275 research outputs found

    Adaptive Real Time Imaging Synthesis Telescopes

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    The digital revolution is transforming astronomy from a data-starved to a data-submerged science. Instruments such as the Atacama Large Millimeter Array (ALMA), the Large Synoptic Survey Telescope (LSST), and the Square Kilometer Array (SKA) will measure their accumulated data in petabytes. The capacity to produce enormous volumes of data must be matched with the computing power to process that data and produce meaningful results. In addition to handling huge data rates, we need adaptive calibration and beamforming to handle atmospheric fluctuations and radio frequency interference, and to provide a user environment which makes the full power of large telescope arrays accessible to both expert and non-expert users. Delayed calibration and analysis limit the science which can be done. To make the best use of both telescope and human resources we must reduce the burden of data reduction. Our instrumentation comprises of a flexible correlator, beam former and imager with digital signal processing closely coupled with a computing cluster. This instrumentation will be highly accessible to scientists, engineers, and students for research and development of real-time processing algorithms, and will tap into the pool of talented and innovative students and visiting scientists from engineering, computing, and astronomy backgrounds. Adaptive real-time imaging will transform radio astronomy by providing real-time feedback to observers. Calibration of the data is made in close to real time using a model of the sky brightness distribution. The derived calibration parameters are fed back into the imagers and beam formers. The regions imaged are used to update and improve the a-priori model, which becomes the final calibrated image by the time the observations are complete

    A Keplerian disk around Orion Source I, a ~15 Msun YSO

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    We report ALMA long-baseline observations of Orion Source I (SrcI) with resolution 0.03-0.06" (12-24 AU) at 1.3 and 3.2 mm. We detect both continuum and spectral line emission from SrcI's disk. We also detect a central weakly resolved source that we interpret as a hot spot in the inner disk, which may indicate the presence of a binary system. The high angular resolution and sensitivity of these observations allow us to measure the outer envelope of the rotation curve of the H2_2O 55,064,35_{5,0}-6_{4,3} line, which gives a mass MI15±2M_I\approx15\pm2 Msun. We detected several other lines that more closely trace the disk, but were unable to identify their parent species. Using centroid-of-channel methods on these other lines, we infer a similar mass. These measurements solidify SrcI as a genuine high-mass protostar system and support the theory that SrcI and the Becklin Neugebauer Object were ejected from the dynamical decay of a multiple star system \sim500 years ago, an event that also launched the explosive molecular outflow in Orion.Comment: Accepted to ApJ. Data at https://zenodo.org/record/1213350, source repository at https://github.com/keflavich/Orion_ALMA_2016.1.00165.

    Orion Source I's disk is salty

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    We report the detection of NaCl, KCl, and their 37^{37}Cl and 41^{41}K isotopologues toward the disk around Orion SrcI. About 60 transitions of these molecules were identified. This is the first detection of these molecules in the interstellar medium not associated with the ejecta of evolved stars. It is also the first ever detection of the vibrationally excited states of these lines in the ISM above v = 1, with firm detections up to v = 6. The salt emission traces the region just above the continuum disk, possibly forming the base of the outflow. The emission from the vibrationally excited transitions is inconsistent with a single temperature, implying the lines are not in LTE. We examine several possible explanations of the observed high excitation lines, concluding that the vibrational states are most likely to be radiatively excited via rovibrational transitions in the 25-35 {\mu}m (NaCl) and 35-45 {\mu}m (KCl) range. We suggest that the molecules are produced by destruction of dust particles. Because these molecules are so rare, they are potentially unique tools for identifying high-mass protostellar disks and measuring the radiation environment around accreting young stars.Comment: Accepted to ApJ. Analysis code at https://github.com/keflavich/Orion_ALMA_2016.1.00165.S, paper source at https://github.com/keflavich/SaltyDisk, and data at https://zenodo.org/record/121335

    Variable Linear Polarization from Sagittarius A*: Evidence for a Hot Turbulent Accretion Flow

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    We report the discovery of variability in the linear polarization from the Galactic Center black hole source, Sagittarius A*. New polarimetry obtained with the Berkeley-Illinois-Maryland Association array at a wavelength of 1.3 mm shows a position angle that differs by 28 +/- 5 degrees from observations 6 months prior and then remains stable for 15 months. This difference may be due to a change in the source emission region on a scale of 10 Schwarzschild radii or due to a change of 3 x 10^5 rad m^-2 in the rotation measure. We consider a change in the source physics unlikely, however, since we see no corresponding change in the total intensity or polarized intensity fraction. On the other hand, turbulence in the accretion region at a radius ~ 10 to 1000 R_s could readily account for the magnitude and time scale of the position angle change.Comment: accepted for publication in ApJ

    The Rotation Measure and 3.5mm Polarization of Sgr A*

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    We report the detection of variable linear polarization from Sgr A* at a wavelength of 3.5mm, the longest wavelength yet at which a detection has been made. The mean polarization is 2.1 +/- 0.1% at a position angle of 16 +/- 2 deg with rms scatters of 0.4% and 9 deg over the five epochs. We also detect polarization variability on a timescale of days. Combined with previous detections over the range 150-400GHz (750-2000 microns), the average polarization position angles are all found to be consistent with a rotation measure of -4.4 +/- 0.3 x 10^5 rad/m^2. This implies that the Faraday rotation occurs external to the polarized source at all wavelengths. This implies an accretion rate ~0.2 - 4 x 10^-8 Msun/yr for the accretion density profiles expected of ADAF, jet and CDAF models and assuming that the region at which electrons in the accretion flow become relativistic is within 10 R_S. The inferred accretion rate is inconsistent with ADAF/Bondi accretion. The stability of the mean polarization position angle between disparate polarization observations over the frequency range limits fluctuations in the accretion rate to less than 5%. The flat frequency dependence of the inter-day polarization position angle variations also makes them difficult to attribute to rotation measure fluctuations, and suggests that both the magnitude and position angle variations are intrinsic to the emission.Comment: Ap.J.Lett. accepte

    The EGNoG Survey: Gas Excitation in Normal Galaxies at z~0.3

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    As observations of molecular gas in galaxies are pushed to lower star formation rate galaxies at higher redshifts, it is becoming increasingly important to understand the conditions of the gas in these systems to properly infer their molecular gas content. The rotational transitions of the carbon monoxide (CO) molecule provide an excellent probe of the gas excitation conditions in these galaxies. In this paper we present the results from the gas excitation sample of the Evolution of molecular Gas in Normal Galaxies (EGNoG) survey at the Combined Array for Research in Millimeter-wave Astronomy (CARMA). This subset of the full EGNoG sample consists of four galaxies at z~0.3 with star formation rates of 40-65 M_Sun yr^-1 and stellar masses of ~2x10^11 M_Sun. Using the 3 mm and 1 mm bands at CARMA, we observe both the CO(1-0) and CO(3-2) transitions in these four galaxies in order to probe the excitation of the molecular gas. We report robust detections of both lines in three galaxies (and an upper limit on the fourth), with an average line ratio, r_31 = L'_CO(3-2) / L'_CO(1-0), of 0.46 \pm 0.07 (with systematic errors \lesssim 40%), which implies sub-thermal excitation of the CO(3-2) line. We conclude that the excitation of the gas in these massive, highly star-forming galaxies is consistent with normal star-forming galaxies such as local spirals, not starbursting systems like local ultra-luminous infrared galaxies. Since the EGNoG gas excitation sample galaxies are selected from the main sequence of star-forming galaxies, we suggest that this result is applicable to studies of main sequence galaxies at intermediate and high redshifts, supporting the assumptions made in studies that find molecular gas fractions in star forming galaxies at z~1-2 to be an order of magnitude larger than what is observed locally.Comment: Accepted for publication in the Astrophysical Journal, to appear January 2013; 18 pages, 10 figures, 6 table

    Sub-arcsec Observations of NGC 7538 IRS 1: Continuum Distribution and Dynamics of Molecular Gas

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    We report new results based on the analysis of the SMA and CARMA observations of NGC 7538\,IRS\,1 at 1.3 and 3.4 mm with sub-arcsec resolutions. With angular resolutions \sim 0\farcs7, the SMA and CARMA observations show that the continuum emission at 1.3 and 3.4 mm from the hyper-compact \ion{H}{2} region IRS\,1 is dominated by a compact source with a tail-like extended structure to the southwest of IRS\,1. With a CARMA B-array image at 1.3 mm convolved to 0\farcs1, we resolve the hyper-compact \ion{H}{2} region into two components: an unresolved hyper-compact core, and a north-south extension with linear sizes of <270<270 AU and \sim2000 AU, respectively. The fine structure observed with CARMA is in good agreement with the previous VLA results at centimeter wavelengths, suggesting that the hyper-compact \ion{H}{2} region at the center of IRS\,1 is associated with an ionized bipolar outflow. We image the molecular lines OCS(19-18) and CH3_3CN(12-11) as well as 13^{13}CO(2-1) surrounding IRS\,1, showing a velocity gradient along the southwest-northeast direction. The spectral line profiles in 13^{13}CO(2-1), CO(2-1), and HCN(1-0) observed toward IRS\,1 show broad redshifted absorption, providing evidence for gas infall with rates in the range of 310×1033-10\times10^{-3} M_\odot yr1^{-1} inferred from our observations.Comment: 19 pages, 14 figure
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