Astronomy and Astrophysics – Astrophysics
Scientific paper
2002-02-10
Astrophys.J. 584 (2003) 797-817
Astronomy and Astrophysics
Astrophysics
43 pages, 23 figures, submitted to ApJ
Scientific paper
10.1086/345807
Far Ultraviolet radiation has been recognized as the main source of heating of the neutral interstellar gas, and, as a consequence, it determines whether the 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 generally originate in associations that form in Giant Molecular Clouds present in the galactic disk. Using McKee & Williams' (1997) distribution of birthrates for OB associations in the Galaxy, we determine the expected behavior of the time-dependent FUV field for random positions in the local ISM. The FUV field is calculated in two bands (912-1100 \AA and 912-2070 \AA) and at the wavelength 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 this band, and quite close to Draine's (1976) value, $G_0=1.7$. Due to attenuation by dust, only associations within about 500 pc contribute significantly to the energy 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 a variety of amplitudes, the larger ones being less frequent. The mean field is about twice the median field because of these fluctuations, or spikes, in the radiation field. These spikes, which last $\sim 30$ Myr, are caused by the infrequent birth of nearby associations. We also model shorter duration spikes caused by runaway OB stars. The presence of a fluctuating heating rate created by the fluctuating FUV field converts CNM to WNM and vice versa.
Hollenbach David J.
McKee Christopher F.
Parravano Antonio
No associations
LandOfFree
Time Dependence of the Ultraviolet Radiation Field in the Local Interstellar Medium does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Time Dependence of the Ultraviolet Radiation Field in the Local Interstellar Medium, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Time Dependence of the Ultraviolet Radiation Field in the Local Interstellar Medium will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-288622