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Observational Studies of roAp Stars

Observational Studies of roAp Stars. Mikhail Sachkov Institute of Astronomy RAS, Moscow. Observational data. I Pho tome ty (time-series, large scale search, continuous ground based, continuous space based) II Interferometry

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Observational Studies of roAp Stars

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  1. Observational Studies ofroAp Stars Mikhail Sachkov Institute of Astronomy RAS, Moscow June 06, 2013Putting A Stars into Context: Evolution, Environment, and Related Stars

  2. Observational data I Photomety (time-series, large scale search, continuous ground based, continuous space based) II Interferometry III Spectroscopy (high resolution spectra, high resolution time-series, large scale search, polarimetry)

  3. roAp stars=Rapidly oscillatingchemically peculiar A stars roAp stars=Rapidly oscillating chemically peculiar A stars The magnetic chemically peculiar (Ap) stars are upper-main-sequence stars with anomaly strong lines of certain (Si, Cr, Sr, Eu) chemical elements in their spectra and strong globally organized magnetic fields. They often show remarkable variations of line strengths, light and magnetic field with periods ranging from a few days to many years. It is believed that this abnormal chemical composition is limited only to the outer stellar envelopes. Chemical diffusion altered by a global magnetic field can produce surface abundance non-uniformities.

  4. roAp stars=Rapidly oscillatingchemically peculiar A stars roAp stars=Rapidly oscillating chemically peculiar A stars • Discovered by D.Kurtz in 1978 • Cool (Te ~ 6400-8500 K) chemically peculiar stars with a strong magnetic field (1-25 kG) • Multiperiodic non-radial puilsations with periods 5.7-23.6 min => key objects for asteroseismology • Photometric amplitudes 0.8 – 15 mmag • RV amplitudes up to 5 km/s • Most of (45) roAp stars are on south hemisphere

  5. Photometric large scale search. I. Cape survey. High-speed photometry using the 50-cm telescope of SAAO (Kurtz & Martinez 2000) : 31 stars

  6. Photometric large scale search. II. Naini Tal - Cape survey High-speed photometry using the 1.04-m Sampurnanand telescope at ARIES New roAp HD 12098 (Girish et al. 2001) Naini Tal - Cape survey: 140 null result (Joshi et al. 2006) Naini Tal - Cape survey: 61 null result (Joshi et al. 2009)

  7. Photometric large scale search. III. The Hvar survey CCD photometry at the 1 m Austrian-Croatian Telescope, Hvar Observatory 20 null result (Paunzen et al. 2012) up to 2 mmag in B Next 45 candidates to be observed

  8. Classical Asteroseismology: frequencies as basic input data asymptotic theory of acoustic pulsations (p-mode for n>>ℓ) : νnℓ≈∆ν(n+ℓ/2+ε) + δν, ∆ν – mean density indicator δν - age indicator  

  9. Main problem of the ground based observations is aliasing + rotational splitting and modulation + beating Uninterrupted (continues) time-series required

  10. Photometric continues ground based observations. Whole Earth Telescope. HR 1217. 0.6 – 2.1 m telescopes, 35 days. Pushing the ground based photometric limit: 14μmag (Kurtz et al. 2005)

  11. Photometric continues space based observations. A double wave modulation with a period of Prot = 4.4792 ± 0.0004 d and a peak-to-peak amplitude of 4mmag: due to spots on the surface => the firstdirect rotation period of the star. Very stable photometry

  12. Interferometric observations. Bruntt et al. 2008 The first detailed interferometric study of roAp star using the Sydney University Stellar Interferometer to measure the angular diameterα Cir to test theoretical pulsation model. With new Hipparcosparallax the radius is 1.967 ± 0.066 (solar R).

  13. Photometric continues space based observations. MOST. (see presentation by Jaymie Matthews) γ Equ. Puzzling amplitude changes: a consequence of limited mode life time or beating frequecies ? (Gruberbauer et al. 2008)

  14. Photometric continues space based observations. MOST. One of the recent paper on HD 9289, HD99563, HD134214: Gruberbauer et al. 2011 Excellent data on frequencies at the level of 0.01 mmag accuracy

  15. Photometric continues space based observations. Kepler See presentations by Ketrien Uytterhoeven and others

  16. Spectral observations. • Only few radial velocity studies were attempted during 1982 – 1998 •  Equ (ampl ~21 m/s) Libbrecht 1988 (Palomar 5-m telescope) HR 1217 (~200 m/s) Matthews 1988 (CFHT) • “Different sections of the spectrum give different radial velocities” : for  Equ from 100 m/s up to 1 km/s (Kanaan&Hatzes 1998)

  17. Spectral observations. Cir: RV upper limit 60 m/s (Hatzes&Kuerster 1994, using iodine cell, 45Å) but some 10Å wavelength bands show up to 1 km/s (Baldry et al. 1998) H line bisector measurements: amplitude and phase variations as a function of depth in the line – the idea of observed radial node (Baldry et al. 1999)

  18. Spectral observations.  Equ: lines of the rare earth elements (PrIII and NdIII) have large RV amplitude up to 1 km/s while lines of BaII and FeII show no detectable RV variations (Malanushenko et al. 1998, Savanov et al. 1999) Line-by-line analysis: amplitude is a function of atmospheric height (Kochukhov&Ryabchikova 2001)

  19. Spectral observations. The van Hoof effect – phase lag between radial velocity curves of lines of different elements and ions – is one of the most interesting phenomena in the roAp stars. It yields a unique possibility for the vertical atmospheric structure analysis.

  20. Spectral observations. Limitations for cross-correlation (as well as iodine cell) RV studies of roAp stars. Balona & Laney 2003

  21. High resolution spectral observations. High – resolution, high signal-to-noise, high time-resolution Spectroscopy. New heights in asteroseismology: “Until recently the idea of using 8- to 10-m telescope to observe some of the brightest stars in the sky was anathema” (D.Kurtz, MNRAS 2003 343 L5) Exoplanets studies helped

  22. High resolution spectral observations. Spectroscopy allows to search for frequencies undetectable photometrically New roAp stars were discovered based on high resolution spectroscopic observations: β CrB(HD137909)- Hatzes & Mkrtichian (2004) HD116114- Elkin et al. (2005) HD154708 – Kurtz et al. (2006) HD75445 – Kochukhov et al. (2008) HD115226 – Kochukhov et al.(2008) …………………………….. HD132205, HD148593, HD151860 – Kochukhov et al. (2013)

  23. roAp/noAp co-exist in the same region of the parameter space (photometric, kinematical, abundances, magnetic field). (Hubrig et al. 2000)

  24. Abundance anomaly as roAp indicator (spectroscopic signature) (Ryabchikova et al. 2004) There is no real physical difference between roAp and noAp stars (???)

  25. High resolution spectral observations. 15 (!)independent mode frequencies Large spacing 64.1 μHz for which models give best agreement for M=1.530.03sol Age 1.50.1 Gyr

  26. High resolution spectral observations. • Discovery of magnetic field variations with the 12.1-minute pulsation period of the roAp star Equulei (Leone & Kurtz 2003): SARG with polarimeter at TNG 240±37 G • Variations with amolitude of 200 G (Savanov et al. 2003): MAESTRO at 2-m Terscol Obs.

  27. High resolution spectral observations. • No pulsational variations of the surface magnetic field at the level of 40-60 G (Kuchukhov et al. 2004): NES at BTA Zeeman-resolved profile of Fe II 6149 and Fe I 6173 lines • No pulsational variations at the level of 10 G (Kochukhov et al. 2004): Gecko coude spectrograph at CFHT • No pulsational variations at the level of 10 G (Savanov et al. 2006) CES at ESO 3.6 m • No pulsational variations at the level of 40 G for 6 roAp (Hubrig et al. 2004): FORS1 at VLT

  28. High resolution spectral observations. Shock waves in the roAp atmospheres (Shibahashi et al. 2008): features in the lines appear to move smoothly from blue to red, but return to the blue discontinuously

  29. High resolution spectral observations. LPV in roAp stars: resolution of the enigma? (Kochukhov et al. 2007) superposition of two types of variability:the usual time-dependent velocity fielddue to an oblique low-order pulsation mode and anadditional line width modulation, synchronized with the changes of stellar radius

  30. Polarimetric observations (see presentation by Lüftinger)

  31. Zeeman Doppler Imaging (HD 24712) (Luftiner et al. 2007)

  32. 3D tomography of roAp atmospheres The peculiar atmospheres of magnetic roAp stars provide the unique possibility to build a complete 3D model of a pulsating stellar atmosphere. “Clouds" of rare earth elements are located at various heights within the atmosphere. 3D = +

  33. High resolution spectral observations. Sachkov et al. 2006: Saio’s (2005) model for the roAp star HD24712 roughly explains amplitudes and phases up to log  5000 = -4: amplitude and phase increase towards the outer layers => phases and amplitudes of pulsation reflect features of propagating wave through the stellar atmosphere.

  34. High resolution spectral observations. Sachkov et al. 2007: the “phase – amplitude” diagram as a first step of the interpretation of roAp pulsational observations. Such approach has an advantage of being suitable to compare behaviour of different elements, which is impossible for studies of phase/amplitude dependence on line intensity.

  35. High resolution spectral observations. 10 Aql Nodal zone

  36. Photometry and High Resolution Spectroscopy A combination of simultaneous spectroscopy and photometry represents the most sophisticated asteroseismic dataset for any roAp star. An observed phase lag between luminosity and RV variations is an important parameter for a first step towards modelling the stellar structure. mag RV HJD

  37. Photometry and High Resolution Spectroscopy Intense observing campaigns, that combined ground-based spectroscopy with space photometry obtained with the MOST satellite: HD24712 (Ryabchikova et al. 2007) 10 Aql (Sachkov et al. 2008) 33 Lib (Sachkov ey al. 2011)  Equ (still in preparation) Modulation(!) for phase lag

  38. Pulsations for lines identification As in roAp stars mainly lines of the rare-earth elements show high amplitude RV pulsational variations this can serve to identify unknown lines in roAp stars' spectra (Sachkov et al. 2006).

  39. roAp studies roAp “golden decade” (1998-2008)

  40. Future of roAp studies • (ex/in)tensive roAp High-resolution spectroscopic sets (e.g. for mode stability) • Kepler’s legacy • Next generation space projects: WSO-UV, THEIA

  41. Some roAp stars – “champions” • HD154708 – the strongest magnetic field (24.51 kG) • HD177765 – the longest pulsation period (23.6 min) •  Equ – the longest rotation period (92 years – see Poster by Savanov et al.) • HD 101065 - the richest p-mode frequency spectrum (15 freq.) • HD134214 - the shortest pulsation period (5.7 min) • HD213637 - the lowest T eff 6400K (or HD101065 with 6300K) • HD137949 – the largest abundance anomaly (2.2 dex fro Pr III-II and Nd III-II)

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