Cosmic sneezes

Astrophysicists describe evolution of jets of gas 10 light-years in length

The image that appears on the computer screen is reminiscent of the movement of a comet – a huge bright and colored trail, which seems to rip the skies. But this is the only similarity between these clouds of ice and dust that travel around the Sun and the supersonic astrophysical jets – or clouds of gases violently ejected by stars in formation and by the nucleuses of distant and very bright galaxies called quasars. The jets released by the protostars – stars in formation – travel at a speed close to 400 kilometers a second, equivalent to dozens of times the speed of sound in the interstellar medium, and they can reach a length of as much as ten light-years (one light-year corresponds to 9.5 trillion kilometers).

Then there are the extragalactic jets, which move at speeds close to the speed of light (300,000 kilometers a second) and extend for millions of light-years, which is equivalent to dozens of the times the diameter of our galaxy, the Milky Way. “The jets bear the fingerprints of embryonic stars”, explains Elisabete de Gouveia Dal Pino. She is one of the coordinators of a team from the Astronomy, Geophysics and Atmospheric Sciences Institute of the University of São Paulo (IAG/USP), which for ten years has been studying the origin, the behavior and the peculiarities of these celestial bodies. “As we unveil their enigmas, we succeed in understanding better how stars are formed.”

And, do not forget, it is the fusion of chemical elements in the insides of stars that spawns the raw material of every living being. Accordingly, investigating the jets and the stars means looking for our remotest origins. Coordinated by Elisabete and by Jorge Horvath, the group from USP has been achieving pioneer results in the study of high-energy astrophysical phenomena, which include the mechanism of the explosions of supernovas, neutron stars, and bursts of gamma rays. Last year, the São Paulo researchers surprised the Europeans and Americans who work in the same area by concluding the first hydrodynamic simulation on a computer of the HH-34, a giant protostellar jet that has been much studied, located to the south of the Orion nebula, some 1,600 light-years from the Earth. The universe recreated on the computer has revealed that the HH-34 is one of that kind of immense jets, measuring about 10 light years from end to end.

An even larger giant
This confirmed the results achieved a few years previously by another research group. Between 1994 and 1997, astronomers John Bally, David Devine, Bo Reipurth and Steve Heathcote, of the United States, observed a chain of small knots of gas, bright and aligned with the HH-34 jet, in those days still regarded as small. It used to be believed that these nodules were independent, but physicists showed that they were actually ejected by the same protostar, forming what appeared to be a gigantic jet.

“Our numerical simulations reproduced quite accurately the observations and left no doubt that we are in fact dealing with single structure, one mega-jet”, notes Elisabete, one of the authors of the article with the detailed results, published in July 2002 in the Astrophysical Journal. This gigantic jet, with its sinuous shape, had already caused an about-face in the studies on the formation of stars. In accordance with what was known, the jets from protostars seemed to be less than one tenth of the size of the HH-34, even when it appeared on its own and isolated from the nodules which are today regarded as part of its body.

Supported by FAPESP, this work represents a thorough look at astrophysical jets, starting with the HH-34. It is now known that there are nodules in its structure that researchers attribute to the fact that the star, when being formed, releases gas in an intermittent, non-continuous manner. The study details the evolution of the jet from the initial moments of its formation until being expelled and interacting with the gas of the interstellar medium, during its 10,000 years of life. With the benefit of this detailing, the HH-34 has become a scientific landmark for the behavior of astrophysical jets.

The group from USP is concentrating on the study of the jets from protostars for one basic reason: this kind of jet is quite common in regions close to formations of stars, inside our own galaxy. They are therefore more easily observable than the second group of jets, the extragalactic ones, expelled by quasars – nucleuses of distant galaxies with black holes with some 100 million times the mass of the Sun. “In spite of the differences in origins, size and velocity, the extragalactic jets show very similar shapes and behavior as those of the protostar jets”, the researcher says. “The information obtained in the study of the protostar jets helps us a lot to understand the phenomena that occur in those that are produced in remote extragalactic regions.”

Their life history is not all that distinct. In the case of the protostars, the cloud of primordial gases – such as hydrogen, helium and oxygen – condenses under the action of gravity and makes a central core, which will give rise to the star, surrounded by a gas disc, which spins with an increasingly faster rotation. When the rotation meets a speed limit, to the point of preventing the condensation from continuing, the star violently expels clouds of gas from its axis of rotation. This is how the jet is formed, and, at the same time, reduces the speed of rotation both of the cloud and of the star. As a consequence, the condensation of the protostar continues, resulting in a mature star like the Sun.

But the course of the jet that left the star is not always a tranquil one. If it collides with other clouds of gas, the jet forms supersonic impact waves, known as bowshocks, interacts with the interstellar medium, and deposits in that region part of the material that is brought from the star – a spectacle of brightness and colors. At this moment, the jets act like bees that are carrying star seeds and pollen to regions far away from the spot where they were generated. According to the researcher, the jet can induce the birth of a star, when it collides with a cloud with sufficient mass to implode – or, in technical language, to go into gravitational collapse. Larger jets may even destroy the giant cloud that houses them. “It is possible for that to happen, but there has been no direct observational evidence”, the researcher says with caution.

It was Elisabete herself, in 1993, when she was doing postdoctoral studies at Harvard University, in the United States, who developed one of the techniques used in computer simulation of astrophysical jets, in partnership with the Swiss physicist Willy Benz, in those days a visiting researcher at Harvard. Until then, the calculations were based on the numerical integration of the hydrodynamic equations for the jet, with the computational domain – a virtual space that simulates real space – divided into a network of fixed points. This new technique, called SPH (Smoothed Particle Hydrodynamics), replaced the network by particles that move with the fluid. This change made the simulations quicker and more feasible on more modest computers, like the ones the researcher had to go back to using after returning to Brazil, that same year.

The recreation of the universe
Virtual reality is managing to reproduce what is happening in the universe with increasing accuracy. On the computer, the gas in the interstellar medium is represented by a box in the shape of a cobblestone, filled with gas. At the base, there is a hole, through which the jet from the protostar is injected. Inside it, the jet is accelerated supersonically, forms the nodules of gas that move at a supersonic speed, and interacts with the gas in the interstellar medium. Obtained in conjunction with the National Autonomous University of Mexico; Elena Masciadri, from the Max Planck Institute for Astronomy, in Heidelberg, Germany; and Adriano Cerqueira, State University of Santa Cruz, in Bahia, the results are next compared with the astronomical observations, above all those obtained from the European ESO telescope, located in La Silla, in Chile, and by the Hubble space telescope, the group’s two usual sources of information.

As knowledge about astrophysical jets becomes consolidated, the team is diving into the disc of gas that forms the jets, in search of particulars of the phenomena that go on in the regions closest to the star in formation. The results that the researchers are arriving at emerge at a moment that new information on the universe itself is bubbling over.

The astrophysicists are working intensely on the wealth of raw material captured by the Wilkinson Microwave Anisotropy Probe (WMAP), the American satellite that in February, in its first batch of results, gave the universe an age of 13.7 billion years, a sure fate – expanding for ever – and limits estimated today at 15 billion light-years. “After hundreds of years in which the observations with telescopes only managed to go as far as the back yard of our galaxy, we are seeing further and further”, the researcher points out. “Astronomy has come out of its adolescence and reached maturity.”

The project
Investigation of High-Energy Astrophysical Phenomena (nº 97/13084-3); Modality Thematic project; Coordinator Elisabete de Gouveia Dal Pino – IAG/USP; Investment R$ 294,713.73