arXiv:0906.2827v1 [astro-ph.GA] 16 Jun 2009 huh ob le ihprilyinzdcodesof cloudlets ionized partially with temperature filled now is be cavity to the Instead, thought local supernova degree required. old million presently little) an is very plasma is (or Cavity no found. which Local in be remnant the Top’ to which ‘Hot the remains in of that model, gas is explanation cooler possible second and The mechanism dimmer heating the a tra- the model, for with Bubble Although difficulties Local several hot believed. solve ditional previously can as model a hot hot lower such as its a not and has is thought, plasma plane, previously than galactic pressure emission the X-ray thermal soft in the detected of 50% currently except only present Bubble, for accounts Hot the it Local to that the similar of is model scenario accepted widely possible interstel- local One the gas. of state lar physical mod- the speculative describe new that two els forward we plasma, local the plasma. that local model hot accepted a generally requires the of led re-assessment has a soft heliosphere, to the wind appreciable within solar an signal background generate of X-ray discovery can Lo- that hot the exchange a particular, charge of In concept the Bubble. that with cal observations conflict in numerous plasma. seemingly hot listed are K have degree we million However, emission local sample presumed the that from observations par- current to paying region, the reference interstellar ticular summarize back- local We this X-ray on soft knowledge emission. the of diffuse presence ex- ground can ubiquitous that the framework a plain providing as accepted widely Abstract

ar .Welsh Y. Bubble Barry Local the with Trouble The O 10.1007/s DOI Science Space and Astrophysics 2 USA 94720, CA Berkeley, Way, 1 tes A362 USA 30602, GA Athens, ar .WlhadRbnL Shelton L. Robin and Welsh Y. Barry c eateto hsc n srnm,Uiest fGeorgia of University Astronomy, and Physics Gau of 7 Department California, of University Laboratory, Sciences Space Springer-Verlag nodrt xli h aoiyo bevtosof observations of majority the explain to order In h oe faLclHtBbl a been has Bubble Hot Local a of model The ••••• ∼ 00Kta r uruddb lower by surrounded are that K 7000 •••• - ••• 1 n oi .Shelton L. Robin and - •••• - • 2 ss , ue o tr ihnadsac of deduced distance initially a mea- within was extinction stars interstellar for and of sured values years placement been low very has 40 Sun’s the density from over The gas low for unusually Cavity. known subsequently of Local region shall a the 1999; within we as al. et which to Sfeir in- refer 2003), rarefied 1998; al. a that al. et such et learn Lallement (Welsh within N44 to cavity located unsurprising itself terstellar therefore (e.g. finds is Sun Clouds It the Magellanic Small N19). within and and and Eri- superbubbles) the Large I (e.g. the Loop present galaxy and Way ubiquitously Cygnus, Milky danus, are own our They within both 1977). al. et (Weaver aiis hc r fe eerdt s“interstel- of densities as gas to neutral low typically referred have are often These bubbles”, are lar cavities. large which of cavities, creation the medium a including interstellar have (ISM), surrounding stars the on early-type effect massive profound asso- of are clusters that with events supernovae ciated and winds stellar The Introduction 1 X-ray Keywords ultra-soft Be and B the bands. in emis- the observed or levels explanation sion cannot satisfactory we a model, provide Bubble yet) Hot (as Local were the that with observations conflict the in of many for explanation ural temperature of gas ∼ photo-ionized of envelopes density tr Bhi 95adtelc fseta hardening spectral nearby of lack of the measurements 1975)and (Bohlin absorption stars 1978). H Ly-alpha Lucke small 1968; from the (Fitzgerald addition, galaxy distant more In the the of for derived regions reddening the with pared 000K lhuhti e cnropoie nat- a provides scenario new this Although K. 20,000 aay eea ISM — general Galaxy: I ∼ oundniisderived densities column 0 ci imtrand diameter in pc 100 n ∼ ( H 0 c scom- as pc, 100 ) ∼ 0 . 1cm 01 − 3 2 in the observed soft X-ray background signal, which The observed anticorrelation of the soft X-ray back- requires a low value of local neutral gas absorption ground emission intensity with the total sight-line neu- (Bowyer et al. 1968), also added credence to the belief tral hydrogen column density led to two main models that the solar neighborhood resides within a region of that attempted to explain the observed spatial distribu- low neutral hydrogen density. tion of the X-ray emitting gas. In the absorption model During the 1970’s observations of a ubiquitous 0.25 the ubiquitous soft X-ray background emission was keV soft X-ray background signal by the Wiscon- thought to originate outside the Galaxy and to be par- sin group (Sanders et al. 1977; Williamson et al. 1974) tially absorbed by Galactic material, although the ab- and the detection of the O VI (1032) absorption line to- sorption cross sections could not produce the observed wards several OB stars located within the solar neigh- dynamic ranges (McCammon et al. 1976) or band ra- borhood by the Copernicus satellite (Jenkins 1978) tios (Bloch et al. 1986; Juda et al. 1991). In the provided strong supporting evidence that the Lo- widely accepted displacement model of the Local Hot cal Cavity was filled with a highly ionized and hot Bubble (Sanders et al. 1977; Cox & Reynolds 1987), (million degree K), low density plasma. In addi- the observed soft X-ray background brightness was tion, new theoretical models of the general ISM pre- thought to be proportional to the path length through dicted the presence of a pervasive hot gas-phase com- the hot Local Bubble gas, which is roughly anticor- ponent (Cox & Smith 1974; McKee & Ostriker 1977). related with the total hydrogen gas column density. Given that other superbubbles, such as those of Although neither model could fully explain all of the Cygnus (Cash et al. 1980) and the Orion-Eridanus re- observations of the soft X-ray background, the displace- gion (Reynolds & Ogden 1979), were also being de- ment model was stronger. The prevailing thought was tected through their X-ray emission properties and that the soft X-ray background emission signal observed that Heiles (1979) had just compiled an extensive list along any sight-line is composed of 4 components: (i) of galactic bubbles/supershells observed in the radio emission from the Local Hot Bubble of interstellar 21cm H Iline, the observational and theoretical basis plasma located in the galactic disk near to the Sun, for believing that hot gas resided in the cavity seemed (ii) emission from a more distant (halo) source of soft well substantiated (Cox & Reynolds 1987). This hot X-rays located at high galactic latitudes, (iii) emission interstellar plasma was dubbed the Local Hot Bubble. from an isotropic extragalactic background and (iv) an At the time of the the Local Bubble & Beyond emission component arising in the Galactic disk at dis- Conference (Breitschwerdt et al. 1998) the widely ac- tances beyond the absorbing wall of the Local Cavity cepted picture was one in which the Sun is located (Kuntz & Snowden 2000). within an extremely low neutral density (n(HI) Such a model could account for the relative scarcity −3 6 ∼ 0.005 cm ), high thermal temperature (T 10 K) of neutral gas and dust for interstellar sight-lines that ∼ and highly ionized local plasma with dimensions of extend < 100 pc (Frisch & York 1983). Furthermore, at least 80pc in many galactic directions. This high cold and dense high latitude clouds could block emis- temperature plasma produces half to all of the ob- sion originating from a more distant halo and/or extra- served soft X-ray background intensity in most galac- galactic origin, thus creating observable ‘shadows’ in tic directions. The hot gas seen in low latitude di- the maps of the diffuse soft X-ray background emission rections must be local because there are nearby op- (Burrows & Mendenhall 1991; Snowden et al. 1993). tically thick walls of H I (the neutral boundary to By analyzing these shadows, it has been possible to the Local Cavity) towards which a non-zero 1/4 keV isolate the contribution of foreground Local Bubble flux is observed (Snowden et al. 2000). Lying within emission from more distant emission. These shadow- the rarefied and hot bubble are many low density ing experiments have provided the strongest evidence −3 −3 (N(HI) 0.1 cm ), partially ionized (ne 0.1cm ) for a million degree plasma located within a hot Local ∼ ∼ −6 and warm (T 7000 K) cloudlets, the most well- Bubble, with a typical contribution of 400 10 R12 ∼ ∼−1 × −2 studied being the local interstellar cloud, within which (i.e. ROSAT R1 and R2 band) counts s arcmin to the Sun is thought to reside (Lallement & Bertin 1992). the total 1/4 keV soft X-ray background signal in most The origin of the Local Hot Bubble was thought to directions (Snowden et al. 2000). be linked to an explosive event in the nearby (d We discuss additional properties of the hot local gas ∼ 150 pc) Sco-Cen OB association that occurred some in Section 2. Several discrepancies between the model 2 Myr ago (Frisch 1981), or one or more nearby SN of the Local Hot Bubble and the observations are also (Cox & Anderson 1982; Edgar & Cox 1993), such that discussed in this section. The most difficult problem the local region can essentially be considered as an old is that posed by the discovery of solar wind X-rays, cooling SNR or superbubble structure. which appear to account for a significant fraction of The Trouble with the Local Bubble 3 the flux that had been attributed to a million degree limit is in agreement with the inferred average H I col- local plasma. Taking into account the solar wind X-ray umn density throughout the Local Cavity as derived contribution, we outline possible revisions to the Local from Na I observations along sight-lines within 60pc Hot Bubble model in Sections 3 & 4. Section 3 describes (Welsh et al. 1994). However, as discussed in Section the least drastic revision, that in which the emission 2.4, the shape of the Local Hot Bubble, which is cal- from a hot local plasma is simply dimmer in soft X- culated from the assumption of uniform emissivity, is rays than previously believed. Section 4 describes the difficult to reconcile with the shape of the surrounding other extreme, a new model called the Hot Top Model, structure. The ROSAT and Wisconsin X-ray broad- in which the solar wind X-rays account for all of the band data are generally consistent with collisional ion- local X-rays in the Galactic plane. izational equilibrium (CIE) emission models for a gas at 1 million moK. Using higher resolution spectra of the hot emitting gas the Chandra, XMM-Newton, 2 Details of, and Problems with, the Local Hot and Suzaku satellites have also found that the data in Bubble Model the 0.4 to 0.7 keV band can be fit with CIE model spectra (Markevitch et al. 2003; Galleazzi et al. 2007; In the following subsections we briefly discuss each of Henley et al. 2007).The actual spectrum of the soft X- the major physical properties of the standard Local ray background in this energy range is seen to be dom- Hot Bubble model (i.e. temperature, pressure, size, inated by emission from the O VII and O VIII lines. etc). Over the past decade observational evidence has However, CIE models that generate the observed sur- emerged that has raised serious questions concerning face brightness of the O VII line are found to predict the standard model of a hot local plasma. Therefore, too much lower energy 1/4 keV band emission for the we also make note of observations that would seem to MBM 12 direction (Smith et al. 2007). contradict several of the shibboleths concerning how the In a few cases theoretical models in which the X- local ISM is currently perceived. ray emitting gas was slightly out of equilibrium such that the plasma is slightly underionized could not be 2.1 The Spectrum of the 106 K Plasma disallowed (Smith et al. 2005; Henley et al. 2007). We note that models having a highly non-equilibrium ion- Diffuse emission arising from a seemingly ubiquitous ization (NEI) condition in which the plasma is thought 0.25 keV soft X-ray background (soft X-ray back- to be strongly over-ionized due to a previous event ground) signal has been widely observed in the Be, such as a supernova (Breitschwerdt & Schmutzler 1999; B, and C bands during numerous suborbital sound- Breitschwerdt 2001) are seemingly disallowed by sev- ing rocket flights by the (McCammon & Sanders 1990; eral observations. Recent FUSE observations of the McCammon et al. 2002) and during the ROSAT soft emission intensity of both O VI (Shelton 2003) and C III X-ray background all-sky survey (Snowden et al. 1998). (Welsh et al. 2003) cannot be reconciled with the NEI The galactic distribution of this emission, as recorded model of the Local Hot Bubble (Breitschwerdt 2001) by this latter survey, is shown in Figure 1. which implies line intensities far greater than those In the Galactic plane, apart from directions towards which are observed. SNRs, superbubbles, or the Galactic center, observa- Additionally, observations of the diffuse extreme ul- tions record only the emission that originates within traviolet (EUV) sky background spectral emission us- CHIPS EURD the Local Hot Bubble. X-ray shadowing observations ing both the (Hurwitz et al. 2005) and (Edelstein et al. 2001) satellites are also in contradic- using high latitude clouds have also isolated this hot tion with the predictions of the NEI model. We also and local component. The observed ratio of flux in note that strong Si III (1206A)˚ interstellar absorp- the B band relative to that in the C band and the tion has been reported for several sight-lines towards ratio of flux in the ROSAT R1 band relative to that nearby stars (Holberg et al. 1999; Nehme et al. 2008). in the ROSAT R2 band are indicators of the plasma’s This ion has a relatively high rate coefficient for temperature and are roughly constant across the sky. charge exchange with neutral hydrogen and its pres- The prevailing explanation for these observations is ence within the local interstellar medium is unlikely to that the X-ray emission arises in a pool of hot gas be linked to the remnant of some past ionization event (T 106 K) which surrounds the Sun and whose ∼ such as a supernova shock (Lyu & Bruhweiler 1996; radius varies with direction but whose properties do Breitschwerdt & Schmutzler 1999). Depending on the not. There is a 2-sigma upper limit on the intermixed details of the local ionizing radiation field, a high hy- H I column density within the Local Hot Bubble of drogen ionization fraction (95%) is required in order to N(HI) < 6.6 1018 cm−2 (Juda et al. 1991). This × sustain the presently observed amounts of Si III. 4 Two sets of high spectral resolution observations interstellar region that opens into the Loop I superbub- of the soft X-ray background have been made thus ble at a distance of 90 pc (Welsh & Lallement 2005). ∼ far (Sanders et al. 2001; McCammon et al. 2002). The We shall return later to the significance these latter two 148 to 284 eV observations by Sanders et al. found features in Section 3. a blend of emission lines, whose spectrum cannot be UV and optical absorption measurements towards reproduced by a solar abundance CIE plasma. This stellar targets within 100pc of the Sun have re- energy region is the most sensitive to the effects of ab- vealed the presence of many partially ionized and sorption and the observed region was in the galactic warm diffuse cloudlets located within the Local Cav- plane, therefore the emission must originate in a lo- ity (Lallement et al. 2003; Redfield & Linsky 2008). cal source of hot gas. We note that a major difficulty These cloudlets typically have hydrogen column den- 17−18 −2 with the CIE emission model is that a million degree sities, log N(HI) 10 cm and temperatures of ∼ local plasma should also be bright at extreme ultra- < 7000 K and possess bulk motions generally differing −1 violet (EUV) wavelengths, but attempts to detect the byonly 5- 10kms . Their presence within the Local associated EUV line emission have not been success- Hot Bubble is thought to provide natural sites for tran- ful (Jelinsky et al. 1995; 1998; Hurwitz et al. 2005). sition temperature layers where ions such as O VI could Although adherents to a Local Hot Bubble can ex- reside (Slavin & Frisch 2002). The most well-studied of plain these non-detections as being due to the local these cloudlets is the local interstellar cloud, partially hot gas having a very low gas-phase iron abundance within which the Sun is thought to reside. The cloudlets (thus making the detection of the expectedly strong within the Local Cavity should provide very weak ab- Fe IX line at 171A˚ problematic), we note that that Fe sorption sites for the more distant soft X-ray emission, is unlikely to be depleted in typical hot ISM plasma thus not interfering with the X-ray spectroscopic and (Yao et al. 2006).. shadowing studies discussed previously. However, only a very few cold (T < 1000 K) and dense gas clouds and 2.2 Cool Gas Inside and Outside the Local Hot no molecular clouds, which can more efficiently absorb Bubble soft X-rays, are known to exist within the Local Cavity (Lallement et al. 2003). The bubble of local hot gas is assumed to fit within the Local Cavity, whose location is defined by the “walls” of 2.3 Transition Temperature Gas: O VI cold and neutral material surrounding the local region. The size and shape of the Local Cavity is best estimated One of the earliest pieces of observational evidence for a from high spectral resolution optical studies of the NaI hot and highly ionized local plasma came from the de- D-lines (5890) seen in absorption towards many sight- tection of the O VI ion seen in the UV absorption spec- Coper- lines within 300pc. The NaI ion is a good tracer of cold tra of nearby (d < 150 pc) B-type stars by the nicus (T < 1000 K) and neutral (I.P. < 5.1 eV) interstellar satellite (Jenkins & Meloy 1974; Jenkins 1978). gas and has been widely used as a surrogate for local These findings have been substantiated by the re- cent FUSE surveys of O VI absorption detected to- H I absorption (Ferlet et al. 1985; Welsh et al. 1994). wards hot white dwarf stars within 200pc of the Sun In a recent NaI absorption survey of 1005 sight-lines (Oegerle et al. 2005; Savage & Lehner 2006). We note within 250pc (Lallement et al. 2003), 3-D maps of the that O VI (1032A)˚ is a tracer of transition-temperature density distribution of cold gas have revealed the Local gas at 300, 000 K, and because of its quick cooling Cavity to be an irregularly shaped rarefied region of 50 ∼ rate, theoretical production models require a source of - 80pc radius in most directions in the Galactic plane. hotter gas for its formation and continual replenishe- Figure 2 shows that the Local Cavity is connected to ment. The O VI could exist in a transition zone between adjacent low density interstellar regions (such as the the hot million K Local Hot Bubble plasma and cooler Pleiades bubble and Loop III) through narrow inter- gas clouds. The prevailing interpretation of these data stellar tunnels, as predicted by theoretical models of the is that the O VI resides on the periphery of the Lo- ISM (Cox & Smith 1974). These maps of the distribu- cal Cavity and/or on evaporating interfaces located be- tion of cold and neutral local gas reveal 3 interesting tween the (numerous) partially ionized cloudlets which interstellar features: (i) a 200pc long by 50pc wide ex- are embedded within the Local Hot Bubble. The FUSE tension of the Local Cavity towards the direction of the O VI detections were only made on local sight-lines with star Beta CMa, (ii) extensions of the Local Cavity into distances > 35 pc and with typical values of column the lower galactic halo through both of the open-ends of −2 density of log N(OV I) 13.0 cm . a rarefied Local Chimney feature (Welsh et al. 1999), ∼ However, recent re-examinations of O VI data for- see Figure 3 and (iii) a connection through a disturbed ward evidence that argues against the presence of this The Trouble with the Local Bubble 5 ion being located within the rarefied confines of the Lo- interfaces. Jenkins (2008) has proposed that local cal Hot Bubble, whose contours have been discussed clouds could mutually shield each other from the ef- in Section 2.2. Firstly, for the 5 B-stars with (Hip- fects of a hot Local Bubble plasma, such that a con- parcos) distances < 75 pc (i.e. the nominal dis- duction front suitable for the formation of OVI only tance to the edge of the Local Cavity in most di- occurs at the very periphery of the cloud grouping. rections) the Copernicus data reveal only one (albeit Alternately, others have have shown the importance tentative) detection of interstellar O VI absorption of magnetic fields in their ability to inhibit conduc- (Jenkins 1978; Bowen et al. 2008). We note that there tion (Slavin 1989; Cox & Helenius 2003). Although the is a large variation between the (one) measured O VI strength of the magnetic field in the case of the local in- column density and the (four) upper limit values for terstellar cloud has been estimated to be of a low value these very local sight-lines, which is unlikely to be due of 2 µG (Wood et al. 2007), if the magnetic field is ∼ to any O VI absorption that would exist with a million tangled then this could account for the low values of K plasma. Instead, this effect has been attributed to a O VI ion production along certain sight-lines. variation in the character of the transition temperature zones surrounding individual clouds. 2.4 Size and Shape of the Local Hot Bubble A new detailed analysis of the radial velocities of interstellar and (when present) stellar absorption lines The approximate size and shape of the emitting re- in a sample of 100 absorption spectra recorded by gion of the local hot plasma has been estimated ∼ FUSE for hot white dwarfs within 400pc of the Sun under the assumption that the emissivity per vol- has recently been made available (Barstow et al. 2008; ume is constant throughout the Local Hot Bubble Barstow et al. 2009). For 2/3 of the sight-lines having (Snowden et al. 1990; Snowden et al. 1998). This as- significant OVI features the velocity of the OVI ab- sumption implies that the Local Hot Bubble’s surface sorption is found to lie within the velocity uncertainty brightness in any given direction is linearly propor- (i.e. 14 km s−1) of features that Barstow et al deemed tional to its radius. The actual distance scaling was to be photospheric in origin, and thus an interstellar determined from observations aimed toward the shad- origin cannot be confirmed. In most of the remaining owing molecular cloud MBM 12, which appeared to be cases the sight-lines with confirmed interstellar OVI ab- placed within or just beyond the edge of the Local Hot sorption extend to or just beyond the neutral boundary Bubble. The initial calculations assumed a distance of 65pc to this molecular cloud. However, more recent of the Local Cavity and are at high galactic latitudes. ∼ estimates of the distance to MBM 12 now place it at a Similar findings for the location of local O VI absorp- tion, but at lower latitudes, have also been made using distance > 200pc (Luhman 2001; Chauvin et al. 2002), a small sample of B-type stars located > 80pc from the such that the main absorber of the more distant emis- Sun (Welsh & Lallement 2008). sion in this direction is instead provided by the neutral Since the local interstellar cloud is widely thought gas wall to the Local Cavity which is located at a dis- tance of 100pc. Thus, the revised radius of the hot to be (at least partially) surrounded by a hot Local ∼ bubble cavity should be about 1.5 times greater than Bubble plasma, it could therefore be a prime evap- the 50 pc to 100 pc range found previously. orative transition region for the production of high Both the (previous and revised) estimated shape and ions. Several authors have therefore constructed theo- size of the Local Hot Bubble derived from X-ray obser- retical models that provide predictions for the num- vations do not match the detailed contours of the Local bers of associated O VI ions, which if spread over Cavity derived by the NaI mapping method and shown most of the clouds surface should be detected in the in Figures 2 and 3. If the Local Cavity is uniformly absorption spectra recorded along most more dis- filled with hot emitting plasma then the significant ex- tant sight-lines (Slavin 1989; Borkowski et al. 1990; tension of the cavity to a distance of 200pc in the Slavin & Frisch 2002; Cox & Helenius 2003). Most ∼ direction of the star β CMa would be expected to show models predict absorption column densities in the − an enhanced level of soft X-ray background emission, log N(OV I) = 12.5 to 13.0 cm 2 range, which is which is not seen in the data (Snowden et al. 1998). marginally consistent with what is detected by the However, there is at least some concordance that the FUSE O VI surveys along local sight-lines. Since one first 50pc in all directions it contains very little cold might expect to encounter more than one conductive and dense neutral gas (although there are several par- cloud interface over the > 40 pc sight-lines in which tially ionized clouds within this volume). Furthermore, FUSE has detected O VI, then the observations sug- the ROSAT soft X-ray maps find that the Local Hot gest that something may be reducing the effective- Bubble is unusually bright in two high latitude re- ness of thermal conduction in these O VI-producing mo gions that are offset by 45 from the north and ∼ 6 south poles (Snowden et al. 1998). These directions such that a lower value of P/k for the hot gas pressure have been found to be well aligned with the openings of is now obtained (i.e. P/k 12,250 cm−3 K due to the ∼ the Local Chimney into the halo (Welsh et al. 1999), as revision of the clear pathlength in the MBM 12 direc- shown in Figure 3. Additionally, it is well-known that tion from 65 pc to 100 pc, the distance to the Local there is a dipole effect in the distribution the soft X-ray Cavity boundary), and/or (ii) any remaining pressure background emission at mid-plane latitudes, with the difference could be accounted for by interstellar mag- emission being warmer, of greater intensity and with a netic and/or turbulent gas pressure. However, even larger spatial coverage in directions towards the galactic with conservative revisions to these data, the spectre center than sight-lines towards the galactic anti-center of a pressure imbalance still remains. A more thorough (Snowden et al. 2000). The present model of the Local discussion of the issues that arise from this apparent Hot Bubble provides no substantial explanation for this pressure imbalance can be found in Jenkins (2008). effect. However, this effect can be explained through inspection of the NaI density map shown in Figure 2, 2.6 Kinematics of the Local Cavity Gas which clearly shows that there are many more gaps in the neutral boundary of the Local Cavity at mid-plane ◦ ◦ Although the main physical properties of the local hot galactic latitudes between 190 < l < 330 (that could gas have been determined from soft X-ray background potentially allow the passage of more distant generated observations, the limited spectral resolution of these soft X-rays) than in the opposite galactic direction. emission-line data preclude a detailed kinematic assess- Finally, we note that an alternative view has been ment of gas velocity. However the motions of lower tem- forwarded in which the Local Hot Bubble is smaller perature gas residing within the Local Cavity can be than the Local Cavity, leaving a space in the interstel- derived from the absorption profiles of UV and optical lar medium between the inner hot gas and the outer lines observed at high spectral resolution along sight- dense gas of the cavity walls. The properties of the lines towards nearby stars (Redfield & Linsky 2008; material within this space are unknown, and how such Savage & Lehner 2006; Crawford et al. 1998). a situation could have arisen or be sustained has not For the case of the numerous partially ionized gas been adequately explained. clouds of temperature 104 K that are known to re- ∼ side within 50 pc of the Sun, their velocity vec- 2.5 Thermal Pressure of the Hot Gas ∼ tors are roughly parallel with a general flow originat- The derived pressure of the local hot gas has been ing in the Scorpio-Centaurus association. The veloc- a longstanding puzzle, together with the global prob- ity amplitudes encompass a wide range of values be- lem of the apparent pressure imbalance between galac- tween clouds, suggesting that interstellar shocks may tic disk gas and that of the overlying galactic halo be prevalent. The turbulent velocity in some of these (Cox & Reynolds 1987). Supporting the halo requires clouds is high, suggesting the presence or recent ex- −3 a gas pressure of P/k = 7,000 to 10,000 cm K istence of shocks, which could result in the return of (Shelton et al. 2007), suggesting that the Local Hot metals to the gas phase through the shock dissipation Bubble should have a similar pressure. More recent of dust grains (Redfield & Linsky 2008). −3 local pressure estimates of P/k 15, 000 cm K For higher temperature (T 105 K) gas located ∼ ∼ have been derived from soft X-ray and EUV shad- within the Local Cavity, most of the local O VI ab- owing observations towards several nearby cold inter- sorption velocities correlate well with the lower tem- stellar clouds (Snowden et al. 1993; Kuntz et al. 1997; perature ion velocities (Savage & Lehner 2006). This Burrows & Guo 1998; Berghofer et al. 1998). In con- is to be expected if O VI is formed in a condensing in- trast, measurements of thermal pressures of several dif- terface between cool and hot Local Cavity gas. There fuse gas clouds residing inside the Local Hot Bubble re- are also some sight-lines in which the O VI absorption sult in far lower pressure values (Jenkins 2002). There velocity is displaced to positive velocities from the lower clearly seems to be a pressure imbalance between the temperature gas, which suggests in these cases that the hot and colder gas within the LISM . In particular such O VI may be tracing an evaporative flow from interface an imbalance would tend to crush the cool clouds resid- regions. ing within the Local Hot Bubble and also then to try Measurements of the absorption velocity of the NaI and expand the size of Local Cavity, neither of these neutral gas associated with the nominal boundary to effects has been observed. the Local Cavity do not suggest that the Local Cav- However, this apparent local pressure anomaly could ity wall is expanding or contracting with any signif- be accounted for in that (i) the previous distance esti- icant gross motions (Lallement, private communica- mates to the cold clouds used in these shadow experi- tion). This would be expected if the Local Cavity is ments have now been revised (see Lallement et al 2003) The Trouble with the Local Bubble 7 an old SNR whose expansion has been significantly occurring 0.5 Myr ago. In a similar supernova driven ∼ slowed down by its interaction with the surrounding formation model the last re-heating episode is placed dense ISM. Also, if the Local Cavity is collapsing (such at 3 million years ago (Smith & Cox 2001), based on ∼ 60 as being due to a marked pressure imbalance) then it the excess abundance of Fe found in the Earth’s deep would collapse at the sound speed, which is not ob- ocean crust (Fields et al. 2005). served. Finally we note that there are two major dynami- 2.8 Solar Wind Charge Exchange (SWCX) cal movements of gas inflow into the Local Cavity re- gion. The first, as mentioned earlier, is a flow of gas At the Local Bubble conference in 1997, Freyberg clouds into the Local Cavity from the adjacent Loop I (1998) and Cox (1998) separately recognized that superbubble (i.e. the Sco-Cen association). This inflow the then-unknown X-ray production mechanism which has been traced using both low (CaII, SII, FeII) and made Comet Hyakutake surprisingly X-ray bright may high ionization (CIV, SiIV, NV) species, all of which operate elsewhere in the solar system. They warned possess significant negative velocity motions for sight- astronomers that these X-rays could pose a significant lines of distance > 90 pc in the general direction of problem for interpretations of the soft X-ray emission l = 330◦, b = +18◦ (Welsh & Lallement 2005). Sec- from the Local Hot Bubble. It was soon realized that ondly, for galactic directions close to the axis of the the source of such cometary X-rays was the deexcita- Local Chimney that extend into the lower halo, in- tion of solar wind ions following charge exchange with terstellar gas clouds with inflow velocities in the -20 neutral material (Cravens 1997; Cravens 2000). Very to -60 km/s range have been detected along sight- highly ionized and fast moving solar wind ions have a lines spanning both open-ends of the Local Chimney large cross section for interacting with neutral material which appear to be falling towards the galactic disk in cometary and planetary atmospheres, as well as neu- (Welsh et al. 2004; Crawford et al. 2002). tral material that has been swept into the heliosphere. During the interaction, called solar wind charge ex- 2.7 Origin and Age of the Local Hot Bubble and the change (sometimes referred to as SWCX), an electron Local Cavity from the neutral material is transferred into a highly excited state in the solar wind ion followed by a decay The history and age of the Local Hot Bubble are much that produces one or more high energy X-ray photons debated. It is widely believed that the Local Hot Bub- (Wargelin et al. 2008). ble and Local Cavity are the result of one or more su- Coronal mass ejections, which enhance the solar pernova (SN) explosions (Cox & Anderson 1982), but wind density in localized regions for hours to days, in an alternate scenario this superbubble picture is dis- are probably responsible for the long term enhance- ROSAT puted, and it is instead the rarefied Local Cavity that ments seen in the data and nominally removed ROSAT is thought to be caused by star formation epochs in from the All Sky Survey (Snowden et al. 1997; the Scorpius-Centaurus OB association as regulated by Snowden et al. 1998). The Suns slow and fast winds, which are less sporadic but vary in relative contribution the nearby spiral arm configuration (Bochkarev 1987; during the 12 year solar cycle, create a slowly vary- Frisch 1995; Fuchs et al. 2006; Maiz-Apellaniz 2001). ing component which has not been removed from the Other suggested Local Cavity formation scenarios in- ROSAT All Sky Survey. Since solar wind charge ex- volve an association with the young and massive change X-rays are produced locally, they also behave as stars of the Gould Belt (Grenier 2000) or the pas- a local component in all shadowing observations. This sage of a high velocity cloud through the galactic signal level has generally been attributed to emission as- disk (Comeron & Torra 1994). The present day mor- sociated with the Local Hot Bubble. Lallement (2004), phology of the Local Cavity seems to be defined Cravens et al (2001) and Robertson & Cravens (2003) by the ‘void within the ISM created by the outer have assessed the potential level of contamination to the shells of nearby overlapping superbubble structures ROSAT All Sky Survey by solar wind charge exchange (Welsh et al. 1994; Frisch 1995). in the slow and fast winds. Robertson & Cravens Recent modeling of the joint evolution of the Loop I (2003)) found that 0.1 to 1.0 keV X-rays produced in and Local Hot Bubble superbubbles involve 20 SNe ∼ the heliosphere by the steady slow and fast winds could occurring in the moving group of OB stars of the Sco- account for 1/2 of the soft X-rays in the Galac- Cen cluster which passed through the present day lo- ∼ tic plane and a smaller fraction of the high latitude cal cavity (Breitschwerdt & de Avillez 2006). In such locally-made soft X-rays. Similarly, Lallement (2004) a scenario the formation age of the Local Hot Bubble calculated the steady, heliospheric solar wind charge ex- is constrained to 14.5 Myr, with the last re-heating ∼ change X-ray intensity in the 1/4 keV band, the band 8 in which the local X-ray emission is most prominent, (distant) emission from the hot and ionized million de- finding that solar wind charge exchange could explain gree K halo. Both sets of the soft X-ray background most of the 1/4 keV intensity that was previously at- measurements cannot explain all of the observed 1/4 tributed to emission from the Local Hot Bubble. Both keV emission and even less of the B-band emission sets of estimates are uncertain, due to uncertainties in recorded by the Wisconsin surveys, in addition to being the neutral ion density, solar wind density, interaction in conflict with the EUV spectral data. Thus, the Lo- cross section, and solar wind charge exchange spectrum, cal Hot Bubble spectrum is required to be softer then making precise estimates difficult. Even when the up- previously thought, and the soft X-ray emission short- per limits are used in these global predictions, the level fall could be filled by a warm (T <106K) emitting gas of predicted solar wind charge exchange X-ray emission within the Local Cavity. cannot explain the excess of local X-rays observed at high latitudes. In addition, the maps of Roberston & Cravens (2003) show significant structure at low lati- 3 Ramifications for the ‘Accepted Local Hot tudes. The ROSAT maps do not show corresponding Bubble Model’ dim regions, suggesting that the Local Hot Bubble’s X- ray emission has compensated for inhomogeneities in It is clear from the preceding sections that several im- the solar wind charge exchanges. However, an alter- portant problems plague the traditional model of the nate conclusion is that the solar wind charge exchange Local Hot Bubble. The most important of these is X-rays are more evenly distributed than in the Robert- the contamination by solar wind charge exchange X- son & Cravens maps. rays. The predicted severity of the 1/4 and 3/4 keV Koutroumpa et al. (2006; 2007; 2008) have made X-ray contamination by this heliospherically generated more detailed calculations of the solar wind charge ex- emission for low latitude sight-lines ranges from 1/2 ∼ change spectrum, finding it to be somewhat harder than in the model of Robertson & Cravens (2003) to ap- that attributable to the nominal Local Hot Bubble. As proximately 100% from the calculations of Koutroumpa a result, they conclude that solar wind charge exchange (2009). Thus, new models of the local region range from can explain all of the local 3/4 keV emission found from those that are 1/2 as bright in X-rays as the tra- ∼ the ROSAT All Sky Survey and all of the local O VII ditional Local Hot Bubble models to those that have 570 eV and O VIII 650 eV line emission seen in pointed no local hot gas in the Galactic plane. In either case, observations by Chandra, XMM-Newton, and Suzaku. the model must include X-ray producing gas located More recent calculations by Koutroumpa et al. (2009) at high latitudes because solar wind charge exchange that compare the calculated solar wind charge exchange models cannot explain all of the observed high latitude spectra with soft X-ray shadow fields observed with X-ray emission. In this subsection, we consider the first ROSAT reveal that the solar wind charge exchange in- case in which the Local Hot Bubble is half as bright in tensity is approximately independent of latitude. The the plane as previously believed. In the following sec- intensity of the solar wind charge exchange emission in tion, we consider the other extreme, that in which gas the ROSAT 1/4 keV band is, on average, 332 10−6 in the local ISM produces none of the soft X-rays seen −1 −2 ∼ × counts s arcmin , which is of the same order as at low latitudes. the soft X-ray background intensity that was previously Firstly, if emission from a Local Hot Bubble is dimin- attributed to emission from the Local Hot Bubble at ished relative to traditional models, it is diminished at low galactic latitudes. However in high latitude regions all latitudes by a similar amount ( 330 10−6 counts o −1 −2 ∼ × (b > 40 ) the calculated levels of solar wind charge s arcsec in the ROSAT 1/4 keV band) accord- ± exchange emission cannot explain the high levels of 1/4 ing to Koutroumpa et al. (2008). Subtracting a con- keV emission observed by ROSAT and thus a hot emit- stant intensity from the observed anisotropic distribu- ting gas in the galactic halo is required. tion of locally produced X-rays leaves a more extremely The calculations of Koutroumpa et al suggest that anisotropic distribution of X-rays which can then be the solar wind charge exchange spectrum may be harder attributed to the Local Hot Bubble. If we make the than previously thought. This notion agrees with the standard assumption that the X-ray emissivity is con- observations of the soft X-ray background spectrum stant throughout this emitting region, then the Local (Sanders et al. 2001; McCammon et al. 2002). How- Hot Bubble must be extremely distorted, with strong ever, we note that these latter data were collected from lobes in the northern and southern hemispheres and a a large area of the sky centered close to the axis of tight waist in the Galactic plane. Secondly, the model the Local Chimney at high northern galactic latitudes solar wind charge exchange spectra are harder than the and thus presumably sampled significant amounts of observed spectrum. We conclude this because the solar The Trouble with the Local Bubble 9 wind charge exchange predictions of Koutroumpa et al. once the spatial distribution of Local Hot Bubble X-ray account for a greater fraction of the observed X-rays in emission is better known. In addition, given that the the 1/4 keV band than in the Wisconsin B band (130 bubble’s X-ray intensity is less than previously thought, to 188 eV). In order to compensate for the hardness the source and history of the Local Hot Bubble must of the solar wind charge exchange spectrum, the Local be reworked. A single or multiple supernova may have Hot Bubble spectrum must be softer than previously heated the gas, but such events were probably fewer believed. Thus, the local emitting plasma would need and possibly happened further back in the past than to be cooler than previously believed. currently believed. Any revisions to the X-ray brightness and tempera- As previously noted, some of the observed physical ture require that the electron density be recalculated. parameters of the LHB appear to be consistent with If the temperature and path length are unaffected, the characteristics of a fossil SNR. The Local Cavity is then a reduction in the Local Hot Bubble emission in- not expanding at the expense of its surroundings, just tensity by a factor of 2 would imply a reduction in as fossil remnant cavities are no longer driven by the the electron density by a factor of 1/√2, resulting in over-pressure of expanding gas within them. However, −3 ne 0.005 cm . This serves as a reasonable start- the hot gas once contained within the Local Cavity may ∼ ing point. However, once the solar wind charge ex- have escaped through the Local Chimney into the galac- change contamination is better understood, this esti- tic halo, or it may have cooled due to turbulent mixing mate can be improved upon by an improved derivation or conduction with entrained gas or gas close to the of the temperature of the Local Hot Bubble plasma galactic plane. The Local Cavity structure may now be (and thus the emissivity) and the intensity of Local gently photo-ablating in the residual UV field. Shelton Hot Bubble X-rays in the direction of MBM12. Be- (1998) has discussed the possibility that the Local Cav- cause the electron density and temperature are less than ity may be an aging SNR. In her model, she notes that previously believed, the thermal pressure must also be once a cool expansion shell forms any OVI should form less. The reduction in the electron density by 1/√2 within and at the periphery of the hot bubble, which is alone, brings the estimated thermal pressure down from moving far slower than the SN shock front. Once the 12,250 K cm−3 to 8, 700 K cm−3. Any revision to SNR shell forms, the soft X-ray emission (from the rem- ∼ the local plasma temperature will will lower the gas nant hot gas) ceases to be edge-brightened and its emis- pressure further. sion diminishes by an order of magnitude. In addition, These revisions reduce or resolve some of the prob- she finds that the X-ray productivity of older SNRs de- lems posed by observations. Firstly, it may now be pends critically on the ambient non-thermal pressure, possible for the hot gas in the Local Cavity to be in thus allowing the possibility that an ancient SNR in a pressure balance with the embedded warm clouds, es- diffuse environment may possess a higher pressure than pecially if the expected downward revisions to the Lo- previously thought. Because such a model does not cal Hot Bubble’s thermal pressure are severe and if the explain the observed local spatial distribution of OVI clouds are somewhat supported by non-thermal pres- which appears to be mostly formed at the periphery of sures such as magnetic pressure and turbulence. Fur- the cavity as opposed to being distributed within the thermore, the reduction in the Local Hot Bubble X-ray cavity itself, we suggest that most of the observed O VI is due to the interface between the cavity gas and the intensity may help to explain the low fluxes of the Fe IX, hot halo gas. In addition, without an additional source Fe X, and Fe XI photons observed by in ultra-soft X-ray spectra (McCammon et al. 2002; Hurwitz et al. 2005). of heating, old remnants tend to collapse such that the material around a fossil SNR will have moved back to The precise estimates of the expected Fe IX, Fe X, and fill in the low pressure space, and there will not be an Fe XI intensities, however, depend on the Local Hot observable cavity. Bubble’s (newer lower) temperature, which is not yet We now turn to a speculative model of the Local known. Cavity in which solar wind charge exchange explains These revisions also re-open existing inquiries. The all of the observed soft X-ray emission in the Galactic heating mechanism for the hot gas must be re-evaluated plane, i.e. the local interstellar medium is devoid of now that the gas is found to be dimmer and cooler, hot gas in the plane. In addition, the cavity is filled and the spatial distribution of the hot gas must also with warm (20,000K) photo-ionized gas, some of which be reconsidered. The previously estimated shape of the may have entered the aging cavity from the Sco-Cen emitting cavity, which was determined from the 1/4 keV bubble. Although such a model represents a radically X-ray intensity and the assumption that the intensity different view, it goes a long way in resolving many of scaled with the pathlength, disagreed with the shape of the outstanding issues that currently bedevil the gen- the Local Cavity, but such disagreement may evaporate erally accepted model of a Local Hot Bubble. 10 4 The ‘Hot Top Model’ Local Cavity within the thick galactic disk. A simple schematic of the model is shown in Figure 3. The following is a possible scenario for the physical Our assumption that the Local Cavity is mostly filled state of the Local Cavity. It is not based on a rig- with warm and photoionized gas is based on obser- orous theoretical framework but is, instead, driven di- vations of the Ar I, N II, C III and Si III ions that rectly by recent observational evidence. Although it have been widely observed throughout the local ISM is far from complete, it attempts to accommodate the (Lehner et al. 2003) and by the recognition that the ramifications of the recent observations of solar wind shortage of neutral argon in nearby clouds can be bet- charge exchange, OVI and the EUV background emis- ter explained by photoionization rather than collisional sion. Although referred to here as a ‘model’, it is really ionization (Sofia & Jenkins 1998). In addition, photo- a descriptive framework designed to stimulate theorists ionization conditions are observed to vary considerably into re-assessing the currently accepted models of a hot throughout the LISM (Lehner et al. 2003). For exam- Local Bubble. In addition, it has testable predictions ple, Wolff et al. (1999) have found evidence for an that future observations in the UV and/or soft X-ray ionization gradient in the local gas with an ionization regimes will be able to confirm or refute. Bearing these increase along the general direction to the Canis Major caveats in mind, we assume the following: extension to the Local Cavity. The size and shape of the Local Cavity is defined Most back-of-the-evelope calculations assume that by the neutral contours derived from NaI absorption the plasma is in collisional ionization equilibrium measurements (Lallement et al 2003), such that the throughout the Local Cavity, although there is no absorbing wall that surrounds most of the Local Cav- real consensus on this matter and such models do ity can be categorized by log N(HI) > 19.3 cm−2. not fit the observed soft X-ray background spectrum A possible new picture of the Local Cavity is one in (Sanders et al. 2001; Breitschwerdt 2001). Bruhweiler which it is mostly filled with a very diffuse and pho- & Cheng (1988) and Vallerga (1998) have taken a dif- toionized and/or collisionally ionized gas of tempera- ferent approach by considering the photoionizing ef- ture 20, 000 K that is in rough pressure equilibrium fects of nearby hot stars. Photoionization from the ∼ −3 (P/k 3000 cm ) with the numerous partially ion- local EUV radiation field (which is dominated by the ∼ ized and warm (T 7000 K) small cloudlets that are EUV flux from the two B stars ǫ and β CMa, to- ∼ known to exist within the LISM. The average electron gether with a few nearby hot white dwarfs) can ac- density of the ionized Local Cavity ‘filler gas’ is as- count for the high interstellar gas electron density mea- −3 sumed to be ne 0.04 cm (derived from pressure sured towards nearby stars, although the spectrum is ∼ equilibrium with the partially ionized LISM cloudlets) too soft to explain the apparent over-ionization of he- and there is also a very low average neutral gas den- lium with respect to hydrogen measured in the local sity of n(H) < 0.01 cm−3 within the Local Cavity cloud (Vallerga & Slavin 1998). Several authors have (Dupin & Gry 1998; Welsh et al. 1994). We note that invoked additional (non-equilibrium) sources of ioniza- there may also be a contribution from magnetic pres- tion to explain this anomaly (Slavin & Frisch 2002). In sure such that the total pressure of both the local in- one model, the shocks that resulted from a local su- terstellar cloud and ‘filler gas’ may be higher than we pernova event that may have occurred 3.6 million 1 ∼ have assumed. years ago are the main cause of the time-dependent Our model also requires that the major source of ionization at the Sun, although most of the photons local non-solar wind charge exchange generated soft X- with wavelengths > 500A˚ are due to the stellar spectra rays occurs at high galactic latitudes (b > +/ 35o), (Lyu & Bruhweiler 1996). Thus, the supposed shock − which is probably due to emission from a hot and ionization event, from which the gas has yet to return ionized gas viewed through the openings of the Lo- to its equilibrium state, is supplemented by the present cal Chimney in to the overlying halo. Also there is stellar radiation field. The ionization rate for this is a highly absorbed component generated in surrounding Γ(HI) = 1.1 x 10−15s−1 (Vallerga & Welsh 1995). We hot (X-ray emitting) superbubble cavities that abut the note the long recombination time of hydrogen in the ISM, which for 10% ionization and n(HI) 0.1 cm−3 is 7 ∼ about 10 years (Chassefiere et al. 1986). The times- 1Although we use the term ‘filler’ gas, in reality the photo-ionized regions will be the rarefied ionized envelopes surrounding cooler pan since the supposed supernova explosion also agrees, interstellar cloud cores. Since the Local Cavity has numerous roughly, with the data gained from the ferromanganese cloudlets of this type, these outer cloud envelopes will probably ocean layer crust of the Earth, for which a local super- merge into a photo-ionized medium that can best be described nova event is required to have happened 3 Myrs ago as a ‘filler’ gas. ∼ (Knie et al. 2004; Fields et al. 2005). Thus, although The Trouble with the Local Bubble 11 we have invoked photo- (and some collisional) ioniza- major disturbances in the local magnetic field, as also tion in our new model, the long time-scales for H re- concluded by Tinbergen (1982). However, for sight- combination and the current availability of sufficiently lines in the 50 to 100 pc range there is a clear change strong stellar photo-ionization sources seems adequate in both the magnitude and direction of the E-vectors to sustain both the ionization and temperature of our which rise up from the galactic plane to high galactic theorized Local Cavity filler gas. latitudes spanning the galactic longitude range ℓ = 300 From the various solar wind charge exchange calcu- to +30o. Additionally, using radio polarization data, lations of Koutroumpa et al, Lallement, and Robertson Wolleben (2007) has formulated a new model for the & Cravens, we know that there may be far less X-ray Loop I radio feature that involves two X-ray emitting emitting interstellar gas in the local region than pre- shells. The region of overlap of these two expanding viously expected. As noted in Section 2.8, simulations shells of Loop I matches most of the disturbed region of the contribution of solar wind charge exchange spec- of E-vectors in the 50 -100pc range noted by Mathew- tra in ROSAT R1 and R2 observations of shadow field son & Ford (1970). A similarly disturbed region is also sight-lines by Koutroumpa et al (2008; 2009) indicate predicted by Wolleben at the southern opening of the that the level of solar wind charge exchange flux is not Local Chimney. strongly latitude dependent, but can account for almost The idea for our new picture of the Local Cavity was all of the observed R1 and R2 emission at low galactic largely driven by an interpretation of the results from a latitudes (i.e. within the Local Cavity). However, an recent re-analysis of O VI absorption profiles detected unabsorbed 1/4 keV soft X-ray background component by the FUSE satellite towards local hot white dwarfs is seen to be well correlated with the absolute galac- (Barstow et al. 2008; Barstow et al. 2009). Apart tic latitude for galactic latitudes of b > 35◦. Thus, from a few instances, the new Barstow et al. results ± a high latitude interstellar component exists and the largely agree with and extend upon previous mea- spatial distribution of its emission seems directionally surements which contained a smaller sample of hot correlated with sight-lines viewed through both open- white dwarf targets whose spectra were reduced us- ings of the Local Chimney into the lower galactic halo ing an earlier version of the FUSE data processing (Welsh et al. 1999). pipeline (Oegerle et al. 2005; Savage & Lehner 2006). We therefore conjecture that at high latitudes the In Figure 4 we show the spatial distribution of the observed soft X-ray background emission (which is 1/4 keV R1 and R2 soft X-ray background emis- thought to arise in a hot and highly ionized halo) is sion (Snowden et al. 1998) together with the galac- only weakly absorbed when viewed through the low tic positions of local (d < 200pc) hot white dwarfs H I column sight-lines that are contained within the that have undisputed detections (and non-detections) ‘openings’ of the Local Chimney, and thus the soft X- of interstellar O VI absorption (i.e. those sight- ray background is detected with a greater intensity in lines in which the velocity difference between inter- these directions. For sight-lines into the halo that do stellar and photospheric OVI absorption is greater not pass through these low opacity openings, the soft than the FUSE spectral resolution). In addition we X-ray background emission signal is highly absorbed have similarly plotted OVI data from the 4 sight-lines by the higher H I density gas in the galactic plane. within 120pc (Savage & Lehner 2006) ) that are not This is in accord with the ROSAT shadow observations part of the new OVI data set (Barstow et al. 2009). which argued against a single emission component from Finally, for completeness, OVI detections and non- a hot halo of large scale-height (Snowden et al. 2000), detections towards B-type stars with distances < but instead required a low scale-height variable (in both 120pc are also included on the plot (Bowen et al. 2008; temperature and intensity) emission component, which Welsh & Lallement 2008) . could also be considered as the superposition of multi- It is clear from Figure 4 that there is a remarkable ple, spatially distinct emitting components. Addition- spatial co-incidence between detections of local O VI ally, the ends of the Local Cavity near the Chimney absorption (d < 80pc) and the highest levels of soft X- openings may be filled with hot gas that has flowed ray background emission. This is to be expected if O VI into it from above. absorption is formed in a transition region between a We know that there is anomalous structure in the hot (T 106 K) and cooler (T 20, 000 K) plasma. ∼ ∼ direction of the northern chimney opening because of The spatial distribution of O VI absorption shown in polarization data. Mathewson & Ford (1970) have mea- Figure 4 can be explained by our presently proposed sured the polarization to nearby stars and found that model if local O VI absorption (d< 100 pc) only occurs for sight-lines < 50 pc the E-vectors have low polariza- in only two environments: (i) at the interfaces between tion values and are also well ordered with no apparent the Local Cavity and the overlying galactic halo (at the 12 ‘top’ and ‘bottom’ of the Local Chimney) and (ii) at the the inflow of gas from both the adjacent Sco-Cen/Loop neutral walls between the Local Cavity and nearby hot I cavity and from the lower galactic halo. Interestingly, bubbles. The former distribution of O VI absorption is it has been found that positive velocity OVI is mostly confirmed by the new OVI results in which O VI is only found at high latitudes in the northern galactic hemi- detected at high latitudes for sources with distance > sphere, which has been interpreted as possibly being 60pc (Barstow et al. 2008; Barstow et al. 2009). The caused by an evaporative flow from a young interface distribution of O VI at low latitudes is consistent with between warm gas and a hot exterior (halo) medium its formation at the interfaces between distant (d > (Savage & Lehner 2006). This scenario is similar to 80pc) bubble cavities and their surrounding neutral gas our Hot Top model in which the evaporative outflows shells (Bowen et al. 2008; Welsh & Lallement 2008). of OVI are formed at the interface between the warm We conjecture that the above model can also explain photo-ionized filler gas of the Local Cavity and that of many of the problems associated with the widely held the hotter infalling exterior gas of the inner halo located hot gas model of the Local Hot Bubble listed in Section at the two openings of the Local Chimney. 2. In particular, this new model provides the following explanations for many of the problems outlined previ- 4.2 Size and Shape of the Local Cavity ously. In the Hot Top Model, the size and shape of the 4.1 The Local Cavity Pressure Problem rarefied Local Cavity are simply defined by the neu- tral gas walls (as traced by NaI absorption). In pre- There are now only two types of gas contained within vious explanations for the Local Hot Bubble, either the confines of the Local Cavity, i.e. a strongly photo- the hot gas was assumed to fill the Local Cavity and ionized ionized ‘filler gas’ at T 20, 000 K and nu- thus take on its size and shape, or to fill a smaller ∼ merous partially ionized cloudlets of T 7000 K, both region within the cavity such that the bubble’s ra- ∼ of which may be in approximate pressure equilibrium. dius in any given direction scaled with the observed mo The million K, soft X-ray emitting gas is now con- local X-ray intensity. The Hot Top Model easily re- fined to high latitude sight-lines at distances of at least solves the difficulty with the first version of the Local 50 - 100pc from the Sun. The hot, rarefied gas in the Hot Bubble model and its inability to explain why, if halo has a lower mass density than that in the disk, the Local Cavity were filled with hot gas, the β CMa 3 but it has a greater thermal pressure of 7to 10x 10 tunnel direction wasn’t found be brighter in soft X- −3 ∼ K cm for OVI-rich gas (Shelton et al. 2007). Ironi- ray emission than neighboring directions. This is no cally, in re-conceptualizing the local region, the thermal longer problematic if the tunnel is instead filled with pressure has gone from being larger than that of the lower temperature photo/collisionally ionized gas, as hot halo gas to being smaller. However, thermal pres- verified by several UV absorption studies of this re- 3 −3 sures on the order of 3 x 10 K cm are common gion (Dupin & Gry 1998; Gry & Jenkins 2001). In ad- ∼ throughout the disk, making the discrepancy between dition, our model removes the somewhat contrived ver- disk and halo pressure a widespread problem. Also, it sion of the Local Hot Bubble model which required a is commonly assumed that other forms of pressure (in- gap between the soft X-ray emitting hot gas and the cluding magnetic, cosmic ray and turbulent pressure) neutral gas walls of the Local Cavity. can make significant contributions to the total pres- sure of the ISM. The magnetic field strength of the 4.3 Origin and Age neutral wall surrounding much of the Local Cavity in the disk (estimated at 8 µG, Andersson & Potter We note that interstellar structures similar to the ∼ 2 (2006), resulting in a magnetic pressure of B / 8πk of Local Cavity and the Local Chimney have been de- 18,000 K cm−3) demonstrates that it is possible for tected elsewhere in both our own and other galaxies ∼ large magnetic pressures to exist. A magnetic field of (McClure-Griffiths et al. 2003). Therefore, the pres- this strength is very similar to that theorized for mag- ence of such a low-density cavity in the ISM that is netic flux tubes that were originally invoked to con- linked to an overlying galactic halo is clearly not a rar- strain the number of production sites for interstellar ity. Since our new model does not require the presence OVI in the local ISM (Cox & Helenius 2003). of significant amounts of local million degree K gas, In addition, we note that global dynamic pressure then this suggests that either the Local Cavity is an old equilibrium may not actually exist within the Local (fossil) SNR (T > 106 years) in which any hot gas has Cavity for several reasons. Firstly, the cavity is not long since cooled and/or recombined, or that the Local fully pressure constrained by surrounding neutral in- Cavity was formed by some other manner, perhaps that terstellar gas walls and also there is clear evidence for which created Gould’s Belt (Lallement 2008). The Trouble with the Local Bubble 13 The origin and evolution of local blow-outs from Bowen et al. 2008). Clearly more distant (d > 200pc) the galactic disk have been detailed using hydrody- O VI absorption can also occur in more distant halo namic simulations (Breitschwerdt & de Avillez 2006; regions, as well as in distant hot superbubble cavities de Avillez & Breitschwerdt 2007). If the pressure of throughout the galactic plane. overlying halo gas is lower than the mean pressure in the disk, then interstellar bubbles (caused by the ac- 4.6 Soft X-ray background and EUV Emission & tion of either stellar winds or supernova events) could Remaining Problems blow out of the disk in the polar direction. Such struc- tures would cool rapidly in the equatorial regions and Clearly the most radical proposition in our new model is OVI could arise in the upward traveling shock, and in the lack of a requirement for a ubiquitous million degree the interfaces of the shocked cloudlets being ejected soft X-ray emitting gas contained within the confines of into the halo. We note that the numerous neutral and the Local Cavity, as delineated by its neutral absorbing partially ionized gas clouds detected at both ends of walls. Although the contribution of solar wind charge the Local Chimney have mostly negative velocities, exchange generated emission may account for a large whereas the OVI bearing gas has mostly positive ve- fraction of the observed soft X-ray background (espe- locities (Savage & Lehner 2006). Thus, the Hot Top cially at low galactic latitudes), some of the models Model is really an observational description of an aging still require some contribution from a million degree gas SNR whose hot gas has escaped and it is now filled present at low latitudes. This contribution is thought with warm and photo-ionized gas, which explains the to be small (< 200 10−6 ROSAT counts s−1 arcmin−2) × observed local dearth of HI. This warm gas may have and we propose that if it exists then its intrinsic varia- entered the aging cavity through the adjacent Loop I tion in intensity with (galactic) sight-line sampled has interface, having being expelled as stellar winds from previously been masked by the far larger contribution the Sco-Cen OB association. to the total soft X-ray background signal generated by the foreground solar wind charge exchange emission. 4.4 Kinematics of the Local Cavity Gas Therefore, if such hot gas does exist, then at present we know very little about its properties and previous The motions of both ionized and partially ionized gas models of the Local Hot Bubble require substantial re- within the Local Cavity (as measured by UV-visible visions. absorption lines towards nearby stars) is governed by Although the new model attempts to provide ex- inflow from the adjacent interstellar bubbles that abutt planations for many of the observational characteris- the Local Cavity (i.e. Loop I, Loop III, Pleiades etc). tics of soft X-ray, EUV, UV and visible observations No expansion is required (due to any pressure imbal- of the local interstellar gas, there is one set of obser- ance). The inflow of gas from the inner halo is seen vations that cannot be accounted for. From sound- only at z > 100 pc and the lack of any observed gen- ing rocket data gained over 25 years ago numerous | | eral inflow from this infalling gas into the very inner observers have found that the B/Be band ratio is es- regions of Local Cavity is most probably due to such sentially constant, irrespective of the sight-line probed. an effect being masked by the gas flow vectors from the However, present calculations of the solar wind charge adjacent interstellar cavities. exchange emission at these very low energies suggests that very little emission arises within the B-band. We 4.5 Absorption Column Densities note that satellite observations of the diffuse EUV background (Jelinsky et al. 1995) require limits for the In order for O VI absorption to occur, an interface be- plasma emission measure of a factor of 5-10 below the tween hot and cooler gas is required. In the new model B-band emission measured over a temperature range this can occur at locations within the Local Chimney from 105.7−6.4 K. Also, the Be-band measurements by where infalling highly ionized million degree halo gas Cash, Malina & Stern (1976), in agreement with the meets the cooler photoionized filler gas of the Local EUV satellite data, place a strict temperature up- 5 Cavity. The new O VI results support this scenario per limit of 3 x 10 K to any emission from a lo- ∼ for sight-line distances < 100 pc (Barstow et al. 2008; cal plasma. Such conclusions are in direct conflict Barstow et al. 2009). For sight-lines in the galactic with other measurements gained at these soft energies disk that extend beyond the confines of the Local (Bloch et al. 1986; Juda et al. 1991). Thus, the origin Cavity, O VI absorption can occur at interfaces be- and physical nature of this near-isottropic flux, in the tween more distant hot bubble cavities and their sur- context of our new model, still remains to be found. rounding neutral gas shells (Welsh & Lallement 2008; However, we note that it must be due to a local hot gas, 14 but with a lower temperature than 1 million mo K. One equilibrium (P/k 3000 cm−3) with the numerous par- ∼ intriguing possibility is that such emission could arise in tially ionized and warm (T 7000 K) small cloudlets ∼ the terrestrial magnetosheath via charge exchange pro- that are present within the local ISM. However, our cesses between heavy solar wind ions and geocoronal new model does require the presence of a million de- neutrals (Robertson & Cravens 2002). gree emitting gas at high galactic latitudes, such that Clearly, new high spectral resolution observations of the associated soft X-ray emission is only detectable the background emission in the 0.1 to 0.25 keV band when viewed along sight-lines that extend beyond both will be required in order to solve this outstanding issue. ends of the Local Chimney feature into the lower galac- Such observations are non-trivial since they will require tic halo. The model also predicts that locally, the OVI knowledge of the separate contributions to the total ion should be preferentially formed in interface regions emission signal from solar wind charge exchange and a between warm photo-ionized gas and the hot million presumed more distant emitting hot gas. degree gas of the halo. This scenario seems to be the case for OVI absorbers observed over sight-lines < 80pc from the Sun. The new model also provides an expla- 5 Conclusion nation for several observed parameters such as local gas pressure, the observed ‘dipole’ effect in maps of the soft The interstellar region surrounding the Sun to a dis- X-ray background and the spatial concordance between tance of 80pc in all directions has historically been the highest levels of soft X-ray emission and the spatial ∼ termed the Local Hot Bubble. Many of the physical distribution of local OVI absorbers. properties of this gas, as revealed by observations of Although the Hot Top model can explain many ob- the diffuse soft X-ray background (soft X-ray back- servational conundrums, it presently provides no ex- ground) emission, have long been described by emis- planation for observations of the emission observed by sion from a million degree gas in thermal equilibrium. sounding rockets in the B and Be-band X-rays. Such In this paper we have presented a wide variety of ob- data are also in contradiction with observations of the servational evidence gained over the last decade that EUV diffuse background emission recorded by satellite would appear to contradict the expectations of a hot instrumentation. Future high resolution spectral ob- Local Bubble model. In particular, the recently real- servations with an X-ray microcalorimeter at ultra-soft ized non-negligible contribution to the soft X-ray back- X-ray wavelengths could provide important constraints ground signal from solar wind charge exchange X-rays on the (still) disputed physical nature of interstellar gas generated within the heliosphere has required a criti- within 100pc of the Sun. cal re-thinking of the various physical components that constitute the soft X-ray background spectrum. Acknowledgements BYW acknowledges financial We have provided arguments that would imply that, support from NASA grant # NNX07AE33G. RLS at the very least, the intensity of the soft X-ray back- acknowledges financial support from NASA grant # ground emission arising solely from a local million de- NNH072DA001N. Both authors would like to thank gree hot plasma must be reduced by 50%. We have Don Cox, Steve Snowden, John Vallerga, Kip Kuntz, ∼ outlined the ramifications of such a reduction in the Martin Barstow, Ed Jenkins and Rosine Lallement X-ray brightness of a Local Hot Bubble and, although for many interesting discussions concerning this topic, several observations that have seemed at odds with the most of which took place while musing over a mug of notion of an emitting Local Hot Bubble are now ac- fermented hops. We would also like to thank both the counted for, the heating mechanism for the hot plasma editor and referee for their help in improving this paper. must be critically re-evaluated now that the gas is found to be dimmer and cooler. In addition, the spatial dis- tribution of the hot million degree gas within the Local Cavity also requires reconsideration. Finally we present a new model of the Local Cavity (i.e. the ‘Hot Top’ model) in which solar wind charge exchange can explain all of the observed soft X-rays in the Galactic Plane, and thus the local interstellar medium is devoid of million degree plasma at mid-plane latitudes. Instead, the Local Cavity is filled with a very diffuse and photoionized and/or collisionally ionized gas of temperature 20, 000 K that is in rough pressure ∼ The Trouble with the Local Bubble 15

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Fig. 1 The galactic distribution of the summed R1 (0.11 - 0.284 keV) and R2 (0.14 - 0.284 keV) band emission as recorded by the ROSAT all-sky survey (Snowden et al. 1998). Note that the highest intensities are generally found at high galactic latitudes.

Fig. 3 The distribution of cold and neutral gas within ±250pc of the Sun in the plane perpendicular to the galac- tic plane as revealed by interstellar NaI absorption measure- ments (Lallement et al. 2003). Dark contour regions repre- −2 sent neutral gas with log N(HI) > 19.3 cm ; white regions −2 are of low gas density typically of log N(HI) < 18.3 cm . This is a view looking side-on to the galaxy with the Sun at the center of the plot. Note the extension of the Local Cav- ity into the overlying halo via the low density Local Chimney feature, which is tilted at an angle that points towards the regions of highest soft X-ray background emission. Fig. 2 The distribution of cold and neutral gas within ±250pc of the Sun in the galactic plane as revealed by interstellar NaI absorption measurements (Lallement et al. 2003). Dark contour regions represent neutral gas with log N(HI) > 19.3 cm−2; white regions are −2 of low gas density typically of log N(HI) < 18.3 cm . This is a view looking down onto the galaxy with the Sun at the center of the plot. Note the narrow extensions of the low density Local Cavity into surrounding interstellar cavi- ties such as the Beta CMa tunnel, the Pleiades and Loop I bubbles. 18

Fig. 5 Spatial distribution of local O VI absorption to- wards local hot white dwarfs (Barstow et al 2009, Savage and Lehner 2006) and B-type stars within 120pc (Bowen et al. 2008, Welsh & Lallement 2008) superposed on a map of the distribution of the soft X-ray background R12 emission (Snowden et al. 1998). Sight-lines with distances < 80pc are circles, sight-lines with distances > 80pc are squares. Filled black circles or squares are detections of O VI, whereas non- Fig. 4 Simple 3-D schematic of the ‘Hot Top’ Model. The detections are filled white circles or squares. Note that there figure shows the spatial distribution of low neutral density are no detections of interstellar O VI along sight-lines < 80pc photo-ionized gas contained within the Local Cavity sur- that are located near to the galactic plane. Also note that rounded by its cold and neutral gas boundary wall and its the O VI detections are well correlated spatially with the openings into the halo through the Local Chimney as viewed regions of highest soft X-ray background emission intensity in the meridian plane (Lallement et al. 2003). The entire at highly positive or negative galactic latitudes. The figure region is subject to incident soft X-ray emission, arising in has been adapted from Barstow et al. (2009). a million degree hot halo, that impinges on infalling gas clouds at high galactic latitudes.