The Square Kilometre Array (SKA)

Abstract:

In this work, we study the circumstellar material (CSM) around massive stars, and the mass-loss rates depositing this CSM, using a large sample of radio observations of 325 core-collapse supernovae (CCSNe; only ~22% of them being detected). This sample comprises both archival data and our new observations of 99 CCSNe conducted with the AMI-LA radio array in a systematic approach devised to constrain the mass loss at different stages of stellar evolution. In the supernova (SN)–CSM interaction model, observing the peak of the radio emission of an SN provides the CSM density at a given radius (and therefore the mass-loss rate that deposited this CSM). On the other hand, limits on the radio emission, and/or on the peak of the radio emission provide a region in the CSM phase space that can be ruled out. Our analysis shows a discrepancy between the values of mass-loss rates derived from radio-detected and radio-nondetected SNe. Furthermore, we rule out mass-loss rates in the range of 2 × 10−6–10−4 M⊙ yr−1 for different epochs during the last 1000 yr before the explosion (assuming wind velocity of 10 km s−1) for the progenitors of ~80% of the Type II supernovae (SNe II) in our sample. In addition, we rule out the ranges of mass-loss rates suggested for red supergiants for ~50% of the progenitors of SNe II in our sample. We emphasize here that these results take a step forward in constraining mass loss in winds from a statistical point of view.

Abstract:

Nuclear star clusters (NSCs) are commonly found in the centers of galaxies, but their dominant formation mechanisms remain elusive. We perform a consistent analysis of stellar populations of 97 nearby NSCs, based on spectroscopic data from the Very Large Telescope. The sample covers a galaxy stellar mass range of 107–1011 M⊙ and is more than 3 times larger than any previous study. We identify three galaxy stellar mass regimes with distinct NSC properties. In the low-mass regime of logMhost ≲ 8.5 (Mhost is in units of M⊙), nearly all NSCs have metallicities lower than their circum-NSC host but similar to those of typical red globular clusters (GCs), supporting the GC inspiral–merger scenario of NSC formation. In the high-mass regime of logMhost ≳ 9.5, nearly all NSCs have higher metallicities than their circum-NSC host and red GCs, suggesting significant contributions from in situ star formation. In the intermediate-mass regime, a comparable fraction of NSCs have higher or lower metallicities than their circum-NSC host and red GCs, with no clear dependence on NSC mass, suggesting intermittent in situ star formation. The majority of NSCs with higher metallicities than their host exhibit a negative age–metallicity correlation, providing clear evidence of long-term chemical enrichment. The average metallicity difference between NSC and host peaks broadly around logMhost∼9.8 and declines toward both higher and lower galaxy masses. We find that the efficiency of dynamical-friction-driven inspiral of GCs observed in present-day galaxies can explain the NSC mass at logMhost≲9.5 but falls short of observed ones at higher galaxy mass, reinforcing our conclusions based on stellar population analysis.