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The Local Bubble: some like it Hot or Not ?

The Local Bubble: some like it Hot or Not ?. Barry Welsh : SSL/UC Berkeley. The Case for a Hot Million °K Cavity.

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The Local Bubble: some like it Hot or Not ?

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  1. The Local Bubble:some like it HotorNot ? Barry Welsh : SSL/UC Berkeley

  2. The Case for a Hot Million °K Cavity • Observations of the soft X-ray B, Be and C-bands can only be explained if there is a source of a million degree K emitting plasma located within a nearby region of very low neutral density • This view is re-enforced by ROSAT R1 & R2 soft X-ray band observations & line emission from the OVI and OVII ions. • The observed spatial distribution of this emission is best modeled under the assumption of 3 contributing components: - emitting local hot gas in the galactic plane - emitting hot gas in the galactic halo - an extragalactic source of emitting hot gas • A model in which the total emission intensity is proportional to the path-length traveled through the ISM is favored (`displacement’ model) • Small-scale “shadow” features are probably due to absorption by many diffuse interstellar gas clouds located within 100pc

  3. Other supporting evidence for a hot cavity • There may be several small gas clouds within 100pc that exhibit weak levels of OVI absorption • OVI is thought to form in a conductive interface between hot (106K) and cooler (104K) gas. • If the path length to the OVI absorber lies within the Local Cavity then the LC must contain million °K gas • The Local cavity is an old SNR adjacent to the Loop I cavity that is known to possess million °K gas. • The LC must be filled with ionized gas otherwise it would collapse, or if it is not filled with gas at high pressure then the Local Cloud would be expanding…….it isn’t!

  4. THE SOFT X-RAY DATA • ROSAT 0.25keV sky-survey reveals an extensive diffuse emission signal from a million degree gas: (Snowden et al 1998).

  5. The Case for a “Cold” (T ~ 104 K) Local Cavity • UV absorption lines of MgII, SiI, CaII & FeII are detected ubiquitously within 100pc. They are formed in numerous diffuse gas clouds of known distance with T < 2 x 104K. • The temperature of the LIC gas is 7500K and in a few local clouds weak NaI has been detected such that T < 1000K • The higher ionization (and higher temperature) species of SiIV, CIV and NV have not been detected within 100pc • EUV emission lines expected from a million °K plasma have not been detected by EUVE or CHIPS • The pressure derived for several clouds within the LC agree with that derived (with great confidence) for the LIC (T ~ 7500K) and other clouds in the general ISM p/k << 10,000 cm-3 K

  6. Points of Contention • Why is there a large difference in the derived SXR value for the pressure of LC gas? • Can solar-wind charge exchange explain ALL of the SXRB signal in the galactic plane? • If OVI absorption is found within the Local Cavity, is this sufficient proof of the existence of a 106K plasma? • Why is there so little OVI emission from the Local Cavity gas? • Why is there no SXRB enhancement towards Beta CMa? • Can a highly non-equilibrium Local Cavity explain everything? • Is it possible to have a (small) hot bubble located within a larger & cooler Local Cavity? • Can X-ray shadow data really tell us anything given that the foreground signal varies by an ‘unknown’ amount?

  7. More Points of Contention • Does anyone (apart from Don Cox and Dieter Breitschwerdt) really understand: (i) the role of magnetic fields in the ISM (ii) the real consequences of non-equlibrium interstellar conditions (iii) the ability of ‘cold” clouds to exist within a million degree plasma and my personal “beef” (a) why is the Sun near the “center” of the LC? …….and finally….just for Don….. a new picture of the “R-B-E”

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