Misure di asteroidi con tecniche di alta risoluzione

L’argomento principale di questa tesi consiste nell’analisi di osservazioni di asteroidi effettuate mediante il telescopio nazionale Galileo situato nell’isola di La Palma, nell’arcipelago delle isole Canarie, effettuate mediante tecniche di ottica adattiva e speckle interferometry. Lo scopo è quello di dimostrare come il costante miglioramento dei rivelatori e dei sistemi ottici dei moderni telescopi renda oggi possibile effettuare misure dirette delle dimensioni angolari apparenti di oggetti che fino a pochi anni fa restavano al di là del potere risolutivo dei migliori telescopi basati a Terra. Questo permette oggi di ottenere misure dirette delle dimensioni e delle forme complessive per un campione via via crescente di oggetti sufficientemente grandi e brillanti. Tutto ciò permette di calibrare efficacemente le misure di diametro ottenute in passato per un vasto campione di oggetti sulla base dei risultati di tecniche indirette e soggette a considerevoli errori. Esiste inoltre sempre la possibilità, utilizzando tecniche evolute di imaging remoto, di poter risolvere possibili sistemi binari, come già avvenuto in diversi casi in questi anni. Lo studio delle caratteristiche dinamiche e fisiche degli asteroidi riveste un ruolo fondamentale per comprendere la formazione e l’evoluzione del nostro sistema solare, e dei sistemi planetari extrasolari. Gli asteroidi, e più in generale i corpi minori del sistema solare (comete, oggetti transnettuniani) sono molto importanti per diversi motivi: innanzitutto le più recenti teorie interpretano i piccoli corpi come residui in larga misura intatti dei planetesimi progenitori dei pianeti che oggi osserviamo. Il loro studio permette dunque di ricavare informazioni sulle proprietà fisiche del disco protoplanetario che ha dato origine al sistema solare, e che si ritiene sia in generale presente quando un sistema planetario si forma intorno ad una stella. Inoltre, la parentela tra i corpi minori e le meteoriti è fondamentale per la comprensione della loro origine ed evoluzione fisica e dinamica. Gli asteroidi, in particolare, sono interessanti da molti punti di vista. Essi sono oggetti di transizione tra la regione dei pianeti rocciosi e quella dei pianeti gassosi, e sono anche oggetti di transizione da corpi aventi una struttura determinata essenzialmente dalle forze di stato solido, a corpi suf- ficientemente massicci per i quali l’autogravitazione inizia a giocare un ruolo predominante. Bisogna poi ricordare che esistono asteroidi con orbite che possono intersecare quella terrestre, con un conseguente pericolo d’impatto. Essendo piccoli (il più grande di essi, Cerere, raggiunge a malapena i 1000 km di diametro, ma la stragrande maggioranza della popolazione non arriva a 50 km) e poco luminosi, gli asteroidi sono relativamente difficili da osservare, e la conoscenza che abbiamo di molte delle loro proprietà fisiche, anche quelle più fondamentali, è in gran parte sommaria, essendo dedotta per lo più dai risultati di tecniche indirette; l’applicazione delle tecniche di osservazione ad alta risoluzione, divenuta possibile solamente in tempi relativamente recenti, apre nuovi possibilità per quanto riguarda la misura diretta delle dimensioni degli oggetti, e per la scoperta di sistemi binari, come quelli precedentemente trovati mediante esperimenti radar o per mezzo dell’esplorazione diretta mediante sonde spaziali. Questa tesi presenta i risultati di un certo numero di osservazioni di imaging diretto effettuate recentemente al Telescopio Nazionale Galileo. L’analisi dei dati è articolata in tre fasi principali: selezione delle immagini disponibili dell’asteroide e di una stella di riferimento, in base a considerazioni del rapporto segnale rumore; sottrazione del rumore utilizzando opportune immagini di cielo; deconvoluzione del segnale e della cosiddetta Point Spread Function per ricavare l’immagine risolta dell’asteroide. In particolare il problema della deconvoluzione è stato affrontato con due diverse procedure, che sono state poi confrontate tra loro per verificare la stabilità dei risultati ottenuti: una delle due procedure segue un classico metodo di deconvoluzione (metodo di Richardson-Lucy), che partendo dalle immagini osservate dell’asteroide e della stella di riferimento e senza formulare ipotesi a priori sul tipo di immagine asteroidale, effettua la deconvoluzione mediante una tecnica numerica iterativa. La seconda procedura ha come punto di partenza un modello del disco apparente dell’asteroide basato su un’ellisse apparente di luminosità non costante, ma con un effetto di oscuramento al bordo descritto mediante un classico modello di Minnaert. Questa immagine teorica dipende da un certo numero di parametri. Effettuando una convoluzione con una Point Spread Function ricavata dalle osservazioni della stella di riferimento, si ottiene un risultato che va poi direttamente confrontato con l’immagine osservata. Una procedura iterativa permette quindi di convergere ad un set di parametri di partenza (semiassi ed inclinazione dell’ellisse apparente, coefficiente di oscuramento al bordo) che producono un residuo minimo con l’immagine osservata. Sono state analizzate le immagini di sei asteroidi osservati con diversi filtri, e nella maggior parte dei casi le due procedure hanno fornito risutati compatibili. I risultati ottenuti sono apprezzabili, dato che costituiscono un campione importante di oggetti con dimensioni apparenti misurate direttamente. Si è effettuata inoltre la riduzione di un certo numero di dati ottenuti mediante la tecnica della speckle interferometry, sempre al TNG. In questo caso, la procedura di riduzione si è rivelata molto più complessa, ed ha fornito per lo più risultati molto incerti. La ragione di questo parziale insuccesso va imputata probabilmente ad un complesso di circostanze osservative avverse, includendo in ciò delle condizioni di cielo lontane dalle condizioni ideali, e un numero di osservazioni per oggetto spesso insufficiente a fornire un segnale analizzabile in modo soddisfacente. Questo esercizio si è rivelato comunque utile anche in vista di nuovi tentativi in futuro di applicare la camera speckle del TNG allo studio degli asteroidi

Coma environment of comet C/2017 K2 around the water ice sublimation boundary observed with VLT/MUSE

We report a new imaging spectroscopic observation of Oort-cloud comet C/2017 K2 (hereafter K2) on its way to perihelion at 2.53 au, around a heliocentric distance where H2O ice begins to play a key role in comet activation. Normalized reflectances over 6 500--8 500 AA for its inner and outer comae are 9.7+/-0.5 and 7.2+/-0.3 % (10^3 AA)^-1, respectively, the latter being consistent with the slope observed when the comet was beyond the orbit of Saturn. The dust coma at the time of observation appears to contain three distinct populations: mm-sized chunks prevailing at <~10^3 km; a 10^5-km steady-state dust envelope; and fresh anti-sunward jet particles. the dust chunks dominate the continuum signal and are distributed over a similar radial distance scale as the coma region with redder dust than nearby. they also appear to be co-spatial with OI1D, suggesting that the chunks may accommodate H2O ice with a fraction (>~1 %) of refractory materials. The jet particles do not colocate with any gas species detected. The outer coma spectrum contains three significant emissions from C2(0,0) Swan band, OI1D, and CN(1,0 red band, with an overall deficiency in NH2. Assuming that all OI1D flux results from H2O dissociation, we compute an upper limit on the water production rate Q_H2O of ~7 x 10^28 molec s^-1 (with an uncertainty of a factor of two). the production ratio log[Q_C2/Q_CN] of K2 suggests that the comet has typical carbon-chain composition, with the value potentially changing with distance from the Sun. Our observations suggest that water ice-containing dust chunks (>0.1 mm) near K2's nucleus emitted beyond 4 au may be responsible for its very low gas rotational temperature and the discrepancy between its optical and infrared lights reported at similar heliocentric distances.Comment: Accepted for publication in Astronomy & Astrophysic

The volatile composition of comet C/2017 K2 (PanSTARRS)

We present high resolution spectra of comet C/2017 K2 (PanSTARRS) (hereafter 17K2), obtained using the upgraded high resolution spectrometer of the VLT, CRIRES+. We will show our findings in the (2.8 - 5.3) μm range, searching for primary volatiles (e.g., H2O, HCN, NH3, CO, C2H6, CH4, ...) and studying their evolution as the comet approach the Sun. 17K2 is a long period comet, very active already at record heliocentric distances of 16 au, and represents a unique opportunity to study the composition of a mostly unaltered comet.Comets formed from the material surrounding the proto-Sun about 4.6 billion years ago, and after their formation they were scattered into their current reservoirs [1,2], where the frozen nuclei have preserved most of the chemical and mineralogical properties linked to their formation site until today. Probing the chemical diversity in comets may thus unveil the processes that were in effect within the mid-plane of our proto-planetary disk, and test the hypothesis that comets may have contributed in delivering water and prebiotics to the early Earth [3].Among other techniques, the composition of active comets can be studied from ground based telescopes using high resolution spectroscopy in the infrared (IR - 3 to 5 μm), where it is possible to observe emission lines produced by solar-pumped fluorescence of primary species, i.e., molecules released directly from the nucleus. High spectral and spatial resolutions are necessary to resolve different molecular species in the spectra, to study their distribution within the coma and to separate emission lines of the comet from their counterpart in the atmosphere.Comet 17K2 is in excellent observing conditions in 2022, allowing infrared high resolution studies. The comet shows already activity, probably driven by CO and other hyper-volatiles that can sublimate at distances from the Sun larger than 5 au [4,5]. Discovered in 2017 at about 16 au from the Sun [6], it is most likely entering the inner solar system for the first time, and its observation offers a unique opportunity to study its mostly unaltered material.We will present the results obtained from different spectra acquired using CRIRES+ at ESO-VLT at various epochs. We acquired comprehensive high-resolution spectra of the comet as it progressively moved towards the Sun, with the goal of monitoring the evolution of sublimating material with the heliocentric distance. In particular, we have granted time at the beginning of May, beginning of July, and end of August 2022, with the Sun-17K2 distance varying from about 3.5 to 2.3 au. In this heliocentric range, the comet is crossing the CO to H2O ice sublimation regime [7].Data are reduced using custom semi-automated procedures (see [8] and references therein) that allow a fast analysis of the spectra. Spectral calibration and compensation for telluric absorption are achieved by comparing the data with highly accurate atmospheric radiance and transmittance models obtained with PUMAS/PSG [9]. Flux calibration is obtained using the spectra of a standard star observed closely in time with the comet, and reduced with the same algorithms. Production rates and relative abundances (i.e. mixing ratios with respect to water) of different primary species in the coma are obtained using state-of-the-art fluorescence models (see for example [10] and [11]).The molecular abundances found in this comet will be compared to reference median values retrieved for the comet population [12] and with the abundances found in other Oort Cloud Comets

Pre-perihelion high resolution optical spectroscopy of the long period comet C/2017 K2 (PanSTARRS) with UVES at the VLT

The long-period comet C/2017 K2 (PanSTARRS) was discovered in 2017 at a large heliocentric distance of 16 au (Wainscoat et al. 2017). Pre-discovery images from 2013 show that K2 was even active at a record distance of ~24 au from the Sun (Jewitt et al. 2017) well beyond the snow line, indicating that, most probably, CO and CO2 ices - the most abundant species after water - might drive its activity. CO was indeed detected in K2's coma in the sub-mm range at a heliocentric distance of 6.7 au (Yang et al. 2021) and K2 was claimed to be a CO-rich comet.Detecting comets at such large distances is becoming more frequent, but it is still a rare occasion to study a well preserved comet surface coming directly from the Oort Cloud or on a several million years orbit, and especially if it is of a rare type. K2 will reach its perihelion on 2022 December 19 (Rh=1.8 au, Δ=2.5 au) and become a bright target in automn with good observing conditions from the Southern hemisphere. We have started an observing campaign on May 8 (Rh=3.2 au), 2022 with UVES at the ESO VLT to obtain high resolution and good SNR optical spectra to characterize the detailed coma composition of its daughter species before and after K2 perihelion. We report here about the first epochs before perihelion.UVES was setup with a slit width of 0.45" (length of 10") to provide a resolving power of 80.000, and we selected two different settings (DIC#1 346/580 and DIC2 437/860) to cover the whole optical range (304-1040 nm) at each epoch in only two long exposures on the same night. These spectra will allow us to compare K2 - characterized by its unusual distant activity - to other well studied comets in the optical and particularly using the same instrument since 20 years by the Liège comet team. These spectra will allow us to measure the detailed composition of its coma: the production rates of the daughter species (OH, CN, C2 etc.) to check among other things if the comet is a C-chain depleted or normal comet (A'Hearn et al. 1995), to link those production rates with those from the parent species observed in the IR (see CRIRES+ poster by Lippi et al.), to search for CO+ and CO2+ lines to check if K2 is a CO-rich comet like the unique CO-N2-rich blue comet C/2016 R2 (PanSTARRS) (Opitom et al. 2019), to measure the ratio of the [OI] lines to estimate the CO/H2O ratio (Decock et al. 2015), and if the comet is bright enough to measure the isotopic ratios of the light elements (12C/13C and 14N/15N from the CN isotopologues), to measure the ortho- to para- ratio of NH2, and search for faint FeI and NiI lines which are a new and puzzling component of the cometary coma (Manfroid et al. 2021)

Ground-Based Infrared Detections of CO in the Centaur-comet 29P/Schwassmann-Wachmann 1 at 6.26 AU from the Sun

We observed Comet 29P/Schwassmann-Wachmann 1 (hereafter, 29P) in 2012 February and May with CRIRES/VLT and NIRSPEC/Keck-II, when the comet was at 6.26 AU from the Sun and about 5.50 AU from Earth. With CRIRES, we detected five CO emission lines on several nights in each epoch, confirming the ubiquitous content and release of carbon monoxide from the nucleus. This is the first simultaneous detection of multiple lines from any (neutral) gaseous species in comet 29P at infrared wavelengths. It is also the first extraction of a rotational temperature based on the intensities of simultaneously measured spectral lines in 29P, and the retrieved rotational temperature is the lowest obtained in our infrared survey to date. We present the retrieved production rates (~3 × 10^(28) molecules s^(–1)) and remarkably low (~5 K) rotational temperatures for CO, and compare them with results from previous observations at radio wavelengths. Along with CO, we pursued detections of other volatiles, namely H_2O, C_2H_6, C_2H_2, CH_4, HCN, NH_3, and CH_3OH. Although they were not detected, we present sensitive upper limits. These results establish a new record for detections by infrared spectroscopy of parent volatiles in comets at large heliocentric distances. Until now considered to be a somewhat impossible task with IR ground-based facilities, these discoveries demonstrate new opportunities for targeting volatile species in distant comets

Synergies between ground-based and space-based observations in the solar system and beyond

The goal of this white paper is to provide examples where ground-based and space-based observations are combined, and used to obtain understanding or constrain parameters beyond what the separate measurements could yield

The Comet Interceptor Mission

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum ΔV capability of 600 ms−1. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule