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Register a new userPhysical Conditions in Shocked Interstellar Gas Interacting with the Supernova Remnant IC 443
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Adam M. Ritchey
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We present the results of a detailed investigation into the physical conditions in interstellar material interacting with the supernova remnant IC 443. Our analysis is based on a comprehensive examination of high-resolution far-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope of two stars behind IC 443. One of our targets (HD 43582) probes gas along the entire line of sight through the supernova remnant, while the other (HD 254755) samples material located ahead of the primary supernova shock front. We identify low velocity quiescent gas in both directions and find that the densities and temperatures in these components are typical of diffuse atomic and molecular clouds. Numerous high velocity components are observed in the absorption profiles of neutral and singly-ionized atomic species toward HD 43582. These components exhibit a combination of greatly enhanced thermal pressures and significantly reduced dust-grain depletions. We interpret this material as cooling gas in a recombination zone far downstream from shocks driven into neutral gas clumps. The pressures derived for a group of ionized gas components at high positive velocity toward HD 43582 are lower than those of the other shocked components, pointing to pressure inhomogeneities across the remnant. A strong very high velocity component near -620 km/s is seen in the absorption profiles of highly-ionized species toward HD 43582. The velocity of this material is consistent with the range of shock velocities implied by observations of soft thermal X-ray emission from IC 443. Moderately high-velocity gas toward HD 254755 may represent shocked material from a separate foreground supernova remnant.
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We present near-infrared (2.5 - 5.0 um) spectra of shocked H2 gas in the supernova remnant IC 443, obtained with the satellite AKARI. Three shocked clumps-known as B, C, and G-and one background region were observed, and only H2 emission lines were detected. Except the clump B, the extinctioncorrected level population shows the ortho-to-para ratio of ~ 3.0. From the level population of the clumps C and G-both AKARIs only and the one extended with previous mid-infrared observations-we found that the v = 0 levels are more populated than the v > 0 levels at a fixed level energy, which cannot be reproduced by any combination of H2 gas in Local Thermodynamic Equilibrium. The populations are described by the two-density power-law thermal admixture model, revised to include the collisions with H atoms. We attributed the lower (n(H2)=10^(2.8-3.8) cm-3) and higher (n(H2)=10^(5.4-5.8) cm-3) density gases to the shocked H2 gas behind C-type and J-type shocks, respectively, based on several arguments including the obtained high H I abundance n(H I)/n(H2)=0.01. Under the hierarchical picture of molecular clouds, the C-type and J-type shocks likely propagate into clumps and clouds (interclump media), respectively. The power-law index b of 1.6 and 3.5, mainly determined by the lower density gas, is attributed to the shock-velocity diversity, which may be a natural result during shock-cloud interactions. According to our results, H2 v = 1 - 0 S(1) emission is mainly from J-shocks propagating into interclump media. The H2 emission was also detected at the background region, and this diffuse H2 emission may originate from collisional process in addition to the ultraviolet photon pumping.
The physical origin of the overionized recombining plasmas (RPs) in supernova remnants (SNRs) has been attracting attention because its understanding provides new insight into SNR evolution. However, the process of the overionization, although it has been discussed in some RP-SNRs, is not yet fully understood. Here we report on spatially resolved spectroscopy of X-ray emission from IC~443 with {it XMM-Newton}. We find that RPs in regions interacting with dense molecular clouds tend to have lower electron temperature and lower recombination timescale. These tendencies indicate that RPs in these regions are cooler and more strongly overionized, which is naturally interpreted as a result of rapid cooling by the molecular clouds via thermal conduction. Our result on IC~443 is similar to that on W44 showing evidence for thermal conduction as the origin of RPs at least in older remnants. We suggest that evaporation of clumpy gas embedded in a hot plasma rapidly cools the plasma as was also found in the W44 case. We also discuss if ionization by protons accelerated in IC~443 is responsible for RPs. Based on the energetics of particle acceleration, we conclude that the proton bombardment is unlikely to explain the observed properties of RPs.
We present evidence that the very-high-energy (VHE, E > 100 GeV) gamma-ray emission coincident with the supernova remnant IC 443 is extended. IC 443 contains one of the best-studied sites of supernova remnant/molecular cloud interaction and the pulsar wind nebula CXOU J061705.3+222127, both of which are important targets for VHE observations. VERITAS observed IC 443 for 37.9 hours during 2007 and detected emission above 300 GeV with an excess of 247 events, resulting in a significance of 8.3 standard deviations (sigma) before trials and 7.5 sigma after trials in a point-source search. The emission is centered at 06 16 51 +22 30 11 (J2000) +- 0.03_stat +- 0.08_sys degrees, with an intrinsic extension of 0.16 +- 0.03_stat +- 0.04_sys degrees. The VHE spectrum is well fit by a power law (dN/dE = N_0 * (E/TeV)^-Gamma) with a photon index of 2.99 +- 0.38_stat +- 0.3_sys and an integral flux above 300 GeV of (4.63 +- 0.90_stat +- 0.93_sys) * 10^-12 cm^-2 s^-1. These results are discussed in the context of existing models for gamma-ray production in IC 443.
We present near-infrared (2.5-5.0 {mu}m) spectral studies of shocked H2 gas in the two supernova remnants IC 443 and HB 21, which are well known for their interactions with nearby molecular clouds. The observations were performed with Infrared Camera (IRC) aboard the AKARI satellite. At the energy range 7000 K <= E(v,J) <= 20000 K, the shocked H2 gas in IC 443 shows an ortho-to-para ratio (OPR) of 2.4+0.3-0.2, which is significantly lower than the equilibrium value 3, suggesting the existence of non-equilibrium OPR. The shocked gas in HB 21 also indicates a potential non-equilibrium OPR in the range of 1.8-2.0. The level populations are well described by the power-law thermal admixture model with a single OPR, where the temperature integration range is 1000-4000 K. We conclude that the obtained non-equilibrium OPR probably originates from the reformed H2 gas of dissociative J-shocks, considering several factors such as the shock combination requirement, the line ratios, and the possibility that H2 gas can form on grains with a non-equilibrium OPR. We also investigate C-shocks and partially-dissociative J-shocks for the origin of the non-equilibrium OPR. However, we find that they are incompatible with the observed ionic emission lines for which dissociative J-shocks are required to explain. The difference in the collision energy of H atoms on grain surfaces would make the observed difference between the OPRs of IC 443 and HB 21, if dissociative J-shocks are responsible for the H2 emission. Our study suggests that dissociative J-shocks can make shocked H2 gas with a non-equilibrium OPR.
We present the detections of shocked molecular hydrogen (H2) gas in near- and mid-infrared and broad CO in millimeter from the mixed-morphology supernova remnant (SNR) HB~3 (G132.7+1.3) using Palomar WIRC, the Spitzer GLIMPSE360 and WISE surveys, and HHSMT. Our near-infrared narrow-band filter H2 2.12 micron images of HB~3 show that both Spitzer IRAC and WISE 4.6 micron emission originates from shocked H2 gas. The morphology of H2 exhibits thin filamentary structures and a large scale of interaction sites between the HB~3 and nearby molecular clouds. Half of HB~3, the southern and eastern shell of the SNR, emits H2 in a shape of a butterfly or W, indicating the interaction sites between the SNR and dense molecular clouds. Interestingly, the H2 emitting region in the southeast is also co-spatial to the interacting area between HB~3 and the H~II regions of the W3 complex, where we identified star-forming activity. We further explore the interaction between HB~3 and dense molecular clouds with detections of broad CO(3-2) and CO(2-1) molecular lines from the southern and southeastern shells along the H2 emitting region. The widths of the broad lines are 8-20 km/s; the detection of such broad lines is unambiguous, dynamic evidence of the interactions between the SNR and clouds. The CO broad lines are from two branches of the bright, southern H2 shell. We apply the Paris-Durham shock model to the CO line profiles, which infer the shock velocities of 20 - 40 km/s, relatively low densities of 10^{3-4} cm^{-3} and strong (>200 micro Gauss) magnetic fields.
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