The Filament Intersections and Cold Dense Cores in Orion A North (submitted)

ABSTRACT We studied the filament structures in OMC-2,3 region in Orion A North dense molecular cloud using a 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 more than 170 intersections and 128 candidate dense cores. The dense cores are all displaced from the infrared point sources (possible young stars). The cores exhibit a concentration around the intersections, that 101 ones are near the intersections while only 27 ones are on the long single-path filaments or isolated from the filament. Towards the intersections, there is also an increasing trend for the 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 αvir = Mvir/Mgas < 1.0, suggesting that they are bounded by the self gravity. In the mean time, only about one third of the cores have critical mass ratio of αcrit = Mcrit/Mgas < 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 are in a currently stable and gravitationally bound state. The future core collapse may still require continuous core-mass growth or additional perturbations.

DATA REDUCTION AND PROCESSING

Area : OMC-2,3 Telescope : ALMAtracer : N2H+ (1-0) ∆Vchan = 0.1 km s

FWHM beam size : 3” × 3”The rms level : 7 mJy/beam Tb = 0.11 K

Dashed circles : primary beam of each pointing center

The five sub-areas are labeled with boxes.

The N2H+ emission in each sub-area (false-color image). The contour levels are 10, 30, 50, 70, 90 percent of the local maximum.

Yellow dotted lines : DisPerSE-modeled filament paths Yellow and cyan filled circles : real and pseudo intersections Plus : local emission peaks (plus)Stars : YSOs

PV-1: P-V diagrams along the major directionsPV-2: P-V diagrams along the minor directionsThe black dashed lines : velocity components and velocity gradients The red dotted lines : velocity range of ±0.25 km/s and average line width of 0.5 km/s

Intersections: Velocity Distribution

The Column-density Probability Distribution Function

The distribution of average Ntot and N-PDF power law tailindex over the filaments path in OMC-3.

• MMS-2,3 area has noticeably higher Ntot as well as α values than the other gas structures in OMC-3 region.

(a) around the intersections there are also a large amount of gas with relatively low column density ---- the intersected filaments are a necessary but not sufficient condition for assembling the dense gas and cores

(b) The high-Ntot gas in the mean time also has higher α values ---- the gas therein could be more intensely bounded by the self- gravity

The column density profiles Ntot(r) over the local emission peaks.

• Ntot distribution rapidly declines towards the outer part and reaches the lowest level

• Average σv(r)profiles exhibita slight decrease ---- candidate dense cores tend to have comparable or even smaller velocity dispersion than the surrounding gas.

Compared to the intersections, the filament paths have more drastic decline over the transition point, their N-PDF drops to much lower level than the intersections when log Ntot > 23.2.

The Gravitational Instabilities

Dense Cores: Column Density and Velocity Dispersion

αvir = Mvir/Mgas < 1.0 ---- bounded by the self gravityone third of the cores αcrit = Mcrit/Mgas < 1.0 ---- unstable against core collapse