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تسجيل مستخدم جديدStar clusters with dual red clumps
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Leo Girardi
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A few star clusters in the Magellanic Clouds exhibit composite structures in the red-clump region of their colour-magnitude diagrams. The most striking case is NGC419 in the SMC, where the red clump is composed of a main blob as well as a distinct secondary feature. This structure is demonstrated to be real and corresponds to the simultaneous presence of stars which passed through electron degeneracy after central-hydrogen exhaustion and those that did not. This rare occurrence in a single cluster allows us to set stringent constraints on its age and on the efficiency of convective-core overshooting during main-sequence evolution. We present a more detailed analysis of NGC419, together with a first look at other populous LMC clusters which are apparently in the same phase: NGC1751, NGC1783, NGC1806, NGC1846, NGC1852 and NGC1917. We also compare these Magellanic Cloud cases with their Galactic counterparts, NGC752 and NGC7789. We emphasise the extraordinary potential of these clusters as absolute calibration marks on the age scale of stellar populations.
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For a general understanding of the physics involved in the star formation process, measurements of physical parameters such as temperature and density are indispensable. The chemical and physical properties of dense clumps of molecular clouds are str
ongly affected by the kinetic temperature. Therefore, this parameter is essential for a better understanding of the interstellar medium. Formaldehyde, a molecule which traces the entire dense molecular gas, appears to be the most reliable tracer to directly measure the gas kinetic temperature.We aim to determine the kinetic temperature with spectral lines from formaldehyde and to compare the results with those obtained from ammonia lines for a large number of massive clumps.Three 218 GHz transitions (JKAKC=303-202, 322-221, and 321-220) of para-H2CO were observed with the 15m James Clerk Maxwell Telescope (JCMT) toward 30 massive clumps of the Galactic disk at various stages of high-mass star formation. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured para-H2CO 322-221/303-202and 321-220/303-202 ratios. The gas kinetic temperatures derived from the para-H2CO (321-220/303-202) line ratios range from 30 to 61 K with an average of 46 K. A comparison of kinetic temperature derived from para-H2CO, NH3, and the dust emission indicates that in many cases para-H2CO traces a similar kinetic temperature to the NH3 (2,2)/(1,1) transitions and the dust associated with the HII regions. Distinctly higher temperatures are probed by para-H2CO in the clumps associated with outflows/shocks. Kinetic temperatures obtained from para-H2CO trace turbulence to a higher degree than NH3 (2,2)/(1,1) in the massive clumps. The non-thermal velocity dispersions of para-H2CO lines are positively correlated with the gas kinetic temperature. The massive clumps are significantly influenced by supersonic non-thermal motions.
Context: Precise chemical abundances coupled with reliable ages are key ingredients to understand the chemical history of our Galaxy. Open Clusters (OCs) are useful for this purpose because they provide ages with good precision. Aims: The aim of th
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