Welcome to the homepage of the Ultracool Subdwarfs splinter session for the Cool Stars 15 conference, held July 21-25, 2008 in St. Andrews, Scotland. The objective of this session was to provide a forum for discussing current research on metal-poor and halo members of the latest-type M, L and T dwarf classes.

The two hour splinter session consisted of seven 12 minute talks (+ 5 minutes for questions) presenting recent research results. The session was organized into topics on Discoveries, Classification and Fundamental Parameters. Abstracts for the presentations are given below, along with links to PDF versions of some of the presentations. You can also now download the submitted conference proceedings (PDF file, 0.3 Mb).

NEW! Conference Proceedings (PDF 0.3 Mb)

Discoveries

Ralf-Dieter Scholz (Potsdam)
An Extremely Wide and Very Low-Mass Common Proper Motion Pair - Representatives of a Nearby Halo Stream?

[PDF Presentation (8.8 Mb)]

In a high proper motion survey of the southern sky we used multi-epoch positions and photometry from the SuperCOSMOS Sky Surveys. New nearby ultracool dwarf and subdwarf candidates were selected among the faint and red high proper motion objects, and subsequently confirmed by low-resolution classification spectroscopy. A late-type (M7) dwarf and an ultracool subdwarf (sdM7) sharing the same very large proper motion of about 860 mas/yr were found in the same sky region with an angular separation of about 6 degrees. Their proper motions were further improved by involving additional epochs from the Two Micron All Sky Survey and from the DEep Near-Infrared Survey. From the individually estimated spectroscopic distances of about 38 pc and 72 pc, respectively for the M7 dwarf and the sdM7 subdwarf, and in view of the accurate agreement in their large proper motions we assume a common distance of about 50 pc and a projected physical separation of about 5 pc. The mean heliocentric space velocity of the pair (U,V,W)=(-232,-170,+74) km/s is typical of the Galactic halo population. It relies on a preliminary radial velocity measurement and on the assumption of a common distance and velocity vector. The large separation and the different metallicities of dwarfs and subdwarfs make a common formation scenario as a wide binary (later disrupted) improbable. It seems more likely that this wide pair is part of an old halo stream. We suggest a few other candidate stream members identified among known Solar neighbourhood stars with similarly large velocity components.

Philippe Delorme (Grenoble)
The first T subdwarf candidate from Canada-France Brown Dwarfs Survey

We present the latest results of the Canada France Brown Dwarf Survey concerning subdwarfs. CFDBS is an i'-z' wide field search for ultracool brown dwarf which uses the MegaCam wide-field camera on the Canada-France-Hawaii Telescope(CFHT). We present here several L dwarfs whose extreme photometry points to sub-solar metallicity and also one T subdwarf candidate. We obtained a complete near-infrared spectrum of the last object which exhibits strong evidence for low metallicity. This hypothesis is strengthened by its very high proper motion that suggests halo membership. If confirmed, this would be one of the first T subdwarfs ever identified.

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Classification

Jao Wei-Chun (Georgia State)
A Spectral Sequence of K- and M-type Subdwarfs

Using new spectra of 88 K and M-type subdwarfs, we consider novel methods for assigning their spectral types and take steps toward developing a comprehensive spectral sequence for subdwarf types K3.0 to M6.0. The types are assigned based on the overall morphology of spectra covering 6000A to 9000 A. The types and sequence presented link the spectral types of cool subdwarfs to their main sequence counterparts, with emphasis on the relatively opacity-free region from 8200-9000 A. When available, supporting abundance, kinematic, and trigonometric parallax information is used to provide more complete portraits of the observed subdwarfs. We find that the CaHn (n = 1-3) and TiO5 indices often used for subdwarf spectral typing are affected in complicated ways by combinations of subdwarfs' temperatures, metallicities, and gravities, and we use model grids to evaluate the trends in all three parameters. Because of the complex interplay of these three characteristics, it is not possible to identify a star as an "extreme" subdwarf simply based on very low metallicity, and we suggest that the modifiers "extreme" or "ultra" only outline locations on spectroscopic indices plots, and should not be used to imply low or very low metallicity stars. In addition, we propose that "VI" be used to identify a star as a subdwarf, rather than the confusing "sd" prefix, which is also used for hot O and B subdwarfs that are unrelated to the cool subdwarfs discussed in this paper.

Sebastien Lepine (American Museum of Natural History)
Spectroscopic sequences of cool and ultra-cool subdwarfs (sdM/esdM/usdM) from the Sloan Digital Sky Survey

[PDF Presentation (2.0 Mb)]

The spectroscopic database of the Sloan Digital Sky Survey is found to be teeming with spectra from low-mass, metal poor stars (cool and ultra-cool subdwarfs). We use the spectra of 4700 confirmed late-K and M subdwarfs to establish classification sequences for each of the three proposed subclasses (subdwarfs - "sd", extreme subdwarfs - "esd", and ultrasubdwarfs - "usd"). Templates covering the 4000 A - 9000 A wavelength range are generated for each subclass, for spectroscopic subtypes K5 to M8.5 with a 0.5 subtype resolution. We argue that, to first order, the two-parameter classification system (subclass+subtype) represents a mass-metallicity grid. The possibility of having "third parameter" effects that could be due to, e.g., gravity or alpha-element abundance, are investigated.

Adam Burgasser (MIT)
L Subdwarfs: Classification, Distance Scale and Low-Metallicity Condensate Formation

[PDF Presentation (1.3 Mb)]

L subdwarfs are metal-poor, halo population objects with low masses extending down to the (metallicity-dependent) hydrogen fusing minimum mass limit. First identified in 2003, there are now six L subdwarfs known, enabling initial examinations of their properties as a population. In this talk I will briefly describe first attempts at determining a red optical spectral classification scheme for L subdwarfs, which tracks with the existing L dwarf scheme of Kirkpatrick et al. and naturally extends off the end of the M subdwarf sequence. In addition, I report the first astrometric parallax measurement of an L subdwarf, the sdL7 2MASS 0532+8246, which provides a first look into absolute magnitude, luminosity and temperature scales at the end of the metal-poor main sequence. This measurement also constrains the wavelength-dependent surface brightnesses of 2MASS 0532+8246, critical for understanding chemistry and condensate formation in cool, metal-poor atmospheres.

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Fundamental Parameters

Brandon Swift (U. Arizona)
Theoretical Modeling of L Subdwarf Spectra

We present fits of atmospheric models of varying metallicity to the published optical and near-infrared spectrum and IRAC photometry of the mid-L subdwarf 2MASS J05325346+8246465. This object is the best-observed member of a growing population of what are suspected to be metal-poor ultracool dwarfs, and the only one with a parallax distance. The model selection is made using a goodness-of-fit statistic in conjunction with Monte Carlo simulated data to account for uncertainties in the absolute flux; this statistic is then examined in various bands to study systematic issues in the models and to produce a concordance fit for the properties of this object in Teff, log g, and [M/H] space.

Michael Cushing (U. Hawaii)
Astrometric Observations of LSR 1610-0040

[PDF Presentation (1.2 Mb)]

Since its discovery by Lepine and collaborators, LSR 1610-0040 has defied explanation. Its red optical spectrum and high proper motion suggest that LSR 1610 is an early-type L subdwarf yet its near-infrared spectrum indicates that it is a mid-type dM or sdM, albeit with numerous peculiar spectral features. I present new trgionometric parallax astrometry and BVI photometry of LSR 1610 carried out by C. Dahn and collaborators at the U.S. Naval Observatory. Although its position in color-color and color-magnitude diagrams is unremarkable (its positions are roughly consistent with it being mildly metal poor), it does have a remarkably red B-V color of ~3.3. Most interesting, however, is that the astrometry of LSR 1610 indicates that it is a binary with an eccentricity of ~0.4 and a semi-major axis of 0.3 A.U. I will discuss how this new information alters our understanding of LSR 1610 and speculate on how such an interesting object formed.

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Additional Information

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