Doctoral (PhD) Dissertation Defense Announcement Notification

Final Doctoral Defense for: Mr.Tesfaye Dagne Muhie

Department: Astronomy and Astrophysics Research and Development Department

Program: Stellar Astrophysics

DATE: November, 18, 2021

TIME: 9:30 AM

Venue : ESSTI main office

Supervisor: Prof. Leonid Berdnikov

Co-supervisor: Dr. Kniazev Alexei

DISSERTATION TITLE:RR Lyrae Stars as Tracers of Galactic Halo Structure and Kinematics


This thesis presents results from photometric and statistical-parallax analysis of a sample of 850 field RR Lyrae (RRL) variables. The photometric and spectroscopic data for sample RRLs are obtained from (1) our new spectroscopic observations (for 448 RRLs) carried out with the Southern African Large Telescope (SALT); (2) our photometric observations using the 1.0-m telescope of the South African Astro- nomical Observatory (SAAO), and (3) literature. These are combined with accurate proper motion and photometry data from the second release of Gaia mission (DR2). This study primarily determines the velocity distribution of solar neighborhood RRLs, and it also calibrates the zero points of the RRLs visual V-band luminosity-metallicity (LZ or M V −[Fe/H]) relation and their period-luminosity metallicity (PLZ) relations in the WISE W 1 − and 2MASS Ks−band. Furthermore, it calibrates RRLs (V − W 1) 0 and (V − Ks) 0 intrinsic colours in terms of period and metallicity from the photometric analysis along with accurate 3D extinction model for our sample.

We find the bulk velocity of the halo RRLs relative to the Sun to be (U 0, V 0, W 0) Halo = (−16 ± 7, −219 ± 7, −6 ± 5) km s −1 in the direction of Galactic center, Galactic rotation, and North Galactic pole, respectively, with velocity-dispersion ellipsoids (σV R, σV φ, σV θ) Halo = (153 ± 7, 106 ± 4, 101 ±4) km s −1. The corresponding parameters for the disk component are found to be (U 0, V 0, W 0) Disk = (−19 ± 5, −46 ± 5, −14 ± 3) km s −1 and (σV R, σV φ, σV θ) Disk = (49 ± 4, 38 ± 4, 25 ± 3) km s −1. The calibrated PLZ in W 1 −, Ks−band, and V band LZ relations are h M W 1 i = −0.824 + 0.124[F e/H] − 2.381 log P F, hM Ks i =−0.804 + 0.101[F e/H] − 2.33 log P F and h M V i = 1.041 + 0.232[F e/H], respectively; while the related (V −W 1) 0 and (V −Ks) 0 intrinsic colour relations become (V − W 1) 0 = 1.865(±0.022) + 0.108(±0.014)[F e/H] + 2.381(±0.097) log P F and (V −Ks) 0 = 1.834(±0.028)+0.131(±0.018)[F e/H]+2.33 log P F, respectively. The calibrated PLZ and LZ relations are used to estimate the Galactic Center distance and the distance modulus of the Large Magellanic Cloud (LMC) which are found to be 7.99±0.49 kpc and 18.46±0.09 mag, respectively. All our results are in excellent agreement with available literature based on statistical parallax analysis, but are considerably more accurate and precise. Moreover, the zero-points of our calibrated PLZ and LZ relations are quite consistent with current results found by other techniques and yield a LMC distance modulus that is within 0.04 mag of the current most precise estimate.

Note: You are cordially invited to attend the defense of this PhD Dissertation.

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