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Register a new userAstrometry and Photometry for Cool Dwarfs and Brown Dwarfs
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Trigonometric parallax determinations are presented for 28 late type dwarfs and brown dwarfs, including eight M dwarfs with spectral types between M7 and M9.5, 17 L dwarfs with spectral types between L0 and L8, and three T dwarfs. Broadband photometry at CCD wavelengths (VRIz) and/or near-IR wavelengths (JHK) are presented for these objects and for 24 additional late-type dwarfs. Supplemented with astrometry and photometry from the literature, including ten L and two T dwarfs with parallaxes established by association with bright, usually HIPPARCOS primaries, this material forms the basis for studying various color-color and color-absolute magnitude relations. The I-J color is a good predictor of absolute magnitude for late-M and L dwarfs. M_J becomes monotonically fainter with I-J color and with spectral type through late-L dwarfs, then brightens for early-T dwarfs. The combination of zJK colors alone can be used to classify late-M, early-L, and T dwarfs accurately, and to predict their absolute magnitudes, but is less effective at untangling the scatter among mid- and late-L dwarfs. The mean tangential velocity of these objects is found to be slightly less than that for dM stars in the solar neighborhood, consistent with a sample with a mean age of several Gyr. Using colors to estimate bolometric corrections, and models to estimate stellar radii, effective temperatures are derived. The latest L dwarfs are found to have T_eff ~ 1360 K.
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Observations of brown dwarfs provide important feedback on theories of atmospheres and inner structure of substellar objects. Brown dwarf binary systems furthermore offer the unique opportunity to determine the mass of individual brown dwarfs, which is one of the fundamental astrophysical quantities.
Europes Gaia spacecraft will soon embark on its five-year mission to measure the absolute parallaxes of the complete sample of 1,000 million objects down to 20 mag. It is expected that thousands of nearby brown dwarfs will have their astrometry determined with sub-milli-arcsecond standard errors. Although this level of accuracy is comparable to the standard errors of the relative parallaxes that are now routinely obtained from the ground for selected, individual objects, the absolute nature of Gaias astrometry, combined with the sample increase from one hundred to several thousand sub-stellar objects with known distances, ensures the uniqueness of Gaias legacy in brown-dwarf science for the coming decade(s). We shortly explore the gain in brown-dwarf science that could be achieved by lowering Gaias faint-end limit from 20 to 21 mag and conclude that two spectral-type sub-classes could be gained in combination with a fourfold increase in the solar-neighbourhood-volume sampled by Gaia and hence in the number of brown dwarfs in the Gaia Catalogue.
Accurate parallax measurements allow us to determine physical properties of brown dwarfs, and help us to constrain evolutionary and atmospheric models, break the age-mass degeneracy and reveal unresolved binaries. We measured absolute trigonometric parallaxes and proper motions of 6 cool brown dwarfs using background galaxies to establish an absolute reference frame. We derive the absolute J-mag. The six T brown dwarfs in our sample have spectral types between T2.5 and T7.5 and magnitudes in J between 13.9 and 18.0, with photometric distances below 25 pc. The observations were taken in the J-band with the Omega-2000 camera on the 3.5 m telescope at Calar Alto, during a time period of 27 months, between March 2011 and June 2013. The number of epochs varied between 11 and 12 depending on the object. The reduction of the astrometric measurements was carried out with respect to the field stars. The relative parallax and proper motions were transformed into absolute measurements using the background galaxies in our fields. We obtained absolute parallaxes for our six brown dwarfs with a precision between 3 and 6 mas. We compared our results in a color-magnitude diagram with other brown dwarfs with determined parallax and with the BT-Settl 2012 atmospheric models. For four of the six targets we found a good agreement in luminosity with objects of similar spectral types. We obtained an improved accuracy in the parallaxes and proper motions in comparison to previous works. The object 2MASS J11061197+2754225 is more than 1 mag overluminous in all bands pointing to binarity or high order multiplicity.
The I-Z colour has been recently shown to be a good temperature indicator for M dwarfs. We present the first IZ photometry of a small sample of L dwarfs ranging in spectral type from L0.5V to L6.0V. We find that the I-Z colour is not a good temperature indicator for objects between L1V and L5V, such objects having colours that overlap with mid M dwarfs. We attribute this to the reduction in the strength of the TiO and VO bands in the L dwarfs which are the dominant opacity source in the I band for late M dwarfs. Beyond L5V, I-Z appears to be a reasonable indicator. This has important implications for the planning of optical surveys for cool objects in clusters and the field. For example I-Z will cease to be a good method of discriminating brown dwarfs in the Pleiades below around 0.04 Msolar, and at around 0.075 Msolar in the Hyades and Praesepe.
The survey of the mid-infrared sky by the Wide-field Infrared Survey Explorer (WISE) led to the discovery of extremely cold low-mass brown dwarfs, classified as Y dwarfs, which extend the T class to lower temperatures. Twenty-four Y dwarfs are known at the time of writing. Here we present improved parallaxes for four of these, determined using Spitzer images. We give new photometry for four late-type T and three Y dwarfs, and new spectra of three Y dwarfs, obtained at Gemini Observatory. We also present previously unpublished photometry taken from HST, ESO, Spitzer and WISE archives of 11 late-type T and 9 Y dwarfs. The near-infrared data are put on to the same photometric system, forming a homogeneous data set for the coolest brown dwarfs. We compare recent models to our photometric and spectroscopic data set. We confirm that non-equilibrium atmospheric chemistry is important for these objects. Non-equilibrium cloud-free models reproduce well the near-infrared spectra and mid-infrared photometry for the warmer Y dwarfs with 425 <= T_eff K <= 450. A small amount of cloud cover may improve the model fits in the near-infrared for the Y dwarfs with 325 <= T_eff K <= 375. Neither cloudy nor cloud-free models reproduce the near-infrared photometry for the T_eff = 250 K Y dwarf W0855. We use the mid-infrared region, where most of the flux originates, to constrain our models of W0855. We find that W0855 likely has a mass of 1.5 - 8 Jupiter masses and an age of 0.3 - 6 Gyr. The Y dwarfs with measured parallaxes are within 20 pc of the Sun and have tangential velocities typical of the thin disk. The metallicities and ages we derive for the sample are generally solar-like. We estimate that the known Y dwarfs are 3 to 20 Jupiter-mass objects with ages of 0.6 to 8.5 Gyr.
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