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Magnetic activity on rapidly rotating stars I: Surface flux distributions. Activity proxies Surface coverage of active regions Polar spots Diffusion and advection of surface magnetic fields Filling factors and flux emergence rates. Andrew Collier Cameron, Moira Jardine,

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magnetic activity on rapidly rotating stars i surface flux distributions
Magnetic activity on rapidly rotating stars I: Surface flux distributions
  • Activity proxies
  • Surface coverage of active regions
  • Polar spots
  • Diffusion and advection of surface magnetic fields
  • Filling factors and flux emergence rates.

Andrew Collier Cameron, Moira Jardine,

John Barnes, Sandra Jeffers, Duncan Mackay, Kenny Wood

(University of St Andrews)

Jean-François Donati (Obs. Midi-Pyrenees, Toulouse).

Meir Semel (Obs. de Paris, Meudon)

overview
Overview
  • Why are rapidly rotating stars useful?
  • We can:
    • map their surfaces!
    • determine the latitude distribution of active regions
    • estimate the flux emergence rate and spot lifetime
    • map the magnetic polarity distribution in the network
  • What does this all tell us about dynamos?
    • Spectral-type dependence of surface flux distribution
    • Spectral-type dependence of differential rotation
    • Cyclic behaviour: spot coverage, differential rotation
    • Meridional circulation?
magnetic activity proxies
Magnetic activity proxies
  • Broad-band optical modulation
    • => dark starspots

Collier Cameron et al 1999

Kürster et al 1997

magnetic activity proxies1
Magnetic activity proxies
  • Emission cores in strong UV/optical lines
    • => chromospheres

Sun in Ca II 393.3 nm filter

Linsky et al 1979

magnetic activity proxies2
Magnetic activity proxies
  • Emission cores in strong UV/optical lines
    •  chromospheres
    •  rotation periods
    •  activity cycles

Vaughan et al 1981

magnetic activity proxies3
Magnetic activity proxies
  • Emission cores in strong UV/optical lines
    •  chromospheres
    •  rotation periods
    •  activity cycles
    •  differential rotation?
    • Secular changes in Ω

 range of surface

rotation rates.

    • Period-DR relation:
    • BUT: No reliable latitude

information

Donahue, Saar & Baliunas 1996

magnetic activity proxies4
Magnetic activity proxies
  • Emission cores in strong UV/optical lines
    •  chromospheres
    •  rotation periods
    •  activity cycles
    •  dynamos?

Noyes et al 1984

magnetic activity proxies5
Magnetic activity proxies
  • Soft X-ray emission
    •  magnetically confined coronal plasma

XMM spectrum and light-curve of star in IC2391 (Marino et al 2003)

magnetic activity proxies6
Magnetic activity proxies
  • Soft X-ray emission
    •  “Saturation” …

Vilhu 1984

magnetic activity proxies7
Magnetic activity proxies
  • Soft X-ray emission
    •  “Saturation” …
    •  and “super-saturation”

Stauffer et al. 1997

Prosser et al. 1996, alpha Persei cluster

magnetic activity proxies8
Magnetic activity proxies
  • Decrease in rotation with age
    • Ultra-fast rotators found in young clusters only
    • Earlier spectral types spin faster after ~0.3 Gyr
    • => Hot magnetically channelled winds dΩ/dt ~ -Ω3

Barnes, S. 2001

slide12
Barnes, S. 2001

Barnes, S. 2001

convection and rotation
Convection and rotation
  • F, G, K, M spectral types
    •  outer convective zones
  • Activity indicators increase with rotation
    •  Rotation drives activity
  • Evidence of differential rotation: can we map it?
  • Spindown rates depend on spectral type
    •  Convection zone depth is important
  • Do young stars really have up to 50% starspot occupancy?
  • For the fastest rotators Lx decreases with Ω !
evidence for dense spot coverage
Evidence for dense spot coverage
  • TiO bands occur in spots only.

O’Neal, Neff & Saar 1996

evidence for dense spot coverage1
Evidence for dense spot coverage
  • TiO bands occur in spots only.
  • 7055Å/8860Å band ratio gives spot temperature.

O’Neal, Neff & Saar 1998

evidence for dense spot coverage2
Evidence for dense spot coverage
  • TiO bands occur in spots only.
  • 7055Å/8860Å band ratio gives spot temperature.
  • Band strength gives spot covering fraction.

Normalised

photospheric

spectrum

Normalised

spot

spectrum

Composite

model

spectrum

Continuum

brightness

ratio

Spot

filling

factor

O’Neal, Neff & Saar 1998

evidence for dense spot coverage3
Evidence for dense spot coverage
  • TiO bands occur in spots only.
  • 7055Å/8860Å band ratio gives spot temperature.
  • Band strength gives spot covering fraction.
  • Active stars have filling factors fs~20% to 40%

O’Neal, Neff & Saar 1998

measuring spot coverage with hst
Measuring spot coverage with HST
  • Eclipsing binary SV Cam
  • G0V + K5V
  • Edge-on orbit
  • K5V transits primary
  • Light-curve analysis  radii
  • Measure missing-flux spectrum at mid eclipse
  • Use HIPPARCOS parallax to get solid angle  surface brightness

Jeffers et al. 2004

eclipsed flux deficiency in sv cam
Eclipsed-flux deficiency in SV Cam
  • Eclipsed flux is ~30% less than best-fit Teff indicates.
  • fS~40%

Jeffers et al. 2004

evolutionary effects of flux blocking
Evolutionary effects of flux blocking
  • Star expands slightly
  • Photospheric Teff increases
    •  significant effects on HR diagrams of young open clusters, e.g. Pleiades

Spruit & Weiss 1986

Stauffer et al 2003

imaging of stellar surfaces
Imaging of stellar surfaces
  • Direct imaging?
  • Stellar Imager mission concept:
    • Goal is 50,000 km resolution on a Sunlike star 4 pc away
    • Requires angular resolution 60-120 µas
    • 0.5-km space-based UV-optical interferometer array ?
rotational broadening of photospheric lines
Rotational broadening of photospheric lines

Stauffer et al 1997

  • Rotational Doppler shift dominates broadening of stellar photospheric lines in rapid rotators.
  • Rotation profile contains information about surface features (Goncharsky et al 1977, Vogt & Penrod 1983)
starspot bumps in spectral lines
A

A

Intensity

Intensity

-v sin i

v(spot)

v sin i

-v sin i

v(spot)

v sin i

Velocity

Velocity

Starspot “bumps” in spectral lines
imaging of stellar surfaces on a budget
Imaging of stellar surfaces on a budget
  • Combine profiles of all recorded photospheric lines to boost S:N.
  • Compute synthetic line profiles from trial image.
  • Iterate to target c2 at maximum entropy.
  • Get simplest image that fits data.
  • Nearly always get a dark polar cap.

-v sin i +v sin i

Starspot

signatures in

photospheric lines

example speedy mic k3v
Example: Speedy Mic (K3V)
  • Spots present at all latitudes including polar regions.

Barnes et al 2004

example hde 283572
Example: HDE 283572
  • Strassmeier et al 1998: WTTS, v sin i = 78 km s–1
polar fields
Polar fields
  • Schrijver & Title (2002) modelled flux emergence on stars of different rotation rates.
  • Rapid rotators develop rings of opposite polarity at poles.
  • Note reversal of polar fields over cycle.
  • Also Schüssler (1997) modelled buoyant flux tube emergence. Flux tubes deflected to high latitudes on rapid rotators.
magnetic activity on rapidly rotating stars ii temporal evolution
Magnetic activity on rapidly rotating stars II:Temporal evolution
  • Tracking starspots
  • Time-varying differential rotation
  • Differential rotation along the main sequence
  • Stellar magnetograms
  • 3D coronal structure

Andrew Collier Cameron, Moira Jardine, Duncan Mackay, Kenny Wood,

John Barnes, Sandra Jeffers

(University of St Andrews)

Jean-François Donati (Obs. Midi-Pyrenees, Toulouse).

Meir Semel (Obs. de Paris, Meudon)

what else can we learn from stellar surface maps
What else can we learn from stellar surface maps?
  • Snapshots:
    • Unpolarized: Latitude distributions of spots
    • Locations of slingshot prominence complexes
    • Circularly polarized: Magnetic topology of corona
  • Days-weeks timescale:
    • starspots trace surface differential rotation and meridional flows
  • Weeks-months:
    • Lifetimes of individual spots and magnetic regions
  • Years:
    • Stellar butterfly diagram: Dynamo cycles
    • Polarity reversals?
polar spots and convective zone depth
Polar spots and convective-zone depth
  • LQ Lup (G2)
  • Donati et al (2000)
polar spots and convective zone depth1
Polar spots and convective-zone depth
  • HE 699 (G2-3V; alpha Per G dwarf)
  • Jeffers et al (2002)
polar spots and convective zone depth2
Polar spots and convective-zone depth
  • HK Aqr (M1)
  • Barnes et al (2004)
polar spots and convective zone depth3
Polar spots and convective-zone depth
  • RE J1816+541 (M1)
  • Barnes et al (2001)
polar spots and convective zone depth4
Polar spots and convective-zone depth

G3V G6V G8V

K0V K3V

M1V M1V

surface brightness 1996 dec 23 29
Surface brightness: 1996 Dec 23 - 29
  • Equator rotates

faster than pole

    • solar-like shear
    • Prot ~ 0.5 d
    • Equator laps pole by

1 cycleevery ~ 120d

surface shear 1996 december 23 29
Surface shear: 1996 December 23 - 29
  • CCF for surface-brightness images
  • CCF for magnetic images:
starspots as flow tracers
Starspots as flow tracers
  • Individual spot trails have their own recurrence periods.
  • Velocity amplitude of sinusoid:

Rotation rate

at latitude q

Axial

inclination

Stellar

radius

matched filter analysis
Spot velocity amplitude:Matched-filter analysis
  • Travelling gaussian:

Spot rotation rate:

Intrinsic line width

Foreshortening and limb darkening

Spot phase angle

relative to observer’s

meridian

Inclination

Latitude

optimal scaling
Optimal scaling

Scale factor:

Badness of fit:

xij (phase binned on trial period)

c2

gij: equatorial spot at phase 0.5

optimal scaling1
Optimal scaling

Scale factor:

Badness of fit:

xij (phase binned on trial period)

c2

gij: equatorial spot at phase 0.5

differential rotation 1988 2001
Differential rotation 1988-2001
  • Differential rotation rate doubled in 3 years from 1988 Dec to 1992 Jan.
  • As equator speeds up, polar regions slow down.
  • Rotation rate at q ~ 40o remains ~ constant.
slide50
Impact on convective zones
  • Angular rotation in convective zone
  • Plot estimates in Ωeq-dΩ plane
  • Interpret differences as:
  • distinct anchoring depths
  • of tracers within CZ
  • temporal changes in angular
  • rotation profile within CZ
impact on convective zones
Impact on convective zones
  • Angular rotation in convective zone
  • plot estimates in Ωeq-dΩ plane
  • changes in differential rotation:
  •  powered with a few % of L*
  •  correspond to a field of
  • ≈10 kG in the whole CZ
  •  sufficient to generate
  • orbital period fluctuations
  • of binary stars
  • (Applegate 1992)
differential rotation along the main sequence
Differential rotation along the main sequence

Barnes et al. 2004

zeeman doppler imaging
Zeeman Doppler Imaging
  • Zeeman effect:
    •  component: Linear polarization, no  shift
    •  components: Elliptical polarization,  ~ ± gB
  • Field orientation and line-of-sight:
    • Circular polarization ( cpt) strongest when B // line of sight.
  • How stellar rotation helps:
    • Rotational Doppler effect separates magnetic regions in velocity space.
    • Field orientation relative to line of sight changes as magnetic region crosses disc.

Landé

g-factor

for line

Local magnetic

field strength

the semel polarimeter
AAT

f/8 Cass

focus

The Semel Polarimeter

Semel et al 1993: A&A 278, 231

Optical axis switched

at ±45o relative to

beamsplitter axes

/4 plate

Aberration-free

linear polarizing

beamsplitter

Dual beams analyzed

for opposite circular

polarization states

Bowen-

Walraven

image slicer

at UCLES

slit position

U

C

L

E

S

Focal

reducer

Dual fibre feed

to UCLES slit area

stokes v weak field approximation
Stokes V: weak-field approximation

I ()

I ()

Left

Right

Difference

V ()

V ()

High g

Low g

multi line imaging
=

*

Multi-line imaging
  • Essential for ZDI
    • Stokes V signature is typically < 10–4 times continuum.
    • Typical S:N is 300 in continuum.
    • Weighted least-squares deconvolution procedure recovers profile information from up to 4600 images of 2700 lines.
  • Nice for Stokes I too
    • Huge sensitivity gain – turns the AAT into a 160-m telescope!
    • Allows use of full time and wavelength resolution.
    • Unprecedented amounts of surface detail recoverable.

Weight =g-factor * depth

detecting magnetic fields
Detecting magnetic fields
  • Zeeman effect in spectral lines
  • circular and linear
  • polarisation in line profiles
  • amplitude usually < 0.1%
detecting magnetic fields1
Detecting magnetic fields
  • Zeeman effect in spectral lines
  • circular and linear
  • polarisation in line profiles
  • amplitude usually < 0.1%
  • Stack line profiles with

Least-squares

Deconvolution

stokes v time series spectra
Stokes V time-series spectra

Stokes I & V dynamic spectra of AB Dor

  • Demonstrates rotational

modulation of Zeeman

signature

  • Yields location of

magnetic regions &

orientation of field lines

the shape of a stellar corona
The shape of a stellar corona
  • Altshuler & Newkirk (1969):
    • fitted potential field models to solar surface magnetograms.
    • Mimic transition from closed corona to solar wind by imposing a “source surface” at several solar radii. Field beyond source surface is radial.
  • Can we do this for other stars?
coronal topology and x ray emission
Coronal topology and X-ray emission
  • Jardine, Wood, Cameron, Donati & Mackay(2002) MNRAS, 336:
  • Potential field.
    • 1995 Dec 7-11 + 1996 Dec 23-29 magnetograms
  • AB Dor rotation rate.
  • Isothermal corona
    • T=107 K
  • Base pressure µ B2 .
    • But p=0 on open lines
  • Soft X-ray emissivity µ ne2 .
  • Monte Carlo RT code .
  • Centrifugal compression/stripping:
    • p=0 on field lines where p > B2 / 2µ (cf. Mestel & Spruit 1987)
centrifugal stripping and supersaturation
Centrifugal stripping and supersaturation

P=0.17 d

P = 0.51 d

Jardine 2004

centrifugal stripping and supersaturation1
Centrifugal stripping and supersaturation
  • Co-rotation radius shrinks as Ω increases
  • Loops near co-rotation burst open
  • Coronal X-ray emission measure decreases

Jardine 2004

slingshot prominences signatures
-v sin i +v sin i

-v sin i +v sin i

Starspot

signatures in

photospheric lines

Absorption

transients in

H alpha

Slingshot prominences: signatures
  • AB Dor, AAT/UCLES, 1996 Dec 29
  • Donati et al 1999
coronal condensations single stars
Coronal condensations: single stars
  • Detected in 90% of young (pre-) main sequence stars with Prot<1 day
    • AB Dor (K0V): Collier Cameron &Robinson 1989
    • HD 197890 =“Speedy Mic” (K0V): Jeffries 1993
    • 4 G dwarfs in Per cluster: Collier Cameron & Woods 1992
    • HK Aqr = Gl 890 (M1V): Byrne, Eibe & Rolleston 1996
    • RE J1816+541: Eibe 1998
    • PZ Tel: Barnes et al 2000 (right) Prot = 1 day (slowest yet)
    • Pre-main sequence G star RX J1508.6-4423 (Donati et al 2000) --prominences in emission!
emission signatures
Emission signatures
  • Seen only in the most rapidly-rotating, early G dwarfs, e.g. RX J1508.6 -4423 (Donati et al 2000):

Star is viewed at low inclination; uneclipsed Ha-emitting clouds trace out sinusoids

tomographic back projection
Tomographic back-projection
  • Clouds congregate near co-rotation radius (dotted).
  • Little evidence of material inside co-rotation radius.
  • Substantial evolution of gas distribution over 4 nights.
latitude dependence
Latitude dependence
  • AB Dor prominences need to be anchored at high latitude to cross stellar disk, since i = 60 degrees.
  • What about other stars with different inclinations?
    • BD+22 4409: Low inclination, no transients found: Jeffries et al 1994

where do bipoles emerge on young stars
Where do bipoles emerge on young stars?
  • Solar-type star
  • Bipoles emerge at solar-like latitudes as cycle progresses.
  • Solar transport coeffs.
  • Flux emergence rate 30 times solar.
  • Solar meridional flow rate (11 m/sec)
  • 2 cycles per movie loop.
  • Van Ballegooijen code, modified by Duncan Mackay.
  • Confirms earlier work by Schrijver & Title for similar flux emergence rate.
where do bipoles emerge on young stars1
Where do bipoles emerge on young stars?
  • Solar-type star
  • Bipoles emerge at latitudes 70 deg to 10 deg as cycle progresses.
  • Solar transport coeffs.
  • Flux emergence rate 30 times solar.
  • Solar meridional flow rate (11 m/sec)
  • 2 cycles per movie loop.
  • Flux vanishes at all latitudes around activity minimum.
  • Mostly unipolar caps.
where do bipoles emerge on young stars2
Where do bipoles emerge on young stars?
  • Solar-type star
  • Bipoles emerge at latitudes 70 deg to 10 deg as cycle progresses.
  • Solar transport coeffs.
  • Flux emergence rate 30 times solar.
  • 9 x Solar meridional flow rate (100 m/sec)
  • 2 cycles per movie loop.
  • Flux vanishes at all latitudes around activity minimum.
  • Multipolar caps.
where do bipoles emerge on young stars3
Where do bipoles emerge on young stars?
  • Solar-type star
  • Bipoles emerge at range of latitudes around 35 degrees (no butterfly diagram).
  • Solar transport coeffs.
  • Flux emergence rate 30 times solar.
  • 9 x Solar meridional flow rate (100 m/sec)
  • 2 cycles per movie loop.
  • Flux vanishes at all latitudes around activity minimum.
  • Multipolar caps.
where do bipoles emerge on young stars4
Where do bipoles emerge on young stars?
  • Solar-type star
  • Bipoles emerge at range of latitudes around 35 degrees (no butterfly diagram).
  • Solar transport coeffs.
  • Flux emergence rate 30 times solar.
  • 9 x Solar meridional flow rate (100 m/sec)
  • Slow-motion action replay.
  • Flux vanishes at all latitudes around activity minimum.
  • Multipolar caps.
slingshot prominences and polar fields
Slingshot prominences and polar fields
  • McIvor et al 2003
  • Possible polar field configurations:
slingshot prominences and polar fields1
Slingshot prominences and polar fields
  • McIvor et al 2003
  • Corresponding coronal field configurations:
  • Unipolar cap supports fewer high-latitude prominences.
summary and prospects
Summary and prospects
  • Rotational shear ∆Ω
    • Decreases strongly with increasing convective-zone depth
    • Increases weakly with increasing stellar rotation rate.
  • Differential rotation rate shows year-to-year variability
    • Consistent with Applegate (1992) mechanism for binaries.
  • Polar spot activity appears stronger in shallow convective zones.
  • Advection and diffusion of emergent bipoles can give rise to flux pile-up at polar caps.
  • Prominence distribution suggests mixed-polarity caps.
  • Enhanced poleward meridional flows may be needed.
    • Should be detectable if present.
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