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Scientific Report 2007-2009
Astronomy & Astrophysics
A3. Advanced evolutionary phases of high mass stars
The details of post-MS evolution of massive stars
are still poorly understood: the intermediate H-burning
phases of an O-type star (typical mass up to 150 solar
masses), are crucial as during them the star has to
loose a huge quantity of its mass until it reaches its W-R
phase (not in excess of 30 solar masses) during which it
completely sheds its H envelope. This relatively short
phase is thought to be represented by the extremely rare
Luminous Blue Variables (LBVs) residing at the top of
the HR diagram. The spectral and photometric characteristics
of LBVs and related stars indicate that their
evolution is driven by copious variable stellar winds. As
a consequence their temporal light curves display irregular
variability of 1-2 mag. Some of these stars have
experienced ejection of significant shells with strong and
rapid luminosity variations from the X-ray to the optical
range. For most LBVs such events have been witnessed
very rarely, but the presence of extended circumstellar
nebulae suggests that they are a common aspect of LBV
behavior.
Figure 1: Spectral type oscillations of the LBV GR290 in
the HeI vs HeII diagram
To date only about a dozen Galactic candidates have
been confirmed, while there are more known extragalactic
LBVs, in part thanks to the less obscured view of
these extragalactic populations. As part of a spectrophotometric
investigation of very massive stars we have
studied the seven confirmed LBVs in the galaxy M33.
May be the most intriguing is VarA, known to have formerly
presented an M-type spectrum. Our most recent
data indicated warmer color indexes successively confirmed
by the spectral evolution towards an intermediate
G-type . At the opposite edge of the temperature
range, GR290 reached the hottest phase so far detected
in an LBV [1]; If this phase would persist, we may be
witnessing the rst case of transition from an LBV stage
to a more stable WolfRayet one (see fig 1).
However, for apparent low luminosity levels there remain
serious limitations in present instrumental capabilities,
essentially in the X-ray range, so the narrow
Galactic sample still remains the baseline for comparative
characteristics and analysis of the class in general.
For the prototype eta Car our observations with the X-
ray satellite BeppoSAX revealed a constant non thermal
excess luminosity between 13 and 20KeV in contrast
with the variable thermal emission visible from the soft-
X to the IR which is linked to the stellar wind. In a detailed
spectroscopic analysis of AG Carinae, one of the
galactic LBV prototypes, we unexpectedly found that
the bolometric luminosity decreases as the star moves
toward the maximum flux in the V band,contrary to the
common assumption; this discovery allowed us to speculate
about the amount of mass involved in the S-Doradus
type instabilities which appear to be failed Giant Eruption,
with several solar masses never becoming unbound
from the star [2].
New discoveries would greatly advance the knowledge
of evolutionary connection between LBV and other
intermediate phases in the life of very massive stars,
the duration of the LBV phase, the origin of their
nebulae which show evidence for different wind regions.
Puzzling is the presence of circumstellar dust, which has
not been previously thought to exist around stars of this
temperature and luminosity range. In a recent paper
we presented a list of new members and candidates, for
some of which we found evidence of binarity. Actually
from a spectroscopic monitoring of an LBV candidate,
apparently an intrinsecally very luminous B[e] star,
we determined a periodical displacement of the photospheric
absorption lines; the orbital period of about
30 days is compatible with a binary system composed
by a massive B star and a collapsed object [3] We thus
suggest that all the stars of this class are components
of binary systems that have experienced strong mass
transfer, responsible for the formation of extended
gaseous and dusty envelopes. The spectroscopically
confirmed LBV candidates discovered only require that
variability be demonstrated to become actual LBVs.
This last step can be achieved using both archival data
and concerted long term monitoring .
References
1. R.F. Viotti et al., A&A 464, 53 (2007).
2. J.H. Groh et al., ApJ 698, 1698 (2009).
3. G.Muratorio et al., A&A 487, 637 (2008).
Authors
C.Rossi
Sapienza Università di Roma 150 Dipartimento di Fisica