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					                                                                                     The Atacama Large Millimeter Array

                                                                                                                      ALMA Specifications
                                                                                                                                                                                                                                  ALMA Median Sensitivity
                                                                                                 Antennas:          64                                                                                                               (1 minute; AM=1.3; PWV= 1.5mm)
                                                                                                                    12m diameter
                                                                                                                    <25 m rss                                                                       Freq                     Continuum Line 1 km s-1                                    Line 1 km s-1
                               ALMA Timeline                                                                        <0”.6 pointing                                                                   (GHz)                      (mJy)       (mJy)                                            (mJy)
 Design and Development Phase Jun 1998 - Dec 2001                                               Collecting Area: >7000 m2                                                                           35                       0.015               3.9                                    0.77
                                                                                                 Resolution:        0”.02  (mm)
            International partnership established 1999                                          Receivers:         10 bands,                                                                        110                      0.026               3.9                                    0.78
            Prototype antenna contract Dec 99                                                                      Trx 20-50K mm
            ALMA/NA delivered to VLA site Q2 2002                                                                  Trx 3*h/k mm to 0.6mm
                                                                                                                                                                                                     230                      0.042               4.3                                    0.86
            ALMA/EU delivery Q2 2003                                                                               Trx 4-6h/k mm to 0.3mm                                                     345                      0.11                9.3                                    1.9
 Construction 2002-2010                                                                         Correlator:        2016 Baselines                                                                   675                      3.0                 180.                                   36.
            Production antenna contract Q4 2004                                                 Bandwidth:         16 GHz per baseline
                                                                                                 Spectral channels: 4096 per IF
            Production antenna in Chile Q4 2005
            Interim operations fourth quarter 2007
            Construction ends 2010
                                                                                          Formation of Stars

                                                                     •Paradigm: material falls through a rotating circumstellar disk
                                                                     onto a forming star from more extensive envelope, fuelling a
                                                                     bipolar flow which allows loss of angular momentum (see
                                                                     HH30 disk, far right at best current resolution).
                                                                     •Without sufficient resolution, separation of these motions is
                                                                     difficult
                                                                     •A key observation, not currently achievable, would be to
                                                                     observe the infalling gas in absorption against the background
                                                                     protostar. As molecular depletion may occur in the densest
                                                                     regions (c.f. NH3 in IRAM04191 at left) sensitivity is critical to
                                                                     detection; ALMA will easily provide the sensitivity for this.
                                                                     •In the bipolar flow, shock waves process envelope molecules,                                                                                                      HH 30: Overlay of the integrated 13CO 2-1 emission (contours) on the
                                                                                                                                                                                                                                        HST/WFPC2 image (color). A cross marks the position of the 1.3 mm continuum
                                                                                                                                                       CO(2-1) contours superimposed on an HST image of HH 30. The HST
                                                                     providing a rich chemistry--ALMA will be able to observe the                      observations in false colors (from Burrows et al. 1996) show the                 source. Stapelfeldt and Padgett (2001) inWootten, A., ASP Conf. Ser. 235: Science
                                                                                                                                                                                                                                        with the Atacama Large Millimeter Array, 163.
                                                                                                                                                       optical continuum emission tracing the reflected light in the flared
                                                                     progress of these shocks in real time and study how their                         circumstellar disk, together with the emission of bright atomic lines
                                                                                                                                                       ([SII], Ha, [OI]), tracing a highly collimated jet, perpendicular to the
                                                                     composition changes.                                                              disk. The contours represent the CO(2-1) emission, as observed with
                                                                                                                                                       the IRAM Plateau de Bure interferometer with an angular resolution
                                                                                                                                                       of 1.2”×0.7” by Gueth et al. in prep. Only the channel map at a
                                                                                                                                                       velocity of 11 km/s is plotted (contours are 80 mJy/beam). It shows
                                                                                                                                                       the conical molecular outflow emanating out of the disk and
      IRAM04191. Green: NH3 (1,1) VLA; Red/Blud: 12CO 2-1 NRAO 12m                                                                                     surrounding the jet. The cross indicates the position of the peak of the
                                                                                                                                                       1.3 mm continuum emission.




                                                              Debris Disks                                                                                                                                          Protoplanetary Disks


                                                                     •Tdust at 1 AU = 350 K
                                                                     •Tdust power law index q = 0.45
                                                                     •Surface density power law index = 1.3
                                                                     •Inclination = 45 degrees

                                                                     Feature         Amplitude            Radius (AU) Width (AU) PA (deg)
                                                                     Dark Ring            1.0                    7                     2
                                                                     Dark Ring            2.0                    16                    4
                                                                     Planet Debris        1.5                    40                    5       45
                                                                     Planet Debris        3.0                    60                    9       155




           The model is a simulated modestly-bright debris disk at a distance of 12 pc located around a Sun-like star. The
           observing frequency is 345 GHz, at which the total emission is 10 mJy. The disk has an inner radius at 3 AU and
           an outer radius at 125 AU, with a mass of roughly 0.4 lunar masses of dust. This is a fairly dusty system, of
           which perhaps a dozen might be available.
                                                                                                                                                                                                                                                                   Modeling: Lee Mundy


                                                                                                                                                                  ALMA will be able to trace the chemical evolution of star-forming regions over an
                                                                                                                                                                  unprecedented scale from cloud cores to the inner circumstellar disk. At spatial
                                                                                                                                                                  resolution of 5 AU, it will determine the nature of dust-gas interactions the extent of
                                      ALMA Simulation of Debris Disk Image: Fidelity                                                                              the resulting molecular complexity, and the major reservoirs of the biogenic
                                                                                                                                                                  elements. Angular resolution will exceed that of the HST. On the right above, a
                                                                                                                                                                  model image; on the left a simulation of how ALMA will image the model.




                                                                                                                                                                           Simulation: Structural Details




                                                                                                                                                 Simulations of an ALMA observation of the debris disk using multi-scale CLEAN in the
                                                                                                                                                 aips++ package. On the left, an observation with the compact array, stretched to show the
                                                                                                                                                 structures in the disk in a four hour integration. On the right, a 4 hour observation with the
                                                                                                                  ALMA Memo No. 386 & 387
                                                                                                                                                 450m array, which achieves higher resolution. Thermal noise limits sensitivity. A
     The debris disk model is spread over several primary beamwidths of the ALMA antenna. Imaging the disk would pose a problem for              combination of these two observations would afford the best representation of the original
     current interferometers, which do not recover short spacing data from the antennas operating as single units. ALMA will incorporate         image. Clearly, in one transit ALMA would be able to constrain 1) the photospheric flux of
     this data to provide high fidelity images. The simulation results shown above use software developed at IRAM with image                     the central star (not resolved from inner dust in these compact configurations), 2) the general
     reconstruction using a CLEAN technique. The simulations is done for a frequency of 230 GHz with ALMA in its most compact
                                                                                                                                                 structure of the disk—suggesting the presence of planets and 3) the total dust mass of the
     configuration, so the resolution provided is a bit over one arcsecond, insufficient to show the fine detail in the model. Thermal noise
     has not been included in this simulation. Image fidelity is the ratio of the model to the difference (model – simulated) image, so          disk, as all of the flux is recovered in the image.
     higher numbers reflect more accurate quality. On the right, cumulative fidelity is plotted and evaluated for four fidelity medians.
     For a wide range of medians, the fidelity measure lies near 100, showing that ALMA images will be of quite high quality indeed.
     Further improvement of the images is possible by the addition to ALMA of a small (~12) array of smaller (7-8m) antennas, outside
     the scope of the current project but a likely enhancement should a third partner join ALMA.

				
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posted:2/21/2010
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