Maximum mass of compact stars from gravitational wave events with finite-temperature equations of state

Armen Sedrakian ( Frankfurt Institute for Advanced Studies)

We conjecture and verify a set of universal relations between global parameters of hot and fast-rotating compact stars, including a relation connecting the masses of the mass-shedding (Kepler) and static configurations. We apply these relations to the GW170817 event by adopting the scenario in which a hypermassive compact star remnant formed in a merger evolves into a supramassive compact star that collapses into a black hole once the stability line for such stars is crossed. We deduce an upper limit on the maximum mass of static, cold neutron stars 2.15+0.100.07MTOV2.24+0.120.10 for the typical range of entropy per baryon 2S/A3 and electron fraction Ye=0.1 characterizing the hot hypermassive star. Our result implies that accounting for the finite temperature of the merger remnant relaxes previously derived constraints on the value of the maximum mass of a cold, static compact star.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

Black hole scalarization

Stoytcho Yazadjiev ( Eberhard Karls University of T ̈ubingen & Sofia University)

In extended scalar-tensor theories, such as
scalar-Gauss-Bonnet gravity, the black holes can undergo spontaneous
scalarization – a strong gravity phase transition triggered by a
tachyonic instability due to the non-minimal coupling between the
scalar field(s) and the spacetime curvature. This very interesting
phenomenon is the only known dynamical mechanism for endowing black
holes (and other compact objects) with scalar hair without altering
the predictions in the weak-field limit. In this talk, I will present
the basic theoretical ideas behind the spontaneous scalarization and
will review some of the recent achievements in the field.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078

Black holes with scalar hair and astrophysical implications

Daniela Doneva ( Eberhard-Karls-Universitat Tuebingen)

Even though the Kerr black hole fits very well in the
interpretation of various astrophysical observations, there is a
number of yet untested modifications of general relativity that can
endow it with hair. The rapid advance of observational astrophysics
gives us the unique opportunity to test the existence of beyond-Kerr
black holes and eventually to constraint the strong-field regime of
gravity. A particular widely studied case is a Kerr-like black hole
endowed with scalar hair that can form for example in the presence of
time-varying complex scalar field or in the more general context of
tensor-multi-scalar theories. We will discuss these solutions and
their astrophysical manifestations. We will put a special emphasis on
the accretion discs around such objects that can have fundamentally
different properties compared to pure GR.

Zoom Meeting ID: 854 3210 3337
Passcode: 959078