Solar and Stellar Astrophysics (astro-ph.SR)

Abstract: CN Cha is a slow symbiotic nova characterized by a three-years-long optical flat peak followed by a rapid decline. We present theoretical light curves for CN Cha, based on hydrostatic approximation, and estimate the white dwarf (WD) mass to be $\sim 0.6 ~M_\odot$ for a low metal abundance of Z = 0.004. This kind of flat peak novae are border objects between classical novae having a sharp optical peak and extremely slow novae, the evolutions of which are too slow to be recognized as a nova outburst in human timescale. Theoretically, there are two types of nova envelope solutions, static and optically-thick wind, in low mass WDs ($\lesssim 0.7 ~M_\odot$). Such a nova outburst begins first in a hydrostatic manner, and later it could change to an optically-thick wind evolution due to perturbation by the companion star in the nova envelope. Multiple peaks are a reflection of the relaxation process of transition. CN Cha supports our explanation on the difference between long-lasted flat peak novae like CN Cha and multiple peak novae like V723 Cas, because the companion star is located far outside, and does not perturb, the nova envelope in CN Cha.

Abstract: In this work, we study the accuracy that can be achieved when inferring the atmospheric information from realistic numerical magneto-hydrodynamic simulations that reproduce the spatial resolution we will obtain with future observations made by the 4m class telescopes DKIST and EST. We first study multiple inversion configurations using the SIR code and the Fe I transitions at 630 nm until we obtain minor differences between the input and the inferred atmosphere in a wide range of heights. Also, we examine how the inversion accuracy depends on the noise level of the Stokes profiles. The results indicate that when the majority of the inverted pixels come from strongly magnetised areas, there are almost no restrictions in terms of the noise, obtaining good results for noise amplitudes up to 1$\times10^{-3}$ of $I_c$. At the same time, the situation is different for observations where the dominant magnetic structures are weak, and noise restraints are more demanding. Moreover, we find that the accuracy of the fits is almost the same as that obtained without noise when the noise levels are on the order of 1$\times10^{-4}$of $I_c$. We, therefore, advise aiming for noise values on the order of or lower than 5$\times10^{-4}$ of $I_c$ if observers seek reliable interpretations of the results for the magnetic field vector reliably. We expect those noise levels to be achievable by next-generation 4m class telescopes thanks to an optimised polarisation calibration and the large collecting area of the primary mirror.

Abstract: The forecasting of solar energetic particles (SEPs) is a prominent area of space weather research. Numerous forecasting models exist that predict SEP event properties at proton energies <100MeV. One of these models is the SPARX system, a physics-based forecasting tool that calculates >10MeV and >60MeV flux profiles within minutes of a flare being detected. This work describes SPARX-H, the extension of SPARX to forecast SEP events above 300MeV . SPARX-H predicts fluxes in three high energy channels up to several hundred MeV. Correlations between SEP peak flux and peak intensity of the associated solar flare are seen to be weak at high energies, but improved when events are grouped based on the field polarity during the event. Initial results from this new high energy forecasting tool are presented here and the applications of high energy forecasts are discussed. Additionally, the new high energy version of SPARX is tested on a set of historic SEP events. We see that SPARX-H performs best when predicting peak fluxes from events with source locations in well-connected regions, where many large SEP events tend to originate.

Abstract: We report on a detailed abundance study of six bright, mostly southern, slowly rotating late B stars: HD~1279 (B8III), HD~99803 (B9V), HD~123445 (B9V), HD~147550 (B9V), HD~171961 (B8III) and HD~202671 (B5II/III), hitherto reported as normal stars. We compare them to the two classical HgMn stars $\mu$ Lep and $\beta$ Scl and to the superficially normal star, $\nu$ Cap. In the spectra of the six stars, the \ion{Hg}{2} line at 3984 \AA\ line is clearly seen and numerous lines of P, Ti, Mn, Fe, Ga, Sr, Y, and Zr appear to be strong absorbers. A comparison of newly acquired and archival spectra of these objects with a grid of synthetic spectra for selected unblended lines reveals large overabundances of P, Ti, Cr, Mn, Sr, Y, Zr, Ba, Pt and Hg and underabundances of He, Mg, Sc and Ni. The effective temperatures, surface gravities, low projected rotational velocities and the peculiar abundance patterns of the six investigated stars show that they are new chemically peculiar stars, mostly new HgMn stars, and are reclassified as such. The evolutionary status of these stars has been inferred and their ages and masses estimated. The two most massive objects, HD~1279 and HD~202671, might have evolved away from the main-sequence recently, the other stars are main-sequence objects. HD~99803A is a sharp lined HgMn star with grazing eclipses; from TESS and MASCARA photometry we determine an orbital period of $P_{\rm orb} = 26.12022 \pm 0.00004$\,d.

Abstract: Nuclear reactions are key to our understanding of stellar evolution, particularly the $^{12}\rm{C}\left({\alpha},{\gamma}\right)^{16}\!\rm{O}$ rate, which is known to significantly influence the lower and upper ends of the black hole (BH) mass distribution due to pair-instability supernovae (PISNe). However, these reaction rates have not been sufficiently determined. We use the $\texttt{MESA}$ stellar evolution code to explore the impact of uncertainty in the $^{12}\rm{C}\left({\alpha},{\gamma}\right)^{16}\!\rm{O}$ rate on PISN explosions, focusing on nucleosynthesis and explosion energy by considering the high resolution of the initial mass. Our findings show that the mass of synthesized radioactive nickel ($^{56}{\rm Ni}$) and the explosion energy increase with $^{12}\rm{C}\left({\alpha},{\gamma}\right)^{16}\!\rm{O}$ rate for the same initial mass, except in the high-mass edge region. With a high (about twice the $\texttt{STARLIB}$ standard value) rate, the maximum amount of nickel produced falls below 70 $M_\odot$, while with a low rate (about half of the standard value) it increases up to 83.7 $M_\odot$. These results highlight that carbon burning plays a crucial role in PISNe by determining when a star initiates expansion. The initiation of expansion competes with collapse caused by helium photodisintegration, and the maximum mass that can lead to an explosion depends on the $^{12}\rm{C}\left({\alpha},{\gamma}\right)^{16}\!\rm{O}$ reaction rate.