Scientific requirements of ALMA, and its capabilities for key-projects: Galactic John Richer, Cavendish Laboratory, Cambridge.

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