18Hearing the First Cries of Newly Born Stars
Spectroscopy lets us investigate the composition of gas clouds floating in deep space. It also lets us track their movements. Various molecules in gas clouds emit radio waves, which reveal forms in space that cannot be observed by using visible light. The interstellar medium permeates the Galaxy and consists of gas and dust. Stars, such as our Sun, are formed in a molecular cloud core which is a condensation of interstellar medium gathered by the gravity. A molecular cloud core has a diameter of approximately 0.1 light years, a mass several times that of our Sun, and a density of several hundreds of thousands of hydrogen atoms per cubic centimeter. When this core contracts further due to gravity, an infant protostar is formed at the cloud's center. Protostars are difficult to study in detail with visible light observations because a thick cloud of interstellar medium surrounds them.
That's where radio observations step in. This very cold interstellar medium emits radio waves, which we can use to investigate what's happening near the protostar. Also, when gas is moving, the Doppler Effect causes slight shifts in the radio wave frequency. By measuring these Doppler shifts very precisely, we can see the details of how gas swirls around the protostar. We can also observe high-speed jets of gas (called bipolar outflows) that are ejected from the protostar. When the new star at the center begins to shine, this heats up the surrounding gas and provides energy for a wide range of chemical reactions.
Bipolar outflows can travel at velocities over 100 km/s. When they collide with the surrounding interstellar medium, the temperature of the dust rises. The collisions knock various molecules from the surfaces of the dust particles and cause shockwaves that break down the dust. In regions where this occurs, certain molecules, such as silicon monoxide (SiO), are found with abundance many millions of times that found in interstellar space. Of course, the exact scale and velocity of bipolar outflows, as well as the chemical reactions that occur, depend on the environment of the protostar and its surroundings. Molecular spectra elegantly capture the diversity of star-forming regions.
： “ Searching for the root cause of protostar gas release ”
Protostars emit various kinds of gas outflows. Some outflows are wide and travel at several kilometers per second. Others are tight streams that travel at over 100 km/s and extend to distances of several light years. Some protostars eject hot jets of ionized gas. Researchers think that the magnetic field surrounding a protostar is responsible for the formation of the narrow high-speed jets, but we haven't figured out all of the details quite yet. Also, some protostars have high-speed molecular flows, but others don't. Is this because the stars are in different stages of evolution? Or perhaps could it be due to differences in the speed at which the gas revolves around the protostar and/or in the molecular cloud core? We don't know yet. One reason why answers remain elusive is that the molecular flows originate very close to the protostar (between about 0.01 and 30 astronomical units), and our telescopes don't yet have the spatial resolution to peer that closely. This situation may soon be resolved thanks to ALMA; this telescope has a resolution dozens of times greater than that of previous radio telescopes. So we may soon uncover the mechanisms by which molecular flows occur.
