Publications of the Astronomical Society of the Pacific

Vereš, Peter; Farnocchia, Davide; Jedicke, Robert; Spoto, Federica

doi: 10.1086/676676pmid: N/A

We study the time evolution of the impact probability for synthetic, but realistic, impacting and close-approaching asteroids detected in a simulated all-sky survey. We use the impact probability to calculate the impact warning time (tw) as the time interval between when an object reaches a Palermo Scale value of -2 and when it impacts Earth. A simple argument shows that tw ∝ Dx, with the exponent in the range [1.0,1.5], and our derived value was x = 1.3 ± 0.1. The low-precision astrometry from the single simulated all-sky survey could require many days or weeks to establish an imminent impact for asteroids larger than 100 m in diameter that are discovered far from Earth. Most close-approaching asteroids are quickly identified as not being impactors, but a size-dependent percentage, even for those larger than 50 m diameter, have a persistent impact probability of greater than 10-6 on the day of closest approach. Thus, a single all-sky survey can be of tremendous value in identifying Earth-impacting and close-approaching asteroids in advance of their closest approach, but it can not solve the problem on its own: high-precision astrometry from other optical or radar systems is necessary to rapidly establish an object as an impactor or close approacher. We show that the parallax afforded by surveying the sky from two sites is only of benefit for a small fraction of the smallest objects detected within a couple days before impact: probably not enough to justify the increased operating costs of a two-site survey. Finally, the survey cadence within a fixed time span is relatively unimportant to the impact probability calculation. We tested three different reasonable cadences and found that one provided ∼10 times higher (better) value for the impact probability on the discovery night for the smallest (10 m diameter) objects, but the consequences on the overall impact probability calculation are negligible.

Halverson, Samuel; Mahadevan, Suvrath; Ramsey, Lawrence; Hearty, Fred; Wilson, John; Holtzman, Jon; Redman, Stephen; Nave, Gillian; Nidever, David; Nelson, Matt; Venditti, Nick; Bizyaev, Dmitry; Fleming, Scott

Maier, D.; Tenzer, C.; Santangelo, A.

doi: 10.1086/676820pmid: N/A

Linear polarization measurements provide access to two quantities, the degree (DOP) and the angle of polarization (AOP). The aim of this work is to give a complete and concise overview of how to analyze polarimetric measurements. We review interval estimations for the DOP with a frequentist and a Bayesian approach. Point estimations for the DOP and interval estimations for the AOP are further investigated with a Bayesian approach to match observational needs. Point and interval estimations are calculated numerically for frequentist and Bayesian statistics. Monte Carlo simulations are performed to clarify the meaning of the calculations. Under observational conditions, the true DOP and AOP are unknown, so that classical statistical considerations—based on true values—are not directly usable. In contrast, Bayesian statistics handles unknown true values very well and produces point and interval estimations for DOP and AOP directly. Using a Bayesian approach, we show how to choose DOP point estimations based on the measured signal-to-noise ratio. Interval estimations for the DOP show great differences in the limit of low signal-to-noise ratios between the classical and Bayesian approach. AOP interval estimations that are based on observational data are presented for the first time. All results are directly usable via plots and parametric fits.

Petit, P.; Louge, T.; Théado, S.; Paletou, F.; Manset, N.; Morin, J.; Marsden, S. C.; Jeffers, S. V.

doi: 10.1086/676976pmid: N/A

PolarBase is an evolving database that contains all stellar data collected with the ESPaDOnS and NARVAL high-resolution spectropolarimeters, in their reduced form, as soon as they become public. As of early 2014, observations of 2000 stellar objects throughout the Hertzsprung-Russell diagram are available. Intensity spectra are available for all targets, and the majority of the observations also include simultaneous spectra in circular or linear polarization, with the majority of the polarimetric measurements being performed only in circularly polarized light (Stokes V). Observations are associated with a cross-correlation pseudoline profile in all available Stokes parameters, greatly increasing the detectability of weak polarized signatures. Stokes V signatures are detected for more than 300 stars of all masses and evolutionary stages, and linear polarization is detected in 35 targets. The detection rate in Stokes V is found to be anticorrelated with the stellar effective temperature. This unique set of Zeeman detections offers the first opportunity to run homogeneous magnetometry studies throughout the H-R diagram. The Web interface of PolarBase is available at http://polarbase.irap.omp.eu.

McKinnon, M. M.

doi: 10.1086/676975pmid: N/A

The radio pulse from a pulsar can be temporally broadened by multipath scattering in the interstellar medium and by instrumental effects within the radio telescope. The observed pulse shape is a convolution of the intrinsic one with the impulse responses of the scattering medium and instrumentation. Until recently, common methods used to model the observed shape made assumptions regarding the intrinsic pulse shape and impulse responses, computed the convolution numerically, and solved for the pulse width and scattering timescale iteratively. An analytical solution is shown to exist for the specific case of the temporal broadening of a Gaussian-shaped pulse by a thin scattering screen. The solution is applied to multi-frequency observations of PSR B1834–10 to characterize the frequency dependence of its intrinsic pulse width and scattering timescale.

Kornilov, V.; Safonov, B.; Kornilov, M.; Shatsky, N.; Voziakova, O.; Potanin, S.; Gorbunov, I.; Senik, V.; Cheryasov, D.

doi: 10.1086/676648pmid: N/A

We present the results of the atmospheric optical turbulence (OT) measurements performed atop Mount Shatdzhatmaz at the installation site of new 2.5 m telescope of the Sternberg Astronomical Institute. Nearly 300,000 vertical OT profiles from the ground up to an altitude of 23 km were obtained in the period of 2007 November–2013 June 2 with the combined multiaperture scintillation sensor (MASS) and differential image motion monitor (DIMM) instrument. The medians of the main OT characteristics, computed over the whole dataset are as follows: the integral seeing β0 = 0.96″, the free-atmosphere seeing βFA = 0.43″, and the isoplanatic angle θ0 = 2.07″. The median atmospheric time constant is τ0 = 6.57 ms. The revealed long-term variability of these parameters on scales of months and years implies the need to take it into account in astroclimatic campaign planning. For example, the annual variation in the monthly θ0 estimate amounts to 30%, while the time constant τ0 changes by a factor of 2.5. Evaluation of the potential of Mount Shatdzhatmaz in terms of high angular resolution observations indicates that in October–November, this site is as good as the best of studied summits in the world.

Guo, Di-Fu; Hu, Shao-Ming; Chen, Xu; Gao, Dong-Yang; Du, Jun-Ju

doi: 10.1086/676819pmid: N/A

In this paper, a total of about 28000 images in V and R band obtained over 161 nights using the one-meter optical telescope at Weihai Observatory (WHO) of Shandong University from 2008 to 2012 have been processed to measure the sky brightness. They provide us with an unprecedented database, which can be used to study the variation of the sky brightness with the sky position, the moonlight contribution, and the twilight sky brightness. The darkest sky brightness is about 19.0 and 18.6 mag arcsec-2 in V and R band, respectively. An obvious darkening trend is found at the first half of the night at WHO, and the variation rate is much larger in summer than in other seasons. The sky brightness variation depends more on the azimuth than on the altitude of the telescope pointing for WHO. Our results indicate that the sky brightness at WHO is seriously influenced by urban light.