The Institut d'Astronomie et d'Astrophysique in the media



[16/04/2024] Sleeping giant surprises Gaia scientists

[10/01/2024] Thibault Merle receives the Paul & Marie Stroobant Prize 2023

[20/05/2023] Daily Science (10/10/2023) about Gaia Focused Data Releases

[13/05/2022] Press release : Four-Star Merry-Go-Round

A spectroscopic quadruple as a possible progenitor of sub-Chandrasekhar type Ia supernovae, Nature Astronomy, May 12 2022, Thibault Merle et al.

Stars like to be in company, and unlike our Sun, most of the stars in the Galaxy have one or more stellar companions. Binary stars are now recognized to play a major role in a large range of astrophysical events, and mergers of binaries are in particular the cause of the recent gravitational wave emission detection. In addition, binary stars allow us to derive fundamental stellar parameters like masses, radii and luminosities with a better accuracy compared to single stars. They represent the gems on which various astrophysics topics rely. While binaries have received much attention so far, higher-order stellar systems have remained aside until recently, despite the fact that they show a wide variety of interactions, especially in tight systems. Stellar quadruples only represent a marginal fraction (a few percent) of all multiple systems. The complex evolution of such high-order multiples involves mass transfer and collisions, leading to mergers that are also possible progenitors of thermonuclear supernovae. These supernovae represent standard candles for fixing the Universe distance scale, despite the fact that the evolutionary channel(s) leading to the progenitors of such supernova explosions are still highly debated. A spectroscopic quadruple (HD 74438) was discovered in 2017 in the Gaia-ESO Survey. The Gaia-ESO Survey is a public spectroscopic survey providing a detailed overview of the stellar content of the Milky Way by characterizing more than 100 000 stars. Subsequent follow-up spectroscopic observations of HD 74438 were obtained with high-resolution spectrographs at University of Canterbury Mount John Observatory in New Zealand and at the Southern African Large Telescope in South Africa. These observations allowed us to determine that this stellar quadruple is made of 4 gravitationally-bound stars: a short-period binary orbiting another short-period binary on a longer orbital period (2+2 configuration). Its membership in the open cluster IC 2391 makes it the youngest (43 million of years) spectroscopic quadruple discovered so far and among the quadruple systems with the shortest outer orbital period (6 years). Thanks to the spectroscopic analysis it was possible to show that this quadruple system is undergoing dynamical effects on long time scales compared to the orbital periods. Indeed, one of the inner binaries should have evolved into a circular orbit whereas it has an eccentric one. This is explained by the gravitational effect of the distant binary companion which can pump up the eccentricity. State-of-the art simulations of this system's future evolution show that such gravitational dynamics can lead to one or multiple collisions and merger events producing white dwarfs with masses just below the Chandrasekhar limit. A star like our Sun will end its life as a white dwarf, and the mass of white dwarfs cannot go above the so-called Chandrasekhar limit. If it does, as a result of mass transfer or merger events, it does collapse and produces a thermonuclear supernova. Interestingly, 70 to 85% of all thermonuclear supernovae are now suspected to result from the explosion of white dwarfs with sub-Chandrasekhar masses. The evolution of stellar quadruples such as HD 74438 thus represents a new promising channel to form them. Trajectories within the quadruple stellar system HD 74438: the two close pairs, having orbital periods of 21 and 4 days, orbit around each other in 6 years. At some point, the two pairs could merge into white dwarfs giving rise to a thermonuclear supernova, as illustrated in the bottom right.

Reference T. Merle, A. S. Hamers, S. Van Eck, A. Jorissen, M. Van der Swaelmen, K. Pollard, R. Smiljanic, D. Pourbaix, Tomaž Zwitter, G. Traven, G. Gilmore, S. Randich, A. Gonneau, A. Hourihane , G. Sacco and C. C. Worley
A spectroscopic quadruple as a possible progenitor of sub-Chandrasekhar type Ia supernovae Nature Astronomy, May 12 2022 Four-Star Merry-Go-Round



[20/05/2022] When the interests of private companies jeopardize astronomical science ... and humanity

Press Release of the Belgian National Astronomy Committee

Daily Science (20/5/2022)



[22/11/2021] Succès scientifiques et images époustouflantes : l’héritage du télescope spatial Hubble The Conversation, Alain Jorissen & Thibault Merle

[08/06/2021] Ana Escorza receives the Spanish Society of Astronomy Thesis Award 2021

The Spanish Society of Astronomy (SEA) has just announced the winners of the Thesis Awards 2021, which recognize the doctoral theses with the greatest present and future impact, both in astronomy research and in the area of instrumentation, computing and technological development in astronomy. The SEA Thesis 2021 Prize in Astronomy and Astrophysics has been awarded to Ana Escorza, for the thesis “Barium stars as tracers of binary evolution in the Gaia era”, carried out at the KU Leuven and the Université Libre de Brussels (ULB) and supervised by Prof. Hans Van Winckel and Dr. Lionel Siess. The awards ceremony will be held at the opening ceremony of the next Scientific Meeting of the Spanish Society of Astronomy, in La Laguna in September 2022. Ana Escorza graduated in Physics from the University of Zaragoza, studying for a year at the University of Southampton. She completed the Master in Astronomy and Astrophysics at the KU Leuven and from 2016 to 2020 she has been a PhD student of the Research Foundations Flanders (FWO). Her doctoral thesis studies a family of stars with chemical peculiarities called "barium stars". Using broadband photometry, high-resolution spectroscopy, results from the Gaia mission and state-of -the-art evolution models, she has been able to determine the properties of these stars with unprecedented accuracy. She also has built the largest catalog of barium dwarf systems published to date. She currently has a postdoctoral position as a Fellow of the European Southern Observatory (ESO) in Santiago de Chile, where she is an instrument fellow for the UVES and ESPRESSO spectrographs, mounted on the Very Large Telescope at ESO's Cerro Paranal Observatory.

A report in Nature Astronomy by Chiao May of our A&A Letter (Kravchenko et al.) on the possible cause of the dimming of Betelgeuse: preprint here

ULB astronomers contribute to ESA's Gaia astrometric mission, which released data for 1.4 billion stars today.

Astrophysicist working at ULB, formerly research associate of the Fund for Scientific Research (FRS-FNRS), Sophie Van Eck contributed to the imaging of pi Gruis, a giant star located 530 pc away in the southern constellation Gruis, using European Southern Observatory Very Large Telescope Interferometer.

  • [22/12/2017] Giant Bubbles on Red Giant Star’s Surface


Astronomers using ESO’s Very Large Telescope have for the first time directly observed granulation patterns on the surface of a star outside the Solar System — the ageing red giant π1 Gruis. This remarkable new image from the PIONIER instrument reveals the convective cells that make up the surface of this huge star, which has 350 times the diameter of the Sun. Each cell covers more than a quarter of the star’s diameter and measures about 120 million kilometres across. These new results are being published this week in the journal Nature.

Located 530 light-years from Earth in the constellation of Grus (The Crane), π1 Gruis is a cool red giant. It has about the same mass as our Sun, but is 350 times larger and several thousand times as bright. Our Sun will swell to become a similar red giant star in about five billion years.

An international team of astronomers led by Claudia Paladini (ESO) used the PIONIER instrument on ESO’s Very Large Telescope to observe π1 Gruis in greater detail than ever before. They found that the surface of this red giant has just a few convective cells, or granules, that are each about 120 million kilometres across — about a quarter of the star’s diameter. Just one of these granules would extend from the Sun to beyond Venus. The surfaces — known as photospheres — of many giant stars are obscured by dust, which hinders observations. However, in the case of π1 Gruis, although dust is present far from the star, it does not have a significant effect on the new infrared observations.

When π1 Gruis ran out of hydrogen to burn long ago, this ancient star ceased the first stage of its nuclear fusion programme. It shrank as it ran out of energy, causing it to heat up to over 100 million degrees. These extreme temperatures fueled the star’s next phase as it began to fuse helium into heavier atoms such as carbon and oxygen. This intensely hot core then expelled the star’s outer layers, causing it to balloon to hundreds of times larger than its original size. The star we see today is a variable red giant. Until now, the surface of one of these stars has never before been imaged in detail.

By comparison, the Sun’s photosphere contains about two million convective cells, with typical diameters of just 1500 kilometres. The vast size differences in the convective cells of these two stars can be explained in part by their varying surface gravities. π1 Gruis is just 1.5 times the mass of the Sun but much larger, resulting in a much lower surface gravity and just a few, extremely large, granules.

While stars more massive than eight solar masses end their lives in dramatic supernovae explosions, less massive stars like this one gradually expel their outer layers, resulting in beautiful planetary nebulae. Previous studies of π1 Gruis found a shell of material 0.9 light-years away from the central star, thought to have been ejected around 20 000 years ago. This relatively short period in a star's life lasts just a few tens of thousands of years – compared to the overall lifetime of several billion – and these observations reveal a new method for probing this fleeting red giant phase.

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  • [20/10/2017] Cosmic Crashes Forging Gold: ULB and Max Planck Institute astrophysicists did predict it back in 2011!


The observation, made public on October 16 2017, that neutron-star mergers emit gravitational waves and visible light with signatures of heavy elements validates a prediction made by scientists of the Max Planck Institute for Astrophysics (MPA) and of the Free University of Brussels (ULB) back in 2011. They had identified that ejected matter from neutron stars merging in a violent collision provides ideal conditions to produce the heaviest chemical elements such as lead or gold.



  • [20/09/2017] Ageing Star Blows Off Smoky Bubble


Astronomers have used the ALMA radio telescope array to capture a strikingly beautiful view of a delicate bubble of expelled material around the exotic red star U Antliae. These observations will help astronomers to better understand how stars evolve during the later stages of their life-cycles.

In the faint southern constellation of Antlia (The Air Pump) the careful observer with binoculars will spot a very red star, which varies slightly in brightness from week to week. This very unusual star is called U Antliae and new observations with the Atacama Large Millimeter/submillimeter Array (ALMA) are revealing a remarkably thin spherical shell around it.

This research was presented in a paper entitled “Rings and filaments. The remarkable detached CO shell of U Antliae”, by F. Kerschbaum et al., to appear in the journal Astronomy & Astrophysics. The team includes C. Paladini (Université Libre de Bruxelles, Belgium).

ESO press release

[12/08/2020] La conquête spatiale, un enjeu géopolitique? Mag international, LN24 (Alain Jorissen)

[27/05/2020] About SpaceX and Starlink Le grand débat, LN24 (Alain Jorissen)

[21/05/2019] Alain Jorissen, Pint of Science

Festival Pint of Science in Brussels, about Gaia, the billion-star surveyor, live on Facebook

[12/04/2018] Sophie van Eck, la chasseuse belge d’étoiles

Astrophysicienne à l’ULB où elle enseigne actuellement à la faculté des sciences et anciennement chercheuse qualifiée FNRS pendant 20 ans, Sophie Van Eck a participé à une découverte capitale concernant une étoile géante située à 530 années lumière de notre planète, dans la Constellation de la Grue. Elle a eu le privilège d’avoir accès aux télescopes de l’Observatoire Européen Austral au Chili.

[23/2/2017] Comment on the 7-planets discovery

Dimitri Pourbaix commenting on the discovery of 7 exoplanets by a University of Liège team on Tele Bruxelles

[12/1/2016] Parallax and masses of alpha Centauri revisited

Despite thorough works previously devoted to our close neighbour alpha Centauri, its distance is still far from being carved in stone. Any derivation of the individual masses is therefore uncertain, if not questionable. And yet, that does not prevent this system from being used for calibration purpose in several studies. By combining measurements of the binary separation with 10 years of radial-velocity data obtained with the HARPS spectrograph at ESO, Pourbaix and Boffin were able to derive a full 3D orbit of alpha Cen.
They locate alpha Cen right where the ESA Hipparcos satellite had put it. The masses of the setellar components could be derived accurately, and appear to be a bit more massive than previously thought (1.13 and 0.97 Msun for A and B respectively). These values are now in excellent agreement with the latest asteroseismologic results.

[8/1/2015] The temperature and chronology of heavy-element synthesis in low-mass stars



Our study, published in the January 8, 2015, issue of Nature, presents high-resolution spectra obtained from a sample of 23 evolved stars — 17 'S-type' and 6 'M-type'. S-type stars are red giants in which the slow-neutron-capture or 's' process is synthesizing heavy elements, and M-type stars are similar giant stars with no s-process enhancement. Using the new spectrographic data (obtained with the HERMES/Mercator spectrograph) and dedicated model atmospheres, our study derives accurate technetium, zirconium and niobium abundances. Comparison with predictions from state-of-the-art stellar evolution and nucleosynthesis calculations points to a synthesis temperature of less than about 250 million kelvin in the S-type stars and supports carbon-13 as the s-process neutron source and suggest that and that heavy element synthesis began one million to three million years ago.



The Orion constellation, well visible from the Northern hemisphere. The surface temperature of stars can easily be determined from their color: for exemple, Betelgeuse (3600 degrees) looks red while Bellatrix (21 000 degrees) blue. On the contrary, measuring the temperature inside stars is a real challenge. The Orion constellation precisely hosts two red giant stars of the S spectral type studied in the present paper. The first one, V1261 Orionis, led to the temperature determination of the synthesis of elements heavier than iron (roughly 100 millions degrees). The other one, o1 Orionis, has been producing such heavy elements for 1.3 million years. (Infography designed thanks to Aladin, Centre de Données Astronomiques de Strasbourg)


[24/11/2014] Seeing into the Heart of Mira A and its Partner



Studying red giant stars tells astronomers about the future of the Sun — and about how previous generations of stars spread the elements needed for life across the Universe. One of the most famous red giants in the sky is called Mira A, part of the binary system Mira which lies about 400 light-years from Earth. In this image ALMA reveals Mira’s secret life.
Mira A is an old star, already starting to throw out the products of its life’s work into space for recycling. Mira A’s companion, known as Mira B, orbits it at twice the distance from the Sun to Neptune.
Mira A is known to have a slow wind which gently moulds the surrounding material. ALMA has now confirmed that Mira’s companion is a very different kind of star, with a very different wind. Mira B is a hot, dense white dwarf with a fierce and fast stellar wind.
New observations show how the winds from the two stars have created a fascinating, beautiful and complex nebula. The remarkable heart-shaped bubble at the centre is created by Mira B’s energetic wind inside Mira A’s more relaxed outflow. The heart, which formed some time in the last 400 years or so, and the rest of the gas surrounding the pair show that they have long been building this strange and beautiful environment together.
By looking at stars like Mira A and Mira B scientists hope to discover how our galaxy’s double stars differ from single stars in how they give back what they have created to the Milky Way’s stellar ecosystem. Despite their distance from one another, Mira A and its companion have had a strong effect on one another and demonstrate how double stars can influence their environments and leave clues for scientists to decipher.
Other old and dying stars also have bizarre surroundings, as astronomers have seen using both ALMA and other telescopes. But it’s not always clear whether the stars are single, like the Sun, or double, like Mira. Mira A, its mysterious partner and their heart-shaped bubble are all part of this story.
The new observations of Mira A and its partner are presented in this paper published in the European Journal Astronomy & Astrophysics, including authors A. Jorissen and C. Paladini from IAA-ULB.
They complement the previous study made by the same team with the Herschel satellite.

Credit: ESO/S. Ramstedt (Uppsala University, Sweden) & W. Vlemmings (Chalmers University of Technology, Sweden)

[1/6/2011] A cosmic firewheel blown by the wind



The star o Ceti was named Mira, the wonderful, by Johannes Hevelius in 1662 who was stunned by the appearance and disappearance of the star on the night sky. Meanwhile the star became a well-studied object due to its distance of only 300 light-years and today a whole class of late-type stars with long periodic light variations are named after Mira. The star was observed in many wavelengths and revealed different interesting features. In the present picture two previous images that were both Astronomy Picture Of The Day (2007-08-17 and 2006-07-22) and a new observation are combined. On the left, a GALEX observation in the ultra-violet by Martin et al. (2007) shows an extended 13 light-years long tail structure of Mira flying through the interstellar medium and on the right an X-ray observation with Chandra reveals Mira's binary nature (Karovska 2006). Late-type stars undergo massive mass loss in form of dusty winds which are visible in the infrared. New observations at far-infrared wavelengths with the PACS instrument on board the Herschel Space Observatory reveal arc-like structures in the expelled matter of Mira. They are interpreted as resulting from the perturbation of Mira's wind by the binary companion, combined with the 110 km/s motion of Mira through the interstellar medium.
A. Mayer, A. Jorissen, F. Kerschbaum, S. Mohamed, S. Van Eck, et al. MESS consortium,
Astronomy & Astrophysics Letter, 2011, 531, L4
Panel (a): Deconvolved PACS/Herschel image at 70 µm. The arrow indicates the space motion and the position in 500 yrs; (b) is the same for the 160 µm band and (c) is the 70 µm deconvolved PACS image with contours and arcs labelled as A, B, C, D and E. The lowest contour is the 3σ threshold (and is depicted in white), the arrow labelled with Vspace shows the direction of the space motion, the bar labelled PA orbit gives the orientation of the major axis of the apparent orbit, orthogonal to the node line. Panel (d) results from a paraboloidal cut (representing the bow shock at the wind - ISM interface) in Mira hydrodynamical simulations of Mohamed & Podsiadlowski (2007, 2011).

[24/1/2013] Plumbing Neutron Stars to New Depths



Nuclear fusion reactions in stars produce many elements found on Earth, but only those with atomic numbers up to that of iron. Heavier elements may have been created during previous supernova explosions of massive stars, or they may have been produced from the decompression of matter ripped from the crust of neutron stars, the super-dense remnants that those explosions left behind. If neutron stars indeed explain the origin of heavy elements, the composition of their crust (which varies rapidly with depth) should be reflected in the observed nuclear abundances. In Physical Review Letters, an international team of researchers, including Nicolas Chamel and Stephane Goriely from the Institute of Astronomy and Astrophysics at ULB, report new precision measurements of the mass of short-lived zinc nuclides. The new mass measurement supports a revised model of neutron-star crusts in which the zinc-82 is in fact no longer present, replaced by nickel-78, and allows us to "drill" deeper into the crust than before.




[2/1/2013] "Glitch" in Pulsar's Models



Sudden pulsar spin-ups have long been thought to be the manifestation of a neutron ocean permeating the crustal layers of these dead stars. In a paper published in Physical Review letters, Nicolas Chamel from the Institute of Astronomy and Astrophysics at ULB shows that this interpretation fails to explain the large observed "glitches" in the emblematic Vela pulsar. This conclusion has been independently confirmed by researchers at the University of Southampton. At the endpoint of stellar evolution, pulsars are spinning extremely rapidly with periods ranging from milliseconds to seconds and delays of a few milliseconds per year at most, thus providing the most accurate clocks in the universe. Nevertheless, some pulsars exhibit sudden decreases of their spin period. It has been widely believed that the free neutrons in the crust could rotate faster than the rest of the star until a critical lag is reached leading to the observed glitches. However, calculations indicate that this scenario is unrealistic because neutrons are very strongly coupled to the crust due to non-dissipative entrainment effects.




[9/11/2011] Cosmic Crashes Forging Gold



The cosmic site where the heaviest chemical elements such as lead or gold are formed is likely to be identified: Ejected matter from neutron stars merging in a violent collision provides ideal conditions. In detailed numerical simulations, scientists of the Max Planck Institute for Astrophysics (MPA) and of the Free University of Brussels (ULB) have verified that the relevant reactions of atomic nuclei do take place in this environment, producing the heaviest elements in the correct abundances.

[23/6/2011] The flames of Betelgeuse: New image reveals vast nebula around famous supergiant star

Using the VISIR instrument on ESO’s Very Large Telescope (VLT), astronomers (among which A. Chiavassa, from IAA-ULB) 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.
ESO Press release
Astronomy Picture of the Day

[6/6/2011] Nicolas Chamel honoured by the Adolphe Wetrems prize 2011 for Mathematical and Physical Sciences from the Royal Academy of Belgium for his study of extreme conditions in the interiors of supernovae and neutron stars

[22/3/2010] A. Jorissen's & T. Dermine's interview about Printemps des Sciences 2010

[1/10/2009] Conspiration between the distributions of dark and visible matter in galaxies

[6/3/2009] The new dome is brought to its final location, atop building D

[31/10/2008] B. Famaey discusses dark matter issues with Nobel laureate G. 't Hooft

[6/2007] B. Famaey comments upon Modified Newtonian Dynamics (MOND)

[31/1/2006] Le coté obscur de la force

[13/12/2005] L'Etoile mystérieuse (Le Soir, 13/12/2005, C. Du Brulle)

[20/10/2004] ESA's Hipparcos finds rebel stars with a cause: Full story

[22/8/2001] Lead stars: