scholarly journals Dynamical evolution of fractal structures in star-forming regions

2020 ◽  
Vol 493 (4) ◽  
pp. 4925-4935
Author(s):  
Emma C Daffern-Powell ◽  
Richard J Parker

ABSTRACT The $\mathcal {Q}$-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 the minimum spanning tree, $\bar{m}$, to the average length within the complete graph, $\bar{s}$. The interpretation of the $\mathcal {Q}$-parameter often relies on comparing observed values of $\mathcal {Q}$, $\bar{m}$, and $\bar{s}$ to idealized synthetic geometries, where there is little or no match between the observed star-forming regions and the synthetic regions. We measure $\mathcal {Q}$, $\bar{m}$, and $\bar{s}$ 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 ratios and fractal dimensions. We find that the dynamical evolution of idealized fractal geometries can account for the observed $\mathcal {Q}$, $\bar{m}$, and $\bar{s}$ 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 different 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 $\bar{m}{-}\bar{s}$ plot as it evolves. We caution that the observed $\mathcal {Q}$-parameter should not be directly compared to idealized 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.

2020 ◽  
Vol 644 ◽  
pp. A141
Author(s):  
T. Roland ◽  
C. M. Boily ◽  
L. Cambrésy

Context. Young stellar clusters harbour complex spatial structures emerging from the star formation process. Identifying stellar over-densities is a key step in better constraining how these structures are formed. The high accuracy of distances derived from Gaia DR2 parallaxes still do not allow us to locate individual stars within clusters of ≈1 pc in size with certainty. Aims. In this work, we explore how such uncertainty on distance estimates can lead to the misidentification of membership of sub-clusters selected by the minimum spanning tree (MST) algorithm. Our goal is to assess how this impacts their estimated properties. Methods. Using N-body simulations, we build gravity-driven fragmentation models that self-consistently reproduce the early stellar configurations of a star forming region. Stellar groups are then identified both in two and three dimensions by the MST algorithm, representing respectively an inaccurate and an ideal identification. We compare the properties derived for these resulting groups in order to assess the systematic bias introduced by projection and incompleteness. Results. We show that in such fragmented configurations, the dynamical mass of groups identified in projection is systematically underestimated compared to those of groups identified in 3D. This systematic error is statistically of 50% for more than half of the groups and reaches 100% in a quarter of them. Adding incompleteness further increases this bias. Conclusions. These results challenge our ability to accurately identify sub-clusters in most nearby star forming regions where distance estimate uncertainties are comparable to the size of the region. New clump-finding methods need to tackle this issue in order to better define the dynamical state of these substructures.


2006 ◽  
Vol 2 (S237) ◽  
pp. 132-140
Author(s):  
Philippe André

AbstractSeveral (sub)millimeter-wave studies of nearby star-forming regions have revealed self-gravitating prestellar condensations that seem to be the direct progenitors of individual stars and whose mass distribution resembles the IMF. In a number of cases, small internal and relative motions have been measured for these condensations, indicating they are much less turbulent than their parent cloud and do not have time to interact before evolving into protostars and pre-main sequence stars. These findings suggest that the IMF is at least partly determined by pre-collapse cloud fragmentation and that one of the keys to understanding the origin of stellar masses lies in the physical mechanisms responsible for the formation and decoupling of prestellar cores within molecular clouds.


2010 ◽  
Vol 718 (2) ◽  
pp. 610-619 ◽  
Author(s):  
Sergio Dzib ◽  
Laurent Loinard ◽  
Amy J. Mioduszewski ◽  
Andrew F. Boden ◽  
Luis F. Rodríguez ◽  
...  

2019 ◽  
Vol 14 (S351) ◽  
pp. 216-219
Author(s):  
Steven Rieder ◽  
Clare Dobbs ◽  
Thomas Bending

AbstractWe present a model for hydrodynamic + N-body simulations of star cluster formation and evolution using AMUSE. Our model includes gas dynamics, star formation in regions of dense gas, stellar evolution and a galactic tidal spiral potential, thus incorporating most of the processes that play a role in the evolution of star clusters.We test our model on initial conditions of two colliding molecular clouds as well as a section of a spiral arm from a previous galaxy simulation.


2009 ◽  
Vol 5 (H15) ◽  
pp. 771-771
Author(s):  
Paul C. Clark

We review the properties of the discs that form around ‘sink particles’ in smoothed particle hydrodynamics (SPH) simulations of cluster formation, similar to those of Bate et al. (2003) and Bonnell et al. (2004), and compare them to the observed properties of discs in nearby star-forming regions. Contrary to previous suggestions, discs can form and survive in such an environment, despite the chaotic effects of competitive accretion. We find the discs are typically massive, with ratios of disc mass to central object mass of around 0.1, or higher, being typical. Naturally, the evolution of these discs is dominated by gravitational torques, and the more massive examples exhibit strong m=2 spiral modes. We also find that they can continuously grow over a period of 100,000 years, provided the central object is a single sink particle and the local density of sink particles is low. Discs that form around sink particles in the very centres of clusters tend to be shorter lived, but a single star can lose and gain a disc several times during the main accretion phase. However due to the nature of the turbulence in the cluster, the disc orientation can change dramatically over this time period, since disc-sink systems can accrete from counter-rotating envelopes. Since the competitive accretion process brings in material from large distances, the associated angular momentum can be higher than one would expect for an isolated star formation model. As such, we find that the discs are typically several hundred of AUs in extent, with the largest keplerian structures having radii of ~ 2000AU.


2016 ◽  
Vol 826 (2) ◽  
pp. 201 ◽  
Author(s):  
Sergio A. Dzib ◽  
Gisela N. Ortiz-León ◽  
Laurent Loinard ◽  
Amy J. Mioduszewski ◽  
Luis F. Rodríguez ◽  
...  

2018 ◽  
Vol 610 ◽  
pp. A9 ◽  
Author(s):  
Miwa Goto ◽  
Jeffrey D. Bailey ◽  
Seyit Hocuk ◽  
Paola Caselli ◽  
Gisela B. Esplugues ◽  
...  

Context. Spectroscopic studies of ices in nearby star-forming regions indicate that ice mantles form on dust grains in two distinct steps, starting with polar ice formation (H2O rich) and switching to apolar ice (CO rich). Aims. We test how well the picture applies to more diffuse and quiescent clouds where the formation of the first layers of ice mantles can be witnessed. Methods. Medium-resolution near-infrared spectra are obtained toward background field stars behind the Pipe Nebula. Results. The water ice absorption is positively detected at 3.0 μm in seven lines of sight out of 21 sources for which observed spectra are successfully reduced. The peak optical depth of the water ice is significantly lower than those in Taurus with the same AV. The source with the highest water-ice optical depth shows CO ice absorption at 4.7 μm as well. The fractional abundance of CO ice with respect to water ice is 16-6+7%, and about half as much as the values typically seen in low-mass star-forming regions. Conclusions. A small fractional abundance of CO ice is consistent with some of the existing simulations. Observations of CO2 ice in the early diffuse phase of a cloud play a decisive role in understanding the switching mechanism between polar and apolar ice formation.


Fractals ◽  
1993 ◽  
Vol 01 (04) ◽  
pp. 908-916 ◽  
Author(s):  
Z.Y. YUE ◽  
B. ZHANG ◽  
G. WINNEWISSER ◽  
J. STUTZKI

Two-dimensional compressible turbulence in a self-gravitating, magnetic interstellar medium is calculated as an initial value problem. It is shown that even if the initial density distribution is homogeneous and the initial velocity distribution contains only a few Fourier components, the nonlinear interaction among the Fourier components will generate more and more Fourier components and lead to a turbulent and fractal structure in the interstellar medium. The calculations are carried out for three different initial states. In order to see the time evolution, detailed density distributions and fractal dimensions of the density contours are calculated at three moments of time for each of the initial states. The results show that the fractal dimension remains almost the same (~1.4–1.5), although the detailed density distribution has changed considerably. The insensibility of the fractal dimension of density contours to both the initial conditions and the evolution time is in good agreement with observations of molecular clouds in the interstellar medium.


2018 ◽  
Vol 614 ◽  
pp. A130 ◽  
Author(s):  
K. George ◽  
P Joseph ◽  
P. Côté ◽  
S. K. Ghosh ◽  
J. B. Hutchings ◽  
...  

Context. The tidal tails of post-merger galaxies exhibit ongoing star formation far from their disks. The study of such systems can be useful for our understanding of gas condensation in diverse environments. Aims. The ongoing star formation in the tidal tails of post-merger galaxies can be directly studied from ultraviolet (UV) imaging observations. Methods. The post merger galaxy NGC7252 (“Atoms-for-Peace” galaxy) is observed with the Astrosat UV imaging telescope (UVIT) in broadband NUV and FUV filters to isolate the star-forming regions in the tidal tails and study the spatial variation in star formation rates. Results. Based on ultraviolet imaging observations, we discuss star-forming regions of ages <200 Myr in the tidal tails. We measure star formation rates in these regions and in the main body of the galaxy. The integrated star formation rate (SFR) of NGC7252 (i.e., that in the galaxy and tidal tails combined) without correcting for extinction is found to be 0.81 ± 0.01 M⊙ yr−1. We show that the integrated SFR can change by an order of magnitude if the extinction correction used in SFR derived from other proxies are taken into consideration. The star formation rates in the associated tidal dwarf galaxies (NGC7252E, SFR = 0.02 M⊙ yr−1 and NGC7252NW, SFR = 0.03 M⊙ yr−1) are typical of dwarf galaxies in the local Universe. The spatial resolution of the UV images reveals a gradient in star formation within the tidal dwarf galaxy. The star formation rates show a dependence on the distance from the centre of the galaxy. This can be due to the different initial conditions responsible for the triggering of star formation in the gas reservoir that was expelled during the recent merger in NGC7252.


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