Observations of the companion to the pulsar PSR B1718-19

The role of tidal circularisation

¹ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands e-mail: T.Janssen@phys.uu.nl
² Department of Astronomy and Astrophysics, University of Toronto, 60 St George Street, Toronto, Ontario M5S 3H8, Canada e-mail: mhvk@astro.utoronto.ca

Received: 3 January 2005
Accepted: 6 April 2005

Abstract

We present optical and infrared observations taken with the Very Large Telescope of the eclipsing binary pulsar system PSR B1718-19. The candidate companion of the pulsar, identified earlier in Hubble Space Telescope observations, has been detected in all three bands, R, I, and J. These detections allowed us to derive constraints on temperature, radius, and mass, pointing to a companion that has expanded to a radius between one of a main sequence star and one at the Roche-limit. We focus on the role of tidal circularisation in the system, which will have transformed the initially eccentric orbit expected from formation scenarios into the nearly circular orbit presently observed. Based on simple energy balance arguments, we are able to draw a picture of the companion's evolution resulting from the energy deposition in the star due to circularisation. In this picture, our measurement of the companion's parameters is consistent with the expected initial eccentricity. However, with the present understanding of tidal dissipation, it remains difficult to account for the short time in which the system was circularised.