Loading in 2 Seconds...
Loading in 2 Seconds...
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
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
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)
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
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: H109αν≈5 GHz) Continuum:bremsstrahlung emission
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)
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 (νfew Hz). The signal is then demodulated by a lock-in amplifier.
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
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)
Data analysis:Flux calibration Observations of planets (Drift Scan) Sabbatini et al., 2007, submitted Rayleigh-Jeans approximation:
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
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
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)
COCHISE January 2007:Installation @ Dome C
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)
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
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
See also: Chamberlin, 2001 (Typical PWVSP0.7mm in January) Burova, 1986 Townes & Melnick, 1990 (as low as PWVVostok0.1 mm) Lawrence, 2004 PWV January 1997 January 2007 Valenziano et al. , 1997 Valenziano & Dall’Oglio, PASA, 1999 Sabbatini et al., 2007, in prep
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…
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