Evolved Stars with Weak Paschen Series Absorption and Small Proper Motions in the HALO7D Survey

We have analyzed a small (~10) subsample of unusual stars in the Milky Way’s stellar halo that were identified from a larger sample of stars that were observed as part of the Halo Assembly in Lambda-CDM: Observations in 7 Dimensions (HALO7D), a survey designed to develop a better understanding of the accretion history of our galaxy. The HALO7D dataset includes deep spectra obtained with the Keck II 10-m telescope and DEIMOS instrument and deep multi-epoch images obtained with the Hubble Space Telescope (HST). The unusual stars that are the focus of this study are characterized by two things: (1) Paschen series absorption in their spectra, typically quite weak, and (2) small HST-based proper motion magnitudes. We classified these stars by determining the strength of their Paschen series, through a method of quantifying the strength of multiple equivalent widths across a set of spectral lines. Based on this kind of spectral analysis and HST-based photometry, we were able to draw rough inferences about other characteristics of these stars such as their colors and effective temperature, surface gravity, chemical composition, and luminosity. It is likely that a couple of these stars are on the horizontal branch and some may even be RR Lyrae stars. This would imply that their luminosities are significantly higher than that of the more typical main sequence turnoff stars in the HALO7D survey, which would put them at a significantly larger Galactocentric distance and make them particularly interesting tracers of the Milky Way’s outer stellar halo.