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Register a new userThe C$^{18}$O core mass function toward Orion A: Single-dish observations
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We have performed an unbiased dense core survey toward the Orion A Giant Molecular Cloud in the C$^{18}$O ($J$=1--0) emission line taken with the Nobeyama Radio Observatory (NRO) 45-m telescope. The effective angular resolution of the map is 26, which corresponds to $sim$ 0.05 pc at a distance of 414 pc. By using the Herschel-Planck H$_2$ column density map, we calculate the C$^{18}$O fractional abundance and find that it is roughly constant over the column density range of $lesssim$ 5 $times$ 10$^{22}$ cm$^{-3}$, although a trend of C$^{18}$O depletion is determined toward higher column density. Therefore, C$^{18}$O intensity can follow the cloud structure reasonably well. The mean C$^{18}$O abundance in Orion A is estimated to be 5.7$times$10$^{-7}$, which is about 3 times larger than the fiducial value. We identified 746 C$^{18}$O cores with astrodendro and classified 709 cores as starless cores. We compute the core masses by decomposing the Herschel-Planck dust column density using the relative proportions of the C$^{18}$O integrated intensities of line-of-sight components. Applying this procedure, we attempt to remove the contribution of the background emission, i.e., the ambient gas outside the cores. Then, we derived mass function for starless cores and found that it resembles the stellar initial mass function (IMF). The CMF for starless cores, $dN/dM$, is fitted with a power-law relation of $M^alpha$ with a power index of $alpha = -$2.25$pm$ 0.16 at the high-mass slope ($gtrsim$ 0.44 $M_odot$). We also found that the ratio of each core mass to the total mass integrated along the line of sight is significantly large. Therefore, in the previous studies, the core masses derived from the dust image are likely to be overestimated at least by a factor of a few. Accordingly, such previous studies may underestimate the star formation efficiency of individual cores.
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We present the core mass function (CMF) of the massive star-forming clump G33.92+0.11 using 1.3 mm observations obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). With a resolution of 1000 au, this is one of the highest resolution CMF measurements to date. The CMF is corrected by flux and number incompleteness to obtain a sample that is complete for gas masses $Mgtrsim2.0 M_odot$. The resulting CMF is well represented by a power-law function ($dN/dlog Mpropto M^Gamma$), whose slope is determined using two different approaches: $i)$ by least-squares fitting of power-law functions to the flux- and number-corrected CMF, and $ii)$ by comparing the observed CMF to simulated samples with similar incompleteness. We provide a prescription to quantify and correct a flattening bias affecting the slope fits in the first approach, which is caused by small-sample or edge effects when the data is represented by either classical histograms or a kernel density estimate, respectively. The resulting slopes from both approaches are in good agreement each other, with $Gamma=-1.11_{-0.11}^{+0.12}$ being our adopted value. Although this slope appears to be slightly flatter than the Salpeter slope $Gamma=-1.35$ for the stellar initial mass function (IMF), we find from Monte Carlo simulations that the CMF in G33.92+0.11 is statistically indistinguishable from the Salpeter representation of the stellar IMF. Our results are consistent with the idea that the form of the IMF is inherited from the CMF, at least at high masses and when the latter is observed at high-enough resolution.
Applying dendrogram analysis to the CARMA-NRO C$^{18}$O ($J$=1--0) data having an angular resolution of $sim$ 8, we identified 692 dense cores in the Orion Nebula Cluster (ONC) region. Using this core sample, we compare the core and initial stellar mass functions in the same area to quantify the step from cores to stars. About 22 % of the identified cores are gravitationally bound. The derived core mass function (CMF) for starless cores has a slope similar to Salpeters stellar initial mass function (IMF) for the mass range above 1 $M_odot$, consistent with previous studies. Our CMF has a peak at a subsolar mass of $sim$ 0.1 $M_odot$, which is comparable to the peak mass of the IMF derived in the same area. We also find that the current star formation rate is consistent with the picture in which stars are born only from self-gravitating starless cores. However, the cores must gain additional gas from the surroundings to reproduce the current IMF (e.g., its slope and peak mass), because the core mass cannot be accreted onto the star with a 100% efficiency. Thus, the mass accretion from the surroundings may play a crucial role in determining the final stellar masses of stars.
Similarity in shape between the initial mass function (IMF) and the core mass functions (CMFs) in star-forming regions prompts the idea that the IMF originates from the CMF through a self-similar core-to-star mass mapping process. To accurately determine the shape of the CMF, we create a sample of 8,431 cores with the dust continuum maps of the Cygnus X giant molecular cloud complex, and design a procedure for deriving the CMF considering the mass uncertainty, binning uncertainty, sample incompleteness, and the statistical errors. The resultant CMF coincides well with the IMF for core masses from a few $M_{odot}$ to the highest masses of 1300 $M_{odot}$ with a power-law of ${rm d}N/{rm d}Mpropto M^{-2.30pm0.04}$, but does not present an obvious flattened turnover in the low-mass range as the IMF does. More detailed inspection reveals that the slope of the CMF steepens with increasing mass. Given the numerous high-mass star-forming activities of Cygnus X, this is in stark contrast with the existing top-heavy CMFs found in high-mass star-forming clumps. We also find that the similarity between the IMF and the mass function of cloud structures is not unique at core scales, but can be seen for cloud structures of up to several pc scales. Finally, our SMA observations toward a subset of the cores do not present evidence for the self-similar mapping. The latter two results indicate that the shape of the IMF may not be directly inherited from the CMF.
We present an initial overview of the filamentary structure in the Orion A molecular cloud utilizing a high angular and velocity resolution C$^{18}$O(1-0) emission map that was recently produced as part of the CARMA-NRO Orion Survey. The main goal of this study is to build a credible method to study varying widths of filaments which has previously been linked to star formation in molecular clouds. Due to the diverse star forming activities taking place throughout its $sim$20 pc length, together with its proximity of 388 pc, the Orion A molecular cloud provides an excellent laboratory for such an experiment to be carried out with high resolution and high sensitivity. Using the widely-known structure identification algorithm, DisPerSE, on a 3-dimensional (PPV) C$^{18}$O cube, we identified 625 relatively short (the longest being 1.74 pc) filaments over the entire cloud. We study the distribution of filament widths using FilChaP, a python package that we have developed and made publicly available. We find that the filaments identified in a 2 square degree PPV cube do not overlap spatially, except for the complex OMC-4 region that shows distinct velocity components along the line of sight. The filament widths vary between 0.02 and 0.3 pc depending on the amount of substructure that a filament possesses. The more substructure a filament has, the larger is its width. We also find that despite this variation, the filament width shows no anticorrelation with the central column density which is in agreement with previous Herschel observations.
We present sensitive high angular resolution ($sim$ 0.1$$ -- 0.3$$) continuum ALMA (The Atacama Large Millimeter/Submillimeter Array) observations of the archetypal hot core located in Orion-KL. The observations were made in five different spectral bands (bands 3, 6, 7, 8, and 9) covering a very broad range of frequencies (149 -- 658 GHz). Apart of the well-know millimeter emitting objects located in this region (Orion Source I and BN), we report the first submillimeter detection of three compact continuum sources (ALMA 1-3) in the vicinities of the Orion-KL hot molecular core. These three continuum objects have spectral indices between 1.47 to 1.56, and brightness temperatures between 100 to 200 K at 658 GHz suggesting that we are seeing moderate optically thick dust emission with possible grain growth. However, as these objects are not associated with warm molecular gas, and some of them are farther out from the molecular core, we thus conclude that they cannot heat the molecular core. This result favours the hypothesis that the hot molecular core in Orion-KL core is heated externally.
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