VIEWS: 10 PAGES: 30 POSTED ON: 2/1/2013
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 know There are known unknowns. That is to say, We know there are some things We do not know. But there are also unknown unknowns, The ones we don’t know We 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) (submillimeter-wave rotational spectrum) “Beware the weeds, my observers! The torsions that bite, the congestion that catches…” 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- + hn III. EVOLUTION, HETEROGENEITY AND DYNAMICS ALMA: the future…….following BIMA, CARMA, SMA…. (T05) IIIA. STAR FORMATION Cold Core Low-mass Star Pre-stellar Core Formation stellar Isothermal collapse Diffuse n = 104 cm-3 T = 10 K Exotic molecules adiabatic Protostar Star + Disk Cold envelope hot corino 100 K Normal organic molecules High-Mass Star Formation ??? Hot core (300 K) HII region IR dark cloud IIIB. INDIVIDUAL SOURCES Chemistry, heterogeneity, dynamics The Case of TMC-1 CO 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 – 10 yr ?) Hot Core/Corinos T05 (Sgr B2(N-LMH), Ori KL, IRAS 16293 2422) T=10-30 K Warm-up to 100-300 K evaporation Gas: unsaturated species Surface Saturated gas- chemistry phase chemistry Surface: more saturated species (e.g. to more complex CH3OH) species 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+, z, H3+, 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 A ULIRG galaxy…… Molecules such as HCN and CH2NH claimed in Arecibo 1.1-10 GHz survey (Minchin et al. 2008 AJ?) The Future • Known Unknowns: • New molecules, new kinetics, more structure and dynamics, more detailed chemical models, more knowledge of stellar formation • Unknown unknowns ????????????? The Far-Infrared The soon-to-be Herschel Space Observatory NO SHORTAGE OF CHEMICAL, PHYSICAL, ASTRONOMICAL PROBLEMS WAITING TO BE SOLVED!!!!!!!!!!!!!
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