Spitzerobservations of Supernova Remnants. Ii. Physical Conditions and Comparison With Hh7 and Hh54

We have studied the shock-excited molecular regions associated with four supernova remnants (SNRs)—IC443C, W28, W44, and 3C391—and two Herbig-Haro objects—HH7 and HH54—using Spitzer 's Infrared Spectrograph (IRS). The physical conditions within the observed areas (roughly ∼1' × 1' in size) are inferred from spectroscopic data obtained from IRS and from the Short and Long Wavelength Spectrometers on board the Infrared Space Observatory ( ISO ), together with photometric data from Spitzer 's Infrared Array Camera. Adopting a power-law distribution for the gas temperature in the observed region, with the mass of gas at temperature T to T + dT assumed proportional to T − b dT , the H 2 S (0) to S (7) spectral line maps obtained with IRS were used to constrain the gas density, yielding estimated densities n (H 2 ) in the range of ∼ (2–4) × 10 3 cm −3 . The excitation of H 2 S (9) to S (12) and high- J CO pure rotational lines, however, require environments several times denser. The inconsistency among the best-fit densities estimated from different species can be explained by density fluctuations within the observed regions. The best-fit power-law index b is smaller than the value 3.8 predicted for a paraboloidal C-type bow shock, suggesting that the shock front has a "flatter" shape than that of a paraboloid. The best-fit parameters for SNRs and Herbig-Haro objects do not differ significantly between the two classes of sources, except that for the SNRs the ortho-to-para ratio (OPR) of hot gas ( T > 1000 K) is close to the local thermal equilibrium value 3, while for HH7 and HH54 even the hottest gas exhibits an OPR smaller than 3; we interpret this difference as resulting from environmental differences between these classes of sources, molecular material near SNRs being subject to stronger photodissociation that results in faster para-to-ortho conversion. Finally, we mapped the physical parameters within the regions observed with IRS and found that the mid-lying H 2 emissions— S (3) to S (5)—tend to trace the hot component of the gas, while the intensities of S (6) and S (7) are more sensitive to the density of the gas compared to S (3) to S (5).