The Thermostat Problem Rok Roskar Nick Cowan December 9 th 2004 Rok Roskar Nick Cowan December 9 th 2004

Outline The Hot Phase of the ISM The Thermostat Problem Early Models The McKee-Ostriker Model The Slavin-Cox Model Conclusions The Hot Phase of the ISM The Thermostat Problem Early Models The McKee-Ostriker Model The Slavin-Cox Model Conclusions

Why we need a Hot Phase A “Galactic Corona” invoked by Spitzer in 1956 to confine observed high- altitude ISM clouds.

The Hot Phase of the ISM Mechanically heated by supernova shocks and Wolf-Rayet winds. Hot: T > 10 5 K Diffuse:  < cm -3 Collisionally Ionized Not in Equilibrium

Supernovae happen in groups Stars form in clusters Only massive stars can go supernova (>10M sun ) Massive stars don’t live very long (<10 Myrs)  All the supernova in a cluster (barring Type Ia) will occur within 10 Myrs of each other.  This produces a superbubble.

Eridanus Superbubble

Collisional Ionization This is different from photoionization. X r + e -  X r+1 + 2e - -IP xr The inverse 3-body R x is suppressed. Recombination is unlikely since electrons are moving too fast: ~100 eV Radiative recombination happens. Dielectronic recombination dominates. This is different from photoionization. X r + e -  X r+1 + 2e - -IP xr The inverse 3-body R x is suppressed. Recombination is unlikely since electrons are moving too fast: ~100 eV Radiative recombination happens. Dielectronic recombination dominates.

How do we Detect Hot Phase? Diffuse, soft X-ray emission  = 2.7x erg cm -3 s -1 {[P/(1.5x dyn cm -2 )] 2 }/T 5/6 At the fiducial pressure,  is rather low. OVI absorption lines … detecting the Hot Phase is hard! Diffuse, soft X-ray emission  = 2.7x erg cm -3 s -1 {[P/(1.5x dyn cm -2 )] 2 }/T 5/6 At the fiducial pressure,  is rather low. OVI absorption lines … detecting the Hot Phase is hard!

Spectrum of Hot, Thin Plasma Free-free continuum Bound-free continuum 2-photon continuum Permitted recombination lines Collisionally excited forbidden lines Permitted resonance lines Free-free continuum Bound-free continuum 2-photon continuum Permitted recombination lines Collisionally excited forbidden lines Permitted resonance lines

ROSAT Soft X-ray Emission

UV Absorption Lines In the Hot Phase, atoms have lost most (if not all) of their electrons. SiIV, CIV, SVI, NV, OVI, etc are present. OVI can survive up to 300,000 K. It has an ionization potential IP > 100 eV. It is easy to model since it is hydrogenic. Strong doublet resonance line (10 -7 m) In the Hot Phase, atoms have lost most (if not all) of their electrons. SiIV, CIV, SVI, NV, OVI, etc are present. OVI can survive up to 300,000 K. It has an ionization potential IP > 100 eV. It is easy to model since it is hydrogenic. Strong doublet resonance line (10 -7 m)

The Thermostat Problem A consequence of the Cox & Smith (1974) model of the hot phase is the so-called thermal runaway. For  3 x 10 5 K, radiative cooling is not effective. Such hot regions should keep growing and getting heated by supernovae. This doesn’t appear to happen: What gives? A consequence of the Cox & Smith (1974) model of the hot phase is the so-called thermal runaway. For  3 x 10 5 K, radiative cooling is not effective. Such hot regions should keep growing and getting heated by supernovae. This doesn’t appear to happen: What gives?

Proposed Solutions to the Thermostat Problem Galactic Fountain (Shapiro & Field 1976) Just wait ‘till there enough hot gas for it to radiate Galactic Wind Turbulent Mixing Photoevaporative Flows Thermal Conduction Galactic Fountain (Shapiro & Field 1976) Just wait ‘till there enough hot gas for it to radiate Galactic Wind Turbulent Mixing Photoevaporative Flows Thermal Conduction

Galactic Fountain

Problems with the Galactic Fountain (according to Cox) SN don’t radiate most of their energy in x-rays The fountains can’t be more than 10 6 K They don’t get very high The Galactic Disk is pretty thick Sounds more like superbubbles! SN don’t radiate most of their energy in x-rays The fountains can’t be more than 10 6 K They don’t get very high The Galactic Disk is pretty thick Sounds more like superbubbles!

Problems with the Galactic Wind It doesn’t carry away much of the SN energy It should carry away at least as much energy in the form of cosmic rays and this is not observed It doesn’t carry away much of the SN energy It should carry away at least as much energy in the form of cosmic rays and this is not observed

After  30 years of debate… … “there are many conceptions of the ISM, all flawed” (Cox, 2003)

McKee & Ostriker Model of the ISM Character of ISM dominated by supernova remnants Supernovae often occur in close proximities, such that SNR’s can overlap This results in the majority of the volume dominated by a hot, ionized component SNR cooled through evaporation of colder clouds Character of ISM dominated by supernova remnants Supernovae often occur in close proximities, such that SNR’s can overlap This results in the majority of the volume dominated by a hot, ionized component SNR cooled through evaporation of colder clouds

McKee & Ostriker Model of the ISM HIM fills most of the space f HIM  0.8 (T  K, n  cm -3) CNM contributes f CNM  (T  K, n  cm -3 ) CNM is embedded in a photoionized corona - the WM consisting of two regions, WIM and WNM; f WM  0.2 (T  8000 K) HIM fills most of the space f HIM  0.8 (T  K, n  cm -3) CNM contributes f CNM  (T  K, n  cm -3 ) CNM is embedded in a photoionized corona - the WM consisting of two regions, WIM and WNM; f WM  0.2 (T  8000 K) “So, for me, the bottom line is that the ideas are useful, the geometry Is wrong, and the details trouble me…” (Cox, 2004)

M-O Model of the ISM

MO Solution to Cox’s “Thermostat Problem” SN shocks encounter colder, denser clouds as they expand Gas is unable to cool until the clouds are evaporated This adds mass and therefore increases the density -> we have cooling Runaway heating is avoided Evaporated material deposited onto other clouds and “recycled” SN shocks encounter colder, denser clouds as they expand Gas is unable to cool until the clouds are evaporated This adds mass and therefore increases the density -> we have cooling Runaway heating is avoided Evaporated material deposited onto other clouds and “recycled”

Issues with the MO model Random, homogeneous distribution of small clouds - does not match observations SNR only encounter thermal pressure in their expansion - should also consider magnetic pressure Enormous size of the SNR when it finally cools –Up to 180 pc –Much larger than the scale of inhomogeneities observed in the ISM The OVI absorption problem Random, homogeneous distribution of small clouds - does not match observations SNR only encounter thermal pressure in their expansion - should also consider magnetic pressure Enormous size of the SNR when it finally cools –Up to 180 pc –Much larger than the scale of inhomogeneities observed in the ISM The OVI absorption problem “The principle purpose of this paper is to show that [problems with the ISM] were not resolved in 1977” (Slavin & Cox, 1993)

OVI absorption Important to consider because it is the key to the thermostat problem In MO model, it comes from the evaporative interface –However, there are more clouds per line of sight than OVI features! –Maybe clouds with large OVI densities are the exception? –Alternatively, there isn’t that much hot gas to begin with -> OVI from local disturbances Important to consider because it is the key to the thermostat problem In MO model, it comes from the evaporative interface –However, there are more clouds per line of sight than OVI features! –Maybe clouds with large OVI densities are the exception? –Alternatively, there isn’t that much hot gas to begin with -> OVI from local disturbances

OVI absorption Cox believes it should be observed from the rest of the HIM as well! Reanalysis of Copernicus satellite data reveals –Significant contribution from the Local Bubble –Other isolated areas of absorption, consistent with Cox model Cox believes it should be observed from the rest of the HIM as well! Reanalysis of Copernicus satellite data reveals –Significant contribution from the Local Bubble –Other isolated areas of absorption, consistent with Cox model

The Porosity Debate What is it? Can describe the volume filling fraction of the remnants If q > 1, remnants overlap If q < 1, remnants isolated & q can be an estimate for their filling fraction MO predict q > 3, but Cox & Slavin predict q  0.18 What is it? Can describe the volume filling fraction of the remnants If q > 1, remnants overlap If q < 1, remnants isolated & q can be an estimate for their filling fraction MO predict q > 3, but Cox & Slavin predict q  0.18

The Porosity Debate What does this mean? MO model –SNR’s very disruptive –Quickly heat up & ionize the medium they expand into and therefore they overlap CS model –SNR limited by magnetic fields, much more confined –Mostly warm medium (similar to the one postulated by the early 2-phase model) What does this mean? MO model –SNR’s very disruptive –Quickly heat up & ionize the medium they expand into and therefore they overlap CS model –SNR limited by magnetic fields, much more confined –Mostly warm medium (similar to the one postulated by the early 2-phase model) “It is our conviction that despite extreme approximations we cannot do worse than McKee & Ostriker” (Slavin & Cox 1992)

Magnetic Pressure CS expect strong fields Large effect on the evolution of SNR - anisotropies in the magnetic field cause asymetries in the remnant CS expect strong fields Large effect on the evolution of SNR - anisotropies in the magnetic field cause asymetries in the remnant

Cox’s final word…? SNRs form isolated cavities, constricted by magnetic fields Can be connected by tunnels Solution to the runaway problem: don’t avoid it, embrace it! Many problems still (Spitzer notes that the warm medium too homogeneous) No solution, but many ideas are out there SNRs form isolated cavities, constricted by magnetic fields Can be connected by tunnels Solution to the runaway problem: don’t avoid it, embrace it! Many problems still (Spitzer notes that the warm medium too homogeneous) No solution, but many ideas are out there

Canonical ISM Assumptions SNR’s dominate ISM physics Actually, porosity is pretty low Thermal pressure balance Actually, P hot ≈ 26P warm Mass transfer between hot and cold phase Actually, magnetic fields inhibit this SNR’s dominate ISM physics Actually, porosity is pretty low Thermal pressure balance Actually, P hot ≈ 26P warm Mass transfer between hot and cold phase Actually, magnetic fields inhibit this

References Cox, The Diffuse Interstellar Medium (2004) Cox, The Devil’s in the Details (2003) Ferriere, Rev. Mod. Phys., Vol 73 (2001) Heiles, The McKee/Ostriker Model: Paradigm? (2000) Shelton & Cox, ApJ 434: (1994) Slavin & Cox, ApJ 417: (1993) Slavin & Cox, ApJ 392: (1992) McKee & Ostriker, ApJ 218: (1977) Shapiro & Field, ApJ 205: (1976) Cox & Smith, ApJ 189:L105 (1974) Pogge, Astronomy 871 Course Notes Cox, The Diffuse Interstellar Medium (2004) Cox, The Devil’s in the Details (2003) Ferriere, Rev. Mod. Phys., Vol 73 (2001) Heiles, The McKee/Ostriker Model: Paradigm? (2000) Shelton & Cox, ApJ 434: (1994) Slavin & Cox, ApJ 417: (1993) Slavin & Cox, ApJ 392: (1992) McKee & Ostriker, ApJ 218: (1977) Shapiro & Field, ApJ 205: (1976) Cox & Smith, ApJ 189:L105 (1974) Pogge, Astronomy 871 Course Notes