NEW SURVEY OF PLEIADES SUGGESTS THAT BROWN DWARFS ARE RARE
CAMBRIDGE, MA -- The new survey of the Pleiades open star cluster suggests that brown dwarfs, often cited as a missing link between planets and stars, may not be the long-sought repository of dark matter in the Universe.
Indeed, as John R. Stauffer of the Harvard-Smithsonian Center for Astrophysics reported today at the American Astronomical Society meeting, it is more likely that brown dwarfs are relatively rare and thus do not contribute significantly to the mass of the Universe.
Brown dwarfs seemed very plausible candidates for the "missing mass." Since they are expected to be extremely faint, a large number could exist without ever being detected except by their gravitational influence. Several efforts have been made in recent years to search for brown dwarfs and assess their part in explanations of the missing mass.
Young open clusters of stars, such as the Pleiades, were attractive targets for brown dwarf searches because theoretical models predict that brown dwarfs should be warmest and brightest -- and most easily detected -- when they are young. Also, because young open clusters have formed large number of low-mass stars, it seemed plausible they should have formed a significant number of brown dwarfs.
Because of its youth (70 million years), proximity (400 light years from Earth), and richness (> 500 known stellar members), the Pleiades cluster has been the most popular target. Unfortunately, results of various surveys have been contradictory. Two groups reported detecting more than two dozen brown dwarf candidates after surveying only very small regionswithin the cluster, a result that implies brown dwarfs are both plentiful and a significant fraction of the Pleiades' mass. On the other hand, two other groups have also identified a number of candidate Pleiades brown dwarfs, but their surveys covered a much larger portion of the cluster, and they thus concluded that brown dwarfs are relatively rare.
In an attempt to reconcile these disparate results, a team of astronomers led by Dr. Stauffer reanalyzed all of the Pleiades brown dwarf surveys.
Brown dwarfs have a specific definition - they are objects that do not have sufficient mass to stably burn hydrogen in their cores, which according to theoretical models means objects with masses less than about 0.075 solar masses. The difficulty with attempting to identify objects as brown dwarfs in open clusters is that the surveys do not directly measure mass but instead they measure the brightness and color of sources. Empirical relations are then used to convert the observed quantities to stellar luminosities and temperatures and then those luminosities and temperatures are compared to theoretical model calculations to determine the predicted mass of the object at the age of the Pleiades. However, the empirical conversion between color and stellar surface temperature is a subject of current controversy and also different theoretical models predict differing luminosities and temperatures for a given object mass and age. Therefore, it has been possible for one group to identify an object as a plausible brown dwarf candidate in the Pleiades whereas another group would identify it as simply a low mass star.
To improve this situation, Stauffer and his colleagues compared all existing Pleiades brown dwarf candidates to the same theoretical model, using a uniform conversion from the observational plane (brightness and color) to the theoretical plane (luminosity and surface temperature). The result of this systematic analysis showed that only a small percentage of the objects suggested as brown dwarf members of the Pleiades were actually good candidates. The majority of these objects proved more likely to be either stellar members of the Pleiades (i.e. with masses greater than 0.075 solar masses), or not members of the Pleiades at all, but rather low-mass field stars seen in projection towards the Pleiades.
Oral Presentation 51.09, 184th Meeting of the American Astronomical Society, Wednesday, June 1, 1994. "A Conservative Appraisal of Brown Dwarf Candidates in the Pleiades," John Stauffer, Harvard-Smithsonian Center for Astrophysics.