Molecular hydrogen line emission from the reflection nebula Parsamyan 18

Abstract

The newly commissioned University of New South Wales Infrared Fabry-Perot (UNSWIR F) has been used to image molecular hydrogen emission at 2.12 and 2.25 m in the reflection nebula Parsamyan 18. P 18 is known to exhibit low values of the (1-0) /(2-1) S(1) ratio, suggestive of UV-pumped fluorescence rather than thermal excitation by shocks. Our line ratio mapping reveals the full extent of this fluorescent emission from extended arc-like features, as well as a more concentrated thermal component in regions closer to the central exciting star. We show that the emission morphology, line fluxes and gas density are consistent with the predictions of photodissociation region (PDR ) theory. Those regions with the highest intrinsic 1-0 S(1) intensities also tend to show the highest (1-0) /(2-1) S(1) line ratios. Furthermore, variations in the line ratio can be attributed to intrinsic fluctuations in the incident radiation field and /or the gas density, through the self-shielding action of H 2 . An isolated knot of emission discovered just outside P 18, and having both an unusually high (1-0) /(2-1) S(1) ratio and relative velocity, provides additional evidence for an outflow source associated with P 18.

POLYCYCLIC AROMATIC HYDROCARBON AND H2EMISSIONS IN THE ULTRAVIOLET-DOMINATED REGION IN NGC 2316

“…The combined effect of the efficient formation of H 2 on PAHs, as discussed below, and the weakening UV radiation field gives rise to the observed limb-brightened H 2 emission shell. The H 2 line ratios in higher transitions 1-0 S(1) and 2-1 S(1) are discussed by Ryder et al (1998), and our results are consistent with their interpretation of the H 2 emission from the PDR around the B3 star. Here, we limit our discussion to the H 2 emission in relation to the observed distribution of the PAH emissions.…”

“…In Figure 5, we identify prominent limb brightening in the H 2 emission, suggesting an H 2 emission shell around the UV source at radial distances 10 -20 (0.055-0.11 pc) away from it. This H 2 shell, detected in the pure rotational transitions (0-0) S(1), S(2), and S(3), is consistent with earlier detections of H 2 in higher transitions at 2.12 and 2.25 µm by Ryder et al (1998). It is interesting to note that a similar limb brightening is also evident in our PAH band emissions.…”

“…The H 2 emissions from PDRs are well explained by using UV-induced chemistry models (see Kaufman et al 2005). Ryder et al (1998) estimate a UV flux of ∼5000G 0 in the H 2 shell about 15 away from the UV source (G 0 = 1 represents the average interstellar radi- ation field 1.6 × 10 −3 erg cm Habing 1968). Using this radiation field and the observed H 2 0-0 S(3)/0-0 S(1) line ratio, we estimate from the PDR models ( Figure 10 in Kaufman et al 2005) a hydrogen nucleus number density, n ∼ 3 × 10 4 cm −3 .…”

“…Turning on the UV source modifies the number distribution of the PAH components emitting in a given spectral feature. For our empirical model fitting, we assume a certain form for the variation of PAH survival fraction against UV destruction with radial distance such that the PAH survival fraction has a constant value close to the UV source (at r < 6 ) and increases outward radially as the inverse of the UV intensity, reaching a value of unity (as in a no-UV case) near the H 2 shell (near r ∼ 15 , or r ∼ 0.08 pc, where the UV field ∼5000G 0 (Ryder et al 1998)). We assume the intensity of PAH emission to be proportional to the PAH survival against UV destruction and to the UV intensity.…”

High Spectral and Spatial Resolution Observations of Shocked Molecular Hydrogen at the Galactic Center

“…-8 -Observations of the 2.122 µm line of H 2 were also obtained with the Anglo-Australian Telescope (AAT) in June 1998, using the IRIS 1-2.5µm camera in conjunction with the UNSWIRF 1 Fabry-Perot etalon (Ryder et al 1998). With a FWHM spectral resolution of ∼ 75 kms −1 , a pixel size of 0.77 ′′ and a 100 ′′ circular field of view, the etalon is scanned through a spectral line of interest with a 40 kms −1 plate spacing, and with an 'off-line' setting chosen to provide continuum subtraction.…”