How do massive stars originate in the Universe? Are they the result of the processes of gravitational collapse and gas and dust accretion? The discovery of a collapse adjacent to the luminous massive star G24.78+0.08 A1, recently published in the journal Nature by an international team provides a further boost to theoretical predictions on massive star formation by accretion through a disk or torus, an identical process to low-mass star formation.

The study is a step towards constraining theoretical models on massive stellar body formation. It was produced by an international team led by the researcher on the Juan de la Cierva program Maria Teresa Beltrán, of the Departament d'Astronomia i Meteorologia at the UB. The project has also had the active support of Riccardo Cesaroni, Claudio Codella, Leonardo Testi and Luca Olmi, of l'Osservatorio Astrofisico di Arcetri (Italy), ), and Ray Furuya, of the Subaru Telescope at the National Astronomical Observatory of Japan.

Collapse, expansion and rotation play a fundamental role in the process of star formation. This at least is the scenario for low-mass stars like the Sun, formed by a process of gas and dust accretion, in which both observational evidence and theory point to the existence of circumstellar matter having fallen onto the newly-formed star. The formation of a star eight times more massive than the Sun through accretion has to overcome major obstacles. The strong stellar wind and the star's intense radiation pressure may stop accretion on the central star and therefore prevent the development of its mass. This is a fundamental problem in the field of massive star formation, and presents us with a paradox: stars with more than eight solar masses should not exist. Recently some theoretical research has put forward a possible scenario to solve the problem: massive stars are formed through nonspherical accretion (that is, accretion through a disk or torus). According to this theory, part of the stellar photons can escape along the axis of rotation, where the density has reduced considerably.

This article presents significant evidence of the collapse in a circumstellar torus in rotation around the star G24.78+0.08 A1, which is twenty times more massive than the Sun. This exceptional astronomical discovery is one of the few direct detections of a collapse in a young high-density star reported to date. It also reveals for the first time a torus simultaneously rotating and collapsing, a bipolar molecular outflow from the star, and an ultra-compact ionised region around the star. The discovery was made through observations of the ammonia molecule at a 1.3 cm wavelength, carried out using a network of twenty-seven antennas of the Very Large Array of the US National Radio Astronomy Observatory.

The area studied, G24.78+0.08, is a region of massive star formation located at an approximate distance of 7.7 kiloparsecs. The scientific team has studied the region in depth from different observatories and have published their findings on many occasions in specialised journals. The region contains a cloud of young stellar objects, some of which are associated with a rotating torus. One of these young objects, G24.78+0.08 A1, is generating a strong high-speed bipolar outflow and, in addition, contains an ultra-compact ionised region of roughly 1,500 astronomic units in diameter, located at the centre of the torus. The spectrum in the continuum of this object would correspond to that of a zero-age main-sequence star of spectral type O9.5 (that is, approximately twenty times more massive than the Sun), and with a high luminosity (33,000 times that of the Sun).

Since the ionised region is very bright at centimetric wavelengths and its temperature surpasses 2000º K, the molecular gas – cooler, at 100º K - in absorption is easy to observe above the continuous spectrum emission. If the rotating torus were also collapsing towards the central star, one would expect to observe this absorption diverted to speeds positive with respect to the stellar velocity, due to a Doppler redshift effect.

Indeed, the observations of ammonia in G24.78+0.08 A1 have revealed that the emission of this molecule is in absorption towards the region ionised by the star, with the maximum absorption – observed at the exact centre of the rotating torus – redshifted at approximately 2 km/s with respect to the stellar velocity.

According to the authors, the gas detected in absorption is approaching the ultra-compact ionised region at an approximate speed of 2 km/s. This confirms the existence of a collapse onto the O type star located at the centre of the torus. In agreement with the properties of ammonia, the accretion rate of the material over the central star is high enough to overcome the star's radiation pressure occurs, thus enabling the star to continue gaining mass by accreting the surrounding material.