There are currently two open questions in white dwarf physics: why are massive dwarfs observed less often in astronomical surveys, and why have not any super-Chandrasekhar white dwarfs been found despite the discovery of more than a dozen peculiar, overly-luminous type Ia supernovae in about a couple of decades? According to different research, magnetic fields appear to somewhat resolve these issues, but stability remains a concern. For the first time, we investigate how modified gravity affects the specific heat of electrons and ions, the crystallization process, and the cooling mechanism in white dwarfs. We demonstrate it for the Ricci-based gravity. We show that massive white dwarfs fade faster and conclude that it could be a physical reason, apart from the presence of high magnetic fields, both for finding fewer massive white dwarfs and the lack of direct detection of super-Chandrasekhar white dwarfs.

Spain

Aneta Wojnar

South Africa

Surajit Kalita

India

Lupamudra Sarmah

Effects of modified gravity on microscopic properties and cooling timescale of white dwarfs

We investigate the application of an equation of state that incorporates corrections derived from the Snyder model (and the Generalized Uncertainty Principle) to describe the behaviour of matter in a low-mass star. Remarkably, the resulting equations exhibit striking similarities to those arising from modified Einstein gravity theories. By modeling matter with realistic considerations, we are able to more effectively constrain the theory parameters, surpassing the limitations of existing astrophysical bounds. The bound we obtain is  $$\beta _0 \le 4.5 \times 10^{47}$$  <math> <mrow> <msub> <mi>β</mi> <mn>0</mn> </msub> <mo>≤</mo> <mn>4.5</mn> <mo>×</mo> <msup> <mn>10</mn> <mn>47</mn> </msup> </mrow> </math>  . We underline the significance of realistic matter modeling in order to enhance our understanding of effects arising in quantum gravity phenomenology and implications of quantum gravitational corrections in astrophysical systems.

Norway

Anna Pachoł

Constraining Snyder and GUP models with low-mass stars

In this work, the early evolution of low-mass fully convective stars is studied in the context of DHOST (degenerate higher order scalar-tensor) theories of gravity. Although it is known that the hydrostatic equilibrium equation is modified for scalar-tensor gravity, the consequent modifications to the early evolution phases of a star were not explored in this framework. With this in mind, we consider three evolutionary phases—contraction to the main sequence, lithium burning and entrance to the main sequence—and investigate how each of these phases is affected by the theory’s parameter. Taking these effects into account, we are able to show, among other things, that the Hayashi tracks are shifted and the star’s age is considerably modified.

Estonia

Débora Gomes

Early evolution of fully convective stars in scalar–tensor gravity

We study three aspects of the early-evolutionary phases in low-mass stars within Eddington-inspired Born–Infeld (EiBI) gravity, a viable extension of General Relativity. These aspects are concerned with the Hayashi tracks (i.e. the effective temperature-luminosity relation); the minimum mass required to belong to the main sequence; and the maximum mass allowed for a fully convective star within the main sequence. Using analytical models accounting for the most relevant physics of these processes, we find in all cases a dependence of these quantities not only on the theory’s parameter, but also on the star’s central density, a feature previously found in Palatini f(R) gravity. Using this, we investigate the evolution of these quantities with the (sign of the) EiBI parameter, finding a shift in the Hayashi tracks in opposite directions in the positive/negative branches of it, and an increase (decrease) for positive (negative) parameter in the two masses above. We use these results to elaborate on the chances to seek for traces of new physics in low-mass stars within this theory, and the limitations and difficulties faced by this approach.

Merce Guerrero

Diego Rubiera-Garcia

Pre-main sequence evolution of low-mass stars in Eddington-inspired Born–Infeld gravity

We present the relativistic hydrostatic equilibrium equations for a wide class of gravitational theories possessing a scalar–tensor representation. It turns out that the stellar structure equations can be written with respect to the scalar–tensor invariants, allowing to interpret their physical role.

Poland

Aleksander Kozak

Invariant quantities of scalar–tensor theories for stellar structure

Two exact solutions for $$n=0$$ <math><mrow><mi>n</mi><mo>=</mo><mn>0</mn></mrow></math>  and $$n=1$$ <math><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow></math>  of the Palatini-modified Lane–Emden equation are found. We have employed these solutions to describe a Palatini–Newtonian neutron or low mass brown dwarf star and compared the result with the pure Newtonian counterpart. It turned out that for the negative parameter of the Starobinsky model the star is heavier and larger.

Czechia

Artur Sergyeyev

Brazil

The Palatini star: exact solutions of the modified Lane–Emden equation

We have derived a modified Lane–Emden equation for the Starobinsky model in Palatini gravity which is numerically solvable. Comparing the results to the ones provided by General Relativity we observe a significant difference depending on the theory parameter for the $$M-R$$ <math><mrow><mi>M</mi><mo>-</mo><mi>R</mi></mrow></math>  relations.

Polytropic stars in Palatini gravity

We formulate the generalized Tolman–Oppenheimer–Volkoff equations for the $$f(\hat{R})$$ <math><mrow><mi>f</mi><mo>(</mo><mover><mi>R</mi><mo>^</mo></mover><mo>)</mo></mrow></math>  Palatini gravity in the case of static and spherical symmetric geometry. We also show that a neutron star can be a stable system independently of the form of the functional $$f(\hat{R})$$ <math><mrow><mi>f</mi><mo>(</mo><mover><mi>R</mi><mo>^</mo></mover><mo>)</mo></mrow></math> .

On stability of a neutron star system in Palatini gravity

We propose a new formula to explain circular velocity profiles of spiral galaxies obtained from the Starobinsky model in the Palatini formalism. It is based on the assumption that the gravity can be described by two conformally related metrics: one of them is responsible for the measurement of distances, while the other, the so-called dark metric, is responsible for a geodesic equation and therefore can be used for the description of the velocity profile. The formula is tested against a subset of galaxies taken from the HI Nearby Galaxy Survey (THINGS).

Romania

Ciprian Sporea

Andrzej Borowiec

Galaxy rotation curves via conformal factors

We study static, spherically symmetric equilibrium configurations in extended theories of gravity (ETG) following the notation introduced by Capozziello et al. We calculate the differential equations for the stellar structure in such theories in a very generic form i.e., the Tolman–Oppenheimer–Volkoff generalization for any ETG is introduced. Stability analysis is also investigated with special focus on the particular example of scalar–tensor gravity.