Transcript isml1
ISM & Astrochemistry Lecture 1 Interstellar Matter – Astrochemistry is the study of the synthesis of molecules in space and their use in determining the properties of Interstellar Matter, the material between the stars. • Comprises Gas and Dust • Dust absorbs and scatters (extinguishes) starlight Top row – optical images of B68 Bottom row – IR images of B68 Dust extinction is less efficient at longer wavelengths Interstellar Gas • HII (H+) Region – T ~ 104 K, n ~ 10-2 cm-3 Surround hot stars Photoionised by stellar UV photons Major ion is H+. Other ions: He+, C+, N+, O+, etc No photons with energy > IP(H) = 13.6 eV permeate the ISM. Triffid Nebula (M20) Interstellar Gas • Coronal Gas - T ~ 106 K, n ~ 10-2 cm-3 Detected in UV observations of highly ionised atoms such as OVI (O5+) which has IP = 114 eV. Fills about 20% of volume of the Galaxy. Thought to be produced by interacting supernova remnants. SN Remnant in the Large Magellanic Cloud Diffuse Interstellar Clouds Temperature: 80-100K Density: 102 cm-3 Slab-like, thickness ~ 1019 cm Clouds permeated by UV radiation - with photon energies less than IP(H) Carbon is photoionised f(e-) ~ 10-4 Cloud mostly atomic f(H2) < 0.3 Few simple diatomics – CO, OH, CH, CN, CH+ f(M) ~ 10-6-10-8 The Pleiades Interstellar Gas • Giant Molecular Clouds (GMCs) T ~ 10-50 K, n ~ 105 - 107 cm-3, <n> ~ 6 102 cm-3 Material is mostly molecular. About 100 molecules detected. Most massive objects in the Galaxy. Masses ~ 1 million solar masses, size ~ 50 pc Typically can form thousands of low-mass stars and several high-mass stars. Example – Orion Molecular Cloud, Sagittarius, Eagle Nebula 1 pc = 3.1 1018 cm = 3.26 light years Interstellar Gas The Orion Nebula The Constellation of Orion Image of the Orion Molecular Cloud in Carbon Monoxide – size 30 light years (2 times larger in the sky than the full moon Optical picture of Orion Nebula (M42) Star-Forming Hot Cores Density: 106 - 108 cm-3 Temperature: 100-300 K Very small UV field Small saturated molecules: NH3, H2O, H2S, CH4 Large saturated molecules: CH3OH, C2H5OH, CH3OCH3 Large deuterium fractionation Few molecular ions - low ionisation ? f(CH3OH) ~ 10-6 Dark Interstellar Clouds • Dark Clouds - T ~ 10 K, n ~ 1010 - 1012 m-3 Not penetrated by optical and UV photons. Little ionisation. Material is mostly molecular, dominant species is H2. Over 60 molecules detected, mostly via radio astronomy. Masses 1 – 500 solar masses, size ~ 1-5 pc Typically can form 1 or a couple of low-mass (solar mass) stars. B68: A dark cloud imaged in the IR by the VLT Dark Interstellar Clouds Infrared and radio telescopes are best used to study star formation Infrared image Interstellar Dust • Interstellar extinction - absorption plus scattering - UV extinction implies small (100 nm) grains - Vis. Extinction implies normal (1000 nm) grains - n(a)da ~ a-3.5da - Silicates plus carbonaceous grains - Mass dust/Mass gas ~ 0.01 - Dense gas – larger grains with icy mantles - Normal – nd/n ~ 10-12 - Within interstellar clouds, characterise extinction of UV photons by the visual extinction, AV, measured in magnitudes - Iλ = I0λexp(-Aλ) The interstellar extinction curve Interstellar Ices Mostly water ice Substantial components: - CO, CO2, CH3OH Minor components: - HCOOH, CH4, H2CO Ices are layered - CO in polar and non-polar ices Sensitive to f > 10-6 Solid H2O, CO ~ gaseous H2O, CO Interstellar Gas Phase Abundances H (D He C N O S Mg, Si, Fe, 1.0 1.6e-5) 0.1 0.000073 0.00002 0.00018 <1e-6 < 1e-9 IS Gas is oxygen-rich – O/C > 1 Evolved Stars IRC+10216 (CW Leo) • Nearby (~130 pc) high mass-loss carbon star (AGB) • Brightest object in the sky at 2 microns – optically invisible • Carbon dust envelope detected out to 200’’ = 25,000 AU ( ~ 1 lt yr) • Molecular shells at ~ 1000 4000 AU • >60 molecules detected: CO, C2H2, HC9N ... • Newly discovered anions C8H-, C6H-, C4H- , C3N- , C5N-, CN• Recent detections of H2O, OH and H2CO Figures from Leao et al. (2006) Lucas and Guelin et al. (1999) Protoplanetary Disks Observed directly around low-mass protostars PPD Schematic Protoplanetary Disks Thin accretion disks from which protostar forms Inflow from large radii (300 AU) onto central protostar Temperature of outer disk is cold (10 K) n(H2) ~ 105 – 1014 cm-3 Molecular gas is frozen on to dust grains in outer disk Temperature of inner disk is ~ 100 K at 10 AU, ~1000 K at 1 AU Ices evaporate in inner disk