Anaëlle Maury

Who are the progenitors of planet-forming disks ?

Protostellar disks are disks of gas and dust that surround "protostars", stars that are forming, and in which planets will be later built. The first phases of the evolution of protoplanetary disks are still poorly understood because, even for those closest to us, their observation requires the ability to distinguish details below the arc second (the size of a coin seen from 4 kilometres away) through a telescope. Because of their low temperature and their embedded nature, these disks do not emit visible light but so-called "millimetre" radiation that can be observed by radio-telescopes. Their understanding is of paramount importance to better reconstruct the origin of our own solar system.

When do disks form ? What is their initial size distribution ? How do they grow and evolve during the main accretion phase ? Are protostellar disks surrounding solar-type precursors massive enough to fragment and form multiple systems and planets ? I have been adressing these questions in a series of papers (Maury et al. 2010, 2014, 2015, 2019).

Maury et al. (2019): high angular observations of the youngest protostars shows that a majority of disks where planets will be formed are born much smaller than expected ! See the CEA press release here.

Our CALYPSO Large Observing program provided us with deep and high-resolution images of the dust thermal emission in a large sample of the youngest protostars in the Galaxy. The angular resolution and sensitivity allow to probe distances less than 50 AU of the central star (1UA = the average Earth-Sun distance, about 150 million kilometers), a distance corresponding to the size of the most central part of the Solar System, similar to the distance from Pluto to the Sun. We analyzed the spatial distribution of the dust emission to look for disk structures. We reached the surprising conclusion that, while disk formation is concomitant with the formation phase of the protostar, it occurs predominantly (>75%) at much smaller scales than those predicted by hydrodynamic models of the collapse of protostellar cores, and that the effect of magnetic fields could be a key element in explaining these smaller disks.

The role of magnetic field during the star and planet formation processes

The interstellar medium, where stars are born, is permeated by large-scale magnetic fields. These magnetic fields are observed everywhere in the clouds where stars form, and were suggested to have a crucial role in supporting the high density medium against its own gravity that ultimately produce low mass stars as our own Sun. Star formation models suggest that these magnetic fields could not only help support protostellar cores against their own gravitational potential, but also that they could significantly affect the rotation of matter in these cores, ultimately affecting the formation of the youngest circumstellar disks, and thus the formation of planets in these disks.

What is the intensity and the structure of magnetic field in star-forming cores and disks ? Are they important for disk formation and evolution ? As they tend to align with the magnetic field, dust grains generate polarized radiation (both in absorption and emission), which can be used to trace the structure of the magnetic field within the dense parts of the interstellar medium. However, how well the alignment mechanisms work to trace faithfully the magnetic field, and how to use these datasets to assess the exact role of the magnetic field during the star and planet formation process, are yet widely open questions.

Maury et al. (2018): ALMA unveils a magnetically-regulated collapse scenario for the formation of solar-type stars. See the CEA press release here.

We used the exquisite sensitivity and resolution of the ALMA interferometer to capture the polarized emission of dust in the circumstellar material around a young protostar, B335. We could map the polarized emission from a radius of 50 AU up to 1000 AU (astronomical unit = Earth-Sun distance = 150 million kilometers), showing that this young star in formation undergoes the effect of the field over a very large area. We have shown that the magnetic field structure in B335 has a strikingly ordered topology in the core, with a transition from a large-scale poloidal magnetic field, in the outflow direction, to strongly pinched in the equatorial direction. We carried out a comparison with predictions of hydrodynamical collapse and have shown that only models where the magnetic field is dynamically relevant could reproduce our observations.

Galametz et al. (2018): all the youngest protostars seem to be magnetized !

We investigated the polarized dust emission, tracing the magnetic field topology at envelope scales in 12 very young protostars using the sub-mm interferometer SMA, located in Hawaii. We report the detection of magnetic field at scales 600-5000 astronomical units (one unit is the distance from the Sun to Earth) in all the circumstellar envelopes of our stellar embryos ! Our analysis shows that the envelope-scale magnetic field might preferentially observed either aligned or perpendicular to the outflow direction. Interestingly, our results suggest for the first time a relation between the orientation of the magnetic field and the rotational energy of envelopes, with a larger occurrence of misalignment in sources in which strong rotational motions are detected at hundreds to thousands of au scales. We also show thatthe best agreement between the magnetic field and outflow orientation is found in sources showing no small-scale multiplicity and no large disks at∼100 au scales.

Probing the cosmic dust
fate, from the ISM to planets

The dust grains solid phase represents about 1% of the interstellar matter mass of the Galaxy. However, in spite of this modest abundance, this component plays an important role in many respects. For example, dusty grains constitute the primordial building blocks of rocky planetesimal that will eventually end in terrestrial planets, giant planet cores, satellites, meteorites and comets. They are the essential element structuring the planetary and small bodies formation. In the current paradigm of planet formation, the dust inherited from the ISM coagulates into planets once incorporated during the T-Tauri phase, in protoplanetary disks, thus a million year after the onset of the star formation process. Observations of these disks reveal sub-mm dust emissivities β < 1.0, a signature that grains have grown beyond millimeter sizes (Testi et al. 2014).

Recent ALMA observations even suggest that most of these disks could already host young planets, or even Jupiter-mass gas giants (ALMA partnership 2015, Isella et al. 2016, Pinte et al. 2019). This would mean that the formation of planets occurs much earlier than previously assumed, and that the mm dust observed in T-Tauri disks may actually be a secondary product of the collision and fragmentation of larger bodies (Turrini et al. 2019).

In the MagneticYSOs project, we use observations and models to track the evolution of dust in star-forming cores. Our ALMA and IRAM/PdBI observations of the dust thermal emission try to put constraints on grains size distributions, growth and planetesimal formation.

Valdivia et al. (2019): models of dust alignment suggest sub-mm grains in dense cores.

In Valdivia et al. (2019) we used the standard ISM dust model to calculate the radiative transfer of dust thermal emission from our magneto-hydrodynamical (MHD) simulations of protostellar formation. The synthetic observations of the dust emission and its polarization we produced showed that relatively large (>10 μm) dust grains are required to reproduce mm polarized dust emission at fractions similar to those currently observed in Class 0 envelopes (e.g. 2-10%, observations from Maury et al. 2018, Galametz et al. 2018, Le Gouellec et al. 2019).

Galametz et al. (2019):observational signatures of early grain growth in the youngest protostars ?

We investigated the dust properties in 12 very young protostars using the mm PdBI CALYPSO survey. We find that most protostars show extremely low spectral index throughout their envelope, with decreasing dust emissivity (β) values toward small scales (we probed scales from 200 to 2000 au). Such low indices could be the signature of large dust grains. Recent studies on various interstellar dust analogues confirmed that β values <1, like those observed in protostars, could only be produced by grains larger than 100μm (Ysard et al. 2019).

Our findings severely contradict the current planetary formation paradigm, and call for a revised model of planet formation, in which the first pebbles form early around stellar embryos, within the first 0.1 Myr. Hence they need to be robustly investigated. One key question remains especially on the validity of the dust models used by the community and the classical assumptions they rely on (Ossenkopf & Henning 1994; Natta & Testi 2004). To address these issues, an innovative dust model THEMIS (Jones et al. 2017) has been developed to take into account various grain composition and evolution from the diffuse to the dense ISM. It provides predictions for more realistic grains with additional carbon mantles, ices, different size and shape distribution or chemical composition (Kohler et al. 2015, Ysard et al. 2018; 2019). We are currently working at implementing this new model in our radiative transfer treament so we can compare its predictions against our observations, together with the dust experts at IAS (Ysard, Brauer, Jones).