G213.0-0.6, a true supernova remnant or just an H II region?

Abstract

G213.0-0.6 is a faint extended source situated in the anticentre region of the Galactic plane. It has been classified as a shell-type supernova remnant (SNR) based on its shell-like morphology, steep radio continuum spectrum, and high ratio of [S II]/Hα. With new optical emission line data of Hα, [S II], and [N II] recently observed by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, the ratios of [S II]/Hα and [N II]/Hα are re-assessed. The lower values than those previously reported put G213.0-0.6 around the borderline of SNR-H II region classification. We decompose the steep-spectrum synchrotron and the flat-spectrum thermal free-free emission in the area of G213.0-0.6 with multi-frequency radio continuum data. G213.0-0.6 is found to show a flat spectrum, in conflict with the properties of a shell-type SNR. Such a result is further confirmed by TT-plots made between the 863-MHz, 1.4-GHz, and 4.8-GHz data. Combining the evidence extracted in both optical and radio continuum, we argue that G213.0-0.6 is possibly not an SNR, but an H II region instead. The V_LSR pertaining to the Hα filaments places G213.0-0.6 approximately 1.9 kpc away in the Perseus Arm.