Sean J. Carey

The Physical Properties of the Midcourse Space Experiment Galactic Infrared-dark Clouds

The SPIRIT III infrared telescope on the Midcourse Space Experiment (MSX) satellite has provided an unprecedented view of the mid-infrared emission (8-25 μm) of the Galactic plane. An initial analysis of images from MSX Galactic plane survey data reveals dark clouds seen in silhouette against the bright emission from the Galactic plane (Egan et al.). These clouds have mid-infrared extinctions in excess of 2 mag at 8 μm. We probed the physical properties of 10 of these MSX dark clouds using millimeter-wave molecular rotational lines as an indicator of dense molecular gas. All 10 clouds were detected in millimeter spectral lines of H2CO, which confirms the presence of dense gas. The distances to these clouds range from 1 to 8 kiloparsecs and their diameters from 0.4 to 15.0 pc. Excitation analysis of the observed lines indicates that the clouds are cold (T 105 cm-3]. Some of the clouds have nearby H II regions, H2O masers, and other tracers of star formation at comparable spectral line velocities; however, only one cloud contains embedded centimeter or infrared sources. The lack of mid- to far-infrared emission associated with these clouds suggests that they are not currently forming high-mass stars. If star formation is present in these clouds, it is clearly protostellar class 0 or earlier.

Submillimeter Observations of Midcourse Space Experiment Galactic Infrared-Dark Clouds

We present 850 and 450 μm continuum images of infrared-dark clouds (IRDCs) taken with the Submillimeter Common-User Bolometer Array (SCUBA) submillimeter camera at the James Clerk Maxwell Telescope. The IRDCs are large (1-10 pc diameter) molecular cores with gas densities ~106 cm-3 and temperatures ≈15 K. We detected strong submillimeter sources with peak flux densities of ≈1 Jy beam-1 at 850 μm in all eight clouds that were observed. The submillimeter emission generally lies within the envelope of the mid-infrared extinction where dense gas has been detected using H2CO as a tracer. The dust temperatures in the bright, compact sources are calculated to lie in the range 10-25 K. The masses of these sources are estimated to be in the range of several tens up to about a thousand solar masses. The corresponding gas column densities range over an order of magnitude, up to about 1023 cm-2. Several of the sources are detected in emission at both 850 and 8 μm. Two of the sources have HCO+ line profiles characteristic of molecular infall. It is likely that the bright, compact sources seen in the SCUBA images are in various early stages of star formation, from preprotostellar cores to class I objects.

Large-Scale Structure of the Carina Nebula

Observations obtained with the Midcourse Space Experiment (MSX) satellite reveal for the first time the complex mid-infrared morphology of the entire Carina Nebula (NGC 3372). On the largest size scale of approximately 100 pc, the thermal infrared emission from the giant H ii region delineates one coherent structure: a (somewhat distorted) bipolar nebula with the major axis perpendicular to the Galactic plane. The Carina Nebula is usually described as an evolved H ii region that is no longer actively forming stars, clearing away the last vestiges of its natal molecular cloud. However, the MSX observations presented here reveal numerous embedded infrared sources that are good candidates for sites of current star formation. Several compact infrared sources are located at the heads of dust pillars or in dark globules behind ionization fronts. Because their morphology suggests a strong interaction with the peculiar collection of massive stars in the nebula, we speculate that these new infrared sources may be sites of triggered star formation in NGC 3372.

The MSX galactic plane survey submillimeter results

Abstract The Midcourse Space Experiment (MSX) surveyed the Galactic plane within 5° latitude in four mid-infrared spectral bands. A set of full resolution (20″) 1.5°×1.5° images on 6″ pixel centers has been created in each spectral band by co-adding all the survey data. A lower (1.2′) resolution atlas of 10°×10° images provide large-scale panoramas of the plane. Both sets of images are valuable resources for identifying interesting objects for further study at other wavelengths. The low-resolution maps are ideally suited for comparison with molecular line surveys and one such comparison probes the star formation rate in the inner Galaxy. A new class of objects has been identified in the images, infrared dark clouds, which are silhouetted against the mid-infrared background emission from the interstellar medium in the Galactic plane. These clouds are dark out to 100 μm as evinced on the IRAS IRSA plates. Submillimeter emission traces the form of the dark cloud and reveals cores indicative of class 0 protostars.

Using the Spitzer IRAC science archive for instrument trending

We present a database of reduced data for all staring mode observations taken with the Infrared Array Camera (IRAC) during the Spitzer warm mission to monitor instrument performance, predict future instrument performance, and facilitate exoplanet and brown dwarf science. Our motivation is to be informed so that we can mitigate the impact of changing thermal conditions on science. Monitoring current trends allows us to predict future instrument performance and to adjust our recommended suite of best practices and calibrations accordingly. From this database we show that instrumental effects detrimental to high precision photometry either remain stable or improve. A uniform reduction of all IRAC light curves has never before been published, and will enable powerful science including accurate comparative studies of exoplanets and brown dwarfs. IRAC has been performing well throughout the warm mission and we expect performance to remain excellent.

Correcting distortions in the infrared array camera during the cryogenic mission of the Spitzer Space Telescope

We describe our ongoing efforts to model the field distortions of the Infrared Array Camera (IRAC) during the cryogenic portion of the Spitzer Space Telescope’s operations. We have compared over two million measured source positions in ~35,000 IRAC images with their positions in Gaia Data Release 1. Fitting 3rd and 5th order polynomials to the measured offsets, we find systematic uncertainties in IRAC-measured positions that are in the 50-60 milliarcsecond range for the 3.6 micron array, and 120-150 milliarcsecond range for the 4.5 micron array. A 5th-order fit does not appear to significantly improve the results over a 3rd order fit. However, this may be due at least partly to the failure of our current centroiding technique to account for variations in the Point Response Functions across each detector. We anticipate making several improvements in our continuing analysis, including (i) the refitting of the positions and position angles of each IRAC image using the Gaia catalog, (ii) making use of a less position-sensitive centroiding algorithm, (iii) correcting where possible for the proper motions of detected sources, and (iv) significantly increasing the number of source position measurements. Once finalized, the resulting distortion corrections will be incorporated into the headers of the archived images.

Overview of the Origins Space telescope: science drivers to observatory requirements

The Origins Space Telescope (OST) mission concept study is the subject of one of the four science and technology definition studies supported by NASA Headquarters to prepare for the 2020 Astronomy and Astrophysics Decadal Survey. OST will survey the most distant galaxies to discern the rise of metals and dust and to unveil the co-evolution of galaxy and blackhole formation, study the Milky Way to follow the path of water from the interstellar medium to habitable worlds in planetary systems, and measure biosignatures from exoplanets. This paper describes the science drivers and how they drove key requirements for OST Mission Concept 2, which will operate between ~5 and ~600 microns with a JWST sized telescope. Mission Concept 2 for the OST study optimizes the engineering for the key science cases into a powerful and more economical observatory compared to Mission Concept 1.

Lessons learned in extended-extended Spitzer Space Telescope operations

The Spitzer Space Telescope is executing the ninth year of extended operations beyond its 5.5-year prime mission. The project anticipated a maximum extended mission of about four years when the first mission extension was proposed. The robustness of the observatory hardware and the creativity of the project engineers and scientists in overcoming hurdles to operations has enabled a substantially longer mission lifetime. This has led to more challenges with an aging groundsystem due to resource reductions and decisions made early in the extended mission based on a shorter planned lifetime. We provide an overview of the extended mission phases, challenges met in maintaining and enhancing the science productivity, and what we would have done differently if the extended mission was planned from the start to be nearly twice as long as the prime mission.