Astronomy & Astrophysics最新文献

{"title":"X-ray spectro-polarimetry analysis of the weakly magnetized neutron star X-ray binary GX 9+1","authors":"A. Tarana, F. Capitanio, A. Gnarini, S. Fabiani, F. Ursini, S. Bianchi, C. Ferrigno, M. Parra, M. Cocchi, R. Farinelli, G. Matt, P. Soffitta, A. Bobrikova, P. Kaaret, M. Ng, J. Poutanen, S. Ravi","doi":"10.1051/0004-6361/202554083","DOIUrl":"https://doi.org/10.1051/0004-6361/202554083","url":null,"abstract":"We present an X-ray spectro-polarimetric study of the weakly magnetized neutron star low-mass X-ray binary GX 9+1, utilizing data from the Imaging X-ray Polarimetry Explorer (IXPE), alongside simultaneous NuSTAR, NICER, and INTEGRAL observations. GX 9+1, located in the Galactic bulge, is a persistently bright atoll source known for its spectral variability along the color–color diagram. Our spectral analysis during the soft state confirms that the emission is dominated by a soft blackbody and thermal Comptonization components, with no evidence of a hard X-ray tail. Moreover, these observations suggest a relatively low-inclination system (23° < <i>i<i/> < 46°) with a weak reflection component, consistent with emission from the accretion disk and neutron star boundary layer. Spectro-polarimetric analysis reveals no significant polarization in the 2–8 keV range, with a 3<i>σ<i/> level upper limit on the polarization degree of 1.9%. However, marginal evidence of polarization is detected in the 2–3 keV band at the 95.5% confidence level (2<i>σ<i/>), suggesting potential contributions from scattering effects in the individual spectral components (disk, reflection, and Comptonization) that may cancel each other out due to the different orientations of their polarization angles. This behavior aligns with other atoll sources observed by IXPE, which typically exhibit lower and less variable polarization degrees compared to Z–class sources.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"97 13 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144320025","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Discovery of a kinematically distinct component in the central region of the collisional ring galaxy AM0644-741","authors":"Chayan Mondal, Sudhanshu Barway","doi":"10.1051/0004-6361/202451984","DOIUrl":"https://doi.org/10.1051/0004-6361/202451984","url":null,"abstract":"We present the discovery of a peculiar central stellar structure in the collisional ring galaxy AM0644-741 using HST imaging and MUSE integral field unit (IFU) data. We identified two Sérsic components with a Sérsic index of 1.72 (inner part) and 1.11 (outer part) in the HST F814W band optical image using GALFIT. We utilized the MUSE data cube to construct stellar line-of-sight velocity (<i>V<i/><sub>LOS<sub/>), velocity dispersion (<i>σ<i/><sub>LOS<sub/>), <i>h<i/><sub>3<sub/> and <i>h<i/><sub>4<sub/> velocity moments, and stellar population age maps using the GIST pipeline for further investigating both Sérsic components, which have a difference of ∼60° in their position angle. The inner component, with an effective radius of ∼1 kpc, shows a strong anticorrelation between <i>V<i/><sub>LOS<sub/>/<i>σ<i/><sub>LOS<sub/> and <i>h<i/><sub>3<sub/>, indicating the presence of a rotating stellar structure. In addition, the inner component shows a higher velocity dispersion (average values reaching up to ∼240 km s<sup>−1<sup/>) along with disky isophotes and a stronger Mg <i>b<i/> line strength, which all together highlight a peculiar dynamical state of AM0644-741’s central region. Our analysis suggests that the recent encounter has had a smaller impact on the stellar orbits within the inner component. In contrast, it has specifically affected the stellar orbits of the progenitor’s outer disk when forming the star-forming ring. The Baldwin, Phillips and Terlevich (BPT) analysis of the unresolved nuclear source shows a low-ionization nuclear emission-line region (LINER) type ionization, hinting at active galactic nucleus (AGN) activity in the galaxy. Our study projects the dynamical evolution of collisional systems and provides scope for simulations to explore the central region in greater detail.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"38 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144320002","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Discerning internal conditions of pulsating hot subdwarf B-type stars","authors":"Hamed Ghasemi, Murat Uzundag, Cole Johnston, Conny Aerts","doi":"10.1051/0004-6361/202554183","DOIUrl":"https://doi.org/10.1051/0004-6361/202554183","url":null,"abstract":"<i>Context.<i/> The frequencies of gravity-mode oscillations are determined by the chemical, thermal, and structural properties of stellar interiors, which facilitates the study of internal mixing mechanisms in stars. We investigated the impact of discontinuities in the chemical composition induced by the formation of an adiabatic semi-convection region during the core helium (He)-burning phase of evolution of hot subdwarf B-type (sdB) stars.<i>Aims.<i/> This study delves into the progression of convective core evolution, using a numerical approach to model the emergence of a semi-convection zone. We scrutinize the asteroseismic attributes of the evolutionary stages and assess the core He-burning phase by evaluating the parameter linked to the average interval between the deep trapped modes in both sdB evolutionary models and the observations of KIC 10001893.<i>Methods.<i/> We performed evolutionary and asteroseismic analyses of sdB stars using MESA and GYRE to examine the properties of the semi-convection region. Additionally, we computed parameters related to gravity-mode period spacings and the interval between deep trapped modes to characterize the core He-burning phase at different stages of sdB evolution.<i>Results.<i/> Using a numerical scheme in MESA to model the development of the semi-convection zone, we illustrate the evolution of the convective core in sdB stars. Our study addresses the challenges of relying solely on the average interval between oscillation mode periods with consecutive radial orders to identify the core He-burning stage. To improve identification, we propose a new parameter that represents the average interval between deep trapped modes during some of the stages of sdB evolutionary models. Additionally, we find that integrating convective penetration with convective premixing improves our models and yields comparable outcomes without the need for additional model parameters.<i>Conclusions.<i/> Our results can advance the development of detailed evolutionary models for sdB stars by refining internal mixing schemes, increasing the accuracy of pulsation predictions, and improving alignment with observational data.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"36 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144320027","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Enlightening the Universe behind the Milky Way bulge","authors":"Daniela Galdeano, Georgina Coldwell, Sol Alonso, Fernanda Duplancic, Philip W. Lucas, Julia Fernandez, Noelia Perez, Luis Pereyra, Valeria Mesa, Dante Minniti, Leigh C. Smith, Francisco M. Zarate","doi":"10.1051/0004-6361/202554268","DOIUrl":"https://doi.org/10.1051/0004-6361/202554268","url":null,"abstract":"<i>Context<i/>. The detection of extragalactic sources beyond the Milky Way is limited by the location of the Solar system. In the zone of avoidance (ZOA), optical observations are hindered by stellar crowding and Galactic absorption. To fully understand the background galaxy population in this region, observations at longer wavelengths are needed.<i>Aims<i/>. The objective of this study is to determine the characteristics of galaxies that are situated behind the Milky Way bulge through the use of near-infrared (NIR) data obtained from the VISTA Variables in Vía Láctea (VVV) survey.<i>Methods<i/>. The galaxy candidates were visually confirmed, and a morphological classification of all sources was performed based on their appearance, by differentiating between elliptical, spiral, undefined, and doubtful galaxies. Additionally, the surrounding area was classified based on the number of sources that are detected in the same target field. Isolated galaxies, galaxies with close companions, and galaxies in denser environments were catalogued.<i>Results<i/>. Following a visual inspection, 14 480 galaxy candidates were classified morphologically: 3702 as elliptical, 1704 as spiral, 6425 as undefined, and 2649 as doubtful. The environmental classification showed that 73.9% were isolated, 12.4% had close companions, and 13.7% were in denser environments. Our catalogue shows slower proper motions than star catalogues. The majority of galaxies that are classified as elliptical and spiral manifest SExtractor parameters that are indicative of this category of sources. Additionally, elliptical galaxies in denser environments appear to be redder than spiral galaxies in isolated environments.<i>Conclusions<i/>. A total of 14 480 galaxy candidates were subjected to a morphological classification in the region of the Milky Way bulge. This result verifies the efficacy of the VVV/VVVX survey in identifying and characterising a diverse range of galaxies and extragalactic structures that are obscured by the Milky Way. The survey thereby contributes to a more profound understanding of the Universe in the ZOA.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"24 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144320028","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Helical magnetic field structure in 3C 273","authors":"Teresa Toscano, Sol N. Molina, José L. Gómez, Ai-Ling Zeng, Rohan Dahale, Ilje Cho, Kotaro Moriyama, Maciek Wielgus, Antonio Fuentes, Marianna Foschi, Jan Röder, Ioannis Myserlis, Emmanouil Angelakis, Anton Zensus","doi":"10.1051/0004-6361/202453542","DOIUrl":"https://doi.org/10.1051/0004-6361/202453542","url":null,"abstract":"We present a study on rotation measure (RM) of the quasar 3C 273. This analysis aims to discern the magnetic field structure and its temporal evolution. The quasar 3C 273 is one of the most studied active galactic nuclei due to its high brightness, strong polarisation, and proximity, which enables the resolving of the transverse structure of its jet in detail. We used polarised data from 2014, collected at six frequencies (5, 8, 15, 22, 43, 86 GHz) with the Very Long Baseline Array, to produce total and linear polarisation-intensity images, as well as RM maps. Our analysis reveals a well-defined transverse RM gradient across the jet, indicating a helical, ordered magnetic field that threads the jet and likely contributes to its collimation. Furthermore, we identified temporal variations in the RM magnitude when compared with prior observations. These temporal variations show that the environment around the jet is dynamic, with changes in the density and magnetic field strength of the sheath that are possibly caused by interactions with the surrounding medium.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"13 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144320220","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Dips in the sub-TeV gamma-ray light curves from central parts of galaxies due to transiting luminous stars","authors":"W. Bednarek, J. Sitarek, M. Ulatowski","doi":"10.1051/0004-6361/202452419","DOIUrl":"https://doi.org/10.1051/0004-6361/202452419","url":null,"abstract":"<i>Context.<i/> The GeV-TeV <i>γ<i/>-ray emission was observed from the direction of the source Sgr A<sup>⋆<sup/>, which is identified with the supermassive black hole (SMBH) in the centre of our Galaxy. According to some models this <i>γ<i/>-ray emission might originate in the very compact central region identified with the direct surrounding of the SMBH. Sgr A<sup>⋆<sup/> is surrounded by a massive nuclear star cluster, composed of the S stars, also including OB-type stars. Occasionally these stars might pass close to the line of sight of the observer, resulting in partial absorption of the sub-TeV <i>γ<i/>-ray emission.<i>Aims.<i/> We investigated the conditions at which an absorption feature appears in the <i>γ<i/>-ray light curves from the Galactic Centre or nuclei of other galaxies containing SMBHs. The detection of such features would allow us to obtain constraints on the emission site of <i>γ<i/> rays in active galaxies.<i>Mehods.<i/> We calculated the optical depths for <i>γ<i/> rays in the radiation of individual massive stars, or from the whole population of stars for different parameters of the star cluster.<i>Results.<i/> We show that the observer with a line of sight close to the orbital plane of the star can register a <i>γ<i/>-ray absorption dip lasting from a fraction of a day up to a few tens of days. The combined effect of the bulk absorption on the whole population of stars instead can produce a flickering of the observed emission of a red-noise type in the power spectrum of the emission.<i>Conclusions.<i/> Predicted absorption features in the sub-TeV <i>γ<i/>-ray light curves from galaxies with SMBHs should be easily detectable by the Large-Sized Telescopes of the Cherenkov Telescope Array Observatory. The discovery of such absorption features provides a unique indication that the <i>γ<i/>-ray production is occurring in a compact region close to the horizon of the SMBH.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"600 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144320023","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Photoionization of small neutral polycyclic aromatic hydrocarbons","authors":"H. R. Hrodmarsson, L. Verstraete, E. Dartois, J. Fréreux, O. Lacinbala, L. Nahon, G. A. Garcia, T. Pino, P. Bréchignac","doi":"10.1051/0004-6361/202554245","DOIUrl":"https://doi.org/10.1051/0004-6361/202554245","url":null,"abstract":"<i>Context<i/>. In the interstellar medium (ISM) and circumstellar environments, photoionization, or the photoelectric effect, emerges as a prevalent phenomenon. In regions exposed to UV photons, either stellar or secondary photons induced by cosmic rays, polycyclic aromatic hydrocarbons (PAHs) liberate electrons through the photoelectric effect, efficiently contributing to the heating budget of the gas. In contrast to shielded areas, neutral and cationic PAHs can scavenge electrons and retain a significant portion of the cloud’s electronic charge. This dual behavior of PAHs not only influences the gas thermal and dynamical behavior but also plays a pivotal role in shaping the chemistry of the environments.<i>Aims<i/>. Our aim was to measure the photoelectron kinetic energy distribution (KED) of PAHs of varying sizes, symmetries, and C:H ratios to describe the photoelectron KED with a law that can be implemented in astrophysical photoelectric models that describe gas heating.<i>Methods<i/>. We used a double-imaging photoelectron photoion coincidence spectrometer coupled with the DESIRS VUV beamline at the SOLEIL synchrotron to record the gas-phase spectra of a series of sublimated PAHs with different sizes and structures in the 13–20 C atom range. We then compared our data to current astrophysical dust photoelectric models used to describe the PAH charge and gas photoelectric heating in the ISM. In particular, we extended the Kimura 2016, (MNRAS, 459, 2751) model (eK16) to take into account the KED of the photoelectron and its interaction with the grain.<i>Results<i/>. We show that although subtle differences between the molecules in our dataset arise from individual electronic structures, the photoelectron KED of PAHs of different sizes and symmetry display remarkable similarities. A general law can thus be implemented in sophisticated ISM astrochemical models to describe their photoelectron KED behavior. We find that the eK16 photoelectric model closely reproduces the present photoionization cross sections of neutral, small PAHs as well as literature data obtained on cationic PAHs. It is noteworthy that the eK16 model, unlike former models, matches the absolute scale of the measured photoionization cross sections. We show that the eK16 model predicts a maximum photoelectric efficiency significantly lower than the previous models, implying a lower interstellar gas temperature and less emission.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"4 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-06-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144296044","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Determining the origin of the X-ray emission in blazars through multiwavelength polarization","authors":"Ioannis Liodakis, Haocheng Zhang, Stella Boula, Riccardo Middei, Jorge Otero-Santos, Dmitry Blinov, Iván Agudo, Markus Böttcher, Chien-Ting Chen, Steven R. Ehlert, Svetlana G. Jorstad, Philip Kaaret, Henric Krawczynski, Abel L. Peirson, Roger W. Romani, Fabrizio Tavecchio, Martin C. Weisskopf, Pouya M. Kouch, Elina Lindfors, Kari Nilsson, Callum McCall, Helen E. Jermak, Iain A. Steele, Ioannis Myserlis, Mark Gurwell, Garrett K. Keating, Ramprasad Rao, Sincheol Kang, Sang-Sung Lee, Sanghyun Kim, Whee Yeon Cheong, Hyeon-Woo Jeong, Emmanouil Angelakis, Alexander Kraus, Francisco José Aceituno, Giacomo Bonnoli, Víctor Casanova, Juan Escudero, Beatriz Agís-González, Daniel Morcuende, Alfredo Sota, Rumen Bachev, Tatiana S. Grishina, Evgenia N. Kopatskaya, Elena G. Larionova, Daria A. Morozova, Sergey S. Savchenko, Ekaterina V. Shishkina, Ivan S. Troitskiy, Yulia V. Troitskaya, Andrey A. Vasilyev","doi":"10.1051/0004-6361/202554747","DOIUrl":"https://doi.org/10.1051/0004-6361/202554747","url":null,"abstract":"The origin of the high-energy emission in astrophysical jets from black holes is a highly debated issue. This is particularly true for jets from supermassive black holes, which are among the most powerful particle accelerators in the Universe. So far, the addition of new observations and new messengers have only managed to create more questions than answers. However, the newly available X-ray polarization observations promise to finally distinguish between emission models. We use extensive multiwavelength and polarization campaigns as well as state-of-the-art polarized spectral energy distribution models to attack this problem by focusing on two X-ray polarization observations of blazar BL Lacertae in flaring and quiescent <i>γ<i/>-ray states. We find that, regardless of the jet composition and underlying emission model, inverse-Compton scattering from relativistic electrons dominates at X-ray energies.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"11 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-06-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144296041","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"State-to-state rotational rate coefficients for the OCS+H2 collision at low temperatures","authors":"Otoniel Denis-Apizar, Ignacio A. Murúa-Molina, Cristian Guerra, Rodrigo Urzúa-Leiva","doi":"10.1051/0004-6361/202555275","DOIUrl":"https://doi.org/10.1051/0004-6361/202555275","url":null,"abstract":"<i>Context.<i/> The physicochemical conditions of interstellar regions with low densities, (e.g., typical molecular clouds), should be analyzed using non-LTE models. In such models, the collisional rate coefficients of the observed molecules with H<sub>2<sub/>, He, and H are critical inputs. In the case of OCS, the only set of rate coefficients available for the collision with H<sub>2<sub/> was computed in the seventies, using a potential energy surface (PES) based on an electron gas model for the collision with He. Furthermore, in a recent study on OCS+He, a mass-scaled approximation for the rates was considered, and different propensity rules were found.<i>Aims.<i/> The main goal of this study is to compute a new set of rotational de-excitation rate coefficients of OCS in collision with H<sub>2<sub/> at low temperatures.<i>Methods.<i/> An averaged PES over the orientation of H<sub>2<sub/> is developed from a large grid of ab initio energies computed at the CCSD(T)/aug-cc-pVQZ level of theory. This surface is employed in close-coupling calculations for studying the collision of OCS with para-H<sub>2<sub/>(<i>j<i/> = 0). Furthermore, an available 4D PES was also used in close-coupling calculations to confirm the results of our first approximation.<i>Results.<i/> The agreement between the cross sections for the OCS+para-H<sub>2<sub/> computed using the reduced and 4D PES was very good. The state-to-state rotational de-excitation rate coefficients for the lowest 30 rotational states of OCS by para-H<sub>2<sub/> are computed from these data. However, the rate coefficients show different behavior with published data; particularly, a different propensity rule, Δ<i>j<i/> = 1, is found. Furthermore, similarities between the rates with para- and ortho-H<sub>2<sub/> are found. Finally, the astrophysical implications of the new rate coefficients are explored from non-LTE radiative transfer calculations.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"3 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-06-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144296039","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Peekaboo galaxy: New SALT spectroscopy and implications of archive HST data","authors":"A. Y. Kniazev, S. A. Pustilnik","doi":"10.1051/0004-6361/202554911","DOIUrl":"https://doi.org/10.1051/0004-6361/202554911","url":null,"abstract":"<i>Context.<i/> The dwarf galaxy Peekaboo (HIPASSJ1131–31) was recently identified as a local volume (LV) gas-rich and extremely metal-poor (XMP) dIrr. Its gas metallicity is <i>Z<i/> ∼ Z<sub>⊙<sub/>/50, with a ±1<i>σ<i/> uncertainty range of [Z<sub>⊙<sub/>/72–Z<sub>⊙<sub/>/35]). Its ‘tip of the red-giant branch’ distance is 6.8 ± 0.7 Mpc. The Hubble Space Telescope (HST) data for its individual stars revealed that its older red-giant-branch stars comprise a smaller part of the galaxy, while the majority of visible stars have ages of less than one to a few gigayears. Thus, the Peekaboo dwarf can be considered as the nearest record-low <i>Z<i/> dwarf. As such, the galaxy deserves a deeper multi-method study that examines the properties of its young massive stars and the fainter older population as well as its ionised gas and the dominant baryonic component of HI gas.<i>Aims.<i/> We aim to obtain the higher S-to-N SALT optical spectra of two H II regions in Peekaboo in order to improve the accuracy of its gas O/H and to determine abundances of Ne, S, N, and Ar. With archive HST images, we aim to identify the hot massive stars, including exciting the two H II regions (i.e. east and west), and the XMP supergiants as important targets for follow-up studies of their evolution with upcoming extremely large telescopes.<i>Methods.<i/> We used the direct (T<sub>e<sub/>) method for the east H II region in which a [O III]<i>λ<i/>4363 Å line is well detected in order to estimate its parameter 12 + log(O/H). In the west H II region, the line [OIII]<i>λ<i/>4363 Å is not detected, so we estimated its O/H via the empirical ‘strong-line’ method of Izotov et al. (2019, A&A, 523, A40). The resulting value of O/H is very close to that in the east H II region.<i>Results.<i/> The new spectroscopy of the Peekaboo dwarf allowed us to substantially improve the accuracy of its direct O/H estimate, and we obtained 12 + log(O/H) = 6.99 ± 0.06 dex. The new data reveal that emission lines in the east region consist of two components with a velocity difference of ∼65 km s<sup>−1<sup/>. The fainter approaching component could be related to a fast-moving WR star thrown from a cluster or a binary system. Using the HST <i>V<i/> magnitudes and colour <i>V<i/> − <i>I<i/>, we identified tentative O-type and very hot candidate WO stars, which are likely the ionising stars of the studied H II regions.<i>Conclusions.<i/> With the new optical spectra, the Peekaboo galaxy is confirmed as the lowest-metallicity dwarf in the LV and as a valuable object for in-depth multi-method studies. We separate its most luminous stars for follow-up ground-based brightness monitoring and spectroscopy.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"10 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-06-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144296045","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}