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See detailThe Enigma of B-type Pulsators in the SMC
Salmon, Sébastien ULg; Montalban Iglesias, Josefa ULg; Miglio, Andrea ULg et al

in AIP Conference Proceedings (2009, September 01), 1170

Since the early nineties it is accepted that the excitation mechanism of B-type pulsators on the main sequence is due to the opacity peak in the iron-group elements at T~200,000 K. The Fe content plays ... [more ▼]

Since the early nineties it is accepted that the excitation mechanism of B-type pulsators on the main sequence is due to the opacity peak in the iron-group elements at T~200,000 K. The Fe content plays then a major role in the excitation of β Cep and SPB pulsations. While theoretical non-adiabatic computations predict no β Cep pulsators and only a small number of SPBs for low metallicity environments such as that of the Magellanic Clouds (MCs), recent variability surveys of B stars in the SMC have reported the detection of a significant number of SPB and β Cep candidates. Since the SMC is the metal poorest (Z~0001-0.004) of the MCs, it constitutes an interesting object for investigating the disagreement between theory and observations. We approach the problem by calling into question some of the hypotheses made in previous studies: given the different chemical evolution of the SMC compared with our local galactic environment, is it appropriate to describe the chemical composition of SMC B stars by scaling the solar mixture to lower Z? Is that composition uniform in space and time? In this paper we present the results of a stability analysis of B-type stellar models computed with a revised chemical composition and metallicity specific to the SMC. [less ▲]

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See detailThe evolution of magnetic fields in early B-type stars
Hubrig, S.; Briquet, Maryline ULg; Scholler, M. et al

in Revista Mexicana de Astronomia y Astrofisica Conference Series (2009, August 01)

To date, only a small number of O and early B-type stars have been investigated for magnetic fields, and as a result, only about a dozen magnetic early B-type stars and only few magnetic O stars are known ... [more ▼]

To date, only a small number of O and early B-type stars have been investigated for magnetic fields, and as a result, only about a dozen magnetic early B-type stars and only few magnetic O stars are known. The lack of information on the existence, origin and role of magnetic fields in massive stars is especially disturbing because magnetic fields may have paramount influence on the stellar evolution of high-mass stars. Our study focuses on the magnetic fields in early B-type stars with a stronger emphasis on beta Cep and SPB stars. Weak longitudinal magnetic fields (up to Ë 300 G) have been recently detected using FORS 1 in a few beta Cep and SPB stars, proving that these types of massive B-type stars can no longer be considered as non-magnetic. [less ▲]

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See detailSpace observations of B stars with CoRoT
Degroote, P.; Miglio, Andrea ULg; Debosscher, J. et al

in Communications in Asteroseismology (2009), 158

We present the preliminary results of the exploration of pulsating B stars observed with the CoRoT space mission. The previously known group of Slowly Pulsating B stars gains a substantial amount of new ... [more ▼]

We present the preliminary results of the exploration of pulsating B stars observed with the CoRoT space mission. The previously known group of Slowly Pulsating B stars gains a substantial amount of new candidates, offering the opportunity to test stellar models beyond individual cases. Besides these well-defined stars, the analysis of other B star candidate pulsators hints towards the presence of different variability behaviour, co-existing in the same space in terms of the timescale of the variations and location in the (Teff, logg) diagram. [less ▲]

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See detailGround-based observations of the beta Cephei CoRoT main target HD 180642
Briquet, Maryline ULg; Uytterhoeven, K.; Aerts, C. et al

in Communications in Asteroseismology (2009), 158

We present the preliminary results of a detailed study of ground-based photometric and spectroscopic observations dedicated to the β Cephei CoRoT main target HD~180642. Besides the non-linear dominant ... [more ▼]

We present the preliminary results of a detailed study of ground-based photometric and spectroscopic observations dedicated to the β Cephei CoRoT main target HD~180642. Besides the non-linear dominant radial mode several low-amplitude modes are detected in both kinds of datasets. Our aim is to derive the wavenumbers (â ,m) of these modes, as additional constraints to the CoRoT pulsation frequencies, for forthcoming asteroseismic modelling of the star. [less ▲]

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See detailAbundances of massive stars: some recent developments
Morel, Thierry ULg

in Communications in Asteroseismology (2009), 158

The last few years, we have witnessed a large increase in the amount of abundance data for early-type stars, which have been very useful in various fields of astrophysics (e.g. mixing processes in stars ... [more ▼]

The last few years, we have witnessed a large increase in the amount of abundance data for early-type stars, which have been very useful in various fields of astrophysics (e.g. mixing processes in stars, chemical evolution of galaxies). Two intriguing results emerging since the last reviews on this topic (Herrero 2003; Herrero & Lennon 2004) will be discussed: (a) nearby OB stars exhibit metal abundances generally lower than the solar/meteoritic estimates; (b) evolutionary models of single objects including rotation are largely unsuccessful in explaining the CNO properties of stars in the Galaxy and in the Magellanic clouds. [less ▲]

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See detailUncertainties in the chemical composition of B-type stars: effects on the opacity and on the excitation of pulsation modes
Montalban Iglesias, Josefa ULg; Miglio, Andrea ULg; Morel, Thierry ULg

in Communications in Asteroseismology (2009), 158

Recent determinations of element abundances in early-B stars have shown a significant discrepancy with those derived for the Sun. β Cep pulsators are early-B stars whose pulsations are driven by the ... [more ▼]

Recent determinations of element abundances in early-B stars have shown a significant discrepancy with those derived for the Sun. β Cep pulsators are early-B stars whose pulsations are driven by the opacity mechanism operating at the Fe-group opacity bump (e.g. Dziembowski & Pamyatnykh 1993). Since each element contributes differently to the stellar opacity in the Z-bump region, differences in the metal mixture may affect the pulsation properties of these early-B stars. Here we study the role of different elements in the stellar opacity of B-type stars, and the consequences of the uncertainties in the composition of the metal mixture for the instability strip of β~Cep stars. [less ▲]

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See detailNew magnetic field measurements of beta Cephei stars and slowly pulsating B stars
Hubrig, Swetlana; Briquet, Maryline ULg; De Cat, P. et al

in IAU Symposium (2009, April 01)

We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B-type stars consisting of confirmed or candidate β Cephei stars and Slowly Pulsating B stars ... [more ▼]

We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B-type stars consisting of confirmed or candidate β Cephei stars and Slowly Pulsating B stars. Roughly one third of the studied β Cephei stars have detected magnetic fields. The fraction of magnetic Slowly Pulsating B and candidate Slowly Pulsating B stars is found to be higher, up to 50%. We find that the domains of magnetic and non-magnetic pulsating stars in the H-R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence. [less ▲]

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See detailNew magnetic field measurements of beta Cephei stars and slowly pulsating B stars
Hubrig, S.; Briquet, Maryline ULg; De Cat, P. et al

in Astronomische Nachrichten (2009), 330

We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B-type stars consisting of confirmed or candidate β Cephei stars and Slowly Pulsating B (hereafter ... [more ▼]

We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B-type stars consisting of confirmed or candidate β Cephei stars and Slowly Pulsating B (hereafter SPB) stars, along with a small number of normal B-type stars. A weak mean longitudinal magnetic field of the order of a few hundred Gauss was detected in three β Cephei stars and two stars suspected to be β Cephei stars, in five SPB stars and eight stars suspected to be SPB stars. Additionally, a longitudinal magnetic field at a level larger than 3Ï has been diagnosed in two normal B-type stars, the nitrogen-rich early B-type star HD 52089 and in the B5 IV star HD 153716. Roughly one third of β Cephei stars have detected magnetic fields: Out of 13 β Cephei stars studied to date with FORS 1, four stars possess weak magnetic fields, and out of the sample of six suspected β Cephei stars two show a weak magnetic field. The fraction of magnetic SPBs and candidate SPBs is found to be higher: Roughly half of the 34 SPB stars have been found to be magnetic and among the 16 candidate SPBs eight stars possess magnetic fields. In an attempt to understand why only a fraction of pulsating stars exhibit magnetic fields, we studied the position of magnetic and non-magnetic pulsating stars in the H-R diagram. We find that their domains in the H-R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence. It is possible that stronger fields tend to be found in stars with lower pulsating frequencies and smaller pulsating amplitudes. A somewhat similar trend is found if we consider a correlation between the field strength and the v sin i-values, i.e. stronger magnetic fields tend to be found in more slowly rotating stars. Based on observations obtained at the European Southern Observatory, Paranal, Chile (ESO programmes 078.D-0140(A), 078.D-0330(A), 079.D-0241(A), and 080.D-0383(A)). [less ▲]

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See detailStudying the Magnetic Properties of Upper Main-sequence Stars with FORS1
Hubrig, Swetlana; Scholler, Markus; Briquet, Maryline ULg et al

in The Messenger (2009), 135

We summarise the results of our recent magnetic field studies in upper main-sequence stars, which have exploited the spectropolarimetric capability of FORS1 at the VLT extensively.

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See detailRadial velocities with the Gaia RVS spectrometer
Viala, Y. P.; Blomme, R.; Damerdji, Yassine ULg et al

in Charbonnel, C.; Combes, F.; Samadi, R. (Eds.) SF2A-2008; Annual Meeting of the French Society of Astronomy (2008, November 01)

Four different method are used to derive radial velocities from spectra observed by the Gaia Radial Velocity Spectrometer (RVS). They are briefly presented here together with very preliminary results.

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See detailIs neon a viable solution to the solar model problem? Insights from nearby B-type stars
Morel, Thierry ULg; Butler, Keith

in Journal of Physics Conference Series (2008), 118

The recent downward revision of the solar CNO photospheric abundances now leads to severe inconsistencies between theoretical models for the Sun's internal structure and the results of helioseismology ... [more ▼]

The recent downward revision of the solar CNO photospheric abundances now leads to severe inconsistencies between theoretical models for the Sun's internal structure and the results of helioseismology. There have been claims that the solar Ne abundance may be underestimated and that an increase in this ill-defined quantity could alleviate (or even completely solve) this problem. To address the validity of this hypothesis, we present a fully homogeneous non-LTE abundance analysis of the optical Ne I/II lines in a sample of 18 nearby B-type stars. Our very preliminary results point towards a mean neon abundance slightly higher than the new solar value, but apparently insufficient by itself to restore the agreement between the solar models and the helioseismological constraints. [less ▲]

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See detailClose-up of primary and secondary asteroseismic CoRoT targets and the ground-based follow-up observations
Uytterhoeven, K.; Poretti, E.; Rainer, M. et al

in Journal of Physics Conference Series (2008), 118

To optimise the science results of the asteroseismic part of the CoRoT satellite mission a complementary simultaneous ground-based observational campaign is organised for selected CoRoT targets. The ... [more ▼]

To optimise the science results of the asteroseismic part of the CoRoT satellite mission a complementary simultaneous ground-based observational campaign is organised for selected CoRoT targets. The observations include both high-resolution spectroscopic and multicolour photometric data. We present the preliminary results of the analysis of the ground-based observations of three targets. A line-profile analysis of 216 high-resolution FEROS spectra of the δ Sct star HD 50844 reveals more than ten pulsation frequencies in the frequency range 5-18 d[SUP]-1[/SUP], including possibly one radial fundamental mode (6.92 d[SUP]-1[/SUP]). Based on more than 600 multi-colour photometric datapoints of the β Cep star HD 180642, spanning about three years and obtained with different telescopes and different instruments, we confirm the presence of a dominant radial mode ν[SUB]1[/SUB] = 5.48695 d[SUP]-1[/SUP], and detect also its first two harmonics. We find evidence for a second mode ν[SUB]2[/SUB] = 0.3017 d[SUP]-1[/SUP], possibly a g-mode, and indications for two more frequencies in the 7-8 d[SUP]-1[/SUP] domain. From Stromgren photometry we find evidence for the hybrid 5 Sct/γ Dor character of the F0 star HD 44195, as frequencies near 3 d[SUP]-1[/SUP] and 21 d[SUP]-1[/SUP] are detected simultaneously in the different filters. [less ▲]

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See detailNew insights into the nature of the peculiar star theta Carinae
Hubrig, S.; Briquet, Maryline ULg; Morel, Thierry ULg et al

in Astronomy and Astrophysics (2008), 488

Context: θ Carinae belongs to a group of peculiar early-type stars (OBN) with enhanced nitrogen and carbon deficiency. It is also known as a binary system, but it is not clear yet whether the chemical ... [more ▼]

Context: θ Carinae belongs to a group of peculiar early-type stars (OBN) with enhanced nitrogen and carbon deficiency. It is also known as a binary system, but it is not clear yet whether the chemical anomalies can be explained by mass transfer between the two components. On the basis of the previously reported spectral variability of a few metal lines it may be expected that θ Car possesses a weak magnetic field. <BR />Aims: A study of the physical nature of this hot massive binary which is furthermore a well-known blue straggler lying ~2 mag above the turnoff of the young open cluster IC 2602 is important to understand the origin of its strong chemical anomalies. <BR />Methods: We acquired high resolution spectroscopic and low resolution spectropolarimetric observations to achieve the following goals: a) to improve the orbital parameters to allow a more in-depth discussion on the possibility of mass transfer in the binary system; b) to carry out a non-local thermodynamic equilibrium (NLTE) abundance analysis; and c) to search for the presence of a magnetic field. <BR />Results: The study of the radial velocities using CORALIE spectra allowed us to significantly improve the orbital parameters. A comparative NLTE abundance analysis was undertaken for θ Car and two other early B-type stars with recently detected magnetic fields, Ï Sco and ξ[SUP]1[/SUP] CMa. The analysis revealed significantly different abundance patterns: a one-order-of-magnitude nitrogen overabundance and carbon depletion was found in θ Car, while the oxygen abundance is roughly solar. For the stars ξ[SUP]1[/SUP] CMa and Ï Sco the carbon abundance is solar and, while an N excess is also detected, it is of much smaller amplitude (0.4-0.6 dex). Such an N overabundance is typical of the values already found for other slowly-rotating (magnetic) B-type dwarfs. For θ Car, we attribute instead the chemical peculiarities to a past episode of mass transfer between the two binary components. The results of the search for a magnetic field using FORS 1 at the VLT consisting of 26 measurements over a time span of ~1.2 h are rather inconclusive: only few measurements have a significance level of 3Ï . Although we detect a periodicity of the order of ~8.8 min in the dataset involving the measurements on all hydrogen Balmer lines with the exception of the Hα and Hβ lines, these results have to be confirmed by additional time-resolved magnetic field observations. Based on observations collected with the CORALIE spectrograph attached to the Euler Telescope of the Geneva Observatory located at La Silla (Chile), at the European Southern Observatory, Paranal, Chile (ESO programmes 67.D-0239(A), 072.D-0377(A), 078.D-0080(A) and 278.D-5056(A)), and at the Complejo Astronómico El Leoncito (CASLEO), Argentina. [less ▲]

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See detailThe neon content of nearby B-type stars and its implications for the solar model problem
Morel, Thierry ULg; Butler, K.

in Astronomy and Astrophysics (2008), 487

The recent downward revision of the solar photospheric abundances now leads to severe inconsistencies between the theoretical predictions for the internal structure of the Sun and the results of ... [more ▼]

The recent downward revision of the solar photospheric abundances now leads to severe inconsistencies between the theoretical predictions for the internal structure of the Sun and the results of helioseismology. There have been claims that the solar neon abundance may be underestimated and that an increase in this poorly-known quantity could alleviate (or even completely solve) this problem. Early-type stars in the solar neighbourhood are well-suited to testing this hypothesis because they are the only stellar objects whose absolute neon abundance can be derived from the direct analysis of photospheric lines. Here we present a fully homogeneous NLTE abundance study of the optical Ne I and Ne II lines in a sample of 18 nearby, early B-type stars, which suggests log É (Ne) = 7.97 ± 0.07 dex (on the scale in which log É [H] = 12) for the present-day neon abundance of the local interstellar medium (ISM). Chemical evolution models of the Galaxy only predict a very small enrichment of the nearby interstellar gas in neon over the past 4.6 Gyr, implying that our estimate should be representative of the Sun at birth. Although higher by about 35% than the new recommended solar abundance, such a value appears insufficient by itself to restore the past agreement between the solar models and the helioseismological constraints. Appendices A and B are only available in electronic form at http://www.aanda.org Table [see full text] is also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/487/307 [less ▲]

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See detailGround-based observations of the beta Cephei CoRoT main target HD 180642
Briquet, Maryline ULg; Uytterhoeven, K.; Aerts, C. et al

Conference (2008, July)

We present the preliminary results of a detailed study of ground-based photometric and spectroscopic observations dedicated to the β Cephei CoRoT main target HD 180642. Besides the non-linear dominant ... [more ▼]

We present the preliminary results of a detailed study of ground-based photometric and spectroscopic observations dedicated to the β Cephei CoRoT main target HD 180642. Besides the non-linear dominant radial mode several low-amplitude modes are detected in both kinds of datasets. Our aim is to derive the wavenumbers (ℓ,m) of these modes, as additional constraints to the CoRoT pulsation frequencies, for forthcoming asteroseismic modelling of the star. [less ▲]

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See detailThe stellar population of the Rosat North Ecliptic Pole survey. II. Spectral analysis
Affer, L.; Micela, G.; Morel, Thierry ULg

in Astronomy and Astrophysics (2008), 483

Context: X-ray surveys allow to identify young, main-sequence stars in the solar neighborhood. Young, stellar samples, selected according to their activity, can be used to determine the stellar birthrate ... [more ▼]

Context: X-ray surveys allow to identify young, main-sequence stars in the solar neighborhood. Young, stellar samples, selected according to their activity, can be used to determine the stellar birthrate in the last billion years. The ROSAT North Ecliptic Pole survey (NEP), with its moderately deep sensitivity (fluxes â 10[SUP]-14[/SUP] erg cm[SUP]-2[/SUP] s[SUP]-1[/SUP]), is the best survey, to date, able to sample the intermediate-age (10^8{-}10[SUP]9[/SUP] years) nearby population. The identification process of NEP X-ray sources resulted in 144 X-ray sources having a normal stellar counterpart, with an excess of yellow stars with respect to model predictions. <BR />Aims: We want to determine if these X-ray active stars are young or intermediate-age stars, or active binaries. <BR />Methods: We acquired high-resolution, high signal-to-noise ratio optical spectra, to determine the age and physical properties of the NEP X-ray-detected stellar sources. We measure the (i) lithium abundance using the Li I 6707.8 à line, which is an excellent, youth indicator for our age range of interest; (ii) rotational and radial velocities (through cross-correlation methods); and (iii) chromospheric emission (from Hα and Na I D{_1} and D{_2} lines). <BR />Results: The radial velocities distribution is consistent with that of a young field star population of age 4à 10{^9} yrs, or younger. Rotational velocity measurements imply that our sample is dominated by relatively young or intermediate-age stars, as confirmed by our lithium measurements. <BR />Conclusions: Most of the detected stars probably belong to a young or intermediate-age population. Our measurements suggest that a burst in the stellar birthrate of a factor of four occurred in the last 10[SUP]8[/SUP] years. We cannot, however, exclude the possibility that a small fraction of sources, amongst the fastest of the K-rotators, are old binary systems with tidally-locked rotation. [less ▲]

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See detailMagnetic fields in massive stars
Hubrig, S.; Scholler, M.; Briquet, Maryline ULg et al

in Contributions of the Astronomical Observatory Skalnaté Pleso (2008), 38

We review the recent discoveries of magnetic fields in different types of massive stars and briefly discuss strategies for spectropolarimetric observations to be carried out in the future.

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See detailNitrogen enrichment, boron depletion and magnetic fields in slowly-rotating B-type dwarfs
Morel, Thierry ULg; Hubrig, S.; Briquet, Maryline ULg

in Astronomy and Astrophysics (2008), 481

Evolutionary models for massive stars, accounting for rotational mixing effects, do not predict any core-processed material at the surface of B dwarfs with low rotational velocities. Contrary to ... [more ▼]

Evolutionary models for massive stars, accounting for rotational mixing effects, do not predict any core-processed material at the surface of B dwarfs with low rotational velocities. Contrary to theoretical expectations, we present a detailed and fully-homogeneous, NLTE abundance analysis of 20 early B-type dwarfs and (sub)giants that reveals the existence of a population of nitrogen-rich and boron-depleted, yet intrinsically slowly-rotating objects. The low-rotation rate of several of these stars is firmly established, either from the occurrence of phase-locked UV wind line-profile variations, which can be ascribed to rotational modulation, or from theoretical modelling in the pulsating variables. The observational data presently available suggest a higher incidence of chemical peculiarities in stars with a (weak) detected magnetic field. This opens the possibility that magnetic phenomena are important in altering the photospheric abundances of early B dwarfs, even for surface field strengths at the one hundred Gauss level. However, further spectropolarimetric observations are needed to assess the validity of this hypothesis. [less ▲]

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See detailThe Chemical Composition of B-type Pulsators: Some Unexpected Results
Morel, Thierry ULg; Briquet, Maryline ULg; Aerts, C.

in Precision Spectroscopy in Astrophysics (2008)

We present a project aimed at self-consistently deriving the physical parameters and chemical composition of massive pulsators (β, Cephei stars, Slowly Pulsating B stars) based on high-resolution optical ... [more ▼]

We present a project aimed at self-consistently deriving the physical parameters and chemical composition of massive pulsators (β, Cephei stars, Slowly Pulsating B stars) based on high-resolution optical spectra. Such data will be essential for a proper theoretical interpretation of their oscillation spectrum, but may also contribute in a broader context to our understanding of mixing and diffusion processes in B-type stars. As an illustration, our first results reveal the existence of core-processed material at the surface of some slowly-rotating β, Cephei stars which is not predicted by current evolutionary models including rotation. [less ▲]

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See detailDeep Mixing in Slowly-rotating B Dwarfs: the Possible Role Played by Magnetic Fields
Morel, Thierry ULg

in Unsolved Problems in Stellar Physics: A Conference in Honor of Douglas Gough (2007, November 01)

Evolutionary models for massive stars which take into account rotational mixing effects do not predict any core-processed material at the surface of B dwarfs with low rotational velocities. We present a ... [more ▼]

Evolutionary models for massive stars which take into account rotational mixing effects do not predict any core-processed material at the surface of B dwarfs with low rotational velocities. We present a detailed and fully homogeneous non-LTE abundance analysis of a sample of largely unevolved B stars which points to the existence of a population of intrinsically slowly-rotating, yet nitrogen-rich objects contrary to theoretical expectations. These observations are discussed in relation to the boron content and the (weak) magnetic field detected in some targets. [less ▲]

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