Discovery of new nearby L and late-M dwarfs at low Galactic latitude from the DENIS data base

Abstract

International audienceWe report on new nearby L and late-M dwarfs (dphot <= 30 pc) discovered in our search for nearby ultracool dwarfs (I-J >= 3.0, later than M8.0) at low Galactic latitude (|b| < 15°) over 4800 deg2 in the Deep Near Infrared Survey of the Southern Sky (DENIS) data base. We used late-M (>= M8.0), L and T dwarfs with accurate trigonometric parallaxes to calibrate the MJ versus I-J colour-luminosity relation. The resulting photometric distances have standard errors of ~15 per cent, which we used to select candidates dphot <= 30 pc. We measured proper motions from multi-epoch images found in the public archives ALADIN, DSS, 2MASS and DENIS, with at least three distinct epochs and time baselines of 10-21 yr. We then used a maximum reduced proper motion cut-off to select 28 candidates as ultracool dwarfs (M8.0-L8.0) and to reject one as a distant red star. No T dwarf candidates were found in this search, which required an object to be detected in all three DENIS bands. Our low-resolution optical spectra confirmed that 26 were indeed ultracool dwarfs, with spectral types from M8.0 to L5.5. Two contaminants and one rejected by the maximum reduced proper motion cut-off were all reddened F-K main sequence stars. 20 of these 26 ultracool dwarfs are new nearby ultracool dwarf members, three L dwarfs within 15 pc with one L3.5 at only ~10 pc. We determine a stellar density of dwarfs pc-3 mag-1 over 11.1 <= MJ <= 13.1 based on this sample of M8-L3.5 ultracool dwarfs. Our ultracool dwarf density value is in good agreement with the measurement by Cruz et al. of the ultracool dwarf density at high Galactic latitude

Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Scorpii OB association

“…Column 3: absolute J-band magnitude estimated from the absolute magnitude vs. I − J color relationship given in Phan-Bao et al (2008). Column 4: spectrophometric distance in parsecs.…”

“…Their spectrophotometric distances, and those of all other old UDs in our sample, are given in Table 5. We used our adopted spectral types, the photometry given in Table 1, and the absolute J-band magnitude estimated from the absolute magnitude versus I − J color relationship given in Phan-Bao et al (2008). This relationship is not valid for young BDs, and thus we do not provide spectrophotometric distances from them.…”

“…Henry et al (2002) estimated that more than half of the stellar systems within 25 pc are still unknown, and most of them are probably ultracool dwarfs (UDs; defined as those with spectral types M6-M9, L and T; Martín et al 1996Martín et al , 1997Martín et al , 1999Kirkpatrick et al 1999Kirkpatrick et al , 2000, which include a mixture of very low-mass stars and substellar-mass brown dwarfs. Many UDs within 25 pc have been found in the last few years (Phan-Bao et al 2008;Reid et al 2008).…”

X-ray emission from the M9 dwarf 1RXS J115928.5-524717

“…Observations with VLT/UVES do not show any Li and indicate a relatively fast rotation of V sin i ≈ 25 km s −1 (Hambaryan et al 2005). Recently, Phan-Bao et al (2008) describe the same star as DENIS-P J115927.4-524718 with a spectral type again of M9 and derive a distance of 10.2 ± 1.7 pc. Obviously in the solar vicinity the populations of older, very low-mass stars and young brown dwarfs overlap in the regime of late M dwarfs (see e.g.…”

Astrometric planet search around southern ultracool dwarfs

“…The primary is a very-low-mass star or brown dwarf with an optical spectral type of M8.5 (Phan-Bao et al 2008), and the companion responsible for the astrometric orbit is a brown dwarf.…”