Tuesday, May 24, 2011, 7:00 PM - 8:00 PM 223 Invited Session America South
|223.01
The Sun's Magnetic Surface
Hendrik Spruit1
1MPI for Astrophysics, Germany.
7:00 PM - 8:00 PM
America South
An overview is given of the history and present status of observations and theory of magnetic fields as observed at the solar surface: their structure, their evolution, and their effect on the Sun's brightness. By a fortunate coincidence, the photosphere is the region most accessible to direct numerical MHD simulations. The spectacular level of qualitative and quantitative realism now possible is illustrated by comparisons with the equally remarkable advances high-resolution observations achieved in recents years. These comparisons are now yielding confident physical interpretations of many of the observed properties of the Sun's surface magnetic fields, including, for example the bewildering detail of sunspot structure. The controversial question of a possible connection between climate and brightness variations over the Sun's magnetic cycle will be discussed in the light of recent observational and numerical results.
Wednesday, May 25, 2011, 8:00 AM - 7:00 PM 322 Variable Stars Poster Session
322.01
Hα Monitoring of Early-Type Emission Line Stars
Steven P. Souza1, E. Boettcher2, S. Wilson1, M. Hosek1
1Williams College, 2Haverford College.
8:00 AM - 7:00 PM
Essex Ballroom
We have begun a narrowband imaging program to monitor Hα emission in early-type stars in young open clusters and associations. A minority of early-type stars, particularly Be stars, show Hα in emission due to extended atmospheres and non-equilibrium conditions. Emission features commonly vary irregularly over a range of timescales (Porter, J.M. & Rivinus, T., P.A.S.P. 115:1153-1170, 2003). Some of the brightest such stars, e.g. γ Cas, have been spectroscopically monitored for Hα variability to help constrain models of the unstable disk, but there is relatively little ongoing monitoring in samples including fainter stars (Peters, G., Be Star Newsletter 39:3, 2009). Our program uses matched 5nm-wide on-band (656nm) and off-band (645nm) filters, in conjunction with the Hopkins Observatory 0.6-m telescope and CCD camera. Aperture photometry is done on all early-type stars in each frame, and results expressed as on-band to off-band ratios. Though wavelength-dependent information is lost compared with spectroscopy, imaging allows us to observe much fainter (and therefore many more) objects. Observing young clusters, rather than individual target stars, allows us to record multiple known and candidate emission line stars per frame, and provides multiple "normal" reference stars of similar spectral type. Observations began in the summer of 2010. This project has the potential to produce significant amounts of raw data, so a semi-automated data reduction process has been developed, including astrometric and photometric tasks. Early results, including some preliminary light curves and recovery of known Be stars at least as faint as R=13.9, are presented. We gratefully acknowledge support for student research through an REU grant to the Keck Northeast Astronomy Consortium from the National Science Foundation, and from the Division III Research Funding Committee of Williams College.
322.02
Many Flares Make a Corona
Steven H. Saar1, V. Kashyap1, J. Drake1, K. Reeves1, A. Connors2
1Harvard-Smithsonian, CfA, 2Eureka Scientific.
8:00 AM - 7:00 PM
Essex Ballroom
It is well known that solar flare energies have a self-similar distribution. The number of flares, N, of any given energy, E, follows a power-law distribution, dN/dE ~ E^(-alpha), over many orders of magnitude, with alpha ~ 1.8. A similar distribution holds for stellar coronae, but in this case, typically alpha > 2. The value alpha=2 is important because it represents a threshold beyond which it is possible to ascribe all of the coronal luminosity to increasingly weaker, but more numerous, flares.
Current methods to evaluate the flare distribution index alpha for stars are limited by two factors: they either depend on explicit detections of flares, or if the flare distribution itself is being modeled, then they are highly computation intensive and are thus slow. We have developed analytical methodology that substitutes for Monte Carlo simulations over a majority of the latter calculations. This causes improvements in computational speed of over 100x. We describe these methods below, and apply it to some simulated and observed data.
This work was supported by CXC NASA contract NAS8-39073 and Chandra grant AR0-11001X.
322.03
Variable Stars in the 3.6 Year DIRBE Near-Infrared Light Curve Archive
Kathleen E. Kraemer1, S. D. Price1, B. J. Smith2, T. A. Kuchar3, D. R. Mizuno3, J. Webb2
1Air Force Research Lab, 2East Tennessee State University, 3Boston College.
8:00 AM - 7:00 PM
Essex Ballroom
The 3.6 year light curve archive created by Price et al. (2010) from the cryo+post-cryo Diffuse Infrared Background Experiment (DIRBE) mission contains a wealth of variable star information at 1.25, 2.2, 3.5, and 4.9 microns. Of the ~2700 objects in the archive, over 500 show strong variability and another 75 show potential variability. We have combined visible observations obtained during the DIRBE extended mission with the infrared archive to investigate wavelength-dependent phase lags between the visible and the near-IR maxima, extending the study of Smith et al. (2006) to those stars with periods longer than the ~300 day cryo mission. Of those 518 stars exhibiting strong near-infrared variability, ~200 have visible light curves in the American Association of Variable Star Observers database during the DIRBE mission. Because viewing geometry for both the visible observers and DIRBE mission limited the opportunities for observing the stars, the light curves were inspected to determine if the peaks were defined well enough to determine phase lags among the five wavebands. For those objects that have sufficient data, we investigate a number of methods to best estimate the peaks and thus find the phase lags, if any. We have also examined the differences in phase dependence on variable type, e.g. Miras, SRa's, SRb's, and carbon stars. The DIRBE light curve data are available to the community through the Vizier service at the Centre de Donnees Astronomique de Strasbourg.
322.04
Analyzing Dust Spectra Of Oxygen-rich Agb Stars Using Spatially Resolved Spectroscopy.
Suklima Guha Niyogi1, A. K. Speck1, K. Volk2
1University of Missouri, 2Space Telescope Science Institute.
8:00 AM - 7:00 PM
Essex Ballroom
We present Gemini/ Michelle observational data of 7 oxygen-rich asymptotic giant branch (AGB) stars. Using spatially resolved spectroscopy, we investigate the changes in infrared (IR) spectral dust features with radial distance from the central star. We compare the results to those expected from the hypothetical dust condensation sequence, stellar pulsation and dust formation. In particular, the spatially resolved spectral features allow us to determine which features are correlated and help us to determine the true dust carrier(s) of observed spectral features around O-rich AGB stars.
322.05
Red-Eye Astronomy: 15 Years of V-band and Near-IR Tio Photometry of the Red Supergiants Alpha Orionis And TV Geminorum.
Richard P. Wasatonic1, E. Guinan1, S. Engle1
1Villanova Univ..
8:00 AM - 7:00 PM
Essex Ballroom
V-band and narrow to intermediate-band Wing TiO-band (719 nm, 754 nm), and near-IR 1024 nm pseudo-bolometric photometric observations of the SRc M2 Iab supergiants Alpha Orionis and TV Geminorum have been conducted for the past 15 years. The goals are to monitor brightness and temperature-dependent TiO-band variations, ascertain any resulting periodicities and amplitudes, and estimate variations of basic stellar parameters such as temperature, luminosity, and radius.
Preliminary results for both stars indicate similar long-term V-mag periods
of ~6.5 years with imposed shorter-term V-mag periods of ~1.2 years. The V-magnitude amplitudes were 0.8 for Alpha Ori and 1.3 for TV Gem. For both stars the temperature and luminosity variations correlate well with the V-mag changes.
However, inverse radii correlations with respect to temperature and luminosity variations were not seen in either star. In Alpha Ori the the radii
changes were approximately in direct correlation with the temperature and
luminosity changes. In TV Gem there was
a combination of correlation/inverse correlation effects. Causes for these
variations are speculative, but may be due to highly convective super-granulations occurring
at irregular intervals rather than fundamental mode pulsation or harmonic
oscillations. Based in part on an updated
RSG temperature scale and a new VLA/Hipparchos distance estimate (197 PC), the Alpha Ori temperatures ranged from 3550 K to 3730 K, with ranges in solar luminosities and radii of L = 90,000 to 115,000 and R = 760 to 820, respectively. The slightly more dynamical
TV Gem underwent temperature variations
from 3500 K to 3850 K with ranges in solar luminosity and radii of L = 65,000 to 90,000 and R = 620 to 720 respectively. Discussions of the observations, data reduction methods, and analysis of the data will be presented.
This research is supported by NASA grant
NNX10AI85G and NSF grants AST 10-09903 and AST 05-07542.
322.06
New Evidence for Mass-Loss from δ Cephei Based on HI 21-cm Line Observations
Lynn D. Matthews1, M. Marengo2, N. R. Evans3
1MIT Haystack Observatory, 2Iowa State University, 3Harvard-Smithsonian Center for Astrophysics.
8:00 AM - 7:00 PM
Essex Ballroom
Our recent Spitzer Space Telescope observations of the classical Cepheid archetype δ Cep revealed an extended (~21,000 AU) dusty nebula surrounding this star and its hot companion HD213307 (Marengo et al. 2010, ApJ, 725, 2392). At far infrared wavelengths, the emission resembles a bow shock aligned with the direction of space motion of the star. These observations strongly suggest that δ Cep is currently undergoing mass-loss through a stellar wind. To further investigate the nature of this probable mass-loss, we have used the Very Large Array (VLA) to search for 21-cm emission from neutral atomic hydrogen that may originate directly in the stellar wind and/or be swept up from the interstellar medium. Our VLA data reveal a spatially extended nebula of atomic hydrogen 10' (~0.8 pc) across surrounding the position of δ Cep. Although a portion of the HI line profile is strongly contaminated by Galactic emission along the line-of-sight, the emission detected within the uncontaminated portion of our band is consistent with circumstellar material originating in an optically thin wind with an outflow velocity of ~40 km/s. A portion of the detected HI emission also appears to be swept into a wake trailing the star's space motion. We will discuss the implications of these observations for Cepheid evolution and for resolving the so-called "Cepheid mass discrepancy". This work is supported by grant AST-1009644 from the National Science Foundation.
322.07
A Phase Shift in the Light Curve of Cepheid Variable CF Cas
Timothy M. Taber1, F. R. Chromey1
1Vassar College.
8:00 AM - 7:00 PM
Essex Ballroom
We present CCD photometry in the V and R bands for the Cepheid variable CF Cassiopeiae over three observing seasons, 2008-2010. The light curve shows a phase shift of 0.07 (8.2 hours) compared to values based on the cataloged epoch and period. Our data do not support a change in the period from the catalog value of 4.87522 days.
322.08
Securing the Distance Scale via a Universal VI Wesenheit Template and Deep Infrared ZAMS
Daniel Majaess1, D. Turner1, D. Lane1
1Saint Mary's University, Canada.
8:00 AM - 7:00 PM
Essex Ballroom
HST, VLBA, and HIP geometric distances to SX Phe, Delta Scuti, RR Lyrae, Type II and classical Cepheid variables are used to construct a universal VI Wesenheit template. The template uses the statistical weight of the entire variable star demographic to establish precise (<5%) distances to nearby galaxies, star clusters, etc. The reddening-free nature of the Wesenheit approach obviates the propagation of uncertainties tied to tentative total/differential extinction corrections, ensuring that further calibration may ensue directly from published or forthcoming geometric-based distances (masers, HST, GAIA, SIM). An empirical JHKs ZAMS established from deep 2MASS photometry and precise HIP parallaxes for nearby stars provides a concurrent means of securing absolute Wesenheit magnitudes for variables in stellar clusters (calibrators). The infrared ZAMS is comparatively insensitive to stellar age and [Fe/H], and yields distances to 7 of 9 benchmark open clusters that agree with the van Leeuwen (2009) revised HIP estimates (the Pleiades and Blanco 1 are discrepant cases, but should not detract from the broader consensus). In sum, the distance scale is secured to a geometrically anchored framework that consists of results from several key surveys (OGLE, NOMAD, ASAS, etc.) united by a universal VI Wesenheit template and deep infrared ZAMS. Future research entails populating the universal Wesenheit template with lower-temperature calibrators (variable red giants, Miras, longer period Cepheids, etc.) using observations acquired from the ARO, SRO (AAVSO), OMM, and DAO, and further characterizing insidious photometric contamination associated with variables occupying crowded regions near the cores of globular clusters and galaxies (including the Milky Way).
322.09
The Connection Between SX Phe and Blue Stragglers: Globular Cluster Variables as Period-Luminosity Relation Calibrators
Roger Cohen1, A. Sarajedini1
1Univ. Of Florida.
8:00 AM - 7:00 PM
Essex Ballroom
The number of known SX Phoenicis (SX Phe) pulsators in Galactic globular clusters (GGCs) and the Local Group has more than doubled in recent years. Because all SX Phe in GGCs are likely blue stragglers, it has been suggested that their pulsational properties can constrain their evolutionary histories. We investigate the connection between SX Phe and blue stragglers via period-luminosity and color-magnitude diagrams. In particular, we verify distances to a large set of GGCs by fitting a carefully chosen set of nearby, unevolved subdwarfs to the cluster main sequences. We use these distances to analyze the pulsational properties of all known SX Phe, including SX Phe period-luminosity relations obtained using the SX Phe in these GGCs as calibrators, together with those in Local Group dwarf galaxies and their clusters. Finally, we discuss implications for blue straggler formation and evolution, and address both observational and theoretical avenues for further investigation.
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