A&A 435, 275-287 (2005)
DOI: 10.1051/0004-6361:20041954
O. Chesneau 1,2 - A. Meilland 2 - T. Rivinius 3 - Ph. Stee 2 - S. Jankov 4 - A. Domiciano de Souza 5 - U. Graser 1 - T. Herbst 1 - E. Janot-Pacheco 6 - R. Koehler 1 - C. Leinert 1 - S. Morel 7 - F. Paresce 7 - A. Richichi 7 - S. Robbe-Dubois 4
1 - Max-Planck-Institut für Astronomie, Königstuhl
17, 69117 Heidelberg, Germany
2 -
Observatoire de la Côte d'Azur, CNRS UMR 6203, Avenue Copernic,
Grasse, France
3 -
Landessternwarte Heidelberg, Königstuhl 12, 69117 Heidelberg,
Germany
4 -
Laboratoire Universitaire d'Astrophysique de Nice, France
5 -
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69,
53121 Bonn, Germany
6 -
Instituto de Astronomia, Geofisica e Ciências Atmosféricas
da Universidade de São Paulo (IAG-USP),
CP 9638, 01065-970 São Paulo, Brazil
7 - European Southern
Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching,
Germany
Received 6 September 2004 / Accepted 5 January 2005
Abstract
We present the first VLTI/MIDI observations of the Be
star alpha Ara (HD 158 427), showing a nearly unresolved circumstellar disk
in the N band. The interferometric measurements made use of the UT1
and UT3 telescopes. The projected baselines were 102 and
74 meters with position angles of 7
and 55
,
respectively. These measurements put an upper limit on the envelope
size in the N band under the uniform disk approximation of
mas, corresponding to
14
,
assuming
and the
Hipparcos distance of 74 pc.
On the other hand the disk density must be large enough to produce the observed strong Balmer line emission. In order to estimate the possible circumstellar and stellar parameters we have used the SIMECA code developed by Stee et al. (1995, A&A, 300, 219) and Stee & Bittar (2001, A&A, 367, 532). Optical spectra taken with the échelle instrument H EROS and the ESO-50 cm telescope, as well as infrared ones from the 1.6m Brazilian telescope were used together with the MIDI spectra and visibilities. These observations place complementary constraints on the density and geometry of the alpha Ara circumstellar disk. We discuss the potential truncation of the disk by a companion and we present spectroscopic indications of a periodic perturbation of some Balmer lines.
Key words: techniques: high angular resolution -
techniques: interferometric -
stars: emission-line, Be -
stars: winds, outflows -
stars: individual:
Ara -
circumstellar matter
The thermal infrared domain is an important spectral region where
the transition between the optically thin and thick disk occurs.
The optically thick disk emission increases from 7 to 15 m.
It reaches about half of the total continuum flux at 8
m and
dominates the Spectral Energy Distribution (SED) almost completely
at 15-20
m (Gehrz et al. 1974). The flux from the
central star at these wavelengths is 10 to 50 times fainter
compared to
whereas the ratio
reaches a factor of 4-10. In particular for nearly
edge-on disks, the flux from the optically thick disk is very
sensitive to the inclination angle, since it is proportional to
the emitting surface.
Observations at 20 m by Gehrz et al. (1974)
demonstrated that the mid-IR excess is due to free-free emission
from the hydrogen envelope. The ad-hoc model from Waters
(1986) has been successful explaining near- and far-IR
observations and is coherent with polarization data (Coté &
Waters 1987; Waters & Marlborough 1992;
Dougherty et al. 1994; Coté et
al. 1996). In this model the IR excess originates from
a disk with an opening angle
and a density distribution
seen at an observer's angle i. From
IRAS data of four Be stars Waters found that the far-IR slope of
the SED is not strongly influenced by the opening angle, as long
as
.
Moreover, he found that n=2.4 gives
a good agreement for
Oph,
Cen and
Per if
the disk is truncated to 6.5 R*.
Interferometry in the visible has also been used to study the
circumstellar environment of Be stars (Thom et al. 1986;
Mourard et al. 1989; Quirrenbach et
al. 1994; Stee et al. 1995).
Quirrenbach et al. (1997) gave an upper limit on the
opening angle of about 20
and using spectral Differential
Interferometry (DI), structures within Be disks were monitored
with a high spatial resolution over several years (Vakili et al.
1998; Berio et al. 1999). Nevertheless,
the disk extension at IR wavelengths is still subject of an active
debate since various authors give quite different extensions as a
function of wavelength. For instance Waters (1986) found
a typical IR extension of
from 12, 25 and
60
m observations, Gehrz et al. (1974) found
at 2.3 and 19.5
m, Dougherty et al.
(1994) a disk size larger than
in the
near-IR, and finally Stee & Bittar (2001) obtained that the
Br
emission line and the nearby continuum originates from
a very extended region with a typical size of
.
In order to solve this issue, we have used the VLTI/MIDI
interferometer to obtain the first IR measurements in the N spectral band of a Be star. The selected target,
alpha Ara (HD 158 427, HR 6510, B3 Ve), is one of
the closest Be stars with an estimated distance of pc
derived from the Hipparcos parallax, and is well known as an
emission line star since the discovery of its H
emission
by Pickering (1896, 1896). Alpha Ara is a
2.9 mag star in the Johnson V-band and its IRAS flux at 12 micron is 12.7 Jy. Its color excesses E(V-L) and
are respectively 1.8 and 2.23, being among
the highest of its class. The projected rotational velocity
has been estimated to be 250-300 km s-1 (Yudin
2001; Chauville et al. 2001). An
intrinsic linear polarization of
with a Position
Angle (PA) of 172
has also been detected (McLean & Clarke
1979; Yudin 2001). Since the disk
orientation is expected to be perpendicular to this direction, we
expect the disk major-axis to be around
(Wood
et al. 1996a; Quirrenbach et al.
1997).
The spectral type definition of alpha Ara is constant in the
literature ranging from B2 Ve to B3 Ve. We adopted B3 Ve
following the latest study of Chauville et al. (2001).
The typical stellar radius and effective temperature for this
spectral type are 4.8
and
respectively.
By using IRAS observations of alpha Ara, Waters
(1986) estimated the outer disk radius is estimated as
about 7 ,
i.e. 2.1 mas at a distance of 74 pc. Stee
(2003) predicts the visibility of alpha Ara at 2
m
to be lower than 0.2 with a 60 m baseline, i.e. fully resolved.
Using the same model parameters, a significant visibility loss
should still be detectable in the N band. This paper will show
that at 10
m alpha Ara is in fact unresolved which,
combined with spectroscopic data, puts some constraints on
density, inclination angle, disk orientation and distance of
alpha Ara.
The paper is organized as follows. In Sect. 2 we
present the interferometric MIDI observations and the H-band
spectroscopy. Section 3 briefly describes the SIMECA
code and the envelope parameters used for the modeling of
alpha Ara. The modeling strategy is described in detail. In
Sect. 4 we present our theoretical results and
discuss the possibility for a close but unseen companion to
truncate the disk making alpha Ara unresolved in the N band.
Section 5 presents some spectroscopic
variations of the H
line profile that may be further
evidence of this hypothetical companion. Finally, Sect. 6 draws the main conclusions from these first IR
interferometric measurements of a Be star.
Table 1: Journal of observations.
On the 16th of June 2003, alpha Ara was observed with a 102 m
baseline and a PA = 7
;
on June, 17 the projected baseline was
79 m and PA = 55
.
The observing sequence described extensively in Przygodda et al.
(2003) is summarized hereafter. The chopping
mode (f=2 Hz, angle
)
is used to point and visualize the
star. The number of frames recorded by acquisition file are
generally 2000, the exposure time is by default 4 ms in order to
avoid background saturation. If the result of the acquisition
sequence is not satisfactory, this procedure is re-executed. Then,
the MIDI beam combiner, the wide slit (0
), and the NaCl prism are inserted to disperse
the light and search for the fringes by moving the VLTI delay
lines. The resulting spectra have a resolution
/
.
When searching for the fringe signal, the delay line of the VLTI is moved, while the MIDI internal piezo-driven delay line performs additional scans of the fringe pattern. Once the fringes are found another file is recorded while MIDI tracks the fringes on its own, i.e. by performing a real time estimation of the OPD based on the data continuously recorded. The correction is sent to the VLTI delay lines at a rate of about 1Hz. Finally, the photometric data are recorded in two more files.
In the first file, only one shutter is opened, corresponding to the calibration of the flux from the first telescope, here UT1, and the flux is then divided by the MIDI beam splitter and falls on two different regions of the detector. The total flux is determined separately by chopping between the object and an empty region of the sky, then the source flux is computed by subtraction. Once it has been done, the same procedure is carried out again for the beam arriving from UT3.
We performed the spectral reduction and fringe calibration using a
software developed at the Max-Planck Institut für Astronomie
in Heidelberg, written in IDL. The reader is
referred to Leinert et al. (2004) for an extensive
discussion on MIDI data reduction and error handling.
The photometric datasets used for the calibration of the contrast of the dispersed fringes are also used for the spectrum flux calibration. The frames obtained on the target and the ones obtained on the sky are averaged. Then, the averaged sky frame is subtracted from the averaged target frame. In the resulting integration, the spectral axis is oriented along the horizontal detector axis. For each detector column, a Gaussian function is fitted to the peak. The position of the spectrum in all illuminated columns, as a function of the column number, is fitted by a quadratic polynomial. In a similar way, the width of the spectrum is fitted by a linear function. This procedure is carried out on both photometric datasets, corresponding to telescopes UT1 and UT3, respectively. Both fits are then averaged and used to create a mask consisting of a Gaussian function with the average position and width of the spectra for each column. Finally, a weighting function is computed to create a mask for the extraction.
The photometric masks are also used to extract the fringe information. Each frame of the fringe data is reduced to a one-dimensional spectrum by multiplying it by the mask, integrating in the direction perpendicular to the spectral dispersion and finally subtracting the two output channels of the beam combiner from each other. The spectra from each scan with the piezo-mounted mirrors are collected and Fourier-transformed from optical path difference (OPD) to frequency space. The fringe amplitude at each wavelength is then obtained from the power spectrum. We typically co-added the signal by bins of four pixels in the dispersion direction to improve the signal-to-noise ratio.
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Figure 1:
alpha Ara flux measured by the VLTI/MIDI (8-13 ![]() ![]() ![]() ![]() |
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Table 2: Calibrator star parameters, from the MIDI calibrator catalogue (Van Boekel et al. 2005).
The scans where fringes were actually detected are selected based on the white-light fringe amplitude, i.e. the fringe amplitude that is seen after integrating over all usable wavelengths. The histogram of all white-light fringe amplitudes within a fringe track dataset usually shows a small peak near zero, and a broad peak at higher amplitudes. We interactively set a threshold just below this broad peak, and average the fringe amplitudes of all scans with a white-light fringe amplitude higher than this threshold to give the final fringe amplitude as a function of wavelength. The raw visibility is obtained by dividing the fringe amplitude by the total flux. The calibrated visibility is obtained by dividing the raw visibility of an object by that of a calibrator. The photometrically calibrated flux creating the fringes is called correlated flux.
The interferometric and photometric data are presented in Table 1 and the parameters of the interferometric calibrators are given in Table 2.
The flux calibration has been performed with the data of June 16 only. Alpha Aql was considered as a good calibrator for the absolute flux extraction and we have used the ISO observations of this star as a flux reference (but has not been used as an interferometric calibrator). An independent calibration is performed for the individual spectra from each part of the detector and each telescopes. The airmass is also corrected independently for each of them by using the observations of one calibrator for two different times. Figure 1 shows the VLTI/MIDI Spectral Energy Distribution (SED) with the ISO data and the theoretical SED obtained with the SIMECA code (see hereafter).
The errors on the observed visibilities are mostly systematic. The
statistical signal-to-noise ratio on the white light fringe
amplitudes is 5-10 at minimum and much better after adding the
several hundred scans taken per interferometric measurement. Most
of the time, the dominant error source is the fluctuation of the
photometry which occur between the fringe recording and the
photometry recording from the individual telescopes which are
separated in time by 2 to 10 min. This affects the fringe
signal for object and calibrator measurements. Comparing the raw
visibilities observed for different calibrator stars over one
night, the standard deviation of these values under good
conditions amounts to 5-10% (relative) at the red and blue
end of the spectrum while for adverse conditions these numbers
have to be multiplied by a factor of 2-2.5. These fluctuations are
much larger for instance than the systematic error expected from
the diameters of the calibrators and are mostly achromatic
(Leinert et al. 2004). Yet, the instrumental
visibility curve of MIDI is very stable in shape, i.e. the slope
of the curve typically varies by less than 3-5%. The visibilities
observed on calibrators ("instrumental visibility'') rises from
about 0.4 at 8
m to about 0.7 at 13
m, rather repeatable
from night to night (i.e. it means that, in good atmospheric
conditions, the absolute level is dependant on the photometric
variations and hence the stability of the atmospheric conditions
but the shape of the visibility curves is mostly an instrumental
parameter only weakly affected by the atmosphere).
The data reduction described previously does not consider the variation of the photometry during the 1-2 min needed to record the data and the visibility estimation is usually based on the mean photometric flux. Moreover, under good atmospheric conditions, the statistics of the correlated flux per individual scan follow roughly a Gaussian curve and the mean of this correlated flux is used for the visibility estimation. The main origin of the variability of the correlated flux is the high frequency photometric fluctuations, the atmospheric piston and the quality of the fringe tracking (i.e. the instrumental piston).
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Figure 2:
H![]() ![]() |
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Figure 3:
a) Observed Pa![]() ![]() ![]() ![]() ![]() ![]() |
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During the night of June 17, 2003 thin cirrus clouds passed
through the telescope beams. The seeing was quite stable, below
0.5
during all the night but the standard deviation of
the flux from the target pointed by the (visible) seeing
monitor
oscillated between 0.02 and 0.05 between
3h and 8h (UT time), indicating the passage of thin cirrus clouds.
These fluctuations seem correlated with an acceleration of the
atmosphere turbulence with a coherence time in visible wavelengths
decreasing from
ms at 3h UT to
ms at 7h30 UT. By comparison, the photometric fluctuations of the seeing
monitor calibrator on the 16th of June are below 0.01 during the
entire night although the turbulence was also quite rapid with
ms. The variability of the atmosphere during the June,
17 night was such that a careful inspection of the stability of
the photometry and the correlated flux recorded was necessary. The
passages of the thin cirrus were easily recognized by a rapid drop
of the flux lasting between 0.5 and 2 s during the photometric
record and by a simultaneous photometric and correlated flux drop
in the tracking files. The frames and the scans concerned were
excluded from the data reduction process: this represent about
60% of the photometry and 50% of the scans, i.e. about 800
frames were left from each photometric files and about 100 scans
were used for the visibility estimation. Of course, this
treatment, applied to alpha Ara and its calibrators, is a
first order correction and the standard deviation of the
instrumental visibility curves of the calibrators observed between
2h UT and 9h UT is still large compared to a good observing night,
of the order of 18% whereas the standard deviation of the
previous night is only 8%. Fortunately, the longest baseline of
102 m was used during the best night. The error bars reported in
Figs. 6 and 7 reflect the
standard deviation computed by this mean.
Complementary spectra have also been recorded quasi-simultaneously
with the VLTI run in order to know whether alpha Ara has
shown IR emission lines. We observed alpha Ara in the J2 band
(1.2283-1.2937 m) using the 1.6 m Perkin-Elmer telescope and
Coudé spectrograph (with R=10 000) at the Observatório do Pico
dos Dias, Laboratório Nacional de Astrofísica (LNA),
Itajubá, Brasil. In total, five stellar spectra were recorded
with the Câmara Infravermelho (CamIV) detector, August 13, 2003.
Each spectrum corresponds to a different position of the star
along the spatial axis of the entrance slit, allowing us to use
the median of the five two-dimensional frames, from which the
average bias frame was subtracted, as a sky background. After
dividing the stellar and sky background frames by the average flat
field, the sky was subtracted from two-dimensional stellar
spectra. The extraction of one-dimensional spectra, as well as
wavelength calibration, using an Ar-Ne calibration lamp, was
performed with standard IRAF
packages. Finally, the continuum was normalized to
unity.
The observed Pa
line profile is shown in
Fig. 3. The emission intensity was
,
while the violet-to-red peak height ratio V/Rwas 1.17. Even if the spectroscopic data were not recorded fully
simultaneously with the interferometric ones, available
observations of typical timescales of the spectral changes of
alpha Ara, such as the above-described H EROS data,
justify the assumption that the circumstellar environment of
alpha Ara has not changed significantly between June and
August 2003. This Pa
line profile, combined with the
interferometric data, constrains the physical parameters of the
circumstellar environment of alpha Ara, as it will be shown
in the following sections.
To take into account the photospheric absorption line, we assume
the underlying star to be a normal B3 Ve star with
K and
and synthesize the
photospheric line profiles using the SYNSPEC code by Hubeny
(Hubeny 1988; Hubeny & Lanz 1995). The
resulting line profile is broadened by solid-body rotation and
might be further altered by absorption in the part of the envelope
in the line of sight towards the stellar disk.
The stellar parameters (Table 3) are important
mainly to define the distance and the luminosity of the source
(Fig. 1) and represent the first step of the iterative
process leading to a model of the envelope as described below. In
the scope of this model, we have computed the H,
H
(Fig. 2) and Pa
(Fig. 3) line profiles.
Table 3: Stellar parameters used for the modelling of alpha Ara with SIMECA.
The critical constraint of this study is based on the fact that
the H
and H
lines (Fig. 2), as well as
the the Pa
line (Fig. 3) are strongly in
emission, whereas from Fig. 6 the visibilities are
clearly around unity, i.e. the envelope in the N band is nearly
unresolved. Hence, the circumstellar density must be large enough
to produce Balmer lines in emission while at the same time this
density must be low enough and/or the circumstellar environment
must be well delimited to keep the envelope unresolved. The
emissive lines of many Be stars like alpha Ara exhibit a
characteristic double peaked shape and the "violet'' and "red''
peaks are denoted V and R. This shape provides stringent
constraints on the wind parameters and particularly on the
inclination of the system.
In order to obtain matching parameters derived from both line profiles and interferometric measurements we have adopted the following strategy:
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(2) |
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(3) |
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Figure 4:
Variation of the H![]() |
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Table 4:
Best model parameters for the alpha Ara circumstellar
environment obtained by fitting the visibilities and the 2003
Pa
line profile with SIMECA.
The variability of the circumstellar environment of Ara is
traced by the variations of the H
line equivalent width
(EW) reported in the literature by Dachs et al. (1990),
Hanuschik et al. (1995) and this work. From
Fig. 4 it is evident that we cannot straightforwardly
use parameters derived from the 1999 H
and H
line
profiles to model the Pa
line profile observed in 2003,
i.e. close to our interferometric run. It is most likely that some
physical parameters have changed between these two epochs.
The H
line observed 1999 shows strong emission of
,
which is well reproduced by SIMECA with
the stellar parameters given in Table 3 and the
envelope parameters from Table 4 (Fig. 2).
With a wind base density decreased by 25%, producing a good
agreement for the Pa
line profile (Fig. 3), the
model gives an H
line profile with a
ratio
3 (Fig. 5). Unfortunately, we do not
have H
line profiles taken simultaneously with the VLTI
observations that could confirm such a decrease.
Even though Fig. 3 shows that the Pa
line profile
modelled with SIMECA is in good agreement with the observed one, the
modelled V/R is less than unity, whereas the observed one is
.
SIMECA is based on a radiative wind model for which the
gas is outflowing from the star with a polar and equatorial velocity
of respectively 2000 and 170 km s-1. In order to obtain a
V/R>1, it would be necessary to introduce an asymmetry like a global
one-armed oscillation, as seen by Berio et al. (1999) and Vakili
et al. (1998) for the
Cas and
Tau equatorial
disks. Nevertheless, the global agreement of observation and model
both in profile shape and intensity indicates that, under the general
assumptions of the SIMECA model, the global kinematics and density
distribution within the envelope are well reproduced by the assumed
model parameters.
The computed SED between 2 and 10 m is shown in Fig. 1,
together with the data recorded by VLTI/MIDI (8-13.5
m) and the
ISO satellite (2.5-11
m). The observed SED and the modelled
curve match for a source distance of 105 pc. This distance is
significantly larger than the one obtained from the Hipparcos
satellite (see discussion in Sect. 5). In
Fig. 1 the continuum emitted by the central star is
approximated as a
K black body radiation
plotted as a dashed line. The total, i.e. free-free and free-bound
emission from envelope + star, is indicated by the dotted line. Clearly,
the infrared excess produced by the circumstellar envelope is mandatory
in order to fit the observed data (solid line).
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Figure 5:
H![]() ![]() |
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Figure 6: VLTI/MIDI data for the 79 m ( left) and 102 m ( right) baselines and theoretical visibilities from SIMECA (dashed line) for the distance of 105 pc estimated from our SED fit (see Fig. 1). The error bars are equivalent to one sigma. |
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Figure 7:
VLTI/MIDI data for the 79 m ( left) and 102 m ( right) baselines and theoretical
visibilities from SIMECA (dashed line) for a truncated disk at 22 ![]() |
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The parameters obtained by the above modeling of the emission line
profiles, and the density required to fit the 2003 Pa
line,
can now be used to compute the expected visibility curves for a
distance of 105 pc, as estimated from the fit of the SED. The
resulting visibility curves are plotted in Fig. 6,
clearly showing that the modeled envelope should be well resolved
at 79 m and 102 m (mean visibility
)
whereas the VLTI/MIDI data without any doubt have hardly resolved
the target (
). However, the observed
visibility curves in Fig. 6 may indicate that the
envelope is more resolved for the longer wavelengths. This effect,
if true, is more or less reproduced by our model and is more
obvious for the 102 m baseline where the theoretical visibility is
0.62 at 8
m and 0.57 at 13
m.
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Figure 8:
a) Theoretical visibilities for the H![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Nevertheless, we did not succeed in obtaining model parameters
reproducing both the Balmer lines and the visibility curves at the
same time: at a distance of 105 pc the model gives an envelope
which is clearly resolved, if the constraint that the Paline is in emission with
is met. One
possibility would be to increase the distance of the star up to
122 pc, a distance suggested by Cohen et al. (1998).
However, we consider this value as too high in comparison to the
Hipparcos distance (see also the discussion in
Sect. 5).
The modeled envelope is more or less spherical since the input
parameter m1=0.3 corresponds to an opening angle of 160
(see Stee 2003), producing only a negligible visibility
difference for the envelope seen parallel or perpendicular to the
polar axis. Nevertheless, this small flattening may produce the
small intrinsic linear polarization of
measured
by McLean & Clarke (1979) and Yudin et al. (1998),
who found a polarization angle PA = 172
.
The inner disk
orientation is, therefore, expected to be perpendicular to this
direction at 82
.
This means that the longer baseline was
unfortunately oriented at about 70
from the expected major
axis of the disk. The diagram of the alpha Ara circumstellar
envelope projected onto the sky plane (with an arbitrary
oblateness) and the VLTI baseline positions is given in
Fig. 9.
Considering these difficulties, we have computed a model with a truncated disk. The
radius where the truncation occurs was set to 22 ,
which was
derived using a distance of 105 pc and the fit of the Pa
line
(Fig. 7).
The theoretical visibilities in the H
,
H
,
Pa
and Br
lines are plotted in Fig. 8 for the
two scenarios. For both models it is clear that at shorter
wavelengths, with a 102 m baseline, the envelope of alpha Ara
would be well resolved in the Pa
and Br
lines (
)
even for a truncated disk (
). In H
and H
,
V is 0.6 and 0.85,
respectively, for the full disk and V is 0.65 and 0.94 for the
truncated one. At these wavelengths, the emission comes from the
inner part of the disk, which remains unresolved at a distance of
105 pc. Consequently, such a truncation of the disk would have
only a little impact on the H
and H
emitting
regions, whereas the IR emission, originating from more extended
regions, is more affected.
The fact that alpha Ara is nearly unresolved at 8 m with
a 102 m baseline gives an upper limit for the diameter of the
emitting envelope of
mas, i.e. 14
(for d=74 pc) or 20
(for d=105 pc)
assuming a uniform disk distribution for the star+disk brightness
distribution. If a Gaussian distribution is assumed, the best fit
is obtained with a
mas although the quality if the fit is
poorer than for the uniform disk hypothesis. Such a radial
extension is in good agreement with the one obtained by Waters et
al. (1987) from IRAS Far-IR data, with
.
Indeed, the hypothesis that the
star+envelope flux distribution is unchanged from 8
m to
13.5
m is a crude approximation and these estimated extensions
must be taken as indicative only. It must be stressed out that the
slopes of the visibility curves provided by the model are in good
agreement with the data showing that the flux is more concentrated
at 8
m than at 13
m. Moreover, the angular diameter
estimations are mostly based on the visibilities taken with the
longest projected baseline at PA = 7
to the East, i.e. more
or less along the predicted polar direction (see
Fig. 9).
In Table 4 we present the results based on the best
fit of the alpha Ara August 2003 Pa
line profile and
visibilities in the N band.
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Figure 9:
Diagram of alpha Ara circumstellar envelope projected onto the
sky plane (with arbitrary oblateness) based on polarization (P) measurements
(PA = 172
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The characteristics of the disk around alpha Ara seem to have
been fundamentally stable over the past century. Only the overall
strength of the emission, as traced e.g. by the H
EW
exhibit large variations (see Fig. 4). However, the
compilation of spectroscopic observations and the consecutive
study of this disk by MIDI have revealed a number of interesting
and partly unsuspected characteristics.
Long-term V/R variations are common in Be stars, being
attributed to a one-armed oscillation in the Keplerian disk. Rapid
V/R variations, however, have not been extensively discussed in
the literature. Mennickent (1991) associates fast quasi-periodic
variations ( d) to the smallest disks, i.e. the disks
with the lowest equivalent widths in H
.
Okazaki (1997)
found that for early-type Be stars around B0 radiative effects can
explain the confinement of this one-armed oscillations with
typical periods of about 10 yr. Nevertheless, this model would
have difficulties in explaining more rapid V/R variations.
Another possibility for such variable asymmetry is the presence
of a companion, orbiting at distances larger than the envelope
extension. Then, these quasi-periodic line profile variations
should be related to the orbital parameters of the putative
secondary.
Nevertheless, with respect to the photospheric absorption lines alpha Ara is unusually stable among earlier type Be stars. In a sample of 27 early Be stars observed with H EROS observations, only two stars, including alpha Ara, did not show any detectable line profile variability (Rivinius et al. 2003).
However, this is quite different for the circumstellar emission
lines. As well as slow, possibly cyclic long-term changes typical
of Be stars, there are changes on a shorter timescale. In
particular, the peak-height-ratio of the violet and red peaks of
the higher Balmer lines have made one full cycle of low amplitude
(about 0.95 to 1.05) during the 69 days of the H EROS
observations (see Fig. 10). At the same time,
the radial velocity of the emission component of the Balmer lines
was changing in a cyclic way as well. For instance,
Fig. 11 shows this effect for the central
absorption of the H-emission, but the same is seen if the
radial velocity of the emission peaks is traced, or if other
Balmer lines are investigated. Unfortunately, the available data
are not good enough to claim such a radial velocity change in the
weaker emission lines or in the photospheric absorption lines. The
uncertainty of measuring the radial velocity of such lines is much
higher than the amplitude seen in Fig. 11.
Mennickent et al. (1991) have observed similar changes and gave a
cycle length of 0.13 years, or 45 days. However, their Fig. 2
would also support a cycle length of 70 to 80 days, as present in
the H EROS data of 1999.
Such behavior is known from a few other Be stars as well. In the
cases investigated thoroughly, it was shown to be linked to
binarity, although the mechanisms are not yet known (see for
instance Koubsky et al. 1997). A search for any spectral
contribution of such a hypothetical companion in the phase-binned
H EROS spectra of alpha Ara did not return a positive
result.
![]() |
Figure 10:
Variation of the height ratio of
violet and red emission peaks of the H![]() |
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![]() |
Figure 11:
Radial velocity changes of the
central depression of H![]() |
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Nevertheless, since a companion as a potential origin for an outer
truncation of the disk is important in light of the above results,
the properties of such a system are estimated from the following:
Using the stellar parameters of Table 3, i.e. a
mass of
,
a 70 day period would give a radius
of about
,
assuming a circular orbit of a
companion with negligible mass. With
,
this corresponds to about 32 stellar radii, which is in agreement
with the estimate based on the MIDI/VLTI data for a disk truncated
at 25
,
i.e. somewhat smaller than the companion orbit.
Such an orbital dimension can also be estimated by integrating the
radial velocity amplitude detected in H
of the order of
12-16 km s-1. Assuming a circular orbit, a period of
69 days and testing two inclination angles of
and
,
the mass of the companion would range from
1.4
(F2-4V) and
(A2-4V). The
corresponding Roche lobe radii
are roughly between
15 and 20
although we point out that the radial velocity
curve shown in Fig. 11 would suggest an eccentricity
of about 0.2-0.3, affecting the previous estimation.
However, the radial velocities derived from emission lines cannot be
taken at face value. Depending on the systemic properties, the radial
velocities measured in the central inversion of the circumstellar
emission follow the actual radial velocity curves of the stars only
loosely. The above orbit size and period, again for the circular case
and negligible mass of the companion, result in an orbital velocity of
about 18 km s-1, which is in the same order of magnitude as the
amplitude measured in the emission lines. In some well-investigated
binaries like 59 Cyg (Harmanec et al. 2002; Rivinius & Stefl
2000, d) or
Per (Hummel & Stefl 2001; Stefl
et al. 2000,
d), the emission of the Balmer lines is indeed
in phase with the orbital motion.
Until now, no evidence has been reported for a possible companion
around alpha Ara, but it should be kept in mind that it is
difficult to search for low mass companions around bright stars,
such as Be stars (see for instance the case of Cas,
Harmanec et al. 2000; Miroshnichenko et al. 2002,
period
d). The X-ray flux from alpha Ara is not
peculiar and is similar to that of normal B stars
(
erg s-1, assuming d=122 pc
, Cohen et al. 1998). The proposed
truncation may also help to explain the stringent non-detection of
radio emission from alpha Ara at a level of 0.1 mJy at 3.5 cm
and 6.3 cm (Steele et al. 1998).
Using the NPOI interferometer, Tycner et al. (2004) have recently
studied the disk geometry of the Be star Tau, which is also a
well-investigated spectroscopic binary (
d,
km s-1).
They measured the disk extension quite accurately to be well within the Roche
radius. This suggests also that this disk may be truncated.
![]() |
Figure 12:
Average of the IUE-spectra of alpha Ara
(dotted) compared to a theoretical spectrum with
![]() ![]() |
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The available spectral classifications of the star are all around
B3 Ve, so that it can be assumed that the effective temperature
is well constrained at about 18 000 K. Instead, the main source
of uncertainty in the model may be the radius, which arbitrarily
has been chosen to be 4.8 ,
based on the
statistical value given by Tokunaga in Allen's "Astrophysical
Quantities'' (2000).
However, alpha Ara has a high
of about
270 km s-1 (Dachs et al. 1990; Yudin 2001; Chauville et al.
2001). With such a large projected rotational velocity, the
hypothesis of a uniform disk radius is questionable and the radius
used is most probably underestimated as demonstrated by the recent
observations of alpha Eri (Achernar et al. 2003). This implies an increase of the illuminating area and
strengthens the difficulty of putting the star at the Hipparcos
distance. Without considering any reddening, and keeping the
Hipparcos distance, the radius of alpha Ara would be
unrealistically low (below 3.5
)
or the photosphere
unrealistically cold (
of the order of 15 000 K).
In Table 5 we show selected parameters of the Be
stars alpha Ara, alpha Eri and the "normal'' (i.e. non
Be) star eta UMa which are all classified as B3 V. Comparing
these values, it becomes evident that, if their Hipparcos
distances are correct, the striking visual flux differences
between these three stars cannot be directly related to a distance
difference only.
The reddening of Be stars does not only contain an interstellar term, which is zero for alpha Ara, but also has a circumstellar contribution. This is a model-dependant parameter, explaining the well known difficulty of calibrating the Be stars distances independently of Hipparcos measurements.
For instance, Cohen et al. (1998) have apparently totally
neglected any circumstellar reddening and derived a distance of 122 pc. This difficulty
also affects the present model and an underestimation of the
circumstellar extinction gives a distance too large. On the other
hand, the measured B-V for alpha Ara does not indicate a
circumstellar environment dense enough on the line of sight to
explain the difference, since this should be linked to emission
line properties which are not observed.
A large amount of extinction and reddening of the central star
would have to be due to an edge-on disk, and such an inclination
would lead to a large and deep self-absorption in the Balmer
emission lines, the so-called shell appearance. alpha Ara,
however, has never been seen in a shell phase. Instead, the
inclination angle is relatively well constrained by the shape of
the Balmer line to a value of about 45.
An inclination of about 45
implies an equatorial
rotational velocity of the order of 380 km s-1, since
.
Using the parameters in
Table 3, this value is at about 75% of the
critical velocity. We point out that the equatorial velocity in
the model was considered a free parameter and the best model
provides an equatorial value of 300 km s-1, i.e. 60% of
the critical velocity. With such a high rotational velocity, the
von Zeipel effect becomes important, and thus the continuum
emission must be strongly latitude dependant (see e.g. Townsend
et al. 2004, his Fig. 3). This may also confuse the line profile
diagnostic and affect the angle determination. Such an effect is
not included in the present model. A value of the inclination
closer to 90
,
compatible with the large
observed, may be related to a stronger circumstellar reddening
from the equatorial environment and a lower integrated stellar
flux since the equatorial regions of the star are cooler and
fainter than the polar ones.
Table 5: Stellar parameters of three close B3V stars, alpha Ara, alpha Eri and eta UMa.
![]() |
Figure 13: Schematic view of the alpha Ara circumstellar environment as used in this work (see Table 4). |
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Definitely, the Hipparcos distance, if reliable, is a very
constraining parameter for any model of the circumstellar
environment of alpha Ara. More interferometric observations
are needed, firstly to constrain the angular size and shape of the
hydrogen emission lines, and secondly to evaluate the angular size
and shape of the underlying star. The forthcoming VLTI/AMBER
instrument will be able to address the first point by studying the
fringe properties in Pa
and Br
.
However, baselines
of the order of 150-200 m will be needed to resolve the central
star, becoming available only with the 1.8 m Auxiliary Telescopes.
Their first light is foreseen for 2005.
Acknowledgements
We thank Anne-Marie Hubert for many fruitful discussions and Jean-Louis Falin for checking the quality of the Hipparcos data. We also thank Anatoly Miroshnichenko for his helpful comments. This research has made use of SIMBAD database, operated at CDS, Strasbourg, France. The paper benefited from the careful reading and advice from the referee, Markus Wittkowski.