Astronomical instrumentation

Abstract:

We present high-resolution, near-infrared images of a sample of five low-mass young stellar objects in the Taurus-Auriga dark cloud, mostly observed with the 'shift and add' technique, together with deep polarimetric images. These systems are at the 'Class I' phase of evolution, prior to the T Tauri phase. Monte Carlo models tailored to the individual sources support the view that the flux distribution of the circumstellar nebulosity arises from an envelope with a bipolar cavity, but show that dust in outflows can also be prominent in the near-infrared. The source IRAS 04302 + 2247 is analysed in detail, since it displays a unique quadrupolar morphology, which is explained in terms of a system oriented precisely edge-on with a perpendicular outflow. The masses of the envelopes derived from the models are in good agreement with estimates obtained from submillimetre thermal fluxes and estimates derived from CS(J = 2 - 1) emission. Constraints are placed on envelope and cavity structure, indicating higher density near the equatorial plane, smooth, continuous density profiles and physically thin inner discs. Three of the five systems show evidence for two bipolar cavity/outflow systems, perhaps indicating that these are close binaries. The very high (up to 80 per cent) degrees of polarization observed in the envelopes show that the scattering is dominated by small, interstellar-type dust grains, and the morphologies and high polarizations of the cores show that the protostars themselves are usually obscured at this phase of evolution, even at 3.7 μm. © 1997 RAS.

Abstract:

We present imaging polarimetry of the central 30 arcsec of OMC-1 at 12.5 and 17 μm with arcsecond resolution, together with complementary spectropolarimetry in the ranges 8-13 and 16-22 μm at selected positions, and polarimetry at 800 μm over an approximately 1-arcmin field. The polarization is due to the dichroism of aligned grains in emission in the far-infrared, and predominantly due to absorption in the mid-infrared. The images reveal large variations of polarization fraction and position angle in BNKL, the central region, and these can explain the low fractional polarization observed when this region is unresolved, as in the far-infrared. The mid-infrared polarization indicates that a substantial component of magnetic field is aligned with the plane of the disc-like structures inferred from millimetre-wave studies, and suggests the presence of a toroidal field within the disc. © 1997 RAS.

Abstract:

The ultraluminous infrared Seyfert 1 galaxy Mrk 231 has been spectrally imaged in the K band with the new three-dimensional MPE integral field spectrometer. The combined images of the H2 emission lines show, for the first time in an ultraluminous infrared galaxy, the presence of an extended circumnuclear structure of hot molecular gas. The H2 emitting region has a size of ∼2.4 kpc and a hot molecular gas mass MH2hot ∼ 2 × 104 M⊙. The H2 emission-line ratios indicate that the gas is most likely thermally excited. If as in NGC 7469 star formation is associated with the H2 emission, the starburst would have a far-IR luminosity LFIR ∼ 1 × 1012 L⊙. This value represents an upper limit, since a fraction of the hot molecular gas may be excited by the radiation field emerging from the nucleus. The K-band three-dimensional data cube also shows for the first time the presence of extended narrow Paα emission blueshifted by ∼1400 km s-1 with respect to the systemic velocity, and located ∼0.6 kpc northwest of the nucleus. The detection of CO absorption bands with a spatial distribution peaking on the K-band continuum provides evidence for a central stellar concentration. The low CO spectroscopic index indicates, however, dilution by hot dust emission or by a nonthermal active galactic nucleus. The Paα/Hα ratio confirms previous extinction measurements based on Balmer line ratios, i.e., visual extinction of AV ∼ 2.0-6.6 mag. The quasar-type nucleus of Mrk 231 should then be transparent at 2 μm and also in hard X-rays. A weak nuclear He I λ2.058 μm (He I/Paα = 0.032) is detected, and no detection of [Si VI] λ1.962 μm is made, placing an upper limit of 4 × 10-18 Wm-2 for the coronal gas emission. The ionizing source could either be a far-UV and X-ray quiet quasar or else a nuclear starburst with an upper mass limit ≥60 M⊙. © 1997. The American Astronomical Society. All rights reserved.