On the nature of the bimodal initial velocity distribution of neutron stars

¹ INFN Sezione di Pisa, 56127 Pisa, Italy
² Università di Pisa, Dipartimento di Fisica, via F. Buonarroti 2, 56127 Pisa, Italy e-mail: bombaci@df.unipi.it
³ Università di Padova, Dipartimento di Fisica, via Marzolo 8, 35131, Padova, Italy
⁴ Sternberg Astronomical Institute, Universitetski pr. 13, 119992 Moscow, Russia e-mail: polar@sai.msu.ru

Received: 22 April 2004
Accepted: 12 May 2004

Abstract

We propose that the bimodal nature of the kick velocity distribution of radio pulsars is connected to the dichotomy between hadronic stars (i.e. neutron stars with no quark matter content) and quark stars. Bimodality can appear due to different mechanisms of explosion which lead to the formation of two types of compact stars or due to two different sets of parameters driving a particular kick mechanism. The low velocity maximum (at ~100 km s^-1) is associated with hadronic star formation, whereas the second peak corresponds to quark stars. In the model of delayed collapse of hadronic stars to quark stars [CITE] quark deconfinement leads to a second energy release, and to a second kick, in addition to the kick imparted to the newly formed hadronic star during the supernova explosion. If the electromagnetic rocket mechanism can give a significant contribution to pulsar kicks, then the high velocity peak can be associated with the shorter initial spin periods of quark stars with respect to hadronic stars. We discuss these scenarios.