Very Low Mass Stars and Brown Dwarfs in Taurus‐Auriga (original) (raw)
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The Mass Accretion Rates of Intermediate-Mass T Tauri Stars
Astronomical Journal, 2004
We present Hubble Space Telescope ultraviolet spectra and supporting ground-based data for a sample of nine intermediate-mass T Tauri stars (IMTTSs; 1.5-4 M ). The targets belong to three star-forming regions: T Tau, SU Aur, and RY Tau in the Taurus clouds; EZ Ori, P2441, and V1044 Ori in the Ori OB1c association surrounding the Orion Nebula cluster; and CO Ori, GW Ori, and GX Ori in the ring around k Ori. The supporting groundbased observations include nearly simultaneous UBV(R I ) C photometry, 6 8 resolution spectra covering the range 3900-7000 8, optical echelle observations in the range 5800-8600 8, and K-band near-infrared spectra. We use these data to determine improved spectral types and reddening corrections and to obtain physical parameters of the targets. We find that an extinction law with a weak 2175 8 feature but high values of A UV =A V is required to explain the simultaneous optical-UV data; the reddening laws for two B-type stars located behind the Taurus clouds, HD 29647 and HD 283809, meet these properties. We argue that reddening laws with these characteristics may well be representative of cold, dense molecular clouds. Spectral energy distributions and emission-line profiles of the IMTTSs are consistent with expectations from magnetospheric accretion models. We compare our simultaneous optical-UV data with predictions from accretion shock models to get accretion luminosities and mass accretion rates (Ṁ ) for the targets. We find that the average mass accretion rate for IMTTSs is 3;10Aˋ8MyrAˋ1,afactorof3 ; 10 À8 M yr À1 , a factor of 3;10Aˋ8MyrAˋ1,afactorof5 higher than that for their low-mass counterparts. The new data extend the correlation betweenṀ and stellar mass to the intermediate-mass range. Since the IMTTSs are evolutionary descendants of the Herbig Ae/ Be stars, our results put limits to the mass accretion rates of their disks. We present luminosities of the UV lines of highly ionized metals and show that they are well above the saturation limit for magnetically active cool stars but correlate strongly with accretion luminosity, indicating that they are powered by accretion, in agreement with previous claims but using a sample in which reddening and accretion luminosities have been determined self-consistently. Finally, we find that the relation between accretion luminosity and Br luminosity found for low-mass T Tauri stars extends to the intermediate-mass regime.
New Low-Mass Members of the Taurus Star-forming Region
Astrophysical Journal, 2003
Briceno et al. recently used optical imaging, data from the Two-Micron All-Sky Survey (2MASS), and follow-up spectroscopy to search for young low-mass stars and brown dwarfs in 8 square degrees of the Taurus star-forming region. By the end of that study, there remained candidate members of Taurus that lacked the spectroscopic observations needed to measure spectral types and determine membership. In this work, we have obtained spectroscopy of the 22 candidates that have A_V<=8, from which we find six new Taurus members with spectral types of M2.75 through M9. The new M9 source has the second latest spectral type of the known members of Taurus (~0.02 M_sun). Its spectrum contains extremely strong emission in H_alpha (W~950 A) as well as emission in He I 6678 A and the Ca II IR triplet. This is the least massive object known to exhibit emission in He I and Ca II, which together with the strong H_alpha are suggestive of intense accretion.
The Astrophysical Journal, 2005
Using the largest high-resolution spectroscopic sample to date of young, very low mass stars and brown dwarfs, we investigate disk accretion in objects ranging from just above the hydrogen-burning limit all the way to nearly planetary masses. Our 82 targets span spectral types from M5 to M9.5, or masses from 0.15 M ⊙ down to about 15 Jupiters. They are confirmed members of the ρ Ophiuchus, Taurus, Chamaeleon I, IC 348, R Coronae Australis, Upper Scorpius and TW Hydrae star-forming regions and young clusters, with ages from 1 to ∼10 Myr. The sample contains 41 brown dwarfs (spectral types ≥M6.5). We have previously presented high-resolution optical spectra for roughly half the sample; the rest are new. This is a close to complete survey of all confirmed brown dwarfs known so far in the regions examined, except in ρ Oph and IC 348 (where we are limited by a combination of extinction and distance). We find that: (1) classical T Tauri-like disk-accretion persists in the sub-stellar domain down to nearly the deuterium-burning limit; (2) while an Hα 10% width 200 km s −1 is our prime accretion diagnostic (following our previous work), permitted emission lines of Ca II , O I and He I are also good accretion indicators, just as in CTTs (we caution against a blind use of Hα width alone, since inclination and rotation effects on the line are especially important at the low accretion rates in these objects);
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.
Evidence for a T Tauri Phase in Young Brown Dwarfs
The Astrophysical Journal, 2003
As part of a multi-faceted program to investigate the origin and early evolution of sub-stellar objects, we present high-resolution Keck optical spectra of 14 very low mass sources in the IC 348 young cluster and the Taurus star-forming cloud. All of our targets, which span a range of spectral types from M5 to M8, exhibit moderate to very strong Hα emission. In half of the IC 348 objects, the Hα profiles are broad and asymmetric, indicative of on-going accretion. Of these, IC348-355 (M8) is the lowest mass object to date to show accretion-like Hα. Three of our ∼ M6 IC 348 targets with broad Hα also harbor broad OI (8446Å) and CaII (8662Å) emission, and one shows broad HeI (6678Å) emission; these features are usually seen in strongly accreting classical T Tauri stars. We find that in very low mass accretors, the Hα profile may be somewhat narrower than that in higher mass stars. We propose that low accretion rates combined with small infall velocities at very low masses can conspire to produce this effect. In the non-accretors in our sample, Hα emission is commensurate with, or higher than, saturated levels in field M dwarfs of similar spectral type. Our results constitute the most compelling evidence to date that young brown dwarfs undergo a T Tauri-like accretion phase similar to that in stars. This is consistent with a common origin for most low-mass stars, brown dwarfs and isolated planetary mass objects.
Evidence for high accretion rates in weak-line T Tauri stars?
Monthly Notices of the Royal Astronomical Society, 2004
We have discovered T Tauri stars which show startling spectral variability between observations separated by 20 yr. In spectra published approximately 20 years ago these objects showed very weak Hα emission, broad Ca II absorption and so-called 'composite spectra', where the spectral type inferred from the blue region is earlier than that inferred from the red. We present here new spectroscopy which shows that all four stars now exhibit strong Hα emission, narrow Ca II emission and a spectral type which is consistent at all wavelengths.
The Initial Mass Function in the Taurus Star‐forming Region
Astrophysical Journal, 2002
By combining a deep optical imaging (I; z 0 ) survey of 8 deg 2 in the Taurus star-forming region with data from the Two-Micron All-Sky Survey (2MASS) and follow-up spectroscopy, we have performed a search for low-mass Taurus members that is complete to 0.02 M for reddenings of A V d4. We report the discovery of nine new members with spectral types of M5.75-M9.5, corresponding to masses of 0.1-0.015 M by recent evolutionary models. The new M9.5 member is the least massive brown dwarf found to date in the Taurus star-forming region. We derive an initial mass function (IMF) for the fields surveyed in this work and in our previous studies, which encompass 54% of the known Taurus membership. We compare the Taurus IMF with a similarly derived one for the Trapezium Cluster and to mass functions for the M35 and Pleiades open clusters. While the IMFs in all of these regions flatten near 0.8M,themassfunctioninTaurusismorenarrowandsharplypeakedatthismass.OursurveyindicatesthatTaurushas0.8 M , the mass function in Taurus is more narrow and sharply peaked at this mass. Our survey indicates that Taurus has 0.8M,themassfunctioninTaurusismorenarrowandsharplypeakedatthismass.OursurveyindicatesthatTaurushas2 Â fewer brown dwarfs at 0.02-0.08 M than the Trapezium. We discuss the implications of these results for theories of the IMF, and suggest that the lower frequency of brown dwarfs in Taurus relative to the Trapezium may result from the low-density star-forming environment, leading to larger minimum Jeans masses.
ACCRETION RATES FOR T TAURI STARS USING NEARLY SIMULTANEOUS ULTRAVIOLET AND OPTICAL SPECTRA
The Astrophysical Journal, 2013
We analyze the accretion properties of 21 low mass T Tauri stars using a dataset of contemporaneous near ultraviolet (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph (STIS) and the ground based Small and Medium Aperture Research Telescope System (SMARTS), a unique dataset because of the nearly simultaneous broad wavelength coverage. Our dataset includes accreting T Tauri stars (CTTS) in Taurus, Chamaeleon I, η Chamaeleon and the TW Hydra Association. For each source we calculate the accretion rate (Ṁ ) by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high density, low filling factor accretion spots co-exist with low density, high filling factor spots. By fitting the UV and optical spectra with multiple accretion components, we can explain excesses which have been observed in the near infrared. Comparing our estimates ofṀ to previous estimates, we find some discrepancies; however, they may be accounted for when considering assumptions for the amount of extinction and variability in optical spectra. Therefore, we confirm many previous estimates of the accretion rate. Finally, we measure emission line luminosities from the same spectra used for theṀ estimates, to produce correlations between accretion indicators (Hβ, Ca II K, C II] and Mg II) and accretion properties obtained simultaneously.
Astronomy & Astrophysics, 2007
Context. T Tau stars display different X-ray properties depending on whether they are accreting (classical T Tau stars; CTTS) or not (weak-line T Tau stars; WTTS). X-ray properties may provide insight into the accretion process between disk and stellar surface. Aims. We use data from the XMM-Newton Extended Survey of the Taurus molecular cloud (XEST) to study differences in X-ray properties between CTTS and WTTS. Methods. XEST data are used to perform correlation and regression analysis between X-ray parameters and stellar properties. Results. We confirm the existence of a X-ray luminosity (L X) vs. mass (M) relation, L X ∝ M 1.69 ± 0.11 , but this relation is a consequence of X-ray saturation and a mass vs. bolometric luminosity (L *) relation for the TTS with an average age of 2.4 Myr. X-ray saturation indicates L X = const.L * , although the constant is different for the two subsamples: const. = 10 −3.73 ± 0.05 for CTTS and const. = 10 −3.39 ± 0.06 for WTTS. Given a similar L * distribution of both samples, the X-ray luminosity function also reflects a real X-ray deficiency in CTTS, by a factor of ≈2 compared to WTTS. The average electron temperatures T av are correlated with L X in WTTS but not in CTTS; CTTS sources are on average hotter than WTTS sources. At best marginal dependencies are found between X-ray properties and mass accretion rates or age. Conclusions. The most fundamental properties are the two saturation laws, indicating suppressed L X for CTTS. We speculate that some of the accreting material in CTTS is cooling active regions to temperatures that may not significantly emit in the X-ray band, and if they do, high-resolution spectroscopy may be required to identify lines formed in such plasma, while CCD cameras do not detect these components. The similarity of the L X vs. T av dependencies in WTTS and main-sequence stars as well as their similar X-ray saturation laws suggests similar physical processes for the hot plasma, i.e., heating and radiation of a magnetic corona.
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.