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

Latour, M.; Fontaine, G.; Green, E. M.; Brassard, P.

doi:10.1051/0004-6361/201525999

Cited by 25 publications

(29 citation statements)

References 42 publications

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“…as part of her spectroscopic programmes in recent years (Green et al 2008). A detailed description of the three instrumental set-ups can be found in Latour et al (2015), hence we only describe the main characteristics of the spectra obtained. For each star we have low (∆λ = 8.7 Å) and medium (1.3 Å) resolution spectra taken at the Steward Observatory 2.3 m Bok Telescope on Kitt Peak.…”

Section: Optical Spectroscopymentioning

confidence: 99%

Spectral analysis of four surprisingly similar hot hydrogen-rich subdwarf O stars

Latour

Chayer

Green

et al. 2018

A&A

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Context. Post-extreme horizontal branch stars (post-EHB) are helium-shell burning objects evolving away from the EHB and contracting directly towards the white dwarf regime. While the stars forming the EHB have been extensively studied in the past, their hotter and more evolved progeny are not so well characterized. Aims. We perform a comprehensive spectroscopic analysis of four such bright sdO stars, namely Feige 34, Feige 67, AGK+81• 266, and LS II+18• 9, among which the first three are used as standard stars for flux calibration. Our goal is to determine their atmospheric parameters, chemical properties, and evolutionary status to better understand this class of stars that are en route to become white dwarfs. Methods. We used non-local thermodynamic equilibrium model atmospheres in combination with high quality optical and UV spectra. Photometric data were also used to compute the spectroscopic distances of our stars and to characterize the companion responsible for the infrared excess of Feige 34. Results. The four bright sdO stars have very similar atmospheric parameters with T eff between 60 000 and 63 000 K and log g (cm s−2 ) in the range 5.9 to 6.1. This places these objects right on the theoretical post-EHB evolutionary tracks. The UV spectra are dominated by strong iron and nickel lines and suggest abundances that are enriched with respect to those of the Sun by factors of 25 and 60. On the other hand, the lighter elements, C, N, O, Mg, Si, P, and S, are depleted. The stars have very similar abundances, although AGK+81• 266 shows differences in its light element abundances. For instance, the helium abundance of this object is 10 times lower than that observed in the other three stars. All our stars show UV spectral lines that require additional line broadening that is consistent with a rotational velocity of about 25 km s −1 . The infrared excess of Feige 34 is well reproduced by a M0 main-sequence companion and the surface area ratio of the two stars suggests that the system is a physical binary. However, the lack of radial velocity variations points towards a low inclination and/or long orbital period. Spectroscopic and Hipparcos distances are in good agreement for our three brightest stars. Conclusions. We performed a spectroscopic analysis of four hot sdO stars that are very similar in terms of atmospheric parameters and chemical compositions. The rotation velocities of our stars are significantly higher than what is observed in their immediate progenitors on the EHB, suggesting that angular momentum may be conserved as the stars evolve away from the EHB.

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Section: Optical Spectroscopymentioning

confidence: 99%

Spectral analysis of four surprisingly similar hot hydrogen-rich subdwarf O stars

Latour

Chayer

Green

et al. 2018

A&A

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“…This is a known problem when fitting the spectra of CSPNe and other hot stars with pure HHe models only. It has been shown that He II λ 4686 Å is particularly susceptible to metal line blanketing [15,16]. We expect the systematic effects introduced by neglecting metal-line blanketing effects to be of the order of 0.1 dex on log g and 1 kK on T eff [17], but we will include metals in our models for future analysis of this system to overcome this problem.…”

Section: Spectral Analysismentioning

confidence: 99%

Revealing the True Nature of Hen 2-428

et al. 2018

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Abstract:The nucleus of Hen 2-428 is a short orbital period (4.2 h) spectroscopic binary, whose status as potential supernovae type Ia progenitor has raised some controversy in the literature. We present preliminary results of a thorough analysis of this interesting system, which combines quantitative non-local thermodynamic (non-LTE) equilibrium spectral modelling, radial velocity analysis, multi-band light curve fitting, and state-of-the art stellar evolutionary calculations. Importantly, we find that the dynamical system mass that is derived by using all available He II lines does not exceed the Chandrasekhar mass limit. Furthermore, the individual masses of the two central stars are too small to lead to an SN Ia in case of a dynamical explosion during the merger process.

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“…Indeed, it is thought that analyses based on optical spectroscopy yield systematically cooler temperatures for stars hotter than ∼50 000 K due to the so-called Balmer-line problem (Napiwotzki 1993). For the few cases where both highquality optical and UV data are available and have been exploited for very hot compact evolved stars, the (more reliable) UV spectroscopy yielded temperature estimates over 10 000 K higher than the optical spectra (see Latour et al 2015, for a recent example). We plan to refine the temperatures of the ω Cen variables on the basis of HST-COS spectra in the future, however this is outside of the scope of this study and will be presented in a future publication.…”

Section: The Theoretical Instability Stripmentioning

confidence: 99%

Pulsating hot O subdwarfs inωCentauri: mapping a unique instability strip on the extreme horizontal branch

Randall¹,

Calamida

Fontaine

et al. 2016

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We present the results of an extensive survey for rapid pulsators among Extreme Horizontal Branch (EHB) stars in ω Cen. The observations performed consist of nearly 100 hours of time-series photometry for several off-centre fields of the cluster, as well as low-resolution spectroscopy for a partially overlapping sample. We obtained photometry for some 300 EHB stars, for around half of which we are able to recover light curves of sufficient quality to either detect or place meaningful non-detection limits for rapid pulsations. Based on the spectroscopy, we derive reliable values of log g, T eff and log N(He)/N(H) for 38 targets, as well as good estimates of the effective temperature for another nine targets, whose spectra are slightly polluted by a close neighbour in the image. The survey uncovered a total of five rapid variables with multi-periodic oscillations between 85 and 125 s. Spectroscopically, they form a homogeneous group of hydrogen-rich subdwarf O stars clustered between 48,000 and 54,000 K. For each of the variables we are able to measure between two and three significant pulsations believed to constitute independent harmonic oscillations. However, the interpretation of the Fourier spectra is not straightforward due to significant fine structure attributed to strong amplitude variations. In addition to the rapid variables, we found an EHB star with an apparently periodic luminosity variation of ∼2700 s, which we tentatively suggest may be caused by ellipsoidal variations in a close binary. Using the overlapping photometry and spectroscopy sample we are able to map an empirical ω Cen instability strip in log g − T eff space. This can be directly compared to the pulsation driving predicted from the Montréal "second-generation" models regularly used to interpret the pulsations in hot B subdwarfs. Extending the parameter range of these models to higher temperatures, we find that the region where p-mode excitation occurs is in fact bifurcated, and the well-known instability strip between 29,000-36,000 K where the rapid subdwarf B pulsators are found is complemented by a second one above 50,000 K in the models. While significant challenges remain at the quantitative level, we believe that the same κ-mechanism that drives the pulsations in hot B subdwarfs is also responsible for the excitation of the rapid oscillations observed in the ω Cen variables. Intriguingly, the ω Cen variables appear to form a unique class. No direct counterparts have so far been found either in the Galactic field, nor in other globular clusters, despite dedicated searches. Conversely, our survey revealed no ω Cen representatives of the rapidly pulsating hot B subdwarfs found among the field population, though their presence cannot be excluded from the limited sample.

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

Cited by 25 publications

References 42 publications

Spectral analysis of four surprisingly similar hot hydrogen-rich subdwarf O stars

Spectral analysis of four surprisingly similar hot hydrogen-rich subdwarf O stars

Revealing the True Nature of Hen 2-428

Pulsating hot O subdwarfs inωCentauri: mapping a unique instability strip on the extreme horizontal branch

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