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Register a new userDynamical evolution of star forming regions
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Richard J. Parker n
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We model the dynamical evolution of star forming regions with a wide range of initial properties. We follow the evolution of the regions substructure using the Q-parameter, we search for dynamical mass segregation using the Lambda_MSR technique, and we also quantify the evolution of local density around stars as a function of mass using the Sigma_LDR method. The amount of dynamical mass segregation measured by Lambda_MSR is generally only significant for subvirial and virialised, substructured regions - which usually evolve to form bound clusters. The Sigma_LDR method shows that massive stars attain higher local densities than the median value in all regions, even those that are supervirial and evolve to form (unbound) associations. We also introduce the Q-Sigma_LDR plot, which describes the evolution of spatial structure as a function of mass-weighted local density in a star forming region. Initially dense (>1000 stars pc^{-2}), bound regions always have Q >1, Sigma_LDR > 2 after 5Myr, whereas dense unbound regions always have Q < 1, Sigma_LDR > 2 after 5Myr. Less dense regions (<100 stars pc^{-2}) do not usually exhibit Sigma_LDR > 2 values, and if relatively high local density around massive stars arises purely from dynamics, then the Q-Sigma_LDR plot can be used to estimate the initial density of a star forming region.
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The Q-parameter is used extensively to quantify the spatial distributions of stars and gas in star-forming regions as well as older clusters and associations. It quantifies the amount of structure using the ratio of the average length of a minimum spanning tree, mbar, to the average length within the complete graph, sbar. The interpretation of the Q-parameter often relies on comparing observed values of Q, mbar and sbar to idealised synthetic geometries, where there is little or no match between the observed star-forming regions and the synthetic regions. We measure Q, mbar, and sbar over 10 Myr in N-body simulations which are compared to IC 348, NGC 1333, and the ONC. For each star-forming region we set up simulations that approximate their initial conditions for a combination of different virial rations and fractal dimensions. We find that dynamical evolution of idealised fractal geometries can account for the observed Q, mbar, and sbar values in nearby star-forming regions. In general, an initially fractal star-forming region will tend to evolve to become more smooth and centrally concentrated. However, we show that initial conditions, as well as where the edge of the region is defined, can cause significant differences in the path that a star-forming region takes across the mbar-sbar plot as it evolves. We caution that the observed Q-parameter should not be directly compared to idealised geometries. Instead, it should be used to determine the degree to which a star-forming region is either spatially substructured or smooth and centrally concentrated.
The fraction of star formation that results in bound star clusters is influenced by the density spectrum in which stars are formed and by the response of the stellar structure to gas expulsion. We analyse hydrodynamical simulations of turbulent fragmentation in star-forming regions to assess the dynamical properties of the resulting population of stars and (sub)clusters. Stellar subclusters are identified using a minimum spanning tree algorithm. When considering only the gravitational potential of the stars and ignoring the gas, we find that the identified subclusters are close to virial equilibrium (the typical virial ratio Q_vir~0.59, where virial equilibrium would be Q_vir~0.5). This virial state is a consequence of the low gas fractions within the subclusters, caused by the accretion of gas onto the stars and the accretion-induced shrinkage of the subclusters. Because the subclusters are gas-poor, up to a length scale of 0.1-0.2 pc at the end of the simulation, they are only weakly affected by gas expulsion. The fraction of subclusters that reaches the high density required to evolve to a gas-poor state increases with the density of the star-forming region. We extend this argument to star cluster scales, and suggest that the absence of gas indicates that the early disruption of star clusters due to gas expulsion (infant mortality) plays a smaller role than anticipated, and is potentially restricted to star-forming regions with low ambient gas densities. We propose that in dense star-forming regions, the tidal shocking of young star clusters by the surrounding gas clouds could be responsible for the early disruption. This `cruel cradle effect would work in addition to disruption by gas expulsion. We suggest possible methods to quantify the relative contributions of both mechanisms.
We present analyses of the spatial distributions of stars in the young (1 - 3 Myr) star-forming regions IC348 and NGC1333 in the Perseus Giant Molecular Cloud. We quantify the spatial structure using the $mathcal{Q}$-parameter and find that both IC348 and NGC1333 are smooth and centrally concentrated with $mathcal{Q}$-parameters of 0.98 and 0.89 respectively. Neither region exhibits mass segregation ($Lambda_{rm MSR} = 1.1^{+0.2}_{-0.3}$ for IC348 and $Lambda_{rm MSR} = 1.2^{+0.4}_{-0.3}$ for NGC1333, where $Lambda_{rm MSR} sim 1$ corresponds to no mass segregation), nor do the most massive stars reside in areas of enhanced stellar surface density compared to the average surface density, according to the $Sigma_{rm LDR}$ method. We then constrain the dynamical histories and hence initial conditions of both regions by comparing the observed values to $N$-body simulations at appropriate ages. Stars in both regions likely formed with sub-virial velocities which contributed to merging of substructure and the formation of smooth clusters. The initial stellar densities were no higher than $rho sim 100 - 500$M$_odot$pc$^{-3}$ for IC348 and $rho sim 500 - 2000$M$_odot$pc$^{-3}$ for NGC1333. These initial densities, in particular that of NGC1333, are high enough to facilitate dynamical interactions which would likely affect $sim$10 per cent of protoplanetary discs and binary stars.
Theoretical models suggest that massive stars form via disk-mediated accretion, with bipolar outflows playing a fundamental role. A recent study toward massive molecular outflows has revealed a decrease of the SiO line intensity as the object evolves. The present study aims at characterizing the variation of the molecular outflow properties with time, and at studying the SiO excitation conditions in outflows associated with massive YSOs. We used the IRAM30m telescope to map 14 massive star-forming regions in the SiO(2-1), SiO(5-4) and HCO+(1-0) outflow lines, and in several dense gas and hot core tracers. Hi-GAL data was used to improve the spectral energy distributions and the L/M ratio, which is believed to be a good indicator of the evolutionary stage of the YSO. We detect SiO and HCO+ outflow emission in all the sources, and bipolar structures in six of them. The outflow parameters are similar to those found toward other massive YSOs. We find an increase of the HCO+ outflow energetics as the object evolve, and a decrease of the SiO abundance with time, from 10^(-8) to 10^(-9). The SiO(5-4) to (2-1) line ratio is found to be low at the ambient gas velocity, and increases as we move to high velocities, indicating that the excitation conditions of the SiO change with the velocity of the gas (with larger densities and/or temperatures for the high-velocity gas component). The properties of the SiO and HCO+ outflow emission suggest a scenario in which SiO is largely enhanced in the first evolutionary stages, probably due to strong shocks produced by the protostellar jet. As the object evolves, the power of the jet would decrease and so does the SiO abundance. During this process, however, the material surrounding the protostar would have been been swept up by the jet, and the outflow activity, traced by entrained molecular material (HCO+), would increase with time.
Maser emission plays an important role as a tool in star formation studies. It is widely used for deriving kinematics, as well as the physical conditions of different structures, hidden in the dense environment very close to the young stars, for example associated with the onset of jets and outflows. We will summarize the recent observational and theoretical progress on this topic since the last maser symposium: the IAU Symposium 242 in Alice Springs.
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