The warm and cold neutral phase in the local interstellar medium at absolute value of B greater than or equal to 10 deg
W. G. L. Poppel, P. Marronetti, P. Benaglia;
AaA, 1994, 287, 601
ABSTRACT:We made a systematic separation of both the neutral phases using the atlases of 21-cm profiles of Heiles & Habing (1974) and Colomb et
al.
(1980), complemented with other data.
First, we fitted the emission of the
warm neutral medium (WNM) by means of a broad Gaussian curve (velocity
dispersion sigma approximately 10-14 km/s).
We derived maps of the column
densities NWH and the radial velocities VW of the
WNM.
Its overall distribution appears to be very inhomogeneous with a large
hole in the range b greater than or equal to +50 deg.
However, if the hole is
excluded, the mean latitude-profiles admit a rough cosec absolute value of b-fit
common to both hemispheres.
A kinematical analysis of VW for the
range 10 deg less than or equal to absolute value of b less than or equal to 40 deg
indicates a mean differential rotation with a small nodal deviation.
At
absolute value of b greater than 50 deg VW is negative, with larger
values and discontinuities in the north.
On the mean, sigma increases for
absolute value of b decreasing, as is expected from differential
rotation.
From a statistical study of the peaks of the residual profiles we derived
some characteristics of the cold neutral medium (CNM).
The latter is
generally characterized by a single component of sigma approximately 2-6
km/s.
Additionally we derived the sky-distribution of the column densities
NCH and the radial velocities VC of the CNM within bins of 1.2
deg sec b x 1 deg in l, b.
Furthermore, we focused on the characteristics of
Linblad's feature A of cool gas by considering the narrow ridge of local H I, which
appears in the b-V contour maps at fixed l (e.g.
Schoeber 1976).
The ridge
appears to be the main component of the CNM.
We suggest a scenario for the
formulation and evolution of the Gould belt system of stars and gas on the basis of an
explosive event within a shingle of cold dense gas tilted to the galactic
plane.
The scenario appears to be consistent with the results found for both the
neutral phases, as well as with Danly's (1989) optical and UV observations of
interstellar cool gas in the lower halo.
KEYWORDS: interstellar gas, kinematics, neutral gases, sky surveys (astronomy), solar neighborhood, astronomical maps, h i regions, molecular clouds, normal density functions, radial velocity, star formation
CODE: poppel94