Chasing discs around O-type (proto)stars: Evidence from ALMA observations

Abstract

Context. Circumstellar discs around massive stars could mediate the accretion onto the star from the infalling envelope, and could minimize the effects of radiation pressure. Despite such a crucial role, only a few convincing candidates have been provided for discs around deeply embedded O-type (proto)stars. Aims. In order to establish whether disc-mediated accretion is the formation mechanism for the most massive stars, we have searched for circumstellar, rotating discs around a limited sample of six luminous (>10 5 L ) young stellar objects. These objects were selected on the basis of their IR and radio properties in order to maximize the likelihood of association with disc+jet systems. Methods. We used ALMA with ∼0 . 2 resolution to observe a large number of molecular lines typical of hot molecular cores. In this paper we limit our analysis to two disc tracers (methyl cyanide, CH 3 CN, and its isotopologue, 13 CH 3 CN), and an outflow tracer (silicon monoxide, SiO). Results. We reveal many cores, although their number depends dramatically on the target. We focus on the cores that present prominent molecular line emission. In six of these a velocity gradient is seen across the core, three of which show evidence of Keplerian-like rotation. The SiO data reveal clear but poorly collimated bipolar outflow signatures towards two objects only. This can be explained if real jets are rare (perhaps short-lived) in very massive objects and/or if stellar multiplicity significantly affects the outflow structure. For all cores with velocity gradients, the velocity field is analysed through position-velocity plots to establish whether the gas is undergoing rotation with rot ∝ R −α , as expected for Keplerian-like discs. Conclusions. Our results suggest that in three objects we are observing rotation in circumstellar discs, with three more tentative cases, and one core where no evidence for rotation is found. In all cases but one, we find that the gas mass is less than the mass of any embedded O-type star, consistent with the (putative) discs undergoing Keplerian-like rotation. With the caveat of low number statistics, we conclude that the disc detection rate could be sensitive to the evolutionary stage of the young stellar object. In young, deeply embedded sources, the evidence for discs could be weak because of confusion with the surrounding envelope, while in the most evolved sources the molecular component of the disc could have already been dispersed. Only in those objects that are at an intermediate stage of the evolution would the molecular disc be sufficiently prominent and relatively less embedded to be detectable by mm/submm observations.

Exploring the formation pathways of formamide

“…We observed three high-mass star forming regions with a high luminosity (L bol = 1 − 2 × 10 5 L ). Our sources (shown in Figure 1) are a subset of the sample studied and presented in Cesaroni et al (2017) selected for their potential as O-type (proto)stars harboring circumstellar disks. G17.64+0.16 (hereafter G17, also known as AFGL 2136 and IRAS 18196-1331), shown in the top panel of Figure 1, is located at a distance of 2.2 kpc, has a bolometric luminosity of 1 ×10 5 L and has been well studied from the infrared to the radio.…”

“…G345 has a continuum source associated with strong H30α emission (G345 Main) and a chemically rich region to the northwest of this continuum source (G345 NW spur) (Johnston et al in prep). The other three sources from the dataset described in Cesaroni et al (2017) could not be used in these investigations because the formamide lines were strongly blended with other species.…”

“…The bandwidths for all spectral windows are <2 GHz with the largest being 1.8 GHz. For full details on observations and continuum subtraction see Cesaroni et al (2017). The continuum intensity for subsources showing H30α emission were corrected for free-free emission with direct measurements for G17 from Maud et al (2018), G24 A1 from Moscadelli et al (2018), and calculated for G345 using the spectral index fit from Guzmán et al (2016).…”

Photoprocessing of formamide ice: route towards prebiotic chemistry in space

“…(Min et al 2005), the first comet where formamide was detected. Moreover, dust has been observed in circumstellar envelopes around young stars (Cesaroni et al 2017) and in evolved stars and planetary nebulae (Jäger et al 1998). Among silicates, we chose antigorite (Mg,Fe ++ ) 3 Si 2 O 5 (OH) 4 and Forsterite Mg 2 SiO 4 .…”

The Formation of High-Mass Stars: from High-Mass Clumps to Accretion Discs and Molecular Outflows

“…Despite that, there is still a lack of detections of discs around O-type objects (L bol 10 5 L ⊙ ). Accordingly to Cesaroni et al (2017), discs might be detectable towards high-mass protostars only during the intermediate stages of the pre-Main Sequence (PMS). Such hypothesis arises from the fact that evidence for actively accreting discs might be weak due to confusion with the surrounding envelope at the youngest stages of the high-mass star-forming (HMSF) process.…”