Core dissolution and the dynamics of massive stars in young stellar clusters

We investigate the dynamical effects of rapid gas expulsion from the core of a young stellar cluster. The aims of this study are to determine 1) whether a mass-segregated core survives the gas expulsion and 2) the probable location of any massive stars that have escaped from the core. Feedback from massive stars is expected to remove the gas from the core of the cluster first, as that is where most massive stars are located. We find that gas expulsion has little effect on the core for a core star formation efficiency, of greater than 50%. For lower values of star formation efficiency down to 20%, a reduced core survives containing the majority of the massive stars while some of them are dispersed into the rest of the cluster. In fact we find that ejected stars migrate from radial to tangential orbits due to stellar encounters once they leave the core. Thus, the location of massive stars outside of the core does not exclude their forming in the dense cluster core. Few massive stars are expected to remain in the core for a star formation efficiency lower than 20%.Comment: 8 pages, 7 figures, accepted for publication in MNRA

Massive star formation: Nurture, not nature

We investigate the physical processes which lead to the formation of massive stars. Using a numerical simulation of the formation of a stellar cluster from a turbulent molecular cloud, we evaluate the relevant contributions of fragmentation and competitive accretion in determining the masses of the more massive stars. We find no correlation between the final mass of a massive star, and the mass of the clump from which it forms. Instead, we find that the bulk of the mass of massive stars comes from subsequent competitive accretion in a clustered environment. In fact, the majority of this mass infalls onto a pre-existing stellar cluster. Furthermore, the mass of the most massive star in a system increases as the system grows in numbers of stars and in total mass. This arises as the infalling gas is accompanied by newly formed stars, resulting in a larger cluster around a more massive star. High-mass stars gain mass as they gain companions, implying a direct causal relationship between the cluster formation process, and the formation of higher-mass stars therein.Comment: 8 pages, accepted for publication in MNRAS. Version including hi-res colour postscript figure available at http://star-www.st-and.ac.uk/~sgv/ps/massnurt.ps.g

T Tauri variability in the context of the beat-frequency model

We examine the implications of a beat frequency modulated model of T Tauri accretion. In particular we show that measurements of the variability of accretion generated lines can be used in conjunction with existing photometry to obtain a measurement of the underlying photospheric and disc flux. This provides an independent way of checking spectral energy distribution modelling. In addition, we show how spectroscopy of T Tauri stars can reveal the inclination angle between the magnetic axis and the plane of the disc.Comment: uuencoded compressed postscript. The preprint is also available at http://www.ast.cam.ac.uk/preprint/PrePrint.htm

The hierarchical formation of a stellar cluster

Recent surveys of star forming regions have shown that most stars, and probably all massive stars, are born in dense stellar clusters. The mechanism by which a molecular cloud fragments to form several hundred to thousands of individual stars has remained elusive. Here, we use a numerical simulation to follow the fragmentation of a turbulent molecular cloud and the subsequent formation and early evolution of a stellar cluster containing more than 400 stars. We show that the stellar cluster forms through the hierarchical fragmentation of a turbulent molecular cloud. This leads to the formation of many small subclusters which interact and merge to form the final stellar cluster. The hierarchical nature of the cluster formation has serious implications in terms of the properties of the new-born stars. The higher number-density of stars in subclusters, compared to a more uniform distribution arising from a monolithic formation, results in closer and more frequent dynamical interactions. Such close interactions can truncate circumstellar discs, harden existing binaries, and potentially liberate a population of planets. We estimate that at least one-third of all stars, and most massive stars, suffer such disruptive interactions.Comment: 6 pages, 4 figures, accepted for publication in MNRAS. Version including hi-res colour postscript figure available at http://star-www.st-and.ac.uk/~sgv/ps/clufhier.ps.g

Streaming motions and kinematic distances to molecular clouds

FGR-F and IAB gratefully acknowledge support from the ERC Advanced Grant ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC-2011-ADG.We present high-resolution smoothed particle hydrodynamics simulations of a region of gas flowing in a spiral arm and identify dense gas clouds to investigate their kinematics with respect to a Milky Way model. We find that, on average, the gas in the arms can have a net radial streaming motion of vR ≈ -9 km s-1 and rotate approximate to 6 km s-1 slower than the circular velocity. This translates to average peculiar motions towards the Galaxy centre and opposite to Galactic rotation. These results may be sensitive to the assumed spiral arm perturbation, which is ≈ 3 per cent of the disc potential in our model. We compare the actual distance and the kinematic estimate and we find that streaming motions introduce systematic offsets of ≈ 1 kpc. We find that the distance error can be as large as ± 2 kpc, and the recovered cloud positions have distributions that can extend significantly into the inter-arm regions. We conclude that this poses a difficulty in tracing spiral arm structure in molecular cloud surveys.Publisher PDFPeer reviewe

Large-scale gas flows and streaming motions in simulated spiral galaxies

FGR-F and IAB gratefully acknowledge support from the ERC ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC2011-ADG. FGR-F also acknowledges a St. Leonards Scholarship from the University of St Andrews and support from the Hyperstars project (funded by Région Paris Île-de-France DIMACAV+) at the final stages of this project. This equipment is funded by BIS National EInfrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1.From a galactic perspective, star formation occurs on the smallest scales within molecular clouds, but it is likely initiated from the large scale flows driven by galactic dynamics. To understand the conditions for star formation, it is important to first discern the mechanisms that drive gas from large-scales into dense structures on the smallest scales of a galaxy. We present high-resolution smoothed particle hydrodynamics simulations of two model spiral galaxies: one with a live stellar disc (N-body) and one with a spiral potential. We investigate the large-scale flows and streaming motions driven by the simulated spiral structure. We find that the strength of the motions in the radial direction tends to be higher than in the azimuthal component. In the N-body model, the amplitude of these motions decreases with galactocentric radius whereas for the spiral potential, it decreases to a minimum at the corotation radius, and increases again after this point. The results show that in both simulations, the arms induce local shocks, an increase in kinetic energy that can drive turbulence and a means of compressing and expanding the gas. These are all crucial elements in forming molecular clouds and driving the necessary conditions for star formation.PostprintPeer reviewe

Dynamical Friction on Star Clusters near the Galactic Center

Numerical simulations of the dynamical friction suffered by a star cluster near the Galactic center have been performed with a parallelized tree code. Gerhard (2001) has suggested that dynamical friction, which causes a cluster to lose orbital energy and spiral in towards the galactic center, may explain the presence of a cluster of very young stars in the central parsec, where star formation might be prohibitively difficult owing to strong tidal forces. The clusters modeled in our simulations have an initial total mass of 10^5-10^6 Msun and initial galactocentric radii of 2.5-30 pc. We have identified a few simulations in which dynamical friction indeed brings a cluster to the central parsec, although this is only possible if the cluster is either very massive (~10^6 Msun), or is formed near the central parsec (<~ 5 pc). In both cases, the cluster should have an initially very dense core (> 10^6 Msun pc-3). The initial core collapse and segregation of massive stars into the cluster core, which typically happens on a much shorter time scale than that characterizing the dynamical inspiral of the cluster toward the Galactic center, can provide the requisite high density. Furthermore, because it is the cluster core which is most likely to survive the cluster disintegration during its journey inwards, this can help account for the observed distribution of presumably massive HeI stars in the central parsec.Comment: Accepted for publication in Ap

Variations in Stellar Clustering with Environment: Dispersed Star Formation and the Origin of Faint Fuzzies

The observed increase in star formation efficiency with average cloud density, from several percent in whole giant molecular clouds to ~30 or more in cluster-forming cores, can be understood as the result of hierarchical cloud structure if there is a characteristic density as which individual stars become well defined. Also in this case, the efficiency of star formation increases with the dispersion of the density probability distribution function (pdf). Models with log-normal pdf's illustrate these effects. The difference between star formation in bound clusters and star formation in loose groupings is attributed to a difference in cloud pressure, with higher pressures forming more tightly bound clusters. This correlation accounts for the observed increase in clustering fraction with star formation rate and with the observation of Scaled OB Associations in low pressure environments. ``Faint fuzzie'' star clusters, which are bound but have low densities, can form in regions with high Mach numbers and low background tidal forces. The proposal by Burkert, Brodie & Larsen (2005) that faint fuzzies form at large radii in galactic collisional rings, satisfies these constraints.Comment: 14 pages, 2 figures, ApJ, 672, January 10th 200

Observational Implications of Precessing Protostellar Discs and Jets

We consider the dynamics of a protostellar disc in a binary system where the disc is misaligned with the orbital plane of the binary, with the aim of determining the observational consequences for such systems. The disc wobbles with a period approximately equal to half the binary's orbital period and precesses on a longer timescale. We determine the characteristic timescale for realignment of the disc with the orbital plane due to dissipation. If the dissipation is determined by a simple isotropic viscosity then we find, in line with previous studies, that the alignment timescale is of order the viscous evolution timescale. However, for typical protostellar disc parameters, if the disc tilt exceeds the opening angle of the disc, then tidally induced shearing within the disc is transonic. In general, hydrodynamic instabilities associated with the internally driven shear result in extra dissipation which is expected to drastically reduce the alignment timescale. For large disc tilts the alignment timescale is then comparable to the precession timescale, while for smaller tilt angles $\delta$, the alignment timescale varies as $(\sin \delta)^{-1}$. We discuss the consequences of the wobbling, precession and rapid realignment for observations of protostellar jets and the implications for binary star formation mechanisms.Comment: MNRAS, in press. 10 pages. Also available at http://www.ast.cam.ac.uk/~mbat