Rosette Globulettes and Shells in the Infrared

Abstract

Tiny, dense clumps of sub-solar mass called globulettes form in giant galactic HII regions. The young central clusters compress the surrounding molecular shells which break up into clumps, filaments, and elephant trunks that interact with UV light from the central OB stars. We study the nature of the infrared emission and extinction in the shell and globulettes in the Rosette Nebula (RN) and search for associated newborn stars. We imaged the northwestern quadrant of the RN in the near-infrared (NIR) through JHKs and narrow-band H2 1-0 S(1), Pbeta and continuum filters. NIR images were used to study the surface brightness of the globulettes and associated bright rims. NIR photometry was used to create an extinction map and to search for NIR excess objects. Archival images from Spitzer IRAC and MIPS 24 and Herschel PACS observations were used to further study the region and its stellar population and to examine the structure of the shell and trunks. The globulettes and elephant trunks have bright rims in the Ks band on the sides facing the central cluster. Analysis of 21 globulettes where surface brightness in the H2 1-0 S(1) line is detected shows that about a third of the surface brightness observed in Ks is due to this line: the observed average of the H2/Ks surface brightness is 0.26+-0.02 in the globulettes cores and 0.30+-0.01 in the rims. The estimated H2 1-0 S(1) surface brightness of the rims is 3-8*10^{-8} Wm^{-2}sr^{-1}um^{-1}. The H2/Ks surface brightness ratio supports fluorescence as the H2 excitation mechanism. The globulettes have number densities of n(H2)~10^{-4} cm^{-3} or higher. We confirm the results from previous optical and CO surveys that the larger globulettes contain very dense cores and dense envelopes, and that their masses are sub-solar. Two NIR protostellar objects were found in an elephant trunk and one in the most massive globulette in our study. (abridged)