ALMA Finds a 'Dragon Egg' Core That May Be a Massive Star in Its Earliest Stage

Astronomers using the Atacama Large Millimeter/submillimeter Array have found an unusually small, dense core that may be one of the earliest stages of a massive star's birth. They have nicknamed it the "dragon egg."

The core sits in an infrared-dark region called the Dragon Cloud, about 16,300 light-years away. The object, designated C2c1a, appears to hold roughly 20 to 30 times the mass of the Sun. The clump of gas and dust surrounding it contains about 1,061 solar masses but gives off little energy for its size, which the researchers say points to a very early stage of high-mass star formation. The paper, led by Ashley Thomas Barnes of the European Southern Observatory, has been accepted by Astronomy & Astrophysics and was posted to the arXiv preprint server on Oct. 5.

How stars more than eight times the Sun's mass form is still debated. One idea holds that a dense cloud first condenses into a massive "pre-stellar" core, which then collapses into a single giant star. The other says the star starts small and grows by pulling material from the larger clump around it. Astronomers think that directly spotting a compact, massive, young concentration of mass could tip the argument. Such cores are hard to find, and nearby star formation and stellar activity often muddy their interpretation.

Earlier data could not say whether C2c1a was a single compact object, where its mass was concentrated, or whether a protostar had already formed. The new ALMA Band 6 observations, at 1.3 millimeters and a resolution of 30 milliarcseconds, are among the highest-resolution and most sensitive of their kind for similar targets, the team writes. They show the main region has six components, with emission dominated by the central one, C2c1a(a), meaning most of the glow comes from a tightly concentrated spot. That core is embedded in a much larger reservoir of cold gas and dust of at least about 110 solar masses, which could supply the material needed to build a massive star.

The center of C2c1a(a) shows little glowing gas, only faint, spread-out emission. That could mean the core is cold, or that the dust is so thick it hides the glow. The researchers also found no clear sign of outflows, the jets that young stars blow out. They caution that a weak, deeply embedded or unfavorably oriented outflow cannot be excluded.

The Dragon Cloud's ratio of luminosity to mass is about 0.5, and a ratio below 1 typically marks a young clump. The team says the source's properties fit the initial conditions of "monolithic core-accretion models," in which a single massive core collapses into a star. They stress the data cannot rule out the other path, where the source keeps gaining mass from its surroundings. They also don't know the dust temperature accurately. The object could be a cold, massive core, or a warmer, less massive one inside a larger cold reservoir. Either way, it offers a rare look at the earliest stages of collapse.