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)

Contents

1. Chemical fractionation in prestellar cores and molecular clouds

2. From prestellar cores to protostellar cores

3. Protoplanetary disks … talk by Guilloteau

L1544

dust peakCCS

N2H+

Ohashi et al. (1999)

Tafalla e al (2002 & talk)

Chemical Fractionation in Prestellar CoresDepletion of C-bearing species - destruction of early-phase species (CS,CCS,..) in the gas phase - CO freezes-out onto grains freeze several105 (104 cm-3/nH) yr cf. cont~ several 105 (104 cm-3/nH)1/2 yr

non-depletion of N2H+ and NH3 - depletion of CO, which is the main reactant of N2H+

- slow formation of N2

Aikawa et al. (2001; 2005) see also Maret et al. (2006)

Deuterium enrichment in Prestellar CoresHigh molecular D/H ratios D2CO/H2CO=0.01-0.1 (Bacmann et al. 2003)

N2D+/N2H+=0.2 cf. Elemental abundance: D/H

@L1544 (Caselli et al. 2003)

… talk by Lis

Mechanism of Deuterium EnrichmentExothermic exchange reactions H3

+ + HD H2D+ + H2 + E△ 1

CO)(e)(

HD)(

)H(

)DH(

32

1

3

2

nknk

nk

n

n

CO depletion enhances H2D+ / H3

+

H2D+ + e H2 + D

H2D+ + CO HD + HCO+

Propagation by ion-molecule reactions in the gas phase H2D+ + X XD+ + H2

Deuteration on grain surfaces Hydrogenation with abundant D atoms (originates in H2D+ + e H2 + D) Exchange reactions of CH3OH on grain surfaces (Nagaoka et al. 2005)

CH3OH + D CH2DOH + H, CH2 D OH + D CD2HOH + H, …

L1544

gray: dustsolid: H2D+

Vastel et al. (2006)

HD2+ and D3

+ are produced subsequently

If the core is heated …

H2D+ + H2 H3+ + HD … 104sec @T=50K, n(H2)=106cm-3

H2D+decreases rapidly

Other species (without direct exchange) survive to be observed in protostellar cores

Variation among Cores

L1544

Dynamical evolution

No infall Infall Infall

Chemical evolutionLow D/H ratio Low D/H ratio High D/H ratioCCS central peak CCS central peak CCS central holeNo depletion Small depletion? Significant depletionNo 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

Clumps and chemical differentiation in clouds- Intensity distribution varies with species

Talk by Takakuwa

CH3OHH13CO+

15000 AU

45 m + NMA

clump 2

clump 1

clump 2

clump 3 clump 3

5.52 km s-1 5.62 km s-1 5.72 km s-1

03 00

02 15

26 03 45

04 37 55 53 51 49 04 37 55 53 51 49 04 37 55 53 51 49RIGHT ASCENSION (B1950)

DE

CL

INA

TIO

N (

B1

95

0) 2000 AU

TMC-1C

- Small clumps (2000AU, 0.02Msun) inside cores- Gravitationally unbound- Correlation with physical condition is not yet found

Summary on Prestellar CoresChemical 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 H2D+and HD2

+ in prestellar cores

future: H2D+and HD2+ observation by interferometer

indicator of cores right before star-formation constraints on chemical reaction network

From prestellar cores to protostellar cores

cold prestellar corescompressional heating > cooling (by radiation)

heating byaccretion and a protostar

log r [AU] log r [AU]

log

dens

ity

[g c

m-3]

tem

per

atu

re [

K]

1D radiation hydrodynamicsMasunaga & Inutsuka (2000)

From prestellar cores to protostellar cores

cold prestellar corescompressional heating > cooling (by radiation)

heating byaccretion and a protostar

log r [AU] log r [AU]

log

dens

ity

[g c

m-3]

tem

per

atu

re [

K]

1D radiation hydrodynamicsMasunaga & Inutsuka (2000)

From prestellar cores to protostellar cores

cold prestellar corescompressional heating > cooling (by radiation)

heating byaccretion and a protostar

log r [AU] log r [AU]

log

dens

ity

[g c

m-3]

tem

per

atu

re [

K]

1D radiation hydrodynamicsMasunaga & Inutsuka (2000)

From prestellar cores to protostellar cores

cold prestellar corescompressional heating > cooling (by radiation)

heating byaccretion and a protostar

As the core gets warmer…- Sublimation of ice - CO: 20 K - H2O: 160K - large organic molecules: 100K

CO sublimates at 20 K- CO lines become observable again !- CO kills N2H+ and NH3

benefits CS and HCO+ CO

CS

H2CO

HCN

NH3

N2H+

HCO+

log

n(i)

/nH

-5

-10

-15

log r [pc]-3 -2 -1

Lee et al. (2004)CO sublimation

freeze-out

CO sublimates at 20 K

larger organic species (ex. CH3OH)

first coren 1013 cm-3

second coret=0

t = -770yr

t=9x104yr

Aikawa et al. (in prep) based onMasunaga & Inutsuka (2000)

Sublimation radius

- CO lines become observable again !- CO kills N2H+ and NH3

benefits CS and HCO+

R20K R100K

prestellar~1013cm-3 ~10 AU1st core several 10 AU a few AU2nd core ~100 AU ~10 AU9*104 yrsprotostar several 103AU 100AU

CO sublimation

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 CH3OH abundance

- HCOOH/CH3OH is higher than in high-mass hot core

Bottinelli et al. (2006)

Complex Species in Low-mass Cores- How are they formed ?

Grain-surface reactions e.g. CO CH3OH(Watanabe & Kouchi 2002)

Gas-phase reactions ofsublimatesex. CH3OH2

+ + H2CO HC(OH)OCH3

+ + H2

inefficient (Horn et al. 2004)break-up in the recombination (Geppert et al. 2006)

grain-surface reactions duringwarm-up (Garrod & Herbst 2006)

Molecules freeze-out on grains

grain/ice surfacereaction betweenheavy species

grain/ice surface

hydrogenation

Calculation from a Prestellar to Protostellar CorePhysical model of core contractionand protostar formation (Masunaga & Inutsuka 2000)

Chemical model of gas & grain-surfacereactions (Garrod & Herbst 2006)+

Short warm-up phase: Rwarm/vinfall

T > 20 K … 104yr T > 100K … 102yr

9 x 104yr after 2nd collapse

Distribution of gas and ice at each evolutionary stage

Aikawa et al. in prep

Calculation from a Prestellar to Protostellar CorePhysical model of core contractionand protostar formation (Masunaga & Inutsuka 2000)

Chemical model of gas & grain-surfacereactions (Garrod & Herbst 2006)+

Short warm-up phase: Rwarm/vinfall

T > 20 K … 104yr T > 100K … 102yr

gas phase ice mantle9 x 104yr after 2nd collapse

Distribution of gas and ice at each evolutionary stage

Aikawa et al. in prep

Calculation from a Prestellar to Protostellar Core- Spatial Distribution CH3CN, HCOOH … extends to 1000 AU CH3OH, CH3OCH3 … sharp rise at 100 AU- Formation mechanism CH3OCH3 … formed from CH3OH 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 mantle9 x 104yr after 2nd collapse

Aikawa et al. in prep

Summary on protostellar coresAs 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

ALMA’s contribution on protostellar coreHigh sensitivity detection of weak lines of complex species:

18.5 hr@Nobeyama-45m vs 4 min@ALMA

- How complex the interstellar molecules can be ?- More statistics

ALMA’s 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 104

-2 x 104

-2 x 104 2 x 104 4 x 104-4 x 104

Z [

AU

]

x-y [AU]

Matsumoto & Tomisaka (2004)

high densitywarmslower infall

complex speciesin disk (?)