Interstellar Bubbles Research Articles

A Stellar Life

In this autobiography of 70+ yr of publishing astronomical papers, I summarize some of my major fields of research, such as (1) hfs in the solar spectrum, (2) the atmospheres of supergiants, (3) shock waves in Cepheids, (4) the Crab Nebula, (5) frequencies of binaries, (6) exoplanets, (7) starting Kitt Peak National Observatory, (8) catalogs, (9) China, (10) editing the Astrophysical Journal, (11) blue stragglers, (12) the Local Interstellar Bubble, and (13) publication studies.

Richard (Dick) A. McCray

McCray studied the formation of interstellar bubbles by stellar winds and 'super-bubbles' formed by supernova explosions such as SN 1987A in galactic gas disks. He was also deeply committed to improving undergraduate STEM education.

Tracking down the origin of superbubbles and supergiant shells in the Magellanic Clouds with Minkowski tensor analysis

Aims.We develop an automatic bubble-recognition routine based on Minkowski functionals (MF) and tensors (MT) to detect bubble-like interstellar structures in optical emission line images.Methods.Minkowski functionals and MT are powerful mathematical tools for parameterizing the shapes of bodies. Using the papaya2-library, we created maps of the desired MF or MT of structures at a given window size. We used maps of the irreducible MTψ2, which is sensitive to elongation, to find filamentary regions in Hα, [S II], and [O III] images of the Magellanic Cloud Emission Line Survey. Using the phase ofψ2, we were able to draw lines perpendicular to each filament and thus obtain line-density maps. This allowed us to find the center of a bubble-like structure and to detect structures at different window sizes.Results.The detected bubbles in all bands are spatially correlated to the distribution of massive stars, showing that we indeed detect interstellar bubbles without large spatial bias. Eighteen out of 59 supernova remnants in the Large Magellanic Cloud (LMC) and 13 out of 20 superbubbles are detected in at least one wavelength. The lack of detection is mostly due to surrounding emission that disturbs the detection, a too small size, or the lack of a (circular) counterpart in our emission line images. In line-density maps at larger scales, maxima can be found in regions with high star formation in the past, often inside supergiant shells (SGS). In SGS LMC 2, there is a maximum west of the shell where a collision of large gas clouds is thought to have occurred. In the Small Magellanic Cloud (SMC), bubble detection is impaired by the more complex projected structure of the galaxy. Line maps at large scales show large filaments in the SMC in a north-south direction, especially in the [S II] image. The origin of these filaments is unknown.

Compressed magnetized shells of atomic gas and the formation of the Corona Australis molecular cloud

We present the identification of the previously unnoticed physical association between the Corona Australis molecular cloud (CrA), traced by interstellar dust emission, and two shell-like structures observed with line emission of atomic hydrogen (HI) at 21 cm. Although the existence of the two shells had already been reported in the literature, the physical link between the HI emission and CrA had never been highlighted until now. We used both Planck and Herschel data to trace dust emission and the Galactic All Sky HI Survey (GASS) to trace HI. The physical association between CrA and the shells is assessed based both on spectroscopic observations of molecular and atomic gas and on dust extinction data with Gaia. The shells are located at a distance between ~140 and ~190 pc, which is comparable to the distance of CrA, which we derived as (150.5 ± 6.3) pc. We also employed dust polarization observations from Planck to trace the magnetic-field structure of the shells. Both of them show patterns of magnetic-field lines following the edge of the shells consistently with the magnetic-field morphology of CrA. We estimated the magnetic-field strength at the intersection of the two shells via the Davis-Chandrasekhar-Fermi (DCF) method. Despite the many caveats that are behind the DCF method, we find a magnetic-field strength of (27 ± 8) μG, which is at least a factor of two larger than the magnetic-field strength computed off of the HI shells. This value is also significantly larger compared to the typical values of a few μG found in the diffuse HI gas from Zeeman splitting. We interpret this as the result of magnetic-field compression caused by the shell expansion. This study supports a scenario of molecular-cloud formation triggered by supersonic compression of cold magnetized HI gas from expanding interstellar bubbles.

Supernova Remnants as Sources of Cosmic Rays and Nonthermal Emission

Cosmic ray acceleration by astrophysical shocks in supernova remnants is briefly reviewed. Results of numerical modeling taking into account magnetic field amplification by streaming instability and shock modification are presented. Nonthermal emission produced by accelerated particles in young and old supernova remnants is compared with available data of radio, X-ray, and gamma-ray astronomy. We also discuss a possibility of particle acceleration to PeV energies at supernova shocks propagating in the interstellar bubbles created by stellar winds of supernova progenitors.

Numerical models for the dust in RCW 120

Context. The interstellar bubble RCW 120 seen around a type O runaway star is driven by the stellar wind and the ionising radiation emitted by the star. The boundary between the stellar wind and interstellar medium (ISM) is associated with the arc-shaped mid-infrared dust emission around the star within the HII region. Aims. We aim to investigate the arc-shaped bow shock in RCW 120 by means of numerical simulations, including the radiation, dust, HII region, and wind bubble. Methods. We performed 3D radiation-hydrodynamic simulations including dust using the GUACHO code. Our model includes a detailed treatment of dust grains in the ISM and takes into account the drag forces between dust and gas and the effect of radiation pressure on the gas and dust. The dust is treated as a pressureless gas component. The simulation uses typical properties of RCW 120. We analyse five simulations to deduce the effect of the ionising radiation and dust on both the emission intensity and the shape of the shock. Results. The interaction of the wind and the ionising radiation from a runaway star with the ISM forms an arc-shaped bow shock where the dust from the ISM accumulates in front of the moving star. Moreover, the dust forms a second small arc-shaped structure within the rarefied region at the back of the star inside the bubble. In order to obtain the decoupling between the gas and the dust, it is necessary to include the radiation-hydrodynamic equations together with the dust and the stellar motion. In this work all these elements are considered together, and we show that the decoupling between gas and dust obtained in the simulation is in agreement with the morphology of the infrared observations of RCW 120.

X-Ray Luminosity and Size Relationship of Supernova Remnants in the LMC

Abstract The Large Magellanic Cloud (LMC) has ∼60 confirmed supernova remnants (SNRs). Because of the known distance, 50 kpc, the SNRs’ angular sizes can be converted to linear sizes, and their X-ray observations can be used to assess X-ray luminosities (L X). We have critically examined the LMC SNRs’ sizes reported in the literature to determine the most plausible sizes. These sizes and the L X determined from XMM-Newton observations are used to investigate their relationship to explore the environmental and evolutionary effects on the X-ray properties of SNRs. Our research provides the following three results. (1) Small LMC SNRs, a few to 10 pc in size, are all Type Ia with L X > 1036 erg s−1. The scarcity of small core-collapse (CC) SNRs is a result of CC SNe exploding in the low-density interiors of interstellar bubbles blown by their massive progenitors during their main-sequence phase. (2) Medium-sized (10–30 pc) CC SNRs show bifurcation in L X, with the X-ray-bright SNRs either in an environment associated with molecular clouds or containing pulsars and pulsar-wind nebulae and the X-ray-faint SNRs being located in low-density interstellar environments. (3) Large (size > 30 pc) SNRs show a trend of L X fading with size, although the scatter is large. The observed relationship between L X and sizes can help constrain models of SNR evolution.

Cosmic ray acceleration in magnetic circumstellar bubbles

We consider the diffusive shock acceleration in interstellar bubbles created by powerful stellar winds of supernova progenitors. Under the moderate stellar wind magnetization the bubbles are filled by the strongly magnetized low density gas. It is shown that the maximum energy of particles accelerated in this environment can exceed the ”knee” energy in the observable cosmic ray spectrum.

On the distance to the North Polar Spur and the local CO-H2 factor.

Most models identify the X-ray bright North Polar Spur (NPS) with a hot interstellar (IS) bubble in the Sco-Cen star-forming region at ≃130 pc. An opposite view considers the NPS as a distant structure associated with Galactic nuclear outflows. Constraints on the NPS distance can be obtained by comparing the foreground IS gas column inferred from X-ray absorption to the distribution of gas and dust along the line of sight. Absorbing columns towards shadowing molecular clouds simultaneously constrain the CO-H2 conversion factor. We derived the columns of X-ray absorbing matter N Habs from spectral fitting of dedicated XMM-Newton observations towards the NPS southern terminus (l II ≃ 29°, b II ≃ +5 to +11°). The distribution of the IS matter was obtained from absorption lines in stellar spectra, 3D dust maps and emission data, including high spatial resolution CO measurements recorded for this purpose. N Habs varies from ≃ 4.3 to ≃ 1.3 × 1021 cm-2 along the 19 fields. Relationships between X-ray brightness, absorbing column and hardness ratio demonstrate a brightness decrease with latitude governed by increasing absorption. The comparison with absorption data, local and large-scale dust maps rules out a NPS near side closer than 300 pc. The correlation between N Habs and the reddening increases with the sightline length from 300 pc to 4 kpc and is the tightest with Planck τ 353GHz -based reddening, suggesting a much larger distance. N(H)/E(B-V) τ ≃ 4.1 × 1021 cm-2 mag-1, close to Fermi-Planck determinations. N Habs absolute values are compatible with HI-CO clouds at -5 ≤ V LSR ≤ +25 to +45 km s-1 and a NPS potentially far beyond the Local Arm. A shadow cast by a b=+9° molecular cloud constrains X CO in that direction to ≤ 1.0 × 1020 cm-2 K-1 km-1 s. The average X CO over the fields is ≤ 0.75 × 1020 cm-2 K-1 km-1 s.

Evolution effect of BD+60°2522 to Bubble Nebula NGC 7635

Bubble Nebula is a bubble formed by the interaction between the stellar wind of BD+60°2522 with ambient interstellar gas. We use a web-based stellar evolution code, the EZ- web, to construct the evolution of BD+60°2522. From the evolution, we obtain the age of the system needed for the interstellar bubbles model. Then from the model, we determine parameters such as radius, expansion velocity, luminosity, temperature, and density of the Bubble.

Colliding interstellar bubbles in the direction ofl= 54°

Interstellar bubbles are structures in the interstellar medium with diameters of a few to tens of parsecs. Their progenitors are stellar winds, intense radiation of massive stars, or supernova explosions. Star formation and young stellar objects are commonly associated with these structures. We compare IR observations of bubbles N115, N116 and N117 with atomic, molecular and ionized gas in this region. While determining the dynamical properties of the bubbles, we also look into their ambient environment to understand their formation in a wider context. For finding bubbles in HI (VLA Galactic Plane Survey) and CO data (Galactic Ring Survey), we used their images from Galactic Legacy Infrared Mid-Plane Survey. We manually constructed masks based on the appearance of the bubbles in the IR images and applied it to the HI and CO data. We determined their kinematic distance, size, expansion velocity, mass, original density of the maternal cloud, age and energy input. We identified two systems of bubbles: the first, background system, is formed by large structures G053.9+0.2 and SNR G054.4-0.3 and the infrared bubble N116+117. The second, foreground system, includes the infrared bubble N115 and two large HI bubbles, which we discovered in the HI data. Both systems are independent, lying at different distances, but look similar. They are both formed by two large colliding bubbles with radii around 20-30 pc and ages of a few million years. A younger and smaller (~4 pc, less than a million years) infrared bubble lies at the position of the collision. We found that both infrared bubbles N115 and N116+117 are associated with the collisions of larger and older bubbles. We propose, that such collisions increase the probability of further star formation, probably by squeezing the interstellar material, suggesting that it is an important mechanism for star formation.

A RADIO-POLARIZATION AND ROTATION MEASURE STUDY OF THE GUM NEBULA AND ITS ENVIRONMENT

The Gum Nebula is 36 degree wide shell-like emission nebula at a distance of only 450 pc. It has been hypothesised to be an old supernova remnant, fossil HII region, wind-blown bubble, or combination of multiple objects. Here we investigate the magneto-ionic properties of the nebula using data from recent surveys: radio-continuum data from the NRAO VLA and S-band Parkes All Sky Surveys, and H-alpha data from the Southern H-Alpha Sky Survey Atlas. We model the upper part of the nebula as a spherical shell of ionised gas expanding into the ambient medium. We perform a maximum-likelihood Markov chain Monte-Carlo fit to the NVSS rotation measure data, using the H-halpha data to constrain average electron density in the shell $n_e$. Assuming a latitudinal background gradient in RM we find $n_e=1.3^{+0.4}_{-0.4} {\rm cm}^{-3}$, angular radius $\phi_{\rm outer}=22.7^{+0.1}_{-0.1} {\rm deg}$, shell thickness $dr=18.5^{+1.5}_{-1.4} {\rm pc}$, ambient magnetic field strength $B_0=3.9^{+4.9}_{-2.2} \mu{\rm G}$ and warm gas filling factor $f=0.3^{+0.3}_{-0.1}$. We constrain the local, small-scale (~260 pc) pitch-angle of the ordered Galactic magnetic field to $+7^{\circ}\lesssim\wp\lesssim+44^{\circ}$, which represents a significant deviation from the median field orientation on kiloparsec scales (~-7.2$^{\circ}$). The moderate compression factor $X=6.0\,^{+5.1}_{-2.5}$ at the edge of the H-alpha shell implies that the 'old supernova remnant' origin is unlikely. Our results support a model of the nebula as a HII region around a wind-blown bubble. Analysis of depolarisation in 2.3 GHz S-PASS data is consistent with this hypothesis and our best-fitting values agree well with previous studies of interstellar bubbles.

Wind bubbles within H ii regions around slowly moving stars

Interstellar bubbles around O stars are driven by a combination of the star's wind and ionizing radiation output. The wind contribution is uncertain because the boundary between the wind and interstellar medium is difficult to observe. Mid-infrared observations (e.g., of the H II region RCW 120) show arcs of dust emission around O stars, contained well within the H II region bubble. These arcs could indicate the edge of an asymmetric stellar wind bubble, distorted by density gradients and/or stellar motion. We present two-dimensional, radiation-hydrodynamics simulations investigating the evolution of wind bubbles and H II regions around massive stars moving through a dense (n=3000 cm^{-3}), uniform medium with velocities ranging from 4 to 16 km/s. The H II region morphology is strongly affected by stellar motion, as expected, but the wind bubble is also very aspherical from birth, even for the lowest space velocity considered. Wind bubbles do not fill their H II regions (we find filling factors of 10-20%), at least for a main sequence star with mass M~30 Msun. Furthermore, even for supersonic velocities the wind bow shock does not significantly trap the ionization front. X-ray emission from the wind bubble is soft, faint, and comes mainly from the turbulent mixing layer between the wind bubble and the H II region. The wind bubble radiates <1 per cent of its energy in X-rays; it loses most of its energy by turbulent mixing with cooler photoionized gas. Comparison of the simulations with the H II region RCW 120 shows that its dynamical age is <=0.4 Myr and that stellar motion <=4 km/s is allowed, implying that the ionizing source is unlikely to be a runaway star but more likely formed in situ. The region's youth, and apparent isolation from other O or B stars, makes it very interesting for studies of massive star formation and of initial mass functions.

DYNAMICS AND ENERGY LOSS IN SUPERBUBBLES

Interstellar bubbles appear to be smaller in observations than expected from calculations. Instabilities at the shell boundaries create three-dimensional ef- fects, and are probably responsible for part of this discrepancy. We investigate instabilities and dynamics in superbubbles by 3D hydrodynamics simulations with time-resolved energy input from massive stars, including the supernova explosions. We find that the superbubble shells are accelerated by supernova explosions, coincident with substantial brightening in soft X-ray emission. In between the explosions, the superbubbles lose energy efficiently, approaching the momentum-conserving snowplow limit. This and enhanced radiative losses due to instabilities reduce the expansion compared to the corresponding radiative bubbles in pressure-driven snowplow models with constant energy input. We note generally good agreement with observations of superbubbles and some open issues. In particular, there are hints that the shell velocities in the X-ray-bright phases is underpredicted.

Radiation-pressure-driven dust waves inside bursting interstellar bubbles

Massive stars drive the evolution of the interstellar medium through their radiative and mechanical energy input. After their birth, they form bubbles of hot gas surrounded by a dense shell. Traditionally, the formation of bubbles is explained through the input of a powerful stellar wind, even though direct evidence supporting this scenario is lacking. Here we explore the possibility that interstellar bubbles seen by the Spitzer- and Herschel space telescopes, blown by stars with log(L/L_sun) < 5.2, form and expand due to the thermal pressure accompanying ionization of the surrounding gas. We show that density gradients in the natal cloud or a puncture in the swept up shell lead to an ionized gas flow through the bubble into the general interstellar medium, which is traced by a dust wave near the star, demonstrating the importance of radiation pressure during this phase. Dust waves provide a natural explanation for the presence of dust inside H II bubbles, offer a novel method to study dust in H II regions and provide direct evidence that bubbles are relieving their pressure into the ISM through a champagne flow, acting as a probe of the radiative interaction of a massive star with its surroundings. We create a parameter space connecting the ambient density, the ionizing source luminosity, and the position of the dust wave, while using the well-studied H II bubbles RCW 120 and RCW 82 as benchmarks of our model. Finally, we briefly examine the implications of our study for the environments of super star clusters formed in UltraLuminous InfraRed Galaxies (ULIRGs), merging galaxies and the early Universe, which occur in very luminous and dense environments and where radiation pressure is expected to dominate the dynamical evolution.

A statistical study of gaseous environment of Spitzer interstellar bubbles

The expansion of interstellar bubbles is suggested to be an important mechanism of triggering material accumulation and star formation. In this work, we investigate the gaseous environment of a large sample of interstellar bubbles identified by the Spitzer space telescope, aiming to explore the possible evidence of triggered gas accumulation and star formation in a statistical sense. By cross-matching 6124 Spitzer interstellar bubbles from the Milky Way Project (MWP) and more than 2500 Galactic H ii regions collected by us, we obtain the velocity information for 818 MWP bubbles. To study the gaseous environment of the interstellar bubbles and get rid of the projection effect as much as possible, we constrain the velocity difference between the bubbles and the 13CO(1−0) emission extracted from the Galactic Ring Survey (GRS). Three methods: the mean azimuthally averaged radial profile of 13CO emission, the surface number density of molecular clumps and the angular cross-correlation function of MWP bubbles and the GRS molecular clumps are adopted. Significant over density of molecular gas is found to be close to the bubble rims. 60 per cent of the studied bubbles were found to have associated molecular clumps. By comparing the clump-associated and the clump-unassociated MWP interstellar bubbles, we reveal that the bubbles in associations tend to be larger and thicker in physical sizes. From the different properties shown by the bubble-associated and bubble-unassociated clumps, we speculate that some of the bubble-associated clumps result from the expansion of bubbles. The fraction of the molecular clumps associated with the MWP bubbles is estimated to be about 20 per cent after considering the projection effect. For the bubble-clump complexes, we found that the bubbles in the complexes with associated massive young stellar object(s) (MYSO(s)) have larger physical sizes, hence the complexes tend to be older. We propose that an evolutionary sequence might exist between the relatively younger MYSO-unassociated bubble-clump complexes and the MYSO-associated complexes.

The Interstellar Sight-Line Towards the M33 Galaxy

We present medium resolution ultraviolet absorption measurements recorded with the HST-COS and FUSE spectrographs towards the Wolf-Rayet star UIT-236 belonging to the BCLMP 99 HII region in the M33 galaxy. We have detected at least 5 major absorption systems along the 840 kpc sight-line to UIT-236 with average gas cloud LSR velocities of -205, -170, -135, -32, and +3 km s-1. The three most negative velocity cloud components, which we associate with gas in the M33 galaxy, have been detected in the line profiles of CII, CII∗,CIII, NI, NII, OI, OVI, SiII, SiII∗, SiIII, SiIV, PII, SII, ArI, FeII, and FeIII ions. These profiles have been fit with absorption models to determine ion column densities for each cloud component. For the case of highly ionized gas components, the observed column density ratios of OVI/SiIV are most compatible with the predictions of ionization by turbulent mixing layers or in a shock driven medium. A possible formation scenario is that the SiIV is formed within a photo-ionized stellar wind bubble that is being driven into surrounding gas that collisionally produces the observed OVI ions. Using the column density ratios of the CII/CII∗ lines, we have determined electron density values in the range 1.8–6.0 cm-3 for the ionized M33 gas clouds. Such values are similar to those found for other ionized interstellar bubbles such as N51D (in the Large Magellanic Cloud), γ2 Vel, and NGC 2244 (the Rosette Nebula). We have carried out a gas phase element abundance analysis of the three cloud components, indicating that C, N, O, Si, P, Ar, Fe, and Ni are all depleted relative to their solar abundance values. This pattern is confirmed by the observed N(OI)/N(HI) column density ratios which suggest a low metallicity for all three of the neutral gas components. The presence of interstellar shocks and expanding supershells of gas driven by stellar wind outflows from stars like UIT-236 may well account for the element depletion values observed along the M33 sight-line.

The interstellar medium and the massive stellar content towards the SNR G18.1−0.1 and neighbouring H ii regions

We perform a multiwavelength study toward the SNR G18.1-0.1 and nearby several HII regions (infrared dust bubbles N21 and N22, and the HII regions G018.149-00.283 and G18.197-00.181). Our goal is to provide observational evidence supporting that massive stars usually born in clusters from the same molecular cloud, which then produce, along their evolution, different neighboring objects such as HII regions, interstellar bubbles and supernova remnants. We suggest that the objects analysed in this work belong to a same complex located at the distance of about 4 kpc. Using molecular data we inspected the interstellar medium toward this complex and from optical and X-ray observations we looked for OB-type stars in the region. Analysing public 13CO J=1--0 data we found several molecular structures very likely related to the HII region/SNR complex. We suggest that the molecular gas is very likely being swept and shaped by the expansion of the HII regions. From spectroscopic optical observations obtained with the 2.15 m telescope at CASLEO, Argentina, we discovered three O-type stars very likely exciting the bubbles N21 and N22, and an uncatalogued HII region northward bubble N22, respectively. Also we found four B0-5 stars, one toward the bubble N22 and the others within the HII region G18.149-0.283. By inspecting the Chandra Source Catalog we found two point X-ray sources and we suggest that one of them is an early O-type star. Finally we inspected the large scale interstellar medium around this region. We discovered a big molecular shell of about 70 pc x 28 pc in which the analysed complex appears to be located in its southern border.

Feedback by massive stars and the emergence of superbubbles

We study the evolution of isolated and merging interstellar bubbles of three stars (25, 32 and 60 M\odot) in a homogeneous background medium with a density of 10 mp /ccm via 3D-hydrodynamic simulations with standard ISM thermodynamics (optically thin radiative cooling and photo-electric heating) and time dependent energy and mass input according to stellar evolutionary tracks. We vary the position of the three stars relative to each other to compare the energy response for cases of isolated, merging and initially cospatial bubbles. Due to mainly the Vishniac instability, our simulated bubbles develop thick shells and filamentary internal structures in column density. The shell widths reach tens of per cent of the outer bubble radius, which compares favourably to observations. More energy is retained in the ISM for more closely packed groups, by up to a factor of three and typically a factor of two for intermediate times after the first supernova. Once the superbubble is established, different positions of the contained stars make only a minor difference to the energy tracks. For our case of three massive stars, the energy deposition varies only very little for distances up to about 30 pc between the stars. Energy injected by supernovae is entirely dissipated in a superbubble on a timescale of about 1 Myr, which increases slightly with the superbubble size at the time of the explosion. The Vishniac instability may be responsible for the broadening of the shells of interstellar bubbles. Massive star winds are significant energetically due to their - in the long run - more efficient, steady energy injection and because they evacuate the space around the massive stars. For larger scale simulations, the feedback effect of close groups of stars or clusters may be subsumed into one effective energy input with insignificant loss of energy accuracy.

Distance to northern high-latitude HI shells

A detailed 3D distribution of interstellar matter in the solar neighborhood is increasingly necessary. As part of a 3D mapping program, we aim at assigning a precise distance to the high-latitude HI gas in particular the northern part (b \geq 55^{circ}) of the shell associated with the conspicuous radio continuum Loop I. This shell is thought to be the expanding boundary of an interstellar bubble inflated and recently reheated by the strong stellar winds of the nearby Scorpius-Centaurus OB. We recorded high-resolution spectra of 30 A-type target stars located at various distances in the direction of the northern part of Loop I. Interstellar NaI 5889-5895 and CaII K-H 3934-3968 {\AA} are modeled and compared with the HI emission spectra from the LAB Survey. About two-thirds of our stellar spectra possess narrow interstellar lines. Narrow lines are located at the velocity of the main, low-velocity Loop 1 HI shell ([-6,+1] km/s in the LSR). Using Hipparcos distances to the target stars, we show that the closest boundary of the b geq+70^{\circ} part of this low-velocity Loop I arch is located at of 98 \pm 6 pc. The corresponding interval for the lower-latitude part (55^{\circ} \leq b \leq 70^{\circ}) is 95-157 pc. However, since the two structures are apparently connected, the lower limit is more likely. At variance with this shell, the second HI structure, which is characterized by LSR Doppler velocities centered at -30 km/s, is NOT detected in any of the optical spectra. It is located beyond 200 parsecs or totally depleted in NaI and CaII. We discuss these results in the light of spherical expanding shells and show that they are difficult to reconcile with simple geometries and a nearby shell center close to the Plane. Instead, this high-latitude gas seems to extend the inclined local chimney wall to high distances from the Plane.