* Astronomy

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RE: Betelgeuse

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Betelgeuse braces for a collision

Multiple arcs are revealed around Betelgeuse, the nearest red supergiant star to Earth, in this new image from ESA's Herschel space observatory. The star and its arc-shaped shields could collide with an intriguing dusty 'wall' in 5000 years.
Roughly 1000 times the diameter of our Sun and shining 100 000 times more brightly, Betelgeuse's impressive statistics come with a cost. For this star is likely on its way to a spectacular supernova explosion, having already swelled into a red supergiant and shed a significant fraction of its outer layers.
The new far-infrared view from Herschel shows how the star's winds are crashing against the surrounding interstellar medium, creating a bow shock as the star moves through space at speeds of around 30 km/s.
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Title: CARMA CO(J = 2 - 1) Observations of the Circumstellar Envelope of Betelgeuse
Authors: Eamon O'Gorman, Graham M. Harper, Joanna M. Brown, Alexander Brown, Seth Redfield, Matthew J. Richter, Miguel A. Requena-Torres

We report radio interferometric observations of the 12C16O 1.3 mm J = 2-1 emission line in the circumstellar envelope of the M supergiant Alpha Ori and have detected and separated both the S1 and S2 flow components for the first time. Observations were made with the Combined Array for Research in Millimetre-wave Astronomy (CARMA) interferometer in the C, D, and E antenna configurations. We obtain good u-v coverage (5-280 klambda) by combining data from all three configurations allowing us to trace spatial scales as small as 0.9" over a 32" field of view. The high spectral and spatial resolution C configuration line profile shows that the inner S1 flow has slightly asymmetric outflow velocities ranging from -9.0 km s-1 to +10.6 km s-1 with respect to the stellar rest frame. We find little evidence for the outer S2 flow in this configuration because the majority of this emission has been spatially-filtered (resolved out) by the array. We also report a SOFIA-GREAT CO(J= 12-11) emission line profile which we associate with this inner higher excitation S1 flow. The outer S2 flow appears in the D and E configuration maps and its outflow velocity is found to be in good agreement with high resolution optical spectroscopy of K I obtained at the McDonald Observatory. We image both S1 and S2 in the multi-configuration maps and see a gradual change in the angular size of the emission in the high absolute velocity maps. We assign an outer radius of 4" to S1 and propose that S2 extends beyond CARMA's field of view (32" at 1.3 mm) out to a radius of 17" which is larger than recent single-dish observations have indicated. When azimuthally averaged, the intensity fall-off for both flows is found to be proportional to R^{-1}, where R is the projected radius, indicating optically thin winds with rho \propto R^{-2}.

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Title: Discovery of a detached HI gas shell surrounding alpha Orionis
Authors: T. Le Bertre, L. D. Matthews, E. Gérard, Y. Libert

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We report the detection of the HI line at 21 cm in the direction of alpha Ori with the Nancay Radiotelescope and with the Very Large Array. The observations confirm the previous detection of HI emission centred on alpha Ori, but additionally reveal for the first time a quasi-stationary detached shell of neutral atomic hydrogen ~4 arcmin. in diameter (0.24 pc at a distance of 200 pc). The detached shell appears elongated in a direction opposite to the star's space motion. A simple model shows that this detached atomic gas shell can result from the collision of the stellar wind from alpha Ori with the local interstellar medium (ISM). It implies that alpha Ori has been losing matter at a rate of ~ 1.2x10^-6 solar masses per year for the past 8x10^4 years. In addition, we report the detection of atomic hydrogen associated with the far-infrared arc located 6 arcmin. north-east of alpha Ori, that has been suggested to trace the bow shock resulting from the motion of the star through the ISM. We report also the detection by the Galaxy Evolution Explorer (GALEX) of a far-UV counterpart to this arc.

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alpha Orionis

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RE: Betelgeuse

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Title: 3D Simulations of Betelgeuse's Bow Shock
Authors: S. Mohamed, J. Mackey, N. Langer

Betelgeuse, the bright, cool red supergiant in Orion, is moving supersonically relative to the local interstellar medium. The star emits a powerful stellar wind which collides with this medium, forming a cometary structure, a bow shock, pointing in the direction of motion. We present the first 3D hydrodynamic simulations of the formation and evolution of Betelgeuse's bow shock. The models include realistic low temperature cooling and cover a range of plausible interstellar medium densities and stellar velocities between 0.3 - 1.9 cm-3 and 28 - 73 km/s. We show that the flow dynamics and morphology of the bow shock differ substantially due to the preferential growth of Rayleigh-Taylor or Kelvin-Helmholtz instabilities in the models. The former dominate the models with slow stellar velocities resulting in a clumpy bow shock sub-structure, whereas the latter produce a smoother, more layered sub-structure in the fast models. If the mass in the bow shock shell is low, as seems to be implied by the AKARI luminosities (~0.003 Msun), then Betelgeuse's bow shock is very young and is unlikely to have reached a steady state. The circular nature of the bow shock shell is consistent with this conclusion. Thus, our results suggest that Betelgeuse entered the red supergiant phase only recently.

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Title: The close circumstellar environment of Betelgeuse - II. Diffraction-limited spectro-imaging from 7.76 to 19.50 microns with VLT/VISIR
Authors: Pierre Kervella (LESIA), Guy Perrin (LESIA), Andrea Chiavassa, Stephen T. Ridgway (NOAO), Jan Cami, Xavier Haubois, Tijl Verhoelst

Context: Mass-loss occurring in red supergiants (RSGs) is a major contributor to the enrichment of the interstellar medium in dust and molecules. The physical mechanism of this mass loss is however relatively poorly known. Betelgeuse is the nearest RSG, and as such a prime object for high angular resolution observations of its surface (by interferometry) and close circumstellar environment.
Aims: The goal of our program is to understand how the material expelled from Betelgeuse is transported from its surface to the interstellar medium, and how it evolves chemically in this process.
Methods: We obtained diffraction-limited images of Betelgeuse and a PSF calibrator (Aldebaran) in six filters in the N band (7.76 to 12.81 mic) and two filters in the Q band (17.65 and 19.50 mic), using the VLT/VISIR instrument. Results: Our images show a bright, extended and complex circumstellar envelope at all wavelengths. It is particularly prominent longwards of 9-10 mic, pointing at the presence of O-rich dust, such as silicates or alumina. A partial circular shell is observed between 0.5 and 1.0" from the star, and could correspond to the inner radius of the dust envelope. Several knots and filamentary structures are identified in the nebula, one of the knots, located at a distance of 0.9" west of the star, being particularly bright and compact.
Conclusions: The circumstellar envelope around Betelgeuse extends at least up to several tens of stellar radii. Its relatively high degree of clumpiness indicates an inhomogeneous spatial distribution of the material lost by the star. Its extension corresponds to an important intermediate scale, where most of the dust is probably formed, between the hot and compact gaseous envelope observed previously in the near infrared and the interstellar medium.

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New image reveals vast nebula around famous supergiant star

Using the VISIR instrument on ESO's Very Large Telescope (VLT), astronomers have imaged a complex and bright nebula around the supergiant star Betelgeuse in greater detail than ever before. This structure, which resembles flames emanating from the star, is formed as the behemoth sheds its material into space.
Betelgeuse, a red supergiant in the constellation of Orion, is one of the brightest stars in the night sky. It is also one of the biggest, being almost the size of the orbit of Jupiter - about four and half times the diameter of the Earth's orbit. The VLT image shows the surrounding nebula, which is much bigger than the supergiant itself, stretching 60 billion kilometres away from the star's surface - about 400 times the distance of the Earth from the Sun.
Red supergiants like Betelgeuse represent one of the last stages in the life of a massive star. In this short-lived phase, the star increases in size, and expels material into space at a tremendous rate - it sheds immense quantities of material (about the mass of the Sun) in just 10 000 years.