Statistical Limits of Fourier Transform Imaging in the Gamma-ray Energy Range

Recent advances in imaging techniques and position-sensitive gamma-ray detectors have made feasible hard x-ray and gamma-ray telescopes with arc-second resolution [ 1]. Above an energy of 100 keV, past instrumentation has been limited to a typical angular resolution of a few degrees. A gamma-ray imaging device with 1 arc-second resolution would be a dramatic improvement over conventional, non-imaging instrumentation and have substantial new capabilities for observation of astrophysical gamma-ray sources. The arc-second gamma-ray imager is based on the Fourier transform imaging technique [2]. We briefly describe Fourier transform imaging and its application to hard x-ray and gamma-ray imaging. This description is followed by an analysis of Fourier transform imaging in the statistics limited regime. Computer simulations and laboratory demonstrations of practical gamma-ray imaging systems are presented

Characterizing the Youngest Herschel-detected Protostars I. Envelope Structure Revealed by CARMA Dust Continuum Observations

We present CARMA 2.9 mm dust continuum emission observations of a sample of 14 Herschel-detected Class 0 protostars in the Orion A and B molecular clouds, drawn from the PACS Bright Red Sources (PBRS) sample (Stutz et al.). These objects are characterized by very red 24 \micron\ to 70 \micron\ colors and prominent submillimeter emission, suggesting that they are very young Class 0 protostars embedded in dense envelopes. We detect all of the PBRS in 2.9 mm continuum emission and emission from 4 protostars and 1 starless core in the fields toward the PBRS; we also report 1 new PBRS source. The ratio of 2.9 mm luminosity to bolometric luminosity is higher by a factor of $\sim$5 on average, compared to other well-studied protostars in the Perseus and Ophiuchus clouds. The 2.9 mm visibility amplitudes for 6 of the 14 PBRS are very flat as a function of uv-distance, with more than 50\% of the source emission arising from radii $<$ 1500 AU. These flat visibility amplitudes are most consistent with spherically symmetric envelope density profiles with $\rho$~$\propto$~R$^{-2.5}$. Alternatively, there could be a massive unresolved structure like a disk or a high-density inner envelope departing from a smooth power-law. The large amount of mass on scales $<$ 1500 AU (implying high average central densities) leads us to suggest that that the PBRS with flat visibility amplitude profiles are the youngest PBRS and may be undergoing a brief phase of high mass infall/accretion and are possibly among the youngest Class 0 protostars. The PBRS with more rapidly declining visibility amplitudes still have large envelope masses, but could be slightly more evolved.Comment: Accepted to ApJ, 40 pages, 9 Figures, 4 Table

A map of the day-night contrast of the extrasolar planet HD 189733b

"Hot Jupiter" extrasolar planets are expected to be tidally locked because they are close (<0.05 astronomical units, where 1 AU is the average Sun-Earth distance) to their parent stars, resulting in permanent daysides and nightsides. By observing systems where the planet and star periodically eclipse each other, several groups have been able to estimate the temperatures of the daysides of these planets. A key question is whether the atmosphere is able to transport the energy incident upon the dayside to the nightside, which will determine the temperature at different points on the planet's surface. Here we report observations of HD 189733, the closest of these eclipsing planetary systems, over half an orbital period, from which we can construct a 'map' of the distribution of temperatures. We detected the increase in brightness as the dayside of the planet rotated into view. We estimate a minimum brightness temperature of 973 +/- 33 K and a maximum brightness temperature of 1212 +/- 11 K at a wavelength of 8 microns, indicating that energy from the irradiated dayside is efficiently redistributed throughout the atmosphere, in contrast to a recent claim for another hot Jupiter. Our data indicate that the peak hemisphere-integrated brightness occurs 16$\pm$6 degrees before opposition, corresponding to a hot spot shifted east of the substellar point. The secondary eclipse (when the planet moves behind the star) occurs 120 +/- 24 s later than predicted, which may indicate a slightly eccentric orbit.Comment: To appear in the May 10 2007 issue of Nature, 10 pages, 2 black and white figures, 1 colo

Anomalous CO2 Ice Toward HOPS-68: A Tracer of Protostellar Feedback

We report the detection of a unique CO2 ice band toward the deeply embedded, low-mass protostar HOPS-68. Our spectrum, obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope, reveals a 15.2 micron CO2 ice bending mode profile that cannot modeled with the same ice structure typically found toward other protostars. We develop a modified CO2 ice profile decomposition, including the addition of new high-quality laboratory spectra of pure, crystalline CO2 ice. Using this model, we find that 87-92% of the CO2 is sequestered as spherical, CO2-rich mantles, while typical interstellar ices show evidence of irregularly-shaped, hydrogen-rich mantles. We propose that (1) the nearly complete absence of unprocessed ices along the line-of-sight is due to the flattened envelope structure of HOPS-68, which lacks cold absorbing material in its outer envelope, and possesses an extreme concentration of material within its inner (10 AU) envelope region and (2) an energetic event led to the evaporation of inner envelope ices, followed by cooling and re-condensation, explaining the sequestration of spherical, CO2 ice mantles in a hydrogen-poor mixture. The mechanism responsible for the sublimation could be either a transient accretion event or shocks in the interaction region between the protostellar outflow and envelope. The proposed scenario is consistent with the rarity of the observed CO2 ice profile, the formation of nearly pure CO2 ice, and the production of spherical ice mantles. HOPS-68 may therefore provide a unique window into the protostellar feedback process, as outflows and heating shape the physical and chemical structure of protostellar envelopes and molecular clouds.Comment: Accepted to the Astrophysical Journal, 2013 February 15: 14 pages, 9 figures, 3 table

An Anomalous Extinction Law in the Cep OB3b Young Cluster: Evidence for dust processing during gas dispersal

We determine the extinction law through Cep OB3b, a young cluster of 3000 stars undergoing gas dispersal. The extinction is measured toward 76 background K giants identified with MMT/Hectospec spectra. Color excess ratios were determined toward each of the giants using $V$ and $R$ photometry from the literature, $g$,$r$,$i$ and $z$ photometry from SDSS and $J$, $H$, and $K_{s}$ photometry from 2MASS. These color excess ratios were the used to construct the extinction law through the dusty material associated with Cep OB3b. The extinction law through Cep OB3b is intermediate between the $R_{V} = 3.1$ and $R_{V} = 5$ laws commonly used for the diffuse atomic ISM and dense molecular clouds, respectively. The dependence of the extinction law on line-of-sight $A_{V}$ is investigated and we find the extinction law becomes shallower for regions with $A_{V} > 2.5$ magnitudes. We speculate that the intermediate dust law results from dust processing during the dispersal of the molecular cloud by the cluster.Comment: 31 pages, 10 Figures, 3 Tables, accepted for publication in Ap