Brown dwarfs were initially discovered in open star clusters and in the case of Gl229b as a companion to a nearby star (Nakajima et al. 1995). See Jameson & Hodgkin (1997) for a recent review. More recently numerous L dwarfs have been discovered by the 2MASS and DENIS surveys. L dwarfs are cooler than M dwarfs (T~2200K) and may be either very low mass stars or young brown dwarfs. More T dwarfs, objects like Gl229b, with methane in their atmospheres, have also been found by SDSS. T dwarfs have temperatures T<1300K, and must be brown dwarfs.
The LAS will be 4 magnitudes deeper than 2MASS/DENIS and will thus survey a volume 250 times greater, finding a correspondingly larger number of L/T dwarfs. This will greatly improve the statistics of the L/T dwarf luminosity/mass functions. More importantly by probing 40 times fainter, the LAS will identify intrinsically much fainter sources than 2MASS/DENIS are capable of. Table 2.1 shows how far away brown dwarfs/planets can be seen in the LAS survey as a function of object mass and age, using J-band absolute magnitude predictions models from the models of Burrows et al.
| mass 5M(Jup) | mass 10M(Jup) | mass 40M(Jup) | |
|---|---|---|---|
| t=0.5 Gyr | 7.6pc | 26pc | 171pc |
| t=1.0 Gyr | 2.3pc | 14pc | 101pc |
| t=5.0 Gyr | 0.1pc | 1.8pc | 32pc |
Table 2.1. Distance limit for brown dwarf detections as a function of mass and age, assuming a limit of J=20. Absolute magnitudes taken from the models of Burrows et al.
The LAS will also measure proper motions since these give a handle on the kinematics and hence rough ages for the sources. Ages are vital for determining the masses of isolated brown dwarfs (although the best determination of the mass functions, which require reliable ages, will come from the UKIDSS cluster survey).
The nearest faintest objects. Due to H2, H20, CH4 and NH3 opacities the BDs, and even free floating planets, have their maximum surface brightness in the J band. This surface brightness does not drop too drastically until T(eff)<400K, and all these objects have R~0.1R(sun). As a result, the LAS, probing a large area to J=20, can detect objects that are much cooler than any T dwarf yet discovered (see Table 2.1). The LAS is for example capable of finding objects such as: a free floating planet of age 200 Myr and M=4M(Jup); or a Population II brown dwarf of age 13 Gyr and M=30 M(Jup).
Such very small objects may well be common but so far undiscovered in the solar neighbourhood. Free floating planets M~5-10M(Jup) have been discovered in the sigma-Ori and theta-Ori clusters. Zapatero-Osorio et al. (2000) find the IMF has a slope of -0.8 in the sigma-Ori cluster. Anchoring this to the low mass star MF in the solar neighbourhood (Tinney 1995) suggests 0.03 planets per pc3 in the mass range 5 to 15M(Jup). The LAS could therefore find >10 free floating planets. The identification of such systems in our local neighbourhood would generate immense public interest.
Population II Brown Dwarfs. An important element of star formation theory is the lowest mass object star or brown dwarf that can form. The answer to this question for Population I stars should come from studying open clusters and field objects (see above). For Population II objects it is difficult to use clusters since the nearest globular cluster has a distance of 2.4 kpc so the only hope of finding Pop II brown dwarfs would be to find nearby halo brown dwarfs. Using the ratio of halo to disc stars and assuming a population II mass function slope of -1.0 would predict the LAS survey to find about ten 80M(Jup) brown dwarfs and possibly one of mass 30M(Jup). These would be recognised by their large proper motions. Such a discovery would be of great importance to the issues of the universality of the IMF, and the formation history of the Milky Way.
Very cool white dwarfs with He atmospheres. White dwarfs with pure He atmospheres are known to cool very rapidly since their atmospheres are very transparent. Assuming T=2500K, R=0.01R(sun) and a black body spectrum gives M(Jup)=17. Clearly these could be found by the LAS if they exist. He-rich WDs account for about 50% of the coolest known WDs (Bergeron at al. 2000). Unlike H-rich WDs, He-rich WDs are thought to be red in all colours. However their future cooling rates are uncertain so it is difficult to predict the numbers that the LAS might find.