some chemistry in assorted star-forming regions

Some Chemistry in Assorted Star-forming Regions

Eric Herbst

Some Regions Associated with Star-Formation

pre-stellar cores (L1544)

low mass protostars (IRAS 16293) protoplanetary disks Hot cores PDR’s

A pre-stellar core (cold but with a dense center)

H2D+ - detected by Caselli et al. (2003)

D/H =

1.5 x 10-5

“H2D+ is the main molecular ion in the central..”

L1544 – a prestellar coreCCS – gray scale

Dust emission peak

The model:

multiply deuterated species are now observed in the ISM

observations support the link between high fractionation and CO depletion

we present a pseudo-time-dependent model of deuterium chemistry, including all analogues of H3

+, NH3, CH3OH HD2

+ and D3+ may be important even in

modeling singly deuterated species

Fractionation in the gas-phase….

H3+

CO, N2, O

HCO+

N2H+

OH+

H2D+e-

HD

DCO+, HCO+

N2D+, N2H+

OD+, OH+

CO, N2, O

e-

H2

H H HH2 H

H H D

HD H

H2 D

When species are depleted….

H3+

CO, N2, O

HCO+

N2H+

OH+

H2D+e- HD

DCO+, HCO+

N2D+, N2H+

OD+, OH+

CO, N2, O

e-

H2

H H HH2 H

H H D

HD H

H2 D

At higher densities….

H3+

CO, N2, O

HCO+

N2H+

OH+

H2D+e- HD

DCO+, HCO+

N2D+, N2H+

OD+, OH+

CO, N2, O

e-

H2

H H HH2 H

H H D

HD H

H2 D

Accretion model without HD2+

and D3+:

n(H2) 104 (cm-3) 106 (cm-3)

H2D+/H3+ 0.938 27.37

DCO+/HCO+ 0.217 0.492

N2D+/N2H+ 0.215 0.484

D/H 0.075 0.355

NH2D/NH3 0.313 1.208

HDCO/H2CO 0.133 0.381

Times of peak D/H ratios: 10(6) yr and 2 x 10(4) yr

Deuterium fractionation:

H3+ + HD H2D+ + H2

H2D+ + CO HCO+ + HD 2/3

DCO+ + H2 1/3

• Maximum DCO+/HCO+ ratio is 0.5

Deuterium fractionation:

H2D+ + HD HD2+ + H2

HD2+ + CO HCO+ + D2 1/3

DCO+ + HD 2/3

• DCO+/HCO+ ratio reflects the total degree of deuteration of H3

+

HD2+ + HD D3

+ + H2

D3+ + CO DCO+ + D2 1

Fractional abundances:

Molecular D/H ratios:

A comparison of the homogeneous model with observations of CO and D2CO:observations

model

(Observations from Bacmann et al. 2002; 2003)

Heterogeneous shell model does much better!

Fractionation on Grains

One of the strongest predictions of the pre-stellar core model is that the abundance ratio of D to H atoms in the gas becomes quite high (0.1 – 1.0). In reality, these atoms strike dust particles and react to form both normal and deuterated species!! These species stay on the grains until star formation begins to occur and temperatures rise!

Accretion and Diffusion

DUST

HD

CO

Surface reactions produce

the following molecules:

H2CO, HDCO, D2COCH3OH, CH3OD

CH2DOH, CHD2OH

CH2DOD, CHD2OD

CD3OH, CD3OD

O

H2O, HDO, D2O, CO2, H2, HD, D2

The Protostar IRAS 16293-2422 Temperatures have warmed up to near 100 K

close to the budding star and 50 K somewhat farther removed. The following methanol isotopomers have been detected:

CH3OH, CH3OD, CH2DOH, CHD2OH, CD3OH in addition to HDCO and D2CO.

The belief is that these species have very recently come off grains.

Dust continuum – IRAS 16293

Methanol fractionation from a grain surface chemistry model:

Abundance CH3OH 1 x 10-7

Fractionation CH3OD 0.22

CH2DOH 0.8

CH2DOD 0.16

CHD2OH 0.2

CHD2OD 0.048

CD3OH 0.02

CD3OD 0.004Accreting D/H ratio = 0.4 (Stantcheva & Herbst 2003)

IRAS

0.04

0.9

0.2

0.03

Methanol fractionation from a protostellar model. T=50 K; n(H2)=106cm-3

What happens as the evaporated material ages?

After methanol desorbs from the grains:

CH3OD

CH2DOH

CH3ODH+

CH2DOHH+

CH3OD

CH3OH

CH2DOHH3

+e-

e-

H3+

e-

Osamura et al. 2004

Compared with the observations:

Observations of IRAS 16293-2422 from Parise et al. 2002; 2003

HOT MOLECULAR CORES

Hot cores are regions of warm, quiescent gas near high-mass star-forming regions. Temperatures are 100-300 K and densities are typically 107 K. They are associated with a variety of saturated gas-phase organic molecules: methanol, ethanol, acetaldehyde, methyl formate, acetic acid, glycolaldehyde, ethylene oxide, dimethyl ether, and possibly diethyl ether, glycine, and ethylmethyl ether.

OMC: KL

HOT CORES

HOT MOLECULAR CORES II As in protostellar sources, the chemistry is

associated with evaporation from the dust, although the post-evaporation gas-phase chemistry may be crucial in producing larger species from the precursor methanol.

Key reactions in chain to form methyl formate:

HHCOOCHeOCHHHCO

HOCHHHCOCOHOHCH

33

23223

)(

)(

Ab Initio Calculations

TWO EXPERIMENTS

1) SIFT AT HANSCOM AF BASE dominant product cluster ion (high density) 2) ICR AT WATERLOO, CANADA dominant product CH3OCH2

+ (low density) CONCLUSION: no major channel to produce

protonated methyl formate We don’t know how it is made in hot cores.

There is work left for you to do!!!!!