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Minh, Young Chol,Liu, Hauyu Baobab,Chen, Huei-Ru Vivien The Korean Astronomical Society 2020 Journal of The Korean Astronomical Society Vol.53 No.3
In the molecular cloud G33.92+0.11A, massive stars are forming sequentially in dense cores, probably due to interaction with accreted gas. Cold dense gas, which is likely the pristine gas of the cloud, is traced by DCN line and dust continuum emission. Clear chemical differences were observed in different source locations and for different velocity components in the same line of sight. Several distinct gas components coexist in the cloud: the pristine cold gas, the accreted dense gas, and warm turbulent gas, in addition to the star-forming dense clumps. Filaments of accreted gas occur in the northern part of the A1 and A5 clumps, and the velocity gradient along these features suggests that the gas is falling toward the cloud and may have triggered the most recent star formation. The large concentration of turbulent gas in the A2 clump seems to have formed mainly through disturbances from the outside.
Herczeg, Gregory J.,Johnstone, Doug,Mairs, Steve,Hatchell, Jennifer,Lee, Jeong-Eun,Bower, Geoffrey C.,Chen, Huei-Ru Vivien,Aikawa, Yuri,Yoo, Hyunju,Kang, Sung-Ju,Kang, Miju,Chen, Wen-Ping,Williams, Jo American Astronomical Society 2017 The Astrophysical Journal Vol.849 No.1
<P>Most protostars have luminosities that are fainter than expected from steady accretion over the protostellar lifetime. The solution to this problem may lie in episodic mass accretion -- prolonged periods of very low accretion punctuated by short bursts of rapid accretion. However, the timescale and amplitude for variability at the protostellar phase is almost entirely unconstrained. In 'A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions', we are monitoring monthly with SCUBA-2 the sub-mm emission in eight fields within nearby (<500 pc) star forming regions to measure the accretion variability of protostars. The total survey area of similar to 1.6 sq.deg. includes similar to 105 peaks with peaks brighter than 0.5 Jy/beam (43 associated with embedded protostars or disks) and 237 peaks of 0.125-0.5 Jy/beam (50 with embedded protostars or disks). Each field has enough bright peaks for flux calibration relative to other peaks in the same field, which improves upon the nominal flux calibration uncertainties of sub-mm observations to reach a precision of similar to 2-3% rms, and also provides quantified confidence in any measured variability. The timescales and amplitudes of any sub-mm variation will then be converted into variations in accretion rate and subsequently used to infer the physical causes of the variability. This survey is the first dedicated survey for sub-mm variability and complements other transient surveys at optical and near-IR wavelengths, which are not sensitive to accretion variability of deeply embedded protostars.</P>
Mairs, Steve,Johnstone, Doug,Kirk, Helen,Lane, James,Bell, Graham S.,Graves, Sarah,Herczeg, Gregory J.,Scicluna, Peter,Bower, Geoffrey C.,Chen, Huei-Ru Vivien,Hatchell, Jennifer,Aikawa, Yuri,Chen, Wen American Astronomical Society 2017 The Astrophysical journal Vol.849 No.2
<P>Investigating variability at the earliest stages of low-mass star formation is fundamental in understanding how a protostar assembles mass. While many simulations of protostellar disks predict non-steady accretion onto protostars, deeper investigation requires robust observational constraints on the frequency and amplitude of variability events characterized across the observable SED. In this study, we develop methods to robustly analyze repeated observations of an area of the sky for submillimeter variability in order to determine constraints on the magnitude and frequency of deeply embedded protostars. We compare 850 mu m JCMT Transient Survey data with archival JCMT Gould Belt Survey data to investigate variability over 2-4 year timescales. Out of 175 bright, independent emission sources identified in the overlapping fields, we find seven variable candidates, five of which we classify as Strong, and the remaining two we classify as Extended to indicate that the latter are associated with larger-scale structure. For the Strong variable candidates, we find an average fractional peak brightness change per. year of |4.0|% yr(-1), with a standard deviation of 2.7% yr(-1). In total, 7% of the protostars associated with 850 mu m emission in our sample show signs of variability. Four of the five Strong sources are associated with a known protostar. The remaining source is a good follow-up target for an object that is anticipated to contain an enshrouded, deeply embedded protostar. In addition, we estimate the 850 mu m periodicity of the submillimeter variable source, EC 53, to be 567 +/- 32 days, based on the archival Gould Belt Survey data.</P>