Talks, Seminars, Events

Abstract

Although most models aiming to synthesise the observed exoplanet architectures consider a protoplanetary disc evolving in isolation, this does not reflect the physical reality for many discs. Since stars form in regions of enhanced stellar density, feedback mechanisms play an important role. In particular, UV fields heat the disc and drive a photoevaporative wind, depleting the disc and decreasing the dispersal time-scale. I review the demographics of local star forming regions, and compare them to those regions that are the focus of observational studies of protoplanetary discs, demonstrating that well-studied discs are not typical. I present the evidence for the depletion of discs in various regions by external photoevaporation. Finally, I discuss how, as well as being an important consideration in understanding exoplanet populations, external photoevaporation can be used as a probe of both star formation and disc physics.

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The mass is the most substantial parameter of a star. It defines its temperature, surface gravity and evolution. Currently, relations concerning stellar mass are based on binary stars, where a direct mass measurement is possible.

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Hence, direct measurements of stellar masses for single stars are important.

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Astrometric microlensing is the ideal tool for this purpose. The positional shift of the background star is proportional to the mass of the lens. However, precise astrometric measurements are required. Such measurements are done by Gaia. 

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In this talk, I want to show the potential of the Gaia satellite to determine Stellar masses. 

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Using the second Gaia data release (Gaia DR2), we predicted 501 astrometric microlensing events during the Gaia era (2014.5 - 2024.5). For this events, we simulated the individual Gaia measurements using conservative assumptions, based on the results from  Gaia DR2. We found that Gaia can measure the astrometric shift for 114 events.  For 34 events, the stellar mass can be determined with a significance larger than 3$\sigma$ and up to 15$\sigma$. These results can be further improved by combining the Gaia data with a few external observations. 

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The accretion of Earth is a mysterious era in geologic history without any surviving rock record. However, this is when Earth's bulk geochemistry and geophysical structure was established. Using sophisticated astrophysical-geological modeling, we can reconstruct this era, date important events such as the Moon-forming event, and determine basic characteristics of the nascent protoplanetary disk. By considering the consequences for Earth, we can better understand the stark contrast of Venus--a planet without a planetary magnetic field, without plate tectonics, and without a Moon. This earliest eon which lies at the intersection of astrophysics and geology had profound consequences for life on Earth that are just beginning to be understood.

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In dense regions of the sky ("crowded fields"), the standard Gaia operations scheme cannot cope with such an amount of overlapping stars. This has always been a known, unavoidable property of Gaia. However, around launch it was proposed to create special images from Gaia's Sky Mapper CCDs to fully cover crowded fields down to the detection limit, although at reduced measurement precision. During the development of Gaia, such images had been designed for technical purposes, but never been planned to be used for science. These images were recorded starting July 2014, i.e. right from the start of the scientific mission, and were just stored away for the time being. In early 2019 a small working group was set up within the scientific Gaia consortium (DPAC) to work on this topic. The talk will summarize the first steps and initial software solutions, the current status of the corresponding processing pipeline, as well as an outlook to the scientific results.

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KoCo Signature Speaker

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AGN feedback is now widely considered to be one of the main drivers in regulating the growth of massive galaxies. In my talk I will describe several efforts to understand the power, reach and impact of AGN feedback processes. Using SDSS-IV MaNGA at low-z, we found that AGN signatures can be easily hidden in the integrated spectrum of a galaxy. At higher redshift, we find that outflows can indeed suppress star formation in their hosts, consistent with the AGN having a `negative' impact. However, both star formation and quasar activity peak at z ~ 2-3 where AGN are expected to impact the build-up of stellar mass the most. Our team recently discovered a unique population of luminous high-z quasars (ERQs) with extreme outflow properties. ERQs are ideal to obtain a census of the overall mass and energy budget of both outflow and infall/feeding from the CGM, an essential requirement to probe the detailed and full feedback loop. I will also introduce the JWST ERS Program "Q3D" which we will study the impact of three carefully selected luminous quasars on their hosts. Our program will serve as a pathfinder for JWST science investigations in IFU mode.

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Giant molecular clouds (GMCs) form the communication channel between the galactic environment and the sub-cloud physics of star formation and stellar feedback. Recent observations reveal galaxy-scale trends in GMC properties (e.g. surface densities, velocity dispersions, and turbulent pressures), demanding a systematic and predictive theory of the GMC lifecycle across the parameter space of observable star-forming environments. In this contribution, I will combine a simple analytic theory for the influence of galactic dynamics on GMC lifetimes/evolution with a statistical sample of ~60,000 numerically-simulated GMCs across three isolated disc galaxies in the moving-mesh code Arepo. The analytic predictions depend on just three sets of galactic-scale observables: the rotation curve, surface densities and velocity dispersions of the host galaxy. The numerical simulations incorporate the features required to accurately model the influence of stellar feedback on the ISM, including stochastic star formation, mechanical supernova feedback, pre-supernova feedback from HII regions, and ISM chemistry. Using this combined analytic/numerical framework, I will show (1) that stellar feedback truncates the GMC lifetime across a wide range of galactic environments, and (2) that the lifetimes and properties of GMCs are predicted to follow clear galactic-dynamical trends driven by gravitational instability, galactic shearing, radial orbital perturbations, and cloud-cloud collisions. These results reveal that the complex interplay between galactic dynamics and stellar feedback plays a crucial role in setting the course of GMC evolution, and therefore in determining the star formation rates of the galaxies they live in.

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KoCo Signature Speaker

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We explore the stellar mass density and colour profiles of 118 low redshift, massive, quiescent, central galaxies in order to attempt to find hints of the minor merger activity postulated to be the driver behind the size growth of MASSIVE galaxies since high redshift. We use deep imaging data (down to ~26 mag) in 5 bands from the Subaru Hyper Suprime Cam survey combined with Voronoi binning to maximise signal to noise, and perform SED fitting to construct spatially well-resolved colour and stellar mass density radial profiles, stacking these profiles to utilise the full statistical power of the sample. We find slowly decreasing colour profiles, and an expected smooth, declining stellar mass density profile in the central regions of our sample (~3 Re), however excesses of stellar mass density in the outskirts in the form of bumps in the profile. By visually inspecting the data, we morphologically split the sample, finding that almost 45% of the sample display signs of minor merger activity, 45% display a diffuse stellar envelope, possibly indicative of previous minor merger activity and the rest display no activity. We find that the excesses of stellar mass in the outskirts are driven by those galaxies with clear signs of minor merger activity or a diffuse stellar halo, and that this material only contributes to a few % of the total stellar mass. We also apply similar techniques to the data as in previous studies, finding that these techniques smooth out the signatures of these interactions.

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The presence of a gas volume-density gradient inside star-forming regions allow them to raise their star formation rate (SFR) compared to what they would experience if their gas was of uniform density. I define the “magnification factor 𝝵” as the ratio between the SFR of a centrally-concentrated clump and the SFR that this clump would experience should its gas be uniformly distributed. I show that magnification factors higher than 10 are achieved by power-law gas density profiles with logarithmic slopes steeper than -3. Such steep density profiles describe well the densest regions of the nearby molecular clouds MonR2 and NGC 6334. Clumps with a high magnification factor form stars much faster than expected based on the mean free-fall time of their gas. Therefore, not only does a gas density gradient inflate the clump SFR, it also inflates the star formation efficiency per free-fall time that we measure. The diversity in measured star formation efficiencies per free-fall time thus partly reflects the diversity in star-forming region structures. I provide a method to quantify the contribution of the gas density gradient of a clump to its SFR, thus allowing one to estimate its magnification factor.

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Blocked for talk related to ROSE/IAU OAE.

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