Stunning Images of Dark Cloud L1551 Bring Star Formation Activity to Light

August 3, 2009

Early stages of star formation are usually hidden deep within dark clouds of gas and dust, which curtain off the play of activity that is occurring behind them. Optical telescopes cannot penetrate this cloak, but infrared telescopes can. Observations with the Subaru Telescope, one of the largest optical infrared telescopes in the world, have enabled astronomers to capture clearer images of emissions in a dark cloud known as L1551. The significance of these images extends beyond their clarity and beauty; they contribute to an understanding of star-forming systems and conditions linked to the creation of planets.

L1551, a popular target of observations, interests astronomers because dark clouds are rich contexts where stars and planets are conceived, develop, and are born. The proximity, visibility, and isolation of L1551 make it one of the best objects for studying jets and other outflows coming from low-mass young stars-an early phase in a star's sequence of development. These emissions provide important clues to the developing star's activity.

Sharp images of objects within dark clouds are difficult to secure. When positioned next to a brighter background, dark clouds appear black, because their dense concentration of interstellar gas and dust absorbs light from behind them. Dust grains located in their coldest, densest regions block out the passage of light in optical (visible) wavelengths. As a result, the quality of images of L1551's interior has been poor-until now.

Thanks to its innovative special features, Subaru Telescope has captured stunning infrared images of obscured portions of L1551 and produced clearer, more revealing views of this dark cloud. The telescope's large 8.2 meter primary mirror gives it powerful light-collecting capacity during observations, with a range ten times greater than that of 3-4 meter telescopes. The "infrared eye" of MOIRCS (Multiple Object Infrared Camera and Spectroscope) not only has the widest field of view of any instruments on 8 meter-class telescopes but also has the capacity to view and measure multiple objects at the same time. This technology is so sensitive that it can distinguish fine details of faint emissions. The addition of a narrow band filter on MOIRCS streamlines its capability for producing high quality images and delineating the temperature and density of a very specific area. As a result, the sum of Subaru's technology yields scientific rewards much greater than its individual components alone.

Observations made during a 30 December 2006 engineering test run for a newly installed filter on MOIRCS helped Subaru astronomers Hayashi and Pyo clarify the significance of emission features detected in a region of L1551 containing deeply embedded younger stars. They probed the densest part of this dark cloud to hunt for heavily obscured features of jets and outflows-ejections from young, developing stars. They used the new narrow-band [Fe II] filter as well as another one (H2) to capture images of this veiled star-forming region.

A comparison of the [Fe II] and H2 narrow band images showed differences in the emission features detected by each type of filter and suggested that they arose from different processes. Most of the features revealed by the [Fe II] filter are compact or jet-like and could have developed from fast shocks occurring in the jets' ejected materials. In contrast, features revealed by the H2 filter are more diffuse and widely distributed in outflows, implying that these emissions originate from slower shocks where the ejecta interact with surrounding material. At the very least, Subaru's images of jets and outflows provide scientists with data about activities occurring during the birth and development of stars. Hayashi and Pyo's research on the significance of these emissions raises intriguing questions about how planets are created.

According to one widely accepted model, planetary systems naturally result from star formation. At the beginning of star formation, gravitational energy pulls gas and dust together into a more densely concentrated object, a protostar. Only a portion of the material becomes part of the protostar, and the rest pushes away from it via jets, which disperse the excess. Gaseous disks called circumstellar disks (literally "around a star" disks) develop around young stars and provide materials from the parent molecular cloud to feed their growth. At this stage, the circumstellar disk is referred to as an accretion disk because it feeds the central star with dust and gas.

At a later phase of the accretion process, the disk may function as the original or earliest disk from which a planet or planets develop; it is then called a protoplanetary disk. Dust grains in the disk may collide and stick to form an object called a planetesimal, the smallest original fraction of a planet. As planetismals grow, they can attract each other with their mutual gravity and merge to form larger bodies or planets. What fascinates astronomers is why some of these disks may give rise to planets and others do not. Identification of the specific conditions that foster planet creation is an ongoing focus of astronomical research.

Astronomer Masahiko Hayashi, Director of the Subaru Telescope, is particularly interested in how disks around young stars relate to planet formation. He thinks that understanding jets and other outflows is an important ingredient for identifying the connection. In a recent interview, Hayashi speculated about their significance: "Some think jets are necessary for gas to form on a central star. Jets regulate the mass falling onto the central star, and the formation of planetesimals proceeds. When ice is added to the mass, it makes the planetesimal heavier, and a planet may form this way." Hayashi hopes that further research at Subaru will yield data to support this interpretation.

Composite Color Image of  the L1551 Dark Cloud

Composite Color Image of L1551 NE (p. 586 in Hayashi and Pyo's article.) Enlarge

Zoomable Images: L1551 Dark Cloud

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