Publications of the Astronomical Society of the Pacific

Wallerstein, George; Kovtyukh, V. V.; Andrievsky, S. M.

doi: 10.1086/587533pmid: N/A

High-dispersion spectra were obtained to derive the radial velocity and chemical composition of the distant Cepheid QQ Per. Its location at l = 141°, b = -6.5° allows it to be either a very distant classical Cepheid at 31 kpc from the Galactic center or a type II Cepheid at roughly half that distance. Five spectra at various phases were analyzed for abundances and an additional four spectra were useful for radial velocity. Effective temperatures ranging from 5100 to 5700 K were derived from the five spectra by requiring that there be no dependence of the iron abundance as derived from lines arising from a range of excitation potentials. The metallicity from the mean of five spectra is [Fe/H] = -0.6 ± 0.1. While this allows membership in either the thick disk or the distant thin disk, the presence of H emission at almost all phases indicates that QQ Per is a type II Cepheid of the thick disk. Relative to iron we find a small excess of oxygen, as often seen in metal-poor stars, and a surprising excess of C and N, as derived from high excitation lines. We show that the excess remains so long as no temperature error remains greater than about 200 K. Small deficiencies of the s-process elements appear to be present in common with many metal-poor stars.

Adelman, Saul J.

doi: 10.1086/587701pmid: N/A

New differential Strömgren uvby Four College Automated Photoelectric Telescope (FCAPT) observations of four magnetic CP stars HD 86592, HR 4330, HR 6958, and HR 7786 are presented. These observations are analyzed along with published FCAPT data to improve the periods and the light curves. The period of HD 86592 is that of Babel & North, 2.886669 days. The new periods of HR 4330, HR 6958, and HR 7786 are 3.152, 18.0642, and 8.5295 days, respectively, almost the same as their previous values from FCAPT data. The first three stars can be characterized as having constant light curves and periods. However, the small differences among the seasonal light curves of HR 7786 indicate it is a star whose rotational axis is precessing about its magnetic axis.

Shafter, A. W.; Davenport, J. R. A.; Güth, T.; Kattner, S.; Marin, E.; Sreenivasamurthy, N.

doi: 10.1086/588268pmid: N/A

Time-resolved CCD photometry is presented of the V ∼ 17 optical counterpart of the newly discovered hard-X-ray–emitting polar Swift J2619.4+2619. A total of ∼20 hr of data obtained over five nights in various bandpasses (B, V, R, and I) reveals a strong quasi-sinusoidal modulation in the light curve at a best-fitting period of 0.1254 days (3.01 hr), which we associate with the orbital period of the system (one-day aliases of this period at 0.1114 days and 0.1435 days are considered, but appear to be ruled out by our analysis). The amplitude of the modulation increases with wavelength from in B to ∼1.1 mag in R and I. The increase in amplitude with wavelength is typical of polar systems where the modulated radiation comes from cyclotron emission. The combination of the relatively long orbital period and the emission of hard X-rays suggest that Swift J2619.4+2619 may be a good candidate for an asynchronous polar system.

Weidmann, Walter A.; Díaz, Rubén J.

doi: 10.1086/587788pmid: N/A

We analyze the spatial orientation of a homogenous sample of 440 elongated Planetary Nebulae (PNe) in order to determine the orientation of their apparent major axis with respect to the Milky Way plane. We present some important geometrical and statistical considerations that have been overlooked by previous works on the subject. The global distribution of galactic position angles (GPA) of PNe is quantitatively not very different from a random distribution of orientations in the Galaxy. Nevertheless, we find that there is at least one region on the sky, toward the Galactic center, where a weak correlation may exist between the orientation of the major axis of some PNe and the Galactic equator, with an excess of axes with GPA ∼100°. Therefore, we confirm that “extrinsic” phenomena (i.e., global Galactic magnetic fields, shell compression from motion relative to the Interstellar Medium) do not determine the morphology of PNe on most of the sky, with a possible exception toward the Galactic center.

Bauer, James M.; Choi, Young-Jun; Weissman, Paul R.; Stansberry, John A.; Fernández, Yanga R.; Roe, Henry G.; Buratti, Bonnie J.; Sung, Hyun-Il

doi: 10.1086/587552pmid: N/A

On 2005 December 30, Y.-J. Choi and P. R. Weissman discovered that the formerly dormant Centaur 2000 EC98 was in strong outburst. Previous observations by P. Rousselot et al. spanning a 3-year period indicated a lack of coma down to the 27 mag arcsec-2 level. We present Spitzer Space Telescope MIPS observations of this newly active Centaur—now known as 174P/Echeclus (2000 EC98)—or 60558 Echeclus—taken in 2006 late February. The images show strong signal at both the 24 and 70 μm bands and reveal an extended coma about 2′ in diameter. Analyses yield estimates of the coma signal contribution that are in excess of 90% of the total signal in the 24 μm band. Dust production estimates ranging from 1.7–4 × 102 kg s-1 are on the order of 30 times that seen in other Centaurs. Simultaneous visible-wavelength observations were also obtained with Palomar Observatory's 200-inch telescope, the 1.8-m Vatican Advanced Technology Telescope, the Bohyunsan Optical Astronomy Observatory (BOAO) 1.8-m telescope, and Table Mountain Observatory's 0.6-m telescope, revealing a coma morphology nearly identical to the mid-IR observations. The grain size distribution derived from the data yields a log particle mass power-law with slope parameter α = -0.87 ± 0.07, and is consistent with steady cometary activity, such as that observed during the Stardust spacecraft’s encounter at 81P/Wild 2, and not with an impact-driven event, such as that caused by the Deep Impact experiment.

Greiner, J.; Bornemann, W.; Clemens, C.; Deuter, M.; Hasinger, G.; Honsberg, M.; Huber, H.; Huber, S.; Krauss, M.; Krühler, T.; Küpcü Yoldaş, A.; Mayer-Hasselwander, H.; Mican, B.; Primak, N.; Schrey, F.; Steiner, I.; Szokoly, G.; Thöne, C. C.;

Mittag, M.; Hempelmann, A.; Gonzalez-Perez, J. N.; Schmitt, J. H. M. M.

doi: 10.1086/533478pmid: N/A

A first pointing model was determined during commissioning of the Hamburg Robotic Telescope in 2005 September. Pointing accuracy better than 3″ was achieved with this model in those days. However, in the course of the rest of 2005, a systematic increase of the telescope mispointing mainly in azimuth was observed having been suggested a strong dependence on ambient air temperature. We therefore checked this relation between temperature and pointing accuracy by systematic observations targeted on temperature. We made 16 pointing-model estimates during the year 2006 and correlated the model parameters with temperature. While most of the parameters are either not correlated or merely weakly correlated with temperature we find a clear temperature dependence of a misalignment of the optical axis with the telescope tube. We suggest that the M3 mounting is responsible for this.

Chuss, David T.; Wollack, Edward J.; Moseley, S. Harvey; Withington, Stafford; Saklatvala, George

doi: 10.1086/587494pmid: N/A

Filled arrays of bolometers are currently being employed for use in astronomy from the far-infrared through millimeter portion of the electromagnetic spectrum. Because of the large range of wavelengths for which such detectors are applicable, the size of a pixel (p) relative to the wavelength (λ) will vary according to the specific application of a given available technology. As the pixel size becomes comparable to a wavelength of the absorbed radiation, correlations in the radiation field at the pixel can affect the imaging properties of the system independent of the correlations introduced by the front-end optics. We study the dependence of image fidelity and induced polarization on the size of the pixel by employing a formalism in which diffraction due to the pixel boundary is treated by propagating the second-order statistical correlations of the radiation field through an optical system. We construct simulated polarized images of square pixels for various ratios of p/λ. For the limit in which few modes are supported by the pixel (p/λ ≲ 1), we find that the diffraction due to the pixel edges is nonnegligible and hence must be considered along with the telescope diffraction pattern in modeling the ultimate spatial resolution of an imaging system. For the case in which the pixel is over-moded (p/λ≫1), the geometric limit is approached as expected. This technique provides a quantitative approach to understand and optimize the imaging properties of planar detectors in the few-mode limit.

Cotton, W. D.

doi: 10.1086/586754pmid: N/A

Obit is a development environment for astronomical algorithms. It implements multiple “native” data formats including FITS and AIPS. The shared data model with AIPS means almost complete interoperability, allowing algorithmic development in a production environment. The Obit package supports both prepackaged, compiled tasks and a python interface to the major class functionality to allow rapid prototyping using python scripts. Developed features are predominantly radio astronomical, but support for astronomical images and tables is generic and should allow a wider range of applications. The design is intended to be readily extensible. An example is given that explores the effect of “CLEAN bias” in the VLA Low-Frequency Sky Survey and compares its method of CLEAN to an automated CLEAN window selection scheme that should reduce the bias. This technique shows much promise for reducing this systematic effect.

Miller, J. Patrick; Pennypacker, C. R.; White, Graeme L.

doi: 10.1086/588258pmid: N/A

To detect objects that vary in brightness or spatial coordinates over time, C. Alard and R. H. Lupton in 1998 proposed an “optimal image subtraction” (OIS) method that constructs a convolution kernel from a set of matching stars distributed across the two images to be subtracted. Using multivariable least squares, the kernel is derived and can be designed to vary by pixel coordinates across the convolved image. Local effects in the optics, including aberrations or other spatially sensitive perturbations to a perfect image, can be mitigated. This paper presents the specific systems of equations that originate from the OIS method. Also included is a complete description of the Gaussian components basis vectors used by Alard & Lupton to construct the convolution kernel. An alternative set of basis vectors, called the delta function basis, is also described. Important issues are addressed, including the selection of the matching stars, differential background correction, constant photometric flux, contaminated pixel masking, and alignment at the subpixel level. Computer algorithms for the OIS method were developed, written using the Interactive Data Language (IDL), and applications demonstrating these algorithms are presented.