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Astron. Astrophys. 336, 648-653 (1998)

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1. Introduction

The Red Rectangle is a remarkable bipolar nebula associated with the 9th magnitude visible star HD 44179. The nebula has a rectangular aspect with a marked X-shaped morphology best seen in the red. It has attracted attention ever since it was first studied extensively by Cohen et al. (1975). HD 44179, the central star of the nebula, has been classified as a B9 to A0 III by Cohen et al. (1975), while Kelly & Latter (1995) identified its spectrum as that of an A1 III star.

The nature of the nebula and its central star HD 44179 are not well understood. Zuckerman et al. (1976) argue that the Red Rectangle is a proto-planetary nebula because of its unusual morphology, its carbon-rich nature, and the lack of interstellar material in the region. This interpretation is supported by polarization maps (Perkins et al. 1981) and by radiative transfer simulations (Yusef-Zadeh et al. 1984; Lopez et al. 1995, 1997). The possibility that HD 44179 is in a pre-main-sequence phase was first proposed by Cohen et al. (1975). This interpretation is consistent with the strong PAH emissions, also observed in pre-main-sequence stars (Brooke et al. 1993). On the other hand Waelkens et al. (1992) propose that the Red Rectangle is a post-main-sequence star because helium and/or carbon are over-abundant in HD 44179 relative to the solar values. However, the gas and dust in the circumstellar environment of the Red Rectangle are different from what they are around most carbon-rich red giants which are losing mass.

The Red Rectangle nebula is a powerful infrared source. Its infrared spectrum shows a smooth continuum much broader than that of a blackbody. It is featureless between 1.5 and 2.2 µm (Thronson 1982) and contains the entire family of the so-called unidentified infrared (UIR) emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm (Russell et al. 1978). The width of the strong 3.3 µm feature is narrow near the central star ([FORMULA]2.7 arcsec) but then further away it becomes twice as wide (Geballe et al. 1989). A large variety of amorphous carbon and hydrocarbon solids have been proposed to explain the origin of UIR carriers but no identification definitely matches the observed spectra. This emission band is believed to be the result of absorption of ultraviolet photons followed by vibrational non-equilibrium excitation in the infrared.

High angular resolution speckle techniques have been many times applied to the Red Rectangle. These observations, performed in the infrared using either one-dimensional scans (Low 1979; Dyck et al. 1984; Dainty et al. 1985; Leinert & Haas 1989) or two-dimensional data (Christou et al. 1990; Tessier et al. 1990; Cruzalèbes et al. 1996), show the Red Rectangle to be a clearly extended source in the north-south direction. Dyck et al. (1984) observed in the near-infrared (1.65-4.8 µm) and resolved the central source into two components: an extended (0.9 arcsec) component and a compact one. Dainty et al. (1985) found that the extended emission is elongated (1.05 arcsec) in the north-south direction (vs 0.4 arcsec in the east-west direction) in K, L and M bands. Leinert and Haas (1989) performed a tomographic image reconstruction at K. They resolved the central compact source into two components: a 0.2 arcsec source and an unresolved component offset by 0.15 arcsec to the south, which they identified with the star HD 44179. The K image obtained by Christou et al. (1990) clearly shows two sources with a 0.15 arcsec separation at a position angle of [FORMULA]. Using an adaptive optics technique Roddier et al. (1995) show that the Red Rectangle has an X-shaped morphology within 0.1 arcsec of the central region of the system at 1.65 µm. Recently Osterbart et al. (1997) performed high resolution images at 0.656 µm and at 0.80 µm of the nebula. They have resolved the central part of the nebula into two main lobes, which appear separated by about 0.15 arcsec. Cruzalèbes et al. (1996) found some east-west components in K and M. In addition, Bregman et al. (1993) obtained images of the Red Rectangle in the 3.3 and 11.3 µm UIR emission bands. These images show two different spatial distributions: the 3.3 µm image is slightly extended in the north-south direction and centrally peaked, while the 11.3 µm image shows a north-south bipolar shape with no central peak. Geballe et al. (1989) detected 3.3 µm UIR emission in a 5 arcsec beam centered north of HD 44179. Sloan et al. (1993) showed that the object is spatially extended at the wavelengths of all four UIR features detected between 7 and 14 µm and found that no UIR emission arises in the innermost regions of the source. Images of the Red Rectangle were obtained at UIR emission features and at several continuum wavelengths in the 10 µm and 20 µm atmospheric windows by Hora et al. (1996). The nebula was seen to follow the optical morphology. The "spikes", or brightness enhancements, that give the nebula its rectangular appearance have also been observed at 10 µm. The mid infrared emission appears to be trace material in the bipolar outflow regions of the nebula. Knapp et al. (1995) found an unresolved ([FORMULA]3 arcsec) CO emission associated with the central star which they attribute to ionized gas corresponding to a star earlier than B3. Recently Jura et al. (1997) performed radio VLA observations at 3.6 cm, 2 cm and 1.3 cm of the Red Rectangle. Emission was detected in all three radio wavelengths, and the source was resolved at 2 cm and 1.3 cm. They suggest that the radio emission is produced by large particles confined to a long-lived gravitationally bound disk.

There have been many discussions about the nature of the binarity of the central star of the nebula which has long been considered as a visual binary. Meaburn et al. (1983) estimated the separation to be 0.29 arcsec while Heintz (1990) measured a separation of only 0.14 arcsec. Radial velocity (Van Winckel et al. 1995) and photometric measurements (Waelkens et al. 1996) show that HD 44179 is a spectroscopic binary while adaptive optics measurements (Roddier et al. 1995) provide indirect evidence that the source is a binary star. It seems more probable that the claimed companion seen in visual observations is reflected light of the nebula, as already proposed by Cohen et al. (1975) and by Roddier et al. (1995).

In order to probe the geometry of the circumstellar material in the close environment of the Red Rectangle we have obtained high resolution infrared images at K (2.22 µm), L' (3.87 µm), M (4.75 µm) bands and in the 3.3 µm UIR emission band. The primary goals of this study are to investigate the nature of the circumstellar dust grains and to develop a better understanding of the formation and structure of this source. In Sect. 2, we describe the observations and the data processing. In Sect. 3, we present the results. In Sect. 4 we discuss the intensity and the morphology we have derived from observations and finally, we summarize our conclusions in Sect. 5.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998