arXiv subjects

Felipe H. Navarrete

Publications and source records attributed to Felipe H. Navarrete.

6 recordsLinked to original sources

arXiv2026

Eclipsing time variations in close binaries produced by azimuthal dynamo waves

The nature of eclipsing time variations (ETVs) in post-common-envelope binaries (PCEBs) is still unknown. Circumbinary planets routinely fail the test of time and the Applegate mechanism has energetic constraints and problems in reproducing observations. Based on recent analytic models of magnetically-induced ETVs and stellar dynamo simulations, we aim at explaining ETVs via non-axisymmetric magnetic fields that drift in the azimuthal direction of the star, know as azimuthal dynamo waves (ADWs). We implement a time-varying non-axisymmetric quadrupole moment ($Q$) in a binary system. We solve for the dynamics of the system, compute the resulting eclipsing times, and construct O-C, diagrams. We perform several simulations with different amplitudes of $Q$, periods, stellar masses and binary separations. ADWs naturally give rise to characteristic shapes in the O-C diagram that resemble observations. Depending on how fast $Q$ changes, the solutions can have a sharp decrease in O-C producing amplitudes such as the one obtained in QS Vir, or sinusoidal-like shapes such as in V471 Tau or NN Ser. We also find that the amplitude of the eclipsing times varies from tens to hundreds of seconds. ADWs offer a self-consistent explanation for ETVs as they are expected in dynamo theory. They can explain a variety of features in the observed O-C diagrams. As suggested by dynamo simulations, ADWs are easily excited in rapidly rotating stars, alleviating energetic constrains required in the context of the Applegate mechanism. They produce non-axis $Q$ that in turn produce ETVs that can account for the long-term variation of the O-C, diagrams. We expect in this case that the resulting O-C diagrams are not strictly periodic, unlike explanations based on a third body that would imply a strict periodicity unless additional mechanisms are being invoked.

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arXiv2023

Effects of the centrifugal force in stellar dynamo simulations

The centrifugal force is often omitted from simulations of stellar convection either for numerical reasons or because it is assumed to be weak compared to the gravitational force. However, it might be an important factor in rapidly rotating stars, such as solar analogs, due to its $Ω^2$ scaling, where $Ω$ is the rotation rate of the star. We study the effects of the centrifugal force in a set of 21 semi-global stellar dynamo simulations with varying rotation rates. Included in the set are three control runs aimed at distinguishing the effects of the centrifugal force from the nonlinear evolution of the solutions. We decomposed the magnetic field into spherical harmonics and studied the migration of azimuthal dynamo waves (ADWs), the energy of different large-scale magnetic modes, and differential rotation. In the regime with the lowest rotation rates, $Ω= 5-10Ω_\odot$, where $Ω_\odot$ is the rotation rate of the Sun, we see no marked changes in either the differential rotation or the magnetic field properties. For intermediate rotation, $Ω= 20-25Ω_\odot$, we identify an increase in the differential rotation as a function of centrifugal force. The axisymmetric magnetic energy tends to decrease with centrifugal force, while the non-axisymmetric one increases. The ADWs are also affected, especially in the propagation direction. In the most rapidly rotating set with $Ω=30Ω_\odot$, these changes are more pronounced, and in one case the propagation direction of the ADW changes from prograde to retrograde. The control runs suggest that the results are a consequence of the centrifugal force and not due to the details of the initial conditions or the history of the run. We find that the differential rotation and the ADWs only change as a function of the centrifugal force when rotation is rapid enough.

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arXiv2022

Origin of eclipsing time variations in Post-Common-Envelope binaries: role of the centrifugal force

Eclipsing time variations (ETVs) in post-common-envelope binaries (PCEBs) were proposed to be due to the time-varying component of the stellar gravitational quadrupole moment. This is suggested to be produced by changes in the stellar structure due to an internal redistribution of angular momentum and the effect of the centrifugal force. We examined this hypothesis and present 3D simulations of compressible magnetohydrodynamics (MHD) performed with the {\sc Pencil Code}. We modeled the stellar dynamo for a solar-mass star with angular velocities of 20 and 30 times solar. We included and varied the strength of the centrifugal force and compared the results with reference simulations without the centrifugal force and with a simulation in which its effect is enhanced. The centrifugal force causes perturbations in the evolution of the numerical model, so that the outcome in the details becomes different as a result of nonlinear evolution. While the average density profile is unaffected by the centrifugal force, a relative change in the density difference between high latitudes and the equator of $\sim10^{-4}$ is found. The power spectrum of the convective velocity is found to be more sensitive to the angular velocity than to the strength of the centrifugal force. The quadrupole moment of the stars includes a fluctuating and a time-independent component, which vary with the rotation rate. As very similar behavior is produced in absence of the centrifugal force, we conclude that it is not the main ingredient for producing the time-averaged and fluctuating quadrupole moment of the star. In a real physical system, we thus expect contributions from both components, that is, from the time-dependent gravitational force from the variation in the quadrupole term and from the spin-orbit coupling that is due to the persistent part of the quadrupole.

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arXiv2022

Simulations of fully convective M dwarfs: dynamo action with varying magnetic Prandtl numbers

M dwarfs are low-mass main-sequence stars, the most numerous type of stars in the solar neighbourhood, which are known to have significant magnetic activity. The aim of this work is to explore the dynamo solutions and magnetic fields of fully convective M dwarfs with varying magnetic Prandtl numbers ${\rm Pr_M}$, and a rotation period (${\rm P_{rot}}$) of 43 days. ${\rm Pr_M}$ is known to play an important role in the dynamo action; dynamos for low-$\rm Pr_M$ and large-$\rm Pr_M$ have very different properties. We performed three-dimensional magnetohydrodynamical (MHD) numerical simulations with the ``star-in-a-box'' model using stellar parameters for an M5 dwarf with $0.21 M_{\odot}$. We found that the dynamo solutions are sensitive to ${\rm Pr_M}$. The simulations at this rotation period present periodic cycles of the large-scale magnetic field up to ${\rm Pr_M} \leq 2$; for higher values the cycles disappear and irregular solutions to arise. Our results are consistent with previous studies and suggest that the dynamos operating in fully convective stars behave similarly as those in partially convective stars.

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arXiv2022

Origin of eclipsing time variations: Contributions of different modes of the dynamo-generated magnetic field

The possibility to detect circumbinary planets and to study stellar magnetic fields through eclipsing time variations (ETVs) in binary stars has sparked an increase of interest in this area of research. We revisit the connection between stellar magnetic fields and the gravitational quadrupole moment $Q_{xx}$ and compare different dynamo-generated ETV models with our simulations. We present magnetohydrodynamical simulations of solar mass stars with rotation periods of 8.3, 1.2, and 0.8 days and perform a detailed analysis of the magnetic and quadrupole moment using spherical harmonic decomposition. The extrema of $Q_{xx}$ are associated with changes in the magnetic field structure. This is evident in the simulation with a rotation period of 1.2 days. Its magnetic field has a more complex behavior than in the other models, as the large-scale nonaxisymmetric field dominates throughout the simulation and the axisymmetric component is predominantly hemispheric. This triggers variations in the density field that follow the magnetic field asymmetry with respect to the equator, affecting the $zz$ component of the inertia tensor, and thus modulating $Q_{xx}$. The magnetic field of the two other runs are less variable in time and more symmetric with respect to the equator, such that the variations in the density are weaker, and therefore only small variations in $Q_{xx}$ are seen. If interpreted via the classical Applegate mechanism (tidal locking), the quadrupole moment variations obtained in the current simulations are about two orders of magnitude below those deduced from observations of post-common-envelope binaries. However, if no tidal locking is assumed, our results are compatible with the observed ETVs.

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arXiv2019

Magneto-hydrodynamical origin of eclipsing time variations in post-common-envelope binaries for solar mass secondaries

Eclipsing time variations have been observed for a wide range of binary systems, including post-common-envelope binaries. A frequently proposed explanation, apart from the possibility of having a third body, is the effect of magnetic activity, which may alter the internal structure of the secondary star, particularly its quadrupole moment, and thereby cause quasi-periodic oscillations. Here we present two compressible non-ideal magneto-hydrodynamical (MHD) simulations of the magnetic dynamo in a solar mass star, one of them with three times the solar rotation rate ("slow rotator"), the other one with twenty times the solar rotation rate ("rapid rotator"), to account for the high rotational velocities in close binary systems. For the slow rotator, we find that both the magnetic field and the stellar quadrupole moment change in a quasi-periodic manner, leading to O-C (observed - corrected times of the eclipse) variations of ~0.025 s. For the rapid rotator, the behavior of the magnetic field as well as the quadrupole moment changes become considerably more complex, due to the less coherent dynamo solution. The resulting O-C variations are of the order 0.13 s. The observed system V471~Tau shows two modes of eclipsing time variations, with amplitudes of 151 s and 20 s, respectively. However, the current simulations may not capture all relevant effects due to the neglect of the centrifugal force and self-gravity. Considering the model limitations and that the rotation of V471 Tau is still a factor of 2.5 faster than our rapid rotator, it may be conceivable to reach the observed magnitudes.

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