Globulettes : a new class of very small and dense interstellar clouds

The space between stars is not empty, but filled with a thin gas and microscopic dust grains, together forming the so-called interstellar medium. Matter is concentrated into clouds of very different sizes, ranging from giant molecular cloud complexes to massive isolated dark small isolated cloudlets, called globules. In bright emission regions, surrounding young massive stars, one can find many tiny, isolated and cold objects appearing as dark spots against the background nebulosity. These objects are much smaller and less massive than normal globules. Such small clouds are the topic of the present Licentiate thesis, where they have been baptised globulettes. The analysis is based on H-alpha images of the Rosette Nebula and IC 1805 Nebula, collected with the Nordic Optical Telescope in the years 1999 and 2000. In total 151 globulettes in these two regions were catalogued, measured and analysed. Positions, orientations, sizes, masses, densities and pressures were derived, as well as their present condition with regard to gravitational stability. From these data, their origins and possible evolutionary history were discussed. Most globulettes are sharp-edged and well isolated from the surrounding. The size distributions are quite similar in the two studied nebulae. The masses and densities were derived from the extinction of light and the measured shape of the objects. In a few cases the masses have been estimated earlier by another team, from radio emission of CO gas, and our values are in line with their estimates for these particular globulettes. A majority of the objects have masses < 20 Jupiter masses, and the mass distribution drops rapidly towards higher values. Very few objects have masses above 100 Jupiter masses, which we define as the lower mass limit for normal globules. However, there is no smooth overlap between the two types of clouds, which makes us conclude that globulettes represent a distinct, new class of objects. The column density profile of a typical globulette was found to be rather uniform in the central parts, but flattens at the periphery, as compared to what is expected from a sphere of constant volume density. The virial theorem, including only the kinetic and gravitational energy, indicates that all 133 globulettes are expanding or disrupting. However, other forces, such as outer gas and radiation pressures, can help to confine the globulettes. Our results show that about half of these objects are gravitationally bound and even unstable against contraction, which opens some evolutionary scenarios not expected in the first place. Some massive globulettes could therefore collapse to form stars with very low masses, for instance, so-called brown dwarfs, while the low-mass globulettes could contract to free-floating planets. Globulettes might have been formed either by the fragmentation of larger filaments, or by the disintegration of large molecular clouds originally hosting compact and small cores. At a later stage even the confine globulettes might disrupt because of evaporation form the action of external radiation and gas flows. or evaporate. However, preliminary calculations of their lifetimes show that some might survive for a relatively long time and even longer than their estimated contraction time. No evidence of embedded infrared-emitting sources was found in independent IR studies, but one cannot exclude that globulettes already host low-mass brown dwarfs or planets.Godkänd; 2006; 20070109 (haneit)