Spitzer Infrared Spectrograph Detection of Molecular Hydrogen Rotational Emission towards Translucent Clouds

Using the Infrared Spectrograph on board the Spitzer Space Telescope, we have detected emission in the S(0), S(1), and S(2) pure-rotational (v = 0-0) transitions of molecular hydrogen (H_2) toward six positions in two translucent high Galactic latitude clouds, DCld 300.2–16.9 and LDN 1780. The detection of these lines raises important questions regarding the physical conditions inside low-extinction clouds that are far from ultraviolet radiation sources. The ratio between the S(2) flux and the flux from polycyclic aromatic hydrocarbons (PAHs) at 7.9 μm averages 0.007 for these six positions. This is a factor of about four higher than the same ratio measured toward the central regions of non-active Galaxies in the Spitzer Infrared Nearby Galaxies Survey. Thus, the environment of these translucent clouds is more efficient at producing rotationally excited H_2 per PAH-exciting photon than the disks of entire galaxies. Excitation analysis finds that the S(1) and S(2) emitting regions are warm (T ≳ 300 K), but comprise no more than 2% of the gas mass. We find that UV photons cannot be the sole source of excitation in these regions and suggest mechanical heating via shocks or turbulent dissipation as the dominant cause of the emission. The clouds are located on the outskirts of the Scorpius-Centaurus OB association and may be dissipating recent bursts of mechanical energy input from supernova explosions. We suggest that pockets of warm gas in diffuse or translucent clouds, integrated over the disks of galaxies, may represent a major source of all non-active galaxy H_2 emission

Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1

One aim of modern astronomy is to detect temperate, Earth-like exoplanets that are well suited for atmospheric characterization. Recently, three Earth-sized planets were detected that transit (that is, pass in front of) a star with a mass just eight per cent that of the Sun, located 12 parsecs away. The transiting configuration of these planets, combined with the Jupiter-like size of their host star—named TRAPPIST-1—makes possible in-depth studies of their atmospheric properties with present-day and future astronomical facilities. Here we report the results of a photometric monitoring campaign of that star from the ground and space. Our observations reveal that at least seven planets with sizes and masses similar to those of Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain, such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.1 and 12.35 days) are near-ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inwards. Moreover, the seven planets have equilibrium temperatures low enough to make possible the presence of liquid water on their surfaces

The Transiting Exoplanet Community Early Release Science Program for JWST

The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics of transiting exoplanets in unprecedented detail. However, the high-precision, timeseries observations required for such investigations have unique technical challenges, and prior experience with Hubble, Spitzer, and other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper, we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. We also describe the simulations used to establish the program. The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative data sets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the timeseries modes of all four JWST instruments that have been identified as the consensus highest priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies. The targets have been vetted with previous measurements, will be observable early in the mission, and have exceptional scientific merit. Community engagement in the project will be centered on a two-phase Data Challenge that culminates with the delivery of planetary spectra, timeseries instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission

Improving our understanding of the Spitzer Space Telescope's pointing drifts

Spitzer observations of exoplanets routinely yield photometric accuracies of better than one part in 10,000. However, the attainable precision is limited in part by pointing drifts, which have the effect of moving the target to less stable or less-well characterized regions of Spitzer’s IRAC detector arrays. Here we examine a large sample of observing sequences in an effort to identify the causes of these pointing drifts. We find that short term and higher order drifts are correlated on various time scales to the temperatures of components in and around the spacecraft bus, and are most likely due to very slight angular displacements of the star trackers. Despite the constraints imposed by a limited pool of targets, such pointing drifts are best mitigated by optimal scheduling, minimizing large and/or lengthy excursions in telescope pitch angle within 24 hours of a high-precision photometry sequence. Such an effort is currently being initiated by the Spitzer Science Center

Using drift scans to improve astrometry with Spitzer

The Spitzer Space Telescope Infrared Array Camera (IRAC) is the only space-based instrument currently capable of continuous long duration monitoring of brown dwarfs to detect variability and characterize their atmospheres. Any such studies are limited, however, by the accuracy to which we know the positions and distances to these targets (most of which are newly discovered and therefore do not yet have multiple epochs of astrometric data). To that end, we have begun a new initiative to adapt the astrometric drift scanning technique employed by the Hubble Space Telescope to enhance Spitzer measurements of parallaxes and proper motions of brown dwarfs and other targets. A suite of images are taken with a set of sources scanned across the array. This technique reduces random noise by coaddition, and because each target covers multiple pixels we are able to average over residual instrumental distortion and intra-pixel variations. Although these benefits can be realized with appropriate dithering, scanning is much more effcient because we can take data concurrently with the spacecraft motion, covering many pixels without waiting to reposition and settle. In this contribution we demonstrate that the observing mode works and describe our software for analyzing the observations. We outline ongoing efforts towards simultaneously solving for source position and residual distortion. Initial testing shows a factor of more than 2 improvement in the astrometric precision can be obtained with Spitzer. We anticipate being able to measure parallaxes for sources out to about 50 pc, increasing the volume surveyed by a factor of 100 and enabling luminosity measurements of the young population of brown dwarfs in the β Pictoris moving group. This observing mode will be ready for public use around Winter of 2015

Structure of Diffuse Emission in the Spitzer Galactic First Look Survey

The density and velocity structure of the interstellar medium (ISM) must be understood to address many astrophysical problems. We summarize the spatial statistics of 24 μm emission for 10 diffuse fields in the Spitzer Galactic First Look Survey. In almost all regions we analyzed, the x and y spectral indices are nearly equal, implying that the density structure in emitting regions is isotropic. All but one spectral index we measure is shallower than the index predicted by the theory of Kolmogorov (1941). We discuss the implications of our results in the light of recent simulations of magnetized and non-magnetized hydrodynamic turbulence

Spitzer Infrared Array Camera (IRAC) Pipeline: final modifications and lessons learned

In more than ten years of operations, the Spitzer Space Telescope has conducted a wide range of investigations from observing nearby asteroids to probing atmospheric properties of exoplanets to measuring masses of the most distance galaxies. Observations using the Infrared Array Camera (IRAC) at 3.6 and 4.5um will continue through mid-2019 when the James Webb Space Telescope will succeed Spitzer. In anticipation of the eventual end of the mission, the basic calibrated data reduction pipeline designed to produce flux-calibrated images has been finalized and used to reprocess all the data taken during the Spitzer warm mission. We discuss all final modifications made to the pipeline