Coherent and continuous radio emission from Magnetic Chemically Peculiar stars
C. Trigilio1
P. Leto1, G. Umana1, C.Buemi1, F.Leone2 1 INAF-OACT, 2 UNICT Magnetic Chemically Peculiar stars
• MS B-A type • Anomalous abundance • Magnetic fields
White, 2000; Gudel, 2002 Chemical Peculiarity
Anomalous photospheric abundance (106 Sun) (He-s, He-w, Si, Cr …)
Radiative diffusion (Michaud 1970) Elements with many transitions close to maximum of radiation receive impulse toward the surface
Over/under-abundance in Dependence on Teff photosphere He-s O9-B5
Strong magnetic fields: He-w B5-A0 magnetic freezing, Si A0-A5 concentrations of elements, correlation with orientation of B others A5… Variability of light curve, Beff, lines
Oblique rotator (Babcock, 1949) Dipolar field CU Virginis P=0.52 giorni (Pyper et al. 1998) B misaligned with rotational axis Stellar winds
Magnetic fields + stellar wind
Radio emission? (Kodaira & Fomalont 1970)
driven winds ˙ −10 −1 -1 M <10 M⊗yr , vwind ≈1000 km s from UV obs (Shore et al. 1987, Shore & Brown 1990) € Outflows from magnetic poles Trapped plasma in equatorial belt Radio emission
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 Correlation with 31 % He-s O9-B5 wind/mass loss? 26 % He-w B5-A0 Gyrosynchrotron emission 23 % Si A0-A5 16 18 −1 −1 Radio luminosity L5GHz ≈10 −10 erg s Hz 0 % Others A5…
€ Modulation of radio emission
Radio minima, Beff minima
Oblique rotator model Change of orientation (Leone, Umana 1992) Optically thick source Flat Spectra
Optically thick source α = -0.7, 0.3 Leone, Umana, Trigilio, (1996) Leone, Trigilio, Neri, Umana (2004)
⎛ R ⎞ 3 For a dipole B ∝ B ⎜ * ⎟ P ⎝ R ⎠ ⎛ R ⎞ 3 ν ∝ B, ν ∝ B ⎜ * ⎟ G P ⎝ R ⎠ € High/low ν : close/far from the star € Toward a model
Mass loss from magnetic poles. Trapped plasma in equatorial belt.
1 B2 Wind follows B till ρv 2 ≈ β 2 8π
Current sheets at Alfvén radius Acceleration€ and propagation inwards (middle magnetosphere) Reflection back outwards Gyrosynchrotron emission Figure from Montmerle, 2001 (André et al 1988, model for YSO)
MCP: stable magnetosphere, different orientations Template for other stellar envelopes (thermal/non thermal) 3D model
Trigilio et al (2004), Leto et al (2007)
• Magnetic field and geometry (B, i, β ) • Mass loss, wind velocity, Alfvén radius • Current sheets size −δ • Acceleration efficiency (Nrel) and power law (Nrel ∝ E ) • Absorption by inner magnetosphere plasma
€ Sampling of the magnetosphere
Iν and Fν at different rotational phases Also circular polarization Simulations
Derived parameters
˙ −11 −12 −1 18 cm, 4 cm, 1 cm Mass Loss M ≈10 −10 M⊗yr 12 17 R Ralf − ∗ Inner magnetosphere (T, dens…) Open Questions: Acceleration:€ How radio emission depends −3 −4 Efficiency€ Nrel Nwind ≈10 −10 on Teff, B, Prot? −δ (Need of larger sample) Power law Nrel ∝ E δ ≈ 2
€ € 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 Maser Localization
Cyclotron Maser frequency 6 νB ≈ s⋅ 2.8⋅ 10 BG (Hz) s harmonic number B 3000G pole ≈ νP ≈ 9000 ne (Hz) νB >>νP B ∝ r−3 for a dipole € Pulsar like behaviour B ≈ 500 (s =1); 250 (s = 2) above the€ pole h ≈1.3R* €
€ 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
Electron Cyclotron Maser condition B ∂f (Melrose & Dulk, 1982) > 0 ∂v ⊥ 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 Toward a model for ECME Analogy with auroral planetary emission
Auroral emission: solar wind, acceleration in magnetic tail…
AKR (Auroral Kilometric Radiation) Auroral emission from Earth
Animation: NASA 2011
From Cluster NASA mission: Higly beamed radiation
Localization 1RE above the pole Refraction upward by denser Mutel, 2008 magnetospheric plasma Ring where ν=s νB B Maser amplification where the optical path is longer
Maser radiation in a plane perpendicular to the magnetic axis
(X ) νP nrefr = 1− Plasma ν ν −ν B ≈ 200-300 G ( B ) 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 Acceleration in Current Sheets, regular flow in flux tubes
• Similar geometry (modulation North/South magnetic pole)
• Frequency of the maser How many pulsar style stars can be detected by EMU?
• Dipolar field Acceleration in Current Sheets, regular flow in flux tubes
• Similar geometry (modulation North/South magnetic pole) Magnetic axis ⊥ line of sight About 70 % of MCP • Frequency of the maser How many pulsar style stars can be detected by EMU?
• Dipolar field Acceleration in Current Sheets, regular flow in flux tubes
• Similar geometry (modulation North/South magnetic pole) Magnetic axis ⊥ line of sight About 70 % of MCP • Frequency of the maser EMU Scales as Bpole
500 (G) ν≈ [0.3-1] B pole MHz About 10% of MCP About 7 % of MCP expected • 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 16 -1 -1 With 30 µJy detection limit and Lradio>10 erg s Hz ~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: -19 -2 -1 F(Jup)=10 x(au/10pc)^2=2x10-30 W m Hz =2x10-6 Jy A Hot-Jupiter is about 106 times powerfull Hot-Jup F(HJ)= 2 Jy Solar planets [Zarka,2001] 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