Optical Observations of the Binary Pulsar System PSR B1718−19: Implications for Tidal Circularization

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Abstract
We report on Keck and Hubble Space Telescope optical observations of the eclipsing binary pulsar system PSR B1718-19, in the direction of the globular cluster NGC 6342. These reveal a faint star (mF702W = 25.21 ± 0.07; Vega system) within the pulsar's 05 radius positional error circle. This may be the companion. If it is a main-sequence star in the cluster, it has radius RC 0.3 R☉, temperature Teff 3600 K, and mass MC 0.3 M☉. In many formation models, however, the pulsar (spun-up by accretion or newly formed) and its companion are initially in an eccentric orbit. If so, for tidal circularization to have produced the present-day highly circular orbit, a large stellar radius is required, i.e., the star must be bloated. Using constraints on the radius and temperature from the Roche and Hayashi limits, we infer from our observations that RC 0.44 R☉ and Teff 3300 K. Even for the largest radii, the required efficiency of tidal dissipation is larger than expected for some prescriptions.
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