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)