Knut A. G. Olsen — Page 3

arXiv2001

A Comprehensive Look at LH72 in the Context of Supergiant Shell LMC-4

Stellar spectroscopy, UBV photometry, H$α$ imaging, and analysis of data from the ATCA \ion{H}{1} survey of the LMC are combined in a study of the LMC OB association LH 72 and its surroundings. LH 72 lies on the rim of a previously identified \ion{H}{1} shell, SGS-14, and in the interior of LMC-4, one of the LMC's largest known supergiant shells. Our analysis of the \ion{H}{1} data finds that SGS-14 is expanding with velocity $v_{exp}\sim15$ km s$^{-1}$, giving it an expansion age of $\sim$15 Myr. Through the stellar spectroscopy and photometry, we find similar ages for the oldest stars of LH 72, $\sim15-$30 Myr. We confirm that LH 72 contains an age spread of $\sim15-$30 Myr, similar to the range in ages of stars derived for the entire surrounding supergiant shell. Combining analysis of the O and B stars with H$α$ imaging of the \ion{H}{2} region DEM 228, we find that DEM 228 accounts for only 60% of the available ionizing Lyman continuum photons. Comparing the distribution of ionized gas with that of the \ion{H}{1}, we find that DEM 228 and LH 72 are offset by $\sim1-2\arcmin$ from the peak 21-cm emission, towards the interior of SGS-14. Taken together, these results imply that SGS-14 has cleared its interior of gas and triggered the formation of LH 72. On the basis of our results, we suggest that LMC-4 was not formed as unit but by overlapping shells such as SGS-14, and that LH 72 will evolve to produce a stellar arc similar to others seen within LMC-4.

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arXiv1999

Star Formation Histories from HST Color-Magnitude Diagrams of Six Fields of the LMC

We present results on the analysis of background field stars found in HST WFPC2 observations of six of the old globular clusters of the Large Magellanic Cloud. Treated as contaminants by the globular cluster analysts, we produce V-I,V color-magnitude diagrams of the field stars and use them to explore the LMC's star formation history. The photometry approaches V~26, well below the turnoff of an ancient ~14 Gyr LMC population of stars. We use a maximum-likelihood technique to fit model CMDs drawn from Bertelli et al. (1994) isochrones to the observed CMDs. We constrain the models by the age-metallicity relation derived from LMC clusters, test four IMF slopes, and fit for the reddening, distance modulus, and star formation rate. We find that we can just resolve structure in SFR(t) with time steps of ~0.15 in log age, implying a resolution of ~4x10^8 years at an age of 1 Gyr. For a Salpeter IMF, our derived star formation history for the NGC 1754 field is characterized by an enhanced star formation rate over the past 4 Gyr, qualitatively resembling that derived by others for a variety of LMC fields. The remaining four fields, however, appear to have had high levels of star formation activity as long as 5-8 Gyr ago; these fields lie in the LMC Bar while the NGC 1754 field lies in the disk, suggesting that the inner regions of the LMC contain significantly more older stars than the outer regions. Examining the residuals of the models and observations, we find that the old red giant branches of the models provide a poor fit to the observations, which suggests an error in the model isochrones. The effect of the disagreement appears to be to underestimate the contribution of the old population.

astro-ph↗