Understanding planetesimal formation using debris discs radii

Supervisor: Dr. Nicole Pawellek

Contact information: nicole.pawellek@univie.ac.at

Expected duration: 9 months

Project description & Goals:

Debris discs are – like planets – natural outcomes of planet formation and evolution processes. Like our own Asteroid belt and Edgeworth-Kuiper belt they are rings made of large km-sized planetesimals – the building blocks of planets – which are ground down to dust due to mutual collisions. Studying planetesimal belts gives us an opportunity to analyse physical mechanisms shaping the discs and hence, help us to constrain the processes responsible for the evolution of planetary systems. While the planetesimals themselves remain invisible for our observational instruments we can observe the smaller milli- and micrometre-sized dust particles which are detectable at infrared and sub-mm wavelengths. Over the last years, telescopes like Herschel and ALMA were able to infer the location of a number of dust belts by spatially resolving them. Some studies find a relation between the radius and the stellar luminosity (e.g., Matrà et al. 2018) and others do not (e.g., Pawellek et al. 2014) so that it is still debated whether the location of planetesimal belts is determined by thermal processes such as snowlines or dynamical processes like sculpting by planets. In this project, we will investigate the shaping mechanisms of such discs by analysing the relation between the radius of a debris disc and the stellar parameters (e.g., luminosity) of its host star for a sample of discs. Questions we want to answer are: Is the formation of planetesimal belts determined by thermal processes? Are the disc’s shaping mechanisms different around stars of different spectral types? This study includes the comparison of spatially resolved observations to modelling results from spectral energy distributions (SEDs). An essential part is the analysis of the ratio between disc radius and the radius inferred from SED modelling (‘blackbody radius’) which allows us to estimate the radii of spatially unresolved debris discs.

Working plan & Milestones (including final thesis):

To constrain shaping mechanisms of debris discs we are looking for possible relations between disc and stellar parameters such as disc radius to stellar luminosity. The project starts with the compilation of a homogeneous sample of spatially resolved debris discs. Here, you can focus on either Herschel observations (Pawellek et al. 2014, Marshall et al. 2021) or ALMA observations (Matrà et al. 2018) to infer the disc radius. For each target, the observational data (e.g., flux densities) will be collected to model the spectral energy distributions with different modelling approaches (blackbody or dust population). One aspect here is the differentiation between discs with single and multiple belts. Based on the modelling results, statistical trends of the disc parameters and the properties of the host star will be investigated. Special attention is going to the true disc radius to blackbody radius ratio. This ratio is found to decrease with increasing stellar luminosity for discs around host stars with stellar luminosities larger than 1Lsun (Pawellek & Krivov 2015). (1) For Herschel resolved discs a steeper decrease with increasing stellar luminosity was found compared to the discs resolved with ALMA (Pawellek et al. 2021). A reason might be the different disc geometries applied in the modelling, e.g., narrow multiple or single broad belts. Analysing the ratio might tell us something about the shape of a disc and therefore, indicate the presence of stirring bodies like planets. (2) The ratio can be used to estimate the radius of spatially unresolved debris discs. You can apply it to large (unresolved) disc samples to (a) test whether planetesimal belts form at snowlines in protoplanetary discs indicating the presence of temperature dependend processes. (b) constrain the dust composition of material found in debris discs. (3) The trend does not persist for late-type stars and therefore, we might ask whether the shaping mechanisms of discs around low-mass stars are different from those of more luminous stars. It is possible that the physics of dust removal around late-type stars is dominated by stellar winds and other transport processes which you can test by analysing a sample of debris discs surrounding low-mass stars.

Requirements / special skills: Knowledge of statistical basics Programming skills (preferably C++ but other languages also welcome)

References:

1. Hughes A. M., Duchene G., Matthews B. C.: “Debris Disks: Structure, Composition, and Variability”, 2018, ARA&A, 56, 541
2. Pawellek et al.: “Disk Radii and Grain Sizes in Herschel-resolved Debris Disks”, 2014, ApJ, 792, 65P
3. Pawellek & Krivov: The dust grain sizes – stellar luminosity trend in debris discs”, 2015, MNRAS, 454 3207P
4. Matrà et al.: “An Empirical Planetesimal Belt Radius – Stellar Luminosity Relation”, 2018, ApJ, 859, 72M
5. Marshall et al.: "A search for trends in spatially resolved debris discs at far-infrared wavelengths", 2021, MNRAS, 501, 6168
6. Pawellek et al.: "A ∼75 per cent occurrence rate of debris discs around F stars in the β Pic moving group", 2021, MNRAS, 502, 5390