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Gemini Studies a Plethora of Brown Dwarfs Candidates

July 23, 2015

Figure 1. NIR Spectral type histogram of all known low-gravity dwarfs and those presented in this work. Green bars delimited by dashed lines represent the known population prior to BASS, purple bars delimited by dash-dotted lines represent known dwarfs for which low-gravity features were identified here for the first time, and orange bars delimited by solid lines represent new discoveries from BASS. The BASS survey has contributed significantly in increasing the number of known low-gravity M6–L5 dwarfs.

 

An international team of astronomers from Canada and the United States recently discovered 42 new brown dwarfs using data from the near-infrared imager and spectrograph Flamingos-2 at Gemini South and other telescopes in Chile and Hawai’i. The team used Flamingos-2’s near-infrared spectroscopic capabilities to study a total of 101 targets from 2013 to 2015.

The work, led by Jonathan Gagne, from the University of Montreal, confirmed signs of low-gravity for 42 of the objects with estimated masses between 8 to 75 times that of Jupiter. Further, the team identified previously unrecognized signs of low gravity for 24 known brown dwarfs.

This kind of object has an important role in explaining part of the process of star formation, as current stellar formation models include the production of a non-negligible fraction of free-floating planets. Additionally, the research shows that some objects, that were thought to be brown dwarfs, were indeed much less massive, and similar to planetary mass objects.

Gagne’s research also provides a context for ongoing work with the Gemini Planet Imager (GPI) because these objects are easier to observe without the glare from a nearby host star of GPI targets. “One big question that remains unanswered is whether these isolated planets significantly differ from objects in orbit around stars. There are some reasons to expect differences, but no one has been able to demonstrate a difference to date,” explains Etienne Artigau, an astronomer on the team who is also at the University of Montreal. This work will be published in The Astrophysical Journal Supplement Series, and a preprint is now available.

Abstract:

We present the results of a near-infrared (NIR) spectroscopic follow-up survey of 182 M4–L7 low-mass stars and brown dwarfs (BDs) from the BANYAN All-Sky Survey (BASS) for candidate members of nearby, young moving groups (YMGs). We confirm signs of low-gravity for 42 new BD discoveries with estimated masses between 8–75MJup and identify previously unrecognized signs of low gravity for 24 known BDs. This allows us to refine the fraction of low-gravity dwarfs in the high-probability BASS sample to 82%. We use this unique sample of 66 young BDs, supplemented with 22 young BDs from the literature, to construct new empirical NIR absolute magnitude and color sequences for low-gravity BDs. We show that low-resolution NIR spectroscopy alone cannot differentiate between the ages of YMGs younger than 120 Myr, and that the BTSettl atmosphere models do not reproduce well the dust clouds in field or low-gravity L-type dwarfs. We obtain a spectroscopic confirmation of low-gravity for 2MASS J14252798–3650229, which is a new 27MJup, L4 γ bona fide member of AB Doradus. We identify in this work a total of 19 new low-gravity candidate members of YMGs with estimated masses below 13MJup, seven of which have kinematically estimated distances within 40 pc. These objects will be valuable benchmarks for a detailed atmospheric characterization of planetary-mass objects with the next generation of instruments such as the James Webb Space Telescope. We find 16 strong candidate members of the Tucana-Horologium association with estimated masses between 12.5–14MJup, a regime where our study was particularly sensitive. This would indicate that for this association there is at least one isolated object in this mass range for every 17:5+6:6-5:0 main-sequence stellar member, a number significantly higher than expected based on standard log-normal initial mass function, however in the absence of radial velocity and parallax measurements for all of them, it is likely that this over-density is caused by a number of young interlopers from other moving groups. Finally, as a byproduct of this project, we identify 12 new L0–L5 field BDs, seven of which display peculiar spectroscopic properties.