127 research outputs found
Galactic abundance gradients from Cepheids : On the iron abundance gradient around 10-12 kpc
Context: Classical Cepheids can be adopted to trace the chemical evolution of
the Galactic disk since their distances can be estimated with very high
accuracy. Aims: Homogeneous iron abundance measurements for 33 Galactic
Cepheids located in the outer disk together with accurate distance
determinations based on near-infrared photometry are adopted to constrain the
Galactic iron gradient beyond 10 kpc. Methods: Iron abundances were determined
using high resolution Cepheid spectra collected with three different
observational instruments: ESPaDOnS@CFHT, Narval@TBL and [email protected] ESO/MPG
telescope. Cepheid distances were estimated using near-infrared (J,H,K-band)
period-luminosity relations and data from SAAO and the 2MASS catalog. Results:
The least squares solution over the entire data set indicates that the iron
gradient in the Galactic disk presents a slope of -0.052+/-0.003 dex/kpc in the
5-17 kpc range. However, the change of the iron abundance across the disk seems
to be better described by a linear regime inside the solar circle and a
flattening of the gradient toward the outer disk (beyond 10 kpc). In the latter
region the iron gradient presents a shallower slope, i.e. -0.012+/-0.014
dex/kpc. In the outer disk (10-12 kpc) we also found that Cepheids present an
increase in the spread in iron abundance. Current evidence indicates that the
spread in metallicity depends on the Galactocentric longitude. Finally, current
data do not support the hypothesis of a discontinuity in the iron gradient at
Galactocentric distances of 10-12 kpc. Conclusions: The occurrence of a spread
in iron abundance as a function of the Galactocentric longitude indicates that
linear radial gradients should be cautiously treated to constrain the chemical
evolution across the disk.Comment: 5 tables, 8 figures, Accepted in A&
On the metallicity distribution of classical Cepheids in the Galactic inner disk
We present homogeneous and accurate iron abundances for almost four dozen
(47) of Galactic Cepheids using high-spectral resolution (R40,000) high
signal-to-noise ratio (S/N 100) optical spectra collected with UVES at
VLT. A significant fraction of the sample (32) is located in the inner disk (RG
6.9 kpc) and for half of them we provide new iron abundances. Current
findings indicate a steady increase in iron abundance when approaching the
innermost regions of the thin disk. The metallicity is super-solar and ranges
from 0.2 dex for RG 6.5 kpc to 0.4 dex for RG 5.5 kpc. Moreover,
we do not find evidence of correlation between iron abundance and distance from
the Galactic plane. We collected similar data available in the literature and
ended up with a sample of 420 Cepheids. Current data suggest that the mean
metallicity and the metallicity dispersion in the four quadrants of the
Galactic disk attain similar values. The first-second quadrants show a more
extended metal-poor tail, while the third-fourth quadrants show a more extended
metal-rich tail, but the bulk of the sample is at solar iron abundance.
Finally, we found a significant difference between the iron abundance of
Cepheids located close to the edge of the inner disk ([Fe/H]0.4) and
young stars located either along the Galactic bar or in the nuclear bulge
([Fe/H]0). Thus suggesting that the above regions have had different
chemical enrichment histories. The same outcome applies to the metallicity
gradient of the Galactic bulge, since mounting empirical evidence indicates
that the mean metallicity increases when moving from the outer to the inner
bulge regions.Comment: 10 pages, 5 figures; Corrected typos, corrected Table
On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk
We present homogeneous and accurate iron abundances for 42 Galactic Cepheids
based on high-spectral resolution (R~38,000) high signal-to-noise ratio
(SNR>100) optical spectra collected with UVES at VLT (128 spectra). The above
abundances were complemented with high-quality iron abundances provided either
by our group (86) or available in the literature. We paid attention in deriving
a common metallicity scale and ended up with a sample of 450 Cepheids. We also
estimated for the entire sample accurate individual distances by using
homogeneous near-infrared photometry and the reddening free Period-Wesenheit
relations. The new metallicity gradient is linear over a broad range of
Galactocentric distances (Rg~5-19 kpc) and agrees quite well with similar
estimates available in the literature (-0.060+/-0.002 dex/kpc). We also uncover
evidence which suggests that the residuals of the metallicity gradient are
tightly correlated with candidate Cepheid Groups (CGs). The candidate CGs have
been identified as spatial overdensities of Cepheids located across the thin
disk. They account for a significant fraction of the residual fluctuations, and
in turn for the large intrinsic dispersion of the metallicity gradient. We
performed a detailed comparison with metallicity gradients based on different
tracers: OB stars and open clusters. We found very similar metallicity
gradients for ages younger than 3 Gyrs, while for older ages we found a
shallower slope and an increase in the intrinsic spread. The above findings
rely on homogeneous age, metallicity and distance scales. Finally we found, by
using a large sample of Galactic and Magellanic Cepheids for which are
available accurate iron abundances, that the dependence of the luminosity
amplitude on metallicity is vanishing.Comment: Accepted in A&A, 11 figures, 7 table
New Baade-Wesselink distances and radii for four metal-rich Galactic Cepheids
We provided accurate estimates of distances, radii and iron abundances for
four metal-rich Cepheids, namely V340 Ara, UZ Sct, AV Sgr and VY Sgr. The main
aim of this investigation is to constrain their pulsation properties and their
location across the Galactic inner disk. We adopted new accurate NIR (J,H,K)
light curves and new radial velocity measurements for the target Cepheids to
determinate their distances and radii using the Baade-Wesselink technique. In
particular, we adopted the most recent calibration of the IR surface brightness
relation and of the projection factor. Moreover, we also provided accurate
measurements of the iron abundance of the target Cepheids. Current distance
estimates agree within one sigma with similar distances based either on
empirical or on theoretical NIR Period-Luminosity relations. However, the
uncertainties of the Baade-Wesselink distances are on average a factor of 3-4
smaller when compared with errors affecting other distance determinations. Mean
Baade-Wesselink radii also agree at one sigma level with Cepheid radii based
either on empirical or on theoretical Period-Radius relations. Iron abundances
are, within one sigma, similar to the iron contents provided by Andrievsky and
collaborators, thus confirming the super metal-rich nature of the target
Cepheids. We also found that the luminosity amplitudes of classical Cepheids,
at odds with RR Lyrae stars, do not show a clear correlation with the
metal-content. This circumstantial evidence appears to be the consequence of
the Hertzsprung progression together with the dependence of the topology of the
instability strip on metallicity, evolutionary effects and binaries.Comment: 9 pages, 7 figures, A&A accepte
Multiwavelength Campaign on Mrk 509 X. Lower limit on the distance of the absorber from HST COS and STIS spectroscopy
Active Galactic Nuclei often show evidence of photoionized outflows. A major
uncertainty in models for these outflows is the distance () to the gas from
the central black hole. In this paper we use the HST/COS data from a massive
multi-wavelength monitoring campaign on the bright Seyfert I galaxy Mrk 509, in
combination with archival HST/STIS data, to constrain the location of the
various kinematic components of the outflow. We compare the expected response
of the photoionized gas to changes in ionizing flux with the changes measured
in the data using the following steps: 1) We compare the column densities of
each kinematic component measured in the 2001 STIS data with those measured in
the 2009 COS data; 2) We use time-dependent photionization calculations with a
set of simulated lightcurves to put statistical upper limits on the hydrogen
number density that are consistent with the observed small changes in the ionic
column densities; 3) From the upper limit on the number density, we calculate a
lower limit on the distance to the absorber from the central source via the
prior determination of the ionization parameter. Our method offers two
improvements on traditional timescale analysis. First, we account for the
physical behavior of AGN lightcurves. Second, our analysis accounts for the
quality of measurement in cases where no changes are observed in the absorption
troughs. The very small variations in trough ionic column densities (mostly
consistent with no change) between the 2001 and 2009 epochs allow us to put
statistical lower limits on the distance between 100--200 pc for all the major
UV absorption components at a confidence level of 99%. These results are mainly
consistent with the independent distance estimates derived for the warm
absorbers from the simultaneous X-ray spectra.Comment: Accepted to A&A (06 APR 2012
High-Resolution X-ray Spectroscopy of the Interstellar Medium
The interstellar medium (ISM) has a multiphase structure characterized by
gas, dust and molecules. The gas can be found in different charge states:
neutral, low-ionized (warm) and high-ionized (hot). It is possible to probe the
multiphase ISM through the observation of its absorption lines and edges in the
X-ray spectra of background sources. We present a high-quality RGS spectrum of
the low-mass X-ray binary GS 1826-238 with an unprecedent detailed treatment of
the absorption features due to the dust and both the neutral and ionized gas of
the ISM. We constrain the column density ratios within the different phases of
the ISM and measure the abundances of elements such as O, Ne, Fe and Mg. We
found significant deviations from the proto-Solar abundances: oxygen is
over-abundant by a factor 1.23 +/- 0.05, neon 1.75 +/- 0.11, iron 1.37 +/- 0.17
and magnesium 2.45 +/- 0.35. The abundances are consistent with the measured
metallicity gradient in our Galaxy: the ISM appears to be metal-rich in the
inner regions. The spectrum also shows the presence of warm/hot ionized gas.
The gas column has a total ionization degree less than 10%. We also show that
dust plays an important role as expected from the position of GS 1826-238: most
iron appears to be bound in dust grains, while 10-40% of oxygen consists of a
mixture of dust and molecules
The Dependency of the Cepheid Period-Luminosity Relation on Chemical Composition
The dependency of the Cepheid Period-Luminosity Relation on chemical
composition at different wavelengths is assessed via direct detailed abundance
analysis of Galactic and Magellanic Cepheids, as derived from high resolution,
high signal-to-noise spectra. Our measurements span one order of magnitude in
iron content and allow to rule out at the ~ 9 sigma level the universality of
the Period-Luminosity Relation in the V band, with metal rich stars being
fainter than metal poor ones by ~0.3 mag. The dependency is less pronounced in
the K band. Its magnitude and statistical significance decisively depend on
detailed distance measurements to individual stars, as inferred via the
Infrared Surface Brightness Method.Comment: 3 pages, 2 figures, to be published in "Stellar Pulsation: Challenges
for Theory and Observation" (31 May - 5 June, Santa Fe, New Mexico). Minor
typos fixed in v
Characterisation of the Galactic thin disc with Corot targets
We use kinematical and chemical properties of 754 Corot stars to characterise
the stellar populations of the Milky Way disc in three beams close the Galactic
plane. From the atmospheric parameters derived in Gazzano et al. (2010) with
the Matisse algorithm, we derived stellar distances using isochrones. Combining
these data with proper motions, we provide the complete kinematical description
of stars in three Corot fields. Finally, we used kinematical criteria to
identify the Galactic populations in our sample and study their
characteristics, particularly their chemistry. Comparing our kinematics with
the Besancon Galactic model, we show that, within 3-sigma, simulated and
observed kinematical distributions are in good agreement. We study the
characteristics of the thin disc, finding a correlation that is significant at
a value of 2-sigma between the V-velocity component and the metallicity for two
different radial distance bins (8-9kpc and 9-10kpc; but not for the most inner
bin 7-8kpc, probably because of the uncertainties in the abundances) which
could be interpreted as radial migration evidence. We also measured a radial
metallicity gradient value of -0.097+/-0.015dex/kpc with giant stars, and
-0.053+/-0.015dex/kpc with dwarfs. Finally, we identified metal-rich stars with
peculiar high [alpha/Fe] values in the directions pointing to the inner part of
the Galaxy. Applying the same methodology to the planet-hosting stars detected
by Corot shows that they mainly belong to the thin disc population with normal
chemical and kinematical properties.Comment: Astronomy & Astrophysics, in pres
CoRoT\,102699796, the first metal-poor Herbig Ae pulsator: a hybrid Sct- Dor variable?
We present the analysis of the time series observations of CoRoT\,102699796
obtained by the CoRoT satellite that show the presence of five independent
oscillation frequencies in the range 3.6-5 c/d. Using spectra acquired with
FLAMES@VLT, we derive the following stellar parameters: spectral type F1V,
T=7000200 K, log(g)=, [M/H]=,
sin= km/s, L/L=21. Thus, for the first
time we report the existence of a metal poor, intermediate-mass PMS pulsating
star. Ground-based and satellite data are used to derive the spectral energy
distribution of CoRoT\,102699796 extending from the optical to mid-infrared
wavelengths. The SED shows a significant IR excess at wavelengths greater than
. We conclude that CoRoT\,102699796 is a young Herbig Ae (F1Ve) star
with a transitional disk, likely associated to the HII region [FT96]213.1-2.2.
The pulsation frequencies have been interpreted in the light of the
non-radial pulsation theory, using the LOSC code in conjunction with static and
rotational evolutionary tracks. A minimization algorithm was used to find the
best-fit model with M=1.84 M, T=6900 K which imply an
isochronal age of t2.5 Myr. This result is based on the interpretation of
the detected frequencies as -modes of low-moderate -value. To our
knowledge, this is the first time that such modes are identified in a
intermediate-mass PMS pulsating star. Since CoRoT\,102699796 lies in the region
of the HR diagram where the Sct and Dor instability strips
intersect, we argue that the observed pulsation characteristics are
intermediate between these classes of variables, i.e. CoRoT\,102699796 is
likely the first PMS hybrid Dor- Sct pulsator ever studied.Comment: 13 pages, 7 figures. Accepted for publication in Monthly Notices of
the RA
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