报告题目：A Pan-STARRS1 View of the Taurus Star Formation History in the Substellar Regime
报告人：Zhoujian Zhang（PhD, University of Hawaii）
摘要：Young brown dwarfs are of prime importance to investigate the universality of the initial mass function and to understand the physical connections between substellar and planetary-mass objects. By mining the Pan-STARRS1 3\pi Survey database, we are conducting a proper-motion survey for young brown dwarfs and free-floating planets in the Taurus—Auriga molecular cloud. Our search uses multi-band photometry and astrometry to select candidates and is wider (370 deg2) and deeper (down to ~3 M_Jup) than previous searches. Since extinction in star-forming regions complicates standard spectral classification methods, we develop a new approach to quantitatively determine reddening-free spectral types, extinctions, and gravity classifications for mid-M to late-L ultracool dwarfs (~100−5 M_Jup in Taurus) using low-resolution near-infrared spectra. Applying our method, we have discovered 25 low-gravity (VL-G) and the first 11 intermediate-gravity (INT-G) substellar (M6–L1) members of Taurus. We also reclassify the previously known Taurus brown dwarfs, and altogether our discoveries so far have increased the substellar census by ~40% and added two more L-type members (masses <10 M_Jup). Most notably, our new members reveal an older (>10 Myr) low-mass population in Taurus, in accord with recent studies of the higher-mass stellar members. In addition, a difference in IMF is noticed between the younger and older populations possibly due to the incompleteness of the older stellar members or different ongoing star formation processes. Upon completion, our discoveries will provide a more comprehensive picture of the star formation history of Taurus.