The Future of Astrochemistry

Eric Herbst

Departments of Physics, Astronomy, and Chemistry

The Ohio State University

It’s a molecular universe but there is still much to learn!!!

The Unknown

As we know,There are known knowns.There are things we know we know.We also knowThere are known unknowns.That is to say,We know there are some thingsWe do not know.But there are also unknown unknowns,The ones we don’t knowWe don’t know.

Interstellar Medium

• Gas (99%) and tiny dust particles (1%) mainly in the form of “clouds” (old term “nebulae”)

• Clouds range from diffuse (starlight shines through) to dense

• In “giant” clouds, both diffuse and dense regions exist• Interstellar matter arises from matter expelled from old

stars• Dense interstellar matter collapses to form new stars• Dense clouds are almost entirely molecular!!! Molecules

make good probes, both via spectroscopy and chemical models.

Some Future Prospects

• I) New and interesting molecules in the interstellar gas and grain mantles

• II) Better understanding of relevant chemical processes including surface chemistry

• III) Much better understanding of heterogeneity and dynamics of individual sources, and stellar and planetary formation

• IV) More research on extra-galactic sources

I. NEW MOLECULES

150 + isotopomers already known in gas (2-13 atoms); 10 in ice mantles; PAH’s

Normal, unsaturated, +/- ions, radicals, isomers

Ori KL Survey (CSO; hot cores)

“Beware the weeds, my observers! The torsions that bite, the congestion that catches…”

(submillimeter-wave rotational spectrum)

WEEDS, CONT.

• Mainly internal rotor species (e.g. CH3OH) with thousands of interstellar lines

• Can possibly be removed/accounted for by two methods:– 1. classical spectroscopic techniques of

measuring and analyzing lines, then fitting to a Hamiltonian and predicting new lines etc. (often tabulated in databases) P13

– 2. a radical new technique to account for the intensities of unanalyzed lines T13

Possible New Species

• Small hydrides (LiH)• Unusual molecules (HOCN, HCNO) P08• Biotic species (glycine?) T08, T10• Very large organic species (fullerenes?)

P10 P17,T11-12• Large negative ions (PAH-)• Doubly charged ions (CO2+)• Molecules in ice mantles P01, P15

II. RELEVANT CHEMICAL PROCESSES

Poorly Understood Chemical Processes/Regimes

• Some barrierless reactions T14• Negative ion formation and depletion P02• High temperature chemistry and path to thermal

equilibrium• Formation and chemistry of very large molecules

T12• Non-thermal desorption mechanisms T07• Diffusive and other surface reactive mechanisms• Coagulation, settling of grains T02

Negative Ion Chemistry

• Radiative attachment (Herbst 1981); statistical theory leads to radical ions with large electron affinities and more than 4 atoms; e.g.,

C6H + e C6H- + h

ALMA: the future…….following BIMA, CARMA, SMA…. (T05)

III. EVOLUTION, HETEROGENEITY AND DYNAMICS

IIIA. STAR FORMATION

Cold Core Pre-stellar Core

Protostar

Star + Disk

T = 10 K

n = 104 cm-3

Isothermal collapse

adiabatic

hot corino

100 K

stellar

Diffuse

Cold envelope

Low-mass Star Formation

Exotic molecules

Normal organic molecules

High-Mass Star Formation

IR dark cloud

HII region Hot core (300 K)???

IIIB. INDIVIDUAL SOURCES

Chemistry, heterogeneity, dynamics

The Case of TMC-1CO J=10

TMC-1 Gas-phase Models: the past?

• one-point (0-D) models dominated by ion-molecule reactions with 1000’s of reactions (many not studied); simulations lead to exotic and unsaturated molecules.

• Pseudo-time-dependent: lifetime of perhaps 10(5-6) yr “early time” best

Gas-grain models: The Future?

• Ices build up by accretion and surface chemistry as gas-phase chemistry occurs

• Some major ice features can be reproduced (H2O, CO, CO2?); saturated organic ices predicted

• Stochastic methods needed for quantitative reproduction of surface chemistry but not yet quite useable.

Chemistry and Core Formation

Hear talk T03

The Real TMC-1

Now 6 cores: A, B, C, CP, D, E of different chemical ages (10[5] – 10[7] yr ?)

Hot Core/Corinos T05

T=10-30 K

Gas: unsaturated species

Surface: more saturated species (e.g. CH3OH)

Warm-up to 100-300 K

evaporation

Saturated gas-phase chemistry to more complex species

(Sgr B2(N-LMH), Ori KL, IRAS 16293 2422)

Surface chemistry

Current & Future Models

• One-point models directed at organic chemistry (Garrod & Herbst 2006; Garrod et al. 2008; Hassel et al. 2008) with three phases

• 1-D Hydrodynamic multi-point models (Aikawa et al. 2008)

• Models with non-spherical structure, lots of organic chemistry, leading to disks, etc.

Other Interstellar Sources

• Diffuse interstellar medium (CH+,

polyatomics) P04, T06• Protoplanetary disks (complex molecules,

structure; coagulation) T02, P06• Galactic center clouds (rich in oxygen-

containing organic molecules but not as hot as hot cores)

• Infra-red Dark Clouds

IV. EXTERNAL GALAXIES

Molecules such as HCN and CH2NH claimed in Arecibo 1.1-10 GHz survey (Minchin et al. 2008 AJ?)

A ULIRG galaxy……

The Future

• Known Unknowns:

• New molecules, new kinetics, more structure and dynamics, more detailed chemical models, more knowledge of stellar formation

• Unknown unknowns ?????????????

The soon-to-be Herschel Space Observatory

The Far-Infrared

NO SHORTAGE OF CHEMICAL, PHYSICAL,

ASTRONOMICAL PROBLEMS WAITING TO

BE SOLVED!!!!!!!!!!!!!