HCN, HNC, CN et al. in dense depleted cores

Malcolm Walmsley (Arcetri and Dublin)

With thanks to Marco Padovani andPierre 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 is13CN, 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

With tau optical depth, fc beam filling factor, B Planck function

Intensity = B(Tex) fc (1-exp(-tau))

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