There’s something lurking out there

Friday, 07 December, 2012

Deep in interstellar space there is something lurking. Actually there is lots of something lurking. We know it’s there because of its spectra but we have no idea what it is.

As the concentration of matter in the space between star systems in galaxies is lower than can be achieved by the best vacuum systems on earth, this interstellar space is often considered to be empty. However, this is far from the truth. On average there is about 1 atom per cubic centimetre. (To put this density into perspective, the air we breathe has approximately 30 x 1018 molecules/cm2.)

But astronomical distances are so vast that the total amount of matter out there really adds up. The total material in the interstellar space in the Milky Way is estimated at 510 billion solar masses (one solar mass is about 2 x 1030 kg). This is several per cent of the total mass of visible stars in the galaxy. But what is this material?

Most of the interstellar material is gaseous (99%), and of its mass, about 75% is in the form of hydrogen (either molecular, atomic or ionic), with helium the next most prolific species.

The identity of the chemicals in interstellar space can be determined by examining their spectra.

Atomic spectra are characterised by sharp lines and are an effect of the quantised orbits of electrons around the atom. In other words, a single mechanism, electronic transition, produces atomic spectra.The atomic spectra of interstellar medium (ISM) components, such as H, Na, K and Ca+, have been easily reproduced and matched in the laboratory.

However, the spectra of molecules are much more complex than atomic spectra. They exhibit electronic transitions similar to those of an atom as well as both vibrational transitions and rotational energy states.

Many small gas-phase molecules including CH, CH+, C2 and CO have been identified in the ISM. The first of these small molecules that were discovered, CH and CH+, were identified by comparison of astronomical spectra with electronic spectra recorded in the laboratory.

The development of radiowave spectroscopy has allowed for the identification of other interstellar molecules, beginning with water and ammonia, through the rotational spectra allowed by their permanent dipole moments. Other species have been identified through their infrared emissions, including C3, C2H and C5.

For the last 90 years, astronomers and spectroscopists have been trying to find out what else is in the ISM. We know there is something there because there is a group of several hundred intriguing broad optical spectra called the diffuse interstellar bands (DIBs).

The DIBs are a set of hundreds of absorption lines that are detected from the near-UV to the near-IR in the spectra of so-called reddened stars - meaning there is a lot of interstellar material between us and the star. The DIBs are known to be interstellar, because they do not suffer the periodic Doppler shifts associated with stellar lines in binary star systems.

Diffuse interstellar bands

Diffuse interstellar bands.

The constancy of the absorption wavelengths implies that the carriers are in the gas phase, and the fact that they are broad implies that their carriers are molecular rather than atomic.

While atomic spectra can undergo a number of weak line broadening processes, the three classes of molecular transitions lead to numerous spectral lines superimposed on each other, closely spaced in wavelength and displaying an easily recognisable banded structure. It was the first detection of these substructures in the profiles of several DIBs that pointed to the molecular nature of DIB carriers.

The spectra of countless candidate molecules have been measured but so far none of these spectra have matched the astronomical spectra.

Whatever is causing the DIBs must be widespread in our galaxy and beyond and must be very stable to withstand the harsh conditions of the interstellar medium.

Carbon-based molecules are the current focus of global research, as stellar and galactic chemical evolution models suggest that there is a lot of carbon unaccounted for (~100 carbon atoms per 106 hydrogen atoms). If this ‘missing’ carbon exists in the ISM as molecules which absorb light in the visible region, then carbon-based molecules could be DIB carriers.

Candidate carrier molecules include carbon chains, polycyclic aromatic hydrocarbons (PAHs) and fullerene-type compounds.

There are about 1.2 million different PAH molecules with less than 100 carbon atoms - recording all their spectra in the gas phase and comparing them to the DIBs would be an onerous task. So researchers try to determine the molecular properties of the DIB carriers from their astronomical observations. This eliminates many molecules as potential carriers.

There remains the interesting possibility that some of these spectral features arise from new forms of matter or dust in the ISM and it is notable that new forms of carbon including fullerenes, nanotubes and graphene have only relatively recently become experimentally accessible. In fact, research attempts to simulate DIBs in the laboratory led to the accidental discovery in 1985 of the Buckminsterfullerene, or the ‘buckyball’ carbon 60 molecule, for which the Nobel Prize was awarded in 1996.

Buckyballs possess unique chemical and physical properties that hold an array of possibilities for all the natural sciences. They are an entirely new material providing scientists with information about allotropes of carbon never before conceived. A few areas where buckyballs are proving valuable to research include drug treatments, medical diagnostics, nano scanning tunnelling microscopy, electrical circuitry, lubricants, superconductors and catalysts.

But buckyballs aren’t the answer to what makes the diffuse interstellar bands - so far, we are simply unsure.

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