John S. Richer

HARP/ACSIS: a submillimetre spectral imaging system on the James Clerk Maxwell Telescope

This paper describes a new Heterodyne Array Receiver Programme (HARP) and AutoCorrelation Spectral Imaging System (ACSIS) that have recently been installed and commissioned on the James Clerk Maxwell Telescope (JCMT). The 16-element focal-plane array receiver, operating in the submillimetre from 325 to 375 GHz, offers high (three-dimensional) mapping speeds, along with significant improvements over single-detector counterparts in calibration and image quality. Receiver temperatures are 120 K across the whole band and system temperatures of 300K are reached routinely under good weather conditions. The system includes a single-sideband filter so these are SSB figures. Used in conjunction with ACSIS, the system can produce large-scale maps rapidly, in one or more frequency settings, at high spatial and spectral resolution. Fully-sampled maps of size 1 square degree can be observed in under 1 hour. The scientific need for array receivers arises from the requirement for programmes to study samples of objects of statistically significant size, in large-scale unbiased surveys of galactic and extra-galactic regions. Along with morphological information, the new spectral imaging system can be used to study the physical and chemical properties of regions of interest. Its three-dimensional imaging capabilities are critical for research into turbulence and dynamics. In addition, HARP/ACSIS will provide highly complementary science programmes to wide-field continuum studies, and produce the essential preparatory work for submillimetre interferometers such as the SMA and ALMA.

The Structure of Protostellar Envelopes Derived from Submillimeter Continuum Images

High dynamic range imaging of submillimeter dust emission from the envelopes of eight young protostars in the Taurus and Perseus star-forming regions has been carried out using the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. Good correspondence between the spectral classifications of the protostars and the spatial distributions of their dust emission is observed, in the sense that those with cooler spectral energy distributions also have a larger fraction of the submillimeter flux originating in an extended envelope than in a disk. This results from the cool sources having more massive envelopes rather than warm sources having larger disks. Azimuthally averaged radial profiles of the dust emission are used to derive the power-law index of the envelope density distributions, p (defined by ρ r-p), and most of the sources are found to have values of p consistent with those predicted by models of cloud collapse. However, the youngest protostars in our sample, L1527 and HH 211-mm, deviate significantly from the theoretical predictions, exhibiting values of p somewhat lower than can be accounted for by existing models. For L1527 heating of the envelope by shocks where the outflow impinges on the surrounding medium may explain our result. For HH 211-mm another explanation is needed, and one possibility is that a shallow density profile is being maintained in the outer envelope by magnetic fields and/or turbulence. If this is the case, star formation must be determined by the rate at which the support is lost from the cloud, rather than the hydrodynamical properties of the envelope, such as the sound speed.

Detection of Polarized Millimeter and Submillimeter Emission from Sagittarius A

We report the detection of linear polarization from Sgr A* at 750, 850, 1350, and 2000 µm which confirms the contribution of synchrotron radiation. From the lack of polarization at longer wavelengths, it appears to arise in the millimeter/submillimeter excess. There are large position angle changes between the millimeter and submillimeter results, and these are discussed in terms of a polarized dust contribution in the submillimeter and various synchrotron models. In the model that best explains the data, the synchrotron radiation from the excess is self-absorbed in the millimeter region and becomes optically thin in the submillimeter. This implies that the excess arises in an extremely compact source of approximately 2 Schwarzschild radii.

A SURVEY OF SiO 5 ¿ 4 EMISSION TOWARD OUTFLOWS FROM LOW-LUMINOSITY PROTOSTELLAR CANDIDATES

We have observed the SiO J=5!4 line towards a sample of 25 low-luminosity (L∗ < 10 3 L⊙) protostellar outflow systems. The line was detected towards 7 of the 25 sources, a detection rate of 28 per cent. The majority (5 out of 7) of sources detected were of class 0 status. We detected a higher fraction of class 0 sources compared with the class I and II sources, although given the small numbers involved the significance of this result should be regarded as tentative. Most of the detected sources showed emission either at or close to the central position, coincident with the protostar. In four cases (HH211, HH25MMS, V-380 OriNE and HH212) emission was also detected at positions away from the center, and was stronger than that observed at the centre position. SiO abundances of 10 −8 to 8×10 −7 are derived from LTE analysis. For 2 sources we have additional transitions which we use to conduct statistical equilibrium modeling to estimate the gas density in the SiO-emitting regions. For HH25MMS these results suggest that the SiO emission arises in a higherdensity region than the methanol previously observed. We find that the most likely explanation for the preferential detection of SiO emission towards class 0 sources is the greater density of those environments, reinforced by higher shock velocities. We conclude that while not all class 0 sources exhibit SiO emission, SiO emission is a good signpost for the presence of class 0 sources. Subject headings: stars: formation — ISM: jets and outflows — ISM: molecules — ISM: Herbig-Haro objects — radio lines: ISM

Subarcsecond Imaging of SiO in the HH 211 Protostellar Jet

We present images of the HH 211 molecular jet in the SiOv=0, J =1‐0 line at 43 GHz made with the Very Large Array at approximately 0.5 arcsec resolution. The SiO emission appears to trace primarily internal bowshocks in the outflow, sugges ting that the dust and molecular gas are accelerated via prompt entrainment at internal working surfaces in the jet. There is also some evidence for limb-brightening of the SiO emission, indicating that SiO emission may also arise from entrainment in the jet’s boundary layer. Excitat ion temperatures of & 150‐200 K are inferred from the SiO emission. Enhancements in the SiO abundance of � 10 6 over interstellar values are observed, and the possible origin of the SiO is discussed.

A submillimetre survey of the kinematics of the Perseus molecular cloud – I. Data

We present submillimetre observations of the J = 3 → 2 rotational transition of 12 CO, 13 CO and C 18 O across over 600 arcmin 2 of the Perseus molecular cloud, undertaken with the Heterodyne Array Receiver Programme (HARP), a new array spectrograph on the James Clerk Maxwell Telescope. The data encompass four regions of the cloud, containing the largest clusters of dust continuum condensations: NGC 1333, IC348, L1448 and L1455. A new procedure to remove striping artefacts from the raw HARP data is introduced. We compare the maps to those of the dust continuum emission mapped with the Submillimetre Common-User Bolometer Array (SCUBA; Hatchell et al.) and the positions of starless and protostellar cores (Hatchell et al.). No straightforward correlation is found between the masses of each region derived from the HARP CO and SCUBA data, underlining the care that must be exercised when comparing masses of the same object derived from different tracers. From the 13 CO/C 18 O line ratio the relative abundance of the two species ([ 13 CO]/[C 18 O] ∼ 7) and their opacities (typically τ is 0.02-0.22 and 0.15―1.52 for the C 18 O and 13 CO gas, respectively) are calculated. C 18 O is optically thin nearly everywhere, increasing in opacity towards star-forming cores but not beyond τ 18 ∼ 0.9. Assuming the 12 CO gas is optically thick, we compute its excitation temperature, T ex (around 8-30 K), which has little correlation with estimates of the dust temperature.

The properties of SCUBA cores in the Perseus molecular cloud: the bias of clump-finding algorithms

We present a new analysis of the properties of star-forming cores in the Perseus molecular cloud, identified in SCUBA 850 μm data originally presented by Hatchell et al. Our goal is to determine which core properties can be robustly identified and which depend on the extraction technique. Four regions in the cloud are examined: NGC 1333, IC348/HH211, L1448 and L1455. We identify clumps of dust emission using two popular automated algorithms, CLFIND and GAUSSCLUMPS, finding 85 and 122 clumps in total, respectively. Using the catalogues of Hatchell et al., we separate these clumps into starless, Class 0 and Class I cores. Some trends are true for both populations: clumps become increasingly elongated over time; clumps are consistent with constant surface brightness objects (i.e. M ∝ R 2 ), with an average brightness ≈4-10 0 times larger than the surrounding molecular cloud; the clump mass distribution (CMD) resembles the stellar initial mass function, with a slope α = -2.0 ± 0.1 for CLFIND and α = -3.15 ± 0.08 for GAUSSCLUMPS, which straddle the Salpeter value (α = -2.35). The mass at which the slope shallows (similar for both algorithms at M ≈ 6M ⊙ ) implies a star-forming efficiency of between 10 and 20 per cent. Other trends reported elsewhere depend critically on the clump-finding technique: we find protostellar clumps are both smaller (for GAUSSCLUMPS) and larger (for CLFIND) than their starless counterparts; the functional form, best fitting to the CMD, is different for the two algorithms. The GAUSSCLUMPS CMD is best fitted with a log-normal distribution, whereas a broken power law is best for CLFIND; the reported lack of massive starless cores in previous studies can be seen in the CLFIND but not the GAUSSCLUMPS data. Our approach, exploiting two extraction techniques, highlights similarities and differences between the clump populations, illustrating the caution that must be exercised when comparing results from different studies and interpreting the properties of samples of continuum cores.

A SCUBA survey of compact dark Lynds clouds

We have carried out a survey of optically-selected dark clouds using the bolometer array SCUBA on the James Clerk Maxwell Telescope, at 850 microns wavelength. The survey covers a total of 0.5 square degrees and is unbiased with reference to cloud size, star formation activity, or the presence of infrared emission. Several new protostars and starless cores have been discovered; the protostars are confirmed through the detection of their accompanying outflows in CO(2-1) emission. The survey is believed to be complete for Class 0 and Class I protostars, and yields two important results regarding the lifetimes of these phases. First, the ratio of Class 0 to Class protostars in the sample is roughly unity, very different from the 1:10 ratio that has previously been observed for the rho Ophiuchi star-forming region. Assuming star formation to be a homogeneous process in the dark clouds, this implies that the Class 0 lifetime is similar to the Class I phase, which from infrared surveys has been established to be approximately 200,000 yr. It also suggests there is no rapid initial accretion phase in Class 0 objects. A burst of triggered star formation some 100,000 yr ago can explain the earlier results for rho Ophiuchus. Second, the number of starless cores is approximately twice that of the total number of protostars, indicating a starless core lifetime of approximately 800,000 yr. These starless cores are therefore very short-lived, surviving only two or three free-fall times. This result suggests that, on size scales of 10,000 AU at least, the dynamical evolution of starless cores is probably not controlled by magnetic processes.

Phase correction for ALMA with 183GHz water vapour radiometers

One of the great challenges for ALMA is overcoming the natural limits set by the turbulence in the atmosphere to achieve resolutions as fine as ten milli-arcseconds. A critical component in the strategy to achieve this are mm-wave radiometers on each of the 12 m diameter telescopes that observe the emission from the atmospheric water vapour line at 183 GHz. The information from these radiometers can be used to compute the fluctuations in total water vapour along the line of sight of each telescope, and from this, the fluctuation in effective path to each antenna. The estimates of path fluctuations are then used to phase-rotate recorded visibilities leading to much increased coherence. In this paper we briefly review the design of the radiometers, describe the software processing steps to derive phase corrections and show some of the first results from the ALMA site in Chile.

Measurement of antenna surfaces from in- and out-of-focus beam maps using astronomical sources

We present a technique for the accurate estimation of large-scale errors in an antenna surface using astronomical sources and detectors. The technique requires several out-of-focus images of a compact source and the signal-to-noise ratio needs to be good but not unreasonably high. For a given pattern of surface errors, the expected form of such images can be calculated directly. We show that it is possible to solve the inverse problem of finding the surface errors from the images in a stable manner using standard numerical techniques. To do this we describe the surface error as a linear combination of a suitable set of basis functions (we use Zernike polynomials). We present simulations illustrating the technique and in particular we investigate the effects of receiver noise and pointing errors. Measurements of the 15-m James Clerk Maxwell telescope made using this technique are presented as an example. The key result is that good measurements of errors on large spatial scales can be obtained if the input images have a signal-to-noise ratio of order 100 or more. The important advantage of this technique over transmitter-based holography is that it allows measurements at arbitrary elevation angles, so allowing one to characterise the large scale deformations in an antenna as a function of elevation.