The hot horizontal-branch stars in<i>ω</i> Centauri

Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart, A. V.; Calamida, A.; Nonino, M.

doi:10.1051/0004-6361/201015020

Cited by 55 publications

(132 citation statements)

References 52 publications

Supporting

Mentioning

109

Contrasting

Order By: Relevance

“…As commented in Paper I, the temperature and gravity measured in the two investigations for the seven stars in common cooler than 32 000 K agree well: the mean differences (ours − Moehler et al's) are only Δ(T eff ) = −143 K and Δ(log g) = 0.03 dex. Our mass estimates are on average higher by 0.08 M , as a consequence of the fainter magnitudes of the Castellani et al (2007) catalog, adopted by Moehler et al (2011). On the contrary, the four hottest stars suggest that some systematic could be present between the BH stars.…”

Section: Comparison With Moehler Et Al (2011)mentioning

confidence: 49%

Spectroscopy of horizontal branch stars inω Centauri

Bidin

Villanova

Piotto

et al. 2012

A&A

Self Cite

View full text Add to dashboard Cite

Aims. We analyze the reddening, surface helium abundance and spectroscopic mass of 115 blue horizontal branch (HB) and blue hook (BH) stars in ω Centauri, spanning the cluster HB from the blue edge of the instability strip (T eff = 8000 K) to BH objects with T eff ≈ 50 000 K. Methods. The temperatures, gravities, and surface helium abundances were measured on low-resolution spectra fitting the Balmer and helium lines with a grid of synthetic spectra. From these parameters, the mass and reddening were estimated. Results. The mean cluster reddening is E(B − V) = 0.115 ± 0.004, in good agreement with previous estimates, but we evidence a pattern of differential reddening in the cluster area. The stars in the western half are more reddened than in the southwest quadrant by 0.03−0.04 mag. We find that the helium abundances measured on low-resolution spectra are systematically higher by 0.20−0.25 dex than the measurements based on higher resolution. No difference in surface helium abundance is detected between HB stars in ω Centauri and in three comparison clusters, and the stars in the range 11 500−20 000 K follow a trend with temperature, which probably reflects a variable efficiency of the diffusion processes. There is mild evidence that two families of extreme HB (EHB) cluster stars (T eff ≥ 20 000 K) could exist, as observed in the field, with ∼15% of the objects being helium depleted by a factor of ten with respect to the main population. The distribution of helium abundance above 30 000 K is bimodal, but we detect a fraction of He-poor objects lower than previous investigations. The observations are consistent with these being stars evolving off the HB. Their spatial distribution is not uniform across the cluster, but this asymmetric distribution is only marginally significative. We also find that EHB stars with anomalously high spectroscopic mass could be present in ω Centauri, as previously found in other clusters. The derived temperature-color relation reveals that the HB stars hotter than ∼11 000 K are fainter than the expectations of the canonical models in the U band, while no anomaly is detected in B and V. This behavior, not observed in NGC 6752, is a new peculiarity of ω Centauri HB stars. More investigation is needed to reach a full comprehension of this complex observational picture.

show abstract

Section: Comparison With Moehler Et Al (2011)mentioning

confidence: 49%

Spectroscopy of horizontal branch stars inω Centauri

Bidin

Villanova

Piotto

et al. 2012

A&A

Self Cite

View full text Add to dashboard Cite

show abstract

“…Other authors have realized direct measurements of helium enhancement among the stars of ω Centauri. Moehler et al (2011) andMoni Bidin et al (2012) analysed a large group of bHB and eHB stars describing how helium changes with the temperature. The peculiar pattern described by the authors strongly suggest that the bHB and the eHB is populated by second generation stars and that the phenomenon of the late helium flash mixing is involved in shaping the blue end of the HB locus.…”

Section: Spectroscopic Observationsmentioning

confidence: 99%

The mosaic multiple stellar populations in ω Centauri: the horizontal branch and the main sequence

Tailo

Criscienzo

D’Antona

et al. 2016

Mon. Not. R. Astron. Soc.

View full text Add to dashboard Cite

We interpret the stellar population of ω Centauri by means of a population synthesis analysis, following the most recent observational guidelines for input metallicities, helium and [(C+N+O)/Fe] contents. We deal at the same time with the main sequences, sub-giant and horizontal branch data. The reproduction of the observed color magnitude features is very satisfying and bears interesting hints concerning the evolutionary history of this peculiar stellar ensemble. Our main results are: 1) no significant spread in age is required to fit the colour-magnitude diagram. Indeed we can use coeval isochrones for the synthetic populations, and we estimate that the ages fall within a ∼ 0.5 Gyr time interval; in particular the most metal rich population can be coeval (in the above meaning) with the others, if its stars are very helium-rich (Y∼0.37) and with the observed CNO enhancement ([(C+N+O)/Fe] = + 0.7); 2) a satisfactory fit of the whole HB is obtained, consistent with the choice of the populations providing a good reproduction of the main sequence and sub giant data. 3) the split in magnitude observed in the red HB is well reproduced assuming the presence of two stellar populations in the two different sequences observed: a metal poor population made of stars evolving from the blue side (luminous branch) and a metal richer one whose stars are in a stage closer to the zero age HB (dimmer branch). This modelization also fits satisfactorily the period and the [Fe/H] distribution of the RR Lyrae stars.

show abstract

“…Additional abundances of carbon and nitrogen were then measured in these sdO stars by Hirsch (2009) and Hirsch & Heber (in prep.). The sample of ω Centauri EHB stars in Moehler et al (2011) and Latour et al (2014c) also included a fair amount of sdO stars, but mostly cooler stars found at the transition between the spectral types B and O 2 A comprehensive review of the global properties and characteristics of hot subdwarf stars can be found in Heber (2009). A&A 579, A39 (2015) (i.e., below 40 000 K).…”

Section: Astrophysical Contextmentioning

confidence: 99%

A non-LTE analysis of the hot subdwarf O star BD+28°4211

Latour

Fontaine

Green

et al. 2015

A&A

View full text Add to dashboard Cite

We present the second part of our detailed analysis of the hot subdwarf O (sdO) and spectroscopic standard star BD+28 • 4211, in which we focus on the optical spectrum. This target was selected to revisit the more general question of how reliably the atmospheric parameters of a very hot star can be inferred from optical spectroscopy alone. Given its status as a spectrophotometric standard, spectral data of exceptional quality are available for BD+28 • 4211. In the first part of our study, we determined the abundances of some 11 metals detected in the atmosphere of BD+28 • 4211 using UV and far-UV spectra of the star and corroborated the fundamental parameters estimated in past studies (T eff ∼ 82 000 K, log g ∼ 6.2, and solar N(He)/N(H)). In this work, we aim at rederiving these secured parameters on the sole basis of high-quality optical spectra. A first grid of non-LTE line-blanketed model atmospheres, including C, N, O, Mg, Si, S, Fe, and Ni with the abundances derived from the UV spectrum, does not give satisfactory results when we apply a standard simultaneous fitting procedure to the observed H and He lines of our optical spectra. The line profiles are not finely reproduced and the resulting effective temperatures, in particular, are too low by ∼10 000 K. We next investigate the probable cause of this failure, that is, the importance of missing opacity sources on the atmospheric stratification. We thus compare line profiles computed from models with artificially boosted metallicities, from solar abundances to 15× these values. We find that the structural effects saturate for a metallicity of ∼10× solar, and use this to compute a second full grid of models and synthetic spectra. This metal-enriched grid allows us to achieve significantly improved spectral fits with models having the expected parameters. As an a posteriori test, we compared the detailed profiles of several model spectral lines with high-resolution spectra culled from archived HIRES observations. The agreement between our synthetic lines and the observed lines is very convincing. Our test case thus reveals that there is still a need for models with enhanced metallicity for better estimating the atmospheric parameters of objects such as hot subdwarfs and hot white dwarfs if only optical spectra are available.

show abstract

The hot horizontal-branch stars inω Centauri

Cited by 55 publications

References 52 publications

Spectroscopy of horizontal branch stars inω Centauri

Spectroscopy of horizontal branch stars inω Centauri

The mosaic multiple stellar populations in ω Centauri: the horizontal branch and the main sequence

A non-LTE analysis of the hot subdwarf O star BD+28°4211

Contact Info

Product

Resources

About