Past Interns and Projects: Summer 2008
 
 SAO Summer Intern Program Projects, 2008


Links to:

    List of colloquium talks given during the summer of 2008
    Program of the SAO Summer Intern Symposium, August 13, 2008
    2008 Summer Program Calendars for June , July , and August
    Abstracts for posters presented at the January, 2009 AAS meeting

 



INTERN: India Anderson (Southern University)

ADVISOR: Dr. Leonard Strachan

PROJECT TITLE: Testing Solar Wind Models with data from the SOHO Ultraviolet Coronagraph Spectrometer

Abstract:
One of the major discoveries from the SOHO Ultraviolet Coronagraph Spectrometer is that ions in the solar corona are heated to much higher temperatures than the electrons. This result has had a major impact on what solar physicists think about coronal heating and solar wind acceleration. Now with more than a solar cycle's worth of UVCS data we are in a position to test theoretical solar wind models over a wide range of coronal conditions. This project will test the latest MHD solar wind models for their accuracy and self-consistency in predicting plasma parameters in the corona.


INTERN: Dan D'Orazio (Juniata College)

ADVISORS: Dr. Greg Dobler and Dr. Beth Willman

PROJECT TITLE: Finding Invisible Galaxies with Gravitational Lensing

Abstract:
If an asteroid were traveling through space in a straight line and happened to pass near a massive object like the Earth, it would feel the gravitational pull of the Earth and its trajectory would be bent. A similar phenomenon occurs for light rays (photons), and the bending of a photon path by the gravitational pull of a massive body is called Gravitational Lensing. Gravitational lensing offered the first proof of Einstein's theory of general relativity in 1919 when, during a solar eclipse, the light from stars behind the sun was found to be bent by the gravitational pull of the sun. Gravitational lensing is observed when the light from a very distant object, such as a galaxy or a quasar is bent by the gravity of an (also very distant) object such as a galaxy or cluster of galaxies. For cases of near perfect alignment, i.e. when the background quasar is almost directly behind the foreground galaxy, gravitational lensing can cause the quasar to be split into multiple images. This type of lensing, where the background object is severely distorted by the lens galaxy is called strong lensing. Not only does strong lensing provide striking and spectacular astronomical images, the properties of the images, namely their positions and brightnesses, contain detailed information about the gravitational potential of the lens galaxy. With the gravitational potential, we can infer the distribution of matter in lens galaxies which are otherwise too far away to study with other techniques. In fact, lensing has already been proposed as a tool for studying the missing satellites problem. The Milky Way galaxy is surrounded by many smaller satellite galaxies. However, the problem is that detailed computer simulations of the Milky Way indicate that there should be more than ten times as many satellite galaxies than are actually observed. A possible solution is that the satellites are there but that they do not contain stars or gas and are invisible or dark. At present, gravitational lensing is the only way to detect the presence of dark matter in distant galaxies, and it has been suggested that the brightnesses and positions of images in multiply imaged quasars provide evidence for the existence of the missing satellites in distant galaxies.

My research on the gravitational lensing approach to this missing satellites problem has relied heavily on the available data. Basically I have built models which describe the gravitational potential of the lens galaxy plus satellite galaxies and used the data to constrain the parameters of the model (e.g. how massive are the satellites, where are they located relative to the lensing galaxy, etc.) Presently there are very few multiply imaged quasars that can be used for this purpose, but with upcoming surveys (such as PanSTARRS, in which Harvard-CfA is a partner) this will change dramatically. In the anticipation of the influx of new data, I would like to extend the simple modeling that has been done in the past to include more realistic satellite populations - as determined by the most recent simulations of the Milky Way - and apply these models to the presently available data. The work is theoretical/computational in nature and encompasses a broad range of astrophysics, from the dark matter halos of distant galaxies, to the satellite population derived from simulations, to the physical size of the background "source" quasar.


INTERN: Christene Lynch (Gettysburg College)

ADVISORS: Dr. Gerardo Juan Manuel Luna and Dr. Scott Kenyon

PROJECT TITLE: Time resolved optical spectroscopy of RS Oph after 2006 outburst

Abstract:
Symbiotic stars are binary systems in which a white dwarf accretes from a red giant wind which forms a dense nebula surrounding the system. Observationally, their optical spectra consist of various absorption bands from the red giant's photosphere, a blue continuum from the white dwarf photosphere and various emission lines from the ionized nebula. Some symbiotic system (e.g. RS Oph, T CrB) have massive white dwarf (M~1.35 Msun) as accretors. These systems, known as recurrent novae (Sokoloski et al. 2006, Nature, 442, 276), experience quasi-periodic outburst triggered by the accumulation of material onto the white dwarf surface.

The recurrent nova RS Oph went into outburst in 2-12-2006. During this event ~10^{-7} Msun were ejected from the white dwarf surface with velocities ~5000 km/s. Since then, this material is expanding and its density and temperature dropping. A few days after the 2006 outburst, extensive observational campaigns were performed in various wavelengths ranges, from X-rays to radio. We obtained approximately 200 low-spectral resolution optical observations during ~4 months after the outburst using the TILLINGHAST telescope together with the FAST spectrograph.

Optical spectroscopy of nova shell is used to investigate the chemical composition of the nebula, its ionization state, source of radiation and expansion rate (see e.g. Augusto & Diaz, 2003, AJ, 125,3349). With the available data, the student will be able to measure the evolution of these parameters as the shell is expanding, mapping its different stages and provide answers to questions as: is the accretion disk reconstructed? when?, are the nebular abundances compatible with a normal red giant wind or there was some chemical enrichment during the thermonuclear outburst?, what is the temperature evolution of the accreting white dwarf?


INTERN: Greg Mosby (Yale University)

ADVISORS: Dr. Lori Allen and Dr. Kevin Covey

PROJECT TITLE: Properties and Evolution of young stellar clusters in Orion

Abstract:
We recently used the Spitzer Space Telescope, NASA's premier infrared platform, to completely survey the giant molecular clouds in Orion, the nearest massive star-forming region to the Sun. Our Spitzer data reveal thousands of newly discovered young stars in these clouds. To better characterize these objects, we are using the state-of-the art, multi-fiber (Hectospec) on the 6.5-meter MMT telescope of Mt. Hopkins Arizona to acquire classification spectra. So far we have collected spectra for several hundred stars. These spectra, along with the mid-infrared data from Spitzer and near-infrared photometry from the Two Micron All Sky Survey (2MASS), wil allow us to examine fundamental questions related to the timescales for evolution and dissipation of protoplanetary disks, the characteristic lifetimes of molecular clouds, the predominance of triggered star formation, and the dynamical evolution of clusters.

The interested student will first perform the task of merging the spectroscopic and photometric data into a single database, then will use this information to construct Hertzsprung-Russell diagrams, and, by comparing to pre-main sequence evolutionary models, estimate the ages, masses, and disk properties of the young stars. In the course of this work, the student will learn about star formation, the evolution of protoplanetary disks and the timescales for planet formation. S/he will also gain experience using standard astronomical software such as IRAF, and more general software like IDL.


INTERN: Katherine (Kaylea) Nelson (Colby College)

ADVISOR: Dr. Ewan O'Sullivan

PROJECT TITLE: A combined Chandra/XMM-Newton study of nearby elliptical galaxies

Abstract:
The Chandra and XMM-Newton X-ray observatories have vastly increased our knowledge of nearby galaxies, providing both exceptionally sharp images and the sensitivity needed to probe for faint X-ray emission. However, most studies of ellipticals have focused on the brightest, best known systems, particularly those which reside at the centers of galaxy clusters, whose properties are primarily a product of their surrounding environment rather than their formation history. We have embarked on a project to develop a more representative view of the X-ray properties of elliptical galaxies, using a statistically complete sample of ellipticals observed by Chandra and/or XMM.

The aim of the project is to characterize the X-ray properties of ellipticals in the local Universe, which host three main classes of X-ray source; diffuse gas with temperatures of a few million Kelvin, the active nucleus, and X-ray binaries. With this sample, we hope to be able to address issues such as:

1) What fraction of nearby ellipticals have stable, gravitationally bound gas halos? What is the temperature and metal abundance structure of these halos, and is it correlated with factors such as stellar population age and nuclear activity?

2) How large is the variation in the luminosity of the X-ray binary population, and how is it related to properties such as galaxy mass and globular cluster population?

To address these issues we will measure the relative contributions of the AGN, X-ray binaries and diffuse emission, determine the spectral properties and structure of the hot gas halo, and examine the X-ray binary luminosity function. Comparison with HST imaging data (where available) and with radio and optical data from the literature will provide further information on galaxy structure, the stellar population and AGN activity.

A student working on this project will learn how to analyze X-ray and optical data using existing, well-tested software (CIAO, SAS, HEASOFT, IRAF), and carry out comparisons between Chandra, XMM-Newton and HST data. We have ready-made scripts and tasks to carry out parts of this analysis, which will be used by the student. Since the overall sample is too large to be analyzed in the short period available, the student will focus on a subset of 6 galaxies with good quality Chandra and XMM data, covering the more poorly-know faint end of the optical luminosity range, and including examples of both gas-rich and binary-dominated systems. The sample size can be adjusted depending on the progress of the student and data from a further 10 systems whose XMM observations have already been analyzed can be included in the final comparisons. As well as experiencing the process of analysis and interpretation of scientific data, the student will learn about the relative diagnostic capabilities of different wavebands and instruments, the structure and properties of elliptical galaxies, the different classes of X-ray source they host, and the physical mechanisms behind their emission.


INTERN: Arpita Roy (Franklin & Marshall College)

ADVISOR: Dr. Elizabeth Humphreys

PROJECT TITLE: What does High-Mass Star Formation "Look" like? Accretion and Outflow Close to a Forming High-Mass Star

Abstract:
High-mass star formation is difficult to study relative to the low mass case partly because high-mass stars evolve more rapidly and are rare. Because there are not many examples of high-mass star formation close by, it is important to study the few that there are in detail so that we can determine the key processes involved.

I have two projects relating to the formation of the closest forming high-mass star, Source I in Orion. With collaborators in the Radio and Geoastronomy Division at CfA, we have been working to characterize the nature of the accretion and outflow process of Source I using very high resolution (milliarcsecond) radio interferometry observations of molecular maser emission. The first project within our group is performing modeling to work out the geometry of the forming star (e.g., disk plus outflow) and its 3D orientation. We have programs developed for this use already. The second project is to perform radiative transfer modeling of emission from the source to work out the temperature and density of the gas. Again we already have programs that perform the calculations. The results will be reported in a publication and/or at the AAS.


INTERN: Greg Salvesen (University of Michigan)

ADVISORS: Dr. John Raymond and Dr. Dick Edgar

PROJECT TITLE: Cosmic Ray Pressure in the Cygnus Loop

Abstract:
Models of cosmic ray acceleration in shock waves predict either very efficient or very inefficient acceleration, with a ratio of cosmic ray to gas pressure (P_cr=P_g) near 80% or below 10%. Intermediate values are unstable. P_cr is hard to measure, but it can be inferred because the ram pressure, rho*V^2, nearly equals P_cr + P_g. Halpha filaments of a non-radiative shock delineate the northern 1/3 of the outer edge of the Cygnus Loop supernova remnant. Their proper motion is about 5 arcseconds between the first and second epoch Palomar surveys. An upper limit to the distance to the Cygnus Loop is given by the distance D to a star whose spectrum shows high velocity, high temperature absorption lines from the shocked gas. The combination of distance and proper motion gives V. The temperature can be determined from the X-ray spectrum, and high quality ROSAT PSPC spectra exist for the entire region (Levenson et al. ApJ 526, 874). From the upper limit to V and the lower limit to Tx one can derive an upper limit on P_cr=P_g. We propose that an REU student measure the proper motions for about 50 segments along the northern Cygnus Loop and fit the ROSAT spectra for the corresponding post-shock regions. We have tried this for one segment where we and our collaborators have obtained optical line profiles and UV spectra (Ghavamian et al. ApJ 547, 995; Raymond et al. ApJ 584, 770), and P_cr=P_g < 0.60 for the upper limits on proper motion and D and lower limit on Tx, and P_cr=P_g < 0.06 for the nominal values of proper motion, D and Tx. Measuring 50 independent regions will reduce the uncertainty and probably provide a limit somewhere between these values. To our knowledge, this sort of analysis has never been published for any SNR shock.


INTERN: David Stark (University of Minnesota)

ADVISORS: Dr. Paul Nulsen and Dr. Ralph Kraft

PROJECT TITLE: Jets in Cen A

Abstract:
When matter falls into the black hole at the center of a radio galaxy, some of the energy released is funneled into powerful opposed jets. These flow out and interact with gas surrounding the galaxy, driving it out of the way, creating shocks and inflating lobes of radio emitting plasma. Such effects can be seen in radio and X-ray images of many radio galaxies. A particularly good example is provided by the nearest radio galaxy, Centaurus A (Cen A).

We do not yet know what radio jets are composed of. They emit synchrotron radiation, showing that they contain relativistic electrons and magnetic fields, but there is good reason to believe that these only represent a small component of jets. Positively charged particles are needed to keep jets neutral (otherwise electrostatic forces would prevent them escaping the nucleus). These could be ions (mostly protons) or positrons. The ions may be relativistic or non-relativistic and there may also be other non-relativistic matter swept up in jets.

One step to determining the composition of a jet is to know its equation of state. In contrast to more distant radio galaxies, Cen A is close enough that its eastern radio jet is well resolved in X-ray observations with Chandra. This enables us to measure the size of the jet and its pressure distribution. The aim of this project will be to use the equations for relativistic fluid flow to relate these properties and determine the equation of state of the jet plasma. That information will be used to constrain the composition of the jet.


INTERN: Caleb Wheeler (University of Missouri Columbia)

ADVISOR: Dr. Guillermo Torres

PROJECT TITLE:   Binarity in the Pleiades Cluster

Abstract:
The open star cluster known as the "Seven Sisters" (Pleiades) is one of the most prominent in the northern sky. It has been studied by astronomers for at least a century. We are currently engaged in a long-term radial-velocity survey of more than 200 of its members, and one of the goals of this project is to study the binary population in the cluster. In particular, we are interested in investigating the effects of tidal forces in binaries, which tend to synchronize the rotation of the components to the mean orbital motion, and tend make the orbits circular with time. These processes are not yet fully understood theoretically.

We are seeking a motivated student who is interested in learning about stars and the techniques used to study them. The work will involve the determination of spectroscopic parameters of cluster members (mainly the effective temperatures and projected rotational velocities) on the basis of more than 3000 optical spectra accumulated over more than 20 years with the same instrument. For the binary stars, the student will derive radial velocities using existing software, and will incorporate also historical radial velocities available from the literature, to supplement our own measurements. Spectroscopic orbits based on these velocities will be derived both for single-lined binaries and double-lined binaries. We hope to significantly increase the number of spectroscopic binary systems in the Pleiades. This information will be used to construct the eccentricity versus Log period diagram, one of the most useful diagnostics of the efficiency of orbital circularization. This will be used to infer the circularization timescale for the Pleiades, which can be compared with theoretical predictions.

The spectroscopic parameters for single stars will be used to study the rotational properties of members as a function of spectral type. Their radial velocities, along with the center-of-mass velocities of the binaries, will be used to determine the internal velocity dispersion in the Pleiades. If time permits, we will also use the radial-velocity information to study the kinematics of the cluster.


INTERN: Angie Wolfgang (Cornell College)

ADVISORS: Dr. Paul Green and Dr. Kevin Covey

PROJECT TITLE: Characterizing X-ray Active Objects in the ChaMP Survey

Abstract:
The Chandra Multiwavelength project (ChaMP) is a wide-area X-ray sky survey encompassing nearly 19,000 newly-discovered X-ray sources. New spectroscopy has been obtained for hundreds of optically bright objects. These spectra will help us to measure for the first time the true fraction of nearby galaxies that harbor actively-fed supermassive black holes, and will also allow us to understand the strange excess blue emission from X-ray active stars in our own galaxy. We seek a student to assist in the analysis of spectroscopy from the FAST spectrograph at the FLWO1.5m on Mt Hopkins of optically bright, X-ray active stars, galaxies and active galactic nuclei obtained as part of the ChaMP. This includes analysis and verification of spectra for radial velocities using IRAF, characterization of the object types by comparison to templates, compilation of results in a Sybase database for uniform tabulation and easy retrieval. Final products will be incorporated into the existing ChaMP database, which will become public after journal publication of results.

 
 

Clay Fellow Warren Brown