arXiv subjects

Cathie J. Clarke

Publications and source records attributed to Cathie J. Clarke.

106 records · Page 6Linked to original sources

arXiv2011

On the theory of disc photoevaporation

We discuss a hydrodynamical model for the dispersal of protoplanetary discs around young, low mass (<1.5 M_sun) stars by photoevaporation from the central object's energetic radiation, which considers the far-ultraviolet as well as the X-ray component of the radiation field. We present analytical scaling relations and derive estimates for the total mass-loss rates, as well as discussing the existence of similarity solutions for flows from primordial discs and discs with inner holes. Furthermore, we perform numerical calculations, which span a wide range of parameter space and allow us to provide accurate scalings of the mass-loss rates with the physical parameters of the systems (X-ray and FUV luminosity, stellar mass, disc mass, disc temperature and inner hole radius). The model suggest that the X-ray component dominates the photoevaporative mass-loss rates from the inner disc. The mass-loss rates have values in the range from 10e-11 to 10e-7 M_sun/yr and scale linearly with X-ray luminosity, with only a weak dependence on the other parameters explored. However, in the case of high FUV to X-ray (L_FUV/L_X>100) luminosity ratios, the FUV constricts the X-ray flow and may dominate the mass-loss. Simulations of low mass discs with inner holes demonstrate a further stage of disc clearing, which we call `thermal sweeping'. This process occurs when the mid-plane pressure drops to sufficiently low values. At this stage a bound, warm, X-ray heated region becomes sufficiently large and unstable, such that the remaining disc material is cleared on approximately dynamical time-scales. This process significantly reduces the time taken to clear the outer regions of the disc, resulting in an expected transition disc population that will be dominated by accreting objects, as indicated by recent observations.

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arXiv2011

Testing the accuracy of radiative cooling approximations in SPH simulations

Hydrodynamical simulations of star formation have stimulated a need to develop fast and robust algorithms for evaluating radiative cooling. Here we undertake a critical evaluation of what is currently a popular method for prescribing cooling in SPH simulations, i.e. the polytropic cooling due originally to Stamatellos et al. This method uses the local density and potential to estimate the column density and optical depth to each particle and then uses these quantities to evaluate an approximate expression for the net radiative cooling. We evaluate the algorithm by considering both spherical and disc-like systems with analytic density and temperature structures. In spherical systems, the total cooling rate computed by the method is within around 20 for the astrophysically relevant case of opacity dominated by ice grains and is correct to within a factor of order unity for a range of opacity laws. In disc geometry, however, the method systematically under-estimates the cooling by a large factor at all heights in the disc. For the self-gravitating disc studied, we find that the method under-estimates the total cooling rate by a factor of 200. This discrepancy may be readily traced to the method's systematic over-estimate of the disc column density and optical depth, since (being based only on the local density and potential) it does not take into account the low column density route for photon escape normal to the disc plane. These results raise an obvious caution about the method's use in disc geometry whenever an accurate cooling rate is required, although we note that there are situations where the discrepancies highlighted above may not significantly affect the global outcome of simulations. Finally, we draw attention to our introduction of an analytic self-gravitating disc structure that may be of use in the calibration of future cooling algorithms.

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arXiv2011

The dynamical state of stellar structure in star-forming regions

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.

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arXiv2011

Debris disk size distributions: steady state collisional evolution with P-R drag and other loss processes

We present a new scheme for determining the shape of the size distribution, and its evolution, for collisional cascades of planetesimals undergoing destructive collisions and loss processes like Poynting-Robertson drag. The scheme treats the steady state portion of the cascade by equating mass loss and gain in each size bin; the smallest particles are expected to reach steady state on their collision timescale, while larger particles retain their primordial distribution. For collision-dominated disks, steady state means that mass loss rates in logarithmic size bins are independent of size. This prescription reproduces the expected two phase size distribution, with ripples above the blow-out size, and above the transition to gravity-dominated planetesimal strength. The scheme also reproduces the expected evolution of disk mass, and of dust mass, but is computationally much faster than evolving distributions forward in time. For low-mass disks, P-R drag causes a turnover at small sizes to a size distribution that is set by the redistribution function (the mass distribution of fragments produced in collisions). Thus information about the redistribution function may be recovered by measuring the size distribution of particles undergoing loss by P-R drag, such as that traced by particles accreted onto Earth. Although cross-sectional area drops with 1/age^2 in the PR-dominated regime, dust mass falls as 1/age^2.8, underlining the importance of understanding which particle sizes contribute to an observation when considering how disk detectability evolves. Other loss processes are readily incorporated; we also discuss generalised power law loss rates, dynamical depletion, realistic radiation forces and stellar wind drag.

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arXiv2011

The formation of permanent soft binaries in dispersing clusters

Wide, fragile binary stellar systems are found in the galactic field, and have recently been noted in the outskirts of expanding star clusters in numerical simulations. Energetically soft, with semi-major axes exceeding the initial size of their birth cluster, it is puzzling how these binaries are created and preserved. We provide an interpretation of the formation of these binaries that explains the total number formed and their distribution of energies. A population of weakly bound binaries can always be found in the cluster, in accordance with statistical detailed balance, limited at the soft end only by the current size of the cluster and whatever observational criteria are imposed. At any given time, the observed soft binary distribution is predominantly a snapshot of a transient population. However, there is a constantly growing population of long-lived soft binaries that are removed from the detailed balance cycle due to the changing density and velocity dispersion of an expanding cluster. The total number of wide binaries that form, and their energy distribution, are insensitive to the cluster population; the number is approximately one per cluster. This suggests that a population composed of many dissolved small-N clusters will more efficiently populate the field with wide binaries than that composed of dissolved large-N clusters. Locally such binaries are present at approximately the 2% level; thus the production rate is consistent with the field being populated by clusters with a median of a few hundred stars rather than a few thousand.

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arXiv2010

The imprint of photoevaporation on edge-on discs

We have performed hydrodynamic and radiative transfer calculations of a photoevaporating disc around a Herbig Ae/Be star to determine the evolution and observational impact of dust entrained in the wind. We find that the wind selectively entrains grains of different sizes at different radii resulting in a dust population that varies spatially and increases with height above the disc at radii > 10 AU. This variable grain population results in a 'wingnut' morphology to the dust density distribution. We calculate images of this dust distribution at NIR wavelengths that also show a wingnut morphology at all wavelengths considered. We have also considered the contribution that crystalline dust grains will have in the wind and show that a photoevaporative wind can result in a significant crystallinity fraction at all radii, when the disc is edge-on. However, when the disc's photosphere is unobscured, a photoevaporative wind makes no contribution to the observable crystallinity fraction in the disc. Finally, we conclude that the analysis of extended emission around edge-on discs could provide a new and independent method of testing photoevaporation models.

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arXiv2010

Collisional formation of very massive stars in dense clusters

We investigate the contraction of accreting protoclusters using an extension of n-body techniques that incorporates the accretional growth of stars from the gaseous reservoir in which they are embedded. Following on from Monte Carlo studies by Davis et al., we target our experiments toward populous clusters likely to experience collisions as a result of accretion-driven contraction. We verify that in less extreme star forming environments, similar to Orion, the stellar density is low enough that collisions are unimportant, but that conditions suitable for stellar collisions are much more easily satisfied in large-n clusters, i.e. n ~ 30,000 (we argue, however, that the density of the Arches cluster is insufficient for us to expect stellar collisions to have occurred in the cluster's prior evolution). We find that the character of the collision process is not such that it is a route toward smoothly filling the top end of the mass spectrum. Instead, runaway growth of one or two extreme objects can occur within less than 1 Myr after accretion is shut off, resulting in a few objects with masses several times the maximum reached by accretion. The rapid formation of these objects is due to not just the post-formation dynamical evolution of the clusters, but an interplay of dynamics and the accretional growth of the stars. We find that accretion-driven cluster shrinkage results in a distribution of gas and stars that offsets the disruptive effect of gas expulsion, and we propose that the process can lead to massive binaries and early mass segregation in star clusters.

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arXiv2010

The clearing of discs around late type T Tauri stars: constraints from the infrared two colour plane

We have undertaken SED modeling of discs around low mass T Tauri stars in order to delineate regions of the infrared two colour plane (K - [8] versus K - [24]) that correspond to discs in different evolutionary stages. This provides a ready tool for classifying the nature of star-disc systems based on infrared photometry. In particular we demonstrate the distinct loci followed by discs that undergo `uniform draining' (reduction in surface density by a spatially uniform factor) from those that clear from the inside out. We draw attention to the absence of objects on this `draining locus' in those star forming regions where the 24um sensitivity would permit their detection, as compared with the ~20 objects in these regions with colours suggestive of inner holes. We thus conclude that discs predominantly clear from the inside out. We also apply our classification of the infrared two colour plane to stars of spectral type M3-M5 in the IC 348 cluster and show that some of its members (dubbed `weak excess' sources by Muzerolle et al 2010) that had previously been assumed to be in a state of clearing are instead likely to be optically thick discs in which the dust is well settled towards the mid-plane. Nevertheless, there are many discs in a state of partial clearing in IC 348, with their abundance (relative to the total population of disc bearing stars) being around four times higher than for comparable stars in Taurus. However, the number of partially cleared discs relative to the total number of late type stars is similarly low in both regions (10 and 20 % respectively). We therefore conclude that IC 348 represents a more evolved version of the Taurus population (with more of its discs being highly settled or partially cleared) but that the timescale for clearing is similarly short (a few times 10^5 years) in both cases.

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arXiv2009

X-ray irradiated protoplanetary disk atmospheres II: Predictions from models in hydrostatic equilibrium

We present new models for the X-ray photoevaporation of circumstellar discs which suggest that the resulting mass loss (occurring mainly over the radial range 10-40 AU) may be the dominant dispersal mechanism for gas around low mass pre-main sequence stars, contrary to the conclusions of previous workers. Our models combine use of the MOCASSIN Monte Carlo radiative transfer code and a self-consistent solution of the hydrostatic structure of the irradiated disc. We estimate the resulting photoevaporation rates assuming sonic outflow at the surface where the gas temperature equals the local escape temperature and derive mass loss rates of ~10^{-9} M_sun/yr, typically a factor 2-10 times lower than the corresponding rates in our previous work (Ercolano et al., 2008) where we did not adjust the density structure of the irradiated disc. The somewhat lower rates, and the fact that mass loss is concentrated towards slightly smaller radii, result from the puffing up of the heated disc at a few AU which partially screens the disc at tens of AU. (.....) We highlight the fact that X-ray photoevaporation has two generic advantages for disc dispersal compared with photoevaporation by extreme ultraviolet (EUV) photons that are only modestly beyond the Lyman limit: the demonstrably large X-ray fluxes of young stars even after they have lost their discs and the fact that X-rays are effective at penetrating much larger columns of material close to the star (abridged).

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arXiv2009

The transition disc frequency in M stars

We re-examine the recent suggestion of a high fraction of transition discs (i.e. those with a cleared inner hole) in M stars, motivated by the fact that we expect that, for M stars, even discs without inner holes should exhibit very weak excess shortward of around 10um. Our analysis of spectral energy distribution models suggest that this indeed means that M stars where a detectable excess begins at around 6um may be mis-classified as transition discs when in fact they have optically thick dust extending in to the dust sublimation radius. Consequently, we estimate that the transition disc fraction among M stars in the Coronet cluster is ~15 +/-10 % (rather than the recently claimed value of 50%). This revised figure would imply that the transition disc fraction is not after all markedly higher in later type stars. We suggest that for M stars, transition discs can only be readily identified if they have emission that is close to photospheric out to > 10um.

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arXiv2003

The spectroscopic signature of hot Jupiters in FU Orionis objects

We show that if FU Orionis objects harbour hot Jupiters embedded in their discs, the resulting non-axisymmetric dissipation profile in the disc would be manifest as time-dependent distortions in the absorption line profiles of these objects. In order to affect the infrared line profiles, planets must lie within approximately 0.5 au of the central star, whereas only planets within 0.1 au would influence the optical line profiles. The timescale for modulation of the line profiles is relatively short (months) in each case, so that the effect could not have been discovered from published spectra (which combine data taken in different observing seasons). The detection of hot Jupiters in FU Orionis objects would be in line with the expectations of tidal migration theories (which predict a high incidence of close planets around young stars) and would also lend support to models which link the triggering of rapid rise FU Orionis events to the existence of a close massive planet.

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arXiv2003

The characteristic stellar mass as a function of redshift

We present a model for the star formation process during the initial collapse of dark matter haloes at redshifts z=0-30. We derive a simple expression for the characteristic stellar mass scale during this initial burst of star formation. In our picture, this characteristic scale reflects both the minimum temperature to which the gas can cool (determined by the metallicity and the temperature of the cosmic microwave background) and the pressure of overlying baryons in the collapsing halo. This prescription reproduces both the large mass scales found in simulations of Population III star formation and the near solar values observed for star formation at low redshift.

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arXiv2003

Dispersion in the lifetime and accretion rate of T Tauri discs

We compare evolutionary models for protoplanetary discs that include disc winds with observational determinations of the disc lifetime and accretion rate in Taurus. Using updated estimates for stellar ages in Taurus, together with published classifications, we show that the evolution of the disc fraction with stellar age is similar to that derived for ensembles of stars within young clusters. Around 30 percent of stars lose their discs within 1 Myr, while the remainder have disc lifetimes that are typically in the 1-10 Myr range. We show that the latter range of ages is consistent with theoretical models for disc evolution, provided that there is a dispersion of around 0.5 in the log of the initial disc mass. The same range of initial conditions brackets the observed variation in the accretion rate of Classical T Tauri stars at a given age. We discuss the expected lifetime of discs in close binary systems, and show that our models predict that the disc lifetime is almost constant for separations exceeding 10 au. This implies a low predicted fraction of binaries that pair a Classical T Tauri star with a Weak-lined T Tauri star, and is in better agreement with observations of the disc lifetime in binaries than disc models that do not include disc mass loss in a wind.

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arXiv2002

The Formation of the First Globular Clusters in Dwarf Galaxies Before the Epoch of Reionization

We explore a mechanism for the formation of the first globular clusters, operating during the assembly of dwarf galaxies at high redshifts, z > 10. The substructure in the dark matter and the corresponding potential wells are responsible for setting the cluster scale of ~10^5 M_sun. The second mass scale in the formation problem, the stellar scale of ~1 M_sun, is determined in turn by the processes that cool the gas. We address the origin of the first, cluster scale by means of three-dimensional numerical simulations of the collapsing dark matter and gaseous components. We find that the gas falls into the deepest dark subhalos, resulting in a system of ~ 5 proto-globular clouds. The incipient globular clusters lose their individual dark halos in the process of violent relaxation, leading to the build-up of the general dark halo around the dwarf galaxy.

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arXiv1999

The Formation of Stellar Clusters

We review recent work that investigates the formation of stellar clusters, ranging in scale from globular clusters through open clusters to the small scale aggregates of stars observed in T associations. In all cases, recent advances in understanding have been achieved through the use of state of the art stellar dynamical and gas dynamical calculations, combined with the possibility of intercomparison with an increasingly large dataset on young clusters. Among the subjects that are highlighted are the frequency of cluster-mode star formation, the possible relationship between cluster density and the highest stellar mass, subclustering and the dynamical interactions that occur in compact aggregates, such as binary star formation. We also consider how the spectrum of stellar masses may be shaped by the process of competitive accretion in dense environments and how cluster properties, such as mass segregation and cluster morphology, can be used in conjunction with numerical simulations to investigate the initial conditions for cluster formation. Lastly, we contrast bottom-up and top-down scenarios for cluster formation and discuss their applicability to the formation of clusters on a range of scales.

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arXiv1998

Interpreting the Mean Surface Density of Companions in Star-Forming Regions

We study the interpretation of the mean surface density of stellar companions as a function of separation (or the two point correlation function of stars) in star-forming regions. First, we consider the form of the functions for various simple stellar distributions (binaries, global density profiles, clusters, and fractals) and the effects of survey boundaries. Following this, we study the dependencies of the separation at which a transition from the binary to the large-scale clustering regime occurs. Larson (1995) found that the mean surface density of companions follows different power-law functions of separation in the two regimes. He identified the transition separation with the typical Jeans length in the molecular cloud. However, we show that this is valid only for special cases. In general, the transition separation depends on the volume density of stars, the depth of the star-forming region, the volume-filling nature of the stellar distribution, and on the parameters of the binaries. Furthermore, the transition separation evolves with time. We then apply these results to the Taurus-Auriga, Ophiuchus, and Orion Trapezium star-forming regions. We find that while the transition separation in the Taurus-Auriga star-forming region may indicate a typical Jeans length, this is not true of the Orion Trapezium Cluster. We also show that there is no evidence for sub-clustering or fractal structure in the stars of the Orion Trapezium Cluster. This is consistent with the fact that, if such structure were present when the stars formed, it would have been erased by the current age of the cluster due to the stellar velocity dispersion. [Abstract abridged]

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