The binary Be star δ Sco at high spectral and spatial resolution

Disk geometry and kinematics before the 2011 periastron

by Meilland, A., Delaa, O., Stee, Ph. et al. A&A, 532A, 80M

δ Scorpii is located in the Scorpius constellation, close to the luminous supergiant Antares. δ Sco is intriguing Astronomers since many decades because its brightness is regularly increasing by a factor 2.5 each 10 years. It is a massive star, with a mass of 14 times the solar mass and 8.5 times the sun radius. Moreover, it is a very fast rotating star, spinning at about 350 km/s (2 km/s for our sun), i.e. at about 70% of its “critical” velocity, where the matter can freely leave the stellar surface due to the centrifugal force.

In fact, δ Sco is already surrounded by an extended disk, which was formed by the matter previously ejected by the central star and producing the IR excess and emission lines observed in its spectrum. But its sudden brightness variations must be found elsewhere: δ Sco is in fact a binary star with a companion orbital period of 10 years, which will help the central star to loose its mass when approaching the primary (periastron passage) due to tidal effects, producing the observed brightness variations.

By using simultaneously the two largest interferometers in the world: the VLTI with its AMBER instrument in Chili and the CHARA interferometer and its VEGA instrument on Mt Wilson in the U.S., both instruments designed and built mainly by the Observatoire de la Côte d’Azur in France, we were able to measure with a very high spatial resolution, and for the first time, the disk size around the primary star at different wavelengths. We were thus able to study “in depth” the primary circumstellar disk, which was found to be like “Russian dolls”, i.e. with increasing sizes starting from 4.5 stellar radii (R*) in the Helium line, 5.5 R* in the Br γ line in the IR domain and 9 R* in the Hα line at visible wavelengths. We were also able to evidence that the disk was rotating following the Keplerian rotation (as the planets rotate in our solar system) and that its size has increased by a factor 1.3 between 2005 and 2007.

But the most spectacular phenomenon is to come since we have refined the orbit of the system and found that the companion is also a massive star, with a spectral type between B2V and B4V and that it will be at the closest distance from the primary nearly for the Independence Day (July, 4th 2011). At this date, the companion will be at a distance of 11 R* whereas the primary circumstellar disk size is 10 R* This temporary promiscuity will produce strong tidal effects at the origin of a Cosmic Firework which will be the source of the observed fast brightness variations and will last for many weeks. Thus, many Astronomers all around the world, including the VLTI and CHARA are already pointing their telescopes toward this very bright star (mv=2.2), i.e. observable by amateur Astonomers. Ready to observe ?

Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed IR-excess and emission lines. The influence of binarity on these phenomena remains controversial.

δ Sco is a binary system whose primary suddently began to exhibit the Be phenomenon at the last periastron in 2000. We want to constrain the geometry and kinematics of its circumstellar environment.

We observed the star between 2007 and 2010 using spectrally-resolved interferometry with the VLTI/AMBER and CHARA/VEGA instruments.

We found orbital elements that are compatible with previous estimates. The next periastron should take place around July 5, 2011 (±4 days). We resolved the circumstellar disk in the Hα (FWHM = 4.8 ± 1.5 mas), Brγ (FWHM = 2.9 ± 0.5 mas), and the 2.06μm He I (FWHM = 2.4 ± 0.3 mas) lines as well as in the K band continuum (FWHM ≈ 2.4 mas). The disk kinematics are dominated by the rotation, with a disk expansion velocity on the order of 0.2 km s−1 . The rotation law within the disk is compatible with Keplerian rotation.

As the star probably rotates at about 70% of its critical velocity the ejection of matter doesn’t seems to be dominated by rotation. However, the disk geometry and kinematics are similar to that of the previously studied quasi-critically rotating Be stars, namely α Ara, ψ Per and 48 Per.

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The binary Be star δ Scorpii at high spectral and spatial resolution
II. The circumstellar disk evolution after the periastron

By A. Meilland, Ph. Stee, A. Spang, F. Malbet, F. Massi and D. Schertl, 2013 A&A, 550, L5

Context. Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed infrared (IR) excess and emission lines. The influence of binarity on these phenomena remains controversial.
Aims. We followed the evolution of the environment surrounding the binary Be star δ Scorpii one year before and one year after the 2011 periastron to check for any evidence of a strong interaction between its companion and the primary circumstellar disk. Methods. We used the VLTI/AMBER spectro-interferometric instrument operating in the K band in high (12 000) spectral resolution to obtain information on both the disk geometry and kinematics. Observations were carried out in two emission lines: Brγ (2.172 μm) and He i (2.056 μm).
Results. We detected some important changes in δ Scorpii’s circumstellar disk geometry between the first observation made in April 2010 and the new observation made in June 2012. During the last two years the disk has grown at a mean velocity of 0.2 km s−1 . This is compatible with the expansion velocity previously found during the 2001−2007 period. The disk was also found to be asym- metric at both epochs, but with a different morphology in 2010 and 2012.
Conclusions. Considering the available spectroscopic data showing that the main changes in the emission-line profiles occurred quickly during the periastron, it is probable that the differences between the 2010 and 2012 disk geometry seen in our interferometric data stem from a disk perturbation caused by the companion tidal effects. However, taking into account that no significant changes have occurred in the disk since the end of the 2011 observing season, it is difficult to understand how this induced inhomogeneity has been “frozen” in the disk for such a long period.

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The spinning-top Be star Achernar from VLTI-VINCI

by Domiciano et al. 2003, A&A, 407, L47


Observations with the VLT Interferometer (VLTI) at the ESO Paranal Observatory have allowed to obtain by far the most detailed view of the general shape of the fast-spinning hot star Achernar (Alpha Eridani), the brightest in the southern constellation Eridanus (The River).They find that Achernar is much flatter than expected - its equatorial radius is more than 50% larger than the polar one! In other words, this star is shaped very much like the well-known spinning-top toy, so popular among young children.

VINCI measurements of the spinning-top Be star Achernar.

The high degree of flattening measured for Achernar - a first in observational astrophysics - now poses an unprecedented challenge for theoretical astrophysics. The effect cannot be reproduced by common models of stellar interiors unless certain phenomena are incorporated, e.g. meridional circulation on the surface ("north-south streams") and non-uniform rotation at different depths inside the star. All the story is presented as a ESO Press Release HERE. The paper describing this very nice result is published in A&A, 407, L47 (2003)

Models developed by Domiciano et al. (2003)


Disk and wind evolution of Achernar: the breaking of the fellowship
by Kanaan et al. 2008 published in A&A, 486, 785

We use spectral energy distributions (SEDs), Hα line profiles, and visibilities available in the literature to study Achernar’s envelope geometry and to propose a possible scenario for its circumstellar disk formation and dissipation. We use the SIMECA code to investigate possible geometries of the circumstellar environment by comparing our synthetic results with spectroscopic and high angular resolution data from the VLTI/VINCI instrument. We compute three different kinds of models: an equatorial disk, a polar wind, and a disk+wind model.

We develop a 2D axial symmetric kinematic model to study the variation of the observed Hα line profiles, which provides clear evidence of Achernar’s equatorial disk formation and dissipation between 1991 and 2002. Our model can reproduce the polar-wind extension greater than 10 R* and a possible equatorial disk ( ≤5 R*) but we were unable to estimate the wind opening angle. We reproduce the Hα line-profile variations using an outburst scenario, but the disk final contraction requieres an additional physical effect to be taken into account. The polar stellar wind does not appear to be linked to the presence of a disk or a ring around the star. We test the possibility of a binary companion to Achernar, as found by Kervella & Domiciano de Souza (2007, A&A, 474, L49), but conclude that the VLTI/VINCI visibilities cannot be explained by a rotationally-distorted Be star and a companion alone. The presence of a polar jet provides an important component to reproduce the observations even if it is not excluded that the companion could partially influence the observations. New interferometric observations at short baselines (5 ≤ B ≤ 20 m) are mandatory to constrain Achernar’s circumstellar envelope, as well as spectroscopic long-term follow-up observational programs to link Achernar’s mass-loss episodes with its circumstellar disk formation.

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These results are publised in Kanaan et al. A&A, 486, 785

The environment of the fast rotating star Achernar. II. Thermal infrared interferometry with VLTI/MIDI
by Kervella et al. 2009, A&A, 493, L53

As is the case of several other Be stars, Achernar is surrounded by an envelope, recently detected by near-IR interferometry.

We have searched for the signature of circumstellar emission at distances of a few stellar radii from Achernar, in the thermal IR domain.

We have obtained interferometric observations on three VLTI baselines in the N band (8-13 μm), using the MIDI instrument.

From the measured visibilities, we derive the angular extension and flux contribution of the N band circumstellar emission in the polar direction of Achernar. The interferometrically resolved polar envelope contributes 13.4 ± 2.5% of the photospheric flux in the N band, with a full width at half maximum of 9.9 ± 2.3 mas (≈6 R_star). This flux contribution is in good agreement with the photometric IR excess of 10-20% measured by fitting the spectral energy distribution. Due to our limited azimuth coverage, we can only establish an upper limit of 5-10% for the equatorial envelope. We compare the observed properties of the envelope with an existing model of this star computed with the SIMECA code.

The observed extended emission in the thermal IR along the polar direction of Achernar is well reproduced by the existing SIMECA model. Already detected at 2.2 μm, this polar envelope is most probably an observational signature of the fast wind ejected by the hot polar caps of the star.

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