June 21, 201166th International Symposium on Molecular Spectroscopy

Detection of FeCN (X4Di) in the Circumstellar Envelope of IRC+10216

Lindsay N. Zack, DeWayne T. Halfen, and Lucy M. Ziurys

Department of Chemistry and Biochemistry, Department of Astronomy and Steward Observatory, Arizona Radio Observatory,

University of Arizona

June 21, 201166th International Symposium on Molecular Spectroscopy

• AGB star• Age = 10,000 - 30,000 yr• Distance = 150 pc (31 × 106 AU)• High mass-loss rate (3 × 10-5

Myr-1)

• Most-studied star in mm observations

• Rich chemistry, 60+ molecules

IRC+10216

June 21, 201166th International Symposium on Molecular Spectroscopy

Metal-containing molecules in the ISM

• Seen in circumstellar envelopes (e.g. IRC+10216, VY CMa) and proto-planetary nebulae (e.g. CRL618, CRL2688)

• Halides and Cyanides common carriers in C-rich sources– e.g. NaCl, AlCl, KCl, KCN, MgNC, AlNC

• Oxides and Hydroxides carriers in O-rich sources– AlO, AlOH

• Optical spectra of stars– TiS, TiO, YS, ZrS

June 21, 201166th International Symposium on Molecular Spectroscopy

Cosmic Abundances

Iron PeakNa-, Mg-, Al-, and K-bearing

molecules detected in IRC+10216.

Fe-containing molecules??

June 21, 201166th International Symposium on Molecular Spectroscopy

Iron Abundance

• Cosmic abundance: Fe/H ~ 3 × 10-5

– Order of magnitude greater than Na, Al, and K

• Depletion– Most of iron condensed into grains? (e.g. Savage & Bohlin 1979)

– Only 1% in gas phase in diffuse clouds (Jensen & Snow 2007)

• Fe in IRC+10216 (Mauron & Huggins 2010)

– Gas phase abundance N(Fe I + Fe II)/N(H) ~1.7 × 10-7

June 21, 201166th International Symposium on Molecular Spectroscopy

Previous Searches for Fe-bearing molecules

• FeO– Merer et al. (1982)

• Survey of several stars, molecular clouds, and planetary nebulae

• Upper limit: Ntot < 2 × 1012 cm-2

– Walmsley et al. (2002)• Tentative detection of J = 5 4 transition in W = 4 ladder toward

Sgr B2(M) with IRAM 30 m telescope, but no other transitions observed

• FeCO– Kagi et al. (1997)

• Survey of star-forming regions and IRC+10216

• Upper limits: 8 × 1011 < Ntot < 3 × 1013 cm-2

June 21, 201166th International Symposium on Molecular Spectroscopy

FeCN (X4Di) in the Lab

• 5 year laboratory search by Ziurys group (Mike Flory)– Uncertainty about lowest

energy isomer (FeNC vs. FeCN) and ground state (6Di or 4Di)

• Determined accurate rest frequencies for astronomical search

Flory & Ziurys, submitted

June 21, 201166th International Symposium on Molecular Spectroscopy

Observations

• ARO 12m telescope on Kitt Peak

• Nine transitions from 68-148 GHz

• ~100+ hours per transition

• rms noise level < 1 mK

Search for FeCN began April 2007

KP 12m

… it’s gotta be there…

June 21, 201166th International Symposium on Molecular Spectroscopy

• FeCN definitively detected in IRC+10216• Emission at all nine transitions observed• Three “clean” transitions (84, 100, and 108

GHz)• Confirmation on IRAM 30m in Spain

Zack et al. 2011Cernicharo et al. 2008

ResultsIRAM

FeCN observed at IRAM 30m telescope in Spain

June 21, 201166th International Symposium on Molecular Spectroscopy

First definitive detection of an iron-containing molecule in the

ISM

Abundances of Cyanides in IRC+10216

SpeciesSource Size

(")Observed

Abundance Reference

KCN 30 6 × 10-10 Pulliam et al. 2010

NaCN 5 2 × 10-8 Guelin et al 1993; Highberger & Ziurys 2003

AlNC 15 3 × 10-10 Ziurys et al. 2002

MgCN 40 7 × 10-10 Ziurys et al. 1995

MgNC 40 9 × 10-9 Guelin et al. 1993; Highberger & Ziurys 2003

FeCN 30 2-7 × 10-10 Zack et al. 2011

June 21, 201166th International Symposium on Molecular Spectroscopy

Possible Formation Pathways

Radiative Association / Dissociative Electron Recombination

Fe+ + HCnN FeNCnH+ + hn (k = 6.6 × 10-9(T/300)-0.47 cm3 s-1)

FeNCnH+ + e- FeNC + Cn-1H (followed by isomerization to FeCN)

Strong possibility of FeNC, as well.

Insertion (k = 10-11 cm3 s-1)

Fe+ + HCnN NCFeCn-1H+ + hn

NCFeCn-1H+ + e- FeCN + Cn-1H

Radical-radical (k = 10-11 cm3 s-1):

Fe + MgNC FeCN + Mg

• FeCN formed in outer envelope of IRC+10216• Fast reactions/ low barriers• Ions and radicals

Petrie 1997; Petrie et al. 1996; Dunbar & Petrie 2002; Cordiner & Millar 2009

June 21, 201166th International Symposium on Molecular Spectroscopy

Conclusions

• FeCN is the 6th metal cyanide detected in IRC+10216

• Ntot = 8.6 × 1011 cm-2; Trot = 21 K; [FeCN]/[H2] ~ 2-7 × 10-10

• First transition-metal bearing molecule in ISM• Iron is not completely depleted in the ISM• Other Fe-containing molecules could be detectable• Reaction rates and formation pathways for iron-bearing

molecules need to be studied by theoreticians and experimentalists

June 21, 201166th International Symposium on Molecular Spectroscopy

Acknowledgements

• Prof. Lucy Ziurys• Ziurys Group• ARO telescope operators, engineers, and staff

• NSF and NASA