A Survey of Dense Cores in the Orion A Cloud

Abstract

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.

SIMILARITY BETWEEN THE C18O (J= 1-0) CORE MASS FUNCTION AND THE INITIAL MASS FUNCTION (IMF) IN THE S140 REGION

“…Furthermore, the mass fraction of the cores in S140 is very large: the total mass of the C 18 O cores is 5000 M , 75% of the total mass traced by the C 18 O emission (see Section 3.2). This fraction is twice that of the C 18 O cores in OMC-1 ) but is roughly consistent with the fraction of 60% for the H 13 CO + cores (Ikeda et al 2007. The large massive cores and the high mass fraction of the cores in the S140 region are likely to be caused by high column densities in the region.…”

“…They found a γ value of the C 18 O CMF of 2.3-2.4. The γ value is not only similar to that of the H 13 CO + DCMF (2.2 ± 0.1; Ikeda et al 2007) and the 850 μm dust continuum DCMF (2.2 ± 0.2; Nutter & Ward-Thompson 2007) within the uncertainties, but is also quite consistent with that of the IMF of the Orion Nebula Cluster (2.2; Muench et al 2002), which is associated with the OMC-1 region. The agreement between the C 18 O CMF and IMF γ values suggests that, at least in the OMC-1 region, the power-law form of the IMF with γ 2 has been maintained from the formation time of the tenuous structure with density of ∼10 3 -10 4 cm −3 .…”

“…This is because Williams et al (1994) determined the optimal threshold of the 2σ level for the full-beam sampling case. We also used the additional criteria introduced by Ikeda et al (2007) to reject ambiguous or fake core candidates whose size and velocity width are smaller than the spatial and velocity resolutions, respectively.…”

“…Particularly, the dense core mass function (DCMF) has been considered a key to understanding the stellar initial mass function (IMF). Many authors have investigated the DCMFs by using dense ( 10 4 -10 5 cm −3 ) gas tracers such as (sub)millimeter dust continuum emission, infrared visual extinction, and molecular line emissions having high critical densities, for example, the H 13 CO + (J = 1-0) and N 2 H + (J = 1-0) lines (e.g., Motte et al 1998;Reid & Wilson 2006;Nutter & Ward-Thompson 2007;Rathborne et al 2009;Ikeda et al 2007;Walsh et al 2007;Enoch et al 2008). In nearby ( 500 pc) star-forming regions such as Orion, Ophiuchus, the Pipe Nebula, Perseus, and Serpens, they found that the DCMFs seem to have power-law-like behaviors in high-mass parts as dN/dM ∝ M −γ , whose γ values are very similar to those of the IMF.…”

PHYSICAL PROPERTIES OF DENSE CORES IN THE Ρ OPHIUCHI MAIN CLOUD AND a SIGNIFICANT ROLE OF EXTERNAL PRESSURES IN CLUSTERED STAR FORMATION

“…Similar studies in other star-forming regions have confirmed that such a resemblance between the core mass spectrum and the stellar IMF seems to be common in nearby star-forming regions (Testi & Sargent 1998;Motte et al 2001;Reid & Wilson 2006;Ikeda et al 2007). These observations suggest that the stellar IMF may be determined to a large extent by the core mass distribution (e.g., André et al 2007), although mass accretion from ambient gas onto forming stars may sometimes play an important role in determining the final masses of stars (Bonnell et al 2001;Bate et al 2003;Wang et al 2010).…”