Disparate MgII Absorption Statistics towards Quasars and Gamma-Ray Bursts : A Possible Explanation

We examine the recent report by Prochter et al. (2006) that gamma-ray burst (GRB) sight lines have a much higher incidence of strong MgII absorption than quasar sight lines. We propose that the discrepancy is due to the different beam sizes of GRBs and quasars, and that the intervening MgII systems are clumpy with the dense part of each cloudlet of a similar size as the quasars, i.e. < 10^16 cm, but bigger than GRBs. We also discuss observational predictions of our proposed model. Most notably, in some cases the intervening MgII absorbers in GRB spectra should be seen varying, and quasars with smaller sizes should show an increased rate of strong MgII absorbers. In fact, our prediction of variable MgII lines in the GRB spectra has been now confirmed by Hao et al. (2007), who observed intervening FeII and MgII lines at z=1.48 to be strongly variable in the multi-epoch spectra of z=4.05 GRB060206.Comment: 12 pages, 2 figures; substantially revised model calculation; accepted for publication in Astrophysics & Space Science as a Lette

Biosignatures from Earth-Like Planets Around M Dwarfs

Coupled one-dimensional photochemical-climate calculations have been performed for hypothetical Earth-like planets around M dwarfs. Visible, near-infrared and thermal-infrared synthetic spectra of these planets were generated to determine which biosignature gases might be observed by a future, space-based telescope. Our star sample included two observed active M dwarfs, AD Leo and GJ 643, and three quiescent model stars. The spectral distribution of these stars in the ultraviolet generates a different photochemistry on these planets. As a result, the biogenic gases CH4, N2O, and CH3Cl have substantially longer lifetimes and higher mixing ratios than on Earth, making them potentially observable by space-based telescopes. On the active M-star planets, an ozone layer similar to Earth's was developed that resulted in a spectroscopic signature comparable to the terrestrial one. The simultaneous detection of O2 (or O3) and a reduced gas in a planet's atmosphere has been suggested as strong evidence for life. Planets circling M stars may be good locations to search for such evidence.Comment: 34 pages, 10 figures, Astrobiology, in pres

Interferometric Space Missions for the Search for Terrestrial Exoplanets: Requirements on the Rejection Ratio

The requirements on space missions designed to study Terrestrial exoplanets are discussed. We then investigate whether the design of such a mission, specifically the Darwin nulling interferometer, can be carried out in a simplified scenario. The key element here is accepting somewhat higher levels of stellar leakage. We establish detailed requirements resulting from the scientific rationale for the mission, and calculate detailed parameters for the stellar suppression required to achieve those requirements. We do this utilizing the Darwin input catalogue. The dominating noise source for most targets in this sample is essentially constant for all targets, while the leakage diminishes with the square of the distance. This means that the stellar leakage has an effect on the integration time only for the nearby stars, while for the more distant targets its influence decreases significantly. We assess the impact of different array configurations and nulling profiles and identify the stars for which the detection efficiency can be maximized.Comment: 21 pages, 8 figures; TBP in Astrophysics and Space Science 200

Transit Telescope Designs Optimized for Multiple Object Spectroscopy with Fibers

Instruments to study simultaneously the spectra of many objects in the field of view of a telescope can be made with the aid of fused silica fibers. The spectrograph at the 2.3m telescope of the University of Arizona has been modified for such operation, and is used routinely to investigate the dynamics of clusters of galaxies (Hill et al. 1981). The system presently in use locates up to 40 fibers in the telescope's Cassegrain focal plane, with the aid of plates previously drilled with holes in the configuration of the objects to be studied. Each field must be set up by hand by inserting fibers into a hole plate. An obvious improvement to this method would be to mount each fiber on a mechanical actuator, so new field configurations can be set up by remote control. It is our intention to make such a system with 32 fibers for the 2.3m telescope, over the next two years.</jats:p

Implementation and Use of Wide Fields in Future Very Large Telescopes

AbstractThe full potential of the next generation of larger telescopes will be realized only if they have well instrumented large fields of view. Scientific problems for which very large ground-based optical telescopes will be of most value often will need surveys to very deep limits with imaging and slitless spectroscopy, followed by spectroscopy of faint objects taken many at once over the field. Improved instruments and detectors for this purpose are being developed. Remotely positioned fibers allow the coupling of light from many objects in the field to the spectrograph slit. CCD arrays, operated in the TDI or drift scan mode, will make large area detectors of high efficiency that may supercede photographic plates. An ideal telescope optical design should be based on a fast parabolic primary, have a field of at least 1° with achromatic images &lt; 0.25 arcseconds and have provision for dispersive elements to be used for slitless spectroscopy and compensation of atmospheric dispersion over the full field. A good solution for a general purpose telescope that can satisfy these needs is given by a three element refractive corrector at a fast Cassegrain focus. A specialized telescope dedicated to sky surveys, with better image quality and higher throughput than presently available, might be built as a scaled up Schmidt with very large photographic plates. Better performance in most areas should be obtained with a large CCD mosaic detector operated in the drift scan mode at a telescope with a 2-mirror reflecting corrector.</jats:p

Use of the LBT to study zodiacal emission around other stars

Thermal emission from "planetary" discs is known in other stars such as β Pictoris. The emission, discovered by the IRAS satellite, is comparable in strength to that from the star itself at 60 µm wavelength, indicating a large quantity of material at radius ~ 100 AU and temperature ~ 50 K (ref 1). Such discs are uncommonly bright, far more dense and extensive than the known zodiacal dust in our solar system. Here the emission is concentrated mostly within 1 AU, where the dust temperature is ~ 300K and it is optically very thin, causing an increment of only 10-5 in the 10 µm luminosity of the Sun as seen from afar.</jats:p

Advanced Wide-Field Broad-Passband Refracting Field Correctors for Large Telescopes

AbstractA preliminary 30-arcmin prime focus (f/2.0) refracting field corrector system for the University of California Ten-Meter Telescope (UC TMT) is presented which features 1/4-arcsec images containing more than 80% of the energy, over limited passbands within the wavelength range λ3300Å to λ1.0µ. Provision has been made in this system for an atmospheric dispersion corrector (ADC) but same has not yet been realized. Optical elements herein are small enough that this design could be scaled up to a Fifteen-Meter NNTT/SMT.A compact 40-arcmin internal Cassegrain (f/1.75 hyperbola to f/5.0) broad-passband (λ3300Å to λ1.0µ) corrector, suitable for imaging and multi-object spectroscopy at the UC TMT, is presented which features 1/4-arcsec images containing more than 90% of the energy when averaged over field angle and color.Three 60-arcmin external Cassegrain correctors for 300-inch f/1.8 and f/2.0 parabolic primary mirrors are presented which are suitable for a Fifteen-Meter NNTT/MMT. Image quality is comparable to the UC TMT Cassegrain corrector and it exceeds that of the UC TMT preliminary prime focus corrector system by a substantial margin. Each of these correctors contains an ADC which has been implemented in one example, eliminating 4.0 arcsec of differential atmospheric refraction with an rms residual of +/-0.10 arcsec over the broad passband (λ3300Å to λ1.0µ). A 60-arcmin external Cassegrain (f/1.8 extreme hyperbola to f/4.5) corrector with ADC yields yet a factor two in image quality but said hyperbolic primary mirror would be incompatible with angular field requirements in the thermal infrared.A (300-inch) 40-arcmin external Cassegrain (f/1.0 parabola to f/4.0) broad-passband (λ3300Å to λ1.0µ) corrector with ADC is presented. Image quality is comparable to the previous Cassegrain correctors. The practicality of this design, together with recent advances in optical manufacturing capability of large, fast, nonspherical optics, suggests that relatively inexpensive compact telescopes of very large collecting area may be possible in the near future.</jats:p

Alignment of the optical polarization with the radio structure of QSOs

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