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Unraveling Space Weathering on Planetary and Astrophysical Surfaces

Sede A. Riccò Via Santa Sofia 78, Catania

It is well known that the interaction of energetic ions, electrons and photons with surfaces and interfaces leads to non-thermal desorption via a process typically referred to as desorption induced by electronic transitions (DIET).  When DIET involves either electrons or photons, these processes are generally referred to as electron-stimulated desorption and photon-stimulated desorption (PSD), respectively.  Recent attention has focused on understanding the role of non-thermal “space weathering” in the processing of interstellar grains and ices.
Specifically, there is deep interest in understanding the radiation processing of carbon grains in the solar nebula and unraveling the H2O formation mechanisms in solar nebula and planetary systems, including the Moon. Using graphite grains, the VUV photon-simulated oxidation of carbon grains via reactive scattering of water fragments produced by dissociative electron attachment at the buried interface was examined. The results suggested that VUV PSD at the buried water:carbon grain interface may help control the carbon inventory during planet formation. The inverse process, (i.e, the formation of water) may happen on metal-oxide samples such as mineral grains and lunar regolith samples, that contain or are terminated by hydroxyl groups. Solar wind space weathering experiments of several Apollo lunar samples demonstrated that thermally activated recombinative desorption (RD) can be H2O sources and that electron-stimulated reactive scattering to produce water may also be occurring, especially when the Moon is in the magnetopause.  RD can occur on a diurnal basis on the Moon and is prevalent during meteoroid impacts. The latter is simulated by laser irradiation studies followed by state and velocity resolved detection of the produced water.  Finally, evidence of space weathering and surface alteration has also been revealed using spatially resolved, high-resolution nanoscale Fourier transform infrared imaging/spectroscopy correlated with photoluminescence (PL) on Apollo samples with different origins and history.

Stellar-wind-fed magnetospheres of magnetic massive stars

Sede A. Riccò Via Santa Sofia 78, Catania

A subpopulation (~9%) of hot (OB) stars exhibit strong (B~100-10,000 G), large-scale (often predominantly dipolar) magnetic fields that channel their stellar wind outflows into circumstellar magnetospheres. For young, rapidly rotating B-stars that have not yet been spun down by wind-magnetic braking, wind material can be trapped between the Kepler co-rotation radius (RK) and the Alfven radius (RA), forming then a “Centrifugal Magnetosphere” (CM), with density set at the critical level for “Centrifugal Breakout” (CBO) against the confining magnetic tension. This talk discusses how such CBO controls both the onset and strength of observed H-alpha emission, while the energetics of the associated CBO-driven magnetic reconnection match well the observed scalings of a non-thermal, circularly polarized radio emission from such stars .

Spectroscopic observations and modeling of solar flares: new insights from IRIS

Sede A. Riccò Via Santa Sofia 78, Catania

Recent high-resolution observations from the IRIS satellites have enabled significant advancements in our understanding of the physical mechanisms at play during the impulsive phase of flares, including details of how the non-thermal energy is released and propagated from the corona to the low-atmosphere through accelerated particles. At the same time, the new discoveries have brought to light new unsolved questions and challenges for current models. This talk will provide some examples of the unique contributions to our understanding of flares from IRIS, also in coordination with other solar observatories, and how state-of-the-art heating models of flares can be constrained by the available imaging and spectral diagnostics. I will also discuss some of the outstanding problems in preparation for the next generation of solar missions.

From clouds to fragments: on the multi-scale interplay between gravity and turbulence

Sede A. Riccò Via Santa Sofia 78, Catania

The star formation mechanism occurs in well defined structures that can be identified and studied in great details in our own Galaxy: the process starts in giant molecular clouds, objects extended up to several tens of parsecs, within which elongated sub-structures, called filaments, may form. Inside filaments, round-like condensations extended up to ~1pc in radius, the so-called clumps, are the natural birth site of the pre- and proto- stellar fragments, inside which will origin the future stars.
There are still many open questions in this hierarchical view of the star formation process: are these structures relatively confined from each other, or is the large-scale environment affecting the dynamics of the formation down to clumps and fragments? Is there a continuous interplay of the various forces involved in the process, namely turbulence, gravity (and magnetic fields), at all scales? Or is there a relevant scale at which gravity will start to dominate the collapse, with critical implications on the star-formation mechanism?
After a general overview of the problem, I will present in details some recent results focused on the interplay between gravity and turbulence at the filament, clump and fragment scales. To investigate this interplay at the larger scales, we have combined the dynamics of so-called 70 micron quiet clumps, i.e. very pristine regions not yet strongly affected by feedbacks, with the dynamics of the parent filaments in which they are embedded. At smaller scales, I will discuss the different scenarios of fragments formation in light of the most recent results from the SQUALO (Star formation in QUiescent And Luminous Objects) project. This ALMA survey has been designed to investigate the formation properties in a sample of massive clumps selected to be at various evolutionary stages and with the common feature that they are all accreting at the clump scales.
Our results show that a large scales we observe a continuous interplay between turbulence and gravity, where the former creates structures at all scales and the latter takes the lead above a critical value of the surface density is reached, ~ 0.1 g cm^-2. At the same time, the fragmentation properties show several indications that the fragment are "clump-fed", i.e. the process is dynamical and the gravity dominates the collapse inside our massive clumps.

Lo Spazio come bene comune dell’umanità

Sede A. Riccò Via Santa Sofia 78, Catania

Ad oggi, ci sono in orbita circa 10.000 satelliti operativi (e non meno di 3.000 per così dire “defunti”). Offrono servizi di grande utilità per migliorare la qualità della nostra vita, ma occorre considerare le conseguenze della crescita esplosiva nel loro numero. Benché operino nello spazio, i satelliti hanno infatti un impatto non trascurabile sulla vita e sulle attività che si svolgono sulla Terra e sulla qualità della nostra atmosfera. Lo spazio è parte integrante dell’ecosistema terrestre e prima ce ne renderemo conto meglio sarà.

Status and future of 21-cm cosmology during the first billion years

Sede A. Riccò Via Santa Sofia 78, Catania

The 21-cm hyperfine line of neutral hydrogen is set to revolutionize studies of the first billion years, spanning the cosmic dawn of the first stars and eventual reionization of our Universe. I will discuss the potential of this probe in learning about the unknown astrophysics of the first galaxies as well as physical cosmology. Current upper limits on the cosmic 21-cm power spectrum already provide new insights into the heating of the intergalactic medium, and the X-ray sources in the first galaxies. I will discuss the upcoming steps, including the main challenges, that will eventually lead to the Nobel prize-worthy 3D map of half of our observable Universe with the Square Kilometer Array (SKA) telescope.

How is magnetism affecting the properties of solar and stellar acoustic modes?

Sede A. Riccò Via Santa Sofia 78, Catania

Outside of solar neutrinos, the only way to directly probing solar and stellar interiors is to use seismic techniques, i.e. studying the waves propagating inside them. In the case of the Sun, acoustic waves are excited by turbulent motion in the convective envelope, and propagate towards the interior, creating a variety of standing pressure modes (p modes). By investigating how small perturbations influence the modes parameters, it is possible to probe the structural and dynamical properties of the star such as internal rotation and mixing, chemical composition, density, convection zone depth, etc. In the stellar case, asteroseismology allows the inference of the stellar fundamental parameters such as mass, radius, and age. Although, solar p-modes frequency, amplitude, and energy vary in relation with the solar magnetic cycle and similar variations were observed for other magnetically active solar-type stars, such a variability is often overlooked in stellar modelling. In the context of the preparation of the PLATO mission, whose aim is to characterize Earth-like planets orbiting solar-like stars in part thanks to asteroseismology, we need to better understand the relation between magnetic variations and modes parameters. In this seminar, I will focus on the excitation of the solar p modes using the last 28 years of data from the SoHO/GOLF instrument, with a method gathering a better temporal resolution compared to classical approaches. In this framework, I was able to perform a statistical study of the energy of the modes. Summing the energy of all studied modes, I will demonstrate that there is a discrepancy between the observed excitation rate and the expected rate under the hypothesis of excitation driven entirely by turbulent convection. I will discuss the link between this discrepancy and surface magnetism effects such as flares and coronal mass ejection. In conclusion, I will explain how a better understanding of the relation between the dynamo mechanism and modes properties variation across time would allow us to improve the constraint we have on stellar dynamics and obtained refined stellar fundamental parameters.

Probing Impacts of Stellar Variability within HST WFC3/STIS and Ariel Tier 2 and Tier 3 Observations with Activity Metrics

Sede A. Riccò Via Santa Sofia 78, Catania

Stellar activity produces two main diagnostics within low-resolution exoplanet transmission spectra. Its highly chromatic nature imparts trends in the underlying spectrum that become most noticeable at shorter wavelengths due to the increased contrast between the flux from the active regions (i.e. spots and/or faculae) and the quiet photosphere. The second characteristic is that activity is inherently time-variable, predominantly modulated by stellar rotation as active regions rotate into and out of view but also with contributions from longer timescales of evolution/decay and even long-term activity cycles e.g. maxima and minima (although certain configurations e.g. polar spots and active latitudes will be more resistant to this). This variability can reveal itself through subsequent observations of the system and is both useful and challenging; it can further help us to identify and constrain potential contamination but also means that observations at different epochs may require individual corrections before they can robustly be combined and analysed together which will become increasingly difficult for smaller planets as the SNR of each visit progressively decreases. In this seminar I will present how both diagnostics have been used with archival HST WFC3 and STIS datasets to create two new activity metrics that are highly complementary to existing indicators. I will also show preliminary results surrounding how this work is being extended to Tier 2 and Tier 3 Ariel simulations to explore how stellar variability will impact our ability to stack visits to obtain the required SNR for these tiers.

Non-standard signatures from CMB polarisation with an insight into the new technological challenges

Sede A. Riccò Via Santa Sofia 78, Catania

In this seminar, I will focus on non-standard signatures from CMB polarisation that may indicate the existence of new phenomena beyond the standard models of cosmology and particle physics, from both theoretical and observational perspectives. ESA's Planck mission has observed CMB temperature anisotropies at the cosmic variance limit, but polarisation remains to be further investigated. CMB polarisation data are important not only because they contribute to provide tighter constraints on cosmological parameters but also because they allow the study of physical processes that would be excluded if only the CMB temperature maps were considered. I use polarisation data into account to assess the statistical significance of the anomalies currently observed only in the CMB temperature map, and to constrain the Cosmic Birefringence (CB) effect, which is expected in parity-violating extensions of the standard electromagnetism.
Measuring CMB polarisation is technically challenging because the polarised signal is much fainter than the temperature signal, and accurate polarisation estimates require exquisite control of systematic effects. To investigate the impact of spurious signals in upcoming CMB polarisation experiments, I present a study of the interplay between half-wave plate (HWP) non-idealities and the beams of the instrument for the next generation of CMB experiments, with an insight into how this instrumental contamination affects the measurement of the cosmic birefringence effect.

Line shape modeling for the characterization of magnetized plasmas in fusion research and astrophysics

Sede A. Riccò Via Santa Sofia 78, Catania

In this talk, we report on a selection of issues present in the elaboration of spectroscopic models for magnetized plasma diagnostic. A focus is put on tokamak edge and white dwarf atmosphere plasma conditions. In both cases, the observed spectra exhibit clean lines, either in absorption or in emission, denoting the presence of neutral species (atoms). An analysis of the line shape and intensity yields information on the plasma parameters provided a suitable physical model is used. We first discuss in detail the physical mechanisms underlying the broadening of the spectral lines due to the plasma microfield (Stark effect), and we next report on models and codes that have been developed in our group at the PIIM laboratory. Applications to the fitting of observed spectra – both in magnetic fusion and astrophysics – are presented. Motivated by current research trends, we also discuss specific issues related to the presence of strong external magnetic field (Zeeman effect etc.) and report on some recent works done in atomic physics for the improvement of line shape models.