Publications of the Astronomical Society of the Pacific

Werner, Klaus; Herwig, Falk

doi: 10.1086/500443pmid: N/A

We review the observed properties of extremely hot, hydrogen‐deficient post–asymptotic giant branch (AGB) stars of spectral type [WC] and PG1159. Their H deficiency is probably caused by a (very) late helium‐shell flash or an AGB final thermal pulse, laying bare interior stellar regions that are usually kept hidden below the hydrogen envelope. Thus, the photospheric elemental abundances of these stars allow us to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We summarize the state of the art of stellar evolution models that simulate AGB evolution and the occurrence of a late He‐shell flash. We compare predicted elemental abundances to those determined by quantitative spectral analyses performed with advanced non‐LTE model atmospheres. Good qualitative and quantitative agreement is found. Future work can contribute to an even more complete picture of the nuclear processes in AGB stars.

Davis, Tamara M.; Schmidt, Brian P.; Kim, Alex G.

doi: 10.1086/499116pmid: N/A

To use Type Ia supernovae as standard candles for cosmology, we need accurate broadband magnitudes. In practice the observed magnitude may differ from the ideal magnitude‐redshift relationship either through intrinsic inhomogeneities in the Type Ia supernova population, or through observational error. Here we investigate how we can choose filter bandpasses to reduce the error caused by both these effects. We find that bandpasses with large integral fluxes and sloping wings are best able to minimize several sources of observational error, and are also least sensitive to intrinsic differences in Type Ia supernovae. The most important feature of a complete filter set for Type Ia supernova cosmology is that each bandpass be a redshifted copy of the first. We design practical sets of redshifted bandpasses that are matched to typical high‐resistivity CCD and HgCdTe infrared detector sensitivities. These are designed to minimize systematic error in well‐observed supernovae; final designs for specific missions should also consider signal‐to‐noise ratio requirements and observing strategy. In addition, we calculate how accurately filters need to be calibrated in order to achieve the required photometric accuracy of future supernova cosmology experiments, such as the Supernova /Acceleration Probe (SNAP), which is one possible realization of the Joint Dark Energy Mission (JDEM). We consider the effect of possible periodic miscalibrations that may arise from the construction of an interference filter.

Woolf, Vincent M.; Wallerstein, George

doi: 10.1086/498459pmid: N/A

We report progress in the calibration of a method to determine cool dwarf star metallicities using molecular band strength indices. The molecular band index to metallicity relation can be calibrated using chemical abundances calculated from atomic‐line equivalent width measurements in high‐resolution spectra. Building on previous work, we have measured Fe and Ti abundances in 32 additional M and K dwarf stars to extend the range of temperature and metallicity covered. A test of our analysis method using warm star–cool star binaries shows we can calculate reliable abundances for stars warmer than 3500 K. We have used abundance measurements for warmer binary or cluster companions to estimate abundances in six additional cool dwarfs. Adding stars measured in our previous work and others from the literature provides 76 stars with Fe abundance and CaH2 and TiO5 index measurements. The CaH2 molecular index is directly correlated with temperature. TiO5 depends on temperature and metallicity. Metallicity can be estimated to within ±0.3 dex within the bounds of our calibration, which extends from roughly [Fe/H] = +0.05 to −1.0, with a limited extension to −1.5.

Paulson, Diane B.; Allred, Joel C.; Anderson, Ryan B.; Hawley, Suzanne L.; Cochran, William D.; Yelda, Sylvana

doi: 10.1086/499497pmid: N/A

We present optical spectra of a flare on Barnard’s star. Several photospheric and chromospheric species were enhanced by the flare heating. An analysis of the Balmer lines shows that their shapes are best explained by Stark broadening rather than chromospheric mass motions. We estimate the temperature of the flaring region in the lower atmosphere to be ≥8000 K and the electron density to be ∼1014 cm−3, similar to values observed in other dM flares. Because Barnard’s star is considered to be one of our oldest neighbors, a flare of this magnitude is probably quite rare.

Templeton, M. R.; Leaman, R.; Szkody, P.; Henden, A.; Cook, L.; Starkey, D.; Oksanen, A.; Koppelman, M.; Boyd, D.; Nelson, P. R.; Vanmunster, T.; Pickard, R.; Quinn, N.; Huziak, R.; Aho, M.; James, R.; Golovin, A.;

French, R. G.; McGhee, C. A.; Frey, M.; Hock, R.; Rounds, S.; Jacobson, R.; Verbiscer, A.

doi: 10.1086/499215pmid: N/A

Highly accurate astrometric positions of 14 of Saturn’s satellites have been obtained from 444 Hubble Space Telescope images taken with the Wide Field Planetary Camera 2 (WFPC2) between 1996 and 2005. In all, 1036 satellite positions were measured in Planetary Camera (PC) frames, with a typical uncertainty of σPC = 0.″014 (80 km at Saturn), and 1403 positions from Wide Field (WF) frames, with σWF = 0.″020 (120 km at Saturn). A key part of the reduction involved the application of an improved WFPC2 distortion‐correction scheme (Anderson & King) and precise determination of the relative positions of the PC and WF chips, which varied substantially over the full course of the observation period. The time span covered by the observations is more than twice the nominal duration of the Cassini mission and thus provides an important baseline of measurements that is particularly important for studying time‐variable phenomena such as the orbital exchange of Janus and Epimetheus and the chaotic interactions of Prometheus and Pandora. These results have been incorporated into ephemerides that are being used for planning and analysis of Cassini satellite and ring observations.

Soker, Noam

doi: 10.1086/498829pmid: N/A

An increasing amount of the literature reports the detection of magnetic fields in asymptotic giant branch (AGB) stars and in central stars of planetary nebulae (PNe). These detections lead to claims that the magnetic fields are the main agent shaping the PNe. In this paper, I examine the energy and angular momentum carried by magnetic fields expelled from AGB stars, as well as other physical phenomena that accompany the presence of large‐scale fields, such as those claimed in the literature. I show that a single star cannot supply the energy and angular momentum if the magnetic fields have the large coherent structure required to shape the circumstellar wind. Therefore, the structure of nonspherical planetary nebulae cannot be attributed to dynamically important large‐scale magnetic fields. I conclude that the observed magnetic fields around evolved stars can be understood with respect to locally enhanced magnetic loops, which can have a secondary role in the shaping of the PN. The primary role, I argue, rests with the presence of a companion.

Pasquali, A.; Pirzkal, N.; Larsen, S.; Walsh, J. R.; Kümmel, M.

doi: 10.1086/498731pmid: N/A

The Advanced Camera for Surveys on board the Hubble Space Telescope is equipped with one grism and three prisms for low‐resolution, slitless spectroscopy in the range 1150–10500 Å. The G800L grism provides optical spectroscopy between 5500 Å and >1 μm, with a mean dispersion of 39 and 24 Å pixel−1 (in the first spectral order) when coupled with the Wide Field and the High Resolution Channels, respectively. Given the lack of any on‐board calibration lamps for wavelength and narrowband flat‐fielding, the G800L grism can only be calibrated using astronomical targets. In this paper, we describe the strategy used to calibrate the grism in orbit, with special attention given to the treatment of the field dependence of the grism flat field, wavelength solution, and sensitivity in both channels.

Guidorzi, C.; Monfardini, A.; Gomboc, A.; Mottram, C. J.; Mundell, C. G.; Steele, I. A.; Carter, D.; Bode, M. F.; Smith, R. J.; Fraser, S. N.; Burgdorf, M. J.; Newsam, A. M.

doi: 10.1086/499289

Wizinowich, Peter L.; Le Mignant, David; Bouchez, Antonin H.; Campbell, Randy D.; Chin, Jason C. Y.; Contos, Adam R.; van Dam, Marcos A.; Hartman, Scott K.; Johansson, Erik M.; Lafon, Robert E.; Lewis, Hilton; Stomski, Paul J.; Summers, Douglas M.; Brown, Curtis G.; Danforth, Pamela M.;