58 research outputs found
Titans metal-poor reference stars II. Red giants and CEMP stars
Representative samples of F-, G-, K-type stars located out of the Solar
Neighbourhood has started to be available in spectroscopic surveys. The
fraction of metal-poor ([Fe/H]~~dex) giants becomes increasingly
relevant to far distances. In metal-poor stars, effective temperatures
() based on LTE spectroscopy and on former
colour- relations of still wide use have been reported to be
inaccurate. It is necessary to re-calibrate chemical abundances based on these
scales in the multiple available surveys to bring them to
the same standard scale for their simultaneous use. For that, a complete sample
of standards is required, which so far, is restricted to a few stars with
quasi-direct measurements. We aim at providing a legacy
sample of metal-poor standards with proven accurate atmospheric parameters. We
add 47 giants to the sample of metal-poor dwarfs of Giribaldi et al. 2021,
thereby constituting the Titans metal-poor reference stars.
was derived by 3D non-LTE H modelling, whose accuracy was tested
against interferometry and InfraRed Flux Method (IRFM). Surface gravity (log
) was derived by fitting Mg~I~b triplet lines, whose accuracy was tested
against asteroseismology. Metallicity was derived using Fe II lines, which was
verified to be identical to the [Fe/H] derived from non-LTE spectral synthesis.
from 3D non-LTE H is equivalent to interferometric
and IRFM temperatures within a 46~K uncertainty. We achieved precision of
50~K for 34 stars with spectra with the highest S/N. For log , we
achieved a total uncertainty of 0.15~dex. For [Fe/H], we obtained a total
uncertainty of 0.09~dex. We find that the ionization equilibrium of Fe
lines under LTE is not valid in metal-poor giants.Comment: Accepted in A&
Red Horizontal Branch stars: an asteroseismic perspective
Robust age estimates of red giant stars are now possible thanks to the
precise inference of their mass based on asteroseismic constraints. However,
there are cases where such age estimates can be highly precise yet very
inaccurate. An example is giants that have undergone mass loss or mass transfer
events that have significantly altered their mass. In this context, stars with
"apparent" ages significantly higher than the age of the Universe are
candidates as stripped stars, or stars that have lost more mass than expected,
most likely via interaction with a companion star, or because of the poorly
understood mass-loss mechanism along the red-giant branch. In this work we
identify examples of such objects among red giants observed by
, both at low ([Fe/H] ) and solar metallicity.
By modelling their structure and pulsation spectra, we find a consistent
picture confirming that these are indeed low-mass objects consisting of a He
core of and an envelope of . Moreover, we find that these stars are characterised by a rather
extreme coupling () between the pressure-mode and gravity-mode
cavities, i.e. much higher than the typical value for red clump stars,
providing thus a direct seismic signature of their peculiar structure. The
complex pulsation spectra of these objects, if observed with sufficient
frequency resolution, hold detailed information about the structural properties
of likely products of mass stripping, hence can potentially shed light on their
formation mechanism. On the other hand, our tests highlight the difficulties
associated with measuring reliably the large frequency separation, especially
in shorter datasets, with impact on the reliability of the inferred masses and
ages of low-mass Red Clump stars with e.g. K2 or TESS data.Comment: Accepted for publication in A&A Letter
Photo-chemo-dynamical analysis and the origin of the bulge globular cluster Palomar 6
Context. Palomar 6 (Pal6) is a moderately metal-poor globular cluster projected towards the Galactic bulge. A full analysis of the cluster can give hints on the early chemical enrichment of the Galaxy and a plausible origin of the cluster. Aims. The aim of this study is threefold: a detailed analysis of high-resolution spectroscopic data obtained with the UVES spectro graph at the Very Large Telescope (VLT) at ESO, the derivation of the age and distance of Pal6 from Hubble Space Telescope (HST) photometric data, and an orbital analysis to determine the probable origin of the cluster. Methods. High-resolution spectra of six red giant stars in the direction of Pal6 were obtained at the 8 m VLT UT2-Kueyen telescope equipped with the UVES spectrograph in FLAMES+UVES configuration. Spectroscopic parameters were derived through excitation and ionisation equilibrium of Fe i and Fe ii lines, and the abundances were obtained from spectrum synthesis. From HST photometric data, the age and distance were derived through a statistical isochrone fitting. Finally, a dynamical analysis was carried out for the cluster assuming two different Galactic potentials. Results. Four stars that are members of Pal 6 were identified in the sample, which gives a mean radial velocity of 174.3 ± 1.6 km s−1 and a mean metallicity of [Fe/H] = −1.10 ± 0.09 for the cluster. We found an enhancement of α-elements (O, Mg, Si, and Ca) of 0.29 +0.30, this being evidence of a second stellar population, further confirmed with the NaON-Al (anti)correlations. For the first time, we derived the age of Pal 6, which resulted to be 12.4±0.9 Gyr. We also found a low extinction coefficient RV = 2.6 for the Pal 6 projection, which is compatible with the latest results for the highly extincted bulge populations. The derived extinction law results in a distance of 7.67 ± 0.19 kpc from the Sun with an AV = 4.21 ± 0.05. The chemical and photometric analyses combined with the orbital-dynamical analyses point out that Pal 6 belongs to the bulge component probably formed in the main-bulge progenitor. Conculsions. The present analysis indicates that the globular cluster Pal 6 is located in the bulge volume and that it was probably formed in the bulge in the early stages of the Milky Way formation, sharing the chemical properties with the family of intermediate metallicity very old clusters M 62, NGC 6522, NGC 6558, and HP 1
Using seismic targets as benchmarks for spectroscopic analyses of cool stars
The frequency of maximum oscillation power measured in dwarfs and giants exhibiting solar-like pulsations provides a precise, and potentially accurate, inference of the stellar surface gravity. An extensive comparison for about 40 well-studied pulsating stars with gravities derived by traditional means (ionization balance, pressure-sensitive spectral features or location with respect to evolutionary tracks) supports the validity of this technique and reveals an overall remarkable agreement with mean differences not exceeding 0.05 dex (although with a dispersion of up to ~0.2 dex). It is argued that interpolation in theoretical isochrones may be the most precise way of estimating the gravity by traditional means in nearby dwarfs. The use of seismic targets as benchmarks in the context of forthcoming large-scale surveys (such as the follow up of the Gaia mission) is briefly discussed.Peer reviewe
StarHorse results for spectroscopic surveys + Gaia DR3: Chrono-chemical populations in the solar vicinity, the genuine thick disk, and young-alpha rich stars
The Gaia mission has provided an invaluable wealth of astrometric data for
more than a billion stars in our Galaxy. The synergy between Gaia astrometry,
photometry, and spectroscopic surveys give us comprehensive information about
the Milky Way. Using the Bayesian isochrone-fitting code StarHorse, we derive
distances and extinctions for more than 10 million unique stars observed by
both Gaia Data Release 3 as well as public spectroscopic surveys: GALAH DR3,
LAMOST DR7 LRS, LAMOST DR7 MRS, APOGEE DR17, RAVE DR6, SDSS DR12 (optical
spectra from BOSS and SEGUE), Gaia-ESO DR5 survey, and Gaia RVS part of Gaia
DR3 release. We use StarHorse for the first time to derive stellar age for
main-sequence turnoff and subgiant branch stars (MSTO-SGB), around 2.5 million
stars with age uncertainties typically around 30%, 15% for only SGB stars,
depending on the resolution of the survey. With the derived ages in hand, we
investigate the chemical-age relations. In particular, the and
neutron-capture element ratios versus age in the solar neighbourhood show
trends similar to previous works, validating our ages. We use the chemical
abundances from local subgiant samples of GALAH DR3, APOGEE DR17 and LAMOST MRS
DR7 to map groups with similar chemical compositions and StarHorse ages with
the dimensionality reduction technique t-SNE and the clustering algorithm
HDBSCAN. We identify three distinct groups in all three samples. Their
kinematic properties confirm them to be the genuine chemical thick disk, the
thin disk and a considerable number of young alpha-rich stars. We confirm that
the genuine thick disk's kinematics and age properties are radically different
from those of the thin disk and compatible with high-redshift (z2)
star-forming disks with high dispersion velocities.Comment: 27 pages, 19 figures. Accepted for publication in Astronomy &
Astrophysics. Catalogues can be downloaded at https://data.aip.de
Stellar Astrophysics and Exoplanet Science with the Maunakea Spectroscopic Explorer (MSE)
The Maunakea Spectroscopic Explorer (MSE) is a planned 11.25-m aperture
facility with a 1.5 square degree field of view that will be fully dedicated to
multi-object spectroscopy. A rebirth of the 3.6m Canada-France-Hawaii Telescope
on Maunakea, MSE will use 4332 fibers operating at three different resolving
powers (R ~ 2500, 6000, 40000) across a wavelength range of 0.36-1.8mum, with
dynamical fiber positioning that allows fibers to match the exposure times of
individual objects. MSE will enable spectroscopic surveys with unprecedented
scale and sensitivity by collecting millions of spectra per year down to
limiting magnitudes of g ~ 20-24 mag, with a nominal velocity precision of ~100
m/s in high-resolution mode. This white paper describes science cases for
stellar astrophysics and exoplanet science using MSE, including the discovery
and atmospheric characterization of exoplanets and substellar objects, stellar
physics with star clusters, asteroseismology of solar-like oscillators and
opacity-driven pulsators, studies of stellar rotation, activity, and
multiplicity, as well as the chemical characterization of AGB and extremely
metal-poor stars.Comment: 31 pages, 11 figures; To appear as a chapter for the Detailed Science
Case of the Maunakea Spectroscopic Explore
Fast core rotation in red-giant stars revealed by gravity-dominated mixed modes
When the core hydrogen is exhausted during stellar evolution, the central
region of a star contracts and the outer envelope expands and cools, giving
rise to a red giant, in which convection occupies a large fraction of the star.
Conservation of angular momentum requires that the cores of these stars rotate
faster than their envelopes, and indirect evidence supports this. Information
about the angular momentum distribution is inaccessible to direct observations,
but it can be extracted from the effect of rotation on oscillation modes that
probe the stellar interior. Here, we report the detection of non-rigid rotation
in the interiors of red-giant stars by exploiting the rotational frequency
splitting of recently detected mixed modes. We demonstrate an increasing
rotation rate from the surface of the star to the stellar core. Comparing with
theoretical stellar models, we conclude that the core must rotate at least ten
times faster than the surface. This observational result confirms the
theoretical prediction of a steep gradient in the rotation profile towards the
deep stellar interior.Comment: to appear as a Letter to Natur
The sixth data release of the Radial Velocity Experiment (RAVE). I. Survey description, spectra and radial velocities
The Radial Velocity Experiment (RAVE) is a magnitude-limited (9<I<12)
spectroscopic survey of Galactic stars randomly selected in the southern
hemisphere. The RAVE medium-resolution spectra (R~7500) cover the Ca-triplet
region (8410-8795A). The 6th and final data release (DR6 or FDR) is based on
518387 observations of 451783 unique stars. RAVE observations were taken
between 12 April 2003 and 4 April 2013. Here we present the genesis, setup and
data reduction of RAVE as well as wavelength-calibrated and flux-normalized
spectra and error spectra for all observations in RAVE DR6. Furthermore, we
present derived spectral classification and radial velocities for the RAVE
targets, complemented by cross matches with Gaia DR2 and other relevant
catalogs. A comparison between internal error estimates, variances derived from
stars with more than one observing epoch and a comparison with radial
velocities of Gaia DR2 reveals consistently that 68% of the objects have a
velocity accuracy better than 1.4 km/s, while 95% of the objects have radial
velocities better than 4.0 km/s. Stellar atmospheric parameters, abundances and
distances are presented in subsequent publication. The data can be accessed via
the RAVE Web (http://rave-survey.org) or the Vizier database.Comment: 32 pages, 11 figures, accepted for publication to A
The 16th Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra
This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17)
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