Preliminary Parallaxes of 40 L and T Dwarfs from the US Naval Observatory Infrared Astrometry Program

Abstract

We present preliminary trigonometric parallaxes and proper motions for 22 L dwarfs and 18 T dwarfs measured using the ASTROCAM infrared imager on the US Naval Observatory ( USNO) 1.55 m Strand Astrometric Reflector. The results presented here are based on observations obtained between 2000 September and 2002 November; about half of the objects have an observational time baseline of Át ¼ 1:3 yr and half Át ¼ 2:0 yr. Despite these short time baselines, the astrometric quality is sufficient to produce significant new results, especially for the nearer T dwarfs. Seven objects are in common with the USNO optical CCD parallax program for quality control and seven in common with the European Southern Observatory 3.5 m New Technology Telescope parallax program. We compare astrometric quality with both of these programs. Relative to absolute parallax corrections are made by employing Two Micron All Sky Survey and/or Sloan Digital Sky Survey photometry for reference-frame stars. We combine USNO infrared and optical parallaxes with the best available California Institute of Technology (CIT) system photometry to determine M J , M H , and M K values for 37 L dwarfs between spectral types L0 and L8 and 19 T dwarfs between spectral types T0.5 and T8 and present selected absolute magnitude versus spectral type and color diagrams, based on these results. Luminosities and temperatures are estimated for these objects. Of special interest are the distances of several objects that are at or near the L-T dwarf boundary so that this important transition can be better understood. The previously reported early to mid T dwarf luminosity excess is clearly confirmed and found to be present at J, H, and K. The large number of objects that populate this luminosity-excess region indicate that it cannot be due entirely to selection effects. The T dwarf sequence is extended to M J % 16:9 by 2MASS J041519À0935, which, at d ¼ 5:74 pc, is found to be the least luminous [log (L=L ) ¼ À5:58] and coldest (T eA % 760 K) brown dwarf known. Combining results from this paper with earlier USNO CCD results we find that, in contrast to the L dwarfs, there are no examples of lowvelocity (V tan < 20 km s À1 ) T dwarfs. This is consistent with the T dwarfs in this study being generally older than the L dwarfs. We briefly discuss future directions for the USNO infrared astrometry program.

Physical parameters of T dwarfs derived from high-resolution near-infrared spectra

“…Our best modelled spectra with T eff of 1002 ± 278 K is, taking into account the errors, consistent with the estimate of 1231 K from the relation of Looper et al and the value of 1200 K found by Cushing et al (2008). Our best modelled spectrum of SDSSp J125453.90-012247.4 indicates an effective temperature above 2000 K, which is much higher than that found by Vrba et al (2004) and Cushing et al (2008), but consistent (i.e. within the errors) with a temperature around 1500 K. This value is similar to the estimate of 1370 K in the relation of Looper et al…”

“…The T4.5 dwarf 2MASS J05591914-1404488 has historically been considered enigmatic (see Vrba et al 2004). Our best modelled spectra with T eff of 1002 ± 278 K is, taking into account the errors, consistent with the estimate of 1231 K from the relation of Looper et al and the value of 1200 K found by Cushing et al (2008).…”

Binary frequency of planet-host stars at wide separations

“…We have three very wide companions with spectral types later than M5, whose number of field objects is typically lower than earlier spectral types, suggesting that those are most likely to be associated to th planet-host star (if any of these are). This result is not so surprising and in line with the conclusions of Leggett et al (2000a), Vrba et al (2004), and Kirkpatrick et al (2012). A120, page 8 of 14 separations larger than 8.3 arcmin belonging to the thin disk can not be identified reliably.…”

“…First, we have estimated that the expected number of T dwarfs in the full area of our search (∼150 deg 2 around the full sample of 49 stars) is ∼1.3. We have adopted a density of 5 × 10 −3 objects/pc −3 (Burgasser et al 2003a) and an average absolute magnitude of M J ∼ 15 mag (Vrba et al 2004) for such objects, and considered that our search is limited to J = 17 mag. Secondly, we have estimated the probability that one of this object has a proper motion within 40 mas/yr of the primary's proper motion in each direction.…”

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).…”