How to Blow a Local Bubble

The Sun burns alone in space, closely surrounded by its children (the planets), but on a galactic scale it is isolated. The vast voids between it and its neighbors are pure vacuum, utter emptiness, devoid of all matter. This is the view that c onventional belief and science fiction hold, but how accurate is it, really? Research carried out by Don Cox and others gives us an alternate picture: The Local Bubble. The theory behind the Local Bubble is simple in concept, but complex in background. Put simply, the theory claims that the Sun is in a region of the galactic disk with an amazingly low density and high temperature compared to the rest of the Interstellar Medium (ISM), and that this 'bubble' of low density and high temperature is the re sult of one, possibly several supernovae whose explosions swept this region clean (3, 304). The 'normal' density of the ISM ranges from 1 H atom cm-3 to as much as 10 cm-3 in 'clouds' between the stars, with temperatures of 100 K and 8,000 K respectively , though these figures are a source of much debate (1). Professor Cox, along with Ronald J. Reynolds, are carrying out research to explore the properties and physics of the Local Bubble, and this paper will describe the theory and their work.

The existence of the Local Bubble was suspected due to the soft X-ray background (SXRB). Observations of the sky, starting in 1968 with sounding rockets (6, 71) revealed a diffuse spectrum in the SXR band of .1 to 10 KeV, with normal thermal excitation of hydrogen gas taking place below .25 KeV (5, 659). From this background, the temperature of the local ISM was deduced to be 106 K, with a density of 5 10-3 H atoms per cm-3, and a thermal pressure of 104 cm-3 K. Therefore, this radiation is opticall y thin and at high temperature. Also, the radiation is fairly isotropic, found in all directions, so it was known that its source was from within the galaxy (if it was extragalactic, it would be only visible from above and below the galactic plane, since the vast mass of the galaxy would absorb the radiation in the disk) (1). The best model was emission from hot plasma in a cavity surrounding us, though not filling the cavity entirely. Further observations have revealed a radius of 30 pc in the Galacti c plane, and as much as 200 pc above the disk, resulting in a volume equal to that of a sphere 200 pc in diameter (3, 304).

The origin and age of the cavity is open to several possibilities. The Local Bubble's low density may be explained by the winds and explosions of several average stars acting in unison out of the average ISM (3, 304). These activities would result in a higher concentration of stars that are farther down the main sequence (as is seen in the solar neighborhood), since presumably it is the hotter and younger stars that are so outspokenly violent. This method gives an estimated age of the Local Bubble as 2 107 years (3, 304). The high temperature may be the result of an active shock wave that could define the boundary of the LISM, still constrained in the lower-density area of the Local Bubble. This model needs an ambient ISM density of about 4 10-3 o r 10-2 cm-3, and the energy of one supernova explosion to work, giving us an age of about 105 years, in the vicinity of the Sun (3, 305). Another model for the high temperature is that the Bubble is comprised of pressure-confined hot gas with the most re cent supernova in it occurring at least 106 years ago, probably ten times that or longer. Models of this type of region show a lifetime 100 times longer than the active-wave blast model, so our odds of being within such a region increase by two orders of magnitude. The source of this pressure-confined region may be attributed to about a dozen supernovae occurring 107 years ago in an average density ISM (3, 305). Supernovae are not necessarily lone objects; it is entirely reasonable to assume that a gro up of O and B stars formed at about the same time would all die in a relatively short period (6, 86). Therefore this second option is quite reasonable.

Professor Cox's research is based on assuming the pressure-confined, hot quiescent gas model is the accurate one. The assumption is also made, for various observational (and highly technical) reasons, that the Local Bubble has an actual boundary beyond which there is no SXRB. From these assumptions, one finds two possibilities regarding the Local Bubble, directly quoted from Professor Cox's paper in the 1987 Annual Review of Astrophysics (3, 309):

  1. The LISM is somewhat typical. Its low density is like that of the intercloud medium elsewhere, but its properties resemble those in that small percentage of the general ISM that has recently experienced supernova reheating. Thus, the local value of the intercloud temperature is abnormally high.
  2. The LISM is distinctly unusual, dominated by the presence of a large cavity of exceedingly low density and high temperature, as compared with interstellar norms.
The third possibility, that the Local Bubble is identical to the rest of the ISM and we are wasting our time, is of course ignored for the purposes of this paper and Professor Cox's research. These two possibilities give rise to several sub-possibil ities, as well, to further complicate matters. Under #1 above, the Local Bubble could be (I) an actively expanding supernova remnant, or (II) a gradually-cooling bubble left over from an explosion that has long since completed its growth. In a similar m anner under #2 above, there are the possibilities that (III) the SXRB is due to a young SN remnant reheating the diffuse material in the Local Bubble, (IV) the SXRB results from an ancient SN that has reheated a less dense cavity, or (V) that one explosi on alone both created the cavity and left it so hot (3, 314). I and III can be discussed simultaneously, as can II and V. Option IV is similar to these, but is inertially confined by the dense boundary of the Bubble and is thus exceedingly compli cated, so I will not discuss it here (it is heavily theoretical). Naturally, it is possible that there are phenomena that we don't understand taking place out there, making all these explanations seem useless. Assumptions I and III above depend on the fact that the Local Bubble is a young SN remnant, still actively expanding. These models assume that the cavity of low density existed beforehand, either due to chance or possibly that low density regions are no t uncommon in the galaxy. From various technical methods, involving observed temperature, radius, x-ray surface brightness, etc. it is found that one supernova explosion (51050 ergs) 105 years ago in the local solar neighborhood, in an ISM of 410-3 cm-3 could result in a Bubble that matches observations (3, 315). Note that this short timescale may have had evolutionary influence on primitive human beings. This is a tightly constrained model, mainly due to the assumption that the remnant we call the Loc al Bubble will continue to grow and evolve (as all young things do) for ages to come.

If the Local Bubble is really the result of an ancient SN remnant in a low-density region, (timescale of about 2 105 or 106 years ago), then it is very difficult to model (3, 316). The reason lies with the necessary assumption that the LISM is pressure- confined and old; there would be strong reflected shocks off the denser boundary with effects upon the ambient pressure inside the Bubble. Again, various technical observations and derivations too complex to discuss here make this scenario unlikely. For LISM reheating phenomena taking place even further back than millions of years, a reasonable model can be constructed if one interprets the SXRB as originating from a residual pocket of hot gas in rough equilibrium with the rest of the Local Bubble (3, 3 16). Thus, II and V above are reasonable only if the explosion is incredibly ancient, on the timescale of the mass extinction of the dinosaurs.

Finally, there are also low-density features in the Local Bubble that are something of an anomaly (3, 307). The Sun is inside one of these clouds, termed the Local Fluff, and it was mainly discovered via backscattered solar radiation. These clouds are of slightly higher density than the rest of the Bubble, about .1 cm-3, about 104 K (significantly colder than the rest of the Local Bubble) (3, 329). From various sources of data, like interstellar absorption-line observations of nearby stars, it is pos sible to determine things like the ionization state of the Local Fluff and its distribution around the Sun, and much more no doubt remains to be discovered. One of the greatest difficulties about the Fluff is why it exists at all - if a SN or similar exp losion created the Bubble, it would have swept away all remnants of clouds in its way, so the existence of the Local Fluff is something of a surprise (3, 339). A possible source of the Fluff is planetary nebulae; with careful observations, one could theo retically associate white dwarfs with individual clouds (3, 341).

Professor Cox's work is mainly on the theoretical side, involving data analysis from various sources, notably including observations from the Wisconsin Fabry-Perot spectrometer and the German-British ROSAT satellite. New data obtained in the last year ( as yet unpublished) has revealed several new points of controversy and question. For example, in the direction of galactic longitude 330o at high altitudes, there is an outcropping of the Local Bubble that extends to as much as 200 pc (3, 326). ROSAT da ta seems to indicate that this piece of SXR-emitting bubble is not actually connected to our Local Bubble after all, but is actually 5 times more distant! Instead, a distant source of emission is in blocked by some colder gas, and the 'shadow' caused by this intruder causes us to think the emission source is nearer than it is, in a manner reminiscent of interstellar absorption (2). This raises a problem: why do the x-rays reach us at all, if they actually have to travel five times as far (presumably thr ough normal density ISM)? Possible answers may be that the distant source and the Local Bubble are connected by a cold but low-density tunnel or corridor, or that the source is right next to the shadow-caster. Further observations will be published soon ; much of the work in this field is happening in the Space Sciences department here at the University of Wisconsin, Madison.

Also, there is a problem with variable sources of x-rays in the Local Bubble. The SXRB varies by a large amount due to unknown sources that have a period of days, so at any given time it is difficult to know if the spectrum received is the real spectrum or not (2). Since the variable contributions are positive only, ROSAT and other instruments are focusing on small pieces of the background, and attempting to find the lowest level of emission, under the assumption that the lowest rate is equal to the ac tual rate. In some directions, the SXRB may actually be zero!

Other problems also have arisen from the data. For example, the 30 pc figure quoted earlier may be in error according to recent white dwarf observations. There are large 'barriers' to vision in several directions, that imply a distance to the higher-de nsity 'wall' of the Bubble to be as little as 10 pc by some estimates. This lesser figure would greatly upset many of the working models that are used, since sources that were thought to be X distance away are now three times closer and massive recalibra tion must be carried out (2). Also, if the source of the SXRB is really only hot gas (as all the present models assume), then theory requires Silicon VIII ions present in the spectrum. In fact, Sulfur VIII is observed, which would seem to indicate that there is a significant amount of dust present as well, which should result in pressure and temperature changes (2). There is still a large amount of observations to be carried out, and an equally large amount of analysis to be undertaken, before these an d other problems are resolved.

ROSAT did deliver some good news, as well. An X-ray source, Geminga, was discovered last spring that was in the right place 300,000 years ago to be the progenitor of the SN that formed our Bubble (4, 15). Geminga, observed to be a pulsar with period .2 seconds, lies at present outside the Local Bubble because its explosion imparted a large velocity to it, and by studying its rotation its age was determined. Ground zero for the explosion was in Orion, about 200 pc away - the explosion would have been e asily visible to our ancestors (4, 15).

So what is the real picture of the local solar neighborhood? It seems that the following picture may be more appropriate: The Sun nestles deep inside a cloud of Fluff, one of several adrift inside a vast 'cavern' where it is clean and warm, unlike the looming presence of cold, crowded normal space hundreds of parsecs off. Instead of cold, empty space we are in warm, fluffy space inside a hot, empty space, surrounded by cold, dense space. Though many difficulties are yet to be conquered in the models for the Local Bubble, this theory is the best fit to the observed data yet. Professor Cox is just one of the many researchers trying to unlock the mystery of the SXRB, and Madison is one of many centers of research, but both Madison and Cox are acknowled ged leaders in the field.