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Register a new userProbing the structure of a massive filament: ArTeMiS 350 and 450 micron mapping of the integral-shaped filament in Orion A
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(abridged) Within the Orion A molecular cloud, the integral-shaped filament (ISF) is a prominent, degree-long structure of dense gas and dust, with clear signs of recent and on-going high-mass star formation. We used the ArTeMiS bolometer camera at APEX to map a 0.6x0.2 deg^2 region covering OMC-1, OMC-2, OMC-3 at 350 and 450 micron. We combined these data with Herschel-SPIRE maps to recover extended emission. The combined Herschel-ArTeMiS maps provide details on the distribution of dense, cold material, with a high spatial dynamic range, from our 8 resolution (0.016 pc) up to the size of the map ~10-15 deg. By combining Herschel and ArTeMiS data at 160, 250, 350 and 450 micron, we constructed high-resolution temperature and H2 column density maps. We extracted radial profiles from the column density map in several, representative portions of the ISF, that we fitted with Gaussian and Plummer models to derive their intrinsic widths. We also compared the distribution of material traced by ArTeMiS with that seen in the higher density tracer N2H+(1-0) recently observed with the ALMA interferometer. All the radial profiles that we extracted show clear deviation from a Gaussian, with evidence for an inner plateau, previously not seen using Herschel-only data. We measure intrinsic half-power widths in the range 0.06 to 0.11 pc. This is significantly larger than the Gaussian widths measured for fibers seen in N2H+, which probably traces only the dense innermost regions of the large-scale filament. These half-power widths are within a factor of two of the value of 0.1 pc found for a large sample of nearby filaments in various low-mass star-forming regions, which tends to indicate that the physical conditions governing the fragmentation of prestellar cores within transcritical or supercritical filaments are the same over a large range of masses per unit length.
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We study the fragmentation of the nearest high line-mass filament, the integral shaped filament (ISF, line-mass $sim$ 400 M$_odot$ pc$^{-1}$) in the Orion A molecular cloud. We have observed a 1.6 pc long section of the ISF with the Atacama Large Millimetre/submillimeter Array (ALMA) at 3 mm continuum emission, at a resolution of $sim$3 (1 200 AU). We identify from the region 43 dense cores with masses about a solar mass. 60% of the ALMA cores are protostellar and 40% are starless. The nearest neighbour separations of the cores do not show a preferred fragmentation scale; the frequency of short separations increases down to 1 200 AU. We apply a two-point correlation analysis on the dense core separations and show that the ALMA cores are significantly grouped at separations below $sim$17 000 AU and strongly grouped below $sim$6 000 AU. The protostellar and starless cores are grouped differently: only the starless cores group strongly below $sim$6 000 AU. In addition, the spatial distribution of the cores indicates periodic grouping of the cores into groups of $sim$30 000 AU in size, separated by $sim$50 000 AU. The groups coincide with dust column density peaks detected by Herschel. These results show hierarchical, two-mode fragmentation in which the maternal filament periodically fragments into groups of dense cores. Critically, our results indicate that the fragmentation models for lower line-mass filaments ($sim$ 16 M$_odot$ pc$^{-1}$) fail to capture the observed properties of the ISF. We also find that the protostars identified with Spitzer and Herschel in the ISF are grouped at separations below $sim$17 000 AU. In contrast, young stars with disks do not show significant grouping. This suggests that the grouping of dense cores is partially retained over the protostar lifetime, but not over the lifetime of stars with disks.
The Integral Shaped Filament (ISF) is the nearest molecular cloud with rapid star formation, including massive stars, and it is therefore a star-formation laboratory. We use Gaia parallaxes, to show that the distances to young Class II stars (disks) projected along the spine of this filament are related to the gas radial velocity by $$ v = -{Dovertau} + K;qquad tau = 4,{rm Myr}, $$ where $K$ is a constant. This implies that the ISF is a standing wave, which is consistent with the Stutz & Gould (2016) Slingshot prediction. The $tau=4,{rm Myr}$ timescale is consistent with the Slingshot picture that the Orion Nebula Cluster (ONC) is the third cluster to be violently split off from the Orion A cloud (following NGC 1981 and NGC 1987) at few-Myr intervals due to gravito-magnetic oscillations. We also present preliminary evidence that the truncation of the ISF is now taking place $16^prime$ south of the ONC and is mediated by a torsional wave that is propagating south with a characteristic timescale $tau_{rm torsion} = 0.5,{rm Myr}$, i.e. eight times shorter. The relation between these two wave phenomena is not presently understood.
Abridged. Are all filaments bundles of fibers? To address this question, we have investigated the gas organization within the paradigmatic Integral Shape Filament (ISF). We combined two new ALMA Cycle 3 mosaics with previous IRAM 30m observations to produce a high-dynamic range N$_2$H$^+$(1-0) emission map of the ISF tracing its high-density material and velocity structure down to scales of 0.009 pc. From the analysis of the gas kinematics, we identify a total of 55 dense fibers in the central region of the ISF. Independently of their location, these fibers are characterized by transonic internal motions, lengths of ~0.15 pc, and masses per-unit-length close to those expected in hydrostatic equilibrium. The ISF fibers are spatially organized forming a dense bundle with multiple hub-like associations likely shaped by the local gravitational potential. Within this complex network, the ISF fibers show a compact radial emission profile with a median FWHM of 0.035 pc systematically narrower than the previously proposed universal 0.1 pc filament width. Our ALMA observations reveal complex bundles of fibers in the ISF, suggesting strong similarities between the internal substructure of this massive filament and previously studied lower-mass objects. The fibers show identical dynamic properties in both low- and high-mass regions, and their widespread detection suggests a preferred organizational mechanism of gas in which the physical fiber dimensions (width and length) are self-regulated depending on their intrinsic gas density. Combined with previous works, we identify a systematic increase of the surface density of fibers as a function of the total mass per-unit-length in filamentary clouds. Based on this empirical correlation, we propose a unified star-formation scenario where the observed differences between low- and high-mass clouds emerge naturally from the initial concentration of fibers.
We studied the filament structures and dense cores in OMC-2,3 region in Orion A North molecular cloud using the high-resolution N2H+ (1-0) spectral cube observed with the Atacama Large Millimeter/Submillimeter Array (ALMA). The filament network over a total length of 2 pc is found to contain 170 intersections and 128 candidate dense cores. The dense cores are all displaced from the infrared point sources (possible young stars), and the major fraction of cores (103) are located around the intersections. Towards the intersections, there is also an increasing trend for the total column density Ntot as well as the the power-law index of the column-density Probability Distribution Function (N-PDF), suggesting that the intersections would in general have more significant gas assembly than the other part of the filament paths. The virial analysis shows that the dense cores mostly have virial mass ratio of alpha_vir=M_vir/M_gas<1.0, suggesting them to be bounded by the self gravity. In the mean time, only about 23 percent of the cores have critical mass ratio of alpha_crit=M_crit/M_gas<1.0, suggesting them to be unstable against core collapse. Combining these results, it shows that the major fraction of the cold starless and possible prestellar cores in OMC-2,3 are being assembled around the intersections, and currently in a gravitationally bound state. But more extensive core collapse and star formation may still require continuous core-mass growth or other perturbatio
We determine the magnetic field strength in the OMC 1 region of the Orion A filament via a new implementation of the Chandrasekhar-Fermi method using observations performed as part of the James Clerk Maxwell Telescope (JCMT) B-Fields In Star-Forming Region Observations (BISTRO) survey with the POL-2 instrument. We combine BISTRO data with archival SCUBA-2 and HARP observations to find a plane-of-sky magnetic field strength in OMC 1 of $B_{rm pos}=6.6pm4.7$ mG, where $delta B_{rm pos}=4.7$ mG represents a predominantly systematic uncertainty. We develop a new method for measuring angular dispersion, analogous to unsharp masking. We find a magnetic energy density of $sim1.7times 10^{-7}$ Jm$^{-3}$ in OMC 1, comparable both to the gravitational potential energy density of OMC 1 ($sim 10^{-7}$ Jm$^{-3}$), and to the energy density in the Orion BN/KL outflow ($sim 10^{-7}$ Jm$^{-3}$). We find that neither the Alfv{e}n velocity in OMC 1 nor the velocity of the super-Alfv{e}nic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the $sim$500-year lifetime of the outflow. Hence, we propose that the hour-glass field morphology in OMC 1 is caused by the distortion of a primordial cylindrically-symmetric magnetic field by the gravitational fragmentation of the filament and/or the gravitational interaction of the BN/KL and S clumps. We find that OMC 1 is currently in or near magnetically-supported equilibrium, and that the current large-scale morphology of the BN/KL outflow is regulated by the geometry of the magnetic field in OMC 1, and not vice versa.
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