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An enigma in the Milky Way

International team identifies young stars in our galaxy with the chemical composition of old stars

Uncommon stars: artistic rendering of red giant stars with an unusual chemical composition, identified recently

Source Cristina Chiappini / ILA Uncommon stars: artistic rendering of red giant stars with an unusual chemical composition, identified recentlySource Cristina Chiappini / ILA

The discovery of relatively young stars with a chemical composition typical of old stars proves that a method known as the Milky Way “chemical clock,” used to estimate the age of far-away stars in our galaxy, does not always work. These stars were identified recently by an international team of astronomers coordinated by Brazilian Cristina Chiappini and described in an article in the April 2015 issue of the journal Astronomy and Astrophysics. The origin of these young stars that look old, however, is still a mystery.

A researcher at the Leibniz Institute for Astrophysics in Potsdam, Germany, Chiappini noted the existence of these uncommon celestial objects when her PhD student Friedrich Anders showed her an analysis of 622 stars from various sections of the Milky Way’s disk. Chiappini develops stellar chemical evolution models in order to deduce when and where the galaxy’s stars were born. One of the predictions of these models is that the more iron atoms a star has in relation to “alpha-type” chemical elements, the younger the star is.

057-059_Estrelas novas_232In order to check this prediction, Anders compared the chemical composition of the stars, obtained by astronomers from the Apogee survey, with the age of the stars, calculated by researchers from data obtained from the CoRoT space telescope. Apogee is investigating the evolution of the galaxy using instruments sensitive to infrared light mounted on the 2.5 m telescope at the Sloan Observatory, in New Mexico, United States. The CoRoT, in contrast, is a satellite developed through a French-European-Brazilian collaboration that allows researchers to investigate the internal structure of stars and determine their age.

Anders confirmed that the ages of most of the 622 stars, determined by the CoRoT, agreed with the age range suggested by their chemical composition. However, about 20 of these stars stood out because they had proportionally more alpha chemical elements than iron for their ages. “We thought that something strange was happening,” recalls Chiappini.

Intrigued, Chiappini and Anders asked one of their colleagues on the CoRoT project, astronomer Benoit Mosser, of the Paris Observatory, to re-analyze the data on each of these stars in detail in order to better calculate their ages. The confirmation of the ages of the iron-poor stars was surprising. “They are too young,” says Chiappini. “One of them, for example, has the proportion of chemical elements expected of a star 10 billion years old, but it is only 2 billion years old.”

Except in very special circumstances, astronomers have a hard time calculating the age of Milky Way stars located more than 80 light-years away from the Sun. Most telescopes are unable to determine the properties of stars so far away with the precision necessary to calculate their ages. There is, however, a less precise way to estimate whether a far-away star is very new or very old by examining its chemical elements.

A broken clock
This method is called the “chemical clock,” and is based on the following rationale: the first stars in the galaxy supposedly formed from primordial gas, composed solely of light elements—hydrogen, helium and a bit of lithium—created during the Big Bang, the event from which the universe is believed to have originated. The explosive death of giant stars, with masses 8 to 10 times greater than the Sun’s, would have added heavier chemical elements to the primordial gas, especially what are called the alpha elements: oxygen, magnesium, silicon, calcium and titanium, created from the fusion of helium nuclei inside these stars.

These explosions, known as type II supernovae, are the principal sources of these chemical elements in the galaxy. However, most of the iron in the Milky Way comes from another type of supernova, type Ia. They are white dwarf stars that, after sucking a certain amount of gas from a neighboring giant star, eventually explode, spewing iron atoms across the galaxy.

Type II supernovae take millions of years to explode, while type Ia supernovae take much longer to explode, on the order of billions of years. This difference in supernovae time scales acts as a time marker for estimating the birth date of stars in the Milky Way. Thus, the greater the percentage of alpha elements in a star in relation to iron, the older the star must be.

Until these 20 uncommon stars were identified, the “chemical clock” method always seemed to work. In all cases in which it was possible to take measurements that allowed the calculation of the age of a star, the figure that the astronomers calculated corresponded well to that obtained via the “chemical clock” method.

In 2012, Chiappini and her colleagues drew attention to the fact that one could use the CoRoT space telescope to calculate the ages of stars located more than 80 light-years from the Sun. Previously, there had been no other method available except for the “chemical clock” method. “The CoRoT measures brightness variations from which we can obtain the radius, the mass, and the distance of the star,” she explains. “With this data we can calculate the age.”

Since then, Chiappini has been the liaison for a group of astronomers from specialties that do not tend to collaborate.  Called CoRoGEE, the collaboration is a partnership between researchers from the CoRoT, an instrument better known for its discovery of exoplanets, and researchers involved with Apogee, which also includes Brazilians linked to the Interinstitutional e-Astronomy Laboratory (LIneA) in Rio de Janeiro. It was comparing data for the same stars obtained both by CoRoT and by Apogee that led the researchers to discover the strange stars for which the chemical clock does not seem to work.

Chiappini suggests that “a young star with a high percentage of alpha elements compared to iron could be formed if a quantity of primordial gas that had not been very enriched by type Ia supernovae had been left over in some isolated section of space, and had not participated in the general chemical evolution of the galaxy.” This gas would have had to remain there for billions of years, without interacting with the gas in the rest of the galaxy, and only formed stars later.

CoRoT and Apogee data also suggest that the 20 young stars made of old material originated in a place in the Milky Way’s disk about 20,000 light-years from the center of the galaxy, near a structure called the bar. “In this region, the gas and stars in the disk are thought to rotate at the same speed as the gas and the stars in the bar,” explains Chiappini. “For this reason, it is more unlikely that collisions between gas clouds—needed to form stars—will occur. If this is, indeed, the case, this region could have harbored pockets of gas that maintained primordial characteristics.

Intergalactic gas
Another possibility is that these stars formed from a gas with primordial characteristics that fell into the Milky Way only recently, coming from the intergalactic medium. “But it is hard to understand why this would have happened closer to the center of the galaxy and not everywhere,” says Chiappini.

“This discovery is interesting because it shows that there are several processes occurring in our galaxy, especially near the central bar,” says astrophysicist Beatriz Barbuy, of the University of São Paulo (USP), who studies the chemical evolution of the Milky Way. “We know from observing other galaxies and dynamic models that the bars allow the migration of gas and stars in both directions, from the bar to the disk and from the disk to the bar.”

The researchers need to discover more of these stars in order to understand their origin. They hope that this will be possible by combining the data from the Kepler-2 space mission with that of Apogee-2, the new Milky Way star survey that is being developed by the Sloan Digital Sky Survey project.

Scientific article
CHIAPPINI, C. et al. Young [Alpha/Fe]-enhanced stars discovered by CoRoT and Apogee: What is their origin? Astronomy & Astrophysics. V. 576, L12. Apr. 10, 2015.