Multiplicity and disks within the high-mass core NGC 7538IRS1

Abstract

Context. High-mass stars have a high degree of multiplicity and most likely form via disk accretion processes. The detailed physics of the binary and disk formation are still poorly constrained. Aims. We seek to resolve the central substructures of the prototypical high-mass star-forming region NGC7538IRS1 at the highest possible spatial resolution line and continuum emission to investigate the protostellar environment and kinematics. Methods. Using the Karl G. Jansky Very Large Array (VLA) in its most extended configuration at ∼24 GHz has allowed us to study the NH 3 and thermal CH 3 OH emission and absorption as well as the cm continuum emission at an unprecedented spatial resolution of 0.06 × 0.05 , corresponding to a linear resolution of ∼150 AU at a distance of 2.7 kpc. Results. A comparison of these new cm continuum data with previous VLA observations from 23 yrs ago reveals no recognizable proper motions. If the emission were caused by a protostellar jet, proper motion signatures should have been easily identified. In combination with the high spectral indices S ∝ ν α (α between 1 and 2), this allows us to conclude that the continuum emission is from two hypercompact Hii regions separated in projection by about 430 AU. The NH 3 spectral line data reveal a common rotating envelope indicating a bound high-mass binary system. In addition to this, the thermal CH 3 OH data show two separate velocity gradients across the two hypercompact Hii regions. This indicates two disk-like structures within the same rotating circumbinary envelope. Disk and envelope structures are inclined by ∼33• , which can be explained by initially varying angular momentum distributions within the natal, turbulent cloud. Conclusions. Studying high-mass star formation at sub-0.1 resolution allows us to isolate multiple sources as well as to separate circumbinary from disk-like rotating structures. These data show also the limitations in molecular line studies in investigating the disk kinematics when the central source is already ionizing a hypercompact Hii region. Recombination line studies will be required for sources such as NGC7538IRS1 to investigate the gas kinematics even closer to the protostars.

Long-term Variability of H2CO Masers in Star-forming Regions

“…The formaldehyde masers could be located inside of (or close to) a possible circumbinary envelope that has been proposed by Goddi et al (2015). While the H 2 CO maser Component II seems to reside approximately between two 6.7 GHz methanol maser clusters (A and C, Figure 1; see also Moscadelli & Goddi 2014), Component I is very close (in projection) to the northern 6.7 GHz maser cluster A, which shows small-scale velocity gradients in thermal molecular lines from NH 3 and CH 3 OH (Beuther et al 2017).…”

The first interferometric survey of massive YSOs in the K-band

“…It is mostly because of high angular resolution and infrared interferometry that we are now finally in a position to start revealing and exploring these relatively rare objects in more detail. On the large scales (300-2000 au), millimeter line observations (ALMA, SMA, VLA) reveal Keplerian-like disc structures in cold (T∼50 K) material (Ilee et al 2016(Ilee et al , 2018aJohnston et al 2015Johnston et al , 2020, while substructures down to ∼45 au were recently traced by ALMA (e.g., Beuther et al 2017;Maud et al 2019). At smaller scales (down to a few au), where the accretion onto the star takes place, the disc is traced using near and mid-IR (hot/warm) emission (e.g., Boley et al 2013).…”

“…Direct detections of high-mass binaries covering tighter separations (< 500 au) were obtained serendipitously. In particular, only a handful of such protobinary systems are known, with ranging separations of several hundreds of au (PDS 27: 30 au, PDS 37: 48 au, V921 Sco: 45 au, NGC 7538 IRS1: 430 au, IRAS 17216-3801: 170 au, IRAS 07299-1651: 180 au; Koumpia et al 2019;Kraus et al 2012;Beuther et al 2017;Kraus et al 2017;Zhang et al 2019).…”

A pilot survey of the binarity of Massive Young Stellar Objects withK-band adaptive optics

“…They found that two of their targets showed velocity differences of 90 km/s between the two different epochs, interpreted as being caused by close binary companions. Beuther et al (2017) studied the massive protostar/UCHii region NGC 7538 IRS1 using JVLA data, discovered a binary source at 430 au, and found (misaligned) disks surrounding both objects. Kraus et al (2017) discovered a companion at 58 mas (170 au) from the 20M protostar IRAS 17216-3801.…”