C. Meny

Inverse temperature dependence of the dust submillimeter spectral index

We present a compilation of PRONAOS-based results concerning the temperature dependence of the dust submillimeter spectral index, including data from Galactic cirrus, star-forming regions, dust associated to a young stellar object, and a spiral galaxy. We observe large variations of the spectral index (from 0.8 to 2.4) in a wide range of temperatures (11 to 80 K). These spectral index variations follow a hyperbolic-shaped function of the temperature, high spectral indices (1.6-2.4) being observed in cold regions (11-20 K) while low indices (0.8-1.6) are observed in warm regions (35-80 K). Three distinct effects may play a role in this temperature dependence: one is that the grain sizes change in dense environments, another is that the chemical composition of the grains is not the same in different environments, a third one is that there is an intrinsic dependence of the dust spectral index on the temperature due to quantum processes. This last effect is backed up by laboratory measurements and could be the dominant one.

Far-infrared to millimeter astrophysical dust emission I: A model based on physical properties of amorphous solids

Aims. We propose a new description of astronomical dust emission in the spectral region from the far-infrared to millimeter wavelengths. Methods. Unlike previous classical models, this description explicitly incorporates the effect of the disordered internal structure of amorphous dust grains. Our model is based on results from solid state physics used to interpret laboratory data. The model takes into account the effect of absorption by disordered charge distribution, as well as the effect of absorption by localized two level systems. Results. We review constraints on the various free parameters of the model from theory and laboratory experimental data. We show that, for realistic values of the free parameters, the shape of the emission spectrum will exhibit very broad structures whose shape will change in a non trivial way with the temperature of dust grains. The spectral shape also depends upon the parameters describing the internal structure of the grains. This opens new perspectives for identifying the nature of astronomical dust from the observed shape of the FIR/mm emission spectrum. A companion paper will provide an explicit comparison of the model with astronomical data.

Low-temperature FIR and submillimetre mass absorption coefficient of interstellar silicate dust analogues

Context. Cold dust grains are responsible for the far-infrared and submillimetre (FIR/submm) emission observed by Herschel and Planck. Their thermal emission is usually expressed as a modified black body law in which the FIR/submm dust opacity, or mass absorption coefficient (MAC), is described by the MAC at a given wavelength κλ0 and the temperature- and wavelength-independent emissivity spectral index β. However, numerous data from previous space and balloon-borne missions and recently from Herschel and Planck show that the dust emission is not well understood, as revealed for example by the observed anti-correlation of β with the grain temperature. Aims. The aim of this work is to measure the optical properties of interstellar dust analogues at low temperatures to give astronomers the necessary data for interpreting FIR/submm observations such as those from the Herschel and Planck satellites. Methods. We synthesised, via sol-gel methods, analogues of interstellar amorphous silicate grains, rich in Mg and Ca, and having stoichiometry of olivine and pyroxene. The samples are characterised by various techniques to determine their composition, size, amorphisation degree. All the amorphous samples are annealed at 1100 ◦ C to study the crystallised materials for comparison. We measured the MAC of all the samples in the 2–25 μm range at room temperature and in the 100–1000/1500 μm range for grain temperatures varying from 300 to 10 K. Results. The experimental results show that, for all the amorphous samples, the grain MAC decreases when the grain temperature decreases and that the local spectral index, β, defined as the slope of the MAC curve, is anti-correlated with the grain temperature. These variations, which are not observed in the crystallised samples, are related to the amorphous nature of the samples. In addition, the spectral shape of the MAC is complex and cannot be described by a single spectral index over the 100–1500 μm range. At short wavelengths (λ ≤ 500/700 μm), β is in the range 1.6–2.1 for all grain temperature and grain composition. However, at longer wavelengths (λ ≥ 500/700 μm), β ≤ 2 for samples with a pyroxene stoichiometry and β ≥ 2 for samples with an olivine stoichiometry. Conclusions. The dust properties in the FIR/submm domain and at low temperature are more complicated than expected. The simplifying asymptotic expression based on a single temperature- and wavelength-independent spectral index used by astronomers is not appropriate to describe the dust MAC, hence the dust emission, and may induce significant errors on the derived parameters, such as the dust mass and the dust physical and chemical properties. Instead, dust emission models should use the dust MAC as a function of wavelength and temperature.

SAFIRE-A: Spectroscopy of the Atmosphere Using Far-Infrared Emission/Airborne

A new instrument named SAFIRE-A (Spectroscopy of the Atmosphere using Far-Infrared Emission/Airborne), which can operate on high-altitude platforms, has been developed for the study of the atmospheric composition through limb-scanning emission measurements. The instrument is a polarizing Fourier transform spectrometer that operates in the far infrared with a resolution of 0.004 cm(−1). SAFIRE-A uses efficient photon noise limited detectors and a novel optical configuration, which provide a cold pupil and field stop as well as cold narrow bandpass filters to enhance its sensitivity. The instrument was successfully operated on an M-55 stratospheric research aircraft in the polar regions during the winter 1996–97 Airborne Polar Experiment. The instrument design, aircraft integration, and performances attained in the field campaign are described and discussed. The atmospheric emission spectrum is measured with an rms noise accuracy of 0.5 K (measured in brightness temperature) in each spectral element near 20 cm(−1) with a 30-s measurement time.

Detection and characterization of a 500 μm dust emissivity excess in the Galactic plane using Herschel/Hi-GAL observations

Context. Past and recent observations have revealed unexpected variations in the far-infrared – millimeter (FIR-mm) dust emissivity in the interstellar medium. In the Herschel spectral range, those are often referred to as a 500 μm emission excess. Several dust emission models have been developed to interpret astrophysical data in the FIR-mm domain. However, these are commonly unable to fully reconcile theoretical predictions with observations. In contrast, the recently revised two level system (TLS) model, based on the disordered internal structure of amorphous dust grains, seems to provide a promising way of interpreting existing data. Aims. The newly available Herschel infrared GALactic (Hi-GAL) data, which covers most of the inner Milky Way, offers a unique opportunity to investigate possible variations in the dust emission properties both with wavelength and environment. The goal of our analysis is to constrain the internal structure of the largest dust grains on Galactic scales, in the framework of the TLS model. Methods. By combining the IRIS (Improved Reprocessing of the IRAS Survey) 100 μm with the Hi-GAL 160, 250, 350, and 500 μm data, we model the dust emission spectra in each pixel of the Hi-GAL maps, using both the TLS model and, for comparison, a single modified black-body fit. The effect of temperature mixing along the line of sight is investigated to test the robustness of our results. Results. We find a slight decrease in the dust temperature with distance from the Galactic center, confirming previous results. We also report the detection of a significant 500 μm emissivity excess in the peripheral regions of the plane (35° < |l| < 70°) of about 13–15% of the emissivity, which can reach up to 20% in some HII regions. We present the spatial distributions of the best-fit values for the two main parameters of the TLS model, i.e. the charge correlation length, lc, used to characterize the disordered charge distribution (DCD) part of the model, and the amplitude A of the TLS processes with respect to the DCD effect. These distributions illustrate the variations in the dust properties with environment, in particular the plausible existence of an overall gradient with distance to the Galactic center. A comparison with previous findings in the solar neighborhood shows that the local value of the excess is less than expected from the Galactic gradient observed here.

Far-infrared to millimeter astrophysical dust emission - II. Comparison of the two-level systems (TLS) model with astronomical data

Aims. In a previous paper we proposed a new model for the emission by amorphous astronomical dust grains, based on solid-state physics. The model uses a description of the disordered charge distribution (DCD) combined with the presence of two-level systems (TLS) defects in the amorphous solid composing the grains. The goal of this paper is to compare this new model to astronomical observations of different Galactic environments in the far-infrared/submillimeter, in order to derive a set of canonical model parameters to be used as a Galactic reference to be compared to in future Galactic and extragalactic studies. Methods. We compare the TLS model with existing astronomical data. We consider the average emission spectrum at high latitudes in our Galaxy as measured with FIRAS and WMAP, as well as the emission from Galactic compact sources observed with the Archeops balloon experiment, for which an inverse relationship between the dust temperature and the emissivity spectral index has been shown. Results. We show that, unlike models previously proposed that often invoke two dust components at different temperatures, the TLS model successfully reproduces both the shape of the Galactic spectral energy distribution and its evolution with temperature as observed in the Archeops data. The best TLS model parameters indicate a charge coherence length of � 13 nm and other model parameters in broad agreement with expectations from laboratory studies of dust analogs. We conclude that the millimeter excess emission, which is often attributed to the presence of very cold dust in the diffuse ISM, is very likely caused solely by TLS emission in disordered amorphous dust grains. We discuss the implications of the new model, in terms of mass determinations from millimeter continuum observations and the expected variations in the emissivity spectral index with wavelength and dust temperature. The implications for analyzing the Herschel and Planck satellite data are discussed.

Submillimeter dust emission of the M 17 complex measured with PRONAOS

We map a 50 0 30 0 area in and around the M 17 molecular complex with the French submillimeter balloon-borne telescope PRONAOS, in order to better understand the thermal emission of cosmic dust and the structure of the interstellar medium. The PRONAOS-SPM instrument has an angular resolution of about 3 0 , corresponding to a size of 2 pc at the distance of this complex, and a high sensitivity up to 0.8 MJy/sr. The observations are made in four wide submillimeter bands corresponding to eective wavelengths of 200m, 260m, 360m and 580m. Using an improved map-making method for PRONAOS data, we map the M 17 complex and faint condensations near the dense warm core. We derive maps of both the dust temperature and the spectral index, which vary over a wide range, from about 10 K to 100 K for the temperature and from about 1 to 2.5 for the spectral index. We show that these parameters are anticorrelated, the cold areas (10-20 K) having a spectral index around 2, whereas the warm areas have a spectral index between 1 and 1.5. We discuss possible causes of this eect, and we propose an explanation involving intrinsic variations of the grain properties. Indeed, to match the observed spectra with two dust components having a spectral index equal to 2 leads to very large and unlikely amounts of cold dust. We also give estimates of the column densities and masses of the studied clumps. Three cold clumps (14-17 K) could be gravitationally unstable.

Galactic cold cores

Context. The Galactic Cold Cores project has carried out Herschel photometric observations of 116 fields where the Planck survey has found signs of cold dust emission. The fields contain sources in different environments and different phases of star formation. Previous studies have revealed variations in their dust submillimetre opacity. Aims. The aim is to measure the value of dust opacity spectral index and to understand its variations spatially and with respect to other parameters, such as temperature, column density, and Galactic location. Methods. The dust opacity spectral index β and the dust colour temperature T are derived using Herschel and Planck data. The relation between β and T is examined for the whole sample and inside individual fields. Results. Based on IRAS and Planck data, the fields are characterised by a median colour temperature of 16.1 K and a median opacity spectral index of β = 1.84. The values are not correlated with Galactic longitude. We observe a clear T–β anti-correlation. In Herschel observations, constrained at lower resolution by Planck data, the variations follow the column density structure and β_(FIR) can rise to ~2.2 in individual clumps. The highest values are found in starless clumps. The Planck 217 GHz band shows a systematic excess that is not restricted to cold clumps and is thus consistent with a general flattening of the dust emission spectrum at millimetre wavelengths. When fitted separately below and above 700 μm, the median spectral index values are β_(FIR) ~ 1.91 and β(mm) ~ 1.66. Conclusions. The spectral index changes as a function of column density and wavelength. The comparison of different data sets and the examination of possible error sources show that our results are robust. However, β variations are partly masked by temperature gradients and the changes in the intrinsic grain properties may be even greater.

Modeling and predicting the shape of the far-infrared to submillimeter emission in ultra-compact HII regions and cold clumps

Context. Dust properties are very likely affected by the environment in which dust grains evolve. For instance, some analyses of cold clumps (7-17 K) indicate that the aggregation process is favored in dense environments. However, studying warm (30-40 K) dust emission at long wavelength (λ > 300 µm) has been limited because it is difficult to combine far infrared-to-millimeter (FIR-to-mm) spectral coverage and high angular resolution for observations of warm dust grains. Aims. Using Herschel data from 70 to 500µm, which are part of the Herschel infrared Galactic (Hi-GAL) survey combined with 1.1 mm data from the Bolocam Galactic Plane Survey (BGPS), we compared emission in two types of environments: ultracompact HII (UCHII) regions, and cold molecular clumps (denoted as cold clumps). With this comparison we tested dust emission models in the FIR-to-mm domain that reproduce emission in the diffuse medium, in these two environments (UCHII regions and cold clumps). We also investigated their ability to predict the dust emission in our Galaxy. Methods. We determined the emission spectra in twelve UCHII regions and twelve cold clumps, and derived the dust temperature (T) using the recent two-level system (TLS) model with three sets of parameters and the so-called T-β (temperature-dust emissivity index) phenomenological models, with β set to 1.5, 2 and 2.5. Results. We tested the applicability of the TLS model in warm regions for the first time. This analysis indicates distinct trends in the dust emission between cold and warm environments that are visible through changes in the dust emissivity index. However, with the use of standard parameters, the TLS model is able to reproduce the spectral behavior observed in cold and warm regions, from the change of the dust temperature alone, whereas a T-β model requires β to be known.

Low compensation impurity band photoconductors

Low compensation thin layer of antimony doped silicon impurity band photoconductors doped at the level 1017 - 1018 cm-3 are evaluated in moderate background photon flux in the range of 1012 s-1 with the goal to approach photon noise limitation operation in spectral ranges near 300 cm-1. Blocked impurity band photodetectors based on the same active layer geometry and thickness than the photoconductors were also implemented and measured. Spectral features including cut off wavenumbers specific to impurity band effects are investigated as a function of electric field and temperature. Spectroscopic evidence for a giant gain mechanism for photoelectrons excited from residual impurities in the blocking layer of BIB structure is found. Figures of merit of both IB and BIB elements were measured and physical mechanisms underlying the limitation of their performances are outlined.