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Professor Adrienne Cool has discovered 24 unusual stars in an ancient star cluster in the Milky Way. Made of helium rather than the usual carbon and oxygen, these white dwarf stars appear as faint, pale blue dots as spotted in new Hubble telescope images.

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Title: Helium-Core White Dwarfs in the Globular Cluster NGC 6397
Authors: R. R. Strickler, A. C. Cool, J. Anderson, H. N. Cohn, P. M. Lugger, A. M. Serenelli

We present results of a study of the central regions of NGC 6397 using Hubble Space Telescope's Advanced Camera for Surveys, focusing on a group of 24 faint blue stars that form a sequence parallel to, but brighter than, the more populated sequence of carbon-oxygen white dwarfs (CO WDs). Using F625W, F435W, and F658N filters with the Wide Field Channel we show that these stars, 18 of which are newly discovered, have magnitudes and colours consistent with those of helium-core white dwarfs (He WDs) with masses ~ 0.2-0.3 Msun. Their H-alpha--R625 colours indicate that they have strong H-alpha absorption lines, which distinguishes them from cataclysmic variables in the cluster. The radial distribution of the He WDs is significantly more concentrated to the cluster center than that of either the CO WDs or the turnoff stars and most closely resembles that of the cluster's blue stragglers. Binary companions are required to explain the implied dynamical masses. We show that the companions cannot be main-sequence stars and are most likely heavy CO WDs. The number and photometric masses of the observed He WDs can be understood if ~1-5% of the main-sequence stars within the half-mass radius of the cluster have white dwarf companions with orbital periods in the range ~1-20 days at the time they reach the turnoff. In contrast to the CO WD sequence, the He WD sequence comes to an end at R625 ~ 24.5, well above the magnitude limit of the observations. We explore the significance of this finding in the context of thick vs. thin hydrogen envelope models and compare our results to existing theoretical predictions. In addition, we find strong evidence that the vast majority of the CO WDs in NGC 6397 down to Teff ~ 10,000 K are of the DA class. Finally, we use the CO WD sequence to measure a distance to the cluster of 2.34 0.13 kpc.

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Twenty-four unusual stars, 18 of them newly discovered, have been observed in new Hubble telescope images. The stars are white dwarfs, a common type of dead star, but they are odd because they are made of helium rather than the usual carbon and oxygen. This is the first extensive sequence of helium-core white dwarfs to be observed in a globular cluster, a dense swarm of some of the oldest stars in our galaxy.
A new study suggests that these helium-core white dwarfs have had their lives cut short because of their orbital dance around a partner star.

"Helium-core white dwarfs have only about half the mass of typical white dwarfs, but they are found concentrated in the center of the cluster" - Adrienne Cool, professor of physics and astronomy at San Francisco State University, who co-authored the study with graduate student Rachel R. Strickler.

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