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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The spectra observed at positions (-11'', -78'') and (+33'', +105'') are shown in Figures 1 and 2, respectively. Selected sequences of vibrational-rotational lines are indicated. Table 1 lists all lines that can be identified together with their flux densities. These were determined by Gaussian fitting the spectra with the FWHM set to the width of the instrumental profile and adjusting the amplitude and wavelength as free parameters, using a least-squares fitting algorithm. In many cases lines were blended, in which case multiple Gaussians were fitted, keeping both line width and separations fixed.

However, the accuracy of this procedure can be severely affected by unequal attenuation of the line source at the specific line frequency compared to the atmospheric attenuation of the continuum source, averaged over a resolution element (see Howe, 1992). The atmospheric transmission spectrum from 1-2.5tex2html_wrap_inline439m contains a multitude of unresolved absorption lines, many of which are nearly opaque. Thus, even though the continuum transmission is high, when observed at moderate resolution an individual Htex2html_wrap_inline441 spectral line coincident in wavelength with a narrow atmospheric absorption line could be attenuated severely, resulting in an underestimate of the Htex2html_wrap_inline441 line flux after calibration by the standard. Likewise, the strength of an unresolved line may be overestimated if the continuum transmission is low. To reduce such occurrences we modeled the atmospheric transmission across the pass bands as observed at Mauna Kea using a model developed by Grossman (1989). The transmission at specific Doppler-shifted wavelengths of Htex2html_wrap_inline441 line emission from NGC 2023 (which was redshifted by 19km tex2html_wrap_inline453 and 42km tex2html_wrap_inline453 on Nov 28-Dec 1 and Jan 19, respectively, using a velocity of +10km tex2html_wrap_inline453 with respect to the local standard of rest for the source) was determined using the model and compared to the mean transmission over the spectral resolution of the observation. The reliability of the flux calibration of a particular Htex2html_wrap_inline441 line could then be estimated from its proximity to a telluric feature. In practice, uncertainties in the widths of the telluric features made the correction of the line flux of an affected Htex2html_wrap_inline441 line very uncertain, so line fluxes were only determined if the attenuation was less than 10%.

Over 100 lines were observed and identified. However, of these nearly half were too severely blended to allow a reliable determination of individual amplitudes. Of the remainder, thirteen were at wavelengths where atmospheric absorption features made the calibration unreliable. Thus roughly one third of the lines identified could have their specific intensities accurately determined. These are listed in Table 2, converted to erg tex2html_wrap_inline563tex2html_wrap_inline565 srtex2html_wrap_inline569. Also listed is their level column density, tex2html_wrap_inline907, calculated by
where I is the specific intensity of a transition from level (v,J), with radiative decay rate A (taken from Turner, Kirby-Docken & Dalgarno, 1977) emitting a photon of wavelength tex2html_wrap_inline549. Note that no attempt has been made to correct for extinction, though this is likely to be Atex2html_wrap_inline915 magnitudes (McCartney, 1998).


We note here, for completeness, that there is a calibration error in the 8000Å Htex2html_wrap_inline441 data presented by Burton et al. (1992) for the (-11'', -78'') position in NGC 2023. The intensities quoted in that paper should be divided by 23 to yield surface brightnesses in erg tex2html_wrap_inline563tex2html_wrap_inline565 arcsectex2html_wrap_inline925.

Next Section: DISCUSSION
Title/Abstract Page: Near-IR Fluorescent Molecular Hydrogen
Previous Section: OBSERVATIONS
Contents Page: Volume 15, Number 2

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