41 research outputs found
High-precision elements of double-lined spectroscopic binaries from combined interferometry and spectroscopy. Application to the beta Cephei star beta Centauri
We present methodology to derive high-precision estimates of the fundamental
parameters of double-lined spectroscopic binaries. We apply the methods to the
case study of the double-lined beta Cephei star beta Centauri. We also present
a detailed analysis of beta Centauri's line-profile variations caused by its
oscillations. We point out that a systematic error in the orbital amplitudes,
and any quantities derived from them, occurs if the radial velocities of
blended component lines are computed without spectral disentangling. This
technique is an essential ingredient in the derivation of the physical
parameters if the goal is to obtain a precision of only a few percent. We have
devised iteration schemes to obtain the orbital elements for systems whose
lines are blended throughout the orbital cycle. We find the following
parameters for beta Cen: and , an age of years. We deduce two oscillation
frequencies for the broad-lined primary of beta Centauri with degrees higher
than 2. We propose that our iteration schemes be used in any future derivations
of the spectroscopic orbital parameters of double-lined binaries with blended
component lines to which disentangling can be successfully applied.Comment: 12 pages, 13 figures, accepted for publication in A&
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
Orbital parameters, masses and distance to Beta Centauri determined with the Sydney University Stellar Interferometer and high resolution spectroscopy
The bright southern binary star beta Centauri (HR 5267) has been observed
with the Sydney University Stellar Interferometer (SUSI) and spectroscopically
with the ESO CAT and Swiss Euler telescopes at La Silla. The interferometric
observations have confirmed the binary nature of the primary component and have
enabled the determination of the orbital parameters of the system. At the
observing wavelength of 442 nm the two components of the binary system have a
magnitude difference of 0.15. The combination of interferometric and
spectroscopic data gives the following results: orbital period 357 days,
semi-major axis 25.30 mas, inclination 67.4 degrees, eccentricity 0.821,
distance 102.3 pc, primary and secondary masses M1 = M2 = 9.1 solar masses and
absolute visual magnitudes of the primary and secondary M1V = -3.85 and M2V =
-3.70. The high accuracy of the results offers a fruitful starting point for
future asteroseismic modelling of the pulsating binary components.Comment: 10 pages, 4 figures. Accepted for publication in MNRA
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
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&
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
Detection of frequency spacings in the young O-type binary HD 46149 from CoRoT photometry
Using the CoRoT space based photometry of the O-type binary HD46149, stellar
atmospheric effects related to rotation can be separated from pulsations,
because they leave distinct signatures in the light curve. This offers the
possibility of characterising and exploiting any pulsations seismologically.
Combining high-quality space based photometry, multi-wavelength photometry,
spectroscopy and constraints imposed by binarity and cluster membership, the
detected pulsations in HD46149 are analyzed and compared with those for a grid
of stellar evolutionary models in a proof-of-concept approach. We present
evidence of solar-like oscillations in a massive O-type star, and show that the
observed frequency range and spacings are compatible with theoretical
predictions. Thus, we unlock and confirm the strong potential of this
seismically unexplored region in the HR diagram.Comment: 11 pages, 12 figures, 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
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
An asteroseismic study of the Beta Cephei star Theta Ophiuchi: spectroscopic results
We present the results of a detailed analysis of 121 ground-based
high-resolution high S/N spectroscopic measurements spread over 3 years for the
Beta Cephei star Theta Ophiuchi. We discovered Theta Oph to be a triple system.
In addition to the already known Speckle B5 companion of the B2 primary, we
showed the presence of a low-mass spectroscopic companion and we derived an
orbital period of 56.71 days with an eccentricity of 0.1670. After removing the
orbit we determined two frequencies for the primary in the residual radial
velocities: f1 = 7.1160 c/d and f2 = 7.4676 c/d. We also found the presence of
f3 = 7.3696 c/d by means of a two dimensional frequency search across the Si
III 4567 A profiles. We identified the m-value of the main mode with frequency
f1 by taking into account the photometric identifications of the degrees l. By
means of the moment method and the amplitude and phase variations across the
line profile, we derived (l1,m1) = (2,-1). This result allows us to fix the
mode identifications of the whole quintuplet for which three components were
detected in photometry. This is of particular use for our forthcoming seismic
modelling of the primary. We also determined stellar parameters of the primary
by non-local thermodynamic equilibrium hydrogen, helium and silicon line
profile fitting and we obtained Teff = 24000 K and log g = 4.1, which is
consistent with photometrically determined values.Comment: 8 pages, 6 figure