The size distribution of supernova remnants (SNRs) can help to clarify the various aspects of their evolution and interaction with the interstellar medium (ISM). Since the observed samples of SNRs are a collection of objects with very different ages and origin that evolve in different conditions of the ISM, statistical Monte Carlo methods can be used to model their statistical distributions. Based on very general assumptions on the evolution, we have modeled samples of SNRs at various initial and environmental conditions, which were then compared with observed collections of SNRs. In the evolution of SNRs the pressure of the ISM is taken into account, which determines their maximum sizes and lifetimes. When comparing the modeled and observed distributions, it is very important to have homogeneous observational data free from selection effects. We found that a recently published collection of SNRs in M33 (Long et al. 2010, ApJS,187,495) satisfies this requirement if we select the X-ray SNRs with hardness ratios in a limited range of values. An excellent agreement between distributions of this subset of SNRs and the subset of modeled SNRs was reached for a volume filling-factor of the warm phase of the ISM (partly ionized gas with $n_{\rm H}\sim 0.2-0.5~ \rm {cm}^{-3}; T \sim 8000-10000~K $) in M33 of $\sim\ 90%$. The statistical distributions constructed in this way, which reproduce practically all the statistical properties of observed SNRs, allowed us to obtain one of the important parameters of M33: the birthrate is one SNR every $ {140} - {150}$ yr, and the total number of SNRs with a shock Mach number $M_{s} \geq 2$ is larger than $\sim 1000$.
Unveiling Vela -- Variability of Interstellar Lines in the Direction of the Vela Supernova Remnant III. Na D and Ca II K
2017年至2019年，南部非洲大型望远镜在船帆座超新星遗迹的方向上获得了15颗恒星的高分辨率光谱。本文讨论了星际Ca i H、K和Na i D线。特别是，线配置文件进行比较，在1993-1996年获得的Cha和Sembach的光谱分辨率的配置文件。其中10条视线显示了该视线中一个或多个组件的变化。变化包括在两次观测期间之间的二十年间径向速度的微小变化（1-2公里/秒）和（或）等效宽度的增加/减少。Ca-K线的变化比Na-D线的变化更明显。这些变化归因于超新星喷出物和周围星际介质之间相互作用所扰动的气体。典型的时间尺度可以是20-50年。如前所述，遗迹表面线剖面的小尺度变化表明，相互作用的线性尺度是目前遗迹40 pc大小的一小部分。
Odd Radio Circles as supernovae remnants in the intragroup medium
在最近发现的弧分大小的圆形弥散射电源中，有一部分（$\sim8$ %）被称为奇射电圈（Odd Radio Circles）或ORC，它们可能是本星系群及其近邻星系群内群内介质中的超新星遗迹。这一估计是基于光学探测率的群内超新星事件在附近（$z \sim 0.1-0.2$）星系群。在本星系群及其邻近的星系群中，预计每百万年约有5400颗群内超新星。对于一个约为$10^{4}$年的射电可探测周期，平均每1000平方度1.3个群内中等超新星遗迹预计将在射电勘测中被探测到，其灵敏度导致了ORC的发现。在群内介质中，距离$\sim3$ Mpc的超新星遗迹的角大小、表面亮度和射电通量可以预期与五个已知的ORC相似。群内超新星遗迹并不存在于星系的致密和寒冷的星际介质中，而是在星系晕或更远的低密度（$10^{-4}-10^{-5}$ cm $^{-3}$）温暖介质（$10^{5}-10^{6}$ K）中演化，并且可能在与银河系和其他附近星系周围的各种潮汐流光相关的漫射恒星光中找到它们的祖先。
