arXiv subjects

Derek Ward-Thompson

Publications and source records attributed to Derek Ward-Thompson.

88 records · Page 5Linked to original sources

arXiv2014

The next-generation BLASTPol experiment

The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (BLASTPol) is a suborbital mapping experiment designed to study the role magnetic fields play in star formation. BLASTPol has had two science flights from McMurdo Station, Antarctica in 2010 and 2012. These flights have produced thousands of polarization vectors at 250, 350 and 500 microns in several molecular cloud targets. We present the design, specifications, and progress towards the next-generation BLASTPol experiment (BLAST-TNG). BLAST-TNG will fly a 40% larger diameter primary mirror, with almost 8 times the number of polarization-sensitive detectors resulting in a factor of 16 increase in mapping speed. With a spatial resolution of 22 arcseconds and four times the field of view of BLASTPol, BLAST-TNG will bridge the angular scales between Planck's low resolution all-sky maps and ALMA's ultra-high resolution narrow fields. The new receiver has a larger cryogenics volume, allowing for a 28 day hold time. BLAST-TNG employs three arrays of Microwave Kinetic Inductance Detectors (MKIDs) with 30% fractional bandwidth at 250, 350 and 500 microns. In this paper, we will present the new BLAST-TNG instrument and science objectives.

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arXiv2013

From Filamentary Networks to Dense Cores in Molecular Clouds: Toward a New Paradigm for Star Formation

Recent studies of the nearest star-forming clouds of the Galaxy at submillimeter wavelengths with the Herschel Space Observatory have provided us with unprecedented images of the initial and boundary conditions of the star formation process. The Herschel results emphasize the role of interstellar filaments in the star formation process and connect remarkably well with nearly a decade's worth of numerical simulations and theory that have consistently shown that the ISM should be highly filamentary on all scales and star formation is intimately related to self-gravitating filaments. In this review, we trace how the apparent complexity of cloud structure and star formation is governed by relatively simple universal processes - from filamentary clumps to galactic scales. We emphasize two crucial and complementary aspects: (i) the key observational results obtained with Herschel over the past three years, along with relevant new results obtained from the ground on the kinematics of interstellar structures, and (ii) the key existing theoretical models and the many numerical simulations of interstellar cloud structure and star formation. We then synthesize a comprehensive physical picture that arises from the confrontation of these observations and simulations.

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arXiv2013

Lupus I Observations from the 2010 Flight of the Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry

The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (BLASTPol) was created by adding polarimetric capability to the BLAST experiment that was flown in 2003, 2005, and 2006. BLASTPol inherited BLAST's 1.8 m primary and its Herschel/SPIRE heritage focal plane that allows simultaneous observation at 250, 350, and 500 μm. We flew BLASTPol in 2010 and again in 2012. Both were long duration Antarctic flights. Here we present polarimetry of the nearby filamentary dark cloud Lupus I obtained during the 2010 flight. Despite limitations imposed by the effects of a damaged optical component, we were able to clearly detect submillimeter polarization on degree scales. We compare the resulting BLASTPol magnetic field map with a similar map made via optical polarimetry (The optical data were published in 1998 by J. Rizzo and collaborators.). The two maps partially overlap and are reasonably consistent with one another. We compare these magnetic field maps to the orientations of filaments in Lupus I, and we find that the dominant filament in the cloud is approximately perpendicular to the large-scale field, while secondary filaments appear to run parallel to the magnetic fields in their vicinities. This is similar to what is observed in Serpens South via near-IR polarimetry, and consistent with what is seen in MHD simulations by F. Nakamura and Z. Li.

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arXiv2012

Interferometric Identification of a Pre-Brown Dwarf

It is not known whether brown dwarfs (stellar-like objects with masses less than the hydrogen-burning limit, 0.075 Msun) are formed in the same way as solar-type stars or by some other process. Here we report the clear-cut identification of a self-gravitating condensation of gas and dust with a mass in the brown-dwarf regime, made through millimeter interferometric observations. The level of thermal millimeter continuum emission detected from this object indicates a mass ~ 0.02-0.03 Msun, while the small radius < 460 AU and narrow spectral lines imply a dynamical mass of 0.015-0.02 Msun. The identification of such a pre-brown dwarf core supports models according to which brown dwarfs are formed in the same manner as hydrogen-burning stars.

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arXiv2011

Filaments and ridges in Vela C revealed by Herschel: from low-mass to high-mass star-forming sites

We present the first Herschel PACS and SPIRE results of the Vela C molecular complex in the far-infrared and submillimetre regimes at 70, 160, 250, 350, and 500 um, spanning the peak of emission of cold prestellar or protostellar cores. Column density and multi-resolution analysis (MRA) differentiates the Vela C complex into five distinct sub-regions. Each sub-region displays differences in their column density and temperature probability distribution functions (PDFs), in particular, the PDFs of the `Centre-Ridge' and `South-Nest' sub-regions appear in stark contrast to each other. The Centre-Ridge displays a bimodal temperature PDF representative of hot gas surrounding the HII region RCW 36 and the cold neighbouring filaments, whilst the South-Nest is dominated by cold filamentary structure. The column density PDF of the Centre-Ridge is flatter than the South-Nest, with a high column density tail, consistent with formation through large-scale flows, and regulation by self-gravity. At small to intermediate scales MRA indicates the Centre-Ridge to be twice as concentrated as the South-Nest, whilst on larger scales, a greater portion of the gas in the South-Nest is dominated by turbulence than in the Centre-Ridge. In Vela C, high-mass stars appear to be preferentially forming in ridges, i.e., dominant high column density filaments.

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arXiv2010

The balloon-borne large-aperture submillimeter telescope for polarimetry: BLAST-Pol

The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (BLAST-Pol) is a suborbital mapping experiment designed to study the role played by magnetic fields in the star formation process. BLAST-Pol is the reconstructed BLAST telescope, with the addition of linear polarization capability. Using a 1.8 m Cassegrain telescope, BLAST-Pol images the sky onto a focal plane that consists of 280 bolometric detectors in three arrays, observing simultaneously at 250, 350, and 500 um. The diffraction-limited optical system provides a resolution of 30'' at 250 um. The polarimeter consists of photolithographic polarizing grids mounted in front of each bolometer/detector array. A rotating 4 K achromatic half-wave plate provides additional polarization modulation. With its unprecedented mapping speed and resolution, BLAST-Pol will produce three-color polarization maps for a large number of molecular clouds. The instrument provides a much needed bridge in spatial coverage between larger-scale, coarse resolution surveys and narrow field of view, and high resolution observations of substructure within molecular cloud cores. The first science flight will be from McMurdo Station, Antarctica in December 2010.

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arXiv2009

BIMA N2H+ 1-0 mapping observations of L183 -- fragmentation and spin-up in a collapsing, magnetized, rotating, pre-stellar core

We have used the Berkeley-Illinois-Maryland Array (BIMA) to make deep N2H+ 1-0 maps of the pre-stellar core L183, in order to study the spatial and kinematic substructure within the densest part of the core. Three spatially and kinematically distinct clumps are detected, which we label L183-N1, L183-N2 and L183-N3. L183-N2 is approximately coincident with the submillimetre dust peak and lies at the systemic velocity of L183. Thus we conclude that L183-N2 is the central dense core of L183. L183-N1 and 3 are newly-discovered fragments of L183, which are marked by velocity gradients that are parallel to, but far stronger than, the velocity gradient of L183 as a whole, as detected in previous single-dish data. Furthermore, the ratio of the large-scale and small-scale velocity gradients, and the ratio of their respective size-scales, are consistent with the conservation of angular momentum for a rotating, collapsing core undergoing spin-up. The inferred axis of rotation is parallel to the magnetic field direction, which is offset from its long axis, as we have seen in other pre-stellar cores. Therefore, we propose that we have detected a fragmenting, collapsing, filamentary, pre-stellar core, rotating about its B-field, which is spinning up as it collapses. It will presumably go on to form a multiple protostellar system.

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arXiv2009

The Spitzer survey of interstellar clouds in the Gould Belt. II. The Cepheus Flare observed with IRAC and MIPS

We present Spitzer IRAC (~2 deg^2) and MIPS (~8 deg^2) observations of the Cepheus Flare which is associated with the Gould Belt, at an approximate distance of ~300 pc. Around 6500 sources are detected in all four IRAC bands, of which ~900 have MIPS 24 micron detections. We identify 133 YSO candidates using color-magnitude diagram techniques, a large number of the YSO candidates are associated with the NGC 7023 reflection nebula. Cross identifications were made with the Guide Star Catalog II and the IRAS Faint Source Catalog, and spectral energy distributions (SED) were constructed. SED modeling was conducted to estimate the degree of infrared excess. It was found that a large majority of disks were optically thick accreting disks, suggesting that there has been little disk evolution in these sources. Nearest-neighbor clustering analysis identified four small protostellar groups (L1228, L1228N, L1251A, and L1251B) with 5-15 members each and the larger NGC 7023 association with 32 YSO members. The star formation efficiency for cores with clusters of protostars and for those without clusters was found to be ~8% and ~1% respectively. The cores L1155, L1241, and L1247 are confirmed to be starless down to our luminosity limit of L_bol=0.06 L_sol.

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arXiv2007

The relationship between the prestellar core mass function and the stellar initial mass function

Stars form from dense molecular cores, and the mass function of these cores (the CMF) is often found to be similar to the form of the stellar initial mass function (IMF). This suggests that the form of the IMF is the result of the form of the CMF. However, most stars are thought to form in binary and multiple systems, therefore the relationship between the IMF and the CMF cannot be trivial. We test two star formation scenarios - one in which all stars form as binary or triple systems, and one in which low-mass stars form in a predominantly single mode. We show that from a log-normal CMF, similar to those observed, and expected on theoretical grounds, the model in which all stars form as multiples gives a better fit to the IMF.

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arXiv2006

SCUBA polarisation observations of the magnetic fields in the prestellar cores L1498 and L1517B

We have mapped linearly polarized dust emission from the prestellar cores L1498 and L1517B with the James Clerk Maxwell Telescope (JCMT) using the Submillimetre Common User Bolometer Array (SCUBA) and its polarimeter SCUBAPOL at a wavelength of 850um. We use these measurements to determine the plane-of-sky magnetic field orientation in the cores. In L1498 we see a magnetic field across the peak of the core that lies at an offset of 19 degrees to the short axis of the core. This is similar to the offsets seen in previous observations of prestellar cores. To the southeast of the peak, in the filamentary tail of the core, we see that the magnetic field has rotated to lie almost parallel to the long axis of the filament. We hypothesise that the field in the core may have decoupled from the field in the filament that connects the core to the rest of the cloud. We use the Chandrasekhar-Fermi (CF) method to measure the plane-of-sky field strength in the core of L1498 to be 10 +/- 7 uG. In L1517B we see a more gradual turn in the field direction from the northern part of the core to the south. This appears to follow a twist in the filament in which the core is buried, with the field staying at a roughly constant 25 degree offset to the short axis of the filament, also consistent with previous observations of prestellar cores. We again use the CF method and calculate the magnetic field strength in L1517B also to be 30 +/- 10 uG. Both cores appear to be roughly virialised. Comparison with our previous work on somewhat denser cores shows that, for the denser cores, thermal and non-thermal (including magnetic) support are approximately equal, while for the lower density cores studied here, thermal support dominates.

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arXiv2004

Simulating star formation in molecular cores II. The effects of different levels of turbulence

(Abridged) We explore, by means of a large ensemble of SPH simulations, how the level of turbulence affects the collapse and fragmentation of a star-forming core. All our simulated cores have the same, except that we vary (a) the initial level of turbulence (as measured by the ratio of turbulent to gravitational energy, $α_{\rm turb} \equiv U_{\rm turb}/|Ω| = 0, 0.01, 0.025, 0.05, 0.10 {\rm and} 0.25$) and (b), for fixed $α_{\rm turb}$, the details of the initial turbulent velocity field (so as to obtain good statistics). A low level of turbulence ($α_{\rm turb} \sim 0.05$) suffices to produce multiple systems. As $α_{\rm turb}$ is increased, the number of objects formed and the companion frequency both increase. The mass function is bimodal, with a flat low-mass segment representing single objects ejected from the core before they can accrete much, and a Gaussian high-mass segment representing objects which because they remain in the core grow by accretion and tend to pair up in multiple systems.

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arXiv2004

An explanation for the unusual IMF in Taurus

In comparison with other well studied star formation regions, Taurus is unusual in several respects. (i) Its stellar initial mass function (IMF) peaks at relatively high mass ($\sim 0.8 M_\odot$), but contains very few stars much more massive than $1 M_\odot$, and is relatively deficient in brown dwarfs. (ii) It has a higher binary fraction, particularly at large separations. (iii) Its core mass function is strongly peaked at a few $M_\odot$, and the cores have extended envelopes and relatively low levels of turbulence. We present here the results of an ensemble of hydrodynamic simulations which suggest that the unusual stellar IMF in Taurus is a direct consequence of the unusual properties of the cores there. By following the collapse and fragmentation of cores having properties typical of Taurus, we find that roughly 50% of the objects formed in a core, predominantly the low-mass ones, are ejected from the core to form a population of low-mass stars and brown dwarfs with a flat mass function. The remaining objects form multiple systems within the core, accreting until their masses approach $1 M_\odot$; this produces a population of intermediate-mass stars whose mass function peaks at $\sim 0.8 M_\odot$. Together these two populations reproduce the IMF in Taurus very well. This demonstrates, for the first time, a direct causal link between the core mass function and the stellar IMF in a star formation region.

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arXiv2003

Detailed Studies of Cloud Cores: Probing the Initial Conditions for Protostellar Collapse

Improving our understanding of the earliest stages of star formation is crucial to gain insight into the origin of stellar masses, multiple systems, and protoplanetary disks. We discuss recent advances made in this area thanks to detailed mapping observations at infrared and (sub)millimeter wavelengths. Although ambipolar diffusion appears to be too slow to play a direct role in the formation of dense cores, there is nevertheless good evidence that the gravitational collapse of isolated protostellar cores is strongly magnetically controlled. We also argue that the beginning of protostellar collapse is much more violent in cluster-forming clouds than in regions of distributed star formation.

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arXiv2001

The Structure of Prestellar Cores as derived from ISO Observations

Observations of dark cloud cores have been carried out in the mid-infrared using ISOCAM and in the far-infrared using ISOPHOT, both aboard the Infrared Space Observatory. The cores are in most cases detected in emission at 200 and 170 micron, remain undetected at 90 micron and are seen in absorption against the diffuse mid-infrared background at 7 micron. The observations are consistent with the cores being pre-stellar and not having a central heating source, and yield core temperatures of ~ 11-13 K. We were able to determine the density structure of the cores up to radii that extend beyond the sensitivity limit of previous submillimetre studies. The column density profiles of the cores studied here flatten out in the centre, as shown by previous submillimetre continuum results, and interestingly, in a few cases, the profiles steepen with radius beyond ~ 5000-10000 AU and present sharp edges. This could indicate that these cores are decoupled from their parent molecular cloud and represent finite reservoirs of mass for subsequent accretion.

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arXiv2000

An ISOCAM Absorption Survey of the Structure of Pre-stellar Cloud Cores

We present the results of a mid-infrared (7 micron) imaging survey of a sample of 24 starless dense cores carried out at an angular resolution of 6 arcsec with the ISOCAM camera aboard the Infrared Space Observatory (ISO). The targeted cores are believed to be pre-stellar in nature and to represent the initial conditions of low-mass, isolated star formation. In previous submillimeter dust continuum studies of such pre-stellar cores, it was found that the derived column density profiles did not follow a single power-law such as N[H2] \propto r^(-1) throughout their full extent but flattened out near their center. These submillimeter observations however could not constrain the density profiles at radii greater than ~ 10000 AU. The present absorption study uses ISOCAM's sensitivity to map these pre-stellar cores in absorption against the diffuse mid-infrared background. The goal was to determine their structure at radii that extend beyond the limits of sensitivity of the submillimeter continuum maps and at twiceas good an angular resolution. Among the 24 cores observed in our survey, a majority of them show deep absorption features. The starless cores studied here all show a column density profile that flattens in the center, which confirms the submillimeter emission results. Moreover, beyond a radius of ~ 5000-10000 AU, the typical column density profile steepens with distance from core center and gets steeper than N[H2] \propto r^(-1), until it eventually merges with the low-density ambient molecular cloud. At least three of the cores present sharp edges at R ~ 15000-30000 AU and appear to be decoupled from their parent clouds, providing finite reservoirs of mass for subsequent star formation.

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arXiv1999

From Pre-Stellar Cores to Protostars: The Initial Conditions of Star Formation

The last decade has witnessed significant advances in our observational understanding of the earliest stages of low-mass star formation. The advent of sensitive receivers on large radio telescopes such as the JCMT and IRAM 30m MRT has led to the identification of young protostars at the beginning of the main accretion phase (`Class 0' objects), and has made it possible to probe, for the first time, the inner density structure of pre-collapse cores. Class 0 objects are characterized by strong, centrally-condensed dust continuum emission at submillimeter wavelengths, very little emission shortward of 10 microns, and powerful jet-like outflows. Direct evidence for gravitational infall has been observed toward several of them. They are interpreted as accreting protostars which have not yet accumulated the majority of their final stellar mass. In contrast to protostars, pre-stellar cores have flat inner density profiles, suggesting the initial conditions for fast protostellar collapse depart sometimes significantly from a singular isothermal sphere. In the case of non-singular initial conditions, the beginning of protostellar evolution is expected to feature a brief phase of vigorous accretion/ejection which may coincide with Class 0 objects. In addition, submillimeter continuum imaging surveys of regions of multiple star formation such as Ophiuchus and Serpens suggest a picture according to which each star in an embedded cluster is built from a finite reservoir of mass and the associated IMF is primarily determined at the pre-stellar stage of evolution.

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