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Chemistry in low-mass star forming regions: ALMA ’ s contribution Yuri Aikawa (Kobe Univ.) Collaborators: Hideko Nomura (Kobe Univ.) Hiroshi Koyama (Kobe

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Text of Chemistry in low-mass star forming regions: ALMA ’ s contribution Yuri Aikawa (Kobe Univ.)...

  • Slide 1
  • Chemistry in low-mass star forming regions: ALMA s contribution Yuri Aikawa (Kobe Univ.) Collaborators: Hideko Nomura (Kobe Univ.) Hiroshi Koyama (Kobe Univ.) Valentine Wakelam (Obs de Bordeaux) Robbin Garrod (OSU) Paola Caselli (Arcetri) Eric Herbst (OSU)
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  • Contents 1.Chemical fractionation in prestellar cores and molecular clouds 2. From prestellar cores to protostellar cores 3. Protoplanetary disks talk by Guilloteau
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  • L1544 dust peak CCS N2H+N2H+ Ohashi et al. (1999) Tafalla e al (2002 & talk) Chemical Fractionation in Prestellar Cores Depletion of C-bearing species - destruction of early-phase species (CS,CCS,..) in the gas phase - CO freezes-out onto grains freeze several 10 5 (10 4 cm -3 /n H ) yr cf. cont ~ several 10 5 (10 4 cm -3 /n H ) 1/2 yr non-depletion of N 2 H + and NH 3 - depletion of CO, which is the main reactant of N 2 H + - slow formation of N 2 Aikawa et al. (2001; 2005) see also Maret et al. (2006)
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  • Deuterium enrichment in Prestellar Cores High molecular D/H ratios D 2 CO/H 2 CO=0.01-0.1 (Bacmann et al. 2003) N 2 D + /N 2 H + =0.2 cf. Elemental abundance: D/H @L1544 (Caselli et al. 2003) talk by Lis
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  • Mechanism of Deuterium Enrichment Exothermic exchange reactions H 3 + + HD H 2 D + + H 2 + E 1 CO depletion enhances H 2 D + / H 3 + H 2 D + + e H 2 + D H 2 D + + CO HD + HCO + Propagation by ion-molecule reactions in the gas phase H 2 D + + X XD + + H 2 Deuteration on grain surfaces Hydrogenation with abundant D atoms (originates in H 2 D + + e H 2 + D) Exchange reactions of CH 3 OH on grain surfaces (Nagaoka et al. 2005) CH 3 OH + D CH 2 DOH + H, CH 2 D OH + D CD 2 HOH + H, L1544 gray: dust solid: H 2 D + Vastel et al. (2006) HD 2 + and D 3 + are produced subsequently If the core is heated H 2 D + + H 2 H 3 + + HD 10 4 sec @T=50K, n(H 2 )=10 6 cm -3 H 2 D + decreases rapidly Other species (without direct exchange) survive to be observed in protostellar cores
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  • Variation among Cores L1544 Dynamical evolution No infall Infall Infall Chemical evolution Low D/H ratio Low D/H ratio High D/H ratio CCS central peak CCS central peak CCS central hole No depletion Small depletion? Significant depletion No NH3, N2H + Central NH3, N2H + Central NH3, N2H + L1521B 10000 AU L492 10000 AU (Hirota & Yamamoto 2006, Crapsi et al. 2006, Aikawa et al. 2005, Tafalla & Santiago 2004, Lee at el 2003, Aikawa et al. 2001) CCS
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  • Clumps and chemical differentiation in clouds - Intensity distribution varies with species Talk by Takakuwa CH 3 OH H 13 CO + 15000 AU TMC-1C - Small clumps ( 2000AU, 0.02M sun ) inside cores - Gravitationally unbound - Correlation with physical condition is not yet found
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  • Summary on Prestellar Cores Chemical Fractionation: current: Depletion of CO and non-depletion of N-species Line survey towards CO-depleted cores (Tafalla et al. 2006) future: Deep look at the freeze-out region Statistics - correlation between physical evolution chemical signature - difference between clouds Small clumpy structures - smallest size of clumps ? - correlation with physical structure ? Deuterium Enrichment: current: High D/H ratio towards prestellar/protostellar cores Spatial distribution of H 2 D + and HD 2 + in prestellar cores future: H 2 D + and HD 2 + observation by interferometer indicator of cores right before star-formation constraints on chemical reaction network
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  • From prestellar cores to protostellar cores cold prestellar cores compressional heating > cooling (by radiation) heating by accretion and a protostar log r [AU] log density [g cm -3 ] temperature [K] 1D radiation hydrodynamics Masunaga & Inutsuka (2000)
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  • From prestellar cores to protostellar cores cold prestellar cores compressional heating > cooling (by radiation) heating by accretion and a protostar log r [AU] log density [g cm -3 ] temperature [K] 1D radiation hydrodynamics Masunaga & Inutsuka (2000)
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  • From prestellar cores to protostellar cores cold prestellar cores compressional heating > cooling (by radiation) heating by accretion and a protostar log r [AU] log density [g cm -3 ] temperature [K] 1D radiation hydrodynamics Masunaga & Inutsuka (2000)
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  • From prestellar cores to protostellar cores cold prestellar cores compressional heating > cooling (by radiation) heating by accretion and a protostar As the core gets warmer - Sublimation of ice - CO: 20 K - H 2 O: 160K - large organic molecules: 100K
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  • CO sublimates at 20 K - CO lines become observable again ! - CO kills N 2 H + and NH 3 benefits CS and HCO + CO CS H 2 CO HCN NH 3 N2H+N2H+ HCO + log n(i)/n H -5 -10 -15 log r [pc] -3-2 Lee et al. (2004) CO sublimation freeze-out
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  • CO sublimates at 20 K larger organic species (ex. CH 3 OH) first core n 10 13 cm -3 second core t=0 t = -770yr t=9x10 4 yr Aikawa et al. (in prep) based on Masunaga & Inutsuka (2000) Sublimation radius - CO lines become observable again ! - CO kills N 2 H + and NH 3 benefits CS and HCO + R 20K R 100K prestellar ~10 13 cm -3 ~10 AU 1 st core several 10 AU a few AU 2 nd core ~100 AU ~10 AU 9*10 4 yrs protostar several 10 3 AU 100AU CO sublimation
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  • Complex Species in Low-mass Cores SMA observation of IRAS 16293-2422(Kuan et al. 2004) - Detection of complex species toward IRAS 16293-2422, NGC1333 (talks by van Dishoeck and Sakai) - Abundances varies among cores HCOOH (line contour) Remijan & Hollis (2006) - Some species are confined, some are extended - No evident dependence on CH 3 OH abundance - HCOOH/CH 3 OH is higher than in high-mass hot core Bottinelli et al. (2006)
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  • Complex Species in Low-mass Cores - How are they formed ? Grain-surface reactions e.g. CO CH 3 OH (Watanabe & Kouchi 2002) Gas-phase reactions of sublimates ex. CH 3 OH 2 + + H 2 CO HC(OH)OCH 3 + + H 2 inefficient (Horn et al. 2004) break-up in the recombination (Geppert et al. 2006) grain-surface reactions during warm-up (Garrod & Herbst 2006) Molecules freeze-out on grains grain/ice surface reaction between heavy species grain/ice surface hydrogenation
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  • Calculation from a Prestellar to Protostellar Core Physical model of core contraction and protostar formation (Masunaga & Inutsuka 2000) Chemical model of gas & grain-surface reactions (Garrod & Herbst 2006) + Short warm-up phase: R warm /v infall T > 20 K 10 4 yr T > 100K 10 2 yr 9 x 10 4 yr after 2 nd collapse Distribution of gas and ice at each evolutionary stage Aikawa et al. in prep
  • Slide 18
  • Calculation from a Prestellar to Protostellar Core Physical model of core contraction and protostar formation (Masunaga & Inutsuka 2000) Chemical model of gas & grain-surface reactions (Garrod & Herbst 2006) + Short warm-up phase: R warm /v infall T > 20 K 10 4 yr T > 100K 10 2 yr gas phase ice mantle 9 x 10 4 yr after 2 nd collapse Distribution of gas and ice at each evolutionary stage Aikawa et al. in prep
  • Slide 19
  • Calculation from a Prestellar to Protostellar Core - Spatial Distribution CH 3 CN, HCOOH extends to 1000 AU CH 3 OH, CH 3 OCH 3 sharp rise at 100 AU - Formation mechanism CH 3 OCH 3 formed from CH 3 OH via gas-phase reaction other species combination of gas-phase and grain-surface reactions - The abundances are smaller than observed in IRAS16293-2422, NGC1333 gas phase ice mantle 9 x 10 4 yr after 2 nd collapse Aikawa et al. in prep
  • Slide 20
  • Summary on protostellar cores As the core temperature rises - heavy-element species migrate and react on grain surfaces - ice sublimates - sublimates react with each other in the gas phase formation of larger molecules or destrcution current challenges: Interferometric observation of IRAS 16293-2422 - spatial distribution varies with species why ? Observation of other low-mass YSOs (Talk by Sakai) - when the complex molecules become abundant ? - Difference between low-mass hot cores and high-mass hot cores Fully dynamical model with gas-phase and grain-surface reactions
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  • ALMAs contribution on protostellar core High sensitivity detection of weak lines of complex species: 18.5 [email protected] vs 4 [email protected] - How complex the interstellar molecules can be ? - More statistics
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  • ALMAs contribution on protostellar core - Spherical symmetry ? NO! magnetic fields and rotation High spatial resolution - Derive molecular abundance without beam dilution outflow, disk & envelope - Spatial distribution formation mechanism - connection to disks and planetary systems 2 x 10 4 -2 x 10 4 2 x 10 4 4 x 10 4 -4 x 10 4 Z [AU] x-y [AU] Matsumoto & Tomisaka (2004) high density warm slower infall complex species in disk (?)