The Sch\"{o}nberg-Chandrasekhar limit in presence of small anisotropy and modified gravity

TL;DR

This paper revisits the Sch"{o}nberg-Chandrasekhar limit for stellar cores, incorporating small anisotropies and modified gravity, deriving a new formula that alters the classical quadratic dependence.

Contribution

It introduces a three-parameter formula for the limit considering anisotropy and modified gravity, challenging the traditional quadratic dependence in standard models.

Findings

Derived a master formula for the limit with anisotropy and modified gravity.

Found the classical limit is better fitted by a linear polynomial in 1/α.

Discussed physical implications of the modified limit.

Abstract

The Sch\"{o}nberg-Chandrasekhar limit in post main sequence evolution for stars of masses in the range $1.4\lesssim M/M_{\odot}\lesssim 6$ gives the maximum pressure that the stellar core can withstand, once the central hydrogen is exhausted. It is usually expressed as a quadratic function of $1/\alpha$, with $\alpha$ being the ratio of the mean molecular weight of the core to that of the envelope. Here, we revisit this limit in scenarios where the pressure balance equation in the stellar interior may be modified, and in the presence of small stellar pressure anisotropy, that might arise due to several physical phenomena. Using numerical analysis, we derive a three parameter dependent master formula for the limit, and discuss various physical consequences. As a byproduct, in a limiting case of our formula, we find that in the standard Newtonian framework, the Sch\"{o}nberg-Chandrasekhar limit is best fitted by a polynomial that is linear, rather than quadratic, to lowest order in $1/\alpha$.