Ultra-cool Subdwarfs in the Subaru HSC survey

Poshih Chiang^1*, Chien-Hsiu Lee², Eugene Magnier³, Wen-Ping Chen¹

¹Graduate Institute of Astronomy, National Central University, Taoyuan, Taiwan
²Subaru Telescope, National Astronomical Observatory of Japan, Tokyo, Japan
³Institute of Astronomy, University of Hawaii, Honolulu, USA

* Presenter:Poshih Chiang, email:pschiang@gmail.com

Ultra-cool subdwarfs are metal-poor low-mass stars and brown dwarfs. They are effective tracers to study the history of star formation and chemical enrichment in Milky Way, because of their old ages and long lives. To date, only a handful of confirmed metal-poor brown dwarfs (L type and later) are known. Their faintness, required to identify them reliably in surveys, makes it very challenging to detect these important probes of the early Milky Way. In this study, we are using the ongoing Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) survey to search for ultracool subdwarfs. The HSC-SSP allows us to push the depth in optical with a wide area matched with UKIDSS and ALLWISE in near infrared. We have identified 12 strong candidate L subdwarfs on the basis of optical & near IR photometry and obvious proper motions. Five of the 12 candidates were observed by our Gemini spectroscopic follow-ups and were all confirmed as metal-poor L type subdwarfs. With the successful results, our method will be applied to on-going and future surveys such as VISTA, LSST, and Ultimate Subaru.

Keywords: brown dwarfs, metal-poor stars