Self-Similarity in the Rosette Molecular Cloud

Meredith Hughes
Mentor: Jonathan Williams

The Rosette Molecular cloud is a star-forming region of gas and dust in the Milky Way which exhibits a highly complex structure with regions of varying density and temperature. The purpose of studying the Rosette nebula was to attempt to better understand the process of star formation. Infrared observations show that stars are forming in the cloud, and much is known about how stars form (through the collapse of gravitationally bound matter). The initial mass function of stars, that is, the number of stars that form at a given mass, is also well-determined. However, the processes connecting the conditions in a molecular cloud to the observed properties of young stars are not well understood. We would like to know more about the conditions which make star formation favorable, for example, how massive star-forming regions must be, whether we can observe star formation being influenced by temperature, density perturbations, etc. We want to know why some regions form stars while others don't. We would also like to be able to connect conditions in molecular clouds with the observed properties of young stars, especially the initial mass function.

Our data consisted of two maps of the region, one a 3-D data cube mapping emission from the ¹³CO molecule in position and velocity space, and one a 2-D map of ¹²CO emission over the surface of the cloud. Each data set consisted of 50,000 spectra at a resolution of 50" (0.3 pc) and extended over 3 degrees (60 pc). The two data sets were complementary, since the emission line of the 1 —> 0 rotational transition of the ¹³CO molecule is optically thin, which allows us to trace the location of mass in the cloud, and the ¹²CO emission line is optically thick, which gives a measurement of surface temperature. These pieces of information, when combined, allowed us to determine the masses of various regions in the cloud. We approached this study of the Rosette molecular cloud by dividing the nebula into discrete clumps of emission and studying the physical properties of these clumps, including their size, mass, linewidth, temperature, and other quantities associated with the clump like aspect ratio, degree of virialization, and distance from the HII region (an area of violent star formation activity in the cloud). We then looked for and investigated various trends in the data, and studied the effects of smoothing the map over various ranges to better understand how the cloud is structured over different size scales and how the resolution of observations affects the results obtained. We also located IRAS point sources within the cloud whose colors matched those of regions of recent star formation in an attempt to better understand why the regions associated with these sources should be more likely to form stars.

¹²CO peak temperature map of the Rosette molecular cloud. The cross marks the center of the Rosette nebula and the molecular gas forms a large ring around this. Embedded star forming regions within the cloud, detected at infrared wavelengths are shown by the stars. We studied the structure in the cloud at different scales and the properties of the star forming regions.

Several interesting trends emerged from our analysis of the clumps of material in the Rosette nebula. Density remained roughly constant with the size of the clump, for example, which was also true at all scales of the data. The size-linewidth relation, a well-studied trend in molecular clouds that displays the degree to which the kinematics of the cloud are interconnected on various scales, exhibited a large degree of scatter and a different power law than that generally measured for molecular clouds . This power law relationship remained constant over all scales of the nebula. The influence of the HII region was evident in the temperature of the clumps, but seemed to have little effect on other aspects of the cloud. There was a slight trend for objects near the HII region to be smaller and more dense, which is consistent with expectations for the effects of an expanding bubble of gas, but there was no detectable effect on degree of virialization, which would tend to promote or inhibit star formation. Alpha, the virialization parameter, scaled approximately as M-2/3 over all size scales of the cloud, which is consistent with predictions made by Bertoldi and McKee (1992) for pressure-confined clumps. There was a tendency for the measured alpha to increase with smoothing scale, indicating that size scales are important for virialization. This was supported by the investigation of clumps associated with IRAS sources: those clumps tended to be towards the high-mass, low-alpha end of the spectrum, although most still had α > 1. This indicates that they are gravitationally unbound, contrary to expectations for a star-forming region. This, along with the fact that our ¹³CO observations, sensitive to large-scale properties of the cloud, are unable to reveal differences between these regions, suggests that perhaps the ranges over which differences in star forming and non-star forming regions occur are at smaller scales than 50" (0.3 pc), the resolution of our data. Other than the trend for alpha to increase with smoothing, however, the cloud was remarkably self-similar over a range of size scales, indicating the presence of fractal structure.