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ALMA view on the stellar-merger site in V838 Monocerotis

Shortly after its discovery at the beginning of 2002, V838 Monocerotis (or V838 Mon) became an astronomical sensation. In a manner of weeks, the object increased its brightness by a factor larger than 1000 giving rise to a rare light echo. Pictures of the expanding echo from the Hubble Space Telescope are icons of modern astrophotography. The superluminal light echo resulted from scattering of the light of the eruption on interstellar dust grains. Equally extraordinary was the stellar eruption itself, which was due to a collision of two young stars.  Recent images obtained with ALMA show the merger site some 17 years after the stellar collision and partially explain the circumstances of the stellar cataclysm that took place in 2002.     


At maximum light, V838 Mon was one of the brightest stars in the Local Group of Galaxies. Since it was such a spectacular display of cosmic fireworks, it was all the more frustrating when astronomers could not classify V838 Mon  as a known or well observed stellar eruption. This debate persisted for a few years. The observational characteristics of the eruption excluded the possibility that it was one of classical novae whose eruptions are observed several times a year in our Galaxy. The extraordinarily high brightness of the object and its remarkably red color just after the explosion (easily seen in the center of the light echo images), led Romuald Tylenda from CAMK Toruń, Poland, and his collaborators to propose the idea that the 2002 event was a manifestation of a stellar merger. This hypothesis is widely accepted today, and objects similar to V838 Mon are known as red novae. We know only a dozen or so of such objects in our Galaxy and other nearby galaxies.


Animation: A time sequence of images of the light echo of V838 Mon. The “expansion” is only apparent, that is, the images do not show any movement of matter but rather illustrate changing zones within the interstellar medium where light of the stellar outburst is reflected towards us. The third image in the sequence was captured only in the blue filter. Source: Wikimedia.


Picture: The location of V838 Mon with respect to the Sun and to the Galactic Center. Media source: NASA/ESA.  


V838 Mon is located in the outer parts of the Milky Way, some 5.9 kiloparsecs or 20 thousand light years from the Sun. That is pretty far, especially if one would like to take a good picture of the merger site. In the visual and infrared light, no one had managed to obtain an image with a sufficiently high resolution. However, because the merger remnant is relatively cool,  for an object that has just experienced a stellar explosion, it is possible to observe V838 Mon at millimeter wavelengths (i.e., a type of light between the radio and microwaves regimes). It has been known for a few years already that the vicinity of V838 Mon is rich in molecules and dust, and the ALMA observations from 2019 have shown us the cool merger remnant in its full complexity. In order to obtain the image at a satisfactory resolution, the ALMA antennas had to be spread out to an area of a diameter of 15.2 km.



Picture: The ALMA interferometer at the Chajnantor Plateau in Chile. In the observations described in the text, the outermost antennas were separated by 15.2 km. Media source: ESO/ALMA.

So how does the merger site look like? The new interferometric maps show two stars surrounded by dust. One of the observed sources is identified as the star that formed from the coalesced binary (and is referred to as V838 Mon). At millimeter wavelengths we do not see the star directly but rather material directly heated by its radiation. From optical observations, however, it was found that it looks similar to red supergiants of a temperature of 3500 degrees Kelvin (and is very similar to Betelgeuse). The red supergiant is a source of an outflow or sort of stellar wind, much denser than that of the Sun. The second source spotted by ALMA is related to the distant companion of V838 Mon. It is a hot star of type B (temperature of 18 000 Kelvin). We have known about its existence for years, but this is the first time we directly see its location with respect to the V838 Mon. The companion is located 250 astronomical units (that is, 250 times the average distance of the Earth from the Sun) from the object that underwent the merger. 

It was estimated that the V838 Mon merger happened due to a collision between stars of masses of 8.0 and 0.4 times the mass of the Sun and the third star, the companion, is a star of 8 solar masses. The triple nature of the system is very interesting because it has been long predicted that some triple or higher-multiplicity systems undergo a special kind of orbit instability, caused by the so-called Kozai-Lidov cycles, which can subsequently lead to mergers. It is still uncertain whether this scenario took place in the V838 Mon system, because the companion is at a very wide orbit. If the orbit is circular, the orbital period of the companion is close to 1000 years and strong interactions within the triple systems would be unlikely. However, if the orbit is highly elliptical, strong tidal interaction between the stars might have taken place in the past, possibly eventually leading to the collision of the inner binary. Regardless of the shape of the outer orbit, the case of V838 Mon is the only known collision in the so-called hierarchical triple system. 

Picture: Maps of V838 Mon system obtained with the ALMA interferometer. The left panel shows distribution of dust emission. Warm dust, heated by stellar radiation, reveals the location of V838 Mon and its distant companion (labelled as B). The middle and right panels show molecular emission as color maps (e.g. white and red represent regions with the strongest emission, and blue and purple show regions near the detection limit). Black contours plotted over those maps show dust emission, the same as in the left panel, to illustrate the relative location of the different components. Carbon monoxide (CO) is seen chiefly in the vicinity of V838 Mon, while molecules containing sulfur (sulfur dioxide, SO₂, and hydrogen sulfide, H₂S) are traced closer to the companion, where a shock wave is expected to form.

The data gathered by ALMA also show us the distribution of molecular gas that was dispersed during the 2002 cataclysm. Most details are seen in emission of simple species, such as CO, SiO, SO, SO₂, and AlOH. The matter ejected during the stellar collision moves fast, with velocities of 200 km/s (720 000 km per hour) or higher. From observations obtained prior to the ALMA data described here, it was known that the ejecta must have reached the distant companion some time around 2005. Since the material was abundant in dust, the ejecta completely obscured the blue star so that no visual observations of this component were possible in the last 14 years. Fortunately, ALMA can “see” stars even through thick clouds of dust and, as mentioned above, it is actually due to the presence of warm dust close to this hot star that we could identify its location in ALMA maps. In 2019, the merger ejecta reached out far beyond the orbit of the companion and formed a roughly spherical nebula around V838 Mon. Emission of the mapped molecular species shows that within the nebula some molecules are only present close to the companion, others seem to disappear in its neighborhood. The changes in the chemical composition of the gas are caused by shock waves imposed in the flow of matter by the companion, mainly through its gravity. This effect is known from astrochemical studies, but has rarely been observed directly, as in the V838 Mon system. Because the current chemical models of this kind of interaction lack good predictive power, especially when it comes to the gas composition after the shock passage, the recent ALMA observations can prove helpful in such astrochemical studies.  

The sequence of events in the V838 Mon system was reproduced in the artistic visualization: https://youtu.be/9Wb4bUqt7dY

Two decades ago it seemed impossible to catch a stellar collision in the act. Today not only we study the collisions and mergers of red novae, but also, owing to millimeter-wave interferometry, we can study finer details within merger sites, allowing us to learn more about the circumstances under which stars coalesce. Future research will determine whether newly formed merger products such as V838 Mon are rapid rotators and have strong magnetic fields, as predicted by theoreticians.

The ALMA observations of V838 Mon were made by a team led by T. Kamiński from Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences. The research was funded by the Polish National Science Center. Polish astronomers have access to ALMA through their membership in ESO. 


dr Tomasz Kamiński


Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences

ul. Rabiańska 8, 87-100 Toruń

Original article: V838 Monocerotis as seen by ALMA: a remnant of a binary merger in a triple system, T. Kamiński, R. Tylenda, A. Kiljan, M. Schmidt, K. Lisiecki, C. Melis, A. Frankowski, V. Joshi, & K. M. Menten, Astronomy & Astrophysics

Preprint at: https://www.aanda.org/articles/aa/pdf/forth/aa41526-21.pdf