The Thermal Pressure of the Hot Interstellar Medium Derived from Cloud Shadows in the Extreme Ultraviolet
T. W. Berghoefer, S. Bowyer, R. Lieu, J. Knude;
ApJ, 1998, 500, 838
ABSTRACT:We have used the Deep Survey telescope of the Extreme Ultraviolet Explorer to investigate shadows in the diffuse EUV/soft X-ray background that
are cast by clouds in the interstellar medium.
We confirm the existence of
a shadow previously reported and provide evidence for two new
shadows.
We used IRAS data to identify the clouds producing these shadows and to
determine their optical depth to EUV radiation.
The EUV-absorbing clouds are
optically thick in the EUV, and all EUV emission detected in the direction of these
shadows must be produced from material in front of the clouds.
We obtained new
optical data to determine the distance to these clouds.
We use a new
differential cloud technique to obtain the pressure of the interstellar
medium.
These results do not depend on any zero-level calibration of the data.
Our
results provide evidence that the pressure of the hot interstellar gas is the
same in three different directions in the local interstellar medium and is
at least 8 times higher than that derived for the Local Cloud surrounding
our Sun.
This provides new evidence for large thermal pressure
imbalances in the local ISM and directly contradicts the basic assumption of
thermal pressure equilibrium used in almost all present models of the
interstellar medium.
KEYWORDS: ism: clouds, ultraviolet: ism, x-rays: ism
CODE: berghoefer98