Poster Abstract
Protoplanetary disks are important objects for planet formation. However, their formation process is still unclear. Disks form around protostars as mass and angular momentum are transferred from cores. Hence, revealing the gas kinematics around protostars over wide-spatial scales from a disk (~100 au) to a core (~10,000 au) is essential to understand the disk formation process. We have conducted mapping observations of the protostar L1489 IRS with Atacama Compact Array (ACA or Morita array) and IRAM 30-meter telescope in the C18O 2–1 emission to investigate the kinematics on 1,000–10,000 au scales in addition to our previous ALMA observations, which have revealed the kinematics on 100–1,000 au scales. The C18O 2–1 emission shows an intensity peak at the protostellar position, a velocity gradient in the same direction as that of the disk rotation, and the second intensity peak on the northeast side of the protostar. We have measured rotational velocity as a function of radius on the southwest side on a position-velocity diagram cut along the velocity gradient. A double power-law function is fitted to the measured rotational profile: the power-law index is almost -1 at r<2,800 au, which can be interpreted as rotation of an envelope conserving angular momentum, whereas the velocity slightly increases with radius at r>2,800 au. This suggests a kinematical transition from an infalling envelope to a core. We have also constructed kinematic models of an envelope and compared them to observations. We found that an envelope model with infalling velocity slower than the free-fall velocity by a factor of 0.4 explained observations better than a model with the free-fall velocity. Radial profile of specific angular momentum is calculated from the measured rotational profile. Possible scenario of angular momentum transfer is discussed.
Plain text summary
Page 1 is for title and abstract. I describe background and our observations on page 2. Angular momentum measured in various sources and on various spatial scales suggests that protostars are surrounded by three different kinematic structures: a disk rotating at Keplerian velocity on ~100 au scale, an infalling envelope conserving angular momentum on a scale of ~1,000 au, and a core rotating like a rigid-body or being turbulent on ~10,000 au scale. Confirming this picture in individual sources is required to understand how material is brought from cores to disks. Recent works probing the kinematics on 100–1,000 au scales have revealed the transition from a disk to an envelope in tens of protostellar systems. However, the kinematics on 1,000–10,000 au scales and the transition from an envelope to a core are still unclear. L1489 IRS is a Class I protostar in the Taurus molecular cloud (d~140 pc). We have measured rotational velocity of gas at radii less than 1,000 au from ALMA observations and identified a Keplerian disk and rotation of an envelope conserving angular momentum in a previous work. As a next step, we aim to measure the rotational and infalling velocity on 1,000–10,000 au scales and to reveal a kinematical transition from an infalling envelope and a core in this work. We have conducted mapping observations covering 2’x2’ region around this source with Atacama Compact Array (ACA) and IRAM 30-meter telescope in C18O 2–1. Obtained data were combined using CASA with a task feather and then analyzed. The angular resolution of the combined image is 7.7’’x6.4’’ (-85 deg). I show maps of the combined data and a radial profile of the rotational velocity measured from the maps on page 3. The C18O 2–1 emission shows an intensity peak at the protostellar position, and a velocity gradient in the same direction as that of the disk rotation from northeast to southwest. The emission also exhibits the second intensity peak at a position ~40’’ northeast of the protostellar position. Rotational velocity was measured on the southwest side on a PV diagram cut along the velocity gradient through fitting a Gaussian function to a spectrum at each radius. A radial profile of the measured rotational velocity shows a break at radius of 2,800 au. We fitted a double power-law function: the power-law index of the best fit function is almost -1 inside the radius of 2,800 au, suggesting rotation of an envelope conserving angular momentum. On the other hand, the power-law index is about 0.21 outside the radius. These results suggest a kinematical transition from an infalling envelope to a core. We discuss infalling velocity and angular momentum transfer on page 4. We have constructed kinematical models of a disk and an envelope to characterize the infalling velocity. We introduced a parameter alpha ranging from 0 to 1: radial velocity is multiplied by alpha to examine a case of slower infall than the free-fall. From comparison between models and observations on the southwest side on the PV diagram, we found that an envelope model with α=0.4 reproduces observations better than a model with α=1. A radial profile of specific angular momentum calculated from the measured rotational profile is compared to a simple model calculation. We calculated angular momentum transfer based on the inside-out collapse model (Shu 1977) assuming that specific angular momentum in an initial core proportions to r^1.6. Our calculation suggests that large initial angular momentum is prefered to explain our observations with the Inside-out collapse model. Expected specific angular momentum profile at the age of 1.5x10^5 yr shows good agreement with observations.
Poster Title
A Kinematical Transition from an Infalling Envelope to a Core around the Protostar L1489 IRS
