Planets and Exoplanets

The study of exoplanets and giant planets in our system is crucial to understanding our origin. The goal of planetary evolution is to build models that include thermal and compositional evolution in their interior while maintaining equilibrium for billions of years. 


APPLE Code

I led the development of this new planetary evolution code purpose-built to study the evolution of giant planets. This software tool is tailored specifically to address the emerging complexities of planetary evolution revealed by recent planetary probes. Key to the development has been the incorporation of the most current equations of state for hydrogen, helium, ices, and rock; the inclusion ice/rock cores and metals in the gaseous envelope; the embedding of conservative prescriptions for helium rain and hydrogen/helium immiscibility; the use of detailed atmosphere boundary tables (which also provide self-consistent Bond and geometric albedos and multi-frequency spectra); and options for envelope metal gradients and stably-stratified regions. The pre-calculated atmosphere tables employ the latest molecular opacities and a sophisticated spectral and atmosphere algorithm already validated over the last twenty years of exoplanet and brown dwarf research. By integrating these advanced features from the start, APPLE will facilitate the simulation in an integrated fashion of the complex processes of giant planet evolution that have emerged to be of central importance since the revelations of Juno, Cassini, and Galileo. APPLE is now poised to create a new generation of giant exoplanet and Jovian planet evolutionary models and, hopefully, to elevate this fascinating and important topic to the next level of physical fidelity.



Evolution of a 0.25 Jupiter-mass object : (top) 2D projection (bottom): 3D visualization

Neutron Stars

Neutron stars are some of the most fascinating objects in the universe. They are incredibly dense remnants of massive stars that have undergone supernova explosions, with masses similar to that of our Sun but condensed into a sphere the size of a city. One of the most intriguing features of neutron stars is their incredibly strong magnetic fields, which can be trillions of times stronger than the magnetic field of the Earth. These magnetic fields can have a profound effect on the behaviour of neutron stars, shaping their emission properties, their structure, and their dynamics. In this context, understanding the nature and properties of neutron star magnetic fields is a crucial task in astrophysics, as it can help shed light on the fundamental physics of compact objects and on the complex interplay between magnetic fields, plasma, and gravity in extreme conditions. My research explores the fascinating world of neutron star magnetic fields and their role in shaping the astrophysical phenomena associated with these exotic objects.

Fig. 3D view of the magnetic field from my simulation

Fig. Equatorial view of the magnetic field geomtery in pulsars. The colorscale represents the strength of toroidal field (in units of B~1e13 G).



Magnetic Field configurations from MHD simulations


For both initially predominantly poloidal and toroidal fields, with varying strengths, we find that the magnetic field in neutron stars settles down to a mixed poloidal-toroidal configuration, where the toroidal component contributes between 10% and 20% of the total magnetic energy. In our simulations, however, the average energy of the toroidal and poloidal components is approximately stable, with a complex multipolar surface and a dipolar magnetic field at the star's exterior boundary.





Magnetic field from Grad-Shafranov

I have developed a numerical scheme for solving the Grad-Shafranov equation which calculates the equilibrium magnetic field configurations in pulsars, accounting for superconductivity in the core of the neutron star, and for the Hall effect in the crust. Our numerical code uses a finite-difference method in which the source term appearing in the Grad-Shafranov equation is nonlinear. We have developed our code in both C++ and Python, and our numerical algorithm can further be adapted to solve any nonlinear PDEs appearing in other areas of computational astrophysics. Equilibrium configurations of the internal magnetic field of a pulsar play a key role in modeling astrophysical phenomena, from glitches to gravitational wave emission.

Gravitational waves


Gravitational waves, the elusive ripples in the fabric of spacetime, represent a revolutionary concept in our understanding of the cosmos. Predicted by Albert Einstein's theory of general relativity in 1915, these waves are disturbances caused by the acceleration of massive objects, such as merging black holes or neutron stars. Unlike light or sound, gravitational waves traverse the universe unimpeded, offering a unique way to explore the most violent and energetic events in the cosmos. It wasn't until the historic detection in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) that these ethereal signals were directly observed, ushering in a new era of gravitational wave astronomy and opening a window to previously unseen phenomena in the vast expanse of space.

Gravitational waves from millisecond magnetars


The detection of gravitational waves has revolutionized our understanding of the universe, allowing us to probe some of the most extreme and violent phenomena in the cosmos. One of the most promising sources of gravitational waves is isolated neutron stars, which are dense, compact objects that can emit gravitational waves as they spin or wobble. These gravitational waves carry information about the properties of neutron stars, including their mass, radius, and equation of state, as well as the dynamics of their environment. Detecting these signals is a significant challenge, as they are weak and buried in the noise of the detector. However, recent advancements in gravitational wave detectors, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector, have made it possible to detect these signals with unprecedented sensitivity. In this context, studying gravitational waves from isolated neutron stars represents a crucial opportunity to explore the physics of these exotic objects and to test the predictions of general relativity in extreme conditions. In this article, we will explore the exciting world of gravitational waves from isolated neutron stars and their potential implications for astrophysics and fundamental physics.


Fig: An accreting neutron star showing the formation of two accretion mass columns at its poles.

Cosmological Inference using gravitational-wave standard sirens

The observation of binary neutron star merger GW170817, along with its optical counterpart, provided the first constraint on the Hubble constant H0 using gravitational wave standard sirens. When no counterpart is identified, a galaxy catalog can be used to provide the necessary redshift information. However, the true host might not be contained in a catalog which is not complete out to the limit of gravitational-wave detectability. These electromagnetic and gravitational-wave selection effects must be accounted for. We describe and implement a method to estimate H0 using both the counterpart and the galaxy catalog standard siren methods. We perform a series of mock data analyses using binary neutron star mergers to confirm our ability to recover an unbiased estimate of H0. Our simulations used a simplified universe with no redshift uncertainties or galaxy clustering, but with different magnitude-limited catalogs and assumed host galaxy properties, to test our treatment of both selection effects. We explore how the incompleteness of catalogs affects the final measurement of H0, as well as the effect of weighting each galaxy’s likelihood of being a host by its luminosity. In our most realistic simulation, where the simulated catalog is about three times denser than the density of galaxies in the local universe, we find that a 4.4% measurement precision can be reached using galaxy catalogs with 50% completeness and ∼250 binary neutron star detections with sensitivity similar to that of Advanced LIGO’s second observing run.


Fig. Individual and combined results for MDA0 (known host galaxy or direct counterpart case). The solid thick purple line shows the combined posterior probability density on H0, while the dashed line shows the combined posterior when GW selection effects are neglected. Individual likelihoods (normalized and then scaled by an arbitrary value), for each of the 249 events, are shown as thin lines with shades corresponding to their optimal SNR. The simulated value of H0 is shown as a vertical dashed line.