Galaxy history is marked by a peak of star formation ten billion years ago and a subsequent drop of the star formation rate (SFR) by an order of magnitude. To understand this evolution, it is crucial to probe the gas reservoirs from which stars are formed. With programs observing the molecular gas phase in typical star-forming galaxies at different epochs, I will present how the cosmic evolution of the SFR is mainly driven by that of the molecular gas fraction. The depletion time associated to star formation indeed only weakly changes with redshift, both at galactic and at sub-galactic scales. I will show that the molecular gas content during the winding-down of star formation does not seem to correlate with morphology, suggesting an ongoing supply of molecular gas to compensate for star formation while bulges grow. In contrast, molecular gas reservoirs can be dramatically depleted in extreme environments such as cluster centres. While structure formation is primarily driven by dark matter (DM) dynamics in ΛCDM cosmology, gas processes can in turn affect the DM distribution at galactic scales. Using theoretical modelling and simulations, I will discuss how outflow episodes and gas density fluctuations induced by stellar feedback can expand both the DM and the stellar distributions and hence provide a simple understanding of the formation of DM halo cores and ultra-diffuse galaxies.
Kora Muzic (CENTRA, University of Lisbon, Portugal)
With masses between those of stars and planets, brown dwarfs represent a critical link between the two classes of objects. Understanding their origin has been a major motivation for recent deep studies of star-forming regions and clusters as well as a driver for the development of state-of-the-art simulations. Deep surveys show that brown dwarfs are a ubiquitous outcome of star formation, with about 0.2-0.5 sub-stellar objects formed for each star. One of the big questions in brown dwarf studies is whether the birth environment affects their formation efficiency, as predicted in several formation theories. The expectation is that high gas or stellar densities, as well as the presence of massive OB stars, may be factors that boost the incidence of newly formed brown dwarfs with respect to stars. To test these scenarios, we compare the findings of our decade-long deep survey SONYC, in which we characterized sub-stellar populations of several nearby star-forming regions, with the results of our new investigations of sub-stellar objects in the massive young clusters RCW 38 and NGC 2244, characterized by drastically different star-forming environments. Here I will present the current status of young brown dwarf studies, compare the low-mass Initial Mass Functions in a variety of Milky Way environments, and outline the implications of these results for our understanding of sub-stellar formation processes.
Inverse methods have become a fundamental analysis technique for modelling exoplanetary atmospheres. This technique explores a variety of potential bulk and atmospheric model parameters that combine to best-fit an observed spectrum. TauREx (Tau Retrieval of Exoplanets) is a Bayesian retrieval suite designed to be applied to spectroscopic observations of extrasolar planetary atmospheres. We have adapted TauREx for analysis of near-infrared spectrophotometry from a variety of directly-imaged gas giant exoplanets and brown dwarfs. This includes the HR 8799 system, Beta Pic b, 51 Eri b, and PSO 318 observed using instruments such as SPHERE, GPI and GNIRS. We also perform retrieval analysis on L and T dwarf spectra from the IRTF SpeX Spectral Library. This analysis returns estimates for target mass, radius, surface gravity, temperature-pressure structure and cloud properties, as well as confirming and constraining the presence of a variety of molecular species including H2O, CO and CH4. Inverse techniques have mainly been applied to transit spectroscopy in the past and as a result, this is the first application of retrieval analysis to many directly-imaged targets. This project aims to help bridge directly-imaged exoplanet observations with robust, efficient and precise characterisation. The development and adaptation of this retrieval tool is timely and relevant given the upcoming launch of JWST. In this talk I will present a summary of the novel retrieval results from this project.
Interstellar complex organic molecules (COMs) were first detected toward so called hot cores around high-mass protostars. The high temperatures that prevail in these regions boost gas-phase chemical complexity via thermal desorption of the complex species trapped in dust-grain ice-mantles. The COMs detected in the gas-phase are excellent diagnostic tools of the physical conditions of their environment. In this context, the high sensitivity and high angular resolution provided by the Atacama Large Millimeter/submillimeter Array (ALMA) is a strong asset to investigate the physical and chemical structure of the envelope of young high-mass protostars. I will present the results from the analysis of two ALMA projects, the 3mm imaging line survey Exploring Molecular Complexity with ALMA (EMoCA) and the Search for high-mass Protostars with ALMA up to Kiloparsec Scales (SPARKS), to show that hot cores exhibit a great diversity in their chemical composition and physical structure. I will also show how by combining observations with astrochemical models we can study the building-up of chemical complexity toward high-mass star forming regions.
The formation of stars in galaxies is regulated by the amount of cold gas that can fragment into small clumps. Interstellar dust grains play a key role in this process shielding light and providing sites for molecule formation. Dust grains are created and destroyed during galaxy evolution and how the current dust content relates to other internal properties, and to the environment, has been the aim of recent infrared survey and studies. I will outline the basic scaling relations involving dust and gas in star forming galaxies as well as some surprising recent results that suggest possible variations of the grain physical properties with local conditions in galaxy disks.
Edvige Corbelli
INAF-Osservatorio di Arcetri
(Firenze – Italy)
A broad effort is ongoing with large spectroscopic surveys such as APOGEE, Gaia-ESO, GALAH, LAMOST from which stellar parameters, radial velocities and detailed chemical abundances can be measured for CoRoT, Kepler, and K2 targets. In addition, asteroseismic data of red-giants stars observed by the space missions CoRoT and Kepler allow determination of stellar masses, radii, and can be used to determine the position and ages of stars. This association between spectroscopic and asteroseismic constraints provide a new way to understand Galactic and stellar evolutions.
To exploit all potential of this combination and the large number of observations, it would be crucial to develop our approach of synthetic stellar populations. We develop the Besançon Galactic model (BGM) for which stellar evolution predictions providing the global and asteroseismic properties, as well as the surface chemical abundances along the evolution of low- and intermediate-mass stars are included. For the first time, the BGM can explore the effects of an extra-mixing occurring in red-giant stars. These synthetic populations can be compare with the significant large surveys as APOKASC (APOGEE -Kepler) or CoRoGES (CoRoT-GES).
I will present the evolution of red-giant stars, computed with the stellar evolution code STAREVOL including the effects of different hydrodynamical processes on the surface chemical abundances as well as on the Galactic chemical evolution of helium-3.
I will also present the first comparison between synthetic populations computed with the Besançon Galaxy model and the last release of the Gaia-ESO survey, exploring the efficiency of extra mixing with the stellar metallicity, mass, and age (Lagarde et al. 2019).
We emphasize the usefulness of population synthesis tools to test stellar models and transport processes inside stars. We show that transport processes occurring in red-giant stars should be taken into account in the determination of ages for future Galactic archaeology studies.
Star formation is a key astrophysical process. At small scale, it governs the settling of planetary systems, and the complex chemistry allowing the emergence of life. At large scale, star formation controls the energy budget of galaxies. Our knowledge about star formation is both extensive and extremely coarse. In the one hand, we know well the different steps that lead from a molecular cloud to a single star, but in the other hand, the physical processes behind those steps are, at best, debated. Building on the empirical scenario I will review some key physical processes and show the differences/similarities between low mass, and high-mass star formation.
Titre : MHD non-idéale pour la formation d’étoile
Abstract :
Les effets de MHD non-idéale jouent un rôle primordial lors de la formation d’étoile et de disque. En découplant le champ magnétique du gaz, ils affaiblissent le freinage magnétique et les jets protostellaires, processus réduisant le moment cinétique du système. La force de ces effets dépend de l’ionisation du gaz et des grains, et donc de la chimie à l’oeuvre. Je montrerai l’impact de la distribution en taille de grains sur les effets de MHD non-idéale et sur le moment cinétique du gaz lors de l’effondrement protostellaire. Je présenterai également de nouvelles méthodes permettant de calculer rapidement la coagulation et l’ionization des grains.
Titre : Tides in multi-layer close-in planets
Abstract :
Many rocky exoplanets are detected on close-in orbits. For these planets, tides play an important role by changing their orbit and their rotation. In most studies on the orbital and rotational evolution of planets, simple tidal models have been used. But they are known to have flaws, in particular to correctly describe small rocky planets.
Title: How to detect extraterrestrial life: Laser-based Mass Spectrometry instruments on planetary science missions
Speaker: Dr. Niels F.W. Ligterink (SNSF Ambizione Fellow Space Research & Planetary Sciences division, Physics Institute, University of Bern)
Abstract: Life may exist or have existed on various planets and moons in our Solar System, such as Mars, Europa, or even Venus. Detecting extinct or extant life on these objects is challenging and, among other things, requires extraordinary instruments to do so. In this colloquium I will present the space instruments and methods that we develop at the University of Bern (Switzerland) to detect the chemical signatures of life. I will discuss the functioning of our Laser-based Mass Spectrometers, the types of biosignatures that we will detect with these instruments, and how we will employ them on space exploration missions.