Near-infrared spectroscopy of young brown dwarfs in upper Scorpius

Spectroscopic follow-up is a pre-requisite for studies of the formation and early evolution of brown dwarfs. Here, we present Infrared Telescope Facility/SpeX near-infrared spectroscopy of 30 candidate members of the young Upper Scorpius association, selected from our previous survey work. All 24 high-confidence members are confirmed as young very low mass objects with spectral types from M5 to L1, 15–20 of them are likely brown dwarfs. This high yield confirms that brown dwarfs in Upper Scorpius can be identified from photometry and proper motions alone, with negligible contamination from field objects (<4percent). Out of the six candidates with lower confidence, five might still be young very low mass members of Upper Scorpius, according to our spectroscopy. We demonstrate that some very low mass class II objects exhibit radically different near-infrared (0.6–2.5 µm) spectra from class III objects, with strong excess emission increasing towards longer wavelengths and partially filled in features at wavelengths shorter than 1.25 µm. These characteristics can obscure the contribution of the photosphere within such spectra. Therefore, we caution that near-infrared derived spectral types for objects with discs may be unreliable. Furthermore, we show that the same characteristics can be seen to some extent in all class II and even a significant fraction of class III objects (∼40percent), indicating that some of them are still surrounded by traces of dust and gas. Based on our spectra, we select a sample of objects with spectral types of M5–L1, whose near-infrared emission represents the photosphere only. We recommend the use of these objects as spectroscopic templates for young brown dwarfs in the future.

[1]  G. Rieke,et al.  The interstellar extinction law from 1 to 13 microns. , 1985 .

[2]  M. Skrutskie,et al.  New Young Brown Dwarfs in the Orion Molecular Cloud 2/3 Region , 2008, 0806.2818.

[3]  A.G.A. Brown,et al.  Improved Methods for Identifying Moving Groups , 1997 .

[4]  L. Hartmann,et al.  Optical excess emission in T Tauri stars , 1991 .

[5]  D. Padgett,et al.  New brown dwarf discs in Upper Scorpius observed with WISE , 2012, 1211.4484.

[6]  R. F. Jameson,et al.  Near‐infrared cross‐dispersed spectroscopy of brown dwarf candidates in the Upper Sco association★ , 2007, 0711.1109.

[7]  M. Irwin,et al.  New members in the upper scorpius association from the UKIRT infrared deep sky survey early data release , 2006, astro-ph/0603608.

[8]  Motohide Tamura,et al.  SUBSTELLAR OBJECTS IN NEARBY YOUNG CLUSTERS (SONYC). IV. A CENSUS OF VERY LOW MASS OBJECTS IN NGC 1333 , 2011, 1110.1639.

[9]  W. D. Vacca,et al.  THE INFRARED TELESCOPE FACILITY (IRTF) SPECTRAL LIBRARY: COOL STARS , 2009, 0909.0818.

[10]  Mark J. Pecaut,et al.  A REVISED AGE FOR UPPER SCORPIUS AND THE STAR FORMATION HISTORY AMONG THE F-TYPE MEMBERS OF THE SCORPIUS–CENTAURUS OB ASSOCIATION , 2011, 1112.1695.

[11]  Eduardo L. Martin,et al.  A Survey for Low-Mass Stars and Brown Dwarfs in the Upper Scorpius OB Association , 2000 .

[12]  John T. Rayner,et al.  SpeX: A Medium‐Resolution 0.8–5.5 Micron Spectrograph and Imager for the NASA Infrared Telescope Facility , 2003 .

[13]  Motohide Tamura,et al.  SUBSTELLAR OBJECTS IN NEARBY YOUNG CLUSTERS (SONYC). V. NEW BROWN DWARFS IN ρ OPHIUCHI , 2011, 1110.1640.

[14]  B. Swift,et al.  Discovery of an M9.5 Candidate Brown Dwarf in the TW Hydrae Association: DENIS J124514.1–442907 , 2007, 0709.3290.

[15]  K. Mužić,et al.  SUBSTELLAR OBJECTS IN NEARBY YOUNG CLUSTERS (SONYC). VIII. SUBSTELLAR POPULATION IN LUPUS 3 , 2014, 1403.0813.

[16]  John T. Rayner,et al.  Spextool: A Spectral Extraction Package for SpeX, a 0.8–5.5 Micron Cross‐Dispersed Spectrograph , 2004 .

[17]  E. Young,et al.  A Spitzer Census of the IC 348 Nebula , 2007, 0704.0203.

[18]  James P. Lloyd,et al.  METALLICITY AND TEMPERATURE INDICATORS IN M DWARF K-BAND SPECTRA: TESTING NEW AND UPDATED CALIBRATIONS WITH OBSERVATIONS OF 133 SOLAR NEIGHBORHOOD M DWARFS , 2011, 1112.4567.

[19]  Tenerife,et al.  Multi-fibre optical spectroscopy of low-mass stars and brown dwarfs in Upper Scorpius ,, , 2011, 1101.0919.

[20]  R. F. Jameson,et al.  New brown dwarfs in Upper Sco using UKIDSS Galactic Cluster Survey science verification data , 2007 .

[21]  I. Baraffe,et al.  Theory of Low-Mass Stars and Substellar Objects , 2000 .

[22]  Michael C. Liu,et al.  Characterizing Young Brown Dwarfs Using Low-Resolution Near-Infrared Spectra , 2006, astro-ph/0611408.

[23]  K. Keil,et al.  Protostars and Planets V , 2007 .

[24]  John T. Rayner,et al.  An Infrared Spectroscopic Sequence of M, L, and T Dwarfs , 2004, astro-ph/0412313.

[25]  L. Hillenbrand,et al.  A Large-Area Search for Low-Mass Objects in Upper Scorpius. II. Age and Mass Distributions , 2008, 0809.1436.

[26]  Patrick J. Lowrance,et al.  Discovery of a Very Young Field L Dwarf, 2MASS J01415823-4633574 , 2005 .

[27]  M. Tamura,et al.  SUBSTELLAR OBJECTS IN NEARBY YOUNG CLUSTERS (SONYC): THE BOTTOM OF THE INITIAL MASS FUNCTION IN NGC 1333 , 2009, 0907.2243.

[28]  Catherine Slesnick,et al.  A Large-Area Search for Low-Mass Objects in Upper Scorpius. I. The Photometric Campaign and New Brown Dwarfs , 2006 .

[29]  J. Mathis.  Interstellar dust and extinction , 1987 .

[30]  A. Scholz,et al.  New brown dwarfs in the south part of the Upper Scorpius Association , 2011, 1108.1309.