Coherent and continuous radio emission from Magnetic Chemically

Peculiar stars

P. Leto1, G. Umana1, C.Buemi1, F.Leone2 1 INAF-OACT, 2 UNICT

C. Trigilio1

Magnetic Chemically Peculiar stars

White, 2000; Gudel, 2002

•  MS B-A type •  Anomalous

abundance •  Magnetic fields

Chemical Peculiarity

Radiative diffusion (Michaud 1970)

Elements with many transitions close to maximum of radiation receive impulse toward the surface

Over/under-abundance in photosphere

Dependence on Teff

He-s O9-B5

He-w B5-A0

Si A0-A5

others A5…

Strong magnetic fields: magnetic freezing, concentrations of elements, correlation with orientation of B

Anomalous photospheric abundance (106 Sun) (He-s, He-w, Si, Cr …)

Variability of light curve, Beff, lines

CU Virginis P=0.52 giorni (Pyper et al. 1998)

Oblique rotator (Babcock, 1949)

Dipolar field

B misaligned with rotational axis

driven winds

from UV obs (Shore et al. 1987, Shore & Brown 1990)

Outflows from magnetic poles Trapped plasma in equatorial belt

Stellar winds Magnetic fields + stellar wind

Radio emission? (Kodaira & Fomalont 1970)

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˙ M <10−10M⊗yr−1 , vwind ≈1000 km s-1

Gyrosynchrotron emission

Radio luminosity

Targeted surveys (VLA, ATCA) Drake et al (1987), Willson et al (1987), Linsky el al (1992), Leone Trigilio Umana (1994)

Rate detection 25 %

Correlation with Teff

31 % He-s O9-B5

26 % He-w B5-A0

23 % Si A0-A5

0 % Others A5…

Correlation with

wind/mass loss?

Radio emission

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L5GHz ≈1016 −1018 erg s−1 Hz−1

Oblique rotator model

Change of orientation (Leone, Umana 1992)

Optically thick source

Modulation of radio emission

Radio minima, Beff minima

Optically thick source α = -0.7, 0.3

Leone, Umana, Trigilio, (1996)

Leone, Trigilio, Neri, Umana (2004)

Flat Spectra

For a dipole

High/low ν : close/far from the star €

νG ∝ B, ν ∝ BPR*R

⎛

⎝ ⎜

⎞

⎠ ⎟ 3

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B ∝ BPR*R

⎛

⎝ ⎜

⎞

⎠ ⎟ 3

Toward a model

Mass loss from magnetic poles. Trapped plasma in equatorial belt.

Wind follows B till

Current sheets at Alfvén radius Acceleration and propagation

inwards (middle magnetosphere) Reflection back outwards Gyrosynchrotron emission (André et al 1988, model for YSO)

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12ρv 2 ≈ B2

8π

MCP: stable magnetosphere, different orientations Template for other stellar envelopes (thermal/non thermal)

Figure from Montmerle, 2001

β

• Magnetic field and geometry (B, i, β ) • Mass loss, wind velocity, Alfvén radius • Current sheets size • Acceleration efficiency (Nrel) and power law • Absorption by inner magnetosphere plasma

Sampling of the magnetosphere Iν and Fν at different rotational phases Also circular polarization

3D model

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(Nrel ∝ E −δ )

Trigilio et al (2004), Leto et al (2007)

18 cm, 4 cm, 1 cm

Simulations

Derived parameters

Mass Loss Ralf Inner magnetosphere (T, dens…) Acceleration: Efficiency Power law

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˙ M ≈10−11 −10−12M⊗yr−1

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12 −17 R∗

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Nrel Nwind ≈10−3 −10−4

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Nrel ∝ E −δ δ ≈ 2

Open Questions: How radio emission depends on Teff, B, Prot?

(Need of larger sample)

CU Virginis Discovery of coherent radio emission

Detection of two pulses at 20 cm with VLA (Trigilio et al 2000)

Rotational phase: Beff = 0

High directivity (⊥ magnetic axis)

100% circular polarization (RCP)

Cyclotron Maser above the North magnetic pole

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νB ≈ s⋅ 2.8⋅ 106BG (Hz)

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νP ≈ 9000 ne (Hz)

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Bpole ≈ 3000G

B ∝ r−3

B ≈ 500 s =1( ); 250 s = 2( )h ≈1.3R*

Cyclotron Maser frequency

for a dipole

above the pole

Maser Localization

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νB >>νP

s harmonic number

Pulsar like behaviour

Stability of the Maser

Observations over more than 10 yr show no significant variations (Trigilio et al, 2000, 2008, 2011) (Ravi et al 2010)

Differences: -Intensity of the peaks -Phases of the peaks

Separation is constant Central point star slowing down

Change of Prot

Determination of the rotation period with high accuracy Sudden slowing down of the star ΔP≈1.12 s Similar gap in 1985 by photometric meas (Pyper et al 1998)

• Change of moment of inertia? • Sudden mass loss from magnetosphere? • Interaction thin envelope-inner star

• Unstable region?

No definitive answer yet

Precise method for angular momentum loss measurements

Bandwidth of the Maser

From ATCA, VLA and EVLA obs, ν range: 1300-2000 MHz (Trigilio et at 2008, 2011, Ravi et al 2010)

Dynamical spectra (EVLA obs)

Large bandwidth ν not simultaneous

In the framework of the MCP model

1)  Acceleration in current sheets 2)  Magnetic mirroring 3)  Lack of reflected electrons at low pitch angle 4)  Anisotropy in the v space 5)  Electron cyclotron maser

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∂f∂v⊥

> 0Electron Cyclotron Maser condition (Melrose & Dulk, 1982)

ν=s νB s=1,2,3 x-mode polarization Narrow Δν Emission almost perpendicular to B

From observations: Δν very large, problems with geometry

B

Toward a model for ECME Analogy with auroral planetary emission

Auroral emission: solar wind, acceleration in magnetic tail…

From Cluster NASA mission: Higly beamed radiation Localization 1RE above the pole Refraction upward by denser magnetospheric plasma Mutel, 2008

AKR (Auroral Kilometric Radiation)

Auroral emission from Earth

Animation: NASA 2011

Ring where ν=s νB

Maser amplification where the optical path is longer

Maser radiation in a plane

perpendicular to the magnetic axis

B

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nrefrX( ) = 1− νP

ν ν −νB( )Plasma B ≈ 200-300 G N ≈ 109 cm-3

Refractive index (0.98-0.95) consistent with the observed deviation ψ

(Trigilio et al, 2011, ApJ 739, L10)

How many pulsar style stars can be detected by EMU?

•  Dipolar field

•  Similar geometry (modulation North/South magnetic pole)

•  Frequency of the maser

Acceleration in Current Sheets, regular flow in flux tubes

•  Dipolar field

•  Similar geometry (modulation North/South magnetic pole)

•  Frequency of the maser

How many pulsar style stars can be detected by EMU?

Acceleration in Current Sheets, regular flow in flux tubes

Magnetic axis ⊥ line of sight About 70 % of MCP

•  Dipolar field

•  Similar geometry (modulation North/South magnetic pole)

•  Frequency of the maser

How many pulsar style stars can be detected by EMU?

Scales as Bpole

500 <Bpole< 50 000 G

ν≈ [0.3-1] B(G)pole MHz

About 10% of MCP

About 7 % of MCP expected

EMU

Acceleration in Current Sheets, regular flow in flux tubes

Magnetic axis ⊥ line of sight About 70 % of MCP

<BG>

Conclusions and perspectives

•  Model of MCP other magnetosphere (BD, dMe…) •  Plasma in magnetospheres •  Cyclotron Maser Instability, exoplanets? •  Angular momentum evolution of stars

EMU

3000 MCP within 1 kpc With 30 µJy detection limit and Lradio>1016 erg s-1 Hz-1 ~75% sky with EMU 2200 MCP in EMU

•  25% ~ 550 MCP can be detected •  7% ~ 160 CU Virginis / pulsar-like stars expected

Magnetic Chemically Peculiar stars

Characteristics:

•  MS B-A type •  Anomalous photospheric abundance (106 Sun)

(He-s, He-w, Si, Cr …) •  Strong magnetic fields (103 - 105 G) •  Variability: light curve, Beff, lines… •  P = 0.5 – 10 days

Stability of the Maser

Observations over more than 10 yr show no significant variations (Trigilio et al, 2000, 2008, 2011) Ravi et al (2010)

Differences: -Intensity of the peaks -Phases of the peaks

Toward a model for ECME Analogy with auroral planetary emission

Aurorall emission: solar wind, acceleration in magnetic tail…

Figures from Zarka 1998

At 10 pc Jupiter: F(Jup)=10-19x(au/10pc)^2=2x10-30 W m-2 Hz-1 =2x10-6 Jy

A Hot-Jupiter is about 106 times powerfull F(HJ)= 2 Jy

[Zarka,2001]

Hot-Jup

Solar planets

In the framework of the MCP model

1)  Acceleration in current sheets 2)  Magnetic mirroring 3)  Lack of reflected electrons at low pitch angle 4)  Anisotropy in the v space 5)  Electron cyclotron maser