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