S20A0098abst
S20A0098
This proposal aims at pushing down the observed lower mass limit of star formation processes. In 2012, we performed a deep Z,Y,J survey with VISTA/VIRCam that led to the discovery of tens of 30−5 Jupiter masses brown dwarfs in the nearest OB association to the Sun, Upper Scorpius (145 pc; ∼10 Myr). Our VLT/X-shooter follow-up conducted in 2015 confirmed all of them as spectroscopic members with spectral types of L1−L7 (Teff = 1500−1800 K; M ≤ 10 MJup). We extended our search with a deeper Y-band survey down to 22.5 mag and confirmed spectroscopically new late-L type members but no T-type candidate. Combining the absolute magnitudes of field T dwarfs and the recent model-independent mass determination of two mid-M dwarfs with Kepler light-curve in USco, we conclude that our VISTA survey is deep enough in Y and J but not in Z to probe T-type brown dwarfs in the association. Hence, we request a deep z-band survey of 6 square degrees in Upper Sco down to a completeness limit of z = 25 mag (5σ) to combine with our 2012 first J-band epoch survey for photometric and proper motion selections. The detection (or otherwise) of T-type members will allow us to set an upper limit to the lower mass fragment that star formation processes can form.
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