On the formation of condensations in a gaseous nebula

Abstract

In order to get an insight into the process of formation of condensations in an ionized gas of high kinetic temperature, two main principles are applied:

• (1) that the pressure in the less dense medium 1 should be about the same as in the denser medium 2, which means that, in the denser medium, the kinetic or electron temperature is correspondingly lowered, and

• (2) that this lowering of electron temperature is brought about by electron impacts exciting low level lines. (If the optical depth of the condensations is large, so that the ionizing radiation is nearly completely absorbed, this effect may be enhanced.)

The approximate balancing of pressure is verified in the case of prominences considered as condensations in the solar corona (Section 1).

In order to account for condensations as observed by Baade in the planetary nebula NGC 7293 in Aquarius (Section 2) a very simplified model of a nebula is worked out, consisting of one substance only, occurring as ions A⁺ (for instance N⁺⁺) and “atoms” A (for instance N⁺),the latter having a low metastable level, while the ionization of A into A⁺ is fairly complete. Introducing the liberation temperature $\text{T}$ corresponding to the average energy of the photo-electrons when freed from A, it is shown that separation into two phases 1 and 2 will occur above a certain critical liberation temperature $\text{T}{}_{\mathrm{cr}}$ of about 50,000°, the curves of constant $\text{T}$ having a shape resembling those of the van der Waals theory. For $\text{T}\text{≅ 110,000°}$, or stellar temperature 170,000°, the volumes per unit mass in phases 1 and 2 have the ratio v₁/v₂ = 15, while the electron temperatures are T_ϵ1 = 130,000° and T_ϵ2 = 9000°, the latter being about the value derived in regions of planetary nebulae where forbidden lines are excited. Similar results can be expected for a nebula consisting of hydrogen with the substances A and A⁺ intermixed.

It is suggested that the cometary shape of many of the condensations observed by Baade in NGC 7293 might be due to aggregates of matter already present in interstellar space, serving as nuclei for condensation, the tails being formed by dragging along of matter by the outward moving gases of the nebula.