ALMA: Gravitational instability in the disk around AB Aurigae

[AVAO]

Expert explains evidence for planetary formation through gravitational instability
phys.org | Original release 2024 September 04

Global spirals in the AB Aur disk. Credit: Nature (2024). DOI: 10.1038/s41586-024-07877-0

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The canonical theory for planet formation in circumstellar disks proposes that planets are grown from initially much smaller seeds. The long-considered alternative theory proposes that giant protoplanets can be formed directly from collapsing fragments of vast spiral arms induced by gravitational instability - if the disk is gravitationally unstable. For this to be possible, the disk must be massive compared with the central star: a disk-to-star mass ratio of 1:10 is widely held as the rough threshold for triggering gravitational instability, inciting substantial non-Keplerian dynamics and generating prominent spiral arms. Although estimating disk masses has historically been challenging the motion of the gas can reveal the presence of gravitational instability through its effect on the disk-velocity structure. Here we present kinematic evidence of gravitational instability in the disk around AB Aurigae, using deep observations of 13CO and C18O line emission with the Atacama Large Millimeter/submillimeter Array (ALMA). The observed kinematic signals strongly resemble predictions from simulations and analytic modelling.
[...]

... read more

GOTD4Y Jac

[Ann]

Expert explains evidence for planetary formation through gravitational instability
phys.org | Original release 2024 September 04

Global spirals in the AB Aur disk. Credit: Nature (2024). DOI: 10.1038/s41586-024-07877-0

---

The canonical theory for planet formation in circumstellar disks proposes that planets are grown from initially much smaller seeds. The long-considered alternative theory proposes that giant protoplanets can be formed directly from collapsing fragments of vast spiral arms induced by gravitational instability - if the disk is gravitationally unstable. For this to be possible, the disk must be massive compared with the central star: a disk-to-star mass ratio of 1:10 is widely held as the rough threshold for triggering gravitational instability, inciting substantial non-Keplerian dynamics and generating prominent spiral arms. Although estimating disk masses has historically been challenging the motion of the gas can reveal the presence of gravitational instability through its effect on the disk-velocity structure. Here we present kinematic evidence of gravitational instability in the disk around AB Aurigae, using deep observations of 13CO and C18O line emission with the Atacama Large Millimeter/submillimeter Array (ALMA). The observed kinematic signals strongly resemble predictions from simulations and analytic modelling.
[...]

... read more

GOTD4Y Jac

Interesting, Jac!

Phys.org wrote:

Exoplanets form in protoplanetary disks, a collection of space dust and gas orbiting a star. The leading theory of planetary formation, called core accretion, occurs when grains of dust in the disk collect and grow to form a planetary core, like a snowball rolling downhill.

Core accretion formation of an ice planet.

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A secondary theory of planetary formation is gravitational collapse. In this scenario, the disk itself becomes gravitationally unstable and collapses to form the planet, like snow being plowed into a pile.

Gravitational collapse formation of an ice planet.

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But in a new paper published today in Nature, MIT Kerr-McGee Career Development Professor Richard Teague and his colleagues report evidence that the movement of the gas surrounding the star AB Aurigae behaves as one would expect in a gravitationally unstable disk, matching numerical predictions.

Mass of the disk vs. mass of the star in order to create gravitational collapse that leads to planet formation. Note the planets being formed when the disk mass is 12.5% of the star mass. Credit: James Cadman et al.

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Finally, let's take a look at the visual appearance of the nebula. vdB 131, surrounding AB Aurigae and possibly including yellow star SU Aurigae as well:

Note the dust bunnies surrounding blue reflection nebula vdB 131 and the young stars AB Aurigae, a blue Herbig Ae star (a young star on its way to becoming a main sequence A or B-type star), and SU Aurigae, a yellow T Tauri star (a young star on its way to becoming an F, G, K or M-type main sequence star). Credit: Adam Block.

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SU Aurigae is interesting. It's a T Tauri star with the same surface temperature and probably the same mass as the Sun, but it is some 3-4 times brighter. That is because SU Aur is not powered by core hydrogen fusion like the Sun, but by releasing gravitational energy as this immature star keeps shrinking until its interior gets hot enough to start its core hydrogen fusion, thus becoming a main sequence size star.

Wikipedia wrote:

T Tauri stars comprise the youngest visible F, G, K and M spectral type stars (<2 M☉). Their surface temperatures are similar to those of main-sequence stars of the same mass, but they are significantly more luminous because their radii are larger. Their central temperatures are too low for hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract, while moving towards the main sequence, which they reach after about 100 million years.

Also T Tauri stars burn lithium.

Their spectra show a higher lithium abundance than the Sun and other main-sequence stars because lithium is destroyed at temperatures above 2,500,000 K. From a study of lithium abundances in 53 T Tauri stars, it has been found that lithium depletion varies strongly with size, suggesting that "lithium burning" by the p-p chain during the last highly convective and unstable stages during the later pre–main sequence phase of the Hayashi contraction may be one of the main sources of energy for T Tauri stars.

Oh well!

Ann

[AVAO]

Expert explains evidence for planetary formation through gravitational instability
phys.org | Original release 2024 September 04

Global spirals in the AB Aur disk. Credit: Nature (2024). DOI: 10.1038/s41586-024-07877-0

---

The canonical theory for planet formation in circumstellar disks proposes that planets are grown from initially much smaller seeds. The long-considered alternative theory proposes that giant protoplanets can be formed directly from collapsing fragments of vast spiral arms induced by gravitational instability - if the disk is gravitationally unstable. For this to be possible, the disk must be massive compared with the central star: a disk-to-star mass ratio of 1:10 is widely held as the rough threshold for triggering gravitational instability, inciting substantial non-Keplerian dynamics and generating prominent spiral arms. Although estimating disk masses has historically been challenging the motion of the gas can reveal the presence of gravitational instability through its effect on the disk-velocity structure. Here we present kinematic evidence of gravitational instability in the disk around AB Aurigae, using deep observations of 13CO and C18O line emission with the Atacama Large Millimeter/submillimeter Array (ALMA). The observed kinematic signals strongly resemble predictions from simulations and analytic modelling.
[...]

... read more

GOTD4Y Jac

Interesting, Jac!

Phys.org wrote:

Exoplanets form in protoplanetary disks, a collection of space dust and gas orbiting a star. The leading theory of planetary formation, called core accretion, occurs when grains of dust in the disk collect and grow to form a planetary core, like a snowball rolling downhill.

Core accretion formation of an ice planet.

---

A secondary theory of planetary formation is gravitational collapse. In this scenario, the disk itself becomes gravitationally unstable and collapses to form the planet, like snow being plowed into a pile.

Gravitational collapse formation of an ice planet.

---

But in a new paper published today in Nature, MIT Kerr-McGee Career Development Professor Richard Teague and his colleagues report evidence that the movement of the gas surrounding the star AB Aurigae behaves as one would expect in a gravitationally unstable disk, matching numerical predictions.

Mass of the disk vs. mass of the star in order to create gravitational collapse that leads to planet formation. Note the planets being formed when the disk mass is 12.5% of the star mass. Credit: James Cadman et al.

---

Finally, let's take a look at the visual appearance of the nebula. vdB 131, surrounding AB Aurigae and possibly including yellow star SU Aurigae as well:

Note the dust bunnies surrounding blue reflection nebula vdB 131 and the young stars AB Aurigae, a blue Herbig Ae star (a young star on its way to becoming a main sequence A or B-type star), and SU Aurigae, a yellow T Tauri star (a young star on its way to becoming an F, G, K or M-type main sequence star). Credit: Adam Block.

---

SU Aurigae is interesting. It's a T Tauri star with the same surface temperature and probably the same mass as the Sun, but it is some 3-4 times brighter. That is because SU Aur is not powered by core hydrogen fusion like the Sun, but by releasing gravitational energy as this immature star keeps shrinking until its interior gets hot enough to start its core hydrogen fusion, thus becoming a main sequence size star.

Wikipedia wrote:

T Tauri stars comprise the youngest visible F, G, K and M spectral type stars (<2 M☉). Their surface temperatures are similar to those of main-sequence stars of the same mass, but they are significantly more luminous because their radii are larger. Their central temperatures are too low for hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract, while moving towards the main sequence, which they reach after about 100 million years.

Also T Tauri stars burn lithium.

Their spectra show a higher lithium abundance than the Sun and other main-sequence stars because lithium is destroyed at temperatures above 2,500,000 K. From a study of lithium abundances in 53 T Tauri stars, it has been found that lithium depletion varies strongly with size, suggesting that "lithium burning" by the p-p chain during the last highly convective and unstable stages during the later pre–main sequence phase of the Hayashi contraction may be one of the main sources of energy for T Tauri stars.

Oh well!

Ann

ThanX Ann - You are great!


Just as you like blue, I like the other wavelengths too.
Jac

Click to view full size image 1 or image 2

Original data: NASA/ESA (DSS/HERSCHEL(submillimetre)) jac berne (flickr)

[andreas]

Aur is not powered by core hydrogen fusion like the Sun, but by releasing gravitational energy as this immature star keeps shrinking until its interior gets hot enough to start its core hydrogen fusion
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[AVAO]

Aur is not powered by core hydrogen fusion like the Sun, but by releasing gravitational energy as this immature star keeps shrinking until its interior gets hot enough to start its core hydrogen fusion
spacebar clicker

Right. Good point.


Here another good picture:

Click to view full size image

Click to view full size image

This image of the dust and gas disk around the star AB Aurigae reveals the presence of a growing planetary embryo (f1). Original © ESO/ Boccaletti et al.

"Astronomers have only rarely managed to observe a developing planet. This is because the planetary embryos are usually hidden in dense disks of dust and gas. Sometimes only a gap or turbulence in this protoplanetary disk reveals their existence. There were also initial indications of such a planetary embryo in the star AB Aurigae, around 520 light years away from us in the constellation Aurigae.
Spiral and kink as evidence

Now this suspicion has been confirmed - and this image is the proof. It was created by the SPHERE instrument on the Very Large Telescope of the European Southern Observatory (ESO) in Chile. It shows the disk of matter around AB Aurigae in high resolution and in polarized light, revealing its internal structure. It can be seen that dust and gas form a spiral structure. According to models, such spirals are created by the disruptive influence of forming planets.

But there is a second, even clearer indication: In the inner area of ​​this spiral you can see a bright, yellow gas arm that appears to be noticeably bent. This kink could mark the place where the new planet is growing, as the astronomers led by Anthony Boccaletti from the Sorbonne University in Paris report. The planetary embryo orbits AB Aurigae at a distance that is roughly the same as the distance from the sun to Neptune. (Astronomy & Astrophysics, 2020; doi: 10.1051/0004-6361/202038008)"

Source: ESO