Abstract
Abstract We have measured the gas temperature in the IC 63 photodissociation region (PDR) using the S(1) and S(5) pure rotation lines of molecular hydrogen with SOFIA/EXES. We divide the PDR into three regions for analysis based on the illumination from γ Cas: sunny, ridge, and shady. Constructing rotation diagrams for the different regions, we obtain temperatures of T ex = 562 − 43 + 52 K toward the ridge and T ex = 495 − 25 + 28 K in the shady side. The H2 emission was not detected on the sunny side of the ridge, likely due to the photodissociation of H2 in this gas. Our temperature values are lower than the value of T ex = 685 ± 68 K using the S(1), S(3), and S(5) pure rotation lines, derived by Thi et al. using lower spatial resolution ISO-SWS data at a different location of the IC 63 PDR. This difference indicates that the PDR is inhomogeneous and illustrates the need for high-resolution mapping of such regions to fully understand their physics. The detection of a temperature gradient correlated with the extinction into the cloud, points to the ability of using H2 pure rotational line spectroscopy to map the gas temperature on small scales. We used a PDR model to estimate the FUV radiation and corresponding gas densities in IC 63. Our results shows the capability of SOFIA/EXES to resolve and provide detailed information on the temperature in such regions.
Highlights
Molecular hydrogen is readily detectable in photodissociation regions (PDRs; Nadeau et al 1991; Timmermann et al 1996; van Dishoeck 2004)
We compared this with the direct line integration method and found agreeing flux values
Our results show that Stratospheric Observatory for Infrared Astronomy (SOFIA)/EXES is capable of investigating the spatial variation in temperatures in such regions
Summary
Molecular hydrogen is readily detectable in photodissociation regions (PDRs; Nadeau et al 1991; Timmermann et al 1996; van Dishoeck 2004). In the regions directly affected by FUV radiation, the thermal balance is related to the radiative transfer of the UV photons. The ejection of photoelectrons from the dust grains is the process accountable for heating of the region (Bakes & Tielens 1994; Weingartner & Draine 2001). In addition to this process, the collisional de-excitation of H2 molecules initially excited by UV photons is a mechanism contributing to the heating (Sternberg & Dalgarno 1989). The fine-structure lines of neutral atoms or of singly ionized species provide the cooling of the outer layers and CO rotational lines cool the predominantly molecular inner regions (Allers et al 2005)
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