Fluorescent H₂ Emission Lines from the Reflection Nebula NGC 7023 Observed with IGRINS

Abstract

We have analyzed the temperature, velocity and density of H 2 gas in NGC 7023 with a high-resolution near-infrared spectrum of the northwestern filament of the reflection nebula. By observing NGC 7023 in the H and K bands at R ≃ 45,000 with the Immersion GRating INfrared Spectrograph (IGRINS), we detected 68 H 2 emission lines within the 1 ′′ × 15 ′′ slit. The diagnostic ratios of 2-1 S(1)/1-0 S(1) is 0.41−0.56.In addition, the estimated ortho-to-para ratios (OPR) is 1.63−1.82, indicating that the H 2 emission transitions in the observed region arises mostly from gas excited by UV fluorescence. Gradients in the temperature, velocity, and OPR within the observed area imply motion of the photodissociation region (PDR) relative to the molecular cloud. In addition, we derive the column density of H 2 from the observed emission lines and compare these results with PDR models in the literature covering a range of densities and incident UV field intensities. The notable difference between PDR model predictions and the observed data, in high rotational J levels of ν = 1, is that the predicted formation temperature for newly-formed H 2 should be lower than that of the model predictions. To investigate the density distribution, we combine pixels in 1 ′′ × 1 ′′ areas and derive the density distribution at the 0.002 pc scale. The derived gradient of density suggests that NGC 7023 has a clumpy structure, including a high clump density of ∼10 5 cm −3 with a size smaller than ∼5 × 10 −3 pc embedded in lower density regions of 10 3 −10 4 cm −3 .

High-resolution Near-infrared Spectroscopic Study of Galactic Supernova Remnants. I. Kinematic Distances

“…It is worthwhile to note that its line width is also very small (4.4 ± 0.1 km s −1 ) compared to the other SNRs, whose line widths are larger than 9 km s −1 . It is comparable to the line width (2-6 km s −1 ; Kaplan et al 2017;Le et al 2017;Oh et al 2018) of nearby photodissociation regions (PDRs). We will discuss the nature of this source in Section 4.1.…”

Structure of photodissociation fronts in star-forming regions revealed by Herschel observations of high-J CO emission lines

“…The authors suggested that this is due either to an increase of the photoelectric heating efficiency or a larger H 2 formation rate on grains at high temperatures or to dynamical effects such as advection. The effect of an advancing photodissociation front was also invoked in NGC 7023 NW to account for the non-equilibrium values of the ortho-to-para ratio (Lemaire et al 1999; Fuente et al 1999; Fleming et al 2010; Le et al 2017). Since our models of NGC 7023 and Orion Bar are successful to account for both CO high-J and H 2 line intensities (as well as the ortho-to-para ratio), it is worth to investigate the underlying reason for that.…”

Shocked Molecular Hydrogen and Broad CO lines from the Interacting Supernova Remnant HB 3

“…The line strengths between 2-1 S(1) and 1-0 S(1) are used to distinguish between the two mechanisms. The ratio is ∼ 0.56 for fluorescence of H 2 emission and 0.06 -0.2 for shocked H 2 emission (Burton et al 1989(Burton et al , 1992Le et al 2017;Black & van Dishoeck 1987). Basically the 1-0 S(1) 2.21µm H 2 line is much stronger than the 2-1 S(1) 2.25µm line for shocked gas.…”

“…While the filamentary structures are associated with the SNR HB 3, there is another faint none-filamentary diffuse emission in the area of W 3 complex near the southeastern part of the SNR HB 3 (see Figures 2 and 3). Two mechanisms have been invoked to account for the H 2 line emission: fluorescence induced by ultraviolet pumping (Le et al 2017;Black & van Dishoeck 1987), and radiative cooling following collisional excitation by shock waves (Burton et al 1989). The shocked H 2 characteristically have broad (> 20 km s −1 ) H 2 lines (e.g., see Reach et al 2019), while the widths of fluorescent H 2 lines are expected to be considerably narrower.…”