Constraining external photoevaporative winds of proplyds with high-resolution integral field spectroscopy

Aru, Mari Liis (2025): Constraining external photoevaporative winds of proplyds with high-resolution integral field spectroscopy. Dissertation, LMU München: Fakultät für Physik

Abstract

Astronomers have discovered thousands of planets beyond our solar system—exoplanets, which populate planetary systems of great diversity. To understand the origin of such systems, we look to protoplanetary disks, rotating structures of gas and dust that provide the foundational material for planets. Protoplanetary disks emerge as a consequence of the star formation process, surrounding young stars in the shape of a disk due to angular momentum conservation. Several processes influence how the disks evolve in time. Internally to the system, high-energy photons from the central star can drive photoevaporative winds, while magnetic fields threading the disk can launch magneto-hydrodynamic winds. These mechanisms have been studied thoroughly in nearby star-forming regions. However, disks rarely form in isolation. In massive star clusters, UV photons from OB-type stars heat the gas of the disk outer regions, depleting the disk outside-in. This externally photoevaporative wind influences the size, mass, and lifetime of protoplanetary discs. It is now considered that most stars form in clusters, and evidence points this also as the birth environment of the Sun. External photoevaporation is therefore a key ingredient in planet formation models. Until recently, the same observational diagnostics have been used for studying both internally and externally driven photoevaporative winds. In this thesis, I aim to disentangle the two types of winds by analyzing highly informative and novel observations of externally irradiated disks known as proplyds in the Orion Nebula Cluster (ONC), the closest cluster with massive stars. Whereas proplyds have been studied since the 1990s, integral field spectroscopy introduces novelty: information about a target is not gathered in one band or wavelength (as is the case with imaging and spectroscopy alone), but across an entire wavelength range. Based on data from the MUSE adaptive optics-enabled integral field spectrograph, this thesis presents the morphology of proplyds in the optical, covering a variety of elements and their ionization states. First, the ionization front radii were measured, and the stratification of the ionized oxygen layers was demonstrated. The upper limits of mass-loss rates were also derived, consistent with previous observational estimates. Secondly, the forbidden line of neutral carbon [C I] 8727 A was presented as a diagnostic of external photoevaporation. For the first time, its spatial distribution was compared with [O I] lines and ALMA continuum emission, showing that the [C I] emission is co-spatial with the disk. The absence of the carbon line in isolated disks, in contrast with the ubiquitous detection of [O I] lines, supports its role as a tell-tale tracer of externally driven winds. Finally, the analysis was extended to commonly observed forbidden-line ratios, using a refined background-subtraction method. Among these, the \NII/\SII ratio emerged as a particularly robust diagnostic of external photoevaporation. While MUSE allows to put general constraints on the temperature and electron density of proplyds, a wider range in shorter wavelengths would be more informative. Together, these results establish new tracers uniquely for external photoevaporation. The presented observations provide benchmarks for existing models, and also drive the development of new models that incorporate external winds and its signatures into disk evolution. Establishing these diagnostics in the ONC sets the foundation for future studies in more massive and distant clusters, where disk morphologies cannot be spatially resolved, and where external photoevaporation is expected to play a central role.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: protoplanetary disks, pre-main sequence stars, HII regions, Orion Nebula
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 27. Oktober 2025
• 1. Bericht­erstatter:in: Ercolano, Barbara