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تسجيل مستخدم جديدTW Hya Association Membership and New WISE-detected Circumstellar Disks
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We assess the current membership of the nearby, young TW Hydrae Association and examine newly proposed members with the Wide-field Infrared Survey Explorer (WISE) to search for infrared excess indicative of circumstellar disks. Newly proposed members TWA 30A, TWA 30B, TWA 31, and TWA 32 all show excess emission at 12 and 22 mum providing clear evidence for substantial dusty circumstellar disks around these low-mass, ~8 Myr old stars that were previously shown to likely be accreting from circumstellar material. TWA 30B shows large amounts of self-extinction, likely due to an edge-on disk geometry. We also confirm previously reported circumstellar disks with WISE, and determine a 22 mum excess fraction of 42+/- 9% based on our results.
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We present new photometric and spectroscopic data for the M-type members of the TW Hya association with the aim of a comprehensive study of accretion, disks and magnetic activity at the critical age of ~10 Myr where circumstellar matter disappears.
I report the results of a survey for low-mass (0.030 <~ M <~ 0.013 M_solar) brown dwarfs in the direction of the TW Hya association using 2MASS. Two late-M dwarfs show signs of low surface gravity and are strong candidates to be young, very-low-mass
(M ~ 0.025 M_solar) brown dwarfs related to the TW Hya association. 2MASSW J1207334-393254 is particularly notable for its strong H alpha emission. The number of detected brown dwarfs is consistent with the substellar mass function in richer star formation environments. Newly identified late-M and L dwarfs in the field are also discussed. Unusual objects include an L dwarf with strong H alpha emission, a possible wide M8/M9 triple system, and a possible L dwarf companion to an LHS star.
We present broad-band mid-resolution X-Shooter/VLT spectra for four brown dwarfs of the TW Hya association. Our targets comprise substellar analogs representing the different evolutionary phases in young stellar evolution: For the two diskless brown
dwarfs, TWA-26 and TWA-29, we determine the stellar parameters and we study their chromospheric emission line spectrum. For the two accreting brown dwarfs, TWA-27 and TWA-28, we estimate the mass accretion rates from empirical correlations between emission line luminosities and the accretion luminosity.
To investigate the potential connection between the intense X-ray emission from young, low-mass stars and the lifetimes of their circumstellar, planet-forming disks, we have compiled the X-ray luminosities ($L_X$) of M stars in the $sim$8 Myr-old TW
Hya Association (TWA) for which X-ray data are presently available. Our investigation includes analysis of archival Chandra data for the TWA binary systems TWA 8, 9, and 13. Although our study suffers from poor statistics for stars later than M3, we find a trend of decreasing $L_X/L_{bol}$ with decreasing $T_{eff}$ for TWA M stars wherein the earliest-type (M0--M2) stars cluster near $log{(L_X/L_{bol})} approx -3.0$ and then $log{(L_X/L_{bol})}$ decreases, and its distribution broadens, for types M4 and later. The fraction of TWA stars that display evidence for residual primordial disk material also sharply increases in this same (mid-M) spectral type regime. This apparent anticorrelation between the relative X-ray luminosities of low-mass TWA stars and the longevities of their circumstellar disks suggests that primordial disks orbiting early-type M stars in the TWA have dispersed rapidly as a consequence of their persistent large X-ray fluxes. Conversely, the disks orbiting the very lowest-mass pre-MS stars and pre-MS brown dwarfs in the Association may have survived because their X-ray luminosities and, hence, disk photoevaporation rates are very low to begin with, and then further decline relatively early in their pre-MS evolution.
Context. This paper discusses the sensitivity of water lines to chemical processes and radiative transfer for the protoplanetary disk around TW Hya. The study focuses on the Herschel spectral range in the context of new line detections with the PACS
instrument from the Gas in Protoplanetary Systems project (GASPS). Aims. The paper presents an overview of the chemistry in the main water reservoirs in the disk around TW Hya. It discusses the limitations in the interpretation of observed water line fluxes. Methods. ... (abbreviated) Results. We report new line detections of p-H2O (3_22-2_11) at 89.99 micron and CO J=18-17 at 144.78 micron for the disk around TW Hya. Disk modeling shows that the far-IR fine structure lines ([OI], [CII]) and molecular submm lines are very robust to uncertainties in the chemistry, while the water line fluxes can change by factors of a few. The water lines are optically thick, sub-thermally excited and can couple to the background continuum radiation field. The low-excitation water lines are also sensitive to uncertainties in the collision rates, e.g. with neutral hydrogen. The gas temperature plays an important role for the [OI] fine structure line fluxes, the water line fluxes originating from the inner disk as well as the high excitation CO, CH+ and OH lines. Conclusions. Due to their sensitivity on chemical input data and radiative transfer, water lines have to be used cautiously for understanding details of the disk structure. Water lines covering a wide range of excitation energies provide access to the various gas phase water reservoirs (inside and outside the snow line) in protoplanetary disks and thus provide important information on where gas-phase water is potentially located. Experimental and/or theoretical collision rates for H2O with atomic hydrogen are needed to diminish uncertainties from water line radiative transfer.
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