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1.
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.  相似文献   

2.
We have compiled infrared photometric data from the literature of practically all T Tauri stars found up to date including 444 classical T Tauri stars (CTTSs), 1698 weak-line T Tauri stars (WTTSs) and 1258 not classified T Tauri stars (3400 in total) in addition to 196 post-T Tauri stars (PTTSs). From this data bank we extract the infrared characteristics of the different groups and discuss different origins of the infrared radiation. The observational data are taken from the AKARI, IRAS, WISE and 2MASS missions. We show that in the wavelength range 1–140 μm, all T Tauri stars have infrared excesses. CTTSs have more infrared excess than WTTSs, while PTTSs have little or no infrared excess. We found that in the 1–3 μm wavelength range the infrared emission of T Tauri stars is mainly due to thermal radiation from the photosphere and hot dust grains from circumstellar envelopes. In the 3–140 μm wavelength range the infrared emission of T Tauri stars is mainly due to radiation from dusty/gaseous disks surrounding the stars. In addition, we also make a comparison between T Tauri stars and Herbig AeBe stars (HAeBe). There are some differences between these two kinds of objects in that for HAeBe stars the infrared radiation as a rule originates in dusty/gaseous disks in the 1–3 μm wavelength range, while in the range 3–12 μm it is possibly due to PAH emission for about half of HAeBe stars. In other wavelength ranges both kinds of stars have similar infrared characteristics indicating emission from dusty/gaseous disks.  相似文献   

3.
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.  相似文献   

4.
Long-term BV RI photometric light curves of the pre-main sequence stars V977 Cep and V982 Cep during the period from 2000 October to 2016 August are presented. The stars are located in the vicinity of the reflection nebula NGC 7129. Our photometric data show that both stars exhibit strong photometric variability in all optical passbands, which is typical for Classical T Tauri stars. Using our observational data we analyze the reasons for the observed brightness variations. In the case of V977 Cep we identify previously unknown periodicity in its light curve.  相似文献   

5.
We monitored the light curves of 22 weak-line T Tauri stars (WTTSs) discovered among the X-ray sources in the field of the Taurus-Auriga cloud. For 12 of the 22 WTTSs photometric periodic variability is confirmed and their rational periods are determined using Phase Dispersion Minimization (PDM) and Fourier analysis. Most of them are found to have periods shorter than one day. This gives further evidence for the spin up of solar-type stars predicted by the models of angular momentum evolution of pre-main sequence stars.  相似文献   

6.
The IRAS and 2MASS associations for 193 T Tauri stars are identified in this paper. From the color–color diagrams and spectral index, it is found that the IR excesses for most samples are due to thermal emission from the circumstellar material, as suggested previously. It is also found that the IR excesses at IRAS region for few T Tauri stars and the near-IR excesses for some T Tauri stars are likely attributed to free-free emission or free-bound emission from the circumstellar ionized gas. Moreover, It is found in deredened J–H versus H–K color–color diagram that there is a slight separation in different spectral groups. The T Tauri stars locus equation in J–H versus H–K color–color diagram for our sample is also presented.  相似文献   

7.
The XMM‐Newton Extended Survey of the Taurus Molecular Cloud (XEST) is a survey of the nearest large star‐forming region, the Taurus Molecular Cloud (TMC), making use of all instruments on board the XMM‐Newton X‐ray observatory. The survey, presently still growing, has provided unprecedented spectroscopic results from nearly every observed T Tauri star, and from ≈50% of the studied brown dwarfs and protostars. The survey includes the first coherent statistical sample of high‐resolution spectra of T Tauri stars, and is accompanied by an U ‐band/ultraviolet imaging photometric survey of the TMC. XEST led to the discovery of new, systematic X‐ray features not possible before with smaller samples, in particular the X‐ray soft excess in classical T Tauri stars and the Two‐Absorber X‐ray (TAX) spectra of jet‐driving T Tauri stars. This paper summarizes highlights from XEST and reviews the key role of this large project. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
We present optical photometric and spectroscopic studies of ROSAT X-ray stellar sources in the Rosette Nebula star-forming region. The brightest X-ray sources are either massive stars or active T Tauri stars associated with the open cluster NGC 2244, or are foreground stars. Some of the spectra of the young stars newly identified in the region are presented.  相似文献   

9.
We present the results of a photometric study of X-ray-active weak-lined T Tauri (WTT) stars in the η Chamaeleontis star cluster. Multi-epoch V -band photometric monitoring during 1999 and 2000 of the 10 X-ray-active WTT stars found that all were variable in one or both years, with periods ascribed to rotational modulation of starspots. Comparison between the rotational and X-ray properties of these objects indicates the saturation level,     observed in other studies of X-ray-active pre-main-sequence stars, persists in the η Cha stars from the slow- to the fast-rotator regimes. Cousins VRI photometry of the WTT stars has enabled us to investigate further the photometric properties of these stars. The stars appear sufficiently coeval to distinguish near-equal-mass binaries within the sample. A new Hertzsprung–Russell diagram for these objects suggests ages of 4–9 Myr for M-type RECX primaries using the tracks of D'Antona & Mazzitelli.  相似文献   

10.
I discuss recent observational results on the X-ray properties of young stellar objects, based mostly on Chandra and XMM-Newton observations. The sensitive X-ray data on large, well characterized samples of T Tauri stars (and a number of protostars) allow to study in detail the dependence of magnetic activity on the bulk properties of the young objects and to draw important clues towards the origin of the X-ray emission. The absence of a relation between X-ray activity and rotation for T Tauri stars clearly suggests that their magnetic activity cannot be simply explained by the action of a scaled-up solar-like dynamo. I discuss alternative models for the generation of magnetic fields and also consider the long standing question whether the X-ray properties of the T Tauri stars are related to the presence/absence of circumstellar disks or active accretion.  相似文献   

11.
Do accretion discs regulate the rotation of young stars?   总被引：1，自引：0，他引：1  
We present a photometric study of I -band variability in the young cluster IC 348. The main purpose of the study was to identify periodic stars. In all, we find 50 periodic stars, of which 32 were previously unknown. For the first time in IC 348, we discover periods in significant numbers of lower-mass stars  ( M < 0.25 M⊙)  and classical T Tauri stars. This increased sensitivity to periodicities is a result of the enhanced depth and temporal density of our observations, compared with previous studies. The period distribution is at first glance similar to that seen in the Orion nebula cluster (ONC), with the higher-mass stars  ( M > 0.25 M⊙)  showing a bi-modal period distribution concentrated around periods of 2 and 8 d, and the lower-mass stars showing a uni-modal distribution, heavily biased towards fast rotators. Closer inspection of the period distribution shows that the higher-mass stars show a significant dearth of fast rotators, compared to the ONC, whilst the low-mass stars are rotating significantly faster than those in Orion. We find no correlation between rotation period and K – L colour or Hα equivalent width.
We also present a discussion of our own IC 348 data in the context of previously published period distributions for the ONC, the Orion flanking fields and NGC 2264. We find that the previously claimed correlation between infrared excess and rotation period in the ONC might, in fact, result from a correlation between infrared excess and mass. We also find a marked difference in period distributions between NGC 2264 and IC 348, which presents a serious challenge to the disc-locking paradigm, given the similarity in ages and disc fractions between the two clusters.  相似文献   

12.
We suggest a model based on the representation of the stellar magnetic field as a superposition of a finite number of poloidal and toroidal free decay modes to describe the dynamo action in fully convective stars. For the adopted law of stellar differential rotation, we determined the dynamo number in exceeding which the generation of a cyclically varying magnetic field is possible in stars without a radiative core and derived an expression for the period of the cycle. The dynamo cycles in fully convective stars and in stars with thin convective envelopes are shown to differ qualitatively: first, the distributions of spots in latitude during the cycle are different for these two types of stars and, second, the model predicts a great weakening of the spot formation in fully convective stars at certain phases of the cycle. To compare the theory with observations, we have analyzed the historical light curve for the weak-line T Tauri star V410 Tau and found that its long-term activity is not a well-defined cycle with a definite period—its activity is more likely quasi-cyclic with a characteristic time of ～4 yr and with a chaotic component superimposed. we have also concluded that a redistribution of spots in longitude is responsible for the secular brightness variations in the star. This does not allow the results of photometric observations to be directly compared with predictions of ourmodel, in which, for simplicity, we assumed a symmetry in longitude and investigated the temporal evolution of the spot distribution in latitude. Therefore, we discuss the questions of what and how observations can be compared with predictions of the dynamo theory.  相似文献   

13.
陈培生  王锋 《天文学报》1999,40(4):407-418
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  相似文献   

14.
We monitored 16 X-ray selected young solar-type stars for light variation and found appreciable periodic light variability with amplitudes of a few hundredths of a magni-tude in nine of the objects. Using the method of Phase Dispersion Minimization (PDM) and Fourier analysis (software PERIOD04), the rotation periods of these stars were determined from the photometric data. The rotation periods of all nine stars are shorter than about 3days. It is suggested that, as with the Pleiades cluster, small amplitude light variations are quite common among young solar-type stars with rotation periods around 3 days or less. This gives further evidence for the spin up of solar-type stars predicted by models of angular momentum evolution of pre-main sequence stars.  相似文献   

15.
We present the results of our photometric (V RI) and spectroscopic observations of the young variable star V730 Cep (MisV1147) classified by Uemura et al. (2004) as a Herbig Be star. Our photometry confirms the conclusion of the above authors that this star has a complex pattern of variability including periodic or quasi-periodic brightness variations with a period of about 14 days and deep Algol-like minima typical for UX Ori stars. Our spectroscopy shows that the classification of V730 Cep as a Herbig Be star is wrong. Actually, this star has a much lower temperature and belongs to the family of T Tauri stars. This allows us to explain the nature of the unusual photometric activity of V730 Cep based on a combination of two well-known models of variable circumstellar extinction applied to young stars: AA Tau- and UX Oritype variability. It follows from our observations that the color tracks on the V ?(V ?I) color–magnitude diagram for these models slightly differ: the AA Tau-type variability of circumstellar extinction is caused by larger grains than the UX Ori-type variability. Such a difference can be due to an increase in the characteristic sizes of circumstellar dust as the star is approached and has a simple explanation: small dust grains evaporate faster than large ones.  相似文献   

16.
We consider the problem of dust grain survival in the disk winds from T Tauri and Herbig Ae stars. For our analysis, we have chosen a disk wind model in which the gas component of the wind is heated through ambipolar diffusion to a temperature of ～104 K. We show that the heating of dust grains through their collisions with gas atoms is inefficient compared to their heating by stellar radiation and, hence, the grains survive even in the hot wind component. As a result, the disk wind can be opaque to the ultraviolet and optical stellar radiation and is capable of absorbing an appreciable fraction of it. Calculations show that the fraction of the wind-absorbed radiation for T Tauri stars can be from 20 to 40% of the total stellar luminosity at an accretion rate ? a = 10?8-10?6 M ⊙yr?1. This means that the disk winds from T Tauri stars can play the same role as the puffed-up inner rim in current accretion disk models. In Herbig Ae stars, the inner layers of the disk wind (r ≤ 0.5 AU) are dust-free, since the dust in this region sublimates under the effect of stellar radiation. Therefore, the fraction of the radiation absorbed by the disk wind in this case is considerably smaller and can be comparable to the effect from the puffed-up inner rim only at an accretion rate of the order of or higher than 10?6 M ⊙yr?1. Since the disk wind is structurally inhomogeneous, its optical depth toward the observer can be variable, which should be reflected in the photometric activity of young stars. For the same reason, moving shadows from gas and dust streams with a spiral-like shape can be observed in high-angular-resolution circumstellar disk images.  相似文献   

17.
We consider the problem of the distortion of the photospheric spectrum for a young star as its light is scattered in the inner accretion disk in the dust grain evaporation region. In T Tauri stars, this region is at a distance of the order of several stellar radii and is involved in the large-scale motions of matter with velocities of ～100 km s?1 or higher. The light scattering in such a medium causes the frequency of the scattered radiation to be shifted due to the Doppler effect. We analyze the influence of this effect on the absorption line profiles in the spectra of T Tauri stars using classical results of the theory of radiative transfer. We consider two models of a scattering medium: (i) a homogeneous cylindrical surface and (ii) a cylindrical surface with an azimuth-dependent height (such conditions take place during the accretion of matter onto a star with an oblique magnetic dipole). We show that in the first case, the scattering of the photospheric radiation causes the absorption lines to broaden. If the motion of the circumstellar matter in the dust evaporation region is characterized by two velocity components, then the line profile of the scattered radiation is asymmetric, with the pattern of the asymmetry depending on the direction of the radial velocity. In the second case, the scattered radiation can cause periodic shifts of the absorption line centroid, which can be perceived by an observer as periodic radial-velocity variations in the star. We suggest that precisely this effect is responsible for the low-amplitude radial-velocity variations with periods close to the stellar rotation periods that have recently been found in some of the T Tauri stars.  相似文献   

18.
《New Astronomy》2007,12(4):265-270
Surface lithium abundance and rotation velocity can serve as powerful and mutually complementary diagnostics of interior structure of stars. So far, the processes responsible for the lithium depletion during pre-main sequence evolution are still poorly understood. We investigate whether a correlation exists between equivalent widths of Li (EW(Li)) and rotation period (Prot) for weak-line T Tauri stars (WTTSs). We find that rapidly rotating stars have lower EW(Li) and the fast burning of Li begins at the phase when star’s Prot evolves towards 3 days among 0.9M⊙ to 1.4M⊙ WTTSs in Taurus–Auriga. Our results support the conclusion by Piau and Turch-Chiéze about a model for lithium depletion with age of the star and by Bouvier et al. in relation to rotation evolution. The turn over of the curve for the correlation between EW(Li) and Prot is at the phase of zero-age main sequence (ZAMS). The EW(Li) decreases with decreasing Prot before the star reaches the ZAMS, while it decreases with increasing Prot (decreasing rotation velocity) for young low-mass main sequence stars. This result could be explained as an age effect of Li depletion and the rapid rotation does not inhibit Li destruction among low-mass PMS stars.  相似文献   

19.
A detailed study was performed for a sample of low-mass pre-main-sequence (PMS) stars, previously identified as weak-line T Tauri stars, which are compared to members of the Tucanae and Horologium Associations. Aiming to verify if there is any pattern of abundances when comparing the young stars at different phases, we selected objects in the range from 1 to 100 Myr, which covers most of PMS evolution. High-resolution optical spectra were acquired at European Southern Observatory and Observatório do Pico dos Dias . The stellar fundamental parameters effective temperature and gravity were calculated by excitation and ionization equilibria of iron absorption lines. Chemical abundances were obtained via equivalent width calculations and spectral synthesis for 44 per cent of the sample, which shows metallicities within 0.5 dex solar. A classification was developed based on equivalent width of Li  i 6708 Å and Hα lines and spectral types of the studied stars. This classification allowed a separation of the sample into categories that correspond to different evolutive stages in the PMS. The position of these stars in the Hertzsprung–Russell diagram was also inspected in order to estimate their ages and masses. Among the studied objects, it was verified that our sample actually contains seven weak-line T Tauri stars, three are Classical T Tauri, 12 are Fe/Ge PMS stars and 21 are post-T Tauri or young main-sequence stars. An estimation of circumstellar luminosity was obtained using a disc model to reproduce the observed spectral energy distribution. Most of the stars show low levels of circumstellar emission, corresponding to less than 30 per cent of the total emission.  相似文献   

20.
《New Astronomy》2007,12(6):441-445
The discovery of optical jets immersed in the strong UV radiation field of the Rosette Nebula sheds new light on, but meanwhile poses challenges to, the study of externally irradiated jets. The jet systems in the Rosette are found to have a high state of ionization and show unique features. In this paper, we investigate the evolutionary status of the jet-driving sources for young solar-like stars. To our surprise, these jet sources indicate unexpected near infrared properties with no excess emission. They are bathed in harsh external UV radiation such that evaporation leads to a fast dissipation of their circumstellar material. This could represent a transient phase of evolution of young solar-like stars between classical and weak lined T Tauri stars. Naked T Tauri stars formed in this way have indistinguishable evolutionary ages from those of classical T Tauri stars resulting from the same episode of star formation. However, it would be hard for such sources to be identified if they are not driving an irradiated jet in a photoionized medium.  相似文献   

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