S 266: a ring nebula around a Galactic B[e] supergiant?

Cesar Esteban 1 Matilde Fernandez 0 0 Max Planck Institut fu r Astronomie , Konigstuhl 17, D-69117, Heidelberg , Germany 1 Instituto de Astrof sica de Canarias , 38200 La Laguna, Tenerife , Spain A B S T R A C T A narrow-band Ha image and high-resolution spectroscopy have been obtained in order to investigate the nature of S 266 and its central star MWC 137. The analysis of the stellar and nebular spectra suggests that MWC 137 is a B[e] supergiant located $ 6 kpc away from the Sun and not a Herbig Ae/Be star, as it has been traditionally classified. Moreover, the morphology and other properties of the nebula suggest that S 266 is a ring nebula, probably produced by the interaction of stellar winds with the ambient interstellar medium or unprocessed ejected matter. This result would imply that S 266 is the first ring nebula around a B[e] supergiant known in the Galaxy. I N T R O D U C T I O N The nebula S 266 was discovered by Sharpless (1959) and first described as a possible planetary nebula (PN) with the designation PK 19501 in the catalogue of Perek & Kohoutek (1967). This object has not drawn much interest in the past because of its location towards the Galactic anticentre and its modest optical appearance. The first study devoted to S 266 was by Frogel, Persson & Kleinmann (1972) who obtained near-infrared (NIR) photometric observations and optical spectra of the object and reported the low excitation of the nebula and an excess IR emission around the central star, MWC 137. These authors suggested that S 266 is probably not a PN but a nebulosity associated with an early-type emission-line star originally classified by Merrill & Burwell (1933). Herbig & Rao (1972) included MWC 137 in their catalogue of stars of the Orion population, albeit with a cautionary note that it may not be a member of this class. Allen & Swings (1976) obtained photographic spectra of a sample of peculiar Be stars with IR excess finding a large number of emission lines in the spectrum of MWC 137, including [O i], He i, Fe ii and possibly [Fe ii] among others. These authors classify the star as B[e]. However, since the work by Finkenzeller & Mundt (1984), MWC 137 has traditionally been considered as a Herbig Ae/Be (HAEBE) star in extensive studies of large samples of these objects, (e.g. Hillenbrand et al. 1992; Hamann & Persson 1992; The, de Winter & Perez 1994; Bergner et al. 1995) although its membership in this class has never been discussed in depth. On the other hand, Fich & Blitz (1984) performed a study of the distribution of optical H ii regions in the outer Galaxy, compiling photometric distances and CO velocities. These authors found that S 266 was the most distant H ii region from the centre of the Galaxy in their sample. Fich & Silkey (1991), and more recently Vlchez & Esteban (1996), have studied the chemical abundances of the nebula in the context of the abundance gradient Direct imaging towards the Galactic anticentre, finding consistent results with the behaviour of the expected abundances at large galactocentric distances. We present a new narrow-band image as well as highresolution spectroscopy of the object in order to determine the morphology and dynamics of the nebula and to elucidate the nature of its central star. O B S E R VAT I O N S A N D D ATA R E D U C T I O N Direct images of S 266 were obtained in 1995 February with the 1.5-m telescope of the Observatorio Astronomico Nacional (OAN) at San Pedro Martir (Baja California, Mexico), using a 1024 1024 Thompson CCD. The pixel scale was 0.2 arcsec pixel1 and the seeing was 2.8 arcsec. A narrow-band image in Ha was obtained with a filter of 10 A full width at half maximum (FWHM) from three exposures of 300, 420 and 900 s of integration time. Two exposures of 420 s of the adjacent Ha continuum were also obtained using a filter of 127 A FWHM and centred at 6451 A . Unfortunately, it was not possible to flux-calibrate the images because of bad weather conditions. Echelle spectroscopy These observations were carried out in 1994 October with the 2.1-m telescope of the OAN at San Pedro Martir (Baja California, Mexico). The spectra were obtained using the OAN echelle spectrograph, which gives a reciprocal linear dispersion of 9.738 A mm1 at Ha when used with the UCL camera. A 1024 1024 Thompson CCD detector was used with a 19 mm2 pixel size. At Ha, each pixel corresponds to 0.185 A . The effective spectral resolution was 0.33 A FWHM. The cross-dispersion mode was used to obtain spectra covering from 4500 to 6850 A , including 17 spectral orders. A slit covering 26.6 1 arcsec2 was used in order to avoid overlapping between two consecutive spectral orders in the spatial direction. Two different northsouth slit positions were observed. A first 600s exposure was taken passing through the central star. Two additional 1200-s consecutive exposures were taken 10 arcsec to the east of the central star. positions are shown in Fig. 1. In the case of the high-resolution I (...truncated)