64 research outputs found
G 207-9 and LP 133-144: light curve analysis and asteroseismology of two ZZ Ceti stars
G 207-9 and LP 133-144 are two rarely observed ZZ Ceti stars located in the
middle and close to the blue edge of the ZZ Ceti instability domain,
respectively. We aimed to observe them at least during one observing season at
Konkoly Observatory with the purpose of extending the list of known pulsation
modes for asteroseismic investigations and detect any significant changes in
their pulsational behaviour. We determined five and three new normal modes of G
207-9 and LP 133-144, respectively. In LP 133-144, our frequency analysis also
revealed that at least at three modes there are actually triplets with
frequency separations of ~4 microHz. The rotational period of LP 133-144 based
on the triplets is ~42 h. The preliminary asteroseismic fits of G 207-9 predict
Teff=12 000 or 12 400 K and M*=0.855-0.870 MSun values for the effective
temperature and mass of the star, depending on the assumptions on the spherical
degree (l) values of the modes. These results are in agreement with the
spectroscopic determinations. In the case of LP 133-144, the best-fitting
models prefer Teff=11 800 K in effective temperature and M*>=0.71 MSun stellar
masses, which are more than 0.1 MSun larger than the spectroscopic value.Comment: 12 pages, 11 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (2016 June 30
The CoRoT B-type binary HD50230: a prototypical hybrid pulsator with g-mode period and p-mode frequency spacings
B-type stars are promising targets for asteroseismic modelling, since their
frequency spectrum is relatively simple.
We deduce and summarise observational constraints for the hybrid pulsator,
HD50230, earlier reported to have deviations from a uniform period spacing of
its gravity modes. The combination of spectra and a high-quality light curve
measured by the CoRoT satellite allow a combined approach to fix the position
of HD50230 in the HR diagram.
To describe the observed pulsations, classical Fourier analysis was combined
with short-time Fourier transformations and frequency spacing analysis
techniques. Visual spectra were used to constrain the projected rotation rate
of the star and the fundamental parameters of the target. In a first
approximation, the combined information was used to interpret multiplets and
spacings to infer the true surface rotation rate and a rough estimate of the
inclination angle.
We identify HD50230 as a spectroscopic binary and characterise the two
components. We detect the simultaneous presence of high-order g modes and
low-order p and g-modes in the CoRoT light curve, but were unable to link them
to line profile variations in the spectroscopic time series. We extract the
relevant information from the frequency spectrum, which can be used for seismic
modelling, and explore possible interpretations of the pressure mode spectrum.Comment: 26 pages, 12+6 figures, accepted for publication in Astronomy and
Astrophysic
Mass ratio from Doppler beaming and R{\o}mer delay versus ellipsoidal modulation in the Kepler data of KOI-74
We present a light curve analysis and radial velocity study of KOI-74, an
eclipsing A star + white dwarf binary with a 5.2 day orbit. Aside from new
spectroscopy covering the orbit of the system, we used 212 days of publicly
available Kepler observations and present the first complete light curve
fitting to these data, modelling the eclipses and transits, ellipsoidal
modulation, reflection, and Doppler beaming. Markov Chain Monte Carlo
simulations are used to determine the system parameters and uncertainty
estimates. Our results are in agreement with earlier studies, except that we
find an inclination of 87.0 \pm 0.4\degree, which is significantly lower than
the previously published value. We find that the mass ratio derived from the
radial velocity amplitude (q=0.104 \pm 0.004) disagrees with that derived from
the ellipsoidal modulation (q=0.052 \pm 0.004} assuming corotation). This was
found before, but with our smaller inclination, the discrepancy is even larger
than previously reported. Accounting for the rapid rotation of the A-star is
found to increase the discrepancy even further by lowering the mass ratio to
q=0.047 \pm 0.004. These results indicate that one has to be extremely careful
in using the amplitude of an ellipsoidal modulation signal in a close binary to
determine the mass ratio, when a proof of corotation is not firmly established.
The radial velocities that can be inferred from the detected Doppler beaming in
the light curve are found to be in agreement with our spectroscopic radial
velocity determination. We also report the first measurement of R{\o}mer delay
in a light curve of a compact binary. This delay amounts to -56 \pm 17 s and is
consistent with the mass ratio derived from the radial velocity amplitude. The
firm establishment of this mass ratio at q=0.104 \pm 0.004 leaves little doubt
that the companion of KOI-74 is a low mass white dwarf.Comment: 9 pages, 7 figures, 2 tables; accepted for publication in MNRA
Unresolved Rossby and gravity modes in 214 A and F stars showing rotational modulation
Here we report an ensemble study of 214 A- and F-type stars observed by
\textit{Kepler}, exhibiting the so-called \textit{hump and spike} periodic
signal, explained by Rossby modes (r~modes) -- the \textit{hump} -- and
magnetic stellar spots or overstable convective (OsC) modes -- the
\textit{spike} -- respectively. We determine the power confined in the
non-resolved hump features and find additional gravity~modes (g~modes) humps
always occurring at higher frequencies than the spike. Furthermore, we derive
projected rotational velocities from FIES, SONG and HERMES spectra for 28 stars
and the stellar inclination angle for 89 stars. We find a strong correlation
between the spike amplitude and the power in the r and g~modes, which suggests
that both types of oscillations are mechanically excited by either stellar
spots or OsC modes. Our analysis suggests that stars with a higher power in
r~modes humps are more likely to also exhibit humps at higher azimuthal
orders ( = 2, 3, or 4). Interestingly, all stars that show g~modes humps are
hotter and more luminous than the observed red edge of the Scuti
instability strip, suggesting that either magnetic fields or convection in the
outer layers could play an important role.Comment: 18 pages, 19 figure
Detection of gravity modes in the massive binary V380 Cyg from Kepler spacebased photometry and high-resolution spectroscopy
We report the discovery of low-amplitude gravity-mode oscillations in the
massive binary star V380 Cyg, from 180 d of Kepler custom-aperture space
photometry and 5 months of high-resolution high signal-to-noise spectroscopy.
The new data are of unprecedented quality and allowed to improve the orbital
and fundamental parameters for this binary. The orbital solution was subtracted
from the photometric data and led to the detection of periodic intrinsic
variability with frequencies of which some are multiples of the orbital
frequency and others are not. Spectral disentangling allowed the detection of
line-profile variability in the primary. With our discovery of intrinsic
variability interpreted as gravity mode oscillations, V380 Cyg becomes an
important laboratory for future seismic tuning of the near-core physics in
massive B-type stars.Comment: 5 pages, 4 figures, 2 tables. Accepted for publication in MNRAS
Letter
Discovery of binarity, spectroscopic frequency analysis, and mode identification of the delta Sct star 4CVn
More than 40 years of ground-based photometric observations of the delta Sct
star 4CVn revealed 18 independent oscillation frequencies, including radial as
well as non-radial p-modes of low spherical degree l<=2. From 2008 to 2011,
more than 2000 spectra were obtained at the 2.1-m Otto-Struve telescope at the
McDonald Observatory. We present the analysis of the line-profile variations,
based on the Fourier-parameter fit method, detected in the absorption lines of
4CVn, which carry clear signatures of the pulsations. From a non-sinusoidal,
periodic variation of the radial velocities, we discovered that 4CVn is an
eccentric binary system, with an orbital period Porb = 124.44 +/- 0.03 d and an
eccentricity e = 0.311 +/- 0.003. We firmly detect 20 oscillation frequencies,
9 of which are previously unseen in photometric data, and attempt mode
identification for the two dominant modes, f1 = 7.3764 c/d and f2 = 5.8496 c/d,
and determine the prograde or retrograde nature of 7 of the modes. The
projected rotational velocity of the star, vsini ~ 106.7 km/s, translates to a
rotation rate of veq/vcrit >= 33%. This relatively high rotation rate hampers
unique mode identification, since higher-order effects of rotation are not
included in the current methodology. We conclude that, in order to achieve
unambiguous mode identification for 4CVn, a complete description of rotation
and the use of blended lines have to be included in mode-identification
techniques.Comment: 18 pages, 18 figures (including Appendices), accepted for publication
in A&
Spectroscopic survey of Kepler stars. I. HERMES/Mercator observations of A- and F-type stars
The Kepler space mission provided near-continuous and high-precision photometry of about 207000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Fe i, and Fe ii lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λBoo types. The distribution of the projected rotational velocity, vsin i, is typical for A and F stars and ranges from 8 to about 280kms−1, with a mean of 134kms−
Wandering near the red edge: photometric observations of three cool ZZ Ceti stars
We summarize our findings on three cool ZZ Ceti type pulsating white dwarfs.
We determined eight independent modes in HS 0733+4119, of which seven are new
findings. For GD 154, we detected two new eigenmodes, and the recurrence of the
pulsational behaviour first observed in 1977. We discuss that GD 154 does not
only vary its pulsations between a multiperiodic and a quasi-monoperiodic
phase, but there are also differences between the relative amplitudes of the
near-subharmonics observed in the latter phase. In the complex pulsator, Ross
808, we compared the pre- and post Whole Earth Telescope campaign measurements,
and determined two new frequencies besides the ones observed during the
campaign. Studying these stars can contribute to better understanding of
pulsations close to the empirical ZZ Ceti red edge. All three targets are in
that regime of the ZZ Ceti instability strip where short-term amplitude
variations or even outbursts are likely to occur, which are not well-understood
theoretically.Comment: 15 pages, 13 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
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
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