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Scientific requirements of ALMA, and its capabilities for key-projects: Galactic John Richer, Cavendish Laboratory, Cambridge. ALMA Design Reference Science Plan. Galaxies and Cosmology Previous talk Star and Planet Formation “Initial conditions” Envelopes Disks Chemistry - PowerPoint PPT Presentation
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Scientific requirements of ALMA, and its capabilities for key-projects:
GalacticJohn Richer, Cavendish Laboratory, Cambridge
ALMA Design Reference Science Plan
Galaxies and CosmologyPrevious talk
Star and Planet Formation“Initial conditions”EnvelopesDisksChemistry
Stars and their EvolutionThe Sun, mm-continuum emission from stars, Circumstellar Envelopes, AGB stars, Supernovae,…
Solar SystemPlanetary Atmospheres, Surfaces, Comets, Exosolar Planets, …
ALMA “Level 0” Requirements
Image gas kinematics in protostars and protoplanetary disks around Sun-like stars at 140pc distance, enabling one to study their physical, chemical and magnetic field structures and to detect the gaps created by planets undergoing formation in the disk. Provide precise images at 0.1 arcsec resolution. Precise means representing within the noise level the sky brightness at all points where the brightness is greater than 0.1% of the peak image brightness. This applies to all objects transiting at >20 degree elevation.
ALMA: Current Definition
64 moveable 12-m antennas: ‘100-m class telescope’Baselines from 15m to 15km
Angular resolution ~40 mas at 100 GHz (5mas at 900GHz)Strong implications for atmospheric phase correction scheme
Receivers: low-noise, wide-band (8GHz), dual-polarisation, SSBMany spectral lines per band
Digital correlator, >=8192 spectral channels, 4 Stokesvery high spectral resolution (up to 15kHz)
Short spacing data provided by 12-m antennas in single-dish modeCritical for objects bigger than the primary beam
Requirements for star formation and high-z studies are remarkably similar!
Good match to weather statistics and science
Frequency band capabilities
Band 3: 84-116GHz. FOV = 60 arcsecContinuum: ff/dust separation, optically-thin dust, dust emissivity index, grain sizeSiO maser, low excitation lines CO 1-0 (5.5K), CS 2-1, HCO+ 1-0, N2H+…
Band 6: 211-275GHz. FOV = 25 arcsecDust SEDMedium excitation lines: CO 2-1 (16K), HCN 3-2, …
Band 7: 275-373GHz. FOV = 18 arcsecContinuum: most sensitive band for dust. Wave plate at 345GHz for precision polarimetryMedium-high excitation lines: CO 3-2 (33K), HCN 4-3, N2D+, …
Band 9: 602-720GHz. FOV = 9 arcsecTowards peak of dust SED, away from Rayleigh Jeans; hence T(dust)High excitation lines e.g. CO 6-5 (115K), HCN 8-7 in compact regions
12m
Aperture Synthesis with ALMA
12-m cross-correlations from 60 dishes measure spacings from 12m up to maximum baseline e.g. 10km
Auto-correlations from 4 12-m dishes measure from zero up to ~6m spacings
Extra measurements here help imaging precision:
• Cross-correlations from 7-m dishes, or
• Large single dish observationsUp to 15km
Diffraction limited imaging needs phase correction
Water fluctuations typically 500m-1000m above siteCorrect by Fast Switching of antennas to QSO, plus Water Vapour Radiometry
Initial Conditions: Pre-collapse Cores
L1498: Tafalla et al.
Strong chemical gradients and clumpinessIndicates depletion and chemical evolutionALMA mosaic at 3mm: 100 pointings plus single-dish data neededALMA can resolve 15AU scales in nearby cores, or study cores at 1000AU scales out to 10kpc
Core dynamics: infall
Di Francesco et al (2001)
Small-scaleExtended 0.1 - 0.3 pc
Walsh et al
Starless Core Chemistry: probing the depletion zones
Complete CNO depletion within 2500AU?ALMA can study this region, in objects as far as the GC, in H2D+
CS, CO, HCO+
NH3, N2H+
H2D+
D2H+
Walmsley et al. 2004; Caselli et al 2003
372GHz line8,000AU
2,500AU
15,000AU
Role of Magnetic Fields?
(Figure by A. Chrysostomou)
(Crutcher et al)L1544: Ward-Thompson et al 2000
Star formation in crowded environments
ALMA can resolve 15AU scales at TaurusClump mass function down to 0.1 Jupiter massesOnset of multiplicityBD formationInternal structure of clumpsTurbulence on AU scales
Bate 2002
Protostars and Clumps in Perseus: Hatchell et al 2005.
Cores and Filaments: Are Hydrodynamical Simulations Realistic?
Clump mass spectrumRelation to IMF?Low mass limit?Dependence on age?
Clump structure – transient or bound?Filaments
are they omnipresent?thermal/density structure
Motte et al
Klessen 2004
Molecular Outflows
Origin of flows down to 1.5AU scales10 mas resolution at 345 GHz:
• 24 hours gives 5K rms at 20 km/s resolution
Resolve magnetosphere: X or disk winds?Flow rotation?
Proper motions0.2 arcsec per year for 100km/s at 100pcResolve the cooling length
Resolve multiple outflow regions Beuther et al, 2002
Chandler & Richer 1999
170AU resolution
Spatially-resolved Spectral Surveys
8GHz bandwidth
Schilke et al
Kuan
et a
l 200
4
Protoplanetary disk at 140pc, with Jupiter mass planet at 5AUALMA simulation
428GHz, bandwidth 8GHztotal integration time: 4hmax. baseline: 10km
Contrast reduced at higher frequency as optical depth increasesWill push ALMA to its limits
Wolf, Gueth, Henning, & Kley 2002, ApJ 566, L97
Imaging Protoplanetary Disks
“Debris” disk spectroscopy with Spitzer
Rieke et al 2004
“Debris” Disk imaging with ALMA
Wyatt (2004) model: dust trapped in resonances by migrating planets in diskALMA will revolutionise studies of the large cold grains in other planetary systems
Vega (Holland et al)Fom
alhaut (Greaves et al)
ALMA could map one square degree at 350GHz in 180 hours to0.7mJy sensitivityThis is 0.15 solar masses at 20Kconfusion limited unless resolution high
1 arcsec beam (8500AU) would give ΔT=0.6K at 1 km/s resolution
Possible lines in 2x4GHz passband: USB: SiO 8-7, H13CO+ 4-3, H13CN 4-3, CO 3-2 LSB: CH3CN, CH3OH
OrUSB: HCN 4-3, HCO+ 4-3
LSB: H13CN 4-3, CS 7-6, CO 3-2
Pierce-Price, Richer, et al 2000
SCUBA 850 micron: Pierce-Price et al 2000
SCUBA 450 micron
Star Formation at the Galactic Centre
Final Remarks
ALMA’s unique role will be imaging down to few AU scales in nearby star forming regions with a sensitivity of a few Kelvin
Protostellar and protoplanetary disksAccretion, rotation and outflow deep in the potential wellChemistry and dust properties at high spatial resolutionWill require excellent operation on long baselines
Study star formation across the GalaxyModest resolution observations (0.5 arcsec or so) will be important too
Good brightness sensitivity
ALMA has a narrow field of viewNeed surveys with single dishes to feed ALMA
Many targets extended over several primary beamsNeed high-quality short spacing data to make precise images and for flux ratio experiments