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Star Clusters

T Associations and T Tauri Stars

Interstellar dust clouds actually come as an enigma to scientists, because of the near vacuum medium the particles must travel through to combine and form clouds. Strong forces must play an essential role in their formation. The stellar winds from dying red giant stars are one example of the type of force needed. As these heavier elements cool off, they recombine as molecular dust particles. Over time they become increasingly dense and ionized by the starlight from nearby stars. Gravitational forces continually pull the dust and gas toward the central part of the nebulae. As more particles combine, the more friction is created and thermal heating would begin. This thermal heating represents the beginning of the star formation process. Protostars begin very large and condense through time. As they condense, they give off energy. They have not yet begun to create thermonuclear energy, which stars like our sun generate. These protostars are called T Tauri stars. Since they tend to occur in groups, which are always associated with regions full of interstellar nebulosity and dust, they are often referred to as T associations. A star forming nebula is rather large and can churn out many of these protostars. This is how an open cluster forms. The temperature in protostars keep increasing, until they reach main sequence temperatures, when they illuminate their surface through the process of nuclear fusion. The light from the older stars within the cluster can affect the formation of the protostars. It blasts away dark nebulae, which are the seed
T Tauri Star T Tauri Star of stellar formation. This can cause a smaller star to form. Usually, the stars within a cluster have barely enough gravitational pull to keep them together. This can create a rather large spread of open cluster members. The average cluster exists for approximately 100 million years, because as stars are created they drift further and further from each other. These stars continue drifting until they are no longer within the grasp of the gravitational forces of the clusters. The star constellation Ursa Major is actually part of a vastly spread out (23 degrees) open star cluster.

Figure 06-05a Birth of A Star [view large image]

Figure 06-05b Pre-main Seq. Evolution [view large image]
Figure 06-05a and 06-05b are schematic diagrams showing the pre-main sequence evolution (the birth of a star). When a sufficiently dense core forms inside a molecular cloud, it will begin to collapse. Conservation of angular momentum will ensure that the collapsing cloud forms a flattened, spinning nebula. The central part of the star-forming clound, which collapses faster than its outer parts, froms the protostar. It continues to accumulate mass by accreting material from the outer parts of the cloud. Some of the remaining gas is expelled in two oppositely directed jets, perpendicular to the plane of the disk. It will become a full-blown star once the pressure at the center is high enough to ignite nuclear fusion, the surrounding gas in the disk will be blown away leaving a planetary system around the star.

Table 06-01 shows the characteristic for the three types of star clusters qualitatively. The initial mass of the parent cloud is a key parameter in a 2013 theory. It suggests that the three types of galactic star clusters is determined by the mass of the parent clound. They are the products of two countervailing processes : contraction, caused by the gravity of the parent cloud, and expansion, promoted by stellar winds and ionizing radiation. Although the prediction of molecular cloud contraction has not been observed so far, indirect evidence from star-formation rate implies that it does occur in the early stage of cluster evolution.

Cluster Initial Mass # of Stars Dispersal Time (106yrs) Cloudy
OB Assoication Massive 1000 10 No
Open Cluster Intermediate 100 - 1000 100 No
T Assoication Low 100 1 Yes

Table 06-01 Galactic Star Clusters Characteristic

Massive Star The formation of low-mass stars like the Sun can be explained by the gravitational collapse of a molecular cloud fragment into a protostellar core and the subsequent accretion of gas and dust from the surrounding interstellar medium. Theoretical considerations suggest that the radiation pressure from the protostar on the in-falling material may prevent the formation of stars above 10 solar masses through this mechanism, although some calculations have claimed that stars up to 40 solar masses can in principle be formed via the accretion through a disk. Observation in 2004 show a massive star (in the Omega nebula, Figure 06-06a) being born from a large rotating accretion disk. The protostar has already assembled about 20 solar masses, and accretion process is still going on. The gas reservoir of the circumstellar disk contains at least 100 solar masses of additional gas, providing sufficient fuel for substantial further growth of the forming star.

Figure 06-06a Massive Protostar [view large image]
McNeil's Nebula In January 2004, a new object (now known as McNeil's nebula) was discovered in Orion. A look back at images of this part of the sky revealed that in November 2003, a young low-mass star had brightened suddenly. There was a 50-fold increase in X-ray emissions before the optical outburst. The X-ray emission probably stems from the high-energy accretion. McNeil's nebula is not present on images taken in the 1990s and as early as 1951, but it has been found in mid-1960s, so it may be a variable T Tauri star as indicated in Table 08-02.

[view large image]

Figure 06-06b McNeil's Nebula

HL Tau Taurus Cloud There is a very young T Tauri star by the name of HL Tau (Figure 06-06c) in the Taurus molecular cloud (Figure 06-06d, the blue object in upper left). An image of its protoplanetary disk captured by ALMA was made public in November 2014. It shows a series of concentric bright rings separated by gaps revealing the process of planet formation. This would be a perfect picture to confirm the theory of planetary formation except for its age of less than 100,000 years, which is too young in the computer simulation. The computer model would have to be modified to fit the observation. A pair of jets is also missing in the picture but predicts by some magnetohydrodynamic modeling of such system.

Figure 06-06c HL Tau
[view large image]

Figure 06-06d Taurus Cloud [view large image]

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