Abstract
Anyone seeking to understand the physical processes of star formation must make
reference to the spectrum of masses (mass function) produced by such processes. The
low mass end of this mass function in particular is poorly constrained.
The two leading environments in which to measure the mass function are in open
clusters and star forming regions and in the solar neighbourhood. Studies of some star
forming regions claim to constrain the mass function down to a few tens of Jupiter
masses. However the validity of results in young clusters and star forming regions has
been called into question, with the accuracy of the evolutionary models that all studies
are based on being considered debatable at young ages. Hence a study in the field,
where there is a spread of ages, provides a useful, possibly more robust, m ethod for
measuring the mass function.
Low mass stars and brown dwarfs in the field are usually identified by either infrared
surveys or by proper motion surveys. Here these techniques are combined for the first
time to produce an infrared proper m otion survey. This has produced a sample of over
7000 low mass objects with proper motions greater than 0.1” /y r. It has also found
several com m on proper motion binary systems and SIPS1259-4336, an M8.5 dwarf
within lOpc.
This sample on its own will not yield a constraint on the mass function. In the field
the range of ages and the luminosity evolution of objects leads to the mass function
being intertwined with the stellar birthrate. Here this problem is dealt with by detailed
simulations. By drawing ages from a birthrate and masses from a mass function,
simulated objects can have space positions, velocities and absolute magnitudes assigned
using a simple model of the Galaxy, evolutionary models of low mass objects and
empirical relations. These can then be converted into observables and passed through a survey selection mechanism. By varying the underlying birthrate and mass function
the effect these have 011 the survey results can be found. These results can then be
compared with those of the actual survey to constrain the mass function and birthrate.
Here the mass function parameter a is found to be 0.66 ± 0 .4 4 in the range O.2M⨀ >
m > O.O75M⨀ , that the birthrate parameter ß is —0.01 ±0.10 and that the space density
of objects in the mass range O.O9M⨀ < m < O.1M⨀ is 0.0053 ± 0.0002/pc³. It should
be noted that due to noise in the probability surface, the constraint on ß should be
treated with caution.
Finally the results of the future infrared surveys by UKIDSS are simulated and the
potential constraints that could be set by them outlined.
