Ionization Rate in Diffuse Clouds

Transcription

1 H 3 : A Tracr of th Cosmic Ray Ionization Rat in Diffus Clouds Bn McCall Dpartmnt of Chmistry Dpartmnt of Astronomy

2 Astronomr's Priodic Tabl H C N O N Mg Si S Ar F

3 H : 3 Cornrston of Intrstllar Chmistry C m H n HO 2 OH HCO 2 CHOH 2 5 CH OCH 3 3 CH OH 3 CH CO 2 HO 3 CH OH H 2 CO 3 CH 2 4 CH CO 3 HO CH 4 CH 3 2 HCO 2 3 CH 2 5 CH OH, 3 H 2 HO 2 CO CH OH CO CH 3 O CH 4 3 H CH 4 2 H 2 4 CH C CH 3 3 H 2 CH 3 C CH 2 2 CH 2 3 C CH 4 CH 5 H 2 CH 3 H 2 CH 2 H 2 CH C CO H 2 HCN N H 2 2 H 2 cosmic ray C 4 CH 3 CH CN 3 HCN 3 3 CH 2 HCN NH 3 N C HCO NH 2 CHCNH 2 5 CH CNH 3 HC N 11 HC N 9 HC N 7 HC N 5 HC N 3 CH NH 3 2 HCN 2 CHCN 2 5 CH CN 3 CH2NH CH3NH2 HCN H 2

4 Obsrving Intrstllar H 3 Equilatral triangl No rotational spctrum No lctronic spctrum Vibrational spctrum is only prob ν 1 Absorption spctroscopy against background or mbddd star ν 2

5 Intrstllar Cloud Classification* Dns molcular clouds: H H 2 C CO n(h 2 ) ~ cm -3 T ~ 20 K Diffus clouds: H H 2 C C n(h 2 ) ~ cm -3 [~10-18 atm] T ~ 50 K ζ Prsi Barnard 68 (courtsy João Alvs, ESO) * Snow & McCall, ARAA, 44, 367 (2006) Diffus atomic clouds H 2 << 10% Diffus molcular clouds H 2 > 10% (slf-shildd) Photo: Jos Frnandz Garcia

6 H 3 in Dns Clouds 1.02 R(1,1) u R(1,0) R(1,1) l AFGL AFGL Wavlngth (Å) N(H 3 ) = cm -2 McCall, Gball, Hinkl, & Oka ApJ 522, 338 (1999)

7 Dns Cloud H 3 Chmistry Formation cosmic ray H 2 H 2 - H 2 H 2 H 3 H (fast) Rat = ζ [H 2 ] Dstruction H 3 CO HCO H 2 Rat = k [H 3 ] [CO] McCall, Gball, Hinkl, & Oka ApJ 522, 338 (1999) Stady Stat = = ( s -1 ) ( cm 3 s -1 ) (6700) = 10-4 cm -3 Dnsity Indpndnt!

8 H 3 as a Prob of Dns Clouds Givn n(h 3 ) from modl, and N(H 3 ) from infrard obsrvations: path lngth L = N/n ~ cm ~ 1 pc dnsity n(h 2 ) = N(H 2 )/L ~ cm -3 tmpratur T ~ 30 K Uniqu prob of clouds Consistnt with xpctations 32.9 K confirms dns cloud chmistry (forbiddn) K

9 Diffus Molcular Cloud H 3 Chmistry Formation cosmic ray H 2 H 2 - H 2 H 2 H 3 H Rat = ζ [H 2 ] Dstruction H 3 - H H 2 or 3H Rat = k [H 3 ] [ - ] Stady Stat [H 3 ] ζ [H 2 ] = k [ - ] = ( s -1 ) ( cm 3 s -1 ) (2400) = 10-7 cm -3 Dnsity Indpndnt! 10 3 tims smallr than dns clouds!

10 Lots of H 3 in Diffus Clouds! HD Cygnus OB R(1,1) u R(1,0) Wavlngth (µm) R(1,1) l GC IRS 3 H 3 Column Dnsity (10 14 cm -2 ) HD Cyg OB2 5 HD WR 104 WR 121 HD χ 2 Ori P Cygni HD ζ Oph WR 118 Cyg OB2 12 N(H 3 ) ~ dns clouds n(h 3 ) ~ 1000 tims lss L ~ 1000 tims longr?!? McCall, t al. ApJ 567, 391 (2002) E(B-V) (mag)

11 Big Problm with th Chmistry! >1 ordr of magnitud!! Stady Stat: [H 3 ] ζ [H 2 ] = k [ - ] To incras th valu of [H 3 ], w nd: Smallr lctron fraction [ - ]/[H 2 ] Smallr rcombination rat constant k Highr ionization rat ζ

12 H 3 toward ζ Prsi [ - ]/[H 2 ] not to blam McCall, t al. Natur 422, 500 (2003) N(C ) from HST N(H 2 ) from Coprnicus Savag t al. ApJ 216, 291 (1977) Cardlli t al. ApJ 467, 334 (1996)

13 Big Problm with th Chmistry! Stady Stat: [H 3 ] ζ [H 2 ] = k [ - ] To incras th valu of [H 3 ], w nd: Smallr lctron fraction [ - ]/[H 2 ] Smallr rcombination rat constant k Highr ionization rat ζ

14 H 3 Dissociativ Rcombination Laboratory valus of k hav varid by 4 ordrs of magnitud! Problm: not masuring H 3 in ground stats

15 Storag Ring Masurmnts CRYRING 900 kv 12.1 MV 30 kv H3 20 ns 45 ns lctron bam H, H2 - Vry simpl xprimnt Complt vibrational rlaxation Control H3 - impact nrgy Rotationally hot ions producd No rotational cooling in ring

16 Suprsonic Expansion Ion Sourc Similar to sourcs usd for laboratory spctroscopy Pulsd nozzl dsign Suprsonic xpansion lads to rapid cooling Discharg from ring lctrod downstram Spctroscopy usd to charactriz ions Pinhol flang/ground lctrod Gas inlt 2 atm H V ring lctrod Solnoid valv H 3

17 Spctroscopy of H 3 Sourc Infrard Cavity Ringdown Lasr Absorption Spctroscopy Confirmd that H 3 producd is rotationally cold, as in intrstllar mdium McCall, t al. Natur 422, 500 (2003)

18 CRYRING Rsults Considrabl amount of structur (rsonancs) in th cross-sction k = cm 3 s -1 Factor of two smallr McCall, t al. Phys. Rv. A 70, (2004)

19 Back to th Intrstllar Clouds! Stady Stat: [H 3 ] ζ [H 2 ] = k [ - ] To incras th valu of [H 3 ], w nd: Smallr lctron fraction [ - ]/[H 2 ] Smallr rcombination rat constant k Highr ionization rat ζ

20 Implications for ζ Prsi N(H 3 ) L = [H 3 ] = ζ N(H 2 ) k N( - ) ζ L = k N(H 3 ) N( - ) ( cm 3 s -1 ) ( cm -2 ) ( ) N(H 2 ) ζ L = 5300 cm s -1 (firm) Adopt Adopt n = 215 cm -3 ζ= s -1 L=2.4 pc L = 60 pc ζ= s -1 n = 9 cm -3 (25x highr!)

21 What Dos This Man? Enhancd ionization rat in ζ Prsi Widsprad H 3 in diffus clouds prhaps widsprad ionization nhancmnt? Dns cloud H 3 is "normal" nhancd ionization rat only in diffus clouds low nrgy cosmic-ray flux? cosmic-ray slf-confinmnt? no constraints, asid from chmistry!! Substantial impact on diffus cloud chmistry

22 Nw Astronomical Rsults Rang of ζ from ~ s -1 Biggst uncrtainty is in adoptd n N. Indriolo, T. R. Gball, T. Oka, & B. J. McCall, in prparation

23 Coming Soon Mor obsrvations! Jun 2007: rvisit marginal dtctions, rduc limits Jan 2008?: rvisit Prsus OB2 association Nw lctron rcombination masurmnts Improvd spctroscopy of ion sourc Highr rsolution & highr snsitivity cw cavity ringdown spctroscopy diffrnc frquncy lasr Bttr charactrization of ro-vib distribution Us of pur para-h 2 to produc para-h 3 Singl quantum-stat CRYRING masurmnts

24 Acknowldgmnts Takshi Oka (U. Chicago) Tom Gball (Gmini) Staff of UKIRT (Mauna Ka) Unitd Kingdom InfraRd Tlscop Staff of CRYRING (Stockholm) Chris Grn (Bouldr) Eric Hrbst (Ohio Stat) Mik Lindsay (Eglin AFB) Takamasa Momos (UBC)

25 Acknowldgmnts Nick Indriolo (obsrvations) Brian Tom, Michal Wiczr, & Andrw Mills (lab work) Funding: NSF AMO NSF CHE NASA AFOSR Packard ACS Dryfus UIUC