Susanna C. Finn

Chemistry in Infrared Dark Cloud Clumps: A Molecular Line Survey at 3 mm

We have observed 37 Infrared Dark Clouds (IRDCs), containing a total of 159 clumps, in high-density molecular tracers at 3 mm using the 22 m ATNF Mopra Telescope located in Australia. After determining kinematic distances, we eliminated clumps that are not located in IRDCs and clumps with a separation between them of less than one Mopra beam. Our final sample consists of 92 IRDC clumps. The most commonly detected molecular lines are (detection rates higher than 8%) N2H+, HNC, HN13C, HCO+, H13CO+, HCN, C2H, HC3N, HNCO, and SiO. We investigate the behavior of the different molecular tracers and look for chemical variations as a function of an evolutionary sequence based on Spitzer IRAC and MIPS emission. We find that the molecular tracers behave differently through the evolutionary sequence and some of them can be used to yield useful relative age information. The presence of HNC and N2H+ lines does not depend on the star formation activity. On the other hand, HC3N, HNCO, and SiO are predominantly detected in later stages of evolution. Optical depth calculations show that in IRDC clumps the N2H+ line is optically thin, the C2H line is moderately optically thick, and HNC and HCO+ are optically thick. The HCN hyperfine transitions are blended, and, in addition, show self-absorbed line profiles and extended wing emission. These factors combined prevent the use of HCN hyperfine transitions for the calculation of physical parameters. Total column densities of the different molecules, except C2H, increase with the evolutionary stage of the clumps. Molecular abundances increase with the evolutionary stage for N2H+ and HCO+. The N2H+/HCO+ and N2H+/HNC abundance ratios act as chemical clocks, increasing with the evolution of the clumps.

THE GALACTIC DISTRIBUTION OF INFRARED DARK CLOUDS

CS (2-1) measurements toward a large sample of fourth Galactic quadrant infrared dark clouds (IRDCs) were made with the Australia Telescope National Facility Mopra telescope in order to establish their kinematic distances and Galactic distribution. Due to its large critical density, CS unambiguously separates the dense IRDCs from more diffuse giant molecular clouds. The fourth-quadrant IRDCs show a pronounced peak in their radial galactocentric distribution at R = 6 kpc. The first-quadrant IRDC distribution (traced by 13CO emission) also shows a peak, but at a galactocentric radius of R = 5 kpc rather than 6 kpc. This disparity in the peak galactocentric radius suggests that IRDCs trace a spiral arm which lies closer to the Sun in the fourth quadrant. Indeed, the deduced IRDC distribution matches the location of the Scutum-Centaurus arm in Milky Way models dominated by two spiral arms. Since, in external galaxies, OB stars form primarily in spiral arms, the association of IRDCs with a Milky Way spiral arm supports the idea that high-mass stars form in IRDCs. The first-quadrant IRDC distribution also reveals a second peak near the solar circle, possibly due to the fact that 13CO could trace somewhat lower density IRDCs. The reliability of the MSX IRDC catalog by Simon and coworkers is estimated by using the CS detection rate of IRDC candidates. The overall reliability is at least 58%, and increases to near 100% for high contrasts, Galactic longitudes within ~30° of the Galactic center, and large mid-IR backgrounds. A significant fraction of our IRDC sample (14%) showed two CS velocity components, which probably represent two distinct IRDCs along the same line of sight.

The Millimeter Astronomy Legacy Team 90 GHz (MALT90) pilot survey

We describe a pilot survey conducted with the Mopra 22 m radio telescope in preparation for the Millimeter Astronomy Legacy Team Survey at 90 GHz (MALT90). We identified 182 candidate dense molecular clumps using six different selection criteria and mapped each source simultaneously in 16 different lines near 90 GHz. We present a summary of the data and describe how the results of the pilot survey shaped the design of the larger MALT90 survey. We motivate our selection of target sources for the main survey based on the pilot detection rates and demonstrate the value of mapping in multiple lines simultaneously at high spectral resolution.

Distributed low-mass star formation in the IRDC G34.43+00.24

We have used deep near-infrared observations with adaptive optics to discover a distributed population of low-mass protostars within the filamentary Infrared Dark Cloud G34.43+00.24. We use maps of dust emission at multiple wavelengths to determine the column density structure of the cloud. In combination with an empirically-verified model of the magnitude distribution of background stars, this column density map allows us to reliably determine overdensities of red sources that are due to embedded protostars in the cloud. We also identify protostars through their extended emission in K-band which comes from excited H2 in protostellar outflows or reflection nebulosity. We find a population of distributed low-mass protostars, suggesting that low-mass protostars may form earlier than, or contemporaneously with, high-mass protostars in such a filament. The low-mass protostellar population may also produce the narrow linewidth SiO emission observed in some clouds without high-mass protostars. Finally, we use a molecular line map of the cloud to determine the virial parameter per unit length along the filament and find that the highest mass protostars form in the most bound portion of the filament, as suggested by theoretical models.

Planetary Imaging Concept Testbed Using a Recoverable Experiment-Coronagraph (PICTURE C)

Abstract. An exoplanet mission based on a high-altitude balloon is a next logical step in humanity’s quest to explore Earthlike planets in Earthlike orbits orbiting Sunlike stars. The mission described here is capable of spectrally imaging debris disks and exozodiacal light around a number of stars spanning a range of infrared excesses, stellar types, and ages. The mission is designed to characterize the background near those stars, to study the disks themselves, and to look for planets in those systems. The background light scattered and emitted from the disk is a key uncertainty in the mission design of any exoplanet direct imaging mission, thus, its characterization is critically important for future imaging of exoplanets.

Planet Imaging Coronagraphic Technology Using a Reconfigurable Experimental Base (PICTURE-B): The Second in the Series of Suborbital Exoplanet Experiments

The PICTURE-B sounding rocket mission is designed to directly image the exozodiacal light and debris disk around the Sun-like star Epsilon Eridani. The payload used a 0.5m diameter silicon carbide primary mirror and a visible nulling coronagraph which, in conjunction with a fine pointing system capable of 5milliarcsecond stability, was designed to image the circumstellar environment around a nearby star in visible light at small angles from the star and at high contrast. Besides contributing an important science result, PICTURE-B matures essential technology for the detection and characterization of visible light from exoplanetary environments for future larger missions currently being imagined. The experiment was launched from the White Sands Missile Range in New Mexico on 2015 November 24 and demonstrated the first space operation of a nulling coronagraph and a deformable mirror. Unfortunately, the experiment did not achieve null, hence did not return science results.

Ionospheric imaging using merged ultraviolet airglow and radio occultation data

The Limb-imaging Ionospheric and Thermospheric Extreme-ultraviolet Spectrograph (LITES) and GPS Radio Occultation and Ultraviolet Photometry-Colocated (GROUP-C) experiments are being considered for flight aboard the Space Test Program Houston 5 (STP-H5) experiment pallet to the International Space Station (ISS). LITES is a compact imaging spectrograph that makes one-dimensional images of atmospheric and ionospheric ultraviolet (60-140 nm) airglow above the limb of the Earth. The LITES optical design is advantageous in that it uses a toroidal grating as its lone optical surface to create these high-sensitivity images without the need for any moving parts. GROUP-C consists of two instruments: a nadir-viewing ultraviolet photometer that measures nighttime ionospheric airglow at 135.6 nm with unprecedented sensitivity, and a GPS receiver that measures ionospheric electron content and scintillation with the assistance of a novel antenna array designed for multipath mitigation. By flying together, these two experiments form an ionospheric observatory aboard the ISS that will provide new capability to study low- and mid-latitude ionospheric structures on a global scale. This paper presents the design and implementation of the LITES and GROUP-C experiments on the STP-H5 payload that will combine for the first time high-sensitivity in-track photometry with vertical spectrographic imagery of ionospheric airglow to create high-fidelity images of ionospheric structures. The addition of the GPS radio occultation measurement provides the unique opportunity to constrain, as well as cross-validate, the merged airglow measurements.

End-to-end simulation of high-contrast imaging systems: methods and results for the PICTURE mission family

We describe a set of numerical approaches to modeling the performance of space flight high-contrast imaging payloads. Mission design for high-contrast imaging requires numerical wavefront error propagation to ensure accurate component specifications. For constructed instruments, wavelength and angle-dependent throughput and contrast models allow detailed simulations of science observations, allowing mission planners to select the most productive science targets. The PICTURE family of missions seek to quantify the optical brightness of scattered light from extrasolar debris disks via several high-contrast imaging techniques: sounding rocket (the Planet Imaging Concept Testbed Using a Rocket Experiment) and balloon flights of a visible nulling coronagraph, as well as a balloon flight of a vector vortex coronagraph (the Planetary Imaging Concept Testbed Using a Recoverable Experiment - Coronagraph, PICTURE-C). The rocket mission employs an on-axis 0.5m Gregorian telescope, while the balloon flights will share an unobstructed off-axis 0.6m Gregorian. This work details the flexible approach to polychromatic, end-to-end physical optics simulations used for both the balloon vector vortex coronagraph and rocket visible nulling coronagraph missions. We show the preliminary PICTURE-C telescope and vector vortex coronagraph design will achieve 10-8 contrast without post-processing as limited by realistic optics, but not considering polarization or low-order errors. Simulated science observations of the predicted warm ring around Epsilon Eridani illustrate the performance of both missions.

THE DISTRIBUTION OF INFRARED DARK CLOUDS IN THE FIRST GALACTIC QUADRANT

Infrared dark clouds (IRDCs) are believed to host the earliest stages of high-mass star and cluster formation. Because O stars typically travel short distances over their lifetimes, if IRDCs host the earliest stages of high-mass star formation then these cold, dense molecular clouds should be located in or near the spiral arms in the Galaxy. The Galactic distribution of a large sample of IRDCs should therefore provide information on Galactic structure. Moreover, determination of distances enables mass and luminosity calculations. We have observed a large sample of IRDC candidates in the first Galactic quadrant in the dense gas tracer CS (2-1) using the Mopra telescope in order to determine kinematic distances from the molecular line velocities. We find that the IRDCs are concentrated around a Galactocentric distance of ~4.5 kpc, agreeing with the results of Simon et al. This distribution is consistent with the location of the Scutum-Centaurus spiral arm. The group of IRDCs near the Sun in the first quadrant detected in 13CO (1-0) in Simon et al. is not detected in the CS data. This discrepancy arises from the differences in the critical densities between the 13CO (1-0) and CS (2-1) lines. We determine that the Midcourse Space Experiment selected IRDCs are not a homogeneous population, and 13CO (1-0) traces a population of IRDCs with lower column densities and lower 1.1 mm flux densities in addition to more dense IRDCs detected in CS. Masses of the first quadrant IRDCs are calculated from 13CO (1-0) maps. We find a strong peak in the Galactocentric IRDC mass surface density distribution at R Gal ~ 4.5 kpc.

Optical tolerances for the PICTURE-C mission: error budget for electric field conjugation, beam walk, surface scatter, and polarization aberration

The Planetary Imaging Concept Testbed Using a Recoverable Experiment - Coronagraph (PICTURE-C) mission will directly image debris disks and exozodiacal dust around nearby stars from a high-altitude balloon using a vector vortex coronagraph. Four leakage sources owing to the optical fabrication tolerances and optical coatings are: electric field conjugation (EFC) residuals, beam walk on the secondary and tertiary mirrors, optical surface scattering, and polarization aberration. Simulations and analysis of these four leakage sources for the PICTUREC optical design are presented here.