ANTECEDENTES HISTÓRICOS DEL SEGURO SOCIAL

- 1 2 -14 & d) -16 -18

C O

CH

Time (Days) Time (Days)

l"igure 4.7: S en sitiv ity of species to d e n s ity in th e PA H re a c tio n n e tw o rk . M o d e ls s t a r t e d a t 30 d a y s p o s t - o u t b u r s t w ith d e n s ity of ?i = 10^^ c m “ ^ (solid line); n = 10® c m “ ^ (d a sh e d line); 7? = 10' c m " ^ ( d o t t e d line).

148 CHAPTER 4. LATE CHEMICAL EVOLUTION

and through ‘burning’ with oxygen, for example: C2 4" O —^ CO + C

This is in contrast to the relative insensitivity of C2 to oxygen fraction in the hydrocarbon

chemistry. As with all nova chemistry, most abundances reach some equilibrium within a very few seconds. No m atter w hat the initial abundance of C2H2, it is immediately

reduced to a relative abundance of some 1 0“ ^^ from which it recovers to peak at 1 0“ ^^,

as has already been described. Equilibrium is more or less established by this time and the abundance then falls primarily as a result of the geometry and gradual consumption by the chemistry. A t such low abundances, the rate at which molecules are consumed is also low.

A key species is H2; the rôle of this molecule in guiding the chemistry in its run through

to a steady state has already been noted. This has long term consequences for the local equilibrium state achieved in the ten days after the sta rt of the model. The balance is very delicate and it is found th a t changing the H2 fraction may juggle the abundances

of the larger molecules which lie in the 1 0“ ^° < Xi < region whilst not changing

the overall chem istry to a great extent. Indeed, species such as C4H5S and C4H3U are

enhanced by ju st ~ 1 order of magnitude throughout the model run when the H2 relative

abundance a t 30 days (the model start) is increased by six orders of magnitude from 10~® through to 10“ ^! This is in stark contrast to the work of Rawlings and Williams (1989) where molecular hydrogen was required in very large abundance (AH2I > 1 0“ ^) in order

to form nucléation sites.

W ithout doubt this chemistry is remarkably stable, showing great sensitivity only to density and the presence of a selection of large hydrocarbons in the initial conditions. Such stability is very interesting in the context of an astrophysical environment th a t is observed to be generate dust as efficiently and consistently as a nova,

4.3.5

C on clusions

As a result of the degree of pruning, modification and addition required to transform a complete, but inoperable, PAH formation chemistry into a chemistry th a t worked in this environment, the two chemistries experimented with in this chapter have become very similar. The latter, larger chemistry contains a number of very carefully worked out net­ works with the specific task of building aromatic hydrocarbons. W hilst this chemistry

4.3. THE PAH REACTION SCHEME 149

has been borrowed from cool star research, this is still far more appropriate to the nova than interstellar chemistry! It is found th a t in both cases it is possible to generate stable abundances of large molecules of sufficient size to provide nucléation sites for the develop­ ment of dust grains. The stability of these molecules, in stark contrast to the short lived molecules formed in Rawlings and Williams (1989), is assured as a result of the much greater abundances than in the latter work; once equilibrium is reached, equilibrium re­ actions between the numerous large species maintains a stable population of hydrocarbon species. The simple hydrocarbon chemistry is found to generate high abundances of Ca; (where x =2-8) and five atom molecules such as C2H3 to a relative abundance of ~ 1 0“ ^°.

This is found to provide more than enough nucléation sites to allow relatively inefficient nucléation producing a large number of small grains rather than the small number of large grains th a t would be required given an abundance of nucléation sites much closer to the borderline density of ~ 1 0“ ^^.

The PAH bcLsed reaction scheme results in much lower abundances of very small type molecules but, given a sufficiently dense ejecta, is found capable of producing relative abundances of much larger molecules in the region of This is still 2-3 orders of m agnitude greater than the estim ated critical abundance of nucléation sites required to produce dust on the scale observed in the most efficient novae. As such, once again, a small population of very large grains is not required to explain observations, but a much larger population of lesser grains may be invoked. Presum ably the growth of small grains may be more easily achieved than the rapid development of very large grains.

An appropriate physical environment is key to the successful development of nucléation sites, in particular for the PAH-derived chemistry. Throughout all the work of this thesis, high densities approaching 10^^ cm“ ^ a t ~ 3 days post-outburst are found to be required to sustain the richest chemistry. In contrast to this sensitivity, the tem p eratu re of the ejecta a t all times is of relatively little consequence. This implies th a t the geometric configuration of the ejecta and fluid dynamic effects should be significant in determining the ultim ate product of the chemical evolution of the ejecta. In the case of the PAH chemistry, it is found th a t the requirement th a t the density fall within certain bounds provides a constraint on the overall density of the ejecta based on the tim e after outburst at which dust is observed to form. Nucléation at very early times (e.g. < 100 days) requires a /esser density than for nucléation later on. By contrast, the simple hydrocarbon chemistry operates across a far wider range of densities and as a result is not constrained

150 CHAPTER 4. LATE CHEMICAL EVOLUTION

in the same way.

Both models reach an equilibrium state with high abundances of molecules th a t might be classified as nucléation sites irrespective of the oxygen fraction. This is the first time th a t it is suggested th a t carbon dusts might form in an environment where the oxygen is not bound in the stable CO molecule. Indeed, CO does noi saturate in any of these models no m atter w hat the C :0 ratio. This being the case, a m ajor concern has been eliminated from the nova dust problem.

C h ap ter 5