352 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
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
Direct determination of optimal real-space orbitals for correlated electronic structure of molecules
We demonstrate how to determine nearly numerically exact orthonormal orbitals
that are optimal for evaluation of the energy of arbitrary (correlated) states
of atoms and molecules by minimization of the energy Lagrangian. Orbitals are
expressed in real space using multiresolution spectral element basis that is
refined adaptively to achieve the user-specified target precision while
avoiding the ill-conditioning issues that plague AO basis set expansions
traditionally used for correlated models of molecular electronic structure. For
light atoms the orbital solver in conjunction with a variational electronic
structure model [selected Configuration Interaction (CI)] provides energies of
comparable precision to a state-of-the-art atomic CI solver. The computed
electronic energies of atoms and molecules are significantly more accurate than
the counterparts obtained with the Gaussian AO bases of the same rank, and can
be determined even when linear dependence issues preclude the use of the AO
bases.Comment: 25 pages, 2 figure
MAJOR AREAS OF FOREIGN LANGUAGE SPEECH ACTIVITY OF UNIVERSITY STUDENTS IMPROVEMENT
Purposes: Increased international activity in Russia leads to a steady claim of foreign language knowledge. The list of professional characteristics of a university graduate includes the ability to adapt to new information realities in the professional community, where foreign language speech activities become highly demanded, when not only language skills, but also social and cultural arrangements of the country. Thus, innovative approaches to foreign language teaching are required. The article explored the following aspects of foreign-language speech activity of university students.
Methodology: methodology of teaching foreign-language speech activity; principles of development of foreign language speech activity of students; realization of educational resources for this activity development; model of pedagogical technology of formation of foreign language speech activity, etc.
Result: The results of the pilot work showed the validity of using the principles of the development of foreign language speech activities in the organization of the educational process.
Implications/Applications: This article will help the student to promote the ability to give a speech.
Novelty/Originality: At the same time, being a type of speech activity, foreign language communication in the process of training solves, in turn, educational and development tasks by means of a foreign language sign system, where the whole set of speech actions of the individual is actually present
Two New LBV Candidates in the M33 Galaxy
We present two new luminous blue variable (LBV) candidate stars discovered in
the M33 galaxy. We identified these stars (Valeev et al. 2010) as massive star
candidates at the final stages of evolution, presumably with a notable
interstellar extinction. The candidates were selected from the Massey et al.
(2006) catalog based on the following criteria: emission in Halpha, V<18.5 and
0.35<(B-V)<1.2. The spectra of both stars reveal a broad and strong Halpha
emission with extended wings (770 and 1000 km/s). Based on the spectra we
estimated the main parameters of the stars. Object N45901 has a bolometric
luminosity log(L/Lsun)=6.0-6.2 with the value of interstellar extinction
Av=2.3+-0.1. The temperature of the star's photosphere is estimated as
Tstar~13000-15000K its probable mass on the Zero Age Main Sequence is
M~60-80Msun. The infrared excess in N45901 corresponds to the emission of warm
dust with the temperature Twarm~1000K, and amounts to 0.1% of the bolometric
luminosity. A comparison of stellar magnitude estimates from different catalogs
points to the probable variability of the object N45901. Bolometric luminosity
of the second object, N125093, is log(L/Lsun)=6.3-6.6, the value of
interstellar extinction is Av=2.75+-0.15. We estimate its photosphere's
temperature as Tstar~13000-16000K, the initial mass as M~90-120Msun. The
infrared excess in N125093 amounts to 5-6% of the bolometric luminosity. Its
spectral energy distribution reveals two thermal components with the
temperatures Twarm~1000K and Tcold~480K. The [CaII] lines (7291A and 7323A),
observed in LBV-like stars VarA and N93351 in M33, are also present in the
spectrum of N125093. These lines indicate relatively recent gas eruptions and
dust activity linked with them. High bolometric luminosity of these stars and
broad Halpha emissions allow classifying the studied objects as LBV candidates.Comment: 14 pages, 4 figure
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