Research shows chemical evolution of galaxies
November 30, 2011
By Janaína Simões
Agência FAPESP – Just as the wind blows dust on Earth, the stellar winds blow matter outside the stars throughout the lives of the stars. Stellar winds interest astronomers because it is a preliminary phenomenon to the end of a star’s life cycle.
Discovering the chemical composition of these winds and the influence of this composition in the process of stellar mass loss is the subject of Graziela Keller’s doctoral thesis project, through a FAPESP Fellowship.
The study is part of the Thematic Project “Photoionized nebulae, stars and the chemical evolution of galaxies,” coordinated by Walter Maciel, professor of USP’s Astronomy, Geophysics and Atmospheric Sciences Institute (IAG) and funded by FAPESP.
Maciel is at the helm of a group that studies the chemical evolution of galaxies, or rather, how chemical elements can change with time and their position inside galaxies. In the Thematic Project, the focus is the central stars of planetary nebulae.
“The changes will depend on the evolution over time. Therefore, we need to know their age. We are calculating variations in the chemical composition, but we need to know to what stage of a galaxy’s life they apply,” says Maciel.
“The chemical composition of the Milky Way is different from 5 billion or 10 billion years ago. We need to study objects that have ages corresponding to each of the phases of a galaxy’s lifespan, and to do so, we must calculate the ages of each object in the study,” he explained.
The central stars of the planetary nebulae studied by IAG group are very evolved phases of the lives of stars like the Sun. “They have already lost their entire ‘envelope,’ that is, the planetary nebula that was surrounding them. What they are showing now on the surface is the chemical composition that once was inside the star, something we couldn’t see,” said Keller.
In observing these stars, the researchers that helped to test and perfect the models of evolution and star structure have already been described by science. The loss of matter through stellar winds is related to the luminosity of the stars and basically the decomposition of light, through spectroscopy, which tells what a star is made of. With this scientists calculate the metallicity, or rather, what chemical elements are in its make-up and what quantity.
One scientific hypothesis to explain the winds is radiation pressure: the light generates pressure, pushing the matter of the star’s most external layers. “Depending on the chemical element that is present in that matter, the light will push the wind more or less. If we know what chemical elements are present, we can say that the model is capable of generating the loss of mass we have seen or not,” says Keller.
To study the winds, she used stellar atmospheric codes, developed by other scientists over several years. She spent a year at Johns Hopkins University in the United States to learn how to use a computer program called CMFGEN, which helps conduct calculations and determine the physical characteristics of central stars in planetary nebulae.
“These codes simulate what we are observing. We gave all the characteristics of the star and the code gives us the star’s spectrum, or rather, the division of light in varied colors,” explains Keller.
By comparing all the spectra simulated by the codes with the observed spectrum, one can determine the stellar mass, its surface gravity, luminosity, mass loss rate, wind velocity and chemical composition. “If we know what chemical elements are present in the surface of these stars, we can determine which mass loss mechanisms are capable of accelerating what we observe,” he says.
Keller also studied the instabilities caused by the mechanism of wind acceleration. The quicker the wind, the greater the force of the pull and vice-versa. This process increases the velocity, creating wind collisions that cause so-called inhomogeneity – characteristics of body that does not have the same properties at every point. In the case of wind, movement generates thinner air interspersed with denser regions. The in-homogeneities impact what is observed in the star.
In order to study this aspect of stellar winds, Keller utilized another type of computational code, H-DUST, developed by researcher Alex Carciofi, also of IAG-USP. It serves to stimulate what occurs with light when it passes through a star’s atmosphere, but is tridimensional.
This data can be compared to those generated by the CMFGEN code used by her in the United States, showing that what she adopted as inhomogeneity of the winds in the first half of her doctorate is close to the forecast developed by the Carciofi’s tridimensional code system.
Age of the stars
The Thematic Project coordinated by Maciel also developed two new models to calculate the age of stars located in the center of nebulae. The team had already developed three methods, whose results were published at the beginning of 2010 in the magazine Astronomy and Astrophysics.
Initially, they analyzed a sample of 230 nebulae among the roughly 2,000 existing planetary nebulae in the Milky Way. Now, in the study “Kinematic Ages of The Central Stars of Planetary Nebulae”, published in the October print edition of the Revista Mexicana de Astronomía y Astrofísica, the group is presenting the results of the application of kinematic methods that it developed to calculate the age of stars.
“Under the kinematic method, we can calculate the age based on its movement. The young stars in our galaxy spin around the center of the galaxy, but do not often move in a perpendicular direction. With older stars, it is the opposite: the greater velocity is in the perpendicular direction and the lower speed occurs during rotation. Additionally, the speed of these stars varies with time in a known manner,” explained Maciel.
The researchers calculated the age for two samples, one with 230 stars, chosen by the IAG-USP team, and another 900 stars from an international catalog. In addition to developing new methods, the objective of this phase of the study was to broaden the sample in relation to the previous study to prove the robustness of the method develop by researchers.
Just as in the first study published in 2010, in this one, using a different sample and method, the scientists reached the conclusion that greater majority of central stars in the planetary nebulae are younger than 3 billion years.
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