arXiv subjects

Yves Fremat

Publications and source records attributed to Yves Fremat.

26 records · Page 2Linked to original sources

arXiv2005

B and Be stars in the Magellanic Clouds: rotation, evolution and variability

Thanks to observations at the ESO VLT with the GIRAFFE multifibers spectrograph, we have obtained spectra of 177 and 346 B-type stars in the Large and Small Magellanic Clouds respectively. We have discovered 25 and 90 new Be stars among the 47 and 131 Be stars observed in the LMC and SMC respectively. We have determined the fundamental parameters of these stars and examined the effect of the metallicity, star formation conditions and evolution on the behaviour of the rotational velocities. We discovered that only the future B-type stars with a strong rotational velocity in ZAMS will become a Be star. We have also discovered short term variability in several SMC Be stars thanks to a cross-correlation with the MACHO database.

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arXiv2005

A study of the B and Be star population in the field of the LMC open cluster NGC2004 with VLT-FLAMES

Observations of hot stars belonging to the young cluster LMC-NGC2004 and its surrounding region have been obtained with the VLT-GIRAFFE facilities in MEDUSA mode. 25 Be stars were discovered; the proportion of Be stars compared to B-type stars is found to be of the same order in the LMC and in the Galaxy fields. 23 hot stars were discovered as spectroscopic binaries (SB1 and SB2), 5 of these are found to be eclipsing systems from the MACHO database, with periods of a few days. About 75% of the spectra in our sample are polluted by hydrogen (Halpha and Hgamma), [SII] and [NII] nebular lines. These lines are typical of HII regions. They could be associated with patchy nebulosities with a bi-modal distribution in radial velocity, with higher values (+335 kms^{-1}) preferentially seen inside the southern part of the known bubble LMC4 observed in HI at 21 cm.

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arXiv2005

Evolution of Galactic Field be Stars

Galactic field Be stars were studied by taking into account the effects induced by the fast rotation on their fundamental parameters. Fractional ages $τ/τ\_{\rm MS}$ ($τ\_{\rm MS}$ = time spent in the MS) against stellar mass reveal that: a) Be stars spread over the whole interval $0<τ/τ\_{\rm MS}<1$; b) the Be phenomenon in massive stars ($M>12M\_{\odot}$) is present at smaller age ratios than for less massive stars ($M<12M\_{\odot}$); c) there is a lack of Be stars with $M<7M\_{\odot}$ in the first half of the MS. Low mass fast rotators ($M<7M\_{\odot}$), called Bn stars, could be "becoming" Be stars.

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arXiv2005

On the Frequency of Field Galactic Be Stars

Since Be stars belong to the high velocity tail of a single B star velocity distribution, the appearance of the Be phenomenon must be independent of the stellar mass. In the present paper we show that the shape of the distribution of the number fraction N(Be)/N(Be+B) against the spectral type can be explained in terms of the Balmer line emission efficiency as a function of the effective temperature.

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arXiv2005

Achernar Can be a Differential Rotator

We take advantage of interferometric measurements of Achernar to inquire on its internal rotational law. The reinterpretation of interferome-tric data and the use of fundamental parameters corrected for gravitational darkening effects and models of 2D-models of internal stellar structures, lead us to the conclusion that the star could not be a rigid, near critical, rotator but a differential rotator with the core rotating $\sim3$ times faster than the surface.

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arXiv2005

Zams Rotational Velocities of be Stars

We show that Be stars belong to a high velocity tail of a single B-type star rotational velocity distribution in the main sequence (MS). We studied 127 galactic field Be stars and obtained their true equatorial velocity at the ZAMS using models of stellar evolution with rotation. There is a sharp mass-dependent cut in the ZAMS under which there is no Be star. Velocities above this cut follow a Gaussian-tail distribution. B stars with ZAMS rotational velocities lower than the cut probably cannot become Be.

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arXiv2005

Fast Rotation vs. Metallicity

Fast rotation seems to be the major factor to trigger the Be phenomenon. Surface fast rotation can be favored by initial formation conditions such as metal abundance. Models of fast rotating atmospheres and evolutionary tracks are used to determine the stellar fundamental parameters of 120 Be stars situated in spatially well-separated regions to imply there is between them some gradient of metallicity. We study the effects of the incidence of this gradient on the nature of the studied stars as fast rotators.

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arXiv2005

On the evolutionary status of Be stars. I. Field Be stars near the Sun

A sample of 97 galactic field Be stars were studied by taking into account the effects induced by the fast rotation on their fundamental parameters. All program stars were observed in the BCD spectrophotometric system in order to minimize the perturbations produced by the circumstellar environment on the spectral photospheric signatures. This is one of the first attempts at determining stellar masses and ages by simultaneously using model atmospheres and evolutionary tracks, both calculated for rotating objects. The stellar ages ($τ$) normalized to the respective inferred time that each rotating star can spend in the main sequence phase ($τ\_{\rm MS}$) reveal a mass-dependent trend. This trend shows that: a) there are Be stars spread over the whole interval $0 \la τ/τ\_{\rm MS} \la 1$ of the main sequence evolutionary phase; b) the distribution of points in the ($τ/τ\_{\rm MS},M/M\_{\odot}$) diagram indicates that in massive stars ($M \ga 12M\_{\odot}$) the Be phenomenon is present at smaller $τ/τ\_{\rm MS}$ age ratios than for less massive stars ($M \la 12M\_{\odot}$). This distribution can be due to: $i$) higher mass-loss rates in massive objets, which can act to reduce the surface fast rotation; $ii$) circulation time scales to transport angular momentum from the core to the surface, which are longer the lower the stellar mass.

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