Transcript - Lorentz Center

HCN, HNC, CN et al. in dense
depleted cores
Malcolm Walmsley
(Arcetri and Dublin)
With thanks to Marco Padovani and
Pierre Hily-Blant
Origins
• Attempts to find tracers of kinematics and
density structure in region where CO is
depleted
• Testing use of CN/HCN as a tracer of atomic
oxygen (O destroys CN but not HCN) and
nitrogen
• Understanding the N mystery (why “only”
NH3 and N2H+ survive in depleted regions)
Tafalla et al. L1498 Maps
Hily-Blant et al. 2008 CN data
• This showed however evidence that CN was abundant even
where CO was depleted in L183 and L1544
Red is N2H+ , Black is
13CN, grey continuum
Padovani et al.30m Observations
of L1498,TMC2,L1521
• Cuts in HCN,H13CN,HN13C show that some
HCN and HNC survives into the region where
CO depletes
Recent Results from 30m
(Padovani et al.)
Proxies for H2
• Dust emission or dust extinction (needs
knowledge of optical properties)
• Molecules like CO (or C18O) whose
abundance rel. to H2 is known??
• Molecular excitation which measures collision
rate with H2 and hence local density
(preferable from some points of view)
Measuring collisional rates
• Measure population ratios or excitation temperatures
via Boltzmann
n(u)/n(l)=(gu/gl)exp(-h*nu/kTex)
• The population ratio n(u)/n(l) is determined by
collisions with H2 (LVG models)
• Tex might be derived from
Intensity = B(Tex) fc (1-exp(-tau))
With tau optical depth, fc
beam filling factor, B Planck
function
Hyperfine fits to H13CN and HN13C
• Padovani et al. find that HC13N and HN13C have
sometimes non-negligible optical depths and hence
one can fit the hyperfine structure to determine
optical depth and excitation temperature
Surprise!
They are not thin
Total (not real) optical depths
in L1498
The HCN/HNC test
• Recently, Faure, Lique and collaborators
computed collision rates for HCN,HNC
• This resolved long standing HCN/HNC puzzle
that HN13C more intense than HC13N in
dark clouds
• Explanation is that the HNC-H2 collision rate
is larger than HCN-H2 at low(T=10K)
temperature
Consequence of being able to fit
optical depth for HN13C HC13N
• One can then determine (with reasonable
assumption about covering factor) Tex and
hence n(u)/n(l)
• We can also plot Tex(HN13C) against
Tex(HC13N) and compare with theory using
collision rates
Confirmation of the collisional
rates
• The observed Tex for HCN HNC fall on the
curve predicted for 10 K and range of
densities (RADEX results)
Tex(HCN), Tex(HNC) correlates
with H col.density
• The line excitation temperatures correlate with
col.density from mm dust emission. Thus, col.density
increase is density increase
Conclusions
• One can use a single 3mm transition with
hyperfine structure to derive local density if T
is known.
• We have an astronomical confirmation of
collisional rates
• The dust derived col.density is not fooling us
• The HCN/HNC abundance ratio is close to 1
as predicted from theory