Large-field CO (1--0) observations towards the Galactic historical supernova remnants: Shocked molecular clouds towards the Crab Nebula

The investigation of the interstellar gas surrounding supernova remnants (SNRs) is not only necessary to improve our knowledge of SNRs, but also to understand the nature of the progenitor systems. As part of the Milky Way Imaging Scroll Painting (MWISP) CO line survey, we studied the interstellar gas towards the Galactic historical SNRs. In this work, we present the CO observational results of the Crab Nebula. We performed large-field and high-sensitivity CO (1--0) molecular line observations towards the Crab Nebula using the 3times3 beam Superconducting Spectroscopic Array Receiver (SSAR) at the PMO 13.7,m millimeter telescope. The Gaia optical data were used to measure the distances of molecular clouds detected in this work, while the HI data from the GALFA-HI and the HI4PI surveys were used to compare the distributions of the molecular and atomic gas. The CO observations show molecular clouds towards the Crab Nebula with velocities ranging from about 0 to 16,̨ms. From the CO spectra, we find shocked signatures in the clouds with velocities of roughly ̨ms. These shocked molecular clouds, with an angular distance of ∼,0 towards the Crab Nebula, are located at the shell of a bubble discovered in the HI images in the same velocity range. The dimension of the bubble is roughly 2 and the expansion velocity is ∼,5,̨ms. The kinetic energy released by the supernova is estimated to be about 3.5,times,10^51 erg. The kinetic energy inferred from the shocked molecular clouds, together with the HI bubble, supports the picture that the Crab Nebula belongs to a typical core-collapse SNR. Nevertheless, due to the significant uncertainty in the distance measurement, further observations are needed to verify the physical association between the shocked molecular clouds and the Crab Nebula.