arXiv subjects

D. Ward-Thompson

Publications and source records attributed to D. Ward-Thompson.

At least 127 records · Page 7Linked to original sources

arXiv2007

An Unbiased Survey of 500 Nearby Stars for Debris Disks: A JCMT Legacy Program

We present the scientific motivation and observing plan for an upcoming detection survey for debris disks using the James Clerk Maxwell Telescope. The SCUBA-2 Unbiased Nearby Stars (SUNS) Survey will observe 500 nearby main sequence and sub-giant stars (100 of each of the A, F, G, K and M spectral classes) to the 850 micron extragalactic confusion limit to search for evidence of submillimeter excess, an indication of circumstellar material. The survey distance boundaries are 8.6, 16.5, 22, 25 and 45 pc for M, K, G, F and A stars, respectively, and all targets lie between the declinations of -40 deg to 80 deg. In this survey, no star will be rejected based on its inherent properties: binarity, presence of planetary companions, spectral type or age. This will be the first unbiased survey for debris disks since IRAS. We expect to detect ~125 debris disks, including ~50 cold disks not detectable in current shorter wavelength surveys. A substantial amount of complementary data will be required to constrain the temperatures and masses of discovered disks. High resolution studies will likely be required to resolve many of the disks. Therefore, these systems will be the focus of future observational studies using a variety of observatories to characterize their physical properties. For non-detected systems, this survey will set constraints (upper limits) on the amount of circumstellar dust, of typically 200 times the Kuiper Belt mass, but as low as 10 times the Kuiper Belt mass for the nearest stars in the sample (approximately 2 pc).

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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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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

A SCUBA survey of Orion, the low-mass end of the core mass function

We have re-analysed all of the SCUBA archive data of the Orion star-forming regions. We have put together all of the data taken at different times by different groups. Consequently we have constructed the deepest submillimetre maps of these regions ever made. There are four regions that have been mapped: Orion A North & South, and Orion B North & South. We find that two of the regions, Orion A North and Orion B North, have deeper sensitivity and completeness limits, and contain a larger number of sources, so we concentrate on these two. We compare the data with archive data from the Spitzer Space Telescope to determine whether or not a core detected in the submillimetre is pre-stellar in nature. We extract all of the pre-stellar cores from the data and make a histogram of the core masses. This can be compared to the stellar initial mass function (IMF). We find the high-mass core mass function follows a roughly Salpeter-like slope, just like the IMF, as seen in previous work. Our deeper maps allow us to see that the core mass function (CMF) turns over at ~ 1.3 Mo, about a factor of 4 higher than our completeness limit. This turnover has never previously been observed, and is only visible here due to our much deeper maps. It mimics the turnover seen in the stellar IMF at ~ 0.1 Mo. The low-mass side of the CMF is a power-law with an exponent of 0.35 +/- 0.2, which is consistent with the low-mass slope of the young cluster IMF of 0.3 +/- 0.1. This shows that the CMF continues to mimic the shape of the IMF all the way down to the lower completeness limit of these data at ~ 0.3 Mo.

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arXiv2006

The initial conditions of isolated star formation - VII. Spitzer mapping of pre-stellar cores

We have retrieved Spitzer archive data of pre-stellar cores taken with the Multiband Imaging Photometer for Spitzer (MIPS) at a wavelength of 160 um. Seventeen images, containing eighteen cores, were constructed. Flux densities were measured for each core, and background estimates were made. Mean off-source backgrounds were found to be 48\pm10 MJy/sr in Taurus and 140\pm55 MJy/sr in Ophiuchus. Consistency was found between the MIPS 170um and ISOPHOT 160um calibration. Fourteen cores were detected both by MIPS and our previous submillimetre surveys. Spectral Energy Distributions (SEDs) were made for each core, using additional 24- and 70-um data from the Spitzer data archive, as well as previous infra-red and submillimetre data. Previous temperature estimates were refined, and new temperature estimates were made where no Infrared Space Observatory (ISO) data exist. A temperature range of 8-18K was found for the cores, with most lying in the range 10-13K. We discount recent claims that a large number of pre-stellar cores may have been misclassified and in fact contain low luminosity protostars detectable only by Spitzer. We find no new protostars in our sample other than that previously reported in L1521F. It is shown that this has a negligible effect on pre-stellar lifetime estimates.

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arXiv2006

SCUBA observations of the Horsehead Nebula - what did the horse swallow?

We present observations taken with SCUBA on the JCMT of the Horsehead Nebula in Orion (B33), at wavelengths of 450 and 850 \mum. We see bright emission from that part of the cloud associated with the photon-dominated region (PDR) at the `top' of the horse's head, which we label B33-SMM1. We characterise the physical parameters of the extended dust responsible for this emission, and find that B33-SMM1 contains a more dense core than was previously suspected. We compare the SCUBA data with data from the Infrared Space Observatory (ISO) and find that the emission at 6.75-\mum is offset towards the west, indicating that the mid-infrared emission is tracing the PDR while the submillimetre emission comes from the molecular cloud core behind the PDR. We calculate the virial balance of this core and find that it is not gravitationally bound but is being confined by the external pressure from the HII region IC434, and that it will either be destroyed by the ionising radiation, or else may undergo triggered star formation. Furthermore we find evidence for a lozenge-shaped clump in the `throat' of the horse, which is not seen in emission at shorter wavelengths. We label this source B33-SMM2 and find that it is brighter at submillimetre wavelengths than B33-SMM1. SMM2 is seen in absorption in the 6.75-\mum ISO data, from which we obtain an independent estimate of the column density in excellent agreement with that calculated from the submillimetre emission. We calculate the stability of this core against collapse and find that it is in approximate gravitational virial equilibrium. This is consistent with it being a pre-existing core in B33, possibly pre-stellar in nature, but that it may also eventually undergo collapse under the effects of the HII region.

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arXiv2006

An Observational Perspective of Low Mass Dense Cores II: Evolution towards the Initial Mass Function

We review the properties of low mass dense molecular cloud cores, including starless, prestellar, and Class 0 protostellar cores, as derived from observations. In particular we discuss them in the context of the current debate surrounding the formation and evolution of cores. There exist several families of model scenarios to explain this evolution (with many variations of each) that can be thought of as a continuum of models lying between two extreme paradigms for the star and core formation process. At one extreme there is the dynamic, turbulent picture, while at the other extreme there is a slow, quasi-static vision of core evolution. In the latter view the magnetic field plays a dominant role, and it may also play some role in the former picture. Polarization and Zeeman measurements indicate that some, if not all, cores contain a significant magnetic field. Wide-field surveys constrain the timescales of the core formation and evolution processes, as well as the statistical distribution of core masses. The former indicates that prestellar cores typically live for 2--5 free-fall times, while the latter seems to determine the stellar initial mass function. In addition, multiple surveys allow one to compare core properties in different regions. From this it appears that aspects of different models may be relevant to different star-forming regions, depending on the environment. Prestellar cores in cluster-forming regions are smaller in radius and have higher column densities, by up to an order of magnitude, than isolated prestellar cores. This is probably due to the fact that in cluster-forming regions the prestellar cores are formed by fragmentation of larger, more turbulent cluster-forming cores, which in turn form as a result of strong external compression.

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arXiv2006

A SCUBA survey of L1689 - The dog that didn't bark

We present submillimetre data for the L1689 cloud in the rho-Ophiuchi molecular cloud complex. We detect a number of starless and prestellar cores and protostellar envelopes. We also detect a number of filaments for the first time in the submillimetre continuum that are parallel both to each other, and to filaments observed in the neighbouring L1688 cloud. These filaments are also seen in the 13CO observations of L1689. The filaments contain all of the star-formation activity in the cloud. L1689 lies next to the well studied L1688 cloud that contains the rho Oph-A core. L1688 has a much more active star-formation history than L1689 despite their apparent similarity in 13CO data. Hence we label L1689 as the dog that didn't bark. We endeavour to explain this apparent anomaly by comparing the total mass of each cloud that is currently in the form of dense material such as prestellar cores. We note firstly that L1688 is more massive than L1689, but we also find that when normalised to the total mass of each cloud, the L1689 cloud has a much lower percentage of mass in dense cores than L1688. We attribute this to the hypothesis of Loren (1989) that the star formation in the rho-Ophiuchi complex is being affected and probably dominated by the external influence of the nearby Upper Scorpius OB association and predominantly by sigma-Sco. L1689 is further from sigma-Sco and is therefore less active. The influence of sigma-Sco appears nonetheless to have created the filaments that we observe in L1689. Accepted by MNRAS.

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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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arXiv2005

First ground-based 200-um observing with THUMPER on JCMT - sky characterisation and planet maps

We present observations that were carried out with the Two HUndred Micron PhotometER (THUMPER) mounted on the James Clerk Maxwell Telescope (JCMT) in Hawaii, at a wavelength of 200 um (frequency 1.5 THz). The observations utilise a small atmospheric window that opens up at this wavelength under very dry conditions at high-altitude observing sites. The atmosphere was calibrated using the sky-dipping method and a relation was established between the optical depth, tau, at 1.5 THz and that at 225 GHz: tau_1.5THz = (95 +/- 10)*tau_225GHz. Mars and Jupiter were mapped from the ground at this wavelength for the first time, and the system characteristics measured. A noise equivalent flux density (NEFD) of ~65 +/- 10 Jy (1 sigma 1 second) was measured for the THUMPER-JCMT combination, consistent with predictions based upon our laboratory measurements. The main-beam resolution of 14 arcsec was confirmed and an extended error-beam detected at roughly two-thirds of the magnitude of the main beam. Measurements of the Sun allow us to estimate that the fraction of the power in the main beam is ~15%, consistent with predictions based on modelling the dish surface accuracy. It is therefore shown that the sky over Mauna Kea is suitable for astronomy at this wavelength under the best conditions. However, higher or drier sites should have a larger number of useable nights per year.

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arXiv2005

The initial conditions of isolated star formation - VI. SCUBA mapping of prestellar cores

Observations have been carried out with SCUBA at the JCMT of 52 molecular cloud cores that do not contain any sign of protostellar activity. These are all therefore candidate prestellar cores, which are believed to represent the stage of star formation that precedes the formation of a protostar. 29 of the 52 cores were detected at 850 microns at varying levels of signal-to-noise ratio greater than 3 sigma at peak. The detected cores were split into 'bright' cores and `intermediate' cores, depending on their peak flux density at 850 microns. Cores with peak 850 microns flux densities greater than 170 mJy/beam were designated 'bright' (13 cores), while those flux densities below this value were designated 'intermediate' (16 cores). This dividing line corresponds to A_v~50 under typical assumptions. The data are combined with our previously published ISO data, and the physical parameters of the cores, such as density and temperature, are calculated. Detailed fitting of the bright core radial profiles shows that they are not critical Bonnor-Ebert spheres, in agreement with previous findings. However, we find that intermediate cores, such as B68 (which has previously been claimed to be a Bonnor-Ebert sphere), may in fact be consistent with the Bonnor-Ebert criterion, suggesting perhaps that cores pass through such a phase during their evolution. We make rough estimates of core lifetimes based on the statistics of detections and find that the lifetime of a prestellar core is roughly ~3x10^5 years, while that of a bright core is \~1.5x10^5 years. Comparisons with some magnetic and turbulence regulated collapse models show that no model can match all of the data. Models that are tuned to fit the total prestellar core lifetime, do not predict the relative numbers of cores seen at each stage.

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arXiv2005

The prestellar and protostellar population of R Coronae Australis

We present 450 and 850 um maps of R Coronae Australis. We compare the maps to previous surveys of the region, and shed new light on the previously unknown nature of the protostellar sources at the centre of the cloud. We clarify the nature of two millimetre sources previously discovered in lower resolution data. We identify one new Class 0 protostar that we label SMM 1B, and we measure the envelope masses of a number of more evolved protostars. We identify two new prestellar cores that we call SMM 1A and SMM 6.

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arXiv2004

Turbulence in Class 0 and Class I protostellar envelopes

We estimate the levels of turbulence in the envelopes of Class 0 and I protostars using a model based on measurements of the peak separation of double-peaked asymmetric line profiles. We use observations of 20 protostars of both Class 0 and I taken in the HCO+ (J=3-2) line that show the classic double-peaked profile. We find that some Class 0 sources show high levels of turbulence whilst others demonstrate much lower levels. In Class I protostars we find predominantly low levels of turbulence. The observations are consistent with a scenario in which Class 0 protostars form in a variety of environments and subsequently evolve into Class I protostars. The data do not appear to be consistent with a recently proposed scenario in which Class 0 protostars can only form in extreme environments.

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arXiv2004

The Origin of the IMF from Core Mass Functions

We examine the initial mass functions (IMFs) of stars produced by different molecular core mass functions. Simulations suggest that more massive cores produce more stars, so we propose a model in which the average number of stars formed in a core is equal to the initial number of Jeans masses in that core. Small-$N$ systems decay through dynamical interactions, ejecting low-mass stars and brown dwarfs which populate the low-mass tail of the IMF. Stars which remain in cores are able to competitively accrete more gas and become more massive. We deduce the forms of the core mass functions required to explain the IMFs of Taurus, Orion, IC 348 and NGC 2547. These core mass functions fall into two categories - one which peaks at a few $M_\odot$ to explain Taurus and NGC 2547, and one that peaks at around $0.2 M_\odot$ to explain Orion and IC 348.

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arXiv2004

Radiative transfer models of non-spherical prestellar cores

We present 2D Monte Carlo radiative transfer simulations of prestellar cores. We consider two types of asymmetry: disk-like asymmetry, in which the core is denser towards the equatorial plane than towards the poles; and axial asymmetry, in which the core is denser towards the south pole than the north pole. We limit our treatment to cores with mild asymmetries, which are exposed directly to the interstellar radiation field or are embedded inside molecular clouds. The isophotal maps of a core depend strongly on the viewing angle. Maps at wavelengths longer than the peak of the SED (e.g. 850 micron) essentially trace the column-density. Thus, for instance, cores with disk-like asymmetry appear elongated when mapped at 850 micron from close to the equatorial plane. However, at wavelengths near the peak of the SED (e.g. 200 micron), the emissivity is more strongly dependent on the temperature, and therefore, at particular viewing angles, there are characteristic features which reflect a more complicated convolution of the density and temperature fields within the core. These characteristic features are on scales 1/5 to 1/3 of the overall core size, and so high resolution observations are needed to observe them. They are also weaker if the core is embedded in a molecular cloud (because the range of temperature within the core is then smaller), and so high sensitivity is needed to detect them. Herschel, to be launched in 2007, will in principle provide the necessary resolution and sensitivity at 170 to 250 micron.

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arXiv2003

SCUBA Polarization Measurements of the Magnetic Field Strengths in the L183, L1544, and L43 Prestellar Cores

We have mapped linearly polarized dust emission from L183 with the JCMT SCUBA polarimeter and have analyzed these and our previously published data for the prestellar cores L183, L1544, and L43 in order to estimate magnetic field strengths in the plane of the sky, $B_{pos}$. The analysis used the Chandrasekhar-Fermi technique, which relates the dispersion in polarization position angles to $B_{pos}$. We have used these estimates of the field strengths (neglecting the unmeasured line-of-sight component) to find the mass-to-magnetic flux ratios $λ$ (in units of the critical ratio for magnetic support). Results are $B_{pos} \approx 80$ $μ$G and $λ\approx 2.6$ for L183, $B_{pos} \approx 140$ $μ$G and $λ\approx 2.3$ for L1544, and $B_{pos} \approx 160$ $μ$G and $λ\approx 1.9$ for L43. Hence, without correction for geometrical biases, for all three cores the mass-to-flux ratios are supercritical by a factor of $\sim 2$, and magnetic support cannot prevent collapse. However, a statistical mean correction for geometrical bias may be up to a factor of three; this correction would reduce the individual $λ$'s to $λ_{cor} \approx 0.9$, 0.8, and 0.6, respectively; these values are approximately critical or slightly subcritical. These data are consistent with models of star formation driven by ambipolar diffusion in a weakly turbulent medium, but cannot rule out models of star formation driven by turbulence.

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arXiv2003

The Structure of Cold Molecular Cloud Cores

A brief summary is presented of our current knowledge of the structure of cold molecular cloud cores that do not contain protostars, sometimes known as starless cores. The most centrally condensed starless cores are known as pre-stellar cores. These cores probably represent observationally the initial conditions for protostellar collapse that must be input into all models of star formation. The current debate over the nature of core density profiles is summarised. A cautionary note is sounded over the use of such profiles to ascertain the equilibrium status of cores. The magnetic field structure of pre-stellar cores is also discussed.

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