| Abstract |
Using the Arecibo Observatory, we have obtained neutral hydrogen (HI)
absorption and emission spectral pairs in the direction of 26 background
radio continuum sources in the vicinity of the Perseus molecular cloud.
Strong absorption lines were detected in all cases, allowing us to
estimate spin temperature (Ts ) and optical depth for 107
individual Gaussian components along these lines of sight. Basic
properties of individual H I clouds (spin temperature, optical depth,
and the column density of the cold and warm neutral medium (CNM and
WNM), respectively) in and around Perseus are very similar to those
found for random interstellar lines of sight sampled by the Millennium H
I survey. This suggests that the neutral gas found in and around
molecular clouds is not atypical. However, lines of sight in the
vicinity of Perseus have, on average, a higher total H I column density
and the CNM fraction, suggesting an enhanced amount of cold H I relative
to an average interstellar field. Our estimated optical depth and spin
temperature are in stark contrast with the recent attempt at using
Planck data to estimate properties of the optically thick H I. Only ~15%
of lines of sight in our study have a column density weighted average
spin temperature lower than 50 K, in comparison with >~ 85% of
Planck's sky coverage. The observed CNM fraction is inversely
proportional to the optical depth weighted average spin temperature, in
excellent agreement with the recent numerical simulations by Kim et al.
While the CNM fraction is, on average, higher around Perseus relative to
a random interstellar field, it is generally low, between 10%-50%. This
suggests that extended WNM envelopes around molecular clouds and/or
significant mixing of CNM and WNM throughout molecular clouds are
present and should be considered in the models of molecule and star
formation. Our detailed comparison of H I absorption with CO emission
spectra shows that only 3 of the 26 directions are clear candidates for
probing the CO-dark gas as they have N(H I)>1021
cm-2 yet no detectable CO emission. |