The runaway binary LP 400-22 is leaving the Galaxy

We present optical spectroscopy, astrometry, radio and X-ray observations of the runaway binary LP 400-22. We refine the orbital parameters of the system based on our new radial velocity observations. Our parallax data indicate that LP 400-22 is significantly more distant (3σ lower limit of 840pc) than initially predicted. LP 400-22 has a tangential velocity in excessof 830 km s-1; it is unbound to the Galaxy. Our radio and X-ray observations fail to detect a recycled millisecond pulsar companion, indicating that LP 400-22 is a double white dwarf system. This essentially rules out a supernova runaway ejection mechanism. Based on its orbit, a Galactic Centre origin is also unlikely. However, its orbit intersects the locations of several globular clusters; dynamical interactions between LP 400-22 and other binary stars or a central black hole in a dense cluster could explain the origin of this unusual binary.Instituto de Astrofísica de La Plat

The runaway binary LP 400-22 is leaving the Galaxy

We present optical spectroscopy, astrometry, radio and X-ray observations of the runaway binary LP 400-22. We refine the orbital parameters of the system based on our new radial velocity observations. Our parallax data indicate that LP 400-22 is significantly more distant (3σ lower limit of 840pc) than initially predicted. LP 400-22 has a tangential velocity in excessof 830 km s-1; it is unbound to the Galaxy. Our radio and X-ray observations fail to detect a recycled millisecond pulsar companion, indicating that LP 400-22 is a double white dwarf system. This essentially rules out a supernova runaway ejection mechanism. Based on its orbit, a Galactic Centre origin is also unlikely. However, its orbit intersects the locations of several globular clusters; dynamical interactions between LP 400-22 and other binary stars or a central black hole in a dense cluster could explain the origin of this unusual binary.Instituto de Astrofísica de La Plat

The initial mass function : from Salpeter 1955 to 2005

Fifty years after Ed Salpeter's seminal paper, tremendous progress both on the observational and theoretical sides allow a fairly accurate determination of the Galactic IMF not only down to the hydrogen-burning limit but into the brown dwarf domain. The present review includes the most recent observations of low-mass stars and brown dwarfs to determine this IMF and the related Galactic mass budget. The IMF definitely exhibits a similar behaviour in various environments, disk, young and globular clusters, spheroid. Small scale dissipation of large scale compressible MHD turbulence seems to be the underlying triggering mechanism for star formation. Modern simulations of compressible MHD turbulence yield an IMF consistent with the one derived from observations.Comment: 10 pages. Invited review "IMF@50" in honor of E. Salpete