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Astrophysics

Where stars are born

Unprecedented mapping detects 150 nurseries in the Southern Hemisphere

ALEXANDRE ROMAN LOPES /IAGBehind the clouds: star nursery (blur on the right) of the constellation Norma, 10,400 light years away from Earth ALEXANDRE ROMAN LOPES /IAG

In an unprecedented mapping of the sky of the Southern Hemisphere, São Paulo astronomers have found much more than they were expecting in the so-called star nurseries [RJS12], regions of our galaxy, the Milky Way, which shelter stars in the initial stage of life; they are little studied and difficult to observe, as they are wrapped in dense dark clouds of gas and dust.

In less than one year, the researchers from the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) of the University of São Paulo (USP) detected 157 probable birthplaces of stars, in areas of the skies with dimensions that correspond to one tenth of the surface of the Sun, seen from the Earth. Of the 46 clouds analyzed in the last two months, 32 (70%) were home to rare stars, with very high masses and a few million years in age – veritable babies, compared with the 14.5 billion years of the most ancient in the Universe.

The stars had not been seen before, because of the density and the size of the so called molecular clouds, some with 4 light years in length (one light year corresponds to 9.5 trillion kilometers), about 270,000 times the distance of the Earth from the Sun. For not permitting the passage of light, they are an insurmountable barrier for the most powerful optical telescope. The group from the IAG managed to beat the clouds with an infrared detector camera, able to capture the energy emitted by the young stars in the form of heat – the infrared radiation is not absorbed by the dust in the clouds.

This camera is the only one in the country. Bought for US$ 300,000, it consists of a metal box of 40cm by 30cm, which houses a detector kept in liquid nitrogen, at minus 203ºC, to avoid any interference from the heat of the environment. It works coupled to one of the IAG’s optical telescopes, installed in the National Astrophysics laboratory (LNA), in Brasópolis, Minas Gerais, and refined the information from two other sources, the Infrared Astronomical Satellite (Iras) and the Swedish-ESO (European Southern Observatory) Submillimetre Telescope (Sest), at La Silla, in Chile. It was this that allowed the group from the IAG to be bold. “We wanted to study stars that we didn’t even know if they really existed”, comments Zulema Abraham, the coordinator of the work.

The dozens of stars from these first recently discovered areas have a mass 15 to 50 times greater than the Sun’s 2 octillion (the number 2 followed by 27 zeroes) of tons. They are between four to eight times hotter than the Sun, with a surface temperature varying from 25,000 to 50,000 degrees Celsius, and are located at distances that vary from 600 light years to 18,000 light years from the Earth (the Sun is 8 light minutes away). And they reveal a peculiar characteristic: they die early. In a few million years, they consume the fuel that keeps them active – hydrogen, constantly converted into helium – and they disappear in a sort of implosion, giving rise to a black hole, a region of space with such a high density that it absorbs everything around it, including light.

“If we do not watch them inside the clouds, they may disappear without being known”, comments Alexandre Roman Lopes, from the IAG group. The analysis of these regions will help to estimate if the stars are formed equally in all the regions of the Milky Way, or if there are areas that favor the condensation of the gases that originate them. Up until now, the data corroborates one of the hypotheses of the theory for the evolution of the stars: very dense regions in the insides of clouds of gases and dust contain stars in formation.

Where to look
The group from the IAG achieved relatively quick results because they successfully resolved a primordial issue, without which the infrared detector would be of no use: knowing in which region of space to look for the molecular clouds. These areas where stars are formed, rich in hydrogen, carbon monoxide, ammonia and helium, are concentrated in the direction of the center of the galaxy, but looking for them at random would be an impossible mission. The obvious way was to look for the signature of the recently formed stars: the clouds of gas, ionized by the energy emitted by the stars contained inside them, known as HII regions. But, besides a natural obstacle – the radiation emitted by these regions is absorbed by the molecular cloud -, the team from the IAG intended to observe new regions, still not classified as HII. The innovative solution for hitting the target without wasting time was to associate clues from different sources of observation.

First, Zulema selected 1,427 probable nurseries registered by Iras, which, in ten months of operation, has found more than 320,000 infrared radiation emitting sources, 5,000 of which alone in the Milky Way. Next, it found that it needed one more element to suggest that a given region could contain stars in formation. In search of indirect evidence, the team assessed a map of 873 sources of radio waves, with a wavelength going from a millimeter to hundreds of kilometers, produced by the observatory at La Silla. Emissions in this range of energy are a kind of fingerprint of molecules found in very dense clouds, possible areas for the formation of stars.

After crossing the two kinds of data, the researchers limited even more the probable nurseries, went on to the LNA, and spent 15 nights examining the birthplaces of stars in the range of near infrared – radiation with a wavelength of between 0.8 to 2.5 micrometers, a millionth part of a meter. The work has hardy started: in the Southern Hemisphere alone, there are at least 600 nurseries. It is not known for sure how many exist in the Northern Hemisphere, but the Southern one is probably richer, since it houses the center of the galaxy, where more stars are formed.

By the end of the year, having concluded the analysis of another 111 regions, the astrophysicists from the IAG intend to select a few areas for detailed investigation, with a telescope from the LNA with a mirror of 1.6 meters in diameter, more powerful than the 0.6 meter one used in this work. The next chapters should also be able to count on the telescope of the Itapetinga Radio Observatory, in Atibaia, with an antenna of 13.7 meters on diameter, which will be able to detail the temperature, density and extent of the clouds of gas that surround the star nurseries.

The Project
The Galaxy and the Formation of Stars
MODALITY
Thematic project
COORDINATOR
Zulema Abraham -IAG/USP
INVESTMENT
R$ 3,133,845.64

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