Hunting The Most Distant Stars in the Milky Way: Methods and Initial Results

We present a new catalog of 404 M giant candidates found in the UKIRT Infrared Deep Sky Survey (UKIDSS). The 2,400 deg$^2$ available in the UKIDSS Large Area Survey Data Release 8 resolve M giants through a volume four times larger than that of the entire Two Micron All Sky Survey. Combining near-infrared photometry with optical photometry and proper motions from the Sloan Digital Sky Survey yields an M giant candidate catalog with less M dwarf and quasar contamination than previous searches for similarly distant M giants. Extensive follow-up spectroscopy of this sample will yield the first map of our Galaxy's outermost reaches over a large area of sky. Our initial spectroscopic follow-up of $\sim$ 30 bright candidates yielded the positive identification of five M giants at distances $\sim 20-90$ kpc. Each of these confirmed M giants have positions and velocities consistent with the Sagittarius stream. The fainter M giant candidates in our sample have estimated photometric distances $\sim 200$ kpc (assuming $[Fe/H]$ = 0.0), but require further spectroscopic verification. The photometric distance estimates extend beyond the Milky Way's virial radius, and increase by $\sim 50\%$ for each 0.5 dex decrease in assumed $[Fe/H]$. Given the number of M giant candidates, initial selection efficiency, and volume surveyed, we loosely estimate that at least one additional Sagittarius-like accretion event could have contributed to the hierarchical build-up of the Milky Way's outer halo.Comment: 16 pages, 11 figures, emulateapj format. Accepted by A

The Most Distant Stars in the Milky Way

We report on the discovery of the most distant Milky Way (MW) stars known to date: ULAS J001535.72$+$015549.6 and ULAS J074417.48$+$253233.0. These stars were selected as M giant candidates based on their infrared and optical colors and lack of proper motions. We spectroscopically confirmed them as outer halo giants using the MMT/Red Channel spectrograph. Both stars have large estimated distances, with ULAS J001535.72$+$015549.6 at $274 \pm 74$ kpc and ULAS J074417.48$+$253233.0 at 238 $\pm$ 64 kpc, making them the first MW stars discovered beyond 200 kpc. ULAS J001535.72$+$015549.6 and ULAS J074417.48$+$253233.0 are both moving away from the Galactic center at $52 \pm 10$ km s$^{-1}$ and $24 \pm 10$ km s$^{-1}$, respectively. Using their distances and kinematics, we considered possible origins such as: tidal stripping from a dwarf galaxy, ejection from the MW's disk, or membership in an undetected dwarf galaxy. These M giants, along with two inner halo giants that were also confirmed during this campaign, are the first to map largely unexplored regions of our Galaxy's outer halo.Comment: Accepted and in print by ApJL. Seven pages, 2 figure

Calibrating Ultracool Dwarfs: Optical Template Spectra, Bolometric Corrections, and χ\chi Values

We present optical template spectra, bolometric corrections, and $\chi$ values for ultracool dwarfs. The templates are based on spectra from the Sloan Digital Sky Survey (SDSS) and the Astrophysical Research Consortium 3.5-m telescope. The spectral features and overall shape of the L dwarf templates are consistent with previous spectroscopic standards and the templates have a radial velocity precision of $\sim$10--20 km s$^{-1}$. We calculate bolometric fluxes (accurate to 10--20\%) for 101 late-M and L dwarfs from SDSS, 2MASS, and WISE photometry, SDSS spectra, and BT-Settl model spectra. We find that the $z$- and $J$-band bolometric corrections for late-M and L dwarfs have a strong correlation with $z-J$ and $J-K_S$ colors respectively. The new $\chi$ values, which can be used to convert H$\alpha$ equivalent widths to activity strength, are based on spectrophotometrically calibrated SDSS spectra and the new bolometric fluxes. While the measured $\chi$ values have typical uncertainties of $\sim$20\%, ultracool dwarf models show the continuum surrounding H$\alpha$ can vary by up to an order of magnitude with changing surface gravity. Our semi-empirical $\chi$ values are one to two orders of magnitude larger than previous $\chi$ values for mid- to late-L dwarfs, indicating that the upper limits for H$\alpha$ activity strength on the coolest L dwarfs have been underestimated.Comment: 17 pages, 10 figures, PASP in press. L dwarf templates available at: http://www.astro.washington.edu/users/slh/templates/ltemplates/index.htm

The Brown Dwarf Kinematics Project (BDKP). II. Details on Nine Wide Common Proper Motion Very Low-Mass Companions to Nearby Stars

We report on nine wide common proper motion systems containing late-type M, L, or T companions. We confirm six previously reported companions, and identify three new systems. The ages of these systems are determined using diagnostics for both stellar primaries and low--mass secondaries and masses for the secondaries are inferred using evolutionary models. Of our three new discoveries, the M3+T6.5 pair G 204-39 and SDSS J1758+4633 has an age constrained to 0.5-1.5 Gyr making the secondary a potentially useful brown dwarf benchmark. The G5+L4 pair G 200-28 and SDSS J1416+5006 has a projected separation of ~25,000 AU making it one of the widest and lowest binding energy systems known to date. The system containing NLTT 2274 and SDSS J0041+1341 is an older M4+L0 (>4.5 Gyr) pair which shows Halpha activity in the secondary but not the primary making it a useful tracer of age/mass/activity trends. We find a resolved binary frequency for widely-separated (> 100 AU) low--mass companions (i.e. at least a triple system) which is at least twice the frequency found for the field ultracool dwarf population. The ratio of triples to binaries and quadruples to binaries is also high for this sample: 3:5 and 1:4, respectively, compared to 8-parsec sample values of 1:4 and 1:26. The additional components in these wide companion systems indicates a formation mechanism that requires a third or fourth component to maintain gravitational stability or facilitate the exchange of angular momentum. The binding energies for the nine multiples discussed in this text are among the lowest known for wide low-mass systems, suggesting that weakly bound, low--to--intermediate mass (0.2M_sun < M_tot <1.0M_sun) multiples can form and survive to exist in the field (1-8 Gyr).Comment: 62 pages, 12 figures, 11 Tables, AJ accepted for publicatio

A New Sample of Cool Subdwarfs from SDSS: Properties and Kinematics

We present a new sample of M subdwarfs compiled from the 7th data release of the Sloan Digital Sky Survey. With 3517 new subdwarfs, this new sample significantly increases the number of spectroscopically confirmed low-mass subdwarfs. This catalog also includes 905 extreme and 534 ultra sudwarfs. We present the entire catalog including observed and derived quantities, and template spectra created from co-added subdwarf spectra. We show color-color and reduced proper motion diagrams of the three metallicity classes, which are shown to separate from the disk dwarf population. The extreme and ultra subdwarfs are seen at larger values of reduced proper motion as expected for more dynamically heated populations. We determine 3D kinematics for all of the stars with proper motions. The color-magnitude diagrams show a clear separation of the three metallicity classes with the ultra and extreme subdwarfs being significantly closer to the main sequence than the ordinary subdwarfs. All subdwarfs lie below (fainter) and to the left (bluer) of the main sequence. Based on the average $(U,V,W)$ velocities and their dispersions, the extreme and ultra subdwarfs likely belong to the Galactic halo, while the ordinary subdwarfs are likely part of the old Galactic (or thick) disk. An extensive activity analysis of subdwarfs is performed using H$\alpha$ emission and 208 active subdwarfs are found. We show that while the activity fraction of subdwarfs rises with spectral class and levels off at the latest spectral classes, consistent with the behavior of M dwarfs, the extreme and ultra subdwarfs are basically flat.Comment: 66 pages, 23 figures, accepted in Ap

BOSS Ultracool Dwarfs I: Colors and Magnetic Activity of M and L dwarfs

We present the colors and activity of ultracool (M7-L8) dwarfs from the Tenth Data Release of the Sloan Digital Sky Survey (SDSS). We combine previous samples of SDSS M and L dwarfs with new data obtained from the Baryon Oscillation Sky Survey (BOSS) to produce the BOSS Ultracool Dwarf (BUD) sample of 11820 M7-L8 dwarfs. By combining SDSS data with photometry from the Two Micron All Sky Survey and the Wide-Field Infrared Sky Explorer mission, we present ultracool dwarf colors from $i-z$ to $W2-W3$ as a function of spectral type, and extend the SDSS-2MASS-WISE color locus to include ultracool dwarfs. The $i-z$, $i-J$, and $z-J$ colors provide the best indication of spectral type for M7-L3 dwarfs. We also examine ultracool dwarf chromospheric activity through the presence and strength of H$\alpha$ emission. The fraction of active dwarfs rises through the M spectral sequence until it reaches $\sim$90% at spectral type L0. The fraction of active dwarfs then declines to 50% at spectral type L5; no H$\alpha$ emission is observed in the late-L dwarfs in the BUD sample. The fraction of active L0-L5 dwarfs is much higher than previously observed. The strength of activity declines with spectral type from M7 through L3, after which the data do not show a clear trend. Using one-dimensional chromosphere models, we explore the range of filling factors and chromospheric temperature structures that are consistent with H$\alpha$ observations of M0-L7 dwarfs. M dwarf chromospheres have a similar, smoothly varying range of temperature and surface coverage while L dwarf chromospheres are cooler and have smaller filling factors.Comment: 24 pages and 13 figures, submitted to AJ. A short video describing these results can be found at https://www.youtube.com/watch?v=wwX5WkuJCU

New views of the distant stellar halo

Currently, only a small number of Milky Way (MW) stars are known to exist beyond 100 kpc from the Galactic Centre. Though the distribution of these stars in the outer halo is believed to be sparse, they can provide evidence of more recent accretion events than in the inner halo and help map out the MW's dark matter halo to its virial radius. We have re-examined the outermost regions of 11 existing stellar halo models with two synthetic surveys: one mimicking present-day searches for distant M giants and another mimicking RR Lyra (RRL) projections for the Large Synoptic Survey Telescope (LSST). Our models suggest that colour and proper motion cuts currently used to select M giant candidates for follow-up successfully remove nearly all self-contamination from foreground halo dwarf stars and are useful for focusing observations on distant M giants, of which there are thousands to tens of thousands beyond 100 kpc in our models. We likewise expect that LSST will identify comparable numbers of RRLe at these distances. We demonstrate that several observable properties of both tracers, such as proximity of neighbouring stars, proper motions and distances (for RRLe), could help us separate different accreted dwarf galaxies from one another in the distant MW halo. We also discuss prospects for using ratios of M giants to RRLe as a proxy for accretion time, which in the future could provide new constraints on the recent accretion history of our Galaxy