1. Chemical Evolution During Star Formation
Half a Decade of ALMA: Cosmic Dawns Transformed, Sep , 2016
Chemical Evolution During Star Formation
Jeong-Eun Lee
Kyung Hee University

2. Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee
Outline
Low mass star formation process
Chemical evolution during star formation
ALMA observations of protostars
Episodic accretion and chemical vestige
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

3. Low mass star formation
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

4. Chemical characteristics
Prestellar stage:
Freeze-out of carbon-bearing molecules (e.g. CO & CS) on to grains surfaces due to the cold (<10 K) and dense (> 105 cm-3) condition
Abundant nitrogen-bearing molecules (e.g. N2H+ and NH3) because of the long formation timescale of N2 and depletion of their destroyers (e.g. CO) from gas
High D/H ratios in molecular species (high abundances of DCO+, N2D+, etc.) since the lower ground energy levels of deuterated molecules are favored by the cold condition
Complex molecules forming on grain surfaces
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

5. Chemical characteristics
Prostellar stage:
Evaporation of molecules close to protostars, depending on the binding energies of molecular species (e.g. Teva(CO)=25 K, Teva(H2O)=100 K)
Depletion of ionic species such as N2H+ and HCO+ because of their reactions with evaporated CO and H2O, respectively
Hot corinos with complex organic molecules (e.g. CH3OH, CHOOCH3) formed by the hot core chemistry at the region hot enough to evaporate water ice (e.g. IRAS )
Carbon-chain molecules (e.g. c-C3H2) formed by Warm Carbon Chain Chemistry (WCCC), which is induced by the evaporation of CH4 from grain surfaces (e.g. L1527)
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

6. Chemical characteristics
Protoplanetary disk stage:
Three layers of chemistry
Hot atomic layer (PDR)
Warm molecular layer (Hot Core chemistry)
Cold icy layer (cold prestellar core chemistry)
From Henning & Semenov 2013
One of major questions in astrochemistry is whether the disk material are basically inherited from the ISM, or they are significantly altered within the disk.
To answer this question, we need to observe disks of embedded protostars.
!!! ALMA !!!
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

7. Chemical evolution during star formation
Lee et al. (2004) combined dynamics, dust continuum radiative transfer, gas energetics, chemistry, and line radiative transfer self-consistently in 1-D.
Dynamics from prestellar to protostellar: Bonner-Ebert spheres + Shu’s inside-out collapse model
Dust continuum: DUSTY with an assumed luminosity evolution considering accretion, disk, and protostar
Gas energetics: photoelectric heating, cosmic-ray heating, grain-gas collision, cooling by various emission lines
Chemistry: gas chemistry, gas-grain interactions, no surface chemistry except for H2 formation
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

8. Chemo-Dynamical Model
Evolution Model of Luminosity
n(r,t), v(r,t)
B.-E. spheres + Shu’s collapse model
TD(r,t)
Continuum Radiative Transfer
Tk(r,t)
Gas Energetics
Simulated line profile (r,t)
Line Radiative Tansfer
X(r’,t)  X(r,t)
Chemical evolution
along infalling gas
Lee et al. (2004)
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

9. Chemo-Dynamical Model
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

10. Chemo-Dynamical Model
SiO2 mantle Av,e=0.5 mag
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

11. 2-D Chemodynamical models
Visser et al. (2011) combined a 2-D semi-analytic dynamical model with chemistry to follow parcels’ trajectories in the 2-D space until the parcels accrete to disk to map the distributions of species in the disk.
H2O with different evolution histories
NH3 with different evolution histories
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

12. 2-D chemodynamical models
Hincelin et al. (2016) combined a 2-D radiative MHD simulation with chemistry to study the chemical evolution in envelope, pseudo-disk, disk, and outflow up to the formation of FHSC.
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

13. A protostellar system revealed by ALMA
L1527 IRS
Sakai et al. 2014
Rotating infalling envelope: CCH,
c-C3H2, CS, and H2CO
Centrifugal barrier: SO
Disk: CH3OH
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee 13

14. Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee
New Data
Part of a Cycle 1 program, “Star Formation via Infall: A Definitive Test by ALMA (PI: Neal Evans)”
Observed in 14 June, 2014
Beam size: 0".54 x 0".36 (PA= -3.1 deg)
Velocity resolution: ~0.05 km/s
Sensitivity: 1.1 mJy/beam (cont), ~30 mJy/beam (lines)
Continuum, HCO+ J=4-3, HCN J=4-3, CS J=7-6, H13CN J=4-3
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

15. Rotating infalling envelope, outflow, disk
|v|<2 km/s
High velocity shocked gas
Colors: HCO+
Contours: HCN
IRAC 3.6 ㎛
|v|>7 km/s
shock chemistry model:
n =105 cm-3
Tgas=1000 K during shock (100 yrs)
Tgas=30 K before (105 yrs)
and after (107 yrs) shock
Lee et al. in prep.
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

16. Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee
Modeling of L1527
Density
Temperature
Disk: Tobin et al. (2013)
Envelope: Kristensen et al. (2012)
RADMC-3D using dust properties
from Tobin et al. (2013)
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

17. CASA simulation of C18O J=2-1 for L1527
ΔDEC
(arcsec)
South
North
X(C18O)
colors: observation
contours: model
Rdisk=100 AU
Mstar=0.4 M
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

18. Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee
Luminosity problem
Shu’s continuous accretion model,
Lacc ~ GMṀ/R
R ~ 3 R
M ~ 0.08 M (Protostar/Brown dwarf boundary)
Ṁ ~ 2×10-6 M yr-1 (Shu 1977)
Lacc ~ GMṀ/R ~ 1.6 L
Spitzer Space Telescope discovered 59% of the embedded protostars have luminosity lower than 1.6 L (c2d survey; Evans et al. 2009).
1.6 L
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

19. Episodic Accretion Model
Mass accretion occurs in episodic bursts.
Material piles up on a disk until gravitational instabilities drive mass inward in short lived bursts (Vorobyov & Basu 2005).
Most of the mass accretes in a small fraction of the lifetime.
Protostars stay at low luminosities except accretion bursts.
Dunham & Vorobyov (2012)
Luminosity is not a proxy for protostellar mass.
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

20. Chemical fingerprint of Episodic Accretion
Episodic accretion process heats up the surrounding material during a burst accretion, but it cools down spontaneously right after the bust accretion stops.
Chemistry changes responding to the variation of physical conditions with a much longer timescale, keeping the memory of the hot moment even long after the burst accretion ends.
Therefore, chemical distribution can be used as a fingerprint of episodic accretion.
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

21. Chemical fingerprint of Episodic Accretion
A toy model by Lee et al. (2007):
Use Chemo-dynamical model (Lee et al. 2004)‏
Chemical distribution at the timescale when fits the SED of IRAM04191
assumptions:
constant accretion from envelope to disk
episodic accretion from disk to star
(accretion event every 104 yrs for 103 yrs)‏
Model prameters:
Mcore = 1 M⊙
Rcore = AU
normal accretion rate = 4.8 x 10-6 M⊙/yr
accretion rate (accretion phase) = 10 x normal rate
accretion rate (quiet phase) = 0.01 x normal rate
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

22. Chemical fingerprint of episodic accretion
3000 yrs after a burst accretion (quiescent phase)
Jorgensen et al. (2015)
SMA C18O J=2-1 observations of 16 embedded protostars indicate that the radii of CO emission regions are bigger than what expected from their current luminosities.
Evidence for burst accretions!!!
~1” resolution, d=140 pc
C18O J=2-1
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

23. ALMA for Episodic accretion
X(CH3OH)
X(HCO+)
X(H2O)
IRAS 15398
CH3OH at <1” (red)
H13CO+ at 1–2” (blue)
Surface chemistry included
Only gas chemistry considered
Episodic accretion
Continuous accretion
Jorgensen et al. (2013), ALMA Cycle 0 program
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

24. ALMA for Episodic accretion
5 AU
Effective grain
coagulation
Low opacity
V883 Ori
42 AU
Cieza et al. (2016)
ALMA Cycle 2 & 3 programs with 0.03” resolution
at 218 GHz & GHz
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

25. ALMA for Episodic accretion
A disk model:
Aikawa et al. (2015)
Ṁ =10-4 M yr-1
H2O snow line=125 AU
(following Min et al. 2011)
Keplerian rotation around 0.5 M
D=500 pc
ALMA Cycle 4 programs for FUors disks to study fresh sublimates, which is a unique and direct probe for the ice composition in the pre-eruption
S – FUors in different evolutionary stages and environment
T – when a new FUor detected by various transient surveys
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

26. Environmental and dynamical effects
X(HCN)
Short
prestellar
phase
X(CH4)
X(CH3OH)
warmer
prestellar
phase
Standard model with prestellar timescale of 106 yrs
model with prestellar timescale of 105 yrs
model with prestellar core temperature of 18 K
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

27. Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee
Summary
Star formation is associated with energetic activities such as infall, accretion, jets, and outflows.
These dynamical processes produce energy to affect the physical and chemical conditions of associated material.
In particular, the accretion mechanism has a prime impact on the temperature structures of the inner envelope and disk, which set the initial conditions of planet formation, resulting in inevitable alterations of their chemical distribution.
The high sensitivity and high resolution of ALMA are critical to study the chemical evolution of disks and inner envelopes around protostars.
Half a Decade of ALMA, Sep. 20, 2016, J.-E. Lee

28. 2014-2 SSTF Grant Proposal, J.-E. Lee
Thank you
SSTF Grant Proposal, J.-E. Lee

29. Tracing the Chemical Vestige with ALMA
CASA simulation of the ALMA observation with:
Compact configuration with 50 antenna
1 hour integration
Synthesized beam of 1.9”x 1.7”

30. Tracing the Chemical Vestige with ALMA
Episodic Accretion
Continuous Accretion
Simulation of observations in C18O J=2-1
→ rms nosie
~0.003 Jy/beam
cf.
SMA-extended configuration:
4hr-integration with 0.1 km/s
→ rms nosie
~0.2 Jy/beam

31. 2014-2 SSTF Grant Proposal, J.-E. Lee
surface chemistry explicitly using rate equations
Willacy et al. (2006), Garrod and Herbst (2006), Dodson-Robinson et al. (2009)
SSTF Grant Proposal, J.-E. Lee

32. For exothermic reaction
without an activation energy : P =1
With an activation energy (Ea)
b = 1 Å , μr = reduced mass
SSTF Grant Proposal, J.-E. Lee