The tiny globulettes in the Carina nebula ^⋆,⋆⋆

¹ Applied Physics, Department of Engineering Sciences & MathematicsLuleå University of Technology, 971 87 Luleå, Sweden
e-mail: tiia.grenman@gmail.com
² Stockholm Observatory, AlbaNova University Centre, Stockholm University, 106 91 Stockholm, Sweden

Received: 29 January 2014
Accepted: 18 March 2014

Abstract

Context. Small molecular cloudlets are abundant in many H ii regions surrounding newborn stellar clusters. In optical images these so-called globulettes appear as dark silhouettes against the bright nebular background.

Aims. We aim to make an inventory of the population of globulettes in the Carina nebula complex, and to derive sizes and masses for comparisons with similar objects found in other H ii regions.

Methods. The globulettes were identified from Hα images collected at the Hubble Space Telescope.

Results. We have located close to 300 globulettes in the Carina complex, more than in any other region surveyed so far. The objects appear as well-confined dense clumps and, as a rule, lack thinner envelopes and tails. Objects with bright rims are in the minority, but more abundant than in other regions surveyed. Some globulettes are slightly elongated with their major axes oriented in the direction of young clusters in the complex. Many objects are quite isolated and reside at projected distances >1.5 pc from other molecular structures in the neighbourhood. No globulette coincides in position with recognized pre-main-sequence objects in the area. The objects are systematically much smaller, less massive, and much denser than those surveyed in other H ii regions. Practically all globulettes are of planetary mass, and most have masses less than one Jupiter mass. The average number densities exceed 10⁵ cm^-3 in several objects. We have found a statistical relation between density and radius (mass) in the sense that the smallest objects are also the densest.

Conclusions. The population of small globulettes in Carina appears to represent a more advanced evolutionary state than those investigated in other H ii regions. The objects are subject to erosion in the intense radiation field, which would lead to a removal of any thinner envelope and an unveiling of the core, which becomes more compact with time. We discuss the possibility that the core may become gravitationally unstable, in which case free-floating planetary mass objects can form.