Astronomers have finally obtained detailed observational confirmation of the existence of the "inflated star" – a brief and previously elusive phase in the formation of massive stars. The object IRAS 19520+2759, located approximately 29,339 light years (9 kiloparsecs) from Earth, turns out to be a young O-type star with a mass of 10–15 solar masses, which is roughly 2.2 x 10^33 to 3.3 x 10^33 pounds. This star is in a stage of intensive accretion where the protostar temporarily increases in size and remains comparatively cool.
This research is based on high angular resolution observations conducted using ALMA and EVLA radio interferometers. Their combination enabled a detailed study of the gas and dust structure in close proximity to the object, confirming key predictions of massive star formation theory, which had long lacked direct observational proofs.
IRAS 19520+2759 has long attracted astrophysicists due to its unusual set of properties. With high luminosity exceeding 100,000 times that of the Sun, the object has a comparatively low effective temperature and emits almost nothing in the centimeter radio range. For ordinary O-type stars, such a combination is impossible, but it is precisely what is predicted for the "inflated star" stage when intense accretion suppresses ionizing radiation.
Observations by EVLA revealed an elongated source of radio emission with a spectral index of 0.54 ± 0.04. This value indicates the thermal nature of the emission and suggests the presence of an ionized radio jet – a stream of gas ejected from the protostar’s vicinity. This is a direct sign of ongoing accretion.
Meanwhile, ALMA data showed a compact dust core coinciding with the star’s position, as well as several molecular outflows. The main outflow has a clear bipolar structure, and its orientation matches the direction of the radio jet, indicating a common source of these flows.
The key results are observations of hot molecular gas via sulfur dioxide (SO2) lines. A sustained velocity gradient oriented perpendicularly to the main outflow was detected in the velocity distribution. This geometry is a classic indication of a rotating circumstellar disk, through which material is supplied to the forming star. Keplerian rotation describes how the speed of gas in this disk is determined by the gravitational pull of the central object. Analysis of this disk’s kinematics showed that gas moves in a Keplerian manner, meaning that its speed is dictated by the gravity of the central object. Modeling position-velocity diagrams allowed a direct estimate of the protostar’s mass within the range of 10–15 solar masses – the first direct measurement for an O-type "inflated star" candidate.
The combination of all observations – a massive rotating disk, active radio jet, molecular outflows, coupled with a deficit of radio emissions – provides compelling evidence that IRAS 19520+2759 is indeed in the predicted "inflated star" phase. This object has become a rare natural laboratory, offering insights into the early and most dynamic stage of forming the most massive stars in the universe.
