| 1. |
- Bergemann, Maria, et al.
(författare)
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The Gaia-ESO Survey : Hydrogen lines in red giants directly trace stellar mass
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 594
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Tidskriftsartikel (refereegranskat)abstract
- Red giant stars are perhaps the most important type of stars for Galactic and extra-galactic archaeology: they are luminous, occur in all stellar populations, and their surface temperatures allow precise abundance determinations for many different chemical elements. Yet, the full star formation and enrichment history of a galaxy can be traced directly only if two key observables can be determined for large stellar samples: age and chemical composition. While spectroscopy is a powerful method to analyse the detailed abundances of stars, stellar ages are the missing link in the chain, since they are not a direct observable. However, spectroscopy should be able to estimate stellar masses, which for red giants directly infer ages provided their chemical composition is known. Here we establish a new empirical relation between the shape of the hydrogen line in the observed spectra of red giants and stellar mass determined from asteroseismology. The relation allows determining stellar masses and ages with an accuracy of 10-15%. The method can be used with confidence for stars in the following range of stellar parameters: 4000 < T-eff < 5000 K, 0.5 < log g < 3.5, -2.0 < [ Fe/H] < 0.3, and luminosities log L/L-Sun < 2.5. Our analysis provides observational evidence that the H-alpha spectral characteristics of red giant stars are tightly correlated with their mass and therefore their age. We also show that the method samples well all stellar populations with ages above 1 Gyr. Targeting bright giants, the method allows obtaining simultaneous age and chemical abundance information far deeper than would be possible with asteroseismology, extending the possible survey volume to remote regions of the Milky Way and even to neighbouring galaxies such as Andromeda or the Magellanic Clouds even with current instrumentation, such as the VLT and Keck facilities.
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| 2. |
- Franchini, Mariagrazia, et al.
(författare)
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The Gaia-ESO Survey : Carbon Abundance in the Galactic Thin and Thick Disks
- 2020
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Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 888:2
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Tidskriftsartikel (refereegranskat)abstract
- This paper focuses on carbon, which is one of the most abundant elements in the universe and is of high importance in the field of nucleosynthesis and galactic and stellar evolution. The origin of carbon and the relative importance of massive and low- to intermediate-mass stars in producing it is still a matter of debate. We aim at better understanding the origin of carbon by studying the trends of [C/H], [C/Fe], and [C/Mg] versus [Fe/H] and [Mg/H] for 2133 FGK dwarf stars from the fifth Gaia-ESO Survey internal data release (GES iDR5). The availability of accurate parallaxes and proper motions from Gaia DR2 and radial velocities from GES iDR5 allows us to compute Galactic velocities, orbits, absolute magnitudes, and, for 1751 stars, Bayesian-derived ages. Three different selection methodologies have been adopted to discriminate between thin- and thick-disk stars. In all the cases, the two stellar groups show different [C/H], [C/Fe], and [C/Mg] and span different age intervals, with the thick-disk stars being, on average, older than the thin-disk ones. The behaviors of [C/H], [C/Fe], and [C/Mg] versus [Fe/H], [Mg/H], and age all suggest that C is primarily produced in massive stars. The increase of [C/Mg] for young thin-disk stars indicates a contribution from low-mass stars or the increased C production from massive stars at high metallicities due to the enhanced mass loss. The analysis of the orbital parameters R-med and supports an "inside-out" and "upside-down" formation scenario for the disks of the Milky Way.
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| 3. |
- Franchini, Mariagrazia, et al.
(författare)
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The Gaia-ESO Survey : Oxygen Abundance in the Galactic Thin and Thick Disks*
- 2021
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Ingår i: Astronomical Journal. - : Institute of Physics Publishing (IOPP). - 0004-6256 .- 1538-3881. ; 161:1
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the oxygen abundances of a stellar sample representative of the two major Galactic populations: the thin and thick disks. The aim is to investigate the differences between members of the Galactic disks and contribute to the understanding of the origin of oxygen chemical enrichment in the Galaxy. The analysis is based on the [O i] = 6300.30 A oxygen line in high-resolution spectra (R similar to 52,500) obtained from the Gaia-ESO public spectroscopic Survey (GES). By comparing the observed spectra with a theoretical data set computed in LTE with the SPECTRUM synthesis and ATLAS12 codes, we derive the oxygen abundances of 516 FGK dwarfs for which we have previously measured carbon abundances. Based on kinematic, chemical, and dynamical considerations, we identify 20 thin and 365 thick disk members. We study the potential trends of both subsamples in terms of their chemistry ([O/H], [O/Fe], [O/Mg], and [C/O] versus [Fe/H] and [Mg/H]), age, and position in the Galaxy. The main results are that (a) [O/H] and [O/Fe] ratios versus [Fe/H] show systematic differences between thin and thick disk stars with an enhanced O abundance of thick disk stars with respect to thin disk members and a monotonic decrement of [O/Fe] with increasing metallicity, even at metal-rich regime; (b) there is a smooth correlation of [O/Mg] with age in both populations, suggesting that this abundance ratio can be a good proxy of stellar ages within the Milky Way; and (c) thin disk members with [Fe/H] 0 display a [C/O] ratio smaller than the solar value, suggesting a possibly outward migration of the Sun from lower Galactocentric radii.
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| 4. |
- Tautvaisiene, Grazina, et al.
(författare)
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Abundances of C, N and O as Probes of Mixing in Low-Mass Helium-Core Burning Stars
- 2006
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Ingår i: Convection in Astrophysics. - 1886733953 ; , s. 66-
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Konferensbidrag (populärvet., debatt m.m.)abstract
- Low-mass stars exhibit signatures of extra-mixing processes that require modeling beyond the standard stellar theory and observational data for stars of different masses and metallicity. Abundances of 12C, 13C, N and O chemical elements in atmospheres of 17 low-mass helium-core burning stars were determined from the high-resolution spectra obtained on the Nordic Optical Telescope with the SOFIN spectrograph. The stars belong to the red clump of the Galaxy and according to the analysis are in the metallicity range of ‑0.28 < [Fe/H] < 0.14 and mass range 1.2 < M /M[Sun]< 2.3. Abundances of carbon were studied using the C[2 ]Swan (0,1) band head at 5635.5 Å. The wavelength interval 7980-8130 Å with strong CN features was analysed in order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 Å. In comparison to the chemical composition in field dwarfs, the results show that carbon abundance is depleted and [C/H] is lowered in all the star by about 0.2 dex which is in agreement with results for other stars of this type and the theory of the 1st dredge-up. The results of C/N quite closely follow the C/N vs. mass trend predicted by the 1st dredge-up models. The extra-mixing model by Boothroyd & Sackmann (1999, ApJ, 510, 232) does not predict C/N changes to be noticeable at this metallicity and mass as well. Comparing the results with the same theoretical study by Boothroyd & Sackmann (1999) in the 12C/13C, mass plot the stars separate into two groups, one of which follow the trend of 1st dredge-up with 12C/13C ratio being in the range of 20-30, depending of mass. The 12C/13C ratios of the other group of stars is equal to about 15 which show the evidence of extra-mixing. The analysis of a larger sample of Galaxy red clump stars as well as of metal deficient horizontal branch stars is needed for further progress in revealing of extent of mixing in atmospheres of low mass stars.
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| 5. |
- Tautvaisiene, Grazina, et al.
(författare)
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Abundances of C, N and O as probes of mixing in low-mass helium-core burning stars
- 2007
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Ingår i: Convection in Astrophysics. ; , s. 301-303
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A large population of helium-core-burning clump stars of the Galactic field, revealed by the Hipparcos orbiting observatory, is an excellent target for the analysis of mixing processes in evolved low-mass metal-abundant stars. In this contribution we overview first result of 12C, 13C, N and O abundance determinations in our extensive project which we started with the aim at deriving the fundamental parameters and abundances of more than 20 chemical elements in a large sample of Galactic clump stars.
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| 6. |
- Tautvaisiene, Grazina, et al.
(författare)
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alpha-process Elements in the Galaxy: A Possible GAIA Contribution
- 2002
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Ingår i: Astrophysics and Space Science. ; , s. 143-150
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The sensitivity of stellar spectra to α/Fe abundance changes is investigated with the aim to be detected photometrically and employed for the scientific goals of the GAIA mission. A grid of plane parallel, line blanketed, flux constant, LTE model atm
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| 7. |
- Tautvaisiene, Grazina, et al.
(författare)
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Chemical composition of evolved stars in the open cluster M 67
- 2000
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Ingår i: ASTRONOMY AND ASTROPHYSICS. - 0004-6361. ; 360:2, s. 499-508
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution spectra of six core helium-burning 'clump' stars and three giants in the open cluster M 67 have been obtained with the SOFIN spectrograph on the Nordic Optical Telescope to investigate abundances of up to 25 chemical elements. Abundances o
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| 8. |
- Tautvaisiene, Grazina, et al.
(författare)
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Chemical composition of evolved stars in the open Cluster NGC 7789
- 2005
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 431:3, s. 933-942
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution spectra of six giants and three core-helium-burning “clump” stars in the open cluster NGC 7789 have been obtained with the SOFIN spectrograph on the Nordic Optical Telescope to investigate abundances of up to 20 chemical elements. Abundances of carbon were studied using the C2 Swan (0, 1) band head at 5635.5 Å. The wavelength interval 7980 8130 Å with strong CN features was analysed in order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 Å. The overall metallicity of evolved stars in the cluster was found to be close to solar ([Fe/H]=-0.04±0.05). Compared with the Sun and other dwarf stars of the Galactic disk, mean abundances in the investigated giant stars suggest that carbon is depleted by about 0.2 dex, and nitrogen and oxygen are close to solar. In the clump stars investigated, carbon is depleted by about 0.2 dex, the mean abundance of nitrogen is enhanced by 0.26 dex and oxygen is lower by 0.14 dex. This has the effect of lowering the mean C/N ratios to the value of 1.9±0.5 in the giant stars and to the value of 1.3±0.2 in the clump stars. The mean 12C/13C ratios are lowered to about the same value of 9±1 in the giants and clump stars investigated. Concerning other chemical elements an overabundance of sodium is noticeable and of silicon and calcium one is suspected. Abundances of iron-group and heavier chemical elements in all nine stars were found to be close to solar.
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| 9. |
- Tautvaisiene, Grazina, et al.
(författare)
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Selecting a photometric system for GAIA: C, N, O and alpha-process elements
- 2003
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Ingår i: Baltic Astronomy. - 1392-0049. ; 12:4, s. 532-538
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Tidskriftsartikel (refereegranskat)abstract
- The sensitivity of stellar spectra to C, N, O and alpha-process element abundances is discussed with the aim to be taken into account when selecting a photometric system for the GAIA orbiting observatory. On the basis of a spectrometric, photometric and theoretical study of spectra of evolved first-ascent giants and clump stars in the open cluster NGC 7789 it is demonstrated that evolutionary alterations of carbon and nitrogen abundances can cause noticeable spectral changes and, if not taken into account, may yield misleading photometric [Fe/H] determinations. Carbon features in stellar atmospheres show a particularly complex behavior being dependent on mixing processes in stars, on the stellar surface gravity and on the abundance of oxygen which can also be altered by different reasons. NH bands could serve for the evaluation of mixing processes in stars and the interpretation of carbon dominated spectral regions. Abundances of alpha-process elements can be evaluated photometrically by using the direct indicators -- Ca II H and K lines and Mg I b triplet.
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| 10. |
- Tautvaisiene, Grazina, et al.
(författare)
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Stellar abundance anomalies: open cluster NGC 7789
- 2004
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Ingår i: Stars as Suns: Activity, Evolution and Planets. - 158381163X ; , s. 985-989
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- High-resolution spectra of six giants and three core-helium-burning "clump" stars in the open cluster NGC 7789 have been obtained on the Nordic Optical Telescope to investigate abundances of up to 19 chemical elements and to compare the elemental abundances in differently evolved stars. The overall metallicity of the cluster stars was found to be close to solar ([Fe/H]=-0.04 +- 0.05). Compared with the Sun, mean abundances in the investigated giants and clump stars suggest that carbon is underabundant by approximately the same value of 0.2 dex, the 12C/13C ratios lie in the range from 11 to 7. However, the nitrogen overabundance is larger in the clump stars. The mean C/N ratios are lowered to the values of 1.3 +- 0.2 in the clump stars and to the values of 1.9 +- 0.5 in the giants. Among other chemical elements an overabundance of sodium is noticeable and for some other alpha-elements may be suspected. Abundances of heavy chemical elements in all nine stars were found to be close to solar.
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