Study of exotic stars helps understand formation of solar system
AGÊNCIA FAPESP/DICYT Be stars are such strange objects that even professional astrophysicists are surprised by their description. Nevertheless, these stars are common in our galaxy, and there are several very near the solar system, at distances in the range of 100 light years, which is approximately nothing on the astronomical scale.
Not only are Be stars intrinsically important, but they are particularly worth studying because they are surrounded by disk plasma (atoms, positive ions and electrons) that cannot form planets but can be described in terms of the physical principles that govern protoplanetary disks, such as the one that gave rise to our own solar system.
The research project “Probing the physical characteristics of the disks surrounding Be stars” assembled researchers from the University of São Paulo (USP) in Brazil and the University of Western Ontario (UWO) in Canada to model the plasma disks around Be stars. The project was supported by FAPESP.
An article describing the results of the project was recently accepted for publication by the journal Astronomy & Astrophysics and is expected to appear under the heading “Multi-technique testing of the viscous decretion disk model. I. The stable and tenuous disk of the late-type Be star β CMi”.
“Because these stars spin very fast, the surface material at the equator is weakly bound to the star in gravitational terms and ends up being ejected. This material builds up around the equatorial plane to form the type of disk we studied in collaboration with Canadian colleagues,” said Alex Cavaliéri Carciofi, a professor at the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences (IAG-USP).
Carciofi was principal investigator for the project and one of the authors of the article. The principal investigator abroad was UWO’s Carol Evelyn Jones.
Before the characteristics of plasma disks can be explored, it is necessary to know a little about Be stars and what makes them so peculiar. “Be-type stars are very massive. Some have a mass equivalent to 15 or 20 times the mass of the sun. In addition, they have very rapid rotation periods. This is why they’re oblate, i.e., flattened at the poles rather than spherical. They’re so flattened, in fact, that the distance from the equator to the center can be 50% greater than the distance from either pole to the center,” Carciofi told Agência FAPESP.
The poles and equator also starkly differ in terms of temperature owing to high rotation speed and the resulting deformation. While temperatures at the poles can reach 30,000 degrees, equatorial temperatures are typically approximately 10,000 degrees or less. For comparison, the sun’s surface temperature is estimated at 6,000 degrees (versus 15 million degrees in the core, where nuclear fusion converts hydrogen into helium and generates our star’s energy).
“A possible explanation for the huge temperature gap is that energy is transported from the core to the poles by radiation, whereas convection is responsible for conveying energy to the equator,” Carcioli conjectured. “This would be a reflection of changes in the star’s internal characteristics due to the high rotation speed.” Because of the temperature difference, the poles shine far more brightly than the equator.
Nevertheless, Be stars are very bright overall because their huge mass means that nuclear fusion is intense, so they have short life cycles, lasting around a million years, as opposed to the ten billion years that our sun is expected to last.
Spiral arms of the galaxy
The relative youth explains why such large numbers of Be stars are found in the sun’s vicinity. New stars are formed mainly in a galaxy’s spiral arms, and the solar system is immersed in one arm of the Milky Way.
Massive stars usually end catastrophically, exploding as supernovae, ejecting vast amounts of matter into outer space, and eventually collapsing as black holes.
Well before this spectacular demise, however, Be stars form plasma disks, which can extend out to distances comparable to Earth’s orbit around the sun or even that of Mars.
Plasma disks comprise the same elements as the stars from which their matter is ejected: basically hydrogen and helium, as well as smaller quantities of carbon, nitrogen, oxygen and iron. The intense radiation emitted by Be stars heats their disks to temperatures as high as 10,000-20,000 degrees, so that the disks also emit light.
“Their densities are high by astrophysical standards but lower than the most extreme vacuum that can be produced in a laboratory on Earth,” Carciofi said. “That’s because our atmosphere is ultradense in astronomical terms. As you’d expect, the density of a Be star’s plasma disk decreases sharply from the region contiguous with the star to the outer edge.”
The aim of the research project led by Carciofi was to understand the formation, structure and dynamics of Be stars’ plasma disks, as well as their life cycle. “We studied these disks from the hydrodynamic standpoint, using fluid theory to investigate how they’re formed and organized around Be stars,” he said. “We also studied how radiation from a Be star penetrates its disk, converting gas into plasma, which heats up to such high temperatures that it emits its own light.”
Complex numerical models
The study involved sophisticated physics and complex numerical models. “We made intense use of the Astroinformatics Lab (LAi), which is funded by FAPESP’s Multi-User Equipment Program (EMU). In particular, we used the main facility at LAi, the Alphacrucis supercomputer cluster with 2,304 processing cores operating in an integrated manner,” Carciofi said.
“Using spectroscopy, interferometry and polarimetry, we can detect the presence of a disk around a given star, study its characteristics, and compare observations with theoretical predictions. This shows how good or bad the prevailing theories are.”
A major step in the process of understanding Be stars was conducted by a team of Japanese researchers in the early 1990s [Lee, U., Osaki, Y., & Saio, H. (1991), Monthly Notices of the Royal Astronomical Society, vol. 250, p. 432]. According to the model they proposed at the time, once the matter that forms a disk has been excreted from the star, it is propelled further into outer space by viscous forces. The Brazilian-Canadian research project started from the point reached by the Japanese team.
“We thought the model they proposed was simple enough for us to be able to make predictions based on it,” Carciofi said. “So, we picked Be stars for which there were large numbers of observations and developed predictions relating to disk hydrodynamics and to the comprehensiveness of the model to test whether it was capable of explaining everything indicated by the observations.”
According to Carciofi, the results were exciting. The group developed a new model based on the original one but far more sophisticated, calling it the viscous decretion disk model. “The further we went in comparing observations with this model, the more consistently it explained disk structure,” he said. “In addition, our partnership with colleagues at Western Ontario enabled us to share the numerical models we developed at USP with them.”
Viscous processes are present in various astrophysical systems. Planet formation starts with a viscous accretion disk, for example. As the name implies, however, the matter that forms the star and its planets flows from the outer edge toward the center of an accretion disk. In the case of Be stars, matter flows outward, from the surface of the star to the outer edge.
“Protoplanetary disks and Be star disks are both Keplerian and both viscous, so the physics toolkit developed for Be star disks can also be used to describe protoplanetary disks. This is why it’s so useful to investigate Be star disks in depth. Protoplanetary disks are much harder to study because they’re usually very far away and obscured by dense interstellar matter. Additionally, their chemical composition is much more complex. It’s easier to study Be star disks because they’re closer to us and much simpler from the chemical standpoint,” Carciofi said. The term Keplerian refers to the German astronomer Johannes Kepler (1571-1630). These disks obey Kepler’s laws of motion owing to the dominance of a massive body at their center.