University of Hertfordshire Research Archive

        JavaScript is disabled for your browser. Some features of this site may not work without it.

        Browse

        All of UHRABy Issue DateAuthorsTitlesThis CollectionBy Issue DateAuthorsTitles

        Arkivum Files

        My Downloads
        View Item 
        • UHRA Home
        • University of Hertfordshire
        • PhD Theses Collection
        • View Item
        • UHRA Home
        • University of Hertfordshire
        • PhD Theses Collection
        • View Item

        Discovery and Characterisation of Ultra-Cool Dwarfs in Large Scale Surveys

        View/Open
        Download fulltext (PDF, 38Mb)
        Author
        Zhang, ZengHua
        Attention
        2299/13900
        Abstract
        Ultracool dwarfs including the lowest mass stars and substellar dwarfs (or brown dwarfs) is a rapidly evolving and very active field. In this thesis I present the discovery and characterization of ultracool dwarfs and their binary systems with solar and subsolar abundances and try to answer a few scientific questions related to these ultracool objects. I use different techniques based on photometric and astrometric data of modern large scale surveys to identify ultracool dwarfs and their binaries. I identify around 1000 ultracool dwarfs from SDSS, 2MASS and UKIDSS surveys, including 82 L dwarfs and 129 L dwarf candidates (Chapter 2 and 4). This work largely increases the known number of ultracool dwarfs and aid the statistic study of these objects. Eighteen ultracool dwarfs in my sample are found to be in wide binary systems by common proper motion (Chapter 4 and 5). Wide binary systems are often used to test formation theories of low mass stars and brown dwarfs, which have different predictions of separations and binary fractions. One of these binary systems is the first L dwarf companion to a giant star eta Cancri. The eta Cancri B is clearly a useful benchmark object, with constrained distance, age, and metallicity. Further more, the L3.5 dwarf companion eta Cancri B is found to be a potential L4 + T4 binary. I focus on the studies of low mass stars and brown dwarfs with subsolar abundance referred as red and ultracool subdwarfs. They belong to the older Population II of the Galactic halo contain more information of the formation, early evolution and structure of the Milky Way. Using the most extensive optical survey, the Sloan Digital Sky Survey (SDSS), to select low mass stars with subsolar abundance, referred as red subdwarfs with spectral types of late K and M. I identify about 1800 M subdwarfs including 30 new >M6 subdwarfs and five M ultra subdwarfs with very high gravity as well as 14 carbon enhanced red subdwarfs. I also identify 45 red subdwarf binary systems from my red subdwarf sample. Thirty of them are in wide binary systems identified by common proper motion. Fifteen binaries are partially resolved in SDSS and UKIDSS. I estimate the M subdwarf binary fraction. I fit the relationships of spectral types and absolute magnitudes of optical and near infrared bands for M and L subdwarfs. I also measure $UVW$ space velocities of the my M subdwarf sample (Chapter 5). Our studies of the lowest mass stars and brown dwarfs of the Galactic halo are limited by the lack of known objects. There are only seven L subdwarfs published in the literature. I search for ultracool subdwarfs by a combine use of the most extensive optical and near infrared surveys, the SDSS and the UKIRT Infrared Deep Sky Survey. I identify three new L subdwarfs with spectral types of sdL3, sdL7 and esdL6. I re-examine the spectral types and metal classes of all known L subdwarfs and propose to use 2.3 um CO line as an indicator of L subdwarfs. Two of my new L subdwarfs are found to be candidates of halo brown dwarfs (or substellar subdwarfs). I find four of these known ten L subdwarfs could be halo brown dwarfs. I propose a new name "purple dwarf" for lowest-mass stars and brown dwarfs with subsolar abundance (Chapter 3). Finally I summarize and discuss the thesis project in Chapter 6 and describe future research plans in Chapter 7.
        Publication date
        2013-05-20
        Published version
        https://doi.org/10.18745/th.13900
        https://doi.org/10.18745/th.13900
        Other links
        http://hdl.handle.net/2299/13900
        Metadata
        Show full item record
        Keep in touch

        © 2019 University of Hertfordshire

        I want to...

        • Apply for a course
        • Download a Prospectus
        • Find a job at the University
        • Make a complaint
        • Contact the Press Office

        Go to...

        • Accommodation booking
        • Your student record
        • Bayfordbury
        • KASPAR
        • UH Arts

        The small print

        • Terms of use
        • Privacy and cookies
        • Criminal Finances Act 2017
        • Modern Slavery Act 2015
        • Sitemap

        Find/Contact us

        • T: +44 (0)1707 284000
        • E: ask@herts.ac.uk
        • Where to find us
        • Parking
        • hr
        • qaa
        • stonewall
        • AMBA
        • ECU Race Charter
        • disability confident
        • AthenaSwan