MIT Astrophysics Colloquia - Spring 2001

It has become increasingly clear in recent years that the large variety of supernova types and subtypes is at least partly the result of binary interactions. This talk will briefly review the basic elements of binary evolution theory and discuss how binary interactions affect the presupernova structure of massive stars and the resulting supernovae. SN 1987A was a spectacular, but highly anomalous supernova event; the complex, but essentially axi-symmetric triple-ring nebula provides strong evidence for the binary nature of the progenitor. Dr. Podsiadlowski will present the case for the presently favored scenario in which the progenitor was a member of a massive close binary and merged completely with its companion some 30,000 years ago. He will present the results of recent stellar and hydrodynamical calculations of the merging process and discuss how future observations/calculations can help to test the model.

Gamma-ray burst astronomy is a field maturing at a phenomenal rate. Only three years after the discovery of fading long-wavelength counterparts, we now know these to be the brightest explosions in the Universe, we have distance measurements for dozens of events, we have developed a detailed theoretical framework for understanding the X-ray -- radio afterglow lightcurves. In this talk Professor Harrison will present recent observational evidence connecting the long GRBs to the collapse of massive stars, and she will review what afterglow studies have told us about the physics of the explosions, and the progenitor population.

Eta Carinae is a unique object in a unique environment. The Carina Nebula contains the greatest concentration of very luminous stars within a few kpc of the sun. Eta Car itself has a bolometric luminosity of ~2x10^40 ergs/s. Two short Chandra observations of Eta Car reveal a partial shell of X-ray emission surrounding an unresolved, bright, central source. The spectrum of the central source is strongly absorbed and can be fit with a high-temperature thermal continuum and emission lines. The surrounding shell is coincident with the "Outer Shell" of Eta Car and the X-ray spectrum of the Shell is much softer than that of the central source. ~70 sources in the surrounding nebula are found with X-ray luminosity above 2x10^31 erg/s. Only half of these are associated with bright early stars. Implications for the nature of Eta Car and the structure of the Carina Nebula will be discussed.

The TRACE satellite observes the Solar atmosphere from the photosphere to the low corona with unprecedented spatial and temporal resolution. The interaction between the solar magnetic field and the coronal plasma can now be studied properly for the first time. We are finding that an entirely new vocabulary is needed to describe the structure and dynamics of the corona: instead of loops of confined plasma we have separatrices, flows and interaction regions. The corona is filled with dense, cool absorbing material, in constant motion and intermingled with the hot plasma. Observations of flares and mass ejections show that magnetic reconnection is deeply involved in the energy release.

Important clues to the history of the universe are written in the X-ray sky. Hot baryonic gas collecting into deep potential wells shines brightly in X-rays, enabling us to trace the dark-matter skeleton of the universe. Comparing the statistics of structures seen at z > 0.5 to those seen at z ~ 0 reveals little evolution, strongly supporting a low matter density for the universe. However, these structures are not nearly as luminous as one would expect if their properties were determined by gravity alone. Both the luminosity-temperature relation for clusters and the relatively low level of the ~1 keV background indicate that the process of galaxy formation has established a universal lower limit to the specific entropy of intracluster and intragroup gas. Understanding the interplay between supernova heating and radiative cooling in establishing that entropy floor may yield insight into the feedback processes that govern galaxy formation.

To model the 10-billion year history of a globular star cluster on a star-by-star basis poses severe challenges to hardware and software. The hardware speed of 1 Petaflops, needed to simulate a million particles, is not yet at hand, but with the GRAPE-6, a special-purpose computer built at Tokyo University, we are now coming close, at a speed of 100 Teraflops. The software challenge, to follow encounters with tight binaries with orbits of less than an hour, while still modeling the whole history of a star cluster, has been met in our Starlab environment. The last challenge, to visualize Terabytes of data, is now being met through our use of the Hayden planetarium, at the American Museum of Natural History in New York City. There we have started a project of interactive data mining, using the 100-feet diameter dome as a virtual reality simulator, allowing us to `fly' through the 4D history of the star cluster. This will make it possible to analyze the wide variety of reaction channels for dynamically producing the zoo of stellar objects in a globular cluster, from blue stragglers to binaries containing millisecond pulsars.

The spin rate, magnetic field strength, and velocity of a newly born neutron star provide a unique window on the physics of core collapse and, thus, a precious constraint on models of supernovae and of explosive nucleosynthesis. Evidence is accumulating that our prototypical young pulsar, the Crab, is not very typical at all. I will review recent high-resolution X-ray observations of several young neutron stars and composite supernova remnants which are at last giving us quantitative information on the distributions of neutron star birth parameters as well as suggesting connections between them. I will also describe a new X-ray and radio survey of the Galactic plane designed to provide a definitive census of the Milky Way's young neutron star population.

Some recent work on the formation of soft X-ray transients (SXTs) and their possible connection to gamma-ray bursts (GRBs) will be discussed. Black hole candidates in SXTs are believed to represent the outcome of hypernovae in massive stars - a variant of the failed supernova model originally proposed by Woosley. For a Wolf-Rayet star to evolve into a hypernova, it must maintain its hydrogen envelope during helium core burning. The envelope can only then be lifted off in common envelope evolution, which brings the Helium star into co-rotation, giving the outer parts pressure support while the explosion evolves. It is argued that Nova Scorpii and perhaps other SXTs are relics of GRBs.