The star HD 45677 ( = FS CMa, = , ), is a well studied peculiar Be star with a strong infrared excess. Several attempts were made to classify peculiar Be stars in homogeneous groups. Wackerling (1970) indicated a variety of hot stars with abnormal spectra as BQ, and additionally those with forbidden lines were classified as BQ[ ] (Ciatti et al. 1974). Stars with characteristics such as HD 45677, were named B[e] stars (Allen & Swings 1976) referring to the presence of forbidden lines, e.g. [OI] and [FeII], besides permitted emission lines of which the Balmer lines can be very strong with sometimes indications of stellar winds. Because of their absence in star-forming regions it is often believed that these objects are evolved objects, e.g. being peculiar Be-stars, being in certain stages of the early evolution of planetary nebulae (PN) or being symbiotic (Zickgraf & Schulte-Ladbeck 1989; Halbedel 1989). Evidence for the evolved state of such objects are the B[e] supergiants as located in the Magellanic Clouds (Zickgraf & Schulte-Ladbeck 1989). Most known galactic B[e] stars have unknown luminosity class but the ones with reasonably well-determined luminosity classes seem to be quite heterogeneous in it. Their exact evolutionary state is therefore doubtful. Especially interesting is the case of HD 45677, in the past literature often called the ``prototype'' of the B[e] stellar group but also suggested to be a young stellar object.
The spectral classification of HD 45677 is difficult because of the presence of strong emission lines in its spectrum. It is generally agreed that its spectral type is B2 (e.g. Allen 1973; Feinstein et al. 1976). Its luminosity class however is still uncertain, most probably it is III, IV or V. In the recent catalogue of members and candidate members of the Herbig Ae/Be stellar group (Thé et al. 1994), HD 45677 is classified as an extreme emission line object (EELO). This term only indicates the spectral properties.
Recently, Sitko et al. (1994) argued that the difference in two photometric datasets of HD 45677, obtained in 1980 and in 1992, could be explained by changes in the level of stellar obscuration by circumstellar dust. This dust then acts as a grey absorber, being effective from the UV to the near infrared. Support for this explanation is that they observed a small decrease in total infrared flux, which is then caused by a decrease of the total mass of the star's circumstellar dust envelope. Interesting is that they obtained datasets consisting of UV to IR data at stages when HD 45677 was at minimum brightness (1980) and in a much brighter one (1992). However, their observational interval to obtain these multiwavelength datasets was about 1 month. It is known that HD 45677 varies significantly in brightness on such a time scale. It can therefore be questioned whether the deviations of their differential data from a grey absorber model are real or are due to photometric variations between their data at different wavelengths. Furthermore, modelling of the UV extinction curve of HD 45677 with a multi-scattering non-spherical circumstellar dust shell by Voshchinnikov et al. (1995) showed that it is possible to obtain a flat extinction curve using smaller dust particles, but with a non-standard chemical composition, as well.
Schulte-Ladbeck et al. (1993) demonstrated that the position angle of the intrinsic polarization in the UV is flipped 90 compared to that in the visual. This observation can be perfectly described by two polarization components, one from a bipolar flow with small optical depth, and one from the equatorial disk of large optical depth. At short wavelengths the direct starlight is blocked by the equatorial disk and the polarization of the scattered light from the polar lobs will dominate. Polarization from the equatorial disk will dominate at long wavelengths. The polarization flip occurs where both contributions are equal. Furthermore, the observed UV intrinsic polarization displays a stronger increase than in the red. This observation is explained by two distinct grain-size distributions in both regions or by the variation of the optical depth in the disk with wavelength which effects polarization values for both regions as well. Further support for the first possibility comes from the IRAS LRS spectrum of HD 45677, in which the 9.8 m silicate feature, seen superimposed on a strong dust continuum, is broadened between 9 and 11 m (Fajardo-Acosta & Knacke 1995). The models by Simpson (1991) demonstrate that such broadening of the silicate emission can indeed be due to the presence of different sizes of dust grains.
The presence of high-velocity (+200 to +400 km s), accreting gas in all IUE spectra between 1979 and 1992 led Grady et al. (1993) to extend this picture by suggesting that HD 45677 may be a massive Herbig Be star with an actively accreting gaseous circumstellar disk, viewed nearly edge-on. The polarization data would then be explained by some of the UV light being scattered in a bipolar nebula oriented orthogonally to the dust disk. However, recent observations by Oudmaijer & Drew (1995) showed that the intrinsic polarization in H is also flipped, but only by about 30 compared to that in the continuum. This would indicate the presence of a significant amount of gas located intermediate between the bipolar flow and the disk. Grady et al. (1994) noted that the prominence of the pronounced near-UV emission, mainly due to FeII and MgII, has decreased with increasing system light.
Although there is no obvious nebulosity visible around HD 45677 (Swings 1973) and the star is not located in or near a star forming region, it has been suggested that HD 45677 is a young stellar object (age 10 year) surrounded by a dust shell left over from star formation (Sorrell 1989). This view is supported by the polarization measurements of Coyne & Vrba (1976), who concluded that HD 45677 is surrounded by a ring of patchy clouds of dust with an inner radius of 45 AU, which also explains the strong infrared-excess. Another suggestion for the infrared excess of HD 45677 is a cool companion. This hypothetical companion has been suggested to be a cool giant (Ciatti et al. 1974), or an infrared object similar to the Becklin-Neugebauer object (Low et al. 1970). Searches for CO (1-0) (Nyman et al. 1992), 18 cm OH maser (Le Squeren et al. 1992), and SiO maser (Dickinson et al. 1978) emission from HD 45677 have been made, but all detections were negative. Since the recent CO and OH surveys were quite deep, and HD 45677 is much brighter than most objects that were detected, it is likely that the shell of HD 45677 is either too thin or too small to produce such emission or that the mass loss has been started recently. Therefore, it seems unlikely that HD 45677 is an evolved post-AGB OH/IR-object.
In this paper we will discuss the photometric and spectroscopic behaviour of HD 45677 over the last 25 years, based on both newly obtained data and data from the literature. Photometric variability from the UV to near infrared wavelength ranges on long ( 40 years), intermediate (a few months), and short (hours) time scales will be discussed and explained. Furthermore, we will discuss the star's spectral characteristics, and discuss and explain observed short-term spectroscopic variations in the NaID and H lines. However, a more detailed description of the implications of these data will be given in a forthcoming paper (Israelian et al. 1996). Finally, some remarks will be added to the discussion on the evolutionary phase of HD 45677.