A&A 426, 151-170 (2004)
DOI: 10.1051/0004-6361:20040400
B. Acke1 - M. E. van den Ancker2
1 - Instituut voor Sterrenkunde, KULeuven, Celestijnenlaan 200B,
3001 Leuven, Belgium
2 -
European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748
Garching bei München, Germany
Received 6 March 2004 / Accepted 22 June 2004
Abstract
We have investigated the infrared spectra of all 46 Herbig Ae/Be
stars for which spectroscopic data are available in the ISO data archive.
Our quantitative
analysis of these spectra focuses on the emission bands at 3.3,
6.2, "7.7'', 8.6 and 11.2 micron, linked to polycyclic aromatic
hydrocarbons (PAHs), the nanodiamond-related features at 3.4 and 3.5
micron, the amorphous 10 micron silicate band and the crystalline
silicate band at 11.3 micron. We have detected PAH emission in 57%
of the Herbig stars in our sample. Although for most of these sources
the PAH spectra are similar, there are clear examples of differences in
the PAH spectra within our sample which can be explained by differences
in PAH size, chemistry and/or ionization. Amorphous silicate emission
was detected in the spectra of 52% of the sample stars, amorphous
silicate absorption in 13%. We have detected crystalline silicate
emission in 11 stars (24% of our sample), of which four (9%) also
display strong PAH emission.
We have classified the sample sources according to the strength of their
mid-IR energy distribution.
The systems with stronger mid-infared (20-100 m) excesses relative
to their near-infrared (1-5
m) excess display significantly more
PAH emission than those with weaker mid-infrared excesses.
There are no pronounced differences in the behaviour of the silicate feature
between the two groups.
This provides strong observational support for the disk models by
Dullemond et al. (2001), in which systems with a flaring disk geometry
display a strong mid-infrared excess, whereas those with disks that
are strongly shadowed by the puffed-up inner rim of the disk only
display modest amounts of mid-infrared emission. Since the silicates
are expected to be produced mainly in the warm inner disk regions, no
large differences in silicate behaviour are expected between the
two groups. In contrast to this, the PAH emission is expected to
be produced mainly in the part of the disk atmosphere that is directly
exposed to radiation from the central star. In this model, self-shadowed
disks should display weaker PAH emission than flared disks, consistent
with our observations.
Key words: stars: circumstellar matter - stars: pre-main sequence - stars: planetary systems: protoplanetary disks
Although the optical-sub-mm energy distribution of Herbig Ae/Be (HAEBE) stars has been well explored by previous authors (e.g., Hillenbrand et al. 1992), the chemical and mineralogical composition of the dust remained poorly studied until the 1995 launch of the Infrared Space Observatory (ISO, Kessler et al. 1996). This first possibility to study the complete infrared spectrum of these objects in detail revealed a large variety in dust properties, from small aromatic hydrocarbons to silicate dust. Moreover, some sources were shown to contain partially crystalline dust grains, similar to those found in comets in our own solar system (Waelkens et al. 1996, Malfait et al. 1998; Meeus et al. 2001; van den Ancker et al. 2000b,a; Malfait et al. 1999).
ISO played a major role in opening up the field of infrared spectroscopy.
The emission features at 3.3, 6.2, 7.7, 8.6 and 11.2 micron, found in
ISO spectra of many HAEBE stars and
previously known as the unidentified infrared (UIR) bands, are
generally attributed to polycyclic aromatic hydrocarbons
(PAHs, Allamandola et al. 1989; Léger & Puget 1984). PAHs
are a large family of molecules, for which the fundamental ingredients
are polycyclic benzene rings. PAH molecules are thought to be excited
by far-ultraviolet (UV) photons. The absorption of such a photon induces a
transition of the PAH molecule to an upper electronic state. The
excited molecule then makes rapid transitions to a lower electronic
state, leaving most of the initially absorbed energy in the form of
vibrational energy in the CC and CH bonds. PAHs cool down by infrared
(IR) emission in bands linked to these vibrational modes (Peeters 2002, and
references therein). The 3.3 m band is due to CH
bond stretching vibrations, the 6.2
m band is linked to the CC
stretching mode and the 7.7
m band corresponds to a combination
of the CC stretching and the CH in-plane bending modes. The 8.6 and
11.2
m features are linked to the CH in-plane and out-of-plane
bending modes respectively (Vermeij et al. 2002, and references therein).
In a few sources, emission bands at 3.4 and 3.5 micron are observed. These spectral features are attributed to the CH stretching modes of hydrogenated nanodiamonds (NANs, Guillois et al. 1999; Van Kerckhoven et al. 2002).
The infrared spectra of HAEBEs also contain silicate features. The broad feature at 10 micron has been attributed to the SiO bonds in warm, small amorphous silicate grains like olivine. The feature can appear in emission or in absorption in HAEBE spectra. The spectral signature at 11.3 micron, as well as many features at longer wavelengths which are not included in the analysis presented in this paper, is linked to crystalline silicates (Malfait et al. 1998; van den Ancker et al. 2000b,a; Bouwman et al. 2000,2001; Malfait et al. 1999).
The spectral energy distribution (SED) of HAEBE stars is characterized by the presence of an IR flux excess, due to thermal emission of circumstellar matter. The geometry of the circumstellar dust has been the subject of a long-lasting debate (e.g., Grady et al. 1995; Mannings & Sargent 1997; Böhm & Catala 1994; Grinin et al. 1996; Hartmann et al. 1993; Corcoran & Ray 1998; Hillenbrand et al. 1992; Grinin et al. 1994; Berrilli et al. 1992). For late-B, A and F stars, the evidence for the presence of disk-like geometries is generally accepted. For early-B stars, the matter is less clear. In these systems, the dissipation time scale of the circumstellar spherical envelope is of the order of the pre-main-sequence life time. Disks as well as spherical envelopes might be present.
In their study of the ISO spectra of 14 isolated Herbig Ae/Be stars,
Meeus et al. (2001, henceforth M01) classified their sample
into two groups, based on the shape of the SED.
Group I contains the sources in which a rising mid-IR
(20-100 m) flux excess is observed; these sources have an SED that can
be fitted with a power-law and a black-body continuum. Group
II sources have a more modest mid-IR excess; their SEDs can be
reconstructed by a power-law only. M01 suggest phenomenologically that
this classification represents different geometries of the
circumstellar disk: group I sources have flared disks, group II
members have flat disks.
Dullemond (2002, henceforth D02) and Dullemond & Dominik (2004, henceforth
DD04) have modelled young stellar disks
with a self-consistent model based on 2-D radiative transfer coupled
to the equation of vertical hydrostatics. The model consists of a disk
with an inner hole (0.5 AU), a puffed-up inner rim and an outer
part. The outer part of the disk can be flared (as
in Chiang & Goldreich 1997), but can also lie entirely in the shade of the inner
rim. The SEDs of flared disks display a strong mid-IR flux excess,
while self-shadowed disks have a much more modest mid-IR excess. D02
explains quantitatively the
difference in SED shape in HAEBEs (as expressed by the classification
of M01) as the result of a different disk
geometry; group I sources have flared disks, group II sources have
flat self-shadowed disks.
In this paper we investigate a possible link between the shape of the SED - a proxy for the geometrical distribution of the circumstellar matter - and the strength and profiles of circumstellar infrared emission bands in HAEBE systems. All spectra of HAEBE stars taken by ISO are investigated, amongst others spectra that have never been published before. It is therefore the most complete sample of near-IR spectra of HAEBE stars ever investigated as a whole. Our study is not only qualitative, but contains also, as opposed to many other articles, a quantitative analysis of the infrared spectra of HAEBEs.
Our list of HAEBEs is based on the catalogue of Thé et al. (1994, henceforth
T94). We restricted ourselves to Tables 1 and 2 of that
article. We enlarged the sample with HAEBEs studied by
Malfait et al. (1998, henceforth M98), that satisfy the criteria postulated by T94.
The resulting list was cross-correlated with the ISO data archive; we selected
all objects for which ISO spectra were available with an ISO pointing
within a distance of 5 arcsec of the position given by T94
and M98.
We eliminated some of the remaining sources from the sample; TY CrA
was omitted because the infrared emission emanates from the "TY
CrA bar'' (Siebenmorgen et al. 2000), which is not directly associated
to the star. The object also does not display H
emission, which
indicates that this star is no longer actively accreting.
From a comparison between the ISO-SWS spectrum of MWC 137 and
photometric measurements from the MSX Point Source Catalog
(Egan et al. 1997), it appears that there is an offset of 12
between the
position indicated in T94 and the position of the infrared source.
Only a small portion of the object's flux is seen in
the SWS spectrum. We therefore discarded this spectrum from our
analysis.
A similar problem was encountered for the SWS spectrum of IRAS 12496-7650. A
mispointing of the ISO instrument of about 17
compared to the
2MASS point-source-catalogue position (Cutri et al. 2003) caused an
anomaly between the ISO SWS-spectra and the IR photometry. However,
the PHT-S spectra for this source were not affected due to the larger
aperture of the ISO-PHOT instrument (see Sect. 2.2).
51 Oph and MWC 300 were also removed from the list; these are probably
evolved objects and not HAEBEs (van den Ancker et al. 2001; Molster et al. 2002, respectively).
The resulting sample of 46 sources is tabulated in Table 1. The columns contain respectively the object's name, the type of ISO spectrum (see Sect. 2.2), the ISO Observation Sequence Number (OSN), the right ascension (RA), the declination (Dec) (both epoch 2000 coordinates), the date and starting time (UT start) of the measurement and the total integration time T in seconds. The objects are sorted by increasing RA. The given coordinates are the pointing coordinates of ISO for that source.
We retrieved all spectra of HAEBE stars present in the ISO data
archive.
Three types of ISO data were used in this study: ISO-SWS
(de Graauw et al. 1996), ISO-PHT (Lemke et al. 1996) and ISO-CAM (Cesarsky et al. 1996).
ISO-SWS was the Short Wavelength Spectrometer aboard ISO (2.38-45.2
m). This instrument consisted of 4 sets of 12 detectors. Each set
of detectors covered a different wavelength region ([2.38:4.08],
[4.08:12.0], [12.0:29.0] and [29.0:45.2]
m respectively). Not
only the material of which the
detectors were made differed from band to band, also the aperture
sizes were larger at longer wavelengths (from 14
20
around 3
m to
around 30
m). The data of different detectors and wavelength regions are
calibrated independently.
The source's spectrum was scanned in time, in a way that all 12 detectors of one set scanned through the same wavelength region twice. We only used the SWS data in which the whole spectrum was scanned (scanning mode AOT01). In the standard pipeline reduction, dark currents (residual signal, not related to the actual astrophysical target) are subtracted from the raw data. The detector signal is corrected by multiplication with a responsitivity curve. Furthermore, a wavelength and flux calibration are carried out. We used the resulting product of the latest version of the off-line processing pipeline reduction (OLP 10). After this basic reduction, we manually removed bad data. This includes the suppression of deviating detectors (detectors which measure spectra that are too dissimilar with respect to the spectra observed by the other detectors operating in the same wavelength region) and artefacts like glitches (spikes in intensity, spread out over several data points due to the detector's memory effects). The latter are caused by infalling highly energetic interplanetary particles. A glitch that exceeds the noise level is easily recognizable, since it is only present in one of the two scan directions of the detector and its shape is characteristic.
To diminish the scatter in the data, the calibrated flux
levels in a wavelength interval are averaged and all detectors are
scaled to this
value. The noise distribution is strongly non-Gaussian and
asymmetric, among other reasons because of glitches. Since the data will be rebinned
and averaged, removal of the outliers is needed. Outliers with a
deviation of more than three times the noise in the data were
expelled. In the next step the data are rebinned at a spectral
resolution of 150 for low flux sources (<10 Jy at 10 m) and 500
for high flux sources (>10 Jy at 10
m).
Sometimes small differences in flux level are present between the different wavelength regions in the spectrum. The final phase of the data reduction is to correct for these jumps. Since most sources in the sample have low fluxes, jumps are primarily due to errors in the dark current subtraction. Therefore offsets were applied to correct for the flux differences between the wavelength intervals. The shifting was done by adding a constant term - which was computed to minimize the flux difference in the overlap region between two adjacent wavelength regions - to the flux. We took the interval between 3.02 to 3.52 micron as the flux calibration reference and scaled all parts of the spectrum between 2.38 and 12.0 micron to that level. For the region [12.0:29.0] micron, the interval covering [16.5:19.5] micron was used. We discarded the wavelength range longer than 29 micron, since the flux levels in this part of the spectrum were too low to be reliable for many of the sample stars.
The wavelength region around 12 m is difficult to interpret,
since both the end of the previous and the beginning of the next
wavelength range are very
noisy and therefore unreliable. For a few spectra, we found it useful
to extend the wavelength region of the first interval with respect
to the standard pipeline reduction: from 12 to 12.5
m. This was done for V376 Cas, HD 31648, Z CMa, HD 95881, HD 104237, HD 139614, HD 142666, HD 150193, T CrA, WW Vul,
LkH
224, HD 200775 and SV Cep. Since we
chose to shift the flux levels in
[2.38:12.0]
m, and [12.0:29.0]
m separately, the overlap
region between these two intervals is independent of this
post-reduction flux level correction. In this way, we avoid changing
the shape of spectrum around this wavelength area too much compared to
the original - unshifted - data. Not combining the two intervals in
a direct way also prevents the entry of systematic errors in the shape
and line flux of the broad, amorphous 10 micron silicate feature. We
note that each of the narrower features at 3.3, 3.4, 3.5, 6.2, 7.7, 8.6, 11.2 and
11.3 micron are included within one continuous SWS wavelength
interval. Therefore our measurements of line flux and full width at
half maximum of those features will be independent of the procedure
followed for the alignment of the different wavelength ranges.
For HD 97048, HD 100546, HD 104237, HD 142666, CD-4211721, R CrA, T CrA and
MWC 1080, two ISO-SWS spectra were available. We combined the
spectra after the reduction to increase the
signal-to-noise ratio. For HD 97048 we decided to keep only
one of the SWS spectra (OSN 61801318), because the second spectrum was
affected by severe instrumental artefacts.
The ISO-PHT instrument was a photo-polarimeter, which had a
spectroscopic mode (PHT-S). The spatial resolution of the instrument
was
.
The spectra consist of 127 photometric points,
separated into two wavelength intervals ([2.5:4.9]
m and
[5.8:11.6]
m). For faint sources, PHT-S was able to
obtain spectra with a higher S/N ratio than SWS, albeit at much lower
spectral resolution (
90). We retrieved the resulting spectra from the
latest version of the off-line processing data reduction (OLP 10). For
HD 97048, HD 104237, HD 135344 and WW Vul,
two or three PHT-S spectra were available; we merged the spectra.
The ISO-CAM instrument also had a spectrophotometric mode
(CAM04). This mode made it possible to take a series of images at variable
central wavelengths (between 5 and 15 m). We extracted the
spectrum by applying synthetic photometry to the images with a
circular aperture with a diameter of 15
(comparable with the spatial
resolution of SWS and CAM).
Only 5 CAM04 spectra are included in this analysis. Again we used the
results of the latest version of the off-line processing data
reduction pipeline OLP 10.
The resulting spectra of the data reduction are compiled in Figs. 1-3 for ISO-SWS, ISO-PHT and ISO-CAM, respectively.
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Figure 1:
The reduced 2.4-15 ![]() |
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Figure 2:
The reduced 2-5-11.6 ![]() |
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Figure 3:
The reduced 5-15 ![]() |
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In our analysis, we focused on the infrared emission bands at 3.3, 6.2, 7.7, 8.6 and 11.2 micron, attributed to polycyclic aromatic hydrocarbons and the features at 3.4 and 3.5 micron, linked to nanodiamonds. The amorphous silicate feature around 10 micron and the crystalline silicate feature at 11.3 micron were also included in the analysis. The 11.2 and 11.3 micron bands, when present, are blended. Due to the low spectral resolution, we cannot distinguish between the two. Therefore we consider the blend as one complex, which we call the 11 micron feature.
To characterize these features in the spectra, we measured the line flux, full width at half maximum (FWHM), peak wavelength, peak flux, continuum flux at the peak wavelength and the equivalent width (EW).
We wrote an IDL procedure, measfeat.pro, to
manipulate the spectra
in order to determine the desired measurements. The IDL code allows us to
indicate continuum points by hand. Based on these points a spline function
is fitted to estimate the continuum. Integrating the continuum subtracted
flux, the line flux and EW of the features were computed.
We repeated the procedure, indicating an "extreme'' continuum,
to estimate the systematic error introduced by the continuum
determination. It appears that the uncertainty in the line flux varies from about
10% for the PAH features and the 10 micron feature, up to
30% for the 11 micron feature. Furthermore, the IDL program
determined the peak position and peak flux by fitting a Gaussian
function to the continuum-subtracted line profile.
The peak flux was set equal to the maximum of the fit, the peak
wavelength is the wavelength of this maximum. When the feature was in
absorption, the peak flux was defined as the minimum of the fit. In
the latter case the peak wavelength is the wavelength at which this
minimum occurred. The value of the continuum flux at the peak wavelength
is also recorded.
Note that, because of the possible skewness of the features, the peak
wavelength in general does not coincide with the centroid
wavelength. This may lead to systematic differences between the values
used in this paper and values in the literature.
The FWHM was deduced by computing the width at half the peak flux of the smoothed line profile. The smoothing of the spectrum was done to diminish the influence of the noise in the spectrum on this measurement.
When a feature was not detected, we deduced upper limits for its
line flux and peak flux, and we computed the continuum flux at the
expected central wavelength of the feature. The upper limits were
computed in the following manner; we used the averaged spectrum of
CD-4211721 and HD 97048 (both sources with
fairly typical
PAH spectra; Sect. 3.1) as a template spectrum for
the PAH emission, the spectrum of HD 97048 for the 3.4 and 3.5 micron
NAN emission and
the spectrum of HD 150193 as a template for the 10 micron amorphous
silicate emission. For non-detections, we assumed a peak emission of 5 times the noise on the spectrum at the theoretical central wavelength
of the feature as an upper limit for the peak flux. Scaling the
template spectra to this level, we extracted the upper limits for the
line flux.
This procedure was applied to the SWS, PHT-S and CAM spectra. Since the spectral resolution of the PHT-S and CAM data is not even 100, only the line flux, EW, peak flux and continuum flux at the theoretical central wavelength were determined for the latter; the other measurements are not meaningful in this case.
For 24 sources, SWS as well as PHT-S spectra were available. We independently measured the desired quantities of the features, when present in both spectra. Comparing these two sets of values, we noticed that there is an offset between the two; the computed PHT-S line fluxes are systematically lower than their SWS counterparts. We attribute this difference to the fact that at 2.8 times the Nyquist sample spacing, the PHT-S data are insufficiently sampled in wavelength to recover the full spectral response. The result of this is that we will lose flux around sharp gradients in the spectra. As an example we show in Fig. 4 the PAH 6.2 feature of HD 100546, as it appears in the SWS and PHT-S spectra. The line flux in the PHT-S spectrum is clearly underestimated. In Table 2 we summarize the ratios of the line flux measured in the PHT-S spectrum over the line flux measured in the SWS spectrum (PHT/SWS) for all detected features. Note that most of the ratios (81%) are smaller than unity, which is consistent with our interpretation. The given errors are the statistical errors on the measurements. Ratios that are larger than one can be due to spatially extended emission or intrinsically varying PAH emission.
Table 2:
The ratios of the line flux of the features measured in the
PHT-S spectrum over the line flux measured in the SWS spectrum
LF
/LF
.
The given errors are the statistical errors on
the measurements.
We calculated the mean ratio of the detected SWS line flux over the PHT-S line flux for each feature. The values of these quantities are 1.18, 1.34, 1.17, 1.16, 1.18, 1.08 and 0.58 for PAH 3.3, NAN 3.4, NAN 3.5, PAH 6.2, PAH 7.7, PAH 8.6 and the 11 micron feature respectively. The PHT-S measurements were adjusted by multiplying with these mean values. The factors were also applied to correct the upper limits.
Note that the amorphous 10 micron silicate feature is only partially sampled in the PHT-S spectra, since the wavelength coverage ends at 11.6 micron. Therefore it is difficult to determine the underlying continuum in the PHT-S spectra. Nevertheless, we indicated the continuum in a consistent manner. Similarly to the PAH features, we computed the line flux ratio of detected 10 micron emission features in both the SWS and the PHT-S spectrum, and computed the mean value. By applying this correction factor (0.86), we compensate for this systematical difference between the measurements of the SWS and PHT-S 10 micron feature.
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Figure 4: The PAH feature at 6.2 micron of HD 100546 in the SWS spectrum (dashed line) and in the PHT-S spectrum (full line). |
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The comparison between the CAM and SWS spectra of HD 97048 showed that
the results are equal within a 15% range. No corrections for the CAM
results were applied. We decided not to use the CAM spectra of
HR 5999, HD 179218, HD 200775 and HD 97048
in the analysis; the first two spectra are inhomogeneously covered in
wavelength, which leads to insufficiently sampled features. The CAM
spectrum of HD 200775 is quite different from the SWS spectrum;
the first displays PAH
emission, while the latter does not. We relate this to extended
emission, visible in the CAM spectrum because the (synthetic) aperture
is larger than the aperture for the SWS spectrum at these wavelengths.
The CAM spectrum of BD+463471 is the only spectrum of
this source at our disposal. We added the measurements to our data
set, even though we noticed unusual features in the spectrum, which
could be artefacts (e.g. the "absorption feature'' at 10 micron).
The ISO spectra were compared to space- and ground-based photometric
measurements from the literature; we plotted the photometric points
from the MSX catalog over the reduced spectra. In general, the mean
deviation between the
spectra and the photometry is small and non-systematic. Sources where
the anomaly exceeds the 20% level are HD 104237, HD 142666, MWC 297, BD+404124, V645 Cyg and
MWC 1080. Except for BD+40
4124, the
photometric fluxes in these spectra are higher than the SWS or PHT-S
fluxes. We attribute the differences to spatially extended emission and
different beam/aperture sizes, although we cannot exclude the
possibility of intrinsic infrared variability of (some of) our sources.
After the measurements of the features, we condensed the data set into a final set of numbers: one measurement for each star and examined feature. The presence of the features in the HAEBE spectra is summarized in Table 3. The sources are sorted according to their IR-SED classification (see Sect. 2.6).
When only a SWS, PHT-S or CAM spectrum was availaible for a source,
those data were taken. For sources with a SWS and a PHT-S spectrum,
three possibilities arise; the feature is detected in both spectra,
the feature is undetected in both spectra or the feature is detected
in only one of the two spectra. In the first case, the data are merged
using a weighted average, based on the statistical errors on the
measurements. In the second case, the most stringent upper limit for
the feature is taken. In the third case, when the measured line flux
of the detected feature is lower than the upper limit derived from the
other spectrum, the data are consistent and the values of the
measurement are selected. In the inconsistent case (the upper limit is
lower than the measured
line flux), we set a conservative upper limit for the feature. The
latter only occurred in 4 cases (out of 192 measured
features). This behaviour might be either a result of the different
aperture sizes used by SWS and PHT-S, in which case this could
indicate the existence of spatially extended emission, or a first
indication that the strength of PAH emission may vary with time in
some of our sources. We indicated these measurements in
Table 3 with
and
,
according
to detection in the PHT-S or SWS spectrum respectively.
The computed line fluxes used in the analysis are listed in Table 4. Upper limits are indicated as well. Table 5 summarizes the continuum fluxes at the peak wavelength of each feature.
To characterize the spectral energy distribution (SED) of the sample
sources, several
quantities were determined, based on UV to millimetre (mm) photometry from the
literature. The photometry consists of ANS and IUE ultraviolet data,
ground-based photometry in the Walraven, Johnson/Cousins, and near-IR
JHKLM photometric systems, IRAS and MSX data, and single-dish (sub)-mm
photometry collected from the literature. The effective temperature
of each source was determined from its spectral type listed in
Table 6, using the calibrations from Schmidt-Kaler (1982).
The visual extinction
and absolute luminosity L of the central
star were computed by fitting and integrating a Kurucz (1991) model for
the stellar photosphere to the de-reddened photometry, adopting the
distance estimates listed in Table 6.
The ionising (>13.6 eV) luminosity
and the
UV (2-13.6 eV) luminosity
of the source were computed using this Kurucz model fit. We also computed
the IR-excess
luminosity
,
using a spline fit to the infrared data,
and the absorbed luminosity
of the source, which is the
difference between the theoretical Kurucz model and the reddened model. The
IR-excess flux in the K band (2.2
m), at 60, 850, and 1300 micron
was deduced by subtracting the
Kurucz model at these wavelengths from the observed fluxes. When no
1300 micron photometry was available, we included 1100 micron
measurements, which we then converted into 1300 micron
flux values by multiplying with the average 1300/1100 ratio deduced
from stars for which we had both measurements.
The last parameter used to characterize the SED is the observed bolometric
luminosity
,
which is the total integrated luminosity of the
SED. The latter quantity is not corrected for extinction, as
opposed to the stellar luminosity L. The stellar parameters used in
this study are listed in Table 6.
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Figure 5:
Diagram based on van Boekel et al. (2003). The ratio
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The sample sources were classified into different groups, based on the shape
of their SED. The quantities used to
characterize the infrared spectral energy distribution of HAEBEs are
the ratio of
(the integrated luminosity derived from the
broad-band J, H, K, L and M photometry) and
(the corresponding
quantity derived from IRAS 12, 25 and 60 micron points), and the non-color-corrected
IRAS [12]-[60] color
(Dullemond et al. 2003; Acke et al. 2004; van Boekel et al. 2003). These parameters compare the
near-IR SED, which is expected to be similar in HAEBEs
(Natta et al. 2001; de Winter et al. 1995; Hillenbrand et al. 1992), to the mid-IR SED, where
the major differences in SED shape occur.
M01 group I sources are stronger mid-IR emitters than group II sources.
The luminosity ratio
represents the strength of the
near-IR compared to the mid-IR excess, which is lower for group I than
group II sources. The shape of the mid-IR SED of a group I source is
"double-peaked'' compared to the SED of a group II member. The IRAS[12]-[60] color index provides a quantitative measure for this
difference in SED shape. Group I sources are redder than their group II counterparts.
We use the diagram given in
Fig. 5 to distinguish between group I and group II in
the classification of M01. The dashed line represents
,
which
empirically provides the best separation between the two groups.
Six of our sample stars display the amorphous 10 micron feature in absorption. These objects are believed to possess disks whose luminosity is dominated by viscous dissipation of energy due to accretion, are deeply embedded systems and hence are fundamentally different from the other sample stars. We therefore classify them in a different group: group III.
BD+404124, R CrA and LkH
224 have
not been classified
based on their appearance in Fig. 5. Confusion with
background sources in the IRAS photometry prohibited us from deriving the
quantities needed to plot these objects in the
diagram. BD+40
4124 has been classified as a group I source,
because its SED resembles the SED of HD 200775. R CrA
and LkH
224 are both UX Orionis stars
according to the definition of Dullemond et al. (2003): both sources have
a spectral type later than B9 and
optical variations larger than 1 mag on timescales of days to weeks.
Therefore we classified them as group II members.
The SEDs of all sample sources are displayed in Fig. 6. The group to which the objects belong is indicated as well. The SEDs are overplotted with the ISO spectra. Squares with error bars indicate photometric measurements from the literature, arrows upper limits.
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Figure 6: The spectral energy distributions of the sample stars. The squares are the observed photometric data from the literature with error bars. The solid line is the reddened Kurucz model for the stellar photosphere, fitted to the measured UV-optical photometry. The ISO spectra are plotted in the SEDs as well (noisy solid line). |
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First we investigated the general correlations of the different PAH
features in our spectrum. We compared the line fluxes (LF) of the
different features, which we normalised by the continuum flux at the
peak wavelength (CF), to correct for the
distance-to-the-star dependence of the line flux.
Figure 7 shows the ratio LF/CF for the PAH 7.7
micron feature versus PAH 6.2 micron; Fig. 8 is a
similar plot for the ratio LF/CF of PAH 6.2 micron versus PAH 3.3 micron. The correlation coefficient
in the first plot is 0.90,
while the data in the second plot are less correlated (
). The other PAH features display comparable behaviour. It is
striking though that PAH features that are linked to the CC bonds
(6.2, 7.7 micron) correlate better with each other, as well as the
features linked to the CH bonds (3.3, 7.7, 8.6 micron).
However, as a first order approximation we can state that the LF/CFratios correlate fairly well over a range of a few orders of
magnitude.
Supposing that all HAEBEs have chemically identical PAH molecules, the
PAH spectra of this group of stars should be uniform. Two effects that
could influence the shape of this "unique'' spectrum are the UV field
of the central star and interstellar extinction. If we presume that the
wavelength dependence of the
first does not vary much from source to source, the dominant source of
deviations from a simple linear correlation between the PAH
feature-strengths would be extinction. This will have a stronger effect
on PAH features that are close to the amorphous 10 micron silicate
feature. In Fig. 9 the logarithm of the ratio of the
line fluxes of the PAH feature at 8.6 micron over the feature at 6.2
micron,
,
is plotted versus the logarithm of
the ratio of the line fluxes of the 3.3 micron PAH feature over the
6.2 feature,
(following Hony et al. 2001).
To include the extinction effect into the plot, we
applied the interstellar extinction law of Fluks et al. (1994) to the
line fluxes of MWC 1080 according to the formula
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Figure 7: The LF/CF ratio of PAH 7.7 micron versus PAH 6.2 micron. The filled diamonds with error bars indicate detected features, arrows represent upper limits. |
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Figure 8: Similar plot as Fig. 7. The LF/CFratio of PAH 6.2 micron versus PAH 3.3 micron. |
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Figure 9:
The logarithm of the ratio of the line fluxes of the PAH
features at 8.6 and 6.2 micron
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Table 7:
The mean peak wavelength ,
FWHM and
peak-over-continuum flux ratio of the detected
features.
Only a small minority of stars in this sample display nanodiamond
features in their spectrum. Both examined features (at 3.4 and 3.5 m) were detected in the spectra
of Elias 3-1 and HD 97048 (as thoroughly
described by Van Kerckhoven et al. 2002), but also in the spectrum of
BD+40
4124.
The spectrum of HD 100546 possibly contains the 3.4 micron feature,
while the 3.5 micron feature was detected in the spectrum of MWC 297
(see also Terada et al. 2001).
Tables 7 and 8 contain general information
about the NAN features. Nevertheless, because of the small number of
sources in this sample that display 3.4
or 3.5 micron emission, no stringent conclusions on the conditions
needed to see these features can be
drawn. We are currently carrying out a ground-based 3.2-3.6 m
spectroscopic survey of a much larger sample of HAEBEs, which should
allow us to draw more definitive conclusions on possible correlations
of the NAN and the PAH features in the near future.
In the sample of HAEBEs presented in this paper, most of the sources
(52%) have the amorphous 10 micron silicate feature in emission; 6
out of 46 objects (13%) have the feature in absorption, while it is
undetected in the other cases. We note that at least in some cases,
the presence of a 10 m feature may be masked by the presence
of strong PAH emission in the 8.6 and 11.2 micron bands, so the
true fraction of sources with amorphous silicates is likely
higher than reported here.
The LF/CF correlation coefficients of the 10 micron feature and the PAH features are low. In Fig. 10 the LF/CF ratio of the 10 micron feature is plotted versus that of the PAH 6.2 micron feature. The two ratios correlate poorly, especially when one takes into account the upper limits (arrows) on the left-hand side in the plot. These arrows represent spectra in which the 10 micron feature is present, but where the PAH 6.2 micron feature is undetected. Similar plots in which the other PAH features are plotted against the 10 micron feature show the same result: the strength of the emission of PAHs is uncorrelated with the strength of the amorphous 10 micron silicate feature.
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Figure 10: The LF/CF ratios of the amorphous 10 micron silicate feature (Si 9.7) versus the PAH 6.2 micron feature. Diamonds with error bars indicate detected features, arrows represent upper limits. Sources with the 10 micron feature in absorption have a negative LF/CF ratio. |
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We now consider the peak-over-continuum flux ratio (PF/CF) of the 10 micron feature in our sample. We plotted this dimensionless quantity for all detected 10 micron emission features in the sample in Fig. 11. The histogram peaks around the average value 1.91, which means that the peak of the 10 micron emission is on average twice as high as the underlying continuum. The minimum and maximum value for the ratio in the sample is 1.21 and 2.69 respectively. There does not seem to be a difference between the 10 micron emission feature in group I and group II sources. The average PF/CFratios for the two groups are 1.85 and 2.03 respectively. Strong as well as more modest amorphous 10 micron silicate features appear in both group I and group II sources. The feature can also be absent in both groups.
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Figure 11: The cumulative histogram of the PF/CF for the amorphous 10 micron silicate feature. The 24 sample stars in which this feature is in emission are included in this plot. The lower filled part of the bars represents group I sources, the upper part group II sources. |
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Table 8: The mean line-flux ratios of the examined features. Indicated in the table is the mean LFA/LFB of all sample stars for which both A and B were detected.
The 11 micron feature is detected in 48% of the spectra. It is
unclear how many of these detected features include the PAH 11.2
micron band, since this feature can be blended with the crystalline
silicate feature at 11.3 micron. Figure 12
is similar to Fig. 9. The logarithms of the
ratios
LF11/LF6.2 and
LF3.3/LF6.2 are plotted. Again,
the straight line indicates the variable extinction line, based on the
line fluxes of MWC 1080. Four sources (AB Aur,
Elias 3-1, HD 100546 and HD 179218) lie
significantly to the right of
this line. This suggests that these sources have an 11 micron
feature that contains both PAH and crystalline silicate
emission. The presence of crystalline silicates in HD 100546 and
HD 179218 was already well-established from longer-wavelength
ISO data (Malfait et al. 1998,1999). The detection of
crystalline silicates in AB Aur and Elias 3-1 is a new result
from our quantitative analysis.
We stress that the detection of a stronger than expected
11 micron feature in the four sources mentioned above
does not mean that other sources cannot have crystalline
silicates; it could just be masked by the presence
of strong PAH emission at 11.2 m, or the crystalline
silicates could be cooler than a few hundred K, causing
only very weak emission in our diagnostic band at 11 microns.
When plotting the FWHM of the feature against the ratio
(Fig. 13), the same four
sources appear to have much higher FWHMs. The horizontal line in the plot
indicates the average FWHM (with exclusion of the 4 objects mentioned
before). This also points to the presence of 2 blended emission
features. Even so, the peak position of the 11 micron feature in AB
Aur, Elias 3-1, HD 100546 and HD 179218 is comparable with the average
peak wavelength (see Fig. 14). This might indicate that
the peak position of the crystalline silicate feature varies, which
can be attributed to a variety in its mineorological composition. We stress
however that the uncertainties in these determinations are large.
Sample stars that do not display significant PAH emission at 3.3, 6.2, 7.7 or 8.6 micron are not expected to have significant PAH 11.2 micron emission. Hence a detected 11 micron feature in the spectra of these sources is likely to be solely due to crystalline silicate emission. In the present sample HD 142527 (group I), HD 142666, HD 144432, HR 5999, HD 150193, HD 163296 and VV Ser (all group II) meet these requirements. If these objects indeed display the crystalline silicate band in their spectrum, then the number of sources with crystalline silicates in this sample is at least 11 (24%).
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Figure 12:
Same type of plot as Fig. 9.
The logarithm of the ratio of the line fluxes of the PAH
features at 3.3 and 6.2 micron
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Figure 13:
The FWHM of the 11 micron feature versus the logarithm of
the ratio of the line fluxes of the features at 11 and 6.2 micron
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The 11 micron complex does not correlate with the PAH 6.2 micron feature, and reaches better agreement with other CH bond related PAHs (3.3, 7.7 micron), which is expected, since the 11.2 micron feature is due to a CH bond itself. The correlation with the PAH 8.6 micron feature, on the other hand, is weak.
In Fig. 15 the ratio LF/CF of the amorphous silicate feature versus the 11 micron complex is plotted. There does not seem to be a correlation between the two features. This can be interpreted in two ways. The 11 micron feature could be mostly dominated by PAH emission in this sample of stars, which leads to a poor correlation with the 10 micron feature like for the other PAH features (see Sect. 3.3). The other possibility is that the crystalline 11.3 micron silicate band - when present in the 11 micron complex - does not correlate with the amorphous 10 micron silicate feature. Note that also the four "crystalline silicate stars'' mentioned above do not display an obvious trent. This could be interpreted in terms of the second explanation.
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Figure 14:
The peak position of the detected 11 micron features versus
the ratio of the line fluxes of the 11 micron feature over the 6.2 PAH
feature
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Figure 15: The LF/CF ratios of the amorphous 10 micron silicate feature (Si 9.7) versus the 11 micron feature. Diamonds with error bars indicate detected features, arrows represent upper limits. Sources with the 10 micron feature in absorption have a negative LF/CF ratio. |
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Except for Z CMa, no crystalline silicate absorption or emission is observed in sources which display amorphous silicate absorption (group III). The spectrum of Z CMa does display a local minimum around 10.7-11.5 micron, which might be due to crystalline silicate absorption. In Fig. 16, the spectrum of Z CMa is plotted. For reasons of clearness it was inverted, making the absorption features appear as emission features. Two sources (HD 144432 and HD 163296) that are likely to display a crystalline silicate emission feature at 11.3 micron, are also plotted as a reference.
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Figure 16:
The inverted spectrum of Z CMa, and the emission band
spectra of HD 144432 and HD 163296. For Z CMa,
the figure shows the inverted continuum-divided, continuum-subtracted
flux
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Using the luminosities and effective temperatures computed in
Sect. 2.6, we plot the stars in our sample in a traditional
Hertzsprung-Russel (HR) diagram. There is no
strong correlation between the classification of the sources and the
position in the HR diagram. In Fig. 17, the plotting
symbols are proportional to the observed PAH luminosity (
;
Appendix). In this plot, theoretical
pre-main-sequence (PMS) evolutionary tracks of stars with masses M = 1.5, 2, 3
and 5
are plotted (Bernasconi 1996). Furthermore,
the zero-age main sequence (ZAMS) is indicated by a dashed line. Most
of the stars lie close to the main sequence (MS), which is represented by the
dotted line. Based on this diagram, the masses of the bulk of the sample
stars lie between 1.5 and 3
.
The strength of the PAH bands does not seem to be strongly correlated
with stellar mass or age. Figure 18 is a plot similar to
Fig. 17 for the amorphous 10 micron silicate emission
feature. The plotting symbols are proportional to the
peak-over-continuum flux PF/LF of the feature. The strength of
this solid-state band appears to be also uncorrelated with stellar mass and
age.
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Figure 17:
The Hertzsprung-Russel diagram for the sample stars. The
plotting symbols are proportional to the PAH luminosity. Filled
diamonds indicate detected emission, open symbols represent upper
limits. The full lines indicate the PMS evolutionary tracks of
stars with masses M = 1.5, 2, 3 and 5 ![]() |
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Figure 18: Similar plot as Fig. 17. The plotting symbols are proportional to the peak-over-continuum flux of the amorphous 10 micron silicate emission feature. |
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One of the main characteristics of HAEBE stars is the IR excess due to
thermal emission of circumstellar dust. In Table 6, the ratio of this
excess luminosity
over the stellar luminosity L is
included. Ten of the sample sources have an IR-excess luminosity that
is larger than the stellar luminosity. This means that these
sources cannot have passive reprocessing circumstellar disks.
There does not seem to be a link between the classification of the
sample stars and their
ratio. The average value for the
sources for which the ratio is smaller than unity, is
for group I,
for group II and
for group III.
Half of the group III objects have a ratio bigger than one, though, which is
consistent with the idea that these sources are still actively
accreting systems.
In Fig. 19 the logarithm of the IR-excess luminosity
is plotted versus the logarithm of the absorbed
luminosity
.
The dashed line represents
.
Most of the sample stars lie close to this line.
Exceptions include the group II source in the upper left corner which
represents R CrA and the open diamond at the bottom of the plot,
representing HD 141569. The IR excess luminosity of R CrA is
due to confusion with the nearby source CrA IRS7 (Choi & Tatematsu 2004).
Furthermore, a set of group I sources with low
values, display relatively high
values. An explanation for this
extra IR excess luminosity could be the presence of a
late-type companion. Also the orientation of the system can play a role,
especially in group I sources, where the flaring of the disk implies that the
line of sight to the central star already passes through the disk at
fairly low inclinations (
).
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Figure 19:
The logarithm of the IR-excess luminosity
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The different parameters that characterize the SED were compared to the parameters that describe the infrared solid state bands. In this section we summarize the results.
The UV luminosity of the central star correlates with
the PAH luminosities: the absolute PAH luminosity increases with
increasing UV radiation. The ratio
however
decreases with increasing UV luminosity. In
Fig. 20, the latter correlation is demonstrated. The
dashed line in this figure represents
or equivalently
.
If the emitted PAH luminosity would
increase linearly with increasing UV radiation of the central star
(
), the ratio
would have been constant for all
.
This is obviously not the case. A higher stellar UV
luminosity indeed increases the PAH luminosity, but does this in a
sub-linear way: the PAH luminosity increases with
where p=0.6<1. It appears that either the
efficiency of the
absorption/emission process decreases with increasing UV strength, or
that the increasing hardness of the UV photons plays a role.
Since hotter stars emit not only more UV photons, but also relatively
more high-energy UV photons, the observed trend might be a contrast effect.
Short-wavelength UV photons cannot be
absorbed by PAHs (no increase of
), but do
contribute to the total UV luminosity
,
hence decreasing
the luminosity ratio.
It is quite remarkable that some group II sources, even though they have
comparable UV luminosities as their group I counterparts, do not
display PAH emission. This indicates that the appearance of PAH emission
is linked to the group I/II classification of the sources, and does not
depend on
only.
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Figure 20:
The PAH-over-UV luminosity
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Suppose that the IR emission is due to PAH molecules that
are homogeneously distributed in a spherical, optically thin halo around the star,
which has the radius of the ISO-SWS beam (11
)
at the appropriate
distance. It is then possible to estimate the mean particle density of the PAH
molecules (
;
see Appendix) in that halo. In Table 9 the computed values
are listed. The detected emission of the PAH features at 3.3, 6.2,
7.7 and 8.6 micron has been included. Note that not all emitted PAH
flux (e.g. the PAH 11.2 micron flux) is accounted for. This implies
that - under the given assumptions - the computed molecule density is a
lower limit.
For most objects, the resulting values are high compared to the
typical ISM PAH density (
;
Appendix). We interpret this as an indication that the PAH emission
does not emanate from a halo around the central star. For most stars,
a denser environment is needed to explain the observed emission.
Table 9:
Supposing a homogeneous distribution of the PAH molecules in
a halo around the central star, with a radius of the size of the
ISO-SWS beam (11
)
at the appropriate distance, the PAH particle density
is
computed based on the
ratio. All objects in which PAH
emission is detected are adopted in this table and are listed
according to decreasing PAH-over-UV luminosity.
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Figure 21:
The PAH 7.7 micron luminosity
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Habart et al. (2004) have modelled PAH emission in flared circumstellar disks. In their models, they assume PAH molecules containing 40-100 carbon atoms. For PAHs of these sizes, the influence of ionization and photo-evaporation affects the line-flux ratios of the PAH features, in particular the relative strength of the 3.3 micron feature. Even though a clear dependence of the PAH line fluxes on the stellar UV luminosity was observed in our present sample, no correlations between the relative strength of the PAH features and the UV radiation were noted. The line-flux ratio of the 3.3 and 6.2 micron feature - which ranges from 9 to 94% - is independent of the central star's UV field.
The excesses in the K band, at 60, 850 and 1300 micron were compared to the PAH data. Figure 21 shows the luminosity of the PAH 7.7 feature versus the excess at 60 micron. The sources with the faintest 60 micron excesses (by definition mostly group II objects) are those that have the faintest PAH emission. Other PAH features display comparable behaviour.
Notwithstanding these correlations, in general the luminosities of the IR emission bands do not correlate well with the other measured excesses. The lack of correlation between the 850 or 1300 micron excess and the strength of the PAH features shows that the disk mass has no influence on the resulting PAH spectrum. This is consistent with the idea that PAH emission emanates from the surface layers (atmosphere) of the circumstellar disk, which contains only a small fraction of the disk mass. The FWHMs of the IR PAH bands do not correlate with the excesses either.
The Rayleigh-Jeans part of the SED can be modelled with a power law,
in which n is the
(sub-)mm spectral index. There is no clear correlation between this
index and the strength of
the PAH features. The sub-mm slope of the spectrum can be a proxy for
the grain size
distribution of the cold, large grains in the outer parts and
mid-plane of the disk (Acke et al. 2004, henceforth A04). This
non-correlation indicates that the grain
size distribution of the bulk of the disk's material has no influence
on the PAH emission. This is again in agreement with the hypothesis that PAH
molecules are excited in and radiate from the disk's atmosphere.
The parameters that describe the amorphous silicate 10 micron feature do not correlate with the IR flux excesses. This indicates that the strength of the 10 micron feature does not depend on the disk mass. The LF and FWHM of the silicate band do not correlate with the sub-mm spectral index either. The grain size distribution of the cold particles in the outer parts of the disk and the warm small silicate grains are independent of each other. Nevertheless, if vertical mixing in the disk is efficient, one may expect that only the smallest grains make it to the surface. Hence the amorphous 10 micron silicate feature may be indicative for the smallest grain sizes (still) present in the disk (A04).
We compared the SEDs of the four stars for which the 11 micron feature
is a blend of the PAH
11.2 micron band and the crystalline silicate feature at 11.3 micron
(Elias 3-1, AB Aur, HD 100546 and HD 179218) with the SEDs of the other sample
stars. The sub-mm spectral index n, 10 micron PF/CF ratio and 1300 micron flux excess of these group I sources make them normal
members of this group. The objects AB Aur, HD 100546 and
HD 179218 have steep sub-mm slopes (), high 10
micron PF/CF values (larger than the average value 1.91), and
intermediate 1300 micron excesses (
10 mag).
The circumstellar
material of these sources thus contains cold grains in the outer parts
of the disk, significantly smaller than mm-sized (AV04) and small
(
0.1
m) warm silicate grains (van Boekel et al. 2003; Bouwman et al. 2001).
The estimated disk mass is of the order of 1% of the
stellar mass. Elias 3-1 on the other hand is somewhat more
peculiar, since n is equal to 3.22 and the 10 micron peak-over-continuum flux
ratio is 1.70. The circumstellar matter in this system
seems to be more evolved. The disk mass of this source is also high
compared to the stellar mass (30%). This is close to the theoretical
upper limit for gravitational stability (e.g., Gammie 2001).
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Figure 22:
Similar plot as Fig. 5. The plotting symbols
are scaled proportional to the strength (
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We use plotting diagrams like Fig. 5, and include the strength of the IR spectral features by scaling the plotting symbol proportional to LF/CF. In Fig. 22, the sum of the LF/CF ratios of the PAH features at 3.3, 6.2, 7.7 and 8.6 micron is indicated in this manner. The filled diamonds in the plot represent detected features, while open diamonds stand for upper limits. Squares refer to group III sources. The sizes of the diamonds and squares represent the strength of the PAH features on a logarithmic scale. We specifically indicate group III sources since, because of their location in the diagram, they could be confused with group I sources. That group III sources appear in the group I part of the plot is to be expected, since these sources are believed to be in an earlier evolutionary stage; the objects are still highly embedded in the circumstellar environment, which blocks out the stellar light. This makes their SEDs very red and hence they show up in the lower right part of the diagram.
It is striking that the sources that display the strongest PAH emission are mostly group I sources. The Dullemond (2002) model suggests that these objects have circumstellar disks with a flaring geometry. PAH molecules in the atmosphere of the flaring part of these disks are directly irradiated by the UV photons of the central star. This implies that the molecules will be excited and will radiate in the IR, no matter how far out in the disk, as long as the UV field is strong enough. PAH molecules in the atmosphere of the self-shadowed, geometrically flatter disks of group II sources barely see any direct UV radiation, because the puffed-up inner rim blocks out most of the starlight. Hence, in these systems the IR emission bands are expected to be much fainter. Most objects in our sample are consistent with this hypothesis.
Except for MWC 297, no PAH emission is detected in group III sources. The circumstellar matter of these highly embedded objects can probably accurately shield the stellar UV radiation close to the central star. The volume of excited PAH molecules will likely be too small to be observable. Furthermore, the PAH IR radiation has to travel through the dense circumstellar environment. This prohibits the appearance of PAH emission in the group III spectra.
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Figure 23: Similar plot as Fig. 22 for the amorphous 10 micron silicate feature. The plotting symbols are scaled proportional to the LF/CF ratio of the feature. The filled diamonds indicate stars with a detected 10 micron emission feature, the open diamonds indicate upper limits. |
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In Fig. 23 the LF/CF ratio of the amorphous 10 micron silicate feature is plotted in the
versus
[12]-[60] diagram. Again, the filled diamonds refer to the detected
emission features, while the open diamonds represent upper
limits. Group III sources, which display amorphous silicate absorption,
are not included in this plot. Note that the
silicate emission feature appears in both groups. When present, the
strength of the emission feature seems to be independent of the
classification of the source. Nevertheless, a larger fraction of group I sources have an undetected amorphous silicate feature (10/18 versus 6/22 in
group II). M01 suggested that this is probably a selection effect; group II
objects that do have 10 micron silicate emission are brighter, and
hence are observationally favoured over group II sources without
silicate emission. This selection effect plays a smaller role for
group I sources, since they are by definition brighter IR emitters.
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Figure 24: Similar plot as Fig. 5. The filled symbols indicate UXORs, the open symbols are the other HAEBEs in this sample. The 3 UXOR stars that display clear PAH emission are VX Cas, HD 34282 and RR Tau. |
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According to the definition of Dullemond et al. (2003), 13 of our
sample sources are UX Orionis (UXOR) stars. In their paper, the
authors argue that the disk geometry of this class of stars must be
self-shadowed and hence that UXORs are group II objects. In the
present paper we have shown that PAH emission in group II sources is
weak. Of the 13 UXORs in our sample, only 3 sources
do show significant PAH emission: VX Cas, HD 34282
and RR Tau. They are labelled in
Fig. 24. We note that all three lie close to the
empirical separation line between groups I and II; they might be
transitional objects in which the shadow cast by the inner rim is
relatively small. We remind the reader that
R CrA and LkH
224 are not in the plot, and that
these sources were classified based on their UXOR behaviour.
In Fig. 25, a selection of 14 of our
sample stars shows that the 10 micron emission feature tends to
become broader when the peak-over-continuum flux ratio
decreases. Small warm silicate grains (0.1
m) cause a
distinct feature that
peaks around 9.7 micron, while larger grains (
2
m) induce a
broader, less pronounced feature with a peak towards longer
wavelengths (Bouwman et al. 2001). van Boekel et al. (2003) interpret this
sequence in terms of grain growth in the disk. In
Fig. 25 this sequence is reproduced. Our analysis
confirms observationally the correlation between the shape and the strength of the
amorphous 10 micron silicate band. In their Letter,
van Boekel et al. (2003) only plotted group II sources. In this study we
have shown that the 10 micron feature is independent of the classification of the
objects. Therefore, we also allowed group I sources in Fig. 25.
The correlation plot (Fig. 26) of the FWHM versus the
PF/CF ratio of the
amorphous 10 micron silicate feature shows that indeed the FWHM is
larger when the peak-over-continuum flux ratio is lower. Again, no
difference is observed between group I and group II sources. The
dashed line represents the best fit and has a slope of -0.38 micron.
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Figure 25:
The amorphous 10 micron silicate band for 14 of the sample
stars. The figure shows the continuum-subtracted flux
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Van Boekel et al. (2003) suggested the amorphous 10 micron silicate
emission originates from the disk's surface. It might
appear contradictory that the strength of PAH emission
does depend on the shape of the SED (which represents the disk
geometry), while the 10 micron feature does
not. Nevertheless, the excitation mechanisms for the two
emission sources are not the same: the non-equilibrium PAH emission
occurs during temperature fluctuations after absorption of a UV
photon while the larger silicate grains are in thermal equilibrium
with the radiation field of the central star.
The models of Habart et al. (2004) show that the PAH emission
emanates mostly from the outer parts of the disk (100 AU),
while the warm silicate emission is confined to the innermost disk
regions (
a few AU). This reflects the different excitation
mechanisms of the small PAH molecules and larger silicate grains, and
is in accordance with the observations: spatially resolved PAH
emission, on scales of
10-100 AU, has been detected around HAEBEs
by Geers et al. (2004) and van Boekel et al. (2004).
The results of the present paper on both the PAH emission and the silicate emission are consistent with a scenario in which Group I sources have disks that are flaring, whereas Group II sources have flatter disks that are shadowed by the disk's puffed-up inner rim.
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Figure 26:
The FWHM of the amorphous 10 micron silicate feature versus
the peak-over-continuum flux ratio PF/CF. Filled and open diamonds
refer to group I and group II sources respectively. The dashed line
represents the best fit to the data;
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We summarize the conclusions which were drawn in the previous sections.
The amorphous 10 micron silicate feature in this sample of HAEBE stars displays the characteristics of grain growth, independent of the classification of the sources. The small hot silicate grains from which this feature originates seem to have little interaction with the PAH molecules, even though they are believed to radiate from the same locus: the disk atmosphere. Crystallinity seems to be present in more than 1/4 of the stars in our sample.
The global interpretation of the SED of HAEBEs in terms of disk geometry is in many ways consistent with the observations. Future work with new generation IR instruments (e.g., Spitzer) and interferometers (e.g. MIDI/AMBER on the VLTI) will provide the community with new high-quality observations of HAEBE stars, which will offer a good test for the hypotheses suggested in this paper.
Acknowledgements
The authors would like to thank Drs. C. Dullemond, R. van Boekel and R. Waters for many useful discussions that led to significant improvements in the manuscript. We are also indebted to Dr. B. Vandenbussche for his help with the reduction of the ISO-SWS spectra. This publication makes use of data products from the Two micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has made use of the Simbad data base, operated at CDS, Strasbourg, France.
Table 1: The sample of HAEBEs used in this study, based on Thé et al. (1994) and Malfait et al. (1998). For each object, the observed ISO spectra are given; S01 refers to SWS AOT01, P40 to PHT-S and C04 to CAM04 spectra.
Table 3:
The presence of the PAH features at 3.3, 6.2, 7.7 and 8.6
micron, the NAN features at 3.4 and 3.5 micron, the 11 micron
complex (COMP 11) and the amorphous 10 micron silicate feature (Si 9.7) in the
spectra of the sample stars. The sources are classified in 3
groups.
:
detection; - : no detection; n.s.: no spectrum available
at these wavelengths.
:
tentative detection;
:
tentative detection in the
PHT-S spectrum;
:
feature
undetected in the SWS spectrum;
:
feature undetected in
the PHT-S spectrum (see Sect. 2.5). For the amorphous silicate feature,
E stands for emission and A for absorption.
BD+40
4124, R CrA and LkH
224 have not been classified
based on the diagram in Fig. 5, which is indicated by the
question mark.
Table 4:
The line fluxes LF of the IR features included in this
analysis. a(-b) represents
.
Table 5:
The continuum flux at the peak wavelength of the IR
features included in this analysis. When the amorphous 10 micron
silicate band was present, the PAH features are superimposed on the
band. In this case (flagged by ), the silicate feature is
regarded as being part of the underlying
continuum of the PAH features. Hence the indicated continuum flux
of the PAH band is the flux of the silicate feature at the central
wavelength of that PAH band.
Table 6:
The stellar parameters for the sample sources. The distance
d which we used in the analysis and spectral type (Sp. T.) are
given. The visual extinction ,
the logarithm of the effective
temperature
,
of the stellar luminosity
,
of the stellar UV luminosity
and of
the observed bolometric luminosity
are presented as
well. The latter quantities are estimated based on the photometric
measurements. Furthermore, the ratio of the IR excess luminosity over
the stellar luminosity is computed. Ten of the sample sources have a
ratio larger than unity, hence are not consistent with a passive
reprocessing disk. The last 4 columns contain the IR excess fluxes in
K (2.2
m), at 60, 850 and 1300 micron.
These
stars are UX Orionis objects according to the definition of
Dullemond et al. (2003).
Probable confusion with
the nearby source CrA IRS7.
We estimate the density of PAH molecules in a spherical, optically thin halo of radius R around the central star, supposing a homogeneous distribution of the PAHs. Furthermore, we assume that all UV radiation absorbed by the PAH molecules is re-emitted in the IR bands.
The cross-section of a single PAH molecule with 50 C atoms is
typically
(Li & Draine 2002).
The luminosity of the PAH emission
depends on the number of emitting PAHs
,
the average
energy of the absorped UV photons
,
the
number of UV photons
that pass by per second and
the cross-section of a single PAH molecule over the average surface S of
the sphere:
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