributions in order to put constraints on the
formation and evolution of the bulge. We computed the evolution of several
alpha-elements and Fe and performed several tests by varying different
parameters such as star formation efficiency, slope of the initial mass
function and infall timescale. We also tested the effect of adopting a primary
nitrogen contribution from massive stars. The [alpha/Fe] abundance ratios in
the Bulge are predicted to be supersolar for a very large range in [Fe/H], each
element having a different slope. These predictions are in very good agreement
with most recent accurate abundance determinations. We also find a good fit of
the most recent Bulge stellar metallicity distributions. We conclude that the
Bulge formed on a very short timescale (even though timescales much shorter
than about 0.1 Gyr are excluded) with a quite high star formation efficiency of
about 20 Gyr and with an initial mass function more skewed toward high
masses (i.e. x <= 0.95) than the solar neighbourhood and rest of the disk. The
results obtained here are more robust than previous ones since they are based
on very accurate abundance measurements.Comment: 26 pages, 9 figures, accepted for publication in A&
The Radio - X-ray relation as a star formation indicator: Results from the VLA--E-CDFS Survey
In order to trace the instantaneous star formation rate at high redshift, and
hence help understanding the relation between the different emission mechanisms
related to star formation, we combine the recent 4 Ms Chandra X-ray data and
the deep VLA radio data in the Extended Chandra Deep Field South region. We
find 268 sources detected both in the X-ray and radio band. The availability of
redshifts for of the sources in our sample allows us to derive
reliable luminosity estimates and the intrinsic properties from X-ray analysis
for the majority of the objects. With the aim of selecting sources powered by
star formation in both bands, we adopt classification criteria based on X-ray
and radio data, exploiting the X-ray spectral features and time variability,
taking advantage of observations scattered across more than ten years. We
identify 43 objects consistent with being powered by star formation. We also
add another 111 and 70 star forming candidates detected only in the radio or
X-ray band, respectively. We find a clear linear correlation between radio and
X-ray luminosity in star forming galaxies over three orders of magnitude and up
to . We also measure a significant scatter of the order of 0.4 dex,
higher than that observed at low redshift, implying an intrinsic scatter
component. The correlation is consistent with that measured locally, and no
evolution with redshift is observed. Using a locally calibrated relation
between the SFR and the radio luminosity, we investigate the L_X(2-10keV)-SFR
relation at high redshift. The comparison of the star formation rate measured
in our sample with some theoretical models for the Milky Way and M31, two
typical spiral galaxies, indicates that, with current data, we can trace
typical spirals only at z<0.2, and strong starburst galaxies with
star-formation rates as high as , up to .Comment: 21 pages, 10 figures, 5 table
Comparative analysis of medicinal plants used in traditional medicine in Italy and Tunisia
<p>Abstract</p> <p>Background</p> <p>Italy and Tunisia (Africa for the Romans), facing each other on the opposite sides of the Mediterranean Sea, have been historically linked since the ancient times. Over the centuries both countries were mutually dominated so the vestiges and traces of a mutual influence are still present. The aim of the present study is to conduct a comparative analysis of the medicinal species present in the respective Floras in order to explore potential analogies and differences in popular phytotherapy that have come out from those reciprocal exchanges having taken place over the centuries</p> <p>Methods</p> <p>The comparative analysis based on the respective floras of both countries takes into consideration the bulk of medicinal species mutually present in Italy and Tunisia, but it focuses on the species growing in areas which are similar in climate. The medicinal uses of these species are considered in accordance with the ethnobotanical literature.</p> <p>Results</p> <p>A list of 153 medicinal species belonging to 60 families, present in both floras and used in traditional medicine, was drawn. A considerable convergence in therapeutic uses of many species emerged from these data.</p> <p>Conclusion</p> <p>This comparative analysis strengthens the firm belief that ethno-botanical findings represent not only an important shared heritage, developed over the centuries, but also a considerable mass of data that should be exploited in order to provide new and useful knowledge.</p
Intermediate and extreme mass-ratio inspirals — astrophysics, science applications and detection using LISA
Black hole binaries with extreme (gtrsim104:1) or intermediate (~102–104:1) mass ratios are among the most interesting gravitational wave sources that are expected to be detected by the proposed laser interferometer space antenna (LISA). These sources have the potential to tell us much about astrophysics, but are also of unique importance for testing aspects of the general theory of relativity in the strong field regime. Here we discuss these sources from the perspectives of astrophysics, data analysis and applications to testing general relativity, providing both a description of the current state of knowledge and an outline of some of the outstanding questions that still need to be addressed. This review grew out of discussions at a workshop in September 2006 hosted by the Albert Einstein Institute in Golm, Germany
On the Constancy of the Characteristic Mass of Young Stars
The characteristic mass M_c in the stellar initial mass function (IMF) is
about constant for most star-forming regions. Numerical simulations
consistently show a proportionality between M_c and the thermal Jeans mass M_J
at the time of cloud fragmentation, but no models have explained how it can be
the same in diverse conditions. Here we show that M_J depends weakly on
density, temperature, metallicity, and radiation field in three environments:
the dense cores where stars form, larger star-forming regions ranging from GMCs
to galactic disks, and the interiors of HII regions and super star clusters. In
dense cores, the quantity T^{3/2}n^{-1/2} that appears in M_J scales with core
density as n^{0.25} or with radiation density as U^{0.1} at the density where
dust and gas come into thermal equilibrium. On larger scales, this quantity
varies with ambient density as n^{-0.05} and ambient radiation field as
U^{-0.033} when the Kennicutt-Schmidt law of star formation determines U(n). In
super star clusters with ionization and compression of pre-stellar globules,
M_J varies as the 0.13 power of the cluster column density. These weak
dependencies on n, U, and column density imply that most environmental
variations affect the thermal Jeans mass by at most a factor of ~2.
Cosmological increases in M_J, which have been suggested by observations, may
be explained if the star formation efficiency is systematically higher at high
redshift for a given density and pressure, if dust grains are smaller at lower
metallicity, and so hotter for a given radiation field, or if small pre-stellar
cores are more severely ionized in extreme starburst conditions.Comment: 29 pages, no figures, accepted by Ap
Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. IV. Two bulge populations
[ABRIDGED] Based on high-resolution (R~42000 to 48000) and high
signal-to-noise (S/N~50 to 150) spectra obtained with UVES/VLT, we present
detailed elemental abundances (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y,
and Ba) and stellar ages for 26 microlensed dwarf and subgiant stars in the
Galactic bulge. The analysis is based on equivalent width measurements and
standard 1-D LTE MARCS model stellar atmospheres. We also present NLTE Li
abundances based on line synthesis of the 7Li line at 670.8 nm. We show that
the bulge metallicity distribution (MDF) is double-peaked; one peak at [Fe/H]=
-0.6 and one at [Fe/H]=+0.3, and with a dearth of stars around solar
metallicity. This is in contrast to the MDF derived from red giants in Baade's
window, which peaks at this exact value. A simple significance test shows that
it is extremely unlikely to have such a gap in the microlensed dwarf star MDF
if the dwarf stars are drawn from the giant star MDF. To resolve this issue we
discuss several possibilities, but we can not settle on a conclusive solution
for the observed differences. We further find that the metal-poor bulge dwarf
stars are predominantly old with ages greater than 10\,Gyr, while the
metal-rich bulge dwarf stars show a wide range of ages. The metal-poor bulge
sample is very similar to the Galactic thick disk in terms of average
metallicity, elemental abundance trends, and stellar ages. Speculatively, the
metal-rich bulge population might be the manifestation of the inner thin disk.
If so, the two bulge populations could support the recent findings, based on
kinematics, that there are no signatures of a classical bulge and that the
Milky Way is a pure-disk galaxy. Also, recent claims of a flat IMF in the bulge
based on the MDF of giant stars may have to be revised based on the MDF and
abundance trends probed by our microlensed dwarf stars.Comment: Accepted for publication in A&A, new version with with missing
authors adde