THE END OF THE MACHO ERA, REVISITED: NEW LIMITS ON MACHO MASSES FROM HALO WIDE BINARIES

Abstract

In order to determine an upper bound for the mass of the massive compact halo objects (MACHOs), we use the halo binaries contained in a recent catalog by Allen & Monroy-Rodríguez. To dynamically model their interactions with massive perturbers, a Monte Carlo simulation is conducted, using an impulsive approximation method and assuming a galactic halo constituted by massive particles of a characteristic mass. The results of such simulations are compared with several subsamples of our improved catalog of candidate halo wide binaries. In accordance with Quinn et al., we also find our results to be very sensitive to the widest binaries. However, our larger sample, together with the fact that we can obtain galactic orbits for 150 of our systems, allows a more reliable estimate of the maximum MACHO mass than that obtained previously. If we employ the entire sample of 211 candidate halo stars we, obtain an upper limit of 112 M ☉ . However, using the 150 binaries in our catalog with computed galactic orbits, we are able to refine our fitting criteria. Thus, for the 100 most halo-like binaries we obtain a maximum MACHO mass of 21–68 M ☉ . Furthermore, we can estimate the dynamical effects of the galactic disk using binary samples that spend progressively shorter times within the disk. By extrapolating the limits obtained for our most reliable—albeit smallest—sample, we find that as the time spent within the disk tends to zero, the upper bound of the MACHO mass tends to less than 5 M ☉ . The non-uniform density of the halo has also been taken into account, but the limit obtained, less than 5 M ☉ , does not differ much from the previous one. Together with microlensing studies that provide lower limits on the MACHO mass, our results essentially exclude the existence of such objects in the galactic halo.