T Tauri star

T Tauri stars are a specific class of variable stars. These stars are very young, typically less than ten million years old. They are currently in the pre-main-sequence stage of stellar evolution. This means they are not yet stable like our Sun. Instead, they are in the process of contracting. They move toward the main sequence along a path called the Hayashi track. This track describes a relationship between a star's luminosity and its temperature.

The mechanism that powers these stars is quite different from mature stars. In a main-sequence star, energy comes from hydrogen fusion. However, the central temperatures of T Tauri stars are too low for that process. Instead, they are powered by gravitational energy. As the star contracts and pulls itself inward, it releases this energy. This process continues until the star reaches the main sequence. This transition usually takes about 100 million years. Once they reach this stage, they begin nuclear fusion.

Scientists categorize these stars into several distinct types. Some are known as Classical T Tauri stars (CTTS). Others are classified as Weak-line T Tauri stars (WTTS). A smaller group called Naked T Tauri stars (NTTS) is a subset of the Weak-line group. Roughly half of all T Tauri stars possess circumstellar disks. These are often called protoplanetary disks because they may become planetary systems. These disks are estimated to dissipate within 10 million years.

The history of studying these objects spans over a century. The prototype star, T Tauri, was discovered in 1852. It is located in the Taurus star-forming region. Later, a scientist named Alfred Harrison Joy defined the T Tauri class in 1945. Since then, astronomers have used these stars to understand stellar beginnings. We now know they are often part of binary star systems. Many are also referred to as young stellar objects (YSOs) during various life stages.

These stars exhibit extreme physical properties and high activity levels. They are much more luminous than older stars of the same mass. This is because their radii are significantly larger. They also rotate very rapidly, with periods between one and twelve days. For comparison, the Sun takes about a month to rotate once. Their X-ray and radio emissions are incredibly intense. These emissions can be approximately 1,000 times stronger than those of the Sun.

One fascinating way to study these stars is through lithium abundance. T Tauri stars show higher levels of lithium than main-sequence stars. This happens because lithium is destroyed at temperatures above 2,500,000 K. During the Hayashi contraction, lithium burning can occur via the p-p chain. This process involves protons combining to form lithium. Rapid rotation can increase the transport of lithium into deeper, hotter layers. Because of this, the rate of lithium depletion helps scientists calculate a star's age.

Finally, T Tauri stars are deeply connected to the formation of solar systems. The active magnetic fields and strong stellar winds may transfer angular momentum. This momentum moves from the star to the surrounding protoplanetary disk. This process might be how our own Sun transferred momentum to its planets. We have even discovered planets around these young stars, such as PDS 70b. These stars serve as a window into the chaotic birth of worlds.