Pixel and Micro-lensing with NGST

Within 8 years, the current microlensing surveys of M31 will provide several hundred events affecting unresolved stars. They will thus allow a statistical study of the dark matter in M31's halo. The NGST will resolve these stars and constrain the mass of the corresponding lenses. In case of on-line alerts from ground-based observations, real-time NGST follow-up with high signal-to-noise ratio will provide further constraints on the lenses. In addition, high resolution observations with NGST will complement XMM and the previous optical data and thus enable a closer insight of X-ray binaries within M31 to be obtained. The optimal instrumentation to achieve these scientific goals will be discussed. Last, the study of the dark matter encompassed in the galaxy clusters would be possible with high angular resolution observations on a large field camera and would open a new field of research.Comment: 5 pages -- presented at the NGST Science and Technology Exposition (Hyannis, USA) 13-16 september 1999, published by the PAS

The last gasps of VY CMa: Aperture synthesis and adaptive optics imagery

We present new observations of the red supergiant VY CMa at 1.25 micron, 1.65 micron, 2.26 micron, 3.08 micron and 4.8 micron. Two complementary observational techniques were utilized: non-redundant aperture masking on the 10-m Keck-I telescope yielding images of the innermost regions at unprecedented resolution, and adaptive optics imaging on the ESO 3.6-m telescope at La Silla attaining extremely high (~10^5) peak-to-noise dynamic range over a wide field. For the first time the inner dust shell has been resolved in the near-infrared to reveal a one-sided extension of circumstellar emission within 0.1" (~15 R_star) of the star. The line-of-sight optical depths of the circumstellar dust shell at 1.65 micron, 2.26 micron, and 3.08 micron have been estimated to be 1.86 +/- 0.42, 0.85 +/- 0.20, and 0.44 +/- 0.11. These new results allow the bolometric luminosity of VY~CMa to be estimated independent of the dust shell geometry, yielding L_star ~ 2x10^5 L_sun. A variety of dust condensations, including a large scattering plume and a bow-shaped dust feature, were observed in the faint, extended nebula up to 4" from the central source. While the origin of the nebulous plume remains uncertain, a geometrical model is developed assuming the plume is produced by radially-driven dust grains forming at a rotating flow insertion point with a rotational period between 1200-4200 years, which is perhaps the stellar rotational period or the orbital period of an unseen companion.Comment: 25 pages total with 1 table and 5 figures. Accepted by Astrophysical Journal (to appear in February 1999

A mid infrared imager for the Next Generation Space Telescope: MIRIM, the MIRI imager

MIRI is the mid infrared instrument planned for the NGST. Working in the 5 μm - 28 μm band, it includes 3 units: a spectrograph, an imager and a calibration facility. We describe here briefly MIRIM, the MIRI imager channel as it is at the end of the phase A study. MIRIM provides 3 observing modes: an imaging mode, with a Field Of View of 1.3 arcmin × 1.7 arcmin and a Pixel Field Of View of 0.1 arcsec/pixel, a coronagraphic mode and a low resolution mode for point sources, between 5 μm and 10 μm, with a spectral resolution R = λ/Δλ around 100

Color-differential astrometry with the NGST. Application to the spectroscopy of giant extrasolar planets

The color-differential astrometry method consists in measuring the variations of the photocenter position with wavelength. This yields spectral information on the spatial structure of an unresolved source, and could allow, among other applications, the direct detection and spectroscopy of extrasolar planets. We give some estimates of signal-to-noise ratio for CDA using the the NGST mid-infrared instrument (MIRI) spectrograph, assuming for the planetary luminosity either black-body models or recent synthetic spectra. Giant Planets around nearby Sun-like stars are potentially observable over a wide-range of orbital distances, if the precision on the measurement is limited by the fundamental noises. Space-based CDA would then be complementary, in terms of star-planet separation, to ground based interferometry and to coronography. This assumes, though, a proper chromatic stability of the instrument. This note introduces some possible methods for instrumental internal calibration, and presents a possible spatial design for a calibration by spatial modulation

The wind of the B[e] supergiant Henize S22 viewed through a reflection nebula in DEM L106

Narrowband HST WFPC2 images reveal a bow-shock-like halo around the H II region N30B toward the B[e] supergiant Hen S22 located within the larger DEM L106 nebula in the Large Magellanic Cloud. High-dispersion spectra of N30B show a narrow Halpha emission component from the ionized gas; the velocity variations indicate a gas flow of - 5 to - 10 km s(-1) in the vicinity of the H II regions, which is resultant from interactions with Hen S22's stellar wind and responsible for the bow-shock morphology. Spectra of N30B's halo show broad Halpha profiles extending over more than 1000 km s(-1), similar to that of Hen S22, indicating that the halo is a reflection nebula of Hen S22. Broadband morphologies of N30B's halo are also consistent with the reflection nebula interpretation. We use dust-scattering properties and the observed brightnesses of the reflection nebula and Hen S22 to constrain the reflection geometry. The reflected stellar Halpha emission and absorption vary across the reflection nebula as a result of viewing S22's anisotropic wind from different angles. This reflection nebula, together with the edge-on orientation of Hen S22's disk, provides an invaluable opportunity to study the disk and polar winds of a B[e] supergiant

Optical design for the 5-28mum NGST infrared imager MIRI

International audienc

Optical design for the 5-28mum NGST infrared imager MIRI

International audienc

Optical design for the 5-28mum NGST infrared imager MIRI

International audienc