Transcript Scientific requirements of ALMA, and its capabilities for key

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
Stars and their Evolution
The Sun, mm-continuum emission from stars, Circumstellar
Envelopes, AGB stars, Supernovae,…
Solar System
Planetary 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, SSB
Many spectral lines per band
Digital correlator, >=8192 spectral channels, 4 Stokes
very high spectral resolution (up to 15kHz)
Short spacing data provided by 12-m antennas in single-dish mode
Critical 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 arcsec
Continuum: ff/dust separation, optically-thin dust, dust emissivity index, grain size
SiO maser, low excitation lines CO 1-0 (5.5K), CS 2-1, HCO+ 1-0, N2H+…
Band 6: 211-275GHz. FOV = 25 arcsec
Dust SED
Medium excitation lines: CO 2-1 (16K), HCN 3-2, …
Band 7: 275-373GHz. FOV = 18 arcsec
Continuum: most sensitive band for dust.
Wave plate at 345GHz for precision polarimetry
Medium-high excitation lines: CO 3-2 (33K), HCN 4-3, N2D+, …
Band 9: 602-720GHz. FOV = 9 arcsec
Towards 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
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
12m
Extra measurements here help imaging
precision:
• Cross-correlations from 7-m dishes, or
Up to 15km
• Large single dish observations
Diffraction limited imaging needs phase correction
Water fluctuations typically 500m1000m above site
Correct by Fast Switching of antennas
to QSO, plus Water Vapour Radiometry
Initial Conditions: Pre-collapse Cores
Strong chemical
gradients and
clumpiness
Indicates depletion and
chemical evolution
ALMA mosaic at 3mm:
100 pointings plus
single-dish data needed
ALMA can resolve 15AU
scales in nearby cores,
or study cores at
1000AU scales out to
10kpc
L1498: Tafalla et al.
Core dynamics: infall
Small-scale
Extended 0.1 - 0.3 pc
Walsh et al
Di Francesco et al (2001)
Starless Core Chemistry: probing the depletion zones
CS, CO, HCO+
NH3, N2H+
H2 D +
D2 H +
Complete CNO
depletion within
2500AU?
ALMA can study this
region, in objects as far
as the GC, in H2D+
2,500AU
8,000AU
15,000AU
Walmsley et al. 2004; Caselli et al 2003
372GHz line
Role of Magnetic Fields?
(Figure by A. Chrysostomou)
L1544: Ward-Thompson et al 2000
(Crutcher et al)
Star formation in crowded environments
Bate 2002
Protostars and Clumps in Perseus: Hatchell et al 2005.
ALMA can resolve
15AU scales at
Taurus
Clump mass function
down to 0.1 Jupiter
masses
Onset of multiplicity
BD formation
Internal structure of
clumps
Turbulence on AU
scales
Cores and Filaments:
Are Hydrodynamical Simulations Realistic?
Motte et al
Clump mass spectrum
Relation to IMF?
Low mass limit?
Dependence on age?
Clump structure – transient or bound?
Filaments
are they omnipresent?
thermal/density structure
Klessen 2004
Molecular Outflows
Chandler & Richer 1999
170AU resolution
Origin of flows down to 1.5AU scales
10 mas resolution at 345 GHz:
•
24 hours gives 5K rms at 20 km/s
resolution
Resolve magnetosphere: X or disk
winds?
Flow rotation?
Proper motions
0.2 arcsec per year for 100km/s at
100pc
Resolve the cooling length
Resolve multiple outflow regions
Beuther et al, 2002
Spatially-resolved Spectral Surveys
Kuan et al 2004
8GHz bandwidth
Schilke et al
Imaging Protoplanetary Disks
Protoplanetary disk at 140pc,
with Jupiter mass planet at 5AU
ALMA simulation
428GHz, bandwidth 8GHz
total integration time: 4h
max. baseline: 10km
Contrast reduced at higher
frequency as optical depth
increases
Will push ALMA to its limits
Wolf, Gueth, Henning,
& Kley 2002, ApJ 566, L97
“Debris” disk spectroscopy with Spitzer
Rieke et al 2004
“Debris” Disk imaging with ALMA
Fomalhaut (Greaves et al)
Vega (Holland et al)
Wyatt (2004) model: dust trapped in
resonances by migrating planets in disk
ALMA will revolutionise studies of the
large cold grains in other planetary
systems
Pierce-Price, Richer, et al 2000
Star Formation at the Galactic Centre
SCUBA 850 micron: Pierce-Price et al 2000
ALMA could map one square degree at 350GHz in 180 hours to
0.7mJy sensitivity
This is 0.15 solar masses at 20K
confusion 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
Or
USB: HCN 4-3, HCO+ 4-3
LSB: H13CN 4-3, CS 7-6, CO 3-2
SCUBA 450 micron
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 disks
Accretion, rotation and outflow deep in the potential well
Chemistry and dust properties at high spatial resolution
Will require excellent operation on long baselines
Study star formation across the Galaxy
Modest resolution observations (0.5 arcsec or so) will be important too
Good brightness sensitivity
ALMA has a narrow field of view
Need surveys with single dishes to feed ALMA
Many targets extended over several primary beams
Need high-quality short spacing data to make precise images and for flux ratio
experiments