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Millimetric observations of compact HII regions from Antarctica
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  1. Millimetric observations of compact HII regions from Antarctica Lucia Sabbatini Astronomy PhD student - University “La Sapienza” OASI-COCHISE group – University of Roma Tre SNA - May 2007

  2. HII Regions Interesting problems related to the physical properties of the dust (lack of information in the millimeter range) HII regions are non-variable, bright, compact sources: suitable candidates for calibration and pointing (es: PLANCK)

  3. HII Regions:The structure Final stages of the birth of massive O and B stars (or cluster) Structure of compact HII regions: • Central cavity (radius r1) • Ionized nebula HII (radius rS) • Neutral envelope HI (radius r2) Typical dimensions of neutral envelope: r2 ≈ 5 ± 50 pc Typical dimensions of the ionized nebula:equilibrium between ionization and recombination rates  Strömgren radius: rS ≈ 0.5 ± 10 pc

  4. Low frequencies: τ»1 High frequencies: τ«1 The neutral envelope: modified blackbody emission • Spectral index m (related to composition, grains dimensions, grains structure) • Dust temperature Td HII Regions:The spectrum The ionized nebula: Lines: Lyman (UV), Balmer (visible), Paschen (IR) Lower energy levels (radio: H109αν≈5 GHz) Continuum:bremsstrahlung emission

  5. OASIOsservatorio Antartico Submillimetrico e Infrarosso Dall’Oglio et al., ExA 2, 275 (1992) • The O.A.S.I. telescope @ Terra Nova Bay • Coordinates: LAT. 74° 41’ 42” S LONG. 164° 07’ 23” E • θFWHM = 5.9 arcmin • Detectors: 2 bolometers • Operating temperature: T = 0.3 K (3He refrigerator) ν1 = 240 GHz (λ1=1.25mm) ν2 = 150 GHz (λ2=2.0 mm)

  6. OFF ON Observational techniques: ON-OFF Differential measurement: removal of atmospheric emission (first order). Tracking of the source during Δt: VON (source + atmosphere) Tracking of the blank sky for Δt: VOFF (atmosphere only) The source signal is then the difference: V = VON-VOFF Three fields modulation Double-differential measurement to allow the removal of the linear gradient of temperature in the atmospheric emission. The secondary mirror is modulated (νfew Hz). The signal is then demodulated by a lock-in amplifier.

  7. Data analysis:Baseline removal Right Ascension: evidence of the ON-OFF technique Modulated signal (pre-lockin) Demodulated signal (after lockin): offset varying with time (baseline) Polynomial fit of the OFF part of the data Removal of the baseline Peak signal for every cycle: SPEAK=ViON-ViOFF

  8. G284.3 -0.3 (12 μm) G284.3 -0.3 (6 cm) Data analysis:Source angular dimensions Estimation of sources diameters: gaussian fit along two main axis on IR and radio maps IR maps: IRAS (100, 60, 25 and 12 μm) Radio Maps: Parkes (6 cm) All Sky (408 MHz)

  9. Data analysis:Flux calibration Observations of planets (Drift Scan) Sabbatini et al., 2007, submitted Rayleigh-Jeans approximation:

  10. Results (1) Sabbatini et al., A&A 439,595 (2005) G291.6 -0.5 Distance:7.6 ± 0.8 Kpc Strömgren radius:3 ÷ 5 pc Angular dimensions:10’ x 6.5’ Measured fluxes: F1=367 ± 59 Jy F2=208 ±29 Jy G291.3 -0.7 Distance:3.6 ± 1.0 Kpc Strömgren radis:≈ 0.5 pc Angular dimensions:4.3’ x 4’ Measured fluxes: F1=97 ± 16 Jy F2=68 ±10 Jy

  11. Results (2) G267.9 -1.1 Distance:2.0 ± 0.8 Kpc Strömgren radius:≈ 0.4 pc Angular dimensions:6.5’ x 1.8’ Measured fluxes: F1= 192 ± 23 Jy F2= 123 ± 15 Jy G284.3 -0.3 Distance:6.0 ± 1.2 Kpc Strömgren radius:12 ÷ 15 pc Angular dimensions:11.9’ x 9.0’ Measured fluxes: F1= 223 ± 27 Jy F2= 131 ± 16 Jy

  12. Preliminary results (1)

  13. Preliminary results (1)

  14. Physical parameters • Dust mass: Assuming that the dust cloud is optically thin: Fν: flux density due to dust d: distance from Sun Bν(Td): blackbody at Td kv: dust mass absorption coefficient (@ λ=1.3 mm  kv=0.9 cm2 g-1 cfr. Ossenkopf & Henning 1994) • Bolometric luminosity: integrating fluxes over frequencies (using both literature and our results) • Excitation parameter: calculating the linear dimensions from distance and our estimate of angular dimensions, and using electronic densities from literature: • Lyman flux: number of photons needed to keep the excitation of the source: (Kurtz et al. 1994 ApJ 91, 659) • Number of stars in the cluster: obtained by dividing Nc for the tpical luminosity of a star (eg: O5 V  luminosity 4.9 1049 sec-1 Panagia 1973)

  15. COCHISE January 2007:Installation @ Dome C

  16. Thanks

  17. HII Regions:Selection of sources HII Regions selected for dimensions and flux density (values extrapolated from radio to mm). Sources observed during the XX Campaign: Paladini et al. A&A 397, 213 (2003)

  18. Spectrometer characteristics • Lamellar Grating scheme • Resolution: 0.2 cm-1 • Spectral coverage: 2 – 10 cm-1 • Multi-pixel photometer • Cryogen-free cooling system • Designed to be (eventually) remotely operated

  19. Atmospheric absorption Atmospheric composition: • N2 (78%), O2 (21%) • H2O, CO2, O3 Atmospheric absorption at millimeter wavelengths: O2: 60, 119 GHz H2O: 183, 325 GHz • water vapour content pwv (precipitable water volume) Estimation of the atmospheric transmission in the mm-range Daily variability of the transmission Comparison to atmospheric transmission models

  20. See also: Chamberlin, 2001 (Typical PWVSP0.7mm in January) Burova, 1986 Townes & Melnick, 1990 (as low as PWVVostok0.1 mm) Lawrence, 2004 PWV January 1997 January 2007 Valenziano et al. , 1997 Valenziano & Dall’Oglio, PASA, 1999 Sabbatini et al., 2007, in prep

  21. Spectral hygrometer Taking a pair of simultaneous direct solar irradiance measurements within two narrow spectral intervals centered at nearby wavelengths: • the first in the middle of an infrared water vapour absorption band • the second within a next transparency window of solar spectrum (reference) Prototype model designed by Tomasi and Guzzi (1974) Hygrometric ratio:R=QT1(x)/T2(x) T1, T2: transmission in the two bands λ1 0.940 μm (HBW=0.0122 μm, F(λp)=53.5%) λ2 0.870 μm (HBW=0.0116 μm, F(λp)=55.0%) x: water vapour content R=V(0.940)/V(0.870) Calibration: using radiosoundings (provided by ENEA) • accuracy and reliability (better than radiosounding data) • Possibility of intraday measurements  low costs • easy to be operated at harsh sites • Only for antarctic summer…

  22. Measurements of pwv (1997-2007) December 1996 – January 1997: about 80 intraday measurements (Valenziano et al. 1998) portable near-IR spectral hygrometers portable Volz (1974) sun-photometer for intercomparison tests New calibration (2007): using the monthly mean vertical profiles of pressure, temperature and humidity using 87 radiosoundings performed in 2003 and 2004 (Aristidi et al. 2005) First attempt to characterize the site (pwv content) First instrumental calibration specific for Dome C values (pwv < 1mm) January-February 2007: 16 days, every hour (day time) • More than 100 measurements of pwv • First systematic monitoring of daily variation of pwv • Calibration with radiosoundings of the same period  The instrument is still at Dome C: it is possible to have other measurements at the beginning of next summer season