Understanding the Anomalous Hot Stellar Population in Galactic Globular Clusters

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Hst Proposal Id #8718 Stellar Populations

Scientific paper

We propose to image in mid- {F255W} and near-UV {F336W} the globular clusters NGC5986, NGC6273, NGC6388, and NGC6441. During our HST survey we found that these objects have a horizontal branch {HB} with an anomalously extended blue tail {EBT} and puzzling gaps along it. The survey F439W and F555W data indicate that the EBTs of the proposed targets extend up to T_eff>=40000K, implying that the hottest HB stars have lost almost all their envelope. The EBTs represent the most extreme of the mixed bag of anomalies known as ``second parameter problem'', and likely hold the key for understanding the factors affecting the HB morphology. The UV data, coupled with the existing optical images, will allow us: 1} To isolate the HB stars from other evolutionary sequences, enabling the measurement of the fraction of cluster stars in the EBTs. 2} Measure the main physical parameters {as mass and temperature} of the EBT stars. 3} Locate the gaps on the EBTs and measure their statistical significance 4} to provide tests for the origin of the EBT stars. Also the blue stragglers {BS} are better isolated in UV, allowing us to check if there is any relation between the presence of EBTs and the fraction of BS. Understanding the structure of the EBT stars, and why they are found in some, but not all systems will help to understand the stellar population of distant galaxies, and in particular the intringuing UV-upturn phonomenon in ellipticals.

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