A NICMOS-3 based spectrograph for near infrared astronomy

We describe a design for a compact cooled spectrograph which incorporates two cameras and two collimators to achieve high throughput from 0.8 to 2.5m. The spectrograph utilizes multiple NICMOS 3 arrays, specially mounted to minimize their separation in the spectral dimension. This accommodates the broad wavelength range while providing extensive coverage in the spatial dimension.     

SWIFT: An adaptive optics assisted I/z band integral field spectrograph

SWIFT is an adaptive optics assisted integral field spectrograph covering the I and z astronomical bands (0.7–1.0 μm) at a spectral resolving power R  5000. At its heart is an all-glass image slicer with high throughput based on a novel de-magnifying design allowing a compact instrument. SWIFT profits from two recent developments: (i) the improved ability of second generation adaptive optics systems to correct for atmospheric turbulence in SWIFTS’s bandpass, and (ii) the availability of CCD array detectors with high quantum efficiency at very red wavelengths. It is a dedicated integral field spectrograph, specifically built to address a range of interesting astrophysical questions.     

NAHUAL: a near‐infrared high‐resolution spectrograph for the GTC optimized for studies of ultracool dwarfs

We present the status of an ongoing study for a high‐resolution near‐infrared echelle spectrograph for the 10.4‐m GTC (Gran Telescopio de Canarias) which will soon start operating at the Observatorio del Roque de los Muchachos on the island of La Palma. The main science driver of this instrument, which we have baptized NAHUAL, is to carry out a high precision radial velocity survey of exoplanets around ultracool dwarfs. NAHUAL is being especially designed to achieve the highest possible accuracy for radial velocity measurements. The goal is to reach an accuracy of a few m/s. It is thus required that the instrument is cross‐dispersed and that it covers simultaneously a wide wavelength range. Absorption cells will be placed in front of the slit which will allow a simultaneous self‐reference similar to an iodine‐cell in the optical regime. It is planned to place the instrument at one of the Nasmyth platform of the GTC behind the Adaptive Optics system. Our current design reaches a maximum spectral resolution of λ/Δλ = 50000 with a slit width of 0.175 arcsec, and gives nearly complete spectral coverage from 900 to 2400 nm. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spatially resolved SO2 ice on Io, observed in the near IR

We present equivalent width maps of the 1.98 and 2.13 μm SO₂ ice absorption bands on the surface of Io. The data were taken on 17 April 2006 with the near-infrared mapping spectrometer, OSIRIS at the W.M. Keck Observatory, Hawaii. The maps show significant regional enhancements of SO₂ ice over the Bosphoros, Media, Tarsus and Chalybes Regiones.     

The galactic infrared/submillimeter dust radiation

Summary The unprecedented quality of the IRAS data prompted a series of papers reinvestigating the origin of the galactic infrared/submillimeter radiation and the nature of the heating sources of the interstellar dust. The scope of this paper is to review the main results of these new studies. Sect. 1 contains a general introduction to the subject and an overview of the earlier work. In Sect. 2 and 3, we summarize our current knowledge on dust properties and on the interstellar radiation field. Then, we present the recent interpretations of the infrared emission of our Galaxy, which are based on the IRAS data: in Sect. 4 we present the results from the galactic disk; Sect. 5 covers the question of the contribution from small dust particles, and in Sect. 6 we describe the infrared radiation from the galactic center. The nature of the heating sources and the origin of the galactic infrared radiation is then discussed in Sect. 7. Finally, Sect. 8 presents a comparison of our Galaxy with external galaxies     

OH／IR星的绝对K星等与距离

本文计算了一批ＯＨ／ＩＲ星的绝对Ｋ星等Ｍｋ，对ＯＨ／ＩＲ星的Ｍｋ和距离ｄ作了讨论。发现当Ｋ较亮时，Ｍｋ为一常数；当Ｋ较暗时，Ｍｋ较离散且呈现一定的变化趋势。本文还拟合得到了一个由Ｋ星等求距离的经验公式，并由此公式计算了一批ＯＨ／ＩＲ星的距离。最后，对所得结果进行了解释和讨论。     

Infrared reflectance spectroscopy on thin films: Interference effects

Laboratory simulations of processes on astronomical surfaces that use infrared reflectance spectroscopy of thin films to analyze their composition and structure often ignore important optical interference effects which often lead to erroneous measurements of absorption band strengths and give an apparent dependence of this quantity on film thickness, index of refraction and wavelength. We demonstrate these interference effects experimentally and show that the optical depths of several absorption bands of thin water ice films on a gold mirror are not proportional to film thickness. We describe the method to calculate accurately band strengths from measured absorbance spectra using the Fresnel equations for two different experimental cases, and propose a way to remove interference effects by performing measurements with P-polarized light incident at Brewster's angle.     

The search for extrasolar giant planets using integral field spectroscopy: Simulations

We present here simulations of extrasolar planets detections obtained using a combination of extreme adaptive optics and integral field spectroscopy. The simulation code, written for IDL, provides images and, in particular, spectra, taking into account realistic Speckle Noise, AO correction effects and specific instrumental features. A detailed study has been done for ESO VLT telescopes (8.2 m), within the Phase A of the CHEOPS project, but the code is particularly flexible and can be updated for larger telescope diameters (ELTs) in order to give a realistic estimate of the detection limits, for giant telescopes, in standard conditions of seeing.     

The coating hypothesis for ammonia ice particles in Jupiter: Laboratory experiments and optical modeling

We report laboratory experiments and modeling calculations investigating the effect of a hydrocarbon coating on ammonia ice spectral signatures. Observational evidence and thermochemical models indicate an abundance of ammonia ice clouds in Jupiter's atmosphere. However, spectrally identifiable ammonia ice clouds are found covering less than 1% of Jupiter's atmosphere, notably in areas of strong vertical transport, indicating a short lifetime for the signature of ammonia absorption on condensed ammonia particles [Baines, K.H., Carlson, R.W., Kamp, L.W., 2002. Icarus 159, 74-94]. Current literature has suggested coating of ammonia ice particles by a hydrocarbon haze as a possible explanation for this paradox. The work presented here supports the inference of a coating effect that can alter or suppress ammonia absorption features. In the experiments, thin films of ammonia ices are deposited in a cryogenic apparatus, coated with hydrocarbons, and characterized by reflection-absorption infrared spectroscopy. We have observed the effects on the ammonia ice absorption features near 3 and 9 μm with coverage by thin layers of hydrocarbons. Modeling calculations of these multilayer thin films assist in the interpretation of the experimental results and reveal the important role of optical interference in altering the aforementioned ammonia spectral features. Mie and T-matrix scattering calculations demonstrate analogous effects for ammonia ice particles and investigate the relative effects of ammonia ice particle size, shape, and coating layer thickness on the ice particle spectral signatures.     

Calculation of single-scattering albedos: Comparison of Mie results with Hapke approximations

The grain size of water ice can be determined from its near-infrared spectrum, which has numerous diagnostic absorption bands of different opacities. Models that have been used to determine water ice grain size from infrared spectra of icy outer Solar System objects have shown discrepancies in modeled grain size of a factor of two or more. Here the single-scattering albedo calculated using the commonly used Hapke model given by Roush [Roush, T.L., 1994. Icarus 108, 243-254] is compared with the exact calculation for spheres from a Mie series. An earlier approximation of single-scattering albedo called the Hapke “slab” model is also used in the comparison. All three models are implemented using the same optical constants for water ice at ∼110 K. Results are displayed for a large range of grain sizes from 1 μm to 1 mm. In general neither Hapke model can mimic the Rayleigh effects from particles sized near the wavelength of light that the Mie model predicts. For 10 μm particles, the slab model matches the Mie calculation quite well, but larger sizes are more discrepant. The Hapke/Roush model grain size needs to be ∼2.5 times larger to mimic the Mie results, and there are additional discrepancies in the continuum levels and band strengths. The Mie calculation for spheres is recommended for analysis of unknown remote sensing measurements, as it can mimic the spectra of oblate, prolate, and hollow particles given by equivalent sphere theories.     

New merlin observations of OH/IR stars: OH357.31-1.33 and OH0.9+1.3

We present recent radio interferometer measurements of the OH 1612 MHz maser emission from the OH/IR sources OH0.9+1.3 and OH357.31 obtained with the enhanced MERLIN. Some preliminary results are briefly discussed. These results are part of an on-going observational campaign to obtain the best radio maps of bright OH/IR stars with MERLIN, VLA and VLBA, in order to understand the nature and dynamics of their circumstellar matter and evolution.     

The formation and stability of carbonic acid on outer Solar System bodies

The radiation chemistry, thermal stability, and vapor pressure of solid-phase carbonic acid (H₂CO₃) have been studied with mid-infrared spectroscopy. A new procedure for measuring this molecule’s radiation stability has been used to obtain intrinsic IR band strengths and half-lives for radiolytic destruction. We report, for the first time, measurements of carbonic acid’s vapor pressure (0.290-2.33 × 10⁻¹¹ bar for 240-255 K) and its enthalpy of sublimation (71 ± 9 kJ mol⁻¹). We also report the first observation of a chemical reaction involving solid-phase carbonic acid. Possible applications of these findings are discussed, with an emphasis on the outer Solar System icy surfaces.     

Carbon dioxide segregation in 1:4 and 1:9 CO2:H2O ices

The mid-infrared spectra of mixed vapor deposited ices of CO₂ and H₂O were studied as a function of both deposition temperature and warming from 15 to 100 K. The spectra of ices deposited at 15 K show marked changes on warming beginning at 60 K. These changes are consistent with CO₂ segregating within the ice matrix into pure CO₂ domains. Ices deposited at 60 and 70 K show a greater degree of segregation, as high as 90% for 1:4 CO₂:H₂O ice mixtures deposited at 70 K. As the ice is warmed above 80 K, preferential sublimation of the segregated CO₂ is observed. The kinetics of the segregation process is also examined. The segregation of the CO₂ as the ice is warmed corresponds to temperatures at which the structure of the water ice matrix changes from the high density amorphous phase to the low density amorphous phase. We show how these microstructural changes in the ice have a profound effect on the photochemistry induced by ultraviolet irradiation. These experimental results provide a framework in which observations of CO₂ on the icy bodies of the outer Solar System can be considered.     

Louth crater: Evolution of a layered water ice mound

We report on observations made of the ∼36 km diameter crater, Louth, in the north polar region of Mars (at 70° N, 103.2° E). High-resolution imagery from the instruments on the Mars Reconnaissance Orbiter (MRO) spacecraft has been used to map a 15 km diameter water ice deposit in the center of the crater. The water ice mound has surface features that include roughened ice textures and layering similar to that found in the North Polar Layered Deposits. Features we interpret as sastrugi and sand dunes show consistent wind patterns within Louth over recent time. CRISM spectra of the ice mound were modeled to derive quantitative estimates of water ice and contaminant abundance, and associated ice grain size information. These morphologic and spectral results are used to propose a stratigraphy for this deposit and adjoining sand dunes. Our results suggest the edge of the water ice mound is currently in retreat.     

Simulation of Titan haze formation using a photochemical flow reactor: The optical constants of the polymer

Solar UV is the principal energy source impinging the atmosphere of Titan while the energy from the electrons in Saturn's magnetosphere is less than 0.5% of the UV light. Titan haze analogs were prepared by the photolysis of a mixture of gases that simulate the composition of its atmosphere (nitrogen, methane, hydrogen, acetylene, ethylene, and cyanoacetylene). The real (n) and imaginary (k) parts of the complex refractive index of haze analogs formed from four different gas mixtures were calculated from the spectral properties of the solid polymer in UV-visible, near infrared and infrared wavelength spectral regions. The value of n was constant at 1.6±0.1 throughout the 0.2-2.5 μm region. The variation of k with wavelength for the values derived for Titan has a lower error than the absolute values of k so the more significant comparisons are with the slopes of the k(λ) plots in the UV-VIS region. Three of the photochemical Titan haze analogs had slopes comparable to those derived for Titan from the Voyager data (Rages and Pollack, 1980, Icarus 41, 119-130; McKay and Toon, 1992, in: Proceedings of the Symposium on Titan, in: ESA SP, Vol. 338, pp. 185-190). The slopes of the k(λ) plots for haze analogs prepared by spark discharge (Khare et al., 1984, Icarus 60, 127-137) and plasma discharge (Ramirez et al., 2002, Icarus 156, 515-529) were also comparable to Titan's. These finding show that the k(λ) plots do not differentiate between different laboratory simulations of atmospheric chemistry on Titan in the UV-VIS near IR region (0.2-2.5 microns). There is a large difference between the k(λ) in the infrared between the haze analogs prepared photochemically and analogs prepared using a plasma discharges (Khare et al., 1984, Icarus 60, 127-137; Coll et al., 1999, Planet. Space Sci. 47, 1331-1340; Khare et al., 2002, Icarus 160, 172-182). The C/N ratio in the haze analog prepared by discharges is in the 2-11 range while that of the photochemical analogs is in the 18-24 range. The use of discharges and UV light for initiating the chemistry in Titan's atmosphere is discussed.     

Ethane ices in the outer Solar System: Spectroscopy and chemistry

We report recent experiments on ethane ices made at temperatures applicable to the outer Solar System. New near- and mid-infrared data for crystalline and amorphous ethane, including new spectra for a seldom-studied solid phase that exists at 35-55 K, are presented along with radiation-chemical experiments showing the formation of more-complex hydrocarbons.     

Gregor@night: The future high‐resolution stellar spectrograph for the GREGOR solar telescope

We describe the future night‐time spectrograph for the GREGOR solar telescope and present its science core projects. The spectrograph provides a 3‐pixel resolution of up to R = 87 000 in 45 échelle orders covering the wavelength range 390‐900 nm with three grating settings. An iodine cell can be used for high‐precision radial velocity work in the 500‐630 nm range. The operation of the spectrograph and the telescope will be fully automated without the presence of humans during night‐time and will be based on the successful STELLA control system. Future upgrades include a second optical camera for even higher spectral resolution, a Stokes‐V polarimeter and a link to the laser‐frequency comb at the Vacuum Tower Telescope. The night‐time core projects are a study of the angular‐momentum evolution of “The Sun in Time” and a continuation of our long‐term Doppler imaging of active stars (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Laboratory studies of methane and ethane adsorption and nucleation onto organic particles: Application to Titan's clouds

Titan, Saturn's largest moon, has a thick nitrogen/methane atmosphere. The temperature and pressure conditions in Titan's atmosphere are such that the methane vapor should condense near the tropopause to form clouds. Several ground-based measurements have observed sparse cloud-like features in Titan's atmosphere, while the Cassini mission to Saturn has provided large scale images of the clouds. However, Titan's cloud formation conditions remain poorly constrained. Heterogeneous nucleation (from the vapor phase onto a solid or liquid aerosol surface) greatly enhances cloud formation relative to homogeneous nucleation. In order to elucidate the cloud formation mechanism near the tropopause, we have performed laboratory measurements of the adsorption of methane and ethane onto solid organic particles (tholins) representative of Titan's photochemical haze. We find that monolayers of methane adsorb onto tholin particles at saturation ratios less than unity. We also find that solid methane nucleates onto the adsorbed methane at a saturation ratio of S=1.07±0.008. This implies that Titan's methane clouds should form easily. This is consistent with recent measurements of the column of methane ruling out excessive methane supersaturation. In addition, we find ethane adsorbs onto tholin particles in a metastable phase prior to nucleation. However, ethane nucleation onto the adsorbed ethane occurs at a relatively high saturation ratio of S=1.36±0.08. These findings are consistent with the recent report of polar ethane clouds in Titan's lower stratosphere.     

Saturn’s F ring grains: Aggregates made of crystalline water ice

We present models of the near-infrared (1-5 μm) spectra of Saturn’s F ring obtained by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) at ultra-high phase angles (177.4-178.5°). Modeling this spectrum constrains the size distribution, composition, and structure of F ring particles in the 0.1-100 μm size range. These spectra are very different from those obtained at lower phase angles; they lack the familiar 1.5 and 2 μm absorption bands, and the expected 3 μm water ice primary absorption appears as an unusually narrow dip at 2.87 μm. We have modeled these data using multiple approaches. First, we use a simple Mie scattering model to constrain the size distribution and composition of the particles. The Mie model allows us to understand the overall shapes of the spectra in terms of dominance by diffraction at these ultra-high phase angles, and also to demonstrate that the 2.87 μm dip is associated with the Christiansen frequency of water ice (where the real refractive index passes unity). Second, we use a combination of Mie scattering with Effective Medium Theory to probe the effect of porous (but structureless) particles on the overall shape of the spectrum and depth of the 2.87 μm band. Such simple models are not able to capture the shape of this absorption feature well. Finally, we model each particle as an aggregate of discrete monomers, using the Discrete Dipole Approximation (DDA) model, and find a better fit for the depth of the 2.87 μm feature. The DDA models imply a slightly different overall size distribution. We present a simple heuristic model which explains the differences between the Mie and DDA model results. We conclude that the F ring contains aggregate particles with a size distribution that is distinctly narrower than a typical power law, and that the particles are predominantly crystalline water ice.