T Tauri Stars

The observable characteristics of young solar-mass stars (T Tauri stars) are reviewed. Modern theoretical concepts and models of these stars are discussed briefly.     

Pilgrim dark energy in f(T,T G ) cosmology

We work on the reconstruction scenario of pilgrim dark energy (PDE) in f(T,T _G ). In PDE model it is assumed that a repulsive force that is accelerating the Universe is phantom type with (w _DE f(T,T _G ) models and correspondingly evaluate equation of state parameter for various choices of scale factor. Also, we assume polynomial form of f(T,T _G ) in terms of cosmic time and reconstruct H and w _DE in this manner. Through discussion, it is concluded that PDE shows aggressive phantom-like behavior for s=?2 in f(T,T _G ) gravity.     

T associations in X-rays

The X-ray emission properties of T Tau type stars and the basic results from the ROSAT X-ray mission are presented. The results of optical follow-up observations of ROSAT sources unidentified with known T Tau type stars are given. Surprisingly, some of them are located outside the classical borders of starforming regions, i.e. outside the dark clouds. Their kinematics and formation are discussed. Translated from Astrofizika, Vol. 42, No. 2, pp. 311–326, April–June, 1999.     

A T Tauri Star Database

A T Tauri Star Database was set up with the purpose of designing a user-friendly, graphical database which would provide a quick look into the properties of T Tauri stars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Rotation of T Tauri Stars Revisited

This paper reviews the rotation of T Tauri stars. It summarizes the results of a complete bibliographical survey of all photometric and spectroscopic data available in connection with rotation studies of these stars. It is shown that T Tauri stars are slow rotators. More than half of the stars in the sample have been found to have v sin i < 25 km/s. Furthermore, the histogram for the photometric periods of rotation is clearly bimodal, with a peak at approximately 3 days and another at around 8 days. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evolutionary status of T Tauri stars

The problem of determining the masses and ages of T Tauri star(TTS) using their evolutionary status is discussed. We test four pre-main sequence evolutionary models using well determined observational parameters of 12 binary TTSs and two binary red dwarfs. It is shown that the masses derived using the tracks of all models are in good agreement with the masses obtained from the observations of TTSs with masses M 0.7 M_⊙(mean error ε ~ 10%). Low-mass stars with M ≤ 0.7 M_⊙have significantly greater mean error: ε ~ 50% for the tracks of Bressan et al. and Chen et al., and ε ~ 30% for the other tracks. The isochrones of all tested evolutionary models diverge for stars with masses M ≤ 0.7 M_⊙. The difference increases with the mass decrease and can reach 10% of KelvinHelmholtz time for stars with mass M = 0.2 M_⊙. The ages of most of the considered TTSs are smaller than the Kelvin-Helmholtz time. This confirms their evolutionary status of being pre-main sequence stars.     

金牛T型星的红外光谱

给出２６ 个金牛Ｔ 型星的ＩＲＡＳ低分辨率红外光谱．根据这些光谱讨论了它们有关的性质和所处环境．分析发现硅酸盐尘埃在金牛Ｔ 型星拱星壳层中是常见的物质     

Spectral investigations of T Tauri stars

Basic ideas concerning the nature of young T Tauri stars (TTS) are briefly outlined and some examples of spectral investigations of those stars are considered. The photometric and spectral variability of TTS is believed to be due to circumstellar extinction, magnetic activity, and accretion of matter from the circumstellar disk onto the stellar surface. In the 1990s, a series of high resolution spectra of several TTS were obtained using the SOFIN echelle spectrograph with the Nordic Optical Telescope (NOT). In particular, the emission lines in the spectra of the star RW Aur A were shown to be rotationally modulated with a period of 2.7 days, which was interpreted in terms of the magnetospheric accretion model with an inclined magnetic rotator. The spectra of TTS obtained using the UVES spectrograph with the VLT demonstrated that the effect of veiling the photospheric spectrum, usually attributed to accretion, was largely due to chromospheric extinction. The accretion is suggested to be a complementary heating source in chromospheres of TTS.     

Trapezium systems containing T tauri stars

The results of a spectroscopic investigation of four stars comprising a Trapezium system in the nebula Sh 155 are presented. The stars have a number of emission lines typical of red emission stars and T Tau stars. This system is one of the 13 systems which we have found in the Palomar atlas. It is shown that all of these systems are members of OB associations.Translated fromAstrofizika, Vol. 38, No. 1, pp. 17–24, January–March, 1995.     

Saturn's magnetospheric interaction with Titan as defined by Cassini encounters T9 and T18: New results

We present new results of Cassini's T9 flyby with complementary observations from T18. Based on Cassini plasma spectrometer (CAPS) and Cassini magnetometer (MAG), compositional evidence shows the upstream flow for both T9 and T18 appears composed of light ions (H⁺ and H₂⁺), with external pressures ∼30 times lower than that for the earlier TA flyby where heavy ions dominated the magnetospheric plasma. When describing the plasma heating and sputtering of Titan's atmosphere, T9 and T18 can be considered interactions of low magnetospheric energy input. On the other hand, T5, when heavy ion fluxes are observed to be higher than typical (i.e., TA), represents the limiting case of high magnetospheric energy input to Titan's upper atmosphere. Anisotropy estimates of the upstream flow are 1<T_⊥/T_‖<3 and the flow is perpendicular to B, indicative of local picked up ions from Titan's H and H₂ coronae extending to Titan's Hill sphere radius. Beyond this distance the corona forms a neutral torus that surrounds Saturn. The T9 flyby unexpectedly resulted in observation of two “wake” crossings referred to as Events 1 and 2. Event 2 was evidently caused by draped magnetosphere field lines, which are scavenging pickup ions from Titan's induced magnetopause boundary with outward flux ∼2×10⁶ ions/cm²/s. The composition of this out flow is dominated by H₂⁺ and H⁺ ions. Ionospheric flow away from Titan with ion flux ∼7×10⁶ ion/cm²/s is observed for Event 1. In between Events 1 and 2 are high energy field aligned flows of magnetosphere protons that may have been accelerated by the convective electric field across Titan's topside ionosphere. T18 observations are much closer to Titan than T9, allowing one to probe this type of interaction down to altitudes ∼950 km. Comparisons with previously reported hybrid simulations are made.     

Holographic f(T) gravity model

We try to study the corresponding relation between f(T) gravity and holographic dark energy (HDE). A kind of energy density from f(T) is introduced which has the same role as HDE density. A f(T) model according to the HDE model is calculated. We find out a torsion scalar T based on the scalar factor is assumed by Capoziello et al. (Phys. Lett. B 639:135, 2006). The effective torsion equation of state, deceleration parameter of the holographic f(T)-gravity model are calculated.     

Rotation effects in classical T Tauri stars

Surface temperature inhomogeneities in classical T Tauri stars (CTTS) induced by magnetic activity andmass accretion lead to rotationalmodulation of both photometric and spectroscopic parameters of these stars. Using the extended photometric catalogue byGrankin et al., we have derived the periods and amplitudes of the rotational modulation of brightness and color for 31 CTTS; for six of them, the periods have been revealed for the first time. The inclinations of the rotation axis and equatorial rotational velocities of CTTS have been determined. We show that the known periods of brightness variations for some of the CTTS are not the axial rotation periods but are the Keplerian periods near the inner boundary of the dusty disk. We have found that the angular velocity of CTTS with a mass of 0.3?3M _?? in the Taurus-Auriga complex remains constant in the age range 1?C10 Myr. CTTS on radiative evolutionary tracks rotate faster than completely convective CTTS. The specific angular momentum of CTTS depends on the absolute luminosity in the H?? line.