Like black holes, Buchdahl stars are the most compact non-BH objects

Buchdahl star is the most compact object without an event horizon and is an excellent candidate for a black hole mimicker. It is defined by, while black hole by. Here is the gravitational potential experienced by a particle, radially falling for static and axially for the rotating object. In this talk, we examine the question of extremalization of the Buchdhal star and show that the same result holds good as for the black hole. That is, a non-extremal Buchdahl star cannot be extremalized by test particle accretion. Further since extremal limit for BS is > 1, it could facilitate formation of extremal black holes by neutral and spinless accretion. That is perhaps the only way they could be formed. Unlike black holes, rotating Buchdahl star can be over-extremal with respect to the black hole, sustaining a larger spin. We show that it can also develop an ergosphere above the threshold spin, which allows extraction of its rotational energy. Electromagnetic field around Buchdahl star is also expected to differ from that of black hole in both strength and topology. In this paper, we explore the energetics of Buchdahl star focusing on the magnetic Penrose process in the two magnetic field configurations, i.e., uniform and dipole. Below the threshold spin, Buchdahl star is expected to be quiet, while above the threshold it can be much more efficient than the black hole if a dipolar magnetic field is developed on its surface.