John J. Bochanski

Constraining the Age-Activity Relation for Cool Stars: The Sloan Digital Sky Survey Data Release 5 Low-Mass Star Spectroscopic Sample

We present a spectroscopic analysis of over 38,000 low-mass stars from the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Analysis of this unprecedentedly large sample confirms the previously detected decrease in the fraction of magnetically active stars (as traced by Hα emission) as a function of the vertical distance from the Galactic plane. The magnitude and slope of this effect vary as a function of spectral type. Using simple 1D dynamical models, we demonstrate that the drop in activity fraction can be explained by thin-disk dynamical heating and a rapid decrease in magnetic activity. The timescale for this rapid activity decrease changes according to the spectral type. By comparing our data to the simulations, we calibrate the age-activity relation at each M dwarf spectral type. We also present evidence for a possible decrease in the metallicity as a function of height above the Galactic plane. In addition to our activity analysis, we provide line measurements, molecular band indices, colors, radial velocities, 3D space motions, and mean properties as a function of spectral type for the SDSS DR5 low-mass star sample.

Stellar SEDs from 0.3 to 2.5 μm: Tracing the Stellar Locus and Searching for Color Outliers in the SDSS and 2MASS

The Sloan Digital Sky Survey (SDSS) and Two Micron All Sky Survey (2MASS) are rich resources for studying stellar astrophysics and the structure and formation history of the Galaxy. As new surveys and instruments adopt similar filter sets, it is increasingly important to understand the properties of the ugrizJHKs stellar locus, both to inform studies of ‘normal’ main sequence stars as well as for robust searches for point sources with unusual colors. Using a sample of � 600,000 point sources detected by SDSS and 2MASS, we tabulate the position and width of the ugrizJHKs stellar locus as a function of g i color, and provide accurate polynomial fits. We map the Morgan-Keenan spectral type sequence to the median stellar locus by using synthetic photometry of spectral standards and by analyzing 3000 SDSS stellar spectra with a custom spectral typing pipeline, described in full in an attached Appendix. Having characterized the properties of ‘normal’ main sequence stars, we develop an algorithm for identifying point sources whose colors differ significantly from those of normal stars. This algorithm calculates a point source’s minimum separation from the stellar locus in a seven-dimensional color space, and robustly identifies objects with unusual colors, as well as spurious SDSS/2MASS matches. Analysis of a final catalog of 2117 color outliers identifies 370 white-dwarf/M dwarf (WDMD) pairs, 93 QSOs, and 90 M giant/carbon star candidates, and demonstrates that WDMD pairs and QSOs can be distinguished on the basis of their J Ks and r z colors. We also identify a group of objects with correlated offsets in the u g vs. g r and g r vs. r i color-color spaces, but subsequent follow-up is required to reveal the nature of these objects. Future applications of this algorithm to a matched SDSS-UKIDSS catalog may well identify additional classes of objects with unusual colors by probing new areas of color-magnitude space. Subject headings: surveys — stars:late-type — stars:early-type — Galaxy:stellar content — infrared:stars

The First Hundred Brown Dwarfs Discovered by the Wide-field Infrared Survey Explorer (WISE)

We present ground-based spectroscopic verification of 6 Y dwarfs (see also Cushing et al.), 89 T dwarfs, 8 L dwarfs, and 1 M dwarf identified by the Wide-field Infrared Survey Explorer (WISE). Eighty of these are cold brown dwarfs with spectral types ≥T6, six of which have been announced earlier by Mainzer et al. and Burgasser et al. We present color-color and color-type diagrams showing the locus of M, L, T, and Y dwarfs in WISE color space. Near-infrared and, in a few cases, optical spectra are presented for these discoveries. Near-infrared classifications as late as early Y are presented and objects with peculiar spectra are discussed. Using these new discoveries, we are also able to extend the optical T dwarf classification scheme from T8 to T9. After deriving an absolute WISE 4.6 μm (W2) magnitude versus spectral type relation, we estimate spectrophotometric distances to our discoveries. We also use available astrometric measurements to provide preliminary trigonometric parallaxes to four of our discoveries, which have types of L9 pec (red), T8, T9, and Y0; all of these lie within 10 pc of the Sun. The Y0 dwarf, WISE 1541–2250, is the closest at 2.8^(+1.3)_(–0.6) pc; if this 2.8 pc value persists after continued monitoring, WISE 1541–2250 will become the seventh closest stellar system to the Sun. Another 10 objects, with types between T6 and >Y0, have spectrophotometric distance estimates also placing them within 10 pc. The closest of these, the T6 dwarf WISE 1506+7027, is believed to fall at a distance of ~4.9 pc. WISE multi-epoch positions supplemented with positional info primarily from the Spitzer/Infrared Array Camera allow us to calculate proper motions and tangential velocities for roughly one-half of the new discoveries. This work represents the first step by WISE to complete a full-sky, volume-limited census of late-T and Y dwarfs. Using early results from this census, we present preliminary, lower limits to the space density of these objects and discuss constraints on both the functional form of the mass function and the low-mass limit of star formation.

Low-Mass Dwarf Template Spectra from the Sloan Digital Sky Survey

We present template spectra of low-mass (M0-L0) dwarfs derived from over 4000 Sloan Digital Sky Survey spectra. These composite spectra are suitable for use as medium-resolution (R ~ 1800) radial velocity standards. We report mean spectral properties (molecular band-head strengths, equivalent widths) and use the templates to investigate the effects of magnetic activity and metallicity on the spectroscopic and photometric properties of low-mass stars.

THE LUMINOSITY AND MASS FUNCTIONS OF LOW-MASS STARS IN THE GALACTIC DISK. II. THE FIELD

We report on new measurements of the luminosity function (LF) and mass function (MF) of field low-mass dwarfs derived from Sloan Digital Sky Survey Data Release 6 photometry. The analysis incorporates ~15 million low-mass stars (0.1 ), spread over 8400 deg2. Stellar distances are estimated using new photometric parallax relations, constructed from ugriz photometry of nearby low-mass stars with trigonometric parallaxes. We use a technique that simultaneously measures Galactic structure and the stellar LF from 7 < Mr < 16. We compare the LF to previous studies and convert to an MF using the mass-luminosity relations of Delfosse et?al. The system MF, measured over ?1.0< log <?0.1, is well described by a lognormal distribution with = 0.25 . We stress that our results should not be extrapolated to other mass regimes. Our work generally agrees with prior low-mass stellar MFs and places strong constraints on future theoretical star formation studies.

Cataclysmic Variables from the Sloan Digital Sky Survey. II. The Second Year

The first full year of operation following the commissioning year of the Sloan Digital Sky Survey has revealed a wide variety of newly discovered cataclysmic variables. We show the SDSS spectra of forty-two cataclysmic variables observed in 2002, of which thirty-five are new classifications, four are known dwarf novae (CT Hya, RZ Leo, T Leo and BZ UMa), one is a known CV identified from a previous quasar survey (Aqr1) and two are known ROSAT or FIRST discovered CVs (RX J09445+0357, FIRST J102347.6+003841). The SDSS positions, colors and spectra of all forty-two systems are presented. In addition, the results of follow-up studies of several of these objects identify the orbital periods, velocity curves and polarization that provide the system geometry and accretion properties. While most of the SDSS discovered systems are faint (>18th mag) with low accretion rates (as implied from their spectral characteristics), there are also a few bright objects which may have escaped previous surveys due to changes in the mass transfer rate.The first full year of operation following the commissioning year of the Sloan Digital Sky Survey (SDSS) has revealed a wide variety of newly discovered cataclysmic variables (CVs). We show the SDSS spectra of 42 CVs observed in 2002, of which 35 are new classifications, four are known dwarf novae (CT Hya, RZ Leo, T Leo, and BZ UMa), one is a known CV identified from a previous quasar survey (Aqr1), and two are known ROSAT or FIRST discovered CVs (RX J09445+0357, FIRST J102347.6+003841). The SDSS positions, colors, and spectra of all 42 systems are presented. In addition, the results of follow-up studies of several of these objects identify the orbital periods, velocity curves, and polarization that provide the system geometry and accretion properties. While most of the SDSS discovered systems are faint (greater than 18th magnitude) with low accretion rates (as implied from their spectral characteristics), there are also a few bright objects that may have escaped previous surveys due to changes in the mass transfer rate.

Cataclysmic Variables from Sloan Digital Sky Survey. V. The Fifth Year (2004)

This paper identifies the cataclysmic variables that appear in spectra obtained in 2004 as part of the Sloan Digital Sky Survey. Spectra of 41 objects, including seven systems that were previously known (CC Cnc, DW Cnc, PQ Gem,AR UMa,AN UMa,RXJ1131.3+4322,andUMa6)and34new cataclysmicvariablesarepresented.The positions and ugriz photometry of all 41 systems are given, as well as additional follow-up spectroscopic, photometric, and/or polarimetric observations of eight of the new systems. The new objects include three eclipsing systems, six with prominent He ii emission, and six systems that show the underlying white dwarf.

Clouds in the Coldest Brown Dwarfs: FIRE Spectroscopy of Ross 458C

Condensate clouds are a salient feature of L dwarf atmospheres, but have been assumed to play little role in shaping the spectra of the coldest T-type brown dwarfs. Here we report evidence of condensate opacity in the near-infrared spectrum of the brown dwarf candidate Ross 458C, obtained with the Folded-Port Infrared Echellette (FIRE) spectrograph at the Magellan Telescopes. These data verify the low-temperature nature of this source, indicating a T8 spectral classification, log10 L bol/L ? = ?5.62 ? 0.03, T eff = 650 ? 25 K, and a mass at or below the deuterium burning limit. The data also reveal enhanced emission at the K band associated with youth (low surface gravity) and supersolar metallicity, reflecting the properties of the Ross 458 system (age = 150-800?Myr, [Fe/H] = +0.2 to +0.3). We present fits of FIRE data for Ross 458C, the T9 dwarf ULAS J133553.45+113005.2, and the blue T7.5 dwarf SDSS J141624.08+134826.7B, to cloudless and cloudy spectral models from Saumon & Marley. For Ross 458C, we confirm a low surface gravity and supersolar metallicity, while the temperature differs depending on the presence (635+25 ?35 K) or absence (760+70 ?45 K) of cloud extinction. ULAS J1335+1130 and SDSS J1416+1348B have similar temperatures (595+25 ?45 K), but distinct surface gravities (log g = 4.0-4.5 cgs versus 5.0-5.5 cgs) and metallicities ([M/H] +0.2 versus ?0.2). In all three cases, cloudy models provide better fits to the spectral data, significantly so for Ross 458C. These results indicate that clouds are an important opacity source in the spectra of young cold T?dwarfs and should be considered when characterizing planetary-mass objects in young clusters and directly imaged exoplanets. The characteristics of Ross 458C suggest that it could itself be regarded as a planet, albeit one whose cosmogony does not conform with current planet formation theories.

A Catalog of Spectroscopically Selected Close Binary Systems from the Sloan Digital Sky Survey Data Release Four

We present a spectroscopic sample of 747 detached close binary systems from the Sloan Digital Sky Survey (SDSS) Fourth Data Release. The majority of these binaries consist of a white dwarf primary and a low-mass secondary (typically M dwarf) companion. We have determined the temperature and gravity for 496 of the white dwarf primaries and the spectral type and magnetic activity properties for 661 of the low-mass secondaries. We have estimated the distances for each of the white dwarf-main-sequence star binaries and use white dwarf evolutionary grids to establish the age of each binary system from the white dwarf cooling times. With respect to a spectroscopically identified sample of ~8000 isolated M dwarf stars in the SDSS, the M dwarf secondaries show enhanced activity with a higher active fraction at a given spectral type. The white dwarf temperatures and gravities are similar to the distribution of ~1900 DA white dwarfs from the SDSS. The ages of the binaries in this study range from ~0.5 Myr to nearly 3 Gyr (average age ~0.20 Gyr).

THE BROWN DWARF KINEMATICS PROJECT. 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 Hα activity in the secondary but not the primary making it a useful tracer of age/mass/activity trends. Two of the nine systems have discrepant component ages that emerge from stellar or ultracool diagnostics indicating possible shortcomings in our understanding of the age diagnostics of stars and brown dwarfs. 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 pc 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 (0.2 M_☉ < M_(tot)< 1.0 M_☉) multiples can form and survive to exist in the field (1-8 Gyr).