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On the age of the TW Hydrae Association and 2M1207334-393254

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 نشر من قبل David Barrado y Navascues
 تاريخ النشر 2006
  مجال البحث فيزياء
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We have estimated the age of the young moving group TW Hydrae Association, a cohort of a few dozen stars and brown dwarfs located near the Sun which share the same kinematic properties and, presumably, the same origin and age. The chronology has been determined by analyzing different properties (magnitudes, colors, activity, lithium) of its members and comparing them with several well-known star forming regions and open clusters, as well as theoretical models. In addition, by using medium-resolution optical spectra of two M8 members of the association (2M1139 and 2M1207 -an accreting brown dwarf with a planetary mass companion), we have derived spectral types and measured H(alpha) and lithium equivalent widths. We have also estimated their effective temperature and gravity, which were used to produce an independent age estimation for these two brown dwarfs. We have also collected spectra of 2M1315, a candidate member with a L5 spectral type and measured its H(alpha) equivalent width. Our age estimate for the association, 10 Myr (in the range 3-20 Myr), agrees with previous values cited in the literature. In the case of the two brown dwarfs, we have derived an age of 15 Myr (in the range 5-30 Myr), which also agree with our estimate for the whole group. We compared our results with recent articles published on the same subject using other techniques, and discuss the limits of the age-dating techniques.



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92 - F. J. Low 2005
Spitzer Space Telescope infrared measurements are presented for 24 members of the TW Hydrae association (TWA). High signal-to-noise 24-micron (um) photometry is presented for all of these stars, including 20 stars that were not detected by IRAS. Amon g these 20 stars, only a single object, TWA 7, shows excess emission at 24um and at the level of only 40% above the stars photosphere. TWA 7 also exhibits a strong 70um excess that is a factor of 40 brighter than the stellar photosphere at this wavelength. At 70um, an excess of similar magnitude is detected for TWA 13, though no 24um excess was detected for this binary. For the 18 stars that failed to show measurable IR excesses, the sensitivity of the current 70um observations does not rule out substantial cool excesses at levels 10-40x above their stellar continua. Measurements of two T Tauri stars, TW Hya and Hen 6-300, confirm that their spectacular IR spectral energy distributions (SEDs) do not turn over even by 160um, consistent with the expectation for their active accretion disks. In contrast, the Spitzer data for the luminous planetary debris systems in the TWA, HD 98800B and HR 4796A, are consistent with single-temperature blackbody SEDs. The major new result of this study is the dramatic bimodal distribution found for the association in the form of excess emission at a wavelength of 24um, indicating negligible amounts of warm (>100 K) dust and debris around 20 of 24 stars in this group of very young stars. This bimodal distribution is especially striking given that the four stars in the association with strong IR excesses are >100x brighter at 24um than their photospheres.
Adaptive optics (AO) on 8-10 m telescopes is an enormously powerful tool for studying young nearby stars. It is especially useful for searching for companions. Using AO on the 10-m W.M. Keck II telescope we have measured the position of the brown dwa rf companion to TWA5 and resolved the primary into an 0.055 arcsecond double. Over the next several years follow-up astrometry should permit an accurate determination of the masses of these young stars. We have also re-observed the candidate extrasolar planet TWA6B, but measurements of its motion relative to TWA6A are inconclusive. We are carrying out a search for new planetary or brown dwarf companions to TWA stars and, if current giant planet models are correct, are currently capable of detecting a 1 Jupiter-mass companion at ~1 arcsecond and a 5 Jupiter-mass companion at ~0.5 arcsecon around a typical TWA member.
Context. Circumstellar discs are the places where planets form, therefore knowledge of their evolution is crucial for our understanding of planet formation. The Herschel Space Observatory is providing valuable data for studying disc systems, thanks t o its sensitivity and wavelength coverage. This paper is one of several devoted to analysing and modelling Herschel-PACS observations of various young stellar associations from the GASPS Open Time Key Programme. Aims. The aim of this paper is to elucidate the gas and dust properties of circumstellar discs in the 10 Myr TW Hya Association (TWA) using new far-infrared (IR) imaging and spectroscopy from Herschel-PACS. Methods. We obtained far-IR photometric data at 70, 100, and 160 microns of 14 TWA members; spectroscopic observations centred on the [OI] line at 63.18 microns were also obtained for 9 of the 14. The new photometry for each star was incorporated into its full spectral energy distribution (SED). Results. We detected excess IR emission that is characteristic of circumstellar discs from five TWA members, and computed upper limits for another nine. Two TWA members (TWA 01 and TWA 04B) also show [OI] emission at 63.18 microns. Discs in the TWA association display a variety of properties, with a wide range of dust masses and inner radii, based on modified blackbody modelling. Both transitional and debris discs are found in the sample. Models for sources with a detected IR excess give dust masses in the range from 0.15 Msun to 63 Msun.
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392 - Dagny L. Looper 2007
We report the discovery of a fifth candidate substellar system in the ~5-10 Myr TW Hydrae Association - DENIS J124514.1-442907. This object has a NIR spectrum remarkably similar to that of 2MASS J1139511-315921, a known TW Hydrae brown dwarf, with lo w surface gravity features such as a triangular-shaped H-band, deep H2O absorption, weak alkali lines, and weak hydride bands. We find an optical spectral type of M9.5 and estimate a mass of <24 M_Jup, assuming an age of ~5-10 Myr. While the measured proper motion for DENIS J124514.1-442907 is inconclusive as a test for membership, its position in the sky is coincident with the TW Hydrae Association. A more accurate proper motion measurement, higher resolution spectroscopy for radial velocity, and a parallax measurement are needed to derive the true space motion and to confirm its membership.
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