The Last Stages of Accretion in Young Objects

Abstract

Accretion from protoplanetary disks onto low mass, young stars (T Tauri stars) has been extensively studied for several decades. Theoretical, observational, and modeling efforts show that accretion from the disk onto the star follows the magnetospheric accretion paradigm, in which magnetic field lines truncate the disk and mass flows along the field lines onto the stellar surface. Studies of large samples of stars in many star-forming regions have shown that the fraction of accretors decreases as the population's age increases. Nevertheless, it is still unclear how accretion stops. Understanding the processes shutting off accretion will provide crucial information for studying the properties of the disks in which planets are forming and of the stars that host them.

Here, we present a comprehensive study of stars at the last stages of their accretion phase, aiming to shed some light on the processes driving accretion to stop. By studying the gas in the inner disk using FUV observations and the dust from near-infrared excess, we found that the disks of low accretors are diverse in dust emission, suggesting that the end of accretion is reached in different ways. We also confirmed previous results that stars stop accreting as soon as the inner disk has no gas left.

We showed that the He I line at 10830 Angstrom is more sensitive at detecting accretion than diagnostics using the Balmer alpha line of hydrogen. Using this accretion diagnostics, we re-classify 51 stars previously thought to be non-accretors as accretors. We studied a subset of these stars and found that, at low accretion rates, magnetospheric flows accrete mass in the unstable regime and that the geometry of the accretion region is complex. Based on the relationship between the inferred disk truncation radius, the corotation radius, and the mass accretion rate, we proposed that the dipolar fields in low accretors are weak and that the efficiency for the magnetic fields to truncate the disks is low. We found no low accretors in the propeller regime, suggesting that the propeller is not the primary process inhibiting accretion at the last stages of disk evolution.

We found evidence of a physical lower limit of how much mass can be supplied to the star from its disk, at 10^-10 solar mass per year. This rate is consistent with the EUV-driven photoevaporative mass loss rate, which suggests that the properties of the outer disk control accretion from the inner disk onto the star. However, a possibility of an inner disk mass reservoir cannot be ruled out, as shown by the high mass accretion rate of PDS 70 for low mass transport efficiency.

Lastly, we showed that the magnetospheric accretion model could be applied to the case of accreting giant planets, assuming that they have magnetic field strength on the order of hundreds Gauss. For the planets around PDS 70, we found that mass accreting into planets is much less than that onto the star, suggesting that giant planet formation, although important in creating gaps in the disk, does not directly starve the star of accreted mass.

Our results provided essential constraints in studying protoplanetary disk evolution, star-disk interactions, and the process of planet formation. Including these constraints in simulations will provide a more complete picture of how young stars, protoplanetary disks, and planets form and evolve.