THE THERMALLY UNSTABLE WARM NEUTRAL MEDIUM: KEY FOR MODELING THE INTERSTELLAR MEDIUM

We present 21 cm absorption measurements toward 12 radio continuum sources with previously identified thermally unstable warm neutral medium (WNM). These observations were obtained with the Expanded Very Large Array and were complemented with the Hi emission spectra obtained with the Arecibo Observatory. Out of 12 sources, Hi absorption was detected along five lines of sight (seven new absorption features in total), resulting in a detection rate of similar to 42%. While our observations are sensitive to the WNM with a spin temperature T(s) < 3000 K, we detected only two wide absorption lines with T(s) = 400-900 K. These temperatures lie above the range allowed for the cold neutral medium (CNM) by the thermal equilibrium models and signify the thermally unstable WNM. Several absorption features have an optical depth of only a few x 10(-3). While this is close to or lower than what is theoretically expected for the CNM, we show that these weak lines are important for constraining the fraction of the thermally unstable WNM. Our observations demonstrate that, for the first time, high bandpass stability can be achieved with the Very Large Array, allowing detection of absorption lines with a peak optical depth of similar to 10(-3).