Dissertation

The Imaging and Discovery of M-Dwarf Debris Discs with Alma

Cronin-Coltsmann, Patrick

2022

Overview

Debris discs are an essential piece of planetary system architecture. These exo-asteroid belts and exo-Kuiper belts of stellar systems both comprise that which did not form into planets and are often indelibly marked by that which did. Observing and understanding them is necessary to understanding planetary systems as a whole. M-dwarf debris discs are one of the largest current unknowns within debris disc science. Past surveys have found vanishingly small, if not empty, detection rates among field populations. Only eight discs are currently published in the literature, only four have been resolved, and only the disc of AU Mic has been resolved both thermally and in scattered light. Whether or not M-dwarf discs are significantly less common than the discs of earlier types, or are significantly different in dust properties and planetesimal belt morphology to the discs of earlier types, are unresolved questions. In this thesis I present new resolved ALMA images of two M-dwarf debris discs never before observed at mm-wavelengths or thermally resolved. I also present the first ALMA survey searching for M-dwarf debris discs, identifying two new M-dwarf debris discs and presenting excellent opportunities for follow-up observation. I analyse the newly mm-resolved Fomalhaut C debris disc, now the latest type star to have a resolved debris disc, and find the ring to have a 880 μm flux of 0.9±0.1 mJy, a radius of 26.4±0.6 AU and a narrow full width at half maximum of at most 4.2 AU. I find a 3σ upper limit on the eccentricity of 0.14, neither confirming nor ruling out previous dynamical interactions with Fomalhaut A. Finally, I find that its radius is as expected from previous disc radius–host luminosity trends. I also analyse the newly mm-resolved GSC 07396-00759 debris disc, now the second Mdwarf star to be resolved both thermally and in scattered light, and find the ring to have a 880 μm flux of 1.84±0.22 mJy and a radius of 70.2±4.4 AU. I confirm the total intensity scattered light radius found by Sissa et al. [2018], which is significantly smaller than the radius derived from the polarimetric scattered light observations of Adam et al. [2021], implying complex behaviour in the scattering phase function. I do not recover the brightness asymmetry found in scattered light observations, nor evidence of an extended halo of dust grains, implying that these features observed in scattered light must be limited to the small grain dust and are likely the result of pressure forces acting on the disc, such as stellar wind pressure and interaction with the interstellar medium. Finally, I analyse ALMA observations of 33 M-dwarf systems in the β Pictoris Moving Group, the first such survey conducted with ALMA. I detect two sub-mm excesses that likely constitute new M-dwarf debris discs around GJ 2006 A and AT Mic A and model distributions of the disc fractional luminosities and temperatures. From the science sample of 36 M-dwarfs including AU Mic I find a disc detection rate of 4/36 or 11.1+7.4 -3.3% that rises to 23.1+8.3 -5.3% when adjusted for completeness and conclude that this detection rate is consistent with the detection rate of discs around G and K type stars and that M-dwarf discs are not less likely to host debris discs, but instead require longer wavelength and higher sensitivity observations than have previously been employed.

• Publisher: ProQuest Dissertations & Theses