Periodical light variations of t tauri stars as a result of disk accretion

I argue that temperatures of spots, responsible for observed periodical light variations of T Tauri stars (TTS), are not known with reliable accuracy to discriminate between chromospheric and accretion theories of TTS 's phenomenon. The hypothesis is set up that spots on classical TTS (CTTS) are due to heating of stellar surface by radiation from a collisional accretion shock, whereas spots on weak line TTS (WTTS), at least in some cases, are connected with a collisionless accretion shock rather than chromospheric activity. Possible scenarios of WTTS interaction with circumstellar matter are discussed.     

T Tauri stars: Physical parameters and evolutionary status

Long-term homogeneous photometry for 35 classical T Tauri stars (CTTS) in the Taurus–Auriga star-forming region has been analyzed. Reliable effective temperatures, interstellar extinctions, luminosities, radii, masses, and ages have been determined for the CTTS. The physical parameters and evolutionary status of 35 CTTS from this work and 34 weak-line T Tauri stars (WTTS) from previous studies have been compared. The luminosities, radii, and rotation periods of low-mass (0.3–1.1 M _⊙) CTTS are shown to be, on average, greater than those of low-mass WTTS, in good agreement with the evolutionary status of these two subgroups. The mean age of the younger subgroup of WTTS from our sample (2.3 Myr) essentially coincides with the mean duration of the protoplanetary disk accretion phase (2.3 Myr) for a representative sample of low-mass stars in seven young stellar clusters. The accretion disk dissipation time scale for the younger subgroup of CTTS (<4 Myr) in the Taurus–Auriga star-forming region is shown to be no greater than 0.4 Myr, in good agreement with the short protoplanetary disk dissipation time scale that is predicted by present-day protoplanetary disk evolution models.     

Proper motion study of the ROSAT selected WTTS candidates around Taurus-Auriga

Astrometric study of the T Tauri stars (TTS) candidates and non-TTS X-ray sources around Tau-Aur [Li, Hu 132 (1998) 173], based on the Hipparcos and the ACT Reference Catalogue, is presented in this paper. The ROSAT selected X-ray sources are found to have complicated nature. A few of them are associated with the Tau-Aur or the Orion Star Forming Regions (SFR). Some, with distances similar to that of Tau-Aur but indicating discrepant proper motions, are probable or solid Pleiades supercluster members and other late type young active stars of unresolved nature, the latter are more likely formed in rapidly moving cloudlets, or else have originated from different sites rather than Tau-Aur and dispersed to the present locations. A number of the non-TTS X-ray sources studied are possible Hyades cluster members. Some TTS candidates could be foreground pre-main sequence stars or actually young dwarfs not yet depleted their Lithium.Under the hypothesis that the sources studied are representative of the ROSAT selected TTS candidates discovered in the outskirts of the Tau-Aur region, we conclude that only up to one thirds of the WTTS candidates could be expected to be physically associated with the Tau-Aur association. Along with the parallax and proper motion analysis of the non-TTS X-ray sources around the Tau-Aur SFR, our study suggests that the most majority of the young active X-ray sources, within an angular diameter of about 30^∘ toward the Tau-Aur SFR, are spatially belonging to mainly 4 different subgroups according to the apparent discordance of their distances and/or proper motions.     

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.     

Magnetically active stars in Taurus-Auriga: Photometric variability and basic physical parameters

We have analyzed homogeneous long-term photometric observations of 28 well-known weakline T Tauri stars (WTTS) and 60 WTTS candidates detected by the ROSAT observatory toward the Taurus-Auriga star-forming region. We show that 22 known WTTS and 39 WTTS candidates exhibit periodic light variations that are attributable to the phenomenon of spotted rotational modulation. The rotation periods of these spotted stars lie within the range from 0.5 to 10 days. Significant differences between the long-term photometric behaviors of known WTTS and WTTS candidates have been found. We have calculated accurate luminosities, radii, masses, and ages for 74 stars. About 33% of the sample of WTTS candidates have ages younger than 10 Myr. The mean distance to 24 WTTS candidates with reliable estimates of their radii is shown to be 143 ± 26 pc. This is in excellent agreement with the adopted distance to the Taurus-Auriga star-forming region.     

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.     

X-ray emission as a tracer of circumstellar matter around herbig ae/be stars

We have detected X-ray emission (1 keV) from young intermediate-mass stars (Herbig Ae/Be stars). Since these stars are not supposed to produce intrinsic X-ray emission (no convection, no coronae), we believe that our results suggest that the X-ray emission actually traces the shock interaction of the Ae/Be star stellar winds with remnant circumstellar matter left over from the star formation process, the presence of which is also indicated by far-infrared (IRAS) and submm/mm continuum data.     

Structure and evolution of circumstellar shells formed by massive star winds

We follow the interaction of massive stars with their circumstellar gas over their entire life-times by combining hydrodynamic stellar evolution calculations for 35 and 60M stars and one- and two-dimensional gas dynamical calculations for the circumstellar medium.     

Taurus-Auriga外围区ROSAT定点观测选WTTS的光学证认与研究

1引言金牛T型星（TTaur1）是一类低质量（＜3＊O）光谱晚型（晚F、G、K和M主序前类太阳恒星．依据其H。发射线的强弱可分为两类：经典TTauri星（CTTS）和弱发射线TTauri星（WTTS．一般认为,CTTS的地发射线等值宽度在15A以上,具有强的红外和紫外色余,并有CallH、K发射线,而且其空间分布主要集中在分子云密集区．CTTS具有尘埃包层或吸积盘,有些还伴有分子外向流、HZO脉泽等质能外流,在红外源的分类中山属11型红外源．由于CTTS有着很强的H。发射线,绝大多数＊＊＊s是利用民巡天观测发现的K‘」．而＊＊＊s则没有＊＊＊…     

Accretion,jets and winds: High‐energy emission from young stellar objects – Doctoral Thesis Award Lecture 2010

This article summarizes the processes of high‐energy emission in young stellar objects. Stars of spectral type A and B are called Herbig Ae/Be (HAeBe) stars in this stage, all later spectral types are termed classical T Tauri stars (CTTS). Both types are studied by high‐resolution X‐ray and UV spectroscopy and modeling. Three mechanisms contribute to the highenergy emission from CTTS: 1) CTTS have active coronae similar to main‐sequence stars, 2) the accreted material passes through an accretion shock at the stellar surface, which heats it to a few MK, and 3) some CTTS drive powerful outflows. Shocks within these jets can heat the plasma to X‐ray emitting temperatures. Coronae are already well characterized in the literature; for the latter two scenarios models are shown. The magnetic field suppresses motion perpendicular to the field lines in the accretion shock, thus justifying a 1D geometry. The radiative loss is calculated as optically thin emission. A mixture of shocked and coronal gas is fitted to X‐ray observations of accreting CTTS. Specifically, the model explains the peculiar line‐ratios in the He‐like triplets of Ne IX and O VII. All stars require only small mass accretion rates to power the X‐ray emission. In contrast, the HAeBe HD 163296 has line ratios similar to coronal sources, indicating that neither a high density nor a strong UV‐field is present in the region of the X‐ray emission. This could be caused by a shock in its jet. Similar emission is found in the deeply absorbed CTTS DG Tau. Shock velocities between 400 and 500 km s^–1 are required to explain the observed spectrum (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Detection of accreting circumstellar gas around weak emission-line Herbig Ae/Be stars

We present archival and recent IUE high dispersion spectra of late B stars which reveal the presence of accreting gas with velocities as high as 350 km s^–1, collisional ionization of the accreting gas to temperatures above the stellar T_eff, and column densities intermediate between those observed toward classical Herbig Ae/Be stars and the nearby proto-planetary system Pictoris. One of the stars, HD 176386, while lacking obvious optical signatures of youth, is a member of the R CrA star formation region, and with an inferred age of 2.8 Myr has not yet arrived on the zero-age main sequence (ZAMS). The other object, an isolated, field B star with pronounced IR excess due to warm, circumstellar dust, 51 Oph, exhibits only modest h emission. The combination of high velocity, accreting gas in systems with IR excesses due to circumstellar dust suggests that not only are these objects candidate proto-planetary systems, but that they may represent an extension to higher stellar masses of the weak-emission pre-main sequence (PMS) stars.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Doing physics in star-forming regions with XMM-Newton

Star-forming regions have been observed in X-rays since the first generation of satellites in the late 70s. They are very rich in magnetically-controlled X-ray phenomena: stellar flares and star-disk interactions in hundreds of T Tauri stars, confined winds in massive stars, etc. More recently, in a few low-mass stars, X-ray evidence has been found for accretion shocks. Even if it is not dominant, when it is found the influence of the circumstellar environment on X-ray emission gives precious clues on the magnetic structure in the vicinity of young stars. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

10-µm images and spectra of T Tauri stars

T Tauri stars are young stars usually surrounded by dusty disks similar to the one from which we believe our own Solar System formed. Most T Tauri stars exhibit a broad emission or absorption band between 7.5 and 13.5µm which is attributed to silicate grains in the circumstellar environment. We imaged three spatially resolved T Tauri binaries through a set of broadband filters which include the spectral region occupied by the silicate band. Two of these objects (T Tauri and Haro 6–10) are infrared companion systems in which one component is optically much fainter but contributes strongly in the infrared. Both infrared companions exhibit a deep silicate absorption which is not present in their primaries, indicating that they suffer very strong local extinction which may be due to an edge-on circumstellar disk or to a dense shell. We also took low resolution spectra of the silicate feature of two unresolved T Tauris to look for narrow features in the silicate band which would indicate the presence of specific minerals such as olivine. We observed GK Tau, for which Cohen and Witteborn (1985) reported a narrow emission feature at 9.7µm, but do not find evidence for this feature, and conclude that it is either time-dependent or an artifact of absorption by telluric ozone.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

The galactic evolution of lithium

Measurements of lithium in stars of different galactic populations such as young open clusters ( Per, Pleiades, Praesepe, Coma, Hyades), very young stellar associations (Taurus-Auriga, Chamaeleon, Ophiuchus clouds), intermediate and old open clusters (NGC 752, M 67, NGC 188), old disc stars and halo stars give us the observational framework from which the galactic evolution of lithium has to be inferred. This element is produced mainly via three mechanisms: primordial nucleosynthesis, spallation reactions in the interstellar medium and thermonuclear reactions in some particular stellar evolutionary stages (novae, red giants). The complicated nucleosynthesis and the fact that astration of lithium in stars is not well understood, makes a direct interpretation of the lithium evolutionary abundance curve difficult. The constraints set by recent lithium measurements in very old open clusters and metal-deficient stars on galactic lithium production mechanisms are discussed. Current problems in the determination of the primordial lithium abundance are briefly reviewed.     

The ney-allen nebula: an unambiguous example of circumstellar dust emission

Diffraction-limited array images of the Trapezium/Ney Allen infrared nebula have been obtained at six wavelengths between 7.8 and 12.4 microns, including the 9.7 micron silicate feature. Extended emission from warm dust shows significant differences in structure around each of the four Trapezium stars. The most dramatic infrared source is associated with ¹ Orionis D, where the bright mid-infrared emission is found to be a distinct crescent-shaped ridge or shell, concentric with the O star. This unambiguous relationship between a known type stellar luminosity source and a distinct circumstellar dust cloud of known distance and dimensions provides a unique opportunity to test the predictions of dust grain emission models for circumstellar infrared sources.     

Circumstellar clouds derived from ultraviolet stellar spectra

It is shown that the observed variety in macrostructures of continuous spectra in the ultraviolet (2000–3000 Å) of hot stars is a result of the presence of circumstellar clouds around such stars. A method for calculations of synthetic spectra, originating as a result of passage of central star photospheric radiation through its own circumstellar cloud, is developed. It introduces a new idea of spectral class for circumstellar cloud, and a recommended method for its determination depending from the spectral class of central star and cloud's parameters (Figure 2). The results of calculations of synthetic spectra for the four combinations of system star+cloud are presented (Figures 7-10). The strongest influence of circumstellar cloud in ultraviolet is discovered on A-class stars (Figure 13). Graphic relations are introduced for determination of cloud power by observed parameters of synthetic spectra (Figures 14 and 15).It establishes an important fact for an understanding of the nature of circumstellar clouds and processes occurring in them, according to which the selective absorption in such clouds stimulatesresonance lines only, the largest number of which lies in the ultraviolet in the region of 2100–2600 Å (Figure 1). An absence of visible signs of the effect of circumstellar clouds on continuous spectra of stars in visual region can be explained by a very small number of resonance lines in this region.Lastly, the possibility of determination of physical and geometric parameters of circumstellar clouds from stellar continuous spectra in the ultraviolet is analysed.     

X-ray stars observed in LAMOST spectral survey

X-ray stars have been studied since the beginning of X-ray astronomy. Investigating and studying the chromospheric activity from X-ray stellar optical spectra is highly significant in providing insights into stellar magnetic activity. The big data of LAMOST survey provides an opportunity for researching stellar optical spectroscopic properties of X-ray stars. We inferred the physical properties of X-ray stellar sources from the analysis of LAMOST spectra. First, we cross-matched the X-ray stellar catalogue (12254 X-ray stars) from ARXA with LAMOST data release 3 (DR3), and obtained 984 good spectra from 713 X-ray sources. We then visually inspected and assigned spectral type to each spectrum and calculated the equivalent width (EW) of H\(\alpha\) line using the Hammer spectral typing facility. Based on the EW of H\(\alpha\) line, we found 203 spectra of 145 X-ray sources with H\(\alpha\) emission above the continuum. For these spectra we also measured the EWs of H\(\beta\), H\(\gamma\), H\(\delta\) and Ca ii IRT lines of these spectra. After removing novae, planetary nebulae and OB-type stars, we found there are 127 X-ray late-type stars with H\(\alpha\) line emission. By using our spectra and results from the literature, we found 53 X-ray stars showing H\(\alpha\) variability; these objects are Classical T Tauri stars (CTTs), cataclysmic variables (CVs) or chromospheric activity stars. We also found 18 X-ray stars showing obvious emissions in the Ca ii IRT lines. Of the 18 X-ray stars, 16 are CTTs and 2 are CVs. Finally, we discussed the relationships between the EW of H\(\alpha\) line and X-ray flux.     

Searching for a gas cloud surrounding the WASP-18 planetary system

Near-UV (NUV) Hubble Space Telescope (HST) observations of the extreme hot-Jupiter WASP-12b revealed the presence of diffuse exospheric gas extending beyond the planet’s Roche lobe. Furthermore the NUV observations showed a complete lack of the normally bright core emission of the Mg?ii?h&k resonance lines, in agreement with the measured anomalously low stellar activity index (logR′ _HK ). Comparisons with other distant and inactive stars, and the analysis of radio and optical measurements of the intervening interstellar medium (ISM), led us to the conclusion that the system is surrounded by a circumstellar gas cloud, likely formed of material lost by the planet. Similar anomalous logR′ _HK index deficiencies might therefore signal the presence of translucent circumstellar gas around other stars hosting evaporating planets; we identified five such systems and WASP-18 is one of them. Both radio and optical observations of the region surrounding WASP-18 point towards a negligible ISM absorption along the WASP-18 line of sight. Excluding the unlikely possibility of an intrinsic anomalously low stellar activity, we conclude that the system is probably surrounded by a circumstellar gas cloud, presumably formed of material lost by the planet. Nevertheless only a far-UV spectrum of the star would provide a definite answer. Theoretical modelling suggests WASP-18b undergoes negligible mass loss, in contrast to the probable presence of a circumstellar gas cloud formed of material lost by the planet. The solution might be the presence either of an extra energy source driving mass loss (e.g., the reconnection of the stellar and planetary magnetic fields inside the planet atmosphere) or of an evaporating third body (e.g., moon).     

Observations of Magnetic Fields on T Tauri Stars

We present measurements of magnetic field strength and geometry on the surfaces of T Tauri stars (TTS) with and without circumstellar disks. We use these measurements to argue that magnetospheric accretion models should not assume that a fixed fraction of the stellar surface contains magnetic field lines that couple with the disk. We predict the fractional area of accretion footpoints, using magnetospheric accretion models and assuming field strength is roughly constant for all TTS. Analysis of Zeeman broadened infrared line profiles shows that individual TTS each have a distribution of surface magnetic field strengths extending up to 6 kG. Averaging over this distribution yields mean magnetic field strengths of 1-3 kG for all TTS, regardless of whether the star is surrounded by a disk. These strong magnetic fields suggest that magnetic pressure dominates gas pressure in TTS photospheres, indicating the need for new model atmospheres. The He I 5876 Å emission line in TTS can be strongly polarized, so that magnetic field lines at the footpoints of accretion have uniform polarity. The circular polarization signal appears to be rotationally modulated, implying that accretion and perhaps the magnetosphere are not axisymmetric. Time series spectropolarimetry is fitted reasonably well by a simple model with one magnetic spot on the surface of a rotating star. On the other hand, spectropolarimetry of photospheric absorption lines rules out a global dipolar field at the stellar surface for at least some TTS.     

Stellar Wind as a Stimulator of Accretion Activity in Young Binary Systems

A young binary system is considered, having a mass ratio of components M ₂/M ₁ 1, in which the low-velocity part of the stellar wind of the low-mass component (the so-called disk wind) can be partially captured by the gravitation of the primary component. It is shown that a large-scale redistribution of matter and angular momentum between the inner and outer parts of the gas-dust disk surrounding the binary system occurs as a result, with a consequent increase in the rate of accretion onto the primary component. In cases in which the orbital eccentricity of the secondary component is nonzero, modulation of the rate of accretion onto the primary component should be observed with a period equal to the orbital period, while in the case of a highly elongated orbit the mass accretion acquires a pulsed character. Since dust may be present in the disk wind from the secondary component, the capture of stellar wind will result in an increase in the effective geometrical thickness of the gas-dust disk. For this reason, the infrared (IR) emission excesses of such stars (especially in the near-IR range) and their intrinsic polarization can be considerably greater than in the case of a single star surrounded by a circumstellar disk of the same mass, and a periodic component may also be present in their behavior with time. Moreover, because of disruption of the axial symmetry in the dust distribution in the vicinity of the young binary system, the orbital period may also be present in its brightness variations. The role of these effects in the physics of young stars is discussed.