CONDITIONS IN DENSE INTERSTELLAR CLOUDS Paul F. Goldsmith Jet Propulsion Laboratory with thanks to Ted Bergin, Di Li, the SWAS team, and the Taurus Mapping Project

September 19, 2005 THE HUNT FOR MOLECULES 2 CONDITIONS IN DENSE CLOUDS  Let us take a broad view and include intensive and extensive parametersintensive and extensive parameters chemical and physical quantitieschemical and physical quantities various cloud componentsvarious cloud components  Rather than review numerous results which are probably largely well-known, I would like to describe the TOOL KIT which astronomers have developed to determine these conditions

September 19, 2005 THE HUNT FOR MOLECULES 3 OVERVIEW I will devote most of my time to this task, but cannot resist including some new observations that while undigested indicate that the list of parameters needs to be expanded, and that at least some of the tools to determine them are becoming available I will devote most of my time to this task, but cannot resist including some new observations that while undigested indicate that the list of parameters needs to be expanded, and that at least some of the tools to determine them are becoming available

September 19, 2005 THE HUNT FOR MOLECULES 4 IMPORTANCE OF KNOWING CONDITIONS IN THE DENSE ISM  We wish to understand dense (molecular) clouds for their own sake, but perhaps more importantly as part of cycling of material through different phases of the ISM  Molecular clouds are the starting point for star formation  Conditions in dense clouds may (!) have major effect on formation of new stars

September 19, 2005 THE HUNT FOR MOLECULES 5 PARAMETER TOOL(S) COMMENTS GAS DENSITY & COL. DENS. Virialization + LW +Size MACROValidity Col. Dens. of Trace Species + Frac Abund MESO LTE / nonLTE LOS Average Stat Equilib of Trace Species MICROVariations HINSA N(atomic) Cold H = to of H 2 Nearby Clouds

September 19, 2005 THE HUNT FOR MOLECULES 6 MULTIPLE TRANSITIONS ALLOW DETERMINATION OF CONDITIONS Temperature H 2 density CO column density EXTRAGALACTIC GALACTIC 2 Components

September 19, 2005 THE HUNT FOR MOLECULES 7 SOURCES WITH HINSA FEATURES HI Narrow Self- Absorption Dark Clouds in Taurus Emission from Galactic HI Background OH Emission HI is Very Cold & Produces Very Narrow Lines

September 19, 2005 THE HUNT FOR MOLECULES 8 PARAMETER TOOL(S) COMMENTS DUSTDENSITY IR/SUBMM Emission => N(Dust) + Size Direct Meas. only of N(Dust) MASS Virialization + Line Width M(H 2 ) Validity Dust Mass + Emissivity Dust Properties & Gas/Dust Ratio Uncertain SIZE Map w. Tracer 2D Maps Depends on Species Chosen

September 19, 2005 THE HUNT FOR MOLECULES 9 TAURUS MOLECULAR CLOUD IN 13 CO FCRAO 14m 50” resolution Nyquist sampled map with 1.5 Mpixels; 98 o2 coverage

September 19, 2005 THE HUNT FOR MOLECULES 10 PARAMETER TOOL(S) COMMENTS GAS TEMPERATURE Thermalized Optically Thick Blackbody Emission from CO Where? Thermalized Optically Thin Symmetric Tops - NH 3 CH 3 C 2 H CH 3 CN SubthermalMultitransition High-J CO Complex DUST TEMPERATURE Multiband Submm / FIR Dust Emissivity Law Temp. Gradients

September 19, 2005 THE HUNT FOR MOLECULES 11 TAURUS MOL. CLOUD IN 12 CO 30’ beam (Ungerechts & Thaddeus 1987)

September 19, 2005 THE HUNT FOR MOLECULES 12 TAURUS MOLECULAR CLOUD IN 12 CO FCRAO 14m 50” resolution Nyquist sampled map with 1.5 Mpixels; 98 o2 coverage

September 19, 2005 THE HUNT FOR MOLECULES 13 TEMPERATURE IN CENTRAL PORTION OF ORION USING CH 3 OH Beuther et al SMA 0.9” resolution

September 19, 2005 THE HUNT FOR MOLECULES 14 CONDITIONS IN CEPH A DISK (Patel et al. 2005) Blue: 3.6cm (C) Red: 1.3cm (C) Green: CH 3 CN Color: Dust (C) SOURCE HW2 D = 720 pc

September 19, 2005 THE HUNT FOR MOLECULES 15 Methyl Cyanide Probes Temperature of Gas in Disk => T = 25 – 75 K

September 19, 2005 THE HUNT FOR MOLECULES 16 PARAMETER TOOL(S) COMMENTS VELOCITY FIELD & K.E. DENSITY High Resolution Spectroscopy Mapping + Other inputs incl. Density MAGNETIC FIELD Zeeman Splitting B LINE OF SIGHT LOS Average Emis. & Abs. by Polarized Dust B PLANE OF SKY Direction Only C-F Method Equipartition Assumed CombinationB 3-Dimensions

September 19, 2005 THE HUNT FOR MOLECULES 17

September 19, 2005 THE HUNT FOR MOLECULES 18 PARAMETER TOOL(S) COMMENTS CHEMICALCOMPOSITION Optically Thin Emission Determining Excitation => Multiple Transitions Effectively Optically Thin Emission C*tau/A < 1 H 2 O Validity Absorption Cm & Submm; Translucent Clouds Accurate

September 19, 2005 THE HUNT FOR MOLECULES 19 ISSUES re. CHEMICAL ABUNDANCES 1. Accuracy of Column Densities 2. Averaging Along Line of Sight 3. Appropriate Normalization to use, e.g. N(H 2 ) Considering that all clouds have less well-shielded “outsides” and better shielded “insides” there is possible confusion between SURFACE and VOLUME species Considering that all clouds have less well-shielded “outsides” and better shielded “insides” there is possible confusion between SURFACE and VOLUME species

September 19, 2005 THE HUNT FOR MOLECULES 20 H 2 O ABUNDANCE IN VARIOUS CLOUDS SWAS Dedicated ApJ issue (Aug 2000); Odin Dedicated A&A Issue (May 2003)

September 19, 2005 THE HUNT FOR MOLECULES 21 SWAS: VERY EXTENDED H 2 O EMISSION Melnick et al (in prep.)

September 19, 2005 THE HUNT FOR MOLECULES 22 surface: CN N= GHz volume: C 18 O J=1-0 IS WATER A SURFACE TRACER? Does the water vapor intensity correlate better with (believed) surface tracers than with (believed) volume tracers? Orion molecular cloud:

September 19, 2005 THE HUNT FOR MOLECULES 23 H 2 O Abundance Rel. to H Photo-dissociation Freeze-out AVAV Photo-dissociation= Photo-desorption  n/G 0 Rate  n(t) peak abun. at mag WATER ABUNDANCE WITHIN MOLECULAR CLOUDS

September 19, 2005 THE HUNT FOR MOLECULES 24 CLOUD AGE A “New” Parameter & One Way to Determine It Cold HI in a dense, well-shielded clouds is residual of past history in predominantly atomic form due to lower n and N accompanying larger size As such, N(HI)/N(H 2 ) allows determination of CLOUD AGE “age” = 2x10 6 – 3x10 7 yr

September 19, 2005 THE HUNT FOR MOLECULES 25 TIMESCALES  Of great importance for understanding cloud evolution  Relative values of chemical and physical timescales are particularly significant  A variety of timescales are critical in the process of star formation

September 19, 2005 THE HUNT FOR MOLECULES 26 Integrated Intensity K-km s -1

September 19, 2005 THE HUNT FOR MOLECULES 27 CONDITIONS IN DENSE CLOUDS - CONCLUSIONS   Astronomers have a versatile tool kit for determining key physical and chemical parameters   But new and better tools are always needed!   The validity of tools will have to be checked carefully as research moves into new areas such as very dense, evolved regions (cores,…disks) and very distant regions (other galaxies)   Grain surface chemistry has to be dealt with along with gas phase, and it may be making things very complicated!   High spatial dynamic range studies are essential to unravel the processes that govern molecular cloud structure and evolution