Adventures in the Solar Neighborhood - Sallie Baliunas
The Star Team of the HK Project collects measurements made
nightly at Mount Wilson Observatory in southern California and
Whipple Observatory in southern Arizona. The two telescopes
measure different portions of the electromagnetic spectrum
of stars: the singly-ionizied line of calcium, and the apparent
brightness in the visible. Both measurements reveal information
on surface magnetic activity, similar to sunspots.
The stars we monitor are sun-like stars, that is, stars similar
in mass to the sun. By observing stars like the sun but of
different ages, we can piece together the past, present and future
of the sun's magnetism during its expected 10-billion-year
lifetime. In addition, planets have been reported around several of
the stars we observe. Our measurements give fundamental information,
such as age and rotation, on the stars with detected planets.
The goal of this project will be to study the average surface coverage by
starspots, the rotation period of stars and their ages, and so
detail the history of the sun's magnetism, and its possible
influence on the environmental history of the earth.
How do Molecular Clouds Move? - Alyssa Goodman
Radio telescopes are used to map out the
emission from various trace elements found in the gas between the
stars, which is known as the "interstellar medium" or "ISM." The
trace elements produce spectral lines which offer information on the
location of gas condensations, as well as on the motion of the gas
relative to us (thanks to the Doppler effect).
The proposed project will use data from (existing) large-scale
spectral-line maps of the ISM to study the detailed motions of gas in
regions where new stars are forming. The data will be processed by
computer programs which chart out gradients in gas velocity across the
plane of the sky, and which look for correlations among spectra at
different positions. The information produced by the programs will then be
compared with (existing) computer simulations of interstellar gas in
star-forming regions. The goal of the project is to match a physical
model (which can be input to the simulations) to the spectral-line data.
A student wishing to pursue this project should have some interest in
radio astronomy, computer programming, computer simulation, and the physics
of gas dynamics in the interstellar medium and star formation.
Measuring the Distance to Supernova Explosions - Bob Kirshner
Supernova explosions are the brightest stellar events in the Universe
and can be used to measure the distances to galaxies. Our group uses
the telescopes at Mount Hopkins and elsewhere to gather observations
of supernovae which can be used for this investigation. First, we use
spectra to classify the supernova spectrum: different spectra
correspond to different physical mechanisms for the explosion. Then
we make careful measurements of the brightness and color of the
supernova: within a given supernova type these measurements can be
employed to get the distance. Since we also use spectra to measure
the redshift of the galaxy in which the supernova explodes, each set
of supernova observations helps pin down the Hubble Constant, the
number which measures the rate of cosmic expansion. At present, there
appears to be a conflict between the age of the Universe as measured
from the Hubble Constant and the ages of the oldest stars as determined
from our understanding of stellar evolution.
Supernovae are discovered every month, so a student would have the
opportunity to participate in gathering the data (perhaps remotely)
and in the subsequent measurements of the supernova brightness and
colors. It should be possible to follow the method all the way
through to a new point on the Hubble diagram and a real contribution
to measuring the size and age of the Universe.
The CfA-HEAD Survey for Distant Clusters of Galaxies - Brian
McNamara
Clusters of galaxies are the largest gravitationally bound
objects in the universe. Their abundance and distribution
in space provides constraints on theories for
the formation of structure in the universe following the Big Bang.
Modern theories for the formation of the large scale structure,
which includes clusters of galaxies, indicate that the number
density of clusters in space and the distribution of matter within clusters
of galaxies should evolve in an observable manner within redshifts
of one. To measure and understand this evolution requires a sample of clusters
at high redshifts (greater than 0.5) with well-understood selection criteria.
Over the past two years, we have identified over 200 faint, spatially
extended X-ray sources on images obtained with the ROSAT X-ray
Observatory. X-ray imagery is an excellent method for identifying
distant clusters of galaxies, because all clusters emit X-rays, and
their signatures are easily identified. We have obtained optical
images of over one third of the 200 candidate
clusters using the 48 inch telescope on Mt. Hopkins, near Tucson AZ.
In more than sixty percent of the cases we see faint galaxies at the
location of the extended X-ray emission, which confirms
their identities as clusters of galaxies.
These images are used to accurately locate the cluster
galaxies, for which we then obtain a spectrum using the Multiple
Mirror Telescope, also located on Mt. Hopkins. The spectra provide
redshifts, which are used to measure the cluster's distance.
Using the information provided by our optical and X-ray observations,
we can determine the number of clusters found in distant volume
elements, which probe the universe in a much younger state,
and the degree to which clusters have changed when compared to
similar observations of nearby clusters. These quantities allow us
to measure the degree to which clusters have evolved over cosmic
timescales, and the results can be compared to theoretical predictions
for the formation of clusters. The survey should answer many of our
most fundamental questions regarding the origin and fate of the
universe. The work this summer involves the analysis and
interpretation of optical images of the clusters, and possibly
assisting on an observing run at Mt. Hopkins.
X-Ray Studies of Centrally Bright Supernova Remnants - Pat Slane
The evolutionary cycle of SNRs is typically described in three
relatively distinct phases. The standard picture begins with a
supernova explosion releasing some $10^{50-51}$ ergs of energy into
the surrounding interstellar medium (ISM) in the form of a rapidly
expanding blast wave accompanied by slower moving ejecta. In this
earliest free-expansion phase, the initial explosion energy is
imparted in the form of kinetic energy to the ejected material. As
this supernova remnant (SNR) evolves, the blast wave sweeps up ISM
material forming a shell of $\sim 10^8$~K gas surrounding a tenuous,
hotter interior. The result is a shell of radio emission associated
with magnetic field compression and particle acceleration behind the
primary shock, accompanied by enhanced X-ray emission associated with
the hot swept-up and ejecta components.
However, there also exists a class of middle-aged SNRs for which the
X-ray morphological characteristics appear to be completely opposite
of that suggested by this picture. While the radio emission for these
remnants clearly delineates the shell, the X-ray emission is
concentrated in the central regions with little or no evidence of the
shell itself. A center-filled morphology in X-rays is quite familiar
for such remnants as the Crab or 3C58, but these SNRs are known to be
powered by central pulsars; the X-ray emission is due to a synchrotron
nebula created by energetic electrons accelerated in the pulsar
magnetosphere. For at least some of the center-filled remnants,
however, the X-ray emission is decidedly thermal in origin. A number
of scenarios have been suggested to explain the puzzling
characteristics of some or all of these center-filled SNRs, including
absorption effects, late-phase evolution in which the shell has become
radiative, and evaporation effects in a cloudy ISM.
In order to investigate models for the structure of these remnants, we
have carried out X-ray observations of a characteristic remnant
set. Using available software tools, the student will help compare and
contrast the observed SNR characteristics with models for
centrally-bright morphology. This will include extracting images and
spectra for a remnant and then carrying out spectral analysis to
derive the temperature, absorption, and abundance characteristics of
the emission. These results will then be compared with predicted
temperature and brightness profiles from a series of models. Upon
identification of model parameters which provide a good representation
of the data, remnant properties such as the age, explosion energy, and
total X-ray emitting mass will be derived. This study thus provides
the student with the opportunity to take a single object and carry out
the analysis from the data reduction stage through the analysis and
interpretation phase with the final result providing a detailed look
at the properties of that remnant as well as important input into our
understanding of these centrally-bright SNRs as a class.
Cepheids in the Milky Way and the nearby Galaxies M31/M33 - Nancy Evans
This project is a combination of observational investigations into the
masses of Cepheid variable stars and the discovery of new Cepheids in
external galaxies.
Satellite astronomy has allowed us to make substantial progress in
determining fundamental observational parameters of intermediate mass
stars. These are then compared with the predictions of evolutionary
tracks in order to further refine our understanding of stellar
evolution. Accurate understanding of the evolution of intermediate
mass stars is a step toward tracing the evolution of the most massive
stars which drive galactic chemical and dynamical evolution.
Dynamical masses, of course, can only be determined from binary stars,
and then only for favorable cases. Satellite ultraviolet studies with
the IUE satellite and the Hubble Space Telescope have allowed us to
determine the masses of Cepheid variable (intermediate mass) stars
from the velocities of their hot companions, thus attacking the
long-standing Cepheid mass problem. One star in particular (SU Cygni)
was particularly effectively studied with IUE. We have now obtained
twice as much data as was used in the original mass determination. It
is measuring the new spectra, as well as assessing the improvement in
the reprocessed older data (which has recently become available) that
is the goal of this project. This will substantially increase the
accuracy of what is already the most accurate Cepheid mass.
The second part of the project involves imaging data obtained for the
nearby galaxies M31 and M33. We have monitored 6 fields in M31 and 3
fields in M33 for several months to detect and classify variable
stars. Of particular interest are Cepheids because they allow the
distance to those galaxies to be determined using the Period--
Luminosity relation. Our data set will result in the best Cepheid
distance to those two galaxies. The student would be involved in
reducing the data, searching for variables, and determining periods
for Cepheids. Important concerns are crowding and interstallar
absorption (reddening).
This project would entail the use of modern image processing tools to
reduce and extract both IUE spectra and also ground-based images.
Thus the student would gain experience in two important astronomical
techniques.
The Effects of Supernova Heating on the Properties of Clusters of
Galaxies - Ue-Li Pen
The ten week research project will study the effects of supernova
heating on the properties of clusters of galaxies. The project will
involve controlled experiments with ab initio numerical simulations of
cluster formation. The simulation software is already fully
developed. It evolves a cosmic mixture of gas and dark matter from
first principles. The goal will be to understand how the heat input
from supernovae affects the evolution of the gas. The simulations
will be analyzed to measure the abundance and properties of gas in
clusters of galaxies as one varies the thermal history of the gas.
These can be used to make inferences about the matter density of the
universe, and will provide a quantitative model to interpret the
recently measured evolution of the cluster luminosity function.
Fabrication and Characterization of Multilayer Coatings for Future
X-Ray Missions - Suzanne Romaine
Recent innovations in multilayer coatings allows us to extend the
energy range of grazing incidence optics to 100 keV. We are
investigating the response of multilayer coatings and will be constructing a
prototype telescope for the Hard X-ray Telescope mission. The summer
intern would be exposed to the fabrication and characterization of
multilayer optics, data taking techniques and general laboratory
practices.
The knowledge gained from each of these projects is applicable in a
variety of disciplines, including: physics, astronomy, materials science
and engineering.
ROSAT HRI Studies of the Pleiades - F. Rick Harnden
The 1978 launch of the NASA earth-orbiting satellite known as
"Einstein" marked the advent of imaging technology in extra-solar
X-ray astronomy. For the first time, we learned that X-ray emission
from normal stars throughout the Milky Way was commonplace. In
subsequent years, follow-on missions by NASA's foreign partners have
made it possible to continue the study of stars through this new,
high-energy window on the electromagnetic spectrum. With ROSAT (a
German, U.S., U.K. mission) and ASCA (a Japan/U.S. satellite whose
name is a Japanese pun - see note below), stellar X-ray observations
have become a fertile testing ground for theories of coronal
formation, stellar activity cycles, and stellar magnetic dynamos.
Comparisons among the stars open clusters like the Pleiades make it
possible to study the evolution of stellar X-ray activity as a
function of age. A population of so-called coeval stars, the Pleiades
comprise a "cosmic laboratory" where we can track x-ray activity and
other observed phenomena through time, as the underlying type of star
evolves. The production of magnetic fields in the interior of
convecting and rotating stars is believed to lead to X-ray activity,
and stellar age has emerged as a crucial, observed link between
rotation and activity for late-type stars like those in the Pleiades.
The ROSAT HRI, with its high spatial resolving power, is being used to
study the most crowded regions of the Pleiades, as revealed in
previous Einstein IPC and ROSAT PSPC images. It is particularly
important to separate binary Pleiades stars of types A and B from
their companions in order to determine which stellar types are
produceing the observed X rays.
Our goal will be to complete the analysis of ROSAT HRI, and prepare
plans for the new round of stellar investigations that will be
possible with the 1998 launch of AXAF, NASA's next "Great
Observatory."