Exoplanets and Stellar Physics

Abstract:

The new MPE near infrared imaging spectrometer 3D represents a new generation of astronomical instrumentation. It is based on a 2562 NICMOS-3 Rockwell array and can simultaneously obtain 256 H- or K-band spectra at R= 1100 or 2100 from a square 16×16 pixel field on the sky. Typical pixel scales are 0.3″/pixel or 0.5″/pixel. 3D is a combination of a novel image slicer and a liquid nitrogen cooled long slit spectrometer. It includes high definition on-axis lens optics, a high efficiency directly ruled KRS-5 grism as well as a cold closed-loop piezo-driven tilt mirror allowing full spectral sampling. The instrument efficiency including detector is 15%. Combining the advantages of imaging and spectroscopy increases the observing efficiency on key astronomical objects (e.g. galactic nuclei) by such a large factor over existing grating or Fabry-Perot spectrometers that subarcsecond near-IR spectroscopy of faint Seyferts, starbursts, quasars, or distant galaxy clusters becomes feasible for the first time with 4m-class telescopes. As a portable instrument 3D has already been successfully deployed on several 2 and 4m-class telescopes.

Abstract:

Near-infrared imaging spectroscopy at spatial resolutions of 0.5 arc sec can already be achieved by combining the spatial and spectral resolution of an IR-imaging spectrometer with a first-oreder adaptive-optics system and accurate autoguider. This development makes high-resolution near-IR imaging spectroscopy possible for astronomy.

Abstract:

We report 1″ imaging spectroscopy of the 1.95-2.4 μm wavelength region in the z = 2.284 galaxy IRAS FSC 10214+4724. We find that the rest-frame Hα and [N II] emission have different spatial extents. We also detect broad (ΔνFWZP ≈ 3500 km s^-1) Hα emission. FSC 10214 is a very luminous gravitationally lensed galaxy, which intrinsically contains both a type 1 active galactic nucleus and a more extended star-forming disk. The AGN and circumnuclear star formation both contribute significantly to the total luminosity of ∼ 10¹³ L⊙. © 1996. The American Astronomical Society. All rights reserved.

Abstract:

We study the stability of an accretion disc with an embedded toroidal magnetic field to general perturbations. Disc models are considered in which the equilibrium variables depend on both the radial and vertical coordinates. We consider the full global problem in which the disc may be in the form of a narrow annulus, or occupy a significant radial extent. Perturbations with azimuthal mode number m in the range zero up to the ratio of the radius to disc scmithickness are considered. Discs containing a purely toroidal magnetic field are always found to be unstable. We find spectra of unstable modes using local techniques. In the absence of dissipation, these modes may occupy arbitrarily small scales in the radial and vertical directions. One class of modes is driven primarily by buoyancy, while the other is driven by shear independently of the equilibrium stratification. The first type of instability predominates if the field is large, while the second type predominates if the field is weak and the underlying medium is strongly stable to convection. We also investigate stability by solving the initial value problem for perturbations numerically. We find, for our disc models, that local instabilities predominate over any possible global instability. Their behaviour is in good accord with the local analysis. The associated growth rates become just less than the orbital frequency when the ratio of magnetic energy density to pressure reaches about 10 per cent. Instabilities of the kinds discussed here may provide a mechanism for limiting the growth of toroidal fields in dynamo models of accretion discs.

Abstract:

We describe the characteristics of the BIMA millimeter wave array at Hat Creek, CA. The array is an aperture synthesis instrument consisting of nine 6 m diameter antennas which may be deployed in three different configurations, with spacings ranging from 7 m up to 1.3 km. At an observing frequency of 100 GHz these configurations yield maps with angular resolutions of 5″, 2″, and 0.″4, over a 2′ field. Larger fields may be mapped by using multiple pointings. For all but the oldest telescopes, the surface accuracy is ≤30 μm rms, and the aperture efficiency is 77% at 100 GHz. Background emission from antenna losses and spillover is very low, about 5 K after subtraction of the cosmic B v(2.1 K). Each antenna contains a single dewar which accommodates up to four separate receivers. SIS mixers are cooled to 3.2 K with novel Gifford-McMahon cycle refrigerators. Both the upper and lower sidebands of the first local oscillator are received and separated, providing two bands extending from 70-900 MHz on each side of the first local oscillator. The correlation spectrometer covers a bandwidth of up to 800 MHz, and provides up to 2048 channels for each antenna pair. There are four independently tunable spectral windows (in each sideband), allowing simultaneous observations of several different spectral lines. The spectral resolution ranges from 6 kHz to 3 MHz. For a single 8-hr track in one configuration, the sensitivity is approximately 1 mJy/beam in the 800 MHz wide continuum. Measurements of atmospheric phase fluctuations as functions of both time and baseline have been made; these indicate that routine imaging at angular resolutions of less than 1″ at 100 GHz is possible only if self-calibration or some other means of phase correction can be applied. Examples of a few recent results are included. We note that 30% of the observing time on the array is granted to visitors.