133 research outputs found
Stellar variability in open clusters. I. A new class of variable stars in NGC 3766
Aims. We analyze the population of periodic variable stars in the open
cluster NGC 3766 based on a 7-year multi-band monitoring campaign conducted on
the 1.2 m Swiss Euler telescope at La Silla, Chili.
Methods. The data reduction, light curve cleaning and period search
procedures, combined with the long observation time line, allow us to detect
variability amplitudes down to the milli-magnitude level. The variability
properties are complemented with the positions in the color-magnitude and
color-color diagrams to classify periodic variable stars into distinct
variability types.
Results. We find a large population (36 stars) of new variable stars between
the red edge of slowly pulsating B (SPB) stars and the blue edge of delta Sct
stars, a region in the Hertzsprung-Russell (HR) diagram where no pulsation is
predicted to occur based on standard stellar models. The bulk of their periods
ranges from 0.1 to 0.7 d, with amplitudes between 1 and 4 mmag for the majority
of them. About 20% of stars in that region of the HR diagram are found to be
variable, but the number of members of this new group is expected to be higher,
with amplitudes below our milli-magnitude detection limit.
The properties of this new group of variable stars are summarized, and
arguments set forth in favor of a pulsation origin of the variability, with
g-modes sustained by stellar rotation. Potential members of this new class of
low-amplitude periodic (most probably pulsating) A and late-B variables in the
literature are discussed.
We additionally identify 16 eclipsing binary, 13 SPB, 14 delta Sct and 12
gamma Dor candidates, as well as 72 fainter periodic variables. All are new
discoveries.
Conclusions. We encourage to search for the existence of this new class of
variables in other young open clusters, especially in those hosting a rich
population of Be stars.Comment: Accepted for publication in A&A. Size of pdf file ~7Mo. Figures 12,
13, 14 and in the Appendix are of lower quality. Full quality images
published in A&
On the new late B- and early A-type periodic variable stars
We summarize the properties of the new periodic, small-amplitude, variable stars recently discovered in the open cluster NGC 3766. They are located in the region of the Hertzsprung-Russell diagram between δ Sct and slowly pulsating B stars, a region where no sustained pulsation is predicted by standard models. The origin of their periodic variability is currently unknown. We also discuss how the Gaia mission, launched at the end of 2013, can contribute to our knowledge of those star
New spectroscopic and polarimetric observations of the A0 supergiant HD92207
Our recent search for the presence of a magnetic field in the bright early
A-type supergiant HD92207 using FORS2 in spectropolarimetric mode revealed the
presence of a longitudinal magnetic field of the order of a few hundred Gauss.
However, the definite confirmation of the magnetic nature of this object
remained pending due to the detection of short-term spectral variability
probably affecting the position of line profiles in left- and right-hand
polarized spectra. We present new magnetic field measurements of HD92207
obtained on three different epochs in 2013 and 2014 using FORS2 in
spectropolarimetric mode. A 3sigma detection of the mean longitudinal magnetic
field using the entire spectrum, _all=104+-34G, was achieved in
observations obtained in 2014 January. At this epoch, the position of the
spectral lines appeared stable. Our analysis of spectral line shapes recorded
in opposite circularly polarized light, i.e. in light with opposite sense of
rotation, reveals that line profiles in the light polarized in a certain
direction appear slightly split. The mechanism causing such a behaviour in the
circularly polarized light is currently unknown. Trying to settle the issue of
short-term variability, we searched for changes in the spectral line profiles
on a time scale of 8-10min using HARPS polarimetric spectra and on a time scale
of 3-4min using time series obtained with the CORALIE spectrograph. No
significant variability was detected on these time scales during the epochs
studied.Comment: 10 pages, 11 figures, 2 tables, accepted for publication in
Astronomische Nachrichte
Pulsating B-type stars in the young open cluster h Persei (NGC 869)
We announce the discovery of six Beta Cephei stars and many other variable
stars in the young open cluster h Persei (NGC 869). The cluster seems to be
very rich in variable B-type stars, similarly to its twin, Chi Persei (NGC
884).Comment: 5 pages, 5 figures, Proc. HELAS-II conference, Goettingen, 20-24
August 200
From Hipparcos to Gaia
The measurement of the positions, distances, motions and luminosities of
stars represents the foundations of modern astronomical knowledge. Launched at
the end of the eighties, the ESA Hipparcos satellite was the first space
mission dedicated to such measurements. Hipparcos improved position accuracies
by a factor of 100 compared to typical ground-based results and provided
astrometric and photometric multi-epoch observations of 118,000 stars over the
entire sky. The impact of Hipparcos on astrophysics has been extremely valuable
and diverse. Building on this important European success, the ESA Gaia
cornerstone mission promises an even more impressive advance. Compared to
Hipparcos, it will bring a gain of a factor 50 to 100 in position accuracy and
of a factor of 10,000 in star number, collecting photometric,
spectrophotometric and spectroscopic data for one billion celestial objects.
During its 5-year flight, Gaia will measure objects repeatedly, up to a few
hundred times, providing an unprecedented database to study the variability of
all types of celestial objects. Gaia will bring outstanding contributions,
directly or indirectly, to most fields of research in astrophysics, such as the
study of our Galaxy and of its stellar constituents, the search for planets
outside the solar system.Comment: 6 pages. New Horizons in Time Domain Astronomy Proceedings IAU
Symposium No. 285, 2012, E. Griffin, B. Hanisch & R. Seaman, ed
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
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
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