arXiv subjects

Baltasar Luco

Publications and source records attributed to Baltasar Luco.

4 recordsLinked to original sources

arXiv2026

Very Large Telescope observations of interstellar comet 3I/ATLAS III: High-resolution monitoring of CN and forbidden oxygen emission across the perihelion passage with ESPRESSO

3I/ATLAS is only the third known interstellar object to traverse the Solar System. We obtained high-resolution ($R\simeq 140000$) spectroscopy of 3I/ATLAS on 23 nights with VLT/ESPRESSO ($r_{h}=1.67-2.45$au). CN production rates were derived by fitting the $B^2\Sigma^+-X^2\Sigma^+$ band, using CometSpec, a publicly released Python package for flexible fluorescence modeling. The three forbidden [OI] lines were modeled to derive the green-to-red (G/R) flux ratio and a CO$_2$/H$_2$O proxy. We fit $Q_{CN}=a(r_{h}/1\text{au})^b$ to the production rate versus heliocentric distance relation. The pre-perihelion CN power-law index is $b_{pre}=-4.62^{+1.25}_{-1.22}$, within the range observed for solar-system comets. The CN residuals about the smooth model show substantial scatter that we attribute to a combination of systematics and possible physical drivers that the present cadence cannot disentangle. The G/R ratio decreases from $\sim 0.42\pm 0.14$ at $r_{h}\simeq 2.4$au to $\sim 0.114 \pm 0.004$ near perihelion. The asymmetric piecewise oxygen fit is strongly preferred over the symmetric one, consistent with thermal-inertia models, although denser post-perihelion sampling would be needed to confirm this. The CO$_2$/H$_2$O proxy spans $\sim 0.06-1.06$ and is comparable to the values reported by Subaru/HDS and SPHEREx, and to those measured for 2I/Borisov. We additionally identify NiI and FeI, and report post-perihelion production rates for C$_2$ and CH. Our high-resolution monitoring of 3I/ATLAS reveals a CO$_2$-rich coma that becomes progressively H$_2$O-dominated near perihelion, with a G/R asymmetry consistent with delayed H$_2$O sublimation predicted by thermal-inertia models. Together with the public release of CometSpec and the reduced data, these results provide both an empirical reference and a methodological framework for the next generation of interstellar-object monitoring campaigns.

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arXiv2025

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

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.

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arXiv2025

Very Large Telescope Observations of Interstellar Comet 3I/ATLAS. II. From Quiescence to Glow: Dramatic Rise of Ni i Emission and Incipient CN Outgassing at Large Heliocentric Distances*

We report VLT spectroscopy of the interstellar comet 3I/ATLAS (C/2025~N1) from $r_{\rm h}\!\simeq\!4.4$ to $2.85$~au using X-shooter (300--550\,nm, $R\!\simeq\!3000$) and UVES (optical, $R\!\simeq\!35k-80k$). The coma is dust-dominated with a fairly constant red optical continuum slope ($\sim$21--22\%/1000\AA). We report detection of CN emission and also detect numerous Ni\,\textsc{ii}~lines while Fe\,\textsc{i}~remains undetected, potentially implying efficiently released gas-phase Ni. At $r_{\rm h}\!\simeq\!3.14$~au we derive $3\sigma$ limits of $Q({\rm OH})<{1.48\times10^{26}}\ {\rm s^{-1}}$, but find no indications for [O\,\textsc{i}], C$_2$, C$_3$ or NH$_2$. From our latest X-shooter measurements conducted on 2025-08-21 ($r_{\rm h} = 2.85$\,au) we measure production rates of $\log~Q(\mathrm{CN}) = {24.81\pm 0.01}$ molecules s$^{-1}$ and $\log~Q$(Ni) $= {23.30\pm0.07}$ atoms s$^{-1}$, and characterize their evolution as the comet approaches perihelion.~We observe a steep heliocentric-distance scaling for the production rates $Q(\mathrm{Ni}) \propto r_h^{{-7.7 \pm 1.0}}$ and for $Q(\mathrm{CN}) \propto r_h^{{-6.7 \pm 0.2}}$, and predict a Ni--CO$_{(2)}$ correlation if the Ni\,\textsc{ii}\ emission is driven by the carbonyl formation channel.~Energetic considerations of activation barriers show that this behavior is inconsistent with direct sublimation of canonical metal/sulfide phases and instead favors low--activation--energy release from dust, e.g.~photon-stimulated desorption or mild thermolysis of metalated organics or Ni-rich nanophases, possibly including Ni--carbonyl-like complexes.~These hypotheses are testable with future coordinated ground-based and space-based monitoring as 3I becomes more active during its continued passage through the solar system.

astro-ph.SR↗

arXiv2025

Spectral Characteristics of Interstellar Object 3I/ATLAS from SOAR Observations

Interstellar objects (ISOs) provide unique insights into the building blocks and conditions of extrasolar planetary systems. The newly discovered object, 3I/ATLAS (C/2025 N1), represents the third known ISO after 1I/'Oumuamua and 2I/Borisov. We present initial spectroscopic characterizations of 3I using observations from the Goodman High Throughput Spectrograph on the 4.1 m SOAR Telescope in Chile during the night of July 3rd. The reflectance spectrum of 3I, covering 3700-7000{\AA} reveals a red continuum, comparable to extreme trans-Neptunian objects, with a weak UV-optical turnover indicative of complex carbonaceous and irradiated organics. At the time of observation, when 3I was at a heliocentric distance of 4.4 AU, we detected no discernible gas emission from canonical cometary species (CN, C$_3$, C$_2$, CO$^+$, [OI]). This is in agreement with expectations from our thermal-evolution model, which indicates sublimation-driven activity should commence once 3I/ATLAS approaches smaller heliocentric distances. Nonetheless, the paradoxical situation of early onset coma without evidence of sublimation tracers calls for other dust-liberating mechanisms that ancient ISOs may be subjected to at large heliocentric distances.

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