Post T Tauri stars in the solar neighborhood: isolated or members of young associations and moving groups (original) (raw)
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Identification of Isolated Post T Tauri Stars in the Solar Neighborhood
Astrophysics and Space Science Proceedings, 2010
Post T Tauri stars (PTTS) are late-type stars in the age range between 10 and 100 Myr filling the gap between T Tauri (TTS) and zero-age main sequence phases. This period of evolution remains ambiguous and until now different studies of young stars have failed to find the numbers of PTTS that are expected. In the last years, some PTTS have been identified among the X-ray detected pre-main sequence stars in some star-forming regions. More recently, additional PTTS have been identified in young associations and moving groups (β Pic, TW Hya, Tucana/Horologium, and the AB Dor). However, many isolated PTTS still remain undiscovered. In this contribution, we compiled the PTTS previously identified in the literature, and identified new candidates using the information provided by the high resolution spectra obtained during our surveys of late-type stars possible members to young moving groups, FGK stars in the solar neighborhood. To identify PTTS we applied an ageoriented definition using relative age indicators (Li abundance, chromospheric and coronal emission and the kinematics) as well as color-magnitude diagrams and premain sequence isochrones.
1997
We have used the Hipparcos parallaxes to study the evolutionary status of sample of stars with spectral types from late F to M0 (hereafter \solar-type stars"), selected on the basis of their activity, mainly from Einsteinbased surveys. The parallaxes have been used to place the objects in the H-R diagram, determining their age by comparison with theoretical evolutionary tracks and observational main sequences. This age is compared with age estimates derived from the lithium abundance, the activity level and the presence of circumstellar disks. To complement our sample at the young end we have also studied the Hipparcos-determined distances of a sample of opticallyselected pre-main sequence stars, mostly classical T-Tauri stars (CTTS). Some CTTS appear to be much nearer to us than previously determined, and far away from their putative parent cloud. This implies a signi cantly larger age providing observational evidence for the existence of long-lived T-Tauri disks which could produce the slow rotators on the Zero-Age Main Sequence (ZAMS). None of the above-mentioned age proxies appears to reliably and unambiguously select very young stars in the range of spectral types considered here, with some apparently very young objects e ectively lying onto or very close to the main sequence. The attribution of ages to young solar-type stars on the basis of any of the standard proxies may thus signi cantly under-or overestimate the evolutionary age of the object. Caution must therefore be exercised when attributing ages to individual stars, and claims about the large number of PMS stars found in Xray based surveys may need to be at least in part reconsidered in this light.
HD 98800: A Unique Stellar System of Post–T Tauri Stars
The Astrophysical Journal, 1998
HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have.
Astronomy and Astrophysics Supplement Series, 1997
We present results of intermediate resolution spectroscopy of 131 optical counterparts to 115 ROSAT All-Sky Survey X-ray sources south of the Taurus-Auriga dark cloud complex. These objects have been selected as candidate young stars from a total of 1084 ROSAT sources in a ∼ 300 square degree area. We identify 30 objects as low-mass PMS stars on the basis of the Li i λ6708Å doublet in their spectrum, a signature of their young age. All these stars have a spectral type later than F7 and show spectral characteristics typical of weak-line and post-T Tauri stars. The presence of young objects several parsecs away from the regions of ongoing star formation is discussed in the light of the current models of T Tauri dispersal.
High-resolution X-ray spectroscopy of T Tauri stars in the Taurus-Auriga complex
Astronomy & Astrophysics, 2007
Context. Differences have been reported between the X-ray emission of accreting and non-accreting stars. Some observations have suggested that accretion shocks could be responsible for part of the X-ray emission in classical T Tauri stars (CTTS). Aims. We present high-resolution X-ray spectroscopy for nine pre-main sequence stars in order to test the proposed spectroscopic differences between accreting and non-accreting pre-main sequence stars. Methods. We used X-ray spectroscopy from the XMM-Newton Reflection Grating Spectrometers and the EPIC instruments. We interpret the spectra using optically thin thermal models with variable abundances, together with an absorption column density. For BP Tau and AB Aur we derive electron densities from the O vii triplets. Results. Using the O vii/O viii count ratios as a diagnostic for cool plasma, we find that CTTS display a soft excess (with equivalent electron temperatures of ≈2.5−3 MK) when compared with WTTS or zero-age main-sequence stars. Although the O vii triplet in BP Tau is consistent with a high electron density (3.4 × 10 11 cm −3), we find low density for the accreting Herbig star AB Aur (n e < 10 10 cm −3). The element abundances of accreting and non-accreting stars are similar. The Ne abundance is found to be high (4−6 times the Fe abundance) in all K and M-type stars. In contrast, for the three G-type stars (SU Aur, HD 283572, and HP Tau/G2), we find an enhanced Fe abundance (0.4−0.8 times solar photospheric values) compared to later-type stars. Conclusions. Adding the results from our sample to former high-resolution studies of T Tauri stars, we find a soft excess in all accreting stars, but in none of the non-accretors. On the other hand, high electron density and high Ne/Fe abundance ratios do not seem to be present in all accreting pre-main sequence stars.
The Status of ROSAT X-ray Active Young Stars toward Taurus-Auriga
Chinese Journal of Astronomy and Astrophysics, 2004
We present an astrometric study of the candidates of T Tauri stars (TTS) and non-TTS X-ray sources around Tau-Aur, based on the Hipparcos Catalogue and the ACT Reference Catalogue. The ROSAT selected X-ray sources are found to be a mixed population. A few of them are associated with the Tau-Aur or Orion Star Forming Regions (SFR). Some, with distances similar to that of Tau-Aur but with discrepant proper motions, are probable or sure Pleiades super-cluster members or other late type young active stars with unresolved nature, more likely to originate in rapidly moving cloudlets, or else having originated from different sites other than Tau-Aur and moved to the present locations. A good many of the non-TTS X-ray sources are considered as Hyades cluster members. Some TTS candidates could be foreground pre-main sequence stars or actually young dwarfs not yet depleted of their Lithium. Under the hypothesis that the sources we 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 third of the weak-line TTS 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 result suggests that the vast majority of the young active X-ray sources within an angular diameter of about 30 • of the Tau-Aur SFR, belong to four main subgroups that are spatially separate.
Chromospheric activity of ROSAT discovered weak-lined T Tauri stars
1998
We have started a high resolution optical observation program dedicated to the study of chromospheric activity in weak-lined T Tauri stars (WTTS) recently discovered by the ROSAT All-Sky Survey (RASS). It is our purpose to quantify the phenomenology of the chromospheric activity of each star determining stellar surface fluxes in the more important chromospheric activity indicators (Ca II H & K, H_beta, H_alpha, Ca II IRT) as well as obtain the Li I abundance, a better determination of the stellar parameters, spectral type, and possible binarity. A large number of WTTS have been discovered by the RASS in and around differen star formation clouds.Whether these stars are really WTTS, or post-TTS, or even young main sequence stars is a matter of ongoing debate. However, we have centered our study only on objects for which very recent studies, of Li I abundance (greater than Pleiads of the same spectral type) or radio properties, clearly confirmed their pre-main sequence (PMS) nature. In...
High‐Resolution X‐Ray Spectroscopy of the Post–T Tauri Star PZ Telescopii
The Astrophysical Journal, 2004
We present an analysis of the Chandra High Energy Transmission Grating Spectrometer observation of the rapidly rotating (P rot = 0.94 d) post T Tauri (∼ 20 Myr old) star PZ Telescopii, in the Tucana association. Using two different methods we have derived the coronal emission measure distribution, em(T ), and chemical abundances. The em(T ) peaks at log T = 6.9 and exhibits a significant emission measure at temperatures log T > 7. The coronal abundances are generally ∼ 0.5 times the solar photospheric values that are presumed fairly representative of the composition of the underlying star. A minimum in abundance is seen at a first ionization potential (FIP) of 7-8 eV, with evidence for higher abundances at both lower and higher FIP, similar to patterns seen in other active stars. From an analysis of the He-like triplet of Mg XI we have estimated electron densities of ∼ 10 12 − 10 13 cm −3 . All the coronal properties found for PZ Tel are much more similar to those of AB Dor, which is slightly older than PZ Tel, than to those of the younger T Tauri star TW Hya. These results support earlier conclusions that the soft X-ray emission of TW Hya is likely dominated by accretion activity rather than by a magnetically-heated corona. Our results also suggest that the coronae of pre-main sequence stars rapidly become similar to those of older active main-sequence stars soon after the accretion stage has ended.
An Infrared/X-Ray Survey for New Members of the Taurus Star-Forming Region
The Astrophysical Journal, 2009
We present the results of a search for new members of the Taurus star-forming region using data from the Spitzer Space Telescope and the XMM-Newton Observatory. We have obtained optical and near-infrared spectra of 44 sources that exhibit red Spitzer colors that are indicative of stars with circumstellar disks and 51 candidate young stars that were identified by Scelsi and coworkers using XMM-Newton. We also performed spectroscopy on four possible companions to members of Taurus that were reported by Kraus and Hillenbrand. Through these spectra, we have demonstrated the youth and membership of 41 sources, 10 of which were independently confirmed as young stars by Scelsi and coworkers. Five of the new Taurus members are likely to be brown dwarfs based on their late spectral types (>M6). One of the brown dwarfs has a spectral type of L0, making it the first known L-type member of Taurus and the least massive known member of the region (M ∼ 4-7 M Jup). Another brown dwarf exhibits a flat infrared spectral energy distribution, which indicates that it could be in the protostellar class I stage (star+disk+envelope). Upon inspection of archival images from various observatories, we find that one of the new young stars has a large edge-on disk (r = 2. ′′ 5 = 350 AU). The scattered light from this disk has undergone significant variability on a time scale of days in optical images from the Canada-France-Hawaii Telescope. Using the updated census of Taurus, we have measured the initial mass function for the fields observed by XMM-Newton. The resulting mass function is similar to previous ones that we have reported for Taurus, showing a surplus of stars at spectral types of K7-M1 (0.6-0.8 M ⊙) relative to other nearby star-forming regions like IC 348, Chamaeleon I, and the Orion Nebula Cluster.