Abstract: Recent studies on direct imaging of Type II core-collapse supernova progenitors indicate apossible threshold around Mzams~16-20 Msun, where red supergiants with larger birth masses do not appear to result in supernova explosions, and instead possibly imploding directly into a black hole. We argue that it is not a coincidence this threshold closely matches with the critical transition of central carbon burning in massive stars from convective to radiative regime, afterwich the stellar cores become significantly harder to blow up. This transition is highly sensitive to a number of key input physics, such as the rate of 12C(alpha,gamma)16Oreaction and the overshoot mixing efficiency, and therefore we argue that the upper mass limit of exploding red supergiants could be employed to constrain these uncertainties of massive stellar evolution calculations. We find that only a few of recently published models are in agreement with the estimates inferred from direct imaging studies.
Global properties of circumgalactic medium at high-redshift: A spectroscopic study ofstrong Lyman-a forest absorbers
Abstract: The circumgalactic medium (CGM) is shaped by the complex interplay of gas flows regulating the way stars and galaxies form and evolve. Understanding CGM properties, therefore, is crucial in understanding galaxy evolution. We present a study of the CGM probed by composite spectra constructed from a sample of strong Lyman-a forest absorbers, identified based on their absorption strength and clustering, at redshift ~ 2.7. Single component modelfits to the several Lyman-series lines, detected in our composite spectra, suggest optically thin absorbing gas. Absorption signal from several metals in various ionisation stages are detected,with exquisite precision, in our composite spectra. We go beyond the average properties provided by the composite spectra and forward model the entire metal population for everymetal line. This allows us to incorporate the relative sizes of absorption populations between metal lines. We analyse the measurements from the metal absorption in combination with the H I column densities using various single-phase and multi-phase models and our results imply clumping on scales down to < 100 pc and near-solar metallicities in the circumgalacticsamples (probed primarily by low ionisation species), while high-ionization metal absorption consistent with typical IGM densities and metallicities is present in all samples. Our results provide important clues to the processes shaping CGM at high-redshift using Lya absorbers ina column density regime largely unused so far in CGM studies.