Searcharxiv⌕ Search

arXiv subjects

A. V. Dodin

Publications and source records attributed to A. V. Dodin.

44 records · Page 3Linked to original sources
arXiv2015

Orbital motions and light curves of young binaries XZ Tau and VY Tau

The results of our speckle interferometric observations of young binaries VY Tau and XZ Tau are presented. For the first time, we found a relative displacement of VY Tau components as well as a preliminary orbit for XZ Tau. It appeared that the orbit is appreciably non-circular and is inclined by $i \lesssim 47^o$ from the plane of the sky. It means that the rotation axis of XZ Tau A and the axis of its jet are significantly non-perpendicular to the orbital plane. We found that the average brightness of XZ Tau had been increasing from the beginning of the last century up to the mid-thirties and then it decreased by $ΔB > 2$ mag. The maximal brightness has been reached significantly later on the time of periastron passage. The total brightness of XZ Tau's components varied in a non-regular way from 1970 to 1985 when eruptions of hot gas from XZ Tau A presumably had occurred. In the early nineties the variations became regular following which a chaotic variability had renewed. We also report that a flare activity of VY Tau has resumed after 40 yr pause, parameters of the previous and new flares are similar, and the flares are related with the A component.

astro-ph.SR↗
arXiv2014

Non-LTE Modelling of the Structure and Spectra of the Hot Accretion Spots on the Surface of Young Stars

The paper describes the modelling of the structure and spectra of the hot accretion spots on the surface of young stars with taking into account departures from LTE for hydrogen and helium. It has been found that the existence of the ram pressure of the in-falling gas at the outer boundary of the hot spot leads to the Stark broadening of the hydrogen line profiles up to FWHM of about 1000 km/s at the considered accretion parameters. It is shown that taking into account departures from LTE for atoms and ions of carbon and oxygen does not lead to noticeable changes in the structure of the hot spot.

astro-ph.SR↗
arXiv2013

Results of magnetic field measurements in young stars DO Tau, DR Tau, DS Tau

Results of measurements of the longitudinal magnetic field in a hot accretion spot in three classical T Tauri stars (CTTS) are presented. The magnetic field in the formation region of the narrow component of the emission line HeI 5876 Å was found for each star in our sample at a level of more than 2σ. In case of DS Tau we have found the field in the narrow components of NaI D lines, which was equal to +0.8 \pm 0.3 kG, i.e. it was equal to the field measured on the narrow component of HeI 5876 Å. Our results indicate that the magnetic field in the hot spots can be studied for CTTS down to 13^m that allow in the future to double a number of CTTS with measured field in the accretion zone.

astro-ph.SR↗
arXiv2013

A non-LTE modeling of narrow emission components of He and Ca lines in optical spectra of CTTS

A spectrum of a hot spot, produced by radiation of accretion shock at T Tauri star's surface, has been calculated taking into account non-LTE effects for HeI, HeII, CaI and CaII, using LTE-calculations of spot's atmospheric structure, calculated by Dodin & Lamzin (2012). Assuming that pre-shock gas number density N_0 and its velocity V_0 are the same across the accretion column, we calculated spectra of a system "star + round spot" for a set of N_0, V_0 values and parameters, which characterized the star and the spot. It has been shown that theoretical spectra with an appropriate choice of the parameters reproduce well observed veiling of photospheric absorption lines in optical band as well as profiles and intensities of so-called narrow components of HeII and CaI emission lines in spectra of 9 stars. We found that the accreted gas density N_0>10^{12} cm^{-3} for all considered stars except DK Tau. Observed spectra of 8 stars were succesfully fitted, asuming solar abundance of calcium, but it appeared possible to fit TW Hya spectrum only under assumption that calcium abundance in accreted gas was three times less than solar. We derive spot's parameters by comparison of theoretical and observed spectra, normalised to continuum level, so our results are independent on unknown value of interstellar extinction. We have found that the predicted flux in CaII lines is less than observed one, but this discrepancy can be resolved if not only high-density but also lower density gas falls onto the star. Theoretical equivalent widths as well as relative intensities of He\,I subordinate lines disagree significantly with observations, presumbly due to a number of reasons: necessity to take into account non-LTE thermal structure of upper layers of a hot spot, poorly known collisional atomic data for HeI upper levels and inhomogeneity of the hot spot.

astro-ph.SR↗
arXiv2012

Interpretation of the Veiling of the Photospheric Spectrum for T Tauri Stars in Terms of an Accretion Model

The problem on heating the atmospheres of T Tauri stars by radiation from an accretion shock has been solved. The structure and radiation spectrum of the emerging so-called hot spot have been calculated in the LTE approximation. The emission not only in continuum but also in lines has been taken into account for the first time when calculating the spot spectrum. Comparison with observations has shown that the strongest of these lines manifest themselves as narrow components of helium and metal emission lines, while the weaker ones decrease significantly the depth of photospheric absorption lines, although until now, this effect has been thought to be due to the emission continuum alone. The veiling by lines changes the depth of different photospheric lines to a very different degree even within a narrow spectral range. Therefore, the nonmonotonic wavelength dependence of the degree of veiling r found for some CTTS does not suggest a nontrivial spectral energy distribution of the veiling continuum. In general, it makes sense to specify the degree of veiling r only by providing the set of photospheric lines from which this quantity was determined. We show that taking into account the contribution of lines to the veiling of the photospheric spectrum can cause the existing estimates of the accretion rate onto T Tauri stars to decrease by several times, with this being also true for stars with a comparatively weakly veiled spectrum. Neglecting the contribution of lines to the veiling can also lead to appreciable errors in determining the effective temperature, interstellar extinction, radial velocity, and vsin(i).

astro-ph.SR↗
arXiv2012

Reverse Rotation of the Accretion Disk in RW Aur A: Observations and a Physical Model

Speckle interferometry of the young binary system RW Aur was performed with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences using filters with central wavelengths of 550 nm and 800 nm and pass-band halfwidths of 20 nm and 100 nm, respectively. The angular separation of the binary components was 1.448"{\pm}0.005 and the position angle of the system was 255.9°{\pm}0.3° at the observation epoch (JD 2 454 255.9). We find using published data that these values have been changing with mean rates of +0.002"/yr and +0.02°/yr, respectively, over the past 70 years. This implies that the direction of the orbital motion of the binary system is opposite to the direction of the disk rotation in RW Aur A. We propose a physical model to explain the formation of circumstellar accretion disks rotating in the reverse direction relative to young binary stars surrounded by protoplanetary disks. Our model can explain the characteristic features of the matter flow in RW Aur A: the high accretion rate, small size of the disk around the massive component, and reverse direction of rotation.

astro-ph.SR↗
arXiv2011

Magnetic field of young star RW Aur

Results of longitudinal magnetic field B_z measurements for young star RW Aur A are presented. We found that B_z in the formation region of HeI 5876 line's narrow component varies from -1.47 \pm 0.15 kG to +1.10 \pm 0.15 kG. Our data are consistent with a stellar rotational period of \simeq 5.6^d and with a model of two hotspots with an opposite polarity of magnetic field and with a difference in a longitude about 180^o. The spot with B_z<0 is located at the hemisphere above the midplane of RW Aur's accretion disc and the spot with B_z>0 is below the midplane. The following upper limits for B_z (at 3σ level) were found after averaging of all our observations: 180 G for photosperic lines, 220 G and 230 G for formation regions of H_α and [OI] 6300 lines respectively. Upper limit 600 G were found in the region where broad components of emisson lines form. For two cases out of 11 we observed the field in a formation region of a blue absorption wing of NaI D lines i.e. in an outflow: B_z= -180 \pm 50 G and -810 \pm 80 G. Radial velocity of RW Aur's photospheric lines averaged over all our observations is about + 10.5 km/s what is 5.5 km/s less than value derived ten years earlier by Petrov et al. (2001). In this connection, we discuss a possibility that RW Aur is not a binary but a triple system.

astro-ph.SR↗