Ultra-cool dwarfs: new discoveries, proper motions, and improved spectral typing from SDSS and 2MASS photometric colors

Abstract

Aims. We try to identify ultra-cool dwarfs from the seventh Data Release of the Sloan Digital Sky Survey (SDSS DR7) with SDSS i-z and r-z colors. We also obtain proper motion data from SDSS, 2MASS, and UKIDSS and improve spectral typing from SDSS and 2MASS photometric colors. Methods. We selected ultra-cool dwarf candidates from the SDSS DR7 with new photometric selection criteria, which are based on a parameterization study of known L and T dwarfs. The objects are then cross-identified with the Two Micron All Sky Survey and the Fourth Data Release of the UKIRT Infrared Deep Sky Survey (UKIDSS DR4). We derive proper motion constraints by combining SDSS, 2MASS, and UKIDSS positional information. In this way we are able to assess, to some extent, the credence of our sample using a multi epoch approach, which complements spectroscopic confirmation. Some of the proper motions are affected by short baselines, but, as a general tool, this method offers great potential to confirm faint L dwarfs as UKIDSS coverage increases. In addition we derive updated color-spectral type relations for L and T dwarfs with SDSS and 2MASS magnitudes. Results. We present 59 new nearby M and L dwarfs selected from the imaging catalog of the SDSS DR7, including proper motions and spectral types calculated from the updated color-spectral type relations. and obtain proper motions from SDSS, 2MASS, and UKIDSS for all of our objects.

17 pages, 8 figures

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• A very cool brown dwarf in UKIDSS DR1
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• A Volume-Limited Sample of Ultracool Dwarfs. I. Construction, Space Density, and a Gap in the L/T Transition
• A Search for L/T Transition Dwarfs With Pan-STARRS1 and WISE. II. L/T Transition Atmospheres and Young Discoveries
• Primeval very low-mass stars and brown dwarfs. I. Six new L subdwarfs, classification and atmospheric properties
• Exploring the Role of Sub-micron Sized Dust Grains in the Atmospheres of Red L0 - L6 Dwarfs
• A Brown Dwarf Census from the SIMP Survey
• The Second Arecibo Search for 5 GHz Radio Flares from Ultracool Dwarfs
• A New Sample of Cool Subdwarfs from SDSS: Properties and Kinematics
• PSYM-WIDE: a survey for large-separation planetary-mass companions to late spectral type members of young moving groups
• Discovery And Characterization of Wide Binary Systems With a Very Low Mass Component
• The Infrared Medium-deep Survey. III. Survey of Luminous Quasars at 4.7 $\le$ z $\le$ 5.4
• L dwarfs detection from SDSS images using improved Faster R-CNN
• J-PLUS: Discovery and characterisation of ultracool dwarfs using Virtual Observatory tools II. Second data release and machine learning methodology