Red Supergiant Antares Research Articles

ALMA and VLA reveal the lukewarm chromospheres of the nearby red supergiants Antares and Betelgeuse

We first present spatially resolved ALMA and VLA continuum observations of the early-M red supergiant Antares to search for the presence of a chromosphere at radio wavelengths. We resolve the free-free emission of the Antares atmosphere at 11 unique wavelengths between 0.7 mm (ALMA band 8) and 10 cm (VLA S band). The projected angular diameter is found to continually increase with increasing wavelength, from a low of 50.7 mas at 0.7 mm up to a diameter of 431 mas at 10 cm, which corresponds to 1.35 and 11.6 times the photospheric angular diameter, respectively. All four ALMA measurements show that the shape of the atmosphere is elongated, with a flattening of 15% at a similar position angle. The disk-averaged gas temperature of the atmosphere initially rises from a value of 2700 K at 1.35 R⋆ (i.e., 0.35 R⋆ above the photosphere) to a peak value of 3800 K at ∼2.5 R⋆, after which it then more gradually decreases to 1650 K at 11.6 R⋆. The rise in gas temperature between 1.35 R⋆ and ∼2.5 R⋆ is evidence for a chromospheric temperature rise above the photosphere of a red supergiant. We detect a clear change in the spectral index across the sampled wavelength range, with the flux density Sν ∝ ν1.42 between 0.7 mm and 1.4 cm, which we associate with chromosphere-dominated emission, while the flux density Sν ∝ ν0.8 between 4.3 cm and 10 cm, which we associate with wind-dominated emission. We show that the Antares MOLsphere is transparent at our observed wavelengths, and the lukewarm chromosphere that we detect is therefore real and not just an average of the cool MOLsphere and hot ultraviolet emitting gas. We then perform nonlocal thermal equilibrium modeling of the far-ultraviolet radiation field of another early-M red supergiant, Betelgeuse, and find that an additional hot (i.e., > 7000 K) chromospheric photoionization component with a much smaller filling factor must also exist throughout the chromospheres of these stars.

The convective surface of the red supergiant Antares

Context. Convection is a candidate to explain the trigger of red supergiant star mass loss. Owing to the small size of the convective cells on the photosphere, few of their characteristics are known. Aims. Using near infrared interferometry, we intend to resolve the photosphere of red supergiant stars and to bring new constraints on their modeling. Methods. We observed the nearby red supergiant Antares using the 4-telescope instrument VLTI/PIONIER. We collected data on the three available configurations of the 1.8m telescopes in the H band. Results. We obtained unprecedented angular resolution on the disk of a star (6% of the star angular diameter) that limits the mean size of convective cells, brings new constraints on numerical simulations. Using an analytical model with a distribution of bright spots we determine their effect on the visibility signal. Conclusion. We determine that the interferometric signal on Antares is compatible with convective cells with various sizes from 45% to 5% of the angular diameter. We also conclude that convective cells can strongly affect the angular diameter and limb-darkening measurements. In particular, the apparent angular diameter becomes dependent on the sampled position angles.

Vigorous atmospheric motion in the red supergiant star Antares.

Red supergiant stars represent a late stage of the evolution of stars more massive than about nine solar masses, in which they develop complex, multi-component atmospheres. Bright spots have been detected in the atmosphere of red supergiants using interferometric imaging. Above the photosphere of a red supergiant, the molecular outer atmosphere extends up to about two stellar radii. Furthermore, the hot chromosphere (5,000 to 8,000 kelvin) and cool gas (less than 3,500 kelvin) of a red supergiant coexist at about three stellar radii. The dynamics of such complex atmospheres has been probed by ultraviolet and optical spectroscopy. The most direct approach, however, is to measure the velocity of gas at each position over the image of stars as in observations of the Sun. Here we report the mapping of the velocity field over the surface and atmosphere of the nearby red supergiant Antares. The two-dimensional velocity field map obtained from our near-infrared spectro-interferometric imaging reveals vigorous upwelling and downdrafting motions of several huge gas clumps at velocities ranging from about -20 to +20 kilometres per second in the atmosphere, which extends out to about 1.7 stellar radii. Convection alone cannot explain the observed turbulent motions and atmospheric extension, suggesting that an unidentified process is operating in the extended atmosphere.

Imaging the outward motions of clumpy dust clouds around the red supergiant Antares with VLT/VISIR

We present a 0.5"-resolution diffraction-limited 17.7 micron image of the red supergiant Antares obtained with the VLT mid-infrared instrument VISIR. The VISIR image shows six clumpy dust clouds located at 0.8"--1.8" (43--96 stellar radii = 136--306 AU) away from the star. We also detected compact emission within a radius of 0.5" from the star. Comparison of our VISIR image taken in 2010 and the 20.8 micron image taken in 1998 with the Keck telescope reveals the outward motions of four dust clumps. The proper motions of these dust clumps amount to 0.2"--0.6" in 12 years. This translates into expansion velocities (projected onto the plane of the sky) of 13--40 km/s with an uncertainty of +/-7 km/s. The velocities of the dust clumps cannot be explained by a simple accelerating outflow, implying the possible random nature of the dust cloud ejection mechanism. The inner compact emission seen in the 2010 VISIR image is presumably newly formed dust, because it is not detected in the image taken in 1998. The mass of the dust clouds is estimated to be (3-6)x10^{-9} Msun. These values are lower by a factor of 3--7 than the amount of dust ejected in one year estimated from the (gas+dust) mass-loss rate of 2x10^{-6} Msun/yr, suggesting that the continuous mass loss is superimposed on the clumpy dust cloud ejection.

High spectral resolution imaging of the dynamical atmosphere of the red supergiant Antares in the CO first overtone lines with VLTI/AMBER

We present high spectral resolution aperture-synthesis imaging of the red supergiant Antares (alpha Sco) in individual CO first overtone lines with VLTI/AMBER. The reconstructed images reveal that the star appears differently in the blue wing, line center, and red wing and shows an asymmetrically extended component. The appearance of the star within the CO lines changes drastically within one year, implying a significant change in the velocity field in the atmosphere. Our modeling suggests an outer atmosphere (MOLsphere) extending to 1.2--1.4 stellar radii with CO column densities of (0.5--1)x10^{20} cm^{-2} and a temperature of ~2000 K. While the velocity field in 2009 is characterized by strong upwelling motions at 20--30 km/s, it changed to strong downdrafts in 2010. On the other hand, the AMBER data in the continuum show only a slight deviation from limb-darkened disks and only marginal time variations. We derive a limb-darkened disk diameter of 37.38+/-0.06 mas and a power-law-type limb-darkening parameter of (8.7+/-1.6)x10^{-2} (2009) and 37.31+/-0.09 mas and (1.5+/-0.2)x10^{-1} (2010). We also obtain Teff = 3660+/-120 K and log L/Lsun = 4.88+/-0.23, which suggests a mass of 15+/-5 Msun with an age of 11-15 Myr. This age is consistent with the recently estimated age for the Upper Scorpius OB association. The properties of the outer atmosphere of Antares are similar to those of another well-studied red supergiant, Betelgeuse. The density of the extended outer atmosphere of Antares and Betelgeuse is higher than predicted by the current 3-D convection simulations by at least six orders of magnitude, implying that convection alone cannot explain the formation of the extended outer atmosphere.