Protoplanetary disks are rotating disks of gas and dust surrounding stars. Observations with the ALMA observatory show that these disks have gaps and sometimes spirals, which are believed to be carved by planets. In one of these disks, the presence of a planet has been confirmed by direct imaging with the Very Large Telescope. In another one, a planet has been detected by measuring precisely the gas kinematics around the star: in a protoplanetary disk, the gas is in Keplerian rotation around the star, but planets disturb this motion. These studies demonstrate that protoplanetary disks are the birthplace of planets. Determining how and when these disks are formed is, therefore, important for understanding the formation of planetary systems.
So far the vast majority of protoplanetary disks has been observed around T-Tauri or Herbig-Ae stars, which are a few million years old. However, a few protoplanetary disks have also been detected around Class 0 or Class I protostars, for example in L1527, L1448-N or BHB2007. Class 0 protostars are only a few hundred thousand years old. The detection of disks around these protostars shows that protoplanetary disks may form and grow rapidly. However, it is still unclear whether disks are common around these protostars.
In a paper that has just been accepted to the Astronomy & Astrophysics journal, my colleagues from the CALYPSO team and myself present the results of a survey of protoplanetary disks in a sample of embedded (Class 0) protostars. We used the NOEMA interferometer to map the line emission of two carbon monoxide isotopologs, 13CO and C18O, and the line emission of sulfur oxide (SO). We used these maps to measure the velocity of the gas around the protostars to search for Keplerian rotation.
Figure above: Left panel: C18O (1-0) line intensity map (zeroth-order moment, black contours) and mean velocity (first-order moment, color image) in the L1448-C Class 0 protostar. We detect a velocity gradient (indicated with the black arrow) orthogonal to the direction of the jet (shown by the blue and red lines). Right panel: Rotation curve obtained from the visibilities. The solid curve shows a fit with a power-law, while the dashed curve shows a fit with a Keplerian law.
To measure the gas velocity around the protostars, we computed the first-order moment maps of each line in each protostar of the sample. We used these maps to detect velocity gradients and to measure their amplitude and orientation (see the left panel of the figure above for an example). We find that roughly half of the protostars of our sample have velocity gradients orthogonal to their bipolar jets, as expected if these gradients are due to disks. For each of these disk candidate, we constructed a rotation curve, that is the velocity as a function of the radius. For this, we fitted the centroid position of the emission as a function of velocity directly from the visibilities measured by the interferometer. This technique allowed us to derive the rotation curve down to a radius about 50 au (see the right panel of the figure above). We find that only two protostars have a rotation curve consistent with Keplerian rotation: L1527 and L1448-C. The presence of a disk on L1527 was already known, but this is first detection of a disk in L1448-C.
Our study shows that disks with a radius larger that 50 au (the typical scales probed by our observations) are rare in Class 0 protostars. This is in agreement with magneto-hydrodynamical (MHD) simulations of the formation of disks, which predict disks with radius of a few tens of au. Disks are probably present in all Class 0 protostars, but most of them are too small to be detected with our NOEMA survey. Finding and characterizing Keplerian disks around Class 0 protostars will require observations at higher angular resolution (0.1’’ or better). This can only be achieved by ALMA.
The paper “Searching for kinematic evidence of Keplerian disks around Class 0 protostars with CALYPSO” by Maret et al. is available on arXiv.
Background image credit: NAOJ