Download sachkov_2013 - Putting A Stars into Context

Observational
Studies of roAp
Stars
Mikhail Sachkov
Institute of Astronomy RAS, Moscow
June 06, 2013 Putting A Stars into Context: Evolution, Environment, and Related Stars
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)
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.
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
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)
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 crosscorrelation (as well as
iodine cell) RV studies
of roAp stars.
Balona & Laney 2003
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
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)
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=1.530.03sol
Age 1.50.1 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
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 field due
to an oblique low-order pulsation mode and an additional
line width modulation, synchronized with the changes of
stellar radius
Polarimetric observations (see presentation by
Lüftinger)
Zeeman Doppler Imaging (HD 24712)
(Luftiner et al. 2007)
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 =
+
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.
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.
10 Aql
Nodal zone
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
Photometry and High Resolution Spectroscopy
Intense observing campaigns, that combined groundbased 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
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)
30
25
20
15
N publ
10
5
0
82 86 90 94 98
2
6 10
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 (24.5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)