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Physics Astrophysics

astro-ph Watch Astrophysics

Tuesday 28 October

Earth and Planetary Astrophysics

On The Applicability of Ring-Moon Cycles to Exoplanets

Isabella E. Ward and Matija Ćuk

The presence of rings and moons around exoplanets is likely to be one of the next great discoveries in exoplanet research. Using theories developed for the Solar System, we explore the possibility of coupled ring-moon cycles around exoplanets and what these processes mean for the observability of these features. Around Neptune- and Earth-like planets, we find that ring-moon cycles are capable of producing long-lived rings of comparable and greater relative transit depths than Saturn's. In multi-planet systems, secular spin-orbit resonances can provide the necessary planetary obliquity for these rings to contribute noticeably to transit lightcurves. We model the geometry of a ring's cross-section at various angles in comparison to the cross-section of a transiting planet to determine whether the ring may be detectable during the planet's transit. Ringed planets have also been proposed as an alternative to inflated planetary radii seen in transit, leading to abnormally low observed densities. Ring-moon cycles can produce late-forming and sometimes long-lived rings that can have the potential of explaining at least some of these observations. We also discuss some inconsistencies in the calculation of exoplanet oblateness due to rotation that we have come across in the course of this work.
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Cosmology and Nongalactic Astrophysics

Bridging Simulators with Conditional Optimal Transport

Justine Zeghal, Benjamin Remy, Yashar Hezaveh et al. (+2 more)

We propose a new field-level emulator that bridges two simulators using unpaired simulation datasets. Our method leverages a flow-based approach to learn the likelihood transport from one simulator to the other. Since multiple transport maps exist, we employ Conditional Optimal Transport Flow Matching (COT-FM) to ensure that the transformation minimally distorts the underlying structure of the data. We demonstrate the effectiveness of this approach by bridging weak lensing simulators: a Lagrangian Perturbation Theory (LPT) to a N-body Particle-Mesh (PM). We demonstrate that our emulator captures the full correction between the simulators by showing that it enables full-field inference to accurately recover the true posterior, validating its accuracy beyond traditional summary statistics.
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All authors Justine Zeghal, Benjamin Remy, Yashar Hezaveh, Francois Lanusse, Laurence Perreault Levasseur

Astrophysics of Galaxies

The Impact of Cosmic Ray Transport on the $\gamma$-Ray Luminosity of Diffuse Gas

Roark Habegger, Mateusz Ruszkowski and Ellen G. Zweibel

Observations of $\gamma$-rays from diffuse gas provide the opportunity to study the distribution of high energy particles in different astrophysical environments. In the circumgalactic medium (CGM) and the intracluster medium (ICM), it is expected that relativistic cosmic rays collide with thermal particles and produce $\gamma$-rays through pion decay. The $\gamma$-ray luminosity of a plasma depends on where cosmic rays are: if they are in denser gas, they produce more $\gamma$-rays. In this work, we study how different cosmic-ray transport mechanisms impact the $\gamma$-ray luminosity of a turbulent, multiphase medium formed from an initially diffuse medium. Two quantities set the luminosity: the average cosmic-ray energy density and the correlation of cosmic-ray energy and gas density. Overall, cosmic rays must escape cold dense regions in order to produce less $\gamma$-ray emission and be consistent with observations. Our simulations with fast transport mechanisms (either diffusion or streaming) are degenerate: they each produce a lower $\gamma$-ray luminosity than slow transport simulations by two orders of magnitude. This result means that fast transport (particularly in dense clumps) is necessary for simulations to agree with the dearth of observations of $\gamma$-ray emission from diffuse gas like the CGM and ICM. We also show the significant difference in luminosity is the result of cosmic-ray reacceleration. This reacceleration is different from the turbulent reacceleration described by Ptuskin (1988). Instead, condensing, cold clouds drive a significant increase in the average cosmic-ray energy and, as a result, the $\gamma$-ray luminosity.
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High Energy Astrophysical Phenomena

AT2025ulz and S250818k: Investigating early time observations of a subsolar mass gravitational-wave binary neutron star merger candidate

Xander J. Hall, Malte Busmann, Hauke Koehn et al. (+32 more)

Over the past LIGO–Virgo–KAGRA (LVK) observing runs, it has become increasingly clear that identifying the next electromagnetic counterparts to gravitational-wave (GW) neutron star mergers will likely be more challenging compared to the case of GW170817. The rarity of these GW events, and their electromagnetic counterparts, motivates rapid searches of any candidate binary neutron star (BNS) merger detected by the LVK. We present our extensive photometric and spectroscopic campaign of the candidate counterpart AT2025ulz to the low-significance GW event S250818k, which had a ${\sim} 29\%$ probability of being a BNS merger. We demonstrate that during the first five days, the luminosity and color evolution of AT2025ulz are consistent with both kilonova and shock cooling models, although a Bayesian model comparison shows preference for the shock cooling model, underscoring the ambiguity inherent to early data obtained over only a few days. Continued monitoring beyond this window reveals a rise and color evolution incompatible with kilonova models and instead consistent with a supernova. This event emphasizes the difficulty in identifying the electromagnetic counterparts to BNS mergers and the significant allotment of observing time necessary to robustly differentiate kilonovae from impostors.
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All authors Xander J. Hall, Malte Busmann, Hauke Koehn, Keerthi Kunnumkai, Antonella Palmese, Brendan O'Connor, James Freeburn, Lei Hu, Daniel Gruen, Tim Dietrich, Mattia Bulla, Michael W. Coughlin, Sarah Antier, Marion Pillas, Paul A. Price, Tomás Ahumada, Ariel Amsellem, Igor Andreoni, Jule Augustin, Tom'as Cabrera, Rasika Deshpande, Jennifer Fabà-Moreno, Julius Gassert, Sergey Karpov, Mansi Kasliwal, Ignacio Magaña Hernandez, Rachel Mandelbaum, Felipe Fontinele Nunes, Peter T. H. Pang, Julian Sommer, Robert Stein, Constantin Tabor, Pablo Vega, Thibeau Wouters, Xiaoxiong Zuo

Instrumentation and Methods for Astrophysics · Cross-listed from cs.CL

ReplicationBench: Can AI Agents Replicate Astrophysics Research Papers?

Christine Ye, Sihan Yuan, Suchetha Cooray et al. (+10 more)

Frontier AI agents show increasing promise as scientific research assistants, and may eventually be useful for extended, open-ended research workflows. However, in order to use agents for novel research, we must first assess the underlying faithfulness and correctness of their work. To evaluate agents as research assistants, we introduce ReplicationBench, an evaluation framework that tests whether agents can replicate entire research papers drawn from the astrophysics literature. Astrophysics, where research relies heavily on archival data and computational study while requiring little real-world experimentation, is a particularly useful testbed for AI agents in scientific research. We split each paper into tasks which require agents to replicate the paper's core contributions, including the experimental setup, derivations, data analysis, and codebase. Each task is co-developed with the original paper authors and targets a key scientific result, enabling objective evaluation of both faithfulness (adherence to original methods) and correctness (technical accuracy of results). ReplicationBench is extremely challenging for current frontier language models: even the best-performing language models score under 20%. We analyze ReplicationBench trajectories in collaboration with domain experts and find a rich, diverse set of failure modes for agents in scientific research. ReplicationBench establishes the first benchmark of paper-scale, expert-validated astrophysics research tasks, reveals insights about agent performance generalizable to other domains of data-driven science, and provides a scalable framework for measuring AI agents' reliability in scientific research.
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All authors Christine Ye, Sihan Yuan, Suchetha Cooray, Steven Dillmann, Ian L. V. Roque, Dalya Baron, Philipp Frank, Sergio Martin-Alvarez, Nolan Koblischke, Frank J Qu, Diyi Yang, Risa Wechsler, Ioana Ciuca

Solar and Stellar Astrophysics

Modeling Decadal and Centennial Solar UV Irradiance Changes

Raffaele Reda, Valentina Penza, Serena Criscuoli et al. (+5 more)

Reconstructions of solar spectral irradiance - especially in the ultraviolet (UV) range - are crucial for understanding Earth's climate system. Although total solar irradiance (TSI) has been thoroughly investigated, the spectral composition of solar radiation offers a deeper insight into its interactions with the atmosphere, biosphere, and climate. UV radiation, in particular, plays a key role in stratospheric chemistry and the dynamics of stratospheric ozone. Reconstructing solar irradiance over the past centuries requires accounting for both the cyclic modulation of active-region coverage associated with the 11-year solar cycle and the longer-term secular trends, including their centennial variability. This study utilizes an empirical framework, based on a 1000-year record of Open Solar Flux, to characterize the various temporal components of solar irradiance variability. We then combine these components to reconstruct Solar UV irradiance variations in spectral bands crucial for Earth's atmospheric studies.
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All authors Raffaele Reda, Valentina Penza, Serena Criscuoli, Luca Bertello, Matteo Cantoresi, Lorenza Lucaferri, Simone Ulzega, Francesco Berrilli

Earth and Planetary Astrophysics

A Deep Search for Exomoons Around WISE 0855 With JWST

Mikayla J. Wilson, Mary Anne Limbach, Andrew J. Skemer et al. (+11 more)

JWST is collecting time-series observations of many free-floating planets (FFPs) to study their weather, but these light curves are the ideal datasets to search for exomoons that transit the FFP during observations. In this paper, we present observations of the planetary-mass Y dwarf ($T=250-285K$, $M = 6.5\pm3.5 M_{Jup}$, d = 2.3$\,$pc) WISE J085510.83-071442.5 (WISE 0855), whose proximity and brightness make it ideal for a transiting exomoon search. We examine 11 hours of time-series spectra from the JWST Near-Infrared Spectrograph (NIRSpec) whose sensitivity, in combination with Gaussian process (GP) modeling, allows for the disentanglement of exomoon transits from WISE 0855's variability. We do not find statistically significant evidence of an exomoon transit in this dataset. Using injection and recovery tests of artificial transits for depths ranging between 0.1-1% (0.35-1.12 $R_{\oplus}$) we explore the exomoon parameter space where we could successfully detect transits. For transit depths $\geq 0.5\%$ (1.96$\,R_{\text{Titan}}$), our detection rate is 96%, which, for WISE 0855, corresponds to a moon with a companion-to-host mass ratio similar to that of Titan and Saturn. Given our sensitivity, transit probabilities, and our observational duration, we determine a $\sim$91% probability of detecting a Titan mass analog exomoon after 18 such observations if every observed system hosts a Titan mass analog exomoon in a Galilean-like system. This suggests that JWST observations of dozens of FFPs could yield meaningful constraints on the occurrence rate of exomoons. This paper is the first demonstration that JWST is sensitive to Galilean moon mass analogs around FFPs.
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All authors Mikayla J. Wilson, Mary Anne Limbach, Andrew J. Skemer, Johanna M. Vos, Brittany E. Miles, Melanie J. Rowland, Andrew Vanderburg, Adam C. Schneider, Caroline Morley, Brooke Kotten, Andrew Householder, Roxana Lupu, James Mang, Richard Freedman

Astrophysics of Galaxies

JWST observations of photodissociation regions III. Dust modelling at the illuminated edge of the Horsehead PDR

M. Elyajouri, A. Abergel, N. Ysard et al. (+19 more)

Carbonaceous nano-grains are a significant component of interstellar dust and dominate the mid-infrared emission of photodissociation regions (PDRs). We study the evolution of nano-grains across the illuminated edge of the Horsehead PDR, especially their abundance and size properties. This work is part of the Physics and Chemistry of PDR Fronts program studying dust and gas in PDRs with JWST. We use NIRCam+MIRI photometric bands and NIRSpec+MRS spectroscopy to map the illuminated edge. We model dust emission using the THEMIS dust model with the SOC radiative transfer code. Detailed modeling of high angular resolution JWST data allows us to obtain constraints on nano-grain properties. We find that diffuse ISM dust cannot account for the observed data, requiring evolved grains. A sharp density increase is observed at the illuminated edge, consistent with ALMA observations revealing a sharp transition between molecular and ionized gas. Although the PDR length could not be directly determined, we estimate an upper limit of approximately 0.015 pc. This implies a lower limit on small grain abundance (greater than 0.003), showing small grains are not depleted at the Horsehead edge, unlike in the Orion Bar. Our findings indicate a high-density environment and less steep size distribution for nano-grains at the illuminated edge versus the diffuse ISM. This implies nano-grain destruction mechanisms might be less efficient in the Horsehead's moderate-UV field than in more intense PDRs. These results support a model where nano-grain population recovery is slower in moderate-UV environments, leading to a unique dust size distribution at the edge of the Horsehead Nebula.
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All authors M. Elyajouri, A. Abergel, N. Ysard, E. Habart, T. Schirmer, A. Jones, M. Juvela, B. Tabone, L. Verstraete, K. Misselt, K. D. Gordon, A. Noriega-Crespo, P. Guillard, A. N. Witt, M. Baes, P. Bouchet, B. R. Brandl, O. Kannavou, P. Dellova, P. Klassen, B. Trahin, D. Van De Putte

Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

Black Hole Cold Brew: Fermi Degeneracy Pressure

Wei-Xiang Feng, Hai-Bo Yu and Yi-Ming Zhong

We investigate the dynamical instability of a self-gravitating thermal system in the quantum regime, where Fermi degeneracy pressure becomes significant. Using a truncated Fermi-Dirac distribution and solving the Tolman-Oppenheimer-Volkoff equation, we identify marginally stable configurations following Chandrasekhar's criterion. While Fermi pressure stabilizes a system against gravitational collapse in Newtonian gravity, in general relativity it can instead drive the instability, enabling collapse even at low temperatures. In the low-temperature limit, the critical mass is independent of the boundary temperature. We discuss implications for the formation of massive black holes in the early Universe through the gravothermal collapse of dark matter.
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High Energy Astrophysical Phenomena

XRISM constraints on unidentified X-ray emission lines, including the 3.5 keV line, in the stacked spectrum of ten galaxy clusters

XRISM Collaboration, Marc Audard, Hisamitsu Awaki et al. (+150 more)

We stack 3.75 Megaseconds of early XRISM Resolve observations of ten galaxy clusters to search for unidentified spectral lines in the $E=$ 2.5-15 keV band (rest frame), including the $E=3.5$ keV line reported in earlier, low spectral resolution studies of cluster samples. Such an emission line may originate from the decay of the sterile neutrino, a warm dark matter (DM) candidate. No unidentified lines are detected in our stacked cluster spectrum, with the $3\sigma$ upper limit on the $m_{\rm s}\sim$ 7.1 keV DM particle decay rate (which corresponds to a $E=3.55$ keV emission line) of $\Gamma \sim 1.0 \times 10^{-27}$ s$^{-1}$. This upper limit is 3-4 times lower than the one derived by Hitomi Collaboration et al. (2017) from the Perseus observation, but still 5 times higher than the XMM-Newton detection reported by Bulbul et al. (2014) in the stacked cluster sample. XRISM Resolve, with its high spectral resolution but a small field of view, may reach the sensitivity needed to test the XMM-Newton cluster sample detection by combining several years worth of future cluster observations.
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All authors XRISM Collaboration, Marc Audard, Hisamitsu Awaki, Ralf Ballhausen, Aya Bamba, Ehud Behar, Rozenn Boissay-Malaquin, Laura Brenneman, Gregory V. Brown, Lia Corrales, Elisa Costantini, Renata Cumbee, Maria Diaz Trigo, Chris Done, Tadayasu Dotani, Ken Ebisawa, Megan E. Eckart, Dominique Eckert, Satoshi Eguchi, Teruaki Enoto, Yuichiro Ezoe, Adam Foster, Ryuichi Fujimoto, Yutaka Fujita, Yasushi Fukazawa, Kotaro Fukushima, Akihiro Furuzawa, Luigi Gallo, Javier A. García, Liyi Gu, Matteo Guainazzi, Kouichi Hagino, Kenji Hamaguchi, Isamu Hatsukade, Katsuhiro Hayashi, Takayuki Hayashi, Natalie Hell, Edmund Hodges-Kluck, Ann Hornschemeier, Yuto Ichinohe, Daiki Ishi, Manabu Ishida, Kumi Ishikawa, Yoshitaka Ishisaki, Jelle Kaastra, Timothy Kallman, Erin Kara, Satoru Katsuda, Yoshiaki Kanemaru, Richard Kelley, Caroline Kilbourne, Shunji Kitamoto, Shogo Kobayashi, Takayoshi Kohmura, Aya Kubota, Maurice Leutenegger, Michael Loewenstein, Yoshitomo Maeda, Maxim Markevitch, Hironori Matsumoto, Kyoko Matsushita, Dan McCammon, Brian McNamara, François Mernier, Eric D. Miller, Jon M. Miller, Ikuyuki Mitsuishi, Misaki Mizumoto, Tsunefumi Mizuno, Koji Mori, Koji Mukai, Hiroshi Murakami, Richard Mushotzky, Hiroshi Nakajima, Kazuhiro Nakazawa, Jan-Uwe Ness, Kumiko Nobukawa, Masayoshi Nobukawa, Hirofumi Noda, Hirokazu Odaka, Shoji Ogawa, Anna Ogorzałek, Takashi Okajima, Naomi Ota, Stephane Paltani, Robert Petre, Paul Plucinsky, Frederick S. Porter, Katja Pottschmidt, Kosuke Sato, Toshiki Sato, Makoto Sawada, Hiromi Seta, Megumi Shidatsu, Aurora Simionescu, Randall Smith, Hiromasa Suzuki, Andrew Szymkowiak, Hiromitsu Takahashi, Mai Takeo, Toru Tamagawa, Keisuke Tamura, Takaaki Tanaka, Atsushi Tanimoto, Makoto Tashiro, Yukikatsu Terada, Yuichi Terashima, Yohko Tsuboi, Masahiro Tsujimoto, Hiroshi Tsunemi, Takeshi Tsuru, Aysegül Tümer, Hiroyuki Uchida, Nagomi Uchida, Yuusuke Uchida, Hideki Uchiyama, Shutaro Ueda, Yoshihiro Ueda, Shinichiro Uno, Jacco Vink, Shin Watanabe, Brian J. Williams, Satoshi Yamada, Shinya Yamada, Hiroya Yamaguchi, Kazutaka Yamaoka, Noriko Yamasaki, Makoto Yamauchi, Shigeo Yamauchi, Tahir Yaqoob, Tomokage Yoneyama, Tessei Yoshida, Mihoko Yukita, Irina Zhuravleva, Jean-Paul Breuer, Priyanka Chakraborty, Stefano Ettori, Andrew Fabian, Annie Heinrich, Marie Kondo, Julie HLavacek-Larrondo, Hannah McCall, Paul Nulsen, Tom Rose, Helen Russell, Arnab Sarkar, Evan Scannapieco, Kazunori Suda, Ming Sun, Prathamesh Tamhane, Nhut Truong, Norbert Werner, Congyao Zhang

High Energy Astrophysical Phenomena

Gamma-Ray Burst Polarimetry with the COMCUBE-S CubeSat Swarm – Design and Performance Simulations

Nathan Franel, Vincent Tatischeff, David Murphy et al. (+29 more)

COMCUBE-S (COMCUBE-S (Compton Telescope CubeSat Swarm) is a proposed mission aimed at understanding the radiation mechanisms of ultra-relativistic jets from Gamma-Ray Bursts (GRBs). It consists of a swarm of 16U CubeSats carrying a state-of-the-art Compton polarimeter and a bismuth germanium oxide (BGO) spectrometer to perform timing, spectroscopic and polarimetric measurements of the prompt emission from GRBs. The mission is currently in a feasibility study phase (Phase A) with the European Space Agency to prepare an in-orbit demonstration. Here, we present the simulation work used to optimise the design and operational concept of the microsatellite constellation, as well as estimate the mission performance in terms of GRB detection rate and polarimetry. We used the MEGAlib software to simulate the response function of the gamma-ray instruments, together with a detailed model for the background particle and radiation fluxes in low-Earth orbit. We also developed a synthetic GRB population model to best estimate the detection rate. These simulations show that COMCUBE-S will detect about 2 GRBs per day, which is significantly higher than that of all past and current GRB missions. Furthermore, simulated performance for linear polarisation measurements shows that COMCUBE-S will be able to uniquely distinguish between competing models of the GRB prompt emission, thereby shedding new light on some of the most fundamental aspects of GRB physics.
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All authors Nathan Franel, Vincent Tatischeff, David Murphy, Alexey Ulyanov, Caimin McKenna, Lorraine Hanlon, Prerna Baranwal, Christophe Beigbeder, Arnaud Claret, Ion Cojocari, Nicolas de Séréville, Nicolas Dosme, Eric Doumayrou, Mariya Georgieva, Clarisse Hamadache, Sally Hankache, Jimmy Jeglot, Mózsi Kiss, Beng-Yun Ky, Vincent Lafage, Philippe Laurent, Christine Le Galliard, Joseph Mangan, Aline Meuris, Mark Pearce, Jean Peyré, Arjun Poitaya, Diana Renaud, Arnaud Saussac, Varun Varun, Matias Vecchio, Colin Wade

Instrumentation and Methods for Astrophysics

In-Situ High Performance Visualization for Astronomy & Cosmology

Nicola Tuccari, Eva Sciacca, Yolanda Becerra et al. (+4 more)

The Astronomy & Cosmology (A&C) community is presently witnessing an unprecedented growth in the quality and quantity of data coming from simulations and observations. Writing results of numerical simulations to disk files has long been a bottleneck in high-performance computing. To access effectively and extract the scientific content of such large-scale data sets appropriate tools and techniques are needed. This is especially true for visualization tools, where petascale data size problems cannot be visualized without some data filtering, which reduces either the resolution or the amount of data volume managed by the visualization tool. A solution to this problem is to run the analysis and visualization concurrently (in-situ) with the simulation and bypass the storage of the full results. In particular we use Hecuba, a framework offering a highly distributed database to stream A&C simulation data for on-line visualization. We will demonstrate the Hecuba platform integration with the Changa high performant cosmological simulator and the in-situ visualization of its N-body results with the ParaView and VisIVO tools.
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All authors Nicola Tuccari, Eva Sciacca, Yolanda Becerra, Enric Sosa Cintero, Robert Wissing, Sijing Shen, Emiliano Tramontana

Instrumentation and Methods for Astrophysics

Exascale In-situ visualization for Astronomy & Cosmology

Nicola Tuccari, Eva Sciacca, Yolanda Becerra et al. (+2 more)

Modern simulations and observations in Astronomy & Cosmology (A&C) produce massively large data volumes, posing significant challenges for storage, access and data analysis. A long-standing bottleneck in high-performance computing, especially now in the exascale era, has been the requirement to write these large datasets to disks, which limits the performance. A promising solution to this challenge is in-situ processing, where analysis and visualization are performed concurrently with the simulation itself, bypassing the storage of the simulation data. In this work, we present new results from an approach for in-situ processing based on Hecuba, a framework that provides a highly distributed database for streaming A&C simulation data directly into the visualization pipeline to make possible on-line visualization. By integrating Hecuba with the high-performance cosmological simulator ChaNGa, we enable real-time, in-situ visualization of N-body simulation results using tools such as ParaView and VisIVO.
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All authors Nicola Tuccari, Eva Sciacca, Yolanda Becerra, Enric Sosa Cintero, Emiliano Tramontana

Earth and Planetary Astrophysics · Cross-listed from physics.space-ph

LEVITAS: Levitodynamics for Accurate Individual Particle Sensing in Space

Rafal Gajewski, Ravindra T Desai, James Bateman et al. (+3 more)

Accurately observing the rarefied media of the upper atmosphere, exosphere, and planetary and solar system environments and beyond requires highly sensitive metrological techniques. We present the operating concept and architecture of an in-situ sensing solution based on the dynamics of a levitated nanoparticle (levitodynamics). It can detect and measure impacts of individual particles in rarefied media. Dubbed `LEVITAS', our sensor consists of a dispenser of dielectric nanoparticles and optical trapping of a single nanoparticle in the focus of a laser beam. The trapped nanoparticle constitutes a harmonic oscillator at frequencies in the kilohertz range whose position can be tracked at the standard quantum limit by interferometric detection of the laser photons it scatters. Here, we simulate microcanonical impacts on the nanoparticle and show that the density, velocity, temperature, and composition of the surrounding medium can be estimated accurately. We illustrate the performance of LEVITAS in circumstances ranging from low Earth orbit out to exospheric distances, across which individual impacts can be detected at favourable rates. Furthermore, LEVITAS may be employed to accurately measure highly rarefied neutral distributions within vastly different areas of momentum space. This we demonstrate by simulating the measurement of high-velocity neutral gas particles from the interstellar medium penetrating the heliosphere and flowing through our solar system.
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All authors Rafal Gajewski, Ravindra T Desai, James Bateman, Bengt Eliasson, Daniel K L Oi, Animesh Datta

High Energy Astrophysical Phenomena

Multi-messenger constraints on transient accelerators of ultra-high energy cosmic rays

Antonio Condorelli, Jonathan Biteau, Olivier Deligny et al. (+1 more)

The origin of ultra-high-energy cosmic rays (UHECRs) remains an open questions in astrophysics. We explore two primary scenarios for the distribution of UHECR sources, assuming that their production rate follows either the cosmic star-formation-rate or stellar-mass density. By jointly fitting the UHECR energy spectrum and mass composition measured by the Pierre Auger Observatory above the ankle (10^{18.7} eV), we derive constraints on the acceleration mechanisms, source energetics, and elemental abundances at escape. Using these constraints, we generate sky maps above 40 EeV based on a catalog of over 400,000 galaxies out to 350 Mpc, providing a near-infrared flux-limited sample that maps the two stellar-activity tracers across the full sky. A crucial factor in understanding UHECR propagation is the influence of large-scale cosmic structures, particularly galaxy clusters, the largest gravitationally bound systems in the Universe, which are filled with magnetized diffuse plasma. Intermittent sources hosted in galaxies within such structures, coupled with cosmic magnetic fields, shape the observed UHECR arrival directions and provide insights into the burst rate of the sources. We show that these environments can significantly impact UHECR transport, making them particularly opaque to heavy nuclei. Additionally, we compute the expected secondary neutrino and photon fluxes from UHECR interactions in these environments and compare them with current experimental limits, constraining the maximum energy that particles can achieve. Finally, we assess the compatibility of these constraints with astrophysical candidates, identifying long gamma-ray bursts as the most promising sources.
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All authors Antonio Condorelli, Jonathan Biteau, Olivier Deligny, Remi Adam

Astrophysics of Galaxies

Planetary nebulae as tracers of accreted stellar populations in massive galaxies in groups and clusters

Johanna Hartke

Planetary nebulae (PNe) are valuable spatial and kinematic tracers of nearby galaxies. In this mini-review, I focus on their role in tracing the halo and intra-cluster/intra-group light assembly in groups and clusters of galaxies within 100~Mpc and, in particular, the link between characteristic PN metrics such as the $\alpha$-parameter and the PN luminosity function and changes from the underlying in-situ to ex-situ stellar populations. These results from nearby groups and clusters are placed into context with integral-field spectroscopic surveys of galaxies, which allow the co-spatial measurement of PN and stellar population properties. I provide an outlook on upcoming instrumentation that will provide new opportunities for the study of extragalactic PN populations. I address the challenges of reconciling observations of extragalactic PN populations with predictions from stellar evolution models and how revised late-stellar-evolution models have alleviated some of the tensions between observations and theory.
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Solar and Stellar Astrophysics

Pulsation Mode Identification and Classification of 46 High-Amplitude $\delta$ Scuti Stars from 50 Candidates with TESS observations

Taozhi Yang and Zhaoyu Zuo

Asteroseismic modelling of high-amplitude $\delta$ Scuti (HADS) variables critically depends on their accurate classification, which provides robust constraints on stellar physical parameters. As a foundational step in this direction, we present a detailed analysis of the pulsational behavior of 50 HADS star candidates using high-precision photometric data from the Transiting Exoplanet Survey Satellite (TESS). We confirm 46 as genuine HADS variables, with 40 stars having their dominant frequencies identified for the first time. Among them, 7 pulsate solely in the fundamental mode; 21 exhibit the fundamental mode alongside at least one low-amplitude nonradial mode; 5 are pure double-mode pulsators (the fundamental and first-overtone modes), 13 show double-mode pulsations accompanied by additional low-amplitude nonradial modes. The remaining four stars are classified as other types of variables: two (TIC 69546708 and TIC 110937533) are confirmed hot subdwarfs, one (TIC 8765832) is a cataclysmic variable, and one (TIC 32302937) is a likely hot subluminous star, but it requires further spectroscopic confirmation. We investigate the period-luminosity (PL) relation, also known as the Leavitt law, for these 46 confirmed HADS stars, deriving a revised relation: $M_{V}= (-3.31 \pm0.39)~\mathrm{log}~P+(-1.68 \pm 0.39)$. This result is consistent with previous studies. Their distribution in the Hertzsprung-Russell diagram indicates that HADS stars are not strictly confined to be within a narrow instability strip previously found, but can extend beyond it, with a distribution toward lower temperatures. The refined classifications presented here establish a high-quality sample for precise asteroseismic modelling and enhance the potential for future machine-learning-assisted searches and classifications of HADS stars in large-scale photometric surveys.
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Solar and Stellar Astrophysics · Cross-listed from physics.atom-ph

Comprehensive Inclusion of Higher-order Ca$^+$ Isotope Shifts in the King's Plot Yields an Order Improvement on the $e^-$-$n$ Coupling Limit

Vaibhav Katyal, A. Chakraborty and B. K. Sahoo

By critically evaluating higher-order nonlinear effects to the isotope shifts (ISs) in the low-lying transition frequencies of the singly charged calcium ion, stringent constraint on the electron-neutron coupling due to a hypothetical boson describing physics beyond the Standard Model is inferred. It shows an order magnitude difference compared to the previously reported limit demonstrating importance of higher-order effects in the analysis of nonlinearity in the King's plot. The first-order IS parameters and enhancement factor ($D$) were evaluated using two complementary approaches in the relativistic coupled-cluster theory framework: namely finite-field (FF) and analytical response (AR) approaches. Extraction of the second-order IS parameters in the FF approach show numerical instabilities, so they are determined in the AR approach. Comparison of these factors with previous calculation shows substantial differences in the magnitudes. However, $D$ values from both the FF and AR approaches display excellent agreement. We also show explicitly roles of electron correlation effects in the evaluation of $D$ values accurately.
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High Energy Astrophysical Phenomena · Cross-listed from hep-ph

Probing the nonstrange quark star equation of state with compact stars and gravitational waves

Shu-Peng Wang, Zhen-Yan Lu, Zhi-Jun Ma et al. (+3 more)

A recent study shows that incorporating a new term into the thermodynamic potential density, as required by the thermodynamic consistency criterion, can effectively resolve the thermodynamic inconsistency problems of the conventional perturbative QCD model. This additional term plays a crucial role in resolving inconsistencies at relatively low densities and becomes negligible at extremely high densities. Within this revised perturbative QCD model, we find that if we require only that the energy per baryon of up-down ($ud$) quark matter exceeds 930 MeV so as not to contradict the standard nuclear physics, the maximum mass of an $ud$ quark star allowed by the revised perturbative QCD model can reach up to 2.17 $M_{\odot}$. From this perspective, the observed 2.14 $M_{\odot}$ pulsar PSR J0740+6620 may be an $ud$ quark star. However, if we further impose the constraint that the tidal deformability of a 1.4 $M_{\odot}$ $ud$ quark star must be consistent with the GW170817 event, the maximum mass allowed by the revised perturbative QCD model would decrease to no more than 2.08 $M_{\odot}$. Consequently, our results suggest that the compact object with a mass of 2.50-2.67 $M_{\odot}$, as observed in the GW190814 event, cannot be an $ud$ quark star, according to the revised perturbative QCD model.
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All authors Shu-Peng Wang, Zhen-Yan Lu, Zhi-Jun Ma, Rong-Yao Yang, Jian-Feng Xu, Xiangyun Fu

Instrumentation and Methods for Astrophysics

Optimising Underwater Neutrino Telescopes for All-Flavour Point Source Sensitivity

Iwan Morton-Blake, Fuyudi Zhang, Qichao Chang et al. (+7 more)

High-energy neutrino astronomy has advanced rapidly in recent years, with IceCube, KM3NeT, and Baikal-GVD establishing a diffuse astrophysical flux and pointing to promising source candidates. These achievements mark the transition from first detections to detailed source studies, motivating next-generation detectors with larger volumes, improved angular resolution, and full neutrino-flavour sensitivity. We present a performance study of large underwater neutrino telescopes, taking the proposed TRIDENT array in the South China Sea as a case study, with a focus on comparing the performance of various detector configurations against the TRIDENT baseline design. Both track-like events primarily from muon neutrinos, which provide precise directional information, and cascade events from all flavours, which offer superior energy resolution, diffuse-source sensitivity, and all-sky flavour coverage, are included to achieve a balanced performance across source types. The time to discover potential astrophysical sources with both track- and cascade-like events is used as the figure of merit to compare a variety of detector design choices. Our results show that, for a fixed number of optical modules, simply enlarging the instrumented volume does not inherently lead to improved performance, while taller strings can provide modest gains across all detector channels, within engineering constraints. Distributing dense clusters of strings over a large volume is found to generally worsen discovery potential compared to the baseline layout. Finally, the optical properties of the sea-water emerge as the key factor dictating the optimisation of detector layout, highlighting the need for in-situ measurements and early deployment of optical modules to guide the final array configuration.
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All authors Iwan Morton-Blake, Fuyudi Zhang, Qichao Chang, Shuhua Hao, Weilun Huang, Hualin Mei, Wei Tian, Yingwei Wang, Xin Xiang, Donglian Xu

Astrophysics of Galaxies

Kinematic, Elemental and Structural Dependences on Metallicity in the Galactic Bulge

Haoyang Liu, Cuihua Du, Zhongcheng Li et al. (+2 more)

We selected Bulge stars from APOGEE DR17 cross-matched with astrometric data from Gaia DR3. Bulge stars were divided into sub-samples with line-of-sight velocity dispersion analyzed and the peaks of MDF were detected by both Gaussian Mixture Models (GMM) and scipy.signal.find\_peaks. GMM is also conducted to kinematically distinguish the metal-poor and metal-rich populations. Analyses were put on the Bulge stars (including retrograde stars), their elemental abundances, and the [Mg/Mn]-[Al/Fe] plane to investigate potential accreted components. Finally, the shapes (X-shaped/boxy) of Bulge stars with different metallicities were analyzed through least-squares fitting based on the analytical Bulge models. By studying the kinematic, elemental and structural dependences on metallicity for Bulge stars, our findings are concluded as follows: 1. Six peaks are detected in the Bulge MDF, encompassing values reported in previous studies, suggesting a complex composition of Bulge populations. 2. An inversion relationship is well-observed in metal-rich sub-samples, while absent in metal-poor sub-samples. 3. Metal-poor populations exhibit larger dispersions than metal-rich stars (which is also revealed by GMM decomposition), suggesting that metal-rich stars are kinematically coherent. 4. Retrograde stars are confined to $\sim1$ kpc of the Galactic center, with their relative fraction decreasing at higher [Fe/H] – a trend potentially linked to the “spin-up” process of Galactic disks. 5. Metal-rich Bulge stars with [Al/Fe] $<-0.15$ are likely associated with from disk accreted substructure, while all elemental planes exhibit bimodality but Na abundances rise monotonically with metallicity. 6. In general, stars with all metallicities support a boxy profile.
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All authors Haoyang Liu, Cuihua Du, Zhongcheng Li, Jian Zhang, Mingji Deng

Astrophysics of Galaxies

Half-Light Radius Measurements of Andromeda Dwarf Satellites from the Isaac Newton Telescope Survey Using Exponential, Plummer, and Sérsic Fits

Hedieh Abdollahi

We present half-light radius measurements for the dwarf satellites of Andromeda, based on multi-epoch imaging from the Isaac Newton Telescope (INT) Monitoring Survey of Local Group dwarf galaxies. This analysis is conducted within a larger study to identify long-period variable (LPV) stars in these galaxies. The survey was performed with the Wide Field Camera on the 2.5-m INT and covers multiple epochs obtained between 2015 and 2018 in the i (Sloan) and V (Harris) bands. To determine the half-light radii, we derived surface brightness and number density profiles for each system and fitted them with Exponential, Plummer, and Sérsic models. The resulting half-light radii are in good agreement with literature values but reveal subtle variations linked to differences in stellar distribution and morphology. Distances were independently estimated using the Tip of the Red Giant Branch (TRGB) method, yielding values consistent with previous determinations. The complete photometric and variability catalogs will be made publicly available through CDS/VizieR, providing a valuable resource and foundation for future studies of the structure, stellar populations, and evolution of Andromeda's dwarf companions.
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Cosmology and Nongalactic Astrophysics · Cross-listed from gr-qc

Modified Gravity with Nonminimal Curvature-Matter Couplings: A Framework for Gravitationally Induced Particle Creation

Francisco S. N. Lobo, Tiberiu Harko and Miguel A. S. Pinto

Modified gravity theories with a nonminimal coupling between curvature and matter offer a compelling alternative to dark energy and dark matter by introducing an explicit interaction between matter and curvature invariants. Two of the main consequences of such an interaction are the emergence of an additional force and the non-conservation of the energy–momentum tensor, which can be interpreted as an energy exchange between matter and geometry. By adopting this interpretation, one can then take advantage of many different approaches in order to investigate the phenomenon of gravitationally induced particle creation. One of these approaches relies on the so-called irreversible thermodynamics of open systems formalism. By considering the scalar–tensor formulation of one of these theories, we derive the corresponding particle creation rate, creation pressure, and entropy production, demonstrating that irreversible particle creation can drive a late-time de Sitter acceleration through a negative creation pressure, providing a natural alternative to the cosmological constant. Furthermore, we demonstrate that the generalized second law of thermodynamics holds: the total entropy, from both the apparent horizon and enclosed matter, increases monotonically and saturates in the de Sitter phase, imposing constraints on the allowed particle production dynamics. Furthermore, we present brief reviews of other theoretical descriptions of matter creation processes. Specifically, we consider approaches based on the Boltzmann equation and quantum-based aspects and discuss the generalization of the Klein–Gordon equation, as well as the problem of its quantization in time-varying gravitational fields.
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Astrophysics of Galaxies

Spatial-Kinematic Absorption Models of the Circumgalactic Medium. I. Structures, Orientations, and Kinematics

Christopher W. Churchill

In this two-paper series, we present a straightforward mathematical model for synthesizing quasar absorption line profiles from sight lines through idealized, spatial-kinematic models of the circumgalactic medium (CGM) and their host galaxies. Here, in Paper I, we develop the spatial geometries of multiple galaxy/CGM structures and populate these structures with 3D velocity fields. For arbitrary viewing angles and galaxy-quasar impact parameters, we derive observer coordinate-based expressions for the perceived azimuthal angle and galaxy inclination and a generalized scalar expression for the line-of-sight velocity as a function of position along the line of sight. We motivate and develop four idealized galaxy/CGM spatial-kinematic structures based on empirical data and theoretical predictions: (1) a rotating galactic disk/extra-planar gas, (2) a static or dynamic spherical halo, (3) an outflowing bi-polar galactic wind, and (4) an inward spiraling flared planar accretion. Using a small set of free parameters, the spatial geometries and velocity fields can be adjusted and explored, including velocity gradients, wind stalling, and accretion trajectories. These spatial-kinematic models are designed to be flexible and easily modified and can be tailored for studying individual galaxy-absorber pairs or galaxy group environments; they can be applied to real-world observations or hydrodynamic simulations of the baryon cycle as studied through quasar absorption line systems. These models also serve as tools for developing physical intuition. In Paper II, we will present the formalism for populating the galaxy/CGM structures with multiphase photoionized and collisionally ionized gas and for generating absorption profiles for ions of interest.
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Astrophysics of Galaxies

A possible trigger of the multiple population phenomenon in star clusters

Andrés E. Piatti

Multiple populations (MPs) is a intra-star cluster phenomenon consisting in star-to-star variation of the abundance of some light chemical elements. They have been observed in many star clusters, most of them old globular clusters, populating the Milky Way and other satellite galaxies. Since the study of MPs became more systematic, different astrophysical parameters have been claimed to be the main responsible for its occurrence. However, at the present time, no attempt would seem to have solved this conundrum. This work deals with a potential trigger of the MPs phenomenon, based on the gathered observational evidence of the existence of MPs in some star clusters and the absence of its in others. We found that star clusters with MPs mostly formed during time intervals of intense star formation activity in a galaxy, for instance during the galaxy formation epoch, a close galaxy encounter, etc. At those time intervals where relative peaks in the galaxy star formation rate occur, star clusters with masses above a lower mass limit harbour MPs. This lower star cluster mass limit would marginally depend on the star cluster age.
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Astrophysics of Galaxies

Detailed Abundance Determination of Metal-Poor Stars with X-Shooter I. Unusual Chemistry in Halo Stars

Benjamin D. C. Lowe, Thomas Nordlander, Luca Casagrande et al. (+5 more)

We present a detailed chemical analysis study of 16 candidate metal-poor stars, previously identified with 2dF + AAOmega, using X-Shooter spectra and the Korg 1D local thermodynamic equilibrium spectral synthesis code. We confirm the earlier metallicity estimates and reveal six extremely metal-poor ([Fe/H] $< -3$) stars in the current sample. Two of these stars, including the most metal poor at [Fe/H] = $-3.89 \pm 0.07$, are kinematically associated with the Gaia-Sausage-Enceladus (GSE) accretion event, increasing the number of known GSE stars with [Fe/H] $< -3.5$ to eight. From the X-Shooter spectra we determine abundances for 16 elements, with the element-to-iron abundance ratios generally consistent with high-resolution studies of Galactic halo stars. Within the sample, we identify three peculiar stars: the first is a GSE nitrogen enhanced metal-poor ([N/Fe] $= 1.60 \pm 0.10$ and [C/Fe] $= 0.23 \pm 0.08$) star with unusually high Na ([Na/Fe] $= 2.26 \pm 0.07$) and Li (A(Li)$_{\rm 3DNLTE} = 1.90 \pm 0.08$) abundances, but lacking enhancements in [Al/Fe] or [Mg/Fe]. The second is a halo r-II star significantly enhanced in Sr ([Sr/Ba] $= 0.39 \pm 0.08$), suggesting mixture of r-process and s-process enrichment, uncommon for r-II stars. Whilst the third is a halo star very depleted in N ([N/Fe] $< -1.11$), with low C ([C/Fe] $= -0.33 \pm 0.08$) and otherwise 'normal' [X/Fe] abundances, suggesting enrichment with Type II supernova that proceeds enrichment from massive asymptotic giant branch stars. This study reveals the substantial degree of chemical diversity in the stellar populations which assembled the early Milky Way.
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All authors Benjamin D. C. Lowe, Thomas Nordlander, Luca Casagrande, Gary S. Da Costa, Norbert Christlieb, Sarah E. Aquilina, Tomasz Rozanski, Giacomo Cordoni

High Energy Astrophysical Phenomena

XRISM High-resolution Spectroscopy of SS 433: Evidence of Decreasing Line-of-Sight Velocity Dispersion along the Jet

Megumi Shidatsu, Shogo Kobayashi, Yusuke Sakai et al. (+6 more)

We report on the jet structure in SS 433 based on X-ray high resolution spectroscopy with the XRISM/Resolve. The source was observed over 5 days covering both inside and outside an eclipse of the compact object by the companion star. Doppler-shifted, ionized Fe and Ni K emission lines were resolved, as well as lower-energy lines including Si and S K lines. Time-resolved spectral analysis showed that Fe and Ni K line widths were $1020 \pm 40$ km s$^{-1}$ (corresponding the 1$\sigma$ width) in the eclipse phase, gradually increased during the egress, and reached $1740 \pm 30$ km s$^{-1}$ outside the eclipse. A time-averaged spectrum outside the eclipse confirmed that the Fe and Ni K lines in 5.5-9 keV are significantly broader than the Si and S K$\alpha$ emission lines in 2–4 keV. Specifically, the width in 5.5-9 keV was measured to be $1900 \pm 80$ km s$^{-1}$, whereas the width in 2-4 keV is $1300^{+300}_{-400}$ km s$^{-1}$ for the approaching (blueshifted) jet component. These results indicate that radial velocity dispersion of the jet plasma in SS 433 decreases as it moves outward. We interpret this variation as progressive jet collimation along its axis, as suggested by Namiki et al. (2003), or a decrease in turbulence in the jet plasma flow within the X-ray emitting region. We also detected a clear difference in velocity dispersion between the approaching and receding (redshifted) jet components in the 5.5-9 keV band outside eclipse. The receding jet exhibited a smaller velocity dispersion ($1400 \pm 200$ km s$^{-1}$) than the approaching jet. Since the observation was conducted when the approaching jet was tilted toward the observer, this may suggest that the receding jet was more extensively occulted by the accretion disk.
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All authors Megumi Shidatsu, Shogo Kobayashi, Yusuke Sakai, Toshihiro Takagi, Yuta Okada, Shinya Yamada, Yoshihiro Ueda, Hideki Uchiyama, Robert Petre

Astrophysics of Galaxies

On the Dusty Proximate Damped Lyman-{\alpha} System toward Q2310-3358 at z = 2.40

S. Han, J.-K. Krogager, C. Ledoux et al. (+6 more)

Quasar absorption systems not only affect the way quasars are selected, but also serve as key probes of galaxies, providing insight into their chemical evolution and interstellar medium (ISM). Recently, a method based on Gaia astrometric measurements has aided the selection of quasars reddened by dust hitherto overlooked. We conducted a spectroscopic study using VLT/X-Shooter on one such dust-reddened quasar, Q2310-3358. This quasar, at $z = 2.3909\pm0.0022$, is associated with a damped Lyman-$\alpha$ absorber (DLA) at nearly the same redshift $2.4007\pm0.0003$, with a neutral hydrogen column density of $\log N(HI)=21.214\pm0.003$. The DLA is very metal-rich (close to solar metallicity after correction for depletion on dust grains). Its properties align with the metal-to-dust ratio and the mass-metallicity relation established in previous large samples of DLAs. Surprisingly, given its proximity to the quasar in redshift, the absorber has strong cold gas characteristics, including CI and H$_2$. Based on the derived kinetic temperature of $71^{+28}_{-15}$~K, we infer the presence of a strong UV radiation field, which in turn suggests that the quasar and the DLA are in close proximity, i.e., part of the same galaxy and not just different objects in the same overdensity of galaxies. We used the line ratios of the fine-structure lines to constrain the density of the cold gas, yielding $n_{\rm H} \sim 10^{3}~\mathrm{cm}^{-3}$. Our analysis extends the understanding of $z_{abs} \approx z_{em}$ absorption line systems and provides valuable constraints on the interplay between dust, metals, and neutral gas in the ISM of early galaxies.
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All authors S. Han, J.-K. Krogager, C. Ledoux, G. Ma, K. E. Heintz, S. J. Geier, L. Christensen, P. Møller, J. P. U. Fynbo

Astrophysics of Galaxies

The luminosity function and clustering of bright quasars in the FLAMINGO cosmological simulations

Boyi Ding, Elia Pizzati, Joop Schaye et al. (+3 more)

Cosmological hydrodynamical simulations are essential tools for studying the formation and evolution of galaxies and their central supermassive black holes. While they reproduce many key observed properties of galaxies, their limited volumes have hindered comprehensive studies of the AGN and quasar populations. In this work, we leverage the FLAMINGO simulation suite, focusing on its large $(2.8$ $\mathrm{Gpc})^3$ volume, to investigate two key observables of quasar activity: the quasar luminosity function (QLF) and quasar clustering. FLAMINGO reproduces the observed QLF at low redshift ($z \lesssim 1$) and for faint quasars ($L_\mathrm{bol} \lesssim 10^{45}$ $\mathrm{erg s^{-1}}$), but significantly underpredicts the abundance of bright quasars at $z \approx 1$-$3$. Introducing a 0.75 dex log-normal luminosity scatter to represent unresolved small-scale variability boosts the number of bright quasars by upscattering lower-luminosity systems, thereby improving agreement with observations at the bright end. A decomposition of the QLF by black hole mass reveals that this boost is primarily driven by low-mass black holes radiating above the Eddington limit. Nevertheless, limitations remain in fully reproducing the rise and decline of the bright quasar population over cosmic time and in matching the black hole masses inferred from quasar spectra. Thanks to FLAMINGO's large volume, we can robustly sample rare, luminous quasars and measure their spatial clustering for $\log_{10} L_\mathrm{bol}/\mathrm{erg s^{-1}} \gtrsim 45.5$. The simulation reproduces the observed clustering across $0 \lesssim z \lesssim 3$, and the reduced luminosity dependence introduced by scatter aligns with observational trends. However, it underpredicts the clustering strength at $z \approx 4$, consistent with other models and possibly reflecting high-redshift observational uncertainties.
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All authors Boyi Ding, Elia Pizzati, Joop Schaye, Joseph F. Hennawi, William McDonald, Matthieu Schaller

Solar and Stellar Astrophysics

Eppur si eclissa: Eccentric low-mass companions and time-in-dust selection explain long secondary periods

Leen Decin, Owen Vermeulen, Mats Esseldeurs et al. (+5 more)

[abbreviated] Long Secondary Periods (LSPs) are observed in about one third of pulsating red giants yet remain unexplained. Four key observational constraints anchor the discussion: (i) a roughly 30 percent occurrence rate in semi-regular variable AGB stars (SRVs), with a much lower rate or absence in regularly pulsating Mira-type AGB stars (Miras), (ii) about 50 percent of LSP stars show a secondary mid-infrared minimum, (iii) Keplerian fits to radial-velocity curves favor argument of periastron greater than 180 degrees, and (iv) the radial-velocity to light-curve phase lag clusters around minus 90 degrees. We test whether a close-in, eccentric, low-mass companion that spends only part of its orbit within the giant's dust-formation zone can match all four empirical constraints. Guided by observed radial-velocity amplitudes and periods of about 500 to 1500 days, we adopt companion masses of 0.08 to 0.25 solar masses, orbital separations of 1.5 to 3 AU, eccentricities up to 0.6, and dust condensation radii of about 2.5 to 3 AU for SRVs (larger for Miras via scaling with luminosity). We compute the fraction of the orbit spent outside the condensation radius and apply line-of-sight criteria for detectability. We test the first three empirical constraints analytically, then model the phase offset using three-dimensional hydrodynamical simulations. Our scenario explains the observed excess of systems with argument of periastron greater than 180 degrees and yields LSP detectability of about 27 percent for SRVs and about 2.5 percent for Miras, with a conditional secondary mid-infrared eclipse fraction of about 44 percent. The hydrodynamical models place the optical-depth maximum downstream of the companion near apastron and then shift it forward by roughly 90 to 225 degrees later in the orbit, consistent with the observed radial-velocity to light-curve offsets.
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All authors Leen Decin, Owen Vermeulen, Mats Esseldeurs, Florian Driessen, Camille Landri, Davide Dionese, Lionel Siess, Dorota M. Skowron