5 research outputs found
Surveying the agents of galaxy evolution in the tidally stripped, low metallicity small Magellanic cloud (SAGE-SMC), III: young stellar objects
The Spitzer Space Telescope Legacy Program SAGE-SMC allows global studies of resolved stellar populations in the SMC in a different environment than our Galaxy. Using the SAGE-SMC IRAC (3.6-8.0 mu m) and MIPS (24 and 70 mu m) catalogs and images combined with near-infrared (JHK(s)) and optical (UBVI) data, we identified a population of similar to 1000 intermediate-to high-mass young stellar objects (YSOs) in the SMC (three times more than previously known). Our method of identifying YSO candidates builds on the method developed for the Large Magellanic Cloud by Whitney et al. with improvements based on what we learned from our subsequent studies and techniques described in the literature. We perform (1) color-magnitude cuts based on five color-magnitude diagrams (CMDs), (2) visual inspection of multi-wavelength images, and (3) spectral energy distribution (SED) fitting with YSO models. For each YSO candidate, we use its photometry to calculate a measure of our confidence that the source is not a non-YSO contaminant, but rather a true YSO, based on the source's location in the color-magnitude space with respect to non-YSOs. We use this CMD score and the SED fitting results to define two classes of sources: high-reliability YSO candidates and possible YSO candidates. We found that, due to polycyclic aromatic hydrocarbon emission, about half of our sources have [3.6]-[4.5] and [4.5]-[5.8] colors not predicted by previous YSO models. The YSO candidates are spatially correlated with gas tracers
Three classical Cepheid variable stars in the nuclear bulge of the Milky Way
The nuclear bulge is a region with a radius of about 200 parsecs around the
centre of the Milky Way. It contains stars with ages ranging from a few million
years to over a billion years, yet its star-formation history and the
triggering process for star formation remain to be resolved. Recently, episodic
star formation, powered by changes in the gas content, has been suggested.
Classical Cepheid variable stars have pulsation periods that decrease with
increasing age, so it is possible to probe the star-formation history on the
basis of the distribution of their periods. Here we report the presence of
three classical Cepheids in the nuclear bulge with pulsation periods of
approximately 20 days, within 40 parsecs (projected distance) of the central
black hole. No Cepheids with longer or shorter periods were found. We infer
that there was a period about 25 million years ago, and possibly lasting until
recently, in which star formation increased relative to the period of 30-70
million years ago.Comment: 19 pages, 5 figures, 1 table (including main paper and supplemantary
information