arXiv subjects

A. P. Whitworth

Publications and source records attributed to A. P. Whitworth.

At least 73 records · Page 4Linked to original sources

arXiv2010

Radiation Driven Implosion and Triggered Star Formation

We present simulations of initially stable isothermal clouds exposed to ionizing radiation from a discrete external source, and identify the conditions that lead to radiatively driven implosion and star formation. We use the Smoothed Particle Hydrodynamics code SEREN (Hubber et al. 2010) and the HEALPix-based photoionization algorithm described in Bisbas et al. (2009). We find that the incident ionizing flux is the critical parameter determining the evolution: high fluxes simply disperse the cloud, whereas low fluxes trigger star formation. We find a clear connection between the intensity of the incident flux and the parameters of star formation.

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arXiv2009

Smoothed Particle Hydrodynamics simulations of expanding HII regions. I. Numerical methods and tests

We describe a new algorithm for including the dynamical effects of ionizing radiation in SPH simulations, and we present several examples of how the algorithm can be applied to problems in star formation. We use the HEALPix software to tessellate the sky and to solve the equation of ionization equilibrium along a ray towards each of the resulting tesserae. We exploit the hierarchical nature of HEALPix to make the algorithm adaptive, so that fine angular resolution is invoked only where it is needed, and the computational cost is kept low. We present simulations of (i) the spherically symmetric expansion of an HII region inside a uniform-density, non--self-gravitating cloud; (ii) the spherically symmetric expansion of an HII region inside a uniform-density, self-gravitating cloud; (iii) the expansion of an off-centre HII region inside a uniform-density, non--self-gravitating cloud, resulting in rocket acceleration and dispersal of the cloud; and (iv) radiatively driven compression and ablation of a core overrun by an HII region. The new algorithm provides the means to explore and evaluate the role of ionizing radiation in regulating the efficiency and statistics of star formation.

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arXiv2008

Simulating star formation in molecular cloud cores IV. The role of turbulence and thermodynamics

We perform SPH simulations of the collapse and fragmentation of low-mass cores having different initial levels of turbulence (alpha_turb=0.05,0.10,0.25). We use a new treatment of the energy equation which captures the transport of cooling radiation against opacity due to both dust and gas (including the effects of dust sublimation, molecules, and H^- ions). We also perform comparison simulations using a standard barotropic equation of state. We find that -- when compared with the barotropic equation of state -- our more realistic treatment of the energy equation results in more protostellar objects being formed, and a higher proportion of brown dwarfs; the multiplicity frequency is essentially unchanged, but the multiple systems tend to have shorter periods (by a factor ~3), higher eccentricities, and higher mass ratios. The reason for this is that small fragments are able to cool more effectively with the new treatment, as compared with the barotropic equation of state. We find that the process of fragmentation is often bimodal. The first protostar to form is usually, at the end, the most massive, i.e. the primary. However, frequently a disc-like structure subsequently forms round this primary, and then, once it has accumulated sufficient mass, quickly fragments to produce several secondaries. We believe that this delayed fragmentation of a disc-like structure is likely to be an important source of very low-mass hydrogen-burning stars and brown dwarfs.

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arXiv2008

The direction of outflows from filaments : constraints on core formation

Context: It is generally presumed that the outflows from a YSO are directed close to its rotation axis (i.e. along its angular momentum vector and orthogonal to any attendant accretion disc). Many YSOs are formed from dense prestellar cores embedded in filaments, and therefore the relative orientations of outflows and filaments may place useful constraints on the dynamics of core formation. Aims: We explore this possibility, from the viewpoint of what it may tell us about the angular momentum delivered to a core forming in a filament. We stress that we are not here addressing the issue of the relationship of filaments and outflows to the prevailing magnetic field direction, although this is evidently also an interesting issue. Results: Using the limited data available, and neglecting any possible selection effects or correlations between nearby outflows, we infer that the observed outflows have a tendency to be orthogonal to the filaments that contain their driving sources. Specfically, in the cases where the directions of the filaments and outflows are well defined, we infer statistically that 72% of outflows are within $45^{\rm o}$ of being orthogonal to the filament, and only 28% are within $45^{\rm o}$ of being parallel to the filament. Conclusions: This suggests that the prestellar cores which spawned the YSOs driving the observed outflows had angular momenta which were approximately orthogonal to the filaments out of which the cores formed. We briefly discuss the implications of this for two proposed core formation mechanisms.

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arXiv2007

Radiative transfer and the energy equation in SPH simulations of star formation

We introduce and test a new and highly efficient method for treating the thermal and radiative effects influencing the energy equation in SPH simulations of star formation. The method uses the density, temperature and gravitational potential of each particle to estimate a mean optical depth, which then regulates the particle's heating and cooling. The method captures -- at minimal computational cost -- the effects of (i) the rotational and vibrational degrees of freedom of H2, H2 dissociation, H0 ionisation, (ii) opacity changes due to ice mantle melting, sublimation of dust, molecular lines, H-, bound-free and free-free processes and electron scattering; (iv) external irradiation; and (v) thermal inertia. The new algorithm reproduces the results of previous authors and/or known analytic solutions. The computational cost is comparable to a standard SPH simulation with a simple barotropic equation of state. The method is easy to implement, can be applied to both particle- and grid-based codes, and handles optical depths 0<tau<10^{11}.

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arXiv2007

The James Clerk Maxwell Telescope Legacy Survey of Nearby Star-forming Regions in the Gould Belt

This paper describes a James Clerk Maxwell Telescope (JCMT) legacy survey that has been awarded roughly 500 hrs of observing time to be carried out from 2007 to 2009. In this survey we will map with SCUBA-2 (Submillimetre Common User Bolometer Array 2) almost all of the well-known low-mass and intermediate-mass star-forming regions within 0.5 kpc that are accessible from the JCMT. Most of these locations are associated with the Gould Belt. From these observations we will produce a flux-limited snapshot of star formation near the Sun, providing a legacy of images, as well as point-source and extended-source catalogues, over almost 700 square degrees of sky. The resulting images will yield the first catalogue of prestellar and protostellar sources selected by submillimetre continuum emission, and should increase the number of known sources by more than an order of magnitude. We will also obtain CO maps with the array receiver HARP (Heterodyne Array Receiver Programme), in three CO isotopologues, of a large typical sample of prestellar and protostellar sources. We will then map the brightest hundred sources with the SCUBA-2 polarimeter (POL-2), producing the first statistically significant set of polarization maps in the submillimetre. The images and source catalogues will be a powerful reference set for astronomers, providing a detailed legacy archive for future telescopes, including ALMA, Herschel and JWST.

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arXiv2007

The dust temperatures of the prestellar cores in the rho Oph main cloud and in other star forming regions: consequences for the core mass function

We estimate the dust temperatures of the clumps in the rho Oph main cloud taking into account the 3D geometry of the region, and external heating from the interstellar radiation field and from HD147879,a nearby luminous B2V star, which is believed to dominate the radiation field in the region. We find that the regions where prestellar cores are observed (i.e. at optical visual extinctions >7 mag) are colder than ~10-11K. These dust temperatures are smaller than those which previous studies of the same region have assumed. We use the new dust temperatures to estimate the masses of the prestellar cores in the rho Oph main cloud from mm observations, and we find core masses that are larger than previous estimates by a factor of ~2-3. This affects the core mass function (CMF) of the region; we find that the mass at which the core mass spectrum steepens from a slope alpha~1.5 to a slope alpha~2.5 has moved from ~0.5 Msun to ~1Msun. In contrast with the CMF in other star forming regions (e.g. Orion), there is no indication for a turnover down to the completeness limit (~0.2Msun), but the CMF may flatten at around ~0.4Msun. We generalize our results to the prestellar cores in Taurus and in Orion. In Taurus the ambient radiation field heating the prestellar cores is believed to be weaker than than that in rho Oph. Hence, the dust temperatures of the cores in Taurus are expected to be below ~ 10-11 K.In Orion the radiation field is believed to be 10^3 times stronger than the standard interstellar radiation field. Based on this assumption we estimate that the dust temperatures of the prestellar cores in Orion are around ~20-30K.

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arXiv2007

High-resolution simulations of clump-clump collisions using SPH with Particle Splitting

We investigate, by means of numerical simulations, the phenomenology of star formation triggered by low-velocity collisions between low-mass molecular clumps. The simulations are performed using an SPH code which satisfies the Jeans condition by invoking On-the-Fly Particle Splitting. Clumps are modelled as stable truncated (non-singular) isothermal, i.e. Bonnor-Ebert, spheres. Collisions are characterised by M_0 (clump mass), b (offset parameter, i.e. ratio of impact parameter to clump radius), and M (Mach Number, i.e. ratio of collision velocity to effective post-shock sound speed). The gas subscribes to a barotropic equation of state, which is intended to capture (i) the scaling of pre-collision internal velocity dispersion with clump mass, (ii) post-shock radiative cooling, and (iii) adiabatic heating in optically thick protostellar fragments. The efficiency of star formation is found to vary between 10% and 30% in the different collisions studied and it appears to increase with decreasing M_0, and/or decreasing b, and/or increasing M. For b<0.5 collisions produce shock compressed layers which fragment into filaments. Protostellar objects then condense out of the filaments and accrete from them. The resulting accretion rates are high, 1 to 5 x 10^{-5} M_sun yr^{-1}, for the first 1 to 3 x 10^4 yrs. The densities in the filaments, n >~ 5 x 10^5 cm^{-3}, are sufficient that they could be mapped in NH_3 or CS line radiation, in nearby star formation regions.

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arXiv2007

Adaptive smoothing lengths in SPH

Context: There is a need to improve the fidelity of SPH simulations of self-gravitating gas dynamics. Aims: We remind users of SPH that, if smoothing lengths are adjusted so as to keep the number of neighbours, ${\cal N}$, in the range ${\cal N}_{_{\rm NEIB}}\pmΔ{\cal N}_{_{\rm NEIB}}$, the tolerance, $Δ{\cal N}_{_{\rm NEIB}}$, should be set to zero, as first noted by Nelson & Papaloizou. We point out that this is a very straightforward and computationally inexpensive constraint to implement. Methods: We demonstrate this by simulating acoustic oscillations of a self-gravitating isentropic monatomic gas-sphere (cf. Lucy), using ${\cal N}_{_{\rm TOT}}\sim6,000$ particles and ${\cal N}_{_{\rm NEIB}}=50$. Results: We show that there is a marked reduction in the rates of numerical dissipation and diffusion as $Δ{\cal N}_{_{\rm NEIB}}$ is reduced from 10 to zero. Moreover this reduction incurs a very small computational overhead. Conclusions: We propose that this should become a standard test for codes used in simulating star formation. It is a highly relevant test, because pressure waves generated by the switch from approximate isothermality to approximate adiabaticity play a critical role in the fragmentation of collapsing prestellar cores. Since many SPH simulations in the literature use ${\cal N}_{_{\rm NEIB}}=50$ and $Δ{\cal N}_{_{\rm NEIB}}\geq10$, their results must be viewed with caution.

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arXiv2006

Ionisation-induced star formation I: The collect and collapse model

We conduct Smoothed Particle Hydrodynamics simulations of the `collect and collapse' scenario (Elmegreen and Lada, 1977) for star formation triggered by an expanding HII region. We simulate the evolution of a spherical uniform molecular cloud with an ionising source at its centre. The gas in the cloud is self-gravitating, although the cloud is prevented from globally collapsing. We find that the shell driven by the HII region fragments to form numerous self--gravitating objects. We repeat our calculations at four numerical resolutions to ensure that they are converged. We compare our results to the analytical model of Whitworth et al, 1994 and show that our simulations and the predictions of Whitworth et al are in good agreement in the sense that the shell fragments at the time and radius predicted by Whitworth et al to within 20% and 25% respectively. Most of the fragments produced in our two highest resolution calculations are approximately half the mass of those predicted, but this conclusion is robust against both numerical resolution and the presence of random noise (local fluctuations in density of a factor of $\sim2$) in the initial gas distribution. We conclude that such noise has little impact on the fragmentation process.

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arXiv2006

A VLA search for young protostars embedded in dense cores

Four dense cores, L1582A, L1689A, B133 and B68, classified as prestellar in terms of the absence of detectable NIR emission, are observed at radio wavelengths to investigate whether they nurture very young protostars. No definite young protostars were discovered in any of the four cores observed. A few radio sources were discovered close to the observed cores, but these are most likely extragalactic sources or YSOs unrelated to the cores observed. In L1582A we discovered a weak radio source near the centre of the core with radio characteristics and offset from the peak of the submillimeter emission similar to that of the newly discovered protostar in the core L1014, indicating a possible protostellar nature for this source. This needs to be confirmed with near- and/or mid-infrared observations (e.g. with Spitzer). Hence based on the current observations we are unable to confirm unequivocally that L1582A is starless. In L1689A a possible 4.5-sigma radio source was discovered at the centre of the core, but needs to be confirmed with future observations. In B133 a weak radio source, possibly a protostar, was discovered at the edge of the core on a local peak of the core submm emission, but no source was detected at the centre of the core. Thus, B133 is probably starless, but may have a protostar at its edge. In B68 no radio sources were discovered inside or at the edge of the core, and thus B68 is indeed starless. Four more radio sources with spectral indices characteristic of young protostars were discovered outside the cores but within the extended clouds in which these cores reside. Conclusions: We conclude that the number of cores misclassified as prestellar is probably very small and does not significantly alter the estimated lifetime of the prestellar phase.

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arXiv2006

The minimum mass for star formation, and the origin of binary brown dwarfs

Our first aim is to calculate the minimum mass for Primary Fragmentation in a variety of potential star-formation scenarios, i.e. (i) hierarchical fragmentation of a 3-D medium; (ii) one-shot, 2-D fragmentation of a shock-compressed layer; (iii) fragmentation of a circumstellar disc. Our second aim is to evaluate the role of H2 dissociation in facilitating Secondary Fragmentation and thereby producing close, low-mass binaries. Results: (i)For contemporary, local star formation, the minimum mass for Primary Fragmentation is in the range 0.001-0.004Msun, irrespective of the scenario considered. (ii)Circumstellar discs are only able to radiate fast enough to undergo Primary Fragmentation in their cool outer parts (R>100AU). Therefore brown dwarfs (BDs) should have difficulty forming by Primary Fragmentation at R<30AU, explaining the Brown Dwarf Desert.Conversely, Primary Fragmentation at R>100AU could be the source of brown dwarfs in wide orbits, and could explain why massive discs with Rd>100AU are rarely seen.(iii)H2 dissociation can lead to collapse and Secondary Fragmentation, thereby converting primary fragments into close, low-mass binaries, with semi-major axes a~5AU(Msystem/0.1Msun), in good agreement with observation; in this case, the minimum mass for Primary Fragmentation becomes a minimum system mass, rather than a minimum stellar mass.(iv)Any primary fragment can undergo Secondary Fragmentation, producing a close low-mass binary, provided only that the fragment is spinning. Secondary Fragmentation is therefore most likely in fragments formed in the outer parts of discs, and this could explain why a BD in a wide orbit about a Sun-like star has a greater likelihood of having a BD companion than a BD in the field -as seems to be observed.

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arXiv2006

Star formation in molecular cores III. The effect of the turbulent power spectrum

We investigate the effect of the turbulent power spectrum (P(k) \propto k^{-n}, with n=3, 4 or 5) on the fragmentation of low-mass cores, by means of SPH simulations. We adopt initial density profiles and low levels of turbulence based on observation, and for each n-value we conduct an ensemble of simulations with different initial seeds for the turbulent velocity field, so as to obtain reasonable statistics. We find that when power is concentrated at larger scales (i.e. for larger n), more protostellar objects form and there is a higher proportion of low-mass stars and brown dwarfs. This is in direct contrast with the recent results of Delgado Donate et al., presumably because they adopted much higher levels of turbulence.

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arXiv2006

A dynamical model for the dusty ring in the Coalsack

Lada et al. recently presented a detailed near-infrared extinction map of Globule G2 in the Coalsack molecular cloud complex, showing that this starless core has a well-defined central extinction minimum. We propose a model for G2 in which a rapid increase in external pressure is driving an approximately symmetric compression wave into the core. The rapid increase in external pressure could arise because the core has recently been assimilated by the Coalsack cloud complex, or because the Coalsack has recently been created by two large-scale converging flows. The resulting compression wave has not yet converged on the centre of the core, so there is a central rarefaction. The compression wave has increased the density in the swept-up gas by about a factor of ten, and accelerated it inwards to speeds of order $0.4 {\rm km} {\rm s}^{-1}$. It is shown that even small levels of initial turbulence destroy the ring seen in projection almost completely. In the scenario of strong external compression that we are proposing this implies that the initial turbulent energy in this globule is such that $E_{\rm turb} / E_{\rm grav} \le 2 %$. Protostar formation should occur in about $40,000 {\rm years}$.

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arXiv2005

The formation of brown dwarfs

We review four mechanisms for forming brown dwarfs: (i) turbulent fragmentation (producing very low-mass prestellar cores); (ii) gravitational instabilities in discs; (iii) dynamical ejection of stellar embryos from their placental cores; and (iv) photo-erosion of pre-existing cores in HII regions. We argue (a) that these are simply the mechanisms of low-mass star formation, and (b) that they are not mutually exclusive. If, as seems possible, all four mechanisms operate in nature, their relative importance may eventually be constrained by their ability to reproduce the binary statistics of brown dwarfs, but this will require fully 3-D radiative magneto-hydrodynamic simulations.

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arXiv2005

Resolution requirements for simulating gravitational fragmentation using SPH

Jeans showed analytically that, in an infinite uniform-density isothermal gas, plane-wave perturbations collapse to dense sheets if their wavelength, $λ$, satisfies $λ> λ_{_{\rm JEANS}} = (πa^2 / G ρ_{_0})^{1/2}$ (where $a$ is the isothermal sound speed and $ρ_{_0}$ is the unperturbed density); in contrast, perturbations with smaller $λ$ oscillate about the uniform density state. Here we show that Smoothed Particle Hydrodynamics reproduces these results well, even when the diameters of the SPH particles are twice the wavelength of the perturbation. Our simulations are performed in 3-D with initially settled (i.e. non-crystalline) distributions of particles. Therefore there exists the seed noise for artificial fragmentation, but it does not occur. We conclude that, although there may be -- as with any numerical scheme -- `skeletons in the SPH cupboard', a propensity to fragment artificially is evidently not one of them.

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arXiv2005

Observational characteristics of dense cores with deeply embedded young protostars

Class 0 objects, which are thought to be the youngest protostars, are identified in terms of NIR or radio emission and/or the presence of molecular outflows. We present combined hydrodynamic and radiative transfer simulations of the collapse of a star-forming molecular core, which suggest two criteria for identifying dense cores with deeply embedded very young protostars that may not be observable in the NIR or radio with current telescopes. We find that cores with protostars are relatively warm (T>15K) with their SEDs peaking at wavelengths <170 micron, and they tend to appear circular.

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arXiv2005

How to identify the youngest protostars

We study the transition from a prestellar core to a Class 0 protostar, using SPH to simulate the dynamical evolution, and a Monte Carlo radiative transfer code to generate the SED and isophotal maps. For a prestellar core illuminated by the standard interstellar radiation field, the luminosity is low and the SED peaks at ~190 micron. Once a protostar has formed, the luminosity rises (due to a growing contribution from accretion onto the protostar) and the peak of the SED shifts to shorter wavelengths (~80-100 micron). However, by the end of the Class 0 phase, the accretion rate is falling, the luminosity has decreased, and the peak of the SED shifts back towards longer wavelengths (90-150 micron). In our simulations, the density of material around the protostar remains sufficiently high well into the Class 0 phase that the protostar only becomes visible in the NIR if it is displaced from the centre dynamically. Raw submm/mm maps of Class 0 protostars tend to be much more centrally condensed than those of prestellar cores. However, when convolved with a typical telescope beam, the difference in central concentration is less marked, although the Class 0 protostars appear more circular. Our results suggest that, if a core is deemed to be prestellar on the basis of having no associated IRAS source, no cm radio emission, and no outflow, but it has a circular appearance and an SED which peaks at wavelengths below ~170 micron, it may well contain a very young Class 0 protostar.

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