Presentation on theme: "Jonathan Slavin Harvard-Smithsonian CfA"— Presentation transcript:
1 Jonathan Slavin Harvard-Smithsonian CfA Evaporation and Thermal Balance of Tiny-scale Structure in the Diffuse Interstellar MediumJonathan SlavinHarvard-Smithsonian CfA
2 Types of Small Scale Structure Cold neutral clouds – possibly embedded in a warm (neutral or ionized) envelopeWarm neutral clouds – possibly embedded in an ionized envelopeWarm ionized medium – possibly surrounded by hot ionized mediumDifferent heating/cooling rates apply depending on the temperature and ionization state of the cloud
3 Cloud Creation and Destruction Creation: cooling of small regions that become thermally unstable after compression (e.g. Audit & Hennebelle 2005)Destruction:Turbulent mixingThermal evaporationPhotoionization/photo-evaporationShock heating
4 Destruction of a CNM Cloud by Turbulent Stripping Time evolution of a CNM cloud embedded in a WNM flow. Cold gas is mixed, expands and warms to join the warm gas.5 km/sTimescale for destruction of the cloud is ~106 yr.
5 Thermal Balance and Cloud Destruction Phase properties of CNM cloud being destroyed by turbulent mixing.Points are gas parcels from hydro-dynamicalsimulation at t = 3.5×105 yrthermalequilibriumcooling > heatingwarmmediumcold cloudcold cloudheating > coolingadiabaticexpansion/contraction
6 Thermal Conduction and Cloud Destruction Comparison of CNM cloud destruction with (right) and without (left) thermal conduction. Contours show pressure, colors show log density. Though conduction smears out some of the small scale structure the overall effect is small – turbulence is the dominant destruction mechanism.
7 Evaporation vs. Turbulence for Warm Clouds in Hot Gas Pressure distribution for a warm cloud in a hot medium flow. Contours are density.Cloud with thermal conduction (left) is much less disrupted than cloud with conduction turned off (right).The evaporative outflow prevents instabilities from developing at the interface with the flow. But the less disrupted cloud loses mass faster.
8 Thermal Conductivity - Dependencies Heat flux is carried by electrons in hot/ionized gas; H0 in cold/warm neutral gasMean free path determines temperature dependence, κ ~ T 5/2 for electron conductivity, κ ~ T 0.8 for H0Charge transfer strongly limits H0 mean free path – ionization reduces conductivity in partially ionized warm gasMagnetic field channels electron conductivity along field lines – magnetic topology very important for conduction for clouds in hot gas
9 Dependence of Thermal Conductivity on Temperature and Ionization Conductivity vs. temperature – a moderate amount of ionization ~ 20%, substantially reduces the conductivity in warm gas.Here a photo-ionization rate of10-13 s-1 causes the ionization in gas at a pressure of3000 cm-3 K
10 σ0=3.2(Th /106K)3/[(P /104kB)Rcl(pc)] Heat Flux SaturationLimitation on heat flux – heat can only be transferred as fast as the carriers can diffuse – important for clouds in hot gasSaturation for spherical clouds parameterized by (Cowie & McKee 1977)σ0=3.2(Th /106K)3/[(P /104kB)Rcl(pc)]If σ0> 1 then mass loss rate is reduced relative to “classical” rateSmall clouds have strong saturation, so evaporation rate is far below classical rate
11 Evaporation Timescales The timescale for evaporation is calculated as (cloud mass)/(mass loss rate)For CNM clouds evaporating into WNM envelope, conduction is by H0:τ = 5.8×107 (Rcl/0.1 pc)2 (Tw/104 K)-0.8 (ncl/50 cm-3) yrFor clouds (cold or warm) evaporating into hot gas (electron conduction) in high σ0 limit:τ = 2.2×106 (Rcl/0.1 pc)7/6 (ncl/50 cm-3) (Pcl/104)-5/6 yrFor typical conditions, CNM clouds would evaporate in ~3×107 yr in warm gas and ~5×106 yr when embedded in hot gas
12 Details of Heating and Cooling in Cold and Warm Clouds Dust – ordinary carbonaceous & silicate grains plus PAHs; in WNM/CNM PAHs may dominatePhotoionization – EUV/soft X-ray ionization of H0 and He0 can dominate in partially ionized warm gas (WPIM)Critical factors are dust content and the radiation field. Note: evaporative interface between cloud and hot gas can generate substantial EUV
13 Heating and Cooling (cont’d) [C II] 157μm line is important coolant in both warm and cold gas – though not in highly ionized warm gasMany other IR and optical forbidden lines contribute as well, e.g. [Si II] 34.8μm, [S II] 6731Å depending on temperature, ionizationIf n(e)/n(H0) < 0.3 – 2 % (depending on T ) excitation of C+ by H0 dominates
14 The Local Interstellar Cloud as an Example of Warm Partially Ionized Medium The LIC surrounds the Solar System and is:Warm, T = 6300 KLow density, n = 0.26 cm-3Partially ionized, X(H+) = 20 – 30%Low HI column density, N(HI) = 0.3 – 2×1018 cm-2Is the LIC characteristic of WNM gas? Is the WPIM a significant phase of the ISM?
15 Heating and Cooling in the LIC We have calculated photoionization models using observed and modeled components of the interstellar radiation fieldHeating in the LIC is dominated by H0 and He0 photoionization – dust is minor contributorPrimary coolant is [C II] 157μm line, but only accounts for ~40%; rest is spread among many IR and optical linesC gas phase abundance is high to account for absorption line observation of [C II*] ÅStellar radiation alone cannot account for the heating necessary to balance cooling by C+
16 Local Interstellar Radiation Field EUV from nearby white dwarfs and B starsFUV from O, B, A starsSoft X-rays from hot gas in the Local BubbleEUV/soft X-rays from evaporative boundary of the LIC
17 Photoionization Heating and the LIC Just the line of sight integrated C+ cooling requires more heating than can be provided by stellar sources – without extra heating, cooling time for LIC is ~4×105 yrSoft X-rays from the Local Bubble can provide enough heating – but under restrictive conditionsEmission from the boundary of the cloud, assuming it’s evaporating helps make up the needed EUV flux
18 Phase Evolution in the n-P Plane The local interstellar radiation field is hard, but weak leading to a thermal equilibrium curve that is lower than the one from Wolfire et al. (2003)Dynamical processes lead to departures from equilibrium
19 SummaryTurbulent flow can destroy as well as create CNM clouds – shear causes expansion and mixingThermal conduction is a minor effect for CNM clouds in WNM, but is important for cold/warm clouds in hot gasEvaporation suppresses mixing/disruption of clouds but is slowed by saturation effects in small clouds
20 Summary (cont’d)Primary heating source depends on dust content, ionization, temperature and radiation field – for CNM/WNM it’s dust (especially PAH), for WPIM/WIM it’s photoionizationC+ cooling is primary coolant for CNM/WNM/WPIM but many other lines contributeLIC may be typical example of WPIM/WNM – low carbonaceous dust content and hard radiation field determine thermal equilibriumCreation/destruction of tiny clouds depends on their environment – turbulent velocity field, radiation field, pressure and composition.
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