hcn, hnc, cn et al. in dense depleted cores
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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 - PowerPoint PPT PresentationTRANSCRIPT
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HCN, HNC, CN et al. in dense depleted cores
Malcolm Walmsley (Arcetri and Dublin)
With thanks to Marco Padovani andPierre Hily-Blant
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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)
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Tafalla et al. L1498 Maps
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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 is13CN, grey continuum
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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
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Recent Results from 30m (Padovani et al.)
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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)
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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
With tau optical depth, fc beam filling factor, B Planck function
Intensity = B(Tex) fc (1-exp(-tau))
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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
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Total (not real) optical depths in L1498
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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
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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
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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)
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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
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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