Dust and Gas in Comae of Four Short-Period Comets

Small Solar System bodies are widely regarded as the primary repositories of primordial matter, preserved since the early stages of planetary system formation. Among these, long-period comets (LPCs) represent a unique class of objects; their dynamical trajectories keep them at vast distances from the Sun for most of their existence, ensuring that their volatile and refractory components remain largely unaltered by intense solar radiation. Conversely, the study of short-period comets (SPCs) is indispensable for deciphering the thermal and physical evolution of cometary matter. Having undergone repeated perihelion passages, the coma particles of SPCs exhibit significant morphological and structural transformations. The systematic change in colour indices observed in these objects could serve as a critical diagnostic tool, reflecting the complex interplay of solar heating, devolatilisation, and surface processing that reshapes the dust and gas properties over time.

In this work, we present a comprehensive multiwavelength analysis based on observations of four short-period comets with orbital periods shorter than 10 years. The target objects include comets 4P/Faye, 32P/Comas Solá, and 41P/Tuttle-Giacobini-Kresák (hereafter 4P, 32P, and 41P), which belong to the Jupiter-family class, and 74P/Smirnova-Chernykh (hereafter 74P), classified as an Encke-type comet. The primary objective of this study was to characterise the physical and chemical properties of these bodies during their recent apparitions, with a focus on the comparative analysis of their gas and dust environments.

The observations were carried out using the multimode focal reducer SCORPIO-2 attached to the prime focus of the 6-m BTA telescope. We performed a detailed examination of the photometric, spectral, and polarimetric characteristics of the comets under various observing geometries. Preliminary analysis revealed the presence of prominent emission lines from several molecules, including C2, CN, and NH2, in the spectra of the Jupiter-family comets. These emissions indicate a well-developed gaseous coma with standard volatile sublimating patterns. Conversely, no significant emission features were detected in the spectrum of comet 74P. This is attributed to the comet being observed at a heliocentric distance of nearly 4 au.

Spatial distributions of the linear (for comets 4P and 41P) and circular (for comet 32P) polarisation degrees were constructed. The resulting maps revealed a remarkably homogeneous distribution of polarimetric properties, with no sharp gradients or dramatic variations observed across the coma regions. To enhance the signal-to-noise ratio and identify potential weak morphological features, various digital image processing techniques and filters were applied. Aside from the established coma and tail structures, no additional jet-like features or active shells were detected, pointing towards a relatively uniform surface activity for these nuclei.

Using the photometric and spectral datasets, we estimated the normalised spectral reflectance gradient (S') and the dust activity level parameter (Afρ). These parameters allowed for a rigorous characterisation of the dust component and its evolution with heliocentric distance. Gas production rates for the detected molecular species were calculated using Haser's model, providing a quantitative characterisation of the coma's gas environment. The obtained results indicate the presence of evolutionary changes in the dust and gas properties of the studied comets, consistent with their dynamical classes and observing conditions, and may suggest the influence of repeated perihelion passages on their physical and chemical evolution.

Acknowledgments

The research is supported by the Slovak Academy of Sciences (grant Vega No.2/0067/26) and by the Slovak Research and Development Agency under Contracts No. APVV-24-0076