M. C. H. Wright

The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.

Imaging the HL Tauri Disk at λ = 2.7 Millimeters with the BIMA Array

We have obtained a subarcsecond image of the disk associated with the T Tauri star HL Tau at a wavelength of 2.7 mm using the new high-resolution capability of the BIMA Array. The disk is elongated with a deconvolved Gaussian source size of 10 ± 02 × 05 ± 02, implying a semimajor axis of 70 ± 15 AU for a distance of 140 pc; the minor axis may be unresolved. The position angle of the major axis (125° ± 10°) is orthogonal to the axis of the optical jet. The disk centroid is coincident with the VLA λ = 3.6 cm source position and nearly coincident with recent measurements of the near-infrared emission peak. The λ = 2.7 mm images, along with previous interferometric measurements at λ = 0.87 mm and flux measurements from 10 μm to 1.3 cm, are well fitted by a simple power-law disk model with a shallow radial dependence to the surface density [Σ(r) ∝ r0 to r-1], an outer radius between 90 and 160 AU, and a dust opacity law proportional to ν1.

Formaldehyde in Comets C/1995 O1 (Hale-Bopp), C/2002 T7 (LINEAR), and C/2001 Q4 (NEAT): Investigating the Cometary Origin of H2CO

Observations offormaldehyde (H2CO) have been conducted toward comets C/1995 O1 (Hale-Bopp), C/2001 Q4 (NEAT), and C/2002 T7 (LINEAR) using the Arizona Radio Observatory (ARO) 12 m telescope at 1.2 and 2 mm. Aperture synthesis maps of H2CO at 3 mm were made using the Berkeley-Illinois-Maryland Association (BIMA) interferometer toward comet Hale-Bopp. These data indicate that the production rate of H2CO is � 3:7 ; 10 28 s � 1 at � 1 AU in comet Hale-Bopp, using a simple Monte Carlo model, if a nuclear origin for the molecule is assumed. However, maps of H2CO in Hale-Bopp, in comparison with CO, show an extended distribution (rs � 15,000 km) with small-scale structure oriented roughly along the comet-Sun direction. This result suggests a source of H2CO other than the comet nucleus. The extended source of formaldehyde is probably grains composed of a mixture of silicates and organicmaterial.Theproductionrate forH2CO increasestoQ � 1:4 ; 10 29 s � 1 assuming suchanextended grain source. This value implies a Q/Q(H2O) � 1:4%, which is similar to the production rate ratio of Q/Q(H2O) � 4% derived from in situ measurements of H2CO in comet Halley. Production rates for H2CO toward comets C/2002 T7 (LINEAR) and C/2001 Q4 (NEAT) are 1:4 ; 10 27 and 5:6 ; 10 26 s � 1 , respectively, modeled using the extended grain source. The spectra of H2CO measured toward comet C/2002 T7 (LINEAR) show evidence for a second velocity component, most likely arising from comet fragmentation. Subject headingg astrobiology — comets: individual (Hale-Bopp (C/1995 O1), NEAT (C/2001 Q4), LINEAR (C/2002 T7)) — radio lines: solar system — techniques: interferometric

THE ALLEN TELESCOPE ARRAY Pi GHz SKY SURVEY. I. SURVEY DESCRIPTION AND STATIC CATALOG RESULTS FOR THE BOÖTES FIELD

The Pi GHz Sky Survey (PiGSS) is a key project of the Allen Telescope Array. PiGSS is a 3.1 GHz survey of radio continuum emission in the extragalactic sky with an emphasis on synoptic observations that measure the static and time-variable properties of the sky. During the 2.5 year campaign, PiGSS will twice observe similar to 250,000 radio sources in the 10,000 deg(2) region of the sky with b > 30 degrees to an rms sensitivity of similar to 1 mJy. Additionally, sub-regions of the sky will be observed multiple times to characterize variability on timescales of days to years. We present here observations of a 10 deg(2) region in the Bootes constellation overlapping the NOAO Deep Wide Field Survey field. The PiGSS image was constructed from 75 daily observations distributed over a 4 month period and has an rms flux density between 200 and 250 mu Jy. This represents a deeper image by a factor of 4-8 than we will achieve over the entire 10,000 deg(2). We provide flux densities, source sizes, and spectral indices for the 425 sources detected in the image. We identify similar to 100 new flat-spectrum radio sources; we project that when completed PiGSS will identify 10(4) flat-spectrum sources. We identify one source that is a possible transient radio source. This survey provides new limits on faint radio transients and variables with characteristic durations of months.

SPECTROPOLARIMETRY WITH THE ALLEN TELESCOPE ARRAY: FARADAY ROTATION TOWARD BRIGHT POLARIZED RADIO GALAXIES

We have observed 37 bright, polarized radio sources with the Allen Telescope Array (ATA) to present a novel analysis of their Faraday rotation properties. Each source was observed during the commissioning phase with two to four 100 MHz bands at frequencies ranging from 1 to 2 GHz. These observations demonstrate how the continuous frequency coverage of the ATAs log-periodic receiver can be applied to the study of Faraday rotation measures (RMs). We use RM synthesis to show that wide-bandwidth data can find multiple RM components toward a single source. Roughly a quarter of the sources studied have extra RM components with high confidence (brighter than 40 mJy), when observing with an RM resolution of roughly 100 rad m?2. These extra components contribute 10%-70% of the total polarized flux. This is the first time multiple RM components have been identified in a large sample of point sources. For our observing configuration, these extra RM components bias the measurement of the peak RM by 10-15 rad m?2; more generally, the peak RM cannot be determined more precisely than the RM beam size. Comparing our 1-2 GHz RM spectra to Very Long Baseline Array (VLBA) polarimetric maps shows that both techniques can identify complicated Faraday structures in the sources. However, the RM values and fractional polarization are generally smaller at lower frequencies than in the higher frequency VLBA maps. With a few exceptions, the RMs from this work are consistent with that of earlier, narrow-bandwidth, all-sky surveys. This work also describes the polarimetry calibration procedure and that on-axis ATA observations of linear polarization can be calibrated to an accuracy of 0.2% of Stokes I. Future research directions include studying the time-dependent RM structure in active galactic nuclei and enabling accurate, wide-area RM surveys to test models of Galactic and extragalactic magnetic fields.

Primary Beam and Dish Surface Characterization at the Allen Telescope Array by Radio Holography

The Allen Telescope Array (ATA) is a cm-wave interferometer in California, comprising 42 antenna elements with 6-m diameter dishes. We characterize the antenna optical accuracy using two-antenna interferometry and radio holography. The distortion of each telescope relative to the average is small, with RMS differences of 1% of beam peak value. Holography provides images of dish illumination, characterizing as-built mirror surfaces. Maximal distortions across ~ 2 meter lengths appear to result from mounting stresses or solar radiation. Experimental RMS errors are 0.7 mm at night and 3 mm under worst-case solar illumination. For frequencies 4, 10, and 15 GHz, the nighttime values indicate sensitivity losses of 1, 10 and 20%, respectively. ATAs wide-bandwidth receiver permits observations over a continuous range 0.5-11.2 GHz. We probe the antenna optical gain and beam pattern stability as a function of focus position and observation frequency, concluding that ATA can produce high fidelity images over a decade of simultaneous observation frequencies. We quantify solar heating effects on antenna sensitivity and pointing accuracy. We find that during the day, observations >;5 GHz will suffer some sensitivity loss and it may be necessary to make antenna pointing corrections on a 1-2 hourly basis.

Dust and HCO+ Gas in the Star-Forming Core W3-SE

We report new results from recent Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations of both continuum and HCO + (1–0) line emission at λ3.4 mm from W3-SE, a molecular core of intermediate mass, together with the observations of continuum emission at λ1.1 and λ0.85/λ0.45 mm with the Submillimeter Array (SMA) and the James Clerk Maxwell Telescope, respectively. A continuum emission core elongated from SE to NW, with a size of ∼10 �� , has been observed at the millimeter and submillimeter wavelengths. The dust core has been resolved into a double source with the SMA at λ1.1 mm. The angular separation between the two components is ∼4 �� . Together with the measurements from theSpitzerSpaceTelescope and theMidcourseSpace Experiment at mid-IR wavelengths, we determined the spectral energy distribution (SED) of the continuum emission from W3-SE and fit it with a thermal dust emission model. Our best fitting of the SED suggests the presence of two dust components with different temperatures. The emission at millimeter/submillimeter wavelengths is dominated by a major component that is characterized by a temperature of Td = 41 ± 6Kw ith am ass of 65± 10 M� .I n addition, there is a weaker hot component (∼ 400 K) which accounts for emission in the mid-IR, suggesting that a small fraction of dust has been heated by newly formed stars. We also imaged the molecular core in the HCO + (1–0) line using CARMA at an angular resolution ∼6 �� . In the central region of ∼50 �� , the integrated HCO + (1–0) line emission shows a main component A that coincides with the dust core, as well as two substructures B and C which are located N and SE of the dust core, respectively. With the CARMA observations, we have verified the presence of a blue-dominated double peak profile toward this core. The line profile cannot be explained by infall alone. The broad velocity wings of the line profile suggest that other kinematics such as outflows within the central 6 �� of the core likely dominate the resulting spectrum. The kinematics of the substructures of B and C suggest that the molecular gas outside the main component A appears to be dominated by the bipolar outflow originated from the dust core with a dynamical age of >3 × 10 4 yr. Our analysis, based on the observations at wavelengths from millimeter, submillimeter, to mid-IR, suggest that the molecular core W3-SE hosts a group of newly formed young stars and protostars.

A 2.8 Millimeter Survey of the Hubble Deep Field

We present a continuum survey of the Hubble Deep Field at a wavelength of 2.8 mm made with the BIMA array. A mosaic of seven pointings was constructed to span the entire 47 × 47 area observed by the Hubble Space Telescope with 47 resolution and a nearly uniform 0.71 mJy rms sensitivity. No significant sources of emission were found (>5 σ). We use these observations to place limits on the presence of far-infrared emission from dusty star-forming galaxies at high redshifts. The most stringent limits apply for a critically closed universe (Ω -->0=1), dust temperatures typical of nearby starbursts (40 K), and long-wavelength dust emissivity power-law index β=1. For these parameters, the data rule out systems at z>5 with far-infrared luminosities in excess of 1.3×10 -->13 L☉. The lack of a large population of such luminous systems supports the view that galaxies are assembled in a hierarchical fashion instead of forming the bulk of their stars in single bursts of short duration. For an open universe and/or substantially higher dust temperatures, the limits are higher by an order of magnitude or more and allow for undetected massive starbursts.

Outflow, Infall, and Protostars in the Star-forming Core W3-SE

We report new results on outflow and infall in the star-forming cores W3-SE SMA-1 and SMA-2 based on analysis of ~25 resolution observations of the molecular lines HCN(3-2), HCO+(3-2), N2H+(3-2), and CH3OH(52, 3-41, 3) with the Submillimeter Array (SMA). A high-velocity bipolar outflow originating from the protostellar core SMA-1 was observed in the HCN(3-2) line, with a projected outflow axis at a position angle of 48°. The detection of the outflow is confirmed from other molecular lines. An inverse P-Cygni profile in the HCN(3-2) line toward SMA-1 suggests that at least one of the double cores accretes matter from the molecular core. A filamentary structure in the molecular gas surrounds SMA-1 and SMA-2. Based on the SMA observations, our analysis suggests that the double pre-stellar cores SMA-1 and SMA-2 result from fragmentation in the collapsing massive molecular core W3-SE, and it is likely that they are forming intermediate- to high-mass stars which will be new members of a star cluster in the W3-SE region.

Removal of tropospheric path length variations in very long baseline interferometry with measurement of tropospheric emission

We present our first results of the application of the measurement of atmospheric emission to correct for atmospheric phase variations in very long baseline interferometry. This technique corrects for the fluctuations in the phase path length through the atmosphere by measuring the water vapor content through its emissivity. Two epochs of 3-mm wavelength (λ) observations under different atmospheric conditions show that total power monitoring at Hat Creek and Kitt Peak Observatories can be used to reduce the rms interferometer phase when liquid water is not present in the beam. In the best cases, the root-mean-square (rms) phase is reduced by a factor of 2 to 1 rad. Instability in the receiver gain at the level of one part in 103 creates the 1-rad limit in the rms phase. The technique is currently most successful at eliminating variability on timescales greater than 10 s. However, it is not always successful, and we discuss sources for the residual variability in the phase. We also derive the expected value of the scale factor relating delay to brightness temperature at 86 and 230 GHz. As this technique is improved, future experiments will have greater sensitivity to weak sources, accurate astrometry and geodesy will be possible, and imaging at 1-mm λ may be explored.