187 research outputs found
Protoplanetary gas disks in the far infrared
The physical and chemical conditions in young protoplanetary disks set the
boundary conditions for planet formation. Although the dust in disks is
relatively easily detected as a far-IR photometric ``excess'' over the expected
photospheric emission, much less is known about the gas phase. It seems clear
that an abrupt transition from massive optically thick disks (gas-rich
structures where only ~1% of the total mass is in the form of dust) to tenuous
debris disks almost devoid of gas occurs at ~10^7 years, by which time the
majority of at least the giant planets must have formed. Indeed, these planets
are largely gaseous and thus they must assemble before the gas disk dissipates.
Spectroscopic studies of the disk gas content at different evolutive stages are
thus critical. Far-IR water vapor lines and atomic fine structure lines from
abundant gas reservoirs (e.g., [OI]63um, [SI]56um, [SiII]34um) are robust
tracers of the gas in disks. Spectrometers on board Herschel will detect some
of these lines toward the closest, youngest and more massive protoplanetary
disks. However, according to models, Herschel will not reach the required
sensitivity to (1) detect the gas residual in more evolved and tenuous
transational disks that are potentially forming planets and (2) detect the gas
emission from less massive protoplanetary disks around the most numerous stars
in the Galaxy (M-type and cooler dwarfs). Both are unique goals for
SPICA/SAFARI. Besides, SAFARI will be able to detect the far-IR modes of water
ice at ~44 and ~62um, and thus allow water ice to be observed in many
protoplanetary systems and fully explore its impact on planetary formation and
evolution.Comment: To appear in Proc. Workshop "The Space Infrared Telescope for
Cosmology & Astrophysics: Revealing the Origins of Planets and Galaxies".
Eds. A.M. Heras, B. Swinyard, K. Isaak, and J.R. Goicoeche
Physics based calibration of the Herschel/SPIRE bolometers
The bolometers (and readout circuitry) in the SPIRE instrument on the Herschel Space Observatory are among the best understood and well characterised of any subâmm astronomy instrument to date. SPIRE contains five arrays of NTD germanium spiderweb bolometers with up to 139 pixels per array. Their behaviour has been shown to be extremely stable as seen by repeated measurements in the years between initial array level and final instrument level tests, and can be described extremely well by a simple physical model (the ideal bolometer model). Calibration of the bolometers must take into account the nonâlinear response when viewing bright sources, and the effect of fluctuations in the heat sink temperature. The simple and wellâunderstood behaviour of the detectors, coupled with the stable conditions expected in flight, mean that in contrast to previous subâmm instruments, physical models can be used to improve or possibly replace empirical calibration methods. We describe how this can be done, and use the large amount of data from ground measurements to show that we can use models to accurately calculate the absolute power detected by the bolometers
Carinae's Dusty Homunculus Nebula from Near-Infrared to Submillimeter Wavelengths: Mass, Composition, and Evidence for Fading Opacity
Infrared observations of the dusty, massive Homunculus Nebula around the
luminous blue variable Carinae are crucial to characterize the mass-loss
history and help constrain the mechanisms leading to the Great Eruption. We
present the 2.4 - 670 m spectral energy distribution, constructed from
legacy ISO observations and new spectroscopy obtained with the {\em{Herschel
Space Observatory}}. Using radiative transfer modeling, we find that the two
best-fit dust models yield compositions which are consistent with CNO-processed
material, with iron, pyroxene and other metal-rich silicates, corundum, and
magnesium-iron sulfide in common. Spherical corundum grains are supported by
the good match to a narrow 20.2 m feature. Our preferred model contains
nitrides AlN and SiN in low abundances. Dust masses range from 0.25 to
0.44 but 45 in both cases due to an
expected high Fe gas-to-dust ratio. The bulk of dust is within a 5
7 central region. An additional compact feature is detected at 390 m.
We obtain = 2.96 10 , a 25\% decline from
an average of mid-IR photometric levels observed in 1971-1977. This indicates a
reduction in circumstellar extinction in conjunction with an increase in visual
brightness, allowing 25-40\% of optical and UV radiation to escape from the
central source. We also present an analysis of CO and CO through lines, showing that the abundances are consistent with
expectations for CNO-processed material. The [C~{\sc{ii}}] line is
detected in absorption, which we suspect originates in foreground material at
very low excitation temperatures.Comment: Accepted in Ap
Understanding the Herschel-SPIRE bolometers
Bolometers are very simple devices. In principle, the behaviour of a bolometer can be described by a simple model along with a small number of parameters. The SPIRE instrument for the Herschel Space Observatory contains five arrays of NTD germanium spiderweb bolometers containing up to 139 pixels. We show from characterisation measurements on the ground using the flight read-out system that the bolometers follow the ideal model extremely well, are very stable, and that the read-out system is sufficiently well behaved to take advantage of this. Calibration should be greatly simplified by being able to take advantage of this behaviour
A Demonstration of Spectral and Spatial Interferometry at THz Frequencies
A laboratory prototype spectral/spatial interferometer has been constructed
to demonstrate the feasibility of the double Fourier technique at Far Infrared
(FIR) wavelengths (0.15 - 1 THz). It is planned to use this demonstrator to
investigate and validate important design features and data processing methods
for future astronomical FIR interferometer instruments. In building this
prototype we have had to address several key technologies to provide an end-end
system demonstration of this double Fourier interferometer. We report on the
first results taken when viewing single slit and double slit sources at the
focus of a large collimator used to simulate real sources at infinity. The
performance of the prototype instrument for these specific field geometries is
analyzed to compare with the observed interferometric fringes and to
demonstrate image reconstruction capabilities.Comment: Accepted for publication in Applied Optic
Characterisation of Herschel-SPIRE flight model optical performances
The Spectral and Photometric Imaging Receiver (SPIRE) is one of three scientific instruments on ESA's Herschel Space Observatory. This long wavelength instrument covers 200 to 670ÎŒm with a three band photometric camera and a two band imaging Fourier Transform Spectrometer (IFTS). Following first results reported in a previous paper, we discuss the in-band optical performances of the flight model as measured extensively during several dedicated test campaigns. Complementary to the experimentally probed spectral characteristics of the instrument detailed in an accompanying paper (see L.D. Spencer et al., in these proceedings), attention is focused here on a set of standard but key tests aimed at measuring the spatial response of the Photometer and Spectrometer end-to-end optical chain, including detector. Effects of defocus as well as source size extent, in-band wavelength, and polarization are also investigated over respective Photometer and Spectrometer field-of-views. Comparison with optical modelling, based on instrument design knowledge and some of the internal component measured characteristics, is performed. Beyond the specific characterisation of each effect, this allows estimating in each band where optical behaviour and detector behaviour respectively dominates and also reconstructing some of the contributors to the instrument throughput. Based on this analysis, retrieved optical performances are finally assessed against the related science-driven instrument requirements
Generation of an optimal target list for the Exoplanet Characterisation Observatory (EChO)
The Exoplanet Characterisation Observatory (EChO) has been studied as a space
mission concept by the European Space Agency in the context of the M3 selection
process. Through direct measurement of the atmospheric chemical composition of
hundreds of exoplanets, EChO would address fundamental questions such as: What
are exoplanets made of? How do planets form and evolve? What is the origin of
exoplanet diversity?
More specifically, EChO is a dedicated survey mission for transit and eclipse
spectroscopy capable of observing a large, diverse and well-defined planetary
sample within its four to six year mission lifetime.
In this paper we use the end-to-end instrument simulator EChOSim to model the
currently discovered targets, to gauge which targets are observable and assess
the EChO performances obtainable for each observing tier and time. We show that
EChO would be capable of observing over 170 relativity diverse planets if it
were launched today, and the wealth of optimal targets for EChO expected to be
discovered in the next 10 years by space and ground-based facilities is simply
overwhelming.
In addition, we build on previous molecular detectability studies to show
what molecules and abundances will be detectable by EChO for a selection of
real targets with various molecular compositions and abundances.
EChO's unique contribution to exoplanetary science will be in identifying the
main constituents of hundreds of exoplanets in various mass/temperature
regimes, meaning that we will be looking no longer at individual cases but at
populations. Such a universal view is critical if we truly want to understand
the processes of planet formation and evolution in various environments.
In this paper we present a selection of key results. The full results are
available online (http://www.ucl.ac.uk/exoplanets/echotargetlist/).Comment: Accepted for publication in Experimental Astronomy, 20 pages, 10
figures, 3 table
Relative pointing offset analysis of calibration targets with repeated observations with Herschel-SPIRE Fourier-Transform Spectrometer
We present a method to derive the relative pointing offsets for SPIRE
Fourier-Transform Spectrometer (FTS) solar system object (SSO) calibration
targets, which were observed regularly throughout the Herschel mission. We
construct ratios of the spectra for all observations of a given source with
respect to a reference. The reference observation is selected iteratively to be
the one with the highest observed continuum. Assuming that any pointing offset
leads to an overall shift of the continuum level, then these ratios represent
the relative flux loss due to mispointing. The mispointing effects are more
pronounced for a smaller beam, so we consider only the FTS short wavelength
array (SSW, 958-1546 GHz) to derive a pointing correction. We obtain the
relative pointing offset by comparing the ratio to a grid of expected losses
for a model source at different distances from the centre of the beam, under
the assumption that the SSW FTS beam can be well approximated by a Gaussian. In
order to avoid dependency on the point source flux conversion, which uses a
particular observation of Uranus, we use extended source flux calibrated
spectra to construct the ratios for the SSOs. In order to account for continuum
variability, due to the changing distance from the Herschel telescope, the SSO
ratios are normalised by the expected model ratios for the corresponding
observing epoch. We confirm the accuracy of the derived pointing offset by
comparing the results with a number of control observations, where the actual
pointing of Herschel is known with good precision. Using the method we derived
pointing offsets for repeated observations of Uranus (including observations
centred on off-axis detectors), Neptune, Ceres and NGC7027. The results are
used to validate and improve the point-source flux calibration of the FTS.Comment: 17 pages, 19 figures, accepted for publication in Experimental
Astronom
Herschel SPIRE FTS Relative Spectral Response Calibration
Herschel/SPIRE Fourier transform spectrometer (FTS) observations contain
emission from both the Herschel Telescope and the SPIRE Instrument itself, both
of which are typically orders of magnitude greater than the emission from the
astronomical source, and must be removed in order to recover the source
spectrum. The effects of the Herschel Telescope and the SPIRE Instrument are
removed during data reduction using relative spectral response calibration
curves and emission models. We present the evolution of the methods used to
derive the relative spectral response calibration curves for the SPIRE FTS. The
relationship between the calibration curves and the ultimate sensitivity of
calibrated SPIRE FTS data is discussed and the results from the derivation
methods are compared. These comparisons show that the latest derivation methods
result in calibration curves that impart a factor of between 2 and 100 less
noise to the overall error budget, which results in calibrated spectra for
individual observations whose noise is reduced by a factor of 2-3, with a gain
in the overall spectral sensitivity of 23% and 21% for the two detector bands,
respectively.Comment: 15 pages, 13 figures, accepted for publication in Experimental
Astronom
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