The Most Luminous Protostars in Molecular Clouds: A Hint to Understand the Stellar Initial Mass Function

¹ Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan
² Department of Earth and Life Sciences, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
³ Department Humanity and Environment, Hosei University, Chiyoda-ku, Tokyo 102-8160, Japan
⁴ Institute of Astrophysics and Planetary Science, Ibaraki University, Mito 310-8512, Japan
⁵ IAS, Bât. 121, Campus d'Orsay, 91405 Orsay Cedex, France

Abstract

The maximum luminosity of protostars forming in molecular clouds is investigated as a function of the parent cloud mass on the basis of a rich cloud sample searched for in the literature. In total, we gathered 499 molecular clouds among the published data, out of which 243 clouds are found to be associated with candidates for protostars selected from the IRAS point source catalog. A diagram of the maximum stellar luminosity in each cloud and the parent cloud mass shows that the young stars in the clouds associated with regions are apparently more luminous than those in clouds away from H_II regions over the entire cloud mass range investigated (1 < M_CL/M_⊙ < 10⁶). In addition, we found that there are well-defined upper and lower limits in the maximum stellar luminosity distribution with the lower limit having a steeper dependence on the cloud mass (L_MAX ∝ M_CL^1.5) than the upper one (L_MAX ∝ M_CL^0.8). All these features could be naturally accounted for if we assume that the luminosity function of protostars is controlled by the cloud mass and the external pressure imposed on the cloud surface. We introduce a simple model for the stellar luminosity function as a function of these quantities.