Brazilian Group Unveils ‘new’ Star in Sagittarius

In March 2015, John Seach noticed that a “new” star had emerged in the constellation of Sagittarius observing from Australia. This constellation is formed by stars not very bright, in nights of Full Moon, for example, it is difficult to find it. Then when Seach saw a star shining a little more than its neighbors, he knew he had discovered a new one.

But what is a new one?

Despite the name, a new one is a binary system in which its components are quite old. Among them is a white dwarf, which many people call ‘stellar corpse‘ since it is the final phase of evolution of a star like the Sun. The white dwarf, because of its intense gravity, steals matter from its companion star. The gas, mainly hydrogen, escapes from the companion and is deposited on the surface of the white dwarf. During this transfer process, which can last from hundreds to thousands of years, the hydrogen husk gets dense more and more dense. When the density and temperature reach a critical value, the bark of gas detonates in a thermonuclear explosion, equal to the detonation process of thousands of pumps H. When this happens, the brightness of the system, which is quite low since the pair is not bright even, rises suddenly and can remain so for days, or even several weeks.

But why are old stars called new?

Because in the early days of astronomy there was no technology (nor knowledge) to identify the details of the sudden appearance of a star in the sky.

There are several new types, depending on the mass of the white dwarf, the mass transferred, the transfer time and various other factors. In any case, a new explosion is one of the most intense phenomena in terms of the release of energy that is known. In a particular case, when the white dwarf has mass near a boundary known as the Chandrasekhar boundary, the accumulation of matter may cause it to have a catastrophic thermonuclear explosion in an event known as type Ia supernova.

Going back to 2015, Seach deduced that there was a new event in the constellation of Sagittarius and triggered an international alert. New events are not as rare as that, about 50 per year are expected in the Milky Way, but actually a smaller number is observed because of the obscuration of galactic dust. The New Sagitarii n.02, one of the names that this star received, was highlighted because its brightness far exceeded the average. In one or two days it reached magnitude 4.3, which is the same brightness of the meddlesome star of the Southern Cross. That is, for a few days it was observable by the naked eye even in non ideal conditions. After almost 3 months the new began to fade and its shined fell fast in only a month, as you can see in this light curve, then increase in brightness gradually stabilizing at about 9.5, very weak even for small telescopes.

With so many attractions, the new one has been studied by professionals and amateurs all over the world, both on Earth and in space and it still yields subject! About two years after the event, a group of astronomers from the University of São Paulo led an unprecedented study of this type of event, using the ALMA observatory. To be fair the group includes an astronomer from Holland as well.

The ALMA observatory is a set of 64 mobile antennas located on a plateau in Chile at 5,000 meters altitude. The antennas can be arranged in some pre-set configurations to form a large equivalent antenna. In its more spaced configuration, the antennas working together can achieve resolution equivalent to a 16-km antenna! Resolution of a telescope or antenna is the instrument’s ability to separate and distinguish small structures. Precisely because of this, this was the configuration that the team of astronomers led by Marcos Diaz of USP used to study the New Sagitarii n.02. According to Pedro Paulo Beaklini, also from USP and who participated in the research, the greatest difficulty with the observations was the low signal intensity of the nova.

Watching in radio wavelengths, more specifically the microwaves, Diaz and crew were able to make unprecedented images of the white dwarf’s ejecta shell during the blast. The ejected gas quickly formed dust as soon as it cooled, which explains the sudden drop in brightness. But as gas and dust are expanding at an approximate speed of 650 km / s (2.3 million km / h!) The mixture has dissipated in space enough to allow a little more light to pass in the following July .

Most interesting was the discovery of small structures in the ejected shell. They are small agglomerations of gas and dust with sizes of about 1 billion km, larger than the orbit of Jupiter around the Sun! Several theoretical works predicted the formation of these agglomerations, they have even been identified very marginally in Hubble images or in the largest telescopes on Earth, but they had never been observed in so many details. For example, although the size of the agglomerations is larger than Jupiter’s orbit, the mass of hydrogen they contain is only slightly more than twice the mass of Earth.