Fig 15.24.

Strong Ha emission line from a T Tauri star. [George Herbig, IFA, Hawaii]

observed surface is covered by the spots, and the brightness changes.

(2) The variations may arise in the chromosphere.

(3) The variations may actually result from changes in the opacity of the dust shell surrounding the star.

The emission lines show Doppler-shifted absorption wings, like those in Fig. 15.24. This suggests material both falling into the star and material coming off the star. The infall may be close to the star as the final stage of collapse, while the outflow is a wind (like the solar wind, but stronger) farther away from the star. Alternatively, the infall may be in the form of a disk around the star's equator, while the outflow is along the polar axes.

Fig 15.25.

Far IR image of protostellar core.This is a ground-based image from Mauna Kea, at 850 ^rni.The beam size is shown in the red circle to the lower right, so you can see that the sources are barely resolved. Notice the separation into two sources.The irregular edges of the image are due to problems with the detectors near the edge.This is an example of what is called a Class 0 protostar, which is thought to be the youngest stage, where there is a strong outflow but the surrounding cloud has not been driven away. [Yancy Shirley, University of Texas, Austin, made with SCUBA on the JCMT]

From studies of the spectral lines, we think that the winds may have speeds of about 200 km/s. The mass loss in the wind, dM/dt, is about 10-7 M©/yr. The total luminosity in the

Fig 15.26.

HST images of infrared emission from selected disks around forming stars.All six objects are in Taurus, at a distance of 150 pc. [STScI/NASA]

wind is that rate at which kinetic energy is carried away in the wind,

Using the numbers given, we find a wind luminosity of about 1 L©. That is, the star gives off as much energy per second in its wind as the Sun gives off at all wavelengths. However, the wind phase is a short lived one. The wind does sweep away some of the dust that has collected around the star. We think that a similar wind from the Sun was important in clearing debris out of the early Solar System.

It is now possible to make far IR images of pro-tostellar cores, like that in Fig. 15.25. Far IR emission HST and IR satellites have provided us with some images, such as those in Fig. 15.26, which may be the result of infrared emission from dust shells around recently formed stars. Molecular spectral line observations of these disks will require resolutions achievable using interferometers.

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