Transcript jones
M4 1997 The Proposal Points to keep in mind: 1. M4 was to be built from scratch as a polarimeter. 2. Science was the primary driver for the project. 3. Science goals and technical capabilities had to agree. Primary Science Goals • What is the magnetic field structure in the ISM of the Milky Way? • What role do magnetic fields play in the star formation process? • What magnetic field structures exist in the infrared cirrus? • What is the global magnetic field structure in M31? M4 Sensitivity Estimates for +/- 0.12% Polarization (per pixel) Survey Region longitude latitude Galactic Center Galactic Plane 0 Sco/Oph IR Cirrus M31 Galactic Dust Background (MJy/sr) 20,000 M4 NESB (MJy/sr/ Hz) 0 Zody Background (MJy/sr) 14 12.0 Target Surface Brightness 20,000 Integration Time for S/N=600 zip 25 0 10 2,400 4.1 2,400 1.1s 25 50 Oph core Dark Filaments Bright Cores Filaments (2x2 pixel) Cores Spiral Arms 5 0 9 6 15 15 330 35 40 40 0.8 0.5 0.6 0.6 330 35 300-2,400 5-80 38s 73s 0-1.4s 20s-87min 4.6 4.6 0.2 0.2 0.2 0.2 8 1-3 225s 640s-97min 6.4 6.4 3.7 3.7 0.3 0.3 40 3-14 20s 165s-60min To Move or Not to Move • No moving parts is good. • Rotating spacecraft near the Earth is a problem. • One simple moving part is not impossible, but it is expensive (several million extra). • Not rotating the spacecraft relieves the thermal shielding requirements and improves the observing efficiency somewat. • Rotating a waveplate greatly improves data reduction. What M4 Could Not Do • Resolve the magnetic field geometry in dense cloud cores. • Survey the entire galactic plane. • Measure magnetic field strength. • Work longward of 100 microns. Complimentary with SOFIA • SOFIA will have 12x the spatial resolution but it will be less sensitive than M4. • SOFIA will have to ‘chop’, so it is limited to bright, compact regions, M4 is not. • SOFIA will be unable to connect the MC core to the general ISM, M4 will. • M4 will provide targets for SOFIA. The Task Ahead • Define the science. • Confine the science goals by technical reality. • Articulate the resulting science case as best as possible.