Possible detection of a magnetic field in T Tauri

Medium-resolution $(R\simeq 15000)$ circular spectropolarimetry of T Tauri is presented. The star was observed twice: on November 11, 1996 and January 22, 2002. Weak circular polarization has been found in photospheric absorption lines, indicating a mean surface longitudinal magnetic field $B_{\|}$ of $160\pm 40$ G and $140\pm 50$ G at the epoch of the first and second observations respectively. While these values are near the detection limit of our apparatus, we belive that they are real. In any case one can conclude from our data that $B_{\|}$ of T Tau does not significantly exceed 200 G, which is much less than surface magnetic field strength of the star ($>2.3$ kG) found by Guenther et al. (1999) and Johns-Krull et al. (2000). We discuss possible reasons of this difference.Comment: 5 pages, 3 figure

Flickering in FU Orionis

We analyze new and published optical photometric data of FU Orionis, an eruptive pre-main sequence star. The outburst consists of a 5.5 mag rise at B with an e-folding timescale of roughly 50 days. The rates of decline at B and V are identical, 0.015 +- 0.001 mag per yr. Random fluctuations superimposed on this decline have an amplitude of 0.035 +- 0.005 mag at V and occur on timescales of 1 day or less. Correlations between V and the color indices U-B, B-V, and V-R indicate that the variable source has the optical colors of a G0 supergiant. We associate this behavior with small amplitude flickering of the inner accretion disk.Comment: 19 pages of text, 3 tables, and 6 figures to be published in the Astrophysical Journal, 10 March 200

Force-Free Models of Magnetically Linked Star-Disk Systems

Disk accretion onto a magnetized star occurs in a variety of astrophysical contexts, from young stars to X-ray pulsars. The magnetohydrodynamic interaction between the stellar field and the accreting matter can have a strong effect on the disk structure, the transfer of mass and angular momentum between the disk and the star, and the production of bipolar outflows, e.g., plasma jets. We study a key element of this interaction - the time evolution of the magnetic field configuration brought about by the relative rotation between the disk and the star - using simplified, largely semianalytic, models. We first discuss the rapid inflation and opening up of the magnetic field lines in the corona above the accretion disk, which is caused by the differential rotation twisting. Then we consider additional physical effects that tend to limit this expansion, such as the effect of plasma inertia and the possibility of reconnection in the disk's corona, the latter possibly leading to repeated cycles in the evolution. We also derive the condition for the existence of a steady state for a resistive disk and conclude that a steady state configuration is not realistically possible. Finally, we generalize our analysis of the opening of magnetic field lines by using a non-self-similar numerical model that applies to an arbitrarily rotating (e.g. keplerian) disk.Comment: 75 pages, 22 figures, 2 tables. Submitted to Astrophysical Journa

The analysis of magnetic field measurements of T Tau

It is shown that the existence of hot accretion spots at the surface of T Tau practically has no effect on the accuracy of estimation of its magnetic field strength at photospheric level. We also found that one can interpret results of T Tau's photospheric magnetic field strength measurements carried out via different methods in the frame of the following alternative: 1) if T Tau's inclination angle $i\le 10^\circ,$ then magnetic field of the star may be dipolar with the angle between rotational and magnetic axes is near $85^\circ;$ 2) if it will be found (e.g. from interferometric observations) that $i>10^\circ,$ then magnetic field of T Tau is essentially non-dipolar or/and non-stationary.Comment: 4 figure

The model of dynamo with small number of modes and magnetic activity of T Tauri stars

The model that describes operation of dynamo in fully convective stars is presented. It is based on representation of stellar magnetic field as a superposition of finite number of poloidal and toroidal free damping modes. In the frame of adopted low of stellar differential rotation we estimated minimal value of dynamo number D, starting from which generation of cyclic magnetic field in stars without radiative core is possible. We also derived expression for period of the cycle. It was found that dynamo cycles of fully convective stars and stars with thin convective envelopes differ in a qualitative way: 1) distribution of spots over latitude during the cycle is different in these stars; 2) the model predicts that spot formation in fully convective stars should be strongly suppressed at some phases of the cycle. We have analyzed historical lightcurve of WTTS star V410 Tau and found that long term activity of the star is not periodic process. Rather one can speak about quasi cyclic activity with characteristic time of $\sim 4$ yr and chaotic component over imposed. We concluded also that redistribution of cool spots over longitude is the reason of long term variations of V410 Tau brightness. It means that one can not compare directly results of photometric observations with predictions of our axially symmetric (for simplicity) model which allows to investigate time evolution of spot's distribution over latitude. We then discuss what kind of observations and in which way could be used to check predictions of the dynamo theory.Comment: 18 pages, 5 figures, accepted to Astron. Let

The close T Tauri binary system V4046 Sgr: Rotationally modulated X-ray emission from accretion shocks

We report initial results from a quasi-simultaneous X-ray/optical observing campaign targeting V4046 Sgr, a close, synchronous-rotating classical T Tauri star (CTTS) binary in which both components are actively accreting. V4046 Sgr is a strong X-ray source, with the X-rays mainly arising from high-density (n_e ~ 10^(11-12) cm^(-3)) plasma at temperatures of 3-4 MK. Our multiwavelength campaign aims to simultaneously constrain the properties of this X-ray emitting plasma, the large scale magnetic field, and the accretion geometry. In this paper, we present key results obtained via time-resolved X-ray grating spectra, gathered in a 360 ks XMM-Newton observation that covered 2.2 system rotations. We find that the emission lines produced by this high-density plasma display periodic flux variations with a measured period, 1.22+/-0.01 d, that is precisely half that of the binary star system (2.42 d). The observed rotational modulation can be explained assuming that the high-density plasma occupies small portions of the stellar surfaces, corotating with the stars, and that the high-density plasma is not azimuthally symmetrically distributed with respect to the rotational axis of each star. These results strongly support models in which high-density, X-ray-emitting CTTS plasma is material heated in accretion shocks, located at the base of accretion flows tied to the system by magnetic field lines.Comment: paper accepted by Ap