1. Can masers trace the Galactic magnetic field?
The MAGMO Survey
Project to examine magnetic fields traced by the Zeeman splitting of OH masers
Jimi Green | A Neapolitan of Maser Science 2013 | Sydney
21st May 2013
CSIRO ASTRONOMY AND SPACE SCIENCE

2. Can hydroxyl (OH) masers map the Galactic magnetic field?
Overview
Can hydroxyl (OH) masers map the Galactic magnetic field?
Introduction
Zeeman splitting of OH masers
Star formation environment of OH masers
Magnetic fields of OH masers
Description of MAGMO
Aims
Collaborators
Observations
Results from MAGMO
First results of pilot region
Expected full results
Summary
Three main sections:
-overview of hydroxyl star formation masers – their physics and environment, and some details of magnetic field work with maser Zeeman splitting
-details on the MAGMO project, observations and techniques
-results including that of the pilot study and those expected from the full survey
MAGMO | Jimi Green | Page 2

3. Zeeman splitting of OH masers
Introduction
Paramagnetic molecule
Large splitting factors for maser transitions
Narrow linewidths (typically <1 kms-1)
Field strengths of the level of a few mG
Green et al. 2012
Zeeman splitting enables us to determine:
(total) field strength
&
line-of-sight orientation
of the in situ magnetic fields
basically OH masers are great polarimetric tools
OH is a paramagnetic molecule therefore high levels of polarisation are observed, particularly circular polarisation (up to 100%)
OH has a large splitting factor, emission is spectrally narrow, and we have mG field strengths giving clear zeeman pairs
Zeeman splitting enables us to determine the (total) strength and line-of-sight orientation of the in situ magnetic fields.
MAGMO | Jimi Green | Page 3

4. + + + + + + Star formation environment of OH masers Introduction
Fish & Reid 2007
Temperatures:
30 K to >150 K
H2 densities:
106 to 107
OH abundances:
10s to 100s cm-3 + + + + + +
~20 mpc (4000 AU)
Image credit: Robert Hurt (NASA/JPL, IPAC/SSC)
The OH masers we're interested in with this project are those associated with high-mass star formation, thus those within the spiral arms
These masers are known, from VLBI studies, to be located on the edge of UCHII regions surrounding forming high mass stars
As we've heard this morning we know the Galaxy has a large scale ordered field in the diffuse medium, we also have seen that the ionized material of the UCHIIs appear to exhibit the orientation of Galactic magnetic field, this leads us to...
OH masers lie on edge of Ultra-Compact HII regions (from VLBI)
Large scale magnetic field known from rotation measure observations (e.g. Brown et al and van Eck et al. 2011)
Ultra-Compact HII regions known to exhibit orientation of Galactic magnetic field (e.g. Harvey-Smith et al. 2011)
R. Hurt: NASA/JPL-Caltech/SSC
MAGMO | Jimi Green | Page 4

5. + + + + + + Magnetic fields of OH masers Introduction
Fish & Reid 2007
Temperatures:
30 K to >150 K
H2 densities:
106 to 107
OH abundances:
10s to 100s cm-3 + + + + + +
van Eck et al. 2011
~20 mpc (4000 AU)
... the possibility of tracing these large scale fields with OH maser zeeman splitting – again VLBI studies have shown often coherent field direction within a given star forming region and limited studies have found coherence of field orientation across multiple sites of star formation, over scales of a few kpc, however previous studies have been limited by scale, statistics or the inhomogeneity of data, and this leads us to the MAGMO project...
Masers typically exhibit consistent line-of-sight magnetic field orientations with comparable field strengths (of a few mG)
Limited scale studies have found coherence of field orientation across multiple sites of star formation, over scales of a few kpc
[e.g. Davies 1974 , Reid & Silverstein 1990, Fish et al. 2003, Han & Zhang 2007 compilation]
R. Hurt: NASA/JPL-Caltech/SSC
MAGMO | Jimi Green | Page 5

6. Aims of MAGM Project description
Examine magnetic fields pervading regions of high-mass star formation
Measure Zeeman splitting of OH masers to determine the strength and orientation of the in situ magnetic field
Correlate homogeneous observations of 100s of sites of high-mass star formation spread throughout the spiral arms of the Galaxy
Test if the orientations of weak large-scale magnetic fields can be maintained in the contraction (and field amplification) to the high densities of high-mass star formation
Onto details of the project...
the overall aim is to examine the magnetic fields pervading high-mass star formation through Zeeman splitting, increasing previous magnetic field studies of this type by an order of magnitude.
MAGMO | Jimi Green | Page 6

7. Collaborators of MAGM Naomi McClure-Griffiths James Caswell
Project description
Naomi McClure-Griffiths
CSIRO Astronomy & Space Science
James Caswell
Tim Robishaw
NRC-HIA, DRAO
Lisa Harvey-Smith
Sui Ann Mao
University of Wisconsin
+ Methanol Multibeam Collaboration
acknowledge MMB team for target sources
MAGMO | Jimi Green | Page 7

8. Observations of MAGM Project description
Targets: 6.7-GHz methanol masers (from Methanol Multibeam survey), exclusive tracers of high-mass star formation
Utilised new broadband backend and 16cm receivers on the Australia Telescope Compact Array (ATCA)
Ground-state OH transitions (1612, 1665, 1667 & 1720 MHz)
Full Stokes polarisation (I, Q, U, V)
16 tunable ‘zoom’ bands of 1 MHz width with 2048 channels (velocity resolution ~0.1 kms-1) at 2 Intermediate Frequencies.
~30' primary beam, fitted positional accuracy ~0.4''
‘snapshot’ observing mode with 4/5 cuts across 12 hr period,
~50mJy rms channel noise
~30 arcmin primary beam, 6 km array configuration => ~7 arcsec synthesized beam => fitted positional accuracy ~0.4 arcsec
~30mins on source => 50mJy rms channel noise
MAGMO | Jimi Green | Page 8

9. Carina-Sagittarius Arm Tangent
Results (Green et al. 2012, MNRAS, 425, 2530)
+ 6.7-GHz methanol masers (Green et al. 2012)
Hα emission (SHASSA, Gaustad et al. 2001)
Carina-Sagittarius
Crux-Scutum
Perseus
Norma
Here are the pilot observations, made towards the 23 sites of maser emission in the Carina-Sagittarius spiral arm tangent, seen here in longitude-velocity – this is a nice region to look as its straightforward to associate structurally,
we detected 17 sites of OH masers across six regions of star formation (9 new discoveries), seeing both high linear and high circular polarsation, and crucially 11 statistically significant zeeman pairs with positive field strengths in the range of 1 to 8 mG
The probability of this occurring by chance is <2%
Here is the comparison with the RM measurements - positive RM measurement and positive Zeeman however the two conventions mean the first indicates a field towards us, the second away from us, i.e. opposite orientation which has led us to reinvestigate the zeeman splitting convention
Spiral arm colours:
Yellow = Perseus
Orange = Crux-Scutum
Purple = Carina Sagittarius
Blue = Local
Greyscale Halpha to highlight major HII regions.
symbols as per key
17 sites of OH masers across 6 star forming regions, 11 Zeeman pairs Coherent magnetic field orientation observed across ~5 kpc of spiral arm Opposite (!) orientation to that indicated by rotation measures
MAGMO | Jimi Green | Page 9

10. Summary of full survey results
Results (Green et al. in prep)
Observations completed (~500 hrs between 2010 and )
All 6.7-GHz methanol masers between longitudes 186° and 20° observed
Data reduced through python scripted MIRIAD
~500 (SF) 1665-MHz OH masers, ~ MHz
Detection rate implies ~300 Zeeman pair measurements across the Galaxy
Frequent, high % circular polarisation
Infrequent, high % linear polarisation
Coherent field orientations over several kpc => field direction conserved
Observations done (~43 days observations over last 1-2 years)
Preliminary results encouraging
Expected numbers
500 of which approximately 50% will be new detections
More generally, this implies (as per discussion of Han & Zhang 2007) that large-scale magnetic fields either play a dominant role in the development of molecular clouds and the process of star for- mation (in accordance with the recent review by Crutcher, Heiles & Troland 2003), or the fields are unaffected by the process. Either way, they are conserved during the contraction to the small scales and high densities of star formation despite the dynamically disrup- tive processes involved. Theoretical modelling of high-mass star formation concurs, forecasting that the direction of the magnetic field pervading the larger molecular cloud will be conserved on the collapse of material into star-forming regions (e.g. Li & Shu 1996; Allen, Li & Shu 2003).
MAGMO | Jimi Green | Page 10

11. Still to come...
Results
Full analysis of in situ arm fields (combining longitude-velocity with astrometric distances)
Zeeman field orientation versus Rotation Measure orientation
VLBI followup results – high resolution Zeeman pair identification
Associated excited-state OH analysis – Zeeman pair confirmation
MAGMO (targeted survey of star formation OH with I,Q,U,V)
GASKAP & SPLASH (blind survey of all OH with I,V)
POSSUM (rotation measure complement)
northern hemisphere survey (& Galactic plane completion)
reiterate preliminary result
mention synergy with GASKAP and POSSUM
VLBI follow up
Davies 1974
Reid & Silverstein 1990
Fish et al. 2003
Brown et al. 2007
van Eck et al. 2011
MAGMO ?
MAGMO | Jimi Green | Page 11

12. Still to come... All the extras
VLBI follow-up observations - 4 sources observed with Long Baseline Array (LBA) – Zeeman pairs identifications confirmed
Excited-state OH studies – 30 bright sources observed with Australia Telescope Compact Array with full Stokes polarisation, Pi component identification ~15% of Zeeman pairs
Variability studies – long term, high cadence monitoring similar to G12.889
HI Self Absorption study – HI observed simultaneously with maser transitions
Masers associated with evolved stars (serendipitous MHz detections)
Greyscale Halpha to highlight major HII regions.
symbols as per key
MAGMO | Jimi Green | Page 12

13. Thank you Neapolitan of Maser Science 2013 | Sydney
Jimi Green The MAGMO Survey t e
CSIRO ASTRONOMY AND SPACE SCIENCE

14. Zeeman splitting Conventions & definitions
Splitting of otherwise degenerate energy levels in presence of magnetic field.
Maser emission seen from transitions between levels.
Three components: σ+, σ-, π.
MAGMO | Jimi Green | Extra Page