Concept Statement for :
The Milky Way Magnetic Field Mapping Mission: M4
February 8, 1997
Dan Clemens
Top-Level Science Questions:
-----------------------------
1) What is the magnetic field pattern for the dense ISM in the Milky
Way Galaxy?
2) What is the typical magnetic field pattern in a star-forming
molecular cloud?
3) What role do magnetic fields play in the star-formation process?
To what degree do magnetic fields aid in the development
of protostellar and preplanetary disks?
Mission Science Goals:
-----------------------
To measure the magnetic field directions in the dense ISM component
of the Milky Way galaxy, nearby dark clouds, and in the near environments
of protostars.
Technique to be used:
---------------------
Linear imaging polarimetry of the thermal emission from magnetically
aligned warm dust grains embedded in molecular gas.
Mission Data Sets:
-------------------
1) Complete map of the inner Milky Way disk to +/- 50 degrees of longitude
and +/- 1.5 degrees of latitude including the
galactic center (300 square degrees = 61 million pixel map at 8" PFOV)
2) Detailed map of the rho-Oph dark molecular cloud (15 square degrees =
3 million pixel map at 8" PFOV)
3) Deep map of high latitude cirrus region (1.1 square degrees = 225 pixel map
at 4.2' PFOV)
4) Guest Investigator Program comprising 16% of the minimum mission time.
All polarimetric data sets to have < 0.5% linear polarimetric uncertainty
(S/N > 600).
Estimated Time to Acquire Data Sets:
--------------------
1) Milky Way Map: 6 weeks
2) rho-Oph : 2 weeks
3) Cirrus : 2 weeks
4) GI Program : 2 weeks
=====================
Total Minimum Mission Duration = 3 months
Basis of Time Estimates:
-------------------------
Region Gal.Long. Gal.Lat. Zodi Gal NESB Target Time to
Name [deg.] [deg.] ======[MJy/sr]========= S/N=600
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Gal.Ctr. 0 0 22 19000 7.2 19000 0.05s
0 2 23 125 0.64 125 9.5
Gal.Plane 15 0 21 854 1.55 854 1.2
30 2 14 88 0.53 88 13.
45 0 11 164 0.69 164 6.3
45 2 10 62 0.45 62 19.
rho-Oph 0 20 24 5 0.29 8 460.
Lupus 338 16 18 1 0.23 2 4444.
Cirrus 315 -45 7.43 0.6 0.154 0.25 38hours*
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Notes: "Zodi" is the 70 micron Zodiacal Light background, from
IRSKY tool.
"Gal" is an estimate of the large-scale galactic plane dust
background level.
"NESB" is the instrument Noise-Equivalent-Surface-Brightness,
which is the 1-sigma noise level per pixel. More on this
in the instrument section, below. Note that this is
limited by the combination of the Zodiacal and Galactic
backgrounds, not the instrument background.
"Target" is the brightness of the target. For the galactic
plane, the Gal emission IS the target. For the dark
clouds and cirrus, the target fluxes are fainter than
the background.
"Time to S/N=600" is the integration time needed to achieve
S/N=600 for the indicated target. This is an appropriate
level for sub-percent polarimetric uncertainty, yielding
position angle uncertainties below 10 degrees.
Comments: 1) The galactic plane survey will be mostly limited by
observing efficiency (slew and settling times). Short
integrations will be the norm.
2) The rho-Oph dark cloud is much brighter than the Lupus
dark cloud. The target surface brightness for rho-Oph is
for the long streamer dust emission. The star-forming
core has a SB of about 2000 MJy/sr, so it will be easy.
I think we can do Oph pretty well, but a full survey
of Lupus will be hard. A small survey of the Lupus core
should be possible, however.
3*) Cirrus is harder than I had originally though. However,
if we throw out the pixels hitting bright point sources,
it should be possible to average across the entire focal
plane arrays to achieve lower noise levels. This will
yield about 4 arcmin resolution, but will permit about
a factor of 30 increase in speed. That implies we can
achieve S/N=600 for one cirrus pointing in about 1.2-1.5
hours. The Target Surface Brightness level of 0.25 MJy/sr
was measured using IRSKY images of cirrus at the indicated
high latitude position. This level corresponds to about
Av~< 0.05 mag or about 0.5x10^20 HI cm-2 (if I have read
the recent Dwek et al 1997 ApJ article correctly). This is
faint cirrus, as opposed to some bright cirrus knot.
Instrument Plan:
-----------------
The M4 satellite follows on much of the heritage developed as part of
the PIREX SMEX proposal, with some recent ideas developed for SIRTF
application.
Please note, however, that our industrial partner, Ball Aerospace, has
yet to sign up to the details of this satellite concept. Terry Jones
and I will be at Ball the Thursday and Friday preceding the Boston
workshop, and will bring back better knowledge about what M4 will
look like.
Telescope: 1 meter primary + 11 cm secondary
f/20 system
Cooled to 10K
Optics: tilted collimator mirror
wire-grid beam splitter
twin off-axis camera mirrors
Cooled to 2K
Detectors: twin 32x32 Ge:Ga photoconductor arrays
(one for each linear polarization sense)
cooled to 2K
Diffraction Limited Beamsize: 17 arcseconds at 70 microns
Pixel Field of View: 8x8 arcseconds (sub-PSF) to enable
MEM or other superresolution recovery and
multiple "confirms" (ala IRAS nomenclature).
Array Field of View: 256x256 arcsec (4.26x4.26 arcmin)
Calibration: Photometric "Flash" Emitter in secondary
Coolant: 150 Liters of Superfluid Liquid Helium
contained in dewar surrounding focal
plane optics (not telescope tube).
Coolant Lifetime: 3 months guaranteed
Sensitivity Estimates: Program which includes specified
background contributions, emissivity and
reflectivity of real optics, known detector
characteristics. Verified against published
IRAS, SIRTF, ISO values.
For all observing, telescope optics temperatures
at or under 10K lead to background limited
performance (cirrus observing is the strongest
driver due to low backgrounds at high latitudes).
Mission Operations Notes:
-------------------------
Warm launched telescope optics.
Two week cool down to 70-80K before active cooling of telescope optics.
No moving parts in optical path.
Instantaneous measurement of either Stokes "U" or "Q" via normal
imaging. Must roll spacecraft 45 degrees to obtain other Stokes
parameter.
Sun-synchronous orbit. Telescope points approximately normal to the
orbit plane toward the "midnight" sky, and is expected to roll about the
pointing axis for polarimetric data acquisition. Solar panels always
illuminated, no satellite eclipses.
Mapping is in a hybrid pointing-survey mode of fixed pointings, offset
from each other by 1/2 array spacing.
Polarimetric calibration via fast roll-and-acquire on calibration
targets.
Solar panel illumination angle limits and earth avoidance angle
limits will lead to an observing pointing limit to ecliptic latitudes
no larger than about 45-50 degrees (no ecliptic pole observations).
Short mission lifetime and requirement to map inner galaxy leads
to a narrow launch window (about 2-3 weeks each year).
Orion and Taurus region cannot be observed unless mission lifetime
exceeds 6 months (unlikely).
Data rate and data processing requirements are very modest, and
might not require IPAC assistance. Total mission data volume in
final data products < 2Gbyte (about 3-4 CD-ROMs).
Ground station will be the BU/TERRIERS station. Operations control
at BU.
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