The Pulsar/White Dwarf/Planet System in M4: Improved Astrometry
Authors: H.B. Richer, R. Ibata, G.G. Fahlman, M. Huber
A young and undermassive white dwarf has been identified as the possible companion to the millisecond pulsar PSR B1620-26 in M4.
This association is important since it then helps constrain the mass of the third body in the system to be of order a few times that of
Jupiter. The presence of this planet in M4 has critical implications for planetary formation mechanisms in metal-poor environments
such as globular clusters and the early universe. The identification of the white dwarf is purely via the agreement in position between
it and the pulsar and was limited by the accuracy of the pointing of the H ubble Space T elescope, which is +/-0.7". We have
redetermined the position of the white dwarf using ground-based data tied to the USNO-B1.0 catalog and find that the pulsar and
white dwarf are now coincident to within 0.12"+/-0.13", further strengthening the case for an association between the two. We have
also attempted to improve the proper-motion measurement of the white dwarf by a maximum likelihood analysis of the stellar
positions measured over a baseline of 5 years. While the errors are reduced by almost a factor of 6 from our previous work, we still
have not resolved the cluster's intrinsic dispersion in proper motion. Thus, the proper motion of the white dwarf with respect to the
cluster itself is still not known, although it is very small and is within 2 sigma; of that of the cluster's internal dispersion.