Issue |
A&A
Volume 511, February 2010
|
|
---|---|---|
Article Number | A74 | |
Number of page(s) | 14 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200912898 | |
Published online | 12 March 2010 |
Strong near-infrared emission in the
sub-AU disk of the Herbig Ae star
HD 163296: evidence of refractory dust?![[*]](/icons/foot_motif.png)
M. Benisty 1,2 - A. Natta1 - A. Isella3 - J-P. Berger2 - F. Massi1 - J-B. Le Bouquin4 - A. Mérand4 - G. Duvert2 - S. Kraus5 - F. Malbet2,3 - J. Olofsson2 - S. Robbe-Dubois6 - L. Testi7 - M. Vannier6 - G. Weigelt5
1 - INAF-Osservatorio Astrofisico di Arcetri, Largo
E. Fermi 5, 50125 Firenze, Italy
2 - Laboratoire d'Astrophysique de Grenoble, CNRS-UJF UMR 5571, 414 rue
de la piscine, 38400 St Martin d'Hères, France
3 - Caltech, MC 249-17, 1200 East California Blvd, Pasadena, CA 91125,
USA
4 - European Southern Observatory, Casilla 19001, Santiago 19, Chile
5 - Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121
Bonn, Germany
6 - Laboratoire A. H. Fizeau, UMR 6525,
Université de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 02,
France
7 - European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748
Garching, Germany
Received 15 July 2009 / Accepted 3 November 2009
Abstract
We present new long-baseline spectro-interferometric observations of
the Herbig Ae star HD 163296 (MWC 275)
obtained in the H and K bands
with the AMBER instrument at the VLTI. The observations cover a range
of spatial resolutions between 3 and
12 milliarcseconds,
with a spectral resolution of
30. With a total of 1481
visibilities and 432 closure phases, they represent the most
comprehensive (u,v) coverage
achieved so far for a young star. The circumstellar material is
resolved at the sub-AU spatial scale and closure phase measurements
indicate a small but significant deviation from point-symmetry. We
discuss the results assuming that the near-infrared excess in
HD 163296 is dominated by the emission of a circumstellar
disk. A successful fit to the spectral energy distribution,
near-infrared visibilities and closure phases is found with a model in
which a dominant contribution to the H and K band
emission originates in an optically thin, smooth and
point-symmetric region extending from about 0.1 to 0.45 AU. At
a distance of 0.45 AU from the star, silicates condense, the
disk becomes optically thick and develops a puffed-up rim, whose skewed
emission can account for the non-zero closure phases. We discuss the
source of the inner disk emission and tentatively exclude dense
molecular gas as well as optically thin atomic or ionized gas as its
possible origin. We propose instead that the smooth inner emission is
produced by very refractory grains in a partially cleared region,
extending to at least
0.5 AU.
If so, we may be observing the disk of HD 163296 just before
it reaches the transition disk phase. However, we note that the nature
of the refractory grains or, in fact, even the possibility of any grain
surviving at the very high temperatures we require (
2100-2300 K at
0.1 AU from the star) is unclear and should be investigated
further.
Key words: protoplanetary disks - methods: observational - techniques: interferometric - stars: pre-main sequence
1 Introduction
Herbig AeBe stars (HAeBe) are intermediate-mass young stars, surrounded by large amounts of dust and gas. The distribution of this circumstellar material remains actively debated. Various types of models can reproduce the spectral energy distribution (SED) by considering material in geometrically thin accretion disks (Hillenbrand et al. 1992), in a spherical envelope (Miroshnichenko et al. 1997), a puffed-up inner disk rim (Isella & Natta 2005; Dullemond et al. 2001) or a disk plus a halo (Vinkovic et al. 2006). Fitting the SED alone is therefore highly ambiguous.
Near-infrared (NIR) long baseline interferometry has allowed us to directly probe the properties of matter within the innermost astronomical unit (AU), where key quantities for the star-disk-protoplanets interactions are set. The first interferometric studies of HAeBe showed that the NIR characteristic sizes were larger than expected by classical accretion disk models (Millan-Gabet et al. 2001), and were found to be correlated with the stellar luminosity (Monnier & Millan-Gabet 2002). This supports the idea that the NIR emission is dominated by the thermal emission of hot dust heated by stellar radiation. Natta et al. (2001) suggested that an inner, optically thin cavity produced by dust sublimation exists inside the disk. At the edge of this region, where dust condensates, the disk is expected to puff up because of the direct illumination from the star (Isella & Natta 2005; Dullemond et al. 2001), explaining the size-luminosity law derived for Herbig Ae (and late Be) stars by Monnier & Millan-Gabet (2002). Based on a small number of interferometric observations, simple geometrical models were proposed to explain the global morphology of these regions (Millan-Gabet et al. 2001; Monnier et al. 2005; Eisner et al. 2004; Monnier et al. 2006). However, when larger sets of data became available, it became clear that the regions probed by NIR interferometry are much more complex and that a deeper understanding requires the combination of photometric and multi wavelength interferometric measurements at the milli-arcsecond resolution and more sophisticated models.
In this study, we present an analysis of the inner disk
surrounding
the HAe star HD 163296 (MWC275). This isolated
Herbig Ae star is described well by a spectral type of A1, a 30
luminosity,
and a
2.3
mass (Montesinos
et al. 2009; van den Ancker
et al. 1998; Natta et al. 2004). It is
is located at 122
+17-13 pc
and exhibits a NIR excess interpreted as the emission from a
circumstellar disk (Hillenbrand
et al. 1992). A
large-scale disk was detected in scattered light (Grady
et al. 2000), as well as at millimeter wavelengths (Mannings & Sargent 1997).
This inclined disk,
traced out to 540 AU, was found to be in Keplerian rotation,
and
probably evolving towards a debris disk phase (Isella
et al. 2007). In addition,
it also exhibits an asymmetric outflow on large scales (
27'')
perpendicular to the disk, with a chain of six Herbig-Haro knots
(HH409) that traces the history of mass loss
(Devine
et al. 2000; Wassell et al. 2006).
The spectrophotometric observations that probe the intermediate and
small spatial scales are also compatible with the presence of a disk. Doucet et al. (2006)
studied the warm dust emitting in the mid-infrared,
located in the surface layers of the intermediate regions of the disk
(30-100 AU) and concluded that the emission was consistent
with a disk
that has little flaring. This conclusion is consistent with the
classification of HD 163296 by Meeus
et al. (2001) in their Group II, whose
SED can be explained by assuming that the inner part of the disk
shields the outer part from stellar radiation. In the innermost
regions, the
far-UV emission lines have been attributed to optically thin gas
accreting onto the stellar surface, a magnetically confined wind, or
shocks at the base of the jet (Swartz et al. 2005; Deleuil
et al. 2005). Weak X-ray emission (
/
)
was detected on large scales and attributed to the jet (Günther & Schmitt 2009).
In the
NIR, the SED time variability was interpreted to be due to changes in
the inner disk structure, on timescales similar to the generation of
the HH objects (Sitko et al.
2008).
With a disk, signs of accretion, and a bipolar outflow, HD 163296 provides an excellent case study to understand how circumstellar material is distributed on the sub-AU scale. Its NIR disk was resolved by IOTA, PTI, and Keck-Interferometer (Monnier et al. 2005; Millan-Gabet et al. 2001; Eisner et al. 2009), and at milli-arcsecond (mas) resolution with the long CHARA baselines (Tannirkulam et al. 2008) (T08, hereafter). T08 found that their observations could not be reproduced using models where the majority of the K band emission originates in a dust rim, but that an additional NIR emission inside the dust sublimation radius could explain the visibilities and the SED. They interpreted this additional emission as being produced by gas, as suggested for other Herbig AeBe stars (Eisner et al. 2007; Isella et al. 2008; Kraus et al. 2008b). The advent of spectro-interferometry, as provided by the AMBER instrument at VLTI, allows us to simultaneously measure the emission at various NIR wavelengths and consequently, to derive temperature profiles for the emission, bringing additional constraints on its nature. In this paper, we present an observational study of the circumstellar disk around HD 163296 at the sub-AU scale, using the largest interferometric dataset obtained for a young star so far. The paper is organized as follows: in Sect. 2, we describe the spectro-interferometric observations obtained at AMBER/VLTI and the data processing; in Sect. 3, we present the obtained visibilities and closure phases. In Sect. 4, we outline a successful disk model that reproduces all observables and we discuss its physical origin. We summarize our results in Sect. 5.
Table 1: Log of the interferometric observations.
2 Observations and data reduction
2.1 Observations at VLTI
HD 163296 was observed in the NIR with the AMBER instrument
(Petrov et al. 2007),
at the Very Large Telescope Interferometer
(VLTI; Schöller 2007), located
at Cerro Paranal, Chile and operated
by the European Southern Observatory (ESO). The AMBER instrument allows
the
simultaneous combination of three beams in the H
and K bands
(i.e., from 1.6 to 2.5 m) with spatial filtering. The
instrument delivers spectrally dispersed interferometric observables
(e.g., visibilities, closure phases, differential phases) at
spectral resolutions of up to 12 000.
In the following, we present observations taken at the low
spectral
resolution mode ()
with the 1.8 m Auxiliary Telescopes (AT)
and the 8.2 m Unit Telescopes (UTs). The data were obtained
within programs of both guaranteed time and open time observations
(081.C-0794; 081.C-0098; 081.C-0124; 081.C-0851). HD 163296
was
observed with 14 different baselines of 5 VLTI telescope
configurations,
during 8 nights from May to July 2008. The longest baseline is
128 m
corresponding to a maximum resolution of
3.5 mas in the K band, and of
2.7 mas in the H band. In this
paper, we use the
VLTI nomenclature to identify the different configurations. A summary
of the observations can be found in Table 1, including the
weather conditions, the average baseline position angles (
), and projected lengths (
). The projected baseline is
obtained
when the vector between the two telescopes is projected onto the plane
of the sky. Because of the Earth rotation, measurements with the same
physical baseline but at different hour angles correspond to different
projected baselines.
All the observations were performed with three telescopes, except during the night of the 2008 May 26 when only two were available. In addition to HD 163296, three calibrator stars (HD 156897, HD 160915, HD 163955) were observed before and after each measurement on the scientific target to correct for instrumental effects. Their stellar parameters, including their diameters, can be found in Table 2. About 25% of the observations were performed using the VLTI fringe-tracker FINITO that uses 70% of the H band flux to measure the relative optical path difference between the light beams (Le Bouquin et al. 2008).
2.2 Photometry
In addition to this interferometric dataset, we collected photometric data from the literature (Tannirkulam et al. 2008; Sitko et al. 2008).
2.3 Data reduction
The interferometric data reduction was performed following Tatulli et al. (2007), using the amdlib package (release 2.1) and the yorick interface provided by the Jean-Marie Mariotti Center (JMMC). This led to spectrally dispersed raw visibilities and closure phases for all exposures of each observing file. Not all exposures turned out to provide useful data. In several cases, instrumental jitter, insufficient fringe tracking, and unsatisfactory light injection into the instrument led to low contrast interferograms of our rather faint source, and we had to select the good exposures. Various selection thresholds were examined based on the fringe signal-to-noise ratio (SNR) criterion and led to the same absolute values for the interferometric observables. On the other hand, their accuracy changes with varying selections, and the optimal case (i.e., with the smallest errors) was obtained with a 20% and 80% best exposure selections for the squared visibilities and the closure phases, respectively. In addition, data obtained at very high airmass with unstable fringe tracking were removed. No selection based on the optical path difference (i.e., piston) was performed, since the numbers of useful exposures could have been a possible source of bias.
Table 2: Star and calibrator properties.
![]() |
Figure 1: (u,v) plane coverage of the observations in spatial frequencies. The observing nights are plotted with different symbols and the corresponding telescope configurations are reported in the figure. |
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With 8 to 14 spectral channels in the K band
and 5 to 8 in the
H band, the total data set (after
processing) consists of 1000 and 304
spectrally dispersed K band visibilities
and closure phases,
respectively, and 481 and 128 H band
visibilities and closure
phases, respectively. The processed data will be made
available for the community in the OI-FITS format (Pauls
et al. 2005) on
the OLBIN
website in January 2010.
![]() |
Figure 2:
Squared visibilities against spatial frequencies B/ |
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![]() |
Figure 3:
Broad-band squared visibilities against spatial frequencies B/ |
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3 Results
The set of interferometric data presented in this paper is by far the largest for a single pre-main sequence star. The corresponding (u,v) plane coverage is shown in Fig. 1. In this section, we present a summary of the results and describe the main characteristics of the inner region of HD 163296 as measured by NIR interferometry. For the sake of clarity, we separate the H and K band results in most of the figures below.
3.1 Visibilities
Figure 2
presents the spectral visibilities as a function of the
spatial frequency (i.e., the ratio of the projected baseline
length to the wavelength of the observation). Figure 3
similarly shows broad-band visibilities, obtained when averaging over
all spectral
channels, for the K band (left) and the H band
(right). One can
immediately see from the figures that at these spatial resolutions, the
circumstellar
matter around HD 163296 is resolved. Within the error bars,
the H band
visibilities vary with baseline qualitatively in the same way as those
of the K band. However, at spatial
frequencies higher than 20 m/m, the H band
V2 are at least 25% higher
than those in the K band.
The errors in individual points vary significantly from night to night. However, some of the scatter is not caused by the uncertainties in individual measurements, since different observations were performed with baselines of similar projected baselines (equivalently, at similar spatial frequencies) but different position angles, therefore sampling distinct orientations on the sky. This effect means that the source geometry deviates from a circular one, and can instead present an elongated shape, as expected for an inclined disk.
We note that in the H band, at
the shortest spatial frequencies
(below 10 m/m),
the AMBER visibilities do not reach unity. This
can be due to flux from an extended halo as suggested by
Monnier et al. (2006)
from their IOTA H band measurements. All
the IOTA points, except one, are consistent with our
measurements (see Fig. 3,
right). On the other hand, this
effect is not clearly seen in the K band,
where visibilities are close to
unity at short baselines (e.g.,
at a 13.8 m baseline).
The observations show that V2
also depends on wavelength
(Fig. 2).
This can be related to the physical extension
of the emitting region at various wavelengths and the existence of
temperature gradients within it. Since the interferometer resolution
also changes with wavelength, it is natural to visualize this
dependence using a geometrical model to convert the measurements in
angular sizes while taking this effect into account. With this aim, we
fit the V2 in each spectral
channel for each measurement, using a
ring of uniform brightness (with a 20% thickness). Although the
circumstellar material is mainly responsible for the emission in the H
and K bands, the star also
contributes to the measured fluxes and visibilities. We estimate a
contribution from the stellar photosphere of 33% in H
and
14%
in K (see Sect. 4.1 for
details). The flux from the circumstellar matter was then computed for
each AMBER channel as the difference between the observed NIR flux
(T08) and the
photospheric flux. Figure 4
gives an example of the
wavelength dependence of the size obtained on the three baselines of
the
A0-D0-H0 configuration, in the two extremes cases (observed maximum and
minimum variations with wavelength).
For the majority of the measurements (except those obtained at
very
small baselines with the E0-G0-H0 configuration), the size of the
circumstellar matter slightly increases with wavelength. To quantify
the wavelength dependence of the visibility, we studied its variation
over only the K band, H band,
and over both bands together. Across the K
and H band separately, 24% and 31%,
respectively, of
the measurements show a chromaticity above the 2-
level. Over the entire spectral range, a stronger effect is expected
due to
the greater wavelength interval, 62% of the data show a
chromaticity greater than the 2-
level. This is
consistent with the case shown in Fig. 4, where the
characteristic size of the emitting region increases between the Hand
K bands. This trend is also observed in the
spectrally dispersed measurements across the Br
emission line obtained with the Keck-Interferometer (Eisner et al. 2009).
![]() |
Figure 4: Example of wavelength dependence for the squared visibilities obtained on the three baselines during observations with the A0-D0-H0 configuration. In each panel, the characteristic size of the emission is given at each wavelength across the H and K band. These sizes are derived from a ring model of uniform brightness to account for the change of resolution with wavelength. The triangles show the maximum chromaticity over the whole wavelength range, while the circles give the minimum variation. For each measurement, the corresponding projected baseline length and position angle are indicated in the top right corner. |
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Figure 5: Measured closure phases for each configuration (reported in the top left corners) plotted versus maximum spatial frequency. In all panels, except for U1-U2-U4 for which only K band data were useful, the measurements include both the H and K bands. |
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Figure 6: Broad-band closure phases plotted against the maximum spatial resolution achieved for all telescope configurations (K band, left; H band, right). The configurations and the corresponding symbols are reported at the bottom of the left panel. |
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3.2 Closure phases
The closure phase is a quantity that can be derived from interferometric observations with at least 3 telescopes. By combining up the phases of the fringes obtained with 3 telescopes, the atmospheric disturbances are cancelled out. Consequently, the sum of the three phases (the closure phase) is atmosphere-free, i.e., independent of the phase fluctuations. It is related to the degree of asymmetry of the observed brightness distribution: a point-symmetric object will have zero closure phases, while a non-zero measurement is indicative of a deviation from point-symmetry. The sign of the CP is derived from the way that it is calculated (clockwise or counterclockwise). Physically, the CP depends on the directions that are sampled by the individual baselines of the configuration since they probe different asymmetries of the brightness distribution, which in turn strongly depend on the ratio of the star to circumstellar matter flux.
One or more closure phase (CP) measurements were obtained for
each
observing night, except for the night of 2008 May 26.
In total, we
obtained 304 CP in the K band, and 128 in
the H band. This is an
enormous improvement over the existing datasets for a young star. The
full set of measured CP across the K and H bands
is shown in
Fig. 5,
plotted against the maximum spatial frequency
,
where
refers to the projected length of the longest baseline in the
corresponding configuration.
The CP signal depends on the spatial resolution achieved by
the
interferometer (unresolved sources are centro-symmetric).
Figure 6
show the broad-band CP in K and Hplotted
against the maximum spatial resolution
,
achieved with the corresponding
three-telescope configuration. It can be seen that the level of the
CP signal increases with the power of resolution (see, for example, the
configurations of aligned telescopes along the same
,
E0-G0-H0, and A0-D0-H0). Table 3
gives the corresponding broad-band CP averaged over all
the measurements as well as the achieved maximum spatial resolution
for each telescope configuration. Since we checked that all the CP were
calculated in the same way, the relative signs between measurements
obtained with
various configurations are relevant and should be reproduced by model
fitting.
The CP is close to zero for the short linear array (E0-G0-H0):
we do
not detect any meaningful deviations from zero (i.e., any flux
asymmetries) at these low spatial
resolutions (10.8 and 8.5 mas in the K and
H band, respectively).
Monnier et al. (2006)
detected CP signals below
for 12 Herbig AeBe stars with the IOTA-3T
interferometer, including HD 163296 for which they
derived
at a resolution of
11.8 mas. Our observations at
similar resolution provide comparable results.
At higher resolutions, we found that the CP signal is not
zero, i.e., that the emission is no longer centro-symmetric. However,
the departure of the closure phase signals from zero is small, with a
maximum average CP in K band of
and in H band
of
(see Table 3).
As we discuss in the following section, this level of asymmetry
is not compatible with strongly skewed distributions for the
circumstellar material in the innermost regions surrounding the star.
Except in one case (A0-K0-G1), the CP does not vary much with
varying
hour angle (see Fig. 12). This means that,
considering the
change in maximum resolution and position angle that occurs when
varying the hour angle (i.e., when changing the direction and
the projected baseline length ), the level
of asymmetry
does not change much. Consequently, the circumstellar matter must
have a rather smooth azimuthal brightness distribution.
The H band CP are slightly lower
than those measured in the K band,
as expected since the emission in H probably
originates in a more
compact region, as most of our data suggest, than the one emitting the K band
flux. However, considering the large error bars, this effect is
hardly significant. Over the K band and H band
separately, 12%
and 15% of the CP, respectively, show a variation above the 1-
level and all measurements are consistent with variations within the 2-
level. Considering the entire range of wavelength
(H and K together), 45% of the
CP show a variation with
wavelength above the 2-
level.
Table 3: Values of the closure phase as averaged over all the measurements.
4 Modelling the interferometry results
The analysis of visibilities and closure phases requires the assumption of a model for the brightness distribution on the plane of the sky to be compared with the observations. Even when the comparison is successful, it is impossible to know if the solution is unique, and because models are difficult to compute, it is practically impossible to explore all possibilities. Here, we are guided by the current paradigm that the NIR emission in excess of the photospheric one is produced by the inner parts of a circumstellar disk.
4.1 Stellar parameters
To model the interferometry results, one needs to know the unresolved
contribution of the star to the total flux at the wavelength of
interest. We compute it from the observed magnitude in the I band
(i.e., ,
with very little variability over a period of about 20 years (Tannirkulam
et al. 2008; de Winter et al. 2001),
assuming ZAMS colors for a A1 star and an extinction AV=0.25.
The resulting stellar fluxes are about 1.4 Jy in K
and 2.2 Jy in H. The observed NIR
fluxes display a moderate variability (Sitko
et al. 2008), so that the stellar contribution may
be in the interval 14-18% in K and 33-37%
in H, respectively. The effect of
variability on the interferometry results and the desirability of
performing simultaneous photometry were discussed by Sitko et al. (2008).
This, unfortunately, is practically impossible with AMBER/VLTI, and we
do not know the values of the total flux at the time of the
observations. In this paper, we adopt a stellar contribution of
14% and
33% to the
observed K and H band
fluxes, respectively, i.e., on the lower side of the estimates. These
numbers are in agreement with the CHARA and AMBER measurements at long
baselines, where all the circumstellar matter appears to be resolved,
and are similar to the values adopted by
Tannirkulam et al. (2008).
The HD 163296 SED is shown in Figs. 8 and 9.
![]() |
Figure 7:
Model visibilities when considering a rim alone, located
at 0.23, 0.36, and 0.45 AU (dashed red, dotted blue, and full
black
lines respectively), versus effective spatial frequency
|
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4.2 Incoherent flux
In our models, we consider incoherent flux possibly
emitted by an extended halo, as first suggested by Monnier et al. (2006).
They estimated its contribution to be 5% of the total
H band fluxes (i.e., 10% in V2),
a value that T08
also used when modelling their K band
data. Our H band data suggest a slightly
higher value of 8%
(i.e.,
15%
in V2). We adopt this value
when
fitting both the H and K band
interferometric observations. The
precise origin of this emission is unknown and discussing it is beyond
the scope of this paper.
4.3 Disk position angle and inclination
Values of inclination (i) and position angle (PA) of the HD 163296 disk (i.e., its major axis) have been derived at different wavelengths with a variety of techniques and tend to agree: Wassell et al. (2006) (











where


4.4 A disk rim
Early NIR interferometric studies of Herbig Ae stars have shown that standard accretion disks, extending up to the dust sublimation radius do not fit the observations (Millan-Gabet et al. 2007) and that superior fits are obtained by assuming that the disk develops a curved rim, probably (but not necessarily) controlled by dust sublimation and its dependence on gas density (Isella & Natta 2005; Tannirkulam et al. 2007). Not only the disk rim hypothesis is supported by physical calculations (Isella & Natta 2005; Dullemond et al. 2001): the overall properties of HD 163296 are also consistent with this model. The SED appears to be consistent with a rim emission up to 7-8

As in the NIR, the contribution of the disk outside the rim
can be neglected (Isella
et al. 2007), we begin by examining a star + rim
system to model the NIR emission following Isella
& Natta (2005). We adopt a stellar luminosity and
mass of 30
and 2.3
,
respectively, and an effective temperature of 9250 K. The disk
is assumed to be in hydrostatic equilibrium. The dust consists of
silicates with optical properties given by
Weingartner & Draine
(2001). The evaporation temperature, of around
1500 K, depends on the local gas density as in Pollack et al. (1994).
Since the shape of the rim is controlled by the largest grains, we
consider a single size for the silicate dust, which is therefore the
only free parameter in the model. The dependence of the evaporation
temperature on z implies that the distance from the
star at which dust evaporates increases with z,
i.e., that the rim is curved.
As can clearly be seen in Fig. 7, our
interferometric measurements are inconsistent with a circumstellar
emission produced by a rim only, regardless of its location. This
figure
shows the AMBER broad-band visibilities as a function of the effective
spatial frequency .
In addition to the
AMBER data, we include the K band
visibilities obtained at CHARA
(circles; Tannirkulam
et al. 2008) and at Keck-I (triangles; Monnier et al. 2005) as
well as the
H band IOTA data (upside down triangles; Monnier et al. 2006).
We emphasize that all measurements from
these four interferometers are compatible with each other within their
error bars. The results for three rim models computed by assuming a
30
stellar luminosity and silicate grain sizes of 3,
0.6, or 0.3
m,
corresponding to inner rim radii at
0.23, 0.36, and 0.45 AU, respectively, are overplotted. These
models
produce an emission that does not exceed the observed flux at any
wavelength, and, contributes respectively, to about 80, 50, and 28% of
the observed emission in H, and 91, 86, and 66% in K.
All models
show large departures from the observations. In addition, all these
models have very asymmetric emission, and produce a closure phase
signal greater than observed (see Fig. 12, red dashed
lines). Our large data sets, which include H and K
visibilities
and closure phases, reinforce the conclusions of other authors, namely
that there should be an additional, symmetric emission closer to the
star (T08).
![]() |
Figure 8: SED of HD 163296 (Tannirkulam et al. 2008; Sitko et al. 2008) with the predictions of gaseous disk models. The flux emitted by the rim located at 0.45 AU (cyan dashed line) as well as the additional inner disk emission (black dotted line) are added in both panels to allow direct comparisons. Left: the predictions for optically thick and dense layers of gas in LTE (Muzerolle et al. 2004) are overplotted (red full line). In this case, the gas is mostly in a molecular state. Right: the continuum emission as predicted by thin disks of gas in non-LTE, heated from the top by the stellar radiation, are shown. The gas extends from 0.1 to 0.45 AU and has constant surface densities of 0.1, 1, and 6 g/cm2 (red line with squares, blue line with triangles and green line with diamonds, respectively). In these conditions, the gas is mostly atomic. The predictions of a fully ionized layer of gas at 8000 K with a constant surface density of 0.06 g/cm2 are also reported (magenta curve with diamonds). As long as the disk remains optically thin, the emission is proportional to the surface density, and this value was chosen only to display its wavelength dependence. |
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4.5 A bright inner disk
A rim alone cannot reproduce our observations. As a matter of fact, the lack of bounces in the visibilities suggests a continuous and smooth distribution of matter in the inner regions of HD 163296, and a low contribution of the rim to the NIR emission. In the following, we explore the effect of adding emission from an inner disk (inside the rim) to both the star and rim contributions. We fix the rim location to be




We obtain a first estimate of the emission properties of the
inner disk by modeling it as a region of constant surface brightness
between an inner and outer radius. The surface brightness is
constrained by the condition that the integrated flux must be equal to
the flux derived from the SED for the additional component. We compute
visibilities and find that these three-component models reproduce the H
and K band observations quite well over
the entire range of baselines if the smooth, inner emission extends
between 0.10
and
0.45 AU.
The emission can be roughly described as that of a diluted black-body
with temperature
1600 K
and optical depth
0.2,
which decreases as
with increasing wavelength. A
natural interpretation is that the
emission originates in an optically thin region inside the rim. In the
following sections, we discuss the physical nature of this component.
![]() |
Figure 9: Left: radial temperature profiles for graphite (green line with triangles), corundum (red line with crosses), or iron (blue line with squares). The prediction from a black body (i.e., large grains) is overplotted to allow comparisons. The vertical dotted line indicates 0.10 AU. Right: SED of HD 163296 together with the predictions of three models that include an inner disk made of dust in addition to the star and the rim. The additional disk component has structural characteristics that are reported in Table 5 and is made of a single refractory species - either graphite, corundum, or iron. The dotted lines represent the corresponding emission, while the full lines show the total flux. |
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4.5.1 An inner gaseous disk?
Several authors have suggested that NIR interferometers detect the emission of gas within the dust evaporation radius (Isella et al. 2008; Eisner et al. 2007; Kraus et al. 2008b; Tannirkulam et al. 2008). This interpretation presents several difficulties in the case of HD 163296, where the additional component dominates the emission in H and K, and where good observations of high spectral resolution exist over a wide range of wavelengths.
Models of the emission of purely gaseous disks inside the dust
sublimation radius were computed by Muzerolle
et al. (2004) for HAe
stars assuming LTE opacities. For typical HAe accretion rates (
yr-1),
the inner, dust-free, disk gas surface density is higher than 103 g/cm2,
the gas temperature
ranges from a few thousand to a few hundred K, and the gas is
fully
molecular. The NIR emission is sufficiently strong to, in
principle, account for the observations. However, as has been pointed
out by
several authors, the models also predict many strong molecular
bands (mostly water and CO overtone transitions) that are absent in
the HD 163296 spectrum (see Fig. 8, left)
(Najita
et al. 2000,2007). This problem is also
discussed in
Najita et al. (2009),
in connection with the non-detection of CO overtone
and water emission in the Herbig Ae star, MWC480, that also
exhibits a hot
compact NIR excess previously detected with interferometry and
interpreted as resulting from water (Eisner
2007).
The assumption that the gas is in LTE, however, is certainly
inappropriate
at least in the upper disk layers, where the stellar radiation
can penetrate, ionize, and dissociate matter well above the LTE
predictions.
However, these thin gas layers are unlikely to contribute significantly
to the
broadband observed fluxes. We computed the emission from thin layers of
gas, using the code Cloudy (Ferland
et al. 1998). We assumed that the region
inside the rim can be described as a geometrically thin disk, heated
from the outside by the star. The disk extends from 0.1 to
0.45 AU and has a constant surface density. In these
conditions, as long as the disk is optically thin to the
stellar radiation, the gas is mostly atomic and the ionization
fraction is low (e.g., <0.01). In the NIR, H-
dominates
the emission. We show in Fig. 8,
right,
that for surface density values of 0.1, 1, and 6 g/cm2
(all much lower than predicted by the viscous accreting disks modelled
by Muzerolle et al. 2004), the continuum
emission is always too weak to be significant. Increasing it further
would require higher surface densities, in which case LTE conditions
would very likely be reached. In fact, for g/cm2,
the mean
optical depth to the stellar radiation is already of order unity.
Moreover, we note that the wavelength dependence of the non-LTE
continuum is inconsistent with the observations. In particular, the
H-bound-free emission produces a sharp drop at
1.6
m
corresponding to its activation energy, which would be
seen in the HD 163296 high resolution spectra (Sitko et al. 2008) if the
gas emission was higher.
T08 suggested that hot gas is the physical origin of the
additional inner component. We note that their models cannot reproduce
our AMBER/VLTI data, mostly because of their small inner rim radius.
However, we examined the possibility that the gas is heated to higher
temperatures by additional energy sources. We computed the properties
of the same disk model but after fixing the temperature to
8000 K. In this case, the ionization fraction is high (0.5) and the
emission is dominated by bound-free processes. The emission can be very
high, but its wavelength dependence is inconsistent with the
observations (see Fig. 8,
right).
Based on these crude considerations, we tend to exclude that
the NIR flux detected by the interferometers is dominated by the
emission of hot gas inside the dust sublimation radius. However, it is
clear that, before this can be definitely ruled out, one needs more
realistic, non-LTE models that treat the transition from optically thin
to optically thick layers, i.e., from atomic to molecular gas in a
dust-free environment. These models would also be important for the
interpretation of the hydrogen and helium recombination lines, which
appear very strong in the models. Atomic lines - mostly hydrogen and
helium ones - are often interpreted as being emitted in magnetospheric
accretion columns of gas. This, however, is unlikely to be true for
most Herbig Ae stars, based on the results obtained with
spectro-interferometry around the Br
emission line (Kraus
et al. 2008a; Eisner et al. 2009),
since this line seems to be formed in most cases, in disk material
closer to the star than the silicate dust sublimation radius but
outside the corotation radius.
![]() |
Figure 10: Left: visibilities produced by our model that assumes that iron grains are responsible for the additional dusty inner disk emission, in K band ( left) and H band ( right) compared to the observations. The AMBER/VLTI observations (blue full squares) as well as the Keck-I (triangles) and the CHARA (circles) are added. The IOTA H band data (upside down triangles) are plotted together with the AMBER visibilities. |
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4.5.2 An inner dusty disk?
Inside the silicate sublimation radius, not only gas but also
more refractory grains can exist and contribute to the observed NIR
emission. In this section, we explore the possibility that a layer of
refractory grains, extending between an inner and an outer radius
(inside the rim), accounts for the interferometric and photometric
observations of HD 163296. We assume that the layer is
optically thin in the vertical direction with surface density profile
and that its vertical optical depth is proportional to
1/r, where r is the distance
from the star (D'Alessio
et al. 1999). We compute grain temperatures and
emissivity in the H and K bands,
and vary the inner and outer radii (
and
,
respectively) as well as the optical depth of
the layer. We then computed the emission, visibilities, and closure
phases for models that include this inner layer, the star, and the rim.
We consider separately three grain species known to be refractory:
iron, graphite, and corundum (aluminium oxides). The iron and corundum
grain cross-sections are computed for spherical grains from the optical
constants tabulated in Pollack
et al. (1994), Koike
et al. (1995), and Begemann
et al. (1997). We use the graphite cross
sections tabulated by B. Draine
,
based on the optical constants of Laor
& Draine (1993). For relatively small grains, the
radial temperature profile follows
(Fig. 9,
left). For comparison, we also show the temperature profile of very
large grains (which behave as blackbodies), which have flat opacity
from the UV to the IR (
).
These models are very simple, but probably not unrealistic.
The strongest approximation concerns the dust temperature, which we
compute by assuming that each grain is heated by the stellar radiation,
and attenuated by an average optical depth
.
In fact, the attenuation is not constant, since the optical path toward
any individual grain varies not only with r but
also with the incident angle of the stellar radiation. Once the
temperature of the grains is known, then the emission is computed at
all NIR wavelengths using a ray-tracing algorithm. As discussed in the
following, a rim caused by silicate
condensation can form in the low density region that we propose and its
properties may also be affected by refractory grains in the inner disk,
which absorb a small fraction of the stellar radiation. However, a
self-consistent calculation of the rim properties is beyond the purpose
of this paper. In this section, we model the rim following Isella & Natta (2005) and
Isella et al. (2006),
assuming micron-size silicates, an evaporation temperature of
1250 K,
and an effective stellar luminosity of 75%
to account for the shielding by the inner disk. The rim radius is about
0.45 AU and its effective temperature is about
1000 K.
Table 4: Refractory dust model parameters.
We obtain a good fit to the SED, as shown in Fig. 9 (right), for






![]() |
Figure 11: Model images of the circumstellar matter surrounding HD 163296, in the case where the inner disk is consists of refractory iron grains and a rim (outer ring). The colors represent the flux in W s/m2. |
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The gas density of the inner disk can be derived from the
values of
,
once the abundances of iron, carbon, and aluminium
in the solid species are known. Assuming, for example, that 50% of
the iron is in grains, the gas surface density at
0.10 AU will be 0.2 g/cm2
(0.02 g/cm2 if 30% of carbon is in
graphite, or 1.3 g/cm2 if all aluminium
is in corundum). The gas
density can be higher if a lower fraction of the metals is condensed.
However, it seems likely, from the considerations of
Sect. 4.5.1, that the gas density cannot be too high. A
density only a few times higher than the above lower limits is inferred
by the properties of the rim in HD 163296. In particular, both
the large rim radius and its low effective temperature can be
reproduced in a low density disk by silicates of micron size, as
inferred in several HAe stars (Isella
et al. 2006), and do not require very small grains.
Assuming that the rim is produced by the evaporation of silicates and
that all the silicon is in olivine of
m size, we analyzed the rim
properties in disks of increasing (but still low) surface density. For
g/cm2,
the vertical optical depth to the stellar radiation
is
10 for a gas
surface density of 1 g/cm2 at
, large
enough to allow the formation of an optically thick rim that can be
modelled following Isella &
Natta (2005). The low gas density, and the correspondingly
low evaporation temperature, moves the rim radius further from the
star. Assuming a reasonable scale height of
10-3-10-2 AU,
the gas density is
10-11 g/cm3,
and
the silicate evaporation temperature of the order of 1150 K.
The
corresponding rim radius is
0.4-0.5 AU,
and its effective
temperature is about 1000-1100 K, as required to fit the
HD 163296
SED and interferometric data.
The only difficulty in assuming that the inner disk emission
originates in grains within the silicate sublimation
radius, is the need for them to survive at temperatures much higher
than is generally assumed. The three types of grains that we
examined reach temperatures of 2100-2300 K at 0.10 AU
(Fig. 9,
left). While similar values are possible for graphite (Krugel 2003), they are too high
for both iron and corundum in the pressure of our inner disk (Pollack
et al. 1994; Kama et al. 2009; Posch
et al. 2003). However, there is room for discussion (Najita et al. 2009), as
the balance between gas and dust in the conditions of the inner disk
should be reconsidered in detail (Duschl
et al. 1999). We emphasize that only a small amount
of refractory grains need to survive these high
temperatures, i.e., probably a minor fraction of the original
population. The grains that we have considered are likely candidates,
but if other, more refractory species can form, they would certainly
fit the observations equally well.
![]() |
Figure 12: Closure phases versus hour angle. In each panel, the predictions of the disk rim model (dashed red lines) and our model that includes iron grains (full black line) are overplotted. The corresponding telescope configuration and wavelength band are indicated in the upper, left corners. The K band and H band measurements are plotted with circles and triangles, respectively. |
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5 Summary and conclusions
This paper had discussed the largest set of NIR interferometric data
collected so far for a young star. HD 163296 has a well
studied disk at large spatial scale, which motivated our interpretation
of the NIR interferometry using a star + inner disk model. Both
interferometric and photometric data can be accounted for by an
inclined disk with a low density inner region. The NIR continuum
emission is then not dominated by the thermal emission from the dusty
disk rim located at the sublimation radius of astronomical silicates,
but by the additional optically thin component located inside. This
component emits about 32% of the stellar luminosity, but 54% (50%) of
the observed radiation in H (K) band.
Because of the silicate condensation and the strong increase in
opacity, the disk rim forms in this low-density region at 0.45 AU
from the star and emits about 16% (36%) of the H (K) band
flux. The combination of the unresolved stellar radiation, the smooth
and point-symmetric emission of the inner disk region, and the skewed
disk rim emission can successfully explain the visibilities and
non-zero closure phases measured in both bands.
The nature of the emission in the inner disk remains a matter of discussion. We argue against gas being mainly responsible for this continuum emission. A dense, cold disk, as expected for viscous accretion models (Muzerolle et al. 2004), would produce strong molecular lines that are not seen in high-resolution spectra. Non-LTE tenuous gas layers, in an atomic state if only heating by the star is assumed, or fully ionized by additional sources of energy, cannot account for the observed properties of the NIR continuum. However, we emphasize that self-consistent models of dust-free gaseous disks are not currently available, but are needed to exploit the full potential of the interferometric observations.
We suggest instead that a small fraction of refractory grains
survive very
close to the star. We propose models for the optically
thin emission of the innermost region, using various kinds of
refractory grains, distributed from 0.10 AU to
0.45 AU.
The dust surface density provides only a lower limit to the gas surface
density, as we do know neither the exact nature of the
grains nor their abundance. However, we find that a low density region
is consistent with the location and properties of the rim, as
condensation of silicates will occur, and with the lack of molecular
features in the spectrum of HD 163296. We expect the gas in
the inner disk indeed to be mostly atomic, in non-LTE, and although its
continuum emission will be weak, hydrogen lines can be strong. The
models used in Sect. 4.5.2 to argue for the presence of
refractory grains, make a number of crude assumptions, and improved
models that self-consistently compute the grain temperature and
emission in the thin disk as well as the properties of the rim are
being developed. Our study indicates that the inner region of
HD 163296 is quite empty, with a very low surface density that
is inconsistent with a dense accretion disk. For comparison, a surface
density of 1 g/cm2 at 0.10 AU
corresponds, in a standard accretion disk (
), to an accretion rate of
yr-1,
much lower than typical values for Herbig Ae stars (Garcia Lopez et al.
2006). With our data, we do not constrain the outer radius of
this low density region, which can be larger than 0.5 AU.
However, we know that at large radii the HD 163296 disk is
massive and dense, as shown by the millimeter interferometric
observations of Isella
et al. (2007). It seems likely that
HD 163296 has a dense disk with an inner cavity, and that we
observe it just before it reaches the transition disk phase, as
suggested by Sitko et al.
(2008).
Table 5: Parameters of our model with iron grains.
It is fair to emphasize that we make no claim that our interpretation is unique. As stated at the beginning of Sect. 4, we assumed that the NIR emission of HD 163296, at spatial scales of less than 0.5 AU, is dominated by the emission of a circumstellar disk. Moreover, we interpreted the non-zero values of the CP as evidence of the asymmetric emission of a disk rim. While the properties that we derived for the smooth inner emission are probably robust, the existence of the rim is less so. In particular, the lack of visibility bounces at large baselines argues against the presence of a rim. In this case, the observed closure phases may possibly be caused by any asymmetric brightness distribution, such as a symmetric flared disk with a stellar contribution that is off-centered by a few percent of the inner disk radius with respect to the disk (Malbet et al. 2001), by a hot spot on the disk, or by a density discontinuity. Only a larger (u,v) coverage, providing access to more details of the morphology could solve this ambiguity.
Our interpretation of the smooth, inner emission as
originating in refractory grains requires their survival at very high
temperatures (
2100-2300 K), much
higher than expected, even for the most refractory grains, at the
pressure of the low-density inner disk (Pollack et al. 1994; Kama et al.
2009). However, clearing the innermost regions and optically
thin emission from left-over refractory grains could be a common
phenomenon in Herbig Ae stars. Their presence within the first
few tenths of an AU is a promising interpretation of the observed
depletions in refractory dust species - such as iron - in jets of young
stars that are launched from this region (Nisini
et al. 2005; Podio et al. 2006).
Similar interferometric studies, with a large number of measurements in
various wavelength bands simultaneously, should be performed
for a large sample of stars. A higher level of complexity in models is
also needed to account for both the dust and the gas emission in a
self-consistent way. Finally, the advent of the next-generation of
imaging instruments will hopefully provide unambiguous constraints on
these complex environments.
We acknowledge fundings from CNRS and INAF (grant ASI-INAF I/016/07/0). This work was in part performed under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Michelson Fellowship Program. JPL is managed for NASA by the California Institute of Technology. We thank A. Sargent and T. Ray for hosting part of this research, and the VLTI team at Paranal for the help in obtaining these data. We thank J.D. Monnier who provided with the IOTA data, and M. Sitko for discussions about the photometric measurements. We are greatful to A. Crida, M. Desort and S. Renard for fruitful discussions. We acknowledge the anonymous referee for his comments that improved the clarity of the paper.
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Footnotes
- ... dust?
- Based on AMBER observations collected at the VLTI (European Southern Observatory, Paranal, Chile) with Arcetri Guaranteed Time program 081.C-0124, LAOG Guaranteed Time program 081.C-0794 and open time programs 081.C-0851, 081.C-0098.
- ...
website
- http://olbin.jpl.nasa.gov
- ... B. Draine
- http://www.astro.princeton.edu/ draine/dust/dust.diel.html
All Tables
Table 1: Log of the interferometric observations.
Table 2: Star and calibrator properties.
Table 3: Values of the closure phase as averaged over all the measurements.
Table 4: Refractory dust model parameters.
Table 5: Parameters of our model with iron grains.
All Figures
![]() |
Figure 1: (u,v) plane coverage of the observations in spatial frequencies. The observing nights are plotted with different symbols and the corresponding telescope configurations are reported in the figure. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Squared visibilities against spatial frequencies B/ |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Broad-band squared visibilities against spatial frequencies B/ |
Open with DEXTER | |
In the text |
![]() |
Figure 4: Example of wavelength dependence for the squared visibilities obtained on the three baselines during observations with the A0-D0-H0 configuration. In each panel, the characteristic size of the emission is given at each wavelength across the H and K band. These sizes are derived from a ring model of uniform brightness to account for the change of resolution with wavelength. The triangles show the maximum chromaticity over the whole wavelength range, while the circles give the minimum variation. For each measurement, the corresponding projected baseline length and position angle are indicated in the top right corner. |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Measured closure phases for each configuration (reported in the top left corners) plotted versus maximum spatial frequency. In all panels, except for U1-U2-U4 for which only K band data were useful, the measurements include both the H and K bands. |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Broad-band closure phases plotted against the maximum spatial resolution achieved for all telescope configurations (K band, left; H band, right). The configurations and the corresponding symbols are reported at the bottom of the left panel. |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
Model visibilities when considering a rim alone, located
at 0.23, 0.36, and 0.45 AU (dashed red, dotted blue, and full
black
lines respectively), versus effective spatial frequency
|
Open with DEXTER | |
In the text |
![]() |
Figure 8: SED of HD 163296 (Tannirkulam et al. 2008; Sitko et al. 2008) with the predictions of gaseous disk models. The flux emitted by the rim located at 0.45 AU (cyan dashed line) as well as the additional inner disk emission (black dotted line) are added in both panels to allow direct comparisons. Left: the predictions for optically thick and dense layers of gas in LTE (Muzerolle et al. 2004) are overplotted (red full line). In this case, the gas is mostly in a molecular state. Right: the continuum emission as predicted by thin disks of gas in non-LTE, heated from the top by the stellar radiation, are shown. The gas extends from 0.1 to 0.45 AU and has constant surface densities of 0.1, 1, and 6 g/cm2 (red line with squares, blue line with triangles and green line with diamonds, respectively). In these conditions, the gas is mostly atomic. The predictions of a fully ionized layer of gas at 8000 K with a constant surface density of 0.06 g/cm2 are also reported (magenta curve with diamonds). As long as the disk remains optically thin, the emission is proportional to the surface density, and this value was chosen only to display its wavelength dependence. |
Open with DEXTER | |
In the text |
![]() |
Figure 9: Left: radial temperature profiles for graphite (green line with triangles), corundum (red line with crosses), or iron (blue line with squares). The prediction from a black body (i.e., large grains) is overplotted to allow comparisons. The vertical dotted line indicates 0.10 AU. Right: SED of HD 163296 together with the predictions of three models that include an inner disk made of dust in addition to the star and the rim. The additional disk component has structural characteristics that are reported in Table 5 and is made of a single refractory species - either graphite, corundum, or iron. The dotted lines represent the corresponding emission, while the full lines show the total flux. |
Open with DEXTER | |
In the text |
![]() |
Figure 10: Left: visibilities produced by our model that assumes that iron grains are responsible for the additional dusty inner disk emission, in K band ( left) and H band ( right) compared to the observations. The AMBER/VLTI observations (blue full squares) as well as the Keck-I (triangles) and the CHARA (circles) are added. The IOTA H band data (upside down triangles) are plotted together with the AMBER visibilities. |
Open with DEXTER | |
In the text |
![]() |
Figure 11: Model images of the circumstellar matter surrounding HD 163296, in the case where the inner disk is consists of refractory iron grains and a rim (outer ring). The colors represent the flux in W s/m2. |
Open with DEXTER | |
In the text |
![]() |
Figure 12: Closure phases versus hour angle. In each panel, the predictions of the disk rim model (dashed red lines) and our model that includes iron grains (full black line) are overplotted. The corresponding telescope configuration and wavelength band are indicated in the upper, left corners. The K band and H band measurements are plotted with circles and triangles, respectively. |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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