Coherent and continuous radio emission from Magnetic Chemically Peculiar stars

Transcription

1 Coherent and continuous radio emission from Magnetic Chemically Peculiar stars C. Trigilio 1 P. Leto 1, G. Umana 1, C.Buemi 1, F.Leone 2 1 INAF-OACT, 2 UNICT

2 Magnetic Chemically Peculiar stars MS B-A type Anomalous abundance Magnetic fields White, 2000; Gudel, 2002

3 Chemical Peculiarity Anomalous photospheric abundance (10 6 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 photosphere Strong magnetic fields: magnetic freezing, concentrations of elements, correlation with orientation of B Dependence on T eff He-s O9-B5 He-w B5-A0 Si A0-A5 others A5

4 Variability of light curve, B eff, lines Oblique rotator (Babcock, 1949) CU Virginis P=0.52 giorni (Pyper et al. 1998) Dipolar field B misaligned with rotational axis

5 Stellar winds Magnetic fields + stellar wind Radio emission? (Kodaira & Fomalont 1970) from UV obs driven winds M <10 10 M yr 1, v wind 1000 km s -1 (Shore et al. 1987, Shore & Brown 1990) Outflows from magnetic poles Trapped plasma in equatorial belt

6 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 T eff 31 % He-s O9-B5 26 % He-w B5-A0 23 % Si A0-A5 0 % Others A5 Correlation with wind/mass loss? Gyrosynchrotron emission Radio luminosity L 5GHz erg s 1 Hz 1

7 Modulation of radio emission Radio minima, B eff minima Oblique rotator model Change of orientation (Leone, Umana 1992) Optically thick source

8 Flat Spectra Optically thick source α = -0.7, 0.3 Leone, Umana, Trigilio, (1996) Leone, Trigilio, Neri, Umana (2004) For a dipole R B B * P R ν G B, R ν B * P R 3 3 High/low ν : close/far from the star

9 Toward a model Mass loss from magnetic poles. Trapped plasma in equatorial belt. Wind follows B till 1 2 ρv 2 B2 8π β Current sheets at Alfvén radius Acceleration and propagation inwards (middle magnetosphere) Reflection back outwards Gyrosynchrotron emission (André et al 1988, model for YSO) Figure from Montmerle, 2001 MCP: stable magnetosphere, different orientations Template for other stellar envelopes (thermal/non thermal)

10 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 Acceleration efficiency (N rel ) and power law Absorption by inner magnetosphere plasma (N rel E δ ) Sampling of the magnetosphere I ν and F ν at different rotational phases Also circular polarization

11 Simulations Derived parameters Mass Loss R alf Inner magnetosphere (T, dens ) Acceleration: Efficiency Power law M M yr R N rel N wind N rel E δ δ 2 18 cm, 4 cm, 1 cm Open Questions: How radio emission depends on T eff, B, P rot? (Need of larger sample)

12 CU Virginis Discovery of coherent radio emission Detection of two pulses at 20 cm with VLA (Trigilio et al 2000) Rotational phase: B eff = 0 High directivity ( magnetic axis) 100% circular polarization (RCP) Cyclotron Maser above the North magnetic pole

13 Maser Localization Cyclotron Maser frequency B pole 3000G B r 3 for a dipole B 500 ( s =1); 250 s = 2 h 1.3R * ( ) above the pole ν B s B G (Hz) ν P 9000 n e (Hz) s harmonic number ν B >>ν P Pulsar like behaviour

14 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

15 Change of P rot 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

16 Bandwidth of the Maser From ATCA, VLA and EVLA obs, ν range: MHz (Trigilio et at 2008, 2011, Ravi et al 2010) Dynamical spectra (EVLA obs) Large bandwidth ν not simultaneous

17 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 B f v > 0 Electron Cyclotron Maser condition (Melrose & Dulk, 1982) ν=s ν B s=1,2,3 x-mode polarization Narrow Δν Emission almost perpendicular to B From observations: Δν very large, problems with geometry

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

19 B Ring where ν=s ν B Maser amplification where the optical path is longer Maser radiation in a plane perpendicular to the magnetic axis Plasma B G N 10 9 cm -3 ν P ( X n ) refr = 1 ν ν ν B ( ) Refractive index ( ) consistent with the observed deviation ψ (Trigilio et al, 2011, ApJ 739, L10)

20 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

21 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

22 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 B pole 500 <B pole < G ν [0.3-1] B (G) pole MHz About 10% of MCP About 7 % of MCP expected <B G >

23 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 L radio >10 16 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

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25 Magnetic Chemically Peculiar stars Characteristics: MS B-A type Anomalous photospheric abundance (10 6 Sun) (He-s, He-w, Si, Cr ) Strong magnetic fields ( G) Variability: light curve, B eff, lines P = days

26 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

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28 Toward a model for ECME Analogy with auroral planetary emission Aurorall emission: solar wind, acceleration in magnetic tail Figures from Zarka 1998

29 At 10 pc Jupiter: F (Jup) =10-19 x(au/10pc)^2=2x10-30 W m -2 Hz -1 =2x10-6 Jy A Hot-Jupiter is about 10 6 times powerfull F (HJ) = 2 Jy Hot-Jup Solar planets [Zarka,2001]

30 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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