Ilija Medan

About Me

My name is Ilija Medan and I am a VIDA Postdoctoral Fellow in the Department of Physics & Astronomy at Vanderbilt University under the advisement of Dr. Keivan Stassun. Generally, I am interested in studying the structure and evolution of the Milky Way using results from large astronomical surveys. I am doing this by examining the detailed, chemodynamical structure of low-mass stars in the Solar vicinity using data from e.g., Gaia, SDSS, Pan-STARRS, 2MASS and AllWISE. As such a study requires the metallicities of low-mass stars, a large portion of my work revolves around validating and getting accurate estimates of the metallicties of K and M dwarfs.

Outside of my research work, I also serve as the FPS Design Implementation Lead for SDSS-V. Through this position, I am the main contributor to the software package mugatu, which validates planned SDSS-V observations based on a number of science and observing requirements, and then loads these planned observations into a targeting database so they can be accessed at the telescope.

Pre-Graduate Research

After receiving my B.S. in Mechanical Engineering, with a minor in Physics, from Santa Clara University, I worked at the SOFIA Science Center as a research assistant for first Dr. Ravi Sankrit and then Dr. B-G Andersson. With Dr. Sankrit I used HST images of Kepler's Supernova Remnant in order to catalog and study the evolution of the bright, radiative knots, and the stellar properties (color and proper motion) of the stars in the field (230th AAS poster and abstract).

With Dr. Andersson I studied the variations in grain alignment in the wall of the Local Bubble using archival polarimetry, photometry and spectroscopy (231st AAS poster and abstract). The plot above shows a proxy for grain alignment efficiency vs. galactic longitude for the observed grain alignment efficiency in the wall of the Local Bubble (data points). The solid line shown in the plot is the strength of the radiation field from the nearby OB associations at the distance of the Local Bubble wall. As it can be seen, there is a strong correlation between the observed grain alignment efficiency and the nearby radiation field, which supports radiatively driven grain alignment. Additionally with Dr. Andersson, I preformed radiaitve transfer modeling of the circumstellar envelope of the carbon-rich AGB star IRC +10216.

Graduate Research

Bayesian Cross-Matching

During my first year, I have developed a Bayesian cross-matching method to match high proper motion stars from Gaia DR2 and SUPERBLINK to photometric catalogs spanning the UV to the infrared (such as GALEX, SDSS, Pan-STARRS, RAVE, 2MASS and AllWISE). To associate objects across these catalogs that differ in accuracy and magnitude limit, I compare the multidimensional (astrometry+photometry) distribution of all sources in the vicinity of each high proper motion star ("true" distribution) to a reference distribution of random field stars obtained by extracting all sources in a region on the sky displaced 2’ from the high proper motion star ("field" distribution). The 2’ offset preserves the local field stellar density and magnitude distribution, which allows us to characterize the local frequency of chance alignments, and calculate Bayesian statistics for each match. The two panel below shows such distributions, where the left panel shows the true distribution and the right panel the field distribution. It can be seen that the distribution of random field stars (the field distribution) is a component of the true distribution. Because of this, the subtraction of the field from the true distribution will reveal the distribution of actual matches that can be used to calculate Bayesian probabilities. My resulting high Bayesian probability photometry catalog has been shown to return a higher percent of matches then the internal Gaia DR2 cross-match, especially for Pan-STARRS where I have a match rate of ~98% within the footprint of the survey as compared to ~21% for the internal Gaia match. I have presented this work at the 235th AAS meeting, where more details can be found (poster and abstract). Also, this work has been accepted for publication in The Astronomical Journal.

If you would like use my cross-matching method for your research, I have made the code to carry out a macth for stars in Gaia DR2 to another photometric catalog available on my GitHub.

Photometric Metallicity Calibrations

Using the results from my Bayesian Cross-Match, I am interested in utilizing photometry to estimate stellar metallicites of low-mass stars. Using derived metallicities from APOGEE as the basis of a calibration subset, I trained a Gaussian Process Regressor (with multiple colors and absolute magnitudes from Pan-STARRS, 2MASS and AllWISE as inputs) to predict metallicities for K and early M dwarfs with a precision of 0.12 dex. This calibrated relationship is largely free of systematic errors, that were present in past regressors. I have presented this work as a poster for the Royal Astronomical Society Early Career Poster Exhibition and at the 237th AAS meeting. Also, this work has been accepted for publication in The Astronomical Journal.

If you would like to estimate photometric metallicities of stars in a sample for your own research, I have made this Gaussian Process Regressor available on my GitHub.

Chemodynamical Ages of Small-Scale Kinematic Structures

By combining data from Gaia DR3 and the above photometric metallicties, I am able to examine the chemodynamic structures in the vicinity of the Sun (d < 300 pc) in great detail. These chemodynamics are shown in the left panel in the plot below. Here we see a plot of guiding radius vs. U velocity that is color-coded by the median velocity of the stars in the phase space. Overlayed are ellipses that show the location of local kinematic groups, which are identified as over-densities in the kinematic plane. In this plane, we see clear chemical enhancements that are correlated with some of the kinematic groups. If we also look at the dispersion in the W velocity across this kinematic plan (right panel), we also see much lower dispersions inside some of these kinematic groups. As it has been shown that velocity dispersion is correlated with age, I hypothesized these differences are related to changes in the underlying age distribution across the kinematic plane.

To test this, I developed a method to estimate the age distribution of subpopulations of stars. In this method, I use GALAH data to define probability distributions of W versus [M/H] in age bins of 2 Gyr and determine optimal age distributions as the best-fitting weighted sum of these distributions. This process is then validated using the GALAH subset. I then estimated the probable age distribution for regions in the kinematic plane, where I found significant substructure that is correlated with the kinematic groups. Most notably, I found an age gradient across the Hercules streams that is correlated with birth radius. This work is published in MNRAS, and the age and birth radii distributions, along with the code to reproduce them, can be found here.

Curriculum Vitae

Contact

Office:
Science and Engineering Building
#6718
Vanderbilt University

Email:
ilija.medan@vanderbilt.edu

GitHub:
https://github.com/imedan

Coffee Imports in 2019

Using data from the UN Comtrade Database, I have shown where the top 5 coffee importers (brown labels) get their raw coffee from (green labels). The numbers in each of the "chords" in the visualization are in units of kilotons of coffee imported from the green labeled country to the brown labeled country. The interactive visualization can be viewed HERE. This visualization makes use of the Python package, Chord.

Comet Classification

Using the catalog of cometary orbital parameters provided by Rocher 2007, I classifed comets using the Tisserand parameter (Carusi & Valsecchi 1987). The most abundant type of comets in this catalog were the so-called "Jupiter Family" comets, and are shown in red in the below graphic. For scale, the orbiit of Earth (inner dashed line) and Jupiter (outer dashed line) are shown. The large "clumping" of comets whose long-axis of orbit is centered around x=0 are associated with the 71 fragments of Comet 73P/Schwassmann-Wachmann 3, which began to break apart in September of 1995 (Crovisier et al. 1996).

Asteroid Families in PanSTARRS

I extracted the positions, date of observation and colors for all asteroids observed by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). I then associated the Pan-STARRS asteroids with known, numbered asteroids using a catalog of asteroid orbital elements. To determine asteroid families in various regions based on distance from the sun, I first identified background asteroids (i.e. asteroids that did not belong to any family) using a Local Outlier Factor, where the background asteroids as shown as red data points in the graphic below. The remaining asteroids were then clustered into families using Agglomerative Clustering. In the graphic to the left, various families are color-coded and the families' centers in orbital element space are shown with a plus sign.

Elements

i = 0;

while (!deck.isInOrder()) {
    print 'Iteration ' + i;
    deck.shuffle();
    i++;
}

print 'It took ' + i + ' iterations to sort the deck.';