Pre-perihelion observations of the carbon-depleted comet C/2023 A3 (Tsuchinshan-ATLAS). Insights into CN production and molecular upper limits

The study of cometary molecular emissions provides crucial insights into the primordial composition of the Solar System and the physical and chemical processes shaping these icy bodies. Comets, as remnants of the early Solar System, serve as natural archives of volatile compounds that offer a glimpse into the conditions of the protoplanetary disk. In this work, we analyze an optical pre-perihelion spectrum of comet C/2023 A3 (Tsuchinshan-ATLAS), obtained using the DOLORES spectrograph at the Telescopio Nazionale Galileo (TNG) on May 1, 2024. The cometary spectrum was reduced using standard procedures implemented in the IRAF software package. To characterize the volatile inventory of comet C/2023 A3, we derived the production rate of CN, the only detectable molecular emission, and calculated upper limits for undetected species, including C${}_2$, C${}_3$, and NH${}_2$. These constraints were obtained by analyzing the noise level in continuum regions and integrating theoretical line profiles, accounting for instrumental resolution and observational conditions. We calculated a CN production rate of (3.89$\pm 0.21$)$\times 10^{25}$ molec/s. Despite significant dust contamination, which likely obscures weaker molecular emission lines typically associated with cometary activity, we derived upper limits for the production rates of key volatile species: Q${}_{C_3}<3.12\times 10^{24}$ molec/s, Q${}_{C_2}<1.30\times 10^{25}$ molec/s, and Q${}_{N{H}_2}<2.79\times 10^{25}$ molec/s, respectively. We also determined the $Af\rho$ parameter, obtaining a value of $4329\pm 56$ cm, which confirms the high dust production rate previously reported for this comet. The logarithmic ratio of production rates, $log\left(Q\left(C_2\right)/Q\left(\mathrm{CN}\right)\right)<-0.48$, indicates that C/2023 A3 is a carbon-depleted comet, consistent with previous classifications of dynamically new comets. These findings contribute to a deeper understanding of the compositional diversity and evolutionary processes of such objects.