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Astron. Astrophys. 344, 105-110 (1999)

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4. Concluding remarks

In this paper, we have shown that hot flows may produce neutron disks around black holes, where neutron abundance is significant. However, unlike neutron stars, the formation of which is accompanied by the production of neutron rich isotopes, neutron disks do not produce significant neutron rich elements. Some fragile elements, such as deuterium, could be produced in the cooler outflows as follows:

Neutrons and protons may be released in space through winds which are produced in the centrifugal barrier. These winds are common in black hole sources and earlier they have been attributed to the dispersal of magnetic fields to the galactic medium (Daly & Loeb 1990; Chakrabarti et al. 1994). Recently, Chakrabarti (1998) and Das & Chakrabarti (1998), through a first ever self-consistent calculation of outflows out of accretion, found that significant winds can be produced and for low enough accretion rates, disks may even be almost evacuated causing the formation of quiescence and inactive states such as what is observed in V404 Cyg and our Galactic centre. If the temperature of the wind falls off as [FORMULA] and density as [FORMULA] (as is expected from an outflow of insignificant rotation), the deuterium synthesis rate [FORMULA], increases much faster very rapidly than the reverse ([FORMULA]) process. For instance, with density and temperature mentioned as in the earlier section, at [FORMULA], the forward rate ([FORMULA]) is [FORMULA] while the reverse rate is much higher: [FORMULA]. This results in the dissociation of deuterium. However, at [FORMULA], the above rates are [FORMULA] and [FORMULA] respectively and at [FORMULA], the above rates are [FORMULA] and [FORMULA] respectively. Thus a significant deuterium could be produced farther out, say, starting from a distance of [FORMULA]. Ramadurai & Rees (1985) suggested deuterium formation on the surface of ion tori. As we establish here, this process may be feasible, only if these tori are vertically very thick : [FORMULA]. In any case, deuterium would be expected to form in winds and disperse.

In a typical case of a disk with an accretion rate of [FORMULA], the temperature is lower, but the density is higher. In that case, the photo-dissociation of [FORMULA] is insignificant and typically the change in abundances of some of the elements, such as [FORMULA], [FORMULA] etc. could be around [FORMULA] not as high as that of the neutron as in above cases where [FORMULA]. One could estimate the contamination of the galactic metalicity due to nuclear reactions as we do for realistic models. Assume that, on an average, all the N stellar black holes of equal mass M have a non-dimensional accretion rate of around [FORMULA] ([FORMULA]). Let [FORMULA] be the typical change in composition of this matter during the run and let [FORMULA] be the fraction of the incoming flow that goes out as winds and outflows, then in the lifetime of a galaxy (say, [FORMULA] yrs), the total `change' in abundance of a particular species deposited to the surroundings by all the stellar black holes is given by:


We here assume a conservative estimate that there are [FORMULA] such stellar black holes (there number varies from [FORMULA] (van den Heuvel 1992, 1998) to several thousands (Romani, 1998) depending on assumptions made) and the mass of the host galaxy is around [FORMULA] and the lifetime of the galaxy during which such reactions are going on is about [FORMULA]Yrs. We believe that [FORMULA] is quite reasonable for a typical case when [FORMULA] and a fraction of ten percent of matter is blown off as winds. When [FORMULA] or the outflow rate is higher (particularly in presence of strong centrifugal barrier) the contamination would be even higher.

It is to be noted that our assertion of formation of neutron disks around a black hole for very low accretion rate [FORMULA] is different from that of the earlier results (Hogan & Applegate 1987) where [FORMULA] was believed to be the more favourable accretion rate. This is because in last decades the emphasis was on super-Eddington thick accretion tori. More recent computations suggest that advective regions are not as hot when the rates are very high. Another assertion of our work is that [FORMULA] should not be produced in accretion disks at all. This is not in line with earlier suggestions (Jin 1990) also. That is because unlike earlier case where the spallation reaction [FORMULA] +[FORMULA] was dealt with in isolation, we study this in relation to other reactions prevalent in the disk. We find that [FORMULA] could be dissociated much before it can contribute to spallation. However, our work supports Ramadurai & Rees' (1985) conjecture that deuterium may be produced in the outer regions of the disk provided the disk is at least as thick as [FORMULA].

In the process of performing the simulation we were faced with a challenge which was never addressed earlier in the literature. The problem arises because the inflow under consideration is optically thin vertically, but optically thick horizontally. As a result, photons emitted form a power-law spectrum. Question naturally arises, whether these power-law photons are capable of photo-disintegration. We find that the answer is yes and that the calculation of usual photo-disintegration gives a lower limit of the changes in the composition. In the extreme conditions close to the black hole, such processes are sufficiently effective to produce neutron disks around black holes.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999