Near-IR Fluorescent Molecular Hydrogen Emission from NGC 2023

Michael G. Burton, J.E. Howe, T.R. Geballe, P.W.J.L. Brand, PASA, 15 (2), 194
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Next Section: OBSERVATIONS
Title/Abstract Page: Near-IR Fluorescent Molecular Hydrogen
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Contents Page: Volume 15, Number 2

INTRODUCTION

The reflection nebula NGC 2023 is one of the brightest sources of fluorescent molecular hydrogen in the sky, making it a laboratory for the study of Htex2html_wrap_inline441 fluorescence which occurs over a wide range of physical conditions. It is powered by the B1.5V star HD 37903, the most luminous member of a cluster of young stellar objects illuminating the front surface of the Lynds 1630 molecular cloud in Orion B (Depoy et al. 1990). NGC 2023 forms a cavity in the surface of the cloud, some 450 pc from us, producing both a bright visual reflection nebula and a UV-excited photodissociation region. The latter is evident through, for instance, extensive Ctex2html_wrap_inline445 158tex2html_wrap_inline439m line emission (Howe et al. 1991), extended red emission features (Witt & Malin 1989) and IR emission features from PAHs (Joblin et al. 1995).

Fluorescent molecular hydrogen was first discovered through observations of NGC 2023 (Gatley et al. 1987; Hasegawa et al. 1987), on the basis of a 1-0/2-1 S(1) line ratio of tex2html_wrap_inline449. Narrow line profiles (FWHM < 16km tex2html_wrap_inline453, Burton et al. 1990a), and numerous high-excitation emission lines emitted in the far-red (from levels as high as v=7 & 8, Burton et al. 1992) are also evidence for the fluorescent excitation process in this source, as opposed to shocks. Emission line images (Burton et al. 1989; Field et al. 1994) show narrow filamentary structure on top of a diffuse emission nebula, suggestive of emission from limb-brightened undulations in the surface of the molecular cloud. However, Steiman-Cameron et al. (1997) argue that elevated intensities are due, in large, to enhancements in local density rather than limb brightening.

The application of photodissociation region (PDR) models (e.g., Tielens & Hollenbach 1985) to interpreting the emission in sub-mm CO rotational, far-IR fine structure and near-IR Htex2html_wrap_inline441 lines from NGC 2023 (Jaffe et al. 1990; Burton, Hollenbach & Tielens 1990b; Steiman-Cameron et al. 1997) suggests that a far-UV radiation field tex2html_wrap_inline457 times the average interstellar value is incident on a clumpy molecular cloud, with the bulk of the gas at densities of tex2html_wrap_inline459tex2html_wrap_inline461, but with a fraction (tex2html_wrap_inline463) about two orders of magnitude denser. However, we note that Wyrowski et al. (1997), on the basis of C91tex2html_wrap_inline465 radio data, suggest that the beam filling factor for high density gas is much greater than this, albeit with the dense clumps only at 100-300K as opposed to the 750K determined by Steiman-Cameron et al.

  figure33
Figure: Spectra from 1-2.5tex2html_wrap_inline439m observed at the southern location, (-11'', -78''), in NGC 2023. Several series of vibrational-rotational lines of Htex2html_wrap_inline441 are indicated. The wavelength of the lowest excitation line in each branch shown is described by the following illustrative coding:
e.g., 2-0 S(0) +, 1.190tex2html_wrap_inline439m denotes the 2-0 S-branch with + signs, starting from 2-0 S(0) at 1.190tex2html_wrap_inline439m.
Top panel (J-band): 2-0 S(0) +, 1.190tex2html_wrap_inline439m; 2-0 Q(1) tex2html_wrap_inline479, 1.238tex2html_wrap_inline439m; 2-0 O(2) vertical line, 1.293tex2html_wrap_inline439m; 3-1 S(0) triangle, 1.262tex2html_wrap_inline439m; 3-1 Q(1) filled triangle, 1.314tex2html_wrap_inline439m; 3-1 O(2) 3-sided star, 1.373tex2html_wrap_inline439m; 4-2 S(0) square, 1.343tex2html_wrap_inline439m; 4-2 Q(1) filled square, 1.398tex2html_wrap_inline439m; 5-3 S(0) pentagon, 1.433tex2html_wrap_inline439m; 6-4 S(3) hexagon, 1.446tex2html_wrap_inline439m.
Middle panel (H-band): 1-0 S(6) open circle, 1.788tex2html_wrap_inline439m; 3-1 O(5) 3-sided star, 1.522tex2html_wrap_inline439m; 4-2 O(3) tex2html_wrap_inline479, 1.510tex2html_wrap_inline439m; 5-3 Q(3) filled pentagon, 1.506tex2html_wrap_inline439m; 5-3 O(2) 5-sided star, 1.561tex2html_wrap_inline439m; 6-4 S(0) hexagon, 1.537tex2html_wrap_inline439m; 6-4 Q(1) filled hexagon, 1.602tex2html_wrap_inline439m; 6-4 O(2) 6-sided star, 1.675tex2html_wrap_inline439m; 7-5 S(0) 7-gon, 1.658tex2html_wrap_inline439m; 7-5 Q(1) filled 7-gon, 1.729tex2html_wrap_inline439m; 8-6 S(3) 8-star, 1.702tex2html_wrap_inline439m; 10-7 S(3) 10-star, 1.549tex2html_wrap_inline439m.
Bottom panel (K-band): 1-0 S(0) circle, 2.223tex2html_wrap_inline439m; 1-0 Q(1) filled circle, 2.407tex2html_wrap_inline439m; 2-1 S(0) +, 2.356tex2html_wrap_inline439m; 3-2 S(1) 3-gon, 2.386tex2html_wrap_inline439m; 4-3 S(2) 4-star, 2.435tex2html_wrap_inline439m.

  figure38
Figure: Spectra from 1-2.5tex2html_wrap_inline439m observed at the northern location, (+33'', +105''), in NGC 2023. Symbols are as described for Figure 1.

In view of its brightness, the Htex2html_wrap_inline441 emission from NGC 2023 can be subjected to detailed spectral examination, with emission lines from highly-excited vibrational-rotational levels in the ground electronic state measured. Such observations permit stringent tests of fluorescent excitation models (e.g., Black & van Dishoeck 1987; Sternberg & Dalgarno 1989; Burton, Hollenbach & Tielens 1990b; Draine & Bertoldi 1996) for Htex2html_wrap_inline441. Deviations from a pure fluorescent cascade might result from collisional depopulation of excited levels, from formation pumping of newly created molecules, from variations in the injection levels for the cascade and/or from changes in ortho-to-para ratio. Thus they may be used to test our physical models for these processes. This paper reports observations of the 1-2.5tex2html_wrap_inline439m Htex2html_wrap_inline441 emission from two locations in NGC 2023 which can be used for this purpose. Parts of these data have previously been discussed in the PhD Dissertation of John Howe (1992) and in a review on molecular hydrogen emission (Burton, 1992).

   

Line

tex2html_wrap_inline549 Flux Densitytex2html_wrap_inline423
(tex2html_wrap_inline439m) (-11'', -78'')tex2html_wrap_inline425 (+33'', +105'')
tex2html_wrap_inline561 erg tex2html_wrap_inline563tex2html_wrap_inline565tex2html_wrap_inline439mtex2html_wrap_inline569

7-4 S(1)tex2html_wrap_inline571

1.063070 3.1 (0.7)tex2html_wrap_inline427 ...
2-0 S(7)tex2html_wrap_inline571 1.064143 ...
2-0 S(4) 1.099817 2.3 (0.6) <1.0
2-0 S(3) 1.117492 4.0 (0.6) 2.6 (0.5)
8-5 S(3)tex2html_wrap_inline579 1.129215
3-1 S(7)tex2html_wrap_inline579 1.130405 2.6 (0.5) 2.0 (0.6)
2-0 S(2) 1.138238 3.2 (0.5) 1.7 (0.3)
3-1 S(5)tex2html_wrap_inline583 1.151873 4.2 (0.4) 1.2 (0.5)
8-5 S(1)tex2html_wrap_inline583 1.152436
2-0 S(1) 1.162223 2.9 (0.4) 1.8 (0.3)
3-1 S(4) 1.167158 2.1 (0.4) 1.2 (0.3)
3-1 S(3) 1.185697 3.1 (0.5) 1.9 (0.5)
2-0 S(0) 1.189585 1.8 (0.6) 0.4 (0.4)
8-5 Q(1)tex2html_wrap_inline587 1.201711
4-2 S(7)tex2html_wrap_inline587 1.204710
8-5 Q(2)tex2html_wrap_inline587 1.206846
3-1 S(2)tex2html_wrap_inline587 1.207589 2.8 (0.4) 4.0 (0.6)
4-2 S(6)tex2html_wrap_inline595 1.213907
8-5 Q(3)tex2html_wrap_inline595 1.214628 1.5 (0.4) 2.1 (0.5)
4-2 S(5) 1.226300 2.6 (0.4) 1.1 (0.3)
3-1 S(1) 1.232986 3.5 (0.4) 1.8 (0.2)
2-0 Q(1) 1.238343 2.8 (0.5) 1.9 (0.3)
2-0 Q(2)tex2html_wrap_inline599 1.241937 2.4 (0.5) 1.7 (0.3)
4-2 S(4)tex2html_wrap_inline599 1.242159
2-0 Q(3) 1.247324 2.2 (0.4) 0.9 (0.2)
4-2 S(3)tex2html_wrap_inline603 1.261546 5.0 (0.5) 3.2 (0.4)
3-1 S(0)tex2html_wrap_inline603 1.262065
9-6 S(1)tex2html_wrap_inline603 1.262129
2-0 Q(5)tex2html_wrap_inline603 1.263593
4-2 S(2) 1.284625 2.4 (0.4) 1.1 (0.2)
5-3 S(7) 1.289366 1.7 (0.4) 0.6 (0.2)
2-0 O(2) 1.293229 2.2 (0.4) 1.1 (0.3)
5-3 S(5)tex2html_wrap_inline611 1.310673
4-2 S(1)tex2html_wrap_inline611 1.311568 2.4 (0.5) 0.7 (0.4)
3-1 Q(1)tex2html_wrap_inline615 1.314102 4.0 (0.5) 2.5 (0.3)
9-6 Q(1)tex2html_wrap_inline615 1.315821
3-1 Q(2)tex2html_wrap_inline615 1.318072 3.3 (0.4) 1.7 (0.2)
3-1 Q(3)tex2html_wrap_inline621 1.324025 3.7 (0.4) 1.3 (0.3)
5-3 S(4)tex2html_wrap_inline621 1.326966
2-0 O(3) 1.335422 4.0 (0.4) 1.7 (0.2)
4-2 Q(7)tex2html_wrap_inline625 1.459198
4-2 O(2)tex2html_wrap_inline625 1.461130 3.0 (0.4) 1.5 (0.4)
5-3 Q(1) 1.492938 2.3 (0.3) 1.2 (0.3)
5-3 Q(2) 1.497988 1.5 (0.4) 0.7 (0.3)
6-4 S(1) 1.501553 2.3 (0.3) 1.3 (0.3)
5-3 Q(3) 1.505600 2.5 (0.4) 0.6 (0.3)
4-2 O(3) 1.509865 2.8 (0.2) 2.5 (0.4)
5-3 Q(4) 1.515792 1.0 (0.4) 0.3 (0.3)
3-1 O(5)tex2html_wrap_inline629 1.522026 1.9 (0.2) 2.2 (0.3)
7-5 S(5)tex2html_wrap_inline629 1.523598

Table: Htex2html_wrap_inline441 Line Identifications and Flux Densities from Spectra

Line

tex2html_wrap_inline549 Flux Densitytex2html_wrap_inline423
(tex2html_wrap_inline439m) (-11'', -78'')tex2html_wrap_inline425 (+33'', +105'')
tex2html_wrap_inline561 erg tex2html_wrap_inline563tex2html_wrap_inline565tex2html_wrap_inline439mtex2html_wrap_inline569

5-3 Q(5)

1.528641 1.4 (0.2)tex2html_wrap_inline427 1.0 (0.3)
6-4 S(0) 1.536884 1.0 (0.3) 1.0 (0.3)
7-5 S(4) 1.539990 <0.6 1.1 (0.4)
5-3 Q(6) 1.544261 <1.0 0.7 (0.3)
10-7 O(3) 1.548851 <0.8 0.8 (0.3)
5-3 O(2)tex2html_wrap_inline663 1.560730 2.2 (1.0) 1.6 (0.8)
7-5 S(3)tex2html_wrap_inline663 1.561502
5-3 Q(7)tex2html_wrap_inline663 1.562627
4-2 O(4)tex2html_wrap_inline663 1.563516 2.1 (1.0) 1.2 (0.8)
6-4 Q(1) 1.601535 1.7 (0.3) 1.5 (0.3)
6-4 Q(2) 1.607386 1.7 (0.4) 1.4 (0.3)
5-3 O(3) 1.613535 1.6 (0.4) 1.9 (0.5)
6-4 Q(3) 1.616211 2.0 (0.5) 1.5 (0.6)
7-5 S(1) 1.620531 0.0 (0.7) 0.7 (0.6)
4-2 O(5) 1.622292 2.6 (0.7) 0.5 (0.6)
6-4 Q(4) 1.628084 <0.8 1.4 (0.3)
5-3 O(4)tex2html_wrap_inline673 1.671822 0.1 (0.5) 1.0 (0.5)
6-4 O(2)tex2html_wrap_inline673 1.675020 1.7 (0.3) 1.8 (0.4)
6-4 Q(7)tex2html_wrap_inline677 1.682881 1.0 (0.3) 0.6 (0.3)
4-2 O(6)tex2html_wrap_inline677 1.686494 0.5 (0.8) 0.0 (0.6)
11-8 Q(3)tex2html_wrap_inline677 1.687032
1-0 S(9)tex2html_wrap_inline677 1.687721 1.3 (0.9) 0.5 (0.7)
8-6 S(3)tex2html_wrap_inline685 1.701797 1.1 (0.3) 1.1 (0.3)
13-9 O(3)tex2html_wrap_inline685 1.703761
1-0 S(8) 1.714660 1.0 (0.3) <1.0
7-5 Q(1) 1.728779 1.3 (0.2) 1.3 (0.4)
6-4 O(3)tex2html_wrap_inline691 1.732637 1.7 (0.4) 2.3 (0.4)
7-5 Q(2)tex2html_wrap_inline691 1.735734
5-3 O(5)tex2html_wrap_inline691 1.735888 2.4 (0.3) 1.5 (0.6)
7-5 Q(3)tex2html_wrap_inline697 1.746261
1-0 S(7)tex2html_wrap_inline697 1.748035 3.7 (0.3) 1.3 (0.2)
2-1 S(5) 1.944871 0.8 (0.5) 0.8 (0.3)
1-0 S(3) 1.957556 4.5 (0.5) 1.7 (0.4)
2-1 S(4) 2.004072 1.8 (0.4) 0.6 (0.3)
1-0 S(2) 2.033756 3.2 (0.3) 1.6 (0.2)
3-2 S(5)tex2html_wrap_inline701 2.065557 1.1 (0.3) 0.6 (0.2)
2-1 S(3)tex2html_wrap_inline701 2.073510 1.7 (0.3) 0.8 (0.2)
1-0 S(1)tex2html_wrap_inline705 2.121831 7.9 (0.2) 2.9 (0.2)
3-2 S(4)tex2html_wrap_inline705 2.127968 0.5 (0.4) 0.1 (0.3)

Line

tex2html_wrap_inline549 Flux Densitytex2html_wrap_inline423
(tex2html_wrap_inline439m) (-11'', -78'')tex2html_wrap_inline425 (+33'', +105'')
tex2html_wrap_inline561 erg tex2html_wrap_inline563tex2html_wrap_inline565tex2html_wrap_inline439mtex2html_wrap_inline569

2-1 S(2)

2.154225 1.3 (0.2)tex2html_wrap_inline427 0.7 (0.1)
3-2 S(3) 2.201397 1.1 (0.2) 0.8 (0.2)
1-0 S(0) 2.223299 3.6 (0.2) 1.5 (0.1)
2-1 S(1) 2.247721 2.9 (0.2) 2.0 (0.1)
3-2 S(2) 2.287026 0.5 (0.2) 0.4 (0.2)
4-3 S(3) 2.344479 0.5 (0.3) 0.3 (0.2)
2-1 S(0) 2.355629 1.2 (0.3) 0.7 (0.2)
3-2 S(1) 2.386447 1.3 (0.3) 0.7 (0.2)
1-0 Q(1)tex2html_wrap_inline733 2.406594 14.4 (0.5) 3.7 (0.4)
1-0 Q(2)tex2html_wrap_inline733 2.413436 3.1 (0.6) 1.5 (0.4)
1-0 Q(3) 2.423731 5.7 (0.4) 1.9 (0.3)
1-0 Q(4) 2.437491 2.4 (0.5) 1.2 (0.3)
1-0 Q(5) 2.454746 2.8 (0.5) 1.1 (0.4)

tex2html_wrap_inline423Flux densities have not been corrected for atmospheric transmission (see text).
tex2html_wrap_inline425Positions are R.A. and Declination offsets from HD 37903, in arcseconds.
tex2html_wrap_inline427Values in parentheses are the 1-tex2html_wrap_inline743 uncertainties of the line fluxes. Upper limits are 3-tex2html_wrap_inline743.
tex2html_wrap_inline747Indicates sets of blended lines. Fluxes for blends are the integrated flux of the blend (one flux value tabulated), or are the best-fit multiple Gaussian profiles to the blends (two or more values tabulated). Flux values of the line blends are given opposite the strongest line(s) in the blend according to Black & van Dishoeck (1987) model 14.


Next Section: OBSERVATIONS
Title/Abstract Page: Near-IR Fluorescent Molecular Hydrogen
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