Philipp Podsiadlowski

Abstract:

We have computed an extensive grid of binary evolution tracks to represent low- and intermediate mass X-ray binaries (LMXBs and IMXBs). The grid includes 42,000 models which covers 60 initial donor masses over the range of 1-4 solar masses and, for each of these, 700 initial orbital periods over the range of 10-250 hours. These results can be applied to understanding LMXBs and IMXBs: those that evolve analogously to CVs; that form ultracompact binaries with orbital periods in the range of 6-50 minutes; and that lead to wide orbits with giant donors. We also investigate the relic binary recycled radio pulsars into which these systems evolve. To evolve the donor stars in this study, we utilized a newly developed stellar evolution code called "MESA" that was designed, among other things, to be able to handle very low-mass and degenerate donors. This first application of the results is aimed at an understanding of the newly discovered pulsar PSR J1614-2230 which has a 1.97 solar masses neutron star, orbital period = 8.7 days, and a companion star of 0.5 solar mass. We show that (i) this system is a cousin to the LMXB Cyg X-2; (ii) for neutron stars of canonical birth mass 1.4 solar masses, the initial donor stars which produce the closest relatives to PSR J1614-2230 have a mass between 3.4-3.8 solar masses; (iii) neutron stars as massive as 1.97 solar masses are not easy to produce in spite of the initially high mass of the donor star, unless they were already born as relatively massive neutron stars; (iv) to successfully produce a system like PSR J1614-2230 requires a minimum initial neutron star mass of at least 1.6+-0.1 solar masses, as well as initial donor masses and orbital period of ~ 4.25+-0.10 solar masses and ~49+-2 hrs, respectively; and (v) the current companion star is largely composed of CO, but should have a surface H abundance of ~10-15%.

Abstract:

(abridged) PSR J1903+0327, a millisecond pulsar in an eccentric (e = 0.44) 95-day orbit with a (~ 1Msun) companion poses a challenge to our understanding of stellar evolution in binary and multiple-star systems. Here we describe optical and radio observations which rule out most of the scenarios proposed to explain formation of this system. Radio timing measurements of three post-Keplerian effects yield the most precise measurement of the mass of a millisecond pulsar to date: 1.667 +/- 0.021 solar masses (99.7% confidence limit) (...). Optical spectroscopy of a proposed main sequence counterpart show that its orbital motion mirrors the pulsar's 95-day orbit; being therefore its binary companion (...) The optical detection also provides a measurement of the systemic radial velocity of the binary; this and the proper motion measured from pulsar timing allow the determination of the systemic 3-D velocity in the Galaxy. We find that the system is always within 270 pc of the plane of the Galaxy, but always more than 3 kpc away from the Galactic centre. Thus an exchange interaction in a dense stellar environment (like a globular cluster or the Galactic centre) is not likely to be the origin of this system. We suggest that after the supernova that formed it, the neutron star was in a tight orbit with a main-sequence star, the present companion was a tertiary farther out. The neutron star then accreted matter from its evolving inner MS companion, forming a millisecond pulsar. The former donor star then disappears, either due to a chaotic 3-body interaction with the outer star (caused by the expansion of the inner orbit that necessarily results from mass transfer), or in the case of a very compact inner system, due to ablation/accretion by the newly formed millisecond pulsar.