ABSTRACT: Context: OB stars are important in the chemistry and evolution of the
Universe, but the sample of targets well understood from an asteroseismological
point of view is still too limited to provide feedback on the current
evolutionary models. Our study extends this sample with two spectroscopic
binary systems. AIMS. Our goal is to provide orbital solutions, fundamental
parameters and abundances from disentangled high-resolution high
signal-to-noise spectra, as well as to analyse and interpret the variations in
the Kepler light curve of these carefully selected targets. This way we
continue our efforts to map the instability strips of beta Cep and SPB stars
using the combination of high-resolution ground-based spectroscopy and
uninterrupted space-based photometry. Methods: We fit Keplerian orbits to
radial velocities measured from selected absorption lines of high-resolution
spectroscopy using synthetic composite spectra to obtain orbital solutions. We
use revised masks to obtain optimal light curves from the original pixel-data
from the Kepler satellite, which provided better long term stability compared
to the pipeline processed light curves. We use various time-series analysis
tools to explore and describe the nature of variations present in the light
curve. Results: We find two eccentric double-lined spectroscopic binary systems
containing a total of three main sequence B-type stars (and one F-type
component) of which at least one in each system exhibits light variations. The
light curve analysis (combined with spectroscopy) of the system of two B stars
points towards the presence of tidally excited g modes in the primary
component. We interpret the variations seen in the second system as classical g
mode pulsations driven by the kappa mechanism in the B type primary, and
explain the unexpected power in the p mode region as a result of nonlinear
resonant mode excitation.
ABSTRACT: We present an asteroseismic approach to study the dynamics of the stellar
interior in red-giant stars by asteroseismic inversion of the splittings
induced by the stellar rotation on the oscillation frequencies. We show
preliminary results obtained for the red giant KIC4448777 observed by the space
ABSTRACT: Rotation plays a key role in stellar structure and its evolution. Through
transport processes which induce rotational mixing of chemical species and the
redistribution of angular momentum, internal stellar rotation influences the
evolutionary tracks in the Hertzsprung-Russell diagram. In turn, evolution
influences the rotational properties. Therefore, information on the rotational
properties of the deep interior would help to better understand the stellar
evolution. However, as the internal rotational profile cannot be measured
directly, it remains a major unknown leaving this important aspect of models
unconstrained. We can test for nonrigid rotation inside the stars with
asteroseismology. Through the effect of rotational splitting of non-radial
oscillation modes, we investigate the internal rotation profile indirectly. Red
giants have very slow rotation rates leading to a rotational splitting on the
level of a few tenth of a \mu Hz. Only from more than 1.5 years of consecutive
data from the NASA Kepler space telescope, these tiny variations could be
resolved. A qualitative comparison to theoretical models allowed constraining
the core-to-surface rotation rate for some of these evolved stars. In this
paper, we report on the first results of a large sample study of splitting of
individual dipole modes.
ABSTRACT: We used high-quality Kepler photometry and spectroscopic data to investigate
the Kepler binary candidate KIC 5988140. Using the spectrum synthesis method,
we derived the fundamental parameters Teff, log g, [M/H], and v.sini and the
abundances. Frequency analyses of both the photometric and the spectroscopic
data were performed, revealing the same two dominant frequencies (F_1=0.688 and
F_2=0.344 c/d). We also detected in the photometry the signal of nine more,
significant frequencies located in the typical range of Delta Scuti pulsation.
The light and radial velocity curves follow a similar, stable double-wave
pattern which are not exactly in anti-phase but show a relative phase shift of
about 0.1 period between the moment of minimum velocity and that of maximum
light. We considered three different scenarios: binarity, co-existence of both
Gamma Doradus and Delta Scuti pulsations and rotation of the stellar surface
with an axisymmetric intensity distribution. However, none of these scenarios
is capable of explaining all of the characteristics of the observed variations.
We confirm the occurrence of various independent Delta Scuti-type pressure
modes in the Kepler light curve. With respect to the low-frequency content,
however, we argue that the physical cause of the remaining light and radial
velocity variations of this late A-type star remains unexplained by any of the
presently considered scenarios.
ABSTRACT: The space mission Kepler provides us with long and uninterrupted photometric
time series of red giants. We are now able to probe the rotational behaviour in
their deep interiors using the observations of mixed modes. We aim to measure
the rotational splittings in red giants and to derive scaling relations for
rotation related to seismic and fundamental stellar parameters. We have
developed a dedicated method for automated measurements of the rotational
splittings in a large number of red giants. Ensemble asteroseismology, namely
the examination of a large number of red giants at different stages of their
evolution, allows us to derive global information on stellar evolution. We have
measured rotational splittings in a sample of about 300 red giants. We have
also shown that these splittings are dominated by the core rotation. Under the
assumption that a linear analysis can provide the rotational splitting, we
observe a small increase of the core rotation of stars ascending the red giant
branch. Alternatively, an important slow down is observed for red-clump stars
compared to the red giant branch. We also show that, at fixed stellar radius,
the specific angular momentum increases with increasing stellar mass. Ensemble
asteroseismology indicates what has been indirectly suspected for a while: our
interpretation of the observed rotational splittings leads to the conclusion
that the mean core rotation significantly slows down during the red giant
phase. The slow-down occurs in the last stages of the red giant branch. This
spinning down explains, for instance, the long rotation periods measured in
ABSTRACT: Melk Abbey, a marvel of European high baroque architecture, is one of the most frequently visited tourist attractions in Austria, attracting 450 000 visitors each year. The monastery's museum presents selected aspects of Benedictine life in Melk since the monastery's foundation in 1089. After the church, the library is the second-most important room in a Benedictine monastery. Due to the wide scientific interests and contacts of the medieval monks, these libraries also contain manuscripts on mathematics, physics and astronomy. In 2009, the International Year of Astronomy (IYA2009), the annual library exhibition was fully dedicated to astronomical manuscripts and early prints from the past 1000 years. Following earlier research work on astronomical manuscripts in Melk's library, we were invited to organise the exhibition. In addition, we also presented a lecture series and provided more background in an accompanying book. Because of positive feedback from the visitors, the exhibition was extended until March 2011. In the two years of its duration, the exhibition was seen by more than 900 000 visitors. In this article, we describe the background to the scientific project, how the exhibition was organised and lessons learned from this project.
Communication Astronomy with the Public Journal. 05/2012; 12:19.
ABSTRACT: During our campaign of acquiring follow-up photometric data to resolve short period pulsating sdB (EC14026 or V361Hya) stars,
we obtained data on the known pulsator KUV04421+1416 and discovered that it is also in a reflection-effect binary. Here we
present preliminary results of the pulsation analysis and provide some constraints on the companion, which is most likely
an MV star. This makes KUV04421+1416 only the second known system with an EC14026-type pulsator in a reflection-effect binary.
Astrophysics and Space Science 04/2012; 329(1):83-86. · 1.69 Impact Factor
ABSTRACT: We report for the first time a parametric fit to the pattern of the \ell = 1
mixed modes in red giants, which is a powerful tool to identify
gravity-dominated mixed modes. With these modes, which share the
characteristics of pressure and gravity modes, we are able to probe directly
the helium core and the surrounding shell where hydrogen is burning. We propose
two ways for describing the so-called mode bumping that affects the frequencies
of the mixed modes. Firstly, a phenomenological approach is used to describe
the main features of the mode bumping. Alternatively, a quasi-asymptotic
mixed-mode relation provides a powerful link between seismic observations and
the stellar interior structure. We used period \'echelle diagrams to emphasize
the detection of the gravity-dominated mixed modes. The asymptotic relation for
mixed modes is confirmed. It allows us to measure the gravity-mode period
spacings in more than two hundred red giant stars. The identification of the
gravity-dominated mixed modes allows us to complete the identification of all
major peaks in a red giant oscillation spectrum, with significant consequences
for the true identification of \ell = 3 modes, of \ell = 2 mixed modes, for the
mode widths and amplitudes, and for the \ell = 1 rotational splittings. The
accurate measurement of the gravity-mode period spacing provides an effective
probe of the inner, g-mode cavity. The derived value of the coupling
coefficient between the cavities is different for red giant branch and clump
stars. This provides a probe of the hydrogen-shell burning region that
surrounds the helium core. Core contraction as red giants ascend the red giant
branch can be explored using the variation of the gravity-mode spacing as a
function of the mean large separation.
ABSTRACT: The CoRoT mission has provided thousands of red-giant light curves. The
analysis of their solar-like oscillations allows us to characterize their
stellar properties. Up to now, the global seismic parameters of the pressure
modes remain unable to distinguish red-clump giants from members of the
red-giant branch. As recently done with Kepler red giants, we intend to analyze
and use the so-called mixed modes to determine the evolutionary status of the
red giants observed with CoRoT. We also aim at deriving different seismic
characteristics depending on evolution. The complete identification of the
pressure eigenmodes provided by the red-giant universal oscillation pattern
allows us to aim at the mixed modes surrounding the l=1 expected
eigenfrequencies. A dedicated method based on the envelope autocorrelation
function is proposed to analyze their period separation. We have identified the
mixed-mode signature separation thanks to their pattern compatible with the
asymptotic law of gravity modes. We have shown that, independent of any
modelling, the g-mode spacings help to distinguish the evolutionary status of a
red-giant star. We then report different seismic and fundamental properties of
the stars, depending on their evolutionary status. In particular, we show that
high-mass stars of the secondary clump present very specific seismic
properties. We emphasize that stars belonging to the clump were affected by
significant mass loss. We also note significant population and/or evolution
differences in the different fields observed by CoRoT.
ABSTRACT: Stellar interiors are inaccessible through direct observations. For this reason, helioseismologists made use of the Sun's acoustic oscillation modes to tune models of its structure. The quest to detect modes that probe the solar core has been ongoing for decades. We report the detection of mixed modes penetrating all the way to the core of an evolved star from 320 days of observations with the Kepler satellite. The period spacings of these mixed modes are directly dependent on the density gradient between the core region and the convective envelope.
Science 03/2011; 332(6026):205. · 31.20 Impact Factor
ABSTRACT: As a result of the variability survey in χ Persei and NGC 6910, the number of β Cep stars that are members of these two open clusters is increased to twenty stars, nine in NGC 6910 and eleven in χ Persei. We compare pulsational properties, in particular the frequency spectra, of β Cep stars in both clusters and explain the differences in terms of the global parameters of the clusters. We also indicate that the more complicated pattern of the variability among B-type stars in χ Persei is very likely caused by higher rotational velocities of stars in this cluster. We conclude that the sample of pulsating stars in the two open clusters constitutes a very good starting point for the ensemble asteroseismology of β Cep-type stars and maybe also for other B-type pulsators (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Astronomische Nachrichten 11/2010; 331(9‐10):1080 - 1083. · 1.01 Impact Factor
ABSTRACT: As a result of the variability survey in Chi Persei and NGC6910, the number of Beta Cep stars that are members of these two open clusters is increased to twenty stars, nine in NGC6910 and eleven in Chi Persei. We compare pulsational properties, in particular the frequency spectra, of Beta Cep stars in both clusters and explain the differences in terms of the global parameters of the clusters. We also indicate that the more complicated pattern of the variability among B type stars in Chi Persei is very likely caused by higher rotational velocities of stars in this cluster. We conclude that the sample of pulsating stars in the two open clusters constitutes a very good starting point for the ensemble asteroseismology of Beta Cep-type stars and maybe also for other B-type pulsators. Comment: 4 pages, Astronomische Nachrichten, HELAS IV Conference, Arecife, Lanzarote, Feb 2010, submitted
ABSTRACT: Oscillating stars in binary systems are among the most interesting stellar laboratories, as these can provide information on the stellar parameters and stellar internal structures. Here we present a red giant with solar-like oscillations in an eclipsing binary observed with the NASA Kepler satellite. We compute stellar parameters of the red giant from spectra and the asteroseismic mass and radius from the oscillations. Although only one eclipse has been observed so far, we can already determine that the secondary is a main-sequence F star in an eccentric orbit with a semi-major axis larger than 0.5 AU and orbital period longer than 75 days.
The Astrophysical Journal Letters 03/2010; 713(2):L187. · 5.53 Impact Factor
ABSTRACT: Stars with changing Blazhko periods challenge the currently proposed hypotheses for the Blazhko effect. RR Lyr, the prototype of the class, is one of the best-studied Blazhko stars but it keeps on surprising its observers. We present the first results from a photometric follow- up campaign in 2006-2007 of the star. Multicolour data were gathered from 4 different observatories in the northern hemisphere. Our analysis focuses specifically on the period behaviour. We confirm the previously reported decrease of the modulation period.
Communications in Asteroseismology 12/2008; 157:323-324.
ABSTRACT: We studied the $delta$ Scuti star 4 CVn through time-series spectroscopy1 , since photometry alone is insufficient to provide a unique solution to mode identification. However, the combination of multifilter photometry and high-resolution spectroscopy, similar to the data we obtained and analyzed, allows the necessary reliable mode identification. We have obtained 38 nights of time-series high-resolution spectroscopy at the 2.1 m telescope at McDonald Observatory for 4 CVn. We have done mode identification for five independent frequencies detected by spectroscopy, which were previously detected with photometric observations.
Communications in Asteroseismology 12/2008; 157:124-127.
ABSTRACT: We obtained 74 hours of time-resolved CCD photometry of the pulsating DA white dwarf star WD 1524-0030 from three different sites well separated in longitude. We found evidence for amplitude variability with relative changes of plusmn 10% and detected a total of 15 independent and 10 combination frequencies in our light curves. The large number of excited modes, the high amplitudes and nonsinusoidal light curves, the apparent brightness and the equatorial location on the sky make WD 1524-0030 an attractive target for future campaigns with the goal of asteroseismology and nonlinear light curve fitting.
Communications in Asteroseismology 11/2008; 156:18-26.
ABSTRACT: Montgomery  developed a method to probe convection in pulsating white dwarf stars which allows the recovery of the thermal response time of the convection zone by fitting observed nonsinusoidal light curves. He applied this method to two objects; the Whole Earth Telescope (WET) observed the pulsating DB white dwarf GD 358 for just this purpose. Given this WET run's success, it is time to extend Montgomery's method to pulsating DA white dwarf (ZZ Ceti) stars. We present observations of two ZZ Ceti stars, WD 1524mdash0030 and EC 14012mdash1446, both observed from multiple sites. EC 14012mdash1446 seems better suited thAN WD1524mdash0030 for a future WET run because it has more pulsation modes excited and because it pulsation spectrum appears to be more stable in time. We call for participation in this effort to take place in April 2008.
Journal of Physics Conference Series 10/2008; 118(1):012057.
ABSTRACT: In 2005 a photometric observation campaign started on the open cluster $chi$ Persei, involving 13 telescopes spread over the whole northern hemisphere. After two years we gathered almost 1200 hours of data. We present here preliminary results on the variability search, especially from the 60-cm telescope in Bialków (Poland), which show seven confirmed $beta$ Cephei stars, four candidate B-type pulsators and other interesting variable stars.
Journal of Physics Conference Series 10/2008; 118(1):012071.
Communications in Asteroseismology 06/2007; 150:191.
Communications in Asteroseismology 05/2007; 150:191.