The Type II-P Supernova 2019mhm and Constraints on Its Progenitor System

We present pre- and post-explosion observations of the Type II-P supernova (SN~II-P) 2019mhm located in NGC~6753. Based on optical spectroscopy and photometry, we show that SN,2019mhm exhibits broad lines of hydrogen with a velocity of $-8500\pm 200$~km~s${}^{-1}$ and a $111\pm 2$~day extended plateau in its luminosity, typical of the Type II-P subclass. We also fit its late-time bolometric light curve and infer that it initially produced a ${}^{56}$Ni mass of $1.3\times {10}^{-2}$~M${}_{\odot }\pm 5.5\times {10}^{-4}$~$M_{\odot }$. Using imaging from the Wide Field Planetary Camera 2 on the {\it Hubble Space Telescope} obtained 19~years before explosion, we aligned to a post-explosion Wide Field Camera 3 image and demonstrate that there is no detected counterpart to the SN to a limit of $>$24.53~mag in F814W, corresponding to an absolute magnitude limit of $M_{\mathrm{F}814\mathrm{W}}<-7.7$~mag. Comparing to massive-star evolutionary tracks, we determine that the progenitor star had a maximum zero-age main sequence mass $<$17.5~M${}_{\odot }$, consistent with other SN~II-P progenitor stars. SN,2019mhm can be added to the growing population of SNe~II-P with both direct constraints on the brightness of their progenitor stars and well-observed SN properties.