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Empirical instability strip for classical Cepheids
No full textAims. The aim of this study is to determine empirical intrinsic edges of the classical Cepheids instability strip (IS) in the Small Magellanic Cloud (SMC) galaxy; we considered various effects that alter its shape, and compared them with theoretical models and other galaxies.
Methods. We used the data of classical fundamental-mode (F) and first-overtone mode (1O) SMC Cepheids from the OGLE-IV variable star catalog, with the final cleaned sample including 2388 F and 1560 1O Cepheids. The IS borders are determined by tracing the edges of the color distribution along the strip. Based on that, and using evolutionary tracks, the IS crossing times were then calculated.
Results. We obtained the blue and red edges of the IS in V- and I-photometric bands and in the HR diagram, and detected breaks at periods between 1.4 and 3 days. Interestingly, the central SMC Cepheids are redder than those located farther away. A comparison with existing theoretical models showed good agreement for the blue edge and significant differences for the red edge. We also found that the IS of the SMC is wider than that of the Large Magellanic Cloud (LMC), with its red edge being redder despite its lower metallicity. The analysis of crossing times showed that the expected number of Cepheids as a function of period agrees with the observed distribution for P > 1 days, but differs for P < 1 days.
Conclusions. Slope changes along the SMC IS borders are most likely explained by the distribution of metallicity. The behavior of the blue loops at the SMC metallicity is not consistent with observations, and at the LMC metallicity the blue loops are too short for lower-mass stars. A comparison of theoretical edges with our empirical ISs imposes constraints on the models and enables the identification of valid ones. Based on the positions of the breaks, our study also suggests that fundamental-mode Cepheids with periods longer than 3 days should be used for distance determination
Studying the surface effect in Procyon A as an F-type star
No full textContext. Procyon A is an F-type main-sequence star in a binary system. It has been the subject of numerous ground-based and space-based observing campaigns, providing precise classical constraints, including a well-determined mass. It was also among the first stars in which individual frequencies were detected, making it a crucial benchmark for F-type stars.
Aims. Our goal is to investigate the surface effect, namely the discrepancy between observed and model oscillation frequencies due to inadequate modeling of the surface stellar layers, which is especially important in F-type stars. Using Procyon A as a case study, we aim to understand how different surface correction prescriptions impact the inference of the fundamental properties of this star, and to compare the results with those obtained when the surface corrections are neglected.
Methods. We inferred the fundamental stellar properties by employing a grid of models computed with Modules for Experiments in Stellar Astrophysics (MESA), including gravitational settling, radiative accelerations, and turbulent mixing. We selected the best-fit models using the Asteroseismic Inference on a Massive Scale (AIMS) code, taking into account different methods to fit the individual frequencies.
Results. We find that the use of surface corrections can introduce uncertainties of up to 7% in the inferred stellar mass. We obtain the most reliable stellar mass estimates when using frequency ratios, the Sonoi surface correction, or direct fitting of the individual frequencies.
Conclusions. Our results indicate that the surface effects in F-type stars differ from those found in the Sun and in solar-like stars, highlighting the need for caution when considering the surface corrections for these stars
Infrared spectropolarimetry of a C-class solar flare footpoint plasma
No full textWe performed high-spatial-resolution spectropolarimetric observations of the active region NOAA 13363 during a C-class flare with the Gregor Infrared Spectrograph (GRIS) on 16 July 2023. We examined the coupling between the photosphere and the chromosphere, studying the polarimetric signals during a period that encompasses the decaying phase of a C-class flare and the appearance of a new C-class flare at the same location. We focused on the analysis of various spectral lines. In particular, we studied the Si
A survey for radio pulsars and transients in the 10 pc region around Sgr A*
No full textHere we report on a new survey for pulsars and transients in the 10 pc region around Sgr A∗ using the Effelsberg radio telescope at frequencies between 4 and 8 GHz. Our calibrated full-Stokes data were searched for pulsars and transients using PULSARX, TRANSIENTX, and PRESTO. Polarisation information is used in the scoring of the candidates. Our periodicity acceleration and jerk searches allowed us to maintain good sensitivity towards binary pulsars in ≳10-h orbits. In addition, we performed a dedicated search in linear polarisation for slow transients. While our searches yielded no new discovery beyond the redetection of the magnetar SGR J1745−2900, we report on a faint single pulse candidate in addition to several weak periodicity search candidates. After thoroughly assessing our survey’s sensitivity, we determined that it is still not sensitive to a population of millisecond pulsars. Next generation radio interferometers can overcome the limitations of traditional single-dish pulsar searches of the Galactic Centre
Coordinated space-and ground-based monitoring of accretion bursts in a protoplanetary disc: The orbital and accretion properties of DQ Tau
No full textMultiplicity in pre-main-sequence (PMS) systems shapes circumstellar and circumbinary discs, often resulting in morphological features such as inner cavities, spiral arms, and gas streamers that facilitate mass transfer between the disc and stars. Consequently, accretion in eccentric close binaries is highly variable and synchronized with their orbits, producing distinct bursts near periastron passages. In this study, we examine the orbital and accretion properties of the eccentric Classical T-Tauri binary star DQ Tau using medium- to high-resolution spectroscopy obtained using the Very Large Telescope (VLT) X-shooter and UVES instruments. The data have been taken at the time of a monitoring of the inner disc chemistry with JWST, and the results of our analysis are needed for a correct interpretation of the JWST data. We refine the orbital parameters of the system and report an increment in the argument of periastron of ~30º. This apsidal motion can be caused by the massive disc acting as a third body in the system. We also explore the possibility that the resulting apsidal motion is caused by a still not-detected additional (sub-)stellar companion. In this case, we estimate a lower limit of ~15 MJ for the mass of this putative companion at the cavity edge (a = 3abin). We investigate the accretion of the primary and secondary stars in the system using the Ca II 849.8 nm emission line. We observe the primary accretes more at the periastron compared to its previous quiescent phases. The secondary dominates the accretion at post-periastron phases. Additionally, we report an elevated Lacc at apastron, possibly due to the interaction of the stars with irregularly shaped structures near their closest approach to the circumbinary disc. Finally, we derive the accretion luminosity of each star across the disentangled epochs and compare the results to those derived by the UV excess, finding a good overall agreement. The individual Lacc values can be used as an input for the chemical models
Activity correlation and temporal variation of small-scale magnetic fields on young Sun-like stars
No full textAims. We aim to evaluate how well the variation of small-scale magnetic fields on the stellar surface can be monitored with time-series observations. Further, we aim to establish to what extent the measured total unsigned magnetic field traces other activity indicators.
Methods. We measured the total unsigned magnetic field on four young, Sun-like, stars using the Zeeman splitting of magnetically sensitive Ti
Automated Sound Field Estimation combining robotized acoustic measurements and the boundary elements method
No full textThe identification and reconstruction of acoustic fields radiated by unknown structures is usually performed using either Sound Field Estimation or Near-field Acoustic Holography techniques. The latter turns out to be especially useful when data is only available close to the source, but information throughout the whole space is needed.
Yet, the lack of amendable and efficient implementations of state-of-the-art solutions, as well as the laborious and often lengthy deployment of acoustic measurements continue to be significant obstacles to the practical application of such methods.
The purpose of this work is to address both problems. First, a completely automated metrology setup is proposed, in which a robotic arm is used to gather extensive and accurately positioned acoustic data without any human intervention. The impact of the robot on acoustic pressure measurements is cautiously evaluated, and proved to remain limited below 1 kHz.
The Sound Field Estimation is then tackled using the Boundary Element Method, and implemented using the FreeFEM software. Numerically simulated measurements have allowed us to assess the method accuracy, which matches theoretically expected results and proves to remain robust against positioning inaccuracies, provided that the robot is carefully calibrated.
The overall solution has been successfully tested using actual robotized measurements of an unknown loudspeaker, with a reconstruction error of less than 30%
Back from the dead: AT2019aalc as a candidate repeating tidal disruption event in an active galactic nucleus
No full textContext. To date, three nuclear transients have been associated with high-energy neutrino events. These transients are generally thought to be powered by tidal disruption events (TDEs) in stars caused by massive black holes. However, AT2019aalc, hosted in a Seyfert-1 galaxy, has not yet been classified due to a lack of multiwavelength observations. Interestingly, the source re-brightened 4 years after its discovery.
Aims. Our aim is to constrain the physical mechanism responsible for the second optical flare, which may also provide clues to the origin of the initial event.
Methods. We conducted a multiwavelength monitoring program (from radio to X-rays) of AT2019aalc during its re-brightening in 2023–2024.
Results. The observations revealed multiple X-ray flares during the second optical flaring episode of the transient and a uniquely bright UV counterpart. The second flare, similar to the first one, is accompanied by IR dust echo emission. A long-term radio flare was found with an inverted spectrum. Optical spectroscopic observations revealed the presence of Bowen fluorescence lines and strong high-ionization coronal lines, indicating an extreme level of ionization in the system.
Conclusions. The results suggest that the transient can be classified as a Bowen fluorescence flare (BFF), a relatively new sub-class of flaring active galactic nuclei (AGNs). AT2019aalc can be also classified as an extreme coronal line emitter (ECLE). We find that in addition to AT2019aalc, another BFF, AT2021loi, is spatially coincident with a high-energy neutrino event. We propose a repeating TDE scenario within an AGN framework to explain the multiwavelength properties of AT2019aalc and suggest a possible connection among ECLEs, BFFs, and TDEs occurring in AGNs
Extracting the Alcock-Paczyński signal from voids: A novel approach via reconstruction
No full textThe void-galaxy cross-correlation function is a powerful tool to extract cosmological information. Through the void-galaxy cross-correlation function, cosmic voids – the underdense regions in the galaxy distribution – are used for refined deductions of the Universe’s content by correcting apparent geometric distortions. This study proposes a novel procedure for optimally extracting the Alcock-Paczyǹski (AP) signal from cosmic voids through a cosmological reconstruction technique. Employing cosmological reconstruction, specifically using the Zel’dovich approximation, we estimate the true positions of galaxies from their redshift-space locations, reducing distortions introduced by peculiar velocities. Unlike previous analyses, we identify voids and measure the void-galaxy cross-correlation function directly in reconstructed space. This approach enables us, for the first time, to include in our analysis small nonlinear voids, typically discarded in previous studies, thus enhancing the statistical power of void studies and significantly improving their cosmological constraining power. Reconstruction is particularly effective even at small scales for voids, due to their clean and dynamically simple environment. This ability to recover information encoded on small scales significantly enhances the precision of the analysis, leading to a ∼23% improvement in the constraints on the AP parameters compared to previous methods where the analysis is performed in redshift space and, consequently, to a better estimate of the derived cosmological parameters. Our analysis also includes a comprehensive set of consistency checks, demonstrating its robustness. We expect this methodology to yield a substantial gain in constraining power when applied to data from modern large-scale structure surveys
Fourier transform method of a detailed configuration accounting in hot plasma bound-bound opacity calculations
No full textG. Hazak and J. Kurzweil discovered a method of configurational resolution of transition arrays for the Super Transition Arrays approach to the bound-bound opacity calculation. Their method is based on the representation of the photoabsorption coefficient as the Fourier transform, the linearity of the transition energy between configurations with respect to shell occupation numbers, and factorization of the probabilities of configurations on shell occupation numbers. We propose a simplified variant of Hazak–Kurzweil method for the calculations with Detailed Configuration Accounting. The resulting expressions for the bound-bound opacity represent an alternative to the widely used ones and are quite convenient for numerical implementation