Claire J. Chandler

The Expanded Very Large Array: A New Telescope for New Science

Since its commissioning in 1980, the Very Large Array (VLA) has consistently demonstrated its scientific productivity. However, its fundamental capabilities have changed little since 1980, particularly in the key areas of sensitivity, frequency coverage, and velocity resolution. These limitations have been addressed by a major upgrade of the array, which began in 2001 and will be completed at the end of 2012. When completed, the Expanded VLA?the EVLA?will provide complete frequency coverage from 1 to 50?GHz, a continuum sensitivity of typically 1 ?Jy beam?1 (in 9 hr with full bandwidth), and a modern correlator with vastly greater capabilities and flexibility than the VLAs. In this Letter, we describe the goals of the EVLA project, its current status, and the anticipated expansion of capabilities over the next few years. User access to the array through the Open Shared Risk Observing and Resident Shared Risk Observing programs is described. The following papers in this special issue, derived from observations in its early science period, demonstrate the astonishing breadth of this most flexible and powerful general-purpose telescope.

Large dust particles in disks around T Tauri stars

We present 7-mm continuum observations of 14 low-mass pre-main-sequence stars in the Taurus-Auriga star-forming region obtained with the Very Large Array with ∼1.5 resolution and ∼0.3 mJy rms sensitivity. For 10 objects, the circumstellar emission has been spatially resolved. The large outer disk radii derived suggest that the emission at this wavelength is mostly optically thin. The millimetre spectral energy distributions are characterised by spectral indices α mm = 2.3 to 3.2. After accounting for contributions from free-free emission and corrections for optical depth, we determine dust opacity indices β in the range 0.5 to 1.6, which suggest that millimetre-sized dust aggregates are present in the circumstellar disks. Four of the sources with β > 1 may be consistent with submicron-sized dust as found in the interstellar medium. Our findings indicate that dust grain growth to millimetre-sized particles is completed within less than 1 Myr for the majority of circumstellar disks.

Large-Scale Asymmetries in the Transitional Disks of SAO 206462 and SR 21

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations in the dust continuum (690 GHz, 0.45 mm) and ^(12)CO J = 6-5 spectral line emission of the transitional disks surrounding the stars SAO 206462 and SR 21. These ALMA observations resolve the dust-depleted disk cavities and extended gaseous disks, revealing large-scale asymmetries in the dust emission of both disks. We modeled these disk structures with a ring and an azimuthal Gaussian, where the azimuthal Gaussian is motivated by the steady-state vortex solution from Lyra & Lin. Compared to recent observations of HD 142527, Oph IRS 48, and LkHα 330, these are low-contrast (≾ 2) asymmetries. Nevertheless, a ring alone is not a good fit, and the addition of a vortex prescription describes these data much better. The asymmetric component encompasses 15% and 28% of the total disk emission in SAO 206462 and SR 21, respectively, which corresponds to a lower limit of 2 M_(Jup) of material within the asymmetry for both disks. Although the contrast in the dust asymmetry is low, we find that the turbulent velocity inside it must be large (~20% of the sound speed) in order to drive these azimuthally wide and radially narrow vortex-like structures. We obtain residuals from the ring and vortex fitting that are still significant, tracing non-axisymmetric emission in both disks. We compared these submillimeter observations with recently published H-band scattered light observations. For SR 21 the scattered light emission is distributed quite differently from the submillimeter continuum emission, while for SAO 206462 the submillimeter residuals are suggestive of spiral-like structure similar to the near-IR emission.

A FEATURE MOVIE OF SiO EMISSION 20-100 AU FROM THE MASSIVE YOUNG STELLAR OBJECT ORION SOURCE I

We present multi-epoch Very Long Baseline Array imaging of the 28SiO v = 1 and v = 2, J = 1-0 maser emission toward the massive young stellar object (YSO) Orion Source I. Both SiO transitions were observed simultaneously with an angular resolution of ~0.5 mas (~0.2 AU for d = 414 pc) and a spectral resolution of ~0.2 km s–1. Here we explore the global properties and kinematics of the emission through two 19-epoch animated movies spanning 21 months (from 2001 March 19 to 2002 December 10). These movies provide the most detailed view to date of the dynamics and temporal evolution of molecular material within ~20-100 AU of a massive (8 M ☉) YSO. As in previous studies, we find that the bulk of the SiO masers surrounding Source I lie in an -shaped locus; the emission in the south and east arms is predominantly blueshifted, and emission in the north and west is predominantly redshifted. In addition, bridges of intermediate-velocity emission are observed connecting the red and blue sides of the emission distribution. We have measured proper motions of over 1000 individual maser features and found that these motions are characterized by a combination of radially outward migrations along the four main maser-emitting arms and motions tangent to the intermediate-velocity bridges. We interpret the SiO masers as arising from a wide-angle bipolar wind emanating from a rotating, edge-on disk. The detection of maser features along extended, curved filaments suggests that magnetic fields may play a role in launching and/or shaping the wind. Our observations appear to support a picture in which stars with masses as high as at least 8 M ☉ form via disk-mediated accretion. However, we cannot yet rule out that the Source I disk may have been formed or altered following a recent close encounter.

The Coordinated Radio and Infrared Survey for High-mass Star Formation. II. Source Catalog

The CORNISH project is the highest resolution radio continuum survey of the Galactic plane to date. It is the 5 GHz radio continuum part of a series of multi-wavelength surveys that focus on the northern GLIMPSE region (10° < l < 65°), observed by the Spitzer satellite in the mid-infrared. Observations with the Very Large Array in B and BnA configurations have yielded a 1.″5 resolution Stokes I map with a root mean square noise level better than 0.4 mJy beam -1 . Here we describe the data-processing methods and data characteristics, and present a new, uniform catalog of compact radio emission. This includes an implementation of automatic deconvolution that provides much more reliable imaging than standard CLEANing. A rigorous investigation of the noise characteristics and reliability of source detection has been carried out. We show that the survey is optimized to detect emission on size scales up to 14″ and for unresolved sources the catalog is more than 90% complete at a flux density of 3.9 mJy. We have detected 3062 sources above a 7σ detection limit and present their ensemble properties. The catalog is highly reliable away from regions containing poorly sampled extended emission, which comprise less than 2% of the survey area. Imaging problems have been mitigated by down-weighting the shortest spacings and potential artifacts flagged via a rigorous manual inspection with reference to the Spitzer infrared data. We present images of the most common source types found: H II regions, planetary nebulae, and radio galaxies. The CORNISH data and catalog are available online at http://cornish.leeds.ac.uk.

A MULTI-EPOCH STUDY OF THE RADIO CONTINUUM EMISSION OF ORION SOURCE I: CONSTRAINTS ON THE DISK EVOLUTION OF A MASSIVE YSO AND THE DYNAMICAL HISTORY OF ORION BN/KL

We present new λ7 mm continuum observations of Orion BN/KL with the Very Large Array. We resolve the emission from the young stellar objects radio Source I and BN at several epochs. Radio Source I is highly elongated northwest-southeast, and remarkably stable in flux density, position angle, and overall morphology over nearly a decade. This favors the extended emission component arising from an ionized edge-on disk rather than an outwardly propagating jet. We have measured the proper motions of Source I and BN for the first time at 43 GHz. We confirm that both sources are moving at high speed (12 and 26 km s–1, respectively) approximately in opposite directions, as previously inferred from measurements at lower frequencies. We discuss dynamical scenarios that can explain the large motions of both BN and Source I and the presence of disks around both. Our new measurements support the hypothesis that a close (~50 AU) dynamical interaction occurred around 500 years ago between Source I and BN as proposed by Gomez et al. From the dynamics of encounter, we argue that Source I today is likely to be a binary with a total mass on the order of 20 M ☉ and that it probably existed as a softer binary before the close encounter. This enables preservation of the original accretion disk, though truncated to its present radius of ~50 AU. N-body numerical simulations show that the dynamical interaction between a binary of 20 M ☉ total mass (Source I) and a single star of 10 M ☉ mass (BN) may lead to the ejection of both and binary hardening. The gravitational energy released in the process would be large enough to power the wide-angle, high-velocity flow traced by H2 and CO emission in the BN/KL nebula. Assuming that the proposed dynamical history is correct, the smaller mass for Source I recently estimated from SiO maser dynamics (7 M ☉) by Matthews et al., suggests that non-gravitational forces (e.g., magnetic) must play an important role in the circumstellar gas dynamics.

Spiral density waves in a young protoplanetary disk.

Gravitational forces are expected to excite spiral density waves in protoplanetary disks, disks of gas and dust orbiting young stars. However, previous observations that showed spiral structure were not able to probe disk midplanes, where most of the mass is concentrated and where planet formation takes place. Using the Atacama Large Millimeter/submillimeter Array, we detected a pair of trailing symmetric spiral arms in the protoplanetary disk surrounding the young star Elias 2-27. The arms extend to the disk outer regions and can be traced down to the midplane. These millimeter-wave observations also reveal an emission gap closer to the star than the spiral arms. We argue that the observed spirals trace shocks of spiral density waves in the midplane of this young disk.

Imaging the Ionized Disk of the High-Mass Protostar Orion I

We have imaged the enigmatic radio source I (Orion I) in the Orion KL nebula with the VLA at 43 GHz with 34 mas angular resolution. The continuum emission is highly elongated and is consistent with that expected from a nearly edge-on disk. The high brightness and lack of strong molecular lines from Orion I can be used to argue against emission from dust. Collisional ionization and H- free-free opacity, as in Mira variables, require a central star with 105 L☉, which is greater than infrared observations allow. However, if significant local heating associated with accretion occurs, lower total luminosities are possible. Alternatively, photoionization from an early B-type star and p+/e- bremsstrahlung can explain our observations, and Orion I may be an example of ionized accretion disk surrounding a forming massive star. Such accretion disks may not be able to form planets efficiently.

On the structure of the transition disk around TW Hydrae

Context. For over a decade, the structure of the inner cavity in the transition disk of TW Hydrae has been a subject of debate. Modeling the disk with data obtained at di erent wavelengths has led to a variety of proposed disk structures. Rather than being inconsistent, the individual models might point to the di erent faces of physical processes going on in disks, such as dust growth and planet formation. Aims. Our aim is to investigate the structure of the transition disk again and to find to what extent we can reconcile apparent model di erences. Methods. A large set of high-angular-resolution data was collected from near-infrared to centimeter wavelengths. We investigated the existing disk models and established a new self-consistent radiative-transfer model. A genetic fitting algorithm was used to automatize the parameter fitting, and uncertainties were investigated in a Bayesian framework. Results. Simple disk models with a vertical inner rim and a radially homogeneous dust composition from small to large grains cannot reproduce the combined data set. Two modifications are applied to this simple disk model: (1) the inner rim is smoothed by exponentially decreasing the surface density in the inner 3 AU, and (2) the largest grains (>100 m) are concentrated towards the inner disk region. Both properties can be linked to fundamental processes that determine the evolution of protoplanetary disks: the shaping by a possible companion and the di erent regimes of dust-grain growth, respectively. Conclusions. The full interferometric data set from near-infrared to centimeter wavelengths requires a revision of existing models for the TW Hya disk. We present a new model that incorporates the characteristic structures of previous models but deviates in two key aspects: it does not have a sharp edge at 4 AU, and the surface density of large grains di ers from that of smaller grains. This is the first successful radiative-transfer-based model for a full set of interferometric data.

Compact Outflows Associated with TMC-1 and TMC-1A

High spatial resolution observations are presented of the compact outflows associated with the young protostars TMC-1 (IRAS 04381+2540) and TMC-1A (IRAS 04365+2535) in Taurus. Emission in CO(1–0) imaged with the Owens Valley Millimeter Array shows the outflow lobes to be conical close to the star. Analysis of the outflow dynamics indicates that these objects are low-luminosity versions of the energetic outflows more commonly observed. Near-infrared images at H and K bands show a close correspondence between reflection nebulosity and the location of high-velocity gas and suggest the outflow cavity is evacuated, as do position-velocity diagrams of the CO(2-1). Comparison of the J = 1–0 transition with emission in the 2–1 line indicates that the excitation temperature in the high-velocity gas is higher than the surrounding Taurus cloud. We place limits on the inclination of both objects by comparing the data with theoretical outflow models and conclude that i ~ 40°–70° for both objects. The depro-jected opening angles of the outflow cones are then in the range 30°–40°. None of the current outflow models satisfactorily explains the results for TMC-1 and TMC-1A, which are among the youngest class I sources in Taurus. We find their outflow structure shares many similarities with the more obscured and possibly younger class 0 objects, B335 and L1448-C. The main difference is the lower mechanical luminosities of the TMC-1 and TMC-1A outflows, reflecting a factor of 3–4 smaller linear extent and velocity and a factor of 10–20 lower mass than the L1448-C molecular jet source. Taken together, the four protostars share the common properties of (1) conical outflow lobes close to the star, (2) evacuated outflow cavities, and (3) relatively wide 30°-40° opening angles. A successful theory of young stellar outflows must be able to explain these characteristics.