45 research outputs found
A Quantitative Analysis of the Available Multicolor Photometry for Rapidly Pulsating Hot B Subdwarfs
We present a quantitative and homogeneous analysis of the broadband
multicolor photometric data sets gathered so far on rapidly pulsating hot B
subdwarf stars. This concerns seven distinct data sets related to six different
stars. Our analysis is carried out within the theoretical framework developed
by Randall et al., which includes full nonadiabatic effects. The goal of this
analysis is partial mode identification, i.e., the determination of the degree
index l of each of the observed pulsation modes. We assume possible values of l
from 0 to 5 in our calculations. For each target star, we compute a specific
model atmosphere and a specific pulsation model using estimates of the
atmospheric parameters coming from time-averaged optical spectroscopy. For
every assumed value of l, we use a formal chi-squared approach to model the
observed amplitude-wavelength distribution of each mode, and we compute a
quality-of-fit Q probability to quantify the derived fit and to discriminate
objectively between the various solutions. We find that no completely
convincing and unambiguous l identification is possible on the basis of the
available data, although partial mode discrimination has been reached for 25
out of the 41 modes studied. A brief statistical study of these results
suggests that a majority of the modes must have l values of 0, 1, and 2, but
also that modes with l = 4 could very well be present while modes with l = 3
appear to be rarer. This is in line with recent results showing that l = 4
modes in rapidly pulsating B subdwarfs have a higher visibility in the optical
domain than modes with l = 3. Although somewhat disappointing in terms of mode
discrimination, our results still suggest that the full potential of multicolor
photometry for l identification in pulsating subdwarfs is within reach.Comment: 59 pages, 18 figures, accepted for publication in the Astrophysical
Journal Supplement Serie
From the Heart of The Ghoul: C and N Abundances in the Corona of Algol B
Chandra Low Energy Transmission Grating Spectrograph observations of Algol
have been used to determine the abundances of C and N in the secondary star for
the first time. The analysis was performed relative to similar observations of
an adopted "standard" star HR 1099. It is demonstrated that HR 1099 and Algol
are coronal twins in many respects and that their X-ray spectra are very
similar in nearly all details, except for the observed strengths of C and N
lines. The H-like transitions of C and N in the coronae of Algol and HR 1099
demonstrate that the surface abundances of Algol B have been strongly modified
by CN-processing, as shown earlier by Schmitt & Ness (2002). It is found that N
is enhanced in Algol B by a factor of 3 compared to HR 1099. No C lines are
detected in the Algol spectrum, indicating a C depletion relative to HR 1099 by
a factor of 10 or more. These C and N abundances indicate that Algol B must
have lost at least half of its initial mass, and are consistent with
predictions of evolutionary models that include non-conservative mass transfer
and angular momentum loss through magnetic activity. Little or no dredge-up of
material subjected to CN-processing has occurred on the subgiant component of
HR 1099. It is concluded that Fe is very likely depleted in the coronae of both
Algol and HR 1099 relative to their photospheres by 0.5 dex, and C, N and O by
0.3 dex. Instead, Ne is enhanced by up to 0.5 dex.Comment: 17 pages, 4 figures, ApJ accepte
Massive pulsating stars observed by BRITE-Constellation. I. The triple system Beta Centauri (Agena)
This paper aims to precisely determine the masses and detect pulsation modes
in the two massive components of Beta Cen with BRITE-Constellation photometry.
In addition, seismic models for the components are considered and the effects
of fast rotation are discussed. This is done to test the limitations of seismic
modeling for this very difficult case. A simultaneous fit of visual and
spectroscopic orbits is used to self-consistently derive the orbital
parameters, and subsequently the masses, of the components. The derived masses
are equal to 12.02 +/- 0.13 and 10.58 +/- 0.18 M_Sun. The parameters of the
wider, A - B system, presently approaching periastron passage, are constrained.
Analysis of the combined blue- and red-filter BRITE-Constellation photometric
data of the system revealed the presence of 19 periodic terms, of which eight
are likely g modes, nine are p modes, and the remaining two are combination
terms. It cannot be excluded that one or two low-frequency terms are rotational
frequencies. It is possible that both components of Beta Cen are Beta Cep/SPB
hybrids. An attempt to use the apparent changes of frequency to distinguish
which modes originate in which component did not succeed, but there is
potential for using this method when more BRITE data become available. Agena
seems to be one of very few rapidly rotating massive objects with rich p- and
g-mode spectra, and precisely known masses. It can therefore be used to gain a
better understanding of the excitation of pulsations in relatively rapidly
rotating stars and their seismic modeling. Finally, this case illustrates the
potential of BRITE-Constellation data for the detection of rich-frequency
spectra of small-amplitude modes in massive pulsating stars.Comment: 17 pages (with Appendix), 15 figures, accepted for publication in A&
delta Ceti is not monoperiodic: seismic modeling of a beta Cephei star from MOST spacebased photometry
The beta Cephei star delta Ceti was considered one of the few monoperiodic
variables in the class. Despite (or perhaps because of) its apparently simple
oscillation spectrum, it has been challenging and controversial to identify
this star's pulsation mode and constrain its physical parameters seismically.
Broadband time-resolved photometry of delta Ceti spanning 18.7 days with a duty
cycle of about 65% obtained by the MOST (Microvariability & Oscillations of
STars) satellite -- the first scientific observations ever obtained by MOST --
reveals that the star is actually multiperiodic. Besides the well-known
dominant frequency of f1 = 6.205886/d, we have discovered in the MOST data its
first harmonic 2f1 and three other frequencies (f2 = 3.737/d, f3 = 3.673/d and
f4 = 0.318/d), all detected with S/N > 4. In retrospect, f2 was also present in
archival spectral line profile data but at lower S/N. We present seismic models
whose modes match exactly the frequencies f1 and f2. Only one model falls
within the common part of the error boxes of the star's observed surface
gravity and effective temperature from photometry and spectroscopy. In this
model, f1 is the radial (l = 0) first overtone and f2 is the g2 (l = 2, m = 0)
mode. This model has a mass of 10.2+/-0.2 Msun and an age of 17.9+/-0.3 million
years, making delta Ceti an evolved beta Cephei star. If f2 and f3 are
rotationally split components of the same g2 mode, then the star's equatorial
rotation velocity is either 27.6 km/s or half this value. Given its vsini of
about 1 km/s, this implies we are seeing delta Ceti nearly pole-on.Comment: Accepted for publication in The Astrophysical Journal, 12 pages, 8
figures, 2 table
UVSat: a concept of an ultraviolet/optical photometric satellite
Time-series photometry from space in the ultraviolet can be presently done
with only a few platforms, none of which is able to provide wide-field
long-term high-cadence photometry. We present a concept of UVSat, a twin space
telescope which will be capable to perform this kind of photometry, filling an
observational niche. The satellite will host two telescopes, one for
observations in the ultraviolet, the other for observations in the optical
band. We also briefly show what science can be done with UVSat.Comment: 6 pages, 2 figures, accepted for publication in the Proceedings of
the PAS (Proc. of the 2nd BRITE Science conference, Innsbruck
A multisite photometric study of two unusual Beta Cep stars: the magnetic V2052 Oph and the massive rapid rotator V986 Oph
We report a multisite photometric campaign for the Beta Cep stars V2052 Oph
and V986 Oph. 670 hours of high-quality differential photoelectric Stromgren,
Johnson and Geneva time-series photometry were obtained with eight telescopes
on five continents during 182 nights. Frequency analyses of the V2052 Oph data
enabled the detection of three pulsation frequencies, the first harmonic of the
strongest signal, and the rotation frequency with its first harmonic.
Pulsational mode identification from analysing the colour amplitude ratios
confirms the dominant mode as being radial, whereas the other two oscillations
are most likely l=4. Combining seismic constraints on the inclination of the
rotation axis with published magnetic field analyses we conclude that the
radial mode must be the fundamental. The rotational light modulation is in
phase with published spectroscopic variability, and consistent with an oblique
rotator for which both magnetic poles pass through the line of sight. The
inclination of the rotation axis is 54o <i< 58o and the magnetic obliquity 58o
<beta< 66o. The possibility that V2052 Oph has a magnetically confined wind is
discussed. The photometric amplitudes of the single oscillation of V986 Oph are
most consistent with an l=3 mode, but this identification is uncertain.
Additional intrinsic, apparently temporally incoherent, light variations of
V986 Oph are reported. Different interpretations thereof cannot be
distinguished at this point, but this kind of variability appears to be present
in many OB stars. The prospects of obtaining asteroseismic information for more
rapidly rotating Beta Cep stars, which appear to prefer modes of higher l, are
briefly discussed.Comment: 12 pages, 8 figures, MNRAS, in pres
Multisite spectroscopic seismic study of the beta Cep star V2052 Oph: inhibition of mixing by its magnetic field
We used extensive ground-based multisite and archival spectroscopy to derive
observational constraints for a seismic modelling of the magnetic beta Cep star
V2052 Ophiuchi. The line-profile variability is dominated by a radial mode
(f_1=7.14846 d^{-1}) and by rotational modulation (P_rot=3.638833 d). Two
non-radial low-amplitude modes (f_2=7.75603 d^{-1} and f_3=6.82308 d^{-1}) are
also detected. The four periodicities that we found are the same as the ones
discovered from a companion multisite photometric campaign (Handler et al.
2012) and known in the literature. Using the photometric constraints on the
degrees l of the pulsation modes, we show that both f_2 and f_3 are prograde
modes with (l,m)=(4,2) or (4,3). These results allowed us to deduce ranges for
the mass (M \in [8.2,9.6] M_o) and central hydrogen abundance (X_c \in
[0.25,0.32]) of V2052 Oph, to identify the radial orders n_1=1, n_2=-3 and
n_3=-2, and to derive an equatorial rotation velocity v_eq \in [71,75] km
s^{-1}. The model parameters are in full agreement with the effective
temperature and surface gravity deduced from spectroscopy. Only models with no
or mild core overshooting (alpha_ov \in [0,0.15] local pressure scale heights)
can account for the observed properties. Such a low overshooting is opposite to
our previous modelling results for the non-magnetic beta Cep star theta Oph
having very similar parameters, except for a slower surface rotation rate. We
discuss whether this result can be explained by the presence of a magnetic
field in V2052 Oph that inhibits mixing in its interior.Comment: 12 pages, 6 figures and 5 tables; accepted for publication in MNRAS
on 2012 August 1
Catalog of Galactic Beta Cephei Stars
We present an extensive and up-to-date catalog of Galactic Beta Cephei stars.
This catalog is intended to give a comprehensive overview of observational
characteristics of all known Beta Cephei stars. 93 stars could be confirmed to
be Beta Cephei stars. For some stars we re-analyzed published data or conducted
our own analyses. 61 stars were rejected from the final Beta Cephei list, and
77 stars are suspected to be Beta Cephei stars. A list of critically selected
pulsation frequencies for confirmed Beta Cephei stars is also presented. We
analyze the Beta Cephei stars as a group, such as the distributions of their
spectral types, projected rotational velocities, radial velocities, pulsation
periods, and Galactic coordinates. We confirm that the majority of these stars
are multiperiodic pulsators. We show that, besides two exceptions, the Beta
Cephei stars with high pulsation amplitudes are slow rotators. We construct a
theoretical HR diagram that suggests that almost all 93 Beta Cephei stars are
MS objects. We discuss the observational boundaries of Beta Cephei pulsation
and their physical parameters. We corroborate that the excited pulsation modes
are near to the radial fundamental mode in frequency and we show that the mass
distribution of the stars peaks at 12 solar masses. We point out that the
theoretical instability strip of the Beta Cephei stars is filled neither at the
cool nor at the hot end and attempt to explain this observation
Asteroseismology of the β Cephei star ν Eridani - III. Extended frequency analysis and mode identification
Using the large photometric and spectroscopic data sets of the ν Eridani multisite campaign given in our two recent papers (Aerts et al. and Handler et al.), we present an extended frequency analysis and a photometric mode identification. For the extended frequency analysis, we used an improved radial velocity time series, the second-moment time series and the line profiles themselves. In the radial velocity time series, we can now detect an additional pulsation frequency that was previously only found in photometric time series. We also report several new candidate pulsation frequencies. For seven frequencies, the photometric mode identification indicates that they belong to a radial mode and six dipole modes, and for three frequencies the degree l could not be unambiguously determined. We also placed ν Eri in the Hertzsprung-Russell diagram by determining T[SUB]eff[/SUB] using Geneva plus Strömgren photometric calibrations, spectral energy distribution fitting, by non-local thermodynamic equilibrium hydrogen, helium and silicon line profile fitting, and by determining log(L/L[SUB]solar[/SUB]) using the Hipparcos parallax and an Hβ calibration.Peer reviewe