Formation of Ultracompact X Ray Binaries 8/31/05, J ames Lombardi (Vassar College) Vassar: Zach Proulx '07 Kate Dooley '06 Eric Theriault '06 Northwestern: Natasha Ivanova, Fred Rasio '85

Brief Background Ultracompact X ray binaries (UCXBs) have orbital periods P ~10 36 erg/s probably a neutron star accreting from a white dwarf can be formed by colliding a subgiant or red giant with a neutron star

Motivation Normalized number of physical collisions  shown for a star of mass 0.9 M  in a globular cluster. Time axis runs from zero age main sequence to end of red giant branch. Solid curve shows stellar radius R (left axis). Symbols show  (right axis) for these two different initial times..

Sample Visualization ­ ­ Visualization of sample collision between a 1.4 M  neutron star and a 0.9 M  red giant with a core of mass 0.23 M  : – –r p = 3.8R , e o = 1r p = 3.8R , e o = 1

Final parameters for 32 collisions Common Envelope Mass  Neutron Star Mass  Semimajor axis  Eccentricity  Periastron separation of initial orbit 

How long does it take for these binaries to become UCXBs? The gravitational radiation orbital decay time  gw depends on the post-collision semimajor axis a and eccentricity e. Dashed curves are for constant  gw, as labeled, for a 1.4 M  neutron star and a 0.25 M  white dwarf. The orbital period P values on the right axis are for these same masses. The hatched area shows how the  gw = yr curve changes when we vary the binary masses by ~0.1 M . The area of data point is indicative of collision likelyhood.

Concluding Remark Colliding a neutron star into a subgiant or small red giant is a good way to make a highly eccentric UCXB with a helium white dwarf donor.