Transcript Document 7782842

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
 3N 
rS  

 4 
1/ 3
nh 2 / 3
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
S   I d   B Te 1  e  d
S
S
Low frequencies: τ»1
S   B d   2T
S
High frequencies: τ«1
S   B   d   0.15T 0.35
S
The neutral envelope:
m

d
modified blackbody emission
– Spectral index m (related to composition, grains dimensions, grains structure)
– Dust temperature Td
 B (T )
OASI
Osservatorio 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)
O.A.S.I.
(Osservatorio Antartico Submillimetrico e Infrarosso)
Optical configuration
Cassegrain
Primary mirror
D = 2600mm
Focal length
f = 1300 mm
Focal ratio
f/D = 0.5
Secondary mirror
d = 410 mm
Equivalent focal length
F = 10400 mm
Equivalent focal ratio
F/D = 4
Observational techniques: ON-OFF
Differential measurement: removal
atmospheric emission (first order).
of
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
OFF
ON
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.
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
Data analysis:
Source angular dimensions
S tot  S peak   1   2
Estimation of sources diameters:
gaussian fit along two main axis on IR and radio maps
IR maps:
IRAS (100, 60, 25 and 12 μm)
G284.3 -0.3 (12 μm)
Radio Maps:
Parkes (6 cm)
All Sky (408 MHz)
G284.3 -0.3 (6 cm)
Data analysis:
Flux calibration
Observations of planets (Drift Scan)
Rayleigh-Jeans approximation:
FV 
2  10 26  K B  T   S
2
Sabbatini et al., 2007, submitted
Results (1)
100000
G 291.6 -0.5
Letteratura
100000
Sabbatini et al., A&A 439,595 (2005)
G291.3 -0.7
10000
1000
100
10
1E8
1E9
1E10
1E11
1E12
Frequenza (Hz)
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
Letteratura
Questo lavoro
Densità di flusso (Jy)
Densità di flusso (Jy)
Questo lavoro
1E13
10000
1000
100
10
1E8
1E9
1E10
1E11
1E12
Frequenza (Hz)
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
1E13
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
Preliminary results (1)
Preliminary results (1)
Physical parameters
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Dust mass:
•
Bolometric luminosity:
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Excitation parameter:
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F d 2
Md 
k B (Td )
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)
integrating fluxes over frequencies (using both literature and our results)
calculating the linear dimensions from distance and our estimate of angular dimensions,
and using electronic densities from literature:
2 / 3
U  rn e
Lyman flux:
number of photons needed to keep the excitation of the source:
46 0.85
e
N c  8.04 10 T
U
3
(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
(Cosmological Observations at Concordia with
High sensitivity Instrument for Source
Extraction)
Optical configuration
Cassegrain
Primary mirror
D = 2600mm
Focal legth
f = 1300 mm
Focal ratio
f/D = 0.5
Secondary mirror
d = 410 mm
Equivalent focal length
F = 10400 mm
Equivalent focal ratio
F/D = 4
Angular resolution
Few arcmins in
mm range
COCHISE
January 2007: Installation @ Dome C
Thanks
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:
Source
AR (hh mm ss)
DEC (° ‘ “)
Time of observations
Drift Scan
ON-OFF
G267.9 -1.1
08 59 15
-47 32 27
60 m
145 m
G279.4 -31.7
05 38 37
-69 05 00
--
160 m
G284.3 -0.3
10 24 26
-57 48 29
145 m
260 m
G287.4 -0.6
10 43 49
-59 36 56
105 m
255 m
G287.5 -0.6
10 44 58
-59 40 36
--
245 m
G291.3 -0.7
11 12 10
-61 20 28
220 m
140 m
G291.6 -0.5
11 15 18
-61 17 18
255 m
115 m
G298.2 -0.3
12 10 19
-62 51 40
--
100 m
G298.9 -0.4
12 15 42
-63 03 08
75 m
200 m
G305.2 +0.2
13 11 54
-62 35 01
--
120 m
RCW 38
08 59 07
-47 31 01
135 m
--
Paladini et al. A&A 397, 213 (2003)
Spectrometer characteristics
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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
PWV
January 1997
January 2007
1.8
PWV
PWV UL
1.6
1.4
PWV (mm)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
0
2
4
6
8
10
12
14
16
day
Valenziano et al. , 1997
Valenziano & Dall’Oglio, PASA, 1999
Sabbatini et al., 2007, in prep
See also: Chamberlin, 2001 (Typical PWVSP 0.7mm in January)
Burova, 1986
Townes & Melnick, 1990 (as low as PWVVostok  0.1 mm)
Lawrence, 2004
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)
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
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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