Publications of the Astronomical Society of the Pacific

Jolissaint, Laurent

doi: 10.1086/507849pmid: N/A

We present a critical review of 12 methods available today to emulate optical turbulence for the laboratory testing of astronomical adaptive optics systems, in particular, multiconjugate and extreme contrast modes. All published or known optical turbulence generation methods are presented, and their pros and cons are discussed, from static phase screens to fluid chambers. Optical turbulence measurement methods for the testing and characterization of turbulence generators are also reviewed. Results are summarized in a table with references at the end of the paper. The objective of this review is to provide a designer guide for optical turbulence generators.

Smith, Graeme H.

doi: 10.1086/507654pmid: N/A

Nitrogen abundance inhomogeneities are present among main‐sequence stars in globular clusters of the Milky Way. Since N‐rich cluster dwarfs are unlikely to have nucleosynthesized nitrogen within their own interiors, they presumably obtained their excess N from elsewhere. If the abundance inhomogeneities are due to the self‐enrichment of a globular cluster, then the earliest source of nitrogen could have been massive cluster OB stars and Wolf‐Rayet stars of type N. Several obstacles to WN stars' being a viable enrichment mode for globular clusters are discussed: (1) a W‐R phase may have been inhibited for metal‐poor OB stars, due to the low mass‐loss rates expected of their radiatively driven winds, and (2) unless globular clusters had top‐heavy stellar mass functions, their WN stars would have been too few in number to explain the amount of CNO‐processed material in their nitrogen‐rich, low‐mass stars. In contrast, it is pointed out that OB stars in their main‐sequence and supergiant phases of evolution might also eject CNO‐processed material even if they do not evolve through a WN phase, while W‐R stars have been discovered in some metal‐poor dwarf galaxies and starburst galaxies. In addition, low metallicities could favor the production of WN stars over WC stars, and this could explain why the abundance inhomogeneities in Milky Way globular clusters generally involve excesses of CNO‐processed material. One scenario is discussed in which globular clusters form at the interface between massive, colliding gas clouds. If such clouds have masses comparable to dwarf galaxies, with relative velocities characteristic of the Galactic halo, then the collision process could drive gas into an accreting protocluster at a rate that is sufficient to complete cluster formation within several million years. This timescale could allow nitrogen‐rich wind ejecta from WN or OB stars to be incorporated into later cluster star formation. The ram pressure associated with a colliding‐cloud environment might not only confine the stellar wind ejecta, but also funnel it into sites of active star formation within the protocluster. In this picture, globular clusters form as open systems with gas continuing to flow into them from a greater cloud‐merger environment while star formation is occurring. In such a circumstance, not only may cluster WN and OB stars have promoted enrichment, but wind ejecta from massive stars that are external to the protocluster may also have been acquired.

Thorstensen, John R.; Lépine, Sébastien; Shara, Michael

doi: 10.1086/507956pmid: N/A

RBS (ROSAT Bright Source) 0490 is a cataclysmic variable star (CV) with unusually strong emission lines. The strength of the emission lines has led to a suggestion that the object is intrinsically faint and correspondingly nearby (∼33 pc), which, if true, would strongly affect estimates of the CV space density. Here we report astrometry, filter photometry, and time‐series spectroscopy of this object. The astrometry gives an absolute parallax πabs = 4.5 ± 1.5 mas and a relative proper motion of 102 mas yr−1. A Bayesian procedure gives a very uncertain distance estimate of d∼300 pc, and the small parallax alone implies d>133 pc (at 2 standard deviations). The mean V magnitude is 17.4, which implies MV = 10.9 - 5log [d/(200 pc)], neglecting extinction. At 200 pc, the space velocity would be over 90 km s−1 with respect to the local standard of rest. The time‐series spectroscopy shows a possible emission‐line radial velocity period near 46 minutes. This would be unusually short for an orbital period, and it may represent some other clock in the system.

Wilson, D. M.; Enoch, B.; Christian, D. J.; Clarkson, W. I.; Collier Cameron, A.; Deeg, H. J.; Evans, A.; Haswell, C. A.; Hellier, C.; Hodgkin, S. T.; Horne, K.; Irwin, J.; Kane, S. R.; Lister, T. A.; Maxted, P. F. L.; Norton, A. J.; Pollacco, D.;

Bozorgnia, Nassim; Fortney, Jonathan J.; McCarthy, Chris; Fischer, Debra A.; Marcy, Geoffrey W.

doi: 10.1086/508237pmid: N/A

HD 149026b is a short‐period, Saturn‐mass planet that transits a metal‐rich star. The planet's radius, determined by photometry, is remarkably small compared to other known transiting planets, with a heavy‐element core that apparently comprises ∼70% of the total planet mass. Time‐series spectra were obtained at Keck before and during transit in order to model the Rossiter‐McLaughlin effect. Here we make use of these observations to carry out a differential comparison of spectra obtained in and out of transit to search for signatures of neutral atomic lithium and potassium from the planet atmosphere. No signal was detected at the 2% level; we therefore place upper limits on the column density of these atoms.

Mangum, Jeffrey G.; Baars, Jacob W. M.; Greve, Albert; Lucas, Robert; Snel, Ralph C.; Wallace, Patrick; Holdaway, Mark

doi: 10.1086/508298pmid: N/A

The ALMA (Atacama Large Millimeter Array) North American and European prototype antennas have been evaluated by a variety of measurement systems to quantify the major performance specifications. Near‐field holography was used to set the reflector surfaces to 17 μm rms. Pointing and fast‐switching performance was determined with an optical telescope and by millimeter‐wavelength radiometry, yielding 2″ absolute and 0.″6 offset pointing accuracies. Path‐length stability was measured to be ≲20 μm over 10 minute time periods using optical measurement devices. Dynamical performance was studied with a set of accelerometers, providing data on wind‐induced tracking errors and structural deformation. Considering all measurements made during this evaluation, both prototype antennas meet the major ALMA antenna performance specifications.

Hough, J. H.; Lucas, P. W.; Bailey, J. A.; Tamura, M.; Hirst, E.; Harrison, D.; Bartholomew‐Biggs, M.

doi: 10.1086/507955pmid: N/A

We have built and used on several occasions an optical broadband stellar polarimeter, PlanetPol, which employs photoelastic modulators and avalanche photodiodes and achieves a photon‐noise–limited sensitivity of at least 1 in 106 in fractional polarization. Observations of a number of polarized standards taken from the literature show that the accuracy of polarization measurements is ∼1%. We have developed a method for accurately measuring the polarization of altitude‐azimuth mounted telescopes by observing bright nearby stars at different parallactic angles, and we find that the on‐axis polarization of the William Herschel Telescope is typically ∼15 × 10−6, measured with an accuracy of a few parts in 107. The nearby stars (distance less than 32 pc) are found to have very low polarizations, typically a few ×10−6, indicating that very little interstellar polarization is produced close to the Sun and that their intrinsic polarization is also low. Although the polarimeter can be used for a wide range of astronomy, the very high sensitivity was set by the goal of detecting the polarization signature of unresolved extrasolar planets.

Catanzarite, Joseph; Shao, Michael; Tanner, Angelle; Unwin, Stephen; Yu, Jeffrey

doi: 10.1086/504442pmid: N/A

SIM PlanetQuest (formerly the Space Interferometry Mission) is a space‐borne Michelson interferometer for precision stellar astrometry, with a 9 m baseline, currently slated for launch in 2016. One of the principal science goals is the astrometric detection and orbital characterization of terrestrial planets in the habitable zones of nearby stars. Differential astrometry of the target star against a set of reference stars lying within 1° will allow measurement of the target star’s reflex motion with astrometric accuracy of 1 μas in a single measurement. The purpose of the present paper is to quantitatively assess SIM’s capability for detection (as opposed to characterization by orbital determination) of terrestrial planets in the habitable zones of nearby stars. Note that the orbital periods of these planets are generally shorter than the 5 year SIM mission. We formulate a “joint periodogram” as a tool for planet detection from astrometric data. For adequately sampled orbits (i.e., five or more observations per period over a sampling time span longer than the orbital period), we find that the joint periodogram is more sensitive than the χ2 test for the null hypothesis. In our analysis of the problem, we use Monte Carlo simulations of orbit detection, together with realistic observing scenarios, actual target and reference star lists, realistic estimates of SIM instrument performance, and plausible distributions of planetary system parameters.

Vaillancourt, John E.

doi: 10.1086/507472pmid: N/A

The determination of the true source polarization given a set of measurements is complicated by the requirement that the polarization always be positive. This positive bias also hinders construction of upper limits, uncertainties, and confidence regions, especially at low signal‐to‐noise ratios (S/Ns). We take a Bayesian approach to creating confidence regions and upper limits by integrating over the likelihood function for polarization measurements. These regions are valid for both low‐ and high‐S/N measurements.

Killen, Rosemary M.

doi: 10.1086/508070pmid: N/A

Renewed interest in the Hermean sodium exosphere has resulted from the planned encounters of two spacecraft, MESSENGER and BepiColombo, with Mercury. Both of these spacecraft will have UV‐visible spectrometers on board that are capable of detecting the sodium D lines and many other species. The MSASI (Mercury Sodium Atmospheric Spectral Imager) instrument on board the Mercury Magnetospheric Orbiter (MMO), one of two to be launched with the ESA BepiColombo mission to Mercury, is a Fabry‐Pérot spectrometer that has been specifically designed to image the sodium exosphere of Mercury. Because the data rates will be high, a curve‐of‐growth model for the intensity is useful. We show in this paper how the intensity varies not only with column abundance, but also with the observing geometry. The curve of growth is shown to be a strong function of solar incidence angle and planetary phase angle. Thus, the simple curve‐of‐growth model based on the assumption of observation at zero phase angle is inadequate for observations of Mercury’s exosphere. The solar flux reflected from the planet’s surface contributes a small amount to the exospheric intensity. We show that the D2/D1 ratio is inadequate to determine column abundance and has a strong dependence on the surface reflectivity for lines observed over the surface.