Infalling–rotating Motion and Associated Chemical Change in the Envelope of Iras 16293–2422 Source a Studied With Alma

Abstract

We have analyzed rotational spectral line emission of OCS, CH 3 OH, HCOOCH 3 , and H 2 CS observed toward the low-mass Class 0 protostellar source IRAS 16293-2422 Source A at a sub-arcsecond resolution (∼0. 6 × 0. 5) with ALMA. Significant chemical differentiation is found at a 50 AU scale. The OCS line is found to well trace the infalling-rotating envelope in this source. On the other hand, the CH 3 OH and HCOOCH 3 distributions are found to be concentrated around the inner part of the infalling-rotating envelope. With a simple ballistic model of the infalling-rotating envelope, the radius of the centrifugal barrier (a half of the centrifugal radius) and the protostellar mass are evaluated from the OCS data to be from 40 to 60 AU and from 0.5 to 1.0 M , respectively, assuming the inclination angle of the envelope/disk structure to be 60 • (90 • for the edge-on configuration). Although the protostellar mass is correlated with the inclination angle, the radius of the centrifugal barrier is not. This is the first indication of the centrifugal barrier of the infalling-rotating envelope in a hot corino source. CH 3 OH and HCOOCH 3 may be liberated from ice mantles due to weak accretion shocks around the centrifugal barrier, and/or due to protostellar heating. The H 2 CS emission seems to come from the disk component inside the centrifugal barrier in addition to the envelope component. The centrifugal barrier plays a central role not only in the formation of a rotationally-supported disk but also in the chemical evolution from the envelope to the protoplanetary disk.

The ALMA-PILS survey: 3D modeling of the envelope, disks and dust filament of IRAS 16293–2422

“…It is also possible that both disks A and B are transition structures, between an early rotating toroid structure and a later Keplerian disk, perhaps similar to the structure suggested by Oya et al (2016).…”

“…Jørgensen et al (2009) detect compact mm continuum emission around several Class 0 objects, but it is unknown whether these dust components represent rotationally supported Keplerian disks. The exact transition from early density structures to a rotationally supported Keplerian disk is still uncertain, though some tentative observations of the socalled centrifugal barrier, where the infalling, rotating envelope transitions into a rotationally supported disk, have been made (Sakai et al 2014;Oya et al 2016). These inner regions around embedded protostars are also particularly interesting from a chemical point of view.…”

“…Pineda et al (2012) resolve the region around source A with a 2.2 × 1.0 beam and interpret a Position-Velocity (PV) diagram of methyl formate (CH 3 OCHO) and ketene (H 2 CCO) as Keplerian rotation around a central object of 0.53 M and a disk inclination close to edge-on, based on the linewidths (though the inclination is not constrained). Oya et al (2016) Oya et al (2016) of 0.5-1.0 M , is dependent on the inclination and centrifugal barrier radius. The mass of 0.53 M reported by Pineda et al (2012) assumes exactly edge-on configuration and has lower spatial resolution than Oya et al (2016), thus arguably interpreting material undergoing a rotating collapse as part of a Keplerian disk.…”

“…Oya et al (2016) Oya et al (2016) of 0.5-1.0 M , is dependent on the inclination and centrifugal barrier radius. The mass of 0.53 M reported by Pineda et al (2012) assumes exactly edge-on configuration and has lower spatial resolution than Oya et al (2016), thus arguably interpreting material undergoing a rotating collapse as part of a Keplerian disk. Also, the analysis of the velocity gradients on 50 -400 AU scales around source A by Favre et al (2014), shows a rotating structure which cannot be explained by simple Keplerian rotation around a point mass but needs to take the enclosed mass into account.…”

“…The peak flux values were degenerate with different Σ 0 , H 0 , L * and i values and the column density toward source B did not offer any constraints either, as almost all of our investigated parameter space satisfied the N H 2 column density condition. Due to previous observations of infall toward both sources (Caux et al 2011;Chandler et al 2005;Pineda et al 2012;Zapata et al 2013;Oya et al 2016), we considered the rotating toroid model. The dust density distribution of this model can be found semi-analytically by defining discrete streamlines from the rotating envelope, which spiral inwards and impact opposing streamlines in the midplane, creating a disk structure.…”