Abstract

Filamentary structures are closely associated with star-forming cores, but their detailed physical connections are still not clear. We studied the dense gas in OMC-3 MMS-7 region in the Orion A molecular cloud using the molecular lines observed with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Submillimeter Array (SMA). The ALMA N2H+ (1–0) emission has revealed three dense filaments intersected at the centre, coincident with the central core MMS-7, which has a mass of 3.6 M☉. The filaments and cores are embedded in a parental clump with a total mass of 29 M☉. The N2H+ velocity field exhibits a noticeable increasing trend along the filaments towards the central core MMS-7 with a scale of v − vlsr ≃ 1.5 km s−1 over a spatial range of -20 arcsec (8 × 103 au), corresponding to a gradient of 40 km s−1 pc−1. This feature is most likely to indicate an infall motion towards the centre. The derived infall rate (8 × 10−5 M☉ yr−1) and time-scale (3.6 × 105 yr) are much lower than that for a spherical free-fall collapse and more consistent with the contraction of the filament structures. The filaments also exhibit a possible fragmentation. But this does not seem to largely interrupt the gas structure or its contraction towards the centre. Thus, MMS-7 provides an example of filamentary inward motion directly towards a pre-stellar core. The filament contraction could be less intense but more steady than global spherical collapse, and may help generate an intermediate-mass or even high-mass star.