Evaporation and Thermal Evaporation and Thermal Balance of Tiny-scale Balance of Tiny-scale

Structure in the Diffuse Structure in the Diffuse Interstellar MediumInterstellar Medium

Jonathan SlavinJonathan Slavin

Harvard-Smithsonian CfAHarvard-Smithsonian CfA

Types of Small Scale Types of Small Scale StructureStructure

Cold neutral clouds – possibly embedded Cold neutral clouds – possibly embedded in a warm (neutral or ionized) envelopein a warm (neutral or ionized) envelope

Warm neutral clouds – possibly embedded Warm neutral clouds – possibly embedded in an ionized envelopein an ionized envelope

Warm ionized medium – possibly Warm ionized medium – possibly surrounded by hot ionized mediumsurrounded by hot ionized medium

Different heating/cooling rates apply Different heating/cooling rates apply depending on the temperature and depending on the temperature and ionization state of the cloudionization state of the cloud

Cloud Creation and Cloud Creation and DestructionDestruction

Creation: cooling of small regions Creation: cooling of small regions that become thermally unstable after that become thermally unstable after compression (e.g. Audit & compression (e.g. Audit & Hennebelle 2005)Hennebelle 2005)

Destruction:Destruction: Turbulent mixingTurbulent mixing Thermal evaporationThermal evaporation Photoionization/photo-evaporationPhotoionization/photo-evaporation Shock heatingShock heating

Destruction of a CNM Cloud by Destruction of a CNM Cloud by Turbulent StrippingTurbulent Stripping

Time Time evolution of evolution of a CNM cloud a CNM cloud embedded embedded in a WNM in a WNM flow. Cold flow. Cold gas is gas is mixed, mixed, expands expands and warms and warms to join the to join the warm gas.warm gas.

5 km/s

Timescale for destruction of the cloud is ~106 yr.

Thermal Balance and Cloud Thermal Balance and Cloud DestructionDestruction

Phase Phase properties properties of CNM of CNM cloud cloud being being destroyed destroyed by by turbulent turbulent mixing.mixing.

Points are Points are gas gas parcels parcels from from hydro-hydro-dynamicaldynamicalsimulation simulation at t = at t = 3.5×103.5×1055 yryr

cold cloudcold cloud

warmmedium

thermalequilibrium

adiabaticexpansion/contraction

cooling > heating

heating > cooling

Thermal Conduction and Cloud Thermal Conduction and Cloud DestructionDestruction

Comparison of Comparison of CNM cloud CNM cloud destruction with destruction with (right) and without (right) and without (left) thermal (left) thermal conduction. conduction. Contours show Contours show pressure, colors pressure, colors show log density. show log density. Though conduction Though conduction smears out some smears out some of the small scale of the small scale structure the structure the overall effect is overall effect is small – turbulence small – turbulence is the dominant is the dominant destruction destruction mechanism. mechanism.

Evaporation vs. Turbulence for Evaporation vs. Turbulence for Warm Clouds in Hot GasWarm 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.

Thermal Conductivity - Thermal Conductivity - DependenciesDependencies

Heat flux is carried by electrons in Heat flux is carried by electrons in hot/ionized gas; Hhot/ionized gas; H00 in cold/warm neutral gas in cold/warm neutral gas

Mean free path determines temperature Mean free path determines temperature dependence, dependence, κκ ~ ~ T T 5/25/2 for electron for electron conductivity, conductivity, κκ ~ ~ T T 0.8 0.8 for Hfor H00

Charge transfer strongly limits HCharge transfer strongly limits H00 mean free mean free path – ionization reduces conductivity in path – ionization reduces conductivity in partially ionized warm gaspartially ionized warm gas

Magnetic field channels electron Magnetic field channels electron conductivity along field lines – magnetic conductivity along field lines – magnetic topology very important for conduction for topology very important for conduction for clouds in hot gasclouds in hot gas

Dependence of Thermal Conductivity Dependence of Thermal Conductivity on Temperature and Ionizationon Temperature and Ionization

Conductivity vs. temperature – a moderate amount of ionization ~ 20%, substantially reduces the conductivity in warm gas.

Here a photo-ionization rate of 10-13 s-1 causes the ionization in gas at a pressure of 3000 cm-3 K

Heat Flux SaturationHeat Flux Saturation

Limitation on heat flux – heat can only be Limitation on heat flux – heat can only be transferred as fast as the carriers can transferred as fast as the carriers can diffuse – important for clouds in hot gasdiffuse – important for clouds in hot gas

Saturation for spherical clouds Saturation for spherical clouds parameterized by (Cowie & McKee 1977)parameterized by (Cowie & McKee 1977)

σσ00=3.2(=3.2(TTh h /10/1066K)K)33/[(/[(PP /10 /1044kkBB))RRclcl(pc)](pc)] If If σσ00> 1> 1 then mass loss rate is reduced then mass loss rate is reduced

relative to “classical” raterelative to “classical” rate Small clouds have strong saturation, so Small clouds have strong saturation, so

evaporation rate is far below classical rateevaporation rate is far below classical rate

Evaporation TimescalesEvaporation TimescalesThe timescale for evaporation is calculated as The timescale for evaporation is calculated as

(cloud mass)/(mass loss rate) (cloud mass)/(mass loss rate) For CNM clouds evaporating into WNM For CNM clouds evaporating into WNM

envelope, conduction is by Henvelope, conduction is by H00::ττ = 5.8×10 = 5.8×107 7 (R(Rclcl/0.1 pc)/0.1 pc)2 2 ((TTww/10/104 4 K)K)-0.8 -0.8 ((nnclcl/50 cm/50 cm-3-3) yr) yr

For clouds (cold or warm) evaporating into For clouds (cold or warm) evaporating into hot gas (electron conduction) in high hot gas (electron conduction) in high σσ00 limit:limit:

ττ = 2.2×10 = 2.2×106 6 (R(Rclcl/0.1 pc)/0.1 pc)7/6 7/6 ((nnclcl/50 cm/50 cm-3-3) (P) (Pclcl/10/1044))-5/6-5/6 yr yr

For typical conditions, CNM clouds would For typical conditions, CNM clouds would evaporate in ~3×10evaporate in ~3×1077 yr in warm gas and yr in warm gas and ~5×10~5×1066 yr when embedded in hot gas yr when embedded in hot gas

Details of Heating and Cooling Details of Heating and Cooling in Cold and Warm Cloudsin Cold and Warm Clouds

Heating:Heating: Dust – ordinary carbonaceous & silicate grains Dust – ordinary carbonaceous & silicate grains

plus PAHs; in WNM/CNM PAHs may dominateplus PAHs; in WNM/CNM PAHs may dominate Photoionization – EUV/soft X-ray ionization of HPhotoionization – EUV/soft X-ray ionization of H00

and Heand He00 can dominate in partially ionized warm can dominate in partially ionized warm gas (WPIM)gas (WPIM)

Critical factors are Critical factors are dust contentdust content and the and the radiation fieldradiation field. Note: evaporative interface . Note: evaporative interface between cloud and hot gas can generate between cloud and hot gas can generate substantial EUVsubstantial EUV

Heating and Cooling Heating and Cooling (cont’d)(cont’d)

Cooling:Cooling: [C II] 157[C II] 157μμm line is important coolant in m line is important coolant in

both warm and cold gas – though not in both warm and cold gas – though not in highly ionized warm gashighly ionized warm gas

Many other IR and optical forbidden lines Many other IR and optical forbidden lines contribute as well, e.g. [Si II] 34.8contribute as well, e.g. [Si II] 34.8μμm, [S m, [S II] 6731Å depending on temperature, II] 6731Å depending on temperature, ionizationionization

If If nn(e)/(e)/nn(H(H00) < 0.3 – 2 % (depending on ) < 0.3 – 2 % (depending on T T ) excitation of C) excitation of C++ by H by H00 dominates dominates

The Local Interstellar Cloud as an The Local Interstellar Cloud as an Example of Warm Partially Ionized Example of Warm Partially Ionized

MediumMedium

The LIC surrounds the Solar System and is:The LIC surrounds the Solar System and is: Warm, Warm, T T = 6300 K= 6300 K Low density, Low density, nn = 0.26 cm = 0.26 cm-3-3

Partially ionized, X(HPartially ionized, X(H++) = 20 – 30%) = 20 – 30% Low HI column density, Low HI column density, NN(HI) = 0.3 – (HI) = 0.3 –

2×102×101818 cm cm-2-2

Is the LIC characteristic of WNM gas? Is Is the LIC characteristic of WNM gas? Is the WPIM a significant phase of the ISM?the WPIM a significant phase of the ISM?

Heating and Cooling in the Heating and Cooling in the LICLIC

We have calculated photoionization models We have calculated photoionization models using observed and modeled components of using observed and modeled components of the interstellar radiation fieldthe interstellar radiation field

Heating in the LIC is dominated by HHeating in the LIC is dominated by H00 and He and He00 photoionization – dust is minor contributorphotoionization – dust is minor contributor

Primary coolant is [C II] 157Primary coolant is [C II] 157μμm line, but only m line, but only accounts for ~40%; rest is spread among accounts for ~40%; rest is spread among many IR and optical linesmany IR and optical lines

C gas phase abundance is high to account for C gas phase abundance is high to account for absorption line observation of [C II*] 1335.7Åabsorption line observation of [C II*] 1335.7Å

Stellar radiation alone cannot account for the Stellar radiation alone cannot account for the heating necessary to balance cooling by Cheating necessary to balance cooling by C++

Local Interstellar Radiation Local Interstellar Radiation FieldField

EUV from EUV from nearby white nearby white dwarfs and B dwarfs and B starsstars

FUV from O, FUV from O, B, A starsB, A stars

Soft X-rays Soft X-rays from hot gas from hot gas in the Local in the Local BubbleBubble

EUV/soft X-EUV/soft X-rays from rays from evaporative evaporative boundary of boundary of the LICthe LIC

Photoionization Heating and the Photoionization Heating and the

LICLIC Just the line of sight integrated CJust the line of sight integrated C+ + cooling cooling

requires more heating than can be requires more heating than can be provided by stellar sources – without extra provided by stellar sources – without extra heating, cooling time for LIC is ~4×10heating, cooling time for LIC is ~4×1055 yr yr

Soft X-rays from the Local Bubble can Soft X-rays from the Local Bubble can provide enough heating – but under provide enough heating – but under restrictive conditionsrestrictive conditions

Emission from the boundary of the cloud, Emission from the boundary of the cloud, assuming it’s evaporating helps make up assuming it’s evaporating helps make up the needed EUV fluxthe needed EUV flux

Phase Evolution in the n-P Phase Evolution in the n-P PlanePlane

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

SummarySummary Turbulent flow can destroy as well as Turbulent flow can destroy as well as

create CNM clouds – shear causes create CNM clouds – shear causes expansion and mixingexpansion and mixing

Thermal conduction is a minor effect for Thermal conduction is a minor effect for CNM clouds in WNM, but is important for CNM clouds in WNM, but is important for cold/warm clouds in hot gascold/warm clouds in hot gas

Evaporation suppresses Evaporation suppresses mixing/disruption of clouds but is mixing/disruption of clouds but is slowed by saturation effects in small slowed by saturation effects in small cloudsclouds

Summary (cont’d)Summary (cont’d) Primary heating source depends on dust Primary heating source depends on dust

content, ionization, temperature and content, ionization, temperature and radiation field – for CNM/WNM it’s dust radiation field – for CNM/WNM it’s dust (especially PAH), for WPIM/WIM it’s (especially PAH), for WPIM/WIM it’s photoionizationphotoionization

CC++ cooling is primary coolant for cooling is primary coolant for CNM/WNM/WPIM but many other lines CNM/WNM/WPIM but many other lines contributecontribute

LIC may be typical example of WPIM/WNM – LIC may be typical example of WPIM/WNM – low carbonaceous dust content and hard low carbonaceous dust content and hard radiation field determine thermal equilibriumradiation field determine thermal equilibrium

Creation/destruction of tiny clouds depends on Creation/destruction of tiny clouds depends on their environment – turbulent velocity field, their environment – turbulent velocity field, radiation field, pressure and composition.radiation field, pressure and composition.