‘Old smokers’ and ‘screaming newborns’ among hidden stars spotted for first time

“Hidden” stars, including a new type of elderly giant nicknamed “old smoker”, have been spotted for the first time by astronomers. These mysterious objects exist at the heart of our galaxy, the Milky Way, and can lie quietly for decades, becoming almost invisible, before suddenly blowing clouds of smoke, according to a new study published in the journal. Monthly Notices of the Royal Astronomical Society.

An international team of scientists led by Professor Philip Lucas, from the University of Hertfordshire, have made a groundbreaking discovery after observing almost a billion stars in infrared light during a 10-year study of the night sky .

The articles “The most variable VVV sources: eruptive protostars, nuclear disk diving giants and others”, “Spectroscopic confirmation of high amplitude eruptive YSOs and diving giants from the VVV survey” and “On the incidence of episodic accretion in class I YSOs of the VVV”, have all been published in the Monthly Notices of the Royal Astronomical Society.

They also detected dozens of rarely observed newborn stars, called protostars, which undergo extreme explosions over a period of months, years or decades, as part of the formation of a new solar system.

Most of these newly spotted stars are hidden from visible light by large amounts of dust and gas in the Milky Way, but infrared light can pass through, allowing scientists to see them for the first time.

Astronomers from the United Kingdom, Chile, South Korea, Brazil, Germany and Italy carried out their research with the help of the Visible and Infrared Survey Telescope (VISTA), a construction telescope British located in the Chilean Andes at the Cerro Paranal Observatory, located within the European Southern Observatory (ESO).

The team kept a watchful eye on hundreds of millions of stars and analyzed 222 that showed the largest changes in brightness.

Professor Lucas said: “About two thirds of the stars were easy to classify as well-understood events of various types.

“The rest was a little more difficult, so we used ESO’s Very Large Telescope to obtain the spectra of several of them individually. A spectrum shows us how much light we can see at different wavelengths, which gives a much clearer idea of ​​what we are looking at.”

The work was carried out as part of a long-term investigation called “VISTA Variables in the Via Lactea” or VVV.

Dr Zhen Guo, formerly of the University of Hertfordshire and now based at Valparaiso University in Chile, led the work on the spectra.

He said: “Our main goal was to find rarely seen newborn stars, also called protostars, as they undergo a large explosion that can last for months, years or even decades.

“These explosions occur in the slowly rotating disk of material that forms a new solar system. They help the newborn star in the middle grow, but make it harder for planets to form.

“We don’t yet understand why the disks become so unstable.”

The team discovered 32 erupting protostars that increased in brightness at least 40-fold, and in some cases more than 300-fold.

Most of the flares are still ongoing, allowing astronomers to analyze for the first time many of these mysterious events as they progress from the initial quiescent state through peak brightness. and until the decline phase.

However, the study also revealed something completely unexpected.

There were 21 red stars near the center of the Milky Way that showed ambiguous brightness changes over the 10-year survey.

Professor Lucas said: “We didn’t know if these stars were protostars starting a flare, or recovering from a dip in brightness caused by a disk or dust shell in front of the star – or if it was were older giant stars that were spewing out material. in the final stages of their lives. »

Analysis of the spectra of seven of these stars, compared with data from previous surveys, concluded that they were in fact a new type of red giant star.

Professor Dante Minniti of Andrés Bello University, Chile, founder of the VVV survey, said: “These elderly stars sit quietly for years or decades, then blow clouds of smoke in a manner totally unexpected. They appear very dark and red for several years. , to the point that sometimes we don’t see them at all.”

Another clue to this new discovery lies in the location of these fading giant stars. They are heavily concentrated in the innermost part of the Milky Way, known as the nuclear disk, a region where stars tend to be richer in heavy elements than anywhere else.

This should make it easier for dust particles to condense out of the gas in the relatively cold outer layers of red giant stars. However, how this leads to the ejection of the dense puffs of smoke observed by the team remains a mystery.

The researchers said their findings could change what we know about how elements are distributed in space, as Professor Lucas explains.

“Material ejected from old stars plays a key role in the life cycle of elements, helping to form the next generation of stars and planets,” he said.

“This was thought to occur primarily in a well-studied type of star called the Mira variable.

“However, the discovery of a new type of star that sheds matter could have broader significance in terms of the spread of heavy elements in the nuclear disk and metal-rich regions of other galaxies.”