Документ взят из кэша поисковой машины. Адрес оригинального документа : http://star.arm.ac.uk/~csj/papers/conference/IAUC187_summary.ps Дата изменения: Sat Jun 22 13:04:16 2002 Дата индексирования: Tue Oct 2 10:48:46 2012 Кодировка: Поисковые слова: http astrokuban.info astrokuban

**TITLE**
ASP Conference Series, Vol. **VOLUME**, **PUBLICATION YEAR**
**EDITORS**
Exotic stars as challenges to evolution: an evaluation
C. Simon Jeery
Armagh Observatory, College Hill, Armagh BT61 9DG, Northern
Ireland
Abstract. A star is exotic because it, in some way, extends our knowl-
edge of how stars behave. As we make new observations we discover new
phenomena that we do not understand. As we approach an understand-
ing, those phenomena seem less exotic. That is nature. IAU Colloquium
187 set out to discover how far our knowledge of stars now reaches. This
is a personal commentary on the proceedings.
1. Introduction
IAU Colloquium 187 brought together a multidisciplinary, multiwavelength school
of astronomers working on a highly disparate range of objects. In blunt terms,
we address the question \Was the conference worthwhile, and what have we
learnt?". The rst specic question to consider is \What makes a star exotic?"
A sign outside the conference hotel, situated on route A1A a few kilometers
north of Miami, suggested a possible source of information: \The A1A bar -
where the stars hang out all night". Since the venue remained securely closed
all week, delegates were unable to pursue their researches in that direction,
but themselves provided more tangible suggestions. Milone suggested that the
question was \not whether a star is exotic or not, but whether it is perceived to
be exotic". Knapp rened this observation, citing Trimble: \objects (or people)
are only normal when you don't know them very well". Of course, there has to
be an exception, and Tout reported of one object that \the more we know about
this star the more normal it gets".
However, for all of the remarkable stars discussed, there was not one mention
of a star which, because we can observe it in such detail, is perhaps the most
exotic of them all. Were one to claim for a point source the surface structure and
activity exhibited by the Sun, we would indeed have found an exotic beast. That
the Galaxy is most certainly full of such objects in both variety and profusion
simply underlines the relevance and timeliness of this conference. In the following
summary, I oer a personal perspective of the papers presented, together with
some pointers to future work 1 .
1 A conference evaluation is obliged, broadly speaking, to acknowledge the contributions of all of
the participants, to evaluate, in some sense, what we have learnt, to congratulate and encourage
the contributors, and to be brief. Any speaker nally agreeing (or being coerced) to give such
a talk anticipates a low score against such benchmarks. Therefore this summary comes with
an apology to those who have been misrepresented, undervalued or forgotten.
1

2 Jeery
2. The stars themselves
As the objects of intense scrutiny, unusual scientic specimens often adopt in-
dividual persona. This is just as true of stars. Because the objects are diverse,
there is also a need to categorise. The following somewhat anthropomorphic
classication echoes genuine intimacies expressed in our conference.
 Familiar Faces. The brightest stars in the sky have frequently provided
some of the toughest puzzles for astronomers. Examples include  Lyr,
Cas,  Aur,  Car, Sirius, Vega and more. For many of these stars, the
wealth of data available to today's astronomer is staggering, yet the puzzles
seem no nearer to a solution. This sense of frustration was conveyed by
several delegates: Zwitter asked how we might avoid the equation: more
data = less understanding. Harmanec suggested that \too many things
are known about [ Cas]", while Smith was more sanguine about the fact
that \people tend to argue a lot about  Car". Over thirty years after their
rst encounters with HR2142 and V471 Tau, respectively, two delegates
found themselves still trying to solve career-long questions, while Guinan
{ clearly in for the long haul { found that after 2150 years of observing 
Aur, astronomers still nd it puzzling.
 Double Trouble. Binary star systems, by introducing an absolute minimum
of two additional free parameters (secondary mass and orbital separation),
inevitably give rise to a wider range of problems. Their importance was
emphasized by Ivanova who reminded us that binary frequency was in
the range 50-150%. Eggleton demonstrated how binary-star evolution has
given rise to inexplicable cases such as V379 Cep, OWGem and V643 Ori.
Part of the diфculty lies in keeping track of which star was originally more
massive.
Other characters in the cast included RT Lac { \not an exotic star, but a
member of an exotic club" (Milone), TU Mus { \exotic because it is so
remarkably well behaved" (Terrell) and  Aur { in which circumstellar gas
is probed by the hot companion (Griфn).
More problematic are those binary stars in which the companion is unseen
and which included HR2142 (Peters), V505 Mon and companions (Cho-
chol) and the sdB stars (Han). In the case of  Car, a putative unseen
companion allows a wide range of possible solutions, but in this case crit-
ics still argue against the permissiveness of such a \free parameter heaven"
(Smith). The discussion of GRTau (Yamasaki), AH Cnc (Branly) and oth-
ers remind us that our conference barely skimmed the surface of binary
star evolution.
 Three's a Crowd, . . . Stars don't just come in pairs; a sizable fraction are
members of triple systems. Indeed, Eggleton would argue that \if you can't
understand a binary, it's probably a triple". This approach was shown to
be true in several cases including SuWt2 (Bond), AS Cam (Van Hamme),
ф Ori A (Harvin), and V471 Tau (several authors).
 . . . More's a Factory. Emphasizing the point that unusual stars have a
higher probability of production in multiple systems, the conference was

Conference Evaluation 3
reminded how star clusters provide a fertile breeding ground for low-mass
X-ray binaries, millisecond pulsars, blue stragglers and subdwarf B stars,
amongst others (Hurley).
 Stars that go \Bump" in the Night. Up to this point, stellar evolution has
really only considered stars that either lose or gain mass quasi-statically
during relatively long-lived episodes. In recent years there has been in-
creasing evidence for catastrophic collisions and mergers, both between
young massive stars (Ivanova) and between old white dwarfs (Han, Jef-
fery).
 Disks, Spots, Shells, Streams, Jets, ... What tends to make stars really
unusual is not, normally, their internal structure, but what happens either
at or above the surface { where we can see it best! Examples ranged
from very young massive stars with vigorous out ows, through giants and
degenerates around the fringes of the HR diagram, to microquasars in the
stellar graveyard. Consider, for example,  Car (\kinda like a volcano
or geyser": Smith),  Aur (\the mother of all accretion disks": Guinan),
AG Dra (\as symbiotic as it gets": Wilson), and SS433 (Chakrabarti).
Here also the big problems begin to manifest. Where does the UV iron
emission forest come from in  Lyr (Linnell)? Why have there been \over
900 papers [on Cas] since 1930, but few systematic or penetrative anal-
yses" (Harmanec)? If we're lucky, we can resolve the structures that pose
the problems: \There's one gorgeous picture of  Car, there are lots of
lovely pictures of  Pic" (Lissauer).
Extended structures, generally disks, also provide the conditions necess-
sary for objects like Nova Sco (Tout), high-luminosity Be stars (Mirosch-
nichenko), and the possibility of forming icy planets alongside masers in
the disks surrounding young stellar objects (Slysh, Val'tts).
 Geometry. If we can see it, then we have to solve the radiative transfer
problem in order to understand what we see. This is hard enough in a
one-dimensional plane-parallel semi-innite geometry; it gets even harder
in the real world. If nothing else, \exotic stars" emphasized the fact that
few stars are simply single and spherical. That makes them interesting! In
addition to the supercial material mentioned above, rotational distortion,
Roche geometry, radiative distortion (Beer: not often considered before)
and spots may all play a decisive r^ole in governing stellar structure. Stellar
distortions provoke various conundra: \it depends which end of the stars
you're talking about, whether you mean the front end or the back end";
\I'm just trying to imagine a star living through this experience". Stars in
which understanding the geometry represents a cruial step include  Car
(obviously: Smith, Langer, Sch  ler), RY Sct (Smith), V471 Tau (Vaccaro,
Guinan, Bond) and various HST planetary nebulae (Bond). But to make
life really complicated, one should add a magnetic eld which is never
spherical and may do things that have not yet been discovered: \Next
time you discover some peculiar variability, you might want to keep in
mind that it could be one of these magnetically driven torques" (Lai).

4 Jeery
 Is the Chemistry Working? Magnetic elds play another r^ole in creating
unusual stars by provoking chemical peculiarities. Amongst the Ap stars,
for example, the prize for `strangest star in class' was awarded to Przbyl-
ski's star (\don't claim to believe the observations unless you understand
them": Kurtz), but HD965, HD217522 (Hubrig) and o Leo (Griфn) re-
ceived an honourable mention. HD331319 provided the author with a mis-
chievous thought; if [He/Fe]=1.63 (Miroshnichenko), what might [Fe/H]
be? Deciencies in hydrogen are sometimes masked in cooler stars, but
rarely in B stars such as BI Lyn (Jeery), sole representative of several
classes of extremely hydrogen-poor stars. Surprising as the misclassica-
tion of BI Lyn had been, it was harder to understand how the barium star
WeBo1 had originally been designated a planetary nebula nucleus (Bond).
Nevertheless, it was a reminder that extreme chemical peculiarities are
encountered throughout the HR diagram, and many have still to be un-
derstood.
 The Extreme, the Bizarre, the Surprising, the Deviant, the Wrecks. De-
spite the in uence of binary interactions, stellar winds, and magnetic elds,
most stars survive pretty well intact until they reach old age. The really
strange objects are found in the mortuary. Consider, for example, SS 433
{ \the most spectacular image in the X-ray sky" (Katani, Chakrabari)
and the gargantuan carbon star CW Leo { \not a particularly weird or
unusual star { but it does cover the largest extent on the sky" (Knapp).
I was bewitched by the swiss-cheese character of the CW Leo stellar en-
velope indicated by submillimeter data; this seemed exotic enough to me,
but why is it like that? Perhaps it had less to do with the star than with
\the mess it's made of the ISM" (Knapp again). Born-again stars fea-
tured prominently, including FG Sge which (according to Schonberner &
Jeery) is not H-decient, and not particularly . This group also includes
V605 Aql, V4334 Sgr (Hinkle, Lawlor) and V838 Mon (Rauch), which had
appeared just in time for the conference, and the naked white dwarf core
H1504+65. Is the latter a rare O-Ne-Mg white dwarf (Werner)?
 Absent Friends. Despite the number of quite extraordinary stars discussed,
there were a considerable number of absentees. From a personal point of
view, it would have been nice to have heard more about R CrB, V348 Sgr
and other extremely hydrogen-poor relatives. Another missing VIP was
the magnetic Bp star  Ori E, a star with extreme H/He anomalies across
its surface and a prototype of the oblique rotator model.  Sgr received
a number of passing references, but deserved better, together with a host
of evolved binaries (HD128220, AA Dor, GPCom, PG1336-018), pulsars,
low-mass X-ray binaries, cataclysmic variables, and more besides. Sadly,
we only had one week.
3. Solutions?
Having gathered together all of the problems in stellar astronomy, the task ahead
might seem daunting. However the conference suggested three broad approaches
by which progress will undoubtedly be made in the near future. First among

Conference Evaluation 5
these is the acquisition of new data - although not necessarily to the liking of
the delegate who suggested that \to understand stars better, we need less data,
not more". Munari and Zwitter outlined prospective advances to be made with
GAIA and \the grand armada of astrometric missions". Knowing distances and
luminosities for every star in the galaxy will have a revolutionary impact. Grif-
n described a project for reclaiming the archives: stars work in the temporal
domain, of which a substantial record is locked up in deteriorating photographic
archives. The importance of other existing (INTEGRAL, Gimenez) and pro-
posed (WSO/UV) missions was also touched on.
The second approach will be to develop better models. Three-dimensional
stellar evolution with the Lawrence Livermore Djahooti code (Eggleton) promises
much, but there is room for improvement in every eld.
Finally, and a little tongue in cheek, one wonders whether the key to some
exotic stars lies not in observing them ad innitum but to choose a simpler but
similar star or, in other words, to choose a simpler problem. Many problems are
solved by breaking them up into smaller pieces and then joining up the results.
It seems that some of the objects discussed this week, HR2142 for example,
might benet from such a treatment.
4. The truth is out there
Although much of this conference has dwelt upon apparently insoluble problems
in astronomy, there was much to be encouraged by. At the very worst, Eggleton
remarked that \things in the heavens might not get quite as complex as things
here on earth". Faced with an indundation of data from forthcoming projects, we
shall be much exercised to identify what is exotic, \automating the identication
of what is peculiar" (Kotani) will become a necessity, either to identify \another
can of worms I don't want to get into" (Knapp), or to identify the really new
exotic stars that will unlock the mysteries of the old. This leads me to reconsider
our initial question { what makes a star exotic? I have deliberately echoed in
the abstract what I oer here as a useful answer.
A star is exotic because it, in some way, extends our knowledge of how stars
behave. As we make new observations we discover new phenomena that we do
not understand. As we approach an understanding, those phenomena seem less
exotic. That is nature. Our task is to keep the cycle of exploration, discovery
and interpretation turning.
5. Final Impressions
In conclusion, IAU Colloquium 187 has left us with a set of impressions. The
rst is that of a family gathering. We renewed old acquaintances; some stars
are just so exotic that they have deed explanation over the years. In the
meantime, astronomy has turned up a number of new and equally unusual stars
which challenge theories of stellar evolution.
The second was one of viewing a landscape. Listening to the talks and
reading the posters we were presented with a series of snapshots of completely
disparate objects, stellar evolution at all sorts of stages. Somehow they all lie
in the same picture; our task is to gure out the connections.

6 Jeery
We have carried out a a health check. There has been great progress in some
areas of stellar astronomy. It was also evident that there has been comparatively
little in others. This must be addressed.
For this reviewer there was a sense of a network in which there are ties
between dierent types of star that may have been overlooked. It is hard to be
specic, but often stars are classied by certain types of activity. Sometimes it
is the stars we need to compare, and sometimes the activity.
There was clear evidence of a technology shortfall. It is time to translate
the cartoons into 1-2-3d models with hydrodynamics, radiative transfer, etc..
as required. This is no doubt hard, but only by understanding the geometry,
and describing the system with robust physics, will we be able to answer many
questions about stellar evolution.
Finally, I believe that this conference has provided a wake-up call. I was
invigorated by learning something new about a lot of dierent stars. Stars are
really interesting. They present an incredible range of properties, from the swiss-
cheese structure of CW Leo to the enormous X-ray lobes of SS433, from stellar
wrecks like H1504+065 to seemingly straightforward stars like our Sun.
The story of our science is a good story. We have a duty to tell it to our
public. We should also pursue our science with condence, using the fantastic
facilities available to us today and coming online over the next decade.
Acknowledgments. The author is indebted to the International Astro-
nomical Union and the Royal Society, London, for travel grants which made his
attendance at IAU Colloquium 187 possible. All of the delegates are grateful to
Bob Wilson and the scientic organizing committee and particlarly to the real
stars of the show, Walter van Hamme and his team of local organizers, for their
eorts in making a memorable conference run smoothly.