Publications of the Astronomical Society of the Pacific

Fryer, Chris L.; Mazzali, Paolo A.; Prochaska, Jason; Cappellaro, Enrico; Panaitescu, Alin; Berger, Edo; van Putten, Maurice; van den Heuvel, Ed P. J.; Young, Patrick; Hungerford, Aimee; Rockefeller, Gabriel; Yoon, Sung‐Chul; Podsiadlowski, Philipp; Nomoto, Ken’ichi; Chevalier, Roger; Schmidt, Brian; Kulkarni, Shri

doi: 10.1086/523768pmid: N/A

Although there is strong support for the collapsar engine as the power source of long‐duration gamma‐ray bursts (GRBs), we still do not definitively know the progenitor of these explosions. Here we review the current set of progenitor scenarios for long‐duration GRBs and the observational constraints on these scenarios. Examining these models, we find that single stars cannot be the only progenitor for long‐duration GRBs. Several binary progenitors can match the solid observational constraints and also have the potential to match the trends that we are currently seeing in the observations. Type Ib/c supernovae are also likely to be produced primarily in binaries; we discuss the relationship between the progenitors of these explosions and those of the long‐duration GRBs.

Koo, J.‐R.; Kim, S.‐L.; Rey, S.‐C.; Lee, C.‐U.; Kim, Y. H.; Kang, Y. B.; Jeon, Y.‐B.

doi: 10.1086/523113pmid: N/A

V‐band time‐series CCD photometric observations of the intermediate‐age open cluster M11 were performed to search for variable stars. Using these time‐series data, we carefully examined light variations of all stars in the observing field. A total of 82 variable stars were discovered, of which 39 stars had been detected recently by Hargis et al. On the basis of observational properties such as variable period, light‐curve shape, and position on a color‐magnitude diagram, we classified their variable types as 11 δ Scuti–type pulsating stars, two γ Doradus–type pulsating stars, 40 W UMa–type contact eclipsing binaries, 13 Algol‐type detached eclipsing binaries, and 16 eclipsing binaries with long period. Cluster membership for each variable star was deduced from the previous proper‐motion results and position on the color‐magnitude diagram. Many pulsating stars and eclipsing binaries in the region of M11 are probable members of the cluster.

Turner, David G.; Bryukhanov, Ivan S.; Balyuk, Igor I.; Gain, Alexey M.; Grabovsky, Roman A.; Grigorenko, Valery D.; Klochko, Igor V.; Kosa‐Kiss, Attila; Kosinsky, Alexey S.; Kushmar, Ivan J.; Mamedov, Vyacheslav T.; Narkevich, Natalya A.; Pogosyants, Andrey J.; Semenyuta, Andrey S.; Sergey, Ivan M.;

Adelman, Saul J.; Woodrow, Stephanie L.

doi: 10.1086/523900pmid: N/A

If magnetic fields cause elemental abundances to change in the stellar photosphere, then all magnetic chemically peculiar (mCP) stars should exhibit signatures of this effect in both their spectra and flux distributions. Given that all stars rotate, these stars should be magnetic, spectrum, and photometric variables, albeit sometimes of low amplitude. We study differential Strömgren observations from the Four College Automated Photoelectric Telescope (FCAPT) of α Psc, HR 5857, and HR 6709, which are small‐amplitude mCP stars, and AB Cet, HD 15890, and HR 8240, which were thought to be near‐constant mCP stars, to determine the periods and amplitudes of their photometric variability. We construct the photometric amplitude distribution functions of the mCP stars studied by the first author to obtain statistics on their photometric variability in Strömgren photometry. The star α Psc has a period of 0.74552 days, with its largest amplitude in u of 0.030 mag. The largest amplitude (0.035 mag) for HR 5857 is also for u. We confirmed the period of 1.29957 days found by Hatzes using Doppler imaging. For HR 6709 the largest amplitude observed (0.010 mag) is for u photometry. Its period is 1.20352 days. AB Cet might be minimally variable. HD 15980 appears to be minimally variable, with a period of at least 5 yr. HR 8240 is variable with a b amplitude about 0.05 mag and a period that is several years long. Additional observations of these six stars would be useful to confirm their characteristics. Two apparently constant stars studied with FCAPT Strömgren data, HD 11187 and HD 50169, should be checked to see whether they are long‐period variables. If additional observations show their variability, then all mCP stars observed photometrically by the first author with the FCAPT will be found to be variable.

Wallerstein, George; Sandstrom, Karin; Gredel, Roland

doi: 10.1086/521835pmid: N/A

We have obtained high‐resolution spectra at the Dominion Astrophysical Observatory, the Calar Alto Observatory, and the European Southern Observatory of hot stars in the near‐infrared region to study the diffuse interstellar band (DIB) at 8621 Å. Field stars as well as selected members of the Perseus spiral arm, the ρ Ophiuchi complex, and the Cygnus OB2 association were observed and the equivalent widths of the band measured, as well as interstellar K i absorption at 7699 Å. In total we measure the equivalent width of the 8621 band in 64 stars. In a series of figures we show the correlations of the DIB with reddening, polarization, K i, and strengths of other DIBs at 5780, 5797, and 6613 Å. The quality of the correlations are discussed by means of the Spearman rank correlation test. Good correlations are found with reddening and, among the other DIBs, 8621 correlates best with 5780. By examining the three special regions mentioned above we confirm the weakness of the 8621 DIB in the hot stars of the ρ Oph star‐forming region. In the Perseus spiral arm region we find that the DIBs are formed largely in foreground clouds but not in the gas that appears to have been blown away from the young massive stars of the Perseus arm. In Cyg OB2 we find that the equivalent widths of the 8621 line lie above the linear correlation of equivalent width with E(B - V) and their strength does not vary as strongly with reddening. The heavily reddened Cyg OB2 star 12 is the most extreme example of the relative insensitivity of the 8621 equivalent width to reddening in the Cygnus OB2 region.

Baltay, C.; Rabinowitz, D.; Andrews, P.; Bauer, A.; Ellman, N.; Emmet, W.; Hudson, R.; Hurteau, T.; Jerke, J.; Lauer, R.; Silge, J.; Szymkowiak, A.; Adams, B.; Gebhard, M.; Musser, J.; Doyle, M.; Petrie, H.; Smith, R.; Thicksten, R.;

High, F. William; Rhodes, Jason; Massey, Richard; Ellis, Richard

doi: 10.1086/523112pmid: N/A

Weak gravitational lensing can be used to investigate both dark matter and dark energy but requires accurate measurements of the shapes of faint, distant galaxies. Such measurements are hindered by the finite resolution and pixel scale of digital cameras. We investigate the optimum choice of pixel scale for a space‐based mission, using the engineering model and survey strategy of the proposed Supernova Acceleration Probe as a baseline. We do this by simulating realistic astronomical images containing a known input shear signal and then attempting to recover the signal using the Rhodes, Refregier, & Groth algorithm. We find that the quality of shear measurement is always improved by smaller pixels. However, in practice, telescopes are usually limited to a finite number of pixels and operational life span, so the total area of a survey increases with pixel size. We therefore fix the survey lifetime and the number of pixels in the focal plane while varying the pixel scale, thereby effectively varying the survey size. In a pure trade‐off for image resolution versus survey area, we find that measurements of the matter power spectrum would have minimum statistical error with a pixel scale of 0.09″ for a 0.14″ FWHM point‐spread function (PSF). The pixel scale could be increased to ∼0.16″ if images dithered by exactly half‐pixel offsets were always available. Some of our results do depend on our adopted shape measurement method and should be regarded as an upper limit: future pipelines may require smaller pixels to overcome systematic floors not yet accessible, and, in certain circumstances, measuring the shape of the PSF might be more difficult than those of galaxies. However, the relative trends in our analysis are robust, especially those of the surface density of resolved galaxies. Our approach thus provides a snapshot of potential in available technology, and a practical counterpart to analytic studies of pixelation, which necessarily assume an idealized shape measurement method.

McCallon, Howard L.; Fowler, John W.; Laher, Russ R.; Masci, Frank J.; Moshir, Mehrdad

doi: 10.1086/523599pmid: N/A

Position reconstruction for images acquired by the Infrared Array Camera (IRAC), one of the science instruments onboard the Spitzer Space Telescope, is a multistep procedure that is part of the routine processing done at the Spitzer Science Center (SSC). The IRAC instrument simultaneously images two different sky footprints, each with two independent infrared passbands (channels). The accuracy of the initial Spitzer pointing reconstruction is typically slightly better than 1″. The well‐known technique of position matching imaged point sources to even more accurate star catalogs to refine the pointing further is implemented for SSC processing of IRAC data as well. Beyond that, the optimal processing of redundant pointing information from multiple instrument channels to yield an even better solution is also performed at the SSC. Our multichannel data processing approach is particularly beneficial when the star‐catalog matches are sparse in one channel but copious in others. A thorough review of the algorithm as implemented for the Spitzer mission reveals that the mathematical formalism can be fairly easily generalized for application to other astronomy missions. The computation of pointing uncertainties, the interpolation of pointing corrections and their uncertainties between measurements, and the estimation of random‐walk deviations from linearity are special areas of importance when implementing the method. After performing the operations described in this paper on the initial Spitzer pointing, the uncertainty in the observatory pointing history file is reduced 10–15 fold.

Laidler, Victoria G.; Papovich, Casey; Grogin, Norman A.; Idzi, Rafal; Dickinson, Mark; Ferguson, Henry C.; Hilbert, Bryan; Clubb, Kelsey; Ravindranath, Swara

doi: 10.1086/523898pmid: N/A

We describe the TFIT software package to measure galaxy photometry using prior information from high‐resolution observations. Our basic methodology is similar in principle but different in detail from previous procedures for crowded field photometry. We use the spatial positions and morphologies of objects in an image with higher angular resolution to construct object templates, which are then fitted to a lower resolution image, solving for the object fluxes as free parameters. Using extensive experiments on both simulated and real data, we show that this template‐fitting method measures accurate object photometry to the limiting sensitivity of the image. In this limit, our method derives robust flux upper limits for objects fainter than the limiting image surface brightness. We describe the challenges encountered in applying this technique to real data, and methods to cope with some of them.

Subasavage, John P.

doi: 10.1086/523600pmid: N/A