We have carried out an H 13 CO þ (J ¼ 1Y0) core survey in a large area of 1:5 ; 0:5 , covering the whole region of the Orion A molecular cloud, using the Nobeyama 45 m radio telescope with the 25 Beam Array Receiver System (BEARS). This survey is unique in that a large area ($48 pc 2 ) of the cloud was covered with a high spatial resolution of 21 00 (0.05 pc) and with a deep integration (1 $ 0:1 K in T Ã A ), resulting in a core mass detection of 1.6 M . The morphology of the H 13 CO þ (J ¼ 1Y0) emission is very similar to that of the 850 m continuum emission. We identified 236 dense cores from our data with the clumpfind algorithm. The cores are close to virial equilibrium, independent of whether they are thermal or turbulent. We predict an initial mass function (IMF) from the core mass function, considering binary formation and confusion along the line of sight, and find that this IMF agrees well with the Orion Nebula cluster IMF for a star formation efficiency of $40%. Therefore, we suggest that the IMF is determined at the time of the dense core formation. Furthermore, we discovered three cores with large velocity widths, significantly wider than those of the other cores, only toward the M42 H ii region, suggesting that the energy input from the H ii region increases the velocity width. Since the three cores can produce the most massive stars, owing to their large mass accretion rates, massive star formation in the next generation in the Orion A cloud is likely to be caused by nearby stellar activity.

We have developed spectral line On-The-Fly (OTF) observing mode for the Nobeyama Radio Observatory 45-m and the Atacama Submillimeter Telescope Experiment 10-m telescopes. Sets of digital autocorrelation spectrometers are available for OTF with heterodyne receivers mounted on the telescopes, including the focal-plane 5 × 5 array receiver, BEARS, on the 45-m. During OTF observations, the antenna is continuously driven to cover the mapped region rapidly, resulting in high observing efficiency and accuracy. Pointing of the antenna and readouts from the spectrometer are recorded as fast as 0.1 second. In this paper we report improvements made on software and instruments, requirements and optimization of observing parameters, data reduction process, and verification of the system. It is confirmed that, using optimal parameters, the OTF is about twice as efficient as conventional position-switch observing method.