Some suggestions for B-colloquia (hovedfags colloquia):

Colloquium advisor: Uffe Gråe Jørgensen, AO (3532 5998, e-mail uffgj@nbi.dk)

(1) What is the origin of supernovae of type Ia ?

We have a very good understanding about how a massive star evolve through its life and finally explode as a supernova (SN) of type II, as for example the famous SN 1987A. Approximately every second known SN are, however, not of the "SN 1987A type". These are the type I supernova -- they show no trace of hydrogen in their spectrum. The most important group of them is the type SNIa. It is believed that SNIa are white dwarfs (WD) which have accreted so much material that they become more massive than 1.4 msun, and thereby explode. However, almost all known WD have M=0.55 msun, and there is no known mechanism which can bring the WD mass up to 1.4 msun without involving hydrogen. Current suggestions include cataclysmic variables, AM CVn stars, super-soft X-ray sources, and colliding (merging) white dwarfs in binary systems. Large surveys are currently being carried out at ESO in order cast more light on the origin of the supernovae of type Ia.
Literature:
(1) B.Leibundgut 2000, A&A Rev., in press;
(2) N.Langer et al 2000, astro-ph/0008444;
(3) A.V.Tutukov, L.Yungelson 1996, MNRAS 280, 750;
(4) G.Nelemans et al. 2000, A&A in press;
(5) L.R.Yungelson, M.Livio 2000, ApJ 497, 168.
General: (6) B.Warner 1995, Cataclysmic variable stars, Cambridge University Press. About implications for determination of the cosmological constants:
(7) M.Livio 2000, astro-ph/9903264.
(8) B.Leibundgut et al. 1999, in "Looking deep in the southern sky" (ed. R.Morganti, W.Couch) (Springer), p328.

(2) Why do globular clusters have a horizontal branch ?

This seemingly trivial question, is a deep unsolved puzzle in modern stellar astrophysics. Several new scientific papers regularly discuss in detail the so-called second parameter problem: i.e. the fact that otherwise similar clusters (e.g. clusters of the same metallicity, but with something secondarily different which could e.g. be the age) can have very different horizontal branch morphology. However, the following even more fundamental problem doesn't even have a name and is basically never touched upon: If stars evolved according to the standard stellar evolution theory described in the text books, all stars in a given globular cluster would have the same age, and all evolved stars would have basically the same mass. Since the position on the horizontal branch in our present stellar evolution theory is given only by age, metallicity, and mass, the (zero age) horizontal branch should be "a horizontal point" (which it obviously isn't). We could well call this paradox the first parameter problem. Although the problem is not solved, several new discoveries during the last few years have shed important new light on this "forbidden problem" in astrophysics. Obvoiusly also the solution of the so-called "second parameter problem" is likely to find a solution if we could finally (discuss and) solve the "first parameter problem".
Literature:
(1) B.Behr et al. 2000, Ap.J. 528, 849.
(2) R.M.Rich et al. 1997 ApJL 484, L25.
(3) B.Behr et al. 1999, ApJL 517, L135.
(4) Peterson et al. 1995, ApJ 453, 214.
(5) Catelan et al. 1998, ApJ 494, 265.
(6) G.Piotto et al. 1999 AJ 118, 1727.
(7) Rood 1998 in UV Astrophysics -- beyond the IUE fianl archive, p.515.

(3) $omega$Cen -- the most massive globular cluster in our Galaxy.

With a mass of 4 10${^6}$ M$_odot$, $omega$Cen is by far the most massive globular cluster in our Galaxy, and it is also peculiar among the globular clusters in several other aspects. For example, its red giant branch is unusual broad, which could be interpreted as an indication of a long star formation history (as opposed to the other globular clusters, where the whole population of stars is formed at once), or as a large variation in metallicity among its stars. Very recently a high precision photometric survey has been completed at ESO of 220,000 stars in $omega$Cen, and also high resolution spectroscopic abundance analysis of the brightest stars have been performed. The results show that while the bulk of the stars have [Fe/H] approx -1.6, the cluster also house a population with about 5% of the cluster mass which has [Fe/H] = -0.5, and forms a well separated RGB. While all other globular clusters seems to have had only one event of star formation, omegaCen seems to have had an extensive star formation history and a chemical self-enrichment over a length of time, just like a regular galaxy. Some people therefore believe that $omega$Cen may not really be a globular cluster, but rather a dwarf galaxy which was captured by the Milky Way, and during the capturing process only its galactic core was left, which would look like a globular cluster. Another possibility would be that $omega$Cen with its unusual high mass just forms the lower mass boundary of a self-enriching system, just on the border between what we would usually call a globular cluster and a galaxy.
Litterature:
(1) Borris et al 1996, ApJ 462, 241;
(2) Suntzeff & Kraft 1996, AJ 111, 1913;
(3) Hilker & Richtler 2000, A&A in press;
(4) Lee et al 1999, Nature 402, 55;
(5) Pancino et al. 2000, ApJ 534, L83.

(4) Production and destruction of Li (+ Be and B) in stars.

The determination of the primordial Li abundance forms the basic for estimating the baryon to photon number in the universe, which is a critical test of cosmological models, presently in poor agreement with the Big Bang Inflation model. However, determination of the primordial Li abundance from stellar spectra is not straight forward, because Li is also produced and destroyed in sensitive processes in the stars, which both change the observable Li abundance in individual stars during their life-time, as well as increase and decrease the general Galactic Li abundance with time. However, a better understanding of the Li production/destruction processes not only challenge the cosmological models, but also contribute with crucial information about the stellar structure and evolution, because of its high sensitivity to the badly understood, but important, mixing processes in stars. The major Li production is through internal mixing combined with the so-called hot bottom burning in AGB stars. The major Li destruction is through pre-main-sequence mixing and nuclear burning.
Litterature:
(1) Cameron and Fowler 1971,
(2) I.J.Sackmann & A.I.Boothroyd 1992, ApJ 392, L71,
(3) I.Mazzitelli et al 1999, A&A 348, 846, (4) L.Pasquini 2000, IAU Symp.198, in press,
(5) S.Randich et al. 2000 A&A 356, L25,
(6) J.D. Do Nascimiento et al. 2000, A&A 357, 931,
(7) C.Charbonnel & S.Talon 1999, A&A 351, 635
(8) Hill & Pasquini 1999, A&A 348, L21.

(5) Eta Carinae -- the brightest known star.

Eta Carinae is the brightest star known in our Galaxy, and maybe the most massive one ever seen. With a mass maybe of 120 msun, its luminosity is right at the Eddington limit (i.e., it is constantly on the verge of blowing itself apart because of its high radiation). In the middle of the 19th century it was one of the brightest stars in the sky, but after 1860 it faded with 18 magnitudes. Later it has had several minor eruptions again. During the past few years it has experienced an additional brightening of a factor 2, to what seems to be above the Eddington limit, and this may be the beginning of a new major flare up, with an enormous eruption of material. During the last major eruption in the period 1836-1860 $eta$Carinae blew at least 2 msun of dust out, which still enschrouds the central star. The stellar UV radiation which is absorbed in the dust and re-emitted as mid-infrared radiation, makes $eta$Carinae today the brightest infrared (at 10$mu$m) extra solar system object in the sky. Several recent observations seems to indicate that $eta$ Carinae is in reality a binary star enschrouded in a massive disk surrounded by an even bigger cloud of materian called the Homunculus nebula.
Litterature:
(1) R.M.Humphreys et al. 1999, PASP 111, 1124;
(2) Davidson & Smith 2000, Nature 405, 532;
(3) Smith et al. 2000, AJ 120, 920;
(4) various contributions to ASP Conf.Ser. 179: Eta Carinae at the Millennium.
(5) K.Davidson et al. 1999, AJ 118, 1777.
(6)Damineli et al. 2000, Ap.J. 528, L101 and IAU Circl 8100, 27/3/2003.

(6) Sakurai's object -- a re-born star.

Stars with mass less than $approx$ 8 msun (i.e., basically all stars in the universe) end their life with a red giant phase where helium-shell flashes brings material from the interior to the surface, followed by a strong stellar wind and the expelling of a planetary nebula. Finally the star exposes the white dwarf that was created in its interior meanwhile, and what was previously a star is now a hot white dwarf. However, the object can in theory save a final helium-shell flash till the beginning of the white dwarf phase, which will bring it back to the red giant phase one more time -- the star is "re-born". In 1919 such a re-born star, V605Aql, was observed for the first time, but already in 1925 is was below detectable luminosity. In 1985 it was re-discovered -- now as a Wolf Rayet star, which is usually believed to be a completely different kind of object (an extreme high-mass star). During the last few years another such object, Sakurai's object, has appeared, and it is rapidly evolving from one side of the HR-diagram to the opposite. During 1997-1998 it increased its mass loss rate with a factor 40, and during 1999 it faded out to m$_V$ = 20 -- presumably burried in its own dust, which is probably also what happened to V605Aql/A58 in 1925. It can however, still be traced in infrared light, where huge amounts of dust is seen roling out from the star.
Litterature:
(1) Iben et al. 1983 ApJ 265, 605. (2) Kerber et al. 1998, Ap&SS 255, 279.
(3) Pollaco 1999, MNRAS 304, 127. (4) M.Asplund et al. 1999, A&A 343, 507.
(5) Seitter 1987, ESO Messenger 50, 14. (6) Asplund et al. 1997, A&A 321, L17.
(7) Jacoby et al. 1999, IAU Circ.7155. (8) Duerbeck et al. 1996, ApJ 468, L111.