NEUTRINO DECAY AND THE HEAT-BUDGET OF THE WARM INTERSTELLAR-MEDIUM

While neutrino decay photons could be the main source of the widespread ionization of the warm interstellar medium, and so of its cooling via the collisional excitation of C II, they are too soft to be its main heating agent. We rediscuss here the possibility that this agent is photoelectric emission by polycyclic aromatic hydrocarbons (PAHs), induced by stellar UV photons. We use recent HST observations of the UV spectrum of a halo star by Spitzer & Fitzpatrick (SF) to show, from the column density N of C II*, that the cooling rate per H nucleon in each of their four slowly moving warm absorption components has the same value to within 10%, namely 4 x 10(-26) ergs s-1. Other observational evidence suggests that this may be a general result for relatively undisturbed warm regions of the interstellar medium. The PAH heating mechanism would be adequate to compensate for this cooling if 21% of the C in each component were locked up in PAHs. This fraction agrees with that derived from the overall balance between the total heat input into the PAHs and their infrared emission. Together with the observed constancy of N(C II*)/N(H) in the different quiescent components, this suggests that the gas phase density of C in these components may be negligibly depleted. One would then have to halve the constant value of the electron density n(e) in the different components, derived by SF from the excitation of C II. The revised value of n(e) is 0.055 +/- 0.005 cm-3, which agrees well with values previously derived for other occupied regions by Reynolds (1990a, 1991a) and by Sciama (1990b) using Halpha and pulsar dispersion data. Both this constancy of n(e), and the deduction by SF that along their line of sight most of the free electrons and photons are probably completely mixed up with the H I in warm regions opaque to Lyman-continuum photons, provide strong support for the neutrino decay theory. This theory could be stringently tested by HST observations of the UV absorption spectra of warm quiescent regions along lines of sight to other stars.