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Post Info TOPIC: Wolf940B
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Wolf 940 B
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Title: Properties of the T8.5 Dwarf Wolf 940 B
Authors: S. K. Leggett (Gemini Observatory), D. Saumon (LANL), Ben Burningham (University Hertfordshire), Michael C. Cushing (JPL), M. S. Marley (NASA Ames), D. J. Pinfield (University Hertfordshire)

We present 7.5-14.2um low-resolution spectroscopy, obtained with the Spitzer Infrared Spectrograph, of the T8.5 dwarf Wolf 940 B, which is a companion to an M4 dwarf with a projected separation of 400 AU. We combine these data with previously published near-infrared spectroscopy and mid-infrared photometry, to produce the spectral energy distribution for the very low-temperature T dwarf. We use atmospheric models to derive the bolometric correction and obtain a luminosity of log L/Lsun = -6.01 ±0.05. Evolutionary models are used with the luminosity to constrain the values of effective temperature (T_eff) and surface gravity, and hence mass and age for the T dwarf. We further restrict the allowed range of T_eff and gravity using age constraints implied by the M dwarf primary, and refine the physical properties of the T dwarf by comparison of the observed and modelled spectroscopy and photometry. This comparison indicates that Wolf 940 B has a metallicity within 0.2 dex of solar, as more extreme values give poor fits to the data - lower metallicity produces a poor fit at lambda > 2um while higher metallicity produces a poor fit at lambda < 2um. This is consistent with the independently derived value of [m/H] = +0.24 ±0.09 for the primary star, using the Johnson & Apps (2008) M_K:V-K relationship. We find that the T dwarf atmosphere is undergoing vigorous mixing, with an eddy diffusion coefficient K_zz of 10^4 to 10^6 cm^2 s^-1. We derive an effective temperature of 585 K to 625 K, and surface gravity log g = 4.83 to 5.22 (cm s^-2), for an age range of 3 Gyr to 10 Gyr, as implied by the kinematic and H alpha properties of the M dwarf primary. The lower gravity corresponds to the lower temperature and younger age for the system, and the higher value to the higher temperature and older age. The mass of the T dwarf is 24 M_Jupiter to 45 M_Jupiter for the younger to older age limit.

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Blobrana


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RE: Wolf940B
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An international team, led by astronomers at the University of Hertfordshire in the UK, have discovered one of the coolest sub-stellar bodies ever found outside our own solar system, orbiting the red dwarf star Wolf 940, some 40 light years from Earth.

"Although it has a temperature of 300 degrees celcius, which is almost hot enough to melt lead, temperature is relative when you study this sort of thing, and this object is very cool by stellar standards. In fact, this is the first time we've been able to study an object as cool as this in such detail...the fact that it is orbiting a star makes it extra special" - Dr Ben Burningham, of the University of Hertfordshire.

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Blobrana


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Posts: 131433
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Title: The discovery of an M4+T8.5 binary system
Authors: Ben Burningham, D.J.Pinfield, S.K.Leggett, C.G.Tinney, M.C.Liu, D. Homeier, A.A. West, A. Day-Jones, N.Huelamo, T.J.Dupuy, Z.Zhang, D.N.Murray, N. Lodieu, D. Barrado y Navascues, S. Folkes, M.C.Galvez-Ortiz, H.R.A. Jones, P. W. Lucas, M. Morales Calderon, M. Tamura

We report the discovery of a T8.5 dwarf, which is a companion to the M4 dwarf Wolf 940. At a distance of 12.50 (+0.75,-0.67) pc, the angular separation of 32arcsec corresponds to a projected separation of 400 AU. The M4 primary displays no Halpha emission, and we apply the age-activity relations of West et al. to place a lower limit on the age of the system of 3.5 Gyr. Weak Halpha absorption suggests some residual activity and we estimate an upper age limit of 6 Gyr. We apply the relations of Bonfils et al for V-Ks and M_Ks to determine the metallicity, [Fe/H] = -0.06 ±0.20 for Wolf~940A, and by extension the T8.5 secondary, Wolf 940B. We have obtained JHK NIRI spectroscopy and JHKL' photometry of Wolf 940B, and use these data, in combination with theoretical extensions, to determine its bolometric flux, Fbol = 1.75 ±0.18 E-16 Wm^-2 and thus its luminosity log(L*/Lsun) = -6.07 ±0.04. Using the age constraints for the system, and evolutionary structural models of Baraffe et al. we determine Teff = 570 ±25K and log g = 4.75-5.00 for Wolf940B, based on its bolometric luminosity. This represents the first determination of these properties for a T8+ dwarf that does not rely on the fitting of T-dwarf spectral models. This object represents the first system containing a T8+ dwarf for which fiducial constraints on its properties are available, and we compare its spectra with those of the latest very cool BT-Settl models. This clearly demonstrates that the use of the (W_J,K/J) spectral ratios (used previously to constrain Teff and log g) would have over-estimated Teff by ~100K.

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