Identifying low-mass members of nearby star clusters using proper motion & color selection

Abstract

I present a combined kinematic and photometric search for new, low-mass (m 0:2M) members of nearby (d < 300 pc) star clusters. Using both proper motion and color criteria, a total of 33 low-mass objects have been newly recognized as members of the Taurus, Praesepe, and Pleiades clusters. In addition, 18 potential cluster members are noted, and 4 members are recovered from previous member searches. Multi-epoch imaging was performed using i-band Megacam observations unique to this study, combined with archival CFH telescope data in the optical I and Z bands. Near-infrared detections were also acquired from the 2MASS survey. The imaging data were processed using the Pan-STARRS IPP data pipeline software in order to provide high-precision relative astrometry, from which proper motions were extracted. Low-resolution, near-infrared spectroscopy from the IRTF telescope gives confirmation on the membership status of the selected candidates. The addition of proper motion criteria to complement the often-used color selection allows for a more effective identification of low-mass cluster members whose broadband spectral features are similar to the bulk of galactic field objects lying along the line-of-sight. Culling the candidates using proper motion also significantly reduces the amount of candidates that require spectroscopic follow-up, even in the NIR color-space with the highest levels of field contamination. Comparison of the search results to a galactic field model by Robin et al. (2003) provides strong evidence that brighter member candidates in Taurus (i < 17) found to be of mid-M spectral types are highly likely to be clusters members rather than field dwarfs. While the addition of new members to the Praesepe and Pleiades clusters are minor compared to the current known population, there is suggestive evidence that the mass function of Taurus is significantly lacking in mid-M dwarfs, and in fact may actually resemble the mass functions of other similarly-aged clusters. The successful application of this search technique using the IPP software is significant, as the Pan-STARRS survey program is conducting regular imaging of 75% of the total sky over the next 3-4 years.