Publications of the Astronomical Society of the Pacific

Rakos, Karl; Schombert, James

doi: 10.1086/427987pmid: N/A

We present a new technique for the determination of age and metallicity in composite stellar populations using Strömgren filters. Using principal component (PC) analysis on multicolor models, we isolate the range of values necessary to uniquely determine age and metallicity effects. The technique presented here can only be applied to old (τ>3 Gyr) stellar systems composed of simple stellar populations, such as globular clusters and elliptical galaxies. Calibration using new photometry of 40 globular clusters with spectroscopic [Fe/H] values and main‐sequence–fitted ages links the PC values to the Strömgren colors, for an accuracy of 0.2 dex in metallicity and 0.5 Gyr in age.

Ma, Jun; Zhou, Xu; Chen, Jiansheng; Wu, Zhenyu; Yang, Yanbin; Jiang, Zhaoji; Wu, Jianghua

doi: 10.1086/428276pmid: N/A

In this paper we present catalogs of photometric and spectroscopic data for globular clusters (GCs) in M81. The catalogs include B‐ and V‐band photometric and reddening data of 95 GCs, in addition to spectroscopic metallicities of 40 GCs. Using these data, we make some statistical correlations. The results show that the distributions of intrinsic B and V colors and metallicities are bimodal, with metallicity peaks at [Fe/H] ≈ - 1.45 and −0.53, respectively, as has been demonstrated for our Milky Way and M31. The relation between spectroscopic metallicity and intrinsic B and V color also exists as it does for the Milky Way and M31.

Tramposch, Jonica; Homer, Lee; Szkody, Paula; Henden, Arne; Silvestri, Nicole M.; Yirak, Kris; Fraser, Oliver J.; Brinkmann, J.

doi: 10.1086/428271pmid: N/A

We report follow‐up observations of the Sloan Digital Sky Survey cataclysmic variable SDSS J210014.12+004446.0 (hereafter SDSS J2100). We obtained photometry and spectroscopy in both outburst and quiescent states, providing the first quiescent spectrum of this source. In both states, nonsinusoidal photometric modulations suggestive of superhumps are apparent, placing SDSS J2100 in the SU UMa subclass of dwarf novae. However, the periods during outburst and quiescence differ significantly, at 2.099 ± 0.002 and 1.96 ± 0.02 hr, respectively. Our phase‐resolved spectroscopy during outburst yielded an estimate of ∼2 hr for the orbital period, consistent with the photometry. The presence of the shorter period modulation at quiescence is unusual, but not unique. Another atypical feature is the relative weakness of the Balmer emission lines in quiescence. Overall, we find a close similarity between SDSS J2100 and the well‐studied superhump cataclysmic variable V503 Cygni. By analogy, we suggest that the quiescent modulation is due to a tilted accretion disk—producing negative superhumps—and the modulation in outburst is due to positive superhumps from the precession of an elliptical disk.

Zamanov, R.; Gomboc, A.; Bode, M. F.; Porter, J. M.; Tomov, N. A.

doi: 10.1086/428069pmid: N/A

We report on a search conducted for variability in the Hα emission line of the recurrent nova T CrB with a time resolution of 10–15 minutes. This is comparable to the timescale of the photometric flickering observed in this object. This is the first time that observations of the short‐timescale emission‐line variation have been made for this system. On two nights (1999 January 6 and 7), we detected statistically significant variability (at the 99% confidence level) in the Hα line profile. This variability is confined to the central part of the emission line (±100 km s−1), although FWZI(Hα) ∼ 800 km s−1. The variability in the line profile is accompanied by variability of the total equivalent width, EW(Hα): ±8% for 1999 January 6 and ±6% for 1999 January 7 (calculated from the mean EW value). Assuming Keplerian motion, the variability is generated at a distance of ∼20–30 R⊙ from the white dwarf, which is approximately the radius of the ring that the stream of gas forms as it flows away from the L1 Lagrangian point. For three other nights we are only able to put upper limits on the variability, ΔEW(Hα): ±2% for 1998 April 15, ±4% for 1998 August 2, and ±3% for 1998 August 3.

Makovoz, David; Khan, Iffat; Moshir, Mehrdad

doi: 10.1086/428409pmid: N/A

We present an algorithm for achieving background consistency in overlapping images. The algorithm consists of two main steps. First, the images are interpolated to a common grid. Second, the cumulative pixel‐by‐pixel difference between the overlapping areas of all pairs of images is minimized with respect to the unknown constant offsets of the input images. The images are corrected by adding the computed offsets. Optionally, detection and masking of bright objects in the images can be performed prior to minimization. This option is especially useful for images with a high density of cosmic‐ray hits. We use simulated data to optimize the algorithm in order to achieve a trade‐off between the speed of execution and the quality of the background matching. We also show several examples of the application of this algorithm to real Spitzer data. The algorithm has been implemented and successfully used by the Spitzer image processing package MOPEX (Makovoz & Khan) and deployed by the Spitzer automated pipeline.

Tonry, John L.; Howell, Steve B.; Everett, Mark E.; Rodney, Steven A.; Willman, Mark; VanOutryve, Cassandra

doi: 10.1086/428596pmid: N/A

We observed the young open cluster NGC 2301 for 14 nights in 2004 February using the orthogonal transfer CCD camera OPTIC. We used point‐spread function shaping techniques (“square stars”) during the observations, allowing us to obtain a larger dynamic range (4.5 mag) of high photometric precision results (≤2 mmag). These results are better than similar observing campaigns using standard CCD imagers. This paper discusses our observational techniques and presents initial results for the variability statistics found in NGC 2301. Details of the variability statistics as functions of color, variability type, stellar type, and cluster location will appear in Paper II.

Roberts, Lewis C.; Maeda, Riki; Rehder, Karl H.; Glesne, Thomas R.

doi: 10.1086/428270pmid: N/A

Most ground‐based telescope mirror coatings are exposed to hostile environments, as the humidity, dust, and reactive gases in the air corrode and tarnish the coating. As the coating degrades the scatter increases. Eventually, every mirror needs to be recoated. This is a costly, time‐consuming process that requires telescope down time and involves risks to both people and to equipment. The AEOS (Advanced Electro‐Optical System) telescope has a mirror purge system that fills the mirror cell with dry air while the telescope is stowed. We present results conclusively demonstrating that this system extends the lifetime of the AEOS primary mirror coating, reducing the need for burdensome recoating.

Murakami, Naoshi; Baba, Naoshi

doi: 10.1086/428272pmid: N/A

A coronagraph with a four‐quadrant phase mask (FQPM) can perfectly eliminate stellar light under ideal conditions. However, the coronagraphic performance is severely degraded in a telescope with a central obscuration. We show that a high‐performance FQPM coronagraph can be realized with a centrally obscured telescope by using a two‐mirror pupil‐remapping system. We calculate the optimal shapes of the remapping mirrors to transform a centrally obscured pupil into clear circular one. The mirror shapes become parabola‐like, and the remapping mirrors are shown to provide good coronagraphic performance. The expected performance with adaptive optics systems is also evaluated.