Effects of metallicity, star-formation conditions,  and evolution in B and Be
stars

Martayan, Christophe; Frémat, Y.; Hubert, A. M.; Floquet, M.; Zorec, J.; Neiner, C.

doi:10.1051/0004-6361:20053859

Cited by 104 publications

(135 citation statements)

References 39 publications

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“…The present calibration can then provide a first step for the interpretation of spectra emitted by rotating early-type objects, as already shown in Levenhagen et al (2003); Neiner et al (2003); Zorec et al (2005);Vinicius et al (2006);Frémat et al (2006); Martayan et al (2006Martayan et al ( , 2007; Floquet et al (2000Floquet et al ( , 2002.…”

Section: Effects Related To the Rotationsupporting

confidence: 60%

Fundamental parameters of B supergiants from the BCD system

Zorec¹,

Cidale

Arias

et al. 2009

A&A

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Context. Effective temperatures of early-type supergiants are important to test stellar atmosphere-and internal structure-models of massive and intermediate mass objects at different evolutionary phases. However, these T eff values are more or less discrepant depending on the method used to determine them. Aims. We aim to obtain a new calibration of the T eff parameter for early-type supergiants as a function of observational quantities that are: a) highly sensitive to the ionization balance in the photosphere and its gas pressure; b) independent of the interstellar extinction; c) as much as possible model-independent. Methods. The observational quantities that best address our aims are the (λ 1 , D) parameters of the BCD spectrophotometric system. They describe the energy distribution around the Balmer discontinuity, which is highly sensitive to T eff and log g. We perform a calibration of the (λ 1 , D) parameters into T eff using effective temperatures derived with the bolometric-flux method for 217 program stars, whose individual uncertainties are on average |ΔT eff |/T f eff = 0.05. Results. We obtain a new and homogeneous calibration of the BCD (λ 1 , D) parameters for OB supergiants and revisit the current calibration of the (λ 1 , D) zone occupied by dwarfs and giants. The final comparison of calculated with obtained T eff values in the (λ 1 , D) calibration show that the latter have total uncertainties, which on average are T eff /T f eff ±0.05 for all spectral types and luminosity classes. Conclusions. The effective temperatures of OB supergiants derived in this work agree on average within some 2000 K with other determinations found in the literature, except those issued from wind-free non-LTE plane-parallel models of stellar atmospheres, which produce effective temperatures that can be overestimated by up to more than 5000 K near T eff = 25 000 K. Since the stellar spectra needed to obtain the (λ 1 , D) parameters are of low resolution, a calibration based on the BCD system is useful to study stars and stellar systems like open clusters, associations or stars in galaxies observed with multi-object spectrographs and/or spectro-imaging devices.

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Section: Effects Related To the Rotationsupporting

confidence: 60%

Fundamental parameters of B supergiants from the BCD system

Zorec¹,

Cidale

Arias

et al. 2009

A&A

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“…In contrast, investigations in both the LMC and SMC using statistical tests (Martayan et al 2006(Martayan et al , 2007b did not find significantly different rotational velocities in clusters and the field. In the Galaxy, Huang & Gies (2006) found a larger number of slow rotators in the field than in open clusters.…”

Section: Frequency Of Be Stars As a Function Of Local Star Densitymentioning

confidence: 62%

A slitless spectroscopic survey for Hαemission-line objects in SMC clusters

Martayan

Baade

Fabregat³

2010

A&A

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Context.A fair fraction of all single early-type stars display emission lines well before their supergiant phase. Very rapid rotation is necessary for these stars to form rotationally supported decretion disks. However, it is unknown whether and which other parameters may be important. Aims. We assess the roles of metallicity and evolutionary age in the appearance of the so-called Be phenomenon. Methods. Slitless CCD spectra were obtained covering the bulk (about 3 square degrees) of the Small Magellanic Cloud. For Hα line-emission twice as strong as the ambient continuum, the survey is complete to spectral type B2/B3 on the main sequence. About 8120 spectra of 4437 stars were searched for emission lines in 84 open clusters, and 370 emission-line stars were found, among them at least 231 close to the main sequence. For 176 of them, photometry is available from the OGLE database. For comparison with a higher-metallicity environment, the Galactic sample of the photometric Hα survey by McSwain & Gies (2005, ApJS, 161, 118) was used.Results. Among early spectral sub-types, Be stars are more frequent by a factor ∼3-5 in the SMC than in the Galaxy. The distribution with spectral type is similar in both galaxies, i.e., not strongly dependent on metallicity. The fraction of Be stars does not seem to vary with local star density. The Be phenomenon mainly sets in towards the end of the main-sequence evolution (this trend may be more pronounced in the SMC); but some Be stars already form with Be-star characteristics. In small subsamples (such as single clusters), even if they appear identical, the fraction of emission-line stars can deviate drastically from the mean. Conclusions. In all probability, the fractional critical angular rotation rate, Ω/Ω c , is one of the main parameters governing the occurrence of the Be phenomenon. If the Be character is only acquired during the course of evolution, the key circumstance is the evolution of Ω/Ω c , which is not only dependent on metallicity but differently so for different mass ranges. As a result, even if the Be phenomenon is driven basically by a single parameter (namely Ω/Ω c ), it can assume a complex multi-parametric appearance. The large cluster-to-cluster differences, which seem stronger than all other variations, serve as a caveat that this big picture may undergo significant second-order modulations (e.g., pulsations, initial angular momentum, etc.).

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“…However, massive stars winds also depend on metallicity (Ṁ ∝ Z 0.6−0.8 Mokiem et al 2007;Vink et al 2001): the lower the metal content, the weaker the radiative acceleration through lines, and thus the lower the mass loss rate. The observation of higher rotational velocities in the LMC/SMC compared to the Galaxy (Martayan et al 2006;Mokiem et al 2006) is an indirect confirmation of this effect (although the initial distribution of rotational velocities might also be different). Massive stars in low-Z environments are thus more likely to become LGRBs in the scenario where the angular momentum evolution is governed by mass loss (see Petrovic et al 2005b for alternative possibilities).…”

Section: Introductionmentioning

confidence: 82%

Evidence of quasi-chemically homogeneous evolution of massive stars up to solar metallicity

Martins

Depagne

Russeil

et al. 2013

A&A

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Context. Long soft gamma-ray bursts (LGRBs) are usually associated with the death of the most massive stars. A large amount of core angular momentum in the phases preceding the explosion is required to form LGRBs. A very high initial rotational velocity can provide this angular momentum. This velocity strongly influences the way the star evolves: it is mixed in a chemically homogeneous way and evolves directly towards the blue part of the Hertzsprung-Russell (HR) diagram from the main sequence. Aims. We have shown that chemically homogeneous evolution takes place in the Small Magellanic Cloud (SMC) at low metallicity. We want to see whether there is a metallicity threshold above which such an evolution is not possible. Methods. We performed a spectroscopic analysis of H-rich early-type WN stars in the Large Magellanic Cloud (LMC) and the Galaxy. We used the code CMFGEN to determine the fundamental properties (T eff , L) and the surface composition of the target stars. We then placed the stars in the HR diagram and determined their evolution. Results. We show that both the LMC and Galactic WNh stars we selected cannot be explained by standard stellar evolution. They are located on the left of the main sequence but show surface abundances typical of CN equilibrium. In addition, they still contain a large amount of hydrogen. They are thus core-H burning objects. Their properties are consistent with chemically homogeneous evolution. We determine the metallicity of the Galactic stars from their position and Galactic metallicity gradients and conclude that they have 0.6 < Z < 1.0. A moderate coupling between the core and the envelope is required to explain that stellar winds do not extract too much angular momentum to prevent a blueward evolution. Conclusions. We have shown that chemically homogeneous evolution takes place in environments with metallicity up to solar. Since some long gamma-ray bursts appear in (super-)solar environments, such an evolution may be a viable way to form them over a wide range of metallicities.

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Effects of metallicity, star-formation conditions, and evolution in B and Be stars

Cited by 104 publications

References 39 publications

Fundamental parameters of B supergiants from the BCD system

Fundamental parameters of B supergiants from the BCD system

A slitless spectroscopic survey for Hαemission-line objects in SMC clusters

Evidence of quasi-chemically homogeneous evolution of massive stars up to solar metallicity

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