2004 REU Students' AAS Abstracts

AAS 205th Meeting, San Diego, January 2005
[140.10] Self-Similarity in the Rosette Molecular Cloud
A. M. Hughes (Yale University) and J. P. Williams (IfA)

The Rosette Molecular Cloud is an active region of star formation in the Milky Way. We present a study of the cloud using a large scale ¹²CO and ¹³CO map of the cloud obtained at the Five College Radio Astronomy Observatory. The map covers 6 square degrees at a resolution of 50'' (0.3 pc). Using a clump deconvolution algorithm, we divided the cloud into discrete regions of emission and calculated their physical properties including mass, size, linewidth, temperature, aspect ratio and degree of virialization. Clumps with embedded IRAS sources tend to be hotter and somewhat denser than average. The HII region at the cloud's edge produces a pronounced temperature gradient in the cloud (temperature decreasing as the inverse square root of distance) but does not significantly affect other clump properties. We also smoothed the data set to lower resolutions to study the dependence of the derived clump properties on size scale. The cloud structure was found to be self-similar but clump densities and their virialization parameter decreased with smoothing scale.

AAS 205th Meeting, San Diego, January 2005
[147.13] X-Ray Point Source Excesses around MACS Clusters
J. Ruderman (Stanford University) and H. Ebeling (IfA)  

Chandra X-ray images of clusters of galaxies indicate an excess of X-ray point sources, believed to be AGN cluster members, above the background surface density. We study the point source content of the ACIS-I data for 47 clusters primarily from the MACS survey, which range in redshift from 0.3 to 1.0 and have similar x-ray luminosities, and 7 control fields that do not contain clusters.  

We use the Celldetect algorithm, in the 0.5-2.0 keV range, to detect point sources with a three sigma signal-to-noise ratio above the X-ray background, resulting in 789 detections in the cluster fields and 235 point detections in the control fields. Source count rates are converted to unabsorbed energy fluxes (0.5-2.0 keV) using X-Spec software and a scaled power law source spectral model. Based on the resulting logN-logS graphs, we use a flux limit of completeness of 1.1 × 10¹⁴ erg/sec/cm². We separately analyze eight long-exposure fields in the 0.5 to 0.6 redshift range at a deeper flux limit of 5.9 × 10¹⁵ erg/sec/cm². Sources above the flux limit are binned into annuli extending 20 arcmin in radius, centered at the cluster center or observation aimpoints, for the cluster and control fields respectively.  

The cluster observations exhibit a central excess at the 3-6 σ level over the densities observed at the edge of the field. No such excess is observed in the control fields. This point source density in the control fields is consistent with the one observed at the edge of the cluster fields.  

Future work will focus on the optical counterparts of the X-ray point sources from a subset of the clusters. Color-magnitude diagrams combined with spectroscopic information will allow us to classify the sources as foreground stars, likely cluster members, or gravitationally lensed background galaxies.

AAS 205th Meeting, San Diego, January 2005
[13.05] Magnetic Feilds of Young Stars in NGC 752
C. Christensen (Carleton College) and T. Simon (IfA)

The magnetic fields of stars change in direction and strength not only over short time scales, as in the Sun's twenty-two year cycle, but also as the star ages. As characteristics of a star like rotational velocity and depth of convection zone change, the magnetic field must alter as well. To shed light on this ageing process, we studied the magnetic flux of stars 1.78 gigayears old, an age between that of the Hyades and the Sun, both of which have been extensively studied. This study was done using data from a deep-field Chandra observation to find the coronal luminosity, a proxy for the magnetic flux, of a total of 130 stars, many of which are known to be members of NGC 752. Photon count rate was converted to luminosity using the Raymond-Smith model. Convective stars were found to have a range of coronal luminosities averaging only slightly higher then that of the Sun. Stars with B - V < 0.45, however showed a trend towards brighter luminosities. We determined the average coronal luminosity of G-type stars to be 2.98 × 10²⁸ ergs/sec with a range of plus or minus 2.21 × 10²⁸ ergs/sec, which corresponds to an exponential decline in coronal luminosities from those of the Hyades.

DPS 46th Meeting, Louisville, October 2004
[40.08] An Observational Upper Limit on the Number-Density of Interstellar Comets
B. Meinke (U.C. Berkeley), R. Jedicke (IfA), and J. A. Larsen (LPL, Arizona)

Current theories of Solar System formation predict that comets that acrete in a protostellar nebula are later ejected by interactions with the newly formed giant planets. In such a process a large fraction (>99%) of all comets would be ejected from the new planetary system, subsequently creating a large population of Interstellar Objects (ISO) in the interstellar medium. Due to the observational absence of such objects, Whipple (1975) determined the limit on their number-density to be ~10¹³ pc^-3, although he, and others since (e.g. Stern 1990) contend that this liberal upper limit could be reduced by several orders of magnitude given their continued observational absence in a survey covering large areas of sky to faint magnitudes. Since the early 1990s, the University of Arizona's Spacewatch survey at Kitt Peak has conducted such wide-field observations to V ~ 21.7. In the period corresponding to this study Spacewatch covered about 4200 deg².

Given the amount of sky covered by Spacewatch observations, and the detection efficiency for objects as a function of their rates of motion and apparent magnitude, we determined the 97% upper confidence limit on the number of ISOs as a function of the slope parameter alpha . We have parameterized the number density of ISOs as ρ = ρ_o 10^{α(H - Ho)}, where H is the absolute magnitude of an object, α is the slope of the number density as a function of H, and ρ_o is the space density at H_o. We use H_o = 19.1 which corresponds to an ~1 km diameter object with a typical cometary albedo of p = 0.04.

At α = 0.5, corresponding roughly to the expected slope for accreting planetesimals, the 97% upper C.L. on the number density of ISOs is ~ 10¹⁴ pc^-3 or ~ 10^-2 AU^-3.

DPS 46th Meeting, Louisville, October 2004
[33.14] A Study of Jet Morphology and Outburst Evolution in the Coma of Schwassmann-Wachmann 1
C. E. Ellis (Redlands University), Y. R. Fernandez (IfA) and L. M. Woodney (University of Central Florida)

We present results from a continuing project to study the physical and rotational behavior of the comet Schwassmann-Wachmann 1 (SW-1). SW-1 is one of the most unusual comets known due to its orbit (at the edge of the Centaur region) and its activity behavior (with frequent stochastic outbursts). Studying SW-1 may tell us generally how outbursts are triggered and evolve. Our data consists of CCD imaging taken every few weeks over the summer of 2002. We observed SW-1 having two different outburst events, during which the coma showed a rich diversity of coma morphologies. Manipulating images enhances the faint jet features hidden within the coma, and we used 7 different image manipulation techniques to be sure of identifying features vs. artefacts. These enhancements allow us to obtain quantitative information on jet positions and relative brightnesses, and will let us eventually model the nucleus's rotation state and the location of its active areas. Our data also show us different stages of outburst evolution, which has not been often done before. The outbursts lasted for several weeks, whith two jet features located approximately 180 degrees apart. Interestingly the two features did not always stay at a constant orientation, but rather their PAs seemed to vary slightly as the outburst progressed. We present (a) images that show the evolution of the outburst, (b) a description of the location and any apparent movement of the coma features, and (c) a discussion of possible explanations. The continued study of SW-1 will undoubtedly help us better understand the nature of this truly strange comet.