Jon M. Miller, Misaki Mizumoto, Megumi Shidatsu, Ralf Ballhausen, Ehud Behar, María Díaz Trigo, Chris Done, Tadayasu Dotani, Javier A. García, Timothy Kallman, Shogo B. Kobayashi, Aya Kubota, Randall Smith, Hiromitsu Takahashi, Makoto Tashiro, Yoshihiro Ueda, Jacco Vink, Shinya Yamada, Shin Watanabe, Ryo Iizuka, Yukikatsu Terada, Chris Baluta, Yoshiaki Kanemaru, Shoji Ogawa, Tessei Yoshida and Katsuhiro Hayashi
{"title":"XRISM Spectroscopy of the Stellar-mass Black Hole 4U 1630-472 in Outburst","authors":"Jon M. Miller, Misaki Mizumoto, Megumi Shidatsu, Ralf Ballhausen, Ehud Behar, María Díaz Trigo, Chris Done, Tadayasu Dotani, Javier A. García, Timothy Kallman, Shogo B. Kobayashi, Aya Kubota, Randall Smith, Hiromitsu Takahashi, Makoto Tashiro, Yoshihiro Ueda, Jacco Vink, Shinya Yamada, Shin Watanabe, Ryo Iizuka, Yukikatsu Terada, Chris Baluta, Yoshiaki Kanemaru, Shoji Ogawa, Tessei Yoshida and Katsuhiro Hayashi","doi":"10.3847/2041-8213/ade25c","DOIUrl":"https://doi.org/10.3847/2041-8213/ade25c","url":null,"abstract":"We report on XRISM/Resolve spectroscopy of the recurrent transient and well-known black hole candidate 4U 1630−472 during its 2024 outburst. The source was captured at the end of a disk-dominated high/soft state at an Eddington fraction of λEdd ∼ 0.05 (10 M⊙/MBH). A variable absorption spectrum with unprecedented complexity is revealed with the Resolve calorimeter. This marks one of the lowest Eddington fractions at which highly ionized absorption has been detected in an X-ray binary. The strongest lines are fully resolved, with He-like Fe XXV separated into resonance and intercombination components and H-like Fe XXVI seen as a spin–orbit doublet. The depth of some absorption lines varied by almost an order of magnitude, far more than expected based on a 10% variation in apparent X-ray flux and ionization parameter. The velocity of some absorption components also changed significantly. Jointly modeling two flux segments with a consistent model including four photoionization zones, the spectrum can be described in terms of highly ionized but likely failed winds that sometimes show redshifts, variable obscuration that may signal asymmetric structures in the middle and outer accretion disk, and a tentative very fast outflow (v = 0.026–0.033c). We discuss the impact of these findings on our understanding of accretion and winds in stellar-mass black holes and potential consequences for future studies.","PeriodicalId":501814,"journal":{"name":"The Astrophysical Journal Letters","volume":"109 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144645534","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Shuai-Qing Jiang, Dong Xu, Agnes P. C. van Hoof, Wei-Hua Lei, Yuan Liu, Hao Zhou, Yong Chen, Shao-Yu Fu, Jun Yang, Xing Liu, Zi-Pei Zhu, Alexei V. Filippenko, Peter G. Jonker, A. S. Pozanenko, He Gao, Xue-Feng Wu, Bing Zhang, Gavin P Lamb, Massimiliano De Pasquale, Shiho Kobayashi, Franz Erik Bauer, Hui Sun, Giovanna Pugliese, Jie An, Valerio D’Elia, Johan P. U. Fynbo, WeiKang Zheng, Alberto J. Castro-Tirado, Yi-Han Iris Yin, Yuan-Chuan Zou, Adam T. Deller, N. S. Pankov, A. A. Volnova, A. S. Moskvitin, O. I. Spiridonova, D. V. Oparin, V. Rumyantsev, O. A. Burkhonov, Sh. A. Egamberdiyev, V. Kim, M. Krugov, A. M. Tatarnikov, R. Inasaridze, Andrew J. Levan, Daniele Bjørn Malesani, Maria E. Ravasio, Jonathan Quirola-Vásquez, Joyce N. D. van Dalen, Javi Sánchez-Sierras, Daniel Mata Sánchez, Stuart P. Littlefair, Jennifer A. Chacón, Manuel A. P. Torres, Ashley A. Chrimes, Nikhil Sarin, Antonio Martin-Carrillo, Vik Dhillon, Yi Yang, Thomas G. Brink, Rebecca L. Davies, Sheng Yang, Am..
{"title":"EP240801a/XRF 240801B: An X-Ray Flash Detected by the Einstein Probe and the Implications of Its Multiband Afterglow","authors":"Shuai-Qing Jiang, Dong Xu, Agnes P. C. van Hoof, Wei-Hua Lei, Yuan Liu, Hao Zhou, Yong Chen, Shao-Yu Fu, Jun Yang, Xing Liu, Zi-Pei Zhu, Alexei V. Filippenko, Peter G. Jonker, A. S. Pozanenko, He Gao, Xue-Feng Wu, Bing Zhang, Gavin P Lamb, Massimiliano De Pasquale, Shiho Kobayashi, Franz Erik Bauer, Hui Sun, Giovanna Pugliese, Jie An, Valerio D’Elia, Johan P. U. Fynbo, WeiKang Zheng, Alberto J. Castro-Tirado, Yi-Han Iris Yin, Yuan-Chuan Zou, Adam T. Deller, N. S. Pankov, A. A. Volnova, A. S. Moskvitin, O. I. Spiridonova, D. V. Oparin, V. Rumyantsev, O. A. Burkhonov, Sh. A. Egamberdiyev, V. Kim, M. Krugov, A. M. Tatarnikov, R. Inasaridze, Andrew J. Levan, Daniele Bjørn Malesani, Maria E. Ravasio, Jonathan Quirola-Vásquez, Joyce N. D. van Dalen, Javi Sánchez-Sierras, Daniel Mata Sánchez, Stuart P. Littlefair, Jennifer A. Chacón, Manuel A. P. Torres, Ashley A. Chrimes, Nikhil Sarin, Antonio Martin-Carrillo, Vik Dhillon, Yi Yang, Thomas G. Brink, Rebecca L. Davies, Sheng Yang, Am..","doi":"10.3847/2041-8213/addebf","DOIUrl":"https://doi.org/10.3847/2041-8213/addebf","url":null,"abstract":"We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on 2024 August 1 by the Einstein Probe (EP) satellite with a weak contemporaneous signal also detected by the Fermi Gamma-ray Burst Monitor (GBM). Optical spectroscopy of the afterglow, obtained by Gran Telescopio Canarias and Keck, identified the redshift of z = 1.6734. EP240801a exhibits a burst duration of 148 s in X-rays and 22.3 s in gamma rays, with X-rays leading by 80.61 s. Spectral lag analysis indicates that the gamma-ray signal arrived 8.3 s earlier than the X-rays. Joint spectral fitting of EP Wide-field X-ray Telescope and Fermi/GBM data yields an isotropic energy , a peak energy , and a fluence ratio S(25–50 keV)/S , classifying EP240801a as an X-ray flash (XRF). The host-galaxy continuum spectrum, inferred using Prospector, was used to correct its contribution for the observed outburst optical data. Unusual early R-band behavior and EP Follow-up X-ray Telescope observations suggest multiple components in the afterglow. Three models are considered: a two-component jet model, a forward-reverse shock model, and a forward shock model with energy injection. All three provide reasonable explanations. The two-component jet model and the energy injection model imply a relatively small initial energy and velocity of the jet in the line of sight, while the forward-reverse shock model remains typical. Under the two-component jet model, EP240801a may resemble GRB 221009A (BOAT) if the bright narrow beam is viewed on-axis. Therefore, EP240801a can be interpreted as an off-beam (narrow) jet or an intrinsically weak GRB jet. Our findings provide crucial clues for uncovering the origin of XRFs.","PeriodicalId":501814,"journal":{"name":"The Astrophysical Journal Letters","volume":"206 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144652327","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Tushar Suhasaria, Vanessa Leuschner, Cornelia Jäger, Caroline Gieser and Thomas Henning
{"title":"CO2 Infrared Spectra on Silicate Dust Grain Analogs: Implications for JWST Observations","authors":"Tushar Suhasaria, Vanessa Leuschner, Cornelia Jäger, Caroline Gieser and Thomas Henning","doi":"10.3847/2041-8213/ade993","DOIUrl":"https://doi.org/10.3847/2041-8213/ade993","url":null,"abstract":"Carbon dioxide is one of the three most abundant species within the ice mantles around dust grains inside molecular clouds. Since a substantial number of interstellar grains are made of siliceous materials, we have studied the infrared profile of CO2 deposited on top of a bare and ice-coated amorphous silicate (MgFeSiO4) film using reflection absorption infrared spectroscopy (RAIRS). In contrast to a metal surface, the CO2 IR profile shows a relaxation of the metal surface selection rule in the presence of the bare MgFeSiO4 dust grain analog, which brings the IR profile closer to the observational spectra while maintaining the sensitivity of RAIRS. Experiments with the underlying CO and CH4 ices show that their presence facilitates structural changes toward crystalline ice for the deposited CO2 at much lower temperatures than on the polar ice layers. Warming-up experiments of CO2 showed that it tends to stay on the silicate surface for much longer than on the gold surface without the silicate layer. We noticed for the first time a split in the 13CO2 IR feature on the pure or ice-covered silicate grain as a marker for the onset of diffusion. The laboratory 13CO2 profile then closely resembles recent JWST observations of this feature around young and embedded protostars, suggesting that it can be linked to the observed feature.","PeriodicalId":501814,"journal":{"name":"The Astrophysical Journal Letters","volume":"662 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144652328","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Mihailo M. Martinović, Kristopher G. Klein, Rossana De Marco, Daniel Verscharen, Roberto Bruno and Raffaella D’Amicis
{"title":"Impact of Two-population α-particle Distributions on Plasma Stability","authors":"Mihailo M. Martinović, Kristopher G. Klein, Rossana De Marco, Daniel Verscharen, Roberto Bruno and Raffaella D’Amicis","doi":"10.3847/2041-8213/adeaac","DOIUrl":"https://doi.org/10.3847/2041-8213/adeaac","url":null,"abstract":"The stability of weakly collisional plasmas is well represented by linear theory, and the generated waves play an essential role in the thermodynamics of these systems. The velocity distribution functions (VDFs) characterizing kinetic particle behavior are commonly represented as a sum of anisotropic bi-Maxwellians. A three bi-Maxwellian model is commonly applied for the ions, assuming that the VDF consists of a proton core, a proton beam, and a single He (α) particle population, each with its own density, bulk velocity, and anisotropic temperature. Resolving a secondary α-beam component was generally not possible due to instrumental limitations. The Solar Orbiter Solar Wind Analyser Proton and Alpha Sensor (SWA-PAS) resolves velocity space with sufficient coverage and accuracy to consistently characterize secondary α populations. This design makes the SWA-PAS ideal for examining the effects of α-particle beams on the plasma’s kinetic stability. We test the wave signatures observed in the magnetic field power spectrum at ion scales and compare them to the predictions from linear plasma theory, Doppler-shifted into the spacecraft reference frame. We find that taking into account the α-particle beam component is necessary to predict the coherent wave signatures in the observed power spectra, emphasizing the importance of separating the α-particle populations as is traditionally done for protons. Moreover, we demonstrate that the drifts of beam components are responsible for the majority of the modes that propagate in the oblique direction to the magnetic field, while their temperature anisotropies are the primary source of parallel fast magnetosonic modes.","PeriodicalId":501814,"journal":{"name":"The Astrophysical Journal Letters","volume":"14 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144652331","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Fangyi (Fitz) Hu, Adelle Goodwin, Daniel J. Price, Ilya Mandel, Re’em Sari and Kimitake Hayasaki
{"title":"Radio Emission from Tidal Disruption Events Produced by the Collision between Super-Eddington Outflows and the Circumnuclear Medium","authors":"Fangyi (Fitz) Hu, Adelle Goodwin, Daniel J. Price, Ilya Mandel, Re’em Sari and Kimitake Hayasaki","doi":"10.3847/2041-8213/adeb79","DOIUrl":"https://doi.org/10.3847/2041-8213/adeb79","url":null,"abstract":"In this Letter, we simulate the collision between outflows from the tidal disruption of a 1 M⊙ main-sequence star around a 106M⊙ black hole and an initially spherically symmetric circumnuclear cloud. We launch super-Eddington outflows self-consistently by simulating the disruption of stars on both bound and unbound initial orbits using general relativistic smoothed particle hydrodynamics. We find that shocks formed as early as ∼10 days after the initial stellar disruption produce prompt radio emission. The shock radius (≈1017 cm), velocity (∼0.15c), and total energy (∼1051 erg) in our simulations match those inferred from radio observations of tidal disruption events (TDEs). We ray-trace to produce synthetic radio images and spectra to compare with the observations. While the TDE outflow is quasi-spherical, the synchrotron emitting region is aspherical but with reflection symmetry above and below the initial orbital plane. Our synthetic spectra show a continuous decay in peak frequency, matching prompt radio TDE observations. Our model supports the hypothesis that synchrotron radio flares from TDEs result from the collision between outflows and the circumnuclear material.","PeriodicalId":501814,"journal":{"name":"The Astrophysical Journal Letters","volume":"6 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144652334","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Wenshuai Cheng, Ming Xiong, Yiming Jiao, Hao Ran, Liping Yang, Huidong Hu and Rui Wang
{"title":"Inertial-range Turbulence Anisotropy of the Young Solar Wind from Different Source Regions","authors":"Wenshuai Cheng, Ming Xiong, Yiming Jiao, Hao Ran, Liping Yang, Huidong Hu and Rui Wang","doi":"10.3847/2041-8213/adeb8a","DOIUrl":"https://doi.org/10.3847/2041-8213/adeb8a","url":null,"abstract":"We investigate the wavevector and variance anisotropies in the inertial range of the young solar wind observed by the Parker Solar Probe (PSP). Using the first 19 encounters of PSP measurements, we identify the young solar wind from different source regions: coronal hole (CH) interiors, streamers, and low Mach-number boundary layers (LMBLs), i.e., the peripheral region inside CHs. We assess the wavevector anisotropy with the 2D and slab turbulence model for the CH wind and streamer wind and the nearly incompressible MHD turbulence model for the LMBL wind, where Taylor’s hypothesis becomes questionable. Unlike the ∼80% 2D contribution typically reported at 1 au, our results show that only 26% of the inertial range energy is associated with 2D fluctuations in the CH wind, and this fraction increases to 45% in the streamer wind. As a representation of the LMBL wind, similarly, the oblique sub-Alfvénic intervals and near-subsonic intervals are characterized by the dominance of slab fluctuations. All the results suggest that slab fluctuations are more abundant in the young solar wind below 0.3 au than at 1 au. Furthermore, we find a dependence of the variance anisotropy in the inertial range on proton plasma beta βp. The variance anisotropy is the strongest in the LMBL wind with the lowest βp and the weakest in the streamer wind with the highest βp. This contrast can be interpreted as the remnant of fluctuations from the coronal sources.","PeriodicalId":501814,"journal":{"name":"The Astrophysical Journal Letters","volume":"12 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144652423","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Ananya Rawat, Girjesh Gupta, Tom Van Doorsselaere, S. Krishna Prasad and Robertus Erdélyi
{"title":"Understanding the Magnetic Field and Plasma-β along Umbral Fan Loops Traced Using 3 Minute Slow Waves","authors":"Ananya Rawat, Girjesh Gupta, Tom Van Doorsselaere, S. Krishna Prasad and Robertus Erdélyi","doi":"10.3847/2041-8213/ade9be","DOIUrl":"https://doi.org/10.3847/2041-8213/ade9be","url":null,"abstract":"Plasma-β is an important fundamental physical quantity in solar plasma physics, which determines the dominating process in the solar atmosphere, i.e., magnetic or thermodynamic processes. Here, for the first time, we provide variations of magnetic field and plasma-β along magnetically structured loops from the photosphere to the corona. We have selected several fan loops rooted in sunspot umbra observed simultaneously by the Interface Region Imaging Spectrograph and Solar Dynamics Observatory. The 3 minute slow waves enabled us to trace and analyze several fan loops with cross-sectional areas in the lower atmosphere and locate their footpoints at the photosphere. We find the rms magnetic field strengths in the range 1596–2269 G at the photospheric footpoints of the fan loops, which decrease rapidly to 158–236 G at the coronal footpoints. We estimated the plasma-β at the photospheric and coronal footpoints in the range 0.2–0.5 and 0.0001–0.001, respectively. We found plasma-β < 1 along the whole loop, whereas the plasma-β ≈ 1 layer is found to be at subphotospheric heights. We compared our findings for isolated individual fan loops with a previously established model for active regions and found an almost similar pattern in variations with height, but with different plasma-β values. Our results demonstrate the seismological potential of 3 minute slow waves omnipresent in the umbral sunspot atmosphere to probe and map isolated loops and determine magnetic field and plasma-β along these loops. The obtained parameters provide crucial ingredients for the theoretical modeling of the umbral atmosphere and wave dynamics along loops.","PeriodicalId":501814,"journal":{"name":"The Astrophysical Journal Letters","volume":"12 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144652329","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Fengqiu Adam Dong, Kaitlyn Shin, Casey Law, Mason Ng, Ingrid Stairs, Geoffrey Bower, Alyssa Cassity, Emmanuel Fonseca, B. M. Gaensler, Jason W. T. Hessels, Victoria M. Kaspi, Bikash Kharel, Calvin Leung, Robert A. Main, Kiyoshi W. Masui, James W. McKee, Bradley W. Meyers, Obinna Modilim, Ayush Pandhi, Aaron B. Pearlman, Scott M. Ransom, Paul Scholz and Kendrick Smith
{"title":"CHIME/Fast Radio Burst Discovery of an Unusual Circularly Polarized Long-period Radio Transient with an Accelerating Spin Period","authors":"Fengqiu Adam Dong, Kaitlyn Shin, Casey Law, Mason Ng, Ingrid Stairs, Geoffrey Bower, Alyssa Cassity, Emmanuel Fonseca, B. M. Gaensler, Jason W. T. Hessels, Victoria M. Kaspi, Bikash Kharel, Calvin Leung, Robert A. Main, Kiyoshi W. Masui, James W. McKee, Bradley W. Meyers, Obinna Modilim, Ayush Pandhi, Aaron B. Pearlman, Scott M. Ransom, Paul Scholz and Kendrick Smith","doi":"10.3847/2041-8213/adeaab","DOIUrl":"https://doi.org/10.3847/2041-8213/adeaab","url":null,"abstract":"We report the discovery of CHIME J1634+44, a long-period radio transient (LPT) unique for two aspects: it is the first known LPT to emit fully circularly polarized radio bursts, and it is the first LPT with a significant spin-up. Given that high circular polarization (>90%) has been observed in FRB 20201124A and in some giant pulses of PSR B1937+21, we discuss the implications of the high circular polarization of CHIME J1634+44 and conclude its emission mechanism is likely to be “pulsar-like.” While CHIME J1634+44 has a pulse period of 841 s, its burst arrival patterns are indicative of a secondary 4206 s period, probably associated with binary activity. The timing properties suggest it has a significantly negative period derivative of s s−1. Few systems have been known to spin up, most notably transitional millisecond pulsars and cataclysmic binaries, both of which seem unlikely progenitors for CHIME J1634+44. If the period was only associated with the spin of the object, then the spin-up is likely generated by accretion of material from a companion. If, however, the radio pulse period and the orbital period are locked, as appears to be the case for two other LPTs, the spin-up of CHIME J1634+44 could be driven by gravitational-wave radiation.","PeriodicalId":501814,"journal":{"name":"The Astrophysical Journal Letters","volume":"52 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144645468","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Partial Differentiation of Callisto as Possible Evidence for Pebble Accretion","authors":"Yuhito Shibaike","doi":"10.3847/2041-8213/adeb6d","DOIUrl":"https://doi.org/10.3847/2041-8213/adeb6d","url":null,"abstract":"“Planetesimal or pebble” is one of the most fundamental open questions in planet formation theory. Similarly, “satellitesimal or pebble” remains unsettled regarding the formation of the Galilean satellites. I focus on a unique characteristic of Callisto—its interior is estimated to be only partially differentiated based on gravitational field measurements. I robustly demonstrate that such a state is not achievable through satellitesimal accretion, which inevitably leads to significant differentiation, but can be maintained with pebble accretion. Pebbles can release their impact energy at the surface of the satellite, allowing efficient radiative cooling, and their impact velocities can be reduced by aerodynamic drag from the circumplanetary gas disk. If future missions such as JUpiter ICy moons Explorer confirm that Callisto is indeed only partially differentiated, it could provide the first observed evidence for the pebble accretion mechanism—not only in the context of satellite formation but also in the broader framework of planet formation.","PeriodicalId":501814,"journal":{"name":"The Astrophysical Journal Letters","volume":"109 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144652333","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Hao Gu, Jun Cui, Xiaoshu Wu, Xu Huang, Shiqi Wu, Wenlong Li, Jinjin Zhao, Haoyu Lu and Lei Li
{"title":"Hydrogen Loss on Venus Driven by Photochemistry","authors":"Hao Gu, Jun Cui, Xiaoshu Wu, Xu Huang, Shiqi Wu, Wenlong Li, Jinjin Zhao, Haoyu Lu and Lei Li","doi":"10.3847/2041-8213/adec90","DOIUrl":"https://doi.org/10.3847/2041-8213/adec90","url":null,"abstract":"Venus has experienced substantial H loss through hydrodynamic outflow in its early history, transforming from a warm and wet state to the current arid and scorching state. While Venus continues to lose H today, no consensus has been reached regarding the present dominant escape mechanisms. Recently, photochemical escape via HCO+ dissociative recombination (DR) has been proposed as a prevailing process that had previously been overlooked. However, due to uncertainties in the underlying H2 abundance and the solar cycle variations of the input radiative energy, it is essential to explore how these factors influence the modeled H escape flux under different conditions. By combining a photochemical model with a Monte Carlo test particle model, we demonstrate that the H escape flux increases with the underlying H2 concentration over a possible range of 1 × 106–2 × 108 cm−2 s−1, but varies nonmonotonically with solar activity due to the competition between photochemical production and collisional hindrance. While our results confirm the dominant role of HCO+ DR, we find that the ion-neutral reaction makes an additional contribution, which could reach more than 30% of total H escape. Our findings provide valuable insights into the foundational understanding of photochemically driven H escape because the same mechanism should function in a much broader context.","PeriodicalId":501814,"journal":{"name":"The Astrophysical Journal Letters","volume":"52 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144652428","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}