[ Update June.12.2020: What’s Up: June 2020 – NASA JPL:
What are some skywatching highlights you can see in June 2020? Look for the Summer Triangle rising in the east after sundown, keep tabs on the morning planets and June 20 brings the solstice. Additional information about topics covered in this episode of What’s Up, along with still images from the video, and the video transcript, are available at https://solarsystem.nasa.gov/whats-up… Credit: NASA-JPL/Caltech
Though the nights are shorter in June, they are filled with fine sights. Look for the Hercules constellation, which will lead you to a globular star cluster with hundreds of thousands of densely packed stars. You can also spot Draco the dragon, which will point you to the Cat’s Eye Nebula. Keep watching for space-based views of globular star clusters and the nebula.
** What’s in the Night Sky June 2020#WITNS | Solar Eclipse | Milky Way | NLCs – Alyn Wallace
Astronomers using ESO telescopes have discovered giant spots on the surface of extremely hot stars hidden in stellar clusters, called extreme horizontal branch stars. This image shows an artist’s impression of what one of these stars, and its giant whitish spot, might look like. The spot is bright, takes up a quarter of the star’s surface and is caused by magnetic fields. As the star rotates, the spot on its surface comes and goes, causing visible changes in brightness.
Astronomers using European Southern Observatory (ESO) telescopes have discovered giant spots on the surface of extremely hot stars hidden in stellar clusters. Not only are these stars plagued by magnetic spots, some also experience superflare events, explosions of energy several million times more energetic than similar eruptions on the Sun. The findings, published today in Nature Astronomy, help astronomers better understand these puzzling stars and open doors to resolving other elusive mysteries of stellar astronomy.
The team, led by Yazan Momany from the INAF Astronomical Observatory of Padua in Italy, looked at a particular type of star known as extreme horizontal branch stars — objects with about half the mass of the Sun but four to five times hotter.
“These hot and small stars are special because we know they will bypass one of the final phases in the life of a typical star and will die prematurely,” says Momany, who was previously a staff astronomer at ESO’s Paranal Observatory in Chile. “In our Galaxy, these peculiar hot objects are generally associated with the presence of a close companion star.”
Spots on extreme horizontal branch stars (right) appear to be quite different from the dark sunspots on our own Sun (left), but both are caused by magnetic fields. The spots on these hot, extreme stars are brighter and hotter than the surrounding stellar surface, unlike on the Sun where we see spots as dark stains on the solar surface that are cooler than their surroundings. The spots on extreme horizontal branch stars are also significantly larger than sunspots, covering up to a quarter of the star’s surface. While sunspots vary in size, a typical size is around an Earth-size planet, 3000 smaller than a giant spot on an extreme horizontal branch star.
Surprisingly, however, the vast majority of these extreme horizontal branch stars, when observed in tightly packed stellar groups called globular clusters, do not appear to have companions. The team’s long-term monitoring of these stars, made with ESO telescopes, also revealed that there was something more to these mysterious objects. When looking at three different globular clusters, Momany and his colleagues found that many of the extreme horizontal branch stars within them showed regular changes in their brightness over the course of just a few days to several weeks.
“After eliminating all other scenarios, there was only one remaining possibility to explain their observed brightness variations,” concludes Simone Zaggia, a study co-author from the INAF Astronomical Observatory of Padua in Italy and a former ESO Fellow: “these stars must be plagued by spots!”
Spots on extreme horizontal branch stars appear to be quite different from the dark sunspots on our own Sun, but both are caused by magnetic fields. The spots on these hot, extreme stars are brighter and hotter than the surrounding stellar surface, unlike on the Sun where we see spots as dark stains on the solar surface that are cooler than their surroundings. The spots on extreme horizontal branch stars are also significantly larger than sunspots, covering up to a quarter of the star’s surface. These spots are incredibly persistent, lasting for decades, while individual sunspots are temporary, lasting only a few days to months. As the hot stars rotate, the spots on the surface come and go, causing the visible changes in brightness.
Beyond the variations in brightness due to spots, the team also discovered a couple of extreme horizontal branch stars that showed superflares — sudden explosions of energy and another signpost of the presence of a magnetic field.
“They are similar to the flares we see on our own Sun, but ten million times more energetic,” says study co-author Henri Boffin, an astronomer at ESO’s headquarters in Germany. “Such behaviour was certainly not expected and highlights the importance of magnetic fields in explaining the properties of these stars.”
After six decades of trying to understand extreme horizontal branch stars, astronomers now have a more complete picture of them. Moreover, this finding could help explain the origin of strong magnetic fields in many white dwarfs, objects that represent the final stage in the life of Sun-like stars and show similarities to extreme horizontal branch stars.
“The bigger picture though,” says team member, David Jones, a former ESO Fellow now at the Instituto de Astrofísica de Canarias, Spain, “is that changes in brightness of all hot stars — from young Sun-like stars to old extreme horizontal branch stars and long-dead white dwarfs — could all be connected. These objects can thus be understood as collectively suffering from magnetic spots on their surfaces.”
To arrive at this result, the astronomers used several instruments on ESO’s Very Large Telescope (VLT), including VIMOS, FLAMES and FORS2, as well as OmegaCAM attached to the VLT Survey Telescope at Paranal Observatory. They also employed ULTRACAM on the New Technology Telescope at ESO’s La Silla Observatory, also in Chile. The breakthrough came as the team observed the stars in the near-ultraviolet part of the spectrum, allowing them to reveal the hotter, extreme stars standing out bright amongst the cooler stars in globular clusters.
Red Giant stripped of mass during encounter with a black hole, leaving only a white dwarf in orbit. At closest point of its orbit, material from the white dwarf is pulled away by the black hole, resulting in X-ray pulses seen by Chandra X-Ray telescope. Credits: Chandra Observatory via Carnival of Space.
This image shows the disc around the young AB Aurigae star, where ESO’s Very Large Telescope (VLT) has spotted signs of planet birth. Close to the centre of the image, in the inner region of the disc, we see the ‘twist’ (in very bright yellow) that scientists believe marks the spot where a planet is forming. This twist lies at about the same distance from the AB Aurigae star as Neptune from the Sun. The image was obtained with the VLT’s SPHERE instrument in polarised light.
Observations made with the European Southern Observatory’s Very Large Telescope (ESO’s VLT) have revealed the telltale signs of a star system being born. Around the young star AB Aurigae lies a dense disc of dust and gas in which astronomers have spotted a prominent spiral structure with a ‘twist’ that marks the site where a planet may be forming. The observed feature could be the first direct evidence of a baby planet coming into existence.
“Thousands of exoplanets have been identified so far, but little is known about how they form,”
says Anthony Boccaletti who led the study from the Observatoire de Paris, PSL University, France. Astronomers know planets are born in dusty discs surrounding young stars, like AB Aurigae, as cold gas and dust clump together. The new observations with ESO’s VLT, published in Astronomy & Astrophysics, provide crucial clues to help scientists better understand this process.
“We need to observe very young systems to really capture the moment when planets form,”
says Boccaletti. But until now astronomers had been unable to take sufficiently sharp and deep images of these young discs to find the ‘twist’ that marks the spot where a baby planet may be coming to existence.
The new images feature a stunning spiral of dust and gas around AB Aurigae, located 520 light-years away from Earth in the constellation of Auriga (The Charioteer). Spirals of this type signal the presence of baby planets, which ‘kick’ the gas, creating
“disturbances in the disc in the form of a wave, somewhat like the wake of a boat on a lake,”
explains Emmanuel Di Folco of the Astrophysics Laboratory of Bordeaux (LAB), France, who also participated in the study. As the planet rotates around the central star, this wave gets shaped into a spiral arm. The very bright yellow ‘twist’ region close to the centre of the new AB Aurigae image, which lies at about the same distance from the star as Neptune from the Sun, is one of these disturbance sites where the team believe a planet is being made.
The images of the AB Aurigae system showing the disc around it. The image on the right is a zoomed-in version of the area indicated by a red square on the image on the left. It shows the inner region of the disc, including the very-bright-yellow ‘twist’ (circled in white) that scientists believe marks the spot where a planet is forming. This twist lies at about the same distance from the AB Aurigae star as Neptune from the Sun. The blue circle represents the size of the orbit of Neptune. The images were obtained with the SPHERE instrument on ESO’s Very Large Telescope in polarised light.
Observations of the AB Aurigae system made a few years ago with the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, provided the first hints of ongoing planet formation around the star. In the ALMA images, scientists spotted two spiral arms of gas close to the star, lying within the disc’s inner region. Then, in 2019 and early 2020, Boccaletti and a team of astronomers from France, Taiwan, the US and Belgium set out to capture a clearer picture by turning the SPHERE instrument on ESO’s VLT in Chile toward the star. The SPHERE images are the deepest images of the AB Aurigae system obtained to date.
With SPHERE’s powerful imaging system, astronomers could see the fainter light from small dust grains and emissions coming from the inner disc. They confirmed the presence of the spiral arms first detected by ALMA and also spotted another remarkable feature, a ‘twist’, that points to the presence of ongoing planet formation in the disc.
“The twist is expected from some theoretical models of planet formation,”
says co-author Anne Dutrey, also at LAB.
“It corresponds to the connection of two spirals — one winding inwards of the planet’s orbit, the other expanding outwards — which join at the planet location. They allow gas and dust from the disc to accrete onto the forming planet and make it grow.”
ESO is constructing the 39-metre Extremely Large Telescope, which will draw on the cutting-edge work of ALMA and SPHERE to study extrasolar worlds. As Boccaletti explains, this powerful telescope will allow astronomers to get even more detailed views of planets in the making.
“We should be able to see directly and more precisely how the dynamics of the gas contributes to the formation of planets,”
