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doi: 10.1086/658635pmid: N/A
The theory of slow convective cycles in the envelopes of RR Lyrae stars showing the Blazhko effect predicts an anticorrelation between the pulsation period and radius for hot members of the group, contrary to what might be expected from the (period, mean density) relation. This negative correlation has actually been observed by Jurcsik et al. for the hot stars MW Lyr, DM Cyg, and (in part) CZ Lac, the only stars so far measured accurately for radius variations over a Blazhko cycle. Significantly cooler members of the group would be expected to show a normal, positive correlation between pulsation period and radius.
van Dishoeck, E. F.; Kristensen, L. E.; Benz, A. O.; Bergin, E. A.; Caselli, P.; Cernicharo, J.; Herpin, F.; Hogerheijde, M. R.; Johnstone, D.; Liseau, R.; Nisini, B.; Shipman, R.; Tafalla, M.; van der Tak, F.; Wyrowski, F.; Aikawa, Y.;
Catanzarite, Joseph; Shao, Michael
doi: 10.1086/658243pmid: N/A
We estimate the exo-Earth/super-Earth yield of an imaging mission that combines the James Webb Space Telescope (JWST) with a starshade external occulter under a realistic set of astrophysical assumptions. For the purpose of this study, we define “exo-Earth” and “super-Earth” as a planet of mass 1 to 2 M⊕ and 2 to 10 M⊕, respectively, orbiting within the habitable zone (HZ) of a solar-type star. We show that for a survey strategy that relies on a single image as the basis for detection, roughly half of all exo-Earth/super-Earth detections will be false alarms for η⊕ of 0.1, 0.2, and 0.3. Here, a false alarm is a mistaken identification of a planet as an exo-Earth/super-Earth, and we define η⊕ as the frequency of exo-Earth/super-Earths orbiting sunlike stars. We then consider two different survey strategies designed to mitigate the false alarm problem. The first is to require that for each candidate exo-Earth/super-Earth, a sufficient number of detections are made to measure the orbit. When the orbit is known we can determine if the planet is in the habitable zone. With this strategy, we find that the number of exo-Earth/super-Earths found is, on average, 0.9, 1.9, and 2.7 for η⊕ = 0.1, 0.2, and 0.3. There is a ∼40% probability of finding zero exo-Earth/super-Earths for η⊕ = 0.1. A second strategy can be employed if a space-based astrometry mission capable of submicroarcsecond precision has identified and measured the orbits and masses of the planets orbiting nearby stars. In this case, the occulter mission is much more efficient, because it surveys only the stars known to have exo-Earth/super-Earths. We find that with prior knowledge from a space-based astrometric survey of 60 nearby stars, JWST plus an external occulter can obtain spectra, as well as orbital solutions, for the majority (70% to 80%) of the exo-Earth/super-Earths orbiting these 60 stars. The yield of exo-Earth/super-Earths is approximately five times higher than the yield for the JWST plus occulter mission without prior astrometric information. With prior space-based astrometry, the probability that an imaging mission will find zero exo-Earth/super-Earths is reduced to < 1% for the case of η⊕ = 0.1.
Biesiadzinski, T.; Lorenzon, W.; Newman, R.; Schubnell, M.; Tarlé, G.; Weaverdyck, C.
doi: 10.1086/658282pmid: N/A
Flux-dependent nonlinearity (reciprocity failure) in HgCdTe near-infrared detectors can severely impact an instrument’s performance, in particular, with respect to precision photometric measurements. The cause of this effect is presently not understood. To investigate reciprocity failure, a dedicated test system was built. For flux levels between 1 and 50,000 photons s-1, a sensitivity to reciprocity failure of approximately 0.1% decade-1 was achieved. A wavelength-independent nonlinearity due to reciprocity failure of about 0.35% decade-1 was measured in a 1.7 μm HgCdTe detector.
Koch, Patrick M.; Raffin, Philippe; Huang, Yau-De; Chen, Ming-Tang; Han, Chih-Chiang; Lin, Kai-Yang; Altamirano, Pablo; Granet, Christophe; Ho, Paul T. P.; Huang, Chih-Wei L.; Kesteven, Michael; Li, Chao-Te; Liao, Yu-Wei; Liu, Guo-Chin; Nishioka, Hiroaki; Ong, Ching-Long;
Keremedjiev, Mark; Eikenberry, Stephen S.
doi: 10.1086/658356pmid: N/A
The new technique of speckle stabilization has great potential to provide optical imaging data at the highest angular resolutions from the ground. While speckle stabilization was initially conceived for integral field spectroscopic analyses, the technique shares many similarities with speckle imaging (specifically, shift-and-add and lucky imaging). Therefore, it is worth comparing the two for imaging applications. We have modeled observations on a 2.5 m class telescope to assess the strengths and weaknesses of the two techniques. While the differences are relatively minor, we find that speckle stabilization is a viable competitor to current lucky imaging systems. Specifically, we find that speckle stabilization is 3.35 times more efficient (where efficiency is defined as signal-to-noise ratio [S/N] per observing interval) than shift-and-add and is able to detect targets 1.42 mag fainter when using a standard system. If we employ a high-speed shutter to compare with lucky imaging at 1% image selection, speckle stabilization is 1.28 times more efficient and 0.31 mag more sensitive. However, when we incorporate potential modifications to lucky imaging systems, we find that the advantages are significantly mitigated—and even reversed—in the 1% frame-selection cases. In particular, we find that in the limiting case of optimal lucky imaging, that is, zero read noise and photon counting, we find lucky imaging is 1.80 times more efficient and 0.96 mag more sensitive than speckle stabilization. For the cases in between, we find that there is a gradation in advantages to the different techniques, depending on target magnitude, fraction of frames used, and system modifications. Overall, however, we find that the real strength of lucky imaging is in observations of the brightest targets at all frame-selection levels and in observations of faint targets at the 1% level. For targets in the middle, we find that speckle stabilization regularly achieves higher S/N.
Querel, R. R.; Naylor, D. A.; Kerber, F.
doi: 10.1086/658285pmid: N/A
Atmospheric water vapor is the principal source of opacity at infrared wavelengths. Spectral observations of a star with a featureless continuum, such as a white dwarf, provide a method of determining atmospheric absorption along the line of sight to the star. Through fitting a site-specific atmospheric transmission model to high-resolution atmospheric absorption measurements, it is possible to determine the water vapor column abundance expressed in millimeters of precipitable water vapor (PWV). While more challenging in interpretation, emission spectra can also be used to derive PWV. This article describes a general algorithm that we have developed for retrieving PWV from both atmospheric transmission and emission spectra. The retrieved PWV values have been validated by intercomparison with contemporaneous measurements provided by radiosonde balloons and emission radiometers.
Showing 1 to 10 of 15 Articles
doi: 10.1086/658674pmid: N/A
We present a study of the recently discovered intermediate polar 1RXS J070407 + 262501, distinctive for its large-amplitude pulsed signal at P = 480 s. Radial velocities indicate an orbital period of 0.1821(2) days, and the light curves suggest 0.18208(6) days. Time-series photometry shows a precise spin period of 480.6700(4) s, decreasing at a rate of 0.096(9) ms yr-1: i.e., on a time scale . The light curves also appear to show a mysterious signal at P = 0.263 days, which could possibly signify the presence of a superhump in this magnetic cataclysmic variable.
doi: 10.1086/658676pmid: N/A
Water In Star-forming regions with Herschel (WISH) is a key program on the Herschel Space Observatory designed to probe the physical and chemical structures of young stellar objects using water and related molecules and to follow the water abundance from collapsing clouds to planet-forming disks. About 80 sources are targeted, covering a wide range of luminosities—from low (< 1 L⊙) to high (>105 L⊙)—and a wide range of evolutionary stages—from cold prestellar cores to warm protostellar envelopes and outflows to disks around young stars. Both the HIFI and PACS instruments are used to observe a variety of lines of H2O, H218O and chemically related species at the source position and in small maps around the protostars and selected outflow positions. In addition, high-frequency lines of CO, 13CO, and C18O are obtained with Herschel and are complemented by ground-based observations of dust continuum, HDO, CO and its isotopologs, and other molecules to ensure a self-consistent data set for analysis. An overview of the scientific motivation and observational strategy of the program is given, together with the modeling approach and analysis tools that have been developed. Initial science results are presented. These include a lack of water in cold gas at abundances that are lower than most predictions, strong water emission from shocks in protostellar environments, the importance of UV radiation in heating the gas along outflow walls across the full range of luminosities, and surprisingly widespread detection of the chemically related hydrides OH+ and H2O+ in outflows and foreground gas. Quantitative estimates of the energy budget indicate that H2O is generally not the dominant coolant in the warm dense gas associated with protostars. Very deep limits on the cold gaseous water reservoir in the outer regions of protoplanetary disks are obtained that have profound implications for our understanding of grain growth and mixing in disks.
The Ohio State Multi-Object Spectrograph is a new wide-field imager and multiobject spectrograph for the 2.4 m Hiltner Telescope at the MDM Observatory. OSMOS has an all-refractive design that reimages a 20′ diameter field of view onto the 4064 × 4064 MDM4K CCD with a plate scale of 0.273″ pixel-1. Approximately an 18.5′ square region of this field illuminates the detector and is available for spectroscopy, although with reduced wavelength coverage near the edges of the field. Slit masks, filters, and dispersers are all mounted in a series of six-position aperture wheels. These mechanisms rotate between positions in only a few seconds; consequently, the instrument may be rapidly reconfigured between imaging and spectroscopic modes. At present, a low-resolution triple prism (R ∼ 60–400) and a moderate-resolution volume-phase holographic grism (R ∼ 1600) are available.
doi: 10.1086/658675pmid: N/A
Interferometric millimeter observations of the cosmic microwave background and clusters of galaxies with arcminute resolutions require antenna arrays with short spacings. Having all antennas co-mounted on a single steerable platform sets limits to the overall weight. A 25 kg lightweight novel carbon-fiber design for a 1.2 m diameter Cassegrain antenna is presented. The finite element analysis predicts excellent structural behavior under gravity, wind, and thermal load. The primary- and secondary-mirror surfaces are aluminum-coated with a thin TiO2 top layer for protection. A low beam sidelobe level is achieved with a Gaussian feed-illumination pattern with edge taper, designed based on feed-horn antenna simulations and verified in a far-field beam-pattern measurement. A shielding baffle reduces interantenna coupling to below ∼-135 dB. The overall antenna efficiency, including a series of efficiency factors, is estimated to be around 60%, with major losses coming from the feed spillover and secondary blocking. With this new antenna, a detection rate of about 50 clusters yr-1 is anticipated in a 13-element array operation.