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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.
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.  相似文献   

3.
A sample of 70 magnetically active stars toward the Taurus-Auriga star-forming region has been investigated. The positions of the sample stars on the Rossby diagram have been analyzed. All stars are shown to be in the regime of a saturated dynamo, where the X-ray luminosity reaches its maximum and does not depend on the Rossby number. A correlation has been found between the lithium line equivalent width and the age of a solar-mass (from 0.7 to 1.2 M ⊙) pre-main-sequence star. The older the age, the smaller the Li line equivalent width. Analysis of the long-term photometric variability of these stars has shown that the photometric activity of the youngest stars is appreciably higher than that of the older objects from the sample. This result can be an indirect confirmation of the evolution of the magnetic field in premain-sequence stars from predominantly dipole and axisymmetric to multipole and nonaxisymmetric.  相似文献   

4.
Based on high- and medium-resolution spectra, we analyze the population of Be stars in eight young open star clusters. We have found a clear dependence of the relative content of early-type (B0-B3) Be stars on the cluster age. The relative concentration of Be stars of spectral types B0?CB3 gradually increases with cluster age, reaching its maximum value of 0.46 in clusters with ages of 12?C20 Myr. The almost complete absence of Be stars in older clusters can be easily explained by the fact that B stars leave the main sequence. The few emission objects in clusters with ages of 1?C7 Myr are most likely Herbig Be stars. Such a distribution of Be stars in clusters unequivocally points to the evolutionary status of the Be phenomenon. We also briefly consider the causes of this pattern.  相似文献   

5.
Emission-line stars in young open clusters are identified to study their properties, as a function of age, spectral type and evolutionary state. 207 open star clusters were observed using the slitless spectroscopy method and 157 emission stars were identified in 42 clusters. We have found 54 new emission-line stars in 24 open clusters, out of which 19 clusters are found to house emission stars for the first time. About 20 per cent clusters harbour emission stars. The fraction of clusters housing emission stars is maximum in both the 0–10 and 20–30 Myr age bin (∼40 per cent each). Most of the emission stars in our survey belong to Classical Be class (∼92 per cent) while a few are Herbig Be stars (∼6 per cent) and Herbig Ae stars (∼2 per cent). The youngest clusters to have Classical Be stars are IC 1590, NGC 637 and 1624 (all 4 Myr old) while NGC 6756 (125–150 Myr) is the oldest cluster to have Classical Be stars. The Classical Be stars are located all along the main sequence (MS) in the optical colour–magnitude diagrams (CMDs) of clusters of all ages, which indicates that the Be phenomenon is unlikely due to core contraction near the turn-off. The distribution of Classical Be stars as a function of spectral type shows peaks at B1–B2 and B6–B7 spectral types. The Be star fraction [N(Be)/N(B+Be)] is found to be less than 10 per cent for most of the clusters and NGC 2345 is found to have the largest fraction (∼26 per cent). Our results indicate there could be two mechanisms responsible for the Classical Be phenomenon. Some are born Classical Be stars (fast rotators), as indicated by their presence in clusters younger than 10 Myr. Some stars evolve to Classical Be stars, within the MS lifetime, as indicated by the enhancement in the fraction of clusters with Classical Be stars in the 20–30 Myr age bin.  相似文献   

6.
Based on archival Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for the young tidal dwarf galaxy Ho IX. We have determined that star formation in Ho IX began 110 Myr ago and ended 20 Myr ago. We have identified 20 young star clusters in the galaxy with ages from 25 to 100 Myr. For the main star-forming region of Ho IX along one direction, we have determined the change in the number density of stars with three ages: 30, 50, and 90 Myr. A relation has been found between the ages of stars and the spatial sizes of the subsystem formed by them. This relation can be explained by expansion of the stellar subsystems. Under this assumption, the expansion velocity is 9.8 km s?1 as the age changes from 50 to 90 Myr. Edge-on low-mass late-type galaxies have similar relations between the ages and sizes of their stellar subsystems.  相似文献   

7.
The masses and temperatures of young low-mass stars and brown dwarfs in star-forming regions are not yet well established because of uncertainties in the age of individual objects and the spectral type–temperature scale appropriate for objects with ages of only a few Myr. Using multi-object optical spectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium Cluster in Orion have been classified and 44 of these confirmed as bona fide cluster members. The spectral types obtained have been converted to effective temperatures using a temperature scale intermediate between those of dwarfs and giants, which is suitable for young pre-main-sequence objects. The objects have been placed on a Hertzsprung–Russell (HR) diagram overlaid with theoretical isochrones. The low-mass stars and the higher mass substellar objects are found to be clustered around the 1 Myr isochrone, while many of the lower mass substellar objects are located well above this isochrone. An average age of 1 Myr is found for the majority of the objects. Assuming coevality of the sources and an average age of 1 Myr, the masses of the objects have been estimated and range from  0.018 to 0.44 M⊙  . The spectra also allow an investigation of the surface gravity of the objects by measurement of the sodium doublet equivalent width. With one possible exception, all objects have low gravities, in line with young ages, and the Na indices for the Trapezium objects lie systematically below those of young stars and brown dwarfs in Chamaeleon, suggesting that the 820 nm Na index may provide a sensitive means of estimating ages in young clusters.  相似文献   

8.
We present the first results of a new technique to detect, locate, and characterize young dissolving star clusters. Using Hubble Space Telescope/Advanced Camera for Surveys archival images of the nearby galaxy IC2574, we performed stellar PSF photometry and selected the most massive stars as our first test sample. We used a group-finding algorithm on the selected massive stars to find cluster candidates. We then plot the color-magnitude diagrams for each group, and use stellar evolutionary models to estimate their age. So far, we found 79 groups with ages of up to about 100 Myr, displaying various sizes and densities.  相似文献   

9.
This paper reviews the current state of the problem of magnetism in massive Main Sequence stars. Chemically peculiar Bp stars with enhanced silicon lines and anomalous helium lines in their spectra are shown to be the most promising targets for the observational verification of various mechanisms of the formation and subsequent evolution of magnetic fields in CP stars. A catalog of magnetic Bp stars, containing 125 objects is prepared. Applying different criteria, we compiled a variety of magnetic star samples, which were then used to analyze magnetic fields in objects of different ages. The results of this analysis show that massive stars generally have stronger fields in all the samples studied, and thus confirm earlier results based on smaller star samples. No tight relation is observed and the parameters of individual objects show a very large scatter about the mean relation. The strongest and most complex fields are found in the youngest Bp stars with ages below 30 Myr. Magnetic Bp stars generally rotate slower than normal B-type stars, except for the hottest objects with enhanced helium lines, which have normal rotation velocities. No systematic differences are found between the angular rotation velocities of Bp stars with anomalous helium and silicon lines. We discuss various criteria, which can be used to observationally test the alternative mechanisms of formation and evolution of magnetic fields in CP stars and, in particular, to quantitatively compare not only the magnitudes, but also the topology of fields in objects of different ages.  相似文献   

10.
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   

11.
We present the results of our search for the stars kinematically related to the young star HD 166191 surrounded by a warm dust disk. Using data from the GAIA DR1, 2RXS, ALLWISE catalogues and confirming optical spectroscopy, we have identified five stars of age ~10 Myr with similar kinematics in a sky region of radius ~11° on the periphery of the Scorpius–Centaurus OB association. Simulations of their orbits into the past show that the HD 166191 group stars were not formed inside or on the nearest periphery of the Scorpius–Centaurus association and do not belong to the young kinematic groups known in its neighborhoods either. The age and motion of the HD 166191 group in the Galaxy are closest to those of the extended Corona Australis association (ext.CrA). Possible scenarios for the formation of these young associations are discussed.  相似文献   

12.
Intermediate resolution spectroscopy from the European Southern Observatory Very Large Telescope is analysed for 63 photometrically selected low-mass  (0.08–0.30 M⊙)  candidates of the open cluster NGC 2547. We have confirmed membership for most of these stars using radial velocities, and found that lithium remains undepleted for cluster stars with   I > 17.54 ± 0.14  and   Ks > 14.86 ± 0.12  . From these results, several pre–main-sequence evolutionary models give almost model independent ages of 34–36 Myr, with a precision of 10 per cent. These ages are only slightly larger than the ages of 25–35(±5) Myr obtained using the same models to fit isochrones to higher mass stars descending towards the zero-age main-sequence, both in empirically calibrated and theoretical colour–magnitude diagrams. This agreement between age determinations in different mass ranges is an excellent test of the current generation of low-mass pre–main-sequence stellar models and lends confidence to ages determined with either method between 30 and 120 Myr.  相似文献   

13.
The chemically peculiar (CP) stars of the upper main sequence are perfect tracers for several astrophysical processes. Their study especially in open clusters further helps to establish their evolutionary status. The latter is most important to understand the origin and evolution of the CP phenomenon, i.e. the connection between diffusion and a stellar magnetic field. There are two important topics, we cover with this paper. First of all, we investigate the reliability of the CCD Δa photometry for fainter objects in open clusters. The latter method is able to detect CP stars very efficiently, but still a spectroscopic verification is needed to verify the photometric candidates. On the other hand, already published spectral classifications on the basis of photographic plates and prism technology have tobe tested with modern instruments. Classification resolution spectroscopy is presented for thirty five bona‐fide CP candidates. Twenty six of them are located within the boundaries of fourteen open clusters, for which we also investigated their membership probabilities. Apart from five objects, they seem tobe members of the respective clusters. The objects were classified in the framework of a refined Morgan‐Keenan system with the extension of well established CP star spectra. We confirm the CP nature of all but one target. The results of Δa photometry and the spectral classifications are in excellent agreement. For the cluster members we find a continuous sequence of CP stars from 10 to 850 Myr, the whole range of investigated cluster ages (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
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   

15.
We have found that two members of the TW Hydrae association, TW Hydrae and Hen 3-600A, are still actively accreting, based on the ballistic infall signature of their broad Halpha emission profiles. We present the first quantitative analysis of accretion in these objects and conclude that the same accretion mechanisms which operate in the well-studied 1 Myr old T Tauri stars can and do occur in older (10 Myr) stars. We derive the first estimates of the disk mass accretion rate in TW Hya and Hen 3-600A, which are 1-2 orders of magnitude lower than the average rates in 1 Myr old objects. The decrease in accretion rates over 10 Myr, as well as the low fraction of TW Hya association objects still accreting, points to significant disk evolution, possibly linked to planet formation. Given the multiplicity of the Hen 3-600 system and the large UV excess of TW Hya, our results show that accretion disks can be surprisingly long lived in spite of the presence of companions and significant UV ionizing flux.  相似文献   

16.
Based on archival Hubble Space Telescope images, we have performed stellar photometry for eight edge-on spiral and irregular galaxies. We have identified stars with ages of 20, 50, 80, 160, and 500 Myr in the derived Hertzsprung-Russell diagrams and constructed their number density distributions perpendicularly to the plane of the galactic disk. We have determined the sizes of the stellar subsystems and constructed the size-age diagrams for the stars constituting these subsystems. The stellar subsystems have been found to expand in all of the investigated galaxies within the range of ages studied (from 20 to 500 Myr). The expansion velocity of the subsystems decreases as one recedes from the galactic plane. The subsystems with ages of 1.5 and 6 Gyr also exhibit an increase in their sizes with age. The sizes of these subsystems approach those of the thick disk consisting of red giants. Our results confirm the model of thick-disk formation in irregular and low-mass spiral galaxies through thin-disk expansion.  相似文献   

17.
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   

18.
We present optical spectra of the nuclei of seven luminous ( P 178 MHz≳1025 W Hz−1 Sr−1) nearby ( z <0.08) radio galaxies, which mostly correspond to the FR II class. In two cases, Hydra A and 3C 285, the Balmer and λ 4000-Å break indices constrain the spectral types and luminosity classes of the stars involved, revealing that the blue spectra are dominated by blue supergiant and/or giant stars. The ages derived for the last burst of star formation in Hydra A are between 7 and 40 Myr, and in 3C 285 about 10 Myr. The rest of the narrow-line radio galaxies (four) have a λ 4000-Å break and metallic indices consistent with those of elliptical galaxies. The only broad-line radio galaxy in our sample, 3C 382, has a strong featureless blue continuum and broad emission lines that dilute the underlying blue stellar spectra. We are able to detect the Ca  ii triplet in absorption in the seven objects, with good quality data for only four of them. The strengths of the absorptions are similar to those found in normal elliptical galaxies, but these values are consistent both with stellar populations of roughly similar ages (as derived from the Balmer absorption and break strengths) and with mixed young+old populations.  相似文献   

19.
The spectral type is a key parameter in calibrating the temperature which is required to estimate the mass of young stars and brown dwarfs. We describe an approach developed to classify low-mass stars and brown dwarfs in the Trapezium Cluster using red optical spectra, which can be applied to other star-forming regions. The classification uses two methods for greater accuracy: the use of narrow-band spectral indices which rely on the variation of the strength of molecular lines with spectral type and a comparison with other previously classified young, low-mass objects in the Chamaeleon I star-forming region. We have investigated and compared many different molecular indices and have identified a small number of indices which work well for classifying M-type objects in nebular regions. The indices are calibrated for young, pre-main-sequence objects whose spectra are affected by their lower surface gravities compared with those on the main sequence. Spectral types obtained are essentially independent of both reddening and nebular emission lines.
Confirmation of candidate young stars and brown dwarfs as bona fide cluster members may be accomplished with moderate resolution spectra in the optical region by an analysis of the strength of the gravity-sensitive Na doublet. It has been established that this feature is much weaker in these very young objects than in field dwarfs. A sodium spectral index is used to estimate the surface gravity and to demonstrate quantitatively the difference between young (1–2 Myr) objects, and dwarf and giant field stars.  相似文献   

20.
We have established that several prominent β Pic stars are associated to moving groups. Based on this fact, we have estimated their ages. Our estimates are 200±100 Myr for Castor (which contains Vega and Fomalhaut), the same age for β Pic, and 8±2 Myr for HR4796 A. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

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