From: Andy Soos, ENN
Published April 6, 2011 08:04 AM

Star Jets

Astronomers have discovered that two symmetrical jets shooting away from opposite sides of a blossoming star are experiencing a time delay: knots of gas and dust from one jet blast off four-and-a-half years later than identical knots from the other jet. The finding, which required the infrared vision of NASA's Spitzer Space Telescope, is helping astronomers understand how jets are produced around forming stars, including those resembling our sun when it was young.


Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly born stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig-Haro (HH) objects. These jets, in combination with radiation from nearby massive stars, may help to drive away the surrounding cloud in which the star was formed.

"More studies are needed to determine if other jets have time delays," said Alberto Noriega-Crespo of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, who is a co-author of the new study to be published in the April 1 issue of Astrophysical Journal Letters. "Now we know that in at least one case, there appears to be a delay, which tells us that some sort of communication may be going on between the jets that take time to occur."

Jets are an active phase in a young star's life. A star begins as a collapsing, roundish cloud of gas and dust. By ejecting supersonic jets of gas, the cloud slows down its spinning. As material falls onto the growing star, it develops a surrounding disk of swirling material and twin jets that shoot off from above and below the disk, like a spinning top.

The total mass being ejected to form typical HH objects is estimated to be of the order of 1–20 Earth-masses, a very small amount of material compared to the mass of the stars themselves. The temperatures observed in HH objects are typically about 8000–12,000 K, similar to those found in other ionized nebulae such as H II regions and planetary nebulae. They tend to be quite dense, ranging from a few thousand to a few tens of thousands of particles per cm3, compared to generally less than 1000/cm3 in H II regions and planetary nebulae.

HH objects consist mostly of hydrogen and helium, which account for about 75% and 25% respectively of their mass. Less than 1% of the mass of HH objects is made up of heavier chemical elements, and the abundances of these are generally similar to those measured in nearby young stars.

The discovery of the time delay, in the jets called Herbig-Haro 34, has also led the astronomers to narrow in on the size of the zone from which the jets originate. The new Spitzer observations limit this zone to a circle around the young star with a radius of 3 astronomical units. An astronomical unit is the distance between our sun and Earth. This is about 10 times smaller than previous estimates.

In the early 1980s, observations revealed for the first time the jet-like nature of most HH objects. This led to the understanding that the material ejected to form HH objects is highly collimated (concentrated into narrow jets). Stars are often surrounded by accretion disks in their first few hundred thousand years of existence, which form as gas falls onto them, and the rapid rotation of the inner parts of these disks leads to the emission of narrow jets of partially ionized plasma perpendicular to the disk, known as polar jets.

"Where we stand today on Earth was perhaps once a very violent place where high-velocity gas and dust were ejected from the disk circling around our very young sun," said Alex Raga of the Universidad Nacional Autónoma de México, the first author of the paper. "If so, the formation of planets like Earth depends on how and when this phenomenon ended. Essentially, every star like our own sun has gone through a similar cloud-disk-jets formation process."

One of the jets in Herbig-Haro 34 had been studied extensively for years, but the other remained hidden behind a dark cloud. Spitzer's sensitive infrared vision was able to pierce this cloud, revealing the obscured jet in greater detail than ever before. Spitzer images show that the newfound jet is perfectly symmetrical to its twin, with identical knots of ejected material.

This symmetry turned out to be key to the discovery of the jets' time delay. By measuring the exact distances from the knots to the star, the astronomy team was able to figure out that, for every knot of material punched out by one jet, a similar knot is shot out in the opposite direction 4.5 years later. This calculation also depended on the speed of the jets, which was known from previous studies by NASA's Hubble Space Telescope. Other symmetrical jets similar to Herbig-Haro 34 have been observed closely before, but it is not clear if they are also experiencing time delays.

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