An ALMA study of the Orion Integral Filament: I. Evidence for narrow fibers in a massive cloud (1801.01500v1)

Abstract: 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.

An ALMA Study of the Orion Integral Filament: Evidence for Narrow Fibers in a Massive Cloud

The paper of dense gas structures within massive filaments is crucial for understanding the star formation process in the Milky Way. High-mass stars and massive clusters typically form within these filamentary structures, particularly in hub-like associations that exhibit dense networks of gas. To explore the substructure within star-forming regions, the paper leveraged observations from the Atacama Large Millimeter/submillimeter Array (ALMA) alongside prior IRAM 30m observations to paper the Orion Integral Filament (ISF), a prominent feature in the Orion A molecular cloud.

The researchers identified 55 velocity-coherent structures, termed "fibers," characterized by transonic internal motions with lengths of approximately 0.15 pc. The widths of these fibers are notably compact, with a median full-width-half-maximum (FWHM) of 0.035 pc, which is generally narrower than the previously suggested universal 0.1 pc width for filamentary structures. These findings suggest that the radial dimensions of fibers are self-regulated, sensitive to intrinsic gas densities. The ISF fibers are arranged into a dense bundle of substructures and exhibit strong similarities to lower-mass filaments studied in other regions like Musca, Taurus, and Perseus.

Numerical results highlight that almost all identified fibers (98%) show subsonic to transonic non-thermal velocity dispersions, demonstrating a relatively quiescent internal dynamic state. Fiber lengths, mass-per-unit-length, and line-of-sight velocity dispersions are found to be statistically similar across the ISF. Crucially, the mass-per-unit-length values of these fibers lie close to the threshold expected for hydrostatic equilibrium when accounting for the combined thermal and non-thermal support.

The paper proposes that the complex fabric of fibers within massive filaments acts as a scaffold supporting star formation. The alignment of fiber orientation with local gravitational potential suggests that gravitational effects are integral in shaping the internal structure of the ISF. Additionally, the fiber's distribution into hub-like associations resembles structures seen in processes like competitive accretion scenarios for high-mass star formation, where large-scale gravitational collapse influences the local gas dynamics.

This research implies a unifying mechanism governing filamentary structure across different environments, extending the understanding from low-mass filaments to their high-mass counterparts. The systematic correlation observed between fiber surface density and total mass-per-unit-length indicates a potential scaling law akin to that governing core and star formation. In essence, the fiber networks within filaments such as those in Orion may drive the transition from distributed star formation, seen in clouds like Taurus, to concentrated clusters as in Orion, based solely on the density and complexity of their underlying fibrous framework.

Future work could explore whether similar sub-parsec scale fiber structures exist in other high-mass star-forming regions across the galaxy, potentially validating the broader application of the outlined scaling behaviors and density relationships. Such studies could further elucidate the initial conditions impacting star cluster formation and the consequent emergence of high-mass stellar populations.

In conclusion, the findings provide compelling evidence that fibers are fundamental building blocks of star-forming molecular clouds, with their dynamic and structural characteristics playing a pivotal role in star formation across varying mass regimes. The work presents a coherent step towards a unified model of filamentary structure in interstellar environments, significantly enriching our comprehension of star formation processes.