30 research outputs found
The contact binary VW Cephei revisited: surface activity and period variation
Context. Despite the fact that VW Cephei is one of the well-studied contact
binaries in the literature, there is no fully consistent model available that
can explain every observed property of this system.
Aims. Our motivation is to obtain new spectra along with photometric
measurements, to analyze what kind of changes may have happened in the system
in the past two decades, and to propose new ideas for explaining them.
Methods. For the period analysis we determined 10 new times of minima from
our light curves, and constructed a new OC diagram of the system. Radial
velocities of the components were determined using the cross-correlation
technique. The light curves and radial velocities were modelled simultaneously
with the PHOEBE code. All observed spectra were compared to synthetic spectra
and equivalent widths of the H line were measured on their differences.
Results. We have re-determined the physical parameters of the system
according to our new light curve and spectral models. We confirm that the
primary component is more active than the secondary, and there is a correlation
between spottedness and the chromospheric activity. We propose that flip-flop
phenomenon occurring on the primary component could be a possible explanation
of the observed nature of the activity. To explain the period variation of VW
Cep, we test two previously suggested scenarios: presence of a fourth body in
the system, and the Applegate-mechanism caused by periodic magnetic activity.
We conclude that although none of these mechanisms can be ruled out entirely,
the available data suggest that mass transfer with a slowly decreasing rate
gives the most likely explanation for the period variation of VW Cep.Comment: 13 pages, 18 figures, 9 tables, accepted for publication in Astronomy
and Astrophysic
Variability of M giant stars based on Kepler photometry: general characteristics
M giants are among the longest-period pulsating stars which is why their
studies were traditionally restricted to analyses of low-precision visual
observations, and more recently, accurate ground-based data. Here we present an
overview of M giant variability on a wide range of time-scales (hours to
years), based on analysis of thirteen quarters of Kepler long-cadence
observations (one point per every 29.4 minutes), with a total time-span of over
1000 days. About two-thirds of the sample stars have been selected from the
ASAS-North survey of the Kepler field, with the rest supplemented from a
randomly chosen M giant control sample.
We first describe the correction of the light curves from different quarters,
which was found to be essential. We use Fourier analysis to calculate multiple
frequencies for all stars in the sample. Over 50 stars show a relatively strong
signal with a period equal to the Kepler-year and a characteristic phase
dependence across the whole field-of-view. We interpret this as a so far
unidentified systematic effect in the Kepler data. We discuss the presence of
regular patterns in the distribution of multiple periodicities and amplitudes.
In the period-amplitude plane we find that it is possible to distinguish
between solar-like oscillations and larger amplitude pulsations which are
characteristic for Mira/SR stars. This may indicate the region of the
transition between two types of oscillations as we move upward along the giant
branch.Comment: 12 pages, 13 figures, accepted for publication in MNRAS. The
normalized light curves are available upon reques
A photometric monitoring of bright high-amplitude delta Scuti stars. II. Period updates for seven stars
We present new photometric data for seven high-amplitude delta Scuti stars.
The observations were acquired between 1996 and 2002, mostly in the Johnson
photometric system. For one star (GW UMa), our observations are the first since
the discovery of its pulsational nature from the Hipparcos data.The primary
goal of this project was to update our knowledge on the period variations of
the target stars. For this, we have collected all available photometric
observations from the literature and constructed decades-long O-C diagrams of
the stars. This traditional method is useful because of the single-periodic
nature of the light variations. Text-book examples of slow period evolution (XX
Cyg, DY Her, DY Peg) and cyclic period changes due to light-time effect (LITE)
in a binary system (SZ Lyn) are updated with the new observations. For YZ Boo,
we find a period decrease instead of increase. The previously suggested
LITE-solution of BE Lyn (Kiss & Szatmary 1995) is not supported with the new
O-C diagram. Instead of that, we suspect the presence of transient light curve
shape variations mimicking small period changes.Comment: 11 pages, 15 figures, accepted for publication in A&
The Type la Supernova 2001V in NGC 3987
CCD photometry of the type Ia SN 2001V occured in the edge-on spiral galaxy
NGC 3987 is presented. The observations made through Johnson-Cousins BVRI
filters were collected from Feb. 24 (t = -8 days, with respect to B-maximum),
up to May 5 (t = +62 days). The light curves are analyzed with the revised
Multi-Colour Light Curve Shape (MLCS) method by fitting template vectors to the
observed light curves simultaneously. The reddening of SN 2001V is estimated to
be E(B-V)=0.05 mag, while the galactic component is E(B-V) = 0.02 mag,
suggesting that part of the reddening may be due to the ISM in the host galaxy.
The Delta parameter in MLCS converged to -0.47 mag, indicating that this SN was
overluminous relative to the majority of Type Ia SNe. The inferred distance to
its host galaxy, NGC 3987, is 74.5 \pm 5 Mpc, which is in good agreement with
recently determined kinematic distances, based on radial velocity corrected for
Virgo-infall and Hubble constant H_0 = 65 km/s/Mpc.Comment: 6 pages, 4 figures, accepted in Astronomy and Astrophysic
Testing SNe Ia distance measurement methods with SN 2011fe
The nearby, bright, almost completely unreddened Type Ia supernova 2011fe in
M101 provides a unique opportunity to test both the precision and the accuracy
of the extragalactic distances derived from SNe Ia light curve fitters. We
apply the current, public versions of the independent light curve fitting codes
MLCS2k2 and SALT2 to compute the distance modulus of SN 2011fe from
high-precision, multi-color (BVRI) light curves. The results from the two
fitting codes confirm that 2011fe is a "normal" (not peculiar) and only
slightly reddened SN Ia. New unreddened distance moduli are derived as 29.21
+/- 0.07 mag (D ~ 6.95 +/- 0.23$ Mpc, MLCS2k2), and 29.05 +/- 0.07 mag (6.46
+/- 0.21 Mpc, SALT2). Despite the very good fitting quality achieved with both
light curve fitters, the resulting distance moduli are inconsistent by 2 sigma.
Both are marginally consistent (at ~1 sigma) with the HST Key Project distance
modulus for M101. The SALT2 distance is in good agreement with the recently
revised Cepheid- and TRGB-distance to M101. Averaging all SN- and Cepheid-based
estimates, the absolute distance to M101 is ~6.6 +/- 0.5 Mpc.Comment: 8 pages, 7 figures, accepted for publication in A&
The multimode pulsation of the delta Scuti star V784 Cassiopeae
We present an analysis of new Johnson and Stromgren photometric and
medium-resolution spectroscopic observations of the delta Scuti type variable
star V784 Cassiopeae. The data were obtained in three consecutive years between
1999 and 2001. The period analysis of the light curve resulted in the detection
of four frequencies ranging from 9.15 c/d to 15.90 c/d, while there is a
suggestion for more, unresolved frequency components, too. The mean Stromgren
indices and Hipparcos parallax were combined to calculate the following
physical parameters: =7100+-100 K, log g=3.8+-0.1, M_bol=1.50+-0.15 mag.
The position of the star in the HR diagram was used to derive evolutionary mass
and age yielding to a consistent picture of an evolved delta Scuti star with a
mixture of radial plus non-radial modes.Comment: 11 pages, 9 figures, accepted for publication in A&
Efficient Sparse Coding in Early Sensory Processing: Lessons from Signal Recovery
Sensory representations are not only sparse, but often overcomplete: coding units significantly outnumber the input units. For models of neural coding this overcompleteness poses a computational challenge for shaping the signal processing channels as well as for using the large and sparse representations in an efficient way. We argue that higher level overcompleteness becomes computationally tractable by imposing sparsity on synaptic activity and we also show that such structural sparsity can be facilitated by statistics based decomposition of the stimuli into typical and atypical parts prior to sparse coding. Typical parts represent large-scale correlations, thus they can be significantly compressed. Atypical parts, on the other hand, represent local features and are the subjects of actual sparse coding. When applied on natural images, our decomposition based sparse coding model can efficiently form overcomplete codes and both center-surround and oriented filters are obtained similar to those observed in the retina and the primary visual cortex, respectively. Therefore we hypothesize that the proposed computational architecture can be seen as a coherent functional model of the first stages of sensory coding in early vision
Representational Switching by Dynamical Reorganization of Attractor Structure in a Network Model of the Prefrontal Cortex
The prefrontal cortex (PFC) plays a crucial role in flexible cognitive behavior by representing task relevant information with its working memory. The working memory with sustained neural activity is described as a neural dynamical system composed of multiple attractors, each attractor of which corresponds to an active state of a cell assembly, representing a fragment of information. Recent studies have revealed that the PFC not only represents multiple sets of information but also switches multiple representations and transforms a set of information to another set depending on a given task context. This representational switching between different sets of information is possibly generated endogenously by flexible network dynamics but details of underlying mechanisms are unclear. Here we propose a dynamically reorganizable attractor network model based on certain internal changes in synaptic connectivity, or short-term plasticity. We construct a network model based on a spiking neuron model with dynamical synapses, which can qualitatively reproduce experimentally demonstrated representational switching in the PFC when a monkey was performing a goal-oriented action-planning task. The model holds multiple sets of information that are required for action planning before and after representational switching by reconfiguration of functional cell assemblies. Furthermore, we analyzed population dynamics of this model with a mean field model and show that the changes in cell assemblies' configuration correspond to those in attractor structure that can be viewed as a bifurcation process of the dynamical system. This dynamical reorganization of a neural network could be a key to uncovering the mechanism of flexible information processing in the PFC
Binarity and multiperiodicity in high-amplitude delta Scuti stars
We have carried out a photometric and spectroscopic survey of bright
high-amplitude delta Scuti (HADS) stars. The aim was to detect binarity and
multiperiodicity (or both) in order to explore the possibility of combining
binary star astrophysics with stellar oscillations. Here we present the first
results for ten, predominantly southern, HADS variables. We detected the
orbital motion of RS Gru with a semi-amplitude of ~6.5 km/s and 11.5 days
period. The companion is inferred to be a low-mass dwarf star in a close orbit
around RS Gru. We found multiperiodicity in RY Lep both from photometric and
radial velocity data and detected orbital motion in the radial velocities with
hints of a possible period of 500--700 days. The data also revealed that the
amplitude of the secondary frequency is variable on the time-scale of a few
years, whereas the dominant mode is stable. Radial velocities of AD CMi
revealed cycle-to-cycle variations which might be due to non-radial pulsations.
We confirmed the multiperiodic nature of BQ Ind, while we obtained the first
radial velocity curves of ZZ Mic and BE Lyn. The radial velocity curve and the
O-C diagram of CY Aqr are consistent with the long-period binary hypothesis. We
took new time series photometry on XX Cyg, DY Her and DY Peg, with which we
updated their O-C diagrams.Comment: 15 pages, 16 pages, accepted for publication in MNRA