Introduction to Radio Spectroscopy with the 26m telescope at Hartebeesthoek

In the the space between the stars are found clouds of gas, from which the stars formed. These clouds consist mainly of the simplest atom, hydrogen. More massive atoms such as carbon, oxygen and nitrogen are created within stars by nuclear fusion. These atoms are returned to space in the winds given off by stars and when stars explode. The atoms can combine chemically to form many sorts of molecules, such as water (H₂O), carbon monoxide (CO) and ammonia (NH₃).

Several types of atoms and molecules produce emission lines at specific frequencies in the centimetre wavelength band which are observable with the Hartebeesthoek radio telescope .

Here we describe:

• where and how some of these lines occur,
• show some examples observed by means of spectroscopy at Hartebeesthoek, and
• explain why the lines look the way they do.

Formaldehyde (HCHO) is also common in molecular clouds. It is normally seen as an absorption line at 4829 MHz in the 6cm wavelength band. It is found in the denser parts of molecular clouds.

The dense cores of the molecular clouds can collapse under their own weight to form new stars. If the newly formed star has more than eight times the mass of the sun, it produces sufficient radiation to excite very strong stimulated emission at radio wavelengths, known as a maser, from molecules such as hydroxyl in the surrounding clouds.

In addition to intense masers from the four ground state lines at 18cm wavelength, hydroxyl masers are known to occur in lines from excited states. These include the three lines at 4660, 4750 and 4765 MHz in the 6 cm wavelength band and the four lines at 6016, 6030, 6035 and 6049 MHz in the 5 cm wavelength band. These transitions are all observable at Hartebeesthoek.

In 1987 strong masers were also found to be generated by methanol (CH3OH, the simplest alcohol) molecules in star-forming regions, at a frequency of 12178 MHz in the 2.5 cm wavelength band.

The ultraviolet radiation from these massive stars splits the atoms of neutral hydrogen gas (HI) in the surrounding cloud into their constituent protons and electrons, creating a steadily growing ionized hydrogen cloud, or HII region. A few thousand of these objects are known within our galaxy, well-known ones including the Orion and Rosette Nebulae. The ionized hydrogen is hot, with a temperature of 5000-10000 K.

For most of their lives, stars produce no detectable radio emission lines. However, in their old age they become unstable, swell and pulsate slowly. The first time this occurs is as an irregularly variable supergiant star. Later they evolve into asymptotic giant branch stars which have regular pulsations. These are named Miras (miraculous), after Mira (omicron Ceti), the first star in which the large changes in brightness caused by the pulsations was identified. These stars are highly enlarged and relatively cool. They are red hot rather than white hot like the sun, and appear orange or red to the eye.