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Spatial distribution of unidentified infrared bands and Extended Red Emission in the compact galactic H II region Sh 152 [*] [*]

S. Darbon1, A. Zavagno2, C. Savine1, V. Ducci2, J.-M. Perrin1, J.-P. Sivan1

1Observatoire de Haute Provence du CNRS, 04870 Saint-Michel l'Observatoire, France.
2Observatoire de Marseille, 2 Place Le Verrier, 13248 Marseille Cedex 4, France.


To be published in the proceedings of the colloquium "The Universe as seen by ISO"
held in Paris, October 20-23, 1998

Abstract:

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 6.2m. Other images show the near infrared (7-12m) 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


INTRODUCTION

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 3-16m 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.

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 6.2m and four continuum images taken with the ISOCAM circular variable filter (CVF) at 6.911, 8.222, 10.52 and 12.00m. These observations and data reduction are described in [Zavagno & Ducci1998]. In particular, the 3.3 and 6.2m images presented in this paper were corrected for the adjacent continuum emission.
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 $\simeq$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: 6.2m (colors), 3.3m emission bands (dashed contours) and H$\alpha$ emission (solid contours) in Sh 152
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Figure: ERE distribution (colors), 6.2m emission band (dashed contours) and H$\alpha$ emission (solid contours) in Sh 152
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Figure: ERE distribution (colors) in Sh 152 and 12m continuum image (solid contours)
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Figure: Continuum emission in Sh 152 observed with the ISOCAM CVF at 6.911, 8.222, 10.52 and 12.00m
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RESULTS AND DISCUSSION

The 3.3m, 6.2m and H$\alpha$ images of Sh 152 are displayed in Figure  [*]. It can be seen that the 3.3 and 6.2m emission bands have the same spatial distribution over the entire nebula. This suggests their carriers could be the same. Also, it appears from Figure [*] that the infrared emissions arise from regions located outside the ionized regions (traced by H$\alpha$ emission).

Figure [*] presents the spatial distribution of ERE superimposed on the 6.2m band image and the H$\alpha$ image. ERE is found to coincide with the H$\alpha$ emission but significantly differs from that of the 6.2m 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 12m continuum image. It can be seen that the 12m 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 12m emission is thought to be the short wavelength part of a strong thermal emission from cold grains (see, for example, IR spectra of ultra compact 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 12m 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 12.00m taken with ISOCAM. In these images, the flux is normalized to the maximum observed in the LW6 filter, centered at 7.7m. 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 10.52m flux might be contaminated by the [SIV] 10.54m emission line).

CONCLUSION

Visible and infrared images of Sh 152 allowed us to compare the spatial distribution of ERE and UIRBs and to deduce basic physical properties. We found that :
Nevertheless, the exact nature of the ERE carriers will only be constrained using spectroscopic data and comparing the ERE band shape with laboratory experimental data (Darbon et al., in preparation)

References

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About this document ...

SPATIAL DISTRIBUTION OF UNIDENTIFIED INFRARED BANDS AND EXTENDED RED EMISSION IN THE COMPACT GALACTIC HII REGION SH 152

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Footnotes

[*]
ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

[*]
Partly based on observations made at Observatoire de Haute Provence du CNRS



12/16/1998