Special session planned for the Sunday morning directly
preceeding IAU Symposium 212:
** The Masses of the Most Massive Stars and the Omega Limit **
Preamble:
An unsolved problem that has been nagging the massive-star community for
some time is the question of what the actual masses are of stars in the
upper H-R diagram. Given that the most fundamental parameter of a star
is its mass, this is clearly unacceptable! It is also very exciting,
since the most massive stars are also the most luminous and they are
becoming key probes in early cosmology.
The most direct observations of masses, based on Keplerian orbits in
binaries, have never yielded a mass above 60 M(Sun). Less direct
techniques based on spectroscopic analyses tend to give a fairly large
spread of masses. But both of these tend to yield masses that are lower
than those based on evolutionary tracks, which indicate that stars up to
well over 100 M(Sun) exist (e.g. the most massive star in the Galaxy is
often claimed to be the Pistol Star with 250 M(Sun); then there's eta
Car....). In other words, the mass-discrepancy problem, which grows
with increasing mass, is still with us, although perhaps at a reduced
level compared to its original version.
All of these techniques have their problems. In the case of binaries,
selection effects may have prevented us from finding the most massive
stars in a binary and then there's always the question of whether stars
in binaries behave like their single cousins. Spectroscopic analyses
are less direct for getting the masses, but at least one can look then
at single stars. Here, as well as for the case of evolutionary masses,
there's always the question as to how complete and reliable the physics
is that goes into the models. Besides some aspects which have been
neglected until recently and which are able to increase the
spectroscopic masses to a certain extent (inclusion of wind effects and
line blanketing), two recent factors come to mind here: clumping has
reduced mass-loss-rates for WR stars (and maybe others, too!) with
potentially dramatic effects on evolutionary tracks off the
main-sequence, and rotation still has not been included in the most-used
isochrones, probably because it is extremely complex and how to
implement it has not yet been agreed upon.
Presently, a strong debate is still going on concerning the incorporation
of rotational effects in a consistent way, if stars are severely
affected by mass-loss (this clearly includes all the "most massive massive"
stars), and if they have non-negligible Eddington luminosities (again
true for these objects), a debate which can be summarized under the
buzz-word "Omega-Limit". Recent advances seem to provide a "final"
solution for this debate, and one of the objectives of this session is a
discussion whether this is actually the case or whether there remain
additional problems.
Proposed Guideline for the Session
We propose the following topics to be considered. We don't wish to give
any more details here, leaving it up to the interested community to
voluntarily address any of these and/or suggest others that might be
considered in the context (and within the time-limits: approx. 2 hours!)
of this session. However, once we have heard from a suitable number of
participants, we will provide a list of those people and their interests
to all who responded, either actively or passively, well before the
session. We expect vigorous discussions during the session, and intend
to provide enough open time for this purpose!
Please communicate directly via email with either (preferably both) of
the undersigned, and as soon as possible, with your input and
suggestions. Note that power-point will NOT be available; we prefer
limiting this session uniquely to discussions using overheads, sometimes
even written on the fly!
Program plan as of 07/03/02:
Below is a more or less final program for this special session.
However, in such an informal session there is really no such thing as a
"final" program. If you are still interested and have not signed up
either as an active or passive participant, please let the organisers
know as soon as possible please (Tony Moffat - moffat@astro.umontreal.ca or
Joachim Puls - uh101aw@usm.uni-muenchen.de).
ALL IAU 212 PARTICIPANTS ARE WELCOME whether active or not on the topic
of this session! However we must know in advance how many will likely
attend, so if you're not in the list below, please let us know in good
time.
Note that the goal of this session is to provoke lively and informative
discussions via scheduled (see below) and unscheduled (open to anyone)
statements/summaries/comments, EACH OF ONLY A FEW MINUTES in order to
allow the largest number of people to actively participate. There will
be no formal talks per se. A 4-page resume of the session will be
published in the Symposium proceedings.
Programme (Subject to change at any time! Names in parentheses are not
yet confirmed.)
1. Stellar masses in the upper HRD
A. Moffat - Introduction
P. Massey - The upper HRD; masses
D. Schaerer - Parameters of O3 stars and the most massive stars in
metal-rich regions
D. Gies - The issue of masses
R. Hilditch - Determination of masses from binaries
V. Niemela - The need to compare Keplerian and `atmospheric' masses in
binaries
G. Koenigsberger - Do massive stars in binaries behave like single stars
of the same mass?
D. Figer - Comments on the most massive stars
A. Herrero - Comments on the mass discrepancy problem
J. Vink - The mass discrepancy in light of new radiation-driven wind
models
F. Najarro - The solution of the mass discrepancy problem for very
massive O supergiants
T. Abel - The first stars in the Universe must be massive
2. The Omega limit
J. Puls - Introduction
N. Smith - Latitude structure in eta Car's wind
A. Pasquali - Gross wind structures in LBVs
R. Humphreys - The modified Eddington limit/Omega limit and
instabilities in massive stars on both sides of the HR diagram
(J. Hillier) - Gross structures from polarimetry
V. Dwarkadas - Rotation, radiatively driven winds and the formation of
LBV nebulae
N. St-Louis - Rotation of Wolf-Rayet stars
I. Howarth - Gravity darkening
N. Przybilla, K. Venn, D. Lennon - Observational constraints of
chemical mixing in massive stars
S. Owocki - The Eddington limit and how it limits M(up), and a
corollary on the Omega limit
3. Consequences for Stellar evolution in the upper HR
N. Langer - Introduction
A. Maeder - The Omega limit and evolution
G. Meynet - Evolution of rotating massive stars in the upper HRD
M. El Eid - The sensitivity of cluster morphology to mixing inside
massive stars
(A. Heger) - Evolution of very massive stars at low (zero) metallicity
Others attending
C. Cappa
M. Cohen
E. De Donder
(L. Drissen)
C. Foellmi
A. Fullerton
R. Kudritzki
(L. Penny)
J. Pittard
(G. Rauw)
O. Schnurr
Linda Smith
J. Van Bever
K.A. van der Hucht
E. Verdugo
C. Villamariz-Cid
N. Walborn
We look forward to an exciting, fruitful session! If interested, please
contact either of us at your earliest convenience.
Tony Moffat (moffat@astro.umontreal.ca)
Joachim Puls (uh101aw@usm.uni-muenchen.de)