The ALMA-PILS survey: gas dynamics in IRAS 16293−2422 and the connection between its two protostars

Abstract

Context. The majority of stars form in binary or higher order systems. The evolution of each protostar in a multiple system may start at different times and may progress differently. The Class 0 protostellar system IRAS 16293−2422 contains two protostars, "A" and "B", separated by ∼600 au and embedded in a single, 10 4 au scale envelope. Their relative evolutionary stages have been debated. Aims. We aim to study the relation and interplay between the two protostars A and B at spatial scales of 60 au up to ∼10 3 au. Methods. We selected molecular gas line transitions of the species CO, H 2 CO, HCN, CS, SiO, and C 2 H from the ALMA-PILS spectral imaging survey (329-363 GHz) and used them as tracers of kinematics, density, and temperature in the IRAS 16293−2422 system. The angular resolution of the PILS data set allows us to study these quantities at a resolution of 0.5 (60 au at the distance of the source).Results. Line-of-sight velocity maps of both optically thick and optically thin molecular lines reveal: (i) new manifestations of previously known outflows emanating from protostar A; (ii) a kinematically quiescent bridge of dust and gas spanning between the two protostars, with an inferred density between 4×10 4 cm −3 and ∼3×10 7 cm −3 ; and (iii) a separate, straight filament seemingly connected to protostar B seen only in C 2 H, with a flat kinematic signature. Signs of various outflows, all emanating from source A, are evidence of high-density and warmer gas; none of them coincide spatially and kinematically with the bridge. Conclusions. We hypothesize that the bridge arc is a remnant of filamentary substructure in the protostellar envelope material from which protostellar sources A and B have formed. One particular morphological structure appears to be due to outflowing gas impacting the quiescent bridge material. The continuing lack of clear outflow signatures unambiguously associated to protostar B and the vertically extended shape derived for its disk-like structure lead us to conclude that source B may be in an earlier evolutionary stage than source A.

The bridge: a transient phenomenon of forming stellar multiples

“…The two protostars have a projected distance of 705 au indicating formation via turbulent fragmentation such as is seen in our models. The bridge in IRAS 16293 is kinematically quiescent, while the surrounding is kinematically active (Oya et al 2018;van der Wiel et al 2019). This is similar to the bridgelike structure in our model of the forming triple system.…”

“…From an observational point of view, a well-studied example of a young binary system is IRAS 16293-2422 (hereafter IRAS 16293) (Wootten 1989;Mundy et al 1992;Looney et al 2000). The projected distance between the two stars is 705 au (Dzib et al 2018) and both stars are connected via a small, filamentary structure resembling a bridge between sources A A112, page 1 of 13 A&A 628, A112 (2019) and B (Sadavoy et al 2018;van der Wiel et al 2019). Similar arc-and bridge-like structures were also observed around other embedded sources such as IRAS 04191+1523 (Lee et al 2017), SR24 (Fernández-López et al 2017), or L1521F (Tokuda et al 2014).…”

Orbital and Mass Constraints of the Young Binary System IRAS 16293-2422 A

“…Recent ∼100 au resolution water maser observations also show blueshifted emission from A2, moving along the bipolar ejecta direction (Dzib et al 2018). The bipolar ejecta is also aligned with a 0.1 pc scale CO molecular outflow (Mizuno et al 1990;Stark et al 2004), but this molecular outflow has no clear counterpart below ∼700 au (Yeh et al 2008;Girart et al 2014;van der Wiel et al 2019), and the blueshifted lobe is at the opposite side of the blueshifted water maser emission (Dzib et al 2018). Despite the confusion with the molecular outflow, the evidence from the ejecta and the flux coming from thermal dust emission support that the compact structure revealed in the 3 mm observations toward A2 is tracing a dust circumstellar disk with an approximate radius of 12 au (from the FWHM in Table 1).…”

“…On the other hand, Hernández-Gómez et al (2019) recently argued that A1 is the location of a protostar, due to the nearly constant flux of A1 over time, inconsistent with the expectation of a shock feature. Despite this debate, the observation of possibly three outflows powered within A (Mizuno et al 1990;Stark et al 2004;Girart et al 2014;van der Wiel et al 2019) further supported the multiple protostellar nature of source A.…”

Discovery of an M-type companion to the Herbig Ae Star V1787 Ori

“…However, in this section, we discuss all the possible binary formation scenarios and assess which one suits for V1787 Ori. The monolithic collapse of a core and subsequent disc fragmentation was proposed by Wheelwright et al (2011) for the formation of HAeBe binaries. This mechanism is preferred for near-equal mass binaries (Krumholz et al 2009) and this is true for the case of HAeBe binaries as they have high q value of ∼ 0.7.…”