next up previous contents index
Next: ISOCAM Survey of Nearby Up: ORAL TALKS (by order Previous: ISOCAM mapping and spectro-imaging H

Luminosity distribution of young stars

Timo Prusti 

ISO Data Centre, Vilspa, Spain




A luminosity function of a sample of stars depends on three factors: (1) the initial mass function i.e. the relative probability for the creation of stars of different masses at a particular time and location; (2) the star formation rate i.e. the time dependent efficiency of converting interstellar matter into stars at a given location; and (3) the stellar evolution i.e. the luminosity of stars of different masses as a function of time. It is not possible to separate these factors from the observed luminosity function, but one can test the compatibility of a set of hypotheses by making a model luminosity function based on assumptions and by comparing it to the observed one. Clearly, the selected sample of stars limits the choice of realistic assumptions. With a sample of Population II field stars in the solar neighbourhood different astronomical questions can be addressed than with a sample of young stars in a close by molecular cloud. The luminosity distribution of young stars in a selected area depends heavily on the time dependence of the star formation rate or, in other words, of the moment we happen to witness of the full star formation episode of the region.

In order to construct the observed luminosity function of young stars it is essential to obtain infrared observations. There are two reasons for this: (1) due to circumstellar matter a significant (sometimes dominant) fraction of the bolometric luminosity is emitted in the infrared; and (2) due to extinction optical observations provide inventory only of objects at the front side of a star forming cloud. Prior to ISO, the constructed samples had to rely on IRAS and ground based near infrared data. While the longer IRAS wavelengths provide a view practically free of cloud extinction, the sensitivity is not sufficient to address the questions related to very low mass stars. Modern ground based near infrared imaging provides the sensitivity down to the detection level of young brown dwarfs, but in the near infrared the extinction is still not negligible making interpretation very complicated. With the ISO data it is possible to achieve the sensitivity needed for brown dwarfs at wavelengths in which extinction is low enough to require less complex corrections.

The latest ISO results will be reviewed: (1) expansion of the population of known young stars in close by clouds; (2) pushing the completion limit of the luminosity function into the regime of very low mass stars and brown dwarfs; and (3) constraining the evolutionary time scales based on statistical properties of the young star samples.


next up previous contents index
Next: ISOCAM Survey of Nearby Up: ORAL TALKS (by order Previous: ISOCAM mapping and spectro-imaging H
"The Universe as seen by ISO", 20 - 23 October 1998, Paris: Abstract Book