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7. Evolutionary status

The group of B[e] stars, to which HD 45677 belongs, is not very homogeneous. It contains young stellar objects, such as Herbig Be stars, but also evolved objects, such as post-AGB stars, Luminous Blue Variables (LBVs) and planetary nebulae (Thé et al. 1994). Some of these stellar groups even occupy overlapping regions in the HR-diagram, which makes it even more difficult to classify such objects. To obtain a star's position in the HR-diagram we must know its distance, which is very uncertain for HD 45677. Sitko et al. (1994) argued that the UV extinction curve of HD 45677 in 1992 is compatible with interstellar extinction over a distance of 900 pc. At this distance HD 45677 would be at approximately the same distance as the outer ridge of a large molecular complex that stretches from the Mon R2 cloud to the CMa OB1 complex (Maddalena et al. 1986).

However, since our line of sight towards parts of Canis Majoris is known to be almost devoid of interstellar extinction (e.g. Bruhweiler 1994), the use of an average distance-interstellar extinction relation by Sitko et al. (1994) may yield erroneous results. Therefore, we constructed a graph of E(B-V) versus photometric stellar distance for stars from the Hipparcos Input Catalogue, located within an 1tex2html_wrap3563 circle around HD 45677. This plot is shown in Fig. 12 (click here). From this plot we derive a maximum distance of tex2html_wrap_inline3521 1.5 kpc towards HD 45677, by assuming that its E(B-V) of 0tex2html_wrap3565 30 is completely due to interstellar extinction.

Furthermore, we can compute a photometric distance towards HD 45677 by assuming that it is located on the zero-age main-sequence (ZAMS) and by assuming that there is no circumstellar obscuration present at the maximum recorded brightness of HD 45677 by Swings & Swings (1972). If we also assume that its B-V at that time was not very different from the minimum of the range shown in Fig. 2 (click here), and use the appropriate parameters for a B2 V star from Schmidt-Kaler (1982), the resulting ZAMS distance is 550 pc. Under the same assumptions we computed the maximum intrinsic luminosity of HD 45677 by adopting the maximum distance of 1.5 kpc, resulting in tex2html_wrap_inline3529.

A HR-diagram with the position of HD 45677, computed using the ZAMS distance of 550 pc, the distance of 900 pc by Sitko et al. (1994), and the maximum distance of 1.5 kpc is shown in Fig. 13 (click here). In Fig. 13 (click here)a the pre main-sequence evolutionary tracks and the birthline by Palla & Stahler (1993) for a proto-stellar accretion rate of tex2html_wrap_inline3531 are also plotted, whereas in Fig. 13 (click here)b we plotted the post main-sequence evolutionary tracks by Maeder & Meynet (1988). As can be seen from Fig. 13 (click here)a, of the three positions of HD 45677 in the HR diagram, the one computed using the ZAMS distance of 550 pc is the only one which falls below the birthline. This implies that if HD 45677 is a young object, as suggested by Grady et al. (1993), its distance will be close to its ZAMS distance of 550 pc. In that case the star cannot be formed in the molecular cloud suggested by Sitko et al. (1994), leaving us with the question where the star formation region is. However, as shown by Palla & Stahler (1993) a higher proto-stellar accretion rate will result in a birthline which will intersect the ZAMS at considerably higher masses. But for masses like for HD 45677 still not more than 10tex2html_wrap_inline3533 yr is needed to reach the ZAMS.

If we assume that HD 45677 is in fact an evolved object, we do not have these problems. In that case the star might very well have formed in the molecular complex suggested by Sitko et al. (1994), but a few times tex2html_wrap_inline3535 years ago. Let us first consider several options of evolved objects.

Eruptive behaviour is known for massive stars close to the Humphrey-Davidson (HD) limit, such as LBVs (Humphreys et al. 1989). In the case of HD 45677 the temperature did not change as much as seen for LBVs in such a situation, and would be too high to fit a position close to the HD limit within a reasonable distance. Note here that indications of the luminosity class of HD 45677 are III or IV #P&(Pérez et al. 1993). However, the existence of the AG Car ring nebula is also not well explained by instabilities close to the HD limit as its luminosity class is still not exactly known (Humphreys et al. 1989). Other evolved objects that undergo phases of mass ejections leading to the formation of dusty disks as visible in the near- and far-IR are post-AGB stars and Symbiotics. HD 45677 is too hot to be a post-AGB star, but it could be in a certain transition phase of PNs. The presence of a companion could be the origin of the perturbations as detected. At the moment there are not even traces of such a companion.

Forbidden lines of [OI], [NII], [SII] and [FeII] indicate the existence of a low density region. The electron density of this region, tex2html_wrap_inline3537 cmtex2html_wrap_inline3539, is determined by the ratio of the [SII] lines and is high compared to most nebulae (Osterbrock 1974). Because HD 45677 is too ``cool'' to ionize gas that emits lines of [OIII], we derived the electron temperature by the [NII] lines and determine tex2html_wrap_inline3541 K, which is very high. The tex2html_wrap_inline3543 value of 30 is not far from those of some planetary nebulae (Osterbrock 1974). Since the temperature of HD 45677 is relatively low, these values should indicate that an extended region should be visible. However, on our images HD 45677 appears unresolved, so it seems unlikely that HD 45677 is such a planetary nebula. The values also agree with the conclusion of Swings (1973) who mentioned this region as being of considerable density, but with a low temperature based on the absence of some forbidden lines like [SII]. Swings (1973) reported the occurrence of [NII] lines, now the [SII] are also detected, clearly indicating the ongoing evolution of this region.

However, one of the most peculiar lines is the very strong and narrow blueshifted Htex2html_wrap_inline3545 absorption component at 9 km stex2html_wrap_inline3547 relative to the systemic velocity. If we propose that this concentration of considerable density of gas is distributed in a ring-like structure, indicated by its low velocity-dispersion, it might originate from the 1950 event. At a minimum distance of 500 pc, the angular size of this ring will be about 0tex2html_wrap3567 2 and indeed not yet visible.

As the evolved status of HD 45677 is also not clear we return to the hypothesis that HD 45677 is still in a pre-main sequence phase. This option has some support by the new tex2html_wrap_inline3551 value of 70 km stex2html_wrap_inline3553 (Israelian et al. 1996), a value similar to several comparable PMS objects, and the detection of blueshifted HeI emission as is reported for several HAeBe stars by #B&Böhm & Catala (1995). These findings are probably not unique for HAeBes. Furthermore, the evaporation of cometary-like bodies in the vicinity of some Herbig Ae type stars, the UXOR-group, has recently been discovered (Grinin et al. 1994 and 1996, de , Winter et al. 1995 and de Winter 1996) and show some typical behaviour in colour and variation in line profiles. A colour behaviour due to pure extinction, in the minima, or the so-called ``blueing effect'', during the deep minima, as due to revolving dust clouds in a proto-planetary disk, are not seen for HD 45677. Also the UXOR-group shows clear evidence for infalling evaporating comet-like bodies by redshifted absorption components in the NaID lines with co-variations in the HeI and Htex2html_wrap_inline3557 profiles on a daily time scale. The suggested evaporation of a large cometary-like body around 1950 should have produced similar line profiles. However, no dramatic changes in the mentioned lines have been noticed, although the resolution could be a limiting factor. Also, the time scale and amplitude of the minimum is comparable to those for objects in the UXOR-group.

Figure 13: HR-diagram with the position of HD 45677 assuming a distance of 550 pc (bottom), 900 pc (center), and 1.5 kpc (top). a) With the pre main-sequence evolutionary tracks and birthline by Palla & Stahler (1993), b) With the post main-sequence evolutionary tracks by Maeder & Meynet (1988)

Finally, when HD 45677 is indeed a product of a molecular complex related to the Mon R2 cloud and the CMa OB1 complex (Maddalena et al. 1986), it would be interesting to know whether it is still young enough for post-natal circumstellar material to be the origin for the dynamical properties observed. HD 45677 would then be an isolated young object being only some 10tex2html_wrap_inline3559 yrs old, similar to isolated HAe objects, such as HD 104237, HR 5999 and HD 163296. But, in these cases evidences of their relative youth, up to several times 10tex2html_wrap_inline3561 yrs, are still seen in their near surroundings such as T Tauri stars.

To give the final answer to the question of the evolutionary statues of HD 45677, a more accurate distance determination than the one presented here is necessary. If all goes well, this will become available soon, since this object is one of the primary targets for the Hipparcos astrometric satellite.

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