Stars are born in hiding, behind layers of dust, deep within the clouds of molecular gas whose collapse brings them into existence. The younger a star-to-be (or "protostar"), the more difficult it is to observe.
Three of the PACS Bright Red Sources (PBRS) found with the Herschel Space Telescope, which appear to be among the youngest known protostars. The leftmost panel shows an image taken with the Spitzer Space Telescope (at 24 µm) in which the upper two objects are completely invisible, while the lower one is not clearly identifiable as a protostar. The two panels on the right show images taken with the Herschel Space Telescope (at 70 µm) and with the APEX submillimeter telescope (at 350 µm), which allowed for the identification of all three objects as some of the earliest known protostars yet.
Credit: A. M. Stutz (MPIA)
Over the past years, using ever more advanced infrared technology, there has been a veritable race for finding protostars in ever earlier stages of development. Now, a group of astronomers led by Amelia Stutz of the Max Planck Institute for Astronomy has used both the Herschel Space Telescope and the submillimeter telescope APEX to discover and characterize the youngest known protostars yet.
Team member Tom Megeath of the University of Toledo, Ohio, recalls: "The discovery was totally serendipitious. I was looking at images taken with the space telescopes Spitzer and Herschel, showing a recently discovered interesting protostar in Orion, which is varying in brightness. In the first Herschel image I looked at, sure enough, there was the protostar – but there was another object right next to it! That second object hadn't shown up in the images taken at shorter wavelengths with the Spitzer telescope."
For physicists, the fact that an object shines brightly at longer wavelengths, but is invisible at shorter wavelengths, holds specific clues as to the object's temperature. Human beings for instance, with their body temperatures of around 37 degrees Celsius, emit infrared light, but not visible light. For the Herschel image, the researchers concluded that they could be looking at an exceptionally cool protostar. This was an exciting prospect, since at such low temperatures, this would be a protostar in a much earlier developmental stage than anyone had ever seen before!
After this first compelling discovery, Stutz then carefully combed through the data looking for more examples of other similar specimens in Orion. They ended up with a total of 55 of these apparently very cold objects.
But the universe has one additional trick up its sleeve. Very distant cosmic objects appear "redshifted", which can make a very distant ordinary galaxy look like a very cold nearby protostar. Stutz explains: "We needed to separate the protostars from the impostors. And we knew that would only be possible with more data. That's why we went to APEX, which can receive light at even longer wavelengths than Herschel." The APEX antenna is located in the Atacama desert in Chile and operated by the European Southern Observatory (ESO).
With the combined data, and carefully comparing their observations with physical models of protostars and similar objects, Stutz and her colleagues narrowed their list to 15 reliably identified new protostars. They dubbed the reddest sources "PACS Bright Red Sources", PBRS for short, after the Herschel instrument PACS whose images had led to their discovery in the first place. These sources where not identifiable as protostars by the Spitzer telescope due to their low temperature – a number of them are simply invisible in the Spitzer images.
Going by the analysis of Stutz and her colleagues, these are the youngest protostars yet observed: dusty gas envelopes with masses between 1/5 and 2 times that of the Sun, heated to about 20 degrees Celsius above absolute zero (20 K) by a protostar hidden deep inside.
Stutz on the implications of their find: "The earliest stages are where the protostar builds up most of its mass. But they're also hardest to observe. Up until now, when a theorist built a model of star formation, there was no direct way of comparing what the model said about the earliest stages with observation. Now we're closing that gap – and that's always a good thing if you want to know what's really going on."
For Stutz and their colleagues, the next stages in their campaign are already underway: Follow-up observations with Herschel on eight of the PBRS to look for traces of the gas outflows predicted for these early prototypes, and observations using the Green Bank Telescope of light characteristic for denser regions populated by gas molecules. The astronomers also hope for observation time on ALMA, the array of submillimeter antennae currently under construction in the Atacama desert: ALMA should be able to reveal finer details of the envelopes, and allow for more precise measurements of their densities.
Stutz concludes: "It's always exciting to find new kinds of objects such as our PBRS – in particular when they promise information about something as fundamental as the birth of stars. And both our discovery and the potential for future follow-up show that these are interesting times to be an astronomer. Without Herschel, we would have missed these sources altogether. With facilities like ALMA we will be able to examine them in unprecedented detail."
Contact informationAmelia Stutz (first author)
Markus Pössel | Max-Planck-Institut
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