Pub Date : 2026-03-19DOI: 10.3847/1538-4357/ae47bf
Thiago Ferreira, Earl P. Bellinger, Ebraheem Farag and Christopher J. Lindsay
The first stars likely formed from pristine clouds, marking a transformative epoch after the dark ages by initiating reionization and synthesising the first heavy elements. Among these, low-mass Population III (Pop III) stars are of particular interest, as their long lifespans raise the possibility that some may survive to the present day in the Milky Way’s stellar halo or satellite dwarfs. As the first paper in a series, we present hydrodynamic evolutionary models for 0.7–1 M⊙ stars evolved up to the white dwarf phase, utilising the MESA software instrument. We systematically vary mass-loss efficiencies, convective transport, and overshooting prescriptions, thereby mapping (i) how uncertain physics influences nucleosynthetic yields; (ii) surface enrichment, including nitrogen-rich post-main-sequence stars arising from convective shell mergers; (iii) remnant properties, such as low-mass helium or carbon-oxygen white dwarfs (MWD ∼ 0.45−0.55 M⊙) and transient UV-bright phases; and (iv) potential observational signatures, including neutrino emission during shell mergers and helium flashes. These models establish a predictive framework for identifying surviving Pop III stars and their descendants, providing both evolutionary and observational constraints that were previously unexplored.
{"title":"Evolution of Low-mass Population III Stars: Convection, Mass Loss, Nucleosynthesis, and Neutrinos","authors":"Thiago Ferreira, Earl P. Bellinger, Ebraheem Farag and Christopher J. Lindsay","doi":"10.3847/1538-4357/ae47bf","DOIUrl":"https://doi.org/10.3847/1538-4357/ae47bf","url":null,"abstract":"The first stars likely formed from pristine clouds, marking a transformative epoch after the dark ages by initiating reionization and synthesising the first heavy elements. Among these, low-mass Population III (Pop III) stars are of particular interest, as their long lifespans raise the possibility that some may survive to the present day in the Milky Way’s stellar halo or satellite dwarfs. As the first paper in a series, we present hydrodynamic evolutionary models for 0.7–1 M⊙ stars evolved up to the white dwarf phase, utilising the MESA software instrument. We systematically vary mass-loss efficiencies, convective transport, and overshooting prescriptions, thereby mapping (i) how uncertain physics influences nucleosynthetic yields; (ii) surface enrichment, including nitrogen-rich post-main-sequence stars arising from convective shell mergers; (iii) remnant properties, such as low-mass helium or carbon-oxygen white dwarfs (MWD ∼ 0.45−0.55 M⊙) and transient UV-bright phases; and (iv) potential observational signatures, including neutrino emission during shell mergers and helium flashes. These models establish a predictive framework for identifying surviving Pop III stars and their descendants, providing both evolutionary and observational constraints that were previously unexplored.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"120 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-03-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147478839","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}
Pub Date : 2026-03-19DOI: 10.3847/1538-4357/ae4972
Pasquale Temi, Francesco Ubertosi, Fabrizio Brighenti, Alexandros Maragkoudakis, Valeria Olivares, Alexandre Amblard, Massimo Gaspari, Myriam Gitti, Pamela M. Marcum, Kevin Fogarty, Alejandro S. Borlaff and William G. Mathews
This paper investigates the physical and kinematic properties of dust-rich regions in a small sample of group-centered elliptical galaxies, emphasizing their connection with the hot X-ray emitting gas and detailed dust grain characteristics. Comprehensive multiwavelength data—including Hα and CO emission detected by Multi Unit Spectroscopic Explorer and Atacama Large Millimeter/submillimeter Array—demonstrate the presence of dust clouds embedded within complex, hot X-ray atmospheres shaped by active galactic nucleus (AGN) feedback. X-ray images show bubbles and cavities surrounded by bright rims. We find that dust regions containing molecular gas traced by CO are preferentially located at the rims of these X-ray cavities, suggesting that AGN-driven outflows enhance the condensation of cold, dusty gas at these compressive interfaces. Kinematic measurements indicate that molecular and ionized gas phases are dynamically and spatially linked, supporting the framework of a multiphase medium arising from the top-down condensation rain in the hot plasma and related chaotic cold accretion. Crucially, spatial variations in the total-to-selective extinction ratio RV show that regions where dust, CO, and Hα emission coincide exhibit notably smaller RV values, implying steeper extinction curves and the predominance of smaller or less evolved dust grains within these mixed-phase environments. This contrasts with larger RV values found elsewhere in the dust clouds, suggesting grain growth or survival mechanisms within shielded cold gas.
{"title":"Active Galactic Nucleus Feedback and the Development of Dusty Multiphase Gas in X-Ray Emitting Elliptical Galaxies","authors":"Pasquale Temi, Francesco Ubertosi, Fabrizio Brighenti, Alexandros Maragkoudakis, Valeria Olivares, Alexandre Amblard, Massimo Gaspari, Myriam Gitti, Pamela M. Marcum, Kevin Fogarty, Alejandro S. Borlaff and William G. Mathews","doi":"10.3847/1538-4357/ae4972","DOIUrl":"https://doi.org/10.3847/1538-4357/ae4972","url":null,"abstract":"This paper investigates the physical and kinematic properties of dust-rich regions in a small sample of group-centered elliptical galaxies, emphasizing their connection with the hot X-ray emitting gas and detailed dust grain characteristics. Comprehensive multiwavelength data—including Hα and CO emission detected by Multi Unit Spectroscopic Explorer and Atacama Large Millimeter/submillimeter Array—demonstrate the presence of dust clouds embedded within complex, hot X-ray atmospheres shaped by active galactic nucleus (AGN) feedback. X-ray images show bubbles and cavities surrounded by bright rims. We find that dust regions containing molecular gas traced by CO are preferentially located at the rims of these X-ray cavities, suggesting that AGN-driven outflows enhance the condensation of cold, dusty gas at these compressive interfaces. Kinematic measurements indicate that molecular and ionized gas phases are dynamically and spatially linked, supporting the framework of a multiphase medium arising from the top-down condensation rain in the hot plasma and related chaotic cold accretion. Crucially, spatial variations in the total-to-selective extinction ratio RV show that regions where dust, CO, and Hα emission coincide exhibit notably smaller RV values, implying steeper extinction curves and the predominance of smaller or less evolved dust grains within these mixed-phase environments. This contrasts with larger RV values found elsewhere in the dust clouds, suggesting grain growth or survival mechanisms within shielded cold gas.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"6 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-03-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147478886","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}
Pub Date : 2026-03-19DOI: 10.3847/1538-4357/ae4c3c
Francesco Sylos Labini and Matteo Straccamore
We investigate the velocity field derived from H I measurements of the irregular galaxy ESO 358-60 using the velocity ring model (VRM) method. This technique, which assumes a coplanar disk, allows us to reconstruct coarse-grained maps of both radial and tangential velocity components from the observed line-of-sight velocity field. Such maps reveal that tangential motions dominate the inner regions, while radial motions become increasingly significant toward the outskirts. This kinematic behavior contrasts with that inferred from the tilted ring model (TRM), which suggests that radial motions are more prominent in the intermediate disk and negligible in the outskirts and detects a pronounced warp of approximately 20°, with the inner disk nearly edge on and the outer regions inclined by approximately 60°. In contrast, the VRM analysis finds that the disk exhibits a bar-like structure in its central regions. This interpretation is further supported by the intensity and velocity dispersion maps. To test the origin of the TRM-derived warp, we construct a toy model based on the TRM results and analyze it with the VRM technique, finding evidence that the warp is likely an artifact arising from the TRM’s assumptions. Finally, we estimate the galaxy’s mass using both the standard dark matter halo model and a dark matter disk (DMD) model, where all mass lies in the disk plane. The DMD yields a total mass approximately three times lower and provides a slightly better fit to the rotation curve.
{"title":"Kinematic and Dynamics of the Galaxy ESO 358-60","authors":"Francesco Sylos Labini and Matteo Straccamore","doi":"10.3847/1538-4357/ae4c3c","DOIUrl":"https://doi.org/10.3847/1538-4357/ae4c3c","url":null,"abstract":"We investigate the velocity field derived from H I measurements of the irregular galaxy ESO 358-60 using the velocity ring model (VRM) method. This technique, which assumes a coplanar disk, allows us to reconstruct coarse-grained maps of both radial and tangential velocity components from the observed line-of-sight velocity field. Such maps reveal that tangential motions dominate the inner regions, while radial motions become increasingly significant toward the outskirts. This kinematic behavior contrasts with that inferred from the tilted ring model (TRM), which suggests that radial motions are more prominent in the intermediate disk and negligible in the outskirts and detects a pronounced warp of approximately 20°, with the inner disk nearly edge on and the outer regions inclined by approximately 60°. In contrast, the VRM analysis finds that the disk exhibits a bar-like structure in its central regions. This interpretation is further supported by the intensity and velocity dispersion maps. To test the origin of the TRM-derived warp, we construct a toy model based on the TRM results and analyze it with the VRM technique, finding evidence that the warp is likely an artifact arising from the TRM’s assumptions. Finally, we estimate the galaxy’s mass using both the standard dark matter halo model and a dark matter disk (DMD) model, where all mass lies in the disk plane. The DMD yields a total mass approximately three times lower and provides a slightly better fit to the rotation curve.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"11 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-03-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147478952","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}
Pub Date : 2026-03-19DOI: 10.3847/1538-4357/ae4720
Roger E. Cohen, Mario Gennaro, Matteo Correnti, Kristen B. W. McQuinn and Vedant Chandra
The presence (and nature) of variations in the stellar initial mass function (IMF) at substantially subsolar masses and metallicities (m < 0.5 M⊙ and [M/H] ≲ −1, respectively) remains poorly constrained. Predictions from simulations vary widely, while observationally, resolved star studies of ultrafaint dwarf (UFD) galaxies suffer from small sample sizes and background galaxy contamination due to low projected stellar densities. As an alternative metal-poor target, we measure the IMF from resolved stars toward a carefully selected field in the Small Magellanic Cloud, leveraging a plethora of independent constraints on the target field stellar population including distributions of distance, age, and metallicity. We resolve >15,000 stars down to 0.16 M⊙ within a single pointing of NIRCam on board JWST, using an observing strategy that minimizes contamination from point-source-like background galaxies. We explore three different functional forms of the IMF, forward modeling observed color–magnitude diagrams and luminosity functions. We find a best-fit single power law IMF slope of α = −1.61 , consistent with UFDs probed down to similar limiting masses. Fitting a broken power-law IMF, we find low- and high-mass slopes of α1 = −1.44 and α2 = −2.17 , respectively, consistent with solar neighborhood values. Assuming a lognormal IMF, we find a characteristic mass and lognormal width of mc = and σ = 0.61 M⊙, respectively, allowing for characteristic masses lower than local values as seen in some simulations as well as low-metallicity Galactic clusters. Lastly, we quantify the impact of assumptions required in our analysis and discuss potential future improvements.
{"title":"The Stellar Initial Mass Function down To 0.16 M ⊙ toward the Small Magellanic Cloud","authors":"Roger E. Cohen, Mario Gennaro, Matteo Correnti, Kristen B. W. McQuinn and Vedant Chandra","doi":"10.3847/1538-4357/ae4720","DOIUrl":"https://doi.org/10.3847/1538-4357/ae4720","url":null,"abstract":"The presence (and nature) of variations in the stellar initial mass function (IMF) at substantially subsolar masses and metallicities (m < 0.5 M⊙ and [M/H] ≲ −1, respectively) remains poorly constrained. Predictions from simulations vary widely, while observationally, resolved star studies of ultrafaint dwarf (UFD) galaxies suffer from small sample sizes and background galaxy contamination due to low projected stellar densities. As an alternative metal-poor target, we measure the IMF from resolved stars toward a carefully selected field in the Small Magellanic Cloud, leveraging a plethora of independent constraints on the target field stellar population including distributions of distance, age, and metallicity. We resolve >15,000 stars down to 0.16 M⊙ within a single pointing of NIRCam on board JWST, using an observing strategy that minimizes contamination from point-source-like background galaxies. We explore three different functional forms of the IMF, forward modeling observed color–magnitude diagrams and luminosity functions. We find a best-fit single power law IMF slope of α = −1.61 , consistent with UFDs probed down to similar limiting masses. Fitting a broken power-law IMF, we find low- and high-mass slopes of α1 = −1.44 and α2 = −2.17 , respectively, consistent with solar neighborhood values. Assuming a lognormal IMF, we find a characteristic mass and lognormal width of mc = and σ = 0.61 M⊙, respectively, allowing for characteristic masses lower than local values as seen in some simulations as well as low-metallicity Galactic clusters. Lastly, we quantify the impact of assumptions required in our analysis and discuss potential future improvements.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"8 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-03-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147478896","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}
Pub Date : 2026-03-19DOI: 10.3847/1538-4357/ae4743
Therese A. Kucera, Gelu M. Nita, James A. Klimchuk and Gregory D. Fleishman
In order to understand solar atmospheric heating, it is important to test heating models against spatially resolved data from solar active regions. Here, we model a small active region, AR 12760, observed on 2020 April 28, with the GX Simulator package by fitting the extreme-ultraviolet (EUV) intensities in wave bands observed by the Solar Dynamics Observatory’s Atmospheric Imaging Assembly. We assume the temporally and spatially averaged heating rate along a loop has a power-law dependence on loop length, L, and average magnetic field strength along the loop, Bavg. We find that the best-fit heating model for the 211 Å band is erg cm−3 s−1, but that there is a range of parameters that give qualitatively reasonable fits, which we conclude is due to a correlation between Bavg and L. In addition, we find that the models of the bands including cooler emission (131 and 171 Å) greatly underestimate the extent of the emission in the legs of the longer loops at the peripheries of the active region that are the strongest contributors of the emission in those bands. We conclude that this is because the modeling assumes that all transition-region emission is confined to the loop footpoints, but in reality the upper transition region of longer loops extends significantly farther into the loop. It will be important to consider this aspect of the transition region in future efforts to model EUV emission.
{"title":"Modeling of AR 12760 with GX Simulator and Evidence for the Extended Transition Region in Peripheral Active Region Loops","authors":"Therese A. Kucera, Gelu M. Nita, James A. Klimchuk and Gregory D. Fleishman","doi":"10.3847/1538-4357/ae4743","DOIUrl":"https://doi.org/10.3847/1538-4357/ae4743","url":null,"abstract":"In order to understand solar atmospheric heating, it is important to test heating models against spatially resolved data from solar active regions. Here, we model a small active region, AR 12760, observed on 2020 April 28, with the GX Simulator package by fitting the extreme-ultraviolet (EUV) intensities in wave bands observed by the Solar Dynamics Observatory’s Atmospheric Imaging Assembly. We assume the temporally and spatially averaged heating rate along a loop has a power-law dependence on loop length, L, and average magnetic field strength along the loop, Bavg. We find that the best-fit heating model for the 211 Å band is erg cm−3 s−1, but that there is a range of parameters that give qualitatively reasonable fits, which we conclude is due to a correlation between Bavg and L. In addition, we find that the models of the bands including cooler emission (131 and 171 Å) greatly underestimate the extent of the emission in the legs of the longer loops at the peripheries of the active region that are the strongest contributors of the emission in those bands. We conclude that this is because the modeling assumes that all transition-region emission is confined to the loop footpoints, but in reality the upper transition region of longer loops extends significantly farther into the loop. It will be important to consider this aspect of the transition region in future efforts to model EUV emission.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"60 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-03-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147478837","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}
As a byproduct of sub-Neptune formation, planetary embryos with high eccentricity can remain in outer orbits, near 1 au from the star. In this work, we investigate the long-term evolution of systems consisting of close-in sub-Neptunes (SNs) and outer high-eccentricity embryos. Our analysis focuses on collisions between SNs and embryos, in particular their atmospheric mass loss. We performed N-body simulations for various initial eccentricities and numbers of embryos. We analyzed the impact-induced atmospheric loss using post-processing methods, finding that the embryos and SNs collide at high speeds on timescales of several million years, leading to the loss of the SNs’ atmospheres. Depending on the embryos’ eccentricity and the orbital radius of the SNs, the impact velocity can be quite high, ranging from 2 to 5 times the escape velocity. On average, ∼15%–30% of the atmosphere is dissipated per collision, so after ∼3–6 collisions, the atmospheric mass of an SN is reduced to about 1/3 of its initial value. Collisions between SNs and embryos can thus explain the presence of planets within the radius gap. Depending upon the initial eccentricity and the number of remaining embryos, additional collisions can occur, potentially accounting for the formation of the radius gap. This study also indicates that collisions between remaining embryos and SNs may help to explain the observed rarity of SNs with atmospheric mass fractions greater than 10%, commonly termed the “radius cliff.”
{"title":"Long-term Evolution of Close-in Sub-Neptunes and Outer Planetary Embryos: Atmospheric Mass Loss and Origin of Planets Inside and Outside the Radius Gap","authors":"Yaxing He, 亚星 贺, Masahiro Ogihara, Kangrou Guo and 康柔 郭","doi":"10.3847/1538-4357/ae4352","DOIUrl":"https://doi.org/10.3847/1538-4357/ae4352","url":null,"abstract":"As a byproduct of sub-Neptune formation, planetary embryos with high eccentricity can remain in outer orbits, near 1 au from the star. In this work, we investigate the long-term evolution of systems consisting of close-in sub-Neptunes (SNs) and outer high-eccentricity embryos. Our analysis focuses on collisions between SNs and embryos, in particular their atmospheric mass loss. We performed N-body simulations for various initial eccentricities and numbers of embryos. We analyzed the impact-induced atmospheric loss using post-processing methods, finding that the embryos and SNs collide at high speeds on timescales of several million years, leading to the loss of the SNs’ atmospheres. Depending on the embryos’ eccentricity and the orbital radius of the SNs, the impact velocity can be quite high, ranging from 2 to 5 times the escape velocity. On average, ∼15%–30% of the atmosphere is dissipated per collision, so after ∼3–6 collisions, the atmospheric mass of an SN is reduced to about 1/3 of its initial value. Collisions between SNs and embryos can thus explain the presence of planets within the radius gap. Depending upon the initial eccentricity and the number of remaining embryos, additional collisions can occur, potentially accounting for the formation of the radius gap. This study also indicates that collisions between remaining embryos and SNs may help to explain the observed rarity of SNs with atmospheric mass fractions greater than 10%, commonly termed the “radius cliff.”","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"111 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-03-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147478954","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}
Pub Date : 2026-03-18DOI: 10.3847/1538-4357/ae4aa0
Tatsuya Kotani, Tomoharu Oka and Rei Enokiya
We analyzed millimeter-wave data toward the quasar B0218+357 observed with the Atacama Large Millimeter/submillimeter Array and obtained absorption spectra of the J = 2–1 and J = 3–2 rotational transitions of HCN, HCO+, HNC, H13CN, and H13CO+ at the cosmological redshift of z = 0.68. For HCN, HCO+, and HNC, we identified two distinct absorption components that are common to both transitions, whereas a single component was detected in the isotopologue spectra. In this paper, we accurately evaluate the excitation temperatures and their uncertainties from the absorption strengths of these components and use them to determine the cosmic microwave background radiation (CMB) temperature. Uncertainties in the continuum covering factor were propagated into the excitation temperature via Monte Carlo sampling. We further corrected the observed optical depths for biases due to column-density nonuniformity by assuming a lognormal column-density distribution. Under the assumption that the rotational levels are in radiative equilibrium with the CMB, we derived excitation temperature profiles in the optically thin regime. Because the excitation of HCO+ is biased by an additional velocity component and partial collisional excitation, this species was excluded from the final determination of the CMB temperature. From a weighted mean of the excitation temperatures obtained from HCN and HNC, we determined the CMB temperature at z = 0.68 to be 4.50 ± 0.17 K. This constitutes the first measurement of the CMB temperature at z = 0.68 based on a quasar absorption line system and represents the most precise determination at this redshift, highly consistent with the standard Big Bang cosmological model.
{"title":"First Determination of the Cosmic Microwave Background Radiation Temperature at z = 0.68 Using Molecular Absorption Lines","authors":"Tatsuya Kotani, Tomoharu Oka and Rei Enokiya","doi":"10.3847/1538-4357/ae4aa0","DOIUrl":"https://doi.org/10.3847/1538-4357/ae4aa0","url":null,"abstract":"We analyzed millimeter-wave data toward the quasar B0218+357 observed with the Atacama Large Millimeter/submillimeter Array and obtained absorption spectra of the J = 2–1 and J = 3–2 rotational transitions of HCN, HCO+, HNC, H13CN, and H13CO+ at the cosmological redshift of z = 0.68. For HCN, HCO+, and HNC, we identified two distinct absorption components that are common to both transitions, whereas a single component was detected in the isotopologue spectra. In this paper, we accurately evaluate the excitation temperatures and their uncertainties from the absorption strengths of these components and use them to determine the cosmic microwave background radiation (CMB) temperature. Uncertainties in the continuum covering factor were propagated into the excitation temperature via Monte Carlo sampling. We further corrected the observed optical depths for biases due to column-density nonuniformity by assuming a lognormal column-density distribution. Under the assumption that the rotational levels are in radiative equilibrium with the CMB, we derived excitation temperature profiles in the optically thin regime. Because the excitation of HCO+ is biased by an additional velocity component and partial collisional excitation, this species was excluded from the final determination of the CMB temperature. From a weighted mean of the excitation temperatures obtained from HCN and HNC, we determined the CMB temperature at z = 0.68 to be 4.50 ± 0.17 K. This constitutes the first measurement of the CMB temperature at z = 0.68 based on a quasar absorption line system and represents the most precise determination at this redshift, highly consistent with the standard Big Bang cosmological model.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"28 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-03-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147478890","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}
Pub Date : 2026-03-18DOI: 10.3847/1538-4357/ae4481
S. Seadrow, V. Petit, G. A. Wade, D. Bohlender, J. Maíz Apellániz, A. David-Uraz, M. Oksala and J. MacDonald
NGC 1624-2 hosts the strongest surface magnetic field found on an O star thus far. When applied across several epochs of observations, the star’s currently accepted rotation period (157.99 days) does not coherently characterize the variations of spectral lines of magnetospheric origin. We analyze Lomb–Scargle periodograms produced with new and archival multi-instrument spectroscopic time series of Balmer H and He spectral lines. We find that 153.17 ± 0.42 days and 306.56 ± 1.19 days are both equally suitable periods at phasing the spectral and magnetic time series data in a manner consistent with the oblique rotator model. The 306.56-day period implies a magnetic geometry for NGC 1624-2 that is quite different from the previously accepted one, for which both magnetic poles should be observed during a full rotational cycle. If this is the case, the star’s magnetic south pole has yet to be observed, and additional spectropolarimetric observations should be acquired in order to confirm whether or not the south pole is in fact observable.
NGC 1624-2拥有迄今为止在O型恒星上发现的最强的表面磁场。当应用于几个时期的观测时，目前接受的恒星自转周期（157.99天）并不能连贯地表征磁层起源谱线的变化。本文对Balmer H和He谱线的新的和存档的多仪器光谱时间序列产生的Lomb-Scargle周期图进行了分析。我们发现153.17±0.42天和306.56±1.19天都是与斜旋子模型一致的光谱和磁时间序列数据相相位的合适周期。306.56天的周期意味着NGC 1624-2的磁几何形状与之前接受的完全不同，之前接受的是在一个完整的旋转周期内观察到两个磁极。如果是这样的话，这颗恒星的磁南极尚未被观测到，为了确认南极是否实际上是可观测到的，应该获得额外的光谱偏振观测。
{"title":"New Insights from Revisiting the Rotation Period of the Strongly Magnetic O Star, NGC 1624-2","authors":"S. Seadrow, V. Petit, G. A. Wade, D. Bohlender, J. Maíz Apellániz, A. David-Uraz, M. Oksala and J. MacDonald","doi":"10.3847/1538-4357/ae4481","DOIUrl":"https://doi.org/10.3847/1538-4357/ae4481","url":null,"abstract":"NGC 1624-2 hosts the strongest surface magnetic field found on an O star thus far. When applied across several epochs of observations, the star’s currently accepted rotation period (157.99 days) does not coherently characterize the variations of spectral lines of magnetospheric origin. We analyze Lomb–Scargle periodograms produced with new and archival multi-instrument spectroscopic time series of Balmer H and He spectral lines. We find that 153.17 ± 0.42 days and 306.56 ± 1.19 days are both equally suitable periods at phasing the spectral and magnetic time series data in a manner consistent with the oblique rotator model. The 306.56-day period implies a magnetic geometry for NGC 1624-2 that is quite different from the previously accepted one, for which both magnetic poles should be observed during a full rotational cycle. If this is the case, the star’s magnetic south pole has yet to be observed, and additional spectropolarimetric observations should be acquired in order to confirm whether or not the south pole is in fact observable.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"6 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-03-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147470862","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}
Pub Date : 2026-03-18DOI: 10.3847/1538-4357/ae45aa
Swadesh Chand, Andrzej A. Zdziarski, Gulab C. Dewangan and Pragati Sahu
We present a comprehensive broadband spectral and variability study of the newly detected black hole X-ray binary Swift J1727.8–1613 in the intermediate states during its 2023 outburst, using multimission observations from NICER, NuSTAR, AstroSat, and Insight-HXMT. Spectral data up to 78 keV in the hard-intermediate state (HIMS) require models with two Comptonizing regions. In contrast, models with a single Comptonizing region adequately describe the soft-intermediate states (SIMS), implying a significant evolution in the disk-corona geometry between the states. The hard X-ray tail above 100 keV in the HIMS, detected with both AstroSat/CZTI and Insight-HXMT/High-Energy X-ray Telescope, indicates that the electron population in the corona is not purely thermal but rather hybrid, with a power-law distribution above the thermal cutoff. While both the reflection modeling and disk-continuum fitting favor a truncated disk geometry in the HIMS, the disk in the SIMS moves substantially closer to the innermost stable circular orbit, accompanied by a significant rise in disk temperature. This interpretation is further supported by the increase in the quasiperiodic oscillation frequency from ∼1.3 to ∼6.6 Hz. From joint modeling of the disk continuum and reflection component and assuming the distance of 3.4 kpc, we estimate a black hole mass of , spin of , and the disk inclination angle of ∼37°–53°, which match well with the previously reported spectropolarimetric measurements. We find a weakly variable or stable disk and a highly variable Comptonized component.
{"title":"Evolution of the Inner Accretion Flow in Swift J1727.8–1613 across Intermediate States: Insights from Broadband Spectral and Timing Analysis","authors":"Swadesh Chand, Andrzej A. Zdziarski, Gulab C. Dewangan and Pragati Sahu","doi":"10.3847/1538-4357/ae45aa","DOIUrl":"https://doi.org/10.3847/1538-4357/ae45aa","url":null,"abstract":"We present a comprehensive broadband spectral and variability study of the newly detected black hole X-ray binary Swift J1727.8–1613 in the intermediate states during its 2023 outburst, using multimission observations from NICER, NuSTAR, AstroSat, and Insight-HXMT. Spectral data up to 78 keV in the hard-intermediate state (HIMS) require models with two Comptonizing regions. In contrast, models with a single Comptonizing region adequately describe the soft-intermediate states (SIMS), implying a significant evolution in the disk-corona geometry between the states. The hard X-ray tail above 100 keV in the HIMS, detected with both AstroSat/CZTI and Insight-HXMT/High-Energy X-ray Telescope, indicates that the electron population in the corona is not purely thermal but rather hybrid, with a power-law distribution above the thermal cutoff. While both the reflection modeling and disk-continuum fitting favor a truncated disk geometry in the HIMS, the disk in the SIMS moves substantially closer to the innermost stable circular orbit, accompanied by a significant rise in disk temperature. This interpretation is further supported by the increase in the quasiperiodic oscillation frequency from ∼1.3 to ∼6.6 Hz. From joint modeling of the disk continuum and reflection component and assuming the distance of 3.4 kpc, we estimate a black hole mass of , spin of , and the disk inclination angle of ∼37°–53°, which match well with the previously reported spectropolarimetric measurements. We find a weakly variable or stable disk and a highly variable Comptonized component.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"12 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-03-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147471264","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}
Pub Date : 2026-03-18DOI: 10.3847/1538-4357/ae4a12
Darius Modirrousta-Galian and Jun Korenaga
Planetary atmospheres cannot remain hydrostatic at all altitudes because they approach finite density at infinite radius, implying infinite mass. Classical treatments address this in two directions: either retain a hydrostatic structure while allowing particles in the high-velocity tail to decouple and escape in a Jeans-type manner, or promote the gas to a continuum outflow to obtain a transonic Parker-type solution. The usual criterion compares the local mean free path to the sonic point radius. If the mean free path is shorter, the atmosphere is hydrostatic with an imposed Jeans escape flux; if it is longer, the gas is hydrodynamic with Jeans escape neglected. Here, we show that hydrogen-rich atmospheres do not separate cleanly into hydrodynamic and Jeans-escape regimes. At any radius, some particles still collide and behave as a fluid, while others have already experienced their last collision and move collisionlessly on ballistic trajectories. The relative importance of these two behaviors changes smoothly with radius rather than switching at a single boundary. The hydrodynamic channel accelerates and passes through a sonic point, whereas the collisionless channel decelerates under gravity and grows with altitude, removing mass and momentum from the collisional flow. As the collisionless component grows, the bulk flow speed reaches a maximum and then decelerates thereafter, producing profiles similar to Parker breeze solutions even though escape is carried by the collisionless channel. This two-channel framework provides a first step toward a self-consistent treatment that unifies hydrodynamics and kinetics in atmospheric loss models.
{"title":"On the Dual Nature of Atmospheric Escape","authors":"Darius Modirrousta-Galian and Jun Korenaga","doi":"10.3847/1538-4357/ae4a12","DOIUrl":"https://doi.org/10.3847/1538-4357/ae4a12","url":null,"abstract":"Planetary atmospheres cannot remain hydrostatic at all altitudes because they approach finite density at infinite radius, implying infinite mass. Classical treatments address this in two directions: either retain a hydrostatic structure while allowing particles in the high-velocity tail to decouple and escape in a Jeans-type manner, or promote the gas to a continuum outflow to obtain a transonic Parker-type solution. The usual criterion compares the local mean free path to the sonic point radius. If the mean free path is shorter, the atmosphere is hydrostatic with an imposed Jeans escape flux; if it is longer, the gas is hydrodynamic with Jeans escape neglected. Here, we show that hydrogen-rich atmospheres do not separate cleanly into hydrodynamic and Jeans-escape regimes. At any radius, some particles still collide and behave as a fluid, while others have already experienced their last collision and move collisionlessly on ballistic trajectories. The relative importance of these two behaviors changes smoothly with radius rather than switching at a single boundary. The hydrodynamic channel accelerates and passes through a sonic point, whereas the collisionless channel decelerates under gravity and grows with altitude, removing mass and momentum from the collisional flow. As the collisionless component grows, the bulk flow speed reaches a maximum and then decelerates thereafter, producing profiles similar to Parker breeze solutions even though escape is carried by the collisionless channel. This two-channel framework provides a first step toward a self-consistent treatment that unifies hydrodynamics and kinetics in atmospheric loss models.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"17 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-03-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147471267","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}
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