Two Summers in the UCSC Science Internship Program
by Zareen Choudhury
H
NASA, JPL-Caltech, Spitzer Space Telescope
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 final
presentations 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.
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 cutting-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.
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 minimum 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 mentors, but finding the age required measuring the stars’ effective temperatures and surface gravities. We measured these properties by
analyzing data that our mentors had collected on these stars at
Palomar Observatory.
On the Trail of Runaway Stars
Our 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
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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Sept/Oct 2014
Sept/Oct
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2014
Odd Isotopes and Halo Stars
My 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 predict 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 absorption 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
www.cty.jhu.edu/imagine
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.
NASA/ESA, STScI/AURA
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 concluded 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 competition. In addition, I presented a poster of our work at the American
Astronomical Society’s 221st Meeting in Long Beach, California.
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 content, 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 Competition. Additionally, I presented my research as a poster at the American
Astronomical Society’s 223rd Meeting in Washington, D.C.
W
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.
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