How rapidly collapsing parsec-scale massive molecular clumps feed high-mass stars, and how they fragment to form OB clusters, have been outstanding questions in the field of star-formation. In this work, we report the resolved structures and kinemati
cs of the approximately face-on, rotating massive molecular clump, G33.92+0.11. Our high resolution Atacama Large Millimeter/submillimeter Array (ALMA) images show that the spiral arm-like gas overdensities form in the eccentric gas accretion streams. First, we resolved that the dominant part of the $sim$0.6 pc scale massive molecular clump (3.0$^{+2.8}_{-1.4}$$cdot$10$^{3}$ $M_{odot}$) G33.92+0.11 A is tangled with several 0.5-1 pc size molecular arms spiraling around it, which may be connected further to exterior gas accretion streams. Within G33.92+0.11 A, we resolved the $sim$0.1 pc width gas mini-arms connecting with the two central massive (100-300 $M_{odot}$) molecular cores. The kinematics of arms and cores elucidate a coherent accretion flow continuing from large to small scales. We demonstrate that the large molecular arms are indeed the cradles of dense cores, which are likely current or future sites of high-mass star formation. Since these deeply embedded massive molecular clumps preferentially form the highest mass stars in the clusters, we argue that dense cores fed by or formed within molecular arms play a key role in making the upper end of the stellar and core mass functions.
We report JVLA 8-10 GHz ($lambda$=3.0-3.7 cm) monitoring observations toward the YSO cluster R Coronae Australis (R,CrA), taken in 2012, from March 15 to September 12. These observations were planned to measure the radio flux variabilities in timesca
les from 0.5 hours to several days, to tens of days, and up to $sim$200 days. We found that among the YSOs detectable in individual epochs, in general, the most reddened objects in the textit{Spitzer} observations show the highest mean 3.5 cm Stokes textit{I} emission, and the lowest fractional variabilities on $<$200-day timescales. The brightest radio flux emitters in our observations are the two reddest sources IRS7W and IRS7E. In addition, by comparing with observations taken in 1996-1998 and 2005, we found that the radio fluxes of these two sources have increased by a factor $sim$1.5. The mean 3.5-cm fluxes of the three Class I/II sources IRSI, IRS2, and IRS6 appear to be correlated with their accretion rates derived by a previous near infrared line survey. The weakly accreting Class I/II YSOs, or those in later evolutionary stages, present radio flux variability on $<$0.5-hour timescales. Some YSOs were detected only during occasional flaring events. The source R,CrA went below our detection limit during a few fading events.
We observe the 1.2 mm continuum emission around the OB cluster forming region G10.6-0.4, using the IRAM 30m telescope MAMBO-2 bolometer array and the Submillimeter array. Comparison of the Spitzer 24 $mu$m and 8 $mu$m images with our 1.2 mm continuum
maps reveals the ionization front of an HII region, the photon-dominated layer, and several 5 pc scale filaments following the outer edge of the photon-dominated layer. The filaments, which are resolved in the MAMBO-2 observations, show regularly spaced parsec-scale molecular clumps, embedded with a cluster of submillimeter molecular cores as shown in the SMA 0.87 mm observations. Toward the center of the G10.6-0.4 region, the combined SMA+IRAM 30m continuum image reveals several, parsec-scale protrusions. They may continue down to within 0.1 pc of the geometric center of a dense 3 pc size structure, where a 200 M$_{odot}$ OB cluster resides. The observed filaments may facilitate mass accretion onto the central cluster--forming region in the presence of strong radiative and mechanical stellar feedbacks. Their filamentary geometry may also facilitate fragmentation. We did not detect any significant polarized emission at 0.87 mm in the inner 1 pc region with the SMA.
