Download IAU-Perraut-2013 - Putting A Stars into Context

IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Determination of fundamental parameters of
(Chemically Peculiar) A stars
through optical interferometry
Karine ROUSSELET-PERRAUT
Institut de Planétologie et d’Astrophysique de Grenoble
(with contributions of Denis Mourard, Margarida Cunha, Nicolas Nardetto)
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Science drivers
A-F stars are an ideal laboratory for studying physical processes
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
 radiative diffusion, differential gravitational settling, grain accretion,
convection, rotation, magnetic fields, non-radial pulsations
that show their most extreme manifestations in these stars.
These processes are based on fundamental parameters
 Mass M, radius R, luminosity L, abundances
 Effective temperature Teff, surface gravity log g, mean density r
From the measurement of these fundamental parameters and
theoretical evolutionary tracks, one can put into test models

Stellar interiors, evolutionary stages

Magnetic field topology, pulsation excitation
(when coupled with complementary observational data)
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Outline
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd

How can optical interferometry help to better understand
the A stars ?




Principle
Instruments
Results
Prospects
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Interest of High Angular Resolution

◦ photosphere convection cells
◦ dark spots
◦ active areas
◦ chromosphere jets

Clues for better understanding:
◦ internal structure
◦ pulsation modes
0.5º = 1 800"
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd

High Angular Resolution (HAR) is fundamental for understanding
star formation and evolution as well as physical process at play
within stellar objects.
Sun is the best known star since we manage to "resolve" its
surface, i.e. to see details on its surface like:
◦ activity
◦ magnetism
Extreme Ultraviolet Imaging Telescope (EIT), Sept. 1999
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IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Telescope angular resolution
Telescope size D
(m)
Resolution l/D
(millisecond of arc)
8m
(VLT + perfect AO)
18 (l = 0.6 µm)
42 m
(E-ELT + perfect AO)
~ 3 (l = 0.6 µm)
200 m
(with perfect AO)
~ 0.6 (l = 0.6 µm)
Need for optical interferometry
Smallest detail
« seen » on the
stellar surface
Main-sequence
A stars
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Interferometry principle
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Interferometry is a imaging technique with (small) diluted apertures,
which allows a giant mirror to be synthetized.
The longer the distance b between the
apertures, the higher the angular
resolution (given by l/b).
VLTI
This technique is fully mature in radioastronomy and produces high
angular resolution images.
ALMA
R Scu (imaged by ALMA)
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Optical interferometric arrays
Array
Apertures
Maximal
Number / diameter baseline b (m)
l
(µm)
Resolution
(mas)
KECK
2 / 10 m
85
IR
~ 20
VLTI
4/8m
4 / 1.8 m
130
200
NearMid-IR
2 – 15
1.5 – 2.5
CHARA
6/1m
330
Visible
Near-IR
0.3 – 0.5
0.9 – 1.5
NPOI
6 / 0.12
430
Visible
0.25 – 0.4
CHARA
VLTI
NPOI
KECK
Generally we have not enough apertures to obtain images
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Interferometric observables
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
In optical range we generally observe interference fringe patterns
between the different apertures.
The visibility V and the phase  of these fringe
patterns are related to the Fourier transform
of the object brightness (Van-Cittert theorem)
V
V ei = TF{Object} (b/l)
Photocenter p
p
p
We can deduce angular diameter, binary orbit, environment extent, etc.
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How to measure angular diameters?
The VLTI
V
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
bb
combination
b
b



We record fringes for different
telescope separations b.
We compute the visibility V and the
phase  for each fringe pattern
We fit the curve V = f(b) with an
angular diameter model.
V
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Distance d
Bolometric flux fbol
Distance d
Radius R
Hipparcos
Interferometry
Angular diameter LD
Spectro-photometry
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Fundamental parameters’ determination
Luminosity L
Mass M
Age
Gravity log g
Effective temperature Teff
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IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Application to A stars
In addition to their many other peculiarities, the ages of A stars are
poorly known. As main sequence stars, they evolve in Teff and in L/R,
which affords an opportunity to establish their ages through
interferometric means.
Observational data can be compared to models on an H-R diagram,
which will then indicate their ages and masses.



Significant improvement of measurement accuracy
First statistical studies
Application to A-star sub-classes
◦ Exoplanet host stars
 provide R for planetary system modeling
◦ Chemically Peculiar (CP) stars
 provide Teff that is not affected by the abnormal surfaces
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IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Improvement of accuracy
The recent improvement of the accuracy on interferometric
observables has led to an angular diameter precision of typically
1-2% and consequently to an improved determination of stellar
fundamental parameters, which in turn allows to test stellar
models in an independent way.
Procyon
« classical » error box
« interferometric »
error box
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[Kervella et al. A&A, 413, 251(2004)]
Statistical studies
Several surveys of main-sequence stars have been led with optical
interferometry but they mainly consists of G and K targets.
Survey of 44 stars by T.
Boyajian with CHARA
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Average accuracy of 1.5 %
20
15
10
5
0
A
F
G
[Boyajian et al. ApJ, 746, 101 (2012)]
d < 22 pc
Accuracy on R< 3%
[Cunha et al. A&AR, 14, 217 (2007)]
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Statistical studies
Comparison of interferometric
diameters with SED ones
[Boyajian et al. ApJ, 746, 101 (2012)]
[Boyajian et al. ApJ, in prep (2013)]
Effective temperature law
A stars
Relationships useful in extending our knowledge to a larger number
of stars, at distances too far to accurately resolve their sizes.
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Surface-Brightness relationship
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
In the recent distance determination to the Large Magellanic
Cloud (the best anchor point of the cosmic distance scale) with
an accuracy of 2%, the main uncertainty comes from SurfaceBrightness relationships [Pietrzynski et al. 2013, Nature]
+ Di Benedetto
(2005)
+ Boyajian (2012)
+ Brown (1974)
+ Challouf (in prep)
Bright early-type stars (O-A-B)
for distances in Local Group
Late-type stars (F-G)
for LMC distance
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Interest of multi-technique studies

Interferometry + Spectroscopy:
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
R, L, Teff accurate determination requires interferometric angular
diameters, accurate parallaxes and accurate bolometric flux.

Interferometry + Asteroseismology:
[Creevey et al., ApJ, 659, 616 (2007)]
Accurate R allows accurate masses M to be derived
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Case of the roAp stars
Small rotational speed
(< 100 km/s)
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Abundance inhomogeneities
(with a contrast up to 1000)
< 1 mas
Strong magnetic field (up to
~30 kG) large-scale organized
roAp: intersection of MainSequence and instability strip
Pulsations (period of a
few minutes)
Optical interferometric allows to have a direct (and unbiased)
measurement of the linear radius of these tiny stars.
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Example of 10Aql
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
V²
LD = 0.275 0.006 mas
R = 2.317  0.070 R
[CHARA/VEGA]
+ Bolometric flux and parallax
+ Large frequency separation Dn
+ Evolutionary track (CESAM2K)

B/l
M = 1.95  0.05 M
[Perraut et al. A&A, submitted]
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Test of roAp excitation models
[Cunha et al. A&A, in prep]


« Interferometric » fundamental parameters
for 4 roAp (aCir, bCrB, gEqu, 10 Aql)
Stellar interior models
Excited
g Equ
0
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Not excited


Prediction of excitation modes
Comparison with observed modes
a Cir
Observed modes
10 Aql
Predicted modes derived for interferometric parameters ( ) are in
agreement with observed ones but for aCir.
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Go beyond diameter measurement
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Optical interferometry is clearly a powerful means to derive
accurate fundamental parameters through angular diameter
measurement but this technique can also be used to study the
environments of A stars:
Study debris disks around VEGA-like
 Search for companion(s)

Fomalhaut
and coupled with spectrometry for kinematic studies

Study of wind and mass loss


Deneb
A supergiants, (magnetic) Herbig AeBe
Study of limb-darkening
Imaging of stellar surfaces (rotation)
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Debris disks
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
The short-baseline visibilities are lower than expected for the
stellar photosphere alone. The visibility offset of a few percent
is interpreted as a near-infrared excess arising from dust grains
which must be located within several AU of the central star.
V²
bLeo
b (m)
[Akeson et al. ApJ, 691, 1896 (2009)]
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Supergiant wind
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
The line-formation region is extended (∼1.5–1.75 R*) since the
visibility decreases in the Ha line. There is a significant asymmetry
in the line forming region since the phase is not null in the line.
[Chesneau et al. A&A, 521, A5 (2010)]
Ha line
Visibility
Phase
Deneb
l
l
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Rotation

Optical interferometry can image the surface of fast rotators.
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Altair


[Monnier et al. Science, 317, 342 (2008)]
The image clearly reveals the strong effect of gravity darkening on
the highly-distorted stellar photosphere.
Standard models for a uniformly rotating star cannot explain the
results, requiring differential rotation, alternative gravity
darkening laws, or both.
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Conclusion

IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd




Optical interferometry is a powerful means for deriving accurate
fundamental parameters of A stars through accurate angular
diameter determinations.
With long-baseline arrays an angular resolution of about 0.3 mas
is now reachable.
There is a huge potential of combining interferometric (radius and
derived effective temperature) and asteroseismic (large frequency
separation) data to improve the determination of the mass of
pulsating stars.
Coupled with spectroscopy, optical interferometry can allow
kinematics studies of A stars’ environments.
Going beyong angular diameter measurements allows limbdarkening to be derived and besides surfaces of fast rotators to be
imaged.
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IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Prospects
Position of 109 stars with an accuracy of ~20 µas
 Go towards an Angular Diameter Anthology
[Boyajian et al. ApJ, in prep (2013)]
Needs to
 go to higher sensitivity and to smaller targets

Increase accuracy for putting into test stellar models

Go towards surface imaging across spectral lines
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IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Breakthrough for CP stars
HD24712
[Lüftiger et al., A&A, 509, A71 (2010)]
[See poster of D. Shulyak et al. in this conference]
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IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Thanks for your attention
The CHARA Array
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IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
An example of simulation : a2CVn
• Spectra of a2CVn
(CrIIl4824 line)
-0.5
Dl
0.5
• Doppler maps
(Kochukhov,2002)

-0.5
Dl
0.5
Predicted interferometric phases
Interferometric phases provide 2D geometrical constraints
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Abundance study with CHARA/VEGA

IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd

Line for various
stellar phases

Resolved target
Large visibility effects
Observations in the visibility
lobes
CHARA
Abundance spots of a fraction of
stellar diameter can be detected in
the 2nd and 3rd visibility lobes
BUT
CHARA/VEGA + a2CVn models
Imaging may be difficult due to the
small number of telescopes
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Limb-darkening effect
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Limb darkening in an absorption line is expected to be less than
it would be for the continuum at the wavelength of the line
because the line is formed all the way out to the stellar surface.
Sirius
Uniform-disk diameter
(mas)
Ha line
l
[ten Brummelaar et al. MROI meeting (2011)]30
Determination of fundamental parameters of (Chemically Peculiar) A stars
through optical interferometry
IAU Conference « Putting A Stars into Context”
Moscow, 2013 June 3rd
Karine ROUSSELET-PERRAUT (IPAG, France)
The recent improvement of the accuracy on interferometric observables
has led to an angular diameter precision of typically 1-2% and
consequently to an improved determination of stellar fundamental
parameters, which in turn allows to test stellar models in an independent
way.
Procyon
« classical » error box
« interferometric »
error box
31
[Kervella et al. A&A, 413, 251(2004)]