Detection and characterisation of young planetary-mass objects: novel techniques and optimised survey strategies

Young, low-mass brown dwarfs can be similar in size and composition to young, giant exoplanets. Many exist without host stars and are uncontaminated by starlight, making them useful analogues for studying planets in solar systems. Increasing the population of well-studied brown dwarfs and exoplanets will improve our understanding of the underlying distribution of planets, and of which formation scenarios are viable. Young star-forming regions, such as Serpens and Taurus, are ideal targets when looking for populations of planetary-mass brown dwarfs, as they are relatively nearby, young and active in star formation. In this thesis, I present surveys, past and future, of nearby star-forming regions, conducted in the hope of finding new, very low-mass brown dwarf and planetary-mass members. I also focus on the characterisation of newly-identified individual objects, and of populations as a whole. I aim to demonstrate how custom-designed narrowband photometric filters can be incredibly effective at selecting brown dwarf members of young regions for spectroscopic follow-up. In Chapter 2, I present a survey of the Serpens star-forming region using the novel W-band technique. I obtain photometry using the Wide-field Infrared Camera (WIRCAM) on the Canada-France-Hawaii Telescope (CFHT), and the custom-designed W-band filter, which is centred on the 1.45 µm absorption feature present in brown dwarf atmospheres. I then describe a spectroscopic follow-up campaign, covering J−, H− and K−bands. Finally, I describe a subset of observations using the Hubble Space Telescope (HST), obtained to identify possible low-mass companions or binary components. Using this photometric, spectroscopic, and high-resolution imaging data, I identify five likely-members of Serpens Core and Serpens South, four of which are consistent with having spectral types of M5 or later. In Chapter 3, I describe a future direct imaging survey, optimised to detect young, giant planets using a custom filter and a target list informed by our current understanding of the underlying planet distribution. The survey will use the Near Infrared Camera System (NIX), a high-contrast imager, part of the Enhanced Resolution Imager and Spectrograph (ERIS) instrument that has recently been installed at the Very Large Telescope (VLT). I present the ‘spectral shape’ technique, which uses the custom-designed K−peak filter to efficiently identify promising targets for follow-up observations. I discuss possible targets for such a survey, and conclude that a nearby, young star-forming region is an ideal target to maximise the yield of planet and brown dwarf detections. Finally, in Chapter 4 I use an additional W-band data set to investigate the the form of the initial mass function (IMF) in the Taurus star-forming region, and the question of the possible environmental dependence of the IMF. I combine CFHT and Gaia photometry to isolate likely Taurus members from field contaminants. Using the isolated cluster population, I run multiple Monte Carlo Markov Chain simulations to assess the likely form of the IMF. I use different IMF functional forms (broken power law and log-normal) and Taurus star-formation histories, and find evidence for a spread of stellar ages in Taurus from 1–10 Myr. I also find that both functional forms provide a reasonable fit to the data (with a slight preference for the broken power law), and that the best-fit IMF parameters extracted are consistent with literature values for other clusters and the general Galactic population, supporting the theory of a universal IMF.

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