Exoplanets and Stellar Physics

Abstract:

We study the global bending modes of a thin annular disc subject to both an internally generated magnetic field and a magnetic field due to a dipole embedded in the central star with axis aligned with the disc rotation axis. When there is a significant inner region of the disc corotating with the star, we find spectra of unstable bending modes. These may lead to elevation of the disc above the original symmetry plane facilitating accretion along the magnetospheric field lines. The resulting non-axisymmetric disc configuration may result in the creation of hotspots on the stellar surface and the periodic photometric variations observed in many classical T Tauri stars (CTTSs). Time-dependent behaviour may occur including the shadowing of the central source in magnetic accretors even when the dipole and rotation axes are aligned. © 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 MH2^hot ∼ 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.

Abstract:

We extend a previous study of the global stability of a stratified differentially rotating disc containing a toroidal magnetic field to include the effect of a non-zero resistivity η. We consider the situation when the disc is stable to convection in the absence of the magnetic field. The most robust buoyancy driven unstable modes, which occur when the field is strong enough, have low azimuthal mode number m. They grow exponentially, apparently belonging to a discrete spectrum. They exist for the dimensionless ratio η/(H²Ω) smaller than ∼ 10^-2, where Ω is the angular velocity and H is the disc semithickness. In contrast the magnetorotational modes develop arbitrarily small radial scale and show transient amplification as expected from a shearing sheet analysis. The most robust modes of this type are local in all directions. Because of their more global character, the buoyancy driven modes may be important for the generation of large-scale fields and outflows. © 1997 RAS.

Abstract:

We present new near-infrared speckle and adaptive optics imaging and integral field spectroscopy of the nuclear region of NGC 1068. Ninety-four percent of the K-band light in the central 1" originates from a ≤30 milliarcsecond diameter source whose position we determine to coincide within ± 0''15 with the apex of the cone structure seen in the optical narrow emission lines, as well as the location of the flat spectrum radio component SI and the 12 μm emission peak. We interpret the compact source as hot dust near the sublimation temperature within ∼ 1 pc of the true nucleus of the galaxy. The remaining 6% of the light in the central 1″ comes from a moderately extincted stellar core centered on the nuclear position and of intrinsic size ∼50 pc. We show that this nuclear stellar core is probably 5-16 × 108 yr in age and contributes at least 7% of the total nuclear luminosity of ~1 × 1011 L⊙. © 1997. The American Astronomical Society. All rights reserved.

Abstract:

3D is a new type of a highly sensitive near-infrared integral field spectrometer developed at MPE. It has been designed to multiplex spectral as well as spatial information thus obtaining a full data cube in a single integration. At a spectral resolution between 1000 and 2000 and a field of view of 16 x 16 pixels, optimized for subarcsecond spatial resolution imaging spectroscopy, it has a much higher efficiency compared to conventional techniques. Outfitting one of the VLTs with a near-IR 3D type instrument will provide a powerful tool for diffraction-limited integral field spectroscopic research, in particular on faint high-z galaxies in the early universe. The basic design, recent upgrades as well as plans for a possible VLT-3D instrument are presented.