A spectroscopic study of a large sample of L/T transition brown dwarfs
Abstract
In this thesis I present the spectroscopic analysis of a large sample of L and T dwarfs,
in order to constrain the sub-stellar initial mass function and formation history. The
main points I tried to address are the development of a better spectral type to distance
calibration and of a better spectral type to effective temperature calibration, and the
identification of a statistically complete sample of brown dwarf to be used to measure
the luminosity function, and therefore to constrain the initial mass function and
formation history.
To achieve the first goal I conducted the spectroscopic follow-up of brown dwarfs
from the PARallaxes of Southern Extremely Cool objects (PARSEC) program. This is
a large astrometric campaign to measure the parallaxes and proper motions of 120 L
and T dwarfs in the southern hemisphere. I combined the astrometric results with the
near infra-red spectra I obtained using the Ohio State Infra-Red Imager/Spectrometer
(OSIRIS) on the Southern Astrophysical Research telescope (SOAR). That allowed
me to investigate the nature of some unresolved binaries and common proper motion
companion in the sample, as well as sub-dwarfs candidates, and potential members of
young moving groups. Combining the spectra with the astrometric information and the
available photometry I derived the bolometric luminosity and effective temperature for
the targets, and determined a new polynomial conversion between spectral type and
effective temperature of a brown dwarfs. This is a fundamental step to compare the
results of empirical observations to numerical simulations of the sub-stellar luminosity
function.
Once refined the type to temperature calibration, I measured the luminosity function.
In order to do so my collaborators and I have selected a sample of 250 brown
dwarfs candidates from the United Kingdom Deep Infra-red Sky Survey (UKIDSS)
Large Area Survey (LAS) and followed them up with the echelle spectrograph X-shooter
on the Very Large Telescope. I present in this thesis the results of the observations of
196 of the brown dwarfs candidates. Using the X-shooter spectra I determined their
spectral types, and I identified a number of unresolved binary candidates and peculiar
objects.
One of the peculiar objects in the sample, ULAS J222711004547, turned out to be
the reddest brown dwarf observed so far, and I therefore proceeded to analyse further
its spectrum. Applying a de-reddening technique to its spectrum suggests that the
most likely reason for its redness is an excess of dust in its photosphere, and that can
account for the differences seen between objects of similar spectral type.
By comparing the results of the spectroscopic campaign to numerical simulations,
I found that it is currently impossible to constrain robustly the initial mass function
and formation history of sub-stellar objects, because of our limited knowledge of the
binary fraction among brown dwarfs. The sample of binary candidates identified in
this thesis can be used to place a better constraint on the binary fraction, but in order
to do that the candidates need to be followed-up via high resolution imaging or radial
velocity monitoring to confirm their binary nature.
Publication date
2014-09-25Published version
https://doi.org/10.18745/th.14480https://doi.org/10.18745/th.14480