Tomás Müller wrote:
(CCSNe, see this animation
) are explosions coming from massive stars (above 8 solar masses) when they reach the end of their life. A Type II-P Supernova
(SN II-P) is a common type of CCSN which shows a “plateau” in its light curve
, driven by a hydrogen-rich envelope. We know that the progenitors of SN II-P are Red Supergiant
(RSG) stars by looking what was at the explosion site beforehand. Astronomers can make several predictions about the star by comparing these observations with stellar evolution models
. The initial progenitor mass (i.e. the main sequence mass) can be estimated by: i) comparing the luminosity with model predictions, ii) measuring the mass of the hydrogen-rich envelope by modelling the light curve and making some assumptions about the core mass, or iii) measuring spectral lines
of some elements, like oxygen, during the nebular phase. This phase refers to epochs from a few months to a few years after the explosion, where the material is optically thin and the spectrum shows mainly emission lines, which correlate with the initial mass in some models.
Unfortunately, these methods do not generally agree, so we cannot accurately estimate the initial mass of the progenitor of a SN II-P. Bearing this in mind, the authors of today’s article proposed a different way of estimating the initial mass by measuring the surface composition (or surface abundance) of the progenitor star at early epochs (less than 1 day after the explosion). The structure of RSG stars consist of several layers of burning material. Shallower layers are composed of lighter elements but some mixing occurs between the different layers, the amount of mixing depending mainly on the stellar mass. Looking at the early composition of a SN II-P can give us an idea of the progenitor star’s mass. The benefits of looking at early epochs are: firstly, at this stage some spectral features are easy to identify, and secondly, the surface abundance is not expected to suffer from explosive mixing at early times, which would erase any link to the progenitor mass.
The authors test this using the stellar evolution code, MESA
, to model stars of different initial masses evolving. The evolution across the Hertzsprung-Russell
(H-R) diagram is key to understanding the different processes and phases a star goes through. Figure 1
shows that less massive stars cross the H-R diagram more rapidly than more massive stars. This means that more massive stars have more time to dredge up material from their inner layers into the surface (mixing the abundances) before the end of the RSG phase. Additionally, more massive stars lose more mass than the least massive ones, so their outer envelopes are thinner compared to the total size of the star, hence their surface abundances suffer from more mixing. ...