Astronomical instrumentation

Abstract:

Aliphatic hydrocarbons exhibit an absorption feature at 3.4 μm, observed toward sources that sample diffuse regions of the interstellar medium (ISM). The absorbers responsible for this feature are assumed to reside in some component of interstellar dust, but the physical nature of the particles (size, shape, structure, etc.) is uncertain. Observations of interstellar polarization provide discrimination. Since the grains that carry the silicate absorption feature are known to be aligned, polarization across the 3.4 μm hydrocarbon feature can be used to test the silicate-core organic refractory mantle grain theory. Although the 3.4 μm feature has been observed to be devoid of polarization for one line of sight toward the Galactic center, a corresponding silicate polarization measurement for the same line of sight was not available. Here, we present spectropolarimetric observations of GCS 3-11 and GCS 3-IV toward the Galactic center, where the 9.7 μm silicate polarization has been previously observed. We show that polarization is not detected across the 3.4 μm feature to a limit of 0.06% ± 0.13% (GCS 3-II) and 0.15% ± 0.31% (GCS 3-IV), well below the lowest available prediction of polarization on the basis of the core-mantle model. We conclude that the hydrocarbons in the diffuse ISM do not reside on the same grains as the silicates, and likely form a separate population of small grains. © 2006. The American Astronomical Society. All rights reserved.

Abstract:

We report the results of near infrared spectroscopy of 11 luminosity-selected candidate planetary mass objects (PMOs) in the Trapezium Cluster with Gemini South/Gemini Near-Infrared Spectrograph and Gemini North/Near-Infrared Imager. Six have spectral types ≥M9, in agreement with expectations for PMOs. Two have slightly earlier types, and three are much earlier types which are probably field stars. Four of the six sources with types ≥M9 have pseudo-continuum profiles which confirm them as low gravity cluster members. The gravity status of the other cool dwarfs is less clear but these remain candidate PMOs. The derived number fraction of PMOs with M = 3 - 15 MJup is 1-14 per cent, these broad limits reflecting the uncertainty in source ages. However, the number fraction with M < 20 MJup is at least 5 per cent. These detections add significantly to the body of evidence that a planetary mass population is produced by the star formation process. © 2006 The Authors. Journal compilation © 2006 RAS.

Abstract:

We report recent Chandra observations of the ULX in the elliptical galaxy NGC 3379 that clearly detect two flux variability cycles. Comparing these data with the Chandra observation of ∼5 years ago, we measure a flux modulation with a period of ∼12.6 hr. Moreover, we find that the emission undergoes a correlated spectral modulation, becoming softer at low flux. We argue that our results establish this source as a ULX binary in NGC 3379. Given the old stellar population of this galaxy, the ULX is likely to be a soft transient; however, historical X-ray sampling suggests that the current "on" phase has lasted ∼10yr. We discuss our results in terms of ADC and wind-feedback models. If the flux modulation is orbital, we can constrain the donor mass and orbital period at the onset of mass transfer within 1.15-1.4 Ṁ and 12.5-17 hr, respectively. The duration of the mass transfer phase so far is probably ∼ 1 Gyr, and the binary has been a soft X-ray transient throughout this time. These constraints are insensitive to the mass of the accretor. © 2006. The American Astronomical Society. All rights reserved.

Abstract:

This paper describes the design and development of an accurate temperature compliant lens mounting technique being used on the camera of the UK-FMOS near infrared spectrograph for operation at the Subaru Telescope in Hawaii. A series of fused silica lenses of up to 4.4kg, 255mm in diameter and operating at temperatures as low as 70K are supported within flexures cut away from stainless steel outer rings. Intermediate low thermal expansion pads are attached to these flexures and in turn bonded to the glass during the alignment process. This mounting method lends itself to the domino chips type of assembly process which can be carried out on a rotary table to maintain accurate axial alignment. A detailed description of the overall design progression including the methods of manufacture, alignment process, adhesive selection, assembly methods and testing is included.

Abstract:

We present the SWIFT image slicer and its novel de-magnifying design. It is based on the MPE-3D and SPIFFI image slicers, uses plane mirrors to slice the input field, but achieves a considerable de-magnification through the use of a mosaic of spherical lenses. As only plane and spherical surfaces are used in the design, classical polishing techniques can be applied to achieve very high surface accuracy and quality. This reduces aberrations and scattered light, mandatory for an image slicer working at optical wavelengths and behind an adaptive optics system. Except for the lens mosaic, the SWIFT slicer is built entirely from Zerodur and is assembled using optical contacting. We present a detailed description of the design as well as results of the early stages of its fabrication.