Is some of the missing mass in brown dwarf stars or similar sized objects?

This is also the name of a collaboration that is looking for such objects, using a medium size telescope in Australia.

A French collaboration doing the same thing is called EROS.

The MACHO group has looked for MACHOS passing in front of stars in the Large Magellanic Cloud and the Small Magellanic Cloud, and also in front of stars in the galactic bulge.

The data is gathered with the aid of a ccd camera and analyzed by a computer.

The MACHO group must be able to tell the MACHO microlensing signature from a variable star so they have databases containing light curves in two colors for 8 million stars in the LMC and 10 million in the bulge of the Milky Way.

Here is the star whose light was lensed in one of their events.
Here is the light curve for that event.

The MACHO group rules out the hypothesis that 20% or more of the halo mass is made of objects with mass less than about 0.03 M_sun. (As long as the mass of these objects is at least 0.000001 M_sun.

But they do find enough heavier objects to support the idea that a substantial fraction of the missing mass could be in objects with mass heavier than 0.1 M_sun (but lighter than 1 M_sun).

My interpretation is that small stars could account for part of the missing mass problem, but not all.

From a press release of the MACHO group, January 1996:

An international team of scientists has concluded that MACHOs (Massive Compact Halo Objects) may comprise approximately 50 percent of the dark matter in the halo of the Milky Way Galaxy, and that the objects likely are "white dwarfs" - burned-out stars. ... In fact, only three "microlensing" events were observed the first year, too few to support the hypothesis that the bulk of the dark matter is made up on brown dwarfs. Microlensing is the term given to the apparent brightening of a star caused by the gravitational field of a massive object passing almost directly in front of it. The brightening effect is caused by the bending of light rays by gravity. An additional year of data and improved analyses allowed the MACHO team to extend the sensitivity of their dark matter search down to objects less massive than the earth and up to objects as massive as the Sun (0.000001-1.0 solar mass). The improved analysis yielded a total 7 microlensing events from both years that appear to be caused by more massive objects, most likely between 0.1 and 1.0 solar mass. Calculating a hypothetical percentage of dark matter in the halo of the Milky Way is a function of both the number of objects detected and the mass of those objects. Thus, even though the number of microlensing events observed in the first year and second year remained roughly constant, the estimate of dark matter in the halo was increased to approximately 50 percent because the seven events seen in the second year analysis represented - on average - more massive objects than the events seen in the first year analysis.