SETI bioastro: 3 papers on brown dwarfs

From: LARRY KLAES (ljk4_at_msn.com)
Date: Thu Jul 27 2006 - 12:47:26 PDT

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    Astrophysics, abstract
    astro-ph/0607537

    From: Maria Morales Calderon [view email]

    Date: Mon, 24 Jul 2006 13:00:32 GMT (141kb)

    A Sensitive Search for Variability in Late L Dwarfs: The Quest for Weather

    Authors: M. Morales-Calderon, J.R. Stauffer, J.Davy Kirkpatrick, S. Carey,
    C.R. Gelino, D. Barrado y Navascues, L. Rebull, P. Lowrance, M.S. Marley, D.
    Charbonneau, B.M. Patten, S.T. Megeath, D. Buzasi

    Comments: Accepted by ApJ. 16 pages, 6 figures

    We have conducted a photometric monitoring program of 3 field late-L brown
    dwarfs looking for evidence of non-axisymmetric structure or temporal
    variability in their photospheres. The observations were performed using
    Spitzer/IRAC 4.5 and 8 micron bandpasses and were designed to cover at least
    one rotational period of each object. One-sigma RMS (root mean squared)
    uncertainties of less than 3 mmag at 4.5 micron and around 9 mmag at 8
    micron were achieved. Two out of the three objects studied exhibit some
    modulation in their light curves at 4.5 micron - but not 8 micron - with
    periods of 7.4 hr and 4.6 hr and peak-to-peak amplitudes of 10 mmag and 8
    mmag. Although the lack of detectable 8 micron variation suggests an
    instrumental origin for the detected variations, the data may nevertheless
    still be consistent with intrinsic variability since the shorter wavelength
    IRAC bandpasses probe more deeply into late L dwarf atmospheres than the
    longer wavelengths. A cloud feature occupying a small percentage (1-2 %) of
    the visible hemisphere could account for the observed amplitude of
    variation. If, instead, the variability is indeed instrumental in origin,
    then our non-variable L dwarfs could be either completely covered with
    clouds or objects whose clouds are smaller and uniformly distributed. Such
    scenarios would lead to very small photometric variations. Followup IRAC
    photometry at 3.6 and 5.8 micron bandpasses should distinguish between the
    two cases. In any event, the present observations provide the most sensitive
    search to date for structure in the photospheres of late-L dwarfs at mid-IR
    wavelengths, and our photometry provides stringent upper limits to the
    extent to which the photospheres of these transition L dwarfs are
    structured.

    http://arxiv.org/abs/astro-ph/0607537

    Astrophysics, abstract
    astro-ph/0607516

    From: James Lloyd [view email]

    Date: Fri, 21 Jul 2006 20:48:08 GMT (108kb)

    Direct Detection of the Brown Dwarf GJ 802B with Adaptive Optics Masking
    Interferometry

    Authors: James P. Lloyd, Frantz Martinache, Michael J. Ireland, John D.
    Monnier, Steven H. Pravdo, Stuart B. Shaklan, Peter G. Tuthill

    Comments: 4 Pages, 5 figures, emulateapj format, submitted to ApJL

    We have used the Palomar 200" Adaptive Optics (AO) system to directly detect
    the astrometric brown dwarf GJ 802B reported by Pravdo et al. 2005. This
    observation is achieved with a novel combination of aperture masking
    interferometry and AO. The dynamical masses are 0.175$\pm$0.021 M$_\odot$
    and 0.064$\pm$0.032 M$_\odot$ for the primary and secondary respectively.
    The inferred absolute H band magnitude of GJ 802B is M$_H$=12.8 resulting in
    a model-dependent T$_\mathrm{eff}$ of 1850 $\pm$ 50K and mass range of
    0.057--0.074 M$_\odot$.

    http://arxiv.org/abs/astro-ph/0607516

    Astrophysics, abstract
    astro-ph/0607514

    From: Stanimir A. Metchev [view email]

    Date: Fri, 21 Jul 2006 20:06:10 GMT (308kb)

    HD 203030B: an Unusually Cool Young Sub-Stellar Companion near the L/T
    Transition

    Authors: Stanimir A. Metchev (UCLA), Lynne A. Hillenbrand (Caltech)

    Comments: 16 pages, 4 figures, 2 tables. To appear in ApJ November 10, 2006,
    v651 issue

    We present the discovery of a brown-dwarf companion to the 130-400 Myr-old
    G8 V star HD 203030. Separated by 11.9" (487 AU in projection) from its host
    star, HD 203030B has an estimated mass of 0.023 (+0.008;-0.011) solar
    masses. The K-band spectral type of L7.5+/-0.5 places HD 203030B near the
    critical L/T transition in brown dwarfs, which is characterized by the rapid
    disappearance of dust in sub-stellar photospheres. From a comparative
    analysis with well-characterized field L/T transition dwarfs, we find that,
    despite its young age, HD 203030B has a bolometric luminosity similar to the
    >1 Gyr-old field dwarfs. Adopting a radius from current models of
    sub-stellar evolution, we hence obtain that the effective temperature of HD
    203030B is only 1206 (+74;-116) K, markedly lower than the ~1440 K effective
    temperatures of field L/T transition dwarfs. The temperature discrepancy can
    be resolved if either: (1) the ages of field brown dwarfs have been
    over-estimated by a factor of \~1.5, leading to under-estimated radii, or
    (2) the lower effective temperature of HD 203030B is related to its young
    age, implying that the effective temperature at the L/T transition is
    gravity-dependent.

    http://arxiv.org/abs/astro-ph/0607514


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