HI Shell Research Articles

Bubbles in the galactic haloes

We briefly discuss a possible interconnection of vertical HI structures observed in the Milky Way galaxy with large scale blowing out explosions by clustered SNe. We argue that the observed OB associations can produce only about 60 such events, or approximately one chimney per 3 kpc2 within the solar circle. We Btao discuss the overall properties of HI shells in nearby face-on galaxies and the distribution of Ha and dust in edge-on galaxies. We argue that the presence of dust in galactic haloes may indicate that radiation pressure is the most probable mechanism capable of transporting dust to large heights above the galactic plane. In order to make this possible, the galactic magnetic field must have a strong vertical component. We mention that SNe explosions can initiate the Parker instability which in turn creates large scale magnetic loopa with a strong vertical component. Recent observations of nearby edge-on galaxies favour this suggestion.

The HI supershell GS061+00+51 and its neighbours

We describe HI observations of a 4 x 4 degrees field in the Milky Way centered on l=61 deg, b=0 deg made by the Effelsberg radiotelescope. The field contains one previously identified HI supershell, GS061+00+51 (Heiles, 1979); apart from it we find several new structures. We also study the HI distribution in the vicinity of four HII regions, S86, S87, S88 and S89. We confirm the existence of the shell GS061+00+51, and we find that it has two smaller neighbours, spherical shells with a radius around 30 pc. We identify at least one more regular shell at v_LSR = -18 km/s; and one blown-out shell at v_LSR = -54 km/s. In two cases we are able to connect HII regions with features in the HI distribution (S86 and S87), in two other cases no connection is found. Apart from quite regular HI shells we see a number of non-coherent objects, which are probably a result of the turbulence in the interstellar medium.

The peculiar supernova remnant CTB 80

Deep CCD exposures of the peculiar supernova remnant CTB 80 in the light of the H +( Nii), (S ii), (O ii), and (O iii) lters have been obtained. These images reveal signicant shock heated emission in the area of the remnant. An extended bright diuse nebula in the south{east part of CTB 80 overlaps soft X-ray emission from ROSAT but it does not appear to be related to the remnant under study. New diuse and lamentary structures are detected to the south, south{east, and north of PSR 1951+32, most likely associated with CTB 80. Especially, the sulfur line image shows emission in the north along the outer boundary of the IRAS and HI shells. The (O iii) emission is lamentary, suggesting shock velocities greater than 100 km s 1 but its spatial extent is quite limited. Lower shock velocities are expected in the north and north{east areas of the remnant, since (O ii) emission is present, while (O iii) line emission is not detected. The comparison between the (O iii )a nd (Oii) line images further suggest the presence of signicant inhomogeneities in the interstellar medium. The flux{calibrated images do not indicate the presence of incomplete recombination zones, and we estimate that the densities of the preshock clouds should not exceed a few atoms per cm 3 . The typical projected angular widths of the observed laments are30 00 . Typical surface brightness values of the long (O iii) lament in the south are12 10 17 erg s 1 cm 2 arcsec 2 while the (O ii) image is characterized by fluxes of 10{20 10 17 erg s 1 cm 2 arcsec 2 . The area covered by the optical radiation along with the radio emission at 1410 MHz suggest that CTB 80 occupies a larger angular extent than was previously known.

The galactic plane region near $\mathsf{\ell=93^\circ}$

We present new Canadian Galactic Plane Survey radio continuum and 21 cm HI line observations of NRAO 655 (G93.4+1.8), plus radio recombination line observations, and optical H -line observations. The radio spectrum of NRAO 655 conrms its emission as thermal. From the 21 cm HI data we nd an atomic hydrogen cavity associated with this object atv' 71: 5k m s 1 . The cavity corresponds in position and size to the brightest radio continuum emission from NRAO 655. The corresponding kinematic distance is 8.8 kpc, placing NRAO 655 in the Perseus Arm. NRAO 655's linear size is therefore 70 pc 130 pc. To conrm the21 cm HI velocity we present the rst recombination line detection of NRAO 655 (H158 line, v' 71 km s 1 ), and the rst observations of a molecular cloud coinciding with NRAO 655 (at v' 72 km s 1 ). The rst optical detection of 656 nm H emission line features in NRAO 655 is also presented, and the H emission line brightness is determined. We suggest that the eastward extension of this strongly asymmetric object originates in a champagne outflow, and we estimate its age. We show that a single early-type star cannot be responsible for the outflow, whereas a group of later-type stars would suce. A partial HI shell is seen adjacent to the brightest part of NRAO 655; we suggest that it has been formed by dissociation of H2 in the molecular cloud.

Hα Emission Stars in the Cepheus OB3 Region

Hα emission objects were searched for, using the objective-prismSchmidt plates in a 36 deg2 sky area covering the Cepheus OB3 association.Hundred and eight emission stars have been found, 68 of which are newfindings to our knowledge. The properties of the IRAS point sourcesidentified with Hα emission stars indicate that most of theHα emission stars are T Tauri stars or related objects. Hαemission stars associated with IRAS sources and those unassociated havedifferent distributions, suggesting an age sequence among them.Altogether, they show a ring- or shell-like surface distribution, whichalmost coincides with that of heated dusts, and these distributionsappear to be encompassed by an HI shell.

The H [CSC]i[/CSC] Shell G132.6−0.7−25.3: A Supernova Remnant or an Old Windblown Bubble?

Data from the Canadian Galactic Plane Survey reveal an abundance of HI shells and arcs in the disk of our galaxy. While their shape is suggestive of stellar winds or supernovae influence, very few of these structures have been examined in detail thus far. A fine example is an HI shell in the outer Galaxy with no continuum counterpart discovered in the survey's pilot project. Its size and kinematics suggest that it was created by the winds of a single late-type O star which has since evolved off the main sequence or by a supernova explosion. A B1 Ia star at the centre of the shell, in projection, is a possible candidate for energy source if the shell is assumed to be wind-blown. The shell's shape implies a surprisingly small scale height of less than about 30 pc for the surrounding gas if the elongation is due to evolution in a density gradient.

Holes and Shells in the Interstellar Medium of the Nearby Dwarf Galaxy IC 2574

We present HI synthesis observations of the nearby dwarf galaxy IC 2574 (a member of the M81 group of galaxies) made with the NRAO Very Large Array at high spatial and velocity resolution (95 pc x 2.6 km/s). The VLA HI observations show a stunning amount of detail in the form of 48 mostly expanding HI shells and holes in its neutral interstellar medium. These features range in size from about 100 (a limit set by the size of the beam) to about 1000 pc dominating the appearance of the HI surface brightness map. Their dynamics clearly influence the velocity field of IC 2574. Current star formation, as traced by H-alpha emission, is predominantly found along the rims of the larger HI holes, suggesting propagating star formation. On linear scales of about 95 pc, star formation occurs if the HI surface density reaches values higher than 10^21 cm^-2. The scaleheight of the HI layer is found to be about 350 pc, considerably thicker than that in massive disk galaxies. This is due to a lower gravitational potential (for the same observed onedimensional velocity dispersion of about 7 km/s). A comparison with other galaxies shows that the energies needed to create the expanding HI structures are the same for all types of galaxies, at least to first order. The overall statistical properties of the HI holes and shells in galaxies show clear trends with Hubble type (or rather mass), such as in their diameter distribution, expansion velocities and ages.

Triggered star formation in the LMC4/Constellation III region of the Large Magellanic Cloud

The origin of a regular, 600 pc-long arc of young stars and clusters in the Constellation III region of the Large Magellanic Cloud is considered. The circular form of this arc suggests that the prestellar gas was uniformly swept up by a central source of pressure. In the center of the arc are six $\sim30$ My old A-type supergiant stars and a Cepheid variable of similar age, which may be related to the source of this pressure. We calculate the expansion of a bubble around a cluster of this age, and show that it could have triggered the formation of the arc at the right time and place. Surrounding the central old stars and extending well outside the young arc is the LMC4 superbubble and giant HI shell. We show how this superbubble and shell could have formed by the continued expansion of the 15 My old cavity, following star formation in the arc and the associated new pressures. The age sequence proposed here was not evident in the recent observations by Olsen et al. and Braun et al. because the first generation stars in the center of the LMC superbubble are relatively faint and scarce compared to the more substantial population of stars less than 15 My old that formed throughout the region in a second generation. These considerations lead to an examination of the origin of the LMC4/Constellation III region and other large rings in the LMC and other galaxies. Their size and circularity could be the result of low galactic shear and a thick disk, with several generations of star formation in their interiors now too faint to see.

HI Supergiant Shells in the Large Magellanic Cloud

AbstractThe recently completed HI mosaic survey of the Large Magellanic Cloud (Kim et al. 1997) reveals complex structure in the interstellar medium, including filaments, arcs, holes and shells. We have catalogued giant and supergiant HI shells and searched for correlations with Hα emission, using a new image taken with a camera lens mounted on the 16-inch telescope at Siding Spring Observatory.

Cold neutral gas associated with the Gum nebula

Based on HI maps covering the 3 600 square degrees area delimited by , we investigated the neutral hydrogen distribution in the Gum nebula and its environments. The main result is the appearence of a huge disk of neutral gas at a kinematical distance compatible with that of the nebula, and whose angular dimensions coincide with the optical image of the Gum nebula. The disk, roughly centered at , is located at and has a radius of . We propose that this HI disk is the neutral gas component of the optical nebula, hence becoming the first time in which a structure with the dimensions of the optical nebula is detected in other wavelengths. We analyze the fitting of observations with different theoretical models and conclude that the most plausible interpretation is that the Gum nebula is a supershell produced by the action of repeated SN explosions, where the explosions may have originated in the aged association Vela OB2. In this context, the thick HI shell shown by Dubner et al.(1992) near and the Vela SNR are the debris of two of the several outbursts which started in the region about years ago. The southern boundary is distorted due to the presence of a radius shell, which appears to be an interstellar bubble driven by the stellar winds of the O–type star HD 49798.

The Magnetic Field Near the Local Bubble

AbstractThere are almost no direct observational indicators of the magnetic field inside the local bubble. Just outside the bubble, the best tracers are stellar polarization and HI Zeeman splitting. These show that the local field does not follow the large-scale Galactic field. Here we discuss whether the deformation of the large-scale field by the local HI shells is consistent with the observations. We concentrate on the Loop 1 region, and find that the field lines are well-explained by this idea; in addition, the bright radio filaments of Radio Loop 1 delineate particular field lines that are “lit up” by an excess of relativistic electrons.

The Ursa Major Molecular Clouds

The Ursa Major molecular cloud complex lies in the direction of an expanding HI shell known as the North Celestial Pole loop. The NCP loop, which is centered at (l, b) ∼ (138°,30°) and easily seen in IRAS 100 μm emission, is some 60 pc across and 150 pc distant (Meyerdierks et. al 1991). At 100 μm, the Ursa Major clouds appear in projection as “finger” (l ∼ 140, b ∼ 38) which “hangs down” towards the center of the loop and the plane of the Galaxy. Distance estimates to the molecular clouds (Penprase 1993) are consistent with that of the NCP loop, indicating that the clouds are physically associated with the loop.

13CO (J= 1–0) Observations of the Lupus Molecular Clouds

Large-scale 13CO (J = 1–0) observations were carried out toward the Lupus region using the new 4-m millimeter-wave telescope at Nagoya University (HPBW = 2′.7). Four molecular clouds (Lupus 1, 2, and 3 and a newly found cloud) were mapped using an 8′ grid spacing. The masses of the four clouds were estimated to be 1200, 100, 300, and 1000 M⊙, respectively. Around an infrared point source, IRAS 15398–3359, associated with Lupus 1, we discovered a CO molecular outflow with a relatively short dynamical time scale of ∼ 2000 yr, which indicates ongoing star formation in the Lupus region. Including the outflow source, about 50 candidates for young stellar objects can be found in the literature, which are apparently associated with 13CO clouds. Comparing these objects with our 13CO data, we find that the star-formation efficiency of the four clouds varies over a wide range from 0.4 to ≥ 3.8%, which might represent their different stages of star formation, or might be due to a difference in the interaction with the nearby Scorpius HI shell.

HI Bubbles around NGC 6888

The interstellar bubbles are generally the result of the interaction of strong stellar winds with the surrounding material. HI voids and shells have been disclosed around several WR stars in our Galaxy (e.g. Niemela & Cappa de Nicolau 1991, Dubner et al. 1992). We report here an analysis of the interstellar medium in the vicinity of the WR star HD 192163 = WR 136 and the optical ring nebula NGC 6888 based on HI observations. Our aim is to look for the traces left in the cold interstellar gas by the action of the stellar winds of HD 192163.

The Violent Interstellar Medium in Messier 31

ABSTRACT:TAURUS observations in the line of Hα and VLA HI mapping of the HII complex No. 722 in M31, reveal what seems to be a spherical cavity 330 pc in diameter blown out by a stellar association of over 20 × 106 year old. Evidence of induced star formation which was initiated less than 5 × 106 years ago is present in the form of bright HII emission and numerous O, B and Wolf-Rayet stars which are found within the shell surrounding the cavity. The energy necessary to create the HI shell is estimated to be about 5 × 1051 erg.

The Magnetic Field Structure of the CMa R1 Star Formation Region

The CMa R1 association is embedded in a curved region of dark and reflection nebulosity which is part of a larger-scale ring structure of emission nebulosity thought to be a relatively old supernova remnant (Herbst and Assousa 1977, 1978), These investigators and Herbst, Racine, and Warner (1978) have argued that recent star formation in CMa R1 was induced by the supernova. Indeed, data for the emission ring, an associated expanding HI shell, and a nearby runaway star infer an age for the supernova remnant of 3x105 years; consistent with the age of most of the CMa R1 stars.

The Dynamical Evolution of Expanding HI Shells and Initial Mass Function of OB Associations

The evolution of expanding supershells produced by SN explosion in OB associations is considered. The expansion velocities of the shells and OB associations IMF spectral index are obtained. The results agree well with observational data.

Shock-heated gas in the I Per OB association

In response to a suggestion by Phillips & Gondhalekar (1981) that hotgas is receding from the I Per OB association with a velocity of ∼30 km s−1, we have examined the IUE high-dispersion spectra of stars in the association for blueshifted lines of interstellar CIV, SiIV and AlIII. As a result, the high-velocity ions are now known to be present in front of at least five stars. Our analysis of the radial velocities of the ions and of the column density ratio N(CIV)/N(SiIV) in the different sight-lines leads to a conclusion similar to that of Phillips & Gondhalekar, namely that the motion of the highly ionized gas is due to a supernova. We believe, however, that the expansion of the HI shell in front of I Per is unrelated and that it is probably due to a recombining HII region within which the supernova event has occurred.

Three New Pulsars Near Supernova Remnants

A search for pulsars in the directions of twenty Galactic supernova remnants (SNRs) was carried out at 327 MHz with the Ooty Radio Telescope (ORT). We have measured the P for one of the pulsars which is just outside the shell of SNR W44 and discuss here the possibility of its association with the HI shell around W44. The parameters of the remaining two pulsars have not yet been determined with sufficient accuracy to suggest association with the respective SNRs.

A new class of extraordinary HI shell

I consider an ordinary shell to be one which can be produced by ordinary means such as a supernova. Good examples are the HI shells associated with the prominent radio loops, discussed today by Prof. Weaver, and the large HI shell in Eridanus (Heiles, 1976). Figure 1 shows the shell which is associated with radio Loop I, made from a consolidation of data from Martha Cleary of Australia, Raul Colomb and Wolfgang Poppel of Argentina, and myself. It changes size with velocity in the manner expected for an expanding shell. The distance is only 100 pc or so. Similar shells should be easily resolvable in the galactic plane, even with a modest telescope, because differential galactic rotation eliminates the confusing effects of foreground and background gas.