The Astrophysical Journal, 601:1000–1012, 2004 February 1 # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.
MODELING THE CONTINUUM EMISSION FROM THE CIRCUMSTELLAR ENVIRONMENT OF HERBIG Ae/Be STARS D. Elia, F. Strafella, and L. Campeggio Dipartimento di Fisica, Universita` di Lecce, CP 193, I-73100 Lecce, Italy;
[email protected],
[email protected],
[email protected]
T. Giannini, D. Lorenzetti, and B. Nisini Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monte Porzio, Italy;
[email protected],
[email protected],
[email protected]
and S. Pezzuto Istituto di Fisica dello Spazio Interplanetario, Consiglio Nazionale delle Ricerche, via Fosso del Cavaliere 100, I-00133 Rome, Italy;
[email protected] Received 2003 August 8; accepted 2003 October 10
ABSTRACT This paper discusses a model for the continuum emission of the Herbig Ae/Be stars in the light of an updated set of observational data spanning 5 orders of magnitude in wavelength and including the low-resolution spectra obtained with the Short Wavelength Spectrometer and Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The model is used to reproduce the continuum emission of the 36 Herbig Ae/Be stars included in the list by The´ and coworkers and observed by ISO. The circumstellar matter responsible for the observed spectral energy distributions has been investigated by comparing the set of the observations with the model spectra computed for different possible distributions of circumstellar matter. Cases have been considered with the circumstellar regions partially evacuated along the polar axis by the action of the stellar wind, a phenomenon that is quite common in these pre–main-sequence objects. The inclusion of the polar cavities indirectly allows geometries in which a small-scale disklike structure around the central star is present. The possible coexistence of two different density profiles, in the inner and the outer region of the envelope, respectively, has been also considered. The comparison of the computed models with the observed spectral energy distributions selects the parameter values in such a way that the larger dust grains are preferentially associated with the later spectral types. We find that 17 objects are reasonably fitted, eight of which with a purely spherical model and the remaining nine with the inclusion of the polar cavities. For 10 further objects the fit is worse, and for the remaining nine, almost all associated to IR companions, our model is clearly inappropriate. A linear relationship is suggested between the logarithm of the initial density n0 and the exponent p of the power law nðrÞ ¼ n0 ðR =rÞp adopted for the circumstellar density distribution. Subject headings: circumstellar matter — infrared: stars — stars: emission-line, Be — stars: pre–main-sequence
1. INTRODUCTION
hereafter PSL; Waters & Waelkens 1998). Because of this, a general model for the CS environment is still a matter of debate and we still lack an unambiguous picture of the origin and evolution of this class. Among the controversial points there is certainly the spatial distribution (spherical envelopes versus disks) of the CS matter, whose presence is witnessed by the IR excess, a common characteristic shared by the Herbig Ae/Be stars with their lower mass counterparts, the T Tauri stars. While for the T Tauri there is a general consensus about the presence of a circumstellar disk, the case for the Herbig Ae/Be stars is more complex. The disk/envelope debate on these PMS objects has been recently summarized by Natta et al. (2000), who emphasized that, while in the earlier spectral types a disk geometry is unlikely, in many A stars the interferometric observations at millimeter wavelengths show that the emission is only a few arcseconds extended, ruling out the presence of very extended envelopes at least in the nearest objects. Recently Millan-Gabet, Schloerb, & Traub (2001) and more recently Eisner et al. (2003) observed some Herbig Ae/Be stars with unprecedented (a few milliarcseconds) spatial resolution in the near IR, in this way providing the first observational constraints to the small-scale distribution of the dust
A first account on the precursors of the main-sequence stars of intermediate mass (M2 10 M) was given by Herbig (1960), who was able to recognize pre–main-sequence (PMS) objects on the basis of spectroscopic, photometric, and morphological criteria. These criteria were tailored to pick up early-type stellar objects not only associated with a circumstellar (CS) envelope but also located in the neighborhood of dusty clouds. In this way the first 26 members of the family we now know as the Herbig Ae/Be stars were selected. This family has grown with time because of both the increased instrumental sensitivity and the refinement of the selection criteria, so that now it includes more than 100 members, reaching 300 members when we consider also potential candidates (The´, de Winter, & Pe´rez 1994). More recently, redefining the selection criteria, Malfait, Bogaert, & Waelkens (1998) added 14 further objects to this class of young stars. Despite the fact that the Herbig Ae/Be stars show common characteristics, the family members often reveal a high degree of heterogeneity in some other observed properties, notably in those related to the physics and geometry of the envelopes (for a discussion see, e.g., Pezzuto, Strafella, & Lorenzetti 1997, 1000
CONTINUUM EMISSION IN HERBIG Ae/Be stars component. While the first paper suggests an interpretation favoring the spherical structures, the second paper points to an interpretation in terms of disk models. Excluding some favorable cases, such a high spatial resolution is not available at most wavelengths so that we cannot directly observe the projected geometry of the CS environments. Because of this, the theoretical modeling of the spectral energy distributions (SEDs) still remains an important tool to infer the morphology of the CS matter distribution. In spite of its limited spatial resolution, the Infrared Space Observatory (ISO; Kessler et al. 1996) has provided us with spectrophotometric data in the crucial range, between 2 and 200 m, where the Herbig Ae/Be stars emit the bulk of their luminosity. Because of this, the ISO spectra not only allow more tight constraints for the modeling of the continuum emission of the CS environment but also make possible the study of emission features as the molecular and atomic lines (Lorenzetti et al. 1999, 2002; Giannini et al. 1999), the silicates (Bouwman et al. 2001), and the polycyclic aromatic hydrocarbon (PAH) bands (Peeters et al. 2002; Van Kerckhoven, Tielens, & Waelkens 2002). Studies based on the SED fitting of Herbig Ae/Be stars have been already presented in the literature, focusing on particular samples of objects. Among the most recent, we mention Malfait et al. (1998), who used UV, optical, and IR photometric data to discuss a sample of 45 objects, among which 12 are recognized as Herbig Ae/Be stars by The´ et al. (1994, their Table 1), 19 are considered as candidate Herbig Ae/Be stars, and the remaining 14 objects are new Herbig Ae/Be stars selected by the same authors. Subsequently, Meeus et al. (2001) used the Short Wavelength Spectrometer (SWS) spectra along with UV, optical, and radio fluxes to characterize the CS envelopes of 14 isolated Herbig Ae/Be stars or candidates. The ISO spectroscopy was also considered by Chiang et al. (2001) in testing their flaring disk model (Chiang & Goldreich 1997) for three Herbig Ae/Be stars (MWC 480, HD 36112, and CQ Tau) and by Natta et al. (2001) to discuss another set of three Herbig Ae/Be stars (AB Aur, CQ Tau, WW Vul). More recently Dominik et al. (2003) adopted a modified version of the Chiang et al. (2001) disk model for fitting the SED of 13 isolated objects, nine of which are included in Table 1 of The´ et al. (1994). In this work our aim is to scrutinize the available photometric and spectroscopic information to (i) assemble updated SEDs for the whole set of Herbig Ae/Be objects contained in the ISO data archive and (ii) infer on the CS environments of these stars by means of a simple model, taking into account the presence of both a CS envelope and a passive disklike structure. In x 2 we report on the sample of sources observed by ISO and on the procedures followed for data reduction. In x 3 we briefly illustrate the general trends and peculiarities encountered in the analysis of the ISO spectra. The global SEDs have been obtained by complementing the ISO spectra with ground-based photometry from the optical to radio wavelengths. These SEDs have been compared in x 4 with synthetic spectra computed by means of a CS model that, besides considering different density and temperature structures, also takes into account the possible presence of polar cavities as could be produced by a strong stellar wind. Finally, in x 5, we discuss the possible correlations among the model parameters as they emerge from the selection of the fitting models.
1001
2. SAMPLE SELECTION AND DATA REDUCTION To select our sample of Herbig Ae/Be stars, we searched the ISO mission data bank1 for the 108 ‘‘bona fide’’ Herbig Ae/Be stars listed in the catalog of The´ et al. (1994, their Table 1), obtaining as a result 36 objects with at least one SWS or Long Wavelength Spectrometer (LWS) observation of good quality. Two more objects (HD 95881 and VX Cas) are actually present in the ISO archive, but their spectra are completely dominated by noise, so that these objects have been excluded from our analysis. The SWS spectra we consider here were taken in the socalled SWS01 observing mode corresponding to a full spectral scan in the range 2.3–45 m in the low-resolution mode (k=k 250) and with a typical field of view increasing with wavelength from 14 00 20 00 to 20 00 33 00 (De Graauw et al. 1996). The LWS observations were carried out in the AOT01 full grating scan mode (i.e., in the range 43–196.7 m, k=k 200; Clegg et al. 1996). In the LWS spectral range the instrumental beam size is 8000 so that, in some cases (12 objects), the amount of contamination at the long wavelengths due to the local background has been evaluated by pointing also at off-source positions (see Table 1). The log of the observations is summarized in Table 1, in which we report in columns (1) and (2) the source names and in columns (3) and (4) the observation number of the ISO archive for SWS and LWS spectra, respectively. In column (5), the LWS off-source observations are also indicated, when available, with their observation number. Finally, in the last three columns, the presence of known embedded IR companions is also indicated along with the estimated angular distance in arcseconds and the corresponding reference. The raw SWS and LWS data were processed using version 10 of the off-line pipeline, which corrects for the detector responsivity drift and performs wavelength and flux calibration. This pipeline produces series of repeated spectral scans, each composed of 12 and 10 subspectra for SWS and LWS, respectively. These correspond to different spectral ranges, which are arranged to be partially overlapping. The data were further reduced and analyzed with the ISO Spectral Analysis Package (ISAP), version 2.0a. This further reduction step was necessary to remove bad points, as glitches and residual detector responsivity drifts, remaining even after the standard pipeline processing. The many scans obtained on each subspectrum were then averaged and the low-frequency fringes, particularly affecting the LWS spectra (Swinyard et al. 1998), were removed. Despite accurate data reduction, the subspectra obtained appear in many cases not reasonably overlapping in intensity, an effect that is probably due to the variation of the beam size with the wavelength (De Graauw et al. 1996; Swinyard et al. 1998). While the calibration sources (Uranus for LWS and six stars for SWS; see Schaeidt et al. 1996) are not affected by these variations because they are practically pointlike for the ISO spectrometers, the observations of the extended sources are certainly affected by this problem. This is particularly evident when we try to merge together SWS and LWS spectra that, corresponding to largely different beam sizes, often show a strong discontinuity in the overlapping region (13 cases out of the 23 objects with both SWS and LWS observations). We 1
Available at http://isowww.estec.esa.nl.
1002
ELIA ET AL.
Vol. 601
TABLE 1 The Observed Sample of Herbig Ae/Be Stars ISO observation number Source Name (1) LkH 198 ..................... V376 Cas ...................... Elias 3-1 ........................ AB Aur ......................... MWC 480 ..................... HD 34282 ..................... HD 36112...................... CQ Tau.......................... MWC 137 ..................... Z CMa........................... HD 97048 ..................... HD 100546 ................... HD 104237 ................... IRAS 124967650 ....... HD 141569 ................... HD 142666 ................... HD 144432 ................... HR 5999........................ He 3-1191 ..................... HD 150193 ................... CoD 42 11721 ........... HD 163296 ................... MWC 297 ..................... MWC 300 ..................... TY CrA ......................... R CrA............................ HD 179218 ................... WW Vul ........................ BD +40 4124................ LkH 224 ..................... PV Cep.......................... HD 200775 ................... V645 Cyg...................... LkH 234 ..................... LkH 233 ..................... MWC 1080 ...................
Other Designation (2) V633 Cas ... V892 Tau HD 31293 HD 31648 ... MWC 758 HD 36910 V1308 Ori HD 53179 He 3-597 He 3-672 He 3-741 ... ... ... He 3-1141 HD 155568 WRA 15-1484 MWC 863 V921 Sco MWC 275 ... V431 Sct ... He 3-1733 MWC 614 HD 344361 V1685 Cyg V1686 Cyg ... MWC 361 ... V373 Cep V375 Lac ...
SWS (3)
LWS (4)
LWS Off-Source (5)
Infrared Companion(s) (6)
... 43501514 67301306 68001206 83501201 83301240 ... ... 84901804 72201607 14101343 27601036 23300524 23300112 62802937 44901283 45000284 28901506 29901001 08200444 08402527 32901191 70800234 51601005 34801419 14100458 32301321 17600305 35500693 85800502 14302273 33901897 26301850 ... ... 28301459
28201001 28201003 ... 83501304 83500802 86301724 86603207 82301814 84901903 72301626 26101305 10400640 58401328 11500421 62701509 45400311 29600816 ... ... ... 08402426 ... 70800260 ... 34801307 33401939 ... ... 76803008 14201327 25500445 34601108 26301849 45700351 76803311 28301458
28201002 28201002 ... ... ... ... ... ... ... 72301627,8 26101306 ... ... 11500422 ... ... ... ... ... ... 08402426 ... 70800260 ... ... 33401940 ... ... ... ... 25500446 ... 26301849 36700680 ... 28301457
IR; mm ... IR; mm ... ... ... ... ... Two cores at 1.3 mm IR IR One core at 1.3 mm ... ... IR ... ... IR ... IR IR; two cores at 1.3 mm ... IR ... b c
IR; mm ... ... IRd IRd ... IR ... IR ... IR; two cores at 1.3 mm
a (arcsec) (7)
References (8)
6; 19 ... 4; 4 ... ... ... ... ... 26, 32 0.1 35, 37 6 ... ... 6.8, 8 ... ... 1.5 ... 1.1 5, 8, 11, 14; 20, 37 ... 24, 39 ... ... FIRAS . The faintest objects can be, in fact, more easily contaminated by the background emission, which, evaluated and subtracted in the IRAS photometry, is not discerned by the pointed observations carried out with the ISO spectrometers. We also found three objects (IRAS 124967650, BD +40 4124, LkH 224) showing FISO(12 m) significantly lower than FIRAS (12 m), as could be expected dealing with extended sources seen with the different beam sizes of ISO SWS and IRAS. This problem is clearly attenuated at longer wavelengths as a result of the larger beam size of LWS. However, two objects (MWC 297, HD 200775) remain underestimated by ISO at k ¼ 100 m. 4. MODELING The SEDs of the Herbig Ae/Be stars have largely been used to infer the physical conditions and the geometric distribution of the CS matter surrounding these young stars (see, e.g., Hillenbrand et al. 1992; Berrilli et al. 1992; PSL; Chiang et al. 2001; Dominik et al. 2003). This indirect approach has been dictated by the observational difficulties in resolving the characteristics of the CS environments at the small spatial scales. In fact, despite the observational efforts to clarify the spatial distribution of the CS matter around Herbig Ae/Be stars, the situation remains quite controversial (see, e.g., Mannings & Sargent 1997; Millan-Gabet et al. 2001; Monnier & Millan-Gabet 2002; Eisner et al. 2003). At larger spatial scales the dust is colder and the direct view of the PMS environments is better obtained by observing the far-infrared (FIR) and submillimeter emission. This emission, involving sizes of order 1000 AU, subtends approximately 700 at the distance of the Taurus star-forming region, an angular
extension that is comparable with the beam size attainable in the millimetric and submillimetric continuum with modern interferometers. The advent of these instruments is beginning to change the situation allowing beam sizes of a few arcseconds, so that the observers begin to directly probe the large-scale CS spatial structures of the nearest objects (e.g., di Francesco et al. 1997; Kamazaki et al. 2001). 4.1. Continuum Spectrum The preceding discussion implies that the SED fitting still remains an important tool to investigate the geometry and the properties of the CS environments in a large number of cases. A valuable characteristic of this method is that the different spatial scales of the emitting CS envelope contribute to different wavelength ranges in a way that depends on, and then is sensitive to, the specific density and temperature structure. Consequently, this approach appears to be more meaningful the larger and more complete is the spectral range covered by the observations. Guided by these considerations, in the following we shall use a large spectral range, extended from the U band to the radio region, in an attempt to verify the capability of our model to account for the spectra emitted by the Herbig Ae/Be stars when the continuum observed by the ISO spectrometers is also taken into account. To do this, we complemented the SWS and LWS continuum fluxes of our sample objects with the corresponding photometric data obtained from the literature as in PSL. In addition, we also included the submillimetric observations carried out by Sylvester et al. (1996), Henning et al. (1998), and Fuente et al. (1998). The resulting SEDs have subsequently been compared with synthetic spectra computed by means of a model whose details are described in the following. 4.2. The Model We essentially adopt in this work an implementation of the PSL model. Briefly, this model considers both gas and dust arranged in CS spherical shells with temperature and density profiles described by power laws, T / rq and n / rp , respectively. The emergent SED is computed taking into account the contributions due to the star, the free-free and freebound emission of the gaseous photoionized component, and the dust emission. While the stellar component is simply modeled with a blackbody of the appropriate temperature, the gas contribution
1004
ELIA ET AL.
to the continuum emerges from the photoionization of the CS region, which produces both an H ii region and a photodissociation (PD) region. The first is fully ionized by the stellar photons, while the latter is partially ionized because some metals (as, e.g., C, Mg, Fe), with ionization potential lower than the Lyman limit, are still ionized by the stellar photons escaping the H ii region. In these two regions the gas temperature is taken to be constant at an equilibrium value of TH ii ¼ 104 K and TPD ¼ 3500 K, respectively. To evaluate the contribution of the dust component to the continuum emission, we adopted a grain size distribution and composition similar to the standard Mathis, Rumpl, & Nordsiek (1977) mixture, with optical properties taken from Draine & Lee (1984). This choice is quite general and allows us to ‘‘tune’’ the limits of the grain size distribution to obtain extinction coefficients that are compatible with the observed extinction law and with the different values of the total-toselective extinction ratio RV ¼ AV =EðBV Þ. For interstellar (IS) dust the value of RV ¼ 3:1 is observationally well established, while in star-forming regions RV is typically larger (Gordon 1995). In the work of PSL two values were explored, namely, RV ¼ 3:1 and 5.0, and they concluded that the latter value is better suited to obtain a fit of the computed SEDs to the observational data. Because of this, in the present work we shall limit ourselves to considering only dust mixtures with RV ¼ 5:0. This value is also preferred because it is consistent with the generally accepted ideas on the mean size of the dust grains. In fact, the dust particles residing in dense environments, as the star-forming regions, are considered to be larger than those in the diffuse IS medium because the collisional mechanisms favoring the grain growth processes are clearly enhanced in denser regions. In these environmental conditions the grains tend to grow ‘‘fluffy’’ and their absorption properties in the FIR and submillimeter region are better described by an absorption coefficient following a power law / k with 0 < < 2 (see, e.g., Ossenkopf & Henning 1994), i.e., a value lower than that expected for small spherical particles and more typical for the diffuse IS dust. In PSL two values were used, namely, ¼ 2 and 1.2, and in almost all cases studied (33 out of 36) a better fit was obtained with the latter value. Here we restrict ourselves to considering only models with ¼ 1:2, 0.8, and 0.6, exploring in this way the possibility of different populations of large grains characterized by lower values of the exponent . 4.2.1. Differences with Respect to the PSL Model
Besides the three values of we use, two other important differences characterize the model spectra computed here with respect to those discussed by PSL. The first is the model geometry, which has been modified to allow the presence of polar cavities in the CS environment. These are included because they can be produced by the intense stellar wind characterizing the PMS phases that tend to evacuate preferentially the polar directions toward which the wind is expected to be stronger (for the case of Herbig Ae/Be stars see, e.g., Strafella et al. 1998). This heuristic choice is also suggested by the observed association of bipolar outflows with the PMS objects (see, e.g., Bachiller 1996; Churchwell 1997). Incidentally, it is interesting to note that this particular choice allows us to indirectly consider in our simulations also
Vol. 601
Fig. 2.—Grand-scale geometry of the adopted model is spherical, while the inclusion of the two polar cavities produces a flattened structure at the small scale, resembling a flaring disk surrounding the central star. The relative sizes of the cavities and the central star have been exaggerated for the sake of clarity. The sublimation radius of the dust is represented by the beginning of the innermost dark region.
the possible presence of CS disklike structures, even if we do not model the disks at all. This is in fact a pure geometrical consequence of evacuating two polar cavities that, for simplicity, we assume to be spherical as is shown in Figure 2. The second difference refers to the power law describing the density profile that now can be characterized by two different exponents appropriate for the inner and the outer part of the CS envelope, respectively. We recall that in PSL two density laws have already been considered, but always with shallower exponents in the outer region. However, this additional degree of freedom in modeling was not able to improve the fits of the observed SEDs, so that in PSL no cases were selected with this characteristic density profile. Here we reconsider this point but in the other sense, i.e., allowing the slope of the density law to be steeper in the outer than in the inner region of the CS envelope. This can be justified if we consider that, at some stage of the evolution, the accreting part of the CS environment of a young stellar object detaches from the parent cloud and evolves independently, contracting toward the central object. In these circumstances the density in the more external parts of the CS envelope could drop more rapidly than in the inner region. In Figure 2 we show a schematic sketch of the adopted model geometry. A cross section of the model is actually represented, in which one can recognize the two polar cavities, as well as the change in the slope of the density law. These implementations make the model more flexible but, inevitably, introduce new parameters that we tried to constrain by making specific choices. The cavities are assumed to be spheres of radius r ¼ 1000R tangent to the stellar poles, while the boundary between the inner and outer density law has been located at 2000R*, which is somewhat arbitrary but corresponds
No. 2, 2004
CONTINUUM EMISSION IN HERBIG Ae/Be stars
to simulated disklike structures whose sizes are of the order of those suggested for Herbig Ae/Be stars (see, e.g., Pantin, Waelkens, & Lagage 2000; Hinz, Hoffmann, & Hora 2001). 4.3. Model Results To obtain an appropriate coverage of the parameter space, we selected a grid of values that reflects our feeling on what are reasonable ranges for the spectral type, the density (n0, p) and temperature (T0, q) laws, and the dust spectral behavior () at long wavelengths. Given that the gas temperature in the ionized regions has been fixed and that the initial temperature for dust is taken to be T0 ¼ 1500 K, the remaining parameters have been varied so that we considered 13 spectral types from O8 to A5, 12 values for the initial density ranging from n0 ¼ 106 to 3 1011 cm3, 16 values for the density-law exponent in the range 0:2 p 1:7, three values for the temperature exponent q ¼ 0:4, 0.5, 0.6, and three values for the exponent of the dust absorption coefficient in the FIR, namely, ¼ 1:2, 0.8, 0.6. This volume in the parameter space has been explored for three CS matter distributions, namely, (1) purely spherical, (2) with the inclusion of two polar spherical cavities with radius 1000R*, and (3) with both cavities and a relatively steep density-law exponent p ¼ 1:5 in the outer envelope. In this way a large number of synthetic spectra have been computed. 4.4. Photometric Data and Model Selection To compare the model results with the observations, we considered the spectral range between the optical bands and the radio region. All the available photometric information on the sample objects has been collected from the literature and has been scrutinized in an attempt to select the most accurate values. Among the ground-based observations included, we mention, in particular, the 1.3 mm maps of Henning et al. (1998) and Fuente et al. (1998) and the 1.3 mm fluxes of Sylvester et al. (1996), which were not considered by PSL. In this respect we also remark that, as shown by Henning et al. (1998), photometric measurements using the on-on technique could severely underestimate the intrinsic emission from the sources, as a result of the strong background emission that characterizes the locations of many Herbig Ae/Be stars. The possible consequences of such a contamination in the selection of the models have been illustrated in PSL for the case of AS 310. The ISO fluxes have also been included in determining the global SEDs between 2 and 190 m. The selection of the best model for each Herbig Ae/Be star has been done searching the bank of model spectra generated by our exploration of the parameter space. The criterion we applied was to consider the distances required for diluting, at the different wavelengths, the model fluxes to match the observed values. In this way many model distances have been derived for each object, one for each observed wavelength, so that we obtained a sort of statistical distribution, and then a mean distance and a standard deviation. We assumed these two quantities, respectively, as our best estimate and the associated uncertainty for the model’s distance. Only the models producing the narrower distribution of the object distances were selected for further consideration. In our selection procedure we also took into account the angular size of our models when they are considered at a given distance. This is particularly important for the submillimeter spectral region where the emission of the models involves the
1005
largest spatial scale. To correctly compare the model with the observed fluxes in this region, we considered also the diffraction effects produced by the beam sizes of the telescopes used to collect the observational data. This correction has not been applied to the radio fluxes (k > 1:3 mm) because the model emission in this range comes primarily from the relatively small H ii circumstellar region. It is worth noting that in our selection procedure all the computed models are compared with the observed SED and sorted with respect to the distance uncertainty. At the end of this procedure we retain the 10 models with the smallest uncertainties. A further selection among these models is subsequently done by looking also to the compatibility with other observed quantities such as the spectral type and the total visual extinction. In most cases the final choice was made easy because the best models correspond to very similar AV and differ only for a few subclasses in spectral type. The results of this procedure are shown in Figure 3. For each object the solid line represents the model SED and the crosses show where the model fluxes in the submillimeter region are scaled when the model is convolved with the same beam size used by the observers. Note that the observed fluxes are reasonably well matched after this correction. The corresponding parameters of the selected models are reported in Table 2. For each source we give the name (an asterisk flags a model with polar cavities, two asterisks also a double density law), the dust opacity exponent , the exponents q and p of the temperature and density laws, the initial density n0, the spectral type, the extinction in magnitudes AV, and the distance in parsecs. For the spectral type, the extinction, and the distance in Table 2 we quote two values, i.e., the value corresponding to the selected model in the column labeled ‘‘Model’’ and the observational value in the column ‘‘Observation.’’ Considering the extinction, we further split the model value into two contributions: the first intrinsic to the model and due to the CS envelope, the second due to the IS extinction and derived during the fitting procedure in which the IS contribution is varied on a reasonable range to take into account also the effects of the IS extinction. The quoted IS value is consistent with the extinction correction applied in the range 0.3–14 m to the model fluxes shown in Figure 3. The standard IS extinction curve of Rieke & Lebofsky (1985) has been adopted, and the fluxes at wavelengths k > 14 m have been considered as practically unmodified. As far as the derived distances are concerned, we remark that they cannot be considered to be independent determinations because they merely represent the distances required to fit the corresponding models. We recall, however, that in computing the model spectra we adopted main-sequence stellar radii so that the quoted distances are to be taken as lower limits because of the larger stellar radii and then larger stellar luminosities expected in the PMS phase. It can be easily seen that the model distance scales linearly with the adopted stellar radius and then the observed values are generally found to be greater than or equal to those selected by our procedure. 5. DISCUSSION The results of the model selection shown in Figure 3 and reported in Table 2 clearly indicate that the variety of the observed SEDs is such that they cannot be always satisfactorily fitted, despite the large range allowed for the values of
1006 Fig. 3.—Comparison between observed (open symbols) and computed (solid line) SEDs. The upper limits obtained with submillimeter and radio telescopes are indicated with triangles. When available, the IRAS upper limits in the 60 and 100 m bands are also indicated. In the submillimeter region the model fluxes have been rescaled taking into account the diffraction effects, which are important at these wavelengths. Consequently, the rescaled model fluxes have been indicated with crosses. These corrections have been computed considering both the angular extent of the model sources, at the distance given in Table 2, and the telescope beam sizes reported by the observers. In the left-hand panels the whole spectral range is shown, while in the right-hand panels we enlarge the optical–IR range to show, more clearly, the ground-based photometry (open squares) and the ISO spectra (k=k 200).
1007 Fig. 3.—Continued
Fig. 3.—Continued
1009
CONTINUUM EMISSION IN HERBIG Ae/Be stars TABLE 2 Comparison between Fit Parameters and Observables
Source
a
LkH 198 ............. V376 Cas .............. Elias 3-1 ................ AB Aur ................. MWC 480* ........... HD 34282** ......... HD 36112**.......... CQ Tau**.............. MWC 137 ............. Z CMa ** ............. HD 97048* ........... HD 100546** ....... HD 104237 ........... IRAS 124967650 HD 141569** ....... HD 142666** ....... HD 144432** ....... HR 5999................ He 3-1191** ......... HD 150193* ......... CoD 42 11721 ... HD 163296* ......... MWC 297 ............. MWC 300** ......... TY CrA ................. R CrA.................... HD 179218** ....... WW Vul* .............. BD +40 4124........ LkH 224 ............. PV Cep.................. HD 200775 ........... V645 Cyg.............. LkH 234 ............. LkH 233 ............. MWC 1080 ...........
Distance (pc)
AV (mag)
Spectral Type
q
p
n0 (cm3)
Model
Observation
CS
IS
Observation
0.8 1.2 1.2 0.6 0.6 0.6 0.6 0.6 0.8 0.6 0.6 0.6 0.6 0.8 0.8 0.6 0.6 0.6 1.2 0.6 0.8 0.6 0.6 1.2 0.8 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6
0.5 0.5 0.5 0.4 0.6 0.5 0.6 0.6 0.4 0.5 0.5 0.5 0.6 0.5 0.4 0.6 0.6 0.6 0.6 0.6 0.5 0.6 0.5 0.5 0.4 0.4 0.5 0.6 0.4 0.5 0.5 0.4 0.4 0.4 0.4 0.5
0.9 0.8 1.1 1.4 1.3 1.4 1.2 0.8 0.7 1.5 1.1 0.7 1.3 1.0 0.4 1.5 1.3 1.5 0.5 1.5 0.7 1.3 1.1 0.6 0.6 1.3 1.2 1.1 0.7 0.4 1.2 0.8 1.3 0.7 1.2 0.8
109 3 108 109 3 109 3 1010 1011 3 1010 3 109 3 106 1011 3 109 3 108 1010 109 106 3 1011 3 1010 3 1010 108 3 1011 108 3 1010 109 3 108 106 3 1010 3 109 1010 3 106 106 3 1010 3 106 3 1010 3 107 1010 3 107
B6 B5 A0 B9 A3 A3 A3 A5 B9 B5 A0 B9 A5 A0 A0 A5 A3 A3 B1 A3 B8 A3 B6 B3 A0 A5 B9 A3 A0 B5 A3 A0 A0 B8 A5 A0
B3–A5 B5 A0–A6 B9–A0 A2–A4 A0 A3–A5 A1–F5 B0 B5–B8 B9.5–A0 B9 A4 A B9.5–A0 A7–A8 A5–F0 A5–A7 B0 A0–A4 B0–B7 A0–A7 O9–B1.5 09–B1 B8–A5 A1–F7 B9–A0 A3 B2–B5 B2–B5 A5 B1.5–B5 A0–A5b O8–A A5–A7 B0
4 4 3 0 1 1 1.5 2.5 0 1 1 1 1 1 0 1.5 0.5 1 3 1.5 3 1 0.5 3.5 0 3 0 2 0 1.5 6.5 0 3.5 1 2 0.5
1 2.5 5 0.5 0 0 0 0 4.5 3 0.5 0 0 11 0 0 0.5 0 0.5 0 2 0 6.5 0.5 2.5 0 1 0 3.5 5 2 2 2 3 1 6
2.5–5 2.9–5.2 3.9–12.5 0.4–0.65 0.25–0.4 0.6 0.2–0.7 0.1–1.6 4.5 2.4–2.8 1.2–1.4 0.2–1 0.3–1.3 11–17 0.47 1.5 0.6 0.2–0.5 1.9 1.5–1.7 1.9–7.1 0.3 2.9–8.3 3.9 1–3.1 0.9–1.9 1.3 0.6–0.9 3 4.8 0.4–9.5 1.8–2 2.5 b 3.1–3.4 2.2–2.6 4–5.4
Model
Observation2
References
600–950 600 140–160 144–160 131–140 160–547 150–200 100–150 110–1300 180–1150 140–215 103–190 90–116 165–200 99–170 116–200 >200 140–270 ... 140–230 160–2600 122–160 250–530 700 130–150 130–150 240 534–700 1000 1000 500–600 430–600 3500 b 1000 880 500–2500
1–2, 2–3, 2–4 1, 2–1, 2 5–1, 6–7, 7–2 1, 2–1, 8–9 10–11, 8–12, 8–13 14, 14, 8–14 10–13, 8–13, 13–8 8, 12–13, 8–13 10, 2, 8–15 10, 8–1, 8–9 10, 8–16, 16–2 17, 17–18, 8–17 10, 8–19, 17–8 10, 19, 20–21 22–10, 8, 8–23 10, 8, 8 8–24, 8, 8 25, 9–8, 26–4 10, 18 10, 2–4, 4 10–2, 18–27, 2–16 2–4, 2, 8–4 10–28, 29–2, 29–4 1–8, 1, 30 16–2, 2–16, 16–9 8, 2–9, 2–9 10, 8, 8 16, 31–16, 32–16 33, 8, 2 10–2, 2, 2 10, 34–35, 35–36 37–38, 9–2, 8–2 39, 39, 39 33, 9–2, 9 2–1, 9–2, 9 1, 40–1, 9–16
578 718 136 144 101 319 103 103 199 250 174 124 52 132 161 103 115 75 3760 130 136 71 114 1178 108 104 134 210 124 362 263 72 258 500 494 60
19 20 5 3 5 13 2 4 16 9 6 2 1 7 4 3 2 2 110 3 7 4 4 34 4 4 4 8 5 17 7 2 12 20 10 2
a
The first asterisk denotes a model with polar cavities, the second a presence of two slopes in the density law. For V645 Cyg, Cohen 1977 gives a remarkably different estimate of spectral type (O7), AV (4.9 mag), and distance (6000 pc). See text. References.—(1) Cohen & Kuhi 1979; (2) Hillenbrand et al. 1992; (3) Chavarria-K. 1985; (4) Skinner et al. 1993; (5) Wilking, Mundy, & Schwartz 1986; (6) Zinnecker & Preibisch 1994; (7) Elias 1978; (8) van den Ancker, de Winter, & Tjin a Djie 1998; (9) Finkenzeller & Mundt 1984; (10) The´ et al. 1994; (11) Simon, Dutrey, & Guilloteau 2000; (12) Miroshnichenko et al. 1999; (13) Mannings & Sargent 1997; (14) Sylvester et al. 1996; (15) di Francesco et al. 1997; (16) Brooke, Tokunaga, & Strom 1993; (17) Hu, The´, & de Winter 1989; (18) Valenti, Johns-Krull, & Linsky 2000; (19) Henning et al. 1993; (20) Reipurth et al. 1993; (21) Hughes et al. 1991; (22) Jaschek & Jaschek 1992; (23) Penprase 1993; (24) Dunkin, Barlow, & Ryan 1997; (25) Tjin A Djie et al. 1989; (26) Hughes, Hartigan, & Clampitt 1993; (27) de Winter & The´ 1990; (28) Drew et al. 1997; (29) Canto` et al. 1984; (30) Pirzkal et al. 1997; (31) Natta et al. 2001; (32) Friedemann et al. 1993; (33) Shevchenko et al. 1993; (34) Natta et al. 1993; (35) Osterloh & Beckwith 1995; (36) Leinert et al. 1997; (37) Page 1984; (38) Haupt & Schroll 1974; (39) Goodrich 1986; (40) Hou, Jiang, & Fu 1997. b
the model parameters. To overcome this difficulty, encountered also by other authors, it is usual to distinguish and to discuss separately Be and Ae stars (see, e.g., Natta et al. 2000) and/or embedded and isolated objects (see, e.g., Dominik et al. 2003). Further observational difficulties could also affect the SEDs because of the possible presence of IR companions (see Table 1 for known cases) that, not considered in our model, contribute indeed to the observed fluxes. These problems can explain the many papers that, in the recent literature, deal with relatively small samples of Herbig Ae/Be stars conveniently selected with similar characteristics. On the contrary, we preferred to include in our discussion all
the Herbig Ae/Be stars contained in the ISO Data Archive, even if at the cost of obtaining unsatisfactory fits in some cases. In this respect we recognize that the model has been tailored on single central objects surrounded by a CS region that can be approximated with a grand-scale spherical symmetry. This of course represents also a limit of the model, even if a smallscale disklike structure has been allowed because of the consideration of the polar cavities as shown in Figure 2. If we attempt an empirical classification on the basis of the global spectral fits obtained, we find 17 objects showing a better fit (V376 Cas, AB Aur, HD 36112, CQ Tau, HD
1010
ELIA ET AL.
100546, HD 104237, HD 142666, HD 144432, HR 5999, He 3-1191, HD 150193, MWC 300, TY CrA, HD 179218, PV Cep, V645 Cyg, LkH 233). These are followed by 10 more objects whose fit appears to be worse (LkH 198, Elias 3-1, MWC 480, HD 34282, Z CMa, HD 97048, HD 141569, HD 163296, R CrA, WW Vul). The remaining nine cases correspond to more problematic fits (MWC 137, IRAS 12496 7650, CoD 42 11721, MWC 297, BD +40 4124, LkH 224, HD 200775, LkH 234, MWC 1080), suggesting that the adopted model is inadequate for these cases. It is interesting to note that the first group is dominated by single objects, as can be seen in Table 1. In this respect we can compare our results with the work of Dominik et al. (2003), who adopted a disk model to describe the CS environment of isolated Herbig Ae/Be stars. In fact, taking into account the objects in common between our and their sample, we find that in six (HD 100546, HD 142666, HD 144432, HD 150193, HD 163296, HD 179218) out of eight common cases we selected models with presence of polar cavities that give rise to a disklike geometry in the innermost part of the envelope. These objects are late-type isolated Herbig Ae/Be stars, so that this simple consideration gives further support to the hypothesis that for low-mass, more evolved objects, the CS geometry can be similar to that found around the T Tauri stars. Moreover, it is noteworthy that for all these eight cases, the models we select correspond to a dust absorption coefficient exponent ¼ 0:6, which is typical of large dust grains. This is in agreement with the idea that the grain size is expected to increase with time in CS envelopes, as a result of the ongoing growth processes activated in high-density environments. The object V645 Cyg requires more caution because in the literature there are largely different estimates for spectral type and distance as indicated in Table 2. In this case we preferred to use the estimates given by Goodrich (1986), who critically reanalyzed the kinematic distance model previously adopted by Cohen (1977) toward the Cassiopeia-Perseus direction. The second group contains six out of 10 objects associated with IR and millimetric companions, suggesting that the observed fluxes, at least in these six cases, can be contaminated particularly in the IR and millimetric range where the beam sizes of the observations are typically larger. The last group, consisting of the nine worst cases, also contains eight objects with one or more associated IR or millimeter companions, so that we are not surprised that the adopted model is inappropriate. Only for the more extinguished source, IRAS 124967650, do we not have such evidence for companions. By inspecting the corresponding IRAS maps, we note that IRAS 124967650 is superimposed on a background strongly increasing with wavelength. However, in this case the background contamination was evaluated by pointing ISO LWS toward eight surrounding positions and then subtracted. Despite the attempts to be accurate with this procedure, the model fails to reproduce the global spectral characteristics. In Figure 4 we present three histograms showing the distribution of the selected models as a function of the spectral type with respect to the CS geometry. The total number of objects does not allow a truly statistical discussion, so that we only remark that an indication of disklike geometry is found in approximately half of the cases. A trend favoring the spherical geometry can be envisaged for the early-type objects.
Vol. 601
Fig. 4.—Distribution of the selected geometries as a function of the spectral type. In the top panel each bin is normalized to the total number of objects of a given spectral type. The other two panels detail the number of cases best fitted by each geometry investigated. Models with polar cavities can be seen as an indication of a disklike geometry prevailing at the small spatial scales (see text). In the bottom panel the earlier objects (B1, B3, and B5) are He 3-1191, MWC 300, and Z CMa, respectively. Note that He 3-1191 and MWC 300 have not been detected in the LWS range and their models have been selected with relatively less constraints with respect to other objects.
The distribution of the selected models with respect to the grain parameter is presented in Figure 5. The three panels correspond to the three different values adopted to describe the spectral behavior of the dust grains at long wavelengths. Because its value is expected to decrease with increasing grain size, these histograms can be understood considering the grain growth mechanisms. These are the grain-grain sticking collisions, which are clearly favored by the high densities of the CS environments and proceed as long as this environment persists. In fact, in Figure 5 we see that with ¼ 0:6,
No. 2, 2004
CONTINUUM EMISSION IN HERBIG Ae/Be stars
1011
Fig. 5.—Distribution of the selected models with respect to the parameter (see Table 2)
corresponding to the largest grains case, we find almost all the late spectral types whose evolutionary PMS times are sensibly longer than for early types. The other two values of ¼ 0:8 and 1.2 are clearly of lower statistical significance, even if they seem to give further support to the previous indication. Finally, we note that the initial density n0 and the slope p of the adopted density law show a clear relationship, which is apparent in Figure 6. This kind of relation is expected when we consider that the visibility of the central star implies, in the model, a density distribution producing an optical depth that
Fig. 6.—Correlation between the parameters of the density law n ¼ n0 ðR =rÞp adopted to model the CS environment of the Herbig Ae/Be stars. For the selected models the logarithm of the initial density n0 is reported vs. the density-law exponent p (see Table 2).
remains moderate across the CS envelope. This is accomplished just by decreasing the initial density when the slope of the density law decreases as shown in Figure 6. 6. CONCLUSIONS In this work we analyzed the SEDs of all the objects observed with the SWS and LWS of the ISO satellite and recognized to be genuine Herbig Ae/Be stars in the catalog of The´ et al. (1994). Despite the difficulties due to the intercalibration of the spectrometers, a comparison with the IRAS PSC fluxes of the same objects shows a general agreement, with a few exceptions. The comparison between the model spectra and the observed fluxes over a large spectral range has been done, and, in particular, for 21 objects we present the first global fit taking into account the ISO spectroscopy. We find that for 17 objects out of the 36 the global SED is reasonably fitted by our simple model, eight of which with a pure spherical CS geometry and nine by considering a smallscale disklike passive structure. On the other side, there are nine more Herbig Ae/Be stars for which the model fails to reproduce the SED: in eight of these cases the objects have one or more nearby companions that can contaminate the observations. In most cases the dust component is better characterized by a long-wavelength absorption coefficient k / k , with ¼ 0:6 suggesting the presence of large grains.
This work was partly supported by MURST (Italian Ministry for Education and Scientific Research). The authors are grateful to P. Santo, E. Fasanelli, and F. Ricciardi for assistance in solving computational problems.
1012
ELIA ET AL.
REFERENCES Bachiller, R. 1996, ARA&A, 34, 111 Leinert, C., Richichi, A., & Haas, M. 1997, A&A, 318, 472 Berrilli, F., Corciulo, G., Ingrosso, G., Lorenzetti, D., Nisini, B., & Strafella, F. Lorenzetti, D., Giannini, T., Nisini, B., Benedettini, M., Elia, D., Campeggio, 1992, ApJ, 398, 254 L., & Strafella, F. 2002, A&A, 395, 637 Bouwman, J., Meeus, G., de Koter, A., Hony, S., Dominik, C., & Waters, L. B. Lorenzetti, D., et al. 1999, A&A, 346, 604 F. M. 2001, A&A, 375, 950 Malfait, K., Bogaert, E., & Waelkens, C. 1998, A&A, 331, 211 Brooke, T. Y., Tokunaga, A. T., & Strom, S. E. 1993, AJ, 106, 656 Mannings, V., & Sargent, A. I. 1997, ApJ, 490, 792 Cabrit, S., Lagage, P. O., McCaughrean, M., & Olofsson, G. 1997, A&A, 321, Mathis, J. S., Rumpl, W., & Nordsiek, K. H. 1977, ApJ, 217, 425 523 Meeus, G., Waters, L. B. F. M., Bouwman, J., van den Ancker, M. E., Canto`, J., Rodriguez, L. F., Calvet, N., & Levreault, R. M. 1984, ApJ, 282, 631 Waelkens, C., & Malfait, K. 2001, A&A, 365, 476 Chavarria-K., C. 1985, A&A, 148, 317 Millan-Gabet, R., Schloerb, F. P., & Traub, W. A. 2001, ApJ, 546, 358 Chiang, E. I., & Goldreich, P. 1997, ApJ, 490, 368 Miroshnichenko, A., Ivezic´, Zˇ., Vinkovic´, D., & Elitzur, M. 1999, ApJ, 520, Chiang, E. I., Joung, M. K., Creech-Eakman, M. J., Qi, C., Kessler, J. E., L115 Blake, G. A., & van Dishoeck, E. F. 2001, ApJ, 547, 1077 Monnier, J. D., & Millan-Gabet, R. 2002, ApJ, 579, 694 Churchwell, E. 1997, ApJ, 479, L59 Natta, A., Grinin, V. P., & Mannings, V. 2000, in Protostars and Planets IV, ed. Clegg, P. E., et al. 1996, A&A, 315, L38 V. Mannings, A. P. Boss, & S. R. Russel (Tucson: Univ. Arizona Press), 559 Cohen, M. 1977, ApJ, 215, 533 Natta, A., Palla, F., Butner, H. M., Evans, N. J., II, & Harvey, P. M. 1993, ApJ, Cohen, M., & Kuhi, L. V. 1979, ApJS, 41, 743 406, 674 De Graauw, T., et al. 1996, A&A, 315, L49 Natta, A., Prusti, T., Neri, R., Wooden, D., Grinin, V. P., & Mannings, V. 2001, de Winter, D., & The´, P. S. 1990, Ap&SS, 166, 99 A&A, 371, 186 di Francesco, J., Evans, N. J., II, Harvey, P. M., Mundy, L. G., Guilloteau, S., & Ossenkopf, V., & Henning, Th. 1994, A&A, 291, 943 Chandler, C. J. 1997, ApJ, 482, 433 Osterloh, M., & Beckwith, S. V. W. 1995, ApJ, 439, 288 Dominik, C., Dullemond, C. P., Waters, L. B. F. M., & Walch, S. 2003, A&A, Page, A. A. 1984, Spectrum and Magnitude Data Bank of B(e), B(e), and B(pe) 398, 607 Stars (Strasbourg: CDS) Draine, B. T., & Lee, H. M. 1984, ApJ, 285, 89 Pantin, E., Waelkens, C., & Lagage, P. O. 2000, A&A, 361, L9 Drew, J. E., Busfield, G., Hoare, M. G., Murdoch, K. A., Nixon, C. A., & Peeters, E., Hony, S., Van Kerckhoven, C., Tielens, A. G. G. M., Allamandola, Oudmaijer, R. D. 1997, MNRAS, 286, 538 L. J., Hudgins, D. M., & Bauschlicher, C. W. 2002, A&A, 390, 1089 Dunkin, S. K., Barlow, M. J., & Ryan, S. G. 1997, MNRAS, 286, 604 Penprase, P. E. 1993, ApJS, 88, 433 Eisner, J. A., Lane, B. F., Akeson, R. L., Hillenbrand, L. A., & Sargent, A. I. Pezzuto, S., Strafella, F., & Lorenzetti, D. 1997, ApJ, 485, 290 (PSL) 2003, ApJ, 588, 360 Pirzkal, N., Spillar, E. J., & Dyck, H. M. 1997, ApJ, 481, 392 Elias, J. H. 1978, ApJ, 224, 857 Reipurth, B., Chini, R., Krugel, E., Kreysa, E., & Sievers, A. 1993, A&A, 273, Finkenzeller, U., & Mundt, R. 1984, A&AS, 55, 109 221 Friedemann, C., Riemann, H. G., Gurtler, J., & Toth, V. 1993, A&A, 277, 184 Rieke, G. H., & Lebofsky, M. J. 1985, ApJ, 288, 618 Fuente, A., Martı´n-Pintado, J., Bachiller, R., Neri, R., & Palla, F. 1998, A&A, Sandell, G., & Weintraub, D. A. 1994, A&A, 292, L1 334, 253 Schaeidt, S. G., et al. 1996, A&A, 315, L55 Giannini, T., et al. 1999, A&A, 346, 617 Shevchenko, V. S., Grankin, K. N., Ibragimov, M. A., Mel’Nikov, S. Yu., & Goodrich, R. W. 1986, ApJ, 311, 882 Yakubov, S. D. 1993, Ap&SS, 202, 121 Gordon, M. A. 1995, A&A, 301, 853 Simon, M., Dutrey, A., & Guilloteau, S. 2000, ApJ, 545, 1034 Habart, E., Testi, L., Natta, A., & Vanzi, L. 2003, A&A, 400, 575 Skinner, S. L., Brown, A., & Stewart, R. T. 1993, ApJS, 87, 217 Haupt, H. F., & Schroll, A. 1974, A&AS, 15, 311 Stecklum, B., Eckart, A., Henning, Th., & Lo¨we, M. 1995, A&A, 296, 463 Henning, Th., Burkert, A., Launhardt, R., Leinert, Ch., & Stecklum, B. 1998, Strafella, F., Pezzuto, S., Corciulo, G. G., Bianchini, A., & Vittone, A. 1998, A&A, 336, 565 ApJ, 505, 299 Henning, Th., Launhardt, R., Steinacker, J., & Thamm, E. 1994, A&A, 291, Swinyard, B. M., et al. 1998, Proc. SPIE, 3354, 888 546 Sylvester, R. J., Skinner, C. J., Barlow, M. J., & Mannings, V. 1996, MNRAS, Henning, Th., Pfau, W., Zinnecker, H., & Prusti, T. 1993, A&A, 276, 129 279, 915 Herbig, G. H. 1960, ApJS, 4, 337 The´, P. S., de Winter, D., & Pe´rez, M. R. 1994, A&AS, 104, 315 Hillenbrand, L. A., Strom, S. E., Vrba, F. J., & Keene, J. 1992, ApJ, 397, 613 Tjin A Djie, H. R. E., The´, P. S., Andersen, J., Nordstrom, B., Finkenzeller, U., Hinz, P. M., Hoffmann, W. F., & Hora, J. L. 2001, ApJ, 561, L131 & Jankovics, I. 1989, A&AS, 78, 1 Hou, J. L., Jiang, D. R., & Fu, C. Q. 1997, A&A, 327, 725 Valenti, J. A., Johns-Krull, C. M., & Linsky, J. L. 2000, ApJS, 129, 399 Hu, J. Y., The´, P. S., & de Winter, D. 1989, A&A, 208, 213 van den Ancker, M. E., de Winter, D., & Tjin a Djie, H. R. E. 1998, A&A, 330, Hughes, J., Hartigan, P., & Clampitt, L. 1993, AJ, 105, 571 145 Hughes, J. D., Hartigan, P., Graham, J. A., Emerson, J. P., & Marang, F. 1991, van den Ancker, M. E., Wesselius, P. R., & Tielens, A. G. G. M. 2000, A&A, AJ, 101, 1013 355, 194 Jaschek, C., & Jaschek, M. 1992, A&AS, 95, 535 Van Kerckhoven, C., Tielens, A. G. G. M., & Waelkens, C. 2002, A&A, 384, Kamazaki, T., Saito, M., Hirano, N., & Kawabe, R. 2001, ApJ, 548, 278 568 Kessler, M. F., et al. 1996, A&A, 315, L27 Waters, L. B. F. M., & Waelkens, C. 1998, ARA&A, 36, 233 Koresko, C. D., Beckwith, S. V. W., Ghez, A. M., Matthews, K., & Wilking, B. A., McCaughrean, M. J., Burton, M. G., Giblin, T., Rayner, J. T., & Neugebauer, G. 1991, AJ, 102, 2073 Zinnecker, H. 1997, AJ, 114, 2029 Lagage, P. O., Olofsson, G., Cabrit, S., Cesarsky, C. J., Nordh, L., & Rodriguez Wilking, B. A., Mundy, L. G., & Schwartz, R. D. 1986, ApJ, 303, L61 Espinosa, J. M. 1993, ApJ, 417, L79 Zinnecker, H., & Preibisch, Th. 1994, A&A, 292, 152
