Inclination effects in T Tauri star spectra

CONTEXT. Because of the presence of rotation and accretion disks, classical T Tauri stars have symmetry planes that are normally inclined relative to the plane of the sky. The inclination angles affect the observed spectral properties of these objects. AIMS. We study the influence of the inclination angles on classical T Tauri star spectra in an empirical manner. METHODS. Published inclination angles, derived from the stellar photospheric rotation or from spatially resolved circumstellar disk observations, are compared with various observed spectral properties, and correlations are established and investigated. RESULTS. Inclinations derived from the stellar rotation are found to be much less accurate than the published disk inclinations, and no significant correlations between spectral properties and inclinations based on rotation data could be detected. In contrast, significant correlations are found between the disk inclination angles and the apparent velocities observed for the forbidden emission lines and the wind absorption features of permitted lines. These data support the assumption of cone-like polar winds with opening angles smaller than $\approx 45^\circ$. Other spectral features show weaker or no inclination dependence. Using these results, the true (deprojected) flow velocities of the polar winds are derived for the investigated sample of T Tauri stars. Deprojected wind-ejection velocities appear to differ by a factor of two among the stars in our sample, which spans a range of mass-loss rates from $10^{-10}$M$_\odot$/yr to $3 \times 10^{-7}$M$_\odot$/yr.Comment: accepted by Astronomy and Astrophysic

A kinematic study of the Taurus-Auriga T association

Aims: This is the first paper in a series dedicated to investigating the kinematic properties of nearby associations of young stellar objects. Here we study the Taurus-Auriga association, with the primary objective of deriving kinematic parallaxes for individual members of this low-mass star-forming region. Methods: We took advantage of a recently published catalog of proper motions for pre-main sequence stars, which we supplemented with radial velocities from various sources found in the CDS databases. We searched for stars of the Taurus-Auriga region that share the same space velocity, using a modified convergent point method that we tested with extensive Monte Carlo simulations. Results: Among the sample of 217 Taurus-Auriga stars with known proper motions, we identify 94 pre-main sequence stars that are probable members of the same moving group and several additional candidates whose pre-main sequence evolutionary status needs to be confirmed. We derive individual parallaxes for the 67 moving group members with known radial velocities and give tentative parallaxes for other members based on the average spatial velocity of the group. The Hertzsprung-Russell diagram for the moving group members and a discussion of their masses and ages are presented in a companion paper.Comment: accepted for publication by A&

The accretion disk paradigm for young stars

Abstract. Accretion and magnetic fields play major roles in several of the many models put forward to explain the properties of T Tauri stars since their discovery by Alfred Joy in the 1940s. Early investigators already recognized in the 1950s that a source of energy external to the star was needed to account for the emission properties of these stars in the optical range. The opening of new spectral windows from the infrared to the ultraviolet in the 1970s and 1980s showed that the excess emission of T Tauri stars and related objects extends into all wavelength domains, while evidence of outflow and/or infall in their circumstellar medium was accumulating. Although the disk hypothesis had been put forward by Merle Walker as early as 1972 to explain properties of YY Orionis stars and although Lynden-Bell and Pringle worked out the accretion disk model and applied it specifically to T Tauri stars in 1974, the prevailing model for young stellar objects until the mid-1980s assumed that they experienced extreme solar-type activity. It then took until the late 1980s before the indirect evidence of disks presented by several teams of researchers became so compelling that a paradigm shift occurred, leading to the current consensual picture. I briefly review the various models proposed for explaining the properties of young stellar objects, from their discovery to the direct observations of circumstellar disks that have so elegantly confirmed the nature of young stars. I will go on to discuss more modern issues concerning their accretion disk properties and conclude with some results obtained in a recent attempt to better understand the evolution of Taurus-Auriga young stellar objects

The accretion disk paradigm for young stars

Abstract. Accretion and magnetic fields play major roles in several of the many models put forward to explain the properties of T Tauri stars since their discovery by Alfred Joy in the 1940s. Early investigators already recognized in the 1950s that a source of energy external to the star was needed to account for the emission properties of these stars in the optical range. The opening of new spectral windows from the infrared to the ultraviolet in the 1970s and 1980s showed that the excess emission of T Tauri stars and related objects extends into all wavelength domains, while evidence of outflow and/or infall in their circumstellar medium was accumulating. Although the disk hypothesis had been put forward by Merle Walker as early as 1972 to explain properties of YY Orionis stars and although Lynden-Bell and Pringle worked out the accretion disk model and applied it specifically to T Tauri stars in 1974, the prevailing model for young stellar objects until the mid-1980s assumed that they experienced extreme solar-type activity. It then took until the late 1980s before the indirect evidence of disks presented by several teams of researchers became so compelling that a paradigm shift occurred, leading to the current consensual picture. I briefly review the various models proposed for explaining the properties of young stellar objects, from their discovery to the direct observations of circumstellar disks that have so elegantly confirmed the nature of young stars. I will go on to discuss more modern issues concerning their accretion disk properties and conclude with some results obtained in a recent attempt to better understand the evolution of Taurus-Auriga young stellar objects

Diamagnetic Blob Interaction Model of T Tauri Variability

Assuming a diamagnetic interaction between a stellar-spot originated localized magnetic field and gas blobs in the accretion disk around a T- Tauri star, we show the possibility of ejection of such blobs out of the disk plane. Choosing the interaction radius and the magnetic field parameters in a suitable way gives rise to closed orbits for the ejected blobs. A stream of matter composed of such blobs, ejected on one side of the disk and impacting on the other, can form a hot spot at a fixed position on the disk (in the frame rotating with the star). Such a hot spot, spread somewhat by disk shear before cooling, may be responsible in some cases for the lightcurve variations observed in various T-Tauri stars over the years. An eclipse-based mechanism due to stellar obscuration of the spot is proposed. Assuming high disk inclination angles it is able to explain many of the puzzling properties of these variations. By varying the field parameters and blob initial conditions we obtain variations in the apparent angular velocity of the hot spot, producing a constantly changing period or intermittent periodicity disappearance in the models.Comment: 6 pages, 4 figures, aas2pp4 styl

The Vertical Structure of T Tauri Accretion Disks. I. Heating by the Central Star

International audienceThe paper presents the LTE formal solution for the vertical atmospheric temperature structure of an accretion disk illuminated by its central star. It is shown that there are two consequences of irradiation: (1) heating of the photospheric layers resulting in continuum radiation, and (2) heating of the optically thin layers resulting in a chromosphericlike temperature increase that may produce disk emission lines in these regions. These results are illustrated by computing the gray vertical temperature structure of a flat accretion disk. Because of the emergent intensity's anisotropy, the observed disk temperature distribution depends on the disk's view angle

Systèmes solaires futurs

International audienceWidely regarded as pathological variable stars - with erratic photometric and spectroscopic behavior of unknown physical origin - 20 years ago, T Tauri stars (TTSs) turned out in the last decade to be promising laboratories for observing the formation of solar systems such as ours. This is because circumstellar, presumably protoplanetary disks were found to surround a large fraction of them. While evidence for disks was primarily indirect until 1995, recent high resolution imaging confirmed earlier claims that the infrared (IR) and ultraviolet (UV) excesses seen in the spectral energy distribution (SED) of these stars were due to disk emission. The activity displayed by young stellar objects at all wavelengths is due to the interaction between the circumstellar disk and the magnetized star and to non-stationary accretion/ejection phenomena. In this review, we briefly summarize properties of these young solar-type stars and describe their circumstellar disks in some detail, focusing on current optical, infrared and millimeter high angular resolution observations that now allow us to resolve the disks