Large water reservoirs at the dawn of stellar birth

Herschel’s infrared view of part of the Taurus Molecular Cloud, within which the bright cold pre-stellar cloud L1544 can be seen at the lower left. It is surrounded by many other clouds of gas and dust of varying density. The Taurus Molecular Cloud is about 450 light-years from Earth, and it is the nearest large region of star formation. The image covers a field of view of approximately 1 x 2 arcminutes. // Credits: ESA/Herschel/SPIRE

The European Space Agency’s (ESA) Herschel Space Observatory has discovered enough water vapor to fill Earth’s oceans more than 2,000 times in a gas and dust cloud that is on the verge of collapsing into a new Sun-like star.

Stars form within cold, dark clouds of gas and dust — prestellar cores — that contain all the ingredients to make solar systems like ours.

Water, essential to life on Earth, has previously been detected outside our solar system as gas and ice coated onto tiny dust grains near sites of active star formation, and in protoplanetary disks capable of forming alien planetary systems.

The new Herschel observations of a cold prestellar core in the constellation Taurus, known as Lynds 1544, are the first detection of water vapor in a molecular cloud on the verge of star formation.

More than 2,000 Earth ocean’s worth of water vapor were detected, liberated from icy dust grains by high-energy cosmic rays passing through the cloud.

Close-up of L1544 with the water spectrum seen by Herschel, taken from the center of the pre-stellar core. The peak of the graph shows an excess in brightness, or emission, while the trough shows a deficit, or absorption. These characteristics are used to indicate the density and motions of the water molecules within the cloud. Emission arises from molecules that are approaching the center where the new star will form, from the back of the cloud from Herschel’s viewpoint. The amount of emission indicates that these molecules are moving within the densest part of the core, which spans about 1,000 Astronomical Units. The absorption signature is due to water molecules in front of the cloud flowing away from the observer towards the center. These water molecules are in less dense regions much further away from the center. Together, the emission and absorption signatures indicate that the cloud is undergoing gravitational contraction, that is, it is collapsing to form a new star. // Credits: ESA/Herschel/SPIRE/HIFI/Caselli et al

“To produce that amount of vapor, there must be a lot of water ice in the cloud, more than 3 million frozen Earth oceans’ worth,” said Paola Caselli from the University of Leeds in the United Kingdom. “Before our observations, the understanding was that all the water was frozen onto dust grains because it was too cold to be in the gas phase, and so we could not measure it. Now we will need to review our understanding of the chemical processes in this dense region and, in particular, the importance of cosmic rays to maintain some amount of water vapor.”

The observations also revealed that the water molecules are flowing toward the heart of the cloud where a new star will probably form, indicating that gravitational collapse has just started.

“There is absolutely no sign of stars in this dark cloud today, but by looking at the water molecules, we can see evidence of motion inside the region that can be understood as collapse of the whole cloud towards the center,” said Caselli. “There is enough material to form a star at least as massive as our Sun, which means it could also be forming a planetary system, possibly one like ours.”

Some of the water vapor detected in L1544 will go into forming the star, but the rest will be incorporated into the surrounding disk, providing a rich water reservoir to feed potential new planets.

“Thanks to Herschel, we can now follow the ‘water trail’ from a molecular cloud in the interstellar medium, through the star formation process, to a planet like Earth where water is a crucial ingredient for life,” said Göran Pilbratt from ESA.

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