The dust trap

It is becoming increasingly clear that there are planets in the least expected places: floating alone in space (ejected from their systems), around stars like ours (which was the most logical thing to do), around pulsars (the first exoplanets discovered), and around stars smaller than our Sun. We did not see them and now the numbers are getting out of hand, although the ones that interest us the most are the Earth-like ones, of rocky type. But what conditions exist in those debris disks where planets are born to be of one kind or another?

Protoplanetary disk around the star AB Aurigae.

The birth of the planets is the end of a process that involves a lot of condensation and concentration of matter. In the beginning, we have a faint molecular cloud in space, with grains of dust and gas molecules floating randomly. At some point, the matter begins to condense at certain points that, if the necessary conditions are met, will eventually collapse and create stars. The star will have around it the remains of his own formation, the debris, which will end up forming a disk around him.

That excess matter, composed of gas and dust, will be “floating” around the star, generating, over time, discs of material in which these debris are “condensed” and end up forming planets (we have also talked about second generation disks, something intriguing that is still being studied). So, we can say that the stars are the “mothers” of the planets.

Knowing the composition of these disks, their physics and their chemistry, is fundamental to know what a planet needs to form. That is to say: depending on the raw material we have and the conditions that occur, we will have planets or not and, if we have them, they will be rocky or gaseous.

As it could not be otherwise, we are intrigued to know how the Earth was formed, what conditions were given for the emergence of a planet like ours. To try to know more, we studied the disks of young stars, similar to our Sun in its early stages, in order to try to establish certain parallelisms. One such star is AB Aurigae, a Herbig Ae-type star that hosts a well-known protoplanetary disk in which the planet formation phase appears to have started, a stage known as the “transition disk,” a step between that stage of material accumulated in the disk and planetary formation.

Inside the disk, one of the key places when it comes to studying where and how the planetary birth begins is the so-called “dust trap”, the place where we see that there is greater accumulation of dust within a disk of debris.

NOEMA images of the transition disk of AB Aurigae.

A trap from which you will not be able to escape… or maybe yes.

The “dust trap” is so named because the data indicate that the dust grains are trapped for a very long time, which would facilitate the formation of the seeds of the planets [1]. Another interesting aspect is the shape of the disk which, in this case, is slightly altered, which could be an indication that planetary formation has begun: interferometry data indicate that it is horseshoe-shaped [2].

At first, protoplanetary disks have an abundant amount of gas that will be lost over time, as planets form and the disk is “cleaned” of debris. Some of that gas will also be integrated into the planet. In fact, in this work, led by Susana Pacheco-Vázquez and Asunción Fuente, of the National Astronomical Observatory (OAN-IGN), the chemical composition of the gas of the disk of the star AB Auriga has been studied and several simple organic molecules [3] and sulfur monoxide (SO) have been detected.

Sulfur is one of the most abundant elements of the Solar System. However, so far this is the only protoplanetary disk in which SO has been observed. But that’s not the only mystery: the expected amount of SO isn’t found in the dust trap. Almost all molecules have a horseshoe-shaped spatial distribution, just like dust. However, the spatial distribution of the SO looks more like a ring with uniform emission. This would only be understood if the SO were less abundant in the dust trap than in the rest of the disk.

One possible explanation for understanding where the SO that we did not find could go would be that these SO and SO2 molecules, given the high-density conditions that occur in the dust trap [4], were frozen on the surfaces of the dust grains.  And, with observations in the millimeter range (the range in which the coldest objects emit) the molecules in the ice cannot be detected, so we did not find the expected amount of SO.

What does it mean whether or not there is SO in a dust trap? In principle, its presence, absence or even abundance could be used to know if the disk we are studying is approaching the phase in which it begins to create planets. And, since the gas and dust found in protoplanetary disks are the raw material from which planets are born, understanding their chemistry can shed some light on the eternal question:  the origin of life.


[1] The maximum dust emission corresponds to a maximum gas pressure at which the dust particles would be trapped for a long time, about 0.1 Myr (million years).

[2] The transition disk is highly skewed in azimuth, presenting a morphology disproportionate with the maximum to the southwest.

[3] The compounds detected are HCO+, H2CO, HCN, CN, CS and SO.

[4] The team has come to this conclusion after performing detailed calculations on chemistry, excitation and radiative transfer that simulate the physical conditions in the protoplanetary disk and study the chemistry of sulfur within the dust trap.

Comparison between the spectra modeled and those detected by the 30-meter telescope towards the AB Aurigae disk. The blue and magenta lines correspond to the same model with disk tilt angles of 27◦ and 40◦ respectively.


Image 1: Protoplanetary disk surrounding the Star AB Aurigae. Credits:  Hubble Space Telescope/C.A. Grady (NOAO, NASA/GSFC), et al., NASA.

Image 2: NOEMA images of the transition disk of the star AB Aurigae.

Images with high spatial resolution (~1.6”= 231 AU) of the lines of C18O 2->1, H2CO 30.3->20.2 and SO 56 -> 45 obtained with NOEMA. The emission of the molecular lines follows the ring detected in the continuous dust emission (at 1mm). The dust trap is clearly detected in the 1mm continuum and in the C18O image. However, the SO line has an almost uniform emission along the ring with no significant enhancement.

Radiative transfer, chemical and excitation calculations have been performed, simulating the physical conditions of a protoplanetary disk, in order to investigate the chemistry of sulfur in the region of planet formation. Our model shows that the high-density conditions characteristic of the dust trap would lead to a rapid freezing of the SO and SO2 molecules on the grain surfaces. The absence of some volatile molecules such as SO can therefore be used as a chemical diagnosis to detect the existence of an environment in which planets are being born.

Image 3: Comparison between the spectra modeled and those detected by the 30-meter telescope towards the AB Aurigae disk. The blue and magenta lines correspond to the same model with disk tilt angles of 27◦ and 40◦ respectively.

More information:

This work has been published in the paper “High spatial resolution imaging of SO and H2CO in AB Auriga: the first SO image in a transitional disk”, published in the journal “Astronomy and Astrophysics”, and its authors are Susana Pacheco-Vázquez (OAN-IGN), Asunción Fuente (OAN-IGN), Clément Baruteau (CNRS, IRAP), Olivier Berné (CNRS, IRAP), Marcelino Agúndez (ICMM), Roberto Neri  (IRAM), Javier R. Goicoechea (ICMM), José Cernicharo (ICMM) and Rafael Bachiller (OAN).

This work has been carried out with observations of the NOEMA interferometer and the IRAM 30m radio telescope. The observations with the IRAM 30m radio telescope were carried out within the large ASAI program (IRAM chemical survey of sun-like star-forming regions), whose principal investigators are R. Bachiller and B. LeFloch. NOEMA’s observations were made by an international team led by the National Astronomical Observatory (IGN).

Originally published in Spanish on the Naukas website: “La trampa de polvo” (2016/06/03).

Second generation planets?

Planets are born around young stars in formation. They arise from the rotating discs of remains of material, which are left over after the birth of the star itself. Then, the surroundings of the star, already with their planets emerging, are “cleaned” of diffuse material and stay relatively clear. Until a few years ago we thought that discs were exclusive to those early stellar stages, but then it was discovered that no, that stars in advanced phases again had discs of material around them. And that’s where the question arises: could there be second-generation planets?

The Red Rectangle.

When sun-like stars run out the fuel of their nuclei, they start a decline consisting of various stages. One of them is the red giant phase, in which it swells considerably and begins to expel its material in the form of layers, as in a slow wave of gas molecules and dust grains. This is where stellar winds push that material out. The star continues to “get rid” of its layers, reaching the highest mass loss phase, the Asymptotic Giant Branch, or AGB.

Many stars that have already crossed this stage (called post-AGB) in binary systems (pairs of stars orbiting each other) have a disk made up of gas and dust that revolves around both stars. We know that they exist, but we ignore the details of their formation, structure and evolution, although surprising similarities have been found with discs that revolve around young stars. [1]

There is research that considers the possibility that these gravitationally linked dust discs exist in many binary stars in advanced stages. In fact, the data is even used the other way around: the presence of a disk indicates that it can be a binary system with a post-AGB star.

The Red Rectangle

In 2003, a team led by Valentín Bujarrabal, a researcher at the National Astronomical Observatory (OAN-IGN), discovered that the planetary nebula known as the Red Rectangle had a rotating disc [2]. This nebula, studied with  the IRAM interferometer, also launches jets of material at low speed and has a complex structure in which there is a binary star system whose main star is a post-AGB.

Until early 2015, only this rotating disc had been clearly studied and identified. The second disc of this detected type was the one orbiting around other evolved star:  AC Herculis. From this moment on, scientists suspected that these discs play a key role in late stellar evolution and were abundant around evolved stars.

Bujarrabal, principal investigator of both works, states that these, and other published results, are part of a long collaboration maintained by the OAN team with the Institute of Astronomy of Leuven: “We were the first to demonstrate the existence of discs rotating around old stars, as normally the material around them, which has been ejected by them, is expanding. Some of our latest observations, particularly using ALMA and VLTI, are really spectacular and contain a huge amount of information about these amazing objects.

The data that ALMA is providing will be decisive in the future to compare this type of disc with those around young stars. At the moment, thanks to the VLTI interferometer, the most accurate image of a disc around an evolved star, IRAS 08544-4431, formed by a red giant and a less evolved one, has been obtained.

The image is impressive: the dust ring surrounding the stars is clearly visible. From all these observations it has been inferred that the discs surrounding old stars are very similar to the discs around the young stars. And if they’re so similar, could planets form?

Wait a minute, let’s rewind.

Born and die

We are at the moment just before the stars are born (our protagonists are always mid-sized stars). The molecular cloud, laden with gas and dust, is compacted at some points where matter ends up condensing and ignition begins in the stellar nuclei. Around them, these young stars have discs on which planets can eventually form. Once formed, there may even be a “dance” of planets, called planetary migration, that causes some to change orbit around their star until the system stabilizes.

The stars normally live their hydrogen consumption stage in the nucleus, until it is finished and drift begins. The red giant phase is so overwhelming (because of its huge size and impressive increase in luminosity) that the planets will most likely end up slashed, pushed or broken. It will all depend on the distance that separates the planet from its star and, again, the planetary migrations between orbits.

Of course, planets could overcome that phase and still exist (very battered, yes). And this has led to quite a few confusions when it came to determining whether a star (as in BP Piscium’s case) was young or old – the same thing happened with Gomez’s Hamburguer, which was believed to be an old star until it was studied in depth and was seen to be a young star that probably has a protoplanet orbiting around it.

But would a second generation of planets be possible? What specific conditions should be given? Could the remains of first-generation planets, along with the materials of the second disc, form new planets? How long would it take to do so? Could the energy of the dying white dwarf left in the center feed that system?

As for the possible existence of second generation planets, there are already studies related to the first exoplanets detected, discovered in 1992, which also revolved around a pulsar: PSR B1257+12. But in this case, we are not talking about the death of sun-like stars (between one and eight solar masses) but a hypothesis about something that could have happened after the death as a supernova of a much more massive star [3].

Moreover, in the environments of evolved mid-mass stars, in the final stages of their lives, so far, no planets have been found in formation. Scientists theorize about which environment would be most appropriate, which parameters they should meet, but nothing has yet been detected that can confirm these hypotheses.

Meanwhile, researchers like Bujarrabal continue to study these second-generation discs to determine how they form and what their final destination is.


[1] For example, in its mineralogical composition, as is the case of the AC Her  binary system, which contains a post-AGB star, and the protoplanetary disk of the young star HD100546.

[2] Valentin Bujarrabal himself published a beautiful report  entitled “The Colloquium of the Nebulae”, inspired by Cervantes’ “The Colloquium of Dogs”, in which the Rectangle Nebula and the Pumpkin Nebula have a dialectical encounter. You can find it in the yearbook of the National Astronomical Observatory (OAN-IGN) of 2014 and in this link (in Spanish).

[3] Some argue that a millisecond pulsar could explode in the form of a Quark-type nova. This still hypothetical object, the result of a previous explosion of supernova, would generate a disk capable of forming planets. This theory could explain the existence of the planets around the PSR B1257+12 pulsar.


Image 1: Rectangle nebula. Credit:  ESA,  Hubble,  NASA. Link to the image, APOD (Astronomy Picture of the Day), June 14th, 2010.

Originally published in Spanish on the Naukas website: “¿Planetas de segunda generación?” (2016/04/26).