Sally
Robinson
Ph.D. Candidate, Astronomy
&
Astrophysics
Email: ser at ucolick dot
org
Photo
by Cheryl Robinson, 12/26/06
Publications
Research Interests: Solar System
formation, composition of giant planets, astrochemistry, debris disks Latest News: The second
paper from my work on the N2K project, a catalog of atmospheric parameters of
1907 metal-rich stars, was recently accepted by ApJ Supplements. Scroll down to the "Master's Thesis"
section to learn more about this project. ----------------- Sometimes,
when I have a free moment, I look through the online archives of various
chemistry-related journals and try to track down publications by my
grandparents. My grandma was
Professor Elaine Spencer of the Portland State University chemistry department,
and my grandpa was the pharmaceutical chemist and pathologist Carl Spencer. I found an online record of Grandma's
Ph.D. thesis on tyrosinase (an enzyme), but so far no luck with her journal
articles. There are so many
chemistry, biochemistry and medical journals that I hardly know where to start. I found one short journal article by
Grandpa on preparing 4'-aminobenzanilide.
This article is a gem—an entire lab procedure written clearly and
succinctly in only two columns. Download it if you're
curious. Current Project: Ices in Saturn's
core This
project is brand new, but the preliminary results are encouraging. I'd like to have a paper predicting the
bulk composition and mass of Saturn's core, and the Jupiter/Saturn core mass
ratio, out by October 2007. Recent Results: Composition of
Planet Hosts In May 2006, I published a study of
the silicon and nickel content of planet-host stars with co-authors Greg
Laughlin, Peter Bodenheimer and Debra Fischer. Our new discovery is that
planet hosts have statistically higher silicon and nickel abundances than
planetless stars of the same [Fe/H]. The discovery that planet hosts are
silicon-rich is consistent with a simple interpretation of the
core-accretion theory of planet formation, in which all solids are equally
useful for forming planets. The likelihood of planet formation is then
correlated most strongly with the abundance of elements that, like
silicon, make up a large fraction of protoplanetary material. The discovery
that planet hosts are nickel-rich to the same degree of statistical
significance as they are silicon-rich is surprising, as nickel is 19 times less
abundant than silicon in material with Solar composition. If the degree of
silicon and nickel enhancement in planet hosts ascertained by our analysis
persists once more planets have been discovered, it could indicate that planet
formation depends on the presence of specific elements. Why do planets form most easily
around silicon-rich stars? The answer may lie in the Galactic chemical
enrichment patterns of alpha elements. The alpha-chain nuclei are
produced in approximately the same proportions in all Type II supernovae, which
means a star that is silicon-rich would also be carbon-rich and oxygen-rich.
Giant planets are thought to form via the core accretion process, in which an
icy core builds up slowly, through the coalescence of planetesimals, until it
is large enough to begin runaway gas accretion. Some cores never get
massive enough to reach runaway gas accretion before the protoplanetary disk
dissipates: these will become Neptunian planets. The more icy material
present in the protoplanetary disk, the faster core accretion proceeds, and the
more likely runaway gas accretion, necessary for the formation of a Jovian
planet, will occur. Of course, to have ample ice available for core
formation, the disk needs a high oxygen abundance, and a high oxygen abundance
implies that silicon is abundant as well. My collaborators and I have constructed
a Monte Carlo simulation that determines whether a synthetic star-disk system
will form a giant planet based on the mass, lifetime, radius and chemical
composition of the disk. This model reproduces the planet-silicon
correlation we discovered in the Lick/Keck/AAT planet-search data of Valenti
& Fischer (2005). I
presented the results from this study in a poster at the Protostars and Planets
V workshop (October 2005), and in talks at UCSC, NASA JPL and Caltech. In June 2006, the ApJ paper describint
this research was reviewed in the Editor's Choice column of Science magazine. 8/22/06, Madame Tussaud's, Las Vegas Master's Thesis
I developed a method of measuring a star's atmospheric parameters, [Fe/H], Teff
and log g, from low-resolution spectra. This method works on late F,
G and K dwarfs with metallicity -0.95 < [Fe/H] < 0.5 dex, and was
developed as part of the N2K Consortium, which has the goal of finding the
"Next Two Thousand" metal-rich stars. As principal
investigator, I was awarded 28 nights on the KPNO 2.1m telescope in 2004B-2005A
to screen potential planet-search targets for N2K. We screened more than
2000 stars, identifying ~400 with supersolar metallicity ([Fe/H] > 0.2
dex). These 400 stars will feed the next generation of planet
searches—in fact, five planets have already been discovered! The
paper describing the method of measuring atmospheric parameters to high
precision using low-resolution spectra was published in the February 1st, 2006
edition of the Astrophysical Journal. The
catalog of atmospheric parameters for the 1907 stars observed at Kitt Peak was
recently accepted for publication in the ApJ Supplements. Once published,
the data will be hosted by the Michelson Science Center, along with the
high-resolution spectra from the Lick/Keck/AAT planet search. UCSC grad
students Mark Ammons, Jay Strader, Katherine Kretke and Jeremy Wertheimer have also
worked for N2K, both observing at KPNO and providing star metallicity estimates
from broadband photometry. These broadband [Fe/H] estimates were
instrumental in selecting target lists for KPNO—see Mark's paper on the
subject here. Before Grad School
As an undergrad at RIT (Rochester Institute of Technology), I worked with
Elliott Horch in the wonderful world of binary stars. I analyzed speckle
data from the WIYN telescope and, for my senior thesis, developed a Fourier
algorithm for finding the position angle and separation of a secondary
companion from HST Fine Guidance Sensor transfer scans. Between my junior
and senior year, I did the KPNO REU, where I studied the populous cluster
Terzan 7 with Ken Mighell. After I finished college, I spent a year in
Japan teaching English, but also did some astronomy on the side. I
visited the Japanese national observatory in Mitaka (near Tokyo) and the
Nobeyama Millimeter Array, learned how to analyze radio data and studied the intermediate-mass
protostar NGC 7129 FIR 2. It's an interesting object—quite
luminous, with a very high mass accretion rate. I presented this work at
the NAOJ conference in March 2003. Incidentally, my unsolicited advice
to anyone considering grad school is that a year off is a good idea. Grad
school is hard enough without being burned out at the start. I got just
enough of a taste of research in Japan to realize I would enjoy the opportunity
to do it full time, but I spent most of my time doing my "real" job
and just seeing the country. 