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Physics Astrophysics

astro-ph Watch Astrophysics

Wednesday 7 January

Earth and Planetary Astrophysics

The uncertainty in water mass fraction of wet planets

Michael Lozovsky

Planets with masses between Earth and Neptune often have radii that imply the presence of volatiles, suggesting that water may be abundant in their interiors. However, directly observing the precise water mass fraction and water distribution remains unfeasible. In our study, we employ an internal structure code MAGRATHEA to model planets with high water content and explore potential interior distributions. Departing from traditional assumptions of a layered structure, we determine water and rock distribution based on water-rock miscibility criteria. We model {wet planets} with an iron core and a homogeneous mixture of rock and water above it. At the outer regions of the planet, the pressure and temperature are below the rock-water miscibility point (the second critical point), causing the segregation of water and rock. Consequently, a shell of water is formed in the outermost layers. By considering the water-rock miscibility and the vapor state of water, our approach highlights the uncertainty in estimating the water mass fraction of detected exoplanets.
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Instrumentation and Methods for Astrophysics

Polarization rotation through differential transmission in refractive CMB telescopes identified using a hybrid physical optics method

Xiaodong Ren, Rustam Balafendiev and Jon E. Gudmundsson

We identify a polarization rotation systematic in the far field beams of refractive cosmic microwave background (CMB) telescopes caused by differential transmission in anti-reflection (AR) coatings of optical elements. This systematic was identified following the development of a hybrid physical optics method that incorporates full-wave electromagnetic simulations of AR coatings to model the full polarization response of refractive systems. Applying this method to a two-lens CMB telescope with non-ideal AR coating, we show that polarization-dependent transmission can produce a rotation of the far-field polarization angle that varies across the focal plane with a typical amplitude of 0.05-0.5 degrees. If ignored in analysis, this effect can produce temperature to polarization leakage and Stokes Q/U mixing.
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Instrumentation and Methods for Astrophysics

Science with a large field-of-view polarization survey: The Large Array Survey Telescope Polarization Node (LAST-P)

V. Barbosa Martins, N. Jordana Mitjans, S. Garrappa et al. (+14 more)

Optical polarimetry provides information on the geometry of the emitting region, the magnetic field configuration and the properties of dust in astrophysical sources. Current state-of-the-art instruments typically have a small field of view (FoV), which poses a challenge for conducting wide surveys. We propose the construction of the Large Array Survey Telescope Polarization Node (LAST-P), a wide-field array of optical polarimeters. LAST-P is designed for high-cadence ($\lesssim 1$ day) polarization monitoring of numerous astrophysical transients, such as the early phases of gamma-ray bursts, supernovae, and novae. Furthermore, LAST-P will facilitate the creation of extensive polarization catalogs for X-ray binaries and white dwarfs, alongside a large FoV study of the interstellar medium. In survey mode, LAST-P will cover a FoV of 88.8 deg$^2$. With a 15 x 1-minute exposure, the instrument will be capable of measuring polarization of sources as faint as Gaia Bp-magnitude $\sim$20.9. The precision on the linear polarization degree (PD) will reach 0.7%, 1.5%, and 3.5% for sources with magnitudes 17, 18, and 19, respectively, for a seeing of 2.7 arcsec, air mass of about 1 for observations in dark locations. We propose three distinct non-simultaneous survey strategies, among them an active galactic nuclei (AGN) strategy for long-term monitoring of $\sim$200 AGN with $<$1-day cadence. In this paper, we present the predicted sensitivity of the instrument and outline the various science cases it is designed to explore.
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All authors V. Barbosa Martins, N. Jordana Mitjans, S. Garrappa, A. Franckowiak, E. O. Ofek, S. Ben-Ami, J. Borowska-Naguszewska, V. Fallah Ramazani, R. Konno, D. Kuesters, R.D. Parsons, D. Polishook, I. Sadeh, O. Savushkin, E. Segre, N. Strotjohann, S. Weimann

Astrophysics of Galaxies

HST+IGRINS synergy to characterise the newly discovered metal-rich bulge globular cluster Patchick 126

Elisa R. Garro, Davide Massari, José G. Fernández-Trincado et al. (+10 more)

We present the first comprehensive spectroscopic and deep photometric study of the globular cluster (GC) candidate Patchick~126. The spectroscopic analysis is based on high-resolution near-infrared data obtained with the IGRINS spectrograph, while the photometric analysis relies on HST observations from the Hubble Missing Globular Cluster Survey (MGCS). We derived abundances for $\alpha$-(O, Mg, Si, Ca, Ti), light-(C, N), odd-Z (Na, Al), iron-peak (Fe, Co, Cr, Ni, Mn, V), and s-process elements (Ce) for four red giant stars observed in the H and K bands. Our results yield a mean metallicity of $\langle\mathrm{[Fe/H]}\rangle = -0.30\pm0.03$, with no evidence of intrinsic variation, and an $\alpha$-enhancement of $\langle\mathrm{[\alpha/Fe]}\rangle =+0.19\pm0.02$, consistent with the trends of metal-rich Galactic GCs. We detect an intrinsic C-N anti-correlation, but no Na-O or Al-Mg anti-correlations, in agreement with expectations for low-mass, metal-rich clusters. From the HST photometry, we constructed deep CMDs extending $\sim 2-3$ magnitudes below the MSTO. This depth allowed us to provide the first robust age estimate for the cluster. Applying the methods developed within the CARMA project, we derive an age of $11.9^{+0.3}_{-0.4}$~Gyr. We obtain a photometric metallicity of [Fe/H]$=-0.28$, in agreement with the spectroscopic results. The colour excess we derived, E(B-V) = 1.08, confirms that Patchick~126 is a heavily reddened cluster, located at a heliocentric distance of 7.8 kpc. From the orbital parameters, including energy, vertical angular momentum, circularity, and maximum vertical height, we find that Patchick~126 closely follows a disc-like orbit. Taken together, these results confirm that Patchick 126 is an in situ, low-mass globular cluster of the Milky Way, exhibiting properties that lie at the boundary between old-OCs and GCs.
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All authors Elisa R. Garro, Davide Massari, José G. Fernández-Trincado, Edoardo Ceccarelli, Chris Sneden, Fernando Aguado-Agelet, Melike Afşar, Michele Bellazzini, Rafael Guerço, Dante Minniti, Mattia Libralato, Beatriz Barbuy, Bruno Dias

Cosmology and Nongalactic Astrophysics

Towards an optimal extraction of cosmological parameters from galaxy cluster surveys using convolutional neural networks

Iñigo Sáez-Casares, Matteo Calabrese, Davide Bianchi et al. (+4 more)

The possibility to constrain cosmological parameters from galaxy surveys using field-level machine learning methods that bypass traditional summary statistics analyses, depends crucially on our ability to generate simulated training sets. The latter need to be both realistic, as to reproduce the key features of the real data, and produced in large numbers, as to allow us to refine the precision of the training process. The analysis presented in this paper is an attempt to respond to these needs by (a) using clusters of galaxies as tracers of large-scale structure, together with (b) adopting a 3LPT code (Pinocchio) to generate a large training set of $32\,768$ mock X-ray cluster catalogues. X-ray luminosities are stochastically assigned to dark matter haloes using an empirical $M-L_X$ scaling relation. Using this training set, we test the ability and performances of a 3D convolutional neural network (CNN) to predict the cosmological parameters, based on an input overdensity field derived from the cluster distribution. We perform a comparison with a neural network trained on traditional summary statistics, that is, the abundance of clusters and their power spectrum. Our results show that the field-level analysis combined with the cluster abundance yields a mean absolute relative error on the predicted values of $\Omega_{\rm m}$ and $\sigma_8$ that is a factor of $\sim 10 \%$ and $\sim 20\%$ better than that obtained from the summary statistics. Furthermore, when information about the individual luminosity of each cluster is passed to the CNN, the gain in precision exceeds $50\%$.
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All authors Iñigo Sáez-Casares, Matteo Calabrese, Davide Bianchi, Marina S. Cagliari, Marco Chiarenza, Jean-Marc Christille, Luigi Guzzo

Cosmology and Nongalactic Astrophysics

Filtering Interlopers with Photometry and Diagnostic Features: A Machine Learning Framework Validated with CSST Slitless Spectroscopy

Hui Peng, Yu Yu, Yiyang Guo et al. (+13 more)

The slitless spectroscopic method employed by missions such as Euclid and the Chinese Space-station Survey Telescope (CSST) faces a fundamental challenge: spectroscopic redshifts derived from their data are susceptible to emission-line misidentification due to the limited spectral resolution and signal-to-noise ratio. This effect systematically introduces interloper galaxies into the sample. Conventional strict selection not only struggles to secure high redshift purity but also drastically reduces completeness by discarding valuable data. To overcome this limitation, we develop an XGBoost classifier that leverages photometric properties and spectroscopic diagnostics to construct a high-purity redshift catalog while maximizing completeness. We validate this method on a simulated sample with spectra generated by the CSST emulator for slitless spectroscopy. Of the $\sim$62 million galaxies that obtain valid redshifts (parent sample), approximately 43% achieve accurate measurements, defined as $|\Delta z| \leqslant 0.002(1+z)$. From this parent sample, the XGBoost classifier selects galaxies with a selection efficiency of 42.3% on the test set and 42.2% when deployed on the entire parent sample. Crucially, among the retained galaxies, 96.6% (parent sample: 96.5%) achieve accurate measurements, while the outlier fraction ($|\Delta z|>0.01(1+z)$) is constrained to 0.13% (0.11%). We verified that simplified configurations that exclude either spectroscopic diagnostics (except the measured redshift) or photometric data yield significantly higher outlier fractions, increasing by factors of approximately 3.5 and 6.3, respectively, with the latter case also introducing notable catastrophic interloper contamination. This framework effectively resolves the purity-completeness trade-off, enabling robust large-scale cosmological studies with CSST and similar surveys.
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All authors Hui Peng, Yu Yu, Yiyang Guo, Yizhou Gu, Run Wen, Yunkun Han, Jipeng Sui, Hu Zou, Xiaohu Yang, Pengjie Zhang, Xian Zhong Zheng, Hong Guo, Yipeng Jing, Cheng Li, Hu Zhan, Gongbo Zhao

High Energy Astrophysical Phenomena

First detection of VHE gamma-ray signal from the FSRQ TON 0599

MAGIC Collaboration: K. Abe, S. Abe, J. Abhir et al. (+244 more)

TON\,0599 (z=0.7247) belongs to the few flat spectrum radio quasars (FSRQs) detected in the very high energy (VHE, $E > 100$\,GeV) gamma-ray band. Its redshift makes it currently one of the farthest VHE gamma-ray sources. It was detected for the first time with the MAGIC telescopes on 2017 December 15, and observed until December 29. The flux reached a maximum of about 50 per cent of the Crab Nebula flux above 80 GeV on the second night of observation, after which we witnessed a gradual decrease of the flux. The VHE gamma-ray spectrum connects smoothly to the one in the high energy ($E > 100$\,MeV) band obtained from simultaneous observations with {\textit Fermi}-LAT. It features a cut-off at energies around 50\,GeV, indicating the location of the gamma-ray emission zone beyond the broad line region. In addition, we were able to follow the spectral evolution during the fading phase of the flare. Multiwavelength analysis based on observations in optical, near-infrared, and radio bands acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from November to March, as well as observations in X-ray and optical–UV bands with instruments on board the Swift satellite, shows strong correlation between different bands. We model the broadband emission with a simple one-zone leptonic model, where the high-energy peak is predominantly produced by external Compton (EC) scattering of photons from the dusty torus.
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All authors MAGIC Collaboration: K. Abe, S. Abe, J. Abhir, A. Abhishek, V. A. Acciari, A. Aguasca-Cabot, I. Agudo, T. Aniello, S. Ansoldi, L. A. Antonelli, A. Arbet Engels, C. Arcaro, K. Asano, A. Babić, U. Barres de Almeida, J. A. Barrio, L. Barrios-Jiménez, I. Batković, J. Baxter, J. Becerra González, W. Bednarek, E. Bernardini, J. Bernete, A. Berti, J. Besenrieder, C. Bigongiari, A. Biland, O. Blanch, G. Bonnoli, Ž. Bošnjak, E. Bronzini, I. Burelli, A. Campoy-Ordaz, A. Carosi, R. Carosi, M. Carretero-Castrillo, A. J. Castro-Tirado, D. Cerasole, G. Ceribella, Y. Chai, A. Cifuentes, J. L. Contreras, J. Cortina, S. Covino, G. D'Amico, P. Da Vela, F. Dazzi, A. De Angelis, B. De Lotto, R. de Menezes, M. Delfino, J. Delgado, C. Delgado Mendez, F. Di Pierro, R. Di Tria, L. Di Venere, A. Dinesh, D. Dominis Prester, A. Donini, D. Dorner, M. Doro, L. Eisenberger, D. Elsaesser, J. Escudero, L. Foffano, L. Font, S. Fröse, S. Fukami, Y. Fukazawa, M. Garczarczyk, S. Gasparyan, M. Gaug, J. G. Giesbrecht Paiva, N. Giglietto, F. Giordano, P. Gliwny, N. Godinović, T. Gradetzke, R. Grau, D. Green, J. G. Green, P. Günther, D. Hadasch, A. Hahn, T. Hassan, L. Heckmann, J. Herrera Llorente, D. Hrupec, R. Imazawa, D. Israyelyan, I. Jiménez Martínez, J. Jiménez Quiles, J. Jormanainen, S. Kankkunen, D. Kerszberg, M. Khachatryan, J. Konrad, P. M. Kouch, H. Kubo, J. Kushida, M. Láinez, A. Lamastra, E. Lindfors, S. Lombardi, F. Longo, R. López-Coto, M. López-Moya, A. López-Oramas, S. Loporchio, A. Lorini, L. Lulić, E. Lyard, P. Majumdar, M. Makariev, G. Maneva, M. Manganaro, S. Mangano, K. Mannheim, M. Mariotti, M. Martínez, P. Maruševec, A. Mas-Aguilar, D. Mazin, S. Menchiari, S. Mender, D. Miceli, J. M. Miranda, R. Mirzoyan, M. Molero González, E. Molina, H. A. Mondal, A. Moralejo, T. Nakamori, C. Nanci, V. Neustroev, L. Nickel, C. Nigro, L. Nikolić, K. Nilsson, K. Nishijima, K. Noda, S. Nozaki, Y. Ohtani, A. Okumura, J. Otero-Santos, L. Pacciani, S. Paiano, D. Paneque, R. Paoletti, J. M. Paredes, M. Peresano, M. Persic, M. Pihet, F. Podobnik, P. G. Prada Moroni, E. Prandini, G. Principe, M. Ribó, J. Rico, C. Righi, N. Sahakyan, T. Saito, F. G. Saturni, K. Schmitz, F. Schmuckermaier, J. L. Schubert, A. Sciaccaluga, G. Silvestri, J. Sitarek, D. Sobczynska, A. Stamerra, J. Strišković, Y. Suda, H. Tajima, M. Takahashi, R. Takeishi, F. Tavecchio, P. Temnikov, K. Terauchi, T. Terzić, M. Teshima, A. Tutone, S. Ubach, J. van Scherpenberg, S. Ventura, G. Verna, I. Viale, A. Vigliano, C. F. Vigorito, V. Vitale, I. Vovk, R. Walter, F. Wersig, M. Will, T. Yamamoto, J. A. Acosta-Pulido, G. A. Borman, W. Boschin, V. Bozhilov, M. I. Carnerero, D. Carosati, C. Casadio, W. Chamani, W. P. Chen, G. Damljanovic, S. A. Ehgamberdiev, K. E. Ergashev, M. Giroletti, T. S. Grishina, M. A. Gurwell, M. K. Hallum, E. Järvelä, H. E. Jermak, S. G. Jorstad, M. D. Jovanovic, E. N. Kopatskaya, K. Kuratov, O. M. Kurtanidze, S. O. Kurtanidze, A. Lähteenmäki, E. G. Larionova, L. V. Larionova, V. M. Larionov, H. C. Lin, N. Marchili, A. P. Marscher, C. McCall, M. Minev, D. O. Mirzaqulov, I. Myserlis, M. G. Nikolashvili, T. Pursimo, C. M. Raiteri, I. Reva, S. Righini, A. C. Sadun, S. S. Savchenko, E. Semkov, P. S. Smith, I. A. Steele, M. Tornikoski, Yu. V. Troitskaya, I. S. Troitskiy, A. A. Vasilyev, M. Villata, O. Vince, Z. R. Weaver

High Energy Astrophysical Phenomena

Short gamma-ray burst progenitors have short delay times

Matteo Pracchia and Om Sharan Salafia

Short gamma-ray bursts (SGRBs) are thought to be primarily associated with binary neutron star (BNS) mergers. The SGRB population can therefore be scrutinized for signatures of the delay time between the formation of the progenitor massive star binary and the eventual merger, which could produce an evolution of the cosmic rate density of such events whose shape departs from that of the cosmic star formation history. We therefore studied a large sample of SGRBs within a hierarchical Bayesian framework, with a particular focus on the delay time distribution (DTD) of the population. Following previous studies, we modelled the DTD either as a power-law with a minimum time delay or as a log-normal function. We considered two models for the intrinsic SGRB luminosity distribution: an empirical luminosity function with a doubly broken power-law shape and another based on a quasi-universal structured jet model. Regardless of the chosen parametrization, we found average time delays of $10\lesssim \langle \tau_\mathrm{d}\mathrm\rangle/\mathrm{Myr}\lesssim 900$ and a minimum delay time of $\tau_\mathrm{d,min}\lesssim 350\,\mathrm{Myr}$, in contrast with previous studies that found long delay times of a few gigayears. We demonstrate that the cause of the longer inferred time delays in past studies most likely resides in an incorrect treatment of selection effects
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Earth and Planetary Astrophysics

Disc fragmentation. II. Ejection of low mass Free Floating Planets from growing binary systems

Sergei Nayakshin, Luyao Zhang, Aleksandra Ćalović et al. (+4 more)

Observations indicate that disc fragmentation due to Gravitational Instability (GI) is the likely origin of massive companions to stars, such as giant planets orbiting M-dwarf stars, Brown Dwarf (BD) companions to FGK stars, and binary stars with separations smaller than 100 au. Additionally, we have recently showed that disc fragmentation in young rapidly evolving binary systems ejects an abundant population of massive Jupiter-mass Free-Floating Planets (FFPs). In this model, a massive disc around an initially single protostar undergoes GI and hatches a number of fragments; the most massive oligarch grows by runaway accretion into the secondary star. As the system rearranges itself from a single to a binary star configuration, a dramatic "pincer movement" by the binary ejects planets through dynamical interactions with the stars. Here we propose that the same scenario applies to an even more abundant population of smaller FFPs discovered by the microlensing surveys. Although disc fragmentation is usually believed to form only massive objects, several pathways for forming small core-dominated planets at separations of tens of au exist. We present results from three complementary simulation approaches, all of which confirm planet ejection efficiency as high as 0.5 for secondaries more massive than $\sim 10$% of the primary star mass. On the other hand, Jovian mass planets migrate through the region of tens of au too rapidly to eject planets from that region. We discuss implications of this scenario, concluding that microlensing FFPs may be the most convincing evidence yet that disc fragmentation forms planets much less massive than Jupiter.
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All authors Sergei Nayakshin, Luyao Zhang, Aleksandra Ćalović, Hans Lee, Clement Baruteau, Farzana Meru, Lucio Mayer

Instrumentation and Methods for Astrophysics

Modelling spacecraft-emitted electrons measured by SWA-EAS experiment on board Solar Orbiter mission

Š. Štverák, D. Herčík, P. Hellinger et al. (+6 more)

Thermal electron measurements in space plasmas typically suffer at low energies from spacecraft emissions of photo- and secondary electrons and from charging of the spacecraft body. We examine these effects by use of numerical simulations in the context of electron measurements acquired by the Electron Analyser System (SWA-EAS) on board the Solar Orbiter mission. We employed the Spacecraft Plasma Interaction Software to model the interaction of the Solar Orbiter spacecraft with solar wind plasma and we implemented a virtual detector to simulate the measured electron energy spectra as observed in situ by the SWA-EAS experiment. Numerical simulations were set according to the measured plasma conditions at 0.3~AU. We derived the simulated electron energy spectra as detected by the virtual SWA-EAS experiment for different electron populations and compared these with both the initial plasma conditions and the corresponding real SWA-EAS data samples. We found qualitative agreement between the simulated and real data observed in situ by the SWA-EAS detector. Contrary to other space missions, the contamination by cold electrons emitted from the spacecraft is seen well above the spacecraft potential energy threshold. A detailed analysis of the simulated electron energy spectra demonstrates that contamination above the threshold is a result of cold electron fluxes emitted from distant spacecraft surfaces. The relative position of the break in the simulated spectrum with respect to the spacecraft potential slightly deviates from that in the real observations. This may indicate that the real potential of the SWA-EAS detector with respect to ambient plasma differs from the spacecraft potential value measured on board. The overall contamination is shown to be composed of emissions from a number of different sources and their relative contribution varies with the ambient plasma conditions.
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All authors Š. Štverák, D. Herčík, P. Hellinger, M. Popďakunik, G. R. Lewis, G. Nicolaou, C. J. Owen, Yu. V. Khotyaintsev, M. Maksimovic

Solar and Stellar Astrophysics

The massive hot subdwarf binary LAMOST J065816.72+094343.1

F. Mattig, B. N. Barlow, D. Liu et al. (+9 more)

Massive short-period binaries involving hot subdwarf stars (sdO/Bs) are rare but important to constraining pathways for binary star evolution. Moreover, some of the most promising candidate progenitor systems leading to Type Ia supernovae (SNe Ia) involve sdO/Bs. LAMOST J065816.72+094343.1 has been identified as such a candidate. To explore the nature and evolutionary future of LAMOST J065816.72+094343.1, we complemented archival spectroscopic data with additional time series spectra and high-resolution spectroscopy of the object. After combining these with photometric data, we determined the orbital parameters of the system and the mass of the companion. We solved the orbit of the system by analyzing 68 low- and medium-resolution spectra using state-of-the-art mixed local thermodynamic equilibrium (LTE) and non-LTE model atmospheres. Additionally, we gathered nine high-resolution spectra to determine atmospheric parameters and the projected rotational velocity of the sdOB. The inclination angle of the system was constrained assuming tidal synchronization of the sdOB, which was verified via analysis of the ellipsoidal variations in the TESS light curve. We determine LAMOSTJ065816.72+094343.1 to be a binary consisting of a massive $0.82 \pm 0.17 \mathrm{M}_{\odot}$ sdOB component with a $1.30^{+0.31}_{-0.26} \mathrm{M}_{\odot}$ unseen companion. Due to the companion's mass being very close to the Chandrasekhar mass limit and high for a white dwarf, it is unclear whether it is a white dwarf or a neutron star. We find the system to be in a close orbit, with a period of $P=0.31955193 \mathrm{d}$ and an inclination angle of $i = 49.6^{+5.2}_{-4.2} \mathrm{deg}$. While the exact nature of the companion remains unknown, we determine the system to either lead to a SN Ia or an intermediate mass binary pulsar, potentially after a phase as an intermediate-mass X-ray binary.
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All authors F. Mattig, B. N. Barlow, D. Liu, M. Dorsch, S. Geier, M. Pritzkuleit, H. Dawson, B. Wang, V. Schaffenroth, T. Kupfer, C. Derbyshire, S. Barocci-Faul

Solar and Stellar Astrophysics

A Multiband Photometric Study of RR Lyrae Stars in M53 (NGC 5024)

Shantanu A. Gaur, Nitesh Kumar, Anupam Bhardwaj et al. (+1 more)

We present a multiband (UBVRI) time-series photometric study of RR Lyrae (RRL) stars in the globular cluster Messier 53 (NGC 5024) to refine their pulsation properties and determine a precise cluster distance. The archival photometric data includes images taken over 22 years and 3 months using different optical telescopes, providing an excellent time baseline to investigate light curves of variable stars. Using Lomb-Scargle periodogram, we derived accurate periods for 29 fundamental-mode (RRab) and 35 first-overtone (RRc) RRLs. Template-fitting to phase-folded light curves provided robust mean magnitudes and amplitudes. The refined periods confirm M53 as an Oosterhoff II cluster, with a mean period of 0.649 days for RRab and 0.346 days for RRc, and a high RRc fraction (54.7%). Most RRLs align with the horizontal branch in the color-magnitude diagram, while a few outliers result from blending effects. Period-amplitude diagrams show RRab stars following the Oosterhoff II locus. We derived I-band period-luminosity and multi-band period-Wesenheit relations, comparing them with theoretical predictions. A weighted mean distance modulus of 16.242 $\pm$ 0.05 mag yields a cluster distance of 17.717 $\pm$ 0.408 kpc, in agreement with recent estimates based on parallaxes from Gaia data.
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All authors Shantanu A. Gaur, Nitesh Kumar, Anupam Bhardwaj, Aasheesh Raturi

Astrophysics of Galaxies

Identification of solid N2O in interstellar ices using open JWST data

V. Karteyeva, R. Nakibov, I. Petrashkevich et al. (+2 more)

Context. There are only six molecules containing N-O bond that are detected in gaseous phase in interstellar medium. One of those is nitrous oxide (N2O), which was searched for but not found in solid form from as early as Infrared Space Observatory (ISO) mission was launched. The observational capabilities of James Webb Space Telescope (JWST) present a possibility to identify solid interstellar N2O. Aims. We aim to identify nitrous oxide in open JWST spectra of interstellar ices towards a sample of Class 0, 0/I and flat protostars using the relevant laboratory mixtures of N2O-bearing interstellar ice analogues. Methods. A set of laboratory infrared transmission spectra was obtained for the following mixtures: N2O:CO2=1:20, N2O:CO=1:20, N2O:N2=1:20, N2O:CO2:CO=1:15:5, N2O:CO2:N2=1:15:13 at 10-23 K. A search for N2O in JWST NIRSpec spectra towards 50 protostars was performed by fitting the 4.44-4.47 um (2250-2235 cm-1) NN-stretch absorption band with new laboratory mixtures of N2O-bearing ices. Results. We claim the first secure identification of N2O in 16 protostars. The fitting results show that N2O is formed predominantly within the apolar layer of the ice mantles, rich in CO, CO2 and N2. The abundance of solid N2O is estimated as 0.2-2.1% relative to solid CO. We present band strengths for N2O in the mixtures corresponding to the apolar layer. Also, an identification of the C-N stretch band at 4.42 um (2260 cm-1) is reported, which we tentatively assign to HNCO, the simplest C-N bond carrier.
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All authors V. Karteyeva, R. Nakibov, I. Petrashkevich, M. Medvedev, A. Vasyunin

High Energy Astrophysical Phenomena

Characteristics of the High-frequency Humps in the Black hole X-ray Binary Swift J1727.8–1613

Ze-Xi Li, Liang Zhang, Lian Tao et al. (+13 more)

We present a detailed timing analysis of the two high-frequency humps observed in the power density spectrum of Swift J1727.8–1613 up to 100 keV, using data from the Hard X-ray Modulation Telescope (Insight-HXMT). Our analysis reveals that the characteristic frequencies of the humps increase with energy up to $\sim30$ keV, followed by a plateau at higher energies. The fractional rms amplitudes of the humps increase with energy, reaching approximately 15% in the 50-100 keV band. The lag spectrum of the hump is characterized primarily by a soft lag that varies with energy. Our results suggest that the high-frequency humps originate from a corona close to the black hole. Additionally, by applying the relativistic precession model, we constrain the mass of Swift J1727.8–1613 to $2.84 < M / M_{\odot} < 120.01$ and the spin to $0.14 < a < 0.43$ from the full-energy band dataset, using triplets composed of a type-C quasi-periodic oscillation and two high-frequency humps. When considering only the high-energy bands with stable characteristic frequencies, we derive additional constraints of $2.84 < M/M_{\odot} < 13.98$ and $0.14 < a < 0.40$.
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All authors Ze-Xi Li, Liang Zhang, Lian Tao, Zi-Han Yang, Qing-Chang Zhao, Shu-Jie Zhao, Rui-Can Ma, Zi-Xu Yang, Pan-Ping Li, Xiang Ma, Yue Huang, Shu-Mei Jia, Shuang-Nan Zhang, Hua Feng, Jin-Lu Qu, Shu Zhang

Astrophysics of Galaxies

Predicting dust temperature from molecular line data using machine learning

Tenta Dougome, Yoshito Shimajiri, Kazuya Saigo et al. (+4 more)

We conducted experiments with machine learning techniques to construct dust temperature maps from the CO isotopologue molecular line data in the Orion A molecular cloud. In the classical astrophysical methodology, multi-band continuum data are required to derive the dust temperature. The present study aims to investigate the capability and limitations of machine learning techniques to derive dust temperatures in regions without multi-band dust continuum data. We investigated how the number of pixels used for training influences prediction accuracy, and how the dust temperatures sampled in the training area influence prediction accuracy. We found that $\sim$5% of the total number of pixels in the observational region is sufficient for training to obtain accurate predictions. Furthermore, a dust temperature sample within the training area should cover the whole temperature range and have a similar sample distribution to that of the entire observing region for an accurate prediction. The $^{12}$CO / $^{13}$CO ratio is often found to be the most important feature in predicting the dust temperature. As the $^{12}$CO / $^{13}$CO ratio is a tracer of PDR, the machine learning technique could connect the dust temperatures to the PDRs. We also found that the condition of thermal gas-dust coupling is not required for accurate prediction of the dust temperature from the molecular line data, and that machine learning is capable of capturing information more than classical astrophysical concepts.
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All authors Tenta Dougome, Yoshito Shimajiri, Kazuya Saigo, Sanemichi Takahashi, Miyu Kido, Shu Ishibashi, Shigehisa Takakuwa

Earth and Planetary Astrophysics

Protonic thermoelectric effect of Superionic H$_2$O and magnetic field generation in Uranus and Neptune

Daohong Liu, Wei Zhang, Yu He et al. (+3 more)

The origin of the anomalous magnetic fields of Uranus and Neptune, with significant tilts and multi-dipole configurations, is central to understanding the internal structure and evolution of the ice giants. Recent investigations confirmed that superionic H$_2$O ice is thermodynamically stable and constitutes the dominant H$_2$O phase within their icy mantles. Here, we use deep learning and ab initio molecular dynamics to investigate the thermal and protonic transport properties of superionic H$_2$O ice. We demonstrate that superionic H$_2$O ice exhibits a pronounced protonic thermoelectric effect, with a maximum Seebeck coefficient that can reach $\sim 620$ $\mu$V K$^{-1}$ within the interior of Uranus, whereas Neptune's is lower, within the range of 570-585 $\mu$V K$^{-1}$. Consequently, the temperature gradients in the icy mantles can induce proton migration, thereby driving magnetic field generation. Based on this mechanism, and within a dipolar approximation that captures the mean field, the disparities in mean dipolar magnetic field strength between Uranus and Neptune can be explained exclusively by their differing internal temperature gradients, and the predicted mean field strengths are comparable to those observed by Voyager 2.
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All authors Daohong Liu, Wei Zhang, Yu He, Xinzhuan Guo, Chuanyu Zhang, Yang Sun

Solar and Stellar Astrophysics

Investigating the Center-to-Limb Effects in Helioseismic Data Using 3D Radiative Hydrodynamic Simulations

Irina N. Kitiashvili

Full-disk observations from missions such as the SDO and SOHO have enabled comprehensive studies of solar oscillations and dynamics. Interpreting helioseismic and photospheric data is complicated by systematic center-to-limb variations. To explore the physical origin of these variations, we perform local 3D radiative hydrodynamic simulations that include effects of solar rotation to generate 24-hour synthetic time series of continuum intensity and Doppler velocity for nine viewing angles spanning from $-75^{\circ}$ to $75^{\circ}$. The simulations reveal a systematic decrease in 1D oscillation power toward the limbs and a pronounced East-West asymmetry that increases with frequency, primarily due to rotation-induced flows. Analysis of $\ell-\nu$ diagrams shows a decrease in the amplitude and width of the surface gravity ($f$) and resonant pressure ($p$) modes with increasing angular distance from the disk center. The amplitudes of the corresponding pseudo-modes with frequencies above the acoustic cut-off frequency increase in the intensity power spectra and are suppressed in the velocity spectra. The ring-diagram analysis of the simulation data further demonstrates anisotropic broadening of the modes, and the impact of the foreshortening effect on the energy distribution, and distinct differences in background noise and pseudo-mode structure between the intensity and velocity data. These results indicate that the center-to-limb effects arise from both geometric projection and physical factors such as line-formation height and potential effects of the radial differential rotation. The findings provide a framework for correcting helioseismic observations and demonstrate that realistic simulations are a powerful tool for disentangling geometric and physical biases in solar data.
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High Energy Astrophysical Phenomena · Cross-listed from gr-qc

Limits on the mass of compact objects in Hořava-Lifshitz gravity

Edwin J. Son

It is known that there exist theoretical limits on the mass of compact objects in general relativity. One is the Buchdahl limit for an object with an arbitrary equation-of-state that turns out to be the limit for an object with uniform density. Another one is the causal limit that is stronger than the Buchdahl limit and is related to the speed of sound inside an object. Similar theoretical limits on the mass of compact objects in deformed Hořava-Lifshitz (HL) gravity are found in this \paper. Interestingly, the both curves of the uniform density limit and the sound speed limit meet the horizon curve at the minimum of the horizon, where a black hole becomes extremal, i.e., $M=q$, considering the Kehagias-Sfetsos vacuum that is an asymptotic flat solution in the HL gravity.
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Astrophysics of Galaxies

An Enhanced Sample of Galactic Red Supergiants Reveals Spiral Structures

Zehao Zhang, Biwei Jiang and Yi Ren

Red supergiants (RSGs), representing a kind of massive young stellar population, have rarely been used to probe the structure of the Milky Way, mainly due to the long-standing scarcity of Galactic RSG samples. The Gaia BP/RP spectra (hereafter XP), which cover a broad wavelength range, provide a powerful tool for identifying RSGs. In this work, we develop a feedforward neural network classifier that assigns to each XP spectrum a probability of being an RSG, denoted as $\mathrm{P(RSG)}$. We perform ten independent runs with randomly divided training and validation sets, and apply each run to all XP spectra of stars with $G < 12$ mag. By selecting sources with $\mathrm{P(RSG)} \geq 0.9$, ten high-confidence candidate samples are obtained. A star is considered a ture Galactic RSG only if it appears in at least eight of these samples, yielding a final catalog of 2,436 objects. These RSGs show a clear spatial correlation with OB stars and trace the Galactic spiral arms well, confirming the reliability of our classification, and highlighting their potential to serve as powerful tracers of the Milky Way's structure.
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Cosmology and Nongalactic Astrophysics · Cross-listed from hep-th

Real-time Gravitational Wave Response in Thermal Spinning fields

Atsuhisa Ota, Hui-Yu Zhu and Yuhang Zhu

We study how the spin content of the thermal plasmas affects the propagation of gravitational waves in a radiation-dominated universe. As a simple but representative setup, we consider conformal scalar, Weyl fermion, and Maxwell fields that provide the background radiation, and we ask whether the resulting damping and phase shift of gravitational waves retain any memory of their spins. We revisit this question in a real-time quantum-field-theoretic framework, where the stress tensor splits into a background part, a dynamical (history-dependent) response, and local contact terms, with an additional on-shell projection fixed by the Friedmann equation. We find that the dynamical spin-dependent response arises on a short time scale characterized by the radiation temperature, which is exactly canceled by the local responses. As a result, the remaining long-time response is universal and consistent with kinetic theory in the hard thermal limit. Although the underlying mechanism exhibits strong spin dependence, it leaves no observable imprint on the large-scale effective dynamics of gravitational waves in this setup.
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Solar and Stellar Astrophysics · Cross-listed from physics.space-ph

On flying near the base of a pseudo-streamer

Forrest Mozer, Oleksiy Agapitov, Kyungeun Choi et al. (+1 more)

Near the 10 solar radius perihelion of Parker Solar Probe encounter 24, a confined region containing an enhanced plasma density (25,000 per cubic centimeter) and broadband electrostatic waves was encountered. The solar wind velocity (200 kilometers per second) and ion temperature (25 electron Volts) were significantly reduced as compared to their values in the ambient solar wind. Because the polarity of the radial magnetic field did not change sign on the two sides of the crossing and the crossed region contained a double-peaked plasma structure, the spacecraft must have passed near the base of a pseudo-streamer. In the plasma frame, an electric field as large as 400 millivolts per meter was detected during the crossing. This field is not associated with an ExB drift because it is observed in the plasma rest frame. Instead, it is balanced by the Generalized Ohm's Law terms whose current dependent term associated with this electric field was less than 1 milliampere per square meter, corresponding to a drift velocity less than 2.5 kilometers per second. The region also contained a turbulent plasma with density fluctuations as large as 0.3, suggesting that the resistive term in the Generalized Ohm's Law could be significant. In addition, both the density and electric field had non-zero slopes as functions of time, suggesting that the pressure gradient term in the Generalized Ohm's Law also mattered. This large electric field in the plasma rest frame may be the first and largest such field ever reported.
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All authors Forrest Mozer, Oleksiy Agapitov, Kyungeun Choi, Andrii Voshchepynets

Astrophysics of Galaxies

Particle Acceleration and Depolarization in the Protostellar jet knots HH 80 and HH 81

A. G. Cheriyan, S. Vig, Nirupam Roy et al. (+4 more)

Linearly polarized emission is a powerful tracer of magnetic field geometry and particle acceleration in protostellar jets. We present a polarimetric study of the HH objects HH 80 and HH 81 from where non-thermal emission has been confirmed through spectral index measurements at low frequencies. We carried out observations of HH 80 and HH 81 with the Karl G. Jansky Very Large Array in 4-6 GHz. Unlike the inner jet knots, no linear polarization is detected towards the knots HH 80 and HH 81. We place a $3\sigma$ upper limit of $30~\mu$Jy on the polarization intensity, corresponding to fractional polarization limits of $\Pi_{\max}\approx0.02$ and $0.01$ for HH 80 and HH 81, respectively. To interpret this non-detection, we assess the conditions for synchrotron polarization and the impact of depolarization mechanisms. The shock cooling parameter $\chi_\mathrm{s}$ is lower in these outermost HH objects than in the inner knots, indicating that the reverse shocks in HH 80-81 are less efficient at accelerating relativistic electrons compared with the inner knots. Moreover, Faraday depolarization appears severe: the dispersion in the estimated rotation measure $\sigma_{\rm RM}\sim400~\mathrm{rad~m^{-2}}$ is comparable to or larger than observed RM values themselves. This is consistent with strong fluctuations and turbulence. Together with beam depolarization, these effects can suppress the observable fractional polarization flux densities below the detectable thresholds. We conclude that reduced acceleration efficiency (when compared to inner knots) and strong depolarization account for the absence of polarized emission towards HH 80 and HH 81.
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All authors A. G. Cheriyan, S. Vig, Nirupam Roy, Samir Mandal, C. Carrasco-González, A. Rodríguez-Kamenetzky, A. Pasetto

Instrumentation and Methods for Astrophysics · Cross-listed from gr-qc

Constraining Lorentz and parity violations in gravity with multiband gravitational wave observations

Zhi-Xin Jia, Tao Zhu and Zhoujian Cao

This study evaluates the capability of future multi-band observations of gravitational waves emitted from binary black hole coalescences, utilizing joint third-generation ground-based (CE, ET) and space-based (LISA, Taiji, TianQin) detector networks, to constrain parity and Lorentz symmetry violations in the gravitational sector. We model these effects through a parameterized waveform framework that incorporates a set of parameters that quantify potential deviations from general relativity. The frequency-dependence of their effects is described by power-law indices $\beta$ (i.e., $\beta_{\bar \nu}$, $\beta_{\bar \mu}$, $\beta_{\nu}$, and $\beta_{\mu}$). By analyzing events such as a high-signal noise ratio (SNR) "golden event" like GW250114 and a massive binary system like GW231123 (total mass $190-265 M_\odot$) using two networks of ground- and space-based detectors, we demonstrate that multi-band observations can significantly improve the current constraints on Lorentz and parity violations by several order of magnitude, for both high-frequency ($\beta > 0$) and low-frequency ($\beta < 0$) modifications. Our Bayesian analysis reveals that while the exceptional SNR of the GW250114-like event yields superior constraints for high-frequency modifications ($\beta > 0$), the massive nature of GW231123 provides more stringent limits for low-frequency effects ($\beta < 0$). This work highlights the critical value of future multi-band gravitational wave astronomy for conducting precision tests of general relativity across diverse binary populations.
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Solar and Stellar Astrophysics

Electron Cyclotron Maser Emission from Ejected Stellar Prominences on V374 Peg

C. E. Brasseur, M. M. Jardine, S. Daley-Yates et al. (+2 more)

We investigate a possible origin for bursty radio emission observed on the active M dwarf V374 Peg, combining data-driven magnetic field modelling with archival radio light curves. We examine whether stellar prominence ejection can plausibly account for the observed radio bursts that have been attributed to electron cyclotron maser (ECM) emission. Our analysis shows that ejected prominences can produce the required energy range to drive the emission, and that modelled ECM visibility exhibits a rotational phase dependence consistent with the limited observational data (four observed bursts). The results support prominence ejection as a viable mechanism for ECM generation on V374 Peg and motivate further observational campaigns to constrain this process.
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All authors C. E. Brasseur, M. M. Jardine, S. Daley-Yates, J. F. Donati, J. Morin

Solar and Stellar Astrophysics

Perhaps there is no brown dwarf desert? A study of sub-stellar companions with Gaia DR3

A. L. Wallace and A. R. Casey

The brown dwarf desert describes a range of orbital periods (<5 years) in which fewer brown dwarf-mass companions have been observed around Sun-like stars, when compared to planets and low mass stellar companions. It is therefore theorised that brown dwarf companions are unlikely to form or remain in this period range. The Gaia space telescope is uniquely sensitive to companions in this period range, making it an ideal tool to conduct a survey of the brown dwarf desert. In this study, we use Bayesian inference to analyse data from nearby (<200pc) Sun-like stars in Gaia's DR3 catalogue, assuming single companions. From this, we identify 2673 systems (2.41% of the sample) with possible brown dwarf companions in this period range. Accounting for observational biases, we find that 10.4+0.8-0.6% of nearby Sun-like stars have astrometric errors consistent with a brown dwarf-mass companion with a period less than 5 years, significantly higher than previous studies which reported occurrence rates of <1%. However, we acknowledge the limitations of DR3 and are unable to make a definitive statement without epoch data. By simulating epoch data with multiple companions, we find that, while some of the data can be explained by multiple low-mass brown dwarf companions and high-mass planets (>10MJ), high-mass brown dwarfs (>50MJ) in this period range are comparatively rare. Finally, we used our studies of the brown dwarf distribution to predict the number of companions in the brown dwarf desert we can expect to discover in DR4.
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Astrophysics of Galaxies

Jet-torus interaction revealed by sub-parsec SO absorption in NGC 1052

Satoko Sawada-Satoh, Seiji Kameno, Nozomu Kawakatsu et al. (+10 more)

We report the first {\lambda}2-mm very long baseline interferometry (VLBI) observations of the radio galaxy NGC 1052, conducted with the Korean VLBI Network (KVN) using a wide-band recording mode. Leveraging the wide bandwidth covering a velocity range at 2300 km/s, we successfully detect broad (> 700 km/s) multi-component SO J_N = 3_3 - 2_2 absorption against the sub-parsec-scale continuum structure. The absorption profile consists of both redshifted and blueshifted components, including a newly identified blueshifted feature at -412 km/s relative to the systemic velocity. Significant SO absorption is confined to the central components, with no substantial detection toward the outer jet components. This constrains the location of SO gas to a compact region smaller than 0.45 pc in the sub-parsec vicinity of the supermassive black hole (SMBH). Our results support the scenario in which SO molecules are evaporated through shock heating caused by jet-torus interaction. The SO gas clumps are likely driven outward by the jet, with some returning toward the SMBH as inflowing material. Comparison with 321 GHz H2O masers reveals partial similarities in spatial distribution and radial velocity, suggesting that the jet-torus interaction may also trigger the excitation of H2O masers.
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All authors Satoko Sawada-Satoh, Seiji Kameno, Nozomu Kawakatsu, Do-Young Byun, Se-Jin Oh, Sang-Sung Lee, Duk-Gyoo Roh, Chungsik Oh, Jae-Hwan Yeom, Dong-Kyu Jung, Hyo- Ryoung Kim, Young-Sik Kim, Sanghyun Kim

Tuesday 6 January

Instrumentation and Methods for Astrophysics

Charge Migration and Residual Non-Linearity in NIRSpec BOTS Observations

Munazza K. Alam, Leonardo Ubeda, Qinyan Lu et al. (+2 more)

We investigate the effect of charge migration and residual non-linearity on the JWST/NIRSpec G395H NRS1 and NRS2 detectors using Bright Object Time Series (BOTS) observations of the ultra-hot Jupiter WASP-121b. These full-orbit phase curve observations were taken over 37.8 hours (1.57 days), and provide an excellent testbed of the non-linearity behavior of NRS1 and NRS2 over long timescales. For both detectors, our analysis demonstrates charge losses at the center of the spectral trace and charge excesses at the trace edges. We find that the NRS1 detector displays ~3x larger deviations from linearity compared to NRS2. Given the large transit (~1.5%) and eclipse (~0.5%) signals for WASP-121b, we also investigate variations in the distribution of charge throughout the time-series observation. Our results show that charge distribution varies at different planetary orbital phases for NRS1, which manifests as a change in the morphology and shape of the spectral trace over the course of the time-series. The effect of charge distribution on the trace shape is not evident for NRS2.
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All authors Munazza K. Alam, Leonardo Ubeda, Qinyan Lu, Nestor Espinoza, Nikolay Nikolov

High Energy Astrophysical Phenomena

The maximum mass ratio of hierarchical binary black hole mergers may cause the $q$-$\chi_{\rm eff}$ correlation

Aditya Vijaykumar, Amanda M. Farah and Maya Fishbach

Regardless of their initial spins, the merger of two roughly equal mass black holes (BHs) produces a remnant BH of dimensionless spin $0.69$. Such remnants can merge with other BHs in dense stellar environments and produce hierarchical mergers. Analyzing the latest catalog binary black hole (BBH) mergers from the LIGO-Virgo-KAGRA detectors, we identify a subpopulation with primary spins consistent with such hierarchical mergers. Consistent with astrophysical expectations for mergers of second-generation BHs with first-generation BHs, we find that this subpopulation has mass ratios below $0.59^{+0.18}_{-0.23}$. We also infer that $19$-$88\%$ of the BBH population below this mass ratio is consistent with belonging to the hierarchically merged population. Our results offer a natural explanation for the narrowing of the effective inspiral spin distribution with mass ratio observed in other studies.
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High Energy Astrophysical Phenomena

The steep redshift evolution of the hierarchical binary black hole merger rate may cause the $z$-$\chi_{\rm eff}$ correlation

Amanda M. Farah, Aditya Vijaykumar and Maya Fishbach

There is growing evidence from gravitational-wave observations that some merging black holes are created from previous mergers. Using the prediction that these hierarchically merged black holes have dimensionless spin magnitudes of $\chi \approx 0.69$, we identify a subpopulation in the gravitational-wave data consistent with a hierarchical-merger origin in dense star clusters. This subpopulation's primary mass distribution peaks at $17.0^{+18.3}_{-4.4},\mathrm{M}_{\odot}$, which is approximately twice as large as its secondary mass distribution's mode ($10.5^{+29.7}_{-4.7},\mathrm{M}_{\odot}$), and its spin tilt distribution is consistent with isotropy. Our inferred secondary mass distributions imply that isolated binary evolution may still be needed to explain the entirety of the $9\,\mathrm{M}_{\odot}$ peak. Surprisingly, we find that the rate of hierarchical mergers may evolve more steeply with redshift than the rest of the population ($98.0\%$ credibility): the fraction of all binary black holes that are hierarchically formed at $z=0.1$ is $0.03^{+0.05}_{-0.02}$, compared to $0.09^{+0.11}_{-0.07}$ at $z=1$. This provides an explanation for the previously discovered broadening of the effective spin distribution with redshift. Our results have implications for star cluster formation histories, as they suggest the potential existence of a high-redshift population of massive, compact clusters.
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Astrophysics of Galaxies

Time-dependent chemical evolution during cloud formation: H$_2$-regulated chemistry in diffuse molecular cloud

Yuto Komichi, Yuri Aikawa, Kazunari Iwasaki et al. (+1 more)

We investigate the chemical evolution of a forming molecular cloud behind an interstellar shock wave. We conduct three-dimensional magnetohydrodynamics simulations of the converging flow of atomic gas, including a simple chemical network and tracer particles that move along the local velocity field. Then we perform detailed chemical network calculations along the trajectory of each tracer particle. The diffuse part of forming molecular clouds is CO-poor; i.e., H$_2$ and CO abundances do not correlate. In diffuse regions of $n_\mathrm{H}\lesssim 10^{3}\,\mathrm{cm^{-3}}$, we find that the abundances of hydrocarbons and oxygen-bearing molecules are determined by steady-state chemistry reflecting the local H$_2$ abundance, which is determined by the gas density along the trajectory. In denser regions, the abundances are affected by water ice formation, which changes the elemental abundance of carbon and oxygen (i.e., C/O ratio) in the gas phase. Assuming quasi-steady-state chemistry given the abundances of major molecules (e.g., H$_2$) from the simple network, we derive analytic solutions for molecular abundances, which reproduce the calculation results. We also calculate the molecular column densities based on the spatial distribution of tracer particles and their molecular abundances, and compare them with observations of diffuse molecular clouds. We find that the column densities of CH, CCH, and OH are linearly correlated with those of H$_2$, which supports the empirical relation used in the observations. On the other hand, the column density of HCO$^+$ shows non-linear dependence on the H$_2$ column density, reflecting the difference in HCO$^+$ formation paths in CO-poor and CO-rich regions.
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All authors Yuto Komichi, Yuri Aikawa, Kazunari Iwasaki, Kenji Furuya