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Register a new userDense Cores, Filaments and Outflows in the S255IR Region of High Mass Star Formation
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Igor I. Zinchenko
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We investigate at a high angular resolution the spatial and kinematic structure of the S255IR high mass star-forming region, which demonstrated recently the first disk-mediated accretion burst in the massive young stellar object. The observations were performed with ALMA in Band 7 at an angular resolution $ sim 0.1^{primeprime}$, which corresponds to $ sim 180 $ AU. The 0.9 mm continuum, C$^{34}$S(7-6) and CCH $N=4-3$ data show a presence of very narrow ($ sim 1000 $ AU), very dense ($nsim 10^7$ cm$^{-3}$) and warm filamentary structures in this area. At least some of them represent apparently dense walls around the high velocity molecular outflow with a wide opening angle from the S255IR-SMA1 core, which is associated with the NIRS3 YSO. This wide-angle outflow surrounds a narrow jet. At the ends of the molecular outflow there are shocks, traced in the SiO(8-7) emission. The SiO abundance there is enhanced by at least 3 orders of magnitude. The CO(3-2) and SiO(8-7) data show a collimated and extended high velocity outflow from another dense core in this area, SMA2. The outflow is bent and consists of a chain of knots, which may indicate periodic ejections possibly arising from a binary system consisting of low or intermediate mass protostars. The C$^{34}$S emission shows evidence of rotation of the parent core. Finally, we detected two new low mass compact cores in this area (designated as SMM1 and SMM2), which may represent prestellar objects.
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We describe the general structure of the well known S255IR high mass star forming region, as revealed by our recent ALMA observations. The data indicate a physical relation of the major clumps SMA1 and SMA2. The driving source of the extended high velocity well collimated bipolar outflow is not the most pronounced disk-like SMA1 clump harboring a 20 M$_odot$ young star (S255 NIRS3), as it was assumed earlier. Apparently it is the less evolved SMA2 clump, which drives the outflow and contains a compact rotating structure (probably a disk). At the same time the SMA1 clump drives another outflow, with a larger opening angle. The molecular line data do not show an outflow from the SMA3 clump (NIRS1), which was suggested by IR studies of this region.
We report the results of our observations of the S255IR area with the SMA at 1.3 mm in the very extended configuration and at 0.8 mm in the compact configuration as well as with the IRAM-30m at 0.8 mm. The best achieved angular resolution is about 0.4 arcsec. The dust continuum emission and several tens of molecular spectral lines are observed. The majority of the lines is detected only towards the S255IR-SMA1 clump, which represents a rotating structure (probably disk) around the young massive star. The achieved angular resolution is still insufficient for conclusions about Keplerian or non-Keplerian character of the rotation. The temperature of the molecular gas reaches 130-180 K. The size of the clump is about 500 AU. The clump is strongly fragmented as follows from the low beam filling factor. The mass of the hot gas is significantly lower than the mass of the central star. A strong DCN emission near the center of the hot core most probably indicates a presence of a relatively cold ($lesssim 80$ K) and rather massive clump there. High velocity emission is observed in the CO line as well as in lines of high density tracers HCN, HCO+, CS and other molecules. The outflow morphology obtained from combination of the SMA and IRAM-30m data is significantly different from that derived from the SMA data alone. The CO emission detected with the SMA traces only one boundary of the outflow. The outflow is most probably driven by jet bow shocks created by episodic ejections from the center. We detected a dense high velocity clump associated apparently with one of the bow shocks. The outflow strongly affects the chemical composition of the surrounding medium.
The present study aims at characterizing the massive star forming region G35.20N, which is found associated with at least one massive outflow and contains multiple dense cores, one of them recently found associated with a Keplerian rotating disk. We used ALMA to observe the G35.20N region in the continuum and line emission at 350 GHz. The observed frequency range covers tracers of dense gas (e.g. H13CO+, C17O), molecular outflows (e.g. SiO), and hot cores (e.g. CH3CN, CH3OH). The ALMA 870 um continuum emission map reveals an elongated dust structure (0.15 pc long and 0.013 pc wide) perpendicular to the large-scale molecular outflow detected in the region, and fragmented into a number of cores with masses 1-10 Msun and sizes 1600 AU. The cores appear regularly spaced with a separation of 0.023 pc. The emission of dense gas tracers such as H13CO+ or C17O is extended and coincident with the dust elongated structure. The three strongest dust cores show emission of complex organic molecules characteristic of hot cores, with temperatures around 200 K, and relative abundances 0.2-2x10^(-8) for CH3CN and 0.6-5x10^(-6) for CH3OH. The two cores with highest mass (cores A and B) show coherent velocity fields, with gradients almost aligned with the dust elongated structure. Those velocity gradients are consistent with Keplerian disks rotating about central masses of 4-18 Msun. Perpendicular to the velocity gradients we have identified a large-scale precessing jet/outflow associated with core B, and hints of an east-west jet/outflow associated with core A. The elongated dust structure in G35.20N is fragmented into a number of dense cores that may form massive stars. Based on the velocity field of the dense gas, the orientation of the magnetic field, and the regularly spaced fragmentation, we interpret this elongated structure as the densest part of a 1D filament fragmenting and forming massive stars.
(Abridged) Context. Core condensation is a critical step in the star-formation process, but is still poorly characterized observationally. Aims. We have studied the 10 pc-long L1495/B213 complex in Taurus to investigate how dense cores have condensed out of the lower-density cloud material. Results. From the N$_2$H$^+$ emission, we identify 19 dense cores, some starless and some protostellar. They are not distributed uniformly, but tend to cluster with relative separations on the order of 0.25 pc. From the C$^{18}$O emission, we identify multiple velocity components in the gas. We have characterized them by fitting gaussians to the spectra, and by studying the distribution of the fits in position-position-velocity space. In this space, the C$^{18}$O components appear as velocity-coherent structures, and we have identified them automatically using a dedicated algorithm (FIVe: Friends In Velocity). Using this algorithm, we have identified 35 filamentary components with typical lengths of 0.5 pc, sonic internal velocity dispersions, and mass-per-unit-length close to the stability threshold of isothermal cylinders at 10 K. Core formation seems to have occurred inside the filamentary components via fragmentation, with a small number of fertile components with larger mass-per-unit-length being responsible for most cores in the cloud. At large scales, the filamentary components appear grouped into families, which we refer to as bundles. Conclusions. Core formation in L1495/B213 has proceeded by hierarchical fragmentation. The cloud fragmented first into several pc-scale regions. Each of these regions later fragmented into velocity-coherent filaments of about 0.5 pc in length. Finally, a small number of these filaments fragmented quasi-statically and produced the individual dense cores we see today.
We found that in regions of high mass star formation the CS emission correlates well with the dust continuum emission and is therefore a good tracer of the total mass while the N$_2$H$^+$ distribution is frequently very different. This is opposite to their typical behavior in low-mass cores where freeze-out plays a crucial role in the chemistry. The behavior of other high density tracers varies from source to source but most of them are closer to CS. Radial density profiles in massive cores are fitted by power laws with indices about -1.6, as derived from the dust continuum emission. The radial temperature dependence on intermediate scales is close to the theoretically expected one for a centrally heated optically thin cloud. The velocity dispersion either remains constant or decreases from the core center to the edge. Several cores including those without known embedded IR sources show signs of infall motions. They can represent the earliest phases of massive protostars. There are implicit arguments in favor of small-scale clumpiness in the cores.
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