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D. R. Alves (Columbia), K. H. Cook (LLNL)
The first Hubble Space Telescope images of a massive compact halo object, MACHO-LMC-5, have revealed an M4-5 dwarf lens with standardized (Cousins) colors of (V-R) = 1.60 and (R-I) = 1.58. A complete solution of the microlensing event parameters yielded an absolute magnitude of MV = 16.2 ±0.6 for the lens. The lens also appears to rotate with the Galactic disk. These results are described by Alcock et al.~(Nature, in press). We present a color-color diagram showing that the LMC-5 lens is redder than the standard sequence of old disk dwarfs by about 0.15 mag in (V-R), similar to the extreme subdwarfs (esdM) of Gizis (1997, AJ, 113, 806). We also show that the lens lies on the extension of the color-magnitude relation for esdMs (Gizis & Reid 1999, AJ, 117, 508). If the LMC-5 lens is type esdM, it has a low metallicity like the spheroid ([Fe/H] ~-1.6 dex), yet it rotates with the disk. Such stars are rare. In fact, only about 1/3 of nearby stars with this [Fe/H] are associated with the metal-weak thick disk (MWTD), instead of the spheroid (Chiba & Beers 2000, AJ, 119, 2843). We estimate that microlensing by a MWTD star should be about 200 times less likely than microlensing by a thin disk star, and thus to have observed an MWTD lens first is surprising. Finally, we note that the mass of the LMC-5 lens appears to lie below the hydrogen-burning limit (Alcock et al., ibid.), while the masses of extreme subdwarfs are not known.
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This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.