A new study finds every stage of star formation in a single cloud, firmly backing a popular star-formation recipe.
Forming low-mass stars is like baking from a box: stir together a few ingredients (most importantly, a cold gas cloud and its gravity) and then pop the concoction in the oven. But stars with dozens or even hundreds of solar masses are a bit more like the Christmas cookies I bake from scratch every year — they require some finesse or they’ll fall apart.
Nature closely guards the recipe for high-mass stars: they’re rare, and those that form do so extremely quickly and within dense cocoons of dust and gas. Nevertheless, astronomers are starting to piece together the basic steps. Now a team of astronomers has added another significant piece of evidence that confirms what we knew of this recipe.
Cara Battersby (Harvard-Smithsonian Center for Astrophysics and University of Colorado, Boulder) and colleagues zoomed in on a single cloud of gas and dust 18,000 light-years away that’s home to every stage of star formation. Astronomers will never live long enough to watch an individual star form, so obtaining snapshots of many stars’ evolution within a single cloud is key in order to minimize other variables.
“We are building a fossil record of where these giant stars come from,” Battersby says, “and we've just found a full sequence.”
“The ability to observe multiple evolutionary ‘phases’ of star-formation in the same region cuts out the worst assumptions we are often forced to make,” adds Cormac Purcell (University of Sydney, Australia), who was not involved in the study. Each star-forming clump in a cloud shares the same distance from Earth, position in the galaxy, and nursery-cloud properties with its siblings, so the only differences between the clumps will be their stage of evolution.
The current recipe for high-mass star formation calls for a clump of cold gas and dust large enough to collapse under its own gravity, eventually fragmenting into several dense cores. The cores begin to emit winds and outflows, which produce distinct chemical signatures that astronomers can observe even though the protostar itself still hides behind an infrared-dark veil. Finally, the star ignites and shines brightly in the infrared, still accreting gas until it emerges fully from its cocoon.
Using the Very Large Array in New Mexico, the team observed specific radio wavelengths coming from two regions in the gas cloud. Both regions contain thousands of Suns’ worth of gas and dust. But one is quiescent, appearing as a dark shadow in infrared images, while the other, active region glows with infrared emission from protostars that are about to ignite.
The observations show that massive stars require two key ingredients. Ingredient #1: clumpy filaments. The filaments are the source of the gas that’s flowing into the cold, dense cores. Ingredient #2: turbulence. As gravity fights to collapse and fragment these cores, turbulence and thermal pressure must work together to beef up the fragments so that they’re large enough to form massive stars.
In other words, all the observations fit right into the current recipe for high-mass star formation, says Purcell, confirming results from other studies that have pieced together the evolutionary picture from observations of cores in multiple clouds.
But questions remain that this study can’t address: What role does magnetic pressure play in high-mass formation? Do high-mass stars form in isolation or are they always accompanied by low-mass stars? The rough recipe we have now is a good start, but it still needs refinement.
Battersby C. et al. "The Onset of Massive Star Formation: The Evolution of Temperature and Density Structure in an Infrared Dark Cloud," Astrophysical Journal, 2014, Vol. 787, No. 2.