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Benisty, M.

Berger, J.-P.

Dominik, C.

et al.

Kluska, J.

Lazareff, B.

Malbet, F.

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Improving the performances of current optical interferometers & future designs
Proceedings of Haute Provence Observatory Colloquium (23-27 September 2013)
Edited by L. Arnold, H. Le Coroller & J. Surdej
Imaging Young Stellar Objects with VLTi/PIONIER
J. Kluska1, F. Malbet1, J.-P. Berger2, M. Benisty1, B. Lazareff1, J.-B.
Le Bouquin1, F. Baron3, C. Dominik4, A. Isella5, A. Juhasz6, S.
Kraus7, R. Lachaume8, F. Me´nard1,9, R. Millan-Gabet5, J. Monnier10,
C. Pinte9, F. Soulez11, M. Tallon11, W.-F. Thi1, E´. Thie´baut11 & G.
Zins1
1UJF-Grenoble 1 / CNRS-INSU, Institut de Plane´tologie et d’Astrophysique
de Grenoble (IPAG) UMR 5274, Grenoble, F-38041, France
2ESO, Garching, Germany
3Georgia State University, Atlanta, USA
4University of Amsterdam, Amsterdam, The Netherlands
5California Institute of Technology, Pasadena, USA
6Leiden Observatory, Leiden, The Nertherlands
7University of Exeter, UK
8Pontificia Universidad Cato´lica de Chile. Santiago de Chile, Chile
9UMI-FCA, CNRS y U. de Chile, Chile
10University of Michigan, Ann Arbor, USA
11Universite´ de Lyon I, Lyon, France
Abstract. Optical interferometry imaging is designed to help us to reveal
complex astronomical sources without a prior model. Among these complex
objects are the young stars and their environments, which have a typical
morphology with a point-like source, surrounded by circumstellar material
with unknown morphology. To image them, we have developed a numerical
method that removes completely the stellar point source and reconstructs the
rest of the image, using the differences in the spectral behavior between the
star and its circumstellar material. We aim to reveal the first Astronomical
Units of these objects where many physical phenomena could interplay: the
263
264 J. Kluska et al.
Figure 1.: Left : Plot from Vinkovic et al. (2003) showing the halo scenario.
Right : The puffed-up inner rim scenario proposed by Isella & Natta (2005).
dust sublimation causing a puffed-up inner rim, a dusty halo, a dusty wind
or an inner gaseous component. To investigate more deeply these regions,
we carried out the first Large Program survey of HAeBe stars with two main
goals: statistics on the geometry of these objects at the first astronomical
unit scale and imaging their very close environment. The images reveal
the environment, which is not polluted by the star and allows us to derive
the best fit for the flux ratio and the spectral slope. We present the first
images from this survey and the application of the imaging method on other
astronomical objects.
1. Introduction
Direct imaging techniques have revealed complex structures around young stellar
objects (e.g. Rameau et al. 2013) such as spirals, holes, etc. These features are
the consequence of phenomena occurring more closer to the star, like planetary
formation. To reach these regions we need more angular resolution. This is where
optical interferometry is entering the game.
1.1 The inner regions of Young Stellar Objects (YSO).
The inner parts of the Young Stellar Objects are the place of many physical
processes. In the near infrared, one of the most contributing phenomena (in
the broad band) is the dust sublimation. Dust sublimates at 1500K which is
the temperature of the dust at approximately 1 AU of its host star (for Herbig
stars). It creates an excess emission in the near infrared but is not well constrain
geometrically. Vinkovic et al. (2003) proposed a halo model to explain the excess
Imaging Young Stellar Objects with VLTi/PIONIER 265
Figure 2.: The disk wind solution proposed by Bans & Ko¨nigl (2012).
Figure 3.: This is the sketch from Dullemond & Monnier (2010) of the inner
regions of young stellar objects where we expect to see the dust sublimation region,
an inner accretion disk and constrain the planetary formation.
emission in the near infrared and Isella & Natta (2005) proposed a model of a
puffed-up inner rim due to dust sublimation (see Fig. 1). More recently, Bans
& Ko¨nigl (2012) published a wind scenario to explain this excess and said that
optical interferometry is the tool to distinguish between the scenarios (see Fig. 2).
In addition to dust sublimation, gas could create an accretion disk inside
the dust sublimation radius and cause magneto-spheric accretion onto the star.
Planetary formation could also perturb the inner rim. We can also expect mass
ejection through wind and/or jets that are believed to originate in this inner
region. A schematic view is displayed in Fig. 3.
In order to study this complex inner regions and being model independent,
we want to make image reconstruction from optical interferometry data.
266 J. Kluska et al.
Figure 4.: Datasets on YSOs obtained with AMBER. HR5999 data is from
Benisty et al. (2011) and HD163296 data from Renard et al. (2010). We can
clearly see the chromatic effect.
1.2 YSO and chromatism
First reconstructed images of Young Stellar Objects (YSOs) have already been
made. For instance Benisty et al. (2011) made an image of HR5999 and Renard
et al. (2010) an image of HD163296. These images were obtain with monochro-
matic image reconstruction algorithms. As we can see on the visibility curves,
there is a strong chromatic effect in particular in the H-band (see Fig. 4). The
longest wavelengths have clearly lower visibilities than the shortest ones. It is an
astrophysical effect.
Kluska et al. (2012) showed that if we are in a presence of an unresolved star
which is in the Rayleigh-Jeans regime and a resolved environment which is cold
(≈ 1500K) we will have such an effect. In the near infrared, the young stellar
objects SED is shared between the stellar photosphere flux and the environment
one. The changing flux ratio between a resolved environment and a point source
creates the chromatic effect seen in the squared visibilities seen in both AMBER
(Petrov et al. 2007) and PIONIER (Le Bouquin et al. 2011) data.
2. Chromatic method to reconstruct on object
The monochromatic image reconstruction algorithms are not adapted anymore
for such a chromaticity. To face this problem two approaches exist. The first one
is a three dimensional approach that consists in reconstructing a spectral cube of
images adding a trans-spectral regularization term (see Soulez et al.). Another
approach, which will be presented here, is the Semi-Parametric Approach for
image Reconstruction of Chromatic Objects (SPARCO : Kluska et al. 2014).
2.1 Flux parametrization
The chromatic effect needs to be taken into account in a chromatic algorithm.
That is why we need to define the flux evolution of both the photosphere and the
Imaging Young Stellar Objects with VLTi/PIONIER 267
Figure 5.: Left : fluxes versus wavelength across the H-band. In blue : flux of
the photosphere. In red : the flux of the environment. In solid lines : the fluxes
defined by Eq. 1. In dashed line : fluxes defined by a photosphere + environ-
ment model. Vertical color lines : PIONIER spectral channels. Right : squared
visibilities from a ring alone (black) and a ring with a star (color).
environment. As written in Sect.1.2, the photosphere is in the Rayleigh-Jeans
regime and have its flux Fλ ∝ λ
−4 (or a spectral index of −4). The environment
flux could also be described as a power-law with a spectral index denv (for 1500K
denv = 0.8 for H-band). To define the total flux we need the fraction of flux of
the star f0
∗
at λ0 (which is an arbitrary chosen wavelength). We can then write
the total flux ftot(λ) as :
ftot(λ) = f
0
∗
(
λ
λ0
)
−4
+ (1− f0
∗
)
(
λ
λ0
)denv
(1)
where first term is the stellar flux (f∗(λ)) and the second one is the flux of the
environment (fenv). This flux parametrization is valid in a narrow band such as
the H-band (see Fig. 5, Left).
2.2 Visibility computation
With the fluxes definition, we can compute the total visibilities. To do so we
need to set the visibilities for the star and the environment separately. For the
photosphere we assume that it is not resolved and its visibility is V∗ = 1. This
assumption is valid considering that a 5R at 140pc (distance of the Taurus star
forming region) has a visibility of 0.997 for B=100m in the H-band.
The visibility of the environment (V˜env) will be computed from the image
as it is done in monochromatic image reconstruction algorithms. The star is not
appearing in the image anymore.
The equation of the total visibility that is fitted to the data is :
V˜tot
(
b
λ
, λ
)
=
f∗(λ) + fenv(λ)V˜env(
b
λ
)
f∗(λ) + fenv(λ)
(2)
268 J. Kluska et al.
Figure 6.: Left : The model image. The star is represented with a red star in the
middle of the image. The ring is azimuthally modulated and shifted with respect to
the star. Center and Right : squared visiblities and closure phases of the model
taking a actual (u, v)-plan. A gaussian noise was added to this dataset. The
colors represent the spectral channels as on the previous figures.
where (b
λ
) is the spatial frequency (which is the baseline vector divided by the
wavelength).
Thanks to the linearity of the Fourier transform we can separate the star
from the image of the environment. The algorithm is described more specifically
in Kluska et al. (2014). The result of a ring visibility taking into account the
chromatic effect i shown Fig. 5 Right.
2.3 Validation on a model
The algorithm was tested on a chromatic model of a YSO. It is composed of
a star, having 40% of the flux at 1.65µm. The environment is an azimuthally
modulated gaussian ring, shifted to the south with respect to the star. The
environmental spectral index is set to 1. The model image is showed in Fig. 6.
In order to generate a realistic (u, v)-plan we have taken an existing one on an
object already observed by PIONIER. The (u, v)-plan is then not totally optimal.
We then generate the squared visibilities (V 2) and the closure phases (CP). The
whole data set is shown in Fig. 6.
The image reconstructions are shown in Fig. 7. They were performed using
the MiRA (Thie´baut 2008) algorithm. They were made using three methods :
– An image reconstruction with a monochromatic algorithm. The χ2 is not
satisfactory (≈ 10). There are many artifacts in the image due to the
chromaticity present in the data.
– An image reconstruction by subtracting the star from the image but without
taking into account the chromaticity. The χ2 is slightly better but the image
is still not satisfactory.
– An image reconstruction applying the SPARCO method. This method was
tested in both MiRA and MACIM (Ireland et al. 2006) algorithm with a
χ2 = 1.2. The features of the ring are well retrieved and the artifacts are
weak.
Imaging Young Stellar Objects with VLTi/PIONIER 269
Figure 7.: Image reconstructions on the model using different methods. Left
: classical monochromatic image reconstruction. Center Left : monochromatic
image reconstruction subtracting the star. Right : chromatic image reconstruction
subtracting the star with Mira and MACIM. In the right bottom corner : the dirty
beam of the (u, v)-plan.
2.4 Chromatic degeneracy
The reconstructions showed in Sect. 2.3 were made setting the chromatic param-
eters f0
∗
and denv to the right ones (0.4 and 1 respectively).
Without spectrophotometric informations we have no information on these
chromatic parameters. What are the effects on the image reconstruction process
if the chromatic parameters are wrong ? To test that, we have made a bunch
of images that with different values of the chromatic parameters (see Fig. 8).
The image with the good parameters is in the center. We have taken a range
of spectral indexes for the environment that represent temperatures from 1250K
to 1750K (spectral indexes of 2 and 0 respectively). For these temperatures the
image does not vary much for a fixed value of the flux ratio. The effects are
bigger for the flux ratios. If we put too much flux in the star (50% for instance)
the image is filled by artifacts and the ring is dug. But if the stellar to total flux
ratio is too low, some flux will be put at the position of the star degrading the
quality of the image.
We can also make a χ2-map of the imager reconstructions in the plan of the
chromatic parameters. This map is presented in Fig. 9. We can see there is a
smooth valley of χ2 that indicates a clear degeneracy between the two chromatic
parameters. The spectrophotometric data can be used to find the good values of
the chromatic parameters.
3. Imaging real targets
Once the chromatic image reconstruction method is validated we have applied
it on actual datasets. We have conducted a Large Program with the PIONIER
instrument on the VLT interferometer. 55 Herbig Ae/Be stars were observed
during our 30 nights of observation. A dozen of objects are resolved enough to
be imaged. We present images made on two objects in Fig. 10.
These images were obtain by taking the chromatic parameters showing the
minimum χ2 value.
270 J. Kluska et al.
Figure 8.: The mosaic of chromatic parameters. In the x-axis : the spectral index
of the environment (denv). In the y-axis : the stellar to total flux ratio (f
0
∗
). The
good values are in the center. We can see how an error on a chromatic parameter
is affecting the image.
HD45677 is a Be star which is well resolved for the interferometer. We can
clearly see a ring like structure which is brightest in one of his sides. This is most
easily interpreted as a inclination effect.
The second object, HD98922, is also a Be star, supposed to be very young.
We can clearly see its ring in the image which can be interpreted as the dust
sublimation ring.
These images are preliminary and have to be interpreted more precisely.
But we can see that the image reconstruction method is able to retrieve a cor-
rect brightness distribution of these objects by assuming a correct value for the
environment temperature.
Imaging Young Stellar Objects with VLTi/PIONIER 271
Figure 9.: The χ2-map in the space of the chromatic parameters. There is a clear
χ2-valley showing a degeneracy between the two parameters.
Figure 10.: Left : an image reconstruction on HD98922. Right : an image
reconstruction on HD45677
4. Conclusion
We have presented a new method to perform image reconstructions that takes
into account the chromaticity of Young Stellar Objects. To do so it introduces
two chromatic parameters that are the stellar to total flux ratio and the spectral
index of the environment. This technique is adapted to the case where the flux
ratio between the star and its environment is changing across a bandwidth and
if it is the main effect.
Nevertheless this method is showing a degeneracy showing a χ2 valley in
the chromatic parameter space. This is why spectro-photometric information is
272 J. Kluska et al.
necessary to precisely separate the flux of the star of the environment one and to
correctly reconstruct the image.
We showed in the last section the ability of the method to reconstruct images
on actual dataset gathered by PIONIER/VLTi on Herbig Ae/Be Stars.
This method can be applied on other astrophysical objects showing a unre-
solved and a resolved component with different temperatures such as planetary
nebulae, AGB stars or Active Galactic Nuclei.
Acknowledgements. This work is supported by the French ANR POLCA project (Pro-
cessing of pOLychromatic interferometriC data for Astrophysics, ANR-10-BLAN-0511).
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