With a large number of observations and models of type II supernovae, a statistical analysis was performed on the physical properties that most influence the observational diversity of this type of event.
Supernovae (SNs) are very energetic stellar explosions that mark the end of the evolution of some stars and are divided into two main groups according to the presence or absence of hydrogen in their spectra. Type I supernovae show no signs of hydrogen, unlike type II supernovae. The latter ones are the most abundant stellar explosions in the Universe and come from evolved stars in the red supergiant stage, with masses less than 25 times the mass of the Sun and greater than eight times the mass of the Sun.
It is known that type II supernovae show great observational diversity, both photometrical and spectroscopically. Several authors have studied it in recent years, including using some of the Carnegie Supernova Project I (CSP-I) observations. This observational diversity is the product of various physical properties determining the explosion.
Although these types of explosions have not been traditionally used for cosmological distance measurements -for which type Ia SNs are used- several proposed methods use them as potential distance estimators. Therefore, knowing the type of progenitor star and, mainly, the range of parameters – masses, radii, and energies – that give rise to these stellar explosions is critical for several fields of astronomy. However, no comprehensive studies have provided reliable ranges for these parameters.
The goal of a recent investigation, which resulted in three publications in the prestigious scientific journal Astronomy & Astrophysics, was to derive some physical characteristics of stars and type II supernova explosions by comparing observations with explosion models. All based on light curves (optical and infrared) obtained at Las Campanas Observatory with the Swope and du Pont telescopes during the first phase of CSP-I. The study was led by Laureano Martinez, PhD student at the Instituto de Astrofísica de La Plata, Argentina, and involved Mark Phillips, director emeritus of Las Campanas Observatory (LCO); Nidia Morrell, resident astronomer at LCO; and Carlos Contreras, support astronomer.
The sample used in this research is characterized by a large quantity and quality of photometric and spectroscopic data, a high observation rate, and extensive photometric coverage over a wide range of wavelengths.
A sample of 74 type II supernovae was obtained, which, according to Martinez, represents the largest sample of this type of supernovae, which is also homogeneous since the objects were observed by the same instruments and the same observing program.
For the research, each one of the objects was modelled with a code that simulates stellar explosions and physical parameters of their progenitors (masses and radii); and properties of the explosion (energy, quantity and distribution of the radioactive material generated in the explosion) were derived. For the derivation of the parameters, the models were compared with the observations using a statistical method to determine the model that best represents the observations. In total, physical properties were derived for 53 type II supernovae, of which 24 have more reliable results due to the temporal coverage of the observations and/or the quality of the models that best reproduce them.
The research analyzed relationships between different physical parameters and identified the energy of the explosion as the dominant factor determining the shape of the emission of type II SNs.
"With this work, we have been able to enter into the statistical study of the physical properties of the progenitors and the explosion of type II supernovae. We have found that most of the analyzed supernovae are compatible with explosions of stars with relatively small final masses (within the mass range studied). Assuming standard stellar evolution theory, most analyzed type II supernovae come from stars with small initial masses. However, this is incompatible with the known distribution of initial masses of massive stars (or initial mass function)," says Martinez.
The astrophysicist adds that the differences with the observed distributions could indicate how stars lose mass during their evolution.
"We decode this result as an indicator that some physical ingredient in standard stellar evolution must be missing. We postulate that this may occur since stars lose much more mass during their evolution than current evolutionary models predict," says Melina Bersten, a researcher at the Instituto de Astrofísica de La Plata, Argentina co-author of the publications and Martinez's PhD director.
In this work, the authors indicate that the range of physical parameters was derived from the most complete database of type II supernovae in the literature.
Laureano Martinez comments that, for the construction of the models, it was assumed that stars do not rotate and evolve in isolated systems. This omits the effects of stellar rotation and the possible interaction with a companion star within a binary system, which could cause significant modifications in stars' evolution. Because of this, future work considers further analyzing the "problem" found with the masses inferred from modelling SNs.
"We currently find a major discrepancy called IMF incompatibility (or initial mass function incompatibility). This incompatibility is due to the vast number of objects with low masses in our analysis. Since the derived masses are, in fact, the masses of the stars at the time of the explosion, we believe that the problem lies in associating this mass with the star's mass before birth. In order to connect the two masses, it is necessary to assume models that follow the complete evolution of the stars from birth until they explode," says Bersten.
"We intend to study the effect on our results of assuming different values to different physical processes during stellar evolution. Such as changing the mass loss rates, assuming binary evolution instead of isolated evolution as was done in these works, assuming evolutionary models with rotation, assuming different values for the mixing length, or overshooting parameter. The idea is to test each process and see the effect on the results obtained to try to identify the dominant parameter, if any, responsible for the incompatibility found. We would also like to complete our study by including a larger number of objects, either using existing samples in the literature or from new observations," he concludes.
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The Carnegie Institution for Science (carnegiescience.edu) is a private, nonprofit organization based in Washington, D.C., with three research divisions on both coasts. Since its founding in 1902, the Carnegie Institution has been a pioneering force in basic scientific research. Carnegie scientists are leaders in the life and environmental sciences, Earth and planetary sciences, and astronomy and astrophysics.
Las Campanas Observatory is one of the institution's departments and is located in Chile, in the Atacama and Coquimbo regions. It belongs to the Observatories Division.