1202.2850 (David A. Neufeld)
David A. Neufeld
Motivated by recent observations with Herschel/PACS, and the availability of
new rate coefficients for the collisional excitation of CO (Yang et al. 2010),
the excitation of warm astrophysical CO is revisited with the use of numerical
and analytic methods. For the case of an isothermal medium, results have been
obtained for a wide range of gas temperatures (100 to 5000 K) and H2 densities
(1E+3 to 1E+9 cm-3), and presented in the form of rotational diagrams, in which
the logarithm of the column density per magnetic substate, log (N[J]/g[J]), is
plotted for each state, as a function of its energy, E[J]. For rotational
transitions in the wavelength range accessible to Herschel/PACS, such diagrams
are nearly linear when n(H2) > 1E+8 cm-3. When log10(n[H2]) = 6.8 to 8, they
exhibit significant negative curvature, whereas when log10(n[H2]) < 4.8 the
curvature is uniformly positive throughout the PACS-accessible range. Thus, the
observation of a positively-curved CO rotational diagram does not NECESSARILY
require the presence of multiple temperature components. Indeed, for some
sources observed with Herschel/PACS, the CO rotational diagrams show a modest
positive curvature that can be explained by a single isothermal component.
Typically, the required physical parameters are H2 densities in the 1E+4 to
1E+5 cm-3 range and temperatures, T, close to the maximum at which CO can
survive. Other sources exhibit rotational diagrams with more curvature than can
be accounted for by a single temperature component. For the case of a medium
with a power-law distribution of gas temperatures, with dN/dT proportional to T
to the power -b, results have been obtained for H2 densities 1E+3 to 1E+9 cm-3
and power-law indices, b, in the range 1 to 5; such a medium can account for a
CO rotational diagram that is more positively curved than any resulting from an
isothermal medium.
View original:
http://arxiv.org/abs/1202.2850
No comments:
Post a Comment