Pre-perihelion evolution of the NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS. From extreme to normal

TL;DR

The paper investigates the pre-perihelion evolution of the NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS, leveraging high-resolution UVES and X-shooter spectroscopy across $r_h$ = 3.14–1.85 au. FeI is detected only inside $r_h \approx 2.64$ au while NiI is seen at all epochs; production rates are derived via a fluorescence model, revealing an initially extreme NiI/FeI ratio that normalizes to values typical of Solar System comets by $r_h \approx 2$ au. A carbonyl-sublimation scenario, with $\,\mathrm{Ni(CO)_4}$ and $\,\mathrm{Fe(CO)_5}$, explains the differential release and its heliocentric dependence, and suggests the presence of carbonyls in the cometary material. 3I also exhibits an unusually high total metal production, potentially tied to its CO$_2$/H$_2$O and CO/H$_2$O abundances, while remaining C$_2$-depleted. These results illuminate metal-release mechanisms in comets, including interstellar ones, and motivate post-perihelion follow-ups to further test the carbonyl hypothesis and constrain the distribution of releasing regions.

Abstract

Emission lines of FeI and NiI are commonly found in the coma of Solar System comets, even at large heliocentric distances. These atoms are most likely released from the surface of the comet's nucleus or from a short-lived parent. These lines were also found in the interstellar comet 2I/Borisov, which has a NiI/FeI abundance ratio similar to that observed in Solar System comets. Here, we report observations of the interstellar comet 3I/ATLAS, which were carried out with the ESO Very Large Telescope equipped with the UVES and X-shooter spectrographs. Spectra were obtained at heliocentric distances ranging from 3.14 to 1.85 au. NiI was detected at all epochs. FeI was only detected at heliocentric distances smaller than 2.64 au. We estimated the NiI and FeI production rates by comparing the observed line intensities with those produced by a dedicated fluorescence model. Comet 3I first exhibited extreme and unusual NiI/FeI abundance ratios during the initial stages of its activity. However, as its heliocentric distance decreased, this ratio became indistinguishable from those observed in Solar System comets and in comet 2I/Borisov. Comet 3I was found to be C$_2$-depleted, with a NiI/FeI abundance ratio finally consistent with other C$_2$-depleted comets. Nevertheless, comet 3I remains exceptional due to its high, total production rate of NiI and FeI, which is at least one order of magnitude larger than that of other comets. We interpreted these observations assuming that the NiI and FeI atoms were released through the sublimation of Ni(CO)$_4$ and Fe(CO)$_5$ carbonyls. This scenario provides a straightforward explanation for the asymmetric release of NiI and FeI atoms in the cometary coma and how it depends on the heliocentric distance. It also supports the presence of carbonyls in the cometary material.

Figures (10)

• Figure 1: Continuum-subtracted spectra of comet 3I obtained on August 28 and September 3+4 with UVES. Three narrow FeI lines were detected on August 28 in this spectral range, FeI $\lambda$3719.93Å being the brightest iron line detected in our spectra. In the average spectrum obtained in September 3+4 (shifted vertically for clarity), the FeI lines are brighter, and four of them were detected (as is indicated by the vertical red lines). Broad emission features due to background [OII] emission at 3727Å and 3729Å are also observed.
• Figure 2: Ratio $\log_{10} (I_{\rm obs} / I_{\rm mod})$ for the FeI (red diamonds) and NiI (blue squares) lines measured in comet 3I for observations secured with UVES on August 28 (top) and September 12 (bottom). $I_{\rm obs}$ represents the observed line intensities and $I_{\rm mod}$ the intensities computed with the fluorescence model. $I_{\rm obs} / I_{\rm mod}$ is proportional to the column density of the atoms Manfroid2021. The separation between the mean values computed for NiI and FeI gives the NiI/FeI abundance ratio. The ratios have been shifted on the y axis so that the mean value of $\log_{10} (I_{\rm obs} / I_{\rm mod})$ is zero for FeI.
• Figure 3: Spatial profile of the brightest NiI line ($\lambda$ 3458Å) observed with UVES on September 4. The measured surface brightness (SB; normalized to one at the photocenter) is plotted as a function of the projected nucleocentric distance, $p$, in arcsec. The red line represents SB $\propto p^{-1}$ convolved with a 1.5 full-width-at-half-maximum Gaussian to account for the seeing and tracking imperfections.
• Figure 4: Q(NiI)/Q(FeI) as a function of the heliocentric distance, $r_h$, for interstellar and Solar System comets. The measurements obtained at different heliocentric distances are not averaged and shown individually for comets 3I (9 values, pre-perihelion), 2I (2 values, post-perihelion), C/2001 P1 (4 values, pre-perihelion), and C/2017 K2 (2 values, pre-perihelion). The comet with the smallest Q(NiI)/Q(FeI) ratio is C/1965 S1(Ikeya-Seki) observed at 0.14 au from the Sun. The solar Ni/Fe abundance ratio is equal, in logarithm, to $-1.25 \pm 0.04$Asplund2009.
• Figure 5: Q(NiI)/Q(FeI) as a function of the total production rate Q(FeI+NiI). Comet 3I was observed in the range 2.64-1.85 au. Solar System comets with $r_h >$ 1.85 au are not shown, except comet C/2016 R2, which was observed at 2.75 au. Comet 2I was observed at 2.18$\pm$0.14 au.