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Register a new userThe close circumstellar environment of Betelgeuse - V. Rotation velocity and molecular envelope properties from ALMA
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We observed Betelgeuse using ALMAs extended configuration in band 7 (f~340 GHz, {lambda}~0.88 mm), resulting in a very high angular resolution of 18 mas. Using a solid body rotation model of the 28SiO(v=2,J=8-7) line emission, we show that the supergiant is rotating with a projected equatorial velocity of v_eq sin i = 5.47 +/- 0.25 km/s at the equivalent continuum angular radius R_star = 29.50 +/- 0.14 mas. This corresponds to an angular rotation velocity of {omega} sin i = (5.6 +/- 1.3) x 10^(-9) rad/s. The position angle of its north pole is PA = 48.0 +/- 3.5{deg}. The rotation period of Betelgeuse is estimated to P/sin i = 36 +/- 8 years. The combination of our velocity measurement with previous observations in the ultraviolet shows that the chromosphere is co-rotating with the star up to a radius of ~10 au (45 mas or 1.5x the ALMA continuum radius). The coincidence of the position angle of the polar axis of Betelgeuse with that of the major ALMA continuum hot spot, a molecular plume, and a partial dust shell (from previous observations) suggests that focused mass loss is currently taking place in the polar region of the star. We propose that this hot spot corresponds to the location of a particularly strong rogue convection cell, which emits a focused molecular plume that subsequently condenses into dust at a few stellar radii. Rogue convection cells therefore appear to be an important factor shaping the anisotropic mass loss of red supergiants.
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Context: Betelgeuse is one the largest stars in the sky in terms of angular diameter. Structures on the stellar photosphere have been detected in the visible and near-infrared as well as a compact molecular environment called the MOLsphere. Mid-infrared observations have revealed the nature of some of the molecules in the MOLsphere, some being the precursor of dust. Aims: Betelgeuse is an excellent candidate to understand the process of mass loss in red supergiants. Using diffraction-limited adaptive optics (AO) in the near-infrared, we probe the photosphere and close environment of Betelgeuse to study the wavelength dependence of its extension, and to search for asymmetries. Methods: We obtained AO images with the VLT/NACO instrument, taking advantage of the cube mode of the CONICA camera to record separately a large number of short-exposure frames. This allowed us to adopt a lucky imaging approach for the data reduction, and obtain diffraction-limited images over the spectral range 1.04-2.17 $mu$m in 10 narrow-band filters. Results: In all filters, the photosphere of Betelgeuse appears partly resolved. We identify an asymmetric envelope around the star, with in particular a relatively bright plume extending in the southwestern quadrant up to a radius of approximately six times the photosphere. The CN molecule provides an excellent match to the 1.09 mic bandhead in absorption in front of the stellar photosphere, but the emission spectrum of the plume is more difficult to interpret. Conclusions: Our AO images show that the envelope surrounding Betelgeuse has a complex and irregular structure. We propose that the southwestern plume is linked either to the presence of a convective hot spot on the photosphere, or to the rotation of the star.
We study the circumstellar evolution of the binary HD101584, consisting of a post-AGB star and a low-mass companion, which is most likely a post-common-envelope-evolution system. We used ALMA observations of the 12CO, 13CO, and C18O J=2-1 lines and the 1.3mm continuum to determine the morphology, kinematics, masses, and energetics of the circumstellar environment. The circumstellar medium has a bipolar hour-glass structure, seen almost pole-on, formed by an energetic jet, about 150 km/s. We conjecture that the circumstellar morphology is related to an event that took place about 500 year ago, possibly a capture event where the companion spiraled in towards the AGB star. However, the kinetic energy of the accelerated gas exceeds the released orbital energy, and, taking into account the expected energy transfer efficiency of the process, the observed phenomenon does not match current common-envelope scenarios. This suggests that another process must augment, or even dominate, the ejection process. A significant amount of material resides in an unresolved region, presumably in the equatorial plane of the binary system.
Atacama Large Millimetre/sub-millimetre Array (ALMA) observations of the CO(1-0) and CO(2-1) emissions from the circumstellar envelope of the Asymptotic Giant Branch (AGB) star EP Aqr have been made with four times better spatial resolution than previously available. They are analysed with emphasis on the de-projection in space of the effective emissivity and flux of matter using as input a prescribed configuration of the velocity field, assumed to be radial. The data are found to display an intrinsic axi-symmetry with respect to an axis making a small angle with the line of sight. A broad range of wind configurations, from prolate (bipolar) to oblate (equatorial) has been studied and found to be accompanied by significant equatorial emission. Qualitatively, the effective emissivity is enhanced near the equator to produce the central narrow component observed in the Doppler velocity spectra and its dependence on star latitude generally follows that of the wind velocity with the exception of an omni-present depression near the poles. In particular, large equatorial expansion velocities produce a flared disc or a ring of effective emissivity and mass loss. The effect on the determination of the orientation of the star axis of radial velocity gradients and possibly competing rotation and expansion in the equatorial disc is discussed. In general, the flux of matter is found to reach a broad maximum at distances of the order of 500 au from the star. Arguments are given that may be used to prefer one wind velocity distribution to another. As a result of the improved quality of the data, a deeper understanding of the constraints imposed on morphology and kinematics has been obtained.
The HIFI instrument on board of the Herschel Space Observatory (HSO) has been very successful in detecting molecular lines from circumstellar envelopes around evolved stars, like massive red supergiants, Asymptotic Giant Branch (AGB) and post-AGB stars, as well as planetary nebulae. Among others, ammonia has been found in circumstellar envelopes of C-rich AGB stars in amounts that significantly exceeded theoretical predictions for C-rich stars. Few scenarios have been proposed to resolve this problem: formation of ammonia behind the shock front, photochemical processes in the inner part of the envelope partly transparent to UV background radiation due to the clumpy structure of the gas, and formation of ammonia on dust grains. Careful analysis of observations may help to put constraints on one or another mechanism of ammonia formation. Here, we present results of the non-LTE radiative transfer modeling of ammonia transitions including a crucial process of radiative pumping via v$_2$ = 1 vibrational band (at $sim$10 $mu$m) for V Cyg. Only ground-based ammonia transition NH$_{3}$ J = 1$_{0}$ - 0$_{0}$ at 572.5 GHz has been observed by HIFI. Therefore, to determine abundance of ammonia we estimate a photodissociation radius of NH$_{3}$ using chemical model of the envelope consistent with dust grain properties concluded from the spectral energy distribution.
The nearby red supergiant (RSG) Betelgeuse has a complex circumstellar medium out to at least 0.5 parsecs from its surface, shaped by its mass-loss history within the past 0.1 Myr, its environment, and its motion through the interstellar medium (ISM). In principle its mass-loss history can be constrained by comparing hydrodynamic models with observations. Observations and numerical simulations indicate that Betelgeuse has a very young bow shock, hence the star may have only recently become a RSG. To test this possibility we calculated a stellar evolution model for a single star with properties consistent with Betelgeuse. We incorporated the resulting evolving stellar wind into 2D hydrodynamic simulations to model a runaway blue supergiant (BSG) undergoing the transition to a RSG near the end of its life. The collapsing BSG wind bubble induces a bow shock-shaped inner shell which at least superficially resembles Betelgeuses bow shock, and has a similar mass. Surrounding this is the larger-scale retreating bow shock generated by the now defunct BSG winds interaction with the ISM. We investigate whether this outer shell could explain the bar feature located (at least in projection) just in front of Betelgeuses bow shock.
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