Two Summers in the UCSC Science Internship Program

In my sophomore year, I contacted Dr. Puragra Guhathakurta, the

UC Santa Cruz professor in charge of SIP. Having taken AP Computer

Science that year, I hoped to apply computer programming to cut-

ting-edge research in astrophysics. I was excited when I was assigned

to work with Dr. Guhathakurta and Dr. Evan Kirby of Caltech, as

well as another high school student who was my partner.

On the Trail of Runaway StarsOur project focused on the process of star formation, which is still

not understood well. Specifically, we examined the creation of “field

stars,” which are massive young stars that lie very far from stellar

clusters. In conducting background research, I learned that field

stars are unusual: Stars are known to form in clusters, so it is rare

to find a young, recently formed star in isolation. This anomalous

behavior makes field stars an exciting and hotly debated subject.

One theory proposes that field stars do, in fact, form by themselves;

another claims that field stars are runaway stars that escaped from

their original clusters at high velocities. Which theory is correct? My

partner and I set out to unravel this mystery.

While substantial research has been conducted on field stars in

our Milky Way Galaxy, investigations of field stars in other galaxies

have been limited due to the difficulty of measuring velocities of

more distant objects. Working with our mentors, my partner and I

proposed a solution to this problem: Rather than directly measuring

velocities of field stars in other galaxies, we would calculate the min-

imum velocity required for the field star to escape from the nearest

cluster. We would then compare this value with an approximation of

the star’s actual velocity to determine if the field star met the criterion

to be a runaway star.

We analyzed six supergiant stars from the Andromeda galaxy,

which lies about 2.5 million light-years from the Milky Way. We

calculated the stars’ minimum escape velocities by dividing the

distance between the star and its near cluster by the star’s age.

The distance had been previously measured by one of our men-

tors, but finding the age required measuring the stars’ effective tem-

peratures and surface gravities. We measured these properties by

analyzing data that our mentors had collected on these stars at

Palomar Observatory.

18 imagine Sept/Oct 2014

by Zareen Choudhury

ow much dark matter is there in the Universe? What is the mass of the Andromeda Galaxy? Where is Planet X? It was the summer after my freshman year of high school, and I was at UC Santa Cruz for the finalpresentations of the Science Internship Program (SIP), in which a few of my friends were participating.

After listening to students presenting their research on fascinating questions about the Universe, I knew how I wanted to spend the following summer.

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Sept/Oct 2014

runaway star Zeta Ophiuchi, the bright star in the center of this image, is about 20 times more massive than our Sun and is moving toward the left at about 24 kilometers per second. the star’s stellar wind compresses and heats the interstellar medium ahead of it, creating the interstellar bow wave to its left.

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Although we received guidance from Dr. Guhathakurta and Dr.

Kirby, we were granted a great deal of autonomy. I spent most of my

time independently writing programs and debugging them. I kept

detailed records of my progress in order to retrace my steps if I found

unexpected results, and I met with my mentors a few times a week

to provide updates and seek advice if I was stuck.

Our results showed that the stars’ minimum escape velocities

range from ~1.0 to 26.0 km/s, while the typical velocities of stars in

Andromeda are known to range from ~30 to 50 km/s. Since those

velocities are greater than the minimum escape velocities, we con-

cluded that all six stars have high enough velocities to be runaway

stars that likely originated in nearby stellar clusters.

My partner and I wrote a technical paper on our research and were

recognized as regional finalists in the Siemens Competition in Math,

Science & Technology. I was also named a regional finalist in the Junior

Science & Humanities Symposium (JSHS) and received the Lawrence

Hall of Science Award for my oral presentation at the regional compe-

tition. In addition, I presented a poster of our work at the American

Astronomical Society’s 221st Meeting in Long Beach, California.

Odd Isotopes and Halo StarsMy positive experience motivated me to return to SIP the following

year. Working under the guidance of the same mentors, I investigated

a new topic: the creation of elements in the Universe. The stellar

model and classical model are two models of star formation that pre-

dict the quantity of various elements in stars at different times in the

Universe. These models offer similar predictions for most elements,

but differ for the element barium (Ba). The stellar model predicts that

the oldest stars in the galaxy contain roughly equal percentages of

even and odd Ba isotopes, while the classical model predicts a much

higher percentage of odd Ba isotopes.

I sought to determine which model is correct. My goal was to

measure the fraction of even and odd Ba isotopes in 12 Milky Way

halo stars, which are among the oldest stars in the galaxy. To do

this, I analyzed each star’s spectrum, which is a plot of how much

light the star absorbs or emits at each wavelength. Elements in

stars absorb or emit light at specific wavelengths, creating absorp-

tion and emission lines that appear as dips and spikes in the star’s

spectrum. For each star, I examined a specific absorption line in

the spectrum that corresponds to Ba. To measure the Ba isotope

fraction, I created a model spectrum of this absorption line and

adjusted the model spectrum’s Ba isotope fraction until it matched

the actual spectrum of the star.

When I began the project, I did not anticipate how deep I

would have to venture. The project required an understanding

of chemistry topics well beyond the material taught at school,

such as neutron bombardment processes that create elements.

Additionally, I did not realize how precise my calculations had to

be. Adjusting the Ba isotope percentages created extremely subtle

changes in the Ba absorption line and required very meticulous

observations. I found myself repeating lengthy resolution, Ba con-

tent, and isotope fraction measurements countless times.

The results indicated that the 12 halo stars contain an average of

34% odd Ba isotopes. The classical model predicts that halo stars

contain ~70% odd Ba isotopes, while the stellar model allows much

lower percentages of odd isotopes. Thus, I concluded that the 12 stars

are consistent with the stellar model. My results are the first evidence

indicating that halo stars follow the stellar model’s predictions for Ba,

which adds important information to our understanding of chemical

evolution in the Universe. Once again I wrote a technical paper on my

findings and was recognized as a semifinalist in the Siemens Competi-

tion. Additionally, I presented my research as a poster at the American

Astronomical Society’s 223rd Meeting in Washington, D.C.

orking with Dr. Guhathakurta and Dr. Kirby in SIP has

been enriching and eye-opening. I am now far more

knowledgeable in astronomy, physics, and chemistry.

I gained valuable experience in interacting with adults, collaborating

with fellow researchers, and overcoming hurdles. I am proud to have

contributed new knowledge to cutting-edge questions in astronomy

and am eager to apply my experience to future research endeavors. n

Zareen Choudhury graduated from The Harker School in San Jose, CA, where she was captain of the policy debate team, president of WiSTEM (Women in STEM) Club, and a math and science tutor. She was also a member of the National Honor Society and San Jose’s youth Advisory Council. She is now a freshman at MiT.

www.cty.jhu.edu/imagine imagine 19www.cty.jhu.edu/imagine imagine 19

Like the Milky Way, the Sombrero Galaxy is a spiral galaxy with a spherical halo that extends beyond the disk of the galaxy. Spiral galaxies’ halos contain very old stars—the kind of stars Zareen studied in her second summer at SIP.

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