The SN stars - Magnetically controlled stellar winds among the helium-weak stars

Shore, Steven N.; Brown, Douglas; Sonneborn, G.

doi:10.1086/114512

Cited by 38 publications

(44 citation statements)

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“…For silicon we have an abundance map at our disposal. Because the exact continuum placement is uncertain in the UV domain, Shore et al (1987) We calculated the line indices from low-dispersion spectra. The wavelengths in Eq.…”

Section: Variability Of Uv Linesmentioning

confidence: 99%

Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740

Krtička

Janík

Marková

et al. 2013

A&A

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Context. The light variability of hot magnetic chemically peculiar stars is typically caused by the flux redistribution in spots with peculiar abundance. This raises the question why some stars with surface abundance spots show significant rotational light variability, while others do not. Aims. We study the Bp star HD 64740 to investigate how its remarkable inhomogeneities in the surface distribution of helium and silicon, and the corresponding strong variability of many spectral lines, can result in one of the faintest photometric variabilities among the Bp stars. Methods. We used model atmospheres and synthetic spectra calculated for the silicon and helium abundances from surface abundance maps to predict the ultraviolet and visual light and line variability of HD 64740. The predicted fluxes and line profiles were compared with the observed ones derived with the IUE, HST, and Hipparcos satellites and with spectra acquired using the FEROS spectrograph at the 2.2 m MPG/ESO telescope in La Silla. Results. We are able to reproduce the observed visual light curve of HD 64740 assuming an inhomogeneous distribution of iron correlated with silicon distribution. The light variations in the ultraviolet are hardly detectable. We detect the variability of many ultraviolet lines of carbon, silicon, and aluminium and discuss the origin of these lines and the nature of their variations. Conclusions. The maximum abundances of helium and silicon on the surface of HD 64740 are not high enough to cause significant light variations. The detected variability of many ultraviolet lines is most likely of atmospheric origin and reflects the inhomogeneous elemental surface distribution. The variability of the C iv resonance lines of carbon is stronger and it probably results from the dependence of the wind mass-loss rate on the chemical composition and magnetic field orientation. We have not been able to detect a clear signature of the matter trapped in the circumstellar clouds.

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Section: Variability Of Uv Linesmentioning

confidence: 99%

Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740

Krtička

Janík

Marková

et al. 2013

A&A

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“…5 and 6). We included 6 sn stars in our sample (Abt 1979;Mermilliod 1983;Shore et al 1987) and the sn "candidate" HD 74 146 (Mermilliod 1983). For comparison purposes, we also added two non-sn stars (HD 162630 and HD 162 817) that belong to NGC 6475.…”

Section: Observational Materialsmentioning

confidence: 99%

“…Shore & Adelman (1981) reported variable UV lines in the spectrum of He-rich stars, which are attributed to the magnetic control of the stellar mass outflows in these objects. For the case of He-weak stars, Brown et al (1984) started a systematic survey of C IV line variations followed by other studies in the literature (Brown et al 1985;Shore et al 1987). They studied a total of 15 He-weak stars, including both magnetic and non-magnetic stars, rapid and slow rotators.…”

Section: Introductionmentioning

confidence: 99%

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On the nature of sn stars

Saffe

Levato

2014

A&A

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The sn stars were first discoved by Abt & Levato when studying the spectral types in different open clusters. These stars present sharp Balmer lines, sharp metallic lines (C II, Si II, Ca II, Ti II, Fe II), and broad coreless He I lines. Some of the sn stars seem to be related to CP stars. Initially Abt & Levato proposed a shell-like nature to explain the sn stars, although this scenario was subsequently questioned. There is no general agreement about their origin. We aim to derive abundances for a sample of 9 stars, including sn and non-sn stars, to determine the possible relation between sn and CP stars and compare their chemical abundances. That most sn stars belong to open clusters allows us to search for a possible relation with fundamental parameters, including the age and rotation. We also study the possible contribution of different effects to the broad He I lines observed in these stars, such as Stark broadening and the possible He-stratification. Effective temperature and gravity were estimated by Strömgren photometry and then refined by requiring ionization and excitation equilibrium of Fe lines. We derived the abundances by fitting the observed spectra with synthetic spectra using an iterative procedure with the SYNTHE and ATLAS9 codes. We derived metallic abundances of 23 different chemical elements for 9 stars and obtained low projected rotational velocities for the sn stars in our sample (v sin i up to 69 km s −1 ). We also compared 5 stars that belong to the same cluster (NGC 6475) and show that the sn characteristics appear in the 3 stars with the lower rotational velocity. However, the apparent preference of sn stars for objects with the lower v sin i values should be taken with caution due to the small number of objects studied here. We analysed the photospheric chemical composition of sn stars and show that approximately ∼40% of them display chemical peculiarities (such as He-weak and HgMn stars) within a range of temperature of 10 300 K−14 500 K. However, there are also sn stars with solar or nearly-solar (i.e. non-CP) chemical composition. We have studied the possible contribution of different processes to the broad He I lines present in the sn stars. Although NLTE effects could not be completely ruled out, it seems that NLTE is not directly related to the broad He I profiles observed in the sn stars. The broad-line He I 4026 Å is the clearest example of the sn characteristics in our sample. We succesfully fit this line in 4 out of 7 sn stars by using the appropriate Stark broadening tables, while small differences appear in the other 3 stars. Studying the plots of abundance vs. depth for the He I lines resulted in some sn stars probably being stratified in He. However, a further study of variability in the He I lines would help for determining whether a possible non-uniform He superficial distribution could also play a role in these sn stars. We conclude that the broad He I lines that characterize the sn class could be modelled (at least in some of these stars) by the usual radiati...

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“…The unambiguous evidence of magnetospheric plasma is the detection of enhanced, variable C IV and Si IV resonance line absorption in the CP stars (Shore 1987;Shore, Brown and Sonneborn 1987;). This behavior has also been detected in the 0 7 V star 6 l Ori C (Walborn and Nichols 1994).…”

Section: Magnetospheresmentioning

confidence: 99%

Modeling Oblique Rotators: Magnetospheres and Winds

Shore

1999

International Astronomical Union Colloquium

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A b s t r a c t . The upper main sequence chemically peculiar (CP) stars display evidence of trapped circumstellar gas and nonspherical outflows. These stars are also known to possess strong magnetic fields that are often highly inclined to the rotational axis. Their phenomenology can be understood by using the oblique rotator model, which has successfully accounted for the observed behavior of the cooler CP stars. This paper reviews some features of the oblique rotator model, in which the magnetic field is assumed to provide a rigid framework for the structuring of the stellar and circumstellar gas. Corotation of circumstellar plasma is enforced out to the Alfven radius in the magnetic equatorial plane, while for the hotter stars, a radiatively driven wind emerges from the magnetic polar caps. Some observable consequences of the model are discussed, especially the Ha and ultraviolet resonance line absorption and emission periodic variability that has been observed in the He-peculiar stars and nonthermal radio emission. Magnetospheres may also be present in O stars, e.g. 0 1 Ori C, and in the Herbig Ae/Be stars.

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The SN stars - Magnetically controlled stellar winds among the helium-weak stars

Cited by 38 publications

References 0 publications

Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740

Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740

On the nature of sn stars

Modeling Oblique Rotators: Magnetospheres and Winds

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