ALMA Spots Big Stars Eating Stellar Cocoons

Starbirth is a complex process. It doesn’t happen in a vacuum (so to speak). When a star is born, the throes of its creation reverberate through neighboring regions of space. And sometimes a giant star’s actions can destroy any chance for other stars or planets to be formed nearby. That’s the story behind a set of observations made by the Atacama Large Millimeter Array (ALMA) that peered into the Orion Nebula to study the whys and wherefores of star formation. In particular, they studied so-called “death stars” that have lethal effects on their neighbors in the star birth cloud.

The Orion Nebula is the closest star birth region to us, at a distance of about 1,500 light-years. Astronomers have found stars of nearly all masses and luminosities (brightnesses) in the cloud, as well as some brown dwarfs. It’s likely that many of the newborn stars have planetary systems forming around them, although that process is still hidden within the clouds of gas and dust surrounding the stars (called proplyds (short for protoplanetary disks)).

This all sounds neighborly until you look at how O stars form. These are the most massive and luminous of stars, and they take a LOT of material to form (which is why they’re so massive). In some places, they hog all the available material for star formation, starving their close siblings of the gas they need to form.  This chops off the formation of smaller stars.

But there’s another effect O stars have. As they evolve from proto-stars to fully fledged stars and throughout their early lives, these monsters give off tremendously strong stellar winds and ultraviolet radiation (UV).  The UV light is the same thing that burns your skin when you stay out in the sunlight too long, and all stars radiate in the UV (as well as other wavelengths of light). UV has an interesting property: it can tear apart atoms of gas in a cloud. This is called” photodissociation” and it simply means that the photons of UV light pack enough energy to dissociate the gas atoms and molecules.

The clouds of gas and dust where planetary systems form are susceptible to UV radiation, and so the environment around the big O-type stars is a dangerous place to form planetary systems, particularly in the sphere of influence around the star where UV radiation is the strongest (say within a tenth of a light-year). The ALMA  researchers found that any protostar (that is, any newly forming star)  located within the extreme-UV rich region (think of it as an envelope filled with UV radiation) of a massive star would have much of its disk of material (the proplyd) destroyed very quickly. If proplyds did survive in these regions and the action of their nearby giant neighbors, they had less than half of the mass needed to create even one Jupiter-size planet. As you get farther away from the massive star, say a tenth of a light-year, while there was still some UV effect, the researchers found a wide range of disk masses containing anywhere for one to 80 times the mass of Jupiter. This is similar to the amount of dust found in low-mass star forming regions.

Astronomers have studied the Orion region for many years, first discovering proplyds in this giant cloud of gas and dust using visible-light imagery Hubble Space Telescope, back in the late 1990s. Instruments sensitive to infrared and submillimeter wavelengths allowed them to peer even more deeply into the cloud. These latest studies, also in radio wavelengths, reveal the interactions between stars in this busy stellar créche. All of these studies are giving us a bigger picture of star birth as time goes by. It’s far more complex, beautiful — and hazardous to the neighbors —than we ever knew.

You can read more about this study on the National Radio Astronomy Observatory web site.