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Physics Astrophysics

astro-ph Watch Astrophysics

Friday 6 February

Earth and Planetary Astrophysics

AB Aur, a Rosetta stone for studies of planet formation (IV): C/O estimates from CS and SO interferometric observations

Pablo Rivière-Marichalar, Asunción Fuente, Romane le Gal et al. (+5 more)

Context. Protoplanetary disks are the birthplace of planets. As such, they set the initial chemical abundances available for planetary atmosphere formation. Thus, studying elemental abundances, molecular compositions, and abundance ratios in protoplanetary disks is key to linking planetary atmospheres to their formation sites. Aims. We aim to derive the sulfur abundance and the C/O ratio in the AB Aur disk using interferometric observations of CS and SO. Methods. New NOEMA observations of CS 3-2 towards AB Aur are presented. We used velocity-integrated intensity maps to determine the inclination and position angles. Keplerian masks were constructed for all observed species to assess the presence of non-Keplerian motions. We use the CS/SO ratio to study the C/O ratio. We compare our present and previous interferometric observations of AB Aur with a NAUTILUS disk model to gain insight into the S elemental abundance and C/O ratio. Results. We derive an observational CS/SO ratio ranging from 1.8 to 2.6. Only NAUTILUS models with C/O > 1 can reproduce such ratios. The comparison with models points to strong sulfur depletion, with [S/H]=8e-8, but we note that no single model can simultaneously fit all observed species.
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All authors Pablo Rivière-Marichalar, Asunción Fuente, Romane le Gal, Roberto Neri, Gisela Esplugues, Dmitry Semenov, Richard Teague, Alejandro Santamaría Miranda

Instrumentation and Methods for Astrophysics

Joint optical-digital design strategy for adaptive optics systems: application to wavelength selection for satellite imaging

Florian Cheyssial, Laurent Mugnier and Cyril Petit

Adaptive optics can be used to mitigate the effects of atmospheric turbulence on imaging systems, but the correction is only partial, and deconvolution is often required to improve the resolution. This results in entire optical/digital systems, which are traditionally designed sequentially, i.e. , the adaptive optics system is optimised first, and the restoration algorithms are designed a second time. Studies on optical/digital systems have shown that jointly optimizing the whole system is a better alternative. We propose to extend these co-design strategies to the design of an adaptive optics-assisted imaging system. We derive a simple criterion that takes into account the source properties and the entire optical/ digital system performance. To illustrate its interest, we use it to optimize the wavelength distribution between the wavefront sensor and the imaging camera. In addition, we explore the potential of using multiple imaging channels operating at different wavelengths as a means of making an imaging system robust to turbulence strength and source magnitude variations. Later, any parameter of the optical/digital system, if not the entire system itself, could be optimized this way.
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Solar and Stellar Astrophysics

The Non-Eruptive Reconfiguration of a Quiescent Filament After a Nearby Active Region Emergence

James McKevitt, Louise Harra, Gherardo Valori et al. (+13 more)

The unpredictability of solar filament eruptions presents major challenges for forecasting space weather, as such eruptions frequently drive coronal mass ejections (CMEs) that impact the heliosphere. While nearby flux emergence is often linked to their destabilisation, the specific characteristics of both the emerging flux and the filament that determine whether an eruption occurs remain unclear. We report observations of a quiescent filament that did not erupt following the nearby emergence of active region NOAA 13270 and a subsequent C-class flare in April 2023. Our analysis combines multi-viewpoint extreme ultraviolet (EUV) imaging and X-ray imaging with EUV spectroscopy, radio imaging and measurements of, and extrapolations from, the photospheric magnetic field. We identify the formation of a coronal null point and fan-spine topology at the interface between the active region and filament which exhibited persistent slow reconnection, indicated by chromospheric brightenings, persistent radio emission, and plasma upflows. Our results indicate that ongoing reconnection and jets can relieve magnetic stress and enable filament stability, even when under strong perturbation. We suggest that the orientation of emerging flux relative to the ambient field is a critical parameter in filament evolution, and provide observational constraints for models of filament stability and eruption.
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All authors James McKevitt, Louise Harra, Gherardo Valori, Deborah Baker, Nils Janitzek, Stephanie Yardley, Sarah Matthews, Hamish Reid, Alexander W. James, Muriel Stiefel, David H. Brooks, Ryan Dewey, Jim M. Raines, Susan T. Lepri, Liang Zhao, Juan Sebastián Castellanos Durán

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

The canonical energy-momentum currents in cosmology

Tomi S. Koivisto

The parallel theory of relativity predicts conserved energy-momentum currents for an arbitrary metric, without invoking Killing symmetries. By treating the reference frame as an independent variational field and requiring it to carry no energy, the theory naturally unifies Einstein's two formulations of gravity and yields uniquely defined covariant charges. In isotropic and homogeneous cosmology, the canonical time direction selected by the reference frame coincides with the Kodama vector, and the associated Noether energy reproduces the Misner-Sharp mass.
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Instrumentation and Methods for Astrophysics

A Demonstration of a Neural Network as a Bridge Between Galaxy Simulations and Surveys

E. Elson

This paper demonstrates that the stellar masses of galaxies in the Galaxy and Mass Assembly (GAMA) survey, originally derived via stellar population synthesis modelling, can be accurately predicted using only their absolute magnitudes and colour indices. A central contribution of this work is the demonstration that this long-standing inference problem can be solved using an exceptionally simple machine-learning model: a fully connected, feed-forward artificial neural network with a single hidden layer. The network is trained exclusively on synthetic galaxies generated by the SHARK semi-analytic model and is shown to transfer effectively to real observations. Across nearly 3.5 dex in stellar mass, the predicted values closely track the GAMA SED-derived masses, with a typical scatter of ~0.131 dex. These results demonstrate that complex deep-learning architectures are not a prerequisite for robust stellar mass estimation, and that simulation-trained, lightweight machine-learning models can capture the dominant physical information encoded in broad-band photometry. The method is further applied to 17,006 GAMA galaxies lacking SED-derived masses, with photometric uncertainties propagated through the network to provide corresponding error estimates on the inferred stellar masses. Overall, this work establishes a computationally efficient and conceptually transparent pathway for simulation-to-observation transfer learning in galaxy evolution studies.
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Solar and Stellar Astrophysics

The Walkaway Star HP Tau/G2: Evidence for a Stellar Merger

Bo Reipurth, J. Bally, P. Friberg et al. (+6 more)

HP~Tau/G2 is a luminous, short-period, fast-rotating G-type weak-line T Tauri star with a large radius, an oblate shape with gravity-darkening, little circumstellar material, and centered in a slowly expanding cloud cavity. It is an X-ray source and a variable nonthermal radio source. It forms, together with the late-type T Tauri star KPNO 15, a pair of oppositely directed walkaway stars launched when a multiple system broke apart ~5600 yr ago. Momentum conservation indicates a mass of G2 of only ~0.7 Msun, much lower than the ~1.9 Msun determined from evolutionary models. G2 is virtually a twin of FK Com, the prototype of a class of evolved stars resulting from coalescence of W UMa binaries. We suggest that G2 became a very close and highly eccentric binary during viscous evolution in the protostellar stage and with KPNO 15 formed a triple system, which again was part of a larger unstable group including the binary G3 and the single G1. Dynamical evolution led to multiple bound ejections of KPNO 15 before it finally escaped after ~2 Myr. As a result the G2 binary recoiled and contracted 5600 yr ago, became Darwin unstable and merged in a major outburst ~2000 yr ago. The nearby compact triple system G1+G3 was also disturbed, and broke up 4900 yr ago, forming another walkaway pair. The G5 star HD 283572 has similar unusual properties, indicating that G2 is not a pathological case. G2 is now fading towards a new stable configuration. YSO mergers may be rather common and could explain some FUor eruptions.
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All authors Bo Reipurth, J. Bally, P. Friberg, D.M. Faes, C. Briceno, M.S. Connelley, C. Flores, A.M. Cody, H. Zinnecker

Astrophysics of Galaxies

ELUCID-DESI I: A Parallel MPI Implementation of the Initial Condition Solver for Large-Scale Reconstruction Simulations

Wensheng Hong, Xiaohu Yang, Junde Li et al. (+10 more)

We present a highly scalable, MPI-parallelized framework for reconstructing the initial cosmic density field, designed to meet the computational demands of next-generation cosmological simulations, particularly the upcoming ELUCID-DESI simulation based on DESI BGS data. Building upon the Hamiltonian Monte Carlo approach and the FastPM solver, our code employs domain decomposition to efficiently distribute memory between nodes. Although communication overhead increases the per-step runtime of the MPI version by roughly a factor of eight relative to the shared-memory implementation, our scaling tests-spanning different particle numbers, core counts, and node layouts-show nearly linear scaling with respect to both the number of particles and the number of CPU cores. Furthermore, to significantly reduce computational costs during the initial burn-in phase, we introduce a novel “guess” module that rapidly generates a high-quality initial density field. The results of the simulation test confirm substantial efficiency gains: for $256^3$ particles, 53 steps ($\sim$ 54 core hours) are saved, accelerating convergence by a factor of $\sim$ 18; for $1024^3$, 106 steps ($\sim$7500 core hours), achieving a speedup factor of $\sim$ 3. The total core hour gain grows with the number of particles, rendering large-volume reconstructions computationally practical for upcoming surveys, including our planned ELUCID-DESI reconstruction simulation with $4096^3$ particles. We estimate that achieving convergence for this scale (targeting DESI-BGS data) requires about 800 HMCMC steps ($\sim$ 5 million core hours). Our initial guess module will save approximately 360 steps ($\sim$2.3 million core hours), reducing the total computational time by about 45%.
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All authors Wensheng Hong, Xiaohu Yang, Junde Li, Huiyuan Wang, Zhao Chen, Hong-Ming Zhu, Qingyang Li, Yizhou Gu, Youcai Zhang, Feng Shi, Jiaxin Han, Yu Yu, Zhongxu Zhai

Cosmology and Nongalactic Astrophysics

The Massive and Distant Clusters of WISE Survey 2: Detection of splashback radii in galaxy cluster total light stacks

A. Trudeau, Anthony H. Gonzalez, K. Thongkham et al. (+6 more)

The splashback radius, the radius of the apocenter of the first orbit of infalling material, is a measurable quantity marking the boundary between a galaxy cluster and its infalling region. We report detections of splashback radii in total light stacks, i.e. image stacks centered on the cores of galaxy clusters. Our analysis uses Wide-field Infrared Survey Explorer (WISE) W1 and W2 images of 83,345 candidate clusters at $0.5 \lesssim z \lesssim 1.9$ from the Massive and Distant Clusters of WISE Survey 2 (MaDCoWS2). The clusters are organized in stacks by redshift and signal-to-noise ($S\slash N$) ratios. We adopt a statistical approach, using 1000 bootstrap realizations to determine the median projected splashback radius and its confidence interval in a given bin. We compare our splashback radii with the measurements made by K. Thongkham et al. on a similar sample of MaDCoWS2 clusters using galaxy-cluster cross-correlation and find that they are consistent, although our method yields larger error bars. Our main systematic error is the accuracy of the background subtraction, but its impact remains small: the consistency of K. Thongkham et al. and our results suggests that neither method suffers from large systematics. The sensitivity of total light stacking to the contribution of faint galaxies can be advantageous to locate splashback radii when only the brightest galaxies are detected in individual images, such as at high redshifts. We present a potential application of this new technique to probe the evolution of the stellar mass in cluster infalling regions.
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All authors A. Trudeau, Anthony H. Gonzalez, K. Thongkham, M. Brodwin, Thomas Connor, Peter R. M. Eisenhardt, Emily Moravec, S. A. Stanford, D. Stern

Cosmology and Nongalactic Astrophysics

The Massive and Distant Clusters of WISE Survey 2: Splashback Radii to z=1.65 from Galaxy Density Profiles

Khunanon Thongkham, Anthony H. Gonzalez, Mark Brodwin et al. (+6 more)

The Massive and Distant Clusters of WISE Survey 2 (MaDCoWS2) is a WISE-selected catalog of galaxy clusters at $0.1<z<2$ covering an effective area of $>6000$ deg$^2$. In this paper, we derive splashback radii for this cluster ensemble from galaxy density profiles and constrain the mass threshold of the survey as a function of redshift. We use MaDCoWS2 cluster candidates at $0.4\leq z \leq 1.65$ divided into subsamples with different signal-to-noise (S/N$_{\rm P}$) and redshifts, cross-correlated with galaxies from the CatWISE2020 catalog, to obtain average surface density profiles. We perform a Markov Chain Monte Carlo analysis to derive parameter estimates for theoretical models consisting of orbiting and infalling terms. A distinct splashback feature is detected in all subsamples. The measured splashback radii span from $0.89^{+0.02}_{-0.02}h^{-1}$ comoving Mpc/cMpc ($0.61^{+0.02}_{-0.02}h^{-1}$ proper Mpc/pMpc) at $\overline{z}=0.45$ to $1.27^{+0.05}_{-0.05}h^{-1}$ cMpc ($0.53^{+0.04}_{-0.04}h^{-1}$ pMpc) at $\overline{z}=1.54$. We also find that splashback radii increase with $S/N_{\rm P}$ at fixed redshift. The resultant splashback radii constrain the redshift dependence of the mass of MaDCoWS2 clusters at fixed $S/N_{\rm P}$. We calculate $M_{\rm 200m}$ from the radii using a relation based on a cosmological simulation. MaDCoWS2 $M_{\rm 200m}$ values derived from the simulation-based relation are lower than the expected values based on weak-lensing observations. More robust mass constraints will come from calibrating splashback radii derived from galaxy density profiles with weak lensing shear profiles from facilities such as $\textit{Euclid}$, Rubin, and $\textit{Roman}$.
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All authors Khunanon Thongkham, Anthony H. Gonzalez, Mark Brodwin, Ariane Trudeau, Peter Eisenhardt, S. A. Stanford, Emily Moravec, Thomas Connor, Daniel Stern

High Energy Astrophysical Phenomena

Super-knee cosmic rays from interacting supernovae

Nick Ekanger, Shigeo S. Kimura and Kazumi Kashiyama

There is increasing evidence that, in the very late phase of stellar evolution before core collapse, massive stars have winds with large mass loss rates that give rise to a dense circumstellar medium (CSM) surrounding the progenitor star. After core collapse, a shock wave forms when the supernova ejecta interacts with this CSM. In such an interaction, the nuclei in the CSM can undergo diffusive shock acceleration and reach very high energies. We consider such a model, which includes magnetic field amplification from the non-resonant streaming instability, enhancement to the abundance of heavy-ions, and composition-dependent acceleration. Applying this to several supernova subclasses, we find that IIn supernovae can supply a dominant fraction of the observed super-knee cosmic-ray (CR) flux from $\sim{\rm few}\times10^{15}\,{\rm eV}$ to $\sim{\rm few}\times10^{17}\,{\rm eV}$ and is consistent with recent LHAASO measurements above the CR knee. This systematic model also explains the increasingly heavy nuclear composition in this energy range.
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High Energy Astrophysical Phenomena

Primordial Black Hole signatures from femtolensing and spectral fringe of Gamma Ray Bursts

Chang-Yu Dai and Po-Yan Tseng

Femtolensing of gamma-ray bursts (GRBs) is vastly studied to constrain primordial black holes lighter than $10^{-13}$ solar mass and may close the window for PBH dark matter. In this case, wave optics formalism is required and carefully implemented in our analysis. Incorporating the GRB observational data from Swift XRT, we perform the statistical analysis of PBH lensing, comparing it with the null hypothesis where the BAND model is used to parametrize the GRB spectrum. We found a few GRB data manifest the spectral fringe which characterizes the feature of femtolensing by PBHs, and the analysis shows moderate statistical preference in terms of goodness of fit. Conversely, since most of the fits to GRB spectral data do not improve with PBH lensing, we utilize this to obtain an upper bound on the PBH fractional abundance with respect to dark matter. However, the robust constraint cannot be achieved, unless the size of GRBs is smaller than $5\times 10^7$ m for PBH mass around $5\times 10^{-15}$ solar mass.
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Earth and Planetary Astrophysics

Reversal of Spin: Comet 41P/Tuttle-Giacobini-Kresak

David Jewitt

The rotations of cometary nuclei are known to change in response to outgassing torques. The nucleus of comet 41P/Tuttle-Giacobini-Kresak exhibited particularly dramatic rotational changes when near perihelion in 2017 April. Here, we use archival Hubble Space Telescope observations from 2017 December to study the post-perihelion lightcurve of the nucleus and to assess the nucleus size. From both Hubble photometry and non-gravitational acceleration measurements we find a diminutive nucleus with effective radius r = 500+/-100 m. Systematic optical variations are consistent with a two-peaked (i.e., rotationally symmetric) lightcurve with period 0.60+/-0.01 days, substantially different from periods measured earlier in 2017. The spin of the nucleus likely reversed between perihelion in 2017 April and December as a result of the strong outgassing torque. We infer a dimensionless moment arm k = 0.013, about twice the median value in short-period comets. The lightcurve range of 0.4 magnitudes indicates a projected nucleus axis ratio greater than 1.4:1, while the active fraction of the nucleus decreased from 2.4 in 2001 (suggesting augmentation of the gas production by sublimating coma ice grains) to 0.14 in 2017, a result of long-term modification of the surface. We find that the physical lifetime of this small nucleus to spin-up is short compared to the reported 1500 year dynamical time spent in the current orbit. Two limiting reconciliations of this inequality are suggested. The nucleus could be in a state of unusually strong activity, leading us to over-estimate the average mass loss rate and outgassing torque and so to under-estimate the physical lifetime. Alternatively, the nucleus could be the surviving remnant of a once larger body for which outgassing torques were less effective in changing the spin.
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Cosmology and Nongalactic Astrophysics

Characterization of the Polarization Beam Response of SPT-3G Using Point Sources

T. de Haan, M. Archipley, N. Huang et al. (+101 more)

Precise measurements of cosmic microwave background (CMB) polarization require rigorous control of instrumental systematics. For the South Pole Telescope's third-generation camera (SPT-3G), which observes in three bands centered near 95, 150, and 220 GHz, accurate beam characterization is critical for interpreting the polarized mm-wave sky. We present direct measurements of SPT-3G's polarized beam response from observations of 100 bright extragalactic point sources. Previous SPT-3G power spectrum analyses introduced a phenomenological parameter, $\beta_{\rm pol}$, to describe the polarization preserved in beam sidelobes, and found evidence for significant depolarization from the requirement of inter-frequency polarization power spectrum consistency. Our direct measurements yield $\beta_{\rm pol}=0.89\pm0.10$ at 95 GHz, $1.08\pm0.10$ at 150 GHz, and $0.90\pm0.22$ at 220 GHz, indicating minimal sidelobe depolarization. We validate these results with systematic tests of posterior sampling versus bootstrap resampling, real-space versus Fourier-space analysis, temperature-to-polarization leakage handling, covariance determination, and source selection. Compared to values inferred from previous cosmological analyses, our results differ by an effective $1.3\sigma$. This apparent discrepancy is model dependent, because the point source analysis derives much of its $\beta_{\rm pol}$ constraining power from higher multipoles than the power spectrum analysis. These measurements therefore admit three explanations for the frequency-dependent residuals observed in the power spectrum analysis: a statistical fluctuation, the need for more sophisticated polarized beam models, or systematics other than beam depolarization.
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All authors T. de Haan, M. Archipley, N. Huang, A. J. Anderson, B. Ansarinejad, L. Balkenhol, D. R. Barron, K. Benabed, A. N. Bender, B. A. Benson, F. Bianchini, L. E. Bleem, S. Bocquet, F. R. Bouchet, L. Bryant, E. Camphuis, M. G. Campitiello, J. E. Carlstrom, J. Carron, C. L. Chang, P. Chaubal, P. M. Chichura, A. Chokshi, T.-L. Chou, A. Coerver, T. M. Crawford, C. Daley, K. R. Dibert, M. A. Dobbs, M. Doohan, A. Doussot, D. Dutcher, W. Everett, C. Feng, K. R. Ferguson, N. C. Ferree, K. Fichman, A. Foster, S. Galli, A. E. Gambrel, R. W. Gardner, F. Ge, N. Goeckner-Wald, R. Gualtieri, F. Guidi, S. Guns, N. W. Halverson, E. Hivon, A. Y. Q. Ho, G. P. Holder, W. L. Holzapfel, J. C. Hood, A. Hryciuk, F. Keruzore, A. R. Khalife, L. Knox, M. Korman, K. Kornoelje, C.-L. Kuo, K. Levy, Y. Li, A. E. Lowitz, C. Lu, G. P. Lynch, T. J. Maccarone, A. S. Maniyar, E. S. Martsen, F. Menanteau, M. Millea, J. Montgomery, Y. Nakato, T. Natoli, G. I. Noble, Y. Omori, A. Ouellette, Z. Pan, P. Paschos, K. A. Phadke, A. W. Pollak, K. Prabhu, W. Quan, M. Rahimi, A. Rahlin, C. L. Reichardt, M. Rouble, J. E. Ruhl, E. Schiappucci, A. C. Silva Oliveira, A. Simpson, J. A. Sobrin, A. A. Stark, J. Stephen, C. Tandoi, B. Thorne, C. Trendafilova, C. Umilta, J. D. Vieira, A. G. Vieregg, A. Vitrier, Y. Wan, N. Whitehorn, W. L. K. Wu, M. R. Young, J. A. Zebrowski

High Energy Astrophysical Phenomena

An Archival Optical Counterpart Search for Extragalactic Fast X-Ray Transients Discovered by Einstein Probe

Run-Duo Liang, Wen-Xiong Li, Liang-Duan Liu et al. (+50 more)

Extragalactic fast X-ray transients (eFXTs) represent a rapidly growing class of high-energy phenomena, whose physical origins remain poorly understood. With its wide-field, sensitive all-sky monitoring, the Einstein Probe (EP) has greatly increased the discovery rate of eFXTs. The search and identification of the optical counterparts of eFXT are vital for understanding their classification and constraining their physical origin. Yet, a considerable fraction of eFXTs still lack secure classifications due to the absence of timely follow-up observations. We carry out a systematic search of publicly available optical survey data and transient databases (including the Zwicky Transient Facility, ZTF, and the Transient Name Server, TNS) for optical counterparts to eFXT candidates detected by EP. In this paper, we describe our ongoing program and report the first results. Specifically, we identified the eFXT EP240506a to be associated with a UV/optical counterpart, AT 2024ofs. Spectroscopy of its host galaxy with VLT yields a redshift of $z = 0.120 \pm 0.002$. By combining archival survey data with early-time multiwavelength observations, we find that the luminosity and light-curve evolution of AT~2024ofs are consistent with a core-collapse supernova origin. From detectability simulations, we estimate a local event rate density $\rho_{0}=8.8^{+21.2}_{-3.9}\ \mathrm{yr^{-1}\, Gpc^{-3}}$ for EP240506a-like events, and completeness-corrected rate of about $36$–$78\ \mathrm{yr^{-1}\ Gpc^{-3}}$ for EP-detected X-ray transients associated with supernovae. Our results demonstrate the potential of EP to uncover prompt high-energy emission from core-collapse supernovae and underscore the critical importance of timely follow-up of future eFXT events.
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All authors Run-Duo Liang, Wen-Xiong Li, Liang-Duan Liu, Ken Smith, Stephen Smartt, Niu Li, Arne Rau, Ling-Zhi Wang, Armin Rest, Ezequiel Treister, Jia-Sheng Huang, Franz Bauer, Jennifer Chacon, Ning-Chen Sun, Qin-Yu Wu, Seán Brennan, Matt Nicholl, Ting-Wan Chen, Amar Aryan, Sheng Yang, Albert K.H. Kong, Sofia Rest, Qinan Wang, James Gillanders, Dong-Yue Li, An Li, Jun Yang, Qing-Chang Zhao, Hui Sun, Yun-Fei Xu, Zhi-Xing Ling, Thomas J. L. de Boer, Chien-Cheng Lin, Thomas B. Lowe, Ken C. Chambers, Eugene A. Magnier, J. Quirola-Vásquez, Xiaofeng Wang, Jing-Wei Hu, Yong Chen, Chen Zhang, Dong-Hua Zhao, He-Yang Liu, Hua-Qing Cheng, Chen-Zhou Cui, Shu-Mei Jia, Cheng-Kui Li, Ju Guan, Mao-Hai Huang, Hao-Wei Peng, Samaporn Tinyanont, Yuan Liu, Wei-Min Yuan

Earth and Planetary Astrophysics

Simulated LSST Observations of Real Metre-scale Imminent Impactors

Michael A. Frazer, Hadrien A. R. Devillepoix and Sophie E. Deam

As of mid-2026, 11 objects have been discovered prior to impacting the Earth, with warning times between 2 - 20 hours. Using real metre-sized Earth impactors from the last decade, we ask the question: “If the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) had been operating over the last decade, how many imminent impactors would it have observed and discovered pre-impact, and how early would these discoveries have been?” We use the LSST Solar System Survey Simulator Sorcha and a population of real fireballs observed by orbital sensors over the last decade to investigate which events would have been observed pre-impact. We find that the LSST would have observed 30 (13.9%) of the 216 simulated objects, with most objects receiving 2 - 4 observations. Using the default linking algorithm, only two (0.9%) of these objects would have been `discovered' pre-impact. Using a modified linking algorithm better suited to fast moving objects, this increases to eight (3.7%). Based on this, we predict that the LSST will discover 8 +/- 2 imminent impactors over its nominal 10 year survey, at the low end of previous estimations. However, we predict these objects to be discovered 4 days pre-impact, substantially earlier than the current average. This will bring significant opportunities for telescopic follow-up, targeted fireball observations, planetary defence planning, and public engagement. There is also significant potential for precovery for impactors observed by the LSST but discovered by other surveys, instantly lengthening observation arcs and thereby reducing the orbital and impact location uncertainties. In some cases, these observations may also enable the linkage of telescopic observations with observed fireballs post-impact, providing valuable pre-impact astrometric and photometric data. This has significant implications for both asteroid research and planetary defence.
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Astrophysics of Galaxies

Treecode2: The Power of Pluralism. I. Static Tests

Joshua E. Barnes

I describe an `oct-tree' N-body code which randomly shifts, reorients, and resizes the root cell at each time step. Averaging over a plurality of root cell positions and orientations statistically restores translational and rotational invariance. The potentials and forces which result can be much more accurate than those obtained from a single force calculation. In this paper, the principle of averaging is tested on static configurations. The next paper will show how this technique can substantially improve global energy, momentum, and angular momentum conservation at a negligible computational cost.
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Cosmology and Nongalactic Astrophysics

On the Difficulties with Late-Time Solutions for the Hubble Tension

Prakhar Bansal and Dragan Huterer

We explore the notion that cosmological models that modify the late-time expansion history cannot simultaneously fit the SH0ES collaboration's measurements of the Hubble constant, DESI baryon acoustic oscillations data, and Type Ia supernova distances. Adopting a few simple phenomenological models, we quantitatively demonstrate that a satisfactory fit with a model with late-time expansion history can only be achieved if one of the following is true: 1) there is a sharp step in the absolute magnitude of Type Ia supernovae at very low redshift, $z\sim 0.01$, or 2) the distance duality relation, $d_L(z)=(1+z)^2d_A(z)$, is broken. Both solutions are trivial in that they effectively decouple the calibrated SNIa measurements from other data, and this qualitatively agrees with previous work built on studying specific dark-energy models. We also identify a less effective class of late-time solutions with a transition at $z\simeq 0.15$ that lead to a more modest improvement in fit to the data than models with a very low-z transition. Our conclusions are largely unchanged when we include surface brightness fluctuation distance measurements, with their current systematic uncertainties, to our analysis. We finally illustrate our findings by studying a physical model which, when equipped with the ability to smoothly change the absolute magnitude of Type Ia supernovae, partially resolves the Hubble tension.
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Thursday 5 February

Cosmology and Nongalactic Astrophysics · Cross-listed from physics.gen-ph

Model selection with the Pantheon+ Type Ia SN sample

Namit Chandak, Fulvio Melia and Junjie Wei

Recent discoveries, e.g., by JWST and DESI, have elevated the level of tension with inflationary $\Lambda$CDM. For example, the empirical evidence now suggests that the standard model violates at least one of the energy conditions from general relativity, which were designed to ensure that systems have positive energy, attractive gravity and non-superluminal energy flows. In this Letter, we use a recently compiled Type Ia supernova sample to examine whether $\Lambda$CDM violates the energy conditions in the local Universe, and carry out model selection with its principal competitor, the $R_{\rm h}=ct$ universe. We derive model-independent constraints on the distance modulus based on the energy conditions and compare these with the Hubble diagram predicted by both $\Lambda$CDM and $R_{\rm h}=ct$, using the Pantheon+ Type Ia supernova catalog. We find that $\Lambda$CDM violates the strong energy condition over the redshift range $z \subset (0, 2)$, whereas $R_{\rm h}=ct$ satisfies all four energy constraints. At the same time, $R_{\rm h}=ct$ is favored by these data over $\Lambda$CDM with a likelihood of $\sim 89.5\%$ versus $\sim 10.5\%$. The $R_{\rm h}=ct$ model without inflation is strongly favored by the Type Ia supernova data over the currrent standard model, while simultaneously adhering to the general relativistic energy conditions at both high and low redshifts.
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Earth and Planetary Astrophysics · Cross-listed from physics.space-ph

Venting and Outgassing Simulations of Pressurized Lunar Modules: Contamination of the Lunar Environment

Stefano Boccelli, William M. Farrell, Prabal Saxena et al. (+1 more)

One objective of Artemis science is to determine the impact human activities have on the lunar environment, which might compromise science objectives and measurements. We perform a preliminary analysis of the contamination associated with airlock venting and outgassing from a prototype lunar-module geometry intended to host astronauts on the lunar surface. The air flow generated by the depressurization of the airlock, expanding in the lunar exosphere, is studied using the Direct Simulation Monte Carlo (DSMC) method for two different venting configurations and the particle flux on the surface is computed as a function of the distance from the the module. Outgassing from the main body of the module – assumed to be covered with a Multi-Layer Insulation (MLI) blanketing – and from the solar panels is then analyzed using a view-factor method, employing outgassing rates from the literature.Our results give preliminary indications of the distance at which contamination levels fall below the values characteristic of native species in the lunar atmosphere. Scientific measurements targeting 40Ar should be carried farther than 30–100 meters from the module, while the detection of lower-abundance species such as polar-crater water might require to travel up to and beyond 3 km from the module.
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All authors Stefano Boccelli, William M. Farrell, Prabal Saxena, Orenthal J. Tucker

Solar and Stellar Astrophysics

Disk-Regulated Mass Transfer Between Rotating Non-Degenerate Stars: Insights from Be and sdOB Binaries

Zepei Xing, Tassos Fragos, Vicky Kalogera et al. (+3 more)

Mass transfer between non-degenerate stars is a fundamental but still poorly understood process in binary evolution. The commonly used rotationally limited accretion prescription in detailed binary evolution simulations that account for stellar rotation generally yields low accretion efficiencies that are difficult to reconcile with several observational constraints. We present a physically-motivated mass-accretion prescription in which accretion or decretion disks regulate the angular momentum transported to the accretor, thereby allowing for continued accretion at near-critical rotation. The accretion efficiency can be calculated from the conservation of the mass and the angular momentum of the disk. Analytical estimates show that the accretion efficiency depends on stellar rotation and mass ratio for direct impact accretion, and additionally on stellar radius and orbital separation in the disk accretion regime. The overall mass-weighted accretion efficiencies are close to the values expected near the threshold rotation rate, where the accreted specific angular momentum declines sharply. Applying this model to binary evolution simulations, we find that rotationally limited accretion systematically underestimates Be-star masses in Be+subdwarf O/B-type star (sdOB) systems, whereas the disk-star coupling model can produce more massive Be stars that are consistent with observations. The final binary component masses depend not only on accretion efficiency but also core-envelope mass ratio, which itself depends sensitively on the assumed overshooting. We find that our new disk-star coupling model with reduced overshooting yields component masses for Be+sdOB systems that are in closer agreement with observations.
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All authors Zepei Xing, Tassos Fragos, Vicky Kalogera, Seth Gossage, Kyle Akira Rocha, Emmanouil Zapartas

Astrophysics of Galaxies

Dwarf Galaxy Number Counts within 25 Mpc: Predictions from Local Group Analogues in TNG50

Evangela E. Shread, Joanna M. Piotrowska, Daniel R. Weisz et al. (+2 more)

The modern generation of wide-field galaxy surveys, such as LSST, Euclid and Roman, will enable studies of dwarf galaxies $(10^6 \leq M_\ast / M_\odot \leq 10^9)$ beyond the Local Group (LG) in unprecedented detail. Improved theoretical understanding of this population is necessary to guide these observations, since predictions in this regime are generally limited to specific environments like the LG. We present predictions for the population of dwarf galaxies from the TNG50 run of the IllustrisTNG suite of cosmological hydrodynamical simulations, focusing on the environments within $1 < D / \mathrm{Mpc} < 25$ of LG analogues at $z = 0$. In the simulated sample, there are $\sim 1,000$ and $\sim 12,000$ dwarf galaxies within $10$ and $25$ Mpc, respectively. We compare our results with the 50 Mpc Galaxy Catalog and estimate that current observations are highly incomplete at low masses: for $10^6 \leq M_\ast / M_\odot \leq 10^7$ $(-13 \lesssim M_r \lesssim -10)$, we find completeness fractions of $\sim 23 \%$ within $10$ Mpc and $\sim 4 \%$ within $25$ Mpc. The simulated galaxies below the completeness limits of the observations exist in a range of environments, with notable populations of field dwarfs at all distances and satellites around centrals with masses $10^8 \lesssim M_\ast / M_\odot \lesssim 10^{11}$ within $10-25$ Mpc. We find that there are $\sim 8$ times more quiescent dwarf galaxies in the TNG50 sample than are currently cataloged. Our results suggest that upcoming observations should uncover a substantial population of dwarf galaxies, and that $\gtrsim 15 \%$ of these will be red, currently quenched galaxies in the field.
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All authors Evangela E. Shread, Joanna M. Piotrowska, Daniel R. Weisz, Michael Boylan-Kolchin, Fiona A. Harrison

Solar and Stellar Astrophysics

Compact HII Regions as Clocks of Massive-Star Formation: Evidence for Long Formation Timescales

Paolo Padoan and Mark Gieles

We revisit the luminosity function (LF) of compact HII regions in the context of the inertial–inflow model (IIM), in which massive stars assemble over extended, mass-dependent timescales. The comparison of the compact-HII-region LF with that of OB stars has been used to estimate the compact-HII-phase lifetime and is often cited as evidence for the classical “lifetime problem” of HII regions. We show that once stellar growth during the ionizing phase is included, the LF comparison instead constrains massive-star formation timescales, so the lifetime problem turns into evidence for prolonged growth. We illustrate the principle with a simple analytic model, derive revised Galactic LFs for compact HII regions and OB stars from the Red MSX Source survey and the Alma Luminous Star catalogue, and fit the LFs jointly with a deterministic forward model based on stellar evolutionary tracks. The joint LF constraints imply a growth law in which the formation time is about 4 Myr for a $60\,M_\odot$ star, with an approximately square-root dependence on mass, as predicted by the IIM and supported by the numerical simulations from which it was derived. They also require the field stellar initial mass function to be a broken power law, with a slope close to Salpeter's at low masses and significantly steeper above approximately $18\,M_\odot$, as expected from the model prediction that the maximum stellar mass scales with the mass of the parent cloud. We conclude that massive stars in the Milky Way form over Myr timescales that increase with their final mass.
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High Energy Astrophysical Phenomena

Hypersoft X-ray Sources as a Low-Energy Class of Luminous Cosmic Emitters

Mustafa Muhibullah, Jimmy A. Irwin and R. Di Stefano

X-ray binaries, powered by accretion onto black holes, neutron stars, or white dwarfs from companion stars, are among the brightest non-explosive beacons in galaxies, outshining the Sun by millions. Typical X-ray binaries emit primarily above 0.3 keV, whereas cooler sources, peaking in the extreme ultraviolet (EUV), remain challenging to detect due to a long-standing observational blind spot. Here, we report the discovery of a remarkable class of luminous, point-like, non-nuclear X-ray objects–hypersoft X-ray sources–that have largely been missed by previous surveys. Detected primarily or exclusively below 0.3 keV, the most luminous examples radiate near 1E38 erg/s in the narrow X-ray band, with spectral models suggesting even higher bolometric luminosities, most of which emerge in the EUV. They rank among the most energetic sources in galaxies, yet their EUV-peaking spectra evaded earlier detections. We propose that hypersoft sources represent X-ray binaries spanning multiple physical classes, including accreting white dwarfs or post-nova systems–potential progenitors of Type Ia supernovae–and systems hosting accreting black holes. Beyond their elusive nature, they may play a crucial role in ionizing gas within galaxies.
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Astrophysics of Galaxies

On the radial velocity wave in the Galactic disk

Chris Hamilton, Andrew Mummery and Joss Bland-Hawthorn

Stars in the Galactic disk have mean radial velocities $\overline{v}_R$ that oscillate as a function of angular momentum $J_\varphi$. This `$J_\varphi$-${\overline{v}}_R$ wave' signal also exhibits a systematic phase shift when stars are binned by their dynamical temperatures. However, the origin of the wave is unknown. Here we use linear perturbation theory to derive a simple analytic formula for the $J_\varphi$-$\overline{v}_R$ signal that depends on the equilibrium properties of the Galaxy and the history of recent perturbations to it. The formula naturally explains the phase shift, but also predicts that different classes of perturbation should drive $J_\varphi$-$\overline{v}_R$ signals with very different morphologies. Ignoring the self-gravity of disk fluctuations, it suggests that neither a distant tidal kick (e.g., from the Sgr dwarf) nor a rigidly-rotating Galactic bar can produce a qualitatively correct $J_\varphi$-$\overline{v}_R$ wave signal. However, short-lived spiral arms can, and by performing an MCMC fit we identify a spiral perturbation that drives a $J_\varphi$-${\overline{v}}_R$ signal in reasonable agreement with the data. We verify the analytic formula with test particle simulations, finding it to be highly accurate when applied to dynamically cold stellar populations. More work is needed to deal with hotter orbits, and to incorporate the fluctuations' self-gravity and the role of interstellar gas.
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High Energy Astrophysical Phenomena

How Neutron Star Observations Point Towards Exotic Matter: Existing Explanations and a Prospective Proposal

Mauro Mariani and Ignacio F. Ranea-Sandoval

Multi-messenger astronomical observations of neutron stars, together with more precise calculations and constraints coming from dense matter microphysics, are generating tension with regard to equations of state models used to describe neutron star cores. Assuming an abrupt first-order phase transition with a slow conversion speed between phases, we propose different slow stable hybrid star configurations aiming to reconcile all current constraints simultaneously; within this framework, we also introduce a novel non-CSS parametrization to the quark matter equation of state and discuss its strengths and limitations. We analyze our model results in conjunction with a review of other relevant theoretical possibilities existing in the literature. We found that modern neutron star observations seem to favor the existence of some type of exotic matter in the neutron star cores; in particular, our slow stable hybrid star scenario remains a proposal capable of satisfying these constraints. However, due both to the existing skepticism regarding some of the adopted hypotheses in most extreme neutron star measurements and to the precise adjustment needed for the equation-of-state parameters, significant tension and open questions remain.
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Cosmology and Nongalactic Astrophysics

Dynamic Zoom Simulations of structure formation beyond standard cosmology

Riccardo Zangarelli, Marco Baldi, Federico Marinacci et al. (+1 more)

(Abridged) A thorough interpretation of the current and upcoming generation of cosmological observations requires unprecedented large-scale, high-resolution simulations spanning multiple cosmological models and parameters. The realization of these computationally demanding simulations poses a crucial technical challenge. We present beyond - $\Lambda$CDM implementations of the Dynamic Zoom Simulations (DZS) method, a performance-enhancing technique tailored for large-scale simulations that produce lightcone-like outputs. This approach dynamically decreases the resolution of a simulation in the regions that are not in causal connection with the observer, saving computational resources without directly affecting the physical properties within the lightcone. We implemented the DZS algorithm in two state-of-the-art codes supporting non-standard cosmologies, namely modified $f(R)$ gravity in Arepo and dark sector interactions in Gadget4. We analyzed result accuracy and performance gains across resolution, simulation volume and model by comparing runs performed with and without the DZS algorithm. Our DZS reproduce the lightcone halo mass function, sky-projected massmaps, and matter and weak lensing convergence power spectra with an accuracy of $\simeq$ 0.1% or higher in most cases. In terms of performance, DZS runs in our test simulations can save up to $\sim$ 50% runtime compared to the non-DZS counterparts. A scaling to larger simulated volumes suggests that performance gains could improve by an additional $\sim$ 20% at the resolution levels of current state-of-the-art simulations. The validation of the DZS algorithm in non-standard models demonstrates that this technique can enable cost effective, large-scale ($\gtrsim$ 1 cGpc/h) simulations with state-of-the-art resolution, providing the computational framework needed to constrain and help the interpretation of forthcoming data.
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All authors Riccardo Zangarelli, Marco Baldi, Federico Marinacci, Enrico Garaldi

High Energy Astrophysical Phenomena

Ne and Fe abundances in the ISM: Archival Study of Fe-L and Ne-K edges in Chandra and XMM-Newton

D. L. Moutard, L. R. Corrales, I. Psaradaki et al. (+2 more)

The abundance of elements in the interstellar medium (ISM) is a key facet for many fields of astrophysical study. In the soft X-ray spectra, absorption by interstellar gas can result in deep absorption features that affect continuum measurements. In this paper, we focus on measuring the abundance of interstellar iron and neon from the column densities observed in soft spectra from XMM-Newton and Chandra for various low mass X-ray binaries (LMXBs), which allows for a direct probe of elemental abundances. As a noble gas, neon will not deplete into solid form, thus providing a benchmark with abundances determined via UV spectroscopy. We find that, when assuming Fe is 90% depleted into grains, [Fe/Ne]$ = -0.523\pm0.025$, [Fe/H]$ + 12 = 7.482\pm0.016$, and [Ne/H]$ + 12 = 8.012\pm0.022$, which are the tightest observational constraints on these abundances to date, while being consistent with literature which uses protosolar abundances. We also test how depletion into solid grains and scattering affect the results. The choice of depletion fraction can affect the abundance measurement by roughly $5\%$, and that the inclusion of a scattering component can affect abundance measurements by $\sim1-7\%$.
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All authors D. L. Moutard, L. R. Corrales, I. Psaradaki, E. Temple, M. Shi

Astrophysics of Galaxies

The imprint of AGN-driven outflows on the CGM: the case of Ly{\alpha} nebulae around high-z quasars

Silvia C. Rueda-Vargas, Vincenzo Mainieri, Giulia Tozzi et al. (+5 more)

Some cosmological hydrodynamical simulations predict that outflows driven by active galactic nuclei (AGN) play a key role in powering the Ly$\alpha$ nebulae observed around high-redshift quasars. In such simulations, AGN feedback seeded as powerful outflows leads to extended and luminous nebulae whose morphology and surface-brightness profiles accurately reproduce the observations, while suppressing AGN feedback leads to compact and faint nebulae. This link might arise from outflows opening up a channel for Ly$\alpha$ photons to escape from the galactic nucleus to the circumgalactic medium (CGM). The main aim of this paper is to test this theoretical prediction using observations, by comparing the physical properties of outflows and Ly$\alpha$ nebulae. We analyze integral-field unit data obtained with VLT/ERIS and GEMINI/GNIRS to trace the ionized gas in the interstellar medium (ISM) of a sample of six quasars at $z\sim2-3$, using the [O III] emission line. We detect powerful outflows in all the quasars of our sample, with velocities $>1500~\mathrm{km~s^{-1}}$ and kinetic energies $ \gtrsim 2\times10^{43}~\mathrm{erg~s^{-1}}$. Four of our quasars are spatially resolved and show signs of extended [O III] emission out to distances $>2$ kpc from the central supermassive black hole. When excluding one outlier, we find a positive monotonic correlation between the outflow power and the Ly$\alpha$ nebulae size ($\rho=0.89$, $p=0.03$) and luminosity ($\rho=0.6$, $p=0.28$). Additionally, we find evidence of spatial alignment between the ionization cone and the inner and brightest regions of the Ly$\alpha$ nebula. Our results provide tentative evidence in support of the theoretical prediction that AGN-driven outflows at ISM scales open a low-optical-depth path for central Ly$\alpha$ photons to reach the CGM and create extended nebulae.
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All authors Silvia C. Rueda-Vargas, Vincenzo Mainieri, Giulia Tozzi, Darshan Kakkad, Fabrizio Arrigoni-Battaia, Tiago Costa, Emanuele Paolo Farina, Chris M. Harrison

Earth and Planetary Astrophysics

Satellites and small bodies with ALMA: Insights into Solar System formation & evolution

Katherine de Kleer, Michael E. Brown, Martin Cordiner et al. (+1 more)

Our understanding of the formation and evolution of planetary systems has made major advances in the past decade. This progress has been driven in large part by the Atacama Large Millimeter/submillimeter Array (ALMA), which has given us an unprecedented view of Solar System bodies themselves, and of the structure and chemistry of forming exoplanetary systems. Within our own Solar System, ALMA has enabled the detection of new molecules and isotopologues across moons and comets, as well as placing new constraints on the compositions and histories of small bodies through thermal emission observations. In this article, we highlight some key areas where ALMA has contributed to a deeper understanding of our Solar System's formation and evolution, and place these discoveries in the context of our evolving understanding of protoplanetary disks.
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All authors Katherine de Kleer, Michael E. Brown, Martin Cordiner, Richard Teague

Earth and Planetary Astrophysics

Probing habitable regions with SRG/eROSITA

E. Gatuzz, S. Rukdee, S. Freund et al. (+1 more)

Stellar high-energy radiation is a key driver of atmospheric erosion and evolution in exoplanets, directly affecting their long-term habitability. We present a comprehensive study on stellar high-energy radiation and its impact on exoplanetary atmospheres, leveraging data from the SRG/eROSITA all-sky survey. Our sample consists of 3750 main-sequence stars identified by cross-matching with Gaia DR3. Utilizing X-ray spectral fits from the eROSITA catalog, we computed X-ray ($L_X$) and combined extreme-ultraviolet (EUV) luminosities ($L_{\mathrm{EUV}}$), which we used to derive XUV fluxes at the habitable zone ($F_{\mathrm{XUV,HZ}}$). We find that the majority of stars in our sample are significantly more XUV-active than the Sun, with habitable zone fluxes ranging from $10^0$ to $10^5$ erg~cm$^{-2}$~s$^{-1}$. The ratio of $L_{\mathrm{XUV}}/L_{\mathrm{bol}}$ is found to be higher for cooler, magnetically active stars, highlighting their potentially hazardous nature for planetary atmospheres. Applying the energy-limited escape model, we computed atmospheric mass-loss rates for hypothetical earth-like planets located at the habitable zone of each star. We also present local maps for distances up to $500$~pc of the average XUV flux, revealing “hazard zones” where stellar radiation could significantly influence planetary atmospheric evolution. This work demonstrates the power of X-ray surveys in constraining the high-energy environments of exoplanets and underscores the critical role of stellar activity in planetary habitability.
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All authors E. Gatuzz, S. Rukdee, S. Freund, T. Kallman