A far-infrared millimeter-wave study of star formation in the Magellanic Clouds.

摘要

Stars form primarily in giant molecular clouds; therefore, the conditions within these clouds must be understood in any study of star formation. I present far-infrared and carbon monoxide (CO) line emission data from star-forming giant molecular clouds located in two nearby dwarf galaxies: the Large Magellanic Cloud (LMC), and the Small Magellanic Cloud (SMC). These galaxies differ considerably from the Milky Way in heavy element content, dust to gas ratio, and interstellar radiation field strength. The effects of these variations on star formation in 21 clouds in the LMC and 9 in the SMC are investigated. The CO data are used to delineate the molecular clouds and determine their masses using the virial theorem. Far-infrared data from the IRAS and COBE satellites are used to measure the cloud temperatures and luminosities, and to provide an estimate of the stellar content of each molecular cloud.; The 30 clouds range in mass from {dollar}rm2.7times10sp4 Msb{lcub}odot{rcub}{dollar} to {dollar}rm1.0times10sp6 Msb{lcub}odot{rcub},{dollar} and from {dollar}rm1.7times10sp4 Lsb{lcub}odot{rcub}{dollar} to {dollar}rm2.8times10sp6 Lsb{lcub}odot{rcub}{dollar} in far-infrared luminosity. Dust temperatures calculated from the 60 {dollar}mu{dollar}m and 100 {dollar}mu{dollar}m IRAS data are between 15 and 43 K with some dependence on the assumed dust emissivity. External heating due to the interstellar radiation field is estimated to account for anywhere from 2% to 100% of the far-infrared emission from each cloud. The brightest cloud contains from 1-3 O4 stars, the least bright from 6 B0 to 1 O8 stars. The clouds exhibit a wide range of star formation activity, as measured by the ratio of far-infrared luminosity to mass, with little correlation between this quantity and cloud mass. The star formation activity of both the LMC and SMC clouds is similar in range and magnitude to that of inner and outer Milky Way molecular clouds, in spite of a difference of up to an order of magnitude in the heavy element abundance, dust to gas ratio, and radiation field strength between the galaxies. The lack of dependence on these environmental properties has implications for several theories of star formation.