215 research outputs found
Photometric study of selected cataclysmic variables II. Time-series photometry of nine systems
We present time-series photometry of nine cataclysmic variables: EI UMa,
V844Her, V751 Cyg, V516 Cyg, GZ Cnc, TY Psc, V1315 Aql, ASAS J002511+1217.12,
V1315 Aql and LN UMa. The observations were conducted at various observatories,
covering 170 hours and comprising 7,850 data points in total.
For the majority of targets we confirm previously reported periodicities and
for some of them we give, for the first time, their spectroscopic orbital
periods. For those dwarf-nova systems which we observed during both quiescence
and outburst, the increase in brightness was followed by a decrease in the
amount of flickering. Quasi-periodic oscillations have either been discovered,
or were confirmed. For the eclipsing system V1315 Aql we have covered 9
eclipses, and obtained a refined orbital ephemeris. We find that, during its
long baseline of observations, no change in the orbital period of this system
has occurred. V1315 Aql also shows eclipses of variable depth.Comment: 30 pages, 16 figures, 4 tables. Submitted to JA
Long-Term Optical Monitoring of Eta Carinae. Multiband light curves for a complete orbital period
The periodicity of 5.5 years for some observational events occurring in Eta
Carinae manifests itself across a large wavelength range and has been
associated with its binary nature. These events are supposed to occur when the
binary components are close to periastron. To detect the previous periastron
passage of Eta Car in 2003, we started an intensive, ground-based, optical,
photometric observing campaign. We continued observing the object to monitor
its photometric behavior and variability across the entire orbital cycle. Our
observation program consisted of daily differential photometry from CCD images,
which were acquired using a 0.8 m telescope and a standard BVRI filter set at
La Plata Observatory. The photometry includes the central object and the
surrounding Homunculus nebula. We present up-to-date results of our observing
program, including homogeneous photometric data collected between 2003 and
2008. Our observations demonstrated that Eta Car has continued increasing in
brightness at a constant rate since 1998. In 2006, it reached its brightest
magnitude (V ~ 4.7) since about 1860s. The object then suddenly reverted its
brightening trend, fading to V = 5.0 at the beginning of 2007, and has
maintained a quite steady state since then. We continue the photometric
monitoring of Eta Car in anticipation of the next "periastron passage",
predicted to occur at the beginning of 2009.Comment: Accepted by A&A. The paper contains 3 figures and 2 table
Mitigation of ionospheric signatures in Swarm GPS gravity field estimation using weighting strategies
Even though ESA's three-satellite low-earth orbit (LEO) mission Swarm is
primarily a magnetic field mission, it can also serve as a gravity field
mission. Located in a near-polar orbit with initial altitudes of 480 km
for Swarm A and Swarm C and 530 km for Swarm B and equipped with
geodetic-type dual frequency Global Positioning System (GPS) receivers, it is
suitable for gravity field computation. Of course, the Swarm GPS-only gravity
fields cannot compete with the gravity fields derived from the ultra-precise
Gravity Recovery And Climate Experiment (GRACE) K-band measurements. But for
various reasons like the end of the GRACE mission in October 2017, data
gaps in the previous months due to battery aging, and the gap between GRACE
and the recently launched GRACE Follow-On mission, Swarm gravity fields
became important to maintain a continuous time series and to bridge the gap
between the two dedicated gravity missions. By comparing the gravity fields
derived from Swarm kinematic positions to the GRACE gravity fields,
systematic errors have been observed in the Swarm results, especially around
the geomagnetic equator. These errors are already visible in the kinematic
positions as spikes up to a few centimeters, from where they propagate into
the gravity field solutions.
We investigate these systematic errors by analyzing the geometry-free linear
combination of the GPS carrier-phase observations and its time derivatives
using a combination of a Gaussian filter and a Savitzky–Golay filter and the
Rate of Total Electron Content (TEC) Index (ROTI). Based on this, we present
different weighting schemes and investigate their impact on the gravity field
solutions in order to assess the success of different mitigation strategies.
We will show that a combination of a derivative-based weighting approach with
a ROTI-based weighting approach is capable of reducing the geoid rms from
21.6 to 12.0 mm for a heavily affected month and that almost 10 % more
kinematic positions can be preserved compared to a derivative-based
screening.</p
Photometric study of selected cataclysmic variables
We present time-resolved photometry of five relatively poorly-studied
cataclysmic variables: V1193 Ori, LQ Peg, LD 317, V795 Her, and MCT 2347-3144.
The observations were made using four 1m-class telescopes for a total of more
than 250 h of observation and almost 16,000 data points. For LQ Peg WHT
spectroscopic data have been analysed as well.
The light curves show a wide range of variability on different time scales
from minutes to months. We detect for the first time a brightness variation of
0.05 mag in amplitude in V1193 Ori on the same timescale as the orbital period,
which we interpret as the result of the irradiation of the secondary. A 20-min
quasi-periodic oscillation is also detected. The mean brightness of the system
has changed by 0.5 mag on a three-month interval, while the flickering was
halved. In LQ Peg a 0.05 mag modulation was revealed with a period of about 3
h. The flickering was much smaller, of the order of 0.025 mag. A possible
quasi-periodic oscillation could exist near 30 min. For this object, the WHT
spectra are single-peaked and do not show any radial-velocity variations. The
data of LD 317 show a decrease in the mean magnitude of the system. No periodic
signal was detected but this is certainly attributable to the very large
flickering observed: between 0.07 and 0.1 mag. For V795 Her, the 2.8-hour
modulation, thought to be a superhump arising from the precession of the disc,
is present. We show that this modulation is not stable in terms of periodicity,
amplitude, and phase. Finally, for MCT 2347-3144, a clear modulation is seen in
a first dataset obtained in October 2002. This modulation is absent in August
2003, when the system was brighter and showed much more flickering.Comment: 19 pages, 15 figures, 7 tables. Accepted for pubication by A&
Instability of LBV-stars against radial oscillations
In this study we consider the nonlinear radial oscillations exciting in
LBV--stars with effective temperatures 1.5e4 K <= Teff <= 3e4 K, bolometric
luminosities 1.2e6 L_odot <= L <= 1.9e6 L_odot and masses 35.7 M_odot <= M <=
49.1 M_odot. Hydrodynamic computations were carried out with initial conditions
obtained from evolutionary sequences of population I stars (X=0.7, Z=0.02) with
initial masses from 70M_odot to 90 M_odot. All hydrodynamical models show
instability against radial oscillations with amplitude growth time comparable
with dynamical time scale of the star. Radial oscillations exist in the form of
nonlinear running waves propagating from the boundary of the compact core to
the upper boundary of the hydrodynamical model. The velocity amplitude of outer
layers is of several hundreds of km/s while the bolometric light amplitude does
not exceed 0.2 mag. Stellar oscillations are not driven by the kappa-mechanism
and are due to the instability of the gas with adiabatic exponent close to the
critical value Gamma_1 = 4/3 due to the large contribution of radiation in the
total pressure. The range of the light variation periods (6 day <= P <= 31 day)
of hydrodynamical models agrees with periods of microvariability observed in
LBV--stars.Comment: 14 pages, 5 figures, submitted to Astronomy Letter
The puzzle of HD 104994 (WR 46)
Intense coordinated spectroscopic and photometric monitoring of the suspected Wolf-Rayet binary WR 46 in 1999 reveals clear periodic variations, P = 0.329 ± 0.013 days, in the radial velocities of the emission lines of highest ionization potential, O VI and N V, found deepest in the Wolf-Rayet wind and thus least likely to be perturbed by a companion. These are accompanied by coherent variability in the profiles of lines with lower ionization/excitation potential and in the continuum flux. Most probably originating from orbital motion of the Wolf-Rayet component of the binary, this periodic radial velocity signal disappears from time to time, thus creating a puzzle yet to be solved. We show that the entangled patterns of the line profile variability are mainly governed by transitions between high and low states of the system's continuum flux.Facultad de Ciencias Astronómicas y Geofísica
The puzzle of HD 104994 (WR 46)
Intense coordinated spectroscopic and photometric monitoring of the suspected Wolf-Rayet binary WR 46 in 1999 reveals clear periodic variations, P = 0.329 ± 0.013 days, in the radial velocities of the emission lines of highest ionization potential, O VI and N V, found deepest in the Wolf-Rayet wind and thus least likely to be perturbed by a companion. These are accompanied by coherent variability in the profiles of lines with lower ionization/excitation potential and in the continuum flux. Most probably originating from orbital motion of the Wolf-Rayet component of the binary, this periodic radial velocity signal disappears from time to time, thus creating a puzzle yet to be solved. We show that the entangled patterns of the line profile variability are mainly governed by transitions between high and low states of the system's continuum flux.Facultad de Ciencias Astronómicas y Geofísica
Solar-like oscillations in cluster stars
We present a brief overview of the history of attempts to obtain a clear
detection of solar-like oscillations in cluster stars, and discuss the results
on the first clear detection, which was made by the Kepler Asteroseismic
Science Consortium (KASC) Working Group 2.Comment: 4 pages, 7 figures, accepted by Astronomische Nachrichte
Period changes in six contact binaries: WZ And, V803 Aql, DF Hya, PY Lyr, FZ Ori, and AH Tau
Six contact binaries lacking a period analysis have been chosen to search for
the presence of a third body. The O-C diagrams of these binaries were analyzed
with the least-squares method by using all available times of minima. Ten new
minima times, obtained from our observations, were included in the present
research. The Light-Time Effect was adopted for the first time as the main
cause for the detailed description of the long-term period changes. Third
bodies were found with orbital periods from 49 up to 100 years, and
eccentricities from 0.0 to 0.56 for the selected binaries. In one case (WZ
And), a fourth-body LITE variation was also applied. The mass functions and the
minimal masses of such bodies were also calculated and a possible angular
separation and magnitude differences were discussed for a prospective
interferometric discovery of these bodies.Comment: 7 pages, 8 figures, 2009 New Astronomy 14, 12
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