35 research outputs found
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
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
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
A spectroscopic and proper motion search of Sloan Digital Sky Survey : red subdwarfs in binary systems
Red subdwarfs in binary systems are crucial for both model calibration and spectral classification. We search for red subdwarfs in binary systems from a sample of high proper motion objects with Sloan Digital Sky Survey spectroscopy. We present here discoveries from this search, as well as highlight several additional objects of interest. We find 30 red subdwarfs in wide binary systems including: two with spectral type of esdM5.5, 6 companions to white dwarfs and 3 carbon-enhanced red subdwarfs with normal red subdwarf companions. 15 red subdwarfs in our sample are partially resolved close binary systems. With this binary sample, we estimate the low limit of the red subdwarf binary fraction of similar to 10 per cent. We find that the binary fraction goes down with decreasing masses and metallicities of red subdwarfs. A spectroscopic esdK7 subdwarf + white dwarf binary candidate is also reported. 30 new M subdwarfs have spectral type of >= M6 in our sample. We also derive relationships between spectral types and absolute magnitudes in the optical and near-infrared for M and L subdwarfs, and we present an M subdwarf sample with measured U, V, W space velocities.Peer reviewe
The Gaia-ESO Survey : The analysis of high-resolution UVES spectra of FGK-type stars
Date of Acceptance: 01/09/2014Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using FLAMES at the VLT to obtain high-quality medium-resolution Giraffe spectra for about 105 stars and high-resolution UVES spectra for about 5000 stars. With UVES, the Survey has already observed 1447 FGK-type stars. Aims. These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO second internal release and will be part of its first public release of advanced data products. Methods. The final parameter scale is tied to the scale defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. In addition, a set of open and globular clusters is used to evaluate the physical soundness of the results. Each of the implemented methodologies is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted medians of those from the individual methods. Results. The recommended results successfully reproduce the atmospheric parameters of the benchmark stars and the expected Teff-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55 K for Teff, 0.13 dex for log g and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50-100 K for Teff, 0.10-0.25 dex for log g and 0.05-0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex. Conclusions. The Gaia-ESO sample of high-resolution spectra of FGK-type stars will be among the largest of its kind analyzed in a homogeneous way. The extensive list of elemental abundances derived in these stars will enable significant advances in the areas of stellar evolution and Milky Way formation and evolution.Peer reviewe
Giants reveal what dwarfs conceal: Li abundance in lower RGB stars as diagnostic of the primordial Li
The discrepancy between cosmological Li abundance inferred from Population II
dwarf stars and that derived from WMAP/BBNS is still far from being solved.We
investigated, as an alternative route, the use of Li abundances in Population
II lower RGB stars as empirical diagnostic of the cosmological Li. Both theory
and observations suggest that the surface A(Li) in red giants after the
completion of the first dredge-up and before the RGB bump, are significantly
less sensitive to the efficiency of atomic diffusion, compared with dwarf
stars. Standard stellar models computed under different physical assumptions
show that the inclusion of the atomic diffusion has an impact of 0.07dex in the
determination of A(Li)0 (much smaller than the case of MS stars) and it is
basically unaffected by reasonable variations of other parameters
(overshooting, age,initial Y, mixing length). We have determined the surface Li
content of 17 Halo lower RGB stars,in the metallicity range [Fe/H]=-3.4 /-1.4
dex. The initial Li has then been inferred by accounting for the difference
between initial and post-dredge up A(Li) in the appropriate stellar models. It
depends mainly on the used T(eff) scale and is only weakly sensitive to the
efficiency of atomic diffusion,so long as one neglects Li destruction caused by
the process competing with atomic diffusion. Final A(Li)0 span a relatively
narrow range (2.28 /2.46 dex), and is 0.3-0.4 dex lower the WMAP/BBNS
predictions. These values of A(Li)0 are corroborated by the analysis of the GCs
NGC6397, NGC6752 and M4. Our result provides an independent quantitative
estimate of the difference with the Big Bang value and sets a very robust
constraint for the physical processes invoked to resolve this discrepancy.Comment: Accepted for publication in MNRAS (12 pages, 4 figures, 3 tables
M-dwarf metallicities - A high-resolution spectroscopic study in the near infrared
The relativley large spread in the derived metallicities ([Fe/H]) of M dwarfs
shows that various approaches have not yet converged to consistency. The
presence of strong molecular features, and incomplete line lists for the
corresponding molecules have made metallicity determinations of M dwarfs
difficult. Furthermore, the faint M dwarfs require long exposure times for a
signal-to-noise ratio sufficient for a detailed spectroscopic abundance
analysis. We present a high-resolution (R~50,000) spectroscopic study of a
sample of eight single M dwarfs and three wide-binary systems observed in the
infrared J-band. The absence of large molecular contributions allow for a
precise continuum placement. We derive metallicities based on the best fit
synthetic spectra to the observed spectra. To verify the accuracy of the
applied atmospheric models and test our synthetic spectrum approach, three
binary systems with a K-dwarf primary and an M-dwarf companion were observed
and analysed along with the single M dwarfs. We obtain a good agreement between
the metallicities derived for the primaries and secondaries of our test
binaries and thereby confirm the reliability of our method of analysing M
dwarfs. Our metallicities agree well with certain earlier determinations, and
deviate from others. We conclude that spectroscopic abundance analysis in the J
band is a reliable method for establishing the metallicity scale for M dwarfs.
We recommend its application to a larger sample covering lower as well as
higher metallicities. Further prospects of the method include abundance
determinations for individual elements.Comment: 14 pages, 6 figures, 6 tables, accepted for publication in Astronomy
and Astrophysic