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Monday, November 29, 2021

A collision of two stars in a triple system. Polish astronomers explain the outbreak of V838 Monocerotis

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The star V838 Monocerotis in the constellation of the Unicorn fascinates astronomers. It gained attention in 2002, shining the brightest of all objects in the Milky Way. Polish astronomers explained the circumstances of this famous collision.

In 2002, a star known as V838 Monocerotis (abbreviated: V838 Mon) exploded. This object quickly became an astronomical sensation. Its brightness increased a thousand-fold in a few weeks, and the so-called light echo effect (as a result of the light from the flash scattering on the interstellar dust grains) was also observed. Images of this effect taken with the Hubble Space Telescope are one of the icons of modern astrophotography. At its peak, V838 was one of the brightest stars in the entire Local Group of Galaxies to which the Milky Way belongs.

Collision and coalescence of two stars

For several years, there was a debate over the explanation of this cosmic cataclysm, because its course did not match the previously known cosmic explosions. Professor Romuald Tylenda from the Toruń branch of the Astronomical Center Nicholas Copernicus of the Polish Academy of Sciences (CAMK PAN), together with his colleagues, proposed an explanation that there was a collision and coalescence (merging into one object) of two stars. Currently, this hypothesis is widely accepted, and objects of this type are called new red.

– Thanks to the case of V838 Monocerotis and similar objects, we know that mergers of normal stars do indeed occur in our galaxies and other galaxies, that is, in the cosmos in general. And this happens much more often than black hole or neutron star mergeers, explained Professor Tylenda.

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Currently, a team of scientists led by Dr. Tomasz Kamiński (CAMK PAN) has analyzed the state of the V838 explosion site after several years. The large ALMA network of radio telescopes operating on the Chajnantor plateau in Chile and operated by the European Southern Observatory (ESO) was used for this.

The maps obtained with the ALMA interferometer show two stars surrounded by dust. One is a remnant of the collision, and the other is a distant companion that has long been known to exist, but its exact location is unknown. According to ALMA observations, the companion is 250 AU from the remnants of V838 Mon (i.e. 250 times further than the Earth from the Sun).

This means that the two stars actually collided in a triple system. According to the findings of astronomers, the components of the internal binary system, having masses of 8.0 and 0.4 times the mass of the Sun, took part in the collision. The distant companion, on the other hand, has a mass of 8.0 solar mass and is a hot star.

Data from the ALMA telescope also shows the distribution of molecular gas scattered during the 2002 eruption. Details can be seen thanks to the emissions from molecules of carbon monoxide (CO), silicon oxide (SiO), sulfur oxide (SO), sulfur dioxide (SO2), and aluminum hydroxide (AlOH). Matter that was ejected from a star collision travels at speeds of about 200 kilometers per second (72,000 kilometers per hour) or even greater. It is known from earlier observations that some of this matter reached the vicinity of the hot companion as early as 2005. This matter is so rich in dust that it completely obscures the visible light emitted by the companion, so his observations were not possible in the optical range for 14 years. Fortunately, ALMA can “see” stars even through dense dust clouds, and even – paradoxically – it is thanks to the presence of dust that you can now see this star on ALMA maps.

V838 Monocerotis star outburst NASA, ESA

Cosmic explosions. What are red new?

In 2019, previously ejected material had already traveled far beyond its companion’s orbit and formed a spherically symmetrical nebula around V838 Mon. However, if you look closely at the molecules covered by the ALMA observations, you can see that some of them are present only in the vicinity of the companion, and others just disappear there. This is the effect of a change in the chemical composition of the gas, mainly due to the shock waves caused by the star’s gravitational interaction with the matter flowing around it. This effect is known from astrochemical research, but has never been observed so directly.

How is the red new different from other types of cosmic explosions? As Dr. Tomasz Kamiński explains, the energy of the new red explosion, i.e. the energy associated with the significant brightening of the object and the ejection of matter, comes mainly from the gravitational energy of the star system. This distinguishes red new, or more generally, merdzer phenomena from many other types of stellar outbursts, such as new classical or supernovae, where the energy of the blast comes mainly from thermonuclear reactions, which in turn require very high temperatures. The red new ones do not require such high temperatures and that is one of the reasons why they are rich in dust and molecules.

Just two decades ago, it seemed impossible to observe star collisions. Today we know about a dozen objects of this type, both in our and neighboring galaxies, we not only study stellar collisions of red nova, but thanks to the use of millimeter interferometry techniques we can see details in objects shortly after the catastrophic act of stellar cannibalism.

‘We expect to see many more of these objects with a new generation of observatories such as the Vera C. Rubin Observatory,’ said Professor Tylenda. Objects like the V838 Mon give us a glimpse of the most extreme form of interactions between stars, including the so-called common envelope phase. We hope to find out when and why collisions occur by studying the V838 Mon and similar crash cases in detail. It is also important to find out what kind of stars are formed as a result of such collisions, because it is suspected that many known stars were formed in this way, for example Betelgeuse or the star that exploded as Supernova 1987A – added Dr. Tomasz Kamiński.

The research results were published in the journal “Astronomy & Astrophysics”. The research team from Polish centers included: Tomasz Kamiński (CAMK PAN, Toruń), Romuald Tylenda (CAMK PAN, Toruń), Aleksandra Kiljan (Astronomical Observatory of the University of Warsaw), Mirosław Schmidt (CAMK PAN, Toruń), Krzysztof Lisiecki (Institute of Astronomy UMK, Toruń), Adam Frankowski (CAMK PAN, Toruń) and researchers from the USA, India and Germany.

Main photo source: NASA, ESA

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