Supergiant star gains thick dusty waist

New 3D imaging technique reveals velocity field of
baby-like feature in old star close to death

How is it possible that HD 62623, a hot supergiant star at the verge of death, is surrounded by a disc, generally only associated to baby stars? Using long-baseline stellar interferometry at ESO's VLT interferometer, a team led by Florentin Millour from Observatoire de la Côte d'Azur and Anthony Meilland from Max Planck Institute for Radio Astronomy could generate for the first time a three-dimensional high angular and high spectral resolution image of this star and its closest environment. They conclude that a solar-mass companion star is the key for this mystery. To attain their goal, the researchers adapted an imaging technique from radioastronomy that uses interferometric datasets.

Figure 1: 3D images of HD 62623, obtained with the VLTI (left), compared to the model of a rotating disk (right). In the boxes, the gas kinematics is shown (3rd dimension): blue-coloured gas approaches the observer, while red-coloured gas recedes from the observer. The size of the inner gas disk of approx. 2 milli-arcseconds corresponds to 1.3 astronomical units (distance Earth-Sun), while the outer dust ring seen in the images has a radius corresponding to 4 astronomical units, assuming 2100 light years as distance to HD 62623. Images: F. Millour et al.

HD 62623 is an exotic hot supergiant star. Contrary to its well-known twin, the bright star Deneb in the summer triangle, and almost all stars with the same spectral class, this star is surrounded by a dense and complex environment composed of plasma and dust. Hot supergiant stars are very bright stars, so bright, that they push their strong wind with their own photons. Such a wind would normally prevent matter from condensing as dust next to the star. To better understand dust formation processes in the harsh environment of such stars, it is highly desirable to disentangle the geometry of the gas and dust in the surroundings of the central source, but also to access the kinematics of this close environment.

"Thanks to our interferometric observations with Amber we could synthetize a 3D image of HD 62623 as seen through a virtual 130 m-diameter telescope", says Florentin Millour, leading author of the study. "The resolution is an order of magnitude higher compared with the world's largest optical telescopes of 8-10 m diameter." The Amber instrument is located at the Very Large Telescope Interferometer (VLTI) in Chile. The scientists significantly improved the image quality by adapting the so-called "self-calibration" method, which is well-known from radio interferometry. The obtained image combines spatial and velocity information, showing not only the shape of the close environment of HD 62623, but also its kinematics or motion. Up to now, the necessary kinematics information was missing in such images.

"Our new 3D image locates the dust-forming region around HD 62623 very precisely, and it provides evidence for the rotation of the gas around the central star" explains Anthony Meilland. "This rotation is found to be Keplerian, the same way the Solar system planets rotate around the Sun." A nearby companion star, with approximately the mass of our Sun, could be the reason of such a disc around HD 62623. This companion, though not directly detected due to its brightness thousands of times lower than the primary star, is betrayed by a central cavity between the gas disk and the central star. The presence of the companion could explain the exotic characteristics of HD 62623, exactly like the monster among the old stars wthin our Galaxy, Eta Carinae.

The new 3D imaging technique presented in this work is equivalent to integral-field spectroscopy, but gives access to a 15 times larger angular resolution or capacity to detect fine details in the images. "With these new capacities, the VLTI will be able to provide a better comprehension of many sky targets, too small to be resolved by the largest telescopes", concludes Florentin Millour. "We could aim at young stellar disks or jets, or even the central regions of active galaxies."

Figure 2: Four domes of the 1.8 m Auxiliary Telescopes (AT), utilized for the Very Large Telescope Interferometer (VLTI). ESO, Cerro Paranal, Chile. Image: F. Millour, OCA, Nice, France.

The Very Large Telescope Interferometer (VLTI) utilizes telescopes at ESO's Paranal site, either the 8.2 m UTs or the 1.8m ATs (Auxiliary Telescopes). Amber (Astronomical Multi-BEam Recombiner) is one of the science instruments of the VLTI. It is an interferometric beam combiner, sensitive in the near-infrared wavelength range (from 1 to 2.5 microns), built in collaboration with institutes from Grenoble (Laboratoire d'Astrophysique de Grenoble), Nice (Laboratoire d'Astrophysique Universitaire de Nice und Observatoire de la Côte d'Azur), Florence (Observatorio Astrofisico di Arcetri) and Bonn (Max Planck Institute for Radio Astronomy).

Original Paper:

Imaging the spinning gas and dust in the disc around the supergiant A[e] star HD62623 , Florentin Millour, Anthony Meilland, Olivier Chesneau, Philippe Stee, Samer Kanaan, Romain Petrov, Denis Mourard, Stefan Kraus, 2011, Astronomy & Astrophysics, Vol. 526, A107. DOI: 10.1051/0004-6361/201016193 (see also Imaging the spinning gas and dust in the disc around the supergiant A[e] star HD62623 , arXiv:1012.2957v1 [astro-ph.SR]).

Parallel and Earlier Press Releases:

First 3D View from the VLT Interferometer, ESO Announcement, January 26, 2011. Motions in the disc around a supergiant star revealed.

Première image 3D de l'environnement proche d'une étoile supergéante chaude, CNRS, Communiqué de presse national, Paris , 26 Janvier 2011.

Seeing a Stellar Explosion in 3D, ESO1032 - Science Release, August 04, 2010. VLT image of supernova 1987A.

All Stars are Born the Same Way, PRI (MPIfR) 07/2010 (1), July 15, 2010. VLTI and APEX observations reveal the mystery of massive star birth.

The Little Man and the Cosmic Cauldron, ESO0817 - Photo Release, May 27, 2008. VLT images two nebulae in Carina.

Further Information:

Max Planck Institute for Radio Astronomy (MPIfR)

Infrared Interferometry Group at MPIfR

Observatoire de la Cote d'Azur

European Southern Observatory (ESO)

Astronomical Multi-BEam combineR (AMBER)


Contact:

Dr. Anthony Meilland, Max-Planck-Institut für Radioastronomie, Bonn. Fon: +49-228-525-188 E-mail: meilland@mpifr-bonn.mpg.de

Dr. Florentin Millour, Observatoire de la Cote d'Azur, Nice, France. Fon: +33 4 92 00 30 68 E-mail: fmillour@oca.eu

Dr. Norbert Junkes, Public Outreach, Max-Planck-Institut für Radioastronomie, Bonn. Fon: +49-228-525-399 E-mail: njunkes@mpifr-bonn.mpg.de