Infrared reflection nebulae

Fig. 2. The infrared emission spectrum from the reflection nebula HD 44179, the Red Rectangle (a from Ref. [44], b from Ref. [15].)...

A large tenuous cloud surrounds the object and is seen as a reflection nebula illuminated by the starlight that escapes above and below the ring of dust. The discovery of this object has provided dramatic evidence supporting earlier hypotheses that circum-stellar envelopes of infrared stars must be flattened. The large optical depth of the toroid produces a featureless, mid-infrared spectrum (Forrest eit 1976) but the chemical nature of the cloud has been deduced from optical spectroscopy of the reflection nebula (Crampton, Cowley and Humphreys, 1975) and by detection of a molecular cloud association with the source (Lo and Bechis, 1976 and Zuckermann ad 1976). These observations show that the 0.1 M cloud is carbon-rich, and, in fact has led to the suggestion that the source may be the progenitor of a planetary nebula. 

Another object associated with multiple reflection nebulae is CRL 437. The reflection nebula was found by Kleinmann and Lebofsky (1975) in their search for optical counterparts for new AFCRL sources by means of near-infrared photography with image tube cameras at KPNO. Direct observation of the brightest part of the complex nebula on the KPNO 1.3-m indicated at first, that the source was ten times fainter than reported in the AFCRL Catalogue. This cast doubt on the identification of the reflection nebula with... 

Figure 8 The 10-15 )im spectrum of the outflow from a star known as the Red Rectangle because of the rectangular appearance of its associated reflection nebula on red plates. Bands in this spectral region are largely due to the C-H out-of-plane bending mode whose exact peak position is sensitive to the number of adjacent H atoms involved. The boxes underneath the spectra give the ranges where PAHs with the indicated number of adjacent H atoms emit. The arrow labelled 6 indicates the position for benzene. These spectra were obtained with the Short-Wave-length Spectrometer on board the Infrared Space Observatory at a spectral resolution ranging from 250 to 2000.

Dust radiates in the infrared. By Kirchhoff s law, solid material in thermal equilibrium radiates like a blackbody. Most of the incident energy falling on the grain is scattered, hence the blueness of reflection nebulae and the reddening of starlight. The absorbed photons are reradiated at a rate approximated by = where k, ... 

The water maser emission associated with the infrared centers IRS 1 and IRS 3 of the NGC 207 HR star-forming region was studied by Seth, Greenhill, and Holder, 2002 [305]. They used water masers as tracers for protostellar disks. NGC 2027 is a reflection nebula in the constellation of Orion. 

Many carbon rich stars also present an important emission at 11.3 pm associated with solid carbon and some of them present nebulosity of reflection as a consequence of the scattering of the circumstellar grains. There are indications that in the material ejected by these stars, carbon must exist, apart from CO molecules and solid grains, in some other form or species until now unknown, fullerenes are a possibility. Unfortunately, there is very little information about the presence of molecules of intermediate size (between 10 and 106 atoms) in circumstellar regions. There are bands in carbon rich planetary nebulae, for example those of 3.3,6.2,7.7, 8.6 and 11.3 pm which have not been detected in carbon stars but are observable in transition objects evolving between the giant red phase and the planetary nebula as for example, the Egg Nebula (Fig. 1.5) and the Red Rectangle. These infrared bands are normally associated with the vibration modes of materials based on carbon, possibly PAHs. But until now it has not been possible to make a conclusive identification of the carrier. 

Spectroscopic observations of Jupiter and Saturn made with the Infrared Space Observatory are consistent with other estimates of protosolar D/H (see Robert et al. 2000 and references therein), as is the D/H value determined by the Galileo atmospheric entry probe mass spectrometer (26 7 ppm Mahaffy et al. 1998). This result is expected as the jovian planets are thought to have formed by quantitative capture of gas from the solar nebula. On the other hand, the outer solar system planets Uranus and Neptune appear to be significantly enriched in D/H by factors of -3 compared to the protosolar value (Feuchtgruber et al. 1999). This enrichment is interpreted to reflect the mixing of material from the more D-rich icy cores of these planets, which constitutes a significant fraction of their mass (as opposed to the jovian case where the planetary mass is dominated by the gaseous envelope). 

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