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YSO Bulletin
- 1 June 2020 -

YSO's in the news!
Here are some reports from some online science magazines that may be of interest

Formation of pair of baby planets around their parent star captured in remarkable first

Astronomers at the W M Keck Observatory in Hawaii have captured the first-ever images of a pair of giant planets being born around their parent star V1032 Cen, a K7 PMS star, and young at just 5.4 My. They achieved this using a novel infrared pyramid wavefront sensor, which offers adaptive optics correction.

A report in Phys.org notes that this is the first known multi-planetary system in which astronomers have witnessed planet formation in action. The first direct images of PDS 70b (one of the newborn planets orbiting the star, also known as PDS 70), were taken in 2018 and 2019. There was some confusion when the two protoplanets were first photographed, lead author of the study, Dr Jason Wang, said:
"Planet embryos form from a disk of dust and gas surrounding a newborn star. This circumstellar material accretes onto the protoplanet, creating a kind of smokescreen that makes it difficult to differentiate the dusty, gaseous disk from the developing planet in an image."

infrared image of the newborn planet PDS 70 b and its circumplanetary disc. The size of the solar system is given for comparison. Click on the thumbnail for a full-size picture. Source: V Christiaens et al./ESO
For further clarification, the researchers developed a method to disentangle the image signals from the circumstellar disk and the protoplanets. Subsequently, they were able to take pictures of the baby planets and confirm their existence. They knew the disk’s shape should be an asymmetrical ring around the star, whereas a planet should be a single point in the image.
"So even if a planet appears to sit on top of the disk, which is the case with PDS 70c, based on our knowledge of how the disk looks throughout the whole image, we can infer how bright the disk should be at the location of the protoplanet and remove the disk signal. All that’s left over is the planet’s emission" Wang explained.

AB Aur becomes a proud mum!

Astronomers have gazed into what appears to be a planetary maternity ward, observing for the first time within a huge disk of dense gas and dust surrounding a newly formed star a planet in the process of being born. This large young planet is forming around AB Aurigae which is about 2.4 times the mass of the sun and is 520 light-years away. Using ESO's VLT in Chile to spot a spiral structure within the swirling disk around AB Aurigae generated by the presence of a planet, they detected a “twist” pattern of gas and dust in the spiral structure marking where the planet was coalescing.
"It takes several million years for a planet to be in its final stage, so birth is not well defined in time. However, we can say that we were likely able to catch a planet in the process of formation" said Observatoire de Paris astronomer Anthony Boccaletti, who led the research published in Astronomy & Astrophysics. The planet is located about 30 AU from its star - about the distance of the planet Neptune in our solar system. It appears to be a large gas planet, possibly more massive than Jupiter.

Worth pointing out here that AB Aurigae is visible throughout its range of activity with just a pair of binoculars! With a very small scope you can even follow its equally-interesting companion SU Aur. In the same field a larger aperture will show you another YSO, GM Aur.
Stop Press!
Two days after typing the above, another article appeared in the onlineScience Alert to the effect that this may not be the case. The new interpretation has been described in a paper submitted to MNRAS and is awaiting final peer review. "AB Aurigae is a compendium of interesting features," one of the researchers, Pedro Poblete from the Pontificia Universidad Católica de Chile, explained to the magazine.

Massive YSO's

The latest issue of the Star Formation Newsletter carries an article by Maite Beltrán about the high end of the YSO mass scale: Accretion disks are one of the key ingredients of the star formation process. They redistribute angular momentum and, in the case of high-mass stars (M>8M☉), disks would relieve the radiation pressure on the accreting material, in particular in the equatorial direction, by beaming the radiation through the poles of the system and this would allow the accretion to proceed onto the central protostar. In fact, in recent years, all high-mass starforming theories appear to converge to a disk-mediated accretion scenario. But do the observations of high-mass YSOs confirm the theory predictions? Or in other words, do true accretion disks around massive stars really exist? In 2016, Willem-Jan de Wit and Ms Beltrán wrote a review on accretion disks in luminous YSOs to try to answer such questions. They concluded that the clear signatures of rotating and/or accretion disks reported in the literature confirmed the existence of circumstellar disks around stars with masses up to 20–30M☉ or ~105L☉, which would correspond to early B-type or late O-type stars.


GW Ori: Interactions Between a Triple-star System and its Circumtriple Disk in Action

The same issue of the SFN carries an abstract about a study of the bright T Tau object GW Ori. The authors write:
GW Ori is a hierarchical triple system with a rare circumtriple disk. We present Atacama Large Millimeter/submillimeterArray (ALMA) observations of 1.3 mm dust continuum and 12CO J = 2−1 molecular gas emission of the disk. For the first time, we identify three dust rings in the GW Ori disk at ~ 46, 188, and 338 AU, with estimated dust mass of 74, 168, and 245 Earth masses, respectively. To our knowledge, its outermost ring is the largest dust ring ever found in protoplanetary disks. We use visibility modelling of the dust continuum to show that the disk has misaligned parts, and the innermost dust ring is eccentric. The disk misalignment is also suggested by the CO kinematics. We interpret these substructures as evidence of ongoing dynamical interactions between the triple stars and the circumtriple disk.
Another article shows that exciting stuff can also happen around Post-main sequence stars! Gáspár and Rieke from the University of Arizona write "The apparent detection of an exoplanet orbiting Fomalhaut was announced in 2008. However, subsequent observations of Fomalhaut b raised questions about its status: Unlike other exoplanets, it is bright in the optical and non-detected in the infrared, and its orbit appears to cross the debris ring around the star without the expected gravitational perturbations. We revisit previously published data and analyze additional Hubble Space Telescope (HST) data, finding that the source is likely on a radial trajectory and has faded and become extended. Dynamical and collisional modeling of a recently produced dust cloud yields results consistent with the observations. Fomalhaut b appears to be a directly imaged catastrophic collision between two large planetesimals in an extrasolar planetary system. Similar events should be very rare in quiescent planetary systems of the age of Fomalhaut, suggesting that we are possibly witnessing the effects of gravitational stirring due to the orbital evolution of hypothetical planet(s) around the star."