122 research outputs found
Calculs de modèles d'atmosphères hors-ETL avec métaux pour les étoiles de type sdO : le cas particulier de SDSS J1600+0748
Nous présentons nos grilles de modèles d'atmosphères pour les étoiles sous-naines chaudes de type O (sdO) soit : des modèles classiques hors-ETL H, He, des modèles hors-ETL avec, en plus, du C, N, O et finalement des modèles incluant C, N, O, Fe. En utilisant les raies de Balmer et d'hélium dans le domaine du visible, nous avons fait des comparaisons entre les spectres théoriques de nos différentes grilles afin de caractériser les effets des métaux. On trouve que ces effets dépendent à la fois de la température et de la gravité. De plus, l'abondance d'hélium a une influence importante sur les effets des métaux; une abondance d'hélium faible (log N(He)/N(H) < -1,5) occasionne des effets assez importants alors qu'une abondance plus élevée tend à réduire ces mêmes effets. Nous avons aussi trouvé que l'ajout du fer (en abondance solaire) ne cause que des changements relativement faibles à la structure en température et, par le fait même, aux profils des raies d'hydrogène et d'hélium, par rapport aux changements déjà produits par le C, N, O (en abondance solaire). Nous avons utilisé nos grilles pour faire une analyse spectroscopique du spectre à haut signal sur bruit (180) et basse
résolution (9 Å) de SDSS J160043.6+074802.9 obtenu au télescope Bok. Notre meilleure ajustement a été obtenu avec notre grille de spectres synthétiques incluant C, N, O et Fe en quantité solaire, menant aux paramètres suivants : Teff = 68 500 ± 1770 K, log g = 6,09 ± 0,07, and log N(He)/N(H) = -0,64 ± 0,05, où les incertitudes proviennent uniquement de la procédure d'ajustement. Ces paramètres atmosphériques, particulièrement la valeur de l'abondance d'hélium, placent notre étoile dans une région où les effets des métaux ne sont pas très marqués.We present our new grids of model atmospheres and spectra for hot subdwarf O (sdO) stars: standard NLTE H+He models with no metals, NLTE line-blanketed models with C+N+O, and NLTE line-blanketed models with
C+N+O+Fe. Using hydrogen and helium lines in the optical range, we make detailed comparisons between theoretical spectra of different grids in order to characterize the line blanketing effects of metals. We find
these effects to be dependent on both the effective temperature and the surface gravity. Moreover, we find that the helium abundance also influences in an important way the effects of line blanketing on the
resulting spectra: a low helium abundance (log N(He)/N(H) < -1.5) leads to relatively large effects, while a high helium abundance
tends to reduce their magnitudes. We also find that the addition of Fe (solar abundance) leads only to incremental effects on the atmospheric structure and, hence, on the model line profiles of H and He as compared to the case where the metallicity is defined by C+N+O (solar abundances). We use our grids to perform fits on a 9 Å resolution, high S/N (180) optical spectrum of SDSS J160043.6+074802.9, this (currently) unique pulsating sdO, that we gathered at the Bok Telescope. Our best and most reliable result is based on the fit achieved with NLTE synthetic spectra that include C, N, O, and Fe in solar abundances, leading to the following parameters : Teff = 68 500 ± 1770 K, log g = 6.09 ± 0.07, and log N(He)/N(H) = -0.64 ± 0.05 (formal fitting errors only). This combination of parameters, particularly the comparatively high helium abundance, implies that line blanketing effects due to metals are not very large in the atmosphere of this sdO star
Heavy metals in intermediate He-rich hot subdwarfs: The chemical composition of HZ44 and HD127493
Hot subluminous stars can be spectroscopically classified as subdwarf B (sdB)
and O (sdO) stars. While the latter are predominantly hydrogen deficient, the
former are mostly helium deficient. The atmospheres of most sdOs are almost
devoid of hydrogen, whereas a small group of hot subdwarf stars of mixed H/He
composition exists, showing extreme metal abundance anomalies. Whether such
intermediate helium-rich (iHe) subdwarf stars provide an evolutionary link
between the dominant classes is an open question. The presence of strong Ge,
Sn, and Pb lines in the UV spectrum of HZ44 suggests a strong enrichment of
heavy elements in this iHe-sdO star and calls for a detailed quantitative
spectral analysis focusing on trans-iron elements. Non-LTE model atmospheres
calculated with TLUSTY are combined with high-quality optical, UV and FUV
spectra of HZ44 and its hotter sibling HD127493 to determine their
atmospheric parameters and metal abundance patterns. By collecting atomic data
from literature we succeeded to determine abundances of 29 metals in HZ44,
including the trans-iron elements Ga, Ge, As, Se, Zr, Sn, and Pb and provide
upper limits for 10 other metals. This makes it the best described hot subdwarf
in terms of chemical composition. For HD127493 the abundance of 15 metals,
including Ga, Ge, and Pb and upper limits for another 16 metals were derived.
Heavy elements turn out to be overabundant by one to four orders of magnitude
with respect to the Sun. Zr and Pb are among the most enriched elements. The C,
N, and O abundance for both stars can be explained by nucleosynthesis of
hydrogen burning in the CNO cycle along with their helium enrichment. On the
other hand, the heavy-element anomalies are unlikely to be caused by
nucleosynthesis. Instead diffusion processes are evoked with radiative
levitation overcoming gravitational settlement of the heavy elements.Comment: Accepted for publication in Astronomy & Astrophysic
Heavy-metal enrichment in the intermediate He-sdOB pulsator Feige 46
The intermediate He-enriched hot subdwarf star Feige 46 was recently reported
as the second member of the V366 Aqr (or He-sdOBV) pulsating class. Feige 46 is
very similar to the prototype of the class, LS IV14116, not only in terms of
pulsational properties, but also in terms of atmospheric parameters and
kinematic properties. LS IV14116 is additionally characterized by a very
peculiar chemical composition, with extreme overabundances of the trans-iron
elements Ge, Sr, Y, and Zr. In this paper, we investigate the possibility that
the similitude between both pulsators extends to their chemical composition. We
retrieved archived optical and UV spectroscopic observations of Feige 46 and
perform an abundance analysis using model atmospheres and synthetic spectra
computed with TLUSTY and SYNSPEC. In total, we derive abundances for 16
metallic elements and provide upper limits for four additional elements. From
absorption lines in the optical spectrum of the star we measure an enrichment
of more than 10 000 solar for yttrium and zirconium. As for strontium,
the UV spectrum revealed it to be equally enriched. Our results confirm that
Feige 46 is not only a member of the now growing group of "heavy-metal"
subdwarfs, but also has an abundance pattern remarkably similar to that of LS
IV14116.Comment: 12 pages, Accepted for publication in A&
Observations of the ultraviolet-bright star Y453 in the globular cluster M4 (NGC 6121)
We present a spectral analysis of the UV-bright star Y453 in M4. Model fits to the star´s optical spectrum yield Teff ∼ 56,000 K. Fits to the star´s FUV spectrum, obtained with the Cosmic Origins Spectrograph on board the Hubble Space Telescope, reveal it to be considerably hotter, with Teff ∼ 72,000 K. We adopt Teff = 72,000 ± 2000 K and log g = 5.7 ± 0.2 as our best-fit parameters. Scaling the model spectrum to match the star´s optical and near-infrared magnitudes, we derive a mass M∗ = 0.53±0.24 M⊙ and luminosity = log L/L⊙ 2.84±0.05, consistent with the values expected of an evolved star in a globular cluster. Comparing the star with post-horizontal-branch evolutionary tracks, we conclude that it most likely evolved from the blue horizontal branch, departing the asymptotic giant branch before third dredge-up. It should thus exhibit the abundance pattern (O-poor and Na-rich) characteristic of the second-generation (SG) stars in M4. We derive the star´s photospheric abundances of He, C, N, O, Si, S, Ti, Cr, Fe, and Ni. CNO abundances are roughly 0.25 dex greater than those of the cluster´s SG stars, while the Si and S abundances match the cluster values. Abundances of the iron-peak elements (except for iron itself) are enhanced by 1-3 dex. Rather than revealing the star´s origin and evolution, this pattern reflects the combined effects of diffusive and mechanical processes in the stellar atmosphere.Fil: Dixon, William V.. Space Telescope Science Institute; Estados UnidosFil: Chayer, Pierre. Space Telescope Science Institute; Estados UnidosFil: Latour, Marilyn. Friedrich-alexander University Erlangen-nuremberg; Estados UnidosFil: Miller Bertolami, Marcelo Miguel. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - La Plata. Instituto de AstrofÃsica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y GeofÃsicas. Instituto de AstrofÃsica La Plata; ArgentinaFil: Benjamin, Robert A.. University Of Wisconsin Whitewater; Estados Unido
SHOTGLAS I: The ultimate spectroscopic census of extreme horizontal branch stars in Centauri
The presence of extreme horizontal branch (EHB) and blue hook stars in some
Galactic globular clusters (GGCs) constitutes one of the remaining mysteries of
stellar evolution. In this paper, we focus on Cen, a peculiar, massive
GGC that hosts multiple stellar populations. We use non-LTE model atmospheres
to derive atmospheric parameters (Teff, log g and N(He)/N(H)) and spectroscopic
masses for 152 EHB stars in the cluster. This constitutes the largest
spectroscopic sample of EHB stars ever analyzed in a GGC and represents ~20% of
the EHB population of Cen. We also search for close binaries among
these stars based on radial velocity variations. Our results show that the EHB
population of Cen is divided into three spectroscopic groups that are
very distinct in the Teff - helium abundance plane. The majority of our sample
consists of sdOB stars that have roughly solar or super-solar atmospheric
helium abundances. It is these objects that constitute the blue hook at
18.5 mag in the Cen color-magnitude diagram. Interestingly, the
helium-enriched sdOBs do not have a significant counterpart population in the
Galactic field. Another major difference between the EHB stars in Cen
and the field is the fraction of close binaries. From our radial velocity
survey we identify two binary candidates and estimate an EHB close binary
fraction of ~5% in Cen. This low fraction is in line with findings for
other GGCs, but in sharp contrast to the situation in the field, where around
50% of the sdB stars reside in close binaries. Finally, the mass distribution
derived is very similar for all three spectroscopic groups, however the average
mass (0.38 Msun) is lower than that expected from stellar evolution theory.
While this mass conundrum has previously been noted for EHB stars in
Cen, it so far appears to be unique to that cluster.Comment: 25 pages, Accepted for publication in A&
Observations of the Ultraviolet-bright Star Y453 in the Globular Cluster M4 (NGC 6121)
We present a spectral analysis of the UV-bright star Y453 in M4. Model fits to the star's optical spectrum yield Teff ∼ 56,000 K. Fits to the star's FUV spectrum, obtained with the Cosmic Origins Spectrograph on board the Hubble Space Telescope, reveal it to be considerably hotter, with Teff ∼ 72,000 K. We adopt Teff = 72,000 ± 2000 K and log g = 5.7 ± 0.2 as our best-fit parameters. Scaling the model spectrum to match the star's optical and near-infrared magnitudes, we derive a mass M∗ = 0.53±0.24 M⊙ and luminosity = log L/L⊙ 2.84±0.05, consistent with the values expected of an evolved star in a globular cluster. Comparing the star with post-horizontal-branch evolutionary tracks, we conclude that it most likely evolved from the blue horizontal branch, departing the asymptotic giant branch before third dredge-up. It should thus exhibit the abundance pattern (O-poor and Na-rich) characteristic of the second-generation (SG) stars in M4. We derive the star's photospheric abundances of He, C, N, O, Si, S, Ti, Cr, Fe, and Ni. CNO abundances are roughly 0.25 dex greater than those of the cluster's SG stars, while the Si and S abundances match the cluster values. Abundances of the iron-peak elements (except for iron itself) are enhanced by 1-3 dex. Rather than revealing the star's origin and evolution, this pattern reflects the combined effects of diffusive and mechanical processes in the stellar atmosphere.Instituto de AstrofÃsica de La Plat
Kinematic differences between multiple populations in Galactic globular clusters
The formation process of multiple populations in globular clusters is still
up for debate. Kinematic differences between the populations are particularly
interesting in this respect, because they allow us to distinguish between
single-epoch formation scenarios and multi-epoch formation scenarios. We
analyze the kinematics of 25 globular clusters and aim to find kinematic
differences between multiple populations to constrain their formation process.
We split red-giant branch (RGB) stars in each cluster into three populations
(P1, P2, P3) for the type-II clusters and two populations (P1 and P2) otherwise
using Hubble photometry. We derive the rotation and dispersion profiles for
each cluster and its populations by using all stars with radial velocity
measurements obtained from MUSE spectroscopy. Based on these profiles, we
calculate the rotation strength in terms of ordered-over-random motion
evaluated at the half-light radius of the
cluster. We detect rotation in all but four clusters. For NGC~104, NGC~1851,
NGC~2808, NGC~5286, NGC~5904, NGC~6093, NGC~6388, NGC~6541, NGC~7078 and
NGC~7089 we also detect rotation for P1 and/or P2 stars. For NGC~2808, NGC~6093
and NGC~7078 we find differences in between
P1 and P2 that are larger than . Whereas we find that P2 rotates
faster than P1 for NGC~6093 and NGC~7078, the opposite is true for NGC~2808.
However, even for these three clusters, the differences are still of low
significance. We find that the strength of rotation of a cluster generally
scales with its median relaxation time. For P1 and P2, the corresponding
relation is very weak at best. We observe no correlation between the difference
in rotation strength between P1 and P2 and cluster relaxation time. The MUSE
stellar radial velocities that this analysis is based on are made publicly
available
Translation: The relative native
Translation by Julia Sauma and Martin Holbraad of: Viveiros de Castro, Eduardo (2002) "O Nativo Relativo." Mana, vol. 8, no. 1, p. 113-14
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