Can masers trace the Galactic magnetic field?The MAGMO Survey

CSIRO ASTRONOMY AND SPACE SCIENCE

Jimi Green | A Neapolitan of Maser Science 2013 | Sydney21st May 2013

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

OverviewCan hydroxyl (OH) masers map the Galactic magnetic field?

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• Paramagnetic molecule

• Large splitting factors for maser transitions

• Narrow linewidths (typically <1 kms-1)

• Field strengths of the level of a few mG

Zeeman splitting of OH masersIntroduction

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Zeeman splitting enables us to determine:

(total) field strength

&

line-of-sight orientation

of the in situ magnetic fields

Green et al. 2012

Star formation environment of OH masersIntroduction

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Temperatures: 30 K to >150 K

H2 densities:106 to 107

OH abundances: 10s to 100s cm-3

~20 mpc (4000 AU)

+

+

++

++

R. Hurt: NASA/JPL-Caltech/SSC

• 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. 2007 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)

Fish & Reid 2007

Image credit: Robert Hurt (NASA/JPL, IPAC/SSC)

Magnetic fields of OH masersIntroduction

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Temperatures: 30 K to >150 K

H2 densities:106 to 107

OH abundances: 10s to 100s cm-3

~20 mpc (4000 AU)

+

+

++

++

R. Hurt: NASA/JPL-Caltech/SSC

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]

Fish & Reid 2007

van Eck et al. 2011

• 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

Aims of MAGMProject description

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Naomi McClure-GriffithsCSIRO Astronomy & Space Science

James CaswellCSIRO Astronomy & Space Science

Tim RobishawNRC-HIA, DRAO

Lisa Harvey-SmithCSIRO Astronomy & Space Science

Sui Ann MaoUniversity of Wisconsin

+ Methanol Multibeam Collaboration

Collaborators of MAGMProject description

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• 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

Observations of MAGMProject description

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17 sites of OH masers across 6 star forming regions, 11 Zeeman pairsCoherent magnetic field orientation observed across ~5 kpc of spiral arm

Opposite (!) orientation to that indicated by rotation measures

Carina-Sagittarius Arm TangentResults (Green et al. 2012, MNRAS, 425, 2530)

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+ 6.7-GHz methanol masers (Green et al. 2012)

Hα emission (SHASSA, Gaustad et al. 2001)

Carina-Sagittarius Crux-Scutum

Perseus

Norma

• Observations completed (~500 hrs between 2010 and 2012)

• All 6.7-GHz methanol masers between longitudes 186° and 20° observed

• Data reduced through python scripted MIRIAD

• ~500 (SF) 1665-MHz OH masers, ~300 1667-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

Summary of full survey resultsResults (Green et al. in prep)

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• 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)

Still to come...Results

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Brown et al. 2007 van Eck et al. 2011 MAGMO ?Davies 1974 Reid & Silverstein 1990 Fish et al. 2003

• 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 1612-MHz detections)

Still to come...All the extras

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Thank youNeapolitan of Maser Science 2013 | SydneyJimi GreenThe MAGMO Surveyt +61 2 9372 4610e james.green@csiro.au

CSIRO ASTRONOMY AND SPACE SCIENCE

Splitting of otherwise degenerate energy levels in presence of magnetic field.

Maser emission seen from transitions between levels.

Three components: σ+, σ-, π.

Zeeman splittingConventions & definitions

MAGMO | Jimi Green | Extra Page