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Observational constraints on the ERE interpretation [*]

S. Darbon, J.-M. Perrin, J.-P. Sivan

Observatoire de Haute Provence du CNRS,
F-04870 Saint Michel l'Observatoire, France

Astronomy & Astrophysics (Research Note)


Empirical relationships on the properties of the Extended Red Emission (ERE) are presented. They are based on published observational data and on new results obtained on reflection nebulae illuminated by cold stars. The plot of the width versus the central wavelength of the ERE band is in agreement with laboratory properties of the materials commonly proposed as the ERE carriers. But this is not the case for the plot of the ERE band width versus the effective temperature of the nebula illuminating star.

Key Words: ISM: general - ISM: reflection nebulae - ISM: dust,extinction - ISM: HII regions - ISM: planetary nebulae - Galaxies: individual: M82

The Extended Red Emission (ERE) is a continuous emission band observed in the red part (6000-8000 Å) of the spectrum of various astrophysical objects : reflection nebulae (Schmidt, Cohen & Margon 1980; Witt & Boroson 1990), planetary nebulae (Furton & Witt 1992), galactic and extragalactic Hii regions (Perrin & Sivan 1992; Sivan & Perrin 1993; Darbon, Perrin & Sivan 1998), high-latitude galactic cirrus clouds (Szomoru & Guhathakurta 1998), the halo of the galaxy M82 (Perrin, Darbon & Sivan 1995) and the diffuse galactic medium (Gordon, Witt & Friedmann 1998). The ERE band is found to vary significantly both in position and width from an object to an other. The diversity of objects where the ERE is detected and the diversity of the observed band characteristics lead us to search for empirical relationships that should help identification of the ERE carriers.
For all the available observations, we have plotted in Fig. [*] the width as a function of the central wavelength of the ERE bands. For reflection nebulae, we have used the values of Witt & Boroson (1990). For all the other objects, we have made measurements according to the same definitions as Witt & Boroson : the central wavelength splits the band luminosity into two equal parts and the width is measured as the difference between the wavelengths of the first and third quartiles. Note that for planetary nebulae the values derived from the published spectra of Furton & Witt (1992) are underestimated.

Figure 1: This diagram plots the position of the maximum (Å) versus the width (Å) of the ERE band for a number of objects of various types : Hii regions (triangles)(Perrin & Sivan 1992 ; Sivan & Perrin 1993 ; Darbon, Perrin & Sivan 1998, Darbon et al., 1999), planetary nebulae (squares)(Furton & Witt 1992), reflection nebulae (circles)(Witt & Boroson 1990) and the halo of M82 (diamonds)(Perrin, Darbon & Sivan 1995).

The diagram in Fig. [*] reveals a clear correlation between the position of the maximum and the width of the band. We derived a correlation coefficient r=0.68. This result confirms and reinforces the tendency previously noted by Witt & Boroson (1990) from reflection nebulae only : the correlation coefficient was r=0.52. The same effect is observed in laboratory experiments : Hydrogenated Amorphous Carbon (HAC) grains (Furton & Witt 1993) and nanocrystals of silicon (Witt, Gordon & Furton 1998 ; Ledoux et al. 1998) exhibit luminescence bands whose width increases with the maximum wavelength.
Also, it is found that the plotted values in Fig. [*] are split into two groups : reflection nebulae with smaller widths and bluer peaks and Hii regions and planetary nebulae with larger widths and redder peaks (the halo of M82 lies at the border of these two groups). This repartition might be related to the presence or absence of plasma within the nebula, in agreement with laboratory results (Wagner & Lautenschlager 1986; Robertson & O'Reilly 1987)

Figure 2: Width of the ERE band as a function of the logarithm of the effective temperature of the exciting and/or illuminating star as measured on Hii regions (triangles) and reflection nebulae (circles). The exciting star of Sh 152 is embedded in a dusty cocoon so that the effective temperature of the spectral energy distribution that actually illuminates the regions observed by Darbon et al. (1999) is over-estimated.

As a mean, in Fig. [*], the reflection nebulae are illuminated by stars less energetic than the other nebulae. This repartition suggests that the characteristics of the ERE might depend on the spectral distribution of the exciting flux. This has lead us to study the variation of the ERE band width as a function of the effective temperature of the exciting stars. BVI photometric and spectrophotometric observations, conducted respectively by Witt & Schild (1985) and Witt & Boroson (1990), deal with reflection nebulae illuminated by stars with a spectral type earlier than A0 (i.e. Teff >= 10000 K) : most of these nebulae exhibit ERE. But laboratory results show that the ERE can be excited by low energy visible radiation. So, the observation of reflection nebulae illuminated by stars colder than A0 (i.e. with a spectral energy distribution peaking in the visible) should be useful in order to detect the presence or the absence of the ERE in their spectra. One such nebula, illuminated by an M giant star, has been observed by Witt & Rogers (1991). We have observed six additional nebulae illuminated by cold stars. They are listed in Table [*], together with spectral type and the effective temperature of the illuminating stars. We obtained low resolution spectra for these objects. Data reduction were conducted as previously described in Perrin, Darbon & Sivan (1995). None of these nebulae exhibits ERE in its spectrum.

Table:   Reflection nebulae illuminated by cold stars
Nebula Spectral Type (MK) Teff (K) Observation site
VDB003 K0III 4750 OHP (a)
VDB035 G5 5150 OHP (a)
VDB037 (g)M5III 3330 OHP (a)
VDB120 F7II 6310 OHP (a)
VDB133(d) F5Iab 6900 OHP (a)
VHE14B K0III 4750 ESO (b)
IC2220 (g)M1II 3635 UTSO (c)

In Fig. [*], we have plotted the ERE band width as a function of the effective temperature of the exciting star for the reflection nebulae and Hii regions of Fig. [*] and for the nebulae of Table [*]. For the nebulae without ERE, the width value is set to zero. Clearly, no ERE appears for Teff =< 7000 K. On the contrary, for Teff >= 10000 K, most of the plotted objects exhibit the ERE albeit few of them do not. This suggests a cut-off in effective temperature might exist between 7000 and 10000 K (unfortunately no observation is available for this range). This cut-off cannot be accounted for by extinction effects as demonstrated by Fig. [*] : the presence or the absence of the ERE in a nebula does not appear to be correlated to the color excess of its illuminating star.
The fact that no ERE is present for stars whose effective temperature is smaller than 7000 K does not agree with current laboratory data on various materials such as HACs and silicon nanocrystals. These materials exhibit red luminescence features when they are irradiated by photons of low energy, namely of energy smaller than 3 eV (see e.g. Sussmann & Ogden 1980, Lin & Feldman 1981, Fang et al. 1988, Wilson et al. 1993).
Further observations of reflection nebulae illuminated by cold stars would be useful to confirm our finding. We cannot completely rule out that the absence of ERE in these nebulae could be simply due to the absence of any luminescent material.
This is probably the case for the nebulae without ERE belonging to the left-hand part of the diagram in Fig. [*] (Teff >= 10000 K). This suggests that the ERE carriers would not be present everywhere in the interstellar medium.

Figure 3: Color excess as a function of the logarithm of the effective temperature of the illuminating star for : Hii regions with ERE (open triangles), reflection nebulae with ERE (open circles), Hii regions without ERE (filled triangles) and reflection nebulae without ERE (filled circles).

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