High-frequency Carbon Recombination Line as a Probe to Study the
Environment of Ultra-compact HII regions
Anish ROSHI
Raman Research Institute, Bangalore
Résumé :
A recent Arecibo survey of carbon recombination lines (CRLs)
near 9 GHz toward Ultra-compact HII (UCHII) regions have detected lines
toward a majority (11 out of 17 sources; 65\% detection rate) of the
selected sources. Further high-resolution observations made with the VLA
(Very Large Array) toward two UCHII regions -- W48A and W49-North -- show
that the CRLs are detected against the bright continuum emission from the
UCHII regions. The detection against the continuum emission suggests that
the regions responsible for CRL emission are associated with UCHII
regions. CRL emission originates from photo-dissociation regions (PDRs)
formed at the interface between the HII regions and the dense ($> 10^5$
cm$^{-3}$) natal molecular clouds harbouring them. The far-ultraviolet (6 to
13.6 eV) photons from OB stars embedded in the UCHII regions produce these
PDRs where carbon will be ionised. We have developed non-LTE models for
the CRL emission observed toward a subset of UCHII regions selected from
the Arecibo survey and VLA observations. In particular, detailed modelling
has been done toward W48A were both multi-frequency (5, 8, 14 \& 42 GHz)
and high-resolution data were available. In this modelling, the
temperature, electron and carbon ion density profiles as a function of
depth into the PDR from the surface of UCHII region were taken from
one-dimensional PDR model of Le Bourlot et al. (1993). Our studies show
that a multi-frequency, high-resolution CRL data set can be use to : (1)
estimate the physical properties of the PDR material; (2) study the
kinematics of the PDR material relative to the HII region gas and (3)
constrain the magnetic fields in the vicinity of UCHII regions. We further
show that the derived properties of the environment and HII region as well
as their kinematics can be used to address the lifetime problem of UCHII
regions by investigating whether the HII regions are confined in the natal
cloud.
