Presentation #112.02 in the session “Origin of Cores and IMF”.
The neutral hydrogen (HI) structure of a straight segment of an Hα filament discovered by Ogden & Reynolds (1985) has been studied using Effelsberg-Bonn HI survey data. The HI structure is directly associated with the Hα and is found at the same velocity, -62 km/s, and with the same line width, 26 km/s. If interpreted as thermal broadening, a line width of 26 km/s implies a temperature of 15,000 K at which temperature the hydrogen would be ionized and rendered invisible to 21-cm observations. Having cold hydrogen capable of producing 21-cm emission so closely associated with the Hα filament, which is believed to be evidence of a warm ionized medium (8000 K), poses challenges to thermodynamic models. An alternative model for this association will be proposed.