arXiv subjects

James G. Ingalls

Publications and source records attributed to James G. Ingalls.

24 records · Page 2Linked to original sources

arXiv2010

C$_{60}$ in Reflection Nebulae

The fullerene C$_{60}$ has four infrared-active vibrational transitions at 7.0, 8.5, 17.4 and 18.9 $μ$m. We have previously observed emission features at 17.4 and 18.9 $μ$m in the reflection nebula NGC 7023 and demonstrated spatial correlations suggestive of a common origin. We now confirm our earlier identification of these features with C$_{60}$ by detecting a third emission feature at 7.04 $\pm$ 0.05 $μ$m in NGC 7023. We also report the detection of these three C$_{60}$ features in the reflection nebula NGC 2023. Our spectroscopic mapping of NGC 7023 shows that the 18.9 $μ$m C$_{60}$ feature peaks on the central star and that the 16.4 $μ$m emission feature due to polycyclic aromatic hydrocarbons peaks between the star and a nearby photodissociation front. The observed features in NGC 7023 are consistent with emission from UV-excited gas-phase C$_{60}$. We find that 0.1-0.6% of interstellar carbon is in C$_{60}$; this abundance is consistent with those from previous upper limits and possible fullerene detections in the interstellar medium. This is the first firm detection of neutral C$_{60}$ in the interstellar medium.

astro-ph.GA↗

arXiv2004

Structure and Colors of Diffuse Emission in the Spitzer Galactic First Look Survey

We investigate the density structure of the interstellar medium using new high-resolution maps of the 8 micron, 24 micron, and 70 micron surface brightness towards a molecular cloud in the Gum Nebula, made as part of the Spitzer Space Telescope Galactic First Look Survey. The maps are correlated with 100 micron images measured with IRAS. At 24 and 70 micron, the spatial power spectrum of surface brightness follows a power law with spectral index -3.5. At 24 micron, the power law behavior is remarkably consistent from the 0.2 degree size of our maps down to the 5 arcsecond spatial resolution. Thus, the structure of the 24 micron emission is self-similar even at milliparsec scales. The combined power spectrum produced from Spitzer 24 micron and IRAS 25 micron images is consistent with a change in the power law exponent from -2.6 to -3.5. The decrease may be due to the transition from a two-dimensional to three-dimensional structure. Under this hypothesis, we estimate the thickness of the emitting medium to be 0.3 pc.

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arXiv2002

Photoelectric Heating and [CII] Cooling of High Galactic Latitude Translucent Clouds

The (2P3/2 -> 2P1/2) transition of singly--ionized carbon, [CII], is the primary coolant of diffuse interstellar gas. We describe observations of [CII] emission towards nine high Galactic latitude translucent molecular clouds, made with the long wavelength spectrometer on board the Infrared Space Observatory. To understand the role of dust grains in processing the interstellar radiation field (ISRF) and heating the gas, we compare the [CII] integrated intensity with the far-infrared (FIR) integrated surface brightness for the 101 sampled lines of sight. We find that [CII] is linearly correlated with FIR, and the average ratio is equal to that measured with the COBE satellite for all high-latitude Milky Way gas. There is a significant decrease that was not detected with COBE in [CII] emissivity at high values of FIR. Our sample splits naturally into two populations depending on the 60um/100um surface brightness ratio, or color: ``warm'' positions and ``cold'' positions. A transition from sources with warm to those with cold 60/100 colors coincides approximately with the transition from constant to decreasing [CII] emissivity. We model the [CII] and far-infrared emission under conditions of thermal equilibrium, using the simplifying assumptions that, in all regions heated by the ISRF, the most important source of gas heating is the photoelectric effect on grains and the most important source of gas cooling is [CII] emission. The model matches the data well. There are no statistically significant differences in the derived values of the ISRF intensity and the photoelectric heating efficiency for warm and cold sources. The observed variations in the [CII] emissivity and the 60/100 colors can be understood entirely in terms of the attenuation and softening of the ISRF by translucent clouds, not changes in dust properties.

astro-ph↗

arXiv2000

The Antarctic Submillimeter Telescope and Remote Observatory (AST/RO)

AST/RO, a 1.7 m diameter telescope for astronomy and aeronomy studies at wavelengths between 200 and 2000 microns, was installed at the South Pole during the 1994-1995 Austral summer. The telescope operates continuously through the Austral winter, and is being used primarily for spectroscopic studies of neutral atomic carbon and carbon monoxide in the interstellar medium of the Milky Way and the Magellanic Clouds. The South Pole environment is unique among observatory sites for unusually low wind speeds, low absolute humidity, and the consistent clarity of the submillimeter sky. Four heterodyne receivers, an array receiver, three acousto-optical spectrometers, and an array spectrometer are installed. A Fabry-Perot spectrometer using a bolometric array and a Terahertz receiver are in development. Telescope pointing, focus, and calibration methods as well as the unique working environment and logistical requirements of the South Pole are described.

astro-ph↗

arXiv1999

Physical State of Molecular Gas in High Galactic Latitude Translucent Clouds

The rotational transitions of carbon monoxide (CO) are the primary means of investigating the density and velocity structure of the molecular interstellar medium. Here we study the lowest four rotational transitions of CO towards high-latitude translucent molecular clouds (HLCs). We report new observations of the J = (4-3), (2-1), and (1-0) transitions of CO towards eight high-latitude clouds. The new observations are combined with data from the literature to show that the emission from all observed CO transitions is linearly correlated. This implies that the excitation conditions which lead to emission in these transitions are uniform throughout the clouds. Observed 13CO/12CO (1-0) integrated intensity ratios are generally much greater than the expected abundance ratio of the two species, indicating that the regions which emit 12CO (1-0) radiation are optically thick. We develop a statistical method to compare the observed line ratios with models of CO excitation and radiative transfer. This enables us to determine the most likely portion of the physical parameter space which is compatible with the observations. The model enables us to rule out CO gas temperatures greater than 30K since the most likely high-temperature configurations are 1 pc-sized structures aligned along the line of sight. The most probable solution is a high density and low temperature (HDLT) solution. The CO cell size is approximately 0.01 pc (2000 AU). These cells are thus tiny fragments within the 100 times larger CO-emitting extent of a typical high-latitude cloud. We discuss the physical implications of HDLT cells, and we suggest ways to test for their existence.

astro-ph↗

arXiv1998

A semianalytical model for the observational properties of the dominant carbon species at different metallicities

We present a simple, semianalytical approach to compute the line emission of C+, C and CO in Photodissociation Regions of varying metallicity that depends on very few parameters and naturally incorporates the clumpiness of the interstellar medium into the calculations. We use it to study the dependence of the I_[C II]/I_CO and I_[C I]/I_CO line ratios on the metallicity. We show how to modify it to include the effects of density and radiation field, and we have compared it with observational data. We find that the model explains the observed trend of enhanced [C II]/CO line ratio with decreasing metallicity as the natural result of the augmented fraction of photodissociated gas in a clump. We show that enhanced [C II]/CO ratios can be produced by lowering the upper limit of the clump size distribution, as may happen in 30 Doradus. We also find that the available data favors a [C I]/CO intensity ratio essentially independent of metallicity, albeit the paucity of observations does not exclude other possibilities. This is difficult to understand if most of the C is produced by UV photons as is the case with C+. Finally, we study the prediction of the model for the trend of the X_CO factor with metallicity. Comparison with previous observational studies yields good agreement.

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