Showing papers by "William T. Reach published in 2002"

Molecular and Ionic Shocks in the Supernova Remnant 3C 391

Abstract: New observations of the supernova remnant 3C 391 are presented in the near-infrared, using the H2 2.12 μm and [Fe II] 1.64 μm narrowband filters in the Prime Focus Infrared Camera on the Palomar Observatory Hale 200 inch telescope, and in the mid-infrared, using the circular-variable filters in the ISOCAM on the Infrared Space Observatory. Shocked H2 emission was detected from the broad molecular line region in 3C 391 (3C 391:BML) (40'' size), where broad millimeter CO and CS lines had previously been detected. A small H2 clump, 45'' from the main body of 3C 391:BML, was confirmed to have broad CO emission, demonstrating that the near-infrared H2 images can trace previously undetected molecular shocks. The [Fe II] emission has a significantly different distribution, being brightest in the bright radio bar at the interface between the supernova remnant and the giant molecular cloud, and following filaments in the radio shell. The near-infrared [Fe II] image and the mid-infrared 12-18 μm image (dominated by [Ne II] and [Ne III]) are the first images to reveal the radiative shell of 3C 391. The mid-infrared spectrum is dominated by bright ionic lines of [Fe II] 5.5 μm, [Ar II] 6.9 μm, [Ne II] 12.8 μm, and [Ne III] 15.5 μm, as well as the series of pure rotational lines of H2 S(2) through S(7). There are no aromatic hydrocarbons associated with the shocks, nor is there any mid-infrared continuum, suggesting that macromolecules and very small grains are destroyed in the shocks. Comparing 3C 391 with the better studied IC 443, both remnants have molecular- and ionic-dominated regions; for 3C 391, the ionic-dominated region is the interface into the giant molecular cloud, showing that the main bodies of giant molecular clouds contain significant regions with densities of 102-103 cm-3, and a small filling factor of higher density regions. The broad molecular line region 3C 391:BML was imaged in the 1-0 S(1) line at 15 resolution. The molecular shocked region resolves into 16 clumps of H2 emission, with some fainter diffuse emission, but with no associated near-infrared continuum sources. One of the clumps is coincident with a previously detected OH 1720 MHz maser to within our 03 astrometry. These clumps are interpreted as a cluster of prestellar dense molecular cores that are currently being shocked by the supernova blast wave.

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

Abstract: 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.

Photoelectric Heating and [C II] Cooling of High Galactic Latitude Translucent Clouds*

Abstract: The (2P3/2 → 2P1/2) transition of singly ionized carbon, [C II], is the primary coolant of diffuse interstellar gas. We describe observations of [C II] emission toward 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 [C II] integrated intensity with the far-infrared (far-IR) integrated surface brightness for the 101 sampled lines of sight. We find that [C II] is linearly correlated with far-IR, 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 [C II] emissivity at high values of far-IR. Our sample splits naturally into two populations depending on the 60 μm/100 μm surface brightness ratio, or color: "warm" positions with 60/100 > 0.16 and "cold" positions with 60/100 < 0.16. A transition from sources with warm to those with cold 60/100 colors coincides approximately with the transition from constant to decreasing [C II] emissivity. We model the [C II] and far-IR 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 [C II] emission. The model matches the data well, provided the ISRF incident flux is χ0 ≈ 1.6 (in units of the nominal value near the Sun), and the photoelectric heating efficiency is ≈ 4.3%. There are no statistically significant differences in the derived values of χ0 and for warm and cold sources. The observed variations in the [C II] 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.

The Origin of Far-Infrared [C II] Emission in the Large Magellanic Cloud

Abstract: We compare the distribution of H I 21 cm, H II 6563 A, and [C II] 157.7 μm line emission over the entire Large Magellanic Cloud (LMC). Bright [C II] emission is associated with H II regions and their surroundings, with a good correlation between [C II] and Hα filaments. Faint [C II] emission is also detected in regions with no H II emission. We found a reasonably good correlation between the extended [C II] emission and H I emission. Using the slope of the [C II] versus H I correlation in regions with low Hα brightness, 4πI/N = 6.0 ± 3.0 × 10-26 ergs s-1 (H atom)-1, and the total H I column density away from H II regions, the luminosity of [C II] from the atomic medium is 1.14 ± 0.57 × 106 L☉, which amounts to 20% of the total [C II] luminosity from the LMC. The [C II] emission per unit H I column density from the atomic regions of the LMC is similar to, but somewhat higher than, that in the Milky Way, because of the cancelling effects of lower C abundance and higher radiation field in the LMC. Significant deviations between the H I and [C II] in some regions suggest that part of the [C II] emission comes from ionized regions. Subtracting the contribution to the [C II] luminosity from bright H II regions, an upper limit of 46% of the [C II] luminosity could be due to diffuse ionized gas.