We report on FIR measurements of the dust component towards LVC 88+36–2, a dense neutral filament in the Local Hot Bubble. The measurements were performed with ISOPHOT, the photometer on-board ESA’s Infrared Space Observatory ISO. The dust temperature derived is about 18 K and stays constant across the filament. Limb brightening at 60μm is measured in agreement with cirrus clouds not located in the Local Hot Bubble. No evidence for an increased UV radiation field along the boundaries of the cloud is found.

Modeling the Local Bubble is one of those activities fraught with danger. It is very easy to be too naive, to fail to consider the dependence of the model on assumptions about the nearby ambient state, or the likelihood of such a structure. It is similarly easy to become so caught up in the details of the vicinity that it is unclear where to begin a necessarily idealized modeling effort. And finally, it is important to remember that the data we have may in some cases be lying to us, and that we have not yet learned to read their facial expressions quite carefully enough.

That said, I’ve tried in this paper to be helpful to those who may wish to take the risks. I surveyed the very most basic stories that the data seem to tell, and pointed out the standard coincidences that may be telling us a lot about what is happening, but may turn out once again to have been just coincidences. I’ve described 5 distinct conceptions that in one flavor or another pretty well survey the collection of mental images that have so far been carried by those who’ve attempted models. One may be right, or something entirely different may be more appropriate. It’s at least vital to realize that a conception comes first, followed by a simplified model of details. I’ve also included a long list of questions directed at observers. Some have partial answers, some one wouldn’t know today quite how to approach. But it is a list that students of the soft x-ray background, interstellar absorption lines, possible instrumentation, and the heliosphere may wish to review from time to time, just to see whether they can figure out how to be more helpful. There is another list for modelers, things the models must address, however-so-flimsily if necessary, because there are strong observational constraints (and stronger ones coming) on what can and cannot be present in the local ISM. To that I’ve added a few remarks concerning x-ray emission coming from beyond the Local Bubble, and another few on how x-ray emission from within the solar system might be contaminating what we see. That last bit is new, exciting, and possibly wrong, but it is an example of the ongoing wariness I believe one has to take toward the facts in the case. By the way, Dieter, it really was a great meeting.

We have modeled the Local Bubble (LB) using a one-dimensional hydrocode (ODIN) that can simulate multiple supernova remnants, with non-equilibrium ion evolution and dust. Our model assumes that the local interstellar medium was a cool (104 K) gas approximately 5-10 Myr ago; it was then disturbed by 2 or 3 supernovae exploding within 20-30 pc of each other over a period of 2-4 million years. The LB is the leftover hot gas from these explosions. The model predicts the x-ray emission from such a bubble, as well as ionic abundances for hot gas ions such as O VI. These are compared to the soft x-ray data from the Wisconsin all-sky survey and the ROSAT PSPC.

Equations are derived which describe the propagation of strong shocks in the interstellar matter, without any restrictions for symmetry, in a thin layer approximation (2.5 dimensions). Using these equations permits to calculate the propagation of shock waves from nonsymmetric supernovae explosions in a medium with arbitrary density distribution and the formation of superbubbles in galaxies.

The so-called cumulative effect take place in converging conical shock waves arising behind dense obstacles overtaken by incident interstellar shock. A significant part of energy of converging flow of matter swept-up by a radiative conical shock can be transferred to a dense jet-like ejection (“bullet”) directed along the cone axis. Possible applications of this effect for star-forming regions (e.g., OMC-1) and supernova remnants (e.g., Vela SNR) are discussed.

In the paper we present results of numerical simulations of magnetorotational model of explosion of magnetized cloud. For the simulation we used a specially developed 2D implicit numerical scheme on Lagrangian triangular grid with grid reconstruction. Our results show that due to the amplification of toroidal magnetic field component and transfer of angular momentum, a compression wave in the envelope of the cloud appears and moves through rapidly decreasing density background. This wave turns into a shock wave and pushes out part of the envelope of the star. Quantitative estimates of mass and energy carried away are given.

We describe results from high-resolution (Δv = 0.3-1.5 km s−1) observations of absorption lines due to interstellar Na I, Ca II, K I, Ca I, and Ti II. At those resolutions, we can identify and characterize many (but not all) of the individual clouds along a given line of sight; complex spatial and/or velocity structure appears to be the norm. Both the spectra and the statistics of individual cloud properties derived from fits to the observed line profiles suggest that: 1) the five species are not entirely coextensive in the neutral ISM; 2) the typical separation between adjacent components (in Na I and Ca II) is ~1.2 km s−1; 3) the median line widths (FWHM) for components in Na I, K I, and Ca I are ≾ 1.2 km s−1; 4) larger line widths for Ca II, even for “corresponding” components at similar velocities, suggest that Ca II is more widely distributed; 5) line widths for clouds in the Galactic halo and in the local ISM (within 100 pc) are generally somewhat larger than for clouds in the general Galactic disk; and 6) components with very different properties (line width, relative abundances, overall column densities) are often separated by only several km s−1. Observations of multiple systems and lines of sight in restricted regions reveal complex spatial structure and velocity coincidences even within 100 pc. These high-resolution optical spectra can be used to model lower resolution UV spectra of many other neutral and singly ionized species, enabling abundances and physical properties to be derived for individual interstellar clouds.

With new high resolution UV spectra of ϵ CMa we show that the gas column density in this sight-line is less than 4 1017 cm−2, that the neutral gas density is less than 10−5 cm−3 after the first 3 parsecs, and that the Local Cloud seems to be almost undepleted and to extend to no more than 0.6 pc in this direction.

We present new results based on the analysis of ROSAT PSPC observations towards nearby neutral clouds. These clouds are detectable as deep soft X-ray absorption features within the ¼ keV energy band. Towards one of these clouds, optical absorption line measurements determine an upper distance limit of D ≤ 60 pc. The presented data suggest that this filament is only a part of a much more extended local–cloud structure. Here we demonstrate that we most probably detected a local neutral atomic hydrogen “wall” in the direction of a low–velocity arc (Pietz 1994).

In recent years the nature of the low density clouds within the Local Bubble has been increasingly well characterized. These clouds, including the one which surrounds the solar system, are embedded in the hot gas and therefore should be evaporating via thermal conduction. If several evaporation fronts exist within the Local Bubble, the emissivity and spectrum of the hot gas is significantly different from a single temperature, equilibrium ionization plasma. We explore models in which the the temperature, density and ionization in the hot gas are influenced by cloud evaporation and compare the results with the observed Soft X-ray Background.

We present HST observations of the interstellar medium toward the star β CMa known to be located in a low density extension of the Local Bubble. Most of the matter in the sight-line is ionized and clumped in two main components. One of them, as well as one of the components detected toward ϵ CMa, is mostly ionized and only slightly depleted. Their ionization ratios are compatible with collisional ionization at T~25 000 K. These clouds could have been ionized by shocks related to the Local Bubble creation and also responsible of some dust grain sputtering.

The current status of observations of energetic particles in the “local bubble” is reviewed. This includes primarily “direct” measurements of cosmic rays made in the Solar System, but also the “remote sensing” made possible by observing cosmic ray produced γ-rays in the nearby interstellar clouds. Since the energetic events responsible for the formation of our local bubble may also have produced copious amounts of cosmic rays, fossil records are examined to determine whether there is a corresponding signature. The observations show that: 1) the cosmic ray (proton) intensity is fairly homogeneous throughout the local bubble and its adjacent interstellar clouds, 2) there is some evidence for a “recent” local cosmic ray injection about 40,000 years ago, 3) on longer time scales (a few million years) the cosmic ray intensity was constant within a factor two, 4) there is apparently some “activity” in the Orion cloud, as evidenced by low energy γ-ray signatures, and 5) there are two unexplained observations – the variations in the energy spectra, in particular the significantly flatter spectrum of heavy cosmic rays (Fe) and the matter path length variation, which yields consistently larger path lengths for the lighter elements (H, He). It is suggested that these observations are compatible with two cosmic ray populations – an older one in equilibrium with losses from the galaxy and a younger one which is not yet strongly affected by losses. The latter could be a cosmic ray signature of the formation of the local bubble.

The excess of γ-ray emission in the energy range 3–7 MeV discovered by COMPTEL in the direction of the Orion complex may imply that there is a powerful source of low energy cosmic rays in the local galactic medium. Several interpretations of the excess have been suggested. One of them assumes that the emission is identified with nuclear de-excitation lines of excited 16O and 14C. To provide the excess the energy deposit in these nuclei should be as large as ~ 1039 erg s−1. If a comparable part of these nuclei escape into the local galactic medium they can play a significant role in the energy balance there.

If the excess is interpreted as continuous it can be produced by bremsstrahlung emission of fast 10 MeV electrons. Much less energy deposit in these electrons is necessary (~ 1036 erg s−1) to generate the observed Orion flux. The intensity of these electrons even near Earth can be higher than that of the galactic cosmic ray electrons.

Starting from the casual observation that neighbouring lines of sight to stars at similar distances in the LISM show abruptly varying column densities, we developed a technique for measuring properties of single clouds. Fig.l shows an idealized version of this. The steps (Trapero et al. 1992, 1995) are:

1. – Correct for extinction the distances to local stars used, via B-V (where available), or our own measured Nal or KI absorptions.

2. – Measure diametral column density towards A from Nal or KI equivalent width and convert to hydrogen: N(H) [cm−2].

3. – Measure cloud diameter, between E and E’.

4. – Estimate the number density, n(H) = N(H)/d [cm−3].

5. – Use a canonical thermal pressure, nT = 3500 K cmThis at least gives the decision cool (<100K) or warm (>5000K).

6. – Estimate the cloud mass by integrating n(H) over the estimated volume.

Following up, cool cores within warm clouds, or warm envelopes around cool clouds can be identified using distance and velocity as tags. We have found empirical rules of thumb to guide us:

1. – The KI resonance at 7699Å is ideal for finding cool clouds; it is too weak to show up in warm clouds.

2. – NaI D doublet lines weaker than 5mÅ typify warm clouds. There are too many of these, by a factor 50, to come from cool cloud edges. This is confirmed by the “low velocity Routly-Spitzer effect” (see below).

Observations of interstellar Na I in the spectra of 93 stars within 315 pc from the Sun show that it lies in a tunnel of gas moving away from Scorpio-Centaurus and is surrounded by gas moving toward the Galactic center.

Gas approaches the Sun from Scorpio-Centaurus expanding from (r, l, b)=(160 pc, 313°7, +28°2) with LSR velocity 15.3 km s−1. The radius of this shell is 153 pc.

We identify these clouds:

D: velocity vector (υd, ld, bd)=(+7.2 km s−1, 305°1, −13°5), above and below the Galactic plane (GP) in the range of Galactic longitudes 357°–55°.

C: velocity vector (υc, lc, bc)=(+11.5 km s−1, 349°0, −35°2), above and below the GP in the range 30°≤l≤110°.

M: velocity vector (υm, lm, bm)=(+21.9 km s−1, 34°2, +1°5), above and below the GP in the range 100°≤l≤130°.

P: velocity vector (υp, lp, bp)=(+13.8 km s−1, 244°9, +5°4), above and below the GP from l~120° to the limit of our data at l~210°.

E: velocity vector (υe, le, be)=(+16.8 km s−1, 208°4, +6°2) in the range 160°≤l≤185° and −10°≤b≤–35°.

A: velocity vector (υa, la, ba)=(+12.9 km s−1, 73°6, −5°6) towards the Galactic anti-center, below the GP.

I: velocity vector (υi, li, bi)=(+37.7 km s−1, 132°8, −64°3) towards the Galactic anti-center, above the GP.

We present a definition of the local void of neutral gas from observations in the radio frequency window. We question the concept of the Local Bubble in sense of a more or less spherical volume which is surrounded by a shell of denser gas. The concept of the Local Bubble is challenged by the discovery of numerous neutral, dense clouds inside the local void. The search for a “shell” around the suspected Bubble has resulted only in inconclusive findings so far. The sample of high latitude molecular clouds illustrates the situation particularly well. The statistical properties of their spatial distribution, e.g. the mean distance, seem to fit very nicely to the spatial extent of the Local Bubble. But a more detailed investigation shows that the concept of a bubble – in particular an expanding bubble – is not supported. We suggest that the local void is nothing more than a typical place in an interarm region of our Galaxy.

Finally, a discussion of the high latitude boundary of the local void does not give strong evidence for the concept of a bubble, that has once been in rapid expansion and is still showing signs of interaction with its environment. However, indications for interactions of IVCs or HVCs with their surroundings are found. These hint at the presence of a gaseous disk which is much more vertically extended than previously believed, or at a Galactic wind which may be blowing from the Galactic neighbourhood of the Sun.

We present here the results from an extensive scintillation study of twenty pulsars in the dispersion measure (DM) range 3 – 35 pc cm−3 carried out using the Ooty Radio Telescope, to investigate the distribution of ionized material in the local interstellar medium (LISM). Our analysis reveals several anomalies in the scattering strength, which suggest that the distribution of scattering material in the solar neighborhood is not uniform. Our model suggests the presence of a low density bubble surrounded by a shell of much higher density fluctuations. We are able to put some constraints on geometrical and scattering properties of such a structure, and find it to be morphologically similar to the local bubble known from other studies.

A possibility of obtaining information on small scale inhomogeneities of the electron component of the local interstellar medium (LISM) is investigated using interstellar scintillations of extragalactic radio sources. We analyse Culgoora array observational data on variability of 190 extragalactic radio sources, covering most of the sky, at 80 and 160 MHz. The variability at time scales from 1 month to 15 years is interpreted as refractive interstellar scintillations in fast-moving nearby (less than 150 pc) hot gas near shock waves in the LISM. All-sky map of scintillation indices m averaged over 3–5 sources closest to one another shows several m maxima. Two of the 3 most pronounced maxima are probably connected with Loop I; the third one coincides with the soft X-ray (0.1–0.3 keV) background maximum near the South Galactic Pole. Other, less certain, m maxima probably correspond to the Orion star-formation region and to a soft X-ray maximum near the North Galactic Pole. The ”free-of-gas” tunnel in the direction l = 240° corresponds to low values of m. The estimated time scale of interstellar scintillations on the above-mentioned LISM structures is in agreement with that of the observed radio-source variations.

We discuss details in the galactic HI distribution, which are relevant for studies of the Local Bubble. All results are based on the 21 cm Sky survey with the Large Pulkovo Radiotelescope. The antenna temperature scans were decomposed in two components and after the subtraction of the broad one two details in the residual are presented in this paper. The first is an estimate of the distance to the Sancisi-van Woerden filament in the Sco-Oph region. The second is the connection of a 21 cm feature in the southern galactic quadrant II with the negative-velocity elliptical brightness distribution around the South Pole of the Galaxy.

Based on recent observation of the dominance of the flare generated solar wind streams over the co-rotating streams and a significant lowering in the cosmic ray intensity in the abnormal year of the solar activity period, I have predicted the occurrence of discernible changes in magnetic field strength and the thickness of the shell of the local bubble during high sunspots years of the solar cycle period. In order to observe the variation, both in the magnetic field strength and in the thickness of the compressed shell, a proposal of continuous monitor of these two parameters through the observation of Faraday Rotation Measure has been proposed with an emphasize to the observations in the period of next abnormal year of the solar cycle in near future.