1,721,088 research outputs found
Cometary science with CUBES
Full text linkThe proposed CUBES spectrograph for ESO's Very Large Telescope will be an exceptionally powerful instrument for the study of comets. The gas coma of a comet contains a large number of emission features in the near-UV range covered by CUBES (305-400 nm), which are diagnostic of the composition of the ices in its nucleus and the chemistry in the coma. Production rates and relative ratios between different species reveal how much ice is present and inform models of the conditions in the early solar system. In particular, CUBES will lead to advances in detection of water from very faint comets, revealing how much ice may be hidden in the main asteroid belt, and in measuring isotopic and molecular composition ratios in a much wider range of comets than currently possible, provide constraints on their formation temperatures. CUBES will also be sensitive to emissions from gaseous metals (e.g., FeI and NiI), which have recently been identified in comets and offer an entirely new area of investigation to understand these enigmatic objects
Monitoring of the chemical composition of comets in the framework of the TRAPPIST survey
Full text linkComet taxonomies: composition-based classifications and a search for comets in the Main Belt
No full textComets are icy small bodies assumed to have remained mostly unaltered since their formation, making them key tracers of the early stages of the Solar System. While comets show a great diversity of dynamical, physical and chemical properties, efforts have been deployed to establish classifications based on these properties, with the aim of identifying different formation and evolution histories. On the one hand, from a dynamical standpoint, it has been found that comets exist in two main reservoirs before being deflected towards the inner Solar System: the Oort Cloud and the Kuiper Belt. However, a third reservoir has recently been identified as some comets have been found in the Main Asteroid Belt. Blurring the traditional divide between asteroids and comets, too few of these objects are known to understand their origin and properties. On the other hand, by quantifying the composition of the gas produced by comets, it has also been shown that classes could be established based on a high or low C₂-to-CN abundance ratio. However, the lack of a clear correlation between carbon-chain depletion and dynamical origins make this divide puzzling. Moreover, previous authors report a decrease of the measured C₂/CN ratio with the heliocentric distance of comets at the time of observation, suggesting that our understanding of C₂ production in comae might be incomplete and that C₂ based taxonomies could be biased. Since these studies typically cover short heliocentric distances (<2au) and different authors do not use consistent modelling parameters (in particular photodissociation scalelengths) to derive these abundance ratios, it is difficult to compare their findings and assess these effects. This thesis looks to bring new insights into these two challenges to established comet classifications.
First, I present a survey of comet volatiles using optical long-slit spectroscopy, aiming to investigate trends and biases in observed compositions. Spectra were acquired for 35 comets using the Isaac Newton Telescope’s Intermediate Dispersion Spectrograph. Having produced a semi-automated pipeline to reduce and analyse this large volume of data, I calculated production rates and upper limits for the main volatile species visible in the near-UV/optical range: OH, NH, C₂, CN, C₃, CH. I present a more focussed analysis of a few targets of interest such as C/2023 H2 (volatile rich) or 12P (outbursting), as well as ensemble results from the study. From these production rates, derived using a Haser outgassing model and commonly used photodissociation scalelengths, I find C₂/CN ratios consistent with a decreasing trend up to 3.5au, making most comets that were observed beyond 2au fall below the depletion threshold. I show that a correlation with perihelion distance is also possible, although I cannot clearly disentangle these two factors. When possible, I also determine and model the spatial distributions of volatiles as seen along the slit and show that a Haser model using literature scalelengths often does not reproduce the measured C₂ profiles, while CN and C₃ show a better agreement between models and observations. Using adjusted scalelengths yields larger C₂ abundances than using literature values, although it could not be determined whether this eliminated heliocentric trends.
Additionally, this thesis presents an imaging survey searching for activity in targeted Main Belt Asteroids in the hope of finding more Main Belt Comets. Using the Isaac Newton Telescope’s Wide Field Camera, r-band observations of 534 asteroids were conducted. These targets were selected based on their closeness to perihelion at the time, and on a hypothesis from previous authors that Main Belt Comets would more likely be found among objects with a longitude of perihelion close to that of Jupiter. After applying wedge photometry and point-spread function analysis methods to detect activity features via an automated pipeline, I made a candidate tail detection on images of asteroid 2001 NL19 (279870). Follow-up observations were conducted with the Liverpool Telescope at the asteroid’s following perihelion but I did not detect recurring activity, implying that the activity of this objects might not be cometary
Characterising small exoplanets
No full textIt was only thirty years ago that the first extrasolar planet, or exoplanet, orbiting a Sun-like star was discovered. Since then (as of October 2025), 6,022 have been
confirmed across 4,490 planetary systems, 1,013 of which host multiple planets. Whilst these exoplanets have been discovered through a range of methods, transit
photometry and radial velocity measurements have proven the most effective, accounting for∼96% of confirmed exoplanet discoveries. Through these two techniques, planetary radius and mass can be constrained to high precision. From these two parameters, planet density can be derived, enabling estimates of both atmospheric and internal compositions. Characterising small (<4 R⊕) exoplanets in this way is crucial for inferring the frequency of true Earth-analogues and assessing the uniqueness of our own planet.
However, there are several compositional trends for small exoplanets that remain poorly understood. The first is the ‘radius valley’ that separates super-Earths and sub-Neptunes, which has been consistently observed from ∼1.5–2 R⊕, and is largely without planets. Debate currently surrounds the origin of this gap, with proposed scenarios including core-powered mass-loss, photoevaporation, or that these planets are primordially rocky. Interpretations differ on the physical mechanism of atmospheric mass-loss, but the result is the same – primordially accreted atmospheres are removed in such a way that different planets are affected in different ways over different timescales, resulting in a ‘valley’ that separates a population of stripped-core planets (super-Earths) from those that have retained their H/He envelopes (sub-Neptunes). Secondly, the internal structure of sub- Neptunes is not just limited to that of a rocky core surrounded by a gaseous atmosphere, it has been theorised that these planets might hold significant fractions of ices or liquid water. It has been suggested that the radii of planets hotter than 900 K and with masses below 20 M⊕ can be reproduced assuming ice-dominated compositions without significant gaseous envelopes. However, it has also been argued that the existence of small planets with hydrogen atmospheres is consistent with the data, once thermal evolution and mass-loss are properly accounted for. This means that there is a strong degeneracy between water-world and silicate/iron-hydrogen models, and that the characterisation of larger sub-Neptunes in this region of the mass–radius diagram can be used to determine planetary evolution and formation pathways.
With our understanding still limited regarding the origins of these compositional trends, taking steps towards improving characterisation methods of bodies and systems in this size range is vital. Improving our understanding of the origins of the radius valley and the diverse pathways of planetary development will finally help us to ascertain the uniqueness of our own solar system and planets, which is a question that humanity has attempted to answer since the beginning of time
Orbital architectures of multiple-star systems that host transiting planets
No full textMultiple-star systems provide unique opportunities to study the environments in which planets would have formed. This is under the conservative assumption that planets form after stars, and therefore, the stellar orbits that sculpted the protoplanetary environment are the same orbits observed today. One of the characteristics that could give insights into the formation mechanism of planets in multiple systems is the alignment of the planetary and stellar planes. Alignment of these could indicate that close binaries and their disks preferentially begin as coplanar systems. Transiting planets allow this alignment to be investigated as they must be in edge-on orbits in order to transit their host star. The orientation of the stellar planes in comparison to an edge-on orbit can then be used to test the alignment in these systems.
In this thesis, I present the results of a long-term monitoring programme of Kepler Objects of Interest (KOIs) with at least one stellar companion. The survey uses adaptive optics imaging and non-redundant aperture masking from NIRC2 at the Keck Observatory to monitor the position and separation of planet-hosting multiple-star systems over many epochs. In Chapter 2, I detail the reduction and analysis techniques used to go from raw images of the multiple-star systems to measurements of the separation and position angle of the companions relative to the primary star. With this astrometry, I aim to demonstrate the alignment of stellar planes in both binary and triple-star systems compared to the edge-on orbit of transiting planets.
In Chapter 3, I present the results from the orbit survey for close binaries with a separation of less than 1000 au. I demonstrate how the astrometry of partial orbital arcs can be used to calculate an angle γ. This angle can be used as an indication of the alignment, as edge-on stellar orbits have γ ∼ 0° and face-on orbits have γ ∼ 90°. Low values of γ can also be explained by misaligned systems,
such as ones with high eccentricity. For this reason, the alignment is tested with a statistical sample where an overabundance of low values of γ indicates alignment. In this chapter, I present the γ distribution found for the sample of binaries, which indicates an overdensity of γ values close to 0° and hence illustrates alignment between the stellar and planetary orbits. Finally, I describe how this distribution can be compared to simulated γ results for different alignment scenarios, such as if the orbits of the planet and stellar companion are independent, to statistically determine the significance of the results.
In Chapter 4, I highlight the systems in the orbit survey that have two stellar companions, creating the first-ever statistical sample of orbits of planet-hosting triple systems. I describe how the methods differ when a second stellar orbital plane is introduced in these hierarchical systems. I perform a similar analysis to the binaries, but for each triple system, I calculate two γ values, one for the inner stellar binary and one for the outer stellar companion. For the nine most compact triples, I again calculate a γ distribution and compare it to simulated distributions and the distribution found for the binaries. The statistical sample demonstrates that the observed trend of stellar-planetary orbit-orbit alignment in binaries does not appear to extend to higher-order multiples and that the triples are not completely coplanar systems.
In Chapter 5, I describe how the astrometry for partial orbital arcs can be used for full orbital analysis by fitting complete orbits and calculating posterior distributions for all the orbital parameters. The inclination of the stellar plane can be used as an additional separate measure for the planetary-stellar alignment. However, as the position of the ascending node for the planet is unknown, this again needs to be used as a statistical sample looking for an overabundance of low mutual inclinations. I describe and execute this method for both the binaries and the triple-star systems, highlighting the differences in the methods. Finally, I conclude that for both the binaries and the triples, there is more mutual alignment between the stellar and planetary planes than could be explained by random isotropic orbits where the orbital planes are independent of each other
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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