To be published in the proceedings of the colloquium "The Universe as seen by ISO"
held in Paris, October 20-23, 1998
We present visible and near IR images of the compact HII region Sh 152.
Some of these images reveal the presence of Extended Red Emission (ERE) around
698 nm and emission from Unidentified Infra Red Bands (UIRBs) at 3.3 and
6.2Ám. Other images show the near infrared (7-12Ám) continuous
emission of the nebula. The ERE emission is found to coincide with the ionized
region and significantly differ from the UIRBs location. Also some evidence
is found in favor of grains as carriers for ERE.
Key words: HII regions, ERE, UIRBs
Extended red emission (ERE) is a continuous emission band observed in the
red part (600-800 nm) of the spectrum of various astrophysical objects
like reflection nebulae ([Schmidt, Cohen & Margon1980], [Witt & Boroson1990]), planetary nebulae
([Furton & Witt1992]), HII regions ([Sivan & Perrin1993], [Darbon, Perrin & Sivan1998]), high-latitude
galactic cirrus clouds ([Szomoru & Guhathakurta1998]), the halo of the galaxy M82 ([Perrin, Darbon & Sivan1995])
and also in the diffuse galactic interstellar medium ([Gordon, Witt & Friedmann1998]). This
emission can be attributed either to Hydrogenated Amorphous Carbon (HAC)
grains ([Watanabe, Hasegawa & Kurata1982], [Furton & Witt1993]) or silicon grains ([Witt, Gordon & Furton1998],
[Ledoux et al.1998]).
A series of emission bands in the 3-16Ám range, the so-called
UIRBs, is also observed in dusty environments and commonly attributed
to Polycyclic Aromatic Molecules (PAH)([Puget & Léger1989], [Allamandola, Tielens & Barker1989]) and/or
carbonaceous materials ([Papoular et al.1989]).
In particular, the existence (or absence) of a spatial correlation
between IURBs and ERE might be useful to put constrains on the nature of the
carriers. Compact HII regions are bright and dusty objects well
suited for this kind of study.
This is the reason why we have carried out an observational program for imaging compact HII regions at visible and infrared wavelengths in order to detect and to map respectively ERE and UIRBs. This paper reports on the results obtained for Sh 152.
Infrared images of Sh 152 were obtained with ISOCAM in june 1997, during
ISO revolution 563. These include UIRBs images at 3.3
and 6.2Ám and
four continuum images taken with the ISOCAM circular variable filter (CVF) at
6.911, 8.222, 10.52 and 12.00Ám.
These observations and data reduction
are described in [Zavagno & Ducci1998]. In particular, the 3.3 and
6.2Ám images presented in this paper were corrected for the adjacent
Visible images in the 500-850 nm range were obtained, in october 1997, with a 1024x1024 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 with a FWHM 10 nm centered on 528.2, 612.0, 697.5 and 812.5 nm were used. These filters were chosen to isolate the continuum emission of Sh 152 and to avoid nebular and night sky emission lines. For each continuum filter, twenty-five 15 min exposure time frames were obtained and co-added, yielding a resulting image of six hours exposure time. Standard data reduction was performed using ESO-MIDAS software. It includes : dark current subtraction, flat fielding, airmass and interstellar extinction corrections, deconvolution by point spread function. According to spectroscopic observations of ERE in HII regions ([Sivan & Perrin1993]), the emission excess in the 697.5 and/or 812.5 nm filters should be attributed to ERE. Actually the best contrasted results were obtained by making the difference between the 697.5 and 612.0 nm images. The resulting image was considered as giving the spatial distribution of ERE over Sh 152.
Figure presents the spatial distribution of ERE superimposed on the 6.2Ám band image and the H image. ERE is found to coincide with the H emission but significantly differs from that of the 6.2Ám emission band. Hydrogen environment and UV radiation are well suited to induce luminescence from HAC grains ([Furton & Witt1993]).
Figure presents the spatial distribution of ERE
superimposed on a 12Ám continuum image. It can be seen that the
12Ám emission extends over the area where the ERE intensity reaches its
maximum. This coincidence is in favor of grains as carriers of the ERE
because (i) the 12Ám emission is thought to be the short wavelength part
of a strong thermal emission from cold grains (see, for example, IR spectra of
galactic HII regions presented by [Roelfsema et al.1998])and (ii) because such
cold grains can exist in Sh 152 at the distance from the
exciting star where the observed coincidence occurs.
In effect, according to [Lamy & Perrin1997], the temperature of a dust solid particle located at a distance 104 R* = 2.102 AU = 10-3 pc from an O9.5 V star of radius R AU, would be of 200K for a silicate grain or 400K for a carbonaceous grain. The region in Sh 152 where ERE maximum and 12Ám emission coincide is in fact much farther from the star (about 0.2 pc, assuming a distance of 3.5 kpc for Sh 152 ([Heydari-Malayeri & Testor1981])) than in the calculations so that, although the exciting star of Sh 152 is slightly hotter than an O9.5V star ([Hunter & Massey1990]), we can assume that cold grains do exist in the area.
Figure presents the four continuum images of Sh 152 at 6.911, 8.222, 10.52 and 12.00Ám taken with ISOCAM. In these images, the flux is normalized to the maximum observed in the LW6 filter, centered at 7.7Ám. At the location of the ERE maximum, the infrared images show flux values increasing with wavelength : this is in agreement with thermal emission from cold grains (note that the 10.52Ám flux might be contaminated by the [SIV] 10.54Ám emission line).
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