Is the MACHO-LMC-5 Microlens an Extreme Subdwarf?

Details Is the MACHO-LMC-5 Microlens an Extreme Subdwarf? Is the MACHO-LMC-5 Microlens an Extreme Subdwarf?

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aas...199.1613a&link_type=abstract

American Astronomical Society, 199th AAS Meeting, #16.13; Bulletin of the American Astronomical Society, Vol. 33, p.1334

Astronomy and Astrophysics

Astronomy

2

Scientific paper

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. 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.

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