Protoplanetary nebula

A protoplanetary nebula, or PPN, is a short-lived stage in the life of an intermediate-mass star. These stars have a mass between 1 and 8 times that of our Sun. This phase occurs during a period of rapid evolution. It happens between the late asymptotic giant branch (LAGB) phase and the planetary nebula (PN) phase.

During this time, the object emits strong infrared radiation. It also acts as a type of reflection nebula. Understanding this phase helps astronomers track how stars change as they age.

The process begins during the LAGB phase. At this stage, the star loses mass from its hydrogen envelope. The mass of this envelope drops to about 10^-2 solar masses for a core of 0.60 solar masses. As mass loss continues, the envelope reaches about 10^-3 solar masses. This disruption makes further significant mass loss unlikely. The star begins to move toward the blue side of the Hertzsprung–Russell diagram. The PPN phase officially starts when the star's effective temperature reaches 5,000 K.

As the PPN phase progresses, the central star's temperature continues to rise. This rise happens because of mass loss caused by hydrogen shell burning. During this stage, the star is still too cool to ionize the surrounding gas. This gas is a slow-moving circumstellar shell ejected during the earlier AGB phase. However, the star does drive high-velocity, collimated winds. These winds strike and shape the slower shell. They also entrain the slow-moving ejecta to create a fast molecular wind.

These winds change the physical shape of the nebula. The envelope shape shifts from being roughly spherically symmetric to being axially symmetric. This means it develops a specific axis. The resulting shapes can be bipolar, which means they have two lobes. They can also appear as knotty jets or bow shocks. These shocks are similar to Herbig–Haro objects. Observations from 1998 to 2001 show that this phase shapes the final look of the planetary nebula.

The name "protoplanetary nebula" can be confusing for scientists. It is sometimes used to describe protoplanetary disks, which are unrelated. The term "planetary nebula" was originally chosen by early astronomers. They thought the gas looked like planets such as Uranus or Neptune. To prevent confusion, Sahai, Sánchez Contreras, and Morris suggested the term "preplanetary nebula" in 2005. Some scientists also call these objects "post-AGB stars." However, that category includes stars that never ionize their ejected matter.

The PPN phase ends when the star reaches a temperature of 30,000 K. At this heat, the star produces enough ultraviolet radiation to ionize the gas. Once the gas is ionized, it becomes an emission nebula known as a planetary nebula. This transition must happen quickly. It must take place in less than 10,000 years. If the transition is too slow, the gas density falls too low. It must stay above 100 particles per cubic centimeter. If it falls below this, a "lazy planetary nebula" is formed.

Scientists use different models to explain how these fast winds work. An older model suggested radiatively-driven winds. However, research by Bujarrabal in 2001 showed this model could not explain certain observations. The winds had more momentum and energy than the model allowed. Because of this, theorists investigated accretion disk scenarios. These models suggest that binary interactions create an accretion disk. This disk can then use magneto-centrifugal launching to turn gravitational energy into kinetic energy. This process helps explain the highly collimated jets seen in many systems.