35,356 research outputs found
A search for brown-dwarf like secondaries in cataclysmic variables
We present VTL/ISAAC infrared spectroscopy of a sample of short orbital
period cataclysmic variables which are candidates for harboring substellar
companions. We have detected the KI and NaI absorption lines of the companion
star in VY Aqr. The overall spectral distribution in this system is best fit
with a M9.5 type dwarf spectra, implying a distance of pc. VY Aqr
seems to fall far from the theoretical distribution of secondary star
temperatures around the orbital period minimum. Fitting of the IR spectral
energy distribution (SED) was performed by comparing the observed spectrum with
late-type templates. The application of such a spectral fitting procedure
suggests that the continuum shape in the 1.1-2.5 m spectral region in
short orbital period cataclysmic variables may be an useful indicator of the
companion spectral type. The SED fitting for RZ Leo and CU Vel suggests M5 type
dwarf companions, and distances of 340 110 and 150 50 pc,
respectively. These systems may be placed in the upper evolution branch for
short period cataclysmic variables.Comment: accepted for publication in MNRAS, 6 pages, 7 figure
Search for brown-dwarf like secondaries in cataclysmic variables II
We have examined VTL/ISAAC 1-2.5 \umum spectroscopy of a sample of short
orbital period cataclysmic variables which are candidates for harboring
substellar companions. We provide descriptions of the infrared spectrum of
\hbox{EI Psc}, \hbox{V834 Cen}, \hbox{WX Cet}, \hbox{VW Hyi}, \hbox{TY PsA} and
\hbox{BW Scl}. Fitting of the IR spectral energy distribution (SED) was
performed by comparing the observed spectrum with late-type templates.
Absorption features of the secondary star were detected in \hbox{EI Psc} and
\hbox{V834 Cen}, consistent with dwarf secondaries of spectral type K 5 1
and M 8 0.5, respectively. In addition, we report the first detection of
the secondary star in \hbox{VW Hyi}. The SED in this case is well matched by an
L 0 2 type secondary contributing 23 per cent to the overall flux at
= 1.15 \umum. This is a surprising result for a system with a
relatively high mass transfer rate. We discuss the implication of our findings
on the current scenarios for cataclysmic variable star evolution.Comment: accepted for publication in MNRA
Effects of nuclear molecular configurations on the astrophysical S-factor for O + O
The impact of nuclear molecular configurations on the astrophysical S-factor
for O + O is investigated within the realistic two-center shell
model based on Woods-Saxon potentials. These molecular effects refer to the
formation of a neck between the interacting nuclei and the radial dependent
collective mass parameter. It is demonstrated that the former is crucial to
explain the current experimental data with high accuracy and without any free
parameter, whilst in addition the latter predicts a pronounced maximum in the
S-factor. In contrast to very recent results by Jiang et al., the S-factor does
not decline towards extremely low values as energy decreases.Comment: In press in Physics Letters
Nuclear Activity in Circumnuclear Ring Galaxies
We have analyzed the frequency and properties of the nuclear activity in a
sample of galaxies with circumnuclear rings and spirals (CNRs). This sample was
compared with a control sample of galaxies with very similar global properties
but without circumnuclear rings. We discuss the relevance of the results in
regard to the AGN feeding processes and present the following results: (i)
bright companion galaxies seem not to be important for the appearance of CNRs,
which appear to be more related to intrinsic properties of the host galaxies or
to minor merger processes; (ii) the proportion of strong bars in galaxies with
an AGN and a CNR is somewhat higher than the expected ratio of strongly barred
AGN galaxies from the results of Ho and co-workers; (iii) the incidence of
Seyfert activity coeval with CNRs is clearly larger than the rate expected from
the morphological distribution of the host galaxies; (iv) the rate of Sy 2 to
Sy 1 type galaxies with CNRs is about three times larger than the expected
ratio for galaxies without CNRs and is opposite to that predicted by the
geometric paradigm of the classical unified model for AGNs, although it does
support the hy-pothesis that Sy 2 activity is linked to circumnuclear star
formation. The possible selection effects of the sample are discussed, and we
conclude that the detected trends are strong enough to justify high quality
observations of as large as possible sets of galaxies with circumnuclear rings
and their matched control samples.Comment: Submitted to International Journal of Astronomy and Astrophysic
Effect of partial ionization on wave propagation in solar magnetic flux tubes
Observations show that waves are ubiquitous in the solar atmosphere and may
play an important role for plasma heating. The study of waves in the solar
corona is usually based on linear ideal magnetohydrodynamics (MHD) for a fully
ionized plasma. However, the plasma in the photosphere and the chromosphere is
only partially ionized. Here we investigate theoretically the impact of partial
ionization on MHD wave propagation in cylindrical flux tubes in the two-fluid
model. We derive the general dispersion relation that takes into account the
effects of neutral-ion collisions and the neutral gas pressure. We take the
neutral-ion collision frequency as an arbitrary parameter. Particular results
for transverse kink modes and slow magnetoacoustic modes are shown. We find
that the wave frequencies only depend on the properties of the ionized fluid
when the neutral-ion collision frequency is much lower that the wave frequency.
For high collision frequencies realistic of the solar atmosphere ions and
neutrals behave as a single fluid with an effective density corresponding to
the sum of densities of both fluids and an effective sound velocity computed as
the average of the sound velocities of ions and neutrals. The MHD wave
frequencies are modified accordingly. The neutral gas pressure can be neglected
when studying transverse kink waves but it has to be taken into account for a
consistent description of slow magnetoacoustic waves. The MHD waves are damped
due to neutral-ion collisions. The damping is most efficient when the wave
frequency and the collision frequency are of the same order of magnitude. For
high collision frequencies slow magnetoacoustic waves are more efficiently
damped than transverse kink waves. In addition, we find the presence of
cut-offs for certain combinations of parameters that cause the waves to become
non-propagating.Comment: Accepted for publication in A&
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