STRW local: PhD Colloquia

PhD Colloquia

This week's PhD colloquia are highlighted.

11:00Simulating the birth environment of circumstellar discs
Circumstellar discs are the reservoirs of gas and dust that surround young stars and have the potential to become planetary systems. Their evolution will determine the time and material available to form planets. Studying the evolution of circumstellar discs can then help us understand planet formation and the diversity of observed planetary systems. These discs develop almost immediately after star formation, as a direct consequence of the collapse of a molecular cloud and angular momentum conservation. Their surroundings are rich in gas and neighbouring stars, which can be hostile to the discs and affect their evolution in different ways: dynamical encounters with nearby stars can truncate the discs; stellar winds and supernovae explosions can truncate, tilt, or completely destroy the discs; and the presence of bright, massive stars in the vicinity of circumstellar discs can heat their surface enough to evaporate mass from them. This process, known as external photoevaporation, is arguably one of the most important environmental mechanisms in depleting mass from young circumstellar discs. In this talk I will present the results obtained during my PhD research. My work consisted in simulating the early evolution of circumstellar discs in star clusters and the effects of the environment, in particular truncations due to close encounters and photoevaporation. My results show that photoevaporation is extremely efficient in removing mass from the discs, greatly limiting the amount of material and time available to form planets.
Francisca Concha-Ramirez Leiden Observatory
11:00Cold gas in distant galaxies
The formation and evolution of galaxies is fundamentally driven by the formation of new stars out of cold gas. Observations of young stars in distant galaxies in the early universe, such as we can see in the Hubble Ultra Deep Field, have unveiled how the cosmic star formation rate density evolves. Yet, while the effect of star formation (the young stars) has been mapped in ever-increasing detail, the cause (the cold molecular gas that fuels star formation) has been elusive. In this talk, I will present my thesis work, that involved an observational study of the cold interstellar medium of distant galaxies in the early universe, using the most sensitive submillimeter telescope to date, the Atacama Large Millimeter Array, together with new integral-field spectrographs, such as the Multi Unit Spectroscopic Explorer on the Very Large Telescope. I will present the physical properties of star-forming galaxies and their molecular gas reservoirs, and describe the evolution of the cosmic molecular gas density; the fuel for star formation.
Leindert Boogaard Leiden Observatory
 Feedback by massive stars: Velocity-resolved [CII] observations of the Horsehead Nebula and Orion's Dragon
Stellar feedback is a crucial ingredient in the evolution of galaxies. Massive stars disrupt their natal molecular clouds and perturb the ambient interstellar medium, not only when they explode as supernovae but also during their lifetimes by stellar winds and radiation. The irradiated, heated, and stirred gas cools through the emission of fine-structure lines. The far-infrared fine-structure line of ionized carbon is the dominant coolant of warm neutral gas and has been suggested as a powerful tracer of the star-formation rate (a derivative of stellar feedback) in distant galaxies. In this talk I present my thesis work, that explores the origins of the [CII] emission and quantifies stellar feedback observationally in local samples. I have used high-resolution (both spectral and angular) observations of the [CII] line obtained with the upGREAT instrument onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) of the Horsehead Nebula and the Orion Nebula at the surface of the Orion molecular cloud. This nearby template star-forming region allows to study physical properties of the irradiated gas and to precisely determine the amount of kinetic energy deposited in the expanding bubbles surrounding the massive stars.
Cornelia Pabst
14:00Protostellar jets and planet-forming disks: witnessing the formation of Solar System analogues with interferometry
I will present my thesis results, focusing on characterizing components of young protostellar systems, most notably their jets and disks. Using observations with the ALMA and VLA interferometers, we observed the environments where the first stages of star and planet formation occur. We revealed information on crucial chemical tracers of various protostellar systems components. With a particular focus on molecular jets, I show differentiation in chemical composition between the fast jet and the low-velocity outflow. For the first time, we were able to compare dust masses of young disks with older disks. By comparing this information with masses of the extrasolar planets detected so far, I showed that the solid cores of gas giants must form in the first 0.1 Myr of stellar life. That is an important time constraint that pushes the onset of planet formation earlier and highlights the importance of characterization of the youngest protostars in understanding the origin of Solar System and Earth.
Lukasz Tychoniec Leiden Observatory
11:00Focal-plane wavefront sensors for direct exoplanet imaging
The direct imaging and characterization of rocky exoplanets around nearby stars with extremely large telescopes is one of the goals of modern astronomy. For ground-based telescopes this is very challenging due to uncorrected wavefront aberrations caused by the Earth's atmosphere, the telescope's structure and optics in the instrument. To his end, extreme adaptive optics (xAO) systems have been developed, which have been successful in correcting atmospheric wavefront errors. However, current xAO systems have trouble measuring aberrations caused by optics downstream of the wavefront sensor and the "low-wind effect", which are among the current limitations in direct exoplanet imaging. In this colloquium I will show how I use focal-plane wavefront sensing to measure and correct these aberrations. I will start with an introduction on focal-plane wavefront sensing and discuss its specific challenges. New focal-plane wavefront sensors are presented that are fully integrated with other subsystems to enable highly efficient science observations. Finally, I will show successful on-sky tests of these focal-plane wavefront sensors with the SCExAO instrument at the Subaru telescope.
Steven Bos
11:00It’s just a phase: high-contrast imaging with patterned liquid-crystal phase plates to facilitate characterization of exoplanets
The focus of exoplanet research is transitioning from finding exoplanets to characterizing their atmospheres. High-contrast imaging (HCI) enables the study of resolved exoplanet light through spectroscopy, polarimetry, or even photometric variability. These measurements require optics that operate over a broad wavelength range and can accommodate flexible designs. In this talk I will demonstrate how liquid-crystal (LC) optics can meet these requirements and be used to improve the integration of HCI subsystems to facilitate detailed exoplanet characterization. First, I discuss the design, performance, and future development of the liquid-crystal vector-apodizing phase plate (vAPP) coronagraph, five of which have been installed in different instruments since 2016. In addition, I summarize how the vAPP can be adapted for wavefront sensing or improved exoplanet photometry. By using the achromatic nature of the vAPP in combination with the LBT/ALES integral field spectrograph, I obtained the first ever thermal infrared spectrum of the inner three HR 8799 planets. Finally, I show applications of LC technology for aperture masking with improved throughput and low spectral resolution, as well as a LC Zernike wavefront sensor with extreme sensitivity that simultaneously measures phase and amplitude aberrations.
David Doelman Leiden Observatory
11:00Young suns and infant planets: Probing the origins of solar systems
The driving mechanism for the formation of wide-orbit gas giant exoplanets is a highly debated topic. It remains unclear whether these Jovian giants formed (A) in situ via fragmentation processes in the collapsing protostellar cloud or (B) closer to the star by core accretion mechanisms, followed by an outward migration. A third pathway (C) postulates gravitational instabilities in the circumstellar disk that might lead to the collapse of dense regions into planetary cores. As the efficiency of the different formation channels is highly dependent on the host star mass, the Young Suns Exoplanet Survey (YSES) was initiated. YSES is observing a homogeneous sample of 70 solar-mass members of the approximately 16 Myr-old Lower Centaurus-Crux subgroup of the Scorpius-Centaurus association to search for sub-stellar companions. High-contrast imaging observations with VLT/SPHERE/IRDIS revealed (i) a shadowed transition disk around Wray 15-788 that shows significant signs of ongoing planet formation, (ii) a wide-orbit brown dwarf companion of approximately 45 Jupiter masses, (iii) the first directly imaged multi-planet system around a Sun-like star, and (iv) a giant Jovian companion with a physical separation of more than 100au. The detection of these three gas giants provides important implications for the outer architecture of planetary systems and the underlying formation mechanisms. In addition to the SPHERE observations, we identified further companions to our 'Young Suns' outside the instrument's field of view in the third early data release of the Gaia mission. Based on parallaxes and proper motions provided in this catalogue, we detected eight additional sub-stellar companions at separations larger than 500 au amongst our sample. By combining Gaia astrometry with the high-contrast imaging capabilities of SPHERE, our survey will provide a complete census of wide-orbit sub-stellar companions for a statistically highly significant sample of young, solar analogues.
Alexander Bohn Leiden Observatory

For questions and/or suggestions concerning the colloquium series, please contact Simon Portegies Zwart (e-mail )