The mechanism behind overionization in supernova remnants. The study: “Deep XMM-Newton Observations Reveal the Origin of Recombining Plasma in the Supernova Remnant W44” of H. Okon (Kyoto University) recently appeared on ApJ
When a massive star explodes as a supernova, its atmosphere is ejected forming an expanding nebula called “supernova remnant“. Very often, supernova remnants impact pre-existing clouds, which can be made of material ejected by the star before the explosion (circumstellar material) or clouds that happen to be nearby the star (interstellar material). The interaction between the expanding supernova remnant and the surrounding clouds can produce important phenomena and regulate several properties of remnant itself.
The ionization degree is one of the important parameters that describe the physics of supernova remnants. In the generally accepted scenario, the plasma in the remnants is collisionally ionized until the degree of ionization reaches the collisional ionization equilibrium. This means that the degree of ionization is expected to increase until equilibrium is reached. However, as it typically occurs, observations contradict expectations paving the way to new scenarios and discoveries. In fact, in several supernova remnants the observed degree of ionization is higher than the collisional ionization equilibrium. These remnants share a common feature: they are “mixed morphology supernova remnants“, characterized by a radio bright shell surrounding a X-rays bright core. This morphology is the result of the interaction between the expanding remnants and the surrounding medium, suggesting that overionization is somehow a consequence of this interaction. In fact, one of the main hypothesis to explain overionization in remnants is that the plasma is cooled down by the efficient thermal conduction between the supernova remnants and the surrounding medium, and thus its temperature is lower than that expected by the observed ionization degree. Another possibility is that overionization is a consequence of a rapid adiabatic expansion of the remnant.
To better understand the nature of overionization in supernova remnants, the international team led by the astronomer H. Okon (Department of Physics, Kyoto University) analyzed X-ray observations obtained with the ESA observatory XMM-Newton of the supernova remnant W44, about 10000 light years away from us and with the age of about 20 thousand years. W44 is mixed morphology remnant with observed overionization. The researchers demonstrated that the plasma in W44 is colder (at about 2 million degrees, while the peak temperature in the remnant is about 7 million degrees) in the interface between the remnant and the surrounding cloud. Besides, the atomic recombination time is higher in the regions of the remnants where the temperature is lower. This suggests that overinization in W44 is due to the thermal conduction between the remnant and the cloud. As a further confirmation, the atomic recombination time is the longest in correspondence of the most dense clumps of the surrounding cloud. These results are described in the paper: “Deep XMM-Newton Observations Reveal the Origin of Recombining Plasma in the Supernova Remnant W44“, recently appeared on The Astrophysical Journal. The astronomers S. Orlando and F. Bocchino (INAF – Astronomical Observatory of Palermo) and M. Miceli (University of Palermo) are coauthors of the study.
The figure (click here to see the entire figure) shows XMM-Newton images of W44 in different energy bands, with marked the radio emission profile obtained from the Karl G. Jansky Very Large Array observations.
Mario Giuseppe Guarcello ( follow mguarce)