Convergent Filaments Contracting Towards an Intermediate-mass Prestellar
  Core
  Filamentary structures are closely associated with star-forming cores, but
their detailed physical connections are still not clear. We studied the dense
gas in the region of OMC-3 MMS-7 in Orion A molecular cloud using the molecular
lines observed with the Atacama Large Millimeter/submillimeter Array (ALMA) and
the Submillimeter Array (SMA). The ALMA N$_2$H$^+$ (1-0) emission has revealed
three dense filaments intersected at the center, coincident with the central
core MMS-7, which has a mass of $3.6\,M_\odot$. The filaments and cores are
embedded in a parental clump with total mass of $29\,M_\odot$. The N$_2$H$^+$
velocity field exhibits a noticeable increasing trend along the filaments
towards the central core MMS-7 with a scale of $v-v_{\rm lsr} \simeq 1.5$ ${\rm
km\, s^{-1}}$ over a spatial range of $\sim$20 arcsec ($8\times 10^3$ AU),
corresponding to a gradient of $40\,{\rm km\, s^{-1}}\,{\rm pc}^{-1}$. This
feature is most likely to indicate an infall motion towards the center. The
derived infall rate ($8\times 10^{-5}\,M_\odot$ year$^{-1}$) and timescale
($3.6\times 10^5$ years) are much lower than that in a spherical free-fall
collapse and more consistent with the contraction of filament structures. The
filaments also exhibit a possible fragmentation, but it does not seem to
largely interrupt the gas structure or the infall motion towards the center.
MMS-7 thus provides an example of filamentary infall into an individual
prestellar core. The filament contraction could be less intense but more steady
than the global spherical collapse, and may help generate an intermediate- or
even high-mass star.
