Chemistry in low-mass star forming regions: ALMA

1
Chemistry in low-mass star forming regions
ALMAs 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)
2
Contents

• Chemical fractionation in prestellar
• cores and molecular clouds
• 2. From prestellar cores to
• protostellar cores
• 3. Protoplanetary disks
• talk by Guilloteau

3
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
tfreeze several 105 (104 cm-3/nH) yr cf.
tcont 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
Tafalla e al (2002 talk)
Aikawa et al. (2001 2005) see also Maret et al.
(2006)
4
Deuterium enrichment in Prestellar Cores
High molecular D/H ratios
D2CO/H2CO0.01-0.1 (Bacmann et al. 2003)
N2D/N2H0.2 cf. Elemental abundance D/H
10-5
_at_L1544 (Caselli et al. 2003)
talk by Lis
5
Mechanism of Deuterium Enrichment
Exothermic exchange reactions H3 HD ?
H2D H2 ?E1
L1544
HD2 and D3 are produced subsequently
CO depletion enhances H2D / H3
H2D e ? H2 D H2D CO ? HD
HCO
gray dust solid H2D
Vastel et al. (2006)
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,

If the core is heated
H2Ddecreases rapidly
H2D H2 ? H3 HD 104sec _at_T50K,
n(H2)106cm-3
Other species (without direct exchange) survive
to be observed in protostellar cores
6
Variation among Cores
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
CCS
No infall
Infall Infall
(Hirota Yamamoto 2006, Crapsi et al. 2006,
Aikawa et al. 2005, Tafalla Santiago 2004, Lee
at el 2003, Aikawa et al. 2001)
7
Clumps and chemical differentiation in clouds

• Intensity distribution varies with species

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

TMC-1C
Talk by Takakuwa
8
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 H2Dand HD2 in
prestellar cores future H2Dand HD2
observation by interferometer
indicator of cores right before star-formation
constraints on chemical reaction
network
9
From prestellar cores to protostellar cores
heating by accretion and a protostar
compressional heating gt cooling (by radiation)
cold prestellar cores
1D radiation hydrodynamics Masunaga Inutsuka
(2000)
temperature K
log density g cm-3
log r AU
log r AU
10
From prestellar cores to protostellar cores
heating by accretion and a protostar
compressional heating gt cooling (by radiation)
cold prestellar cores
1D radiation hydrodynamics Masunaga Inutsuka
(2000)
temperature K
log density g cm-3
log r AU
log r AU
11
From prestellar cores to protostellar cores
heating by accretion and a protostar
compressional heating gt cooling (by radiation)
cold prestellar cores
1D radiation hydrodynamics Masunaga Inutsuka
(2000)
temperature K
log density g cm-3
log r AU
log r AU
12
From prestellar cores to protostellar cores
heating by accretion and a protostar
compressional heating gt cooling (by radiation)
cold prestellar cores
As the core gets warmer - Sublimation of ice
- CO 20 K - H2O 160K -
large organic molecules 100K
13
CO sublimates at 20 K

• - CO lines become observable again !
• CO kills N2H and NH3
• benefits CS and HCO

CO sublimation
freeze-out
14
CO sublimates at 20 K

• - CO lines become observable again !
• CO kills N2H and NH3
• benefits CS and HCO

Sublimation radius
R20K
R100K prestellar 1013cm-3 10 AU 1st core
several 10 AU a few AU 2nd core 100 AU
10 AU 9104 yrs protostar several
103AU 100AU
t9x104yr
second core
t0
larger organic species (ex. CH3OH)
CO sublimation
first core
n 1013 cm-3
t -770yr
Aikawa et al. (in prep) based on Masunaga
Inutsuka (2000)
15
Complex Species in Low-mass Cores
- Detection of complex species toward IRAS
16293-2422, NGC1333 (talks by van Dishoeck and
Sakai)
- Abundances varies among cores
- 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)
SMA observation of IRAS 16293-2422(Kuan et al.
2004)
16
Complex Species in Low-mass Cores
- How are they formed ?
Molecules freeze-out on grains
Grain-surface reactions e.g. CO ?
CH3OH (Watanabe Kouchi 2002)
grain-surface reactions during warm-up (Garrod
Herbst 2006)
Gas-phase reactions of sublimates ex. CH3OH2
H2CO ? HC(OH)OCH3 H2
inefficient (Horn et al. 2004) break-up in the
recombination (Geppert et al. 2006)
17
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)

? Distribution of gas and ice at each
evolutionary stage
Short warm-up phase Rwarm/vinfall T gt 20 K
104yr T gt 100K 102yr
9 x 104yr after 2nd collapse
Aikawa et al. in prep
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)

? Distribution of gas and ice at each
evolutionary stage
Short warm-up phase Rwarm/vinfall T gt 20 K
104yr T gt 100K 102yr
gas phase
9 x 104yr after 2nd collapse
ice mantle
Aikawa et al. in prep
19
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
9 x 104yr after 2nd collapse
ice mantle
Aikawa et al. in prep
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
21
ALMAs contribution on protostellar core
High sensitivity ? detection of weak lines of
complex species 18.5 hr_at_Nobeyama-45m vs
4 min_at_ALMA - How complex the interstellar
molecules can be ? - More statistics
22
ALMAs contribution on protostellar core
High spatial resolution
- Derive molecular abundance without beam dilution
- Spherical symmetry ? NO! magnetic
fields and rotation
? outflow, disk envelope
- Spatial distribution ? formation mechanism
- connection to disks and planetary systems
Matsumoto Tomisaka (2004)