The high-mass disk candidates NGC 7538IRS1 and NGC 7538S

Abstract

Context. The nature of embedded accretion disks around forming high-mass stars is one of the missing puzzle pieces for a general understanding of the formation of the most massive and luminous stars. Aims. We want to dissect the small-scale structure of the dust continuum and kinematic gas emission toward two of the most prominent high-mass disk candidates. Methods. Using the Plateau de Bure Interferometer at ∼1.36 mm wavelengths in its most extended configuration we probe the dust and gas emission at ∼0.3 , corresponding to linear resolution elements of ∼800 AU.Results. Even at that high spatial resolution NGC 7538IRS1 remains a single compact and massive gas core with extraordinarily high column densities, corresponding to visual extinctions on the order of 10 5 mag, and average densities within the central 2000 AU of ∼2.1 × 10 9 cm −3 that have not been measured before. We identify a velocity gradient across in northeast-southwest direction that is consistent with the mid-infrared emission, but we do not find a gradient that corresponds to the proposed CH 3 OH maser disk. The spectral line data toward NGC 7538IRS1 reveal strong blue-and red-shifted absorption toward the mm continuum peak position. While the blue-shifted absorption is consistent with an outflow along the line of sight, the red-shifted absorption allows us to estimate high infall rates on the order of 10 −2 M yr −1 . Although we cannot prove that the gas will be accreted in the end, the data are consistent with ongoing star formation activity in a scaled-up low-mass star formation scenario. Compared to that, NGC 7538S fragments in a hierarchical fashion into several sub-sources. While the kinematics of the main mm peak are dominated by the accompanying jet, we find rotational signatures from a secondary peak. Furthermore, strong spectral line differences exist between the sub-sources which is indicative of different evolutionary stages within the same large-scale gas clump. Conclusions. NGC 7538IRS1 is one of the most extreme high-mass disk candidates known today. The large concentration of mass into a small area combined with the high infall rates are unusual and likely allow continued accretion. While the absorption is interesting for the infall studies, higher-excited lines that do not suffer from the absorption are needed to better study the disk kinematics. In contrast to that, NGC 7538S appears as a more typical high-mass star formation region that fragments into several sources. Many of them will form low-to intermediate-mass stars. The strongest mm continuum peak is likely capable to form a high-mass star, however, likely of lower mass than NGC 7538IRS1.

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

“…While the naturally weighted NH 3 (1,1) shows a bit more extended emission, one sees a larger scale velocity gradient approximately in eastwest direction. In comparison to that, the higher resolution (robust weighting 0) image rather reveals a velocity gradient in northeast-southwest direction, which is consistent with previous the findings by Beuther et al (2012Beuther et al ( , 2013, Moscadelli & Goddi (2014), and Goddi et al (2015). Since we are mainly interested in the kinematics of the innermost central sources we concentrate in the following on the higher resolution data.…”

“…Peter et al (2012) find companion separations between 400 and several thousand AU, stressing the necessity of high spatial resolution, Furthermore, Sana et al (2012) infer that multiple system interactions dominate the evolution of massive stars. Interferometer studies have revealed that most high-mass starforming regions fragment into multiple objects, suggesting that massive monolithic cores larger than several 1000 AU are rare, however, the degree of fragmentation varies (e.g., Cesaroni et al 2005;Beuther et al 2007bBeuther et al , 2012Zhang et al 2009;Bontemps et al 2010;Wang et al 2011;Rodón et al 2012;Palau et al 2013;Sánchez-Monge et al 2014;Johnston et al 2015). Even regions that remain single continuum sources down to arcsec resolution, mostly fragment on even smaller scales.…”

“…At a distance of ∼2.7 kpc (Moscadelli et al, 2009), the luminosity of the central energy source is estimated to stem from a 30 M O6 star (e.g., Willner 1976;Gaume et al 1995a;Moscadelli et al 2009). The strong emission of this source from near-to mid-infrared wavelengths to the cm-and mm-regime has made it a famous high-mass protostar for over several decades; a summary of the literature can be found in Beuther et al (2012). The recent 0.2 observations at submm wavelengths with the Northern Extended Millimeter Array (NOEMA) revealed fragmentation of the envelope, however, these observations did not allow us to identify a Keplerian accretion disk (Beuther et al 2013, see also high-resolution data by Zhu et al 2013).…”

Fragmentation, infall, and outflow around the showcase massive protostar NGC 7538 IRS1 at 500 AU resolution

“…For the first time, therefore, time we resolve the structures where not only the massive dense cores but also the proposed embedded high-mass disks are expected to fragment (e.g., Krumholz et al 2007a). While the lower resolution 1.36 mm image in Beuther et al (2012a) consisted mainly of an unresolved point source with only very little extended emission at low intensities, the new 843 μm data now reveal substructure within the inner ∼3000 AU of the region. In addition to spatially resolving the central peak into an elongated structure with a subpeak about 0.3 (∼900 AU) north of the main peak ( Fig.…”

“…The dashed contour shows the same negative levels. The thick full line presents the 4σ contour (1σ ∼ 29 mJy beam −1 ) from the 1.36 mm data presented in Beuther et al (2012a). Several potential disk and outflow axes reported in the literature are presented (Davis et al 1998;De Buizer & Minier 2005;Sandell et al 2009;Pestalozzi et al 2004Pestalozzi et al , 2009Sandell & Wright 2010, see Introduction for more details).…”