Rachael Padman

University of Cambridge | Cam · Department of Physics: Cavendish Laboratory

In reply to David Appell’s feature “The invisibility of length contraction” (August pp41–45), which explained why a 3D object moving at relativistic speeds would appear rotated, not Lorentz contracted, when viewed by a stationary observer.

In reply to the Forum article "Modernizing classical physics" (February p19), in which David Nolte discussed reforming the undergraduate physics curriculum.

At the interface between an HII region and a molecular cloud, lies a neutral gas layer which is subject to both an intense radiation field, and to shocks arising from the expansion of the ionisation front of the HII region. The gas in these regions is highly excited, hot, and may be fairly dense. We present the first high resolution images of atomi...

The General Single-Dish Data format (GSDD) was developed in the mid-1980s as a data model to support centimeter, millimeter and submillimeter instrumentation at NRAO, JCMT, the University of Arizona and IRAM. We provide an overview of the GSDD requirements and associated data model, discuss the implementation of the resultant file formats, describe...

In reply to Arti Agrawal's article "Sounding out higher fees" (Forum, July p19) on the impact of raising undergraduate tuition fees at English universities.

In reply to Max Tegmark's article "It's all just mathematics" (Features, February pp22—27).

Making UK undergraduate physics degrees longer must have seemed like a good idea at the time. Back in the early 1990s the standard three-year Bachelor's physics degree (four years in Scotland) was under pressure at both ends. The A-level curriculum – one of the requirements for entry onto a degree course – was being increasingly modularized, and du...

As your news story "Europe's education experiment" (June pp12–13) reported, the UK government may well view the country as now being compliant with the Bologna process, which aims to create a common European higher-education system. This is because the government persuaded the European Higher Education Area (EHEA) to agree that Bologna recognition...

This paper describes a James Clerk Maxwell Telescope (JCMT) legacy survey that has been awarded roughly 500 hr 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 t...

Robert Crease describes how scientists can best deal with activists who give false warnings to the public, referring to such people as "Cassandras"(June p16). But if he wants us to deal effectively with misinformation about scientific research, it would be better if he did not deliberately redefine well known terms to mean their precise opposite. A...

The basic aspiration of the Institute's Women In Physics Group must be that appropriately qualified women, who can make the necessary commitment, should have the same opportunities in physics as men of similar qualifications and commitment. I share this aspiration. However, I am uneasy about the group's apparent motivation for encouraging women and...

The inquiry by the Institute of Physics into undergraduate physics (October 2001 p5) has resulted in a worthy document, but we read it with a strong sense of déja vu. In the 1970s our late colleague, Norman Thompson, first drew attention to the growing shortage of physics teachers in schools. In 1980 he then chaired a joint working party of the Ins...

We present 450- and 800-μm images, made with the James Clerk Maxwell Telescope, of the NGC 2024 molecular ridge. The seven previously known compact cores, FIR1–7, have been detected, and FIR5 has been resolved into a compact object and an associated extended source to the east. The estimated masses of the dense cores vary between 1.6 and 5.1 M⊙ per...

Our new 21-arcsec resolution CO J=2-->1 map of the L43 dark cloud shows a poorly collimated molecular outflow, with little evidence for wings at velocities10 km s^-1. The outflow appears not to be currently driven by a jet: its structure can instead be modelled as a slowly expanding shell. The shell may be compressed either by a wide-angled wind ca...

The authors report the successful operation of an antipodal finline quasi-particle SIS mixer at 230 GHz. The mixer is fed by a waveguide diagonal horn and uses planar circuit technology and integrated tuning. The mixer is tested over the frequency range 213-265 GHz and a receiver noise temperature of ~60 K DSB over the whole of this range is obtain...

Molecular outflows are intimately related to the highly collimated Herbig-Haro jets emanating from young stars. In consequence, the usual dynamical timescale significantly underestimates the true age of an outflow. If we correct for this factor, and assume an intrinsic outflow speed similar to that of the underlying jet, we predict that molecular o...

Molecular outflows driven by Herbig-Haro jets have been well studied, but the initially appealing jet-driven model fails to account for poorly-collimated sources such as L 1551 IRS 5. The outflow from RNO 91, in the L43 dark cloud is another example which shows little evidence for high-velocity wings or bowshock structures, and appears not to be cu...

Molecular outflows are intimately related to the highly collimated Herbig–Haro jets emanating from young stars. In consequence, the usual dynamical timescale significantly underestimates the true age of an outflow. If we correct for this factor, and assume an intrinsic outflow speed similar to that of the underlying jet, we predict that molecular o...

High-resolution images of the NGC 2024 molecular ridge in CS(2-1) and 3 mm continuum emission have been obtained using the Owens Valley millimeter array. The data are used to determine the temperature and dynamics of the ridge, and to ascertain the evolutionary state of the embedded protostars FIR2-7. Dust continuum emission is detected from all si...

We present 21-arcsec-resolution CO observations of the spectacular molecular outflow associated with RNO 43. This outflow extends over nearly 5 pc and is thus one of the largest known. The high-velocity CO emission is broken up into several distinct regions of high excitation. Each of these regions subtends a small angle at the young stellar object...

Finline transmission lines are easy to manufacture and to integrate with other planar components, but they have not, to our knowledge, been used for submillimetre SIS mixers. One of the main problems is the high characteristic impedance of these structures. In this paper we describe a design which overcomes this difficulty by including an antipodal...

Molecular outflows around young stellar objects (YSOs) have been studied extensively. Many models have tried to explain their appearance, and currently the most popular are those which invoke momentum-conserving collimated jets, emanating from the YSO. Two competing models describe the mechanism by which the jet sets the molecular cloud in motion:...

We present maps of C18O and C17O J = 2 --> 1 emission toward the embedded young star L1551 IRS 5. The C17O emission traces a compact region 1900 AU in radius which is centered on the star which contains 0.1 Msun of material. The integrated C17O emission has a crosslike structure aligned in the cardinal directions which is very similar to the struct...

We have undertaken a search for submillimetre (submm) embedded cores associated with known water maser sources. For 44 water maser sources, we found submm continuum emission from 40 (91 per cent) using the James Clerk Maxwell Telescope (JCMT). Some of the sources were then observed further. Maps were made in CO and C18O ($J=2\rightarrow1$) and CS (...

We present high signal-to-noise ratio 16-20 arcsec resolution submillimeter continuum images of the embedded young stellar object L1551 IRS 5. The images are dominated by a compact sources at the position of IRS 5, which has a deconvolved size of 10 +/- 3 arcsec. Low-level emission extends up to 5000 A.U. from IRS 5 and is distributed nonaxisymetri...

We present submillimeter molecular line observations of comet P/Swift-Tuttle (1992t = 1992 XXVIII) which were undertaken on 7 and 8 December 1992 with the James Clerk Maxwell Telescope (JCMT). Three molecules were detected: (1) HCN through its J (4-3) rotational line at 354.505 GHz; (2) H2CO through its 515 - 1414 ortho line at 351.769 GHz and (3)...

Young stars produce both molecular outflows and, at a later evolutionary stage, well-collimated optical jets. The simplest explanation is that the molecular outflows are driven byobscured optical jets, rather than directly, by a disk wind for example, but the optical jets appear to have too small a momentum flux. Recent statistical studies however...

In an earlier paper, we presented an analysis of the small-scale structure of the M17SW molecular cloud based on observations of the optically thick HCO + and HCN J=3→2 transitions. In this paper we present observations of the C 17 O J=3→2 transition. This transition is optically thin, is observed with a slightly smaller beam, and suffers less from...

We present a numerical, maximum entropy technique for reconstructing the distribution of dust mass as a function of temperature from a set of thermal continuum fluxes. The particular formulation used can accommodate an arbitrary wavelength dependence of dust emissivity, but does require the emission to be optically thin. The dust emission spectrum...

In an earlier paper, we presented an analysis of the small-scale structure of the M175W molecular cloud based on observations of the optically thick HCO+ and HCN J = 3→2 transitions. In this paper we present observations of the C17O J=3→2 transition. This transition is optically thin, is observed with a slightly smaller beam, and suffers less from...

We present high-angular resolution maps of the S 140 molecular cloud in various transitions of the ??CO, ??CO and Censuremath 18ensuremath<?supensuremath>O molecules and single-channel observations of the ?P$_1$$rightarrow$H?P$_0$ line of neutral atomic carbon (CI). Velocity channel maps of the ??CO lines show a systematic shift of the emission pea...

The M17SW molecular cloud core has been mapped in continuum emission at 450, 600, 800, 1100 and 1300 µm, using the James Clerk Maxwell Telescope, with an angular resolution of 8 arcsec for the shortest wavelength. The 450- and 600-µm, maps, in particular, show the dust emission to be highly clumped; these clumps are, in some cases, identified with...

An approximate method is presented for describing the transfer of radiation inside a two-phase medium. The medium is assumed to consist of spherical clumps, with uniform size and density, embedded in a more tenuous interclump medium. The transfer of radiation is modelled by treating the clumps as large grains, or `megagrains', with absorption and s...

We discuss large-scale features in the neutral hydrogen (H I) towards Orion, as observed with the DRAO 26-m telescope. We identify features including (i) a clump of emission ≃ 3 × 6 deg 2 in extent, to the west of Orion B, at negative velocities with respect to the LSR, which is possibly related to Barnard's Loop, (ii) an arc in the south-west at n...

We discuss large-scale features in the neutral hydrogen (H I) towards Orion, as observed with the DRAO 26-m telescope. We identify features including (i) a clump of emission ≍ 3 × 6 deg2 in extent, to the west of Orion B, at negative velocities with respect to the LSR, which is possibly related to Barnard's Loop, (ii) an arc in the south-west at ne...

The study presents millimeter and submillimeter images of the bipolar nebula S106 which resolve the structure of the circumstellar material around the exciting star S106 IR. There is a bright, previously undetected FIR point source in the dust lane which is not detected at 20 microns, suggesting that it is an extremely embedded object of moderate l...

The use of high resolution spectral line observations at millimeter and submillimeter wavelengths to determine the structure, kinematics and composition of molecular clouds is considered. Three key issues are addressed: small scale structure, the search for 'protostars' and 'protostellar disks', and the kinematics and dynamics of outflows from youn...

High resolution millimetre and submillimetre wave astronomical spectrometers using hot-electron bolometer mixers as detectors often show marked standing wave patterns in the spectral baseline. LO phase noise contributes through two mechanisms: the phase noise side-bands may be converted to amplitude noise in the source because of the power-frequenc...

A dual-polarization InSb hot-electron bolometer-mixer receiver has been built for the James Clerk Maxwell Telescope, for operation at 461 and 492 GHz (the frequencies of theJ=4→3 rotational transition of CO and of the3P 1→3P 0 transition of neutral carbon). Receiver noise temperatures of 500K have been obtained at 461 GHz, in observing bandwidths o...

Observations of the 21 cm H exp 0 line and of the J = 3-2 transition of HCO(+) in DR 21 are presented. The H exp 0 observations reveal a massive (24 solar-mass) jet which is redshifted by up to 90 km/s, with peak H exp 0 column density of 2 x 10 exp 21/sq cm. This jet is three orders of magnitude more massive than the H exp 0 wind seen in HH 7-11,...

We report 12-arcsec resolution CO observations of the fast unipolar molecular jet in Orion B together with maps of the ambient cloud as traced by CS and C18O emission. The flow (which appears to originate from FIR5) consists of many clumps of gas, with sizes $\lesssim {10}^{15}$ m and velocity dispersions ~ 6 km s–1, which fill an almost cylindrica...

At the interface between an HII region and a molecular cloud, lies a neutral gas layer which is subject to both an intense radiation field, and to shocks arising from the expansion of the ionisation front of the HII region. The gas in these regions is highly excited, hot, and may be fairly dense. We present the first high resolution images of atomi...

This paper reports on far-infrared and submillimeter wavelength observations of low-mass protostellar candidates. The data set comprises emission maps of nine sources observed over a wavelength range 100-800 microns. The emission is extended at all wavelengths longer than 100 microns. The apparent size of the emission regions is weakly correlated w...

At long wavelengths, diffraction effects cause condensing lightpipes to have a significant response in directions within the geometric optics shadow zone. Here, using an analogy with the corrugated “scalar” horns often used in antenna engineering, we point out that a lightpipe with anisotropic surface impedance on its interior surfaces can have sub...

We analyse condensing lightpipes of the form often used to couple an infrared detector to a relatively slow optical system. The response in a given direction is expressed in terms of the radiation patterns of the different waveguide modes propagating in the lightpipe and throat section. Such lightpipes are seen to be just a generalization of the el...

Although carbon is the fourth most cosmically abundant element, it exists only rarely as neutral atoms, because carbon that is cool enough to be neutral rapidly combines with other atoms to form molecules such as CO. Atomic carbon (CI) forms when CO is dissociated by ultraviolet photons, but the dissociation energy is close to the ionization energy...

We present the results of a $^{12}\text {CO}\enspace J = 2-1$ survey of a sample of IRAS sources representative of low-mass young ($\lesssim 2.5\times 10^5$ yr) stellar objects embedded in dark molecular clouds. Several new outflows were identified and mapped. The survey reveals that $\gtrsim70$ per cent of sources have associated outflow activity,...

We discuss the expected appearance of centrifugally supported astrophysical discs observed in optically thin line emission, with particular emphasis on the molecular line emission arising from protostellar discs. If spectra are taken along the (known) long axis of the projected disc, which is assumed to be unresolved perpendicular to this direction...

Multiple reflections in large radiotelescopes used for astronomical spectroscopy cause characteristic modulations of the observed spectrum (“baseline ripple”). For a given mechanism, the magnitude of the effect depends primarily on the reflection coefficient, which for the most important paths is proportional to λ. Although ripple is thus generally...

The authors have show that scattering matrix theory enables Gaussian beam mode analysis to be extended in a straightforward way to take account of multiple reflections and scattering between modes. The use of propagating modes as basis functions, unlike those of fourier optics, allows the treatment of a vastly expanded range of problems. As well as...

This book is the first to provide students and researchers in the field of astrophysical jets with a comprehensive and up-to-date account of current research. An important feature of the book is that it combines discussions of both extragalactic and Galactic jets. There are ten chapters, authored by fourteen active researchers, each of whom is an e...

Your Connections article 'Women and physics: oil and water?' (June p24) described what appeared to be a relatively successful attempt to introduce (or perhaps reintroduce) young women to physics before they burn their bridges and opt for a nonscientific career. I enjoyed the article, but I wonder if the motivation suggested for encouraging young wo...

The power of Gaussian beam mode analysis to accurately describe the propagation of electromagnetic beams and the location of a horn antenna phase-center is illustrated. By way of example, the case of a compensated pyramidal horn fitted with fins to produce a less abruptly tapered E -plane field distribution is discussed, and the results obtained ar...

A complete sample of dark clouds is being observed in order to study the processes of star formation. Bipolar outflows have been found in 7 out of the 14 clouds studied to date while 2 further clouds show high-velocity features. It is concluded that a period of mass-loss is a widespread feature of star formation in dark clouds and could have a sign...

The authors have observed the "disc" of the bipolar nebula S106 in the J = 3→2 lines of HCN and HCO+. MEM restoration of the position-velocity diagram shows evidence for differential rotation consistent with a central point mass of ≥10 Msun.

Observations of the Orion B molecular outflow reveal an unusually well-collimated high velocity component to the flow, lying inside a massive envelope of more slowly outflowing poorly collimated gas. This jet appears to accelerate as it moves away from the exciting star. Observations at this resolution do not suggest that the emission arises in a t...

The first heterodyne observations on the James Clerk Maxwell Telescope in the far sub-millimetre bands were made in 1988 August with the laser-pumped cooled Schottky receiver of the Max-Planck-Institut (Harris et al., 1987) mounted at the Nasmyth focus. Observations were made at 691 GHz (CO 6–5), 797 GHz (HCN 9–8) and 806 GHz (CO 7–6) with the aims...

Discs occur in a wide variety of astronomical contexts, ranging in size from planetary ring systems to galaxies. Because the dynamical problems posed by each type of disc are remarkably similar, the Astronomy Department of the University of Manchester decided to bring together experts in each area for a conference in December 1988. This book is a c...

We present high-resolution CO $J=2\rightarrow1$ and HCO+ $J=3\rightarrow2$ maps of the Orion B molecular outflow source. The outflow appears to be unipolar, and shows evidence of acceleration of molecular gas at up to 0.6 pc from the driving star. The highest velocity material, as well as being furthest from the source, seems to lie close to the ce...

A 220–280 GHz dual polarization receiver has been built for the James Clerk Maxwell Telescope. Schottky diode mixers cooled to 15K by a closed-cycle refrigerator are used to give DSB noise temperatures of 300K and 420K in the two channels. The optical design is based on gaussian-beam optics, and is frequency independent; it allows the significant h...

An analytical technique for deriving the mass and emissivity distribution of a molecular disk from its observed position-velocity (l-v) diagram is briefly described. The governing equation for a simplified model of an optically thin edge-on axisymmetric molecular disk in Keplerian motion around a star is given, and a maximum-entropy-method (MEM) ap...

High-resolution CO J → 1 → 2 observations of the Orion B molecular outflow show that the outflow is unipolar, and that there is evidence of acceleration of molecular gas at up to 0.5pc from the driving star. The highest-velocity material, as well as being furthest from the source, seems to lie close to the central axis of the flow, and is presumabl...

The molecular discs around Young Stellar Objects can be detected through high spatial and spectral resolution observations of their position-velocity diagrams. By mapping a high lying transition of a density-tracing molecule (e.g. HON, HCO + , NH3), the lines should be optically thin and we can expect to see the signature of rotation of the protost...

The authors have mapped the J = 2 - 1 transition of 12CO in a sample of highly opaque molecular clouds, each having at least one associated IRAS source with infrared flux characteristic of an embedded core. Seven clouds from this sample have been mapped so far, five of which display outflow activity. Two of these outflows, in the Lynds dark clouds...

In this paper we consider the maximization of the throughput of a single large antenna, for two possible array configurations: focal plane imaging arrays and aperture plane phased arrays. We discuss trade-offs between the two types of array in terms of field of view, sampling efficiency and time to map, a source. We also discuss limits on the numbe...

We describe an experimental array receiver intended for radioastronomical observations of the J=32 line of carbon monoxide. It consists of six indium antimonide mixers cooled to 2K in a pumped helium cryostat, with rectangular feed horns arranged on a 32 grid at the f/9 focus of the UK Infrared Telescope (UKIRT). Each detector is mounted in a lengt...

Without Abstract

Gaussian mode analysis is a convenient way to characterize long focal length systems, such as the Cassegrain antenna at the secondary focus. We use multimode Gaussian optics to derive several interesting results concerning the aperture efficiencies of Cassegrain antennas fed by corrugated conical horns. The highest efficiency is obtained when the a...

The authors present HCO+ J = 3-2 maps with 60 arcsec resolution of Orion B. They have detected a dense (≡105cm-3) component in the Orion B bipolar outflow. The maps of the cloud core give evidence that the evolution of the outflow has been influenced by the local density gradients in the molecular cloud. The authors have also made a search for H2S(...

Two receiver systems are described. The first is a 220-280 GHz dual-polarization system using cryogenically cooled Schottky-diode mixers pumped with a frequency-multiplied IMPATT local oscillator. The second is a 350 GHz indium antimonide array receiver which uses a novel quasi-optical LO injection scheme.

Observations of the center of OMC1 obtained in the J = 5-4 transitions of CS and C(S-34) using the A heterodyne receiver and the Schottky-diode mixer and frequency multiplier described by Ellison and Vizard (1983) at the f/9 Cassegrain focus of the UK IR Telescope at Mauna Kea on August 17-22, 1983, and in the J = 2-1 transitions using the 11-m tel...

We have observed the HCO+ J=4 → 3 transition in OMC 1 and three other galactic sources which have self-reversed CO profiles. None of the sources show self-absorption in this line and, in NGC 2071 and Mon R2, the line wings, seen in CO and HCO+ 1 → 0, have disappeared. Minimum masses between 200 and l200Msun. are derived for the high-density cores;...

Observations of the J = 2-1 transitions of (C-12)O and (C-13)O at 230 and 220 GHz in 13 molecular clouds near compact H(+) regions have been made at UKIRT using an uncooled Schottky diode mixer and a digital autocorrelation spectrometer. The sources were chosen on the basis of their ammonia emission. A comparison between (C-12)O and (C-13)O spectra...

The J = 4 - 3 transition of HCO(+) was observed in OMCl and in three other galactic sources which have self-reversed CO profiles. None of the sources shows self-absorption in this line and, in NGC2071 and Mon R2, the line wings seen in CO and in HCO(+) 1 - 0 have disappeared. Minimum masses between 200 and 1200 solar mass are derived for the high-d...

The J = 4 → 3 transition of HCN at 354 GHz has been detected in the Orion Molecular Cloud using the UK Infrared Telescope with an InSb heterodyne detector. Both the "spike" and "plateau" sources are seen, and the physical parameters of these are derived from a model of the cloud. On certain plausible assumptions, the extended spike source is estima...

The cross-polarization and reflection properties of grooved dielectric windows are calculated. Measurements on two windows in the range 8-12 GHz are found to be in good agreement with the theory. It is shown that cross-polarization is substantially reduced if the grooves on the front and back faces of the window are orthogonal to each other.

A dielectric lens, designed to be fed by a wide-angle corrugated conical horn, is described. The radiation pattern of the horn-lens combination is similar to that of a much longer conical horn while retaining wideband characteristics.

Two sources of baseline ripple on the 64 m Parkes radio telescope at 5 GHz are described in terms of methods for their reduction. The first, off-source, arises even when the telescope is pointing at a cold sky, and can be fairly easily attenuated by subtracting an off-source reference from the signal spectrum. Attention is given to the second sourc...

A dual-polarization InSb hot-electron bolometer-mixer receiver has been built for the James Clerk Maxwell Telescope, for operation at 461 and 492 GHz (the frequencies of theJ=4$rightarrow$3 rotational transition of CO and of theensuremath 3ensuremath<?supensuremath> P 1$rightarrow$ensuremath 3ensuremath<?supensuremath> P 0 transition of neutral car...

An unresolved uniform isothermal contracting gas cloud has maximum emission in any molecular line over a range of densities defined at the lower end by the requirement for thermalization by collisions and at the other end by the need for the optical depth to be less than or approximately equal to 1. This makes it likely that with relatively low sen...

Thesis (Ph. D.)--University of Cambridge, 1982.