My published abstracts so far:
Zweibel, E. G., Hole, K. T., & Mathieu, R. D. 2006, ApJ, 249, 879 Star-Disk Coupling by a Time-varying Magnetic Field
Observations suggest that stars lose appreciable angular momentum prior to reaching the main sequence. Two principal spin-down mechanisms have been proposed. One is removal of angular momentum by magnetized winds or jets; the other is transfer of angular momentum from the star to its accretion disk through the effects of magnetic fields. In the latter case, spin evolution occurs due to both mass accretion along field lines and torques resulting from coupling of the stellar magnetic field to the disk. In this paper we study the latter torques in the context of a magnetic field varying in time. We find that magnetic variability reduces the efficiency with which the field can wind up, somewhat widening the region of magnetic coupling. Nonetheless, the steady state result-that magnetic torques can be applied only within a thin annulus around the corotation radius-is little changed for what we believe to be realistic physical conditions. These results are generally applicable to disk accretion onto magnetized bodies.
Hole, K. T., Gallagher, J. S., Harris, W. M. 2006, American Astronomical Society Meeting, 208, 0605 Time Series FUSE HIRS Observations of Ly-β Emission From ε Eridani
We present unique time-series spectra of the young solar-type, planet-bearing star ε Eridani in the Lyman-β region. These data were taken with the HIRS aperture of FUSE, which leads to a reduction in the effect of both airglow and "detector walk" on the line profile. Our time series observations show the line to be variable on the scale of days to weeks. The short timescale of these fluctuations indicates they are likely due to changes in the active stellar chromosphere of ε Eridani, rather than its giant planet, which has an orbital period of 6.9 years. We briefly consider other possible sources of variability that could be associated with the planetary system, such as comets, and conclude these are unlikely to contribute to the short term variability seen in our data.Observations obtained through proposal C165 to the FUSE Guest Investigator program; Research based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985.
Hole, K. T., Mathieu, R. D., & Meibom, S. 2003, American Astronomical Society Meeting, 203, 1406 WIYN Open Cluster Study: Radial-Velocity Measurements of the Intermediate Age Open Cluster NGC6819
We present results of a radial-velocity study of the 2.5 Gyr old open cluster NGC 6819. We have 5140 radial velocity measurements of 1140 stars, obtained over the course of approximately 5 years with the WIYN Multiple-Object Spectrograph. The standard deviation of the measurements for a single (formally, non-variable) star is 0.5 km/s. We have measured velocities for the brighter evolved stars and main sequence stars to V=16.5. We are very nearly complete for stars above the main sequence and approximately 50% complete on the main sequence.
We find the cluster mean velocity to be 2.3 km/s with an observed dispersion of 1.5 km/s; we anticipate that the true cluster velocity dispersion is smaller. We discuss the distribution of velocities in the cluster field of view, and related issues of membership selection. We present a CMD of all data, and a "clean" CMD of cluster member stars.
Our later measurements have emphasized velocity-variable stars, and we have orbital solutions for 35 binaries. Our solutions to the orbits of these binaries will allow us to investigate the evolution of dynamical properties of the systems. Of particular interest is the relationship of eccentricity to period, which indicates the tidal circularization cutoff for the cluster.
This research was funded in part by NSF AST 9731302, Wisconsin Space Grant Consortium, and Oberlin College.
Hole, K. T., Sasselov, D. D., Kondratko, P. T., Heinke, C. E., Schnee, S., Bonanos, A. Z., & Ball, G. 2002, American Astronomical Society Meeting, 199, 6111 Stellar Metallicity Enhancement by Exoplanet Consumption
Recent studies have established a correlation between stars with extrasolar planets and enhanced observed metallicity as compared with the general population. One explanation for the enhancement, suggested by the close orbits of the planets so far discovered, is the pollution of convection zones of host stars by the infall of planetary material. The process of planet destruction in the outer layers of the star makes a significant impact on the observed stellar metallicity resulting from the consumption.
We present results of modeling the destruction of both terrestrial and gaseous planets by shock heating and ablation in stars from 0.8 to 1.6 Mo, as well as the predicted metallicity enhancements. Our results show that higher mass stars are more efficiently polluted than those with lower masses, primarily due to slower orbital decay, higher temperatures and the smaller mass of the surface mixing layer. We conclude that the consumption model does not well reproduce the observed distribution of metallicity enhancement of the extrasolar planet host stars.
Stinebring, D. R., Hole, K., & Foster, S. R. 1998, American Astronomical Society Meeting, 192, 4605 How Common are Refractive Effects in Pulsar Scintillation?
Refractive effects have been noticed in pulsar observations for more than twenty years, although they have not always been identified as such. These effects include pulsar flux variations on time scales of days to years, tilted scintles and fringe patterns in dynamic spectra, and elongation of VLBI images of pulsars. Of particular interest are those phenomena, such as fringing or multiple-imaging, that indicate an excess of fluctuation power at the refractive size scale compared to that expected from a Kolmogorov extrapolation from the diffractive size scale. The frequency of occurrence of such events has not been well characterized. Recently, Rickett, Lyne, and Gupta (1997, MNRAS, 287, 739) underscored the remarkable sensitivity of dynamic spectra observations for probing multiple-imaging effects. They found an instance of multiple imaging in which the flux density from the secondary ray path was only 0.0016 times the flux density of the primary ray path, and the separation between the ray directions was more than 10 times larger than the diffractive image size! How prevalent are such remarkable events? We have applied a similar secondary spectrum analysis to a data set of dynamic spectra obtained by one of us (RSF) in 1991-92 at the NRAO 42-m telescope. A total of 22 pulsars were observed at 8 quarterly epochs over these two years at both 800 MHz and 1330 MHz. We have analyzed these data paying particular attention to weak features in the secondary spectra. We will present the results of this analysis in an effort to quantify how often strong refractive effects are present in pulsar scintillation data.
Hole, K., McMillan, R., & Ciardullo, R. 1996, American Astronomical Society Meeting, 189, 4501 Photometry of Supernova Host Galaxies
As part of an ongoing study of the ages of Type Ia and Type II supernova progenitors, we present V and I broadband photometry of the regions underlying more than 50 extragalactic historical supernovae. We find some indication that regions hosting Type Ia supernovae show a broader variation in color than those of Type II supernovae. This is consistent with earlier results from this project and others showing that Type Ia supernovae arise from intermediate-age stars that have had sufficient time to drift away from their formation regions.
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