Transit-spectroscopy of exoplanets is one of the key observational techniques to characterize the extrasolar planet and its atmosphere. The observational challenges of these measurements require dedicated instrumentation and only the space environmen
t allows an undisturbed access to earth-like atmospheric features such as water or carbon-dioxide. Therefore, several exoplanet-specific space missions are currently being studied. One of them is EChO, the Exoplanet Characterization Observatory, which is part of ESAs Cosmic Vision 2015-2025 program, and which is one of four candidates for the M3 launch slot in 2024. In this paper we present the results of our assessment study of the EChO spectrometer, the only science instrument onboard this spacecraft. The instrument is a multi-channel all-reflective dispersive spectrometer, covering the wavelength range from 400 nm to 16 microns simultaneously with a moderately low spectral resolution. We illustrate how the key technical challenge of the EChO mission - the high photometric stability - influences the choice of spectrometer concept and drives fundamentally the instrument design. First performance evaluations underline the fitness of the elaborated design solution for the needs of the EChO mission.
We present a study of the circumstellar environment of IRAS 04158+2805 based on multi-wavelength observations and models. Images in the optical and near-infrared, a polarisation map in the optical, and mid-infrared spectra were obtained with VLT-FORS
1, CFHT-IR, and Spitzer-IRS. Additionally we used an X-ray spectrum observed with Chandra. We interpret the observations in terms of a central star surrounded by an axisymmetric circumstellar disc, but without an envelope, to test the validity of this simple geometry. We estimate the structural properties of the disc and its gas and dust content. We modelled the dust disc with a 3D continuum radiative transfer code, MCFOST, based on a Monte-Carlo method that provides synthetic scattered light images and polarisation maps, as well as spectral energy distributions. We find that the disc images and spectral energy distribution narrowly constrain many of the disc model parameters, such as a total dust mass of 1.0-1.75x10^-4 sollar masses and an inclination of 62-63 degrees. The maximum grain size required to fit all available data is of the order of 1.6-2.8 microns although the upper end of this range is loosely constrained. The observed optical polarisation map is reproduced well by the same disc model, suggesting that the geometry we find is adequate and the optical properties are representative of the visible dust content. We compare the inferred dust column density to the gas column density derived from the X-ray spectrum and find a gas-to-dust ratio along the line of sight that is consistent with the ISM value. To our knowledge, this measurement is the first to directly compare dust and gas column densities in a protoplanetary disc.
