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 $100 \pm 10$ 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 $\mu$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 $\pm$ 110 and 150 $\pm$ 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 $\pm$ 1 and M 8 $\pm$ 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 $\pm$ 2 type secondary contributing 23 per cent to the overall flux at $\lambda$ = 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 16^{16}O + 16^{16}O

The impact of nuclear molecular configurations on the astrophysical S-factor for $^{16}$O + $^{16}$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&