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At least 271 records · Page 15

Dark matter and the age of globular clusters

The ability of weakly interacting particles called cosmions to suppress convection in the cores of horizontal-branch stars is investigated numerically. It is found that such convection-breaking in horizontal-branch stars by cosmions or by any other novel mode of energy transfer induces thermal relaxation oscillations with a period of about 500,000 yr, corresponding to the core Kelvin-Helmholtz time scale. These thermal pulses can be understood analytically in terms of a simple two-zone model. Observationally, the brightness and brightness dispersion of horizontal-branch stars increases, the periods of RR Lyrae stars change over a pulsation time scale, and the duration of central helium burning slightly decreases. None of these effects is in conflict with observations but, on the contrary, point to a speculative resolution of the age problem for globular clusters and to an alternative explanation of the period fluctuations of RR Lyrae stars.

Dearborn, David↗

Open and Globular Cluster Distances for Extragalactic, Galactic, and Stellar Astrophysics

One of the hallmarks of SIM's few-milliarcsecond astrometric precision is its ability to obtain accurate parallax measurements across more than half of the Galaxy. The "open and globular" project obtains parallax distances to a set of star clusters. One important, goal is to pinpoint the zeropoint of the distance scale for main-sequence fitting. Another goal is to improve stellar evolutionary isochrones and integrated light models. Another goal is to use the clusters themselves to address unsolved problems of late-stage stellar evolution and Galactic and extragalactic chemical evolution. The clusters to be observed are chosen to span the widest possible range of abundance and age, to be as rich as possible, and to be as well-studied as possible.

Worthey, Guy S.↗

Observations of a bright, variable X-ray source in a globular cluster

The source 3U 1820-30 in NGC 6624 has been observed with the OSO-7 satellite. It shows variability of up to a factor of 5 in intensity over days and months and of a factor of 2 in about 5-10 minutes, but there are no evident periodicities. The luminosity of the source is 10 to the 38th ergs per sec (1-10 keV). Arguments are examined which suggest that mass accretion onto a neutron star or black hole provides the most likely mechanism of X-ray production.

Canizares, C. R.↗