Time Dependence of the Ultraviolet Radiation Field in the Local Interstellar Medium

Far Ultraviolet radiation has been recognized as the main source of heatingof the neutral interstellar gas, and, as a consequence, it determines whetherthe thermal balance of the neutral gas results in cold ($T\sim 50 - 100 K$)clouds (CNM), warm ($T \sim 10^4 K$) clouds (WNM), or a combination of the two.High FUV fields convert the neutral gas to WNM, while low fields result in CNM.The sources of FUV radiation are the short-lived massive stars that generallyoriginate in associations that form in Giant Molecular Clouds present in thegalactic disk. Using McKee & Williams' (1997) distribution of birthrates for OBassociations in the Galaxy, we determine the expected behavior of thetime-dependent FUV field for random positions in the local ISM. The FUV fieldis calculated in two bands (912-1100 \AA and 912-2070 \AA) and at thewavelength 1400 \AA. Our median value for the [912-2070 \AA] band is $G_0=1.6$times Habing's (1968) value for the radiation field at the solar circle in thisband, and quite close to Draine's (1976) value, $G_0=1.7$. Due to attenuationby dust, only associations within about 500 pc contribute significantly to theenergy density at a given point. The FUV field is asymmetric at a given point,and the asymmetry grows for higher fields. The FUV field fluctuates with avariety of amplitudes, the larger ones being less frequent. The mean field isabout twice the median field because of these fluctuations, or spikes, in theradiation field. These spikes, which last $\sim 30$ Myr, are caused by theinfrequent birth of nearby associations. We also model shorter duration spikescaused by runaway OB stars. The presence of a fluctuating heating rate createdby the fluctuating FUV field converts CNM to WNM and vice versa.Comment: 43 pages, 23 figures, submitted to Ap