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).