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Introduction to Radio Spectroscopy with the 26m telescope at Hartebeesthoek

Atoms, Molecules and Spectroscopy

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:

Molecular Clouds

Hydroxyl (OH) is one of the simplest molecules and is widely found in molecular clouds in interstellar space. It produces four emission lines at frequencies of 1612, 1665, 1667 and 1720 MHz in the 18cm wavelength band. Observations of these lines provide information about the physical conditions within these clouds and the excitation state of the molecules.

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.

generic spectrum Examples of spectra and more information on hydroxyl and formaldehyde from a molecular cloud showing lines both in emission and in absorption are given here.

Masers in Star-forming Regions

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.

Even stronger methanol masers were discovered in 1992. These occur at 6668 MHz in the 4.5 cm wavelength band. Several hundred have been discovered since then, many of them with the Hartebeesthoek telescope.

generic spectrum These spectra show the rapid variations that were discovered in one methanol maser during monitoring with the Hartebeesthoek telescope.

Recombination lines from Ionized Hydrogen

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.

HII regions produce a series of emission lines from the protons and electrons as they recombine to form hydrogen atoms in excited states. These hydrogen recombination lines are generally broad and weak, owing to the high temperature of the gas. They occur at regular intervals throughout the radio spectrum.

generic spectrum Spectra of recombination lines. Lines observable at Hartebeesthoek include the H157alpha line at 1683 MHz, the H141alpha line at 2321 MHz and the H110alpha line at 4874 MHz.

Masers from Evolved Stars

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.

generic spectrumThese spectra show the maser emission that can occur from hydroxyl molecules in the winds blowing away from Mira-type stars.

In the final stages of their lives the stars expel their outer layers, which form planetary nebulae. These are so called because the shell of hot gas around the star can look like a planet through an optical telescope. The remains of the central star collapses to become a white dwarf star. Just before this occurs, the Mira-type pulsations stop and unusual hydroxyl maser profiles are seen from the rapidly evolving shell of gas and dust as it detaches from the star.

For a translation into Russian, see radiospektrkopiyu.