Up: The peculiar B[e]
From an intensive study of the photometric and spectroscopic properties
of HD 45677, including published data as well as own observations, we have detected
many different variations and properties. We have tried to explain these
variations, from short to long time scales.
Different mechanisms must produce the variations, each with
their own characteristics. Furthermore, we have discussed different options
for the evolutionary status of HD 45677.
Although the properties of HD 45677 are disputable
we can make the following concluding remarks:
- Photometric variations and spectroscopic properties prove the
hypothesis of a significant disruption of material around HD 45677.
By extrapolation to the maximum brightness as measured by
Swings & Swings (1972),
before any large photometric changes occured, we determined this event
took place around 1950. The minimum brightness at 8 8 was reached
around 1981. Linear extrapolation of the current increase of brightness
indicates that the maximum brightness of 7 2 will be reached again about
the year 2020.
- During this long time scale obscuring effect we observed
that the colours do not change significantly at both the
obscuration and brightening phases.
To explain this, formation of large grey grains is proposed
which caused the strong obscuration of
HD 45677 just after the 1950 event. The existence of such grains
was also detected by extinction studies of
Sitko et al. (1994) and already
proposed by Low et al. (1970).
We suggest that the origin of this material around 1950 is either due to the
evaporation of a large cometary-like body, equal to less dramatic but
similar as events for UX Ori
(Grinin et al. 1994), or caused by a blow-out of
material. Probably this blow-out was not extremely violent because of a fall
back of material causing accretion effects.
- The relatively small and continuous changes of the colours during the period
1971-1993 are remarkable because of the opposite sign of the blue colours
versus the red colours. If part of the erupted material falls (back)
in the direction of the central star, a more compact inner disk region will
cause higher extinction for the red colours, while the blue
colours can be compensated with the production of small grains
by accretion probably being visible in the bipolar flow.
- Variations on a time scale of days down to several minutes are
detected. These short time scale variations are explained by brightenings
or ``flickerings'' in the brightness as well as in the colour due to accretion.
To understand the colour
variations a production of small grains by accretion is necessary. The
existence of small grain particles, producing scattering, is detected by
an UV-excess (Pérez et al. 1993) which could be due to a bipolar
(Schulte-Ladbeck et al. 1993),
maybe produced but certainly enriched by
the accretion itself.
This accretion supports the explanations for the origin of the long time
scale variations in which the presence of accretion is suggested.
- Photometric variations on an intermediate time scale
are also detected and well known. These variations need more observations to
be well explained. They seem to exist before and after the minimum
in 1981 but their changes of 0 4 within a few years were probably absent before
1930. The intermediate time scale variations could be produced by revolving
``dust clouds'' or by shock fronts. In the latter case the blow-out
hypothesis suggests certain eruptions. The occurrence of larger variations
and the slightly getting brighter just before the strong fading around 1950
would support this. Then the intermediate time scale variations following the
1981 minimum are more probably due to infall. Support for this is that the
brightening seems to occur in certain ``waves'' and dust destruction by
accretion is seen by the higher amplitudes of the short time scale
variations. The latter can vary then up to about 0.1 within a few
days. Obscurations by ``dust clouds'' will also cause completely different
colour effects as well discussed for the HAe UXOR type objects (e.g.
Grinin et al. 1994 and references therein).
- The fact that changes from 4.8 in 1970 to 5.8 in 1981 and
back to 4.4 in 1992 means that the average particle size in the circumstellar
matter responsible for the extinction increased from 1970 to 1982 and
decreased from 1982 until 1992.
Probably the large grains were directly formed
after 1950, due to a blow out or by the evaporation of a
cometary-like body. Then the combination of the ongoing wind/outflow,
coinciding with the previously erupted material and the pressure at the edges
of the shockfront can produce larger grains by collision.
The increase of the average grain size seems then to be most
effective in obscuring the central star.
At a certain point, 1982, the large grains were destroyed again.
Infall of material seems to occur in waves, the intermediate time scale
variations, which stimulates the accretion as seen as the higher amplitudes
of the short time scale variations. As a result, the average grain size will
decrease, producing the observed lower values and giving rise to
brightening of the object again, apart from the effect of an expanding ring
of material. A dramatic colour change on the longer time
scale due to the production of these smaller grains is not necessary when the
material is either effectively evaporated or destroyed and blown away in a
bipolar flow. It supports the conclusion of
Sitko et al. (1994) of a decrease
of the total mass of the star's circumstellar dust envelope.
- Spectroscopically we have noticed
that forbidden lines are narrow compared to
other observed emission lines and are well centered on 20 km s.
This velocity is also valid for the narrow absorption components of
NaID and CaIIK lines and the central velocity of the
FeII spectral lines. We adopted therefore a systemic velocity
of 20 km s. Relative to this velocity in- and out-flow of gaseous hot
and cool material are seen.
- Remarkable blueshifted absorption lines of NaID
are detected at -10 to -20 km s. They are shifted with about
30 to 40 km s compared to the systemic velocity.
They are probably caused by material swept
up by the radiation pressure and stellar wind. This cool grain material will
collide with the outer disk causing extra cogglumeration and obscurations
on short time scales. Although the intermediate time scale variations can be
explained in a similar way, by material moving away from the central star, a more
plausible explanation is accretion activity. During the observations of the
high resolution time series, this activity was decreasing, see Fig. 10 (click here)b.
- H profiles are detected to contain absorption components on the
large emission profile at certain velocity regimes of which the red
components are the most prominent.
The H profiles are therefore interesting as they show
velocity regimes of both infalling and outflowing material. Although the
infalling gaseous material seems to be of higher densities, the strongest
narrow absorption core is blueshifted with 9 km s with respect to
the systemic velocity adopted as 20 km s.
We might expect that especially this very strong H component
originates from a very dense gaseous ring of which the velocity-dispersion
will be as low as is observed. It could be questioned if this ``wave'' might
exist for about 40 years.
- Accretion close to the stellar surface is indicated by the HeI
and H short time scale variations, probably followed by a reaction
of outflowing gas. This dynamical process could replenish the bipolar flow.
It could be questioned if the spectroscopic short time scale variations
show co-variations to the ``flickering'' as being of a similar, daily, time
scale. Furthermore, the line variations are very different from those seen
for many of the UXOR type stars
(Grinin et al. 1994 and 1995) for which
redshifted absorption components of the NaID are detected as
evidence of the evaporation of cometary-like bodies.
Observations as for the HeI lines are also reported by
Israelian et al. (1996),
whose conclusions do support our detection of accreting gas.
- Outflow of cool material is seen by emission lines that are stronger
towards the blue as in MgI and in NaID. The strong
absorption cores of the NaID and CaIIK indicate
the presence of an optically thick disk at zero-velocity. This suggests that
this disk was already present and is now enriched by the 1950 event.
- The presence of a disk before 1950 is also
indicated by other emission lines which are stable throughout the large
and long-term photometric variations and even far before 1950.
In this case the 1950 event is due to an instability which is probably
not the first one. Evidence of ``ancient'' activities, probably close to the
stellar surface, were seen from 1926 on (Swings 1973)
until now by activity
in several of the Balmer lines.
- The evolutionary status of HD 45677 is discussed extensively.
However, no strong conclusions can be drawn about its age. HD 45677 seems
to fit best in the picture of
Zickgraf & Schulte-Ladbeck (1989)
of non-luminous B[e] stars in contradiction to the B[e] supergiants of the
(Zickgraf et al. 1986). Probably in the case of HD 45677
the dust does not occur by a dense, slow expanding equatorial wind, but by
eruptions. HD 45677 could therefore act as a key object to understand the
reason of dust formation rather than to evolve to an early-type emission line
star without dust.
The authors wish to thank Dr. B. Gilbert for providing
the photometric data of HD 45677 in the Geneva system
and R. van Duuren for his assistance to obtain the CAT/CES observations.
We are thankful to Dr. J.P. Swings for his help, inspiration and patience
in tracing some unpublished observations and to Dr. G. Israelian, Dr. C.A.
Grady, Dr. M.R. Pérez and the referee for their many useful comments.
This research has made use of the Simbad data base, operated at CDS,
Up: The peculiar B[e]
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