1 research outputs found
Characterising & classifying the local population of ultracool dwarfs with Gaia DR2 and EDR3
Ultracool dwarfs (UCDs) are the lowest mass products of star formation and span the end of the
stellar main sequence from very-low mass, hydrogen-burning M stars to the coolest brown dwarfs.
In this thesis we characterise and classify the ultracool dwarf population in the solar neighbourhood using the accuracy and precision of data from the Gaia space observatory. Combining
astrometric (in particular parallax) and photometric data from Gaia DR2 and EDR3 with photometry from UKIDSS, SDSS and 2MASS, we prepare some of the largest and most accurate,
near-100% complete volume-limited populations of nearby, field late-M, L and T dwarfs. From
these samples we derive key population characteristics such as colour-absolute magnitude relationships, the stellar luminosity function, the binary fraction and the binary mass ratio. Our
statistical-based approach differs from much of the UCD literature to date which seeks to prepare
meta-catalogues from disparate surveys and individual spectroscopic observations with distance
determined by indirect methods. Our approach offers improvements in scale, completeness, and
distance accuracy. In particular we use Gaia to update the colour-magnitude relations and
derive the stellar luminosity functions in MJ and MG of the UCDs. We calculate the binary
fraction of the late-M and early-L dwarfs as a function of spectral type by carefully modelling the
over-luminous unresolved binary population and show that late-M dwarf binaries reside almost
exclusively in equal-mass pairs or twins.
Given the complex spectral features of UCDs, consistent and accurate classification is challenging. We investigate the current traditional methods of classification and evaluate a range of
alternative techniques including supervised and unsupervised machine learning.
In a separate study we use Gaia data to prepare a large, cylindrical sample of FGK main
sequence dwarf stars to calculate the structure of the vertical density distribution close to the
galactic plane, in fine detail, as a function of colour. Using our derived colour-dependent thin
disk scale height we directly determine the star formation history of the solar neighbourhood by
modelling the evolution of stellar populations using state-of-the-art PARSEC isochrones.Open Acces