A Helium-Carbon Correlation on the Extreme Horizontal Branch in Ω Centauri

Latour, M.; Randall, S. K.; Fontaine, G.; Bono, G.; Calamida, A.; Brassard, P.

doi:10.1088/0004-637x/795/2/106

Cited by 30 publications

(63 citation statements)

References 51 publications

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“…Fig. 11 of Latour et al 2014). It is clear that in order for He-rich sdBs to exist in the numbers observed, diffusion must be slowed down, possibly by internal turbulence or a weak stellar wind, but the details are far from being understood.…”

Section: Resultsmentioning

confidence: 95%

The enigmatic He-sdB pulsator LS IV-14°116: new insights from the VLT

Randall¹,

Bagnulo

Ziegerer

et al. 2015

A&A

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The intermediate helium subdwarf B star LS IV-14 • 116 is a unique object showing extremely peculiar atmospheric abundances as well as long-period pulsations that cannot be explained in terms of the usual opacity mechanism. One hypothesis invoked was that a strong magnetic field may be responsible. We discredit this possibility on the basis of FORS2 spectro-polarimetry, which allows us to rule out a mean longitudinal magnetic field down to 300 G. Using the same data, we derive the atmospheric parameters for LS IV-14 • 116 to be T eff = 35 150 ± 111 K, log g = 5.88 ± 0.02 and log N(He)/N(H) = −0.62 ± 0.01. The high surface gravity in particular is at odds with the theory that LS IV-14 • 116 has not yet settled onto the helium main sequence, and that the pulsations are excited by an mechanism acting on the helium-burning shells present after the main helium flash. Archival UVES spectroscopy reveals LS IV-14 • 116 to have a radial velocity of 149.1±2.1 km s −1 . Running a full kinematic analysis, we find that it is on a retrograde orbit around the Galactic centre, with a Galactic radial velocity component U = 13.23 ± 8.28 km s −1 and a Galactic rotational velocity component V = −55.56 ± 22.13 km s −1 . This implies that LS IV-14 • 116 belongs to the halo population, an intriguing discovery.

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Section: Resultsmentioning

confidence: 95%

The enigmatic He-sdB pulsator LS IV-14°116: new insights from the VLT

Randall¹,

Bagnulo

Ziegerer

et al. 2015

A&A

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“…Depending on the depth of the mixing, stars with more or less helium in the atmospheres and different atmospheric parameters can be formed in this way. Most recently, Latour et al (2014) found a correlation between the carbon and helium abundances of the He-sdOB stars in the globular cluster ω Cen, which is predicted by late hot flasher models. Hirsch (2009) discovered a similar correlation for field helium-rich hot subdwarf (see also Heber & Hirsch 2010).…”

Section: Introductionmentioning

confidence: 84%

“…6) and the atmospheric parameters are quite typical for the field population of He-sdOs (T eff = 40 000−50 000 K, Ströer et al 2007;Nemeth et al 2012). However, quite a number of stars have lower temperatures between 35 000 K and 40 000 K. Those helium-rich sdOBs are rare in the field population, but quite dominant in the globular cluster ω Cen (Latour et al 2014). Following the discussion in Latour et al (2014) this might be related to the age of the parent population, since most of the stars in our sample belong to the old thick disk or halo populations, while most of the bright stars studied by Nemeth et al (2012) belong to the young thin disk population.…”

Section: Irregular Rv Variations Of Helium-rich Hot Subdwarf Starsmentioning

confidence: 97%

“…However, quite a number of stars have lower temperatures between 35 000 K and 40 000 K. Those helium-rich sdOBs are rare in the field population, but quite dominant in the globular cluster ω Cen (Latour et al 2014). Following the discussion in Latour et al (2014) this might be related to the age of the parent population, since most of the stars in our sample belong to the old thick disk or halo populations, while most of the bright stars studied by Nemeth et al (2012) belong to the young thin disk population. The He-sdOs J232757.46+483755.2 and J110215.45+024034.1 seem to be more evolved than the rest of the sample and might also be associated with the helium-rich post-AGB stars.…”

Section: Irregular Rv Variations Of Helium-rich Hot Subdwarf Starsmentioning

confidence: 99%

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The catalogue of radial velocity variable hot subluminous stars from the MUCHFUSS project

Geier

Kupfer

Heber

et al. 2015

A&A

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The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims to find sdBs with compact companions such as massive white dwarfs, neutron stars, or black holes. Here we provide classifications, atmospheric parameters, and a complete radial velocity (RV) catalogue containing 1914 single measurements for a sample of 177 hot subluminous stars discovered based on SDSS DR7; 110 stars show significant RV variability, while 67 qualify as candidates. We constrain the fraction of close massive compact companions of hydrogen-rich hot subdwarfs in our sample to be smaller than ∼1.3%, which is already close to the theoretical predictions. However, the sample might still contain such binaries with longer periods exceeding ∼8 d. We detect a mismatch between the ΔRV max -distribution of the sdB and the more evolved sdOB and sdO stars, which challenges our understanding of their evolutionary connection. Furthermore, irregular RV variations of unknown origin with amplitudes of up to ∼180 km s −1 on timescales of years, days, and even hours have been detected in some He-sdO stars. They might be connected to irregular photometric variations in some cases.

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“…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

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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.

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A Helium-Carbon Correlation on the Extreme Horizontal Branch in Ω Centauri

Cited by 30 publications

References 51 publications

The enigmatic He-sdB pulsator LS IV-14°116: new insights from the VLT

The enigmatic He-sdB pulsator LS IV-14°116: new insights from the VLT

The catalogue of radial velocity variable hot subluminous stars from the MUCHFUSS project

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

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