All the newly identified stars have luminosities within a narrow range of
-
(see Figure10), which roughly corresponds
to initial stellar masses of
-
(see Section 4 of de Jager1998, and references therein).
Local examples of evolved stars in this luminosity range are the
Yellow Hypergiants (YHGs) such as IRC
10420,
Cas and
HR8752 (de Jager & Nieuwenhuijzen1997; Smith & Owocki2006),
many of which are also partially obscured by dust ejecta. There is
no means of cleanly surveying the Galaxy for these objects and they
are so rare that samples in the Galaxy and the Magellanic Clouds
do not provide good statistics for their abundances, life times or
total mass loss. Our well-defined sample of likely extragalactic
analogs provides a means of addressing some of these questions.
If we assume these objects are similar to stars like IRC10420, their
expansion velocities will be more like
km/s than the
km/s
of the typical LBV shell. Hence, it seems more appropriate to scale
the results to
years. This also matches the
estimated age of the phase of dusty mass loss by IRC
10420 (Tiffany et al.2010).
With 18 candidates, this detection period
then leads to a median estimate that
with
. If we associate these with the mass
range from
to
, they represent a fraction of
of massive stars, so the average number of
episodes per star,
with
a possible range of
, although this
does not include the uncertainties in
Figure9 shows that the median mass causing the obscuration is
. The total mass lost in all the eruptions is then of order
, which would be of order
-
.
This implies that the periods of optically thick (dusty) mass loss cannot
dominate the overall
mass loss of the star. To make the mass lost in these phases dominate
either requires that we have grossly overestimated
, or that
the mass range of the stars is much narrower.
A related point is that these phases represent a negligible fraction
of the post-main-sequence life times of the stars, at most lasting
a few thousand years.