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Planetary Systems in the Universe: Observation, Formation and Evolution
ASP Conference Series, Vol. ???, 2000
A. Penny, P. Artymowicz, A.­M. Lagrange, and S. Russell, eds.
Protoplanetary disks around Herbig Ae/Be stars:
Indications from ISO spectroscopy
M.E. van den Ancker
Harvard­Smithsonian Center for Astrophysics, 60 Garden Street, MS
42, Cambridge, MA 02138, USA
Abstract. An analysis of solid­state features in infrared spectra of 46
Herbig Ae/Be stars is presented. The presence of solid­state emission
bands is compared to other indicators of circumstellar material, such
as Hff emission, optical variability and sub­mm continuum fluxes. The
correlation between these different indicators is weak, if present at all, in
our sample. However, a strong dependence on spectral type of the central
star seems to be present: stars with spectral type earlier than B9 show
either amorphous silicate in absorption or infrared spectra dominated
by PAH emission, whereas more than 70% of the stars of later spectral
type show silicate emission. We conclude that the infrared spectrum of
Herbig Be stars is in general dominated by emission from the circumstellar
envelope, whereas the lower­mass Herbig Ae stars show a spectrum that
is dominated by a disk that is passively heated by the central star.
1. Introduction
Herbig Ae/Be stars are young intermediate­mass (2--10 M fi ) stars which are still
surrounded by gas and dust from their natal cloud. Many possess circumstellar
disks which are believed to be the site of on­going planet formation. The dust
in these circumstellar disks, heated by the central star and possibly by viscous
heating of material that is being accreted, shows up as excess emission above
photospheric levels at infrared to sub­mm wavelengths. The circumstellar gas
can be traced in spectral lines, of which Hff is most prominent. Large­amplitude
(? 1 m ) variations in optical brightness are seen in some Herbig Ae stars, and
are commonly ascribed to circumstellar dust clouds moving in and out of our
line of sight towards the central star. Although the optical to sub­mm energy
distribution of Herbig Ae/Be 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 com­
plete 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 (Malfait et al. 1998, 1999; van den
Ancker et al. 1999; Meeus et al., these proceedings). Here, the first inventory
of solid­state features in all Herbig stars observed by the Infrared Space Obser­
1

2 M.E. van den Ancker
Figure 1. HRD of the stars in our sample. Plot symbols indicate
the solid state components present in the ISO spectra (see caption).
Also shown in the figure are the pre­main sequence evolutionary tracks
(solid lines) and the birthline (dashed line) by Bernasconi (1996).
vatory is presented and we investigate their correlation with more traditional
tracers of circumstellar material.
2. Data Analysis
An inspection of the ISO data archive revealed the presence of spectroscopic
data on 46 Herbig Ae/Be stars, obtained with the short­wavelength spectrometer
(SWS) and the spectroscopic mode of the photometer (ISOPHOT). Spectra were
retrieved and reduced, after which they were inspected for the following features:
(a) the emission bands at 3.3, 3.4, 6.2, 7.6, 7.8, 8.6, 11.3 and 12.7 ¯m, often
attributed to polycyclic aromatic hydrocarbons (PAHs), (b) the broad band
around 10 ¯m due to amorphous silicates, and (c) sharper emission bands at
10.2, 11.4, 16.5, 19.8, 23.8, 27.9 and 33.7 ¯m due to crystalline silicates. Using
these data, we investigated the correlation of infrared spectral features with
parameters of the systems from literature (T ?
, L ?
, level of optical variability,
Hff profile, and dust masses as traced by sub­mm fluxes).

Protoplanetary disks around Herbig Ae/Be stars 3
3. Discussion and conclusions
The strongest correlation found is between spectral type of the central star and
silicate emission: Herbig stars of spectral type earlier than B9 show silicate ab­
sorption, whereas a large majority (ú70%) of the Herbig Ae stars of later type
show silicate emission. Since strong optical variability due to variable circum­
stellar extinction is also only found in Herbig stars with spectral type of B9 or
later (van den Ancker et al. 1998), both phenomena may be related. However,
no correlation between the level of optical variability and silicate emission could
be found, perhaps due to our limited sample size.
In most sources with absorption due to amorphous silicates, we also ob­
serve the absorption bands due to H 2 O and CO 2 ice, with a relative strength
comparable to that in the interstellar medium. However, two sources (Z CMa
and V645 Cyg) show strong silicate absorption, but no evidence for water or
CO 2 ice bands, demonstrating the chemical evolution that has taken place in
the circumstellar environment of these objects.
No strong correlation between spectral type and PAH emission could be
found. This is surprising, since the excitation of PAH molecules is thought
to require intense ultraviolet radiation fields. This means that on average the
particles responsible for the PAH emission must be closer to the central star,
and hence suffer less geometric dilution of the stellar radiation field, in Herbig
Ae stars than in Herbig Be stars.
Both the differences in silicate and in PAH behaviour can be explained by
assuming that the infrared spectrum of Herbig Be stars is in general dominated
by their circumstellar envelope rather than a disk. In contrast, the more slowly
evolving Herbig Ae stars have time to disrupt their envelope and their spectrum
may be dominated by thermal emission from the protoplanetary disk.
Crystalline silicates, as are also found in comets in our own solar system,
are visible in 15% of the late­type Herbig stars, all of systems that are relatively
isolated and appear to be relatively old (a few million years). Therefore also
in young stars longevity appears to be a prerequisite for the annealing process.
These systems form a close analog to the young solar system and may provide
the strongest clue to date that the same processes that have led to rocky planets
in our own solar system are also taking place around other stars.
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