Dynamically New Comet C/2025 D1 (Groeller) with Record Perihelion Distance

Abstract

We studied C/2025 D1 (Groeller), a long-period comet with an unprecedented perihelion distance of 14.1 au, using archival observations. The data reveals that it had been active at inbound heliocentric distances $r_{\rm H} \gtrsim 20$ au. Initially, the comet intrinsically brightened at $r_{\rm H} \gtrsim 16$ au, with brightening parameters comparable to those of other long-period comets. However, observations after late 2023 showed a gradual decay, despite the inbound trajectory of the comet. To our knowledge, such behaviours have not been observed for other long-period comets at similar heliocentric distances. We speculate that this might be linked to the onset of CO$_{2}$ sublimation and/or crystallisation processes. Alternatively, the activity source might have been exhausted. The surface brightness profile of the coma indicates a steady-state mass loss, implying supervolatile sublimation as the primary driver of the observed activity. Despite changes in the orbital plane angle, the circularly symmetric coma persisted throughout the observed period, indicative of the dominance of large grains in the coma. Assuming the activity trend is independent of bandpass, we found that comet was redder than many other solar system comets. Our model-dependent constraint estimates the nucleus radius to be $\gtrsim\!0.4$ km. We performed astrometric measurements, refined the orbital solution, and derived the original and future orbits of the comet. Our N-body integration, accounting for the Galactic tide, strongly favours that the comet is dynamically new, with its previous perihelion at $\gtrsim\!60$ au from the Sun $\gtrsim\!6$ Myr ago. It is highly likely that the comet will be lost from our solar system after the current apparition.

Figures (5)

• Figure 1: Selected archival observations of comet C/2025 D1 (Groeller). In some of the panels where the comet appears faint, a white dotted circle is used to mark its location. The white and magenta scale bars represent an angular distance of 10 and a linear distance of $10^5$ km projected at the observer-centric distance of the comet, respectively. J2000 equatorial north is up and east is to the left. In each panel, the antisolar direction (yellow arrow) and the projected negative heliocentric velocity of the comet in the observer's sky plane (dark orange arrow) are also indicated.
• Figure 4: Temporal evolution of (a) apparent magnitude and (b) absolute magnitude of comet C/2025 D1 (Groeller), measured using a $3 \times 10^4$ km radius aperture. The upper axes in panels (a) and (b) mark time with respect to the perihelion passage and heliocentric distance, respectively. Data from different facilities are plotted with distinct symbols, with colours corresponding to the bands to which the photometric reduction was calibrated (see the legends). The dotted lines in panel (b) represent the best-fit linear models to derive the colour indices and activity trends of the comet.
• Figure 5: The colour of C/2025 D1, measured using a $3 \times 10^4$ km radius photometric aperture, compared with various small solar system body populations 2003Icar..163..363D2007MNRAS.377.1393S2012Icar..218..571S2015AJ....150..201J2023PSJ.....4..135M and the Sun 2018ApJS..236...47W. Colours reported in non-SDSS systems have been transformed to the SDSS system following 2006AA...460..339J. Open circles with letters at their centres represent main-belt asteroids of specific taxonomic types. Objects with linear reflectivity gradients in the $g - i$ spectral interval form the reddening line, drawn as the grey dashed curve.
• Figure 6: Radial surface brightness profiles of comet C/2025 D1 on (a) 2022 February 2 from CFHT and (b) 2022 May 30 from HSC. These images have higher S/N for the comet than any other observations we collected. The azimuthally averaged surface brightness is plotted in olive. The best-fit radial surface brightness model is shown as a red dashed line, with the derived logarithmic surface brightness slope indicated in the lower left corner. The boundaries of the fitted region are marked by the two vertical blue dashed lines.
• Figure 7: Orbit of C/2025 D1 at its previous perihelion, derived from the Monte Carlo clones. The left panel shows the orbital uncertainty in the space of perihelion time and distance, with the corresponding probability density distributions for each element presented in the right two panels. The nominal orbit is highlighted in orange. For the N-body integration, we adopted a local mass density in the solar neighbourhood of $\rho_{\ast} = 0.185~M_{\odot}$ pc$^{-3}$1984ApJ...276..169B.