2. Observations and data reduction
Infrared images of Sh 152 were obtained with ISOCAM in June 1997, during ISO revolution 563. These include UIRBs images at 3.3 and 6.2 µm and four continuum images taken with the ISOCAM circular variable filter (CVF) at 6.911, 8.222, 10.52 and 12.00 µm. These observations and data reduction are described in details by Zavagno & Ducci (1999).
Visible images in the 500-850 nm range were obtained in October 1997 with a 1024 1024 thinned back-illuminated Tektronix CCD camera mounted at the Newton focus of the 120 cm telescope of the Observatoire de Haute Provence. Four interference filters, 10 nm wide (FWHM), centered on 528.2, 612.0, 697.5 and 812.5 nm were used to sample the continuum emission of Sh 152, avoiding nebular and night sky emission lines (Fig. 2). For each continuum filter, twenty-four 15 min exposure frames were obtained and co-added, yielding a resulting image of six hours exposure time. Data reduction is described in detail by Darbon et al. (1999a). The nebula ERE map is obtained by subtracting the 612.0 nm image from the 697.5 nm one.
Spectra of Sh 152 were obtained in October 1996 and October 1997 at the Cassegrain focus of the 193 cm telescope of the Observatoire de Haute Provence using the Carelec long-slit spectrograph (Lemaître et al. 1990) equipped with a thinned back-illuminated Tektronix CCD. The spectral domain 4000-9600 Å was covered with a dispersion of 277 Å mm-1. The slit width was .0, which corresponds to a resolution of 21.6 Å. Two north-south slit positions were observed on the nebula, located respectively and east of the exciting star (Fig. 1).
600 s elementary exposures were obtained, keeping the nebula alternatively in the northern or the southern half of the spectrograph slit. This allowed us to observe simultaneously the nebula and the surrounding sky background. By taking an even number of such short exposures, we could subtract (after offset subtracting and flat-fielding) from the nebula spectrum the sky background spectrum observed exactly at the same pixels as the nebula, thus avoiding any instrumental effect. Total exposure times of 8400 s and 12000 s were obtained for slit positions 1 and 2 respectively. A short (600 s) exposure was obtained for a north-south slit position centered on the exciting star.
Each nebula spectrum was spatially divided into two regions free of contamination by stellar spectra (Fig. 1 and Table 1): regions 1 and 3 are located on the component A of the nebula (respectively behind and on the ionization front) whereas regions 2 and 4 are on the component B (Heydari-Malayeri & Testor 1981). One-dimensional spectra were extracted for each region, calibrated and corrected for atmospheric extinction. The same procedure was applied to the spectrum of the exciting star. The spectra were corrected for foreground interstellar extinction using the standard galactic extinction curve as previously described (Sivan & Perrin 1993) and using (Nandy et al. 1976).
Also, as explained in Sivan & Perrin (1993), we calculated for each region the continuous emission arising from the nebula atomic gas assuming a pure hydrogen nebula (i.e. neglecting He contribution) and using the electron temperature and density derived from NII and SII line intensity ratios (Table 2). The calculated atomic spectra were subtracted from the dereddened nebula spectra (Fig. 2).
It should be noted that this subtraction allows us to validate a posteriori the value of the reddening we have used. A variation as small as 10% in modifies the temperature in such a manner that the amplitude of the theoretical Paschen jump no longer matches the observations.
To characterize the scattering and, eventually, the luminescence phenomena, we have divided the dereddened spectrum of each region, corrected for atomic continuum emission, by the dereddened spectrum of the exciting star. The results are shown in Fig. 3. This assumes the same nebular structure as Heydari-Malayeri & Testor (1981): the star S 152.1 is embedded in a quite homogeneous cocoon of dust and gas (Cox, Deharveng & Caplan 1987) so that its spectral energy distribution is the same in the direction of the observer as well as in the direction of regions 1 to 4 in the nebula. Finally, in order to obtain the spectral characteristics of the ERE, the contribution of the scattering was subtracted from these spectra. This contribution was calculated from models of cosmic dust using size distribution laws of grains made with materials of astrophysical interest (see Sect. 4).
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001