432 research outputs found
Seismic modelling of the Cep star HD\,180642 (V1449\,Aql)
We present modelling of the Cep star HD\,180642 based on its
observational properties deduced from CoRoT and ground-based photometry as well
as from time-resolved spectroscopy. We investigate whether present-day
state-of-the-art models are able to explain the full seismic behaviour of this
star, which has extended observational constraints for this type of pulsator.
We constructed a dedicated database of stellar models and their oscillation
modes tuned to fit the dominant radial mode frequency of HD\,180642, by means
of varying the hydrogen content, metallicity, mass, age, and core overshooting
parameter. We compared the seismic properties of these models with those
observed. We find models that are able to explain the numerous observed
oscillation properties of the star, for a narrow range in mass of
11.4--11.8\,M and no or very mild overshooting (with up to 0.05 local
pressure scale heights), except for an excitation problem of the ,
p mode. We deduce a rotation period of about 13\,d, which is fully
compatible with recent magnetic field measurements. The seismic models do not
support the earlier claim of solar-like oscillations in the star. We instead
ascribe the power excess at high frequency to non-linear resonant mode coupling
between the high-amplitude radial fundamental mode and several of the low-order
pressure modes. We report a discrepancy between the seismic and spectroscopic
gravity at the level.Comment: 10 pages, 2 Tables, 6 Figures. Accepted for publication in Astronomy
and Astrophysic
An asteroseismic study of the beta Cephei star beta Canis Majoris
We present the results of a detailed analysis of 452 ground-based
high-resolution high S/N spectroscopic measurements spread over 4.5 years for
beta Canis Majoris with the aim to determine the pulsational characteristics of
this star, and to use them to derive seismic constraints on the stellar
parameters. We determine pulsation frequencies in the SiIII 4553 Angstrom line
with Fourier methods. We identify the m-value of the modes by taking into
account the photometric identifications of the degrees l. To this end we use
the moment method together with the amplitude and phase variations across the
line profile. The frequencies of the identified modes are used for a seismic
interpretation of the structure of the star. We confirm the presence of the
three pulsation frequencies already detected in previous photometric datasets:
f_1 = 3.9793 c/d (46.057 microHz), f_2 = 3.9995 c/d (46.291 microHz) and f_3 =
4.1832 c/d (48.417 microHz). For the two modes with the highest amplitudes we
unambiguously identify (l_1,m_1) = (2,2) and (l_2,m_2) = (0,0). We cannot
conclude anything for the third mode identification, except that m_3 > 0. We
also deduce an equatorial rotational velocity of 31 +/- 5 Km/s for the star. We
show that the mode f_1 must be close to an avoided crossing. Constraints on the
mass (13.5 +/- 0.5 Msun), age (12.4 +/- 0.7 Myr) and core overshoot (0.20 +/-
0.05 H_P) of beta CMa are obtained from seismic modelling using f_1 and f_2.Comment: Accepted for publication in A&
CoRoT high-precision photometry of the B0.5 IV star HD 51756
OB stars are important constituents for the ecology of the Universe, and
there are only a few studies on their pulsational properties detailed enough to
provide important feedback on current evolutionary models. Our goal is to
analyse and interpret the behaviour present in the CoRoT light curve of the
B0.5 IV star HD 51756 observed during the second long run of the space mission,
and to determine the fundamental stellar parameters from ground-based
spectroscopy gathered with the CORALIE and HARPS instruments after checking for
signs of variability and binarity, thus making a step further in mapping the
top of the Beta Cep instability strip. We compare the newly obtained
high-resolution spectra with synthetic spectra of late O-type and early B-type
stars computed on a grid of stellar parameters. We match the results with
evolutionary tracks to estimate stellar parameters. We use various time series
analysis tools to explore the nature of the variations present in the light
curve. Additional calculations are carried out based on distance and historical
position measurements of the components to impose constraints on the binary
orbit. We find that HD 51756 is a wide binary with both a slow (v sin i \approx
28 km s^-1) and a fast (v sin i \approx 170 km s^-1) early-B rotator whose
atmospheric parameters are similar (T_eff \approx 30000 K and log g \approx
3.75). We are unable to detect pulsation in any of the components, and we
interpret the harmonic structure in the frequency spectrum as sign of
rotational modulation, which is compatible with the observed and deduced
stellar parameters of both components. The non-detection of pulsation modes
provides a feedback on the theoretical treatment, given that non-adiabatic
computations applied to appropriate stellar models predict the excitation of
both pressure and gravity modes for the fundamental parameters of this star.Comment: Accepted for publication in Astronomy and Astrophysics on 14/01/2011,
11 pages, 9 figures, 4 table
First HARPSpol discoveries of magnetic fields in massive stars
In the framework of the Magnetism in Massive Stars (MiMeS) project, a
HARPSpol Large Program at the 3.6m-ESO telescope has recently started to
collect high-resolution spectropolarimetric data of a large number of Southern
massive OB stars in the field of the Galaxy and in many young clusters and
associations. In this Letter, we report on the first discoveries of magnetic
fields in two massive stars with HARPSpol - HD 130807 and HD 122451, and
confirm the presence of a magnetic field at the surface of HD 105382 that was
previously observed with a low spectral resolution device. The longitudinal
magnetic field measurements are strongly varying for HD 130807 from -100
G to 700 G. Those of HD 122451 and HD 105382 are less variable with
values ranging from -40 to -80 G, and from -300 to -600 G,
respectively. The discovery and confirmation of three new magnetic massive
stars, including at least two He-weak stars, is an important contribution to
one of the MiMeS objectives: the understanding of origin of magnetic fields in
massive stars and their impacts on stellar structure and evolution.Comment: 4 pages, 2 figures, accepted for publication in A&A Lette
An observational asteroseismic study of the pulsating B-type stars in the open cluster NGC 884
Recent progress in the seismic interpretation of field β Cep stars has resulted in improvements of the physical description in the stellar structure and evolution model computations of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition. We present an observational asteroseismic study based on the discovery of numerous multi-periodic and mono-periodic B-type stars in the open cluster NGC 884 (χ Persei). Our study illustrates the current status of ensemble asteroseismology of this young open cluste
Rotation and Convective Core Overshoot in theta Ophiuchi
(abridged) Recent work on several beta Cephei stars has succeeded in
constraining both their interior rotation profile and their convective core
overshoot. In particular, a recent study focusing on theta$ Oph has shown that
a convective core overshoot parameter of alpha = 0.44 is required to model the
observed pulsation frequencies, significantly higher than for other stars of
this type. We investigate the effects of rotation and overshoot in early type
main sequence pulsators, and attempt to use the low order pulsation frequencies
to constrain these parameters. This will be applied to a few test models and
theta Oph. We use a 2D stellar evolution code and a 2D linear adiabatic
pulsation code to calculate pulsation frequencies for 9.5 Msun models. We
calculate low order p-modes for models with a range of rotation rates and
convective core overshoot parameters. Using these models, we find that the
convective core overshoot has a larger effect on the pulsation frequencies than
the rotation, except in the most rapidly rotating models considered. When the
differences in radii are accounted for by scaling the frequencies, the effects
of rotation diminish, but are not entirely accounted for. We find that
increasing the convective core overshoot decreases the large separation, while
producing a slight increase in the small separations. We created a model
frequency grid which spanned several rotation rates and convective core
overshoot values. Using a modified chi^2 statistic, we are able to recover the
rotation velocity and core overshoot for a few test models. Finally, we discuss
the case of the beta Cephei star theta Oph. Using the observed frequencies and
a fixed mass and metallicity, we find a lower overshoot than previously
determined, with alpha = 0.28 +/- 0.05. Our determination of the rotation rate
agrees well with both previous work and observations, around 30 km/s.Comment: 10 pages, 14 figures. Accepted for publication in A&A
Analysis of MERCATOR data Part I: variable B stars
We re-classified 31 variable B stars which were observed more than 50 times
in the Geneva photometric system with the P7 photometer attached to the
MERCATOR telescope (La Palma) during its first 3 years of scientific
observations. HD89688 is a possible beta Cephei/slowly pulsating B star hybrid
and the main mode of the COROT target HD180642 shows non-linear effects. The
Maia candidates are re-classified as either ellipsoidal variables or spotted
stars. Although the mode identification is still ongoing, all the
well-identified modes so far have a degree l = 0, 1 or 2.Comment: 4 pages, 3 figures. To appear in: Proceedings of JENAM 2005 'Distant
worlds', Communications in Asteroseismolog
An asteroseismic study of the beta Cephei star 12 Lacertae: multisite spectroscopic observations, mode identification and seismic modelling
We present the results of a spectroscopic multisite campaign for the beta
Cephei star 12 (DD) Lacertae. Our study is based on more than thousand
high-resolution high S/N spectra gathered with 8 different telescopes in a time
span of 11 months. In addition we make use of numerous archival spectroscopic
measurements. We confirm 10 independent frequencies recently discovered from
photometry, as well as harmonics and combination frequencies. In particular,
the SPB-like g-mode with frequency 0.3428 1/d reported before is detected in
our spectroscopy. We identify the four main modes as (l1,m1) = (1, 1), (l2,m2)
= (0, 0), (l3,m3) = (1, 0) and (l4,m4) = (2, 1) for f1 = 5.178964 1/d, f2 =
5.334224 1/d, f3 = 5.066316 1/d and f4 = 5.490133 1/d, respectively. Our
seismic modelling shows that f2 is likely the radial first overtone and that
the core overshooting parameter alpha_ov is lower than 0.4 local pressure scale
heights.Comment: 16 pages, 11 figures, accepted in MNRA
On the co-existence of chemically peculiar Bp stars, slowly pulsating B stars and constant B stars in the same part of the H-R diagram
Aims. In order to better model massive B-type stars, we need to understand
the physical processes taking place in slowly pulsating B (SPB) stars,
chemically peculiar Bp stars, and non-pulsating normal B stars co-existing in
the same part of the H-R diagram.
Methods. We carry out a comparative study between samples of confirmed and
well-studied SPB stars and a sample of well-studied Bp stars with known periods
and magnetic field strengths. We determine their evolutionary state using
accurate HIPPARCOS parallaxes and Geneva photometry. We discuss the occurrence
and strengths of magnetic fields as well as the occurrence of stellar pulsation
among both groups. Further, we make a comparison of Geneva photometric
variability for both kinds of stars.
Results. The group of Bp stars is significantly younger than the group of SPB
stars. Longitudinal magnetic fields in SPB stars are weaker than those of Bp
stars, suggesting that the magnetic field strength is an important factor for B
type stars to become chemically peculiar. The strongest magnetic fields appear
in young Bp stars, indicating a magnetic field decay in stars at advanced ages.
Rotation periods of Bp and pulsation periods of SPB stars are of the same order
and the behaviour of Geneva photometric variability of some Bp stars cannot be
distinguished from the variability of SPB stars, illustrating the difficulty to
interpret the observed variability of the order of days for B-type stars. We
consider the possibility that pulsation could be responsible for the
variability among chemically peculiar stars. In particular, we show that a
non-linear pulsation model is not excluded by photometry for the Bp star
HD175362.Comment: Accepted for publication in Astronomy & Astrophysics on 29/01/2007, 8
pages, 9 figure
Discovery of magnetic fields in the beta Cephei star xi^1 CMa and in several Slowly Pulsating B stars
We present the results of a magnetic survey of a sample of eight beta Cephei
stars and 26 Slowly Pulsating B stars with FORS1 at the VLT. A weak mean
longitudinal magnetic field of the order of a few hundred Gauss is detected in
the beta Cephei star xi^1 CMa and in 13 SPB stars. The star xi^1 CMa becomes
the third magnetic star among the beta Cephei stars. Before our study, the star
zeta Cas was the only known magnetic SPB star. All magnetic SPB stars for which
we gathered several magnetic field measurements show a field that varies in
time. We do not find a relation between the evolution of the magnetic field
with stellar age in our small sample. Our observations imply that beta Cephei
stars and SPBs can no longer be considered as classes of non-magnetic
pulsators, but the effect of the fields on the oscillation properties remains
to be studied.Comment: 5 pages, 2 figures, 2 tables, accepted as a letter to MNRA
- …