429 research outputs found
Monitoring and modelling of white dwarfs with extremely weak magnetic fields
Magnetic fields are detected in a few percent of white dwarfs. The number of
such magnetic white dwarfs known is now some hundreds. Fields range in strength
from a few kG to several hundred MG. Almost all the known magnetic white dwarfs
have a mean field modulus >= 1 MG. We are trying to fill a major gap in
observational knowledge at the low field limit (<= 200 kG) using circular
spectro-polarimetry.
In this paper we report the discovery and monitoring of strong, periodic
magnetic variability in two previously discovered "super-weak field" magnetic
white dwarfs, WD2047+372 and WD2359-434. WD2047+372 has a mean longitudinal
field that reverses between about -12 and +15 kG, with a period of 0.243 d,
while its mean field modulus appears nearly constant at 60 kG. The observations
can be intepreted in terms of a dipolar field tilted with respect to the
stellar rotation axis. WD2359-434 always shows a weak positive longitudinal
field with values between about 0 and +12 kG, varying only weakly with stellar
rotation, while the mean field modulus varies between about 50 and 100 kG. The
rotation period is found to be 0.112 d using the variable shape of the Halpha
line core, consistent with available photometry. The field of this star appears
to be much more complex than a dipole, and is probably not axisymmetric.
Available photometry shows that WD2359-434 is a light variable with an
amplitude of only 0.005 mag, our own photometry shows that if WD2047+372 is
photometrically variable, the amplitude is below about 0.01 mag.
These are the first models for magnetic white dwarfs with fields below about
100 kG based on magnetic measurements through the full stellar rotation. They
reveal two very different magnetic surface configurations, and that, contrary
to simple ohmic decay theory, WD2359-434 has a much more complex surface field
than the much younger WD2047+372.Comment: Accepted for publication in Astronomy & Astrophysic
Helicopter Fuselage Drag ─ Combined Computational Fluid Dynamics and Experimental Studies
In this paper, wind tunnel experiments are combined with Computational
Fluid Dynamics (CFD) aiming to analyze the aerodynamics of
realistic fuselage con¦gurations. A development model of the ANSAT
aircraft and an early model of the AKTAI light helicopter were employed.
Both models were tested at the subsonic wind tunnel of KNRTU-KAI
for a range of Reynolds numbers and pitch and yaw angles. The force
balance measurements were complemented by particle image velocimetry
(PIV) investigations for the cases where the experimental force measurements
showed substantial unsteadiness. The CFD results were found to
be in fair agreement with the test data and revealed some §ow separation
at the rear of the fuselages. Once con¦dence on the CFD method was
established, further modi¦cations were introduced to the ANSAT-like
fuselage model to demonstrate drag reduction via small shape changes
VV124 (UGC4879): A new transitional dwarf galaxy in the periphery of the Local Group
We present the first resolved-star photometry of VV124 (UGC4879) and find
that this is the most isolated dwarf galaxy in the periphery of the Local
Group. Based on imaging and spectroscopic follow up observations with the 6m
BTA telescope, we resolve VV124 into 1560 stars down to the limiting magnitude
levels of V~25.6 and I~23.9. The young blue stellar populations and emission
gas are found near the core, but noticeably displaced from the center of the
galaxy as traced by dominant evolved red stars. The mean radial velocity
derived from the spectra of two Blue Supergiant stars, an HII region and
unresolved continuum sources is -80+/-10 km/s. The evolved ``red tangle''
stellar populations, which contains the red giant branch (RGB), are identified
at large galactocentric radii. We use the I-band luminosity function to
determine the distance based on the Tip of RGB method, 1.1+/-0.1 Mpc. This is
~10 times closer than the values usually assumed in the literature, and we
provide revised distance dependent parameters. From the mean (V-I) color of the
RGB, we estimate the mean metallicity as [Fe/H]~-1.37 dex. Despite of its
isolated location, the properties of VV124 are clearly not those of a galaxy in
formation, but rather similar to a transitional dIrr/dSph type.Comment: 4 pages, submitted to MNRAS Letter
Optical observations of "hot" novae returning to quiescence
We have monitored the return to quiescence of novae previously observed in
outburst as supersoft X-ray sources, with optical photometry of the
intermediate polar (IP) V4743 Sgr and candidate IP V2491 Cyg, and optical
spectroscopy of these two and seven other systems. Our sample includes
classical and recurrent novae, short period (few hours), intermediate period
(1-2 days) and long period (symbiotic) binaries. The light curves of V4743 Sgr
and V2491 Cyg present clear periodic modulations. For V4743 Sgr, the modulation
occurs with the beat of the rotational and orbital periods. If the period
measured for V2491 Cyg is also the beat of these two periods, the orbital one
should be almost 17 hours. The recurrent nova T Pyx already shows fragmentation
of the nebular shell less than 3 years after the outburst. While this nova
still had strong [OIII] at this post-outburst epoch, these lines had already
faded after 3 to 7 years in all the others. We did not find any difference in
the ratio of equivalent widths of high ionization/excitation lines to that of
the Hbeta line in novae with short and long orbital period, indicating that
irradiation does not trigger high mass transfer rate from secondaries with
small orbital separation. An important difference between the spectra of RS Oph
and V3890 Sgr and those of many symbiotic persistent supersoft sources is the
absence of forbidden coronal lines. With the X-rays turn-off, we interpret this
as an indication that mass transfer in symbiotics recurrent novae is
intermittent.Comment: In press in Monthly Notices of the Royal Astronomical Societ
Spectral analysis of lbv stars in m31: Af and and Var 15
Copyright © International Astronomical Union 2015. We study spectra of two bona fide LBV stars in M31: AF And and Var 15. The spectra were obtained with the 6-m telescope (Russia) from 2005 to 2012. The model spectra were calculated with the CMFGEN code. We have not found strong changes in the spectra of the LBV stars in that time interval, however a certain variability has been detected. We estimate the star and wind parameters, such as luminosity, temperature, raduis, mass loss rate, escape velocity, hydrogen content, and reddening. We study the stars on the Hertzsprung-Russell diagram and find their initial masses using evolutionary tracks by Meynet et al. (1994)
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