Astrometry and Photometry for Cool Dwarfs and Brown Dwarfs

Abstract

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.Comment: 48 pages, including 7 figures and 6 tables. Accepted for A

Spectral energy distribution simulations of a possible ring structure around the young, red brown dwarf G 196-3 B

“…The luminosity uncertainty is dominated by the distance error. Within the quoted error bar, this absolute luminosity coincides with the bolometric magnitudes reported for L3-type sources by Dahn et al (2002) and with the luminosities of L3 dwarfs given by Vrba et al (2004) and Dieterich et al (2014). However, it contrasts with the higher luminosity of G 196-3 B, L = 1.86 +0.70 −0.51 ×10 −4 L⊙ (see table 6 of Zapatero Osorio et al 2014).…”

Discovery of a Tight Brown Dwarf Companion to the Low Mass Star LHS 2397a

“…We also find temperatures for the two components to be T A = 2630K(2590 − 2660) and T B = 1470K(1400 − 1500) for the primary and secondary respectively. These temperatures are consistent with the independently derived relationships by Dahn et al (2002). Note that the errors quoted for the mass and temperature only take into account observational errors and do not include virtually unquantifiable modelling errors.…”

“…To determine the spectral types of the two components, we compared our calculated absolute magnitudes, in all four bands, with the absolute magnitude vs. spectral type relationships derived by Dahn et al (2002) (see Figure 2). However, in order to avoid the errors associated with converting from Ks to K magnitudes, we used the Ks magnitudes as determined by the second incremental 2MASS data release along with Dahn et al (2002)'s parallaxes for as many of the stars as possible to derive a M Ks vs. spectral type relationship.…”

“…However, in order to avoid the errors associated with converting from Ks to K magnitudes, we used the Ks magnitudes as determined by the second incremental 2MASS data release along with Dahn et al (2002)'s parallaxes for as many of the stars as possible to derive a M Ks vs. spectral type relationship. For every observed bandpass, we confirm that the primary is an M8 and the secondary is consistent with an L7.5.…”

2MASS 1315−2649: A High Proper-Motion L Dwarf with Strong Hα Emission

“…Since the velocity dispersion of disk stars increases with age, the large transverse velocity inferred for 2MASS 1315Ϫ2649 from its proper motion suggests that it is տ2 Gyr old (Dahn et al 2002) and therefore relatively massive for its spectral type. This is consistent with the correlation noted for weaker Ha emitters by Gizis et al (2000), namely, that "active L dwarfs are drawn from an older, more massive population."…”