Probing the structure of a birthplace of intermediate-mass stars: Ammonia cores in Lynds 1340

Abstract

Lynds 1340, a molecular cloud forming intermediate-mass stars, has been mapped in the NH_3(1,1) and (2,2) transitions with the Effelsberg 100m telescope. We observed the whole area of the cloud where C18O emission was detected earlier, at a 40 arcsec grid, with additional positions towards the C18O peaks and optically invisible IRAS point sources. Our observations covered an area of 170 arcmin^2, corresponding to about 5.15 pc^2 at a distance of 600 pc, and revealed 10 ammonia cores. The cores, occupying some 7% of the mapped area, probably represent the highest density regions of L1340. Their total mass is 80 solar mass, about 6% of the mass traced by C18O. Six cores are associated with optically invisible IRAS point sources. Their average nonthermal line width is 0.78 kms^{-1}, while the same quantity for the four starless cores is 0.28 kms^{-1}. We suggest that the narrow-line cores are destined to form low-mass stars, whereas small groups of intermediate-mass stars are being formed in the turbulent cores. The features traced by NH_3, 13CO, C18O and HI obey the line width-size relation log Delta v_{NT} = 0.41(0.06)log R_{1/2}+ 0.12(0.06). Comparison of sizes, densities and nonthermal line widths of ammonia cores with those of C18O and 13CO structures supports the scenario in which core formation has been induced by turbulent fragmentation. The typical physical properties of the ammonia cores of L1340, R_{1/2} =0.08 pc, T_{kin}=13.8 K, Delta v_{total}=0.64 kms^{-1}, and M =9 solar mass are close to those of the high-mass star forming Perseus and Orion B clouds.