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CARMA(Combined Array for Research in Millimeter Astronomy)
Capabilities and Future Prospects
Dick Plambeck
SF/ISM Seminar, 9/5/2006
outline
• background
• capabilities
• what is CARMA good for?
• science example: does the clump mass spectrum in molecular clouds determine the IMF?
+ UChicago SZA 8 3.5-m antennas
Berkeley-Illinois-Maryland array
10 6.1-m diameter antennas
Caltech array 6 10.4-m antennas
CEDAR FLAT
Aug 2005Apr 2004
Cedar Flat
21 Jul 2004 – lifting off the first reflector
panel adjustmentsurface error determined from holography
before adjustment: 127 μm rms
→ 75% loss at 225 GHz
after adjustment: 28 μm rms
→ 7% loss at 225 GHz
all antennas assembled10 Aug 2005
95 GHz continuum map of SS43325 Aug 200625 Aug 2006
rms noise 0.6 mJy/beam
basicsincoming signals accepted:
3mm 80-116 GHz 1mm 220-270 GHz
downconverted (1-5 GHz), amplified signal sent to lab on optical fiber
mostly noise from sky and receiver; source contributes < 10-3
cross-correlation spectrum
correlator
tuning example
• LO1 tunable 85 -115 GHz, or 220 -270 GHz (1)
• receive 4 GHz wide bands above (USB) and below (LSB) LO1 (2)
• 8 independent spectrometers process (USB+LSB) (3)
• USB and LSB signals separated in each spectrometer
12CO 115.27 GHz13CO 110.20 GHz
LO1 112.73 GHz
(1) no 1mm band on OVRO yet; (2) currently 1.5 GHz for BIMA 3mm receivers; (3) currently only 2.5
sky freq
BIMA, OVRO
correlator sections
correlator (spectrometer) modes(for first 3 bands)
correlator modes(for remaining 5 bands)
5 arrays (A,B,C,D,E)57 pads for 15 telescopes
Cedar FlatE-array
(most compact)
synth beam 4.5" at 230 GHz
Highway 168
D-arraysynth beam
1.8"
C-arraysynth beam
0.8"
B-arraysynth beam
0.32"
A-array synth beam
0.13"
a reminder: interferometer acts as a spatial filter
E-array
• BIMA antennas within collision range
• SZA provides even shorter spacings
• combine with single dish measurements from 10.4-m antennas
colliding antennas
what can we do now that we couldn’t do before?
• better site should allow routine observing at 1.3 mm
• much improved sensitivity (3 x collecting area of BIMA, 5 x instantaneous bandwidth)
• high dynamic range imaging owing to more baselines, hence better sampling of u,v plane
225 GHz zenith opacity
% tau mm H2O
SSB Tsys
25 <.12 <1.8 <290
50 <.16 <2.4 <350
75 <.28 <4.3 <520
Tsys computed for 1.5 airmasses, Trcvr(DSB) = 45 K
sensitivity examples
• 5σ detection of dust continuum from .04 Mo clump at 300 pc (5 mJy at 230 GHz)
• 5σ detection of 1-0 CO emission from 2500 Mo cloud in M33 (2.5 K in 3’’ beam, ΔV = 2 km/sec)
tau BW mins
BIMA 0.32 0.8 GHz 3400
CARMA 0.16 1.5 GHz 100
CARMA 0.16 4.0 GHz 40
tau BW mins
BIMA 0.32 0.7 MHz 470
CARMA 0.27 0.7 MHz 60
Comparison with other arrays
CARMA
+ SZASMA IRAM ALMA
elevation 2200 m 4200 2500 5000
antennas 23 8 6 50+
baselines 253 28 15 1225+
diameter 10, 6, 3.5 6 15 12, 7
area 850 m2 226 1060 5600+
max baseline
1900 m 500 m 400 m 14 km
Comparison of u,v coverage6 hr track on source at decl +10º
OVRO E, 15 baselines CARMA D, 105 baselines
Synthesized beams5% contours
problems with poor u,v sampling
• missing Fourier components (u,v spacings) → an infinite number of maps are consistent with the data!
• how can we publish papers? sources with a few compact components are no problem
• CLEAN, max entropy methods are ways of interpolating/extrapolating based on our bias about the sources (e.g., sources consist of a few compact components).
CLEANed map, point source at centerTsys = 0; no atmospheric phase noise
CLEANed map, point source at centerTsys = 0; atmospheric phase noise 150 um
at 100 m; 1% contours
extended source 12 x 6 arcsec FWHM, total flux 1 Jy
integrated flux = 0.77integrated flux = 0.006
scientific examplewhat determines the IMF?
• physics of infall from disk to star
– ‘stars determine their own mass’
• fragmentation of interstellar clouds
– some (approximately fixed) percentage of a clump mass will find its way onto the star
• observational test: measure the mass spectrum of prestellar clumps
– want ~ a few x 103 AU resolution, ~ 5-10’’ in nearby clouds
– an example: Testi and Sargent 1998, OVRO mosaic of 5’x5’ region in Serpens
Serpens mosaic Testi & Sargent 1998
99 GHz continuum
5“ resolution, about 1500 AU
noise level 0.9 mJy/beam
1σ contours beginning at +/- 3σ
anything > 4.5 σ (4 mJy/beam) considered real
strongest sources are ~100 mJy
cumulative mass spectrum of 26 clumps not associated with IR sources
dotted line is best fitting power law, dN/dM ~ M-2.1
dashed line is Salpeter IMF, dN/dM ~ M-2.35
dash-dot line is power law characteristic of larger cores, dN/dM ~ M-1.7 (Williams, Blitz, McKee 1998)
→ mass spectrum of protostellar dust condensations closely resembles local IMF
strongest sources are ~100 mJy
anything > 4 mJy/beam is considered real
→ need dynamic range of 25:1
synthesized beams are particularly ugly near declination 0
configurations, beam pattern for Serpens mosaic
comparison with BIMA mosaic
Lowest contour 2.7 mJy/beam, peak ~105 mJy, beam 5"
Lowest contour 8 mJy/beam, peak 256 mJy, beam ?
simulate: OVRO C,D,E
2 sources (300 mJy and 12 mJy), no atmospheric phase fluctuations
same model, but include atmospheric phase fluctuations of 150 um on 100-m baseline
extended source in the field
CARMA D array
CARMA D
CARMA D array + SZA(23 antennas, 253 baselines)
CARMA DZ
summary
• accurate measurements of clump mass spectrum in complicated regions require not only high sensitivity, but also high dynamic range, image fidelity
• many antennas, ability to get close spacings are critical
• mosaicing many fields necessary to survey sufficiently large regions
• CARMA is an important step in this direction