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Register a new userStar and Stellar Cluster Formation: ALMA-SKA Synergies
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Over the next decade, observations conducted with ALMA and the SKA will reveal the process of mass assembly and accretion onto young stars and will be revolutionary for studies of star formation. Here we summarise the capabilities of ALMA and discuss recent results from its early science observations. We then review infrared and radio variability observations of both young low-mass and high-mass stars. A time domain SKA radio continuum survey of star forming regions is then outlined. This survey will produce radio light-curves for hundreds of young sources, providing for the first time a systematic survey of radio variability across the full range of stellar masses. These light-curves will probe the magnetospheric interactions of young binary systems, the origins of outflows, trace episodic accretion on the central sources and potentially constrain the rotation rates of embedded sources.
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Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and effectively channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.
The rich SMC star cluster NGC419 has recently been found to present both a broad main sequence turn-off and a dual red clump of giants, in the sharp colour-magnitude diagrams (CMD) derived from the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope. In this work, we apply to the NGC419 data the classical method of star formation history (SFH) recovery via CMD reconstruction, deriving for the first time this function for a star cluster with multiple turn-offs. The values for the cluster metallicity, reddening, distance and binary fraction, were varied within the limits allowed by present observations. The global best-fitting solution is an excellent fit to the data, reproducing all the CMD features with striking accuracy. The corresponding star formation rate is provided together with estimates of its random and systematic errors. Star formation is found to last for at least 700 Myr, and to have a marked peak at the middle of this interval, for an age of 1.5 Gyr. Our findings argue in favour of multiple star formation episodes (or continued star formation) being at the origin of the multiple main sequence turn-offs in Magellanic Cloud clusters with ages around 1 Gyr. It remains to be tested whether alternative hypotheses, such as a main sequence spread caused by rotation, could produce similarly good fits to the data.
We present comprehensive characterization of the Galactic open cluster M 36. Some two hundred member candidates, with an estimated contamination rate of $sim$8%, have been identified on the basis of proper motion and parallax measured by the $Gaia$ DR2. The cluster has a proper motion grouping around ($mu_{alpha} cosdelta = -$0.15 $pm$ 0.01 mas yr$^{-1}$, and $mu_{delta} = -$3.35 $pm$ 0.02 mas yr$^{-1}$), distinctly separated from the field population. Most member candidates have parallax values 0.7$-$0.9 mas, with a median value of 0.82 $pm$ 0.07 mas (distance $sim$1.20 $pm$ 0.13 kpc). The angular diameter of 27$$ $pm$ $0farcm4$ determined from the radial density profile then corresponds to a linear extent of 9.42 $pm$ 0.14 pc. With an estimated age of $sim$15 Myr, M 36 is free of nebulosity. To the south-west of the cluster, we discover a highly obscured ($A_{V}$ up to $sim$23 mag), compact ($sim$ $1farcm9 times 1farcm2$) dense cloud, within which three young stellar objects in their infancy (ages $lesssim$ 0.2 Myr) are identified. The molecular gas, 3.6 pc in extent, contains a total mass of (2$-$3)$times$10$^{2}$ M$_{odot}$, and has a uniform velocity continuity across the cloud, with a velocity range of $-$20 to $-$22 km s$^{-1}$, consistent with the radial velocities of known star members. In addition, the cloud has a derived kinematic distance marginally in agreement with that of the star cluster. If physical association between M 36 and the young stellar population can be unambiguously established, this manifests a convincing example of prolonged star formation activity spanning up to tens of Myrs in molecular clouds.
We investigate the formation and early evolution of star clusters assuming that they form from a turbulent starless clump of given mass bounded inside a parent self-gravitating molecular cloud characterized by a particular mass surface density. As a first step we assume instantaneous star cluster formation and gas expulsion. We draw our initial conditions from observed properties of starless clumps. We follow the early evolution of the clusters up to 20 Myr, investigating effects of different star formation efficiencies, primordial binary fractions and eccentricities and primordial mass segregation levels. We investigate clumps with initial masses of $M_{rm cl}=3000:{rm M}_odot$ embedded in ambient cloud environments with mass surface densities, $Sigma_{rm cloud}=0.1$ and $1:{rm g:cm^{-2}}$. We show that these models of fast star cluster formation result, in the fiducial case, in clusters that expand rapidly, even considering only the bound members. Clusters formed from higher $Sigma_{rm cloud}$ environments tend to expand more quickly, so are soon larger than clusters born from lower $Sigma_{rm cloud}$ conditions. To form a young cluster of a given age, stellar mass and mass surface density, these models need to assume a parent molecular clump that is many times denser, which is unrealistic compared to observed systems. We also show that in these models the initial binary properties are only slightly modified by interactions, meaning that binary properties, e.g., at 20 Myr, are very similar to those at birth. With this study we set up the basis of future work where we will investigate more realistic models of star formation compared to this instantaneous, baseline case.
Aims: We resolve the small-scale structure around the high-mass hot core region G351.77-0.54 to investigate its disk and fragmentation properties. Methods: Using ALMA at 690GHz with baselines exceeding 1.5km, we study the dense gas, dust and outflow emission at an unprecedented spatial resolution of 0.06 ([email protected]). Results: Within the inner few 1000AU, G351.77 fragments into at least four cores (brightness temperatures between 58 and 197K). The central structure around the main submm source #1 with a diameter of ~0.5 does not show additional fragmentation. While the CO(6-5) line wing emission shows an outflow lobe in the north-western direction emanating from source #1, the dense gas tracer CH3CN shows a velocity gradient perpendicular to the outflow that is indicative of rotational motions. Absorption profile measurements against the submm source #2 indicate infall rates on the order of 10^{-4} to 10^{-3}M_sun/yr which can be considered as an upper limit of the mean accretion rates. The position-velocity diagrams are consistent with a central rotating disk-like structure embedded in an infalling envelope, but they may also be influenced by the outflow. Using the CH_3CN(37_k-36_k) k-ladder with excitation temperatures up to 1300K, we derive a gas temperature map of source #1 exhibiting temperatures often in excess of 1000K. Brightness temperatures of the submm continuum never exceed 200K. This discrepancy between gas temperatures and submm dust brightness temperatures (in the optically thick limit) indicates that the dust may trace the disk mid-plane whereas the gas could be tracing a hotter gaseous disk surface layer. In addition, we conduct a pixel-by-pixel Toomre gravitational stability analysis of the central rotating structure. The derived high Q values throughout the structure confirm that this central region appears stable against gravitational instability.
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