Description: In 1992, astronomers discovered the first of a population of small bodies orbiting the Sun at distances beyond Neptune. Observations show that these objects are mostly confined within a 10-20 degrees of the plane of the planets, implying that these objects occupy a ring around the Sun. This ring is referred to as the Kuiper Belt.

Stars do not form alone, but in groups, clusters and associations which can contain several "generations" of stars. One of the implications of this gregarious nature, is that the formation of the youngest generation stars of stars are strongly influenced by the previous generation. This process is vividly shown in the well known Hubble Space telescope images of the "Pillars of Creation" (alternatively known as the Eagle Nebula or M16). These images show a star forming molecular cloud being disrupted by the ionizing UV radiation from a previous generation of hot, massive, O-type stars.

To further study the disruption of star forming molecular clouds by hot, O-type stars, we have recently obtained Hubble Space Telescope images of the NGC 281 region. These optical images provide a detailed portrait of the disruption of a highly active star forming molecular cloud. With these images, we will have the capability to study the effect of the disruption on star and planet formation on an extremely fine scale by resolving structures as small as 200 Astronomical Units.

We are currently analyzing XMM/Newton X-ray observations of the nearby radio galaxy Centaurus A. Cen A is both the nearest active galaxy and the nearest massive elliptical to the Milky Way. This permits us to study the features of this galaxy with the highest possible sensitivity and linear resolution. The results of such studies are then applicable to entire classes of astrophysically interesting sources. The X-ray morphology of this object is complex with at least five distinct sources of emission: the active nucleus, an X-ray jet, a population of LMXBs, diffuse emission from the hot ISM, and emission from one of the radio lobes. We anticipate that the summer student will take an important role in both the analysis of some aspect of this data and in the comparison of this data with our previous Chandra observations.

Over the past 20 years there have been significant advances in the field of star formation. Observationally we have identified and characterized several phases of the star formation process beginning with a centrally concentrated core of molecular gas which collapses to form a star surrounded by a proto-planetary disk. Indeed it has been the isolation of objects that have not yet formed stars -- so called pre-stellar cores -- that has allowed us to probe the earliest initial stages of star formation. A large part of this progress has focused solely on the formation of low-mass or sun-like stars, predominantly because these objects can form isolated from other nearby stars which reduces confusion. However, it has now been recoginized that most stars do not form in an isolated fashion. Instead, most stars (over 90%) form together in clusters of > 100 stars in regions that are currently forming massive stars (stars with masses greater than our sun). Progress in understanding the formation of massive stars is difficult because of the natal clouds are more distant than the low-mass counterparts and the formation of many stars close together increases the chances for confusion. Because of this the earlier stages of the process the "pre-stellar massive cores" have yet to be found.

With Chandra we can obtain very sharp images of the central regions of galaxies. This allows us to study the processes which take place there and determine the energy source of luminous galaxies. This project involves the analysis of archival Chandra data for four active galaxies (NGC 4261, NGC4374, NGC 4736 and NGC404). The aim will be to determine, through spectroscopy and high resolution imaging the energy source of these galaxies: star-formation, shocks, or accretion onto a black hole. Comparison of the X-ray data with archival images obtained using the Hubble Space Telescope will be also used to achieve our goal. This will be the starting point for a larger scale study of galaxies with similar properties, for which the data are not available yet. Finally for the most nearby galaxy (NGC4736), we will also study the X-ray source populations (luminosity distribution and spectral properties using hardness ratios) and compare them with other galaxies. All the required background material (models, software and data) will be provided by the advisors.

Study objective: Use the Chandra Aspect Camera to study variability in Chandra guide stars.

The Aspect Camera Assembly (ACA) on the Chandra X-ray Observatory is a 4.4 inch diameter cassegrain telescope with a TK-1024 CCD detector operating at -10 C. It has a 2 square degree field of view, and is used to track typically 5 guide stars in the 6 to 10.5 magnitude range during each Chandra observation. This has resulted in a database of brightness data for several thousand stars, with continuous magnitude measurements at 2 second intervals over timescales ranging from about one hour to tens of hours.

We propose to use this database to study the photometric variability of the Chandra guide stars. The continuous monitoring for many hours at a 2-second time resolution provides a dataset for each star which is very different to that obtained in typical ground-based monitoring programs, which typically have much sparser time coverage, although potentially over longer timescales. An example of ACA brightness data is given in the attached plot, which shows the light curve of HD 33853, an Algol-type eclipsing binary. These data have not been processed for outlier rejection or other filtering, but show that high quality relative photometry can be obtained from the ACA, with 1% accuracy being achievable on timescales of minutes.

A wide range of types of stellar variability can be studied with the ACA data. Examples include eclipsing binaries, as HD 33853 above, which have eclipse periods ranging from minutes to months; flare stars, such as UV Ceti, with flaring on timescales of seconds to minutes; and extra-solar planetary transits, for example HD 209458 (Saurabh et al. 2000), producing approx. 1% deficits over timescales of hours.

Similar studies have been performed using star trackers and guidance sensors on other spacecraft, for example WIRE (Buzasi et al. 2000), and HST (Schneider et al. 1999).

Newton's Equivalence Principle (EP) states that the motion of a body solely under the influence of gravity is independent of the composition and internal structure of the body. This version is now referred to as the Weak Equivalence Principle (WEP). With two additional requirements [i.e., that the outcome of any local nongravitational experiment is independent of (1) the velocity of the (freely falling) apparatus, and (2) where and when in the universe it is performed], it becomes the Einstein Equivalence Principle, which "is at the heart of gravitation theory." (Will, 1981) Tests of the EP have a long history. They are intended to be sensitive to delta-g, the difference of gravitational acceleration experienced by different types of matter. However, they are sensitive also to gravity gradients and non-gravitational forces, both those that we understand and seek to eliminate, and those that are unknown and might thus be discovered.

The experiment under development is a "Galilean test" of the WEP using Test Mass Assemblies (TMA) falling in an evacuated comoving dropping chamber. Our short term objective is to measure delta-g for two types of mass with (delta-g)/g < 10^-11. Long term, we plan to achieve (delta-g)/g < 10^-13.

One of the most important questions addressed by solar physicists deals with heating of the solar corona. One theory is that the solar corona is heating by small reconnection events (nanoflares) that occur on spatial scales of 200 km. Because these scales are below our current resolution, we are unable to directly assess the viability of this theory. We can, however, infer nanoflare characteristics and indirectly assess the viability of this heating theory through observations of large reconnection events, such as explosive events.

Explosive events are small-scale (1500 km), short-lived (60 s) phenomena that are observed as broadened or skewed line profiles. Explosive events are mainly observed in transition region spectral lines. (The transition region is the narrow region between the cool chromospheric plasma and the hot coronal plasma.) Little is know about the morphology of explosive events and their energy release.

In this project, the intern will analyze co-temporal and co-spatial data from TRACE (a narrow-band filtergram) and SUMER (a high resolution spectrometer). The TRACE images are of the lower corona (~ 1 MK) and the SUMER spectra will contain transition region lines formed between .02 MK and .8MK. After co-aligning the two data sets, the student will extract the locations of the explosive events from the SUMER data and then analyze the TRACE images at those locations. The student will address the following questions. What is the area of the explosive events? Is the area dependent on the degree of velocity shift observed in the line profile? Is there evidence for heating (such as a strong brightening) in the TRACE data?