Astronomy

The Search For Molecules In Interstellar Space

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Prior to 1950, no one would readily agree that there could be molecules in space. In 1944 laboratory tests confirmed that neutral hydrogen could emit a frequency under certain conditions. Hendrik Van De Hulst proved that neutral hydrogen could produce radiation at a wavelength of 21 cm (corresponding to 1420 MHz). The 21-cm line is created by the spin flip of the electron that indicates changes in the energy state of neutral hydrogen. When the electron spins in the same direction as the proton, it can spontaneously jump to a lower energy state in which it spins in the opposite direction, with the emission of radiation at a wavelength of 21 cm.

It takes about 10 million years for the flip to occur, nut the immense number of atoms in the interstellar clouds generate enough photons to be observed with radio telescopes. In 1951, the line was detected from space within the radio spectrum (microwave window). Two Harvard scientists, Ewen and Purcelll detected hydrogen in space at 21-cm wavelength. The presence as well as the absence of the line has become an important tool to probe the evolution of the Universe.

With the advent of instruments that could analyze the ultraviolet spectrum in the 1960s, abundant heavy elements such as carbon, nitrogen, and oxygen were also discovered. Water molecules were found in 1968, followed by formaldehyde in 1969 and carbon monoxide in 1970. Most of the discoveries of molecules in interstellar space had been made by 1972, following the new radio telescope in Kitt Peak in Arizon (USA).

When the Raman Research Institute (RRI), Bangalore, started astrophysical research in 1972, its scientists had by then a good idea of the cosmic chemistry, the understanding of which was evolving rapidly. It was well established that molecules out there reveal themselves by their natural emission in certain specific radio frequencies. Shortly after the newly built 10.4 m diameter radio telescope of the Institute began monitoring the outer space in 1985, its scientists began looking at silicon monoxide masers (so called as they are similar to lasers, except they put out microwaves instead of light waves) at 86 GHz.

The masers are found associated with old stars that undergo periodic changes in their size. This leads to a modest variation in their light output. The amount of master emission too follows this variation with a delay. However, the scientists at RRI observed that the delay was much more than the time that light takes to travel from the stellar surface to the maser clouds. Further study of the phenomenon revealed the origin of the delay: The light from the star first heats up its dusty halo, and it is this heat radiation that makes the molecules behave as a maser! Thus, the light variation of the maser was delayed by the round trip travel from the star to its dusty halo and back! The good detective work speaks volumes about the capability of Indian scientists.

The study triggered further curiosity, but the costly detector equipment put a damper on the enthusiasm of the scientist. Recently, however, detector technology has improved dramatically in terms of efficiency and price. Taking advantage of the market supply conditions, RRI has acquired two sophisticated detectors called low-noise amplifiers. They work in the band, 40 to 50 GHz, in temperatures around 20 k instead of in the very cold 4 k demanded by the previous generation of equipment. Alongside, the parabolic mirror of the telescope has been made as smooth as possible.

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With the upgraded radio telescope, innovative measurements can be made to determine the location and luminosity of the silicon monoxide maser (using the 43 GHz line, the twin of the 86 GHz one) in the deep interiors of our Galaxy, the Milky Way.

An important feature to be measured is its radial velocity (the speed at which an object moves away or towards the observer) of the silicon monoxide maser (SMM). The SMMs, being in the environs of old stars, are widely distributed in the interiors of our Galaxy. The pattern of their motion is expected to reveal the total gravitational potential in the interiors of our Galaxy.

The location of these molecular masers could be more accurately determined, if more than one radio telescope is involved in the research. For instance, if the signals received at two or more widely separated telescopes can be brought together and integrated, one could get the benefit of a bigger telescope. Accordingly, the search and further follow-up studies have implications for understanding the spread of the dark matter in the central regions of our Galaxy.

Visible matter constitutes about 4-5 per cent of the total energy density and 25-26 per cent is constituted by the dark matter. The rest~70 per cent is the dark energy. As a fraction of matter content, the visible matter is 0.45/0.3-16 per cent.

It would be a major contribution of a research institute like the RRI if it could indicate the pattern of the total gravitational potential of our galaxy on the basis of studies like the one with silicon monoxide masers, as that would indirectly give valuable clues on the spread of the dark matter. Based on the findings, further studies with very large base telescopes abroad can be launched to refine our understanding.

Moreover, the low noise receiver in the telescope would strengthen the capability of the Institute to monitor not only one molecule like the silicon monoxide maser but five other molecules at the same time. Such simultaneous observations ensure that nothing important is missed out, while the observer is busy with studying emission from one molecule. Another advantage of having more than one molecule under observation at the same time from the same place is the increased accuracy in studying cosmic chemistry.

About 140 different molecules have been discovered in interstellar clouds. There is no accepted theory addressing how interstellar molecules containing more than five atoms are formed. Recently, a 10-atom molecule called propanal was discovered in Sagittarius-B2, some 26,000 light years away. Surely, we are in for some surprises.

There is another aspect to this story of searching for molecules. Some of them include chemical precursors of life like water, ammonia, formaldehyde and hydrogen cynacetylene. If they are well spread out in space, it is reasonable to infer that life forms could have appeared under appropriate conditions. Amino acids of extra-terrestrial origin have been found in meteorites found on the Earth, and claims of detection of glycine in the interstellar space have been made. Nonetheless, the emergence of life is still a mystery!

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