90 research outputs found
Abundances and possible diffusion of elements in M67 stars
We present a spectroscopic study at high resolution, R~50,000, of 14 stars
located on the main sequence, at the turn-off point and on the early subgiant
branch in the cluster M67 in order to investigate its detailed chemical
composition, for comparison with the Sun and solar twins in the solar
neighbourhood, and to explore selective atomic diffusion of chemical elements
as predicted by stellar-structure theory. We have obtained VLT/FLAMES-UVES
spectra and analysed these strictly differentially in order to explore
chemical-abundance similarities and differences between the M67 stars and the
Sun, and among the M67 stars themselves. Individual abundances of 19 different
chemical elements are obtained for the stars. They are found to agree very well
with solar abundances, with abundance ratios closer to solar than those of most
solar twins in the solar neighbourhood. An exception is Li which shows a
considerable scatter among the cluster stars. There is a tendency for the
cluster-star abundances to be depleted relative to the abundances in the field
stars in correlation with the condensation temperature of the elements, a
tendency earlier found also for the Sun. The results support the hypothesis
that the gas of the proto-cluster was depleted by formation and cleansing of
dust before the stars formed. They also add support to the proposal that the
Sun was once formed in a dense stellar environment. Moreover, the observed
minor reductions of heavy elements in the atmospheres of the dwarfs and
turn-off point stars relative to our standard star M67-1194 and the subgiants
seem to suggest that diffusion processes are at work in these stars, although
the evidence is not compelling. Based on theoretical models the
diffusion-corrected initial metallicity of M67 is estimated to be [Fe/H]=+0.06.Comment: 14 pages, 9 figures, 5 tables. Accepted for publication in section 8.
Stellar atmospheres of Astronomy and Astrophysics. Minor language corrections
and an update of section 4.1 as compared to previous publicatio
The Calibration of Stromgren uvby-Hbeta Photometry for Late-Type Stars -- a Model Atmosphere Approach
We aim to test the power of theoretical calibrations based on a new
generation of MARCS models by comparisons with observational photomteric data.
We calculate synthetic uvby-Hbeta colour indices from synthetic spectra. A
sample of 388 field stars as well as stars in globular clusters is used for a
direct comparison of the synthetic indices versus empirical data and for
scrutinizing the possibilities of theoretical calibrations for temperature,
metallicity and gravity. We show that the temperature sensitivity of the
synthetic (b-y) colour is very close to its empirical counterpart, whereas the
temperature scale based upon Hbeta shows a slight offset. The theoretical
metallicity sensitivity of the m1 index (and for G-type stars its combination
with c1) is somewhat larger than the empirical one, based upon spectroscopic
determinations. The gravity sensitivity of the synthetic c1 index shows a
satisfactory behaviour when compared to obervations of F stars. For stars
cooler than the sun a deviation is significant in the c1-(b-y) diagram. The
theoretical calibrations of (b-y), (v-y) and c1 seem to work well for Pop II
stars and lead to effective temperatures for globular cluster stars supporting
recent claims by Korn et al. (2007) that atomic diffusion occurs in stars near
the turnoff point of NGC 6397. Synthetic colours of stellar atmospheres can
indeed be used, in many cases, to derive reliable fundamental stellar
parameters. The deviations seen when compared to observational data could be
due to incomplete linelists but are possibly also due to effects of assuming
plane-parallell or spherical geometry and LTE
An H-band Spectroscopic Metallicity Calibration for M Dwarfs
We present an empirical near-infrared (NIR) spectroscopic method for
estimating M dwarf metallicities, based on features in the H-band, as well as
an implementation of a similar published method in the K-band. We obtained
R~2000 NIR spectra of a sample of M dwarfs using the NASA IRTF-SpeX
spectrograph, including 22 M dwarf metallicity calibration targets that have
FGK companions with known metallicities. The H-band and K-band calibrations
provide equivalent fits to the metallicities of these binaries, with an
accuracy of +/- 0.12 dex. We derive the first empirically calibrated
spectroscopic metallicity estimate for the giant planet-hosting M dwarf GJ 317,
confirming its super-solar metallicity. Combining this result with observations
of eight other M dwarf planet hosts, we find that M dwarfs with giant planets
are preferentially metal-rich compared to those that host less massive planets.
Our H-band calibration relies on strongly metallicity-dependent features in the
H-band, which will be useful in compositional studies using mid to high
resolution NIR M dwarf spectra, such as those produced by multiplexed surveys
like SDSS-III APOGEE. These results will also be immediately useful for ongoing
spectroscopic surveys of M dwarfs.Comment: Accepted to ApJ Letter
Accurate Fundamental Parameters or A, F, and G-type Supergiants in the Solar Neighbourhood
The following parameters are determined for 63 Galactic supergiants in the
solar neighbourhood: effective temperature Teff, surface gravity log g, iron
abundance log e(Fe), microturbulent parameter Vt, mass M/Msun, age t and
distance d. A significant improvement in the accuracy of the determination of
log g and, all parameters dependent on it, is obtained through application of
van Leeuwens (2007) re-reduction of the Hipparcos parallaxes. The typical error
in the log g values is now +-0.06 dex for supergiants with distances d < 300 pc
and +-0.12 dex for supergiants with d between 300 and 700 pc; the mean error in
Teff for these stars is +-120 K. For supergiants with d > 700 pc parallaxes are
uncertain or unmeasurable, so typical errors in their log g values are 0.2-0.3
dex.
A new Teff scale for A5-G5 stars of luminosity classes Ib-II is presented.
Spectral subtypes and luminosity classes of several stars are corrected.
Combining the Teff and log g with evolutionary tracks, stellar masses and ages
are determined; a majority of the sample has masses between 4 Msun and 15 Msun
and, hence, their progenitors were early to middle B-type main sequence stars.
Using Fe ii lines, which are insensitive to departures from LTE, the
microturbulent parameter Vt and the iron abundance log e(Fe) are determined
from high-resolution spectra. The parameter Vt is correlated with gravity: Vt
increases with decreasing log g. The mean iron abundance for the 48 supergiants
with distances d < 700 pc is log e(Fe)=7.48+-0.09, a value close to the solar
value of 7.45+-0.05, and thus the local supergiants and the Sun have the same
metallicity.Comment: 12 pages, 9 figures. Will be published at MNRA
Broadband UBVR_CI_C Photometry of Horizontal-Branch and Metal-poor Candidates from the HK and Hamburg/ESO Surveys. I.
We report broadband UBV and/or BVR_CI_C CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/A, N = 576), or candidate low-metallicity stars (N = 1221), from the HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of additional stars from other samples are also reported. These data are being used for several purposes. In the case of the FHB/A candidates they are used to accurately separate the lower gravity FHB stars from various higher gravity A-type stars, a subsample that includes the so-called blue metal poor stars, halo and thick-disk blue stragglers, main-sequence A-type dwarfs, and Am and Ap stars. These data are also being used to derive photometric distance estimates to high-velocity hydrogen clouds in the Galaxy and for improved measurements of the mass of the Galaxy. Photometric data for the metal-poor candidates are being used to refine estimates of stellar metallicity for objects with available medium-resolution spectroscopy, to obtain distance estimates for kinematic analyses, and to establish initial estimates of effective temperature for analysis of high-resolution spectroscopy of the stars for which this information now exists
POLLUX : a database of synthetic stellar spectra
Synthetic spectra are needed to determine fundamental stellar and wind
parameters of all types of stars. They are also used for the construction of
theoretical spectral libraries helpful for stellar population synthesis.
Therefore, a database of theoretical spectra is required to allow rapid and
quantitative comparisons to spectroscopic data. We provide such a database
offering an unprecedented coverage of the entire Hertzsprung-Russell diagram.
We present the POLLUX database of synthetic stellar spectra. For objects with
Teff < 6 000 K, MARCS atmosphere models are computed and the program
TURBOSPECTRUM provides the synthetic spectra. ATLAS12 models are computed for
stars with 7 000 K <Teff <15 000 K. SYNSPEC gives the corresponding spectra.
Finally, the code CMFGEN provides atmosphere models for the hottest stars (Teff
> 25 000 K). Their spectra are computed with CMF_FLUX. Both high resolution
(R>150 000) optical spectra in the range 3 000 to 12 000 A and spectral energy
distributions extending from the UV to near--IR ranges are presented. These
spectra cover the HR diagram at solar metallicity. We propose a wide variety of
synthetic spectra for various types of stars in a format that is compliant with
the Virtual Observatory standards. A user--friendly web interface allows an
easy selection of spectra and data retrieval. Upcoming developments will
include an extension to a large range of metallicities and to the near--IR high
resolution spectra, as well as a better coverage of the HR diagram, with the
inclusion of models for Wolf-Rayet stars and large datasets for cool stars. The
POLLUX database is accessible at http://pollux.graal.univ-montp2.fr/ and
through the Virtual Observatory.Comment: 9 pages, 5 figures, accepted for publication in Astronomy ans
Astrophysic
M67-1194, an unusually Sun-like solar twin in M67
The rich open cluster M67 is known to have a chemical composition close to
solar, and an age around 4Gyr. It thus offers the opportunity to check our
understanding of the physics and the evolution of solar-type stars in a cluster
environment. We present the first spectroscopic study at high resolution,
R~50,000, of the potentially best solar twin, M67-1194, identified among
solar-like stars in M67. Based on a pre-selection of solar-twin candidates
performed at medium resolution by Pasquini et al. (2008), we explore the
chemical-abundance similarities and differences between M67-1194 and the Sun,
using VLT/FLAMES-UVES. Working with a solar twin in the framework of a
differential analysis, we minimize systematic model errors in the abundance
analysis compared to previous studies which utilized more evolved stars to
determine the metallicity of M67. We find M67-1194 to have stellar parameters
indistinguishable from the solar values, with the exception of the overall
metallicity which is slightly super-solar ([Fe/H]=0.023 +/- 0.015). An age
determination based on evolutionary tracks yields 4.2 +/- 1.6Gyr. Most
surprisingly, we find the chemical abundance pattern to closely resemble the
solar one, in contrast to most known solar twins in the solar neighbourhood. We
confirm the solar-twin nature of M67-1194, the first solar twin known to belong
to a stellar association. This fact allows us to put some constraints on the
physical reasons for the seemingly systematic departure of M67-1194 and the Sun
from most known solar twins regarding chemical composition. We find that
radiative dust cleansing by nearby luminous stars may be the explanation for
the peculiar composition of both the Sun and M67-1194, but alternative
explanations are also possible. The chemical similarity between the Sun and
M67-1194 also suggests that the Sun once formed in a cluster like M67
Search for giant planets in M67 I. Overview
Precise stellar radial velocities are used to search for massive (Jupiter
masses or higher) exoplanets around the stars of the open cluster M67. We aim
to obtain a census of massive exoplanets in a cluster of solar metallicity and
age in order to study the dependence of planet formation on stellar mass and to
compare in detail the chemical composition of stars with and without planets.
This first work presents the sample and the observations, discusses the cluster
characteristics and the radial velocity (RV) distribution of the stars, and
individuates the most likely planetary host candidates. We observed a total of
88 main-sequence stars, subgiants, and giants all highly probable members of
M67, using four telescopes and instrument combinations. We investigate whether
exoplanets are present by obtaining radial velocities with precisions as good
as 10 m/s. To date, we have performed 680 single observations (Dec. 2011) and a
preliminary analysis of data, spanning a period of up to eight years. Although
the sample was pre-selected to avoid the inclusion of binaries, we identify 11
previously unknown binary candidates. Eleven stars clearly displayed larger RV
variability and these are candidates to host long-term substellar companions.
The average RV is also independent of the stellar magnitude and evolutionary
status, confirming that the difference in gravitational redshift between giants
and dwarfs is almost cancelled by the atmospheric motions. We use the subsample
of solar-type stars to derive a precise true RV for this cluster. We finally
create a catalog of binaries and use it to clean the color magnitude diagram
(CMD). As conclusion, by pushing the search for planets to the faintest
possible magnitudes, it is possible to observe solar analogues in open
clusters, and we propose 11 candidates to host substellar companions.Comment: 11 pages, 10 figure
Tracing the evolution of NGC6397 through the chemical composition of its stellar populations
With the aim to constrain multiple populations in the metal-poor globular
cluster NGC6397, we analyse and discuss the chemical compositions of a large
number of elements in 21 red giant branch stars. High-resolution spectra were
obtained with the FLAMES/UVES spectrograph on VLT. We have determined non-LTE
abundances of Na and LTE abundances for the remaining 21 elements, including O,
Mg, Al, alpha, iron-peak, and neutron-capture elements, many of which have not
previously been analysed for this cluster. We have also considered the
influence of possible He enrichment in the analysis of stellar spectra. We find
that the Na abundances of evolved, as well as unevolved, stars show a distinct
bimodality, which suggests the presence of two stellar populations; one
primordial stellar generation with composition similar to field stars, and a
second generation that is enriched in material processed through
hydrogen-burning (enriched in Na and Al and depleted in O and Mg). The cluster
is dominated (75%) by the second generation. The red giant branch show a
similar bimodal distribution in the Stroemgren colour index c_y=c_1-(b-y),
implying a large difference also in N abundance. The two populations have the
same composition of all analysed elements heavier than Al, within the
measurement uncertainty of the analysis, with the possible exception of [Y/Fe].
Using two stars with close to identical stellar parameters, one from each
generation, we estimate the difference in He content, Delta Y=0.01+-0.06, given
the assumption that the mass fraction of iron is the same for the stars.
Finally, we show that winds from fast rotating massive stars of the first
generation can be held responsible for the abundance patterns observed in
NGC6397 second generation long-lived stars and estimate that the initial mass
of the cluster were at least ten times higher than its present-day value.Comment: 13 pages + appendix with two tables. Accepted for publication in A&A.
v2: minor language corrections and Table A.2. correcte
The ability of intermediate-band Stromgren photometry to correctly identify dwarf, subgiant, and giant stars and provide stellar metallicities and surface gravities
[Abridged] Several large scale photometric and spectroscopic surveys are
being undertaken to provide a more detailed picture of the Milky Way. Given the
necessity of generalisation in the determination of, e.g., stellar parameters
when tens and hundred of thousands of stars are considered it remains important
to provide independent, detailed studies to verify the methods used in the
surveys. We evaluate available calibrations for deriving [M/H] from Stromgren
photometry and develop the standard sequences for dwarf stars to reflect their
metallicity dependence and test how well metallicities derived from ugriz
photometry reproduce metallicities derived from the well-tested system of
Stromgren photometry. We use a catalogue of dwarf stars with both Stromgren
uvby photometry and spectroscopically determined iron abundances (in total 451
dwarf stars with 0.3<(b-y)_0<1.0). We also evaluate available calibrations that
determine log g. A larger catalogue, in which metallicity is determined
directly from uvby photometry, is used to trace metallicity-dependent standard
sequences for dwarf stars. We derive new standard sequences in the c_1,0 versus
(b-y)_0 plane and in the c_1,0 versus (v-y)_0 plane for dwarf stars with 0.40 <
(b-y)_0 < 0.95 and 1.10 < (v-y)_0 < 2.38. We recommend the calibrations by
Ramirez & Me'endez (2005) for deriving metallicities from Stromgren photometry
and find that intermediate band photometry, such as Stromgren photometry, more
accurately than broad band photometry reproduces spectroscopically determined
[Fe/H]. Stromgren photometry is also better at differentiating between dwarf
and giant stars. We conclude that additional investigations of the differences
between metallicities derived from ugriz photometry and intermediate-band
photometry, such as Stromgren photometry, are required.Comment: Accepted for publication in A&A, 34 pages, including on-line materia
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