Martin Harwit

Remote Sensing of Planetary Properties and Biosignatures on Extrasolar Terrestrial Planets

The major goals of NASAs Terrestrial Planet Finder (TPF) and the European Space Agencys Darwin missions are to detect terrestrial-sized extrasolar planets directly and to seek spectroscopic evidence of habitable conditions and life. Here we recommend wavelength ranges and spectral features for these missions. We assess known spectroscopic molecular band features of Earth, Venus, and Mars in the context of putative extrasolar analogs. The preferred wavelength ranges are 7-25 microns in the mid-IR and 0.5 to approximately 1.1 microns in the visible to near-IR. Detection of O2 or its photolytic product O3 merits highest priority. Liquid H2O is not a bioindicator, but it is considered essential to life. Substantial CO2 indicates an atmosphere and oxidation state typical of a terrestrial planet. Abundant CH4 might require a biological source, yet abundant CH4 also can arise from a crust and upper mantle more reduced than that of Earth. The range of characteristics of extrasolar rocky planets might far exceed that of the Solar System. Planetary size and mass are very important indicators of habitability and can be estimated in the mid-IR and potentially also in the visible to near-IR. Additional spectroscopic features merit study, for example, features created by other biosignature compounds in the atmosphere or on the surface and features due to Rayleigh scattering. In summary, we find that both the mid-IR and the visible to near-IR wavelength ranges offer valuable information regarding biosignatures and planetary properties; therefore both merit serious scientific consideration for TPF and Darwin.

PACS Evolutionary Probe (PEP) - A Herschel Key Program

Deep far-infrared photometric surveys studying galaxy evolution and the nature of the cosmic infrared background are a key strength of the Herschel mission. We describe the scientific motivation for the PACS Evolutionary Probe (PEP) guaranteed time key program and its role within the entire set of Herschel surveys, and the field selection that includes popular multiwavelength fields such as GOODS, COSMOS, Lockman Hole, ECDFS, and EGS. We provide an account of the observing strategies and data reduction methods used. An overview of first science results illustrates the potential of PEP in providing calorimetric star formation rates for high-redshift galaxy populations, thus testing and superseding previous extrapolations from other wavelengths, and enabling a wide range of galaxy evolution studies.

The deepest Herschel-PACS far-infrared survey: number counts and infrared luminosity functions from combined PEP/GOODS-H observations

We present results from the deepest Herschel-Photodetector Array Camera and Spectrometer (PACS) far-infrared blank field extragalactic survey, obtained by combining observations of the Great Observatories Origins Deep Survey (GOODS) fields from the PACS Evolutionary Probe (PEP) and GOODS-Herschel key programmes. We describe data reduction and theconstruction of images and catalogues. In the deepest parts of the GOODS-S field, the catalogues reach 3σ depths of 0.9, 0.6 and 1.3 mJy at 70, 100 and 160 μm, respectively, and resolve ~75% of the cosmic infrared background at 100 μm and 160 μm into individually detected sources. We use these data to estimate the PACS confusion noise, to derive the PACS number counts down to unprecedented depths, and to determine the infrared luminosity function of galaxies down to L_(IR) = 10^(11) L⊙ at z ~ 1 and L_(IR) = 10^(12) L⊙ at z ~ 2, respectively. For the infrared luminosity function of galaxies, our deep Herschel far-infrared observations are fundamental because they provide more accurate infrared luminosity estimates than those previously obtained from mid-infrared observations. Maps and source catalogues (>3σ) are now publicly released. Combined with the large wealth of multi-wavelength data available for the GOODS fields, these data provide a powerful new tool for studying galaxy evolution over a broad range of redshifts.

The Submillimeter Wave Astronomy Satellite: Science Objectives and Instrument Description

The Submillimeter Wave Astronomy Satellite (SWAS), launched in 1998 December, is a NASA mission dedicated to the study of star formation through direct measurements of (1) molecular cloud composition and chemistry, (2) the cooling mechanisms that facilitate cloud collapse, and (3) the large-scale structure of the UV-illuminated cloud surfaces. To achieve these goals, SWAS is conducting pointed observations of dense [n(H2) > 103 cm-3] molecular clouds throughout our Galaxy in either the ground state or a low-lying transition of five astrophysically important species: H2O, H218O, O2, C I, and 13CO. By observing these lines SWAS is (1) testing long-standing theories that predict that these species are the dominant coolants of molecular clouds during the early stages of their collapse to form stars and planets and (2) supplying previously missing information about the abundance of key species central to the chemical models of dense interstellar gas. SWAS carries two independent Schottky barrier diode mixers—passively cooled to ~175 K—coupled to a 54 × 68 cm off-axis Cassegrain antenna with an aggregate surface error ~11 μm rms. During its baseline 3 yr mission, SWAS is observing giant and dark cloud cores with the goal of detecting or setting an upper limit on the water and molecular oxygen abundance of 3 × 10-6 (relative to H2). In addition, advantage is being taken of SWASs relatively large beam size of 33 × 45 at 553 GHz and 35 × 50 at 490 GHz to obtain large-area (~1° × 1°) maps of giant and dark clouds in the 13CO and C I lines. With the use of a 1.4 GHz bandwidth acousto-optical spectrometer, SWAS has the ability to simultaneously observe either the H2O, O2, C I, and 13CO lines or the H218O, O2, and C I lines. All measurements are being conducted with a velocity resolution less than 1 km s-1.

Implications of Submillimeter Wave Astronomy Satellite Observations for Interstellar Chemistry and Star Formation

A long-standing prediction of steady state gas-phase chemical theory is that H2O and O2 are important reservoirs of elemental oxygen and major coolants of the interstellar medium. Analysis of Submillimeter Wave Astronomy Satellite (SWAS) observations has set sensitive upper limits on the abundance of O2 and has provided H2O abundances toward a variety of star-forming regions. Based on these results, we show that gaseous H2O and O2 are not dominant carriers of elemental oxygen in molecular clouds. Instead, the available oxygen is presumably frozen on dust grains in the form of molecular ices, with a significant portion potentially remaining in atomic form, along with CO, in the gas phase. H2O and O2 are also not significant coolants for quiescent molecular gas. In the case of H2O, a number of known chemical processes can locally elevate its abundance in regions with enhanced temperatures, such as warm regions surrounding young stars or in hot shocked gas. Thus, water can be a locally important coolant. The new information provided by SWAS, when combined with recent results from the Infrared Space Observatory, also provides several hard observational constraints for theoretical models of the chemistry in molecular clouds, and we discuss various models that satisfy these conditions.

The Herschel Multi-Tiered Extragalactic Survey: source extraction and cross-identifications in confusion-dominated SPIRE images

We present the cross-identification and source photometry techniques used to process Herschel SPIRE imaging taken as part of the Herschel Multi-Tiered Extragalactic Survey (HerMES). Cross-identifications are performed in map-space so as to minimize source-blending effects. We make use of a combination of linear inversion and model selection techniques to produce reliable cross-identification catalogues based on Spitzer MIPS 24-mu m source positions. Testing on simulations and real Herschel observations shows that this approach gives robust results for even the faintest sources (S-250 similar to 10 mJy). We apply our new technique to HerMES SPIRE observations taken as part of the science demonstration phase of Herschel. For our real SPIRE observations, we show that, for bright unconfused sources, our flux density estimates are in good agreement with those produced via more traditional point source detection methods (SUSSEXtractor) by Smith et al. When compared to the measured number density of sources in the SPIRE bands, we show that our method allows the recovery of a larger fraction of faint sources than these traditional methods. However, this completeness is heavily dependent on the relative depth of the existing 24-mu m catalogues and SPIRE imaging. Using our deepest multiwavelength data set in the GOODS-N, we estimate that the use of shallow 24-mu m catalogues in our other fields introduces an incompleteness at faint levels of between 20-40 per cent at 250 mu m.

PHOTON ORBITAL ANGULAR MOMENTUM IN ASTROPHYSICS

Astronomical observations of the orbital angular momentum of photons, a property of electromagnetic radiation that has come to the fore in recent years, have apparently never been attempted. Here I show how measurements of this property of photons have a number of astrophysical applications.

Water abundance in molecular cloud cores

We present Submillimeter Wave Astronomy Satellite (SWAS) observations of the 110 → 101 transition of ortho-H2O at 557 GHz toward 12 molecular cloud cores. The water emission was detected in NGC 7538, ρ Oph A, NGC 2024, CRL 2591, W3, W3OH, Mon R2, and W33 and was not detected in TMC-1, L134N, and B335. We also present a small map of the H2O emission in S140. Observations of the H218O line were obtained toward S140 and NGC 7538, but no emission was detected. The abundance of ortho-H2O relative to H2 in the giant molecular cloud cores was found to vary between 6 × 10-10 and 1 × 10-8. Five of the cloud cores in our sample have previous H2O detections; however, in all cases the emission is thought to arise from hot cores with small angular extents. The H2O abundance estimated for the hot core gas is at least 100 times larger than in the gas probed by SWAS. The most stringent upper limit on the ortho-H2O abundance in dark clouds is provided in TMC-1, where the 3 σ upper limit on the ortho-H2O fractional abundance is 7 × 10-8.

Codes for Multiplex Spectrometry

A number of binary cyclic coding schemes for multiplex spectrometry are discussed and evaluated in terms of a linear, least mean square, unbiased estimate. The optical realization of such codes in dispersion instruments is briefly discussed. We show that there are many advantages both in the construction of the instrument and in its operation which accrue from cyclic codes.

Masks for Hadamard transform optics, and weighing designs

This paper gives a brief survey of the design of masks for Hadamard spectrometers and image scanners. Three different criteria are described for judging a mask, as well as techniques for choosing masks that are not too far from the optimum.