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Post Info TOPIC: Loop 1 Superbubble
Blobrana


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Local Bubble
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Title: Is the Sun Embedded in a Typical Interstellar Cloud?
Authors: P. C. Frisch

The physical properties and kinematics of the partially ionised interstellar material near the Sun are typical of warm diffuse clouds in the solar vicinity. The interstellar magnetic field at the heliosphere and the kinematics of nearby clouds are naturally explained in terms of the S1 superbubble shell. The interstellar radiation field at the Sun appears to be harder than the field ionising ambient diffuse gas, which may be a consequence of the low opacity of the tiny cloud surrounding the heliosphere. The spatial context of the Local Bubble is consistent with our location in the Orion spur.

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Blobrana


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Posts: 131433
Date:
The Local Bubble
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We are riding on the edge of a huge void in space, hurtling along at 600,000 miles per hour. Yes, it's true. We're moving along with the expansion of the universe AND with the coalescence of galaxies along filaments, in clusters at places where the filaments intersect. Yet, as we go about our daily lives, we're largely (if not completely) unaware of the ride we're taking through space and time as part of the Milky Way Galaxy. Yet, our motion tracks with the continuing evolution of the universe.

local void
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-- Edited by Blobrana at 19:59, 2007-05-30

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Blobrana


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Posts: 131433
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Title: The Local Bubble and Interstellar Material Near the Sun
Authors: P. C. Frisch
(Version v2)

The properties of interstellar matter (ISM) at the Sun are regulated by our location with respect to the Local Bubble (LB) void in the ISM. The LB is bounded by associations of massive stars and fossil supernovae that have disrupted natal ISM and driven intermediate velocity ISM into the LB interior void. The Sun is located in such a driven ISM parcel. The Local Fluff has a bulk velocity of 19 km/s in the LSR, and an upwind direction towards the center of the gas and dust ring formed by the Loop I supernova remnant interaction with the LB. When the ram pressure of the LIC is included in the total LIC pressure, and if magnetic thermal and cosmic ray pressures are similar, the LIC appears to be in pressure equilibrium with the local hot bubble plasma.

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Blobrana


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Posts: 131433
Date:
The Local Cloud
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A team of astronomers led by Professor Martin Barstow of the University of Leicester have searched for the hot gas thought to be present in the interstellar space around the Sun but found it just isnt there.
Speaking at the Royal Astronomical Society National Astronomy Meeting in Preston on Tuesday 17 April, Professor Barstow will present a map of the local interstellar medium, the gas lying between the stars out to distances of about 300 light years from the Sun, made using the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite.
Professor Barstow and his team used FUSE to observe a group of white dwarf stars (compact remnants of stars like our Sun will be at the end of its life). The scientists intended to probe the structure of interstellar space in the vicinity of the Sun by searching for the imprint of oxygen in the ultraviolet light from the stars. However, all the oxygen detected was found to be in the atmospheres of the stars and no interstellar oxygen was found. This implies that, rather than being full of tenuous ionised gas, as expected, this region of interstellar space (the Local Cavity) is actually empty and was probably swept clear by an ancient supernova explosion a few million years ago.

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Blobrana


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Posts: 131433
Date:
The North Polar Spur
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Title: A New Model For The Loop-I (The North Polar Spur) Region
Authors: M. Wolleben

The North Polar Spur (NPS) is the brightest filament of Loop I, a large circular feature in the radio continuum sky. In this paper, a model consisting of two synchrotron emitting shells is presented that reproduces large-scale structures revealed by recent polarisation surveys. The polarised emission of the NPS is reproduced by one of these shells. The other shell, which passes close to the Sun, gives rise to polarised emission towards the Galactic poles. It is proposed that X-ray emission seen towards the NPS is produced by interaction of the two shells. Two OB-associations coincide with the centres of the shells. A formation scenario of the Loop I region is suggested.

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Blobrana


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Posts: 131433
Date:
The Local Bubble
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Title: An XMM-Newton Observation of the Local Bubble Using a Shadowing Filament in the Southern Galactic Hemisphere
Authors: David B. Henley (1), Robin L. Shelton (1), K. D. Kuntz (2,3) ((1) University of Georgia, (2) Johns Hopkins University, (3) GSFC)

We present an analysis of the X-ray spectrum of the Local Bubble, obtained by simultaneously analysing spectra from two XMM-Newton pointings on and off an absorbing filament in the Southern galactic hemisphere (b ~ -45 deg). We use the difference in the Galactic column density in these two directions to deduce the contributions of the unabsorbed foreground emission due to the Local Bubble, and the absorbed emission from the Galactic halo and the extragalactic background. We find the Local Bubble emission is consistent with emission from a plasma in collisional ionisation equilibrium with a temperature \log T_{LB} = 6.06^{+0.02}_{-0.04} and an emission measure of 0.018 cm^{-6} pc. Our measured temperature is in good agreement with values obtained from ROSAT All-Sky Survey data, but is lower than that measured by other recent XMM-Newton observations of the Local Bubble, which find \log T_{LB} \approx 6.2 (although for some of these observations it is possible that the foreground emission is contaminated by non-Local Bubble emission from Loop I). The higher temperature observed towards other directions is inconsistent with our data, when combined with a FUSE measurement of the Galactic halo O VI intensity. This therefore suggests that the Local Bubble is thermally anisotropic.
Our data are unable to rule out a non-equilibrium model in which the plasma is underionised. However, an overionised recombining plasma model, while observationally acceptable for certain densities and temperatures, generally gives an implausibly young age for the Local Bubble (\la 6  x 10^5 yr).

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Blobrana


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Posts: 131433
Date:
Loop 1 Superbubble
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A study of supernova remnants – material blown out into space during death throes of giant stars – has shown that a bubble of gas enveloping our Solar System is being shoved backwards by the debris of another, more recent, supernova.

Over the last few million years, several stars have exploded within the Milky Way and they have left behind bubbles of expanding, hot gas that radiate low-energy X-rays. The Solar System sits within one of these shells, known as the “Local Hot Bubble”. A study using data from the XMM-Newton Space Telescope has shown that the “Loop 1 Superbubble”, the remnants of some more recent supernova explosions, is expanding faster than the Local Hot Bubble and is compressing an area of cool dense gas, known as the Wall, that lies between the two shells. Although astronomers have known for some time that the Local Hot Bubble has an hourglass shape, pressure and density measurements from this new study provide evidence that Loop 1’s compression of the Wall is causing the hourglass’s “waist”.

"The X-ray radiation from the bubbles is very faint. In order to see them, we’ve had to remove all the light from stars, nebulae and cosmic rays the images, leaving only the weak X-ray signal. It’s the astronomical equivalent of looking at an aquarium, ignoring the fish and looking only at the water" - Michelle Supper, who is presenting the results at the RAS National Astronomy Meeting in Leicester on 5th April.

"We’ve taken long-exposure images of ten small areas of sky in the direction of the Loop 1 Superbubble, then removed all the bright objects and studied what’s left. Each structure emits a unique x-ray signal, like a fingerprint, that reflects its temperature and chemical composition. This means that, when we come to analyse the images, we can tell which bits of radiation originated from Loop 1, the Wall or the Local Hot Bubble" - Michelle Supper.

Together with Dr Richard Willingale, also from the University of Leicester, Supper developed mathematical models to represent each of the structures and then produced a geometrical model from which she could work out the distances to the structure boundaries and the pressure and density of the interstellar plasma within the structures.



Loop 1 is thought to be expanding because it is being inflated by winds originating from a group of stars known as the Scorpius-Centaurus Association. Supper’s measurements of physical properties of the Wall showed that its density increases fourfold, reaching a peak near the most indented region of the Local Hot Bubble. The pressures also peak around this point, indicating that the Wall is pushing into the bubble at in this region. The chemical analysis showed that the highest concentrations of gases are found at the centre of the Loop 1 Superbubble and levels decrease dramatically in the expanding shell of the bubble.

"Not many astronomers are looking at these structures at present but this study has shown there are many more mysteries to solve! We found that X-ray emissions in an area near the galactic plane are much higher in energy than expected but we don’t know yet whether we’ve discovered a new X-ray source or whether its an extension of the very high energy radiation coming from the centre of the galaxy. We hope that this study will also give us an insight into the distribution of the Galactic Halo, a mysterious X-ray signal that can be detected faintly above and below the disc of the Milky Way."- Michelle Supper.

Source RAS

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