Planets and Exoplanets
Planets and Exoplanets
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Studying 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 that was 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 become 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.
Link to the full article: An Evolution Code for Modeling Giant Planets
2D projections of the evolution of various thermodynamical quantities for a 0.25 Jupiter mass planet over 5 Gyr.
3D visualization of the same 0.25 Jupiter-mass planet with helium rain
Fig: Evolution of various physical parameters that fit Jupiter's current observables. Figure from Arevalo, Sur, + 2025
Latest Jupiter Models
In Tejada Arevalo, Sur+2024, we address the issue of which broad set of initial conditions for the planet Jupiter best matches the current presence of a “fuzzy core” of heavy elements, while at the same time comporting with measured parameters such as its effective temperature, atmospheric helium abundance, radius, and atmospheric metallicity. Our focus is on the class of fuzzy cores that can survive convective mixing to the present day and on the unique challenges of an inhomogeneous Jupiter with stably stratified regions now demanded by the Juno gravity data. Hence, using the new code APPLE, we attempt to put a non-adiabatic Jupiter into an evolutionary context. This requires not only a mass density model, the major relevant byproduct of the Juno data, but a thermal model that is subject to interior heat transport, a realistic atmospheric flux boundary, a helium rain algorithm, and the latest equation of state. The result is a good fit to most major thermal, compositional, and structural constraints that still preserve a fuzzy core and that should inform future more detailed models of the current Jupiter in the context of its evolution from birth.
Link to the article: Jupiter Evolutionary Models Incorporating Stably Stratified Regions
The First Accurate Jupiter and Saturn Evolutionary Models with Fuzzy Cores
Link to the article: Simultaneous Evolutionary Fits for Jupiter and Saturn Incorporating Fuzzy Cores
Abstract: With the recent realization that there likely are stably-stratified regions in the interiors of both Jupiter and Saturn, we construct new non-adiabatic, inhomogeneous evolutionary models with the same microphysics for each that result at present in respectable fits for all major bulk observables for both planets. These include the effective temperature, radius, atmospheric heavy-element and helium abundances (including helium rain), and the lower-order gravity moments J2 and J4. The models preserve most of an extended “fuzzy” heavy-element core from birth. Our predicted atmospheric helium mass fraction for Saturn is ∼0.2, close to some measured estimates, but disagrees with some published predictions. To preserve a fuzzy core from birth, the interiors of both planets must start at lower entropies than would be used for traditional “hot start” adiabatic models. However, the initial exterior mantle entropies can range from hot to warm start values. We do not see a helium ocean in Saturn’s interior, and both models have inner envelopes with significant Brunt-V¨ais¨al¨ frequencies; this region for Saturn at the current epoch is more extended and in it, the Brunt is larger. The total heavy-element mass fraction in Jupiter and Saturn is determined to be ∼14% and ∼26%, respectively, though there is some play in these determinations.
Figure: Jupiter Model: (top left) the evolution of the temperature profile; (top middle) the evolution of the helium mass fraction (Y ) profile; (top right) the effective temperature and equatorial radius; (bottom left) the evolution of the entropy profile; (bottom middle) evolution of the Z profile; and (bottom right) evolution of the gravitational moments J2 and J4. This Jupiter model contains 42.5 M⊕of heavy elements, including a compact core of 3 M⊕(Z = 1). The outer metallicity starts at 1.9 Z⊙ and increases to 3.6 Z⊙. Helium rain begins at 4 Gyrs, based on the LHR0911 miscibility curves with a +410 K temperature shift, resulting in outer helium depletion to Y = 0.236, consistent with Galileo entry probe measurements (von Zahn et al. 1998).
Figure: Saturn Model: This model contains 25 M⊕ of heavy elements, including a compact core of 4 M⊕. Helium rain begins at 0.92 Gyr, based on the LHR0911 miscibility curve with a +410 K temperature shift that also fits our Jupiter model, resulting in outer helium depletion to Yatm ∼ 0.205. The top left panel shows the miscibility curve and helium rain region. The Brunt-Vaisala ratio of approximately 1.7 at 4.56 Gyr, closely aligning with the value of 2 predicted by Mankovich & Fuller (2021), is shown in the bottom left panel. Similar to Mankovich & Fuller (2021), whose stably stratified region extends to 0.6 Saturn radii, in order to match Saturn’s current observables best, our model finds that the stable region extends to 0.5 RSat.
Neutron Stars
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. The development of the toroidal field (in colorscale) is shown below in the meridional (top) and equatorial (bottom) planes of the neutron star. The field lines become turbulent after several timesteps.
Link to the articles: paper 1: Magnetic field configurations in neutron stars from MHD simulations
paper 2: Long-term GRMHD simulations of magnetic field in isolated neutron stars
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.
Link to the article:
The impact of superconductivity and the Hall effect in models of magnetized neutron stars
Gravitational waves
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.
Link to the article: Gravitational waves from mountains in newly born millisecond magnetars
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.
Link to the articles:
paper1: Cosmological Inference using Gravitational Wave Standard Sirens: A Mock Data Analysis
Fig. Individual and combined results for 240 (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.
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