Infalling-Rotating Envelopes and Disks around Low-Mass Protostars Yoko Oya U Tokyo, Tokyo, JP
One of the important frontiers in star-formation studies is to understand when and how rotationally-supported disks are formed around young low-mass protostars. This is essentially a problem of conservation and dissipation of the angular momentum of the infalling gas, and is possibly related to the outflow phenomenon. We here report high-spatial-resolution observations toward a few protostars with ALMA to tackle with this problem. L1527 (d = 137 pc) is a low-mass Class 0-I source, which is known as a typical warm-carbon-chain chemistry (WCCC) source characterized by rich carbon-chain molecules. In this source, an infalling-rotating envelope (IRE) is selectively traced by CCH, c-C3H2, and CS, which is beautifully reproduced by the simple ballistic model (Figure a), as shown in Figure (c). The centrifugal barrier of the IRE is thus clearly identified at the radius of 100 AU, which is defined as the perihelion radius in the ballistic model (a half of the centrifugal radius). We also observed the evolved WCCC source, TMC-1A, and found that the above feature of the IRE is seen in this Class I source. Outflow components are also identified in L1527 and the other WCCC source IRAS 15398-3359, whose kinematic structures are essentially explained with a parabolic model. In addition to the WCCC sources, we also analyzed the ALMA archival data toward IRAS 16293-2422 Source A (d = 120 pc), which is known as a typical hot corino source, characterized by rich complex organic molecules (COMs). In this source, OCS is found to trace the IRE, which can also be explained by the simple ballistic model. Although the IREs are traced by different molecular species in the WCCC and hot corino sources, their kinematic structures can be explained by the same IRE model. The analysis of the IRE is of particular importance, because it gives us estimates of the protostellar mass and the specific angular momentum of the IRE. Moreover, the H2CO lines toward L1527 and IRAS 15398-3359 as well as the H2CS lines toward IRAS 16293-2422 Source A have high-velocity components concentrated to the protostar (Figure e), suggesting the existence of rotationally-supported disks inward of the centrifugal barrier. The kinematic structure of H2CS inward of the centrifugal barrier in IRAS 16293-2422 is well explained by the Keplerian model with the protostellar mass (0.75 Msol) derived from the above analysis (Figure f). These results provide us an important clue to understanding physical structures of the transition zone from the IRE to the rotationally-supported disk. It is generally thought that the disk radius is close to the centrifugal radius. However, the high-angular-resolution observations reveals that this expectation is too simplified. The infalling gas seems to keep infalling to the centrifugal barrier to some extent in the above sources. Such a feature of the transition zone may be related to launching mechanisms of molecular outflows through the angular momentum of the infalling gas.
Caption: (a) Schematic illustration of the gas components in a protostellar core. The kinematic structure in the inner envelope component, which is assumed to be cylindrical around the protostar, is analyzed with our infalling-rotating envelope model. (b) Integrated intensity maps of CCH (color) and c-C3H2 (black contours) toward L1527. The area surrounded by a dashed red ellipse corresponds to the inner envelope component in panel (a). (c) Position-velocity diagrams of the two hyperfine components of CCH (color) toward L1527 and the result of the infalling-rotating envelope model (blue contours). The position axis is shown by a blue arrow in panel (b). Blue contours are every 20% from 5% of the peak intensity. (d) Integrated intensity map of OCS (color) and the continuum map (white contours) toward IRAS 16293-2422. Contour levels for the continuum are 10, 20, 40, 80, 160, and 320σ, where σ = 2 mJy beam-1. (e) Position-velocity diagrams of OCS (color) and H2CS (black contours) toward IRAS 16293-2422 Source A. The position axis is shown by a white arrow in panel (d). Black contours are every 20σ from 10σ where σ= 2.0 mJy beam-1. (f) Position-velocity diagrams of H2CS (color) and the results of the infalling-rotating envelope model (blue contours) and the Keplerian model inward of the centrifugal barrier (black contours). The position axis is the same as that in panel (e). Contours for the two models are every 20% from 5% of the peak intensity in the each model.
Collaborators: N. Sakai, IPCR, JP A. Lopez-Sepulcre, U Tokyo, JP Y. Watanabe, U Tokyo, JP B. Lefloch, IPAG, FR C. Ceccarelli, IPAG, FR S. Yamamoto, U Tokyo, JP	Key publication Suggested Session: Disk Formation