Dense molecular cocoons in the massive protocluster W3 IRS5: a test case for models of massive star formation

Abstract

Context. Two competing models describe the formation of massive stars in objects like the Orion Trapezium. In the turbulent core accretion model, the resulting stellar masses are directly related to the mass distribution of the cloud condensations. In the competitive accretion model, the gravitational potential of the protocluster captures gas from the surrounding cloud for which the individual cluster members compete. Aims. With high resolution submillimeter observations of the structure, kinematics, and chemistry of the proto-Trapezium cluster W3 IRS5, we aim to determine which mode of star formation dominates. Methods. We present 354 GHz Submillimeter Array observations at resolutions of 1 -3 (1800-5400 AU) of W3 IRS5. The dust continuum traces the compact source structure and masses of the individual cores, while molecular lines of CS, SO, SO 2 , HCN, H 2 CS, HNCO, and CH 3 OH (and isotopologues) reveal the gas kinematics, density, and temperature. Results. The observations show five emission peaks (SMM1-5). SMM1 and SMM2 contain massive embedded stars (∼20 M ); SMM3-5 are starless or contain low-mass stars (<8 M ). The inferred densities are high, ≥10 7 cm −3 , but the core masses are small, 0.2−0.6 M . The detected molecular emission reveals four different chemical zones. Abundant (X ∼ few 10 −7 to 10 −6 ) SO and SO 2 are associated with SMM1 and SMM2, indicating active sulfur chemistry. A low abundance (5 × 10 −8 ) of CH 3 OH concentrated on SMM3/4 suggest the presence of a hot core that is only just turning on, possibly by external feedback from SMM1/2. The gas kinematics are complex with contributions from a near pole-on outflow traced by CS, SO, and HCN; rotation in SO 2 , and a jet in vibrationally excited HCN. Conclusions. The proto-Trapezium cluster W3 IRS5 is an ideal test case to discriminate between models of massive star formation. Either the massive stars accrete locally from their local cores; in this case the small core masses imply that W3 IRS5 is at the very end stages (1000 yr) of infall and accretion, or the stars are accreting from the global collapse of a massive, cluster forming core. We find that the observed masses, densities and line widths observed toward W3 IRS 5 and the surrounding cluster forming core are consistent with the competitive accretion of gas at rates ofṀ ∼10 −4 M yr −1 by the massive young forming stars. Future mapping of the gas kinematics from large to small scales will determine whether large-scale gas inflow occurs and how the cluster members compete to accrete this material.

Complex molecule formation around massive young stellar objects

“…Two of the sources, NGC 7538 IRS1 HC and W3 IRS5 c2, have been observed previously by others 24,32 . In the case of NGC 7538 IRS1 HC, the derived CH 3 OH, CH 3 CN and CH 3 OCH 3 columns are consistent with previous observations, while the CH 3 CHO and CH 3 CCH columns are at least an order of magnitude higher in our study.…”

“…NGC7538 IRS9 and NGC7538 IRS1 are located in Perseus, and NGC7538 IRS1 is known to house a bright hot core 19 and is situated in the middle of a cluster of continuum peaks and molecular outflows. W3 IRS5 is associated with at least five young stellar objects, two of which are massive [20][21][22][23][24] , and is known to present strong S-bearing molecular lines 25 . In summary all three sources are known to have complicated structures with many different potential origins of complex molecular emission.…”

“…both studies found 'c1' and 'c2' to be defined by SO 2 and HNCO, and by CH 3 OH, respectively. Wang et al (2013) classified the 'c1' peak as a pole-on outflow and the 'c2' peak as a budding hot core. Both studies also found CS and H 2 CS in the NW region, but the discovery of CH 3 CCH is new, and so is the realization that the ridge connecting 'c1' and 'c2' seems to be chemically distinct, and the source of N-bearing COMs.…”

A necklace of dense cores in the high-mass star forming region G35.20−0.74 N: ALMA observations

“…However, from Table 4, one sees that the values of the core masses are typically small, so one may wonder whether such a limited amount of gas is consistent with the formation of a massive star. We note that, as a matter of fact, there are quite a few B-type protostars associated with cores of a few M , such as IRAS 20126+4104 Beltrán et al 2011), W3 IRS5 (Wang et al 2013) and G35.03+0.35 . It is possible that cores that "light" are only the remnants of the original envelopes enshrouding the protostars, while most of the material has already been accreted and/or dispersed.…”

Chemical segregation in hot cores with disk candidates

“…We used the CH 3 CN line ratios for these sources as this species is a known tracer (Wang et al 2013) We find that B2 is the coolest region with an average T kin of 120 K and a range from 90-170 K. Next hottest is B1 with an average T kin of 160 K and a range from 120-220 K. G35.20 A is significantly hotter than these with an average T kin of 285 K and a range from 150-450 K. B3 is consistently the hottest, ranging from 175-490 K with an average T kin of 300 K. The kinetic temperatures in G35.03 are also very high, ranging from 100-450 K with an average T kin of 275 K.…”