Observational Studies of roAp Stars
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Transcript of Observational Studies of roAp Stars
June 06, 2013 Putting A Stars into Context: Evolution, Environment, and Related Stars
Observational Studies of roAp
Stars
Observational Studies of roAp
Stars Mikhail Sachkov
Institute of Astronomy RAS, Moscow
Observational dataObservational 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)
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.
roAp stars=Rapidly oscillating chemically peculiar A stars
roAp stars=Rapidly oscillating chemically peculiar A stars
roAp stars=Rapidly oscillating chemically 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
Photometric large scale search. I. Cape survey.
High-speed photometry using the 50-cm
telescope of SAAO
(Kurtz & Martinez 2000) : 31 stars
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)
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
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
Main problem of the ground based
observations is aliasing
+ rotational splitting and modulation
+ beating
Uninterrupted (continues) time-series
required
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)
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 first
direct rotation period of
the star. Very stable
photometry
Interferometric observations.
Bruntt et al. 2008
The first detailed interferometric study of roAp star using the Sydney University Stellar Interferometer to measure the angular diameter α Cir to test theoretical pulsation model.
With new Hipparcos parallax the radius is 1.967 ± 0.066 (solar R).
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)
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
Photometric continues space based
observations.
Kepler
See presentations by
Ketrien Uytterhoeven and
others
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)
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)
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)
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.
Spectral observations.
Limitations for cross-correlation (as well as iodine cell) RV studies of roAp stars.
Balona & Laney 2003
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)
High resolution spectral observations.
Exoplanets studies helped
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)
High resolution spectral observations.
roAp/noAp co-exist in
the same region of the
parameter space
(photometric,
kinematical,
abundances, magnetic
field). (Hubrig et al.
2000)
Abundance anomaly as roAp indicator (spectroscopic signature)(Ryabchikova et al. 2004)
There is no real physical difference between roAp and noAp stars (???)
High resolution spectral observations.
15 (!)independent mode
frequencies
Large spacing 64.1 μHz for
which models give best
agreement for M=1.530.03sol
Age 1.50.1 Gyr
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.
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
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
LPV in roAp stars: resolution of the enigma? (Kochukhov et al. 2007) superposition of two types of variability: the usual time-dependent velocity field due to an oblique low-order pulsation mode and an additional line width modulation, synchronized with the changes of stellar radius
High resolution spectral observations.
Polarimetric observations (see
presentation by Lüftinger)
Zeeman Doppler Imaging (HD 24712)
(Luftiner et al. 2007)
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 =
3D tomography of roAp atmospheres
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.
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.
High resolution spectral observations.
Nodal zone
10 Aql10 Aql
High resolution spectral observations.
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.
Photometry and High Resolution
Spectroscopy
HJD
RV
mag
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
Photometry and High Resolution
Spectroscopy
Pulsations for lines identification
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).
roAp studies
roAp “golden decade” (1998-2008)
0
5
10
15
20
25
30
82 86 90 94 98 2 6 10
N publ
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
Some roAp stars – “champions”
HD154708 – the strongest magnetic field (24.51 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)