91 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
Distance to the Active Galaxy NGC 6951 via the Type Ia Supernova 2000E
CCD-photometry and low-resolution spectroscopy of the bright supernova SN
2000E in NGC 6951 are presented. Both the light curve extending up to 150 days
past maximum and the spectra obtained at 1 month past maximum confirm that SN
2000E is of Type Ia. The reddening of SN 2000E is determined as
E(B-V)=0.36+/-0.15, its error is mainly due to uncertainties in the predicted
SN (B-V) colour at late epochs. The V(RI)_C light curves are analyzed with the
Multi-Colour Light Curve Shape (MLCS) method. The shape of the late light curve
suggests that SN 2000E was overluminous by about 0.5 mag at maximum comparing
with a fiducial SN Ia. This results in an updated distance of 33+/-8 Mpc of NGC
6951 (corrected for interstellar absorption). The SN-based distance modulus is
larger by about +0.7 mag than the previous Tully-Fisher estimates. However,
possible systematic errors due to ambiguities in the reddening determination
and estimates of the maximum luminosity of SN 2000E may plague the present
distance measurement.Comment: 9 p., 5 figs, accepted for publication in A&A. A reference correcte
Photometric mode identification methods of nonradial pulsations in eclipsing binaries I. -- Dynamic Eclipse Mapping
We present the Dynamic Eclipse Mapping (DEM) method designed specifically to
reconstruct the surface intensity patterns of non-radial stellar oscillations
in eclipsing binaries. The method needs a geometric model of the binary,
accepts the light curve and the detected pulsation frequencies on input, and on
output yields estimates of the pulsation patterns, in form of images -- thus
allowing a direct identification of the surface mode numbers.
Since it has minimal modelling requirements and can operate on photometric
observations in arbitrary wavelength bands, DEM is well suited to analyze the
wide-band time series collected by space observatories.
The method was extensively tested on simulated data, in which almost all
photometrically detectable modes with a latitudinal complexity
were properly restored. Multimode pulsations can be also reconstructed in a
natural manner, as well as pulsations on components with tilted rotation axis
of known direction. It can also be used in principle to isolate the
contribution of hidden modes from the light curve.
Sensitivity tests show that moderate errors in the geometric parameters and
the assumed limb darkening can be partially tolerated by the inversion, in the
sense that the lower degree modes are still recoverable. Tidally induced or
mutually resonant pulsations, however, are an obstacle that neither the eclipse
mapping, nor any other inversion technique can ever surpass.
We conclude that, with reasonable assumptions, Dynamic Eclipse Mapping could
be a powerful tool for mode identification, especially in moderately close
eclipsing binary systems, where the pulsating component is not seriously
affected by tidal interactions so that the pulsations are intrinsic to them,
and not a consequence of the binarity.Comment: Accepted for publication in MNRAS. 16 pages, 11 figures and 5 table
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&
Eclipsing spotted giant star with K2 and historical photometry
Context. Stars can maintain their observable magnetic activity from the pre-main sequence (PMS) to the tip of the red giant branch. However, the number of known active giants is much lower than active stars on the main sequence (MS) since the stars spend only about 10% of their MS lifetime on the giant branch. Due to their rapid evolution it is difficult to estimate the stellar parameters of giant stars. A possibility for obtaining more reliable stellar parameters for an active giant arises when it is a member of an eclipsing binary system. Aims: We have discovered EPIC 211759736, an active spotted giant star in an eclipsing binary system during the Kepler K2 Campaign 5. The eclipsing nature allows us to much better constrain the stellar parameters than in most cases of active giant stars. Methods: We have combined the K2 data with archival HATNet, ASAS, and DASCH photometry, new spectroscopic radial velocity measurements, and a set of follow-up ground-based BVRCIC photometric observations, to find the binary system parameters as well as robust spot models for the giant at two different epochs. Results: We determined the physical parameters of both stellar components and provide a description of the rotational and long-term activity of the primary component. The temperatures and luminosities of both components were examined in the context of the Hertzsprung-Russell diagram. We find that both the primary and the secondary components deviate from the evolutionary tracks corresponding to their masses in the sense that the stars appear in the diagram at lower masses than their true masses. Conclusions: We further evaluate the proposition that traditional methods generally result in higher masses for active giants than what is indicated by stellar evolution tracks in the HR diagram. A possible reason for this discrepancy could be a strong magnetic field, since we see greater differences in more active stars. Data presented in this paper are based on observations obtained at the HAT station at the Submillimeter Array of SAO, and the HAT station at the Fred Lawrence Whipple Observatory of SAO
- …