arXiv subjects

Ricardo Yarza

Publications and source records attributed to Ricardo Yarza.

17 recordsLinked to original sources

arXiv2026

Atmospheric Signatures of Common Envelope Evolution in White Dwarf Planets

The majority of confirmed exoplanets orbit within 1 au of a main-sequence (MS) star. When their stellar hosts evolve off the MS, many of these planets will be engulfed and destroyed, creating empty "forbidden" zones around the stars as they evolve to their final state as a white dwarf (WD). However, several confirmed and candidate WD planets have been found within this forbidden zone. Two formation scenarios have been proposed to explain the existence of these close-in planets: high-eccentricity migration and common envelope evolution (CEE). There are currently few observational tests to distinguish between these pathways. In this study, we investigate whether CEE could leave a detectable atmospheric signature. Using Modules for Experiments in Stellar Astrophysics (MESA) models, we simulate an engulfed planet inspiraling into an AGB star, and allow the planet to accrete mass via Bondi-Hoyle-Lyttleton accretion. Assuming a range of planet masses (1$-$13 M$_{\mathrm{Jup}}$) and accretion efficiencies (0.01$-$1.0), we find that the planet can accrete up to 48% of its initial mass in the most extreme Eddington-limited scenario. Because this accreted material is enriched in hydrogen and helium, we expect it to decrease the planet's bulk metallicity. Using simulated emission spectra, we find that CEE can increase thermal emission by up to 9.0% for a cool planet such as WD 1856 b. For lower accretion efficiencies (0.01$-$0.5), thermal emission increases between 0.1$-$3.6%. This signature may be observable in the most favorable cases, providing a potential new probe for investigating the dynamical history of close-in planets around WDs.

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arXiv2026

Lithium Enrichment in a Subgiant Star with a Brown Dwarf Companion: A Planetary Engulfment Candidate

Theoretical models predict that subgiants within a narrow mass regime can retain detectable lithium enrichment signatures from planetary engulfment. We test this prediction using TOI-5882, selected because it occupies this favorable subgiant parameter space and hosts a massive brown dwarf ($22 \, M_{ \rm J}$, $P=7.1 \,{\rm d}$) companion capable of dynamically perturbing inner planets. We investigate whether: (1) TOI-5882 exhibits lithium enhancement among similar subgiants, (2) planetary material would be deposited in the convective zone, and (3) the required engulfed mass lies within a plausible range for planetary engulfment. Using spectra from the Tillinghast Reflector Echelle Spectrograph, we measured a Li I equivalent width of $75.39 \pm 3.58$ mA and an abundance of A(Li) $=2.49 \pm 0.12$ dex. Comparing these values to a control sample of 61 subgiants from the GALactic Archaeology with HERMES (GALAH) DR4 survey, we find that TOI-5882 ranks in the 98.4th percentile in both metrics, confirming significant lithium enrichment. We evaluate the engulfment scenario by modeling convective zone deposition and estimating the mass required to reproduce the observed enhancement relative to the control sample. We perform an estimate of the engulfed planetary mass incorporating CI chondritic Li abundances, as planets formed via core accretion are enriched in heavy elements and lithium partitions with these metals. This yields a required engulfed mass of $9$-$95\,M_\oplus$--an order of magnitude lower than the $5.6 \, M_{\rm J}$ implied by proto-solar assumptions. TOI-5882's lithium excess can plausibly result from the ingestion of a super-Earth to Neptune-mass planet, motivating further studies to test this scenario.

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arXiv2026

The Landscape of Unstable Mass Transfer in Interacting Binaries and Its Imprint on the Population of Luminous Red Novae

A common-envelope (CE) phase occurs when a star engulfs its companion and is widely considered the primary channel for producing Luminous Red Novae (LRNe). In this study, we combine binary-population synthesis with stellar-evolution calculations to systematically estimate the mass, velocity, and launching radius of ejecta produced during coalescence across a range of binary configurations. Our aim is to quantify how unstable mass-transfer dynamics in binaries at various evolutionary stages shape CE outcomes, enabling a predictive framework for modeling the LRN luminosity function. We find a bimodal distribution of plateau luminosities with significant implications for binary mass stability criteria that can be tested with forthcoming LSST observations. This bimodality emerges from differing mass-ejection outcomes during common-envelope interactions, which can lead either to stellar mergers, often accompanied by tidal disruption of the companion, or to successful envelope ejection. Although our predicted plateau luminosities and timescales broadly match existing observations, the models underpredict the number of LRNe with long-duration plateaus ($t_p \gtrsim 100\, \text{d}$) by about a third. We propose that these long-duration events arise from highly extended progenitors whose envelopes are ejected over multiple orbits (i.e., non-impulsively), producing relatively faint, long-lived transients. By constraining ejecta properties and incorporating pre-outburst progenitor imaging, we show how our models can clarify the physical processes that drive unstable mass transfer in these events. Finally, we argue that common-envelope interactions involving white-dwarf accretors can yield exotic outcomes, including red giants containing embedded white dwarfs that resemble Thorne-Żytków objects (TŻOs), along with calcium-rich supernovae that preserve hydrogen envelopes.

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arXiv2026

Observable Metal Pollution in Main-Sequence Stars: Simulations of Rocky Planets Engulfed by Stars in the $0.5$ to $1.4$ M$_\odot$ Range

The engulfment of planets by their host stars is an expected outcome of various dynamical processes and has been invoked to explain a variety of observed stellar properties, such as rapid rotation, chemical abundance abnormalities, and other transient phenomena. Recent observations support engulfment as the cause of such signatures; however, many engulfment process details remain uncertain. Here, we present a model for determining the chemical signatures produced due to the pollution of main sequence stars by rocky planets, a common engulfment scenario due to the high frequency of observed short-period rocky exoplanets. A key novel element of our model is that we calculate the gradual evaporation of the planet due to drag interactions with the stellar envelope, which can lead to observable pollution on the stellar surface even if the bulk of the planet is only destroyed below the star's outer convective zone. Our results indicate that rocky planet pollution is most easily measurable for stars in the $1.0$ to $1.4$ M$_\odot$ range and that elements such as aluminium, calcium, and vanadium, in addition to lithium, are most suited to detect pollution. We predict that it is also possible to differentiate between the engulfment of one large planet versus several small planets, for the same total pollution mass, for some stellar hosts. We find that rocky planet engulfment events generally take years to decades for most stars. Our results can guide future observational campaigns that may search for sites of past or current engulfment events.

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arXiv2025

Black Hole Survival Guide: Searching for Stars in the Galactic Center That Endure Partial Tidal Disruption

Once per 10,000-100,000 years, an unlucky star may experience a close encounter with a supermassive black hole (SMBH), partially or fully tearing apart the star in an exceedingly brief, bright interaction called a tidal disruption event (TDE). Remnants of partial TDEs are expected to be plentiful in our Galactic center, where at least six unexplained, diffuse, star-like "G objects" have already been detected which may have formed via interactions between stars and the SMBH. Using numerical simulations, this work aims to identify the characteristics of TDE remnants. We take 3D hydrodynamic FLASH models of partially disrupted stars and map them into the 1D stellar evolution code MESA to examine the properties of these remnants from tens to billions of years after the TDE. The remnants initially exhibit a brief, highly luminous phase, followed by an extended cooling period as they return to stable hydrogen burning. During the initial stage (< 100,000 yr) their luminosities increase by orders of magnitude, making them intriguing candidates to explain a fraction of the mysterious G objects. Notably, mild TDEs are the most common and result in the brightest remnants during this initial phase. However, most remnants exist in a long-lived stage where they are only modestly offset in temperature and luminosity compared to main-sequence stars of equivalent mass. Nonetheless, our results indicate remnants will sustain abnormal, metal-enriched envelopes that may be discernible through spectroscopic analysis. Identifying TDE survivors within the Milky Way could further illuminate some of the most gravitationally intense encounters in the Universe.

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arXiv2025

Evolution of the ZTF SLRN-2020 star-planet merger

We model the optical and infrared transient ZTF SLRN-2020, previously associated with a star-planet merger. We consider the scenario in which orbital decay via tidal dissipation led to the merger, and find that tidal heating within the star was likely unobservable in the archival image of the system taken $12\mathrm{yr}$ before the merger. The observed dust formation months before the merger is consistent with a planet of mass $M_\mathrm{p} \gtrsim 5M_\mathrm{J}$ ejecting material as it skims the stellar surface. This interaction gradually intensifies, leading to significant mass ejection on a dynamical timescale ($ \approx $ hours) as the planet plunges into the stellar interior. Part of the recombination transient associated with this dynamical mass ejection might be inaccessible to the optical observations because its duration ($ \approx $ hours) is comparable to the cadence. Correspondingly, the observed duration of the transient $\approx100\mathrm{d}$ is inconsistent with a single episode of dynamical mass ejection. Instead, the transient could be powered by the recombination of $ \approx 3.4\times10^{-5}M_\odot $ of hydrogen in an outflow, or the contraction of an inflated envelope of mass $ \approx 10^{-6}M_\odot $ that formed during the merger. The observed ejecta mass $320\mathrm{d}$ after the peak of the optical transient is $ \approx 1.3\times10^{-4}M_\odot$, consistent with the idea that a fraction of the ejecta might be unobservable in the light curve. Energetically, this post-merger ejecta mass suggests a planet at least as massive as Jupiter. Our results suggest that ZTF SLRN-2020 was the result of a merger between a star close to the main sequence and a planet with mass at least several times that of Jupiter.

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arXiv2024

Repeating Partial Tidal Encounters of Sun-like Stars Leading to their Complete Disruption

Stars grazing supermassive black holes on bound orbits may produce periodic flares over many passages, known as repeating partial tidal disruption events (TDEs). Here, we present 3D hydrodynamic simulations of sun-like stars over multiple tidal encounters. The star is significantly restructured and becomes less concentrated as a result of mass loss and tidal heating. The vulnerability to mass loss depends sensitively on the stellar density structure, and the strong correlation between the fractional mass loss $ΔM/M_*$ and the ratio of the central and average density $ρ_{\mathrm{c}}/\barρ$, which was initially derived in disruption simulations of main-sequence stars, also applies for stars strongly reshaped by tides. Over multiple orbits, the star loses progressively more mass in each encounter and is doomed to a complete disruption. Throughout its lifetime, the star may produce numerous weak flares (depending on the initial impact parameter), followed by a couple of luminous flares whose brightness increases exponentially. Flux-limited surveys are heavily biased toward the brightest flares, which may appear similar to the flare produced by the same star undergoing a full disruption on its first tidal encounter. This places new challenges on constraining the intrinsic TDE rates, which need to take repeating TDEs into account. Other types of stars with different initial density structures (e.g., evolved stars with massive cores) follow distinct evolution tracks, which might explain the diversity of the long-term luminosity evolution seen in recently uncovered repeaters.

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arXiv2024

Rethinking Thorne-Żytkow Object Formation: The Fate of X-ray Binary LMC X-4 and Implications for Ultra-long Gamma-ray Bursts

We present a start-to-end simulation aimed at studying the long-term fate of high-mass X-ray binaries and whether a Thorne-Żytkow object (TŻO) might ultimately be assembled. We analyze results from a 3D hydrodynamical simulation that models the eventual fate of LMC X-4, a compact high-mass X-ray binary system, after the primary fills its Roche lobe and engulfs the neutron star companion. We discuss the outcome of this engulfment within the standard paradigm of TŻO formation. The post-merger angular momentum content of the stellar core is a key ingredient, as even a small amount of rotation can break spherical symmetry and produce a centrifugally supported accretion disk. Our findings suggest the inspiraling neutron star, upon merging with the core, can accrete efficiently via a disk at high rates ($\approx 10^{-2}M_\odot/{\rm s}$), subsequently collapsing into a black hole and triggering a bright transient with a luminosity and duration typical of an ultra-long gamma-ray burst. We propose that the canonical framework for TŻO formation via common envelope needs to be revised, as the significant post-merger accretion feedback will unavoidably unbind the vast majority of the surrounding envelope.

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arXiv2024

The Evolution of Binaries Embedded Within Common Envelopes

Triple stellar systems allow us to study stellar processes that cannot be attained in binary stars. The evolutionary phases in which the stellar members undergo mass exchanges can alter the hierarchical layout of these systems. Yet, the lack of a self-consistent treatment of common-envelope (CE) in triple star-systems hinders the comprehensive understanding of their long-term fate. This letter examines the conditions predicted around binaries embedded within CEs using local 3D hydrodynamical simulations. We explore varying the initial binary separation, the flow Mach number, and the background stellar density gradients as informed by a wide array of CE conditions, including those invoked to explain the formation of the triple system hosting PSR J0337+1715. We find that the stellar density gradient governs the gaseous drag force, which determines the final configuration of the embedded binary. We observe a comparable net drag force on the center of mass but an overall reduction in the accretion rate of the binary compared to the single object case. We find that for most CE conditions, and in contrast to the uniform background density case, the binary orbital separation increases with time, softening the binary and preventing it from subsequently merging. We conclude that binaries spiraling within CEs become more vulnerable to be disrupted by tidal interactions. This can have profound implications on the final outcomes of triple star-systems.

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arXiv2024

The TEMPO Survey II: Science Cases Leveraged from a Proposed 30-Day Time Domain Survey of the Orion Nebula with the Nancy Grace Roman Space Telescope

The TEMPO (Transiting Exosatellites, Moons, and Planets in Orion) Survey is a proposed 30-day observational campaign using the Nancy Grace Roman Space Telescope. By providing deep, high-resolution, short-cadence infrared photometry of a dynamic star-forming region, TEMPO will investigate the demographics of exosatellites orbiting free-floating planets and brown dwarfs -- a largely unexplored discovery space. Here, we present the simulated detection yields of three populations: extrasolar moon analogs orbiting free-floating planets, exosatellites orbiting brown dwarfs, and exoplanets orbiting young stars. Additionally, we outline a comprehensive range of anticipated scientific outcomes accompanying such a survey. These science drivers include: obtaining observational constraints to test prevailing theories of moon, planet, and star formation; directly detecting widely separated exoplanets orbiting young stars; investigating the variability of young stars and brown dwarfs; constraining the low-mass end of the stellar initial mass function; constructing the distribution of dust in the Orion Nebula and mapping evolution in the near-infrared extinction law; mapping emission features that trace the shocked gas in the region; constructing a dynamical map of Orion members using proper motions; and searching for extragalactic sources and transients via deep extragalactic observations reaching a limiting magnitude of $m_{AB}=29.7$\,mag (F146 filter).

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arXiv2024

TESS Hunt for Young and Maturing Exoplanets (THYME) XI: An Earth-sized Planet Orbiting a Nearby, Solar-like Host in the 400Myr Ursa Major Moving Group

Young terrestrial worlds are critical test beds to constrain prevailing theories of planetary formation and evolution. We present the discovery of HD 63433d - a nearby (22pc), Earth-sized planet transiting a young sunlike star (TOI-1726, HD 63433). HD 63433d is the third planet detected in this multiplanet system. The kinematic, rotational, and abundance properties of the host star indicate that it belongs to the young (414 $\pm$ 23 Myr) Ursa Major moving group, whose membership we update using new data from Gaia DR3 and TESS. Our transit analysis of the TESS light curves indicates that HD 63433 d has a radius of 1.1 $R_\oplus$ and closely orbits its host star with a period of 4.2 days. To date, HD 63433 d is the smallest confirmed exoplanet with an age less than 500 Myr, and the nearest young Earth-sized planet. Furthermore, the apparent brightness of the stellar host (V $\approx$ 6.9 mag) makes this transiting multiplanet system favorable to further investigations, including spectroscopic follow-up to probe atmospheric loss in a young Earth-sized world.

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arXiv2023

SN2023ixf in Messier 101: the twilight years of the progenitor as seen by Pan-STARRS

The nearby type II supernova, SN2023ixf in M101 exhibits signatures of early-time interaction with circumstellar material in the first week post-explosion. This material may be the consequence of prior mass loss suffered by the progenitor which possibly manifested in the form of a detectable pre-supernova outburst. We present an analysis of the long-baseline pre-explosion photometric data in $g$, $w$, $r$, $i$, $z$ and $y$ filters from Pan-STARRS as part of the Young Supernova Experiment, spanning $\sim$5,000 days. We find no significant detections in the Pan-STARRS pre-explosion light curve. We train a multilayer perceptron neural network to classify pre-supernova outbursts. We find no evidence of eruptive pre-supernova activity to a limiting absolute magnitude of $-7$. The limiting magnitudes from the full set of $gwrizy$ (average absolute magnitude $\approx$-8) data are consistent with previous pre-explosion studies. We use deep photometry from the literature to constrain the progenitor of SN2023ixf, finding that these data are consistent with a dusty red supergiant (RSG) progenitor with luminosity $\log\left(L/L_\odot\right)$$\approx$5.12 and temperature $\approx$3950K, corresponding to a mass of 14-20 M$_\odot$

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arXiv2023

Hydrodynamics and survivability during post-main-sequence planetary engulfment

The engulfment of substellar bodies (SBs, such as brown dwarfs and planets) by giant stars is a possible explanation for rapidly rotating giants, lithium-rich giants, and the presence of SBs in close orbits around subdwarfs and white dwarfs. We simulate the flow in the vicinity of an engulfed SB in three-dimensional hydrodynamics. We model the SB as a rigid body with a reflective surface because it cannot accrete. This reflective boundary changes the flow morphology to resemble that of engulfed compact objects with outflows. We measure the drag coefficients for the ram pressure and gravitational drag forces acting on the SB, and use them to integrate its trajectory inside the star. We find that engulfment can increase the luminosity of a $1M_\odot$ star by up to a few orders of magnitude. The time for the star to return to its original luminosity is up to a few thousand years when the star has evolved to $\approx10R_\odot$ and up to a few decades at the tip of the red giant branch. No SBs can eject the envelope of a $1M_\odot$ star before it evolves to $\approx10R_\odot$, if the orbit of the SB is the only energy source contributing to the ejection. In contrast, SBs as small as $\approx10M_\text{Jup}$ can eject the envelope at the tip of the red giant branch. The numerical framework we introduce here can be used to study planetary engulfment in a simplified setting that captures the physics of the flow at the scale of the SB.

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arXiv2023

Tidal disruption events from eccentric orbits and lessons learned from the noteworthy ASASSN-14ko

Stars grazing supermassive black holes (SMBHs) on bound orbits may survive tidal disruption, causing periodic flares. Inspired by the recent discovery of the periodic nuclear transient ASASSN-14ko, a promising candidate for a repeating tidal disruption event (TDE), we study the tidal deformation of stars approaching SMBHs on eccentric orbits. With both analytical and hydrodynamics methods, we show the overall tidal deformation of a star is similar to that in a parabolic orbit provided that the eccentricity is above a critical value. This allows one to make use of existing simulation libraries from parabolic encounters to calculate the mass fallback rate in eccentric TDEs. We find the flare structures of eccentric TDEs show a complicated dependence on both the SMBH mass and the orbital period. For stars orbiting SMBHs with relatively short periods, we predict significantly shorter-lived duration flares than those in parabolic TDEs, which can be used to predict repeating events if the mass of the SMBH can be independently measured. Using an adiabatic mass loss model, we study the flare evolution over multiple passages, and show the evolved stars can survive many more passages than main sequence stars. We apply this theoretical framework to the repeating TDE candidate ASASSN-14ko and suggest that its recurrent flares originate from a moderately massive ($M\gtrsim 1 \mathrm{M_\odot}$), extended ($\approx$ a few $\mathrm{R_\odot}$), evolved star on a grazing, bound orbit around the SMBH. Future hydrodynamics simulations of multiple tidal interactions will enable realistic models on the individual flare structure and the evolution over multiple flares.

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arXiv2022

A revised energy formalism for common-envelope evolution: repercussions for planetary engulfment and the formation of neutron star binaries

Common-envelope evolution is a stage in binary system evolution in which a giant star engulfs a companion. The standard energy formalism is an analytical framework to estimate the amount of energy transferred from the companion's shrinking orbit into the envelope of the star that engulfed it. We show analytically that this energy transfer is larger than predicted by the standard formalism. As the orbit of the companion shrinks, the mass it encloses becomes smaller, and the companion is less bound than if the enclosed mass had remained constant. Therefore, more energy must be transferred to the envelope for the orbit to shrink further. We derive a revised energy formalism that accounts for this effect, and discuss its consequences in two contexts: the formation of neutron star binaries, and the engulfment of planets and brown dwarfs by their host stars. The companion mass required to eject the stellar envelope is smaller by up to $50\%$, leading to differences in common-envelope evolution outcomes. The energy deposition in the outer envelope of the star, which is related to the transient luminosity and duration, is up to a factor of $\approx7$ higher. Common-envelope efficiency values above unity, as defined in the literature, are thus not necessarily unphysical, and result at least partly from an incomplete description of the energy deposition. The revised energy formalism presented here can improve our understanding of stellar merger and common-envelope observations and simulations.

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arXiv2022

PATOKA: Simulating Electromagnetic Observables of Black Hole Accretion

The Event Horizon Telescope (EHT) has released analyses of reconstructed images of horizon-scale millimeter emission near the supermassive black hole at the center of the M87 galaxy. Parts of the analyses made use of a large library of synthetic black hole images and spectra, which were produced using numerical general relativistic magnetohydrodynamics fluid simulations and polarized ray tracing. In this article, we describe the PATOKA pipeline, which was used to generate the Illinois contribution to the EHT simulation library. We begin by describing the relevant accretion systems and radiative processes. We then describe the details of the three numerical codes we use, iharm, ipole, and igrmonty, paying particular attention to differences between the current generation of the codes and the originally published versions. Finally, we provide a brief overview of simulated data as produced by PATOKA and conclude with a discussion of limitations and future directions.

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arXiv2018

K2 study of the Magnetic Precataclysmic Variable V1082 Sagittarius

We present a long-term light curve of the precataclysmic variable (CV) V1082 Sgr obtained by the K2-mission over the course of 81 days. We analyze the entire complex light curve as well as explore several sections in detail with a sliding periodogram. The long dataset allows the first detection of the orbital period in the light curve, as well as the confirmation of cyclical variability on a longer timescale of about a month. A portion of the light curve in deep minimum reveals a clean, near-sinusoidal variability attributed to the rotation of the spotted surface of the donor star. We model that portion of the light curve assuming that the donor star grossly under-fills its Roche lobe, has cool spots similar to a chromospherically active, slightly evolved early K-star, and might be irradiated by the X-ray beam from the magnetically accreting white dwarf. The fast variability of the object in the active phases resembles the light curves of magnetic CVs (polars).

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