February 13, 2014
Bipolar Jets Of Gas, Dust Created By Small Planetary Nebulae
April Flowers for redOrbit.com - Your Universe Online
Large stars can end their lives as violently cataclysmic supernovae. Small stars, in contrast, end up as planetary nebulae—colorful, glowing clouds of dust and gas. These nebulae were once thought to be mostly spherical. In the last few decades, however, they have been observed to often emit powerful, bipolar jets of gas and dust. Scientists are unsure how spherical stars evolve to produce highly aspherical planetary nebulae, however.
As these smaller stars run out of hydrogen, they begin to expand, eventually becoming Asymptotic Giant Branch (AGB) stars. The AGB phase of a star's life -- which represents the distended last spherical stage in the lives of low mass stars -- last around 100,000 years. Eventually, the AGB stars become "pre-planetary" nebula, which are aspherical.
"What happens to change these spherical AGB stars into non-spherical nebulae, with two jets shooting out in opposite directions?" asks Eric Blackman, professor of physics and astronomy at Rochester. "We have been trying to come up with a better understanding of what happens at this stage."
AGB stars have to somehow become non-spherical for the jets in the nebulae to form. According to Blackman, astronomers believe this occurs because AGB stars are not single stars at all, but rather part of a binary system. Scientists believe the jets are produced by the ejection of material that is pulled or accreted from one object to the other and swirled into a so-called accretion disk. Not all accretion disks are created the same way; however, there are a range of different scenarios. All of them involve two stars or a star and a massive planet, but scientists have been unable to rule any of them out until this point because the "core" of the AGBs, where the disks form, are too small to be directly resolved by telescopes.
Blackman worked with his student, Scott Lucchini, to determine whether the binaries can be widely separated and weakly interacting, or whether they must be close and strongly interacting.
According to the findings, only two types of accretion models, both of which involve the most strongly acting binaries, could create these jetted pre-planetary nebulae.
The first model type is known as the "Roche lob overflow." The companions in these models are so close the the AGB stellar envelope gets pulled into a disk around the companion.
The second, or "common envelope," model type has the companion even closer and it fully enters the envelope of the AGB star so that the two objects have a "common" envelope. The very high accretion rate disks within the common envelope can either form around the companion from the AGB star material, or the companion can be shredded into a disk around the AGB star core. Either of these possibilities could provide enough energy and momentum to produce the jets that have been observed.
Astronomer William Herschel coined the name planetary nebulae when he was the first to observe them in the 1780s. Herschel thought they were newly forming gaseous planets. Even though we now understand that they are in fact the end state of low mass stars and would only involve planets if a binary companion in one of the accretion scenarios above were in fact a large planet, the name has persisted.
The difference between pre-planetary and planetary nebulae is in the light they produce: pre-planetary nebulae reflect light, whereas mature planetary nebulae shine through ionization (where atoms lose or gain electrons). Two jets of gas and dust shoot out from pre-planetary nebulae, with the dust forming in the jets as the outflows expand and cool. The light produced by the hotter core is reflected from this dust. The core of a planetary nebulae, thought to be the evolved stage of pre-planetary nebulae, is exposed and the hotter radiation it emits ionizes the gas in the now weaker jets, which in turn glow.