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ultracool subdwarfs ( ucsds ) are metal - deficient , very low mass stars and brown dwarfs with late spectral types .
they are the metal - poor analogs of ultracool dwarfs ( spectral types m7 and later ) and represent the low effective temperature ( t@xmath0 @xmath1 3000  k ; @xcite ) extensions of the m subdwarf ( sdm ) and extreme subdwarf ( esdm ) classes @xcite .
cool and ultracool subdwarfs typically exhibit halo kinematics , and were presumably formed early in the galaxy s history .
these low mass objects are important tracers of galactic structure and chemical enrichment history and are representatives of the first generations of star formation .
ucsds encompass the new spectral class of l subdwarfs ( sdl ; @xcite ) and metal - poor t dwarfs ( e.g. , @xcite ) , reaching masses below the hydrogen burning minimum mass .
this contribution provides an update on the state of ucsd research since a first review was made at cool stars 13 by @xcite . here
i focus on observed spectral properties ( @xmath22 ) , new discoveries ( @xmath23 ) and considerations for spectral classification ( @xmath24 ) , and then briefly touch on other issues under investigation ( @xmath25 ) .
like ultracool dwarfs , ucsds exhibit complex optical and near - infrared spectral energy distributions dominated by strong , overlapping molecular absorption bands ; numerous neutral metal line features ; and red optical spectral continua ( see  @xcite ) .
they are distinguished spectroscopically by signatures of metal deficiency , notably enhanced metal hydride and weakened metal oxide absorption bands @xcite , and blue near - infrared colors resulting from collision induced h@xmath3 absorption @xcite .
these spectral peculiarities are more pronounced in lower metallicity subdwarfs ( figure  1 ) .
the reduction in tio and vo opacity at optical wavelengths allows for the emergence of several weaker lines not generally seen in solar metallicity m dwarfs , including ca  i , ca  ii , rb  i , and ti  i. the smooth continuum opacity of h@xmath3 results in damped h@xmath3o and co bands at near - infrared wavelengths , so the latest - type and most metal - poor ucsds exhibit bland spectral energy distributions longward of 1.5 @xmath4 ( * ? ? ?
* ; * ? ? ?
* see figure 1 ) .
@xcite have recently presented high resolution spectra for ucsds , providing the first rotational velocity and accurate radial velocity measurements for these sources .
the high @xmath5 = 65@xmath615  km / s of the l subdwarf 2mass  j0532 + 8246 @xcite suggests that the lowest mass halo stars may not lose their angular momentum as efficiently as more massive stars .
they also find that one of the first l subdwarfs to be identified , lsr  1610 - 0040 @xcite , has mixed m dwarf and subdwarf spectral features , a conclusion also reached by @xcite on the basis of near - infrared spectroscopy .
this source appears to be peculiar , even among the small number of ucsds now known .
@xcite provides a compendium of 16 currently known ucsds , an increase of 60% since cool stars 13 .
this increase has been driven by new proper motion search programs using red optical photographic plate surveys ( e.g. , superblink , @xcite ; and the supercosmos sky survey [ sss ] , @xcite ) and serendipitous discoveries in near - infrared catalogs such as 2mass .
new discoveries include two ultracool extreme subdwarfs @xcite , tripling the number of known ultracool esdms ; and a new l subdwarf , sdss 1256 - 0224 , reported by @xcite .
while red optical proper motion surveys have been successful , the spectral energy distributions of cooler ucsds peak around 1  @xmath4 ( a balance of reduced t@xmath0 and increased absorption by h@xmath3 ; see figure  1 ) , so near - infrared measurements become increasingly useful .
indeed , the ukidss survey with its 1  @xmath4 @xmath7-band should be highly sensitive to ucsds @xcite .
the recent inception of near - infrared proper motion surveys ( e.g.  @xcite ; j.  d.  kirkpatrick , 2007 , in prep . ) will likely expand the known ucsd population substantially in the near term .
spectral types for ucsds are currently based on extrapolations of schemes developed for earlier - type m subdwarfs @xcite , based on tio and cah bands in the 62007300   region .
these features become inadequate in the ucsd regime as tio and cah bands decline due to condensation ( e.g.  @xcite ) ; wide , pressure - broadened na  i and k  i lines suppress adjacent features @xcite ; and the sources themselves become exceedingly faint at optical wavelengths . @xcite and @xcite have suggested the use of the same red optical features used to classify l dwarfs as diagnostics for ucsd classification , and the direct comparison of l subdwarf spectra to l dwarf spectral standards ( see figure  2 ) . at near - infrared wavelengths , useful diagnostics are largely limited to the @xmath8-band region ( figure  1 ) , and work is progressing in this direction ( burgasser et al . , in prep . ) .    an issue to consider in the classification of ucsds
is the division of metallicity classes .
strong , blanketing molecular bands results in greater spectral sensitivity to metallicity and chemistry effects .
the current population of ucsds may already span a fairly broad range of metallicities , as suggested by comparisons to atmospheric models ( * ? ? ?
* see figure  2 above ) ; and the metallicities of early - type sdms may not ( and perhaps need not ) correspond to those of late - type sdms and sdls . in addition , even slightly metal - poor ( [ m / h ] @xmath9 -0.5 ) `` mild '' ucsds are spectrally distinct ( figure  1 ) , and a `` d / sd '' designation for these objects has been suggested .
this group would encompass slightly metal - poor t dwarfs , distinguished by depressed @xmath10-band peaks ( due to h@xmath3 absorption ) and enhanced @xmath7-band peaks ( possibly related to the red wing of the k  i doublet at 0.77  @xmath4 ; @xcite ) ; and several recent discoveries of so - called `` blue '' l dwarfs @xcite that may also be metal - poor .
i briefly summarize other issues that have arisen in recent studies of ucsds .
the interested reader is encouraged to examine the literature for further discussion .    _
dust formation : _ one of the defining properties of l field dwarfs is the formation of condensate dust in their photospheres , driving the depletion of gaseous tio , vo , cah , al  i , ca  i and ti  i in the photosphere @xcite .
however , several late - type subdwarfs exhibit features from these species when it is absent in their dwarf counterparts ( * ? ? ?
* ; * ? ? ?
* ; * ? ? ?
* see figure  2 above ) .
their persistence suggests that condensate formation may be inhibited in metal - poor atmospheres ; chemical modeling of this effect is needed .
_ spectral modeling : _ attempts to derive physical properties for ucsds based on spectral model fits have met with mixed results ( e.g.  @xcite ) .
the difficulties stem from the same issues that arise when modeling ultracool dwarf spectra ; e.g. , complex molecular opacities , and chemistry and condensate grain formation .
there are new concerted efforts to develop better metal - poor atmosphere models in the ucsd regime ( @xcite ; schweitzer , hauschildt & wawrzyn , these proceedings ) . _
binaries : _ binary systems are useful laboratories for studying detailed properties , testing formation theories and finding low temeprature sources as companions ( see splinter session overview by h.  bouy ) . there have been a couple of studies targeting m subdwarfs for companions ( * ? ? ?
* gelino & kirkpatrick , these proceedings ) with as yet no reported ucsd discoveries .
_ distance measurements : _ these are perhaps the most crucial observations needed , as only two ucsds have reported parallaxes .
such measurements make possible the construction of absolute magnitude , luminosity and t@xmath0 scales , and facilitate comparisons to spectral and structural models .
i would like to thank a.  burrows , j.  d.  kirkpatrick , r .- d .
scholz , and a.  schweitzer for discussions in the final preparation of this contribution , and r .- d .
scholz for providing an electronic version of figure  2 .
burgasser , a.  j. , kirkpatrick , j.  d. , & lpine , s. 2005 in the 13th cambridge workshop on cool stars , stellar systems , and the sun ( esa - sp-560 ) , ed .
f.  favata , g.  a.  j.  hussain & b.  battrick ( noordwijk : esa ) , p.  237
in the past few years , astronomers have uncovered several very low - temperature , metal - poor stars with halo or thick disk kinematics and peculiar spectral and photometric properties , so - called ultracool subdwarfs . 
 these include the first examples of l subdwarfs - metal - poor analogs of the l dwarf spectral class - and slightly metal - deficient t dwarfs . 
 ultracool subdwarfs provide useful empirical tests of low temperature atmosphere and evolutionary models , and are probes of the halo mass function down to and below the ( metal - dependent ) hydrogen burning limit . here 
 i summarize the optical and near - infrared spectroscopic properties of these objects , review recent research results , and point out scientific issues of interest in this developing subject .