66 research outputs found
The Radio Corona of AR Lacertae
We present multifrequency VLA and VLBA observations at 8.4 GHz of the RS CVn
system AR Lac, that were performed in autumn 1997 simultaneously with X-ray
observations obtained from Rodono` et al. (1999). Our VLBA data indicate a
resolved source with dimension close to the system separation, while the study
of the flux density curve evidences a small amplitude outside of the eclipse
variability. The derived five-frequencies spectra, combined with the size
information from VLBA data, are compared with gyrosynchrotron emission from a
two component structured source. A comparison with the results of the X-ray
observations allow us to exclude the possibility that thermal gyrosynchrotron
is responsible for the radio emission, but it is compatible with the hypothesis
of co-spatial X-ray and radio emitting sources.Comment: 9 pages, 7 figures. Accepted for publication in A&
Doppler images of II Pegasi for 2004-2010
Aims. We study the spot activity of II Peg during the years 2004-2010 to
determine long- and short-term changes in the magnetic activity. In a previous
study, we detected a persistent active longitude, as well as major changes in
the spot configuration occurring on a timescale of shorter than a year. The
main objective of this study is to determine whether the same phenomena persist
in the star during these six years of spectroscopic monitoring.
Methods. The observations were collected with the high-resolution SOFIN
spectrograph at the Nordic Optical Telescope. The temperature maps were
calculated using a Doppler imaging code based on Tikhonov regularization.
Results. We present 12 new temperature maps that show spots distributed
mainly over high and intermediate latitudes. In each image, 1-3 main active
regions can be identified. The activity level of the star is clearly lower than
during our previous study for the years 1994-2002. In contrast to the previous
observations, we detect no clear drift of the active regions with respect to
the rotation of the star.
Conclusions. Having shown a systematic longitudinal drift of the
spot-generating mechanism during 1994-2002, the star has clearly switched to a
low-activity state for 2004-2010, during which the spot locations appear more
random over phase space. It could be that the star is near to a minimum of its
activity cycle.Comment: Accepted for publication in Astron. and Astrophys., 8 pages, 5
figure
Multiperiodicity, modulations and flip-flops in variable star light curves I. Carrier fit method
The light curves of variable stars are commonly described using simple
trigonometric models, that make use of the assumption that the model parameters
are constant in time. This assumption, however, is often violated, and
consequently, time series models with components that vary slowly in time are
of great interest. In this paper we introduce a class of data analysis and
visualization methods which can be applied in many different contexts of
variable star research, for example spotted stars, variables showing the
Blazhko effect, and the spin-down of rapid rotators. The methods proposed are
of explorative type, and can be of significant aid when performing a more
thorough data analysis and interpretation with a more conventional method.Our
methods are based on a straightforward decomposition of the input time series
into a fast "clocking" periodicity and smooth modulating curves. The fast
frequency, referred to as the carrier frequency, can be obtained from earlier
observations (for instance in the case of photometric data the period can be
obtained from independently measured radial velocities), postulated using some
simple physical principles (Keplerian rotation laws in accretion disks), or
estimated from the data as a certain mean frequency. The smooth modulating
curves are described by trigonometric polynomials or splines. The data
approximation procedures are based on standard computational packages
implementing simple or constrained least-squares fit-type algorithms.Comment: 14 pages, 23 figures, submitted to Astronomy and Astrophysic
Comparing different approaches to model the rotational modulation of the Sun as a star
The space missions MOST, COROT and Kepler are going to provide us with
high-precision optical photometry of solar-like stars with time series
extending from tens of days to several years. They can be modelled to obtain
information on stellar magnetic activity by fitting the rotational modulation
of the stellar flux produced by the brightness inhomogeneities associated with
photospheric active regions. The variation of the total solar irradiance
provides a good proxy for those photometric time series and can be used to test
the performance of different spot modelling approaches. We test discrete spot
models as well as maximum entropy and Tikhonov regularized spot models by
comparing the reconstructed total sunspot area variation and longitudinal
distributions of sunspot groups with those actually observed in the Sun along
activity cycle 23. Appropriate statistical methods are introduced to measure
model performance versus the timescale of variation. The maximum entropy
regularized spot models show the best agreement with solar observations
reproducing the total sunspot area variation on time scales ranging from a few
months to the activity cycle, although the model amplitudes are affected by
systematic errors during the minimum and the maximum activity phases. The
longitudinal distributions derived from the models compare well with the
observed sunspot group distributions except during the minimum of activity,
when faculae dominate the rotational modulation. The resolution in longitude
attainable through the spot modelling is about 60 degrees.Comment: 21 pages, 7 figures, accepted by Astron. Astrophy
The REM Telescope: A robotic multiwavelength facility to promptly follow up GRB afterglows
The REM (Rapid Eye Mount) Telescope, located in la Silla Observatory Chile, is the first moderate (60 cm) aperture robotic telescope able to cover simultaneously both the visible and near-infrared (0.45–2.3 μm) wavelength range. The high-throughput Infrared Camera (REMIR) and the optical imaging spectrograph (ROSS), both equipping the REM telescope, are simultaneously fed by a dichroic and they allow to collect high-S/N data in an unprecedented large
spectral range on a telescope of this size. The wide band covered, the very fast pointing capability (60 degrees in 5 seconds) and its full robotization make REM the ideal experiment for fast transients observation. The REM observatory is an example of a versatile and agile facility necessary to complement large telescopes in fields in which rapid response and/or target pre-screening are necessary. This paper describes the main characteristics and operation modes of the REM observatory and gives an overview of preliminary results obtained during the Science Verification
Phase
REMIR: The REM infrared camera to follow up the early phases of GRBs afterglows
REMIR is a near-infrared camera, covering the 0.95–2.3 μm range with 5 filters (z,J,H,Ks and H2), mounted at one of the Nasmyth foci of the REM (Rapid Eye Mount) telescope. REM is a fully robotic fast-slewing 60 cm telescope,
primarily designed to follow-up the early phases of the afterglow of GRBs detected by dedicated instruments onboard satellites (like SWIFT, a satellite entirely dedicated
to GRBs science launched the 12 November 2004). Moreover REM hosts a slitless spectrograph covering the range 0.45–0.95 μm, with 30 sample points and with the possibility to perform broad-band V,R,I photometry (ROSS, REM Optical Slitless Spectrograph). The main task of REMIR is to perform realtime NIR observations of GRBs detected by
gamma-ray monitors onboard satellites, looking for any possible infrared transient source. As soon as a transient source is detected in the IR images, larger telescopes are promptly alerted to perform early spectroscopy of the afterglow. All the above operations are performed in a fully automatic way and without any human supervision. We present the results of on-site tests that have been done to
characterize the REMIR camera and the performances of the dedicated reduction pipeline AQuA (Automatic Quick Analysis), suited for fast transients detection
RACE-OC Project: Rotation and variability in young stellar associations within 100 pc
Our goal is to determine the rotational and magnetic-related activity
properties of stars at different stages of evolution. We have focussed our
attention on 6 young loose stellar associations within 100 pc and ages in the
range 8-70 Myr: TW Hydrae (~8 Myr), beta Pictoris (~10 Myr), Tucana/Horologium,
Columba, Carina (~30 Myr), and AB Doradus (~70 Myr). Additional data on alpha
Persei and the Pleiades from the literature is also considered. Rotational
periods of stars showing rotational modulation due to photospheric magnetic
activity (i.e. starspots) have been determined applying the Lomb-Scargle
periodogram technique to photometric time-series obtained by the All Sky
Automated Survey (ASAS). The magnetic activity level has been derived from the
amplitude of the V lightcurves. We detected the rotational modulation and
measured the rotation periods of 93 stars for the first time, and confirmed the
periods of 41 stars already known from the literature. For further 10 stars we
revised the period determinations by other authors. The sample was augmented
with periods of 21 additional stars retrieved from the literature. In this way,
for the first time we were able to determine largest set of rotation periods at
ages of ~8, ~10 and ~30 Myr, as well as increase by 150\% the number of known
periodic members of AB Dor.The analysis of the rotation periods in young
stellar associations, supplemented by Orion Nebula Cluster (ONC) and NGC2264
data from the literature, has allowed us to find that in the 0.6 - 1.2 solar
masses range the most significant variations of the rotation period
distribution are the spin-up between 9 and 30 Myr and the spin-down between 70
and 110 Myr. Variations between 30 and 70 Myr are rather doubtful, despite the
median period indicates a significant spin-up.Comment: Accepted by Astronomy and Astrophysic
X-Ray Spectroscopy of Stars
(abridged) Non-degenerate stars of essentially all spectral classes are soft
X-ray sources. Low-mass stars on the cooler part of the main sequence and their
pre-main sequence predecessors define the dominant stellar population in the
galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense,
of X-ray spectra from the solar corona. X-ray emission from cool stars is
indeed ascribed to magnetically trapped hot gas analogous to the solar coronal
plasma. Coronal structure, its thermal stratification and geometric extent can
be interpreted based on various spectral diagnostics. New features have been
identified in pre-main sequence stars; some of these may be related to
accretion shocks on the stellar surface, fluorescence on circumstellar disks
due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot
stars clearly dominate the interaction with the galactic interstellar medium:
they are the main sources of ionizing radiation, mechanical energy and chemical
enrichment in galaxies. High-energy emission permits to probe some of the most
important processes at work in these stars, and put constraints on their most
peculiar feature: the stellar wind. Here, we review recent advances in our
understanding of cool and hot stars through the study of X-ray spectra, in
particular high-resolution spectra now available from XMM-Newton and Chandra.
We address issues related to coronal structure, flares, the composition of
coronal plasma, X-ray production in accretion streams and outflows, X-rays from
single OB-type stars, massive binaries, magnetic hot objects and evolved WR
stars.Comment: accepted for Astron. Astrophys. Rev., 98 journal pages, 30 figures
(partly multiple); some corrections made after proof stag
- …