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The magnetic topology of young Suns
Stephen Marsden Anglo-Australian Observatory
Collaborators: Brad Carter, Ian Waite, Carolyn Brown, Rhodes Hart (USQ, Australia) Sandra Jeffers (Utrecht, Netherlands) Moira Jardine, Nick Dunstone (St. Andrews, Scotland) Meir Semel, Evelyne Alecian (LESIA, France) Julio Ramirez (UNAM, Mexico) Marian Martinez-Gonzalez (IAC, Spain) Jean-Francois Donati (OMP, France) Cosmic Magnetism, Kiama, June 10th 2010
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Why study stellar magnetic fields?
· · ·
Magnetic fields play a crucial role in a star's evolution. The yo ung Sun was MUCH more magnetically active than it is to day. Our key question: How do yo ung Suns generate magnetic fields?

Solar magnetogram (SOHO) Cosmic Magnetism, Kiama, June 10th 2010

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Generating magnetic fields
· ·
Differential rotation an d convection for m a dynamo process. The intensity an d distribution of magnetic fields pro vi des a w in dow onto this un derlying dynamo.

The solar dynamo (NASA)

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Detecting magnetic fields
· ·
High-resolution spectropolarimeters such as SEMPOL on the Anglo-Australian Telescope (AAT), split incoming light into right- and left-hand circularly polarised light. From this we get a Stokes V polarisation spectrum.
Stokes I (Intensity) spectrum

Stokes V (Polarisation) spectrum Donati et al. (1990)
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Cosmic Magnetism, Kiama, June 10th 2010


Zeeman Doppler Imaging (ZDI)
Azimuthal field changes sign Radial field no change

· ·
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Differing magnetic field orientations have different Stokes V signatures as the star rotates. The longitude, latitude and orientation of field lines can be determined.
Cosmic Magnetism, Kiama, June 10th 2010


Zeeman Doppler Imaging (ZDI)
Azimuthal field changes sign Radial field no change

· ·
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Differing magnetic field orientations have different Stokes V signatures as the star rotates. The longitude, latitude and orientation of field lines can be determined.
Cosmic Magnetism, Kiama, June 10th 2010


LSD and ZDI
(or how LSD helps us see the stars)

· ·

Zeeman signatures are typically extremely small, ~0.1% of the continuum level. Therefore we use LSD (Least-Squares Deconvolution, Donati et al. 1997) to increase the S/N of the Zeeman signature.
Cosmic Magnetism, Kiama, June 10th 2010

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Reconstructing the magnetic field
· · ·
A time series of Stokes V LSD profiles is inverted to recover the magnetic topology. The radial, azimuthal, & meridional field can be reconstructed. ZDI recovers the global magnetic field topology of a star.
Magnetic field reconstruction Radial Azimuthal

Observed

Fitted

Cosmic Magnetism, Kiama, June 10th 2010
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Rotational Phase


· · ·

Magnetic fields on young Suns
The young Sun-like star HD 141943 was observed at the AAT with SEMPOL. HD 141943 has a large azimuthal magnetic field on its surface. Could a distributed dynamo be operating in the star?

vsini ~ 35 km/s

Marsden et al. (submitted) Cosmic Magnetism, Kiama, June 10th 2010

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Fares et al. (2009) Radial

Polarity reversals on mature stars
Jan 2008 Jun 2008 Jul 2008 Polarity reversal?

Jun 2007

· ·

Azimuthal

Polarity reversals are reported on the mature (~1 Gyr) late-F star Tau Boo with a magnetic cycle length of ~2 yr. However, Tau Boo hosts a "Hot Jupiter". Could this be influencing the star's magnetic cycle?
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Marsden et al. (2006), Jeffers & Donati (2008), Jeffers et al. (submitted) Radial No reversal vsini ~ 30 km/s Sep 2004 Jun 2005 May 2007 Nov 2007 Ring of pos. field

Young Suns: I

Azimuthal

· ·

HD 171488 is a young (30 ­ 50 Myr) early-G star observed at 4 epochs from Sep 2004 through to Nov 2007. The most striking feature is the polar ring of azimuthal field occurring at all epochs, but no polarity reversal!
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Marsden et al. (in prep.) Radial

Young Suns: II
Polarity reversal?

vsini ~ 50 km/s

Dec 2001

Dec 2007

Dec 2008

Nov 2009 No reversal?

· · ·

Azimuthal

HR 1817 is a young Sun-like star observed at 4 epochs. A ring of azimuthal field is seen, but it weakens in 2007. In 2009 the polar radial field appears to reverse polarity, but not the azimuthal field.
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Magnetic strength

R = 0.95R

Comparison to theory + -

Southern Hemisphere

Northern Hemisphere Time (days) Attempted reversal

Brown et al. (2009)

· · ·

Recent models of young Suns show "wreaths" of azimuthal field, similar to the rings we see, but at a lower latitude. The models also undergo "attempted" magnetic reversals. Could this be what happened on HR 1817 in 2007?
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Differential rotation (DR)
·
We can include a Sunlike differential rotation law in the imaging : (l) = eq ­ dsin2l, eq = equatorial rotation. d = shear from equator to pole (the differential rotation). d and eq are found using 2-minimsation.
HD 141943 d (rad/d)

d ~ 7 times solar value! eq (rad/d) Marsden et al. (submitted)

·

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The differential rotation of young Suns
·
d (rad/d) d (rad/d)

Young Sun-like stars show increasing d with decreasing convective zone depth (i.e. with increasing mass). For young stars more massive than the Sun there appears to be a LARGE jump in the level of differential rotation.

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.1 0.0 0.1
Sun HD 141943

(Donati et (Barnes et (Marsden (Jeffers & (Jeffers &

al. 2008b) al. 2005) et al. 2006) + Donati 2008) Donati 2008)

HD 106506 (Waite et al. 2010)

·

Convective Zone Depth (R ) Marsden et al. (submitted) Dramatic increase Cosmic Magnetism, Kiama, June 10th 2010

0.2 0.3 0.4 0.5 Convective Zone Depth (R*)

0.6

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Summary of results
· · · ·
The young Sun-like stars appear to have a ring of azimuthal field around the pole, similar to the models of Brown et al. (2009) but at higher latitudes. Could these regions of azimuthal field indicate that a distributed dynamo operates in young Suns? No evidence yet that young Suns undergo full polarity reversals as does today's Sun. Young stars heavier than the Sun show a large increase in differential rotation.
Cosmic Magnetism, Kiama, June 10th 2010
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· · · · · · ·
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How could a distributed dynamo operate?

What we don't know!

Do young Suns undergo full (or even "attempted") magnetic polarity reversals? Do all young stars heavier than the Sun have a large differential rotation? Does high differential rotation imply short magnetic cycles? Do young stars even have regular stellar cycles or does their magnetic activity vary chaotically? When/how did the regular solar dynamo develop? ........
Cosmic Magnetism, Kiama, June 10th 2010


Thank you for your patience!

Questions?
Stephen Marsden: scm@aao.gov.au
Cosmic Magnetism, Kiama, June 10th 2010
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