FAN-SHAPED DUST EMISSION FROM THE NUCLEUS OF COMET 3i
ATel #17350; Virginio Oldani (Stazione Astronomica di Sozzago), Federico Manzini (Stazione Astronomica di Sozzago), Andrea Reguitti (INAF-OAPd), Alessandra Mura (INAF-UniPd), Paolo Ochner (INAF-UniPd), Luigi R. Bedin (INAF-OAPd), Andrea Farina (INAF-UniPd)
on 22 Aug 2025; 15:55 UT
Credential Certification: Luigi R. Bedin BEDIN (luigi.bedin@inaf.it)
We analyzed HST images (P.I.: D. Jewitt) and images coming from the two 2-meter telescopes of the LCOGT node at Siding Spring (coj, Australia) and at Haleakala Observatory (ogg, Hawaii) (Group ID: C/2025 N1 and 3I-2m-JoseMPerez-INS_Alcarras, respectively) of the interstellar comet 3I.
Data-Format[Telescope/site;Date;Filter;Prop-ID]=[coj;2025.07.04;g,r,z,i;TSO2025A-001], [HST;2025.07.21;FP350LP;GO-17830], and [ogg;2025.08.13-14-15-16;g,r,z,i;TSO2025B-001].
The coma shows in the HST images and in the subsequent LCO images taken on August 13-16 a prominent elongation in the W quadrant, with a peak brightness at PA 285deg and an estimated amplitude of ~30deg (Figure 1). This appears related to an anisotropic ejection of dust towards the Sun modulated by direct insolation and by radiation pressure, confirming what had been previously reported by Jewitt et al. (arXiv 2508.02934v2, 2025), who observed on the HST image a broad fan along PA 280+/-10deg.
The elongation was already visible in the LCO images of July 4, although hardly identifiable as an anisotropic dust emission (Figure 2a and 2b). In fact, it was initially described as a diffuse tail along PA 280+/-10deg by Jewitt et Luu (ATEL #17263, 2025), who observed it with the Nordic Optical Telescope on July 2, 2025. However, the projected PA of this emission seemed inconsistent with a tail produced by radiation pressure.
Jewitt et al. (arXiv 2508.02934v2, 2025) identified the cometâs tail later on the HST images as a very faint emission in the anti-solar direction. This observation was confirmed by analyzing the 2-m LCO telescope images taken on August 13-16, when the tail became more evident in PA 100deg, consistent with the calculated PsAMV, as the projected length of the tail increased following the increase of the phase angle from 9deg to 19deg(Figure 3a and 3b).
The coma extension visible on all the images appears fan-shaped as a result of the projection onto the plane of the sky of an emission cone emerging from a high-latitude discrete source on the sunlit side of the rotating nucleus. A numerical model of the inner coma reproduced a realistic simulation of the fan on all of the images as a result of the nucleus rotation by setting a discrete source at a latitude of ~75deg, with a radius of the dust particles between 5 and 100 micron-m (assuming a dust density of 10^3 kg m^-3) and corresponding emission velocities of ~25 to 5 m s^-1. The modelling allowed to estimate an orientation of the north pole of the nucleus within a circle of 5deg centered at RA 154deg and Dec +25deg (or RA 334deg and Dec -25deg). Further high-resolution images are, however, needed to confirm these preliminary findings.
With these estimated coordinates, the spin axis orientation should remain relatively stable in the next few weeks. The geometric conditions of the observation will also remain stable, no major modifications in the appearance of the fan-shaped dust emissions are expected.
As the comet approaches perihelion in mid-October, if the direction of the rotation axis is invariant, the pole of the hemisphere where the active source is located should appear from Earth considerably tilted behind the sky plane. At the same time, as a result of the cometâs path on its orbit, the subsolar point on the nucleus will move to increasingly higher latitudes in the hemisphere opposite to the source of the fan, which should then be in umbra, so the fan should disappear.
Figures and captions are available at:
https://web.oapd.inaf.it/bedin/files/PAPERs_eMATERIALs/ATel/C2025N1/
Figures and Captions
