Solar and Stellar Astrophysics (astro-ph.SR)

Abstract: The kinematics of coronal mass ejections (CMEs) are essential for understanding their initiation mechanisms and predicting their planetary impact. Most acceleration and deceleration occur below 4 R$\odot$, which is crucial for initiation understanding. Furthermore, the kinematics of CMEs in the inner corona ($<$ 3 R$_\odot$) are closely related to their propagation in the outer corona and their eventual impact on Earth. Since the CME kinematics are mainly probed using coronagraph data, it is crucial to investigate how imaging cadence affects the precision of data analysis and conclusions drawn and also for determining the flexibility of designing observational campaigns with upcoming coronagraphs. We study ten CMEs observed by the K-Coronagraph of the MLSO. We manually track the CMEs using high cadence (15 s) white-light observations of K-Cor and vary the cadence as 30 s, 1 min, 2 min, and 5 min to study the impact of cadence on the kinematics. We also employed the bootstrapping method to estimate the fitting parameters. Our results indicate that the average velocity of the CMEs does not have a high dependence on the imaging cadence, while the average acceleration shows significant dependence on the same, with the confidence interval showing significant shifts for the average acceleration for different cadences. The decrease in cadence also influences the determination of acceleration onset time. We further find that it is difficult to find an optimum cadence to study all CMEs, as it is also influenced by the pixel resolution of the instrument and the speed of the CME. However, except for very slow CMEs (speeds less than 300 Kms$^{-1}$), our results indicate a cadence of 1 min to be reasonable for the study of their kinematics. The results of this work will be important in the planning of observational campaigns for the existing and upcoming missions that will observe the inner corona.

Abstract: Yttrium (Y), a key s-process element, is commonly used in nucleosynthesis studies and as a Galactic chemical clock when combined with magnesium (Mg). We study the applicability of the previously assumed LTE line formation assumption in Y abundance studies of main-sequence and red giant stars, and probe the impact of NLTE effects on the [Y/Mg] ratio, a proposed stellar age indicator. We derive stellar parameters, ages, and NLTE abundances of Fe, Mg, and Y for 48 solar analogue stars from high-resolution spectra acquired within the Gaia-ESO survey. For Y, we present a new NLTE atomic model. We determine a solar NLTE abundance of A(Y)$_{\rm NLTE}=2.12\pm0.04$ dex, $0.04$ dex higher than LTE. NLTE effects on Y abundance are modest for optical Y II lines, which are frequently used in Sun-like stars diagnostics. NLTE has a small impact on the [Y/Mg] ratio in such stars. For metal-poor red giants, NLTE effects on Y II lines are substantial, potentially exceeding $+0.5$ dex. For the Gaia/4MOST/WEAVE benchmark star, HD 122563, we find the NLTE abundance ratio of [Y/Fe]$_{\rm NLTE}=-0.55\pm0.04$ dex with consistent abundances obtained from different Y II lines. NLTE has a differential effect on Y abundance diagnostics in late-type stars. They notably affect Y II lines in red giants and very metal-poor stars, which are typical Galactic enrichment tracers of neutron-capture elements. For main-sequence stars, NLTE effects on optical diagnostic Y II lines remain minimal across metallicities. This affirms the [Y/Mg] ratio's reliability as a cosmochronometer for Sun-like stars.

Abstract: Context. A characteristic feature of young stellar objects is their variability, which is caused by a variety of different physical processes. High-resolution interferometric observations in the near- and mid-infrared wavelength ranges spanning multiple epochs allow the detailed study of these processes. Aims. We aim at investigating the expected variations of the interferometric observables connected to changes in the measured photometric fluxes of a typical variable accreting central young stellar object with a circumstellar disk. Methods. We calculated visibilities and closure phases as well as the photometric flux of brightness distributions obtained using 3D Monte Carlo radiative transfer simulations for a model of a circumstellar disk with an accreting central star. Results. Changes in the accretion luminosity of the central object, that is, an accreting pre-main-sequence star, can lead to significant variations in the visibility and closure phase of the star-disk system measured with instruments at the Very Large Telescope Interferometer (VLTI) that can be related to changes in the photometric flux. Taking into account additional effects due to baseline variation, interferometric observations can provide valuable contributions to the understanding of the underlying processes. Additionally, we provide the web application VLTI B-VAR that allows the impact of the hour angle on the visibility and closure phase for customized intensity maps to be estimated.