Solar and Stellar Astrophysics (astro-ph.SR)

Abstract: The globular cluster Messier 80 was monitored by the Kepler space telescope for 80 days during the K2 mission. Continuous, high-precision photometry of such an old, compact cluster allows us to study its variable star population in unprecedented detail. We extract light curves for 27 variable stars using differential-image photometry. A search for new variables in the images led to the discovery of two new variable stars: an RR Lyrae and a variable red giant star, respectively. Analysis of the RR Lyrae population reveals multiple RRc stars with additional modes and/or peculiar modulation cycles. We newly classify star V28 as a spotted extreme horizontal branch variable. Despite their faintness, we clearly detect the three SX Phe stars but we did not find new pulsation modes beyond the known ones in them. Spectra taken with the VLT and Magellan Clay telescopes, as well as absolute color-magnitude diagrams of the cluster based on Gaia and Pan-STARRS observations confirm the classification of the peculiar modulated variables as bona-fide RRc stars. We propose that they highlight a subgroup of overtone stars that may have been overlooked before. We fit MESA isochrones to the CMDs to estimate the age and metallicity of the cluster. We confirm that M80 is old and metal-poor, but show that isochrone fitting to old populations comes with numerous uncertainties.

Abstract: Solar flare is one of the most important solar activities which emit all electromagnetic waves in gigantic burst. The radio emission can be used to determine the physical properties of the solar flares. The e-CALLISTO worldwide network is designed to detect the radio emission of the solar flares and this study used the spectroscopic data from the e-CALLISTO system. Among the five types of solar radio bursts, this study was focused on type II radio bursts. The spectroscopic analysis estimated the shock speed of type II radio bursts using the uniform electron density model and the nonuniform electron density model of the sun. The shock speed is proportional to the electron density (Ne) and inversely proportional to the rate of change in electron density with altitude (dNe/dr). The determined shock speed at the altitude of one solar radius is 2131 km/s for uniform model and 766 km/s for non-uniform model. Although the uniform electron density model is widely used we attempted the non-uniform electron density since in the active region of the sun, the electron densities are non-uniform. The estimated shock speeds of uniform density model is relatively high so that it is reasonable to use non-uniform electron density model for shock speed estimation of type II radio bursts.

Abstract: A CALLISTO system was set up at the Arthur C Clarke Institute and connected to the e-CALLISTO global network which observes the solar radio bursts in 24 hours. CALLISTO is the foremost observation facility to investigate celestial objects in radio region in Sri Lanka. The system consists of the CALLISTO spectrometer and controlling software,logarithmic periodic antenna and pre-amplifier. CALLISTO spectrometer is able to detect solar radio bursts in the frequency range of 45 MHz to 870 MHz with a channel resolution of 62.5 kHz.The log-periodic antenna was designed for 7 dBi gain and achieved the voltage standing wave ratio, less than 1.5 which is acquired by the overall impedance of the antenna, 49.3 ohms. The linear polarized antenna is pointing to zenith and the dipoles directed to north-south direction. The system detects solar radio emissions originated by solar flares and corona mass ejections. The radio bursts occurs as emission stripes in the radio spectra and classify from type I to V mainly on drift rate and band width. The system observed a type III solar radio burst on 5th July 2013 and a type II burst on 25th October 2013 which was originated by X1.7 solar flare. The type II bursts characterize with narrow bandwidth and drift slowly from higher to lower frequencies while the main features of type III bursts are high drift rate and broad bandwidth.

Abstract: Within the quiet Sun corona imaged at 1 MK, much of the field of view consists of diffuse emission that appears to lack the spatial structuring that is so evident in coronal loops or bright points. We seek to determine if these diffuse regions are categorically different in terms of their intensity fluctuations and spatial configuration from the more well-studied dynamic coronal features. We analyze a time series of observations from Solar Orbiter's High Resolution Imager in the Extreme Ultraviolet to quantify the characterization of the diffuse corona at high spatial and temporal resolutions. We then compare this to the dynamic features within the field of view, mainly a coronal bright point. We find that the diffuse corona lacks visible structuring, such as small embedded loops, and that this is persistent over the 25 min duration of the observation. The intensity fluctuations of the diffuse corona, which are within +/-5%, are significantly smaller in comparison to the coronal bright point. Yet, the total intensity observed in the diffuse corona is of the same order as the bright point. It seems inconsistent with our data that the diffuse corona is a composition of small loops or jets or that it is driven by discrete small heating events that follow a power-law-like distribution. We speculate that small-scale processes like MHD turbulence might be energizing the diffuse regions, but at this point we cannot offer a conclusive explanation for the nature of this feature.

Abstract: We establish a sample of 370 Mira variables that are likely near the Galactic center (GC). The sources have been selected from the OGLE and BAaDE surveys based on their sky coordinates, OGLE classifications, and BAaDE maser-derived line-of-sight velocities. As the distance to the GC is known to a high accuracy, this sample is a test bed for reddening and extinction studies toward the GC and in Mira envelopes. We calculated separate interstellar- and circumstellar-extinction values for individual sources, showing that there is a wide range of circumstellar extinction values (up to four magnitudes in the K$_s$ band) in the sample, and that circumstellar reddening is statistically different from interstellar reddening laws. Further, the reddening laws in the circumstellar environments of our sample and the circumstellar environments of Large Magellanic Cloud (LMC) Miras are strikingly similar despite the different metallicities of the samples. Period-magnitude relations for the mid-infrared (MIR) WISE and MSX bands are also explored, and in the WISE bands we compare these to period-magnitude relationships derived from Miras in the LMC as it is important to compare these LMC relations to those in a higher metallicity environment. Emission from the envelope itself may contaminate MIR magnitudes altering the relations, especially for sources with thick envelopes.

Abstract: Many supernovae (SNe) imply an interaction of the SN ejecta with matter (CSM) surrounding the progenitor star. This suggests that many massive stars may undergo various degrees of envelope stripping shortly before exploding, and produce a considerable diversity in their pre-explosion CSM properties. We explore a generic set of ~100 detailed massive binary evolution models to characterize the amount of envelope stripping and the expected CSM configurations. Our binary models were computed with the MESA stellar evolution code, considering an initial primary star mass of 12.6 Msun, and focus on initial orbital periods above 500 d. We compute these models up to the time of the primary's iron core collapse. We find that Roche lobe overflow often leads to incomplete stripping of the mass donor, resulting in a large variety of pre-SN envelope masses. Many of our models' red supergiant (RSG) donors undergo core collapse during Roche lobe overflow, with mass transfer and thus system mass loss rates of up to 0.01 Msun/yr at that time. The corresponding CSM densities are similar to those inferred for Type IIn SNe like 1998S. In other cases, the mass transfer turns unstable, leading to a common envelope phase at such late time that the mass donor explodes before the common envelope is fully ejected or the system has merged. We argue that this may cause significant pre-SN variability, as for example in SN 2020tlf. Other models suggest a common envelope ejection just centuries before core collapse, which may lead to the strongest interactions, as in superluminous Type IIn SNe like 1994W, or 2006gy. Wide massive binaries offer a natural framework to understand a broad range of hydrogen-rich interacting SNe. On the other hand, the flash features observed in many Type IIP SNe, like in SN 2013fs, may indicate that RSGs are more extended than currently assumed.

Abstract: Apsidal motion is the precession of the line of apsides in the orbit of a binary star due to perturbations from General Relativity (GR), tides, or third body interactions. The rate of precession due to tidal effects depends on the interior structures of the stars and, as a result, binaries where such precession occurs are of great interest. Apsidal motion is observed through the analysis of eclipse times, revealing small changes in the average interval between successive primary and secondary eclipses, taking all available observed times of eclipse and yielding an estimate of the apsidal rate. Given that this is a single observed quantity, various degeneracies are unavoidably present. Ideally, one would have a model that predicts eclipse times given the orbital and stellar parameters. These parameters for a given binary could then be computed using least squares, provided a suitably large number of eclipse times. Here we use the Eclipsing Light Curve (ELC) program as such a model. The Newtonian equations of motion with extra force terms accounting for GR contributions and tidal distortions are integrated, yielding precise sky positions as a function of time. Times of mid-eclipse and instantaneous orbital elements are computed as a function of time. In this paper we outline the method, and compare numerically computed apsidal rates with standard formulae using a set of 15 binaries based on real systems. For our simulated systems, the derived apsidal rates agree with the standard formula.