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Physics Astrophysics

astro-ph Watch Astrophysics

Monday 2 February

Earth and Planetary Astrophysics

ExoDNN: Boosting exoplanet detection with artificial intelligence. Application to Gaia Data Release 3

A. Abreu, J. Lillo-Box, A. M. Perez-Garcia et al. (+3 more)

We combine Gaia Data Release 3 and artificial intelligence to enhance the current statistics of substellar companions, particularly within regions of the orbital period vs. mass parameter space that remain poorly constrained by the radial velocity and transit detection methods. Using supervised learning, we train a deep neural network to recognise the characteristic distribution of the fit quality statistics corresponding to a Gaia DR3 astrometric solution for a non single star. We generate a deep learning model, ExoDNN, which predicts the probability of a DR3 source to host unresolved companions based on those fit quality statistics. Applying the predictive capability of ExoDNN to a volume limited sample of F,G,K and M stars from Gaia DR3, we have produced a list of 7414 candidate stars hosting companions. The stellar properties of these candidates, such as their mass and metallicity, are similar to those of the Gaia DR3 non single star sample. We also identify synergies with future observatories, such as PLATO, and we propose a follow up strategy with the intention of investigating the most promising candidates among those samples.
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All authors A. Abreu, J. Lillo-Box, A. M. Perez-Garcia, J. Sahlmann, J. H. J. de Bruijne, C. Cifuentes

Solar and Stellar Astrophysics

Resolving the Tachocline using Inversion of Rotational Splitting Derived from Fitting Very Long and Long Time Series

Sylvain G. Korzennik and Antonio Eff-Darwich

We use rotation splittings derived from very long and long time series, namely 25.2, 12.6 and 6.3 year long, computed by Korzennik (2023) independent methodology to characterize the solar tachocline and its variation with latitude and time. We use two different inversion methodologies and a model of the tachocline to derive its position, width and the amplitude of the radial shear. To validate our methodology we present results from simulated rotational splittings, whether including or not random noise commensurable with the current observational precision. We also describe how we leverage the fact that one of our methodologies uses an initial guess that can be chosen to include a priori information. In order to try to resolve the tachocline, we increased the radial density of the inversion grid and showed how it affect the inferences. We also show how the trade off between smoothing and noise magnification affects these, as well as the effectiveness of using an informed initial guess. Results derived from high-precision rotational splittings show clearly that the location of the tachocline at low latitudes is different for its position at high latitudes. The latitudinal variation of its width is not significantly constrained, but our results agree with estimates based on forward modeling. When using splittings derived from somewhat shorter time series, we find temporal variations that are neither definitive nor significant, since we see systematic differences when using different methodologies.
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Solar and Stellar Astrophysics

Validating the Angular Sizes of Red Clump Stars with Intensity Interferometry

Alex G. Kim and Robin Kaiser

The surface-brightness-color (SBC) relationship for Red Clump stars provides a critical foundation for precision distance ladder measurements, including the 1% distance determination to the Large Magellanic Cloud. Current SBC calibrations rely on angular diameter measurements of nearby Red Clump stars obtained through long-baseline optical interferometry using the Very Large Telescope Interferometer. We explore the application of intensity interferometry to measure limb-darkened angular diameters of Red Clump stars, offering a complementary approach to traditional amplitude interferometry. We describe the framework for extracting angular diameters from squared visibility measurements in intensity interferometry, accounting for limb darkening through the stellar atmosphere models. For the Red Clump star HD~17652, we show that intensity interferometry in the $H$ band at baselines matching PIONIER ($\sim$100~m) could achieve $<1$% angular size uncertainties in 2-hour exposures by measuring the primary peak of the visibility function, enabling direct comparison with existing measurements. Critically, observations at shorter wavelengths probe the secondary visibility maximum, providing independent checks of both measurement and systematic errors that are largely insensitive to limb-darkening assumptions. Exploiting the multiplex advantage of simultaneous multi-bandpass observations and the large number of baselines available with telescope arrays such as the Cherenkov Telescope Array Observatory can reduce observing times to practical levels, making intensity interferometry a viable tool for validating the angular sizes for a subset of the Red Clump star calibration sample.
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Astrophysics of Galaxies · Cross-listed from physics.flu-dyn

Direct power spectral density estimation from structure functions without Fourier transforms

Mark A. Bishop, Sean Oughton, Tulasi N. Parashar et al. (+1 more)

Second-order structure functions and power spectral densities are popular tools in the study of statistical properties across scales, particularly for the analysis of turbulent flows. Although intimately related, analyses primarily use one or the other. We introduce a framework for estimating the power spectrum using the second-order structure function without applying Fourier transforms – enabling one to take advantage of the real-space structure function calculations. We validate and showcase this method, comparing it to classical Fourier power spectrum estimates determined from analytical calculations, fractional Brownian motion, turbulence simulations, and space-physics and astrophysical observations of turbulence. We show that this method is able to robustly obtain the expected power law behaviour where we use turbulence ranges as test-cases.
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All authors Mark A. Bishop, Sean Oughton, Tulasi N. Parashar, Yvette C. Perrott

Cosmology and Nongalactic Astrophysics

Probing beyond the Standard Model with gravitational waves from phase transitions

Chiara Caprini

This review article is based on a seminar presented at the Higgs pairs workshop 2025. Stochastic gravitational wave backgrounds can serve as probe of the diverse phenomenology encountered in beyond-Standard-Model scenarios featuring phase transitions in the early Universe. Focussing on gravitational wave production from first-order phase transitions, we present the main results of a recent analysis by the LISA Cosmology Working Group concerning the detectability of such signals with LISA. Strong degeneracies, both among the parameters controlling the phase transition and between these and the parameters of the beyond-Standard-Model scenario underlying the phase transition, complicate the reconstruction of the model from a potential signal. Nonetheless, once a specific scenario is assumed, LISA observations can supply constraints possibly complementary to those obtainable from present and future particle colliders.
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Solar and Stellar Astrophysics

Ca ii 854.2 nm in an enhanced network region simulated with MURaM-ChE

P.A. Ondratschek, D. Przybylski, H.N. Smitha et al. (+2 more)

The Ca ii 854.2 nm line is widely used to study the chromosphere of the Sun. In the quiet Sun, the spatially averaged line profile shows a red asymmetry and a redshift of the line center. It is known that the effect of isotopic splitting must be taken into account in the forward modeling to reproduce the observed asymmetry. So far, no numerical model could match an average observed line profile in terms of the line width and asymmetry. Our goal is to investigate how well a simulation computed with the chromospheric extension of the MURaM code (MURaM-ChE) reproduces the spatially averaged Ca ii 854.2 nm line profile. We aim to determine the contributions from the isotopic splitting versus the dynamics in the atmosphere to the resulting line width and asymmetry. We solve the radiative transfer problem three times, once considering only the most abundant isotope of calcium in the atmosphere, once taking six calcium isotopes into account, and finally using a single composite atom model. We find the forward modeled spatially and temporally averaged spectra to be in good agreement with an average observation of the quiet Sun. In order to match the observed line width, the simulated atmosphere must be sufficiently dynamic. The typical red asymmetry can only be reproduced by taking the isotopic splitting effect into account, as suggested in the literature.
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All authors P.A. Ondratschek, D. Przybylski, H.N. Smitha, R.H. Cameron, S.K. Solanki

Earth and Planetary Astrophysics

Ultra-hot Jupiter atmospheres at high spectral resolution

Stefan Pelletier, Daniel Kitzmann, Valentina Vaulato et al. (+3 more)

Observations of ultra-hot Jupiters offer an unprecedented opportunity to study the physics of some of the most extreme planetary atmospheres known. With exceedingly high amounts of irradiation blasting their upper atmospheres, ultra-hot Jupiters have dayside temperatures comparable to some late type stars enabling refractory metals otherwise condensed in colder planets to exist in the gas phase, all the while still maintaining comparatively cool nightsides. The ensuing intense temperature contrasts can give rise not only to strong day-to-night winds, but also to vastly different chemical and cloud properties on opposing hemispheres. With its ability to resolve spectral features that are unique to individual chemical species, high resolution spectroscopy can unambiguously disentangle atmospheric signals of exoplanetary origin, which follow a well-defined Keplerian motion, from stationary or pseudo-stationary telluric and stellar lines. Combined, the high temperature of ultra-hot Jupiters providing access to refractory metals with narrow spectral features and the ability of high-resolution spectroscopy to resolve said narrow lines provides access to a wealth of information about these atmospheres that would otherwise be unavailable at lower resolving powers or for other types of planets. In this chapter we explore some of the key physical and chemical transitions that differentiate ultra-hot Jupiters from their colder counterparts and highlight the unique opportunities arising from probing their atmospheres using high resolution spectroscopy.
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All authors Stefan Pelletier, Daniel Kitzmann, Valentina Vaulato, Ana Rita Costa Silva, Michal Steiner, David Ehrenreich

Instrumentation and Methods for Astrophysics

Serial Charge Transfer Efficiency in ACS/WFC

Jenna E. Ryon and Norman A. Grogin

We present a dedicated study of CCD serial ($x$-direction) charge transfer efficiency (CTE) in ACS/WFC. Following past studies of parallel ($y$-direction) CTE, we use the serial CTE trails behind hot pixels in calibration dark frames to characterize charge trapping and release in the serial registers of the WFC detectors. Serial CTE trails are sharper and longer than parallel CTE trails. Many fewer charge traps come into play during serial pixel transfers than parallel transfers, which explains why parallel CTE is much worse than serial CTE. We find that serial CTE can cause losses of $\sim$0.005-0.02~mag in stellar photometry and shift stellar centroids by $\sim$0.01-0.035 pixels. The pixel-based algorithm in CALACS that corrects for parallel CTE losses in WFC data has been modified to include a correction for serial CTE losses. The PCTETAB reference file has also been updated to include serial CTE parameters. The pixel-based correction for serial CTE currently runs only on full-frame WFC images obtained after SM4 (May 2009). Shortly following the publication of this report, science data corrected for both parallel and serial CTE will be available in the MAST archive.
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Earth and Planetary Astrophysics

Unraveling the Brown Dwarf Desert: Four New Discoveries and a Unifying, Period-Coded Picture

Ján Šubjak, Rafael Brahm, Jozef Lipták et al. (+30 more)

We present four newly validated transiting brown dwarfs identified through TESS photometry and confirmed with high-precision radial velocity measurements obtained from the FEROS and PLATOSpec spectrographs. Notably, three of these companions exhibit orbital periods exceeding 100 days, thereby expanding the sample of long-period transiting brown dwarfs from two to five systems. The host stars of long-period brown dwarfs show mild subsolar metallicity. These discoveries highlight the expansion of the metal-poor, long-period distribution and help us better understand the brown dwarf desert. In our comparative analysis of eccentricity and metallicity demographics, we utilize catalogues of long-period giant planets, brown dwarfs, and low-mass stellar companions. After accounting for tidal influences, the eccentricity distribution aligns with that of low-mass stellar binaries, presenting a different profile than that observed within the giant planet population. Additionally, the metallicity of the host stars reveals a noteworthy trend: short-period transiting brown dwarfs are predominantly associated with metal-rich stars, whereas long-period brown dwarfs are more often found around metal-poor stars, demonstrating statistical similarities to low-mass stellar hosts. This trend has also been previously observed in studies of hot and cold Jupiters and points to a period-coded mixture of channels. A natural explanation is that most brown dwarfs originate from fragmentation at wider separations, with long-period systems retaining this stellar-like imprint, while only those embedded in massive, long-lived, metal-rich protoplanetary discs are efficiently delivered and stabilised to short orbits.
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All authors Ján Šubjak, Rafael Brahm, Jozef Lipták, Jan Eberhardt, Marcelo Tala Pinto, Sarah L. Casewell, Thomas Henning, Katharine Hesse, Trifon Trifonov, Andrés Jordán, Felipe I. Rojas, Michaela Vítková, Helem Salinas, Gavin Boyle, Vincent Suc, Luca Antonucci, Krzysztof Bernacki, César Briceño, Karen A. Collins, Jorge Fernández Fernández, Samuel Gill, Jan Janík, Nicholas Law, Andrew W. Mann, James McCormac, Adam Popowicz, Daniel Sebastian, Marek Skarka, Ján Václavík, Leonardo Vanzi, Richard G. West, Francis P. Wilkin, Carl Ziegler

Cosmology and Nongalactic Astrophysics · Cross-listed from hep-ph

Cosmological phase transitions: from particle physics to gravitational waves, semi-analytically

S. Pascoli, S. Rosauro-Alcaraz and M. Zandi

Motivated by the recent evidence of a stochastic gravitational wave background found by pulsar timing array experiments, we focus on one of the prime cosmological explanations, i.e. a supercooled first order phase transition. If confirmed, it would offer a unique opportunity to probe early Universe dynamics and the related physics beyond the Standard Model of particles and interactions. However, the prediction of the gravitational wave spectrum from a given particle physics scenario requires theoretically and computationally demanding methods. While several tools have been put forward to reduce uncertainties and automatize these computations, we study here the possibility to perform the full pipeline of computations semi-analytically in the $4D$ theory for a $U(1)^\prime$ conformal extension of the Standard Model, thus avoiding computationally intensive simulations. Our approach yields accurate results that can be used in phenomenological studies and allow for an efficient exploration of the connection between the particle physics models and their cosmological predictions.
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Astrophysics of Galaxies

The approximate gravitational lensing multiple plane mass sheet degeneracy

Luca Teodori

Strong gravitational lensing has to deal with many modeling degeneracies, the most notable being the Mass Sheet Degeneracy (MSD). We review the MSD when one needs to model more lens planes, each one with an internal mass sheet. We take into account the non-linear lens-lens coupling and line of sight effects, the latter treated as external mass sheets with associated shear. If second order shear terms on external and internal mass sheets can be neglected, we show that the MSD is always retained, and the mass sheets influence can be reabsorbed in the redefinition of angular diameter distances. In particular, internal and external mass sheets can be placed on the same footing. The version of the MSD discussed here does not require any particular relation between the internal mass sheets in the different planes. Even when including time delays from all sources, a residual degeneracy involving time delays, mass sheets and $ H_0 $ remains. We develop a framework which shows what can actually be constrained in multiple plane lens systems.
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Astrophysics of Galaxies

Molecular Gas Morphological Analogues for the Milky Way

Neal J. Evans II, Davide Elia, Keith Hawkins et al. (+2 more)

Complete catalogs of molecular clouds in the Milky Way allow analysis of the molecular medium and the star formation properties of the Milky Way that closely follows the method used for nearby galaxies. We explore whether the big dip in the radial distribution of molecular gas in the Milky Way is peculiar and find several other galaxies with similar patterns, all with similar morphological classifications of YClxxGnR, indicating a clearly defined, long bar leading to a grand-design spiral. This category is fairly rare among galaxies in the PHANGS sample, but all galaxies with this classification have some evidence for dips in the radial distribution of CO emission. The lengths of the bars correlate with the extents of the dips. The Milky Way and the other galaxies with dips have similar stellar masses and star formation rates, both lying near the high ends of the distributions for all PHANGS galaxies.
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All authors Neal J. Evans II, Davide Elia, Keith Hawkins, Sophia Stuber, Jiayi Sun

Astrophysics of Galaxies

Probing Dark Matter Halos of High-redshift Quasars via Wide-Field Clustering

Hao Meng, Guangping Ye and Huanian Zhang

High-redshift quasars are powerful tracers of both astrophysical processes and large-scale structure in the early Universe. Using a sample of 1,251 spectroscopically confirmed quasars and 827 highly reliable photometric quasar candidates selected with a machine-learning framework, all at $4.7 \leq z < 6.3$ (excluding $5.4 < z < 5.6$) and with a median luminosity of $M_{1450}\sim -25.5$, we investigate the large-scale environments of quasars near the end of cosmic reionization. We measure the projected auto-correlation function of the quasar population and derive bias parameters of $b=22.11^{+2.17}_{-2.19}$ and $28.55^{+5.76}_{-5.75}$ for the redshift intervals $4.7\leq z<5.4$ and $5.6\leq z<6.3$, respectively. These correspond to characteristic dark matter halo masses of $\log(M_{ h}/M_\odot)=12.66^{+0.10}_{-0.11}$ and $12.62^{+0.21}_{-0.21}$, respectively. We further estimate quasar duty cycles of $0.011^{+0.017}_{-0.007}$ and $0.012^{+0.104}_{-0.011}$ for the two redshift bins. These measurements are consistent with previous studies and the observed $f_{\rm duty}$–$M_{\rm halo}$ relation, indicating that only a small fraction of suitable dark matter halos host optically luminous quasars at any given time. The inferred low duty cycles suggest that a substantial fraction of supermassive black hole growth may occur during obscured accretion phases. Our results provide new constraints on the connection between high-redshift quasars and their host dark matter halos, offering valuable insights into the co-evolution of supermassive black holes and large-scale structure in the early Universe.
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Solar and Stellar Astrophysics

Time Lag between Accretion and Wind Events in the T Tauri Star RY Tau

E.V.Babina, P.P.Petrov, K.N.Grankin et al. (+1 more)

The results of spectroscopic and photometric monitoring of the classical T Tauri star RY Tau are presented. The observation series span 220 nights from 2013 to 2024. During the observation period, the star's brightness varied within the range of V=9-11 mag. The rotation axis of the "star + accretion disk" system is tilted at a large angle, so the line of sight intersects the wind region and accreting flows in the star's magnetosphere. Variability in the short-wavelength wing of the Halpha emission line and the profile of the D NaI resonance doublet are analyzed. It is shown that the wind and accretion flows vary on a time scale of approximately 20 days. When the predominant flow direction changes, a time lag is observed: initially, accretion increases, and after two days, absorption in the line-of-sight wind decreases. It is concluded that the spectral line profiles are formed in the magnetospheric accretion flows and the conical wind originating from the boundary of the star's magnetosphere. The time lag is determined by the tilt of the magnetic dipole and the opening angle of the conical wind. It is assumed that RY Tau operates in an unstable propeller mode, and fluctuations in the accretion and wind flows are caused by density waves in the accretion disk.
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All authors E.V.Babina, P.P.Petrov, K.N.Grankin, S.A.Artemenko

High Energy Astrophysical Phenomena

Cyclotron lines in subcritical X-ray pulsars: Monte Carlo simulations reveal the origin of the observed variability

Prodromos Fotiadis, Nick Loudas, Nikolaos D. Kylafis et al. (+1 more)

Observed cyclotron resonant scattering features (CRSFs) in X-ray pulsars (XRPs) exhibit strong variability. In the subcritical luminosity regime, the centroid energy ($E_{CRSF}$) and line width ($\sigma_{CRSF}$) often show positive correlations with the X-ray luminosity. We investigate the physical origin of the observed variability quantitatively, focusing on the effects of resonant scattering and Doppler shift induced by the plasma flow in the accretion funnel. We developed a relativistic Monte Carlo code to perform detailed radiative transfer calculations in the accretion funnel above the hotspot and derive angle-dependent spectra. Analytical plasma density and velocity profiles were adopted to account for the effects of radiation pressure on the flow. Approximate resonant scattering cross-sections were employed. We varied the accretion luminosity to explore the resulting variability of the CRSF properties. The emergent spectra exhibit a prominent, asymmetric CRSF accompanied by a broad blue wing. The CRSF is systematically redshifted relative to the classical cyclotron energy, with the magnitude of the redshift decreasing at higher luminosities and for larger viewing angles $\theta$. Both $E_{CRSF}$ and $\sigma_{CRSF}$ correlate positively with luminosity for all viewing angles. Their absolute values, however, depend strongly on the viewing angle, indicating substantial variability over the pulse cycle and sensitivity to the system geometry. At fixed luminosity, $E_{CRSF}$ ($\sigma_{CRSF}$) decreases (increases) with increasing $\cos\theta$. Consequently, phase-resolved observations are expected to reveal an anticorrelation between the CRSF centroid energy and width. When applied to the XRP GX 304$-$1, the model reproduces the observed CRSF variability over nearly an order of magnitude in luminosity for geometries in which the accretion funnel is predominantly viewed edge-on.
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All authors Prodromos Fotiadis, Nick Loudas, Nikolaos D. Kylafis, Joachim Trümper

Earth and Planetary Astrophysics

Imaging Venus-like Worlds: Spectral, Polarimetric, and UV Diagnostics for the Habitable Worlds Observatory

Stephen R. Kane, Kimberly M. Bott, Kenneth E. Goodis Gordon et al. (+15 more)

Understanding planetary habitability requires a comparative approach that explores the divergent evolutionary outcomes of Earth and Venus. The Habitable Worlds Observatory (HWO) will be uniquely positioned to conduct a statistical and physical census of terrestrial exoplanets spanning the Venus Zone (VZ) and the Habitable Zone (HZ), enabling the detection and atmospheric characterization of post-runaway greenhouse worlds (“exoVenuses”). We present an updated list of VZ exoplanets, which raises the number of known candidates to 370. We describe a science case and an observing strategy for VZ exoplanets that integrates precursor exoplanet detection data and stellar characterization with HWO direct imaging, spectroscopy across the UV/optical/IR, and spectropolarimetry. Our proposed framework emphasizes a pathway toward the diagnosis of sulfur chemistry (SO$_2$) and aerosol physics (H$_2$SO$_4$ clouds/hazes), planetary redox states (O$_2$/O$_3$ false positives from hydrogen loss), and cloud microphysics detection (rainbow polarization). We quantify implications for HWO requirements, including UV access to 0.2–0.4 $\mu$m, optical/NIR coverage to $\gtrsim$1.5 $\mu$m, inner working angle (IWA) reaching 0.3–1.5 AU around nearby Sun-like stars, and the SNR/resolution needed for key features. Finally, we outline a community-driven path to producing robust demographic inferences and target selection for optimizing HWO observations.
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All authors Stephen R. Kane, Kimberly M. Bott, Kenneth E. Goodis Gordon, Emma L. Miles, Colby M. Ostberg, Paul K. Byrne, Ludmila Carone, Tansu Daylan, Antonio Garcia Munoz, Caleb K. Harada, Renyu Hu, Noam. R. Izenberg, Erika Kohler, Malena Rice, Sabina Sagynbayeva, Manuel Scherf, Edward W. Schwieterman, Peter Woitke

Astrophysics of Galaxies

Search for Faint Lone Double-Peaked H$\alpha$ Lines as IMBH Signatures in the MUSE Deep Field

Jyoti Yadav, Jorge Sánchez Almeida, Casiana Muñoz Tuñón et al. (+1 more)

Double-peaked H$\alpha$ emission profiles can serve as potential signatures of accreting intermediate-mass black holes (IMBHs), particularly those residing outside galactic nuclei. Such features are expected to arise from rotating disk-like structures around black holes and can be used to identify elusive IMBH candidates. \citet{Almeida2022ApJ...934..100S} reported a sample of such double-peaked H$\alpha$ sources in the MUSE-Wide survey, interpreting them as potential signatures of wandering IMBHs after systematically excluding alternative explanations. Their method relied on constructing H$\alpha$ maps around central galaxies and visually identifying compact emission clumps in the surrounding halo regions. In this work, we revisit the analysis using the deeper MUSE Extremely Deep Field (MXDF) data and an automated detection algorithm tailored to identify such features. However, we do not recover any candidate population in MXDF, resulting in a null detection. This outcome is nevertheless informative, as it (1) highlights the inherent challenges in detecting IMBHs, and (2) demonstrates the potential of automated approaches for future systematic searches, even though it did not yield a positive outcome in this case.
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All authors Jyoti Yadav, Jorge Sánchez Almeida, Casiana Muñoz Tuñón, João Calhau

Astrophysics of Galaxies

Connecting the Dots: UV-Bright Companions of Little Red Dots as Lyman-Werner Sources Enabling Direct Collapse Black Hole Formation

Josephine F.W. Baggen, Matthew T. Scoggins, Pieter van Dokkum et al. (+3 more)

We compile a sample of 83 Little Red Dots (LRDs) with JWST imaging and find that a substantial fraction ($\sim$43%, rising to $\gtrsim$85% for the most luminous LRDs) host one or more spatially offset, UV-bright companions at projected separations of $0.5\rm \, kpc \lesssim d\lesssim 5 \rm \,kpc$, with median of $\langle d \rangle = 1.0\,\mathrm{kpc}$. This fraction is even higher when smaller spatial scales are probed at high S/N ratio: we show that the two most strongly lensed LRDs known to date, A383-LRD and the newly discovered A68-LRD, both have UV-bright companions at separations of only $d\sim0.3$ kpc, below the resolution limit of most unlensed JWST samples. We explore whether these ubiquitous red/blue configurations may be physically linked to the formation of LRDs, in analogy with the "synchronized pair" scenario originally proposed for direct-collapse black hole formation. In this picture, ultraviolet radiation from the companions, which typically have modest stellar masses ($M_\ast \sim 10^{8-9}M_\odot$), suppresses molecular hydrogen cooling in nearby gas, allowing nearly isothermal collapse and the formation of extremely compact objects, such as massive black holes or quasi-stars. Using component-resolved photometry and SED modeling, we infer Lyman-Werner radiation fields of $J_{21,LW} \sim 10^{2.5}$-$10^{5}$ at the locations of the red components, comparable to those required in direct-collapse models, suggesting that the necessary photodissociation conditions are realized in many LRD systems. This framework provides a simple and self-consistent explanation for the extreme compactness and distinctive spectral properties of LRDs, and links long-standing theoretical models for early compact object formation directly to a population now observed with JWST in the early universe.
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All authors Josephine F.W. Baggen, Matthew T. Scoggins, Pieter van Dokkum, Zoltán Haiman, Alberto Torralba, Jorryt Matthee

Earth and Planetary Astrophysics

Visual inspection of potential exocomet transits identified through machine learning and statistical methods

D.V. Dobrycheva, I.V. Kulyk, D.R. Karakuts et al. (+4 more)

In this work, we explore several ways to detect possible exocomet transits in the TESS (The Transiting Exoplanet Survey Satellite) light curves. The first one has been presented in our previous work, a machine learning approach based on the Random Forest algorithm. It was trained on asymmetric transit profiles calculated as a result of the modelling of a comet transit, and then applied to real star light curves from Sector 1 of TESS. This allowed us to detect 32 candidates with weak and non-periodic brightness dips that may correspond to comet-like events. The aim of this work is to analyse the events identified by the visual inspection to make sure that the features detected were not caused by instrumental effects. The second approach to detect possible exocomet transits, which is proposed, is an independent statistical method to test the results of the machine learning algorithm and to look for asymmetric minima directly in the light curves. This approach was applied to e̱ta} Pictoris light curves using TESS data from Sectors 5, 6, 32, and 33. The algorithm reproduced nearly all previously known events deeper than 0.03 % of the star flux, showing that it is efficient to detect shallow and irregular flux changes in the different sectors of the TESS data and at the different levels of noise. The combination of machine learning, visual inspection, and statistical analysis facilitates the identification of faint and short-lived asymmetric transits in photometric data. Although the number of confirmed exocomet transits is still small, the growing amount of observations points to their likely presence in many young planetary systems.
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All authors D.V. Dobrycheva, I.V. Kulyk, D.R. Karakuts, M.Yu. Vasylenko, Ya.V. Pavlenko, O.S. Shubina, I.V. Luk'yanyk

Cosmology and Nongalactic Astrophysics

Degeneracies and modelling choices in double-plane time-delay cosmography

Daniel Johnson, Pierre Fleury and Martin Millon

Double-plane gravitational lensing is a rare but increasingly observed phenomenon in which the light from a distant source is lensed by two foreground objects at different redshifts. Such systems can be used to provide simultaneous constraints on the Hubble constant $H_0$ and the dark-energy equation of state, independent of and complementary to other probes. However, just as for single-plane gravitational lenses, the precision of these constraints is limited by the so-called mass-sheet degeneracy (MSD) – a fundamental limit to the knowledge of the mass profiles of lens galaxies and the line of sight that can be obtained from imaging constraints alone. In this work, we show explicitly how contributions from the line of sight appear in double-plane systems. Because these contributions modify angular diameter distances, we argue that cosmological priors should not be used to simply fix the “cosmological scaling factor”, a ratio of angular diameter distances which is key to the modelling of double-plane lenses. Motivated by this fact, we generalise the double-plane MSD to account for this uncertainty in the scaling factor. While this complicates the time-delay function, we show that, using the “unfolding relation”, a geometric relation between distances which holds even in the presence of line-of-sight corrections, the uncertainty in the Hubble constant reduces to the familiar mass-sheet transformation of the first lens plane, and a line-of-sight contribution between the observer and the second lens plane. Our main message is therefore a prescription for reducing the degrees of freedom within double-plane models, while still safely accounting for the MSD in measurements of $H_0$.
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Astrophysics of Galaxies

Overview of The SDSS-V Magellanic Genesis Survey

David L. Nidever, Danny Horta, Steven R. Majewski et al. (+14 more)

The Sloan Digital Sky Survey-V (SDSS-V) Magellanic Genesis survey is a spectroscopic program designed to map the kinematic and chemical structure of the Magellanic Clouds using APOGEE and BOSS spectroscopy. This overview describes the survey's design, target selection, and science goals, and highlights some first results using these data. In the inner regions of the Large and Small Magellanic Clouds (LMC and SMC), the survey obtained high-resolution near-infrared APOGEE spectra (S/N~45) of ~14,000 bright, oxygen-rich asymptotic giant branch (AGB-O) stars. These data provide contiguous spatial coverage of the Clouds' main bodies, enabling detailed chemo-dynamical studies. To explore extended structures, the survey includes BOSS optical spectroscopy of fainter red giant (RG) stars selected with \gaia~DR3 data, reaching G~17.5. Many of these targets extend to the outer regions of the Clouds, which are known to span ~20 deg (LMC) and ~12 deg(SMC) and contain diffuse substructures of unclear origin. BOSS data in the inner regions also complement APOGEE by providing elements inaccessible in the near-infrared and enabling cross-calibration between instruments. The survey further includes APOGEE and BOSS observations of ~300 evolved massive stars and a small sample of symbiotic binaries previously observed by APOGEE-1 and -2, enhancing our understanding of massive stellar evolution and complementing the SDSS-V main-sequence massive star program.
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All authors David L. Nidever, Danny Horta, Steven R. Majewski, Andres Almeida, Joshua T. Povick, Slater J. Oden, Oscar Jimenez-Arranz, Guy Stringfellow, S. Drew Chojnowski, Roeland van der Marel, Lara Cullinane, Bruno Dias, Jennifer Johnson, John Donor, Maria-Rosa Cioni, Juna Kollmeier, Andrew Tkachenko

High Energy Astrophysical Phenomena

An infrared echo from a circumstellar disk in the hydrogen- and helium-poor SN 2024aecx

Samaporn Tinyanont, Kittipong Wangnok, Jennifer E. Andrews et al. (+34 more)

We present near-infrared (NIR) spectroscopy of the hydrogen- and helium-poor (Type Ic) supernova (SN) 2024aecx, which displays a strong NIR excess emerging 32 days post peak. SN 2024aecx is a peculiar SN Ic that exhibited luminous shock-cooling emission at early times, suggestive of close-in circumstellar medium (CSM), unexpected for this class of SNe. Its early NIR spectra are typical for a SN Ic but with strong C I absorption features. By $\sim$32 days post peak, the spectra show a strong NIR excess, while maintaining normal optical colors, unprecedented for SNe Ic. We find that the NIR excess is well fit with a single-temperature, optically thin dust model with declining temperature, increasing mass, and roughly constant luminosity over time. The NIR excess appears too promptly for dust to have formed in the SN ejecta, indicating an IR echo from pre-existing dust in the CSM. The IR echo is likely powered by the relatively slowly evolving SN peak light, and not the brief shock cooling emission, as the latter requires unrealistically high CSM densities to explain the observed dust mass. We consider different potential CSM geometries and find that a thick face-on disk with an inner edge of around $5\times 10^{16} \rm \ cm$ can best explain the dust mass and temperature evolution. In this scenario, the SN shock should start interacting with this CSM $440\pm200$ days post explosion. CSM around SN Ic is rare, and follow-up observations of SN 2024aecx will probe the mass-loss process responsible for removing hydrogen and helium from their progenitor star.
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All authors Samaporn Tinyanont, Kittipong Wangnok, Jennifer E. Andrews, Ryan J. Foley, Methawee Kaewmookda, Jacob E. Jencson, Armin Rest, Katie Auchettl, K. A. Bostroem, David A. Coulter, Poemwai Chainakun, Ryan Chornock, Kyle W. Davis, Ori D. Fox, Lluís Galbany, Thomas R. Geballe, Brian Hsu, Wynn Jacobson-Galán, Saurabh W. Jha, Ravjit Kaur, Mansi M. Kasliwal, Ryan M. Lau, Natalie LeBaron, Raffaella Margutti, Seong Hyun Park, Jeniveve Pearson, Anthony L. Piro, Conor L. Ransome, Aravind P. Ravi, Jeonghee Rho, César Rojas-Bravo, Sam Rose, David J. Sand, Nathan Smith, Manisha Shrestha, Bhagya M. Subrayan, Stefano Valenti

Cosmology and Nongalactic Astrophysics · Cross-listed from hep-th

Consistent Evaluation of the No-Boundary Proposal

Ahmed I. Abdalla, Stefano Antonini, Raphael Bousso et al. (+3 more)

We revisit the Hartle-Hawking no-boundary proposal. To extract probabilities, one must use the gravitational path integral (GPI) to compute not only the no-boundary amplitude, but also the norms by which its square is divided. We find that this dramatically alters predictions: the probability for any closed universe is either nearly 1, or exactly 1. That is, in the Hilbert space of closed universes defined by the GPI, the states of interest in cosmology are all nearly parallel to the Hartle-Hawking state up to nonperturbative corrections in $G_N^{-1}$. We also consider a statistical interpretation of the GPI, as an average of arbitrary products of amplitudes. We find that all amplitudes are exactly 1 in this case, consistent with recent arguments that the statistical approach to the GPI with a closed boundary computes an average over one-dimensional Hilbert spaces. As an example, we illustrate the consistent evaluation of the no-boundary proposal in inflationary cosmology.
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All authors Ahmed I. Abdalla, Stefano Antonini, Raphael Bousso, Luca V. Iliesiu, Adam Levine, Arvin Shahbazi-Moghaddam

High Energy Astrophysical Phenomena

Lightcurve Modelling of 2,205 ZTF DR2 Type~Ia Supernovae: Implications for SN Ia Physics and Cosmology

Nikhil Sarin, Ellen Lindsjö, Lisa Kelsey et al. (+11 more)

We fit the multi-band light curves of 2,205 Type Ia supernovae (SNe~Ia) from the Zwicky Transient Facility DR2 with a one-zone radioactive decay model with a phenomenological addition to include Fe recombination physics. We find a strong correlation between inferred nickel mass and SALT2 stretch, which our simplified modelling links to longer diffusion times in more massive ejecta, offering a physical basis for the brighter-slower relation. SNe~Ia in low-mass hosts ($\log_{10}(M_*/M_\odot) < 10$) produce $\approx 12\%$ more $^{56}$Ni than those in high-mass hosts, linking the host-galaxy mass step to ejecta properties and hinting at metallicity or age-dependent burning efficiencies. A pseudo-bolometric comparison provides lower limits on the nickel masses, highlighting their sensitivity to SED-level assumptions. Injection-and-recovery tests with realistic ZTF sampling and the same model recover the nickel scale but show significant sensitivity to distance and opacity assumptions; individual-event point estimates are therefore model-dependent. Accounting for selection biases and broad individual-event posteriors, hierarchical modelling of 902 SNe ($z \leq 0.06$) gives Gaussian population distributions with $\mu_{\rm ej} = 1.26 \pm 0.01~M_\odot$ ($\sigma_{\rm ej} = 0.33 \pm 0.01~M_\odot$) and $\mu_{\rm Ni} = 0.64 \pm 0.06~M_\odot$ ($\sigma_{\rm Ni} = 0.42 \pm 0.02~M_\odot$). This work provides a step towards physical characterization of the local SN~Ia population while quantifying diversity and environmental dependencies relevant to progenitor physics and precision cosmology.
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All authors Nikhil Sarin, Ellen Lindsjö, Lisa Kelsey, Matthew Grayling, Jesper Sollerman, Steve Schulze, Adam Miller, Madeleine Ginolin, Erin Hayes, Conor Omand, Kaisey Mandel, Aaron Do, Suhail Dhawan, Joel Johansson

Solar and Stellar Astrophysics

White Dwarf Merger Remnants with Cooling Delays on the Q Branch Lack Strong Magnetism

Lou Baya Ould Rouis, J. J. Hermes, Joseph A. Guidry et al. (+8 more)

A population of anomalous ultra-massive white dwarfs discovered with Gaia, often referred to as the Q branch, show high (multi-Gyr) cooling delays produced by exotic physical mechanisms. They are believed to be the products of stellar mergers, but the exact origin and formation channel remain unclear. We obtained a spectroscopically complete, volume-limited sample of the Q branch region within 100 pc, and found significant differences in atmospheric composition and rotation rates as a function of tangential velocity. In particular, we discover that stellar remnants with the longest cooling delays do not show strong magnetism nor detectable short-period rotational variability, as opposed to what is generally believed for double-degenerate mergers. This indicates that either these white dwarfs arise from a formation channel with no strong magnetism induced, or that the magnetism produced from the merger dissipates over the cooling delay timescales. Our follow-up photometry has also discovered pulsations in the second and third hydrogen-dominated DAQ white dwarfs, one hotter than 15,500 K, possibly extending the boundaries of the DAV instability strip for white dwarfs with thin hydrogen layers.
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All authors Lou Baya Ould Rouis, J. J. Hermes, Joseph A. Guidry, Sihao Cheng, Mukremin Kilic, Olivier Vincent, Pierre Bergeron, Simon Blouin, Adam Moss, Isaac D. Lopez, Gracyn Jewett

Astrophysics of Galaxies

Structure and Evolution of Multi-Cluster within Galactic Disc: Gaia DR3 Insights into Eight Open Clusters

A. Ahmed, W. H. Elsanhoury, D. C. Ç{\i}nar et al. (+3 more)

In this study, we present a comprehensive analysis of the structural, astrophysical, and dynamical properties of eight open clusters: NGC 559, NGC 1817, NGC 2141, NGC 7245, Ruprecht 15, Ruprecht 137, Ruprecht 142, and Ruprecht 169, using precise astrometric and photometric data from Gaia Data Release 3. By fitting the King model to the radial density profiles, we determined the structural parameters of the clusters, including core and limiting radii, which were found to range from 3.07 to 16.21 arcmin and from 9.97 to 25.97 arcmin, respectively. Fundamental astrophysical parameters were derived by fitting PARSEC isochrones to the colour-magnitude diagrams. The results show that the clusters have logarithmic ages between 7.95 and 9.34, metallicities in the range 0.007 to 0.015, and heliocentric distances between 1640 and 5203 pc. The total stellar masses of the clusters were estimated to lie between 257 and 1916 solar masses. For most of the clusters, the mass function slopes are consistent with the Salpeter initial mass function. Our dynamical analysis indicates that all clusters, except Ruprecht 15, are dynamically relaxed. In addition, the spatial distribution and the bimodal structure observed in the radial density profile of NGC 7245 provide strong evidence that this object is a binary cluster candidate. Finally, kinematic analysis and orbit integrations demonstrate that all clusters exhibit dynamical properties fully consistent with membership in the Galactic thin disc.
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All authors A. Ahmed, W. H. Elsanhoury, D. C. Ç{\i}nar, A. A. Haroon, M. S. Alenazi, E. A. Alkholy

Cosmology and Nongalactic Astrophysics

Probabilistic inference in very large universes

Feraz Azhar, Alan H. Guth and Mohammad Hossein Namjoo

[Abridged] Some cosmological theories propose that the observable universe is a small part of a much larger universe in which parameters describing the low-energy laws of physics vary from region to region. How can we reasonably assess a theory that describes such a mostly unobservable universe? We propose a Bayesian method based on theory-generated probability distributions for our observations. We focus on basic principles, leaving aside concerns about practicality. (We also leave aside the measure problem, to discuss other issues.) We argue that cosmological theories can be tested by standard Bayesian updating, but we need to use theoretical predictions for "first-person" probabilities – i.e., probabilities for our observations, accounting for all relevant selection effects. These selection effects can depend on the observer, and on time, so in principle first-person probabilities are defined for each observer-instant – an observer at an instant of time. First-person probabilities should take into account everything the observer believes about herself and her surroundings – i.e., her "subjective state". We advocate a "Principle of Self-Locating Indifference" (PSLI), asserting that any real observer should make predictions as if she were chosen randomly from the theoretically predicted observer-instants that share her subjective state. We believe the PSLI is intuitively very reasonable, but also argue that it maximizes the expected fraction of observers who will make correct predictions. Cosmological theories will in general predict a set of possible universes, each with a probability. To calculate first-person probabilities, we argue that each possible universe should be weighted by the number of observer-instants in the specified subjective state that it contains. We also discuss Boltzmann brains, the humans/Jovians parable of Hartle and Srednicki, and the use of "old evidence".
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Astrophysics of Galaxies

IRS 9: The Case for a Dynamically-Ejected Star from the Galactic Center

Matthew Hosek Jr., Tuan Do, Smadar Naoz et al. (+7 more)

Measuring stellar motions at the Milky Way's Galactic center (GC) provides unique insight into the dynamical processes within galactic nuclei. We present proper motion measurements for 23 SiO-maser emitting stars within 45” of SgrA*, including four previously reported to have velocities exceeding their local escape velocities (i.e., they are "locally unbound" from the GC). Derived from 14 epochs of HST WFC3-IR observations (2010 - 2023), our measurements have a median precision of 0.038 mas/yr - up to ~100x more precise then previous constraints for some sources. By combining these proper motions with published radial velocities, we derive updated 3D velocities for the masers and find that only one is locally unbound (IRS 9; v3d = 370 +/- 1.2 km/s). Orbit integrations place the first constraints on the orbit of IRS 9, which is bound to the GC at larger radii with r_peri >= 0.100 +/- 0.005 pc and r_apo >= 5.25 +/- 0.18 pc. IRS 9's high velocity relative to stars at similar radii in the Nuclear Star Cluster makes it a candidate to have experienced a strong dynamical interaction in order to place it on its orbit. We explore the Hills mechanism as a possible origin, but binary evaporation and ejection velocity limits indicate that IRS 9 is unlikely to have experienced such an event in the past 0.4 Myr (the timescale constrained by the orbit integrations). Alternative mechanisms that could produce IRS 9 include binary supernova disruption, two-body interactions, and stellar collisions. Identifying additional stars like IRS 9 will be essential for understanding these various dynamical processes.
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All authors Matthew Hosek Jr., Tuan Do, Smadar Naoz, Sanaea C. Rose, Gregory D. Martinez, Andrea M. Ghez, Rebecca Lewis-Merrill, Jessica R. Lu, Shoko Sakai, Jay Anderson

Instrumentation and Methods for Astrophysics

The Electromagnetically Isolated Global Signal Estimation Platform (EIGSEP)

Christian H. Bye, David R. DeBoer, Matt Dexter et al. (+14 more)

The Electromagnetically Isolated Global Signal Estimation Platform (EIGSEP) is a new instrument designed to measure the global 21-cm signal from Cosmic Dawn and the Epoch of Reionization, redshifted to frequencies below 250 MHz. To reduce spectral structure in the antenna beam associated with ground scattering, EIGSEP uses a shaped bowtie antenna suspended in a canyon 100 m above the ground. We describe the current system design of EIGSEP, including the rotating antenna platform, a transmitter antenna to characterise the beam of the bowtie antenna, and auxiliary ground antennas. We then discuss the EIGSEP calibration scheme, which incorporates traditional Dicke switching in the receiver, and novel approaches that include beam mapping, beam modulation, and interferometric cross-correlation. The instrument has been deployed near Marjum Pass, Utah, for testing and initial data collection. We discuss the site characteristics and present initial field measurements.
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All authors Christian H. Bye, David R. DeBoer, Matt Dexter, Aaron Ewall-Wice, Adam Fahs, Pranav Karthik, Komal Kaur, Bahram Khalichi, Wei Liu, Raul A. Monsalve, Aaron R. Parsons, Reid Parsons, Richard R. Rodriguez, Richard J. Saeed, Charlie G. Tolley, Dominic Vazquez, Dirk Wright

Astrophysics of Galaxies

Cosmic CO and [CII] backgrounds and the fueling of star formation over 12 Gyr

Yi-Kuan Chiang

Molecular gas, modest in mass yet pivotal within the cosmic inventory, regulates baryon cycling as the immediate fuel for star formation. Across most of cosmic history, its reservoir has remained elusive, with only the tip of the iceberg revealed by luminous carbon monoxide (CO) emitting galaxies. Here we report the first detections of the mean cosmic CO background across its rotational ladder at 7$\sigma$, together with ionized carbon ([CII]) at 3$\sigma$, over $0<z<4.2$. This uses tomographic clustering of diffuse broadband intensities with reference galaxies, directly probing aggregate emission in the cosmic web. From CO(1-0) we infer the total molecular gas density, $\Omega_{\rm H_2}$, finding it about twice that resolved in galaxy surveys. The global depletion time is $\sim$1 Gyr, shorter than the Hubble time, requiring sustained inflow. CO excitation links to star-formation surface density and, with depletion time, yields a super-linear Kennicutt-Schmidt law that appears universal. Together these results establish a global picture of galaxy growth fueled by a larger, short-lived molecular reservoir. The CO and [CII] detections mark a turning point for line-intensity mapping, replacing forecasts with empirical line strengths and defining sensitivity requirements for upcoming 3D experiments poised to open new windows on galaxy formation and cosmology.
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