Forum Springer Astron. Astrophys.
Forum Whats New Search Orders

Astron. Astrophys. 339, 194-200 (1998)

Previous Section Next Section Title Page Table of Contents

2. Observations

For this analysis we have used CVF observations on the reflection nebula NGC 7023, the Ophiuchus molecular cloud, and the Helix nebula. These data are part of works published elsewhere. For the data description and processing, and details of the observations, we refer the reader to the original papers (Cesarsky et al. 1996b, Boulanger et al. 1998, Cox et al. 1998).

Subtraction of the zodiacal emission is an aspect of the data reduction which is of importance for the analysis presented in this paper. For this subtraction we made use of the zodiacal spectrum obtained with ISOCAM on a blank piece of sky (Reach et al. 1996). We used data from the Diffuse Infrared Brackground Experiment (DIRBE, Boggess et al. 1992) to estimate the ratio of zodiacal emission intensities of the Ophiuchus and Reach et al. observations. The main uncertainty in this procedure is set by the reproducibility of the ISOCAM detector response between two independent observations. The comparison of DIRBE data with ISOCAM observations of uniform high latitude fields shows that this uncertainty is smaller than 20%.

NGC 7023, one of the most extensively studied reflection nebulae, is a reference object for the study of the UIR bands (Sellgren et al. 1985). It is illuminated by the Herbig B3Ve star HD 200775 at a distance of 440 pc (Whitcomb et al. 1981). The ISOCAM spectrum presented in Fig. 1 is taken form the data presented by Cesarsky et al. (1996b) at a position within the bright emission ridge to the north of HD 200775. The ultraviolet radiation field at this position is estimated to be about 1000 the solar neighborhood value, [FORMULA] (Mathis et al. 1983).

[FIGURE] Fig. 1. ISOCAM CVF spectra for one pixel in the NGC 7023 reflection nebula ([FORMULA]21:01:36.1 and [FORMULA]68:10:46) and another pixel in the Rho-Ophiuchi molecular cloud ([FORMULA], [FORMULA]). The spectra are fitted by a set of 9 Lorentz profiles plus two Gaussian lines associated with the S(3) and S(5) H2 rotational lines (see Sect. 3.1). The symbols are dots for data points, dashed lines for the elementary Lorentz profiles, and solid line for the resulting fit. The linear base used in the fit is shown as a dashed line.

The Rho-Ophiuchi molecular cloud at a distance of 150 pc is a favored target for star formation and interstellar medium studies. The field of our observation ([FORMULA] centered at [FORMULA]16:25:52.2 [FORMULA]-24:19:08), 10 armin N-E of the exciting star HD147889 (B2 IV), is located in the western Photo-Dissociation Region (PDR) of the main molecular cloud (Abergel et al. 1996). Neglecting projection effects and extinction, an upper limit on the ultraviolet radiation density is [FORMULA], or [FORMULA] times the solar neighborhood value. We extracted from the data one spectrum which is displayed in Fig. 1.

The gaseous envelope of the Helix, an old planetary nebula, was found to be dominated by several gas lines from H2 and ions, with no UIR bands (Cox et al. 1998). It is thus an appropriate object to calibrate the instrumental resolution and spectral profile of the ISOCAM CVF. Fig. 2 shows a high signal to noise spectrum of the S(5) H2 line which is very well fitted by a Gaussian profile. The full-width at half-maximum (FWHM) derived from this fit and those of other lines are given in Table 1. CVF spectral images are obtained by scanning step by step the wavelength. The CVF spectra can thus be affected by memory effects of the ISOCAM detector. For the three data sets, we corrected, to a good approximation, these memory effect by using the procedure described in Abergel et al. (1996). Our confidence that the analysis presented here does not suffer from residual memory effects comes from the two following facts. (1) The Helix and Ophiuchus observations were both performed by going from long to short wavelengths and the gas lines in the Helix observation show no asymmetry that would indicate a residual memory effect (see Fig. 2). (2) The NGC 7023 observation was performed by scanning both up and down in wavelength. In the final spectra, the shape of the dust features are identical in the two scan directions.

[FIGURE] Fig. 2. One example of a Gaussian fit to the S(5) H2 line in the Helix Nebula observations (Cox et al. 1998). Similar fits were used for the spectral calibration of the CVF (see Table 1).


Table 1. Gaussian fits to gas lines in the Helix spectrum

Previous Section Next Section Title Page Table of Contents

© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998