Fig. 2a shows the log-log variation of mass fractions of some of the light and intermediate species of our simulation with radial distance (in cm). For clarity, we plot the curves alternately by solid and dotted type. In Fig. 2b, we plot the variation of more complex molecules. Since they start with zero abundances, we plot them from radius cm for clarity. We put a long dashed vertical line at 1AU, the distance of the earth with respect to the sun. On the upper axis, we have put time (in seconds) elapsed since the beginning of collapse at radial distances of cm, cm, cm and cm respectively. Towards the end of the collapse, time spent is negligible.
Because of low initial velocity, H, N, C and O are depleted much before 100AU and HCN, , etc. rises as also the more complex molecules (Fig. 2b) which form out of them. Inside , as matter falls faster and spends lesser time, the depletion of lighter molecules are controlled, until the density and temperature also rises so high that depletion started once more. Inside, cm, the composition changes in a very short timescale Around AU, the mass fractions of adenine, urea and glycine are already significant. At AU, . Since the mass of the earth is around g, this corresponds to g of adenine which could have contaminated the earth at the time of formation (it could be higher since metallic content of earth is much above the average molecular value. Also, one has to include the effect of dust-gas mixture in the molecular cloud) This computation, does not consider the destructions of adenine at a higher temperature region, and it is likely that much of these contaminants are destroyed during collapse and formation of proto-earth. However, comets formed in the inner cloud could carry away these pre-biotic molecules and deposit them during future impacts on planets. It is to be noted that around 1AU, the composition is close to reducing () type (Fig. 2a) which is favourable for the formation of bio-molecules.
Since regions of low density of molecular cloud could have a very long evolution time scale, it may of interest whether pre-biotic molecules could have formed in a very low density isothermal region. To this effect, we choose a cloud of mass density g cm-3 and K and let it evolve for years. Fig. 3 shows the results of this simulation where we plot the variation of abundance with time (in seconds). We find that the abundance of adenine, for instance, is around . This is a very generic condition, and the abundance is significant. We therefore believe that adenine could be produced during the molecular cloud collapse.
© European Southern Observatory (ESO) 2000
Online publication: January 31, 2000