C₂emission features in the Red Rectangle

Abstract

Context. The Red Rectangle proto-planetary nebula (HD 44179) is known for a number of rather narrow emission features superimposed on a broad extended red emission (ERE) covering the 5000-7500 Å regime. The origin of these emission features is unknown. Aims. The aim of the present work is to search for potential carriers by combining new observational and laboratory data. This also allows to interpret spectral emission features in terms of actual physical conditions like temperature and density constraints and to trace chemical processes in the outflows of the Red Rectangle. Methods. Observational spectra have been obtained with the EMMI-NTT at offsets of 3 , 6 , 7 , 11 , 16 and 20 distance to the central star HD 44179. The spectra are compared to the outcome of a time-gated laser induced fluorescence laboratory study of an expanding acetylene plasma using a special supersonic pin-hole discharge source. With this set-up the hydrocarbon chemistry in the Red Rectangle nebula is simulated under laboratory controlled conditions. The plasma source has the unique feature to generate electronically and vibrationally excited species at low rotational temperatures. The comparison is facilitated by a simple model for fluorescent emission in the nebula. Results. Two of the astronomically observed narrow emission bands can be assigned as originating from unresolved rovibronic progressions within the d 3 Π g → a 3 Π u Swan system of the C 2 radical. The band appearance corresponds to a rotational temperature between 200 and 1000 K. The emission is driven by absorption in the C 2 Phillips bands followed by intersystem crossing from the singlet to the triplet state and pumping in the Swan bands. Conclusions. These observations imply an active (photo)chemistry in the ejecta of the Red Rectangle.

The offset dependent behavior of narrow optical emission features in the Red Rectangle proto-planetary nebula

“…-The emission bands at ∼5585 and ∼5635 Å that recently were identified as originating from unresolved rovibronic progressions (0-1 and 1-2) within the d 3 Π → a 3 Π Swan system between 3 and 7 (Sarre 2006;Glinski et al 2009;Wehres et al 2010) do not reveal clear bandshifts for increasing offsets. -The stronger emission bands in Fig.…”

“…was done using MIDAS in the same way as described by Wehres et al (2010). The standard procedure included trimming of the spectra, flat-fielding, bias subtraction, and cosmic hit cleaning.…”

“…In addition, the carrier molecule must be able to emit light. The UV light present around the star is capable of exciting a molecule rovibronically after which fluorescence may take place, but only when other relaxation processes (e.g., internal conversion because of a short lifetime of the excited state, or intersystem crossing) or collisional de-excitation (unlikely in the Red Rectangle outflows, Wehres et al 2010) do not take over. Fluorescence is also enhanced if large geometry changes in the potential energy surface occur between the ground state and the excited state, since this reduces the overlap of the electronic states and consequently, fast internal conversion is not possible.…”

“…The cross-section for photo-destruction, however, indicated that the molecule is rather fragile and is destroyed very efficiently in the harsh radiation field of the central star. Wehres et al (2010) concluded that the C 2 lifetime in the nebula is too short given the slow outflow velocity (Van Winckel et al 1995;Hobbs et al 2004;Witt et al 2009;Wehres et al 2010) in the nebula and a replenishment (e.g., from a PAH reservoir) is needed in order to explain the appearance of the Swan-bands in several spectra. An active photochemistry is needed to account for this e.g., the break down of PAHs in a timescale of 10 3 -10 4 years, producing C 2 H 2 which is then quickly photolysed to C 2 .…”

“…The emission complex around 5800 Å with mainly stronger narrow emission features has been described by Schmidt & Witt (1991), Scarrott et al (1992), Sarre et al (1995), Van Winckel et al (2002), Glinski & Anderson (2002), and Sharp et al (2006). Recently, two of the weaker narrow emission bands have been attributed to C 2 emission features starting from rovibronically excited energy levels (Glinski et al 2009;Wehres et al 2010). The majority of the observed and rotationally unresolved transitions, however, has not been assigned.…”