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Astron. Astrophys. 336, 565-586 (1998)

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3. Results and derived properties

3.1. Detections at 1.3 mm

In Tables 3 (HAEBE stars) and 4 (FU Orionis stars), we present the results of our 1.3 mm mapping survey together with the 1.3 mm peak positions, their deviation [FORMULA] from the optical positions, the beam-deconvolved extents, and the peak as well as the integrated flux densities of the sources. We consider an observation as a detection when the 3 [FORMULA] level was reached. In 18 of our 22 continuum maps we found emission above the 3 [FORMULA] detection limit. We distinguish the morphology of the emission in a core and an envelope contribution. With "core emission" we mean an emission pattern which is more point-like and shows a significant difference in the flux density to a possibly surrounding envelope on a lower flux density level if observed.
Six of the sources (Elias 1, AB Aur, HD 98048, HD 163296, Par 21, V 1331 Cyg) were found to be point-like without additional envelope structure. The emission around AB Aur, HD 163296 and the FU Orionis stars Par 21 and V 1331 Cyg could even not be resolved with the IRAM telescope. With the exception of HD 97048, the positional offsets of the point-like millimetre continuum sources relative to the optical positions are smaller than the pointing accuracy.
Twelve sources (LkH[FORMULA] 198 MM, MWC 137, VY Mon, HD 100546, IRAS 12496-7650, CoD -42o11721, MWC 297, LkH[FORMULA] 225, LkH[FORMULA] 234, MWC 1080, and the FU Orionis stars RNO 1B and Z CMa) show a core/envelope structure. Actually HD 100546 and IRAS 12496-7650 could just be resolved with the [FORMULA] beam of the SEST telescope.
Four sources (HK Ori, HD 250550, LkH[FORMULA] 25, V 1515 Cyg), all with low flux densities in the 12, 25 and 60 µm IRAS bands, could not be detected at 1.3 mm.


[TABLE]

Table 3. Morphology and flux densities of 1.3 mm continuum emission of the HAEBE stars
Notes:
a) Separation between optical position and millimetre peak
b) [FORMULA] = [FORMULA], except MWC 137 and MWC 1080 which were smoothed to 15 and 12", respectively
c) Beam-deconvolved HPBWs of an elliptical Gaussian fit for the core and beam-deconvolved 2 [FORMULA] extents for the envelope emission.
d) The given extent refers to the "average" diameter of the 2 [FORMULA] contour and excludes some distributed low-level emission or filamentary extension. In the case of VY Mon, the envelope emission extending down to DEC [FORMULA] [FORMULA] was assigned to the target object, the remaining part to a globule (see Chapter 3.5). In the case of MWC 1080, the shoe-shaped extension to the south-west is not included in the given envelope size.
e) Because of the marginal detection we do not give flux densities for the envelope component.
f) The "total" flux density includes all extended emission in the map, while the "envelope" flux density includes only the flux from the region described in d). Therefore, the total flux density for these objects is higher than the sum of the core and envelope emission.
g) Central position between the two emission peaks. "Core size" includes both components.
h) "Core" flux density includes the emission of both cores; envelope emission is only the emission of the northern envelope.



[TABLE]

Table 4. Morphology and flux densities of 1.3 mm continuum emission of the FU Orionis objects
Notes:
a) Separation between optical position and millimetre peak.
b) [FORMULA] = [FORMULA]
c) Beam-deconvolved HPBWs of an elliptical Gaussian fit for the core and beam-deconvolved 2 [FORMULA] extents of the envelope emission.
d) The map is obtained from a combination of a 19-channel and a 7-channel map.


The peak flux densities in Tables 3 and 4 refer to the effective beam area ([FORMULA]). For the non-detected sources we list the 3 [FORMULA] values. The total flux densities were determined by integrating the maps within the area defined by the 2 [FORMULA] rms contour. Most of the maps show a compact core plus an extended envelope structure. To distinguish the flux densities and to determine the extents of the core components, we fitted two-dimensional Gaussian functions to both the core and the envelope emission. After subtraction of the "Gaussian" envelope component, we estimated the flux density of the core by integrating the contributions within an area which just includes the core. The flux density of the envelope was then determined by subtracting the core flux from total flux density. The extents of the envelope components were estimated from the 2 [FORMULA] contour level. In the cases where the envelope boundary has a complex shape we give the extents of an equivalent elliptical configuration to get reliable hydrogen number densities in chapter 3.3. For HD 100546 and IRAS 12496-7650, we do not give flux densities for the envelope components because of their low values. In Tables 3 and 4 we present the beam-deconvolved (with [FORMULA]) sizes of both the cores and the envelopes.

In most maps, the core emission could be assigned to the optically bright target stars. In other cases, the 1.3 mm peak positions differ more than [FORMULA] (V 376 Cas/LkH[FORMULA] 198, MWC 137, CoD -42o11721, and V 1685 Cyg/V 1686 Cyg) or a few arcseconds (HD 97048, HD 100546, IRAS 12496-7650) from the optical and NIR positions and/or more than one target star is located inside the cold dust emission (e.g., V 376 Cas/LkH[FORMULA] 198, LkH[FORMULA] 225, RNO 1B). Especially the clusters of NIR stars (MWC 137, CoD -42o11721, V 1685 Cyg/V 1686 Cyg/LkH[FORMULA] 225, LkH[FORMULA] 234, MWC 1080) inside regions of extended millimetre emission demonstrate that it is absolutely necessary to perform mapping in order to get a correct picture of the dust distribution and its relation to the stellar sources. Flux densities from single-channel On-On measurements are often used as a data point in spectral energy distributions to which disk or envelope models are fitted. However, such measurement may miss the actual peak value of the continuum emission. This is obviously the case if the dust emission is not really related to the considered HAEBE/FU Orionis star. A "good" example for the latter case is the R CrA and T CrA region where neither of these two stars is responsible for the millimetre emission (Henning et al. 1994) or the V 376 Cas/LkH[FORMULA]198 and V 1685 Cyg/V 1686 Cyg/LkH[FORMULA] 225 regions discussed in this paper. In the case of On-On measurements with large beams, part of the envelope emission of a nearby embedded object/molecular cloud core may be assigned to an optically visible HAEBE star which actually does not show any strong millimetre continuum emission. In the case of the association of the target objects with extended emission, On-On measurements result in systematically lower flux densities because the On position is chopped against the envelope.

In the following, we will compare the results from our millimetre maps with previous On-On measurements obtained with different telescopes and beam sizes (Henning et al. (SEST-23'', 1994), Hillenbrand et al. (CSO-28'', 1992), Reipurth et al. (SEST-23'', 1993), Mannings (JCMT-20'', 1994), and Osterloh & Beckwith (MRT-12'', 1995)). The millimetre peak flux densities derived from the maps (for the same aperture as in the corresponding single-channel On-On measurements) are generally higher (within the calibration errors) than the flux densities obtained by the On-On measurements. An exception is MWC 1080 where we got only about half the value Hillenbrand et al. (1992) and Mannings (1994) obtained. The deviations between the peak values and the flux densities from On-On measurements are up to a factor of 5. This is the case for V 376 Cas and V 1685 Cyg which are clearly not associated with the strongest millimetre peak as already discussed above (see Fig. 2). Even if we compare the flux densities from the maps derived for the same positions where the On-On measurements were performed, we found discrepancies which are larger than the usually expected calibration errors. This can be up to a factor of 4 in both directions and is probably caused by pointing errors and relatively uncertain calibrations in the single-channel On-On measurements. The largest deviations were found in the case of two measurements of Hillenbrand et al. (1992) for AB Aur and MWC 297. In contrast, we found good agreement in these two cases with the values of Mannings (AB Aur, 1994) and Henning et al. (MWC 297, 1994). The discrepancies discussed here clearly demonstrate how important high-quality mapping data are. We should note that the measured flux densities for AB Aur and HD 163296 are in very good agreement with the millimetre interferometry data obtained by Mannings & Sargent (1997).

3.2. NIR-maps

The K-band images were used to search for young stellar objects (YSOs) which might be associated with the dust condensations or the target stars themselves. The question was if the 1.3 mm emission could be clearly assigned to a HAEBE/FU Orionis star or if there are other YSOs responsible for the millimetre emission (see the discussion in Sect. 3.1). The quality of our NIR images is comparable to those of Hillenbrand (1995), Li et al. (1994) and Testi et al. (1997). A lot of photometric data are available in the literature (e.g., Hillenbrand et al. 1992, Hillenbrand 1995, Berrilli et al. 1992) for our target sources. Therefore, we do not present photometric data in this paper.

In Table 1, we indicate the objects which are associated with companions (LkH[FORMULA] 198, Elias 1, HK Ori, LkH[FORMULA] 225, LkH[FORMULA] 234, RNO 1B, Z CMa) or clusters (MWC 137, CoD -42o11721, V 1685 Cyg/V 1686 Cyg/LkH[FORMULA] 225, LkH[FORMULA] 234, MWC 1080). The NIR environment around MWC 137 was discussed in detail by Hillenbrand (1995) and that around LkH[FORMULA] 225 (V 1685 Cyg, V 1685 Cyg) by Hillenbrand et al. (1995) and Palla et al. (1995), respectively. In the case of AB Aur, VY Mon, and HD 163296 we present NIR images for the first time.

In the cases where the millimetre emission peak does not coincide with the position of an HAEBE star, we did not detect other NIR sources clearly associated with the millimetre peak. Some of the NIR images show near the target objects additional NIR emission which is discussed in Sect. 3.5 for the objects individually. The NIR images of the non-detected sources do not show any interesting structure and are not presented here.

3.3. Derivation of physical parameters from the continuum emission

Starting from the 1.3 mm continuum flux densities, we derived the following physical parameters: the source-averaged hydrogen column densities ([FORMULA]), the source-averaged number densities ([FORMULA]), and the gas masses ([FORMULA]) of the individual components (see Table 5).


[TABLE]

Table 5. Characteristics of the millimetre continuum sources
Notes:
a) "Core" opacity and temperature applied
b) See remarks to Table 3.


In this paper we calculated the specified physical parameters with a standard set of input parameters. These are a mass-averaged dust temperature of [FORMULA]=50 K, a mass absorption coefficient [FORMULA] per gram dust of 1 cm2g-1 for the core emission and a relative metallicity [FORMULA]. For the envelope we assume [FORMULA]=20 K and [FORMULA]=0.5 cm2g-1. This treatment makes it easier to compare the physical parameters of the different sources and prevents confusion caused by too many variations. Because of the higher densities in the cores we expect a stronger coagulation of the dust particles and, therefore, higher opacities than in the envelopes (Henning et al. 1995).

For a detailed discussion of the uncertainties of the dust opacity, we refer to the papers by Ossenkopf & Henning (1994), Henning et al. (1995), and Henning & Stognienko (1996). Following Draine & Lee (1984), we assume a standard hydrogen-to-dust mass ratio of [FORMULA] [FORMULA] 110. Accounting for He and metals, the total gas-to-dust mass ratio is [FORMULA] [FORMULA] [FORMULA].

Below, we give the formula for the calculation of the physical parameters. Here [FORMULA] denotes the Planck function at 1.3 mm, d the distance, [FORMULA] the flux density, [FORMULA] the beam size, [FORMULA] the effective source extent, [FORMULA] the central wavelength of the observation and x = 1.44[FORMULA]104/([FORMULA]/µm)(T/K) is an abbreviation. From the first formula, we obtained the source-averaged hydrogen column densities:

[EQUATION]

The second equation gives the hydrogen number densities under the assumption that the extent of the sources is the same in the direction of the line of sight as in the plane of the sky

[EQUATION]

The total gas mass was estimated with the following formula which is valid for optically thin dust emission at 1.3 mm and for isothermal, uniformly distributed dust grains

[EQUATION]

The average gas mass of the "genuine" point-like millimetre sources amounts to 0.15[FORMULA]0.15 [FORMULA] with the lowest mass found for AB Aur (0.026 [FORMULA]) and the highest value found for V 1331 Cyg (0.48 [FORMULA]). Around AB Aur the presence of a disk was indicated by MIR observations (Marsh et al. 1995) and proved by millimetre interferometry (Mannings & Sargent 1997). Other sources without an additional envelope structure (HD 163296, Elias 1) also show evidence for disks from interferometry data (Di Francesco et al. 1997, Mannings & Sargent 1997). The average total gas mass of the sources with core/envelope structure (without Cod-42o11721) amounts to 80[FORMULA]60 [FORMULA] with the most extreme values found for V 376 Cas and MWC 137 (24 [FORMULA]) and Cod-42o11721 (1100 [FORMULA]). The cores of the core/envelope sources tend to be more massive than the "genuine" point-like sources (5[FORMULA]5 [FORMULA]).

Regarding the calculated masses, we have to take into account that we assumed that the 1.3 mm emission comes completely from an optically thin dust configuration. In some cases, there may be an additional flux contribution by optically thick circumstellar disks close to the stars, not comprised in the mass estimates. This may be the case, e.g., for the FU Orionis stars where the presence of a disk is generally assumed.

For sources with cores not resolved in one or two dimensions (Elias 1, AB Aur, MWC 137, VY Mon, HD 97048, HD 100546, HD 163296, MWC 297, LkH[FORMULA] 234, Par 21, V 1331 Cyg) we used one half of the HPBW to derive the physical parameters. Therefore, the printed values represent lower limits for the number densities. For the non-detected sources, we give upper limits corresponding to the upper flux density limits. The average denities in the cores range from 105 to 108 cm-3. The densities of the extended envelopes are of the order of 104 to 105 cm-3.

In the case of the core/envelope sources most of the mass is located in the envelope structures and the mean ratio of [FORMULA]/[FORMULA] is 0.05 for the HAEBE stars and 0.13 for the two FU Orionis stars. The average extent of the envelopes is 0.36[FORMULA]0.17 pc with the smallest envelope found around MWC 297 (0.09 pc) and the largest envelope found around Cod-42o11721 (1.3 pc). The compact sources (cores and point-like sources) have a mean FWHM of 0.04[FORMULA]0.06 pc. Note, however, that most of these sources are unresolved and the given size is an upper limit.

Based on 1.3 mm continuum On-On measurements, Henning et al. (HAEBE stars, 1994) and André & Montmerle (T Tauri stars, 1994) estimated median flux density values normalized to a fixed distance for class I and II objects. Both for HAEBE and for T Tauri stars smaller values for class II than for class I objects were found. This result was interpreted in terms of an evolutionary effect. The class II objects should have already lost a part of the circumstellar dust envelope. Using our new dataset, we estimated averaged core, envelope and total masses for the class I and II objects. If we do not include the objects Cod-42o11721 (class I) and LkH[FORMULA] 234 (class II), which have comparatively high core and envelope masses, we find similar averaged core, envelope and total masses in the case of class I and II HAEBE stars. Here we should stress again that some millimetre sources are not really associated with a HAEBE star. Whereas, the fraction of sources with core/envelope morphology is the same for class I and II in the case of HAEBE stars, all core/envelope FU Orionis stars (RNO 1B, Z CMa) belong to the class II. Par 21 is the only class I FU Orionis object, detected at 1.3 mm, which was found to be point-like at this wavelength. As we already noted in previous papers (see, e.g., Men'shchikov & Henning 1997), the classification of an object with a circumstellar disk/non-spherical envelope may be influenced by its inclination angle.

3.4. Discussion of individual sources

In this section, we discuss the 1.3 mm emission of our target stars individually. Corresponding to the observed emission pattern, we divide the stars into three groups: stars with point-like emission, with core/envelope structure, and non-detected sources. The 1.3 mm continuum maps are superimposed on K-band images obtained at different telescopes (Figs. 1 to 2). The beam sizes of the 1.3 mm observations are compiled in Tables 3 (HAEBE stars) and 4 (FU Orionis stars). They are indicated by filled circles in the lower right corner of each individual image. The NIR images are scaled in a way that a good comparison with the millimetre maps is possible. This means that not all faint NIR sources are easily visible in the figures. The optical positions are indicated with crosses if they deviate from the NIR positions. The IRAS error ellipses are plotted with dotted lines.

3.4.1. Compact (point-like) sources

In Fig. 1a-f we present all sources with distinct compact (point-like) 1.3 mm continuum emission. The classification "point-like" is correlated both with the beam sizes of the telescopes where the observations were performed and the distances of the objects. To get the correct linear extent of the sources, we have to take this into account. Four HAEBE stars show only a compact core. These are the objects Elias 1, AB Aur, HD 97048, and HD 163296. The two FU Orionis stars with point-like emission are Par 21 and V 1331 Cyg.

[FIGURE] Fig. 1a-f. Maps of the 1.3 mm continuum emission of the detected (point-like) sources. The contour levels are equally spaced between the 3 [FORMULA] and top levels. The IRAS error ellipses are marked with dotted lines. The background grey-scale images are K-band observations. a Elias 1 (3, 9, 15, 21, 28 [FORMULA]), b AB Aur (3, 9, 15 [FORMULA]), c HD 97048 (3, 6, 9, 12 [FORMULA]), d HD 163296 (3, 8, 13, 18, 23 [FORMULA]). e Parsamian 21 (3 [FORMULA]), f V 1331 Cyg (3, 6, 9 [FORMULA]).

It is remarkable that the sources with point-like 1.3 mm emission are preferably of spectral type A, whereas the core/envelope sources are rather of type B. The three point-like HAEBE stars show spectral types of early A. All point-like FU Orionis stars belong to late A or rather early F types. With an average distance of [FORMULA] 250 pc the point-like target objects are on average closer than the core/envelope sources of spectral type B. With the exception of HD 97048 all point-like millimetre sources were observed with the IRAM telescope ([FORMULA]12" beam size). Therefore, the extent of the millimetre emission around these sources is really more compact than that around the core/envelope sources. Grady et al. (1996) performed an investigation of A-shell stars using IUE high-dispersion spectra and found accreting, circumstellar gas in the line of sight of several stars.

Elias 1  (IRAS 04155+2812): In the K-band grey-scale image two bright stars are visible. The star in the centre is Elias 1 (5.64 mag in K) which was first detected in the near-infrared by Elias (1978). The star south-east of Elias 1 is V 410 X-ray 7 (9.16 mag in K) detected by Strom & Strom (1994). Faint emission is present between the two stars. Elias 1 was found to be a binary system by Skinner et al. (1993) and Leinert et al. (1997). The companion star Elias 1 NE is located 4" north-east of Elias 1 and is considered to be a T Tauri star. For a more extensive discussion of its nature, we refer to Leinert et al. (1997). We found unresolved 1.3 mm continuum emission centred on Elias 1. The emission of this object is also unresolved by millimetre interferometry (Di Francesco et al. 1997).

AB Aur  (IRAS 04525+3028): AB Aur is the very bright star (4.4 mag in K) in the K-band image (Fig. 1b). In addition, some faint NIR sources are present. Three NIR sources south-east of AB Aur are also visible in the optical. Note that one of these sources is located outside the field shown here. AB Aur itself lies relatively nearby (144 pc) and shows short-term (days) spectroscopic variability as well as a fast wind, a P Cygni profile in H[FORMULA] (Finkenzeller & Mundt 1984), and a strong near-infrared excess. The strong NIR excess led Hartmann et al. (1993) to the suggestion that AB Aur may have an infrared companion. Such a companion could also be responsible for the X-ray emission found by Zinnecker & Preibisch (1994). However, no evidence was found for a companion by speckle observations (Leinert et al. 1997). Güdel et al. (1989) detected a weak 4 [FORMULA] source 0:006 SE of AB Aur at 3.6 cm wavelength, but this source was not confirmed by Skinner et al. (1993). AB Aur shows the smallest associated mass among the detected stars with point-like 1.3 mm emission. Based on MIR observation, the presence of a disk around AB Aur was suggested by Marsh et al. (1995). However, long-slit spectroscopy of the forbidden [OI] lines at 6300.31 and 6363.79 [FORMULA] by Böhm & Hirth (1997) seemed to exclude the presence of a compact accretion disk. Nevertheless, Mannings & Sargent (1997) proved the existence of a disk with a semi-major axis of 450 AU by millimetre interferometry.

HD 97048  (IRAS 11066-7722):

HD 97048 is located in the Chamaeleon I dark cloud and is associated with the reflection nebula Ced 111. It is one of the few stars which show the unusual 3.43 and 3.52 µm emission features in their spectra, first detected by Whittet et al. (1983). HD 97048 is the only unresolved SEST target source. From our NIR image we find three additional NIR sources. The bright object in the right upper corner of the NIR image is SZ 22 which is also visible at optical wavelengths (Whittet et al. 1987). HD 97048 was found to be extended in MIR observations performed by Prusti et al. (1994). The extended MIR emission is very probably produced by very small grains and PAHs. Adaptive optics observations we performed at NIR wavelengths did not show any evidence for extended emission. The strong total 1.3 mm emission found by mapping is higher than the value of our previous pointed observations (see, e.g., Henning et al. 1994). This value is difficult to reconcile with a pure spherically symmetric model for the source (Henning et al. 1993).

HD 163296  (IRAS 17533-2156): HD 163296 is the fourth HAEBE star in our sample with point-like 1.3 mm emission. This star is sometimes called an isolated HAEBE star. Because of their youth, HAEBE stars should be close to their parental cloud. Isolated HAEBE stars are sources where no associated cloud is found. There is no sign for an extended envelope structure which may point to the parental cloud. This star shows no photometric variability in contrast to many other HAEBE stars, but a strong infrared excess. In our K-band image we found numerous faint NIR sources, the closest with only 6:007 separation in the north-west.

Mitskevich (1995) interpreted the SED of HD 163296 by an inhomogeneous envelope. If HD 163296 is not only projected onto the cold dust emission, its visibility in the optical points more to a disk-like dust distribution (Henning et al. 1994). Mannings & Sargent (1997) proved the existence of a disk with a semi-major axis of 310 AU by millimetre interferometry.

The FU Orionis stars Par 21 and V 1331 Cyg could not be resolved by our 1.3 mm observations, whereas RNO 1B and Z CMa show core/envelope structures.

Par 21  (IRAS 19266+0932): Par 21 is associated with a small dark cloud in Aquila. From the optical to near-infrared wavelengths, a bright cometary nebula is visible (Eiroa & Hodapp 1990; Li et al. 1994). A similar cometary behaviour was found in [S II] 6717, 6730 Å and 7200 Å line observations and in the red optical continuum by Staude & Neckel (1992). They also detected a Herbig-Haro system associated with the source.

We found three faint emission centres in our K-band image located north of Par 21 within the cometary nebula and in a line oriented east-west. The 1.3 mm continuum map presented in Fig. 2 was constructed from 19-channel and 7-channel (On-On) observations and is a 5 [FORMULA] detection. Therefore, we only show the 3 [FORMULA] contour. The distance between the stellar and the millimetre positions is within the pointing accuracy of the telescope. From our map, we can not derive any reason for the cometary shape of Par 21 in the NIR.

[FIGURE] Fig. 2a-l. Maps of the 1.3 mm continuum emission of the detected (core/envelope) sources. The lowest contours are the 3, 6, 9 [FORMULA] contours. Above the 9 [FORMULA] level, the levels are equally spaced up to the peak level. The IRAS error ellipses are marked with dotted lines. The background grey-scale images are K-band observations. a V 376 Cas (3, 6, 9, 13, 17, 21 [FORMULA]), b MWC 137 (3, 6, 9 [FORMULA]), c VY Mon (3, 6, 9, 15, 21, 27 [FORMULA]), d HD 100546 (3, 6, 9 [FORMULA]).

[FIGURE] Fig. 2. (continued) e IRAS 12496-7650 (3, 6 [FORMULA]), f Cod -42o11721 (3, 6, 9, 11.5, 14 [FORMULA]), g MWC 297 (3, 6, 9, 18, 27, 36 [FORMULA]), h LkH[FORMULA] 225 (3, 6, 9, 13, 17, 21, 25 [FORMULA]).

[FIGURE] Fig. 2. (continued) i LkH[FORMULA] 234 (3, 6, 9, 18, 27, 36, 45 [FORMULA]), j MWC 1080 (3, 6, 9, 12, 15, 18, 21 [FORMULA]), k RNO 1B (3, 6, 9, 19, 29, 39, 49 [FORMULA]), l Z CMa (3, 6, 9, 18, 27, 36, 45 [FORMULA]).  

Based on optical polarization maps, Draper et al. (1985) suggested a disk orientated perpendicular to the main axis of the nebula. Bastien & Ménard (1990) derived an inclination angle of 80-85o and a disk size of 30" x 8" from modelling polarization data. According to the model of Bastien & Ménard (1990), scattering in a disk configuration will result in a shift of the emission peak with wavelength. We imaged the object at J, H, and K. Similar to Eiroa & Hodapp (1990) and Li et al. (1994) we found that there is no significant shift between the positions within the near-infrared bands.

V 1331 Cyg  (IRAS 20595+5009): In the case of V 1331 Cyg we do not have our own NIR image. Therefore, we compared our continuum observation with an K'-band image obtained by Hodapp (1994). V 1331 Cyg has a helical extension towards the east in the V and K'-bands. This star is thought to be in a pre-FU Orionis phase (Welin 1976). The object is not resolved in our 1.3 mm observations which may point, because of the optical visibility, to the existence of a circumstellar disk. The presence of such a disk, seen nearly edge-on, was suggested by Weintraub et al. (1991) based on submillimetre observations. Weintraub et al. noted that V 1331 Cyg should be optically thick at all wavelengths [FORMULA] 2 mm. High-resolution ([FORMULA]4") aperture synthesis maps of CO performed by McMuldroch et al. (1993) and associated continuum emission suggest the presence of a massive circumstellar disk surrounded by a flattened gaseous envelope with 6000 x 4400 AU extent. They estimated a disk mass of 0.5 [FORMULA] and an envelope mass of 0.32 [FORMULA], whereas we calculated a value of 0.48 [FORMULA] assuming an optically thin configuration. Hamann et al. (1994) associated [Fe II] line asymmetries with local obscurations in a circumstellar disk with a mass greater than 0.001 [FORMULA] and [FORMULA] 3 1022cm-2 whereas we estimated a column density of [FORMULA] 1.5 1023cm-2.

3.5. Sources with core and envelope structure

Ten regions containing HAEBE stars and the two FU Orionis stars RNO 1B and Z CMa show a core/envelope structure at 1.3 mm. The maps of these regions are shown in Fig. 2.

V 376 Cas and LkH[FORMULA] 198  (IRAS 00087+5833): The region V 376 Cas/LkH[FORMULA] 198 attracted much attention and quite a lot of optical and infrared observation are available (see, e.g., Koresko et al. 1997). Leinert et al. (1991) performed near-infrared speckle observations and polarimetry (see also Piirola et al. 1992, Asselin et al. 1996). They found dust halos with sizes of about [FORMULA] corresponding to several hundred AU around both stars by speckle observations. V 376 Cas appears to be a bipolar nebula, seen nearly edge-on, with evidence for a circumstellar disk from optical data. LkH[FORMULA] 198 shows a complicated structure with evidence for asymmetry on subarcsec scale. Adaptive-optics compensated speckle imaging revealed the presence of a bar-like feature which extends [FORMULA] from the star in either direction (Koresko et al. 1997).

Lagage et al. (1993) found, at 10 µm, a source located [FORMULA] north of LkH[FORMULA] 198. This source is also present in K-band images (Li et al. 1994) and optical images (Corcoran et al. 1995). In addition, speckle NIR observations showed that LkH[FORMULA] 198 is also a close binary (Fischer et al. 1997, priv. communication). CO observations (see, e.g., Nakano et al. 1990) indicate the presence of a low-velocity bipolar molecular outflow. The driving source for this outflow is still under debate.

Our 1.3 mm continuum map shows an envelope structure with not a very pronounced core. However, the millimetre emission peak is clearly shifted from the position of the two stars V 376 Cas and LkH[FORMULA] 198. This was already realized by Sandell & Weintraub (1994) who observed the region with the JCMT at 800 µm and identified the peak as the new protostar LkH[FORMULA] 198 MM. LkH[FORMULA] 198 is located [FORMULA] east of the millimetre peak emission. V 376 Cas has a distance of [FORMULA] to this peak. Both objects are still located in the envelope. However, V 376 Cas is only associated with the 3 [FORMULA] level of the 1.3 mm emission. Our map does not show any evidence for strong millimetre emission coming from a compact circumstellar disk associated with either of these two stars.

MWC 137: MWC 137 is surrounded by a small optical cometary nebula. At optical and NIR wavelengths it appears as a bright star surrounded by a cluster of fainter stars (see also Hillenbrand 1995). It has been mentioned by Herbig & Rao (1972) that this object might not be a pre-main-sequence star. This star shows not only variability of 0.6 mag in JHK photometry (Bergner et al. 1995) but also large polarization (6%) (Jain & Bhatt 1995). Furthermore, the radio observations by Skinner et al. (1993) point to the presence of a non-thermal component. The real nature of this object remains unclear.

The IRAS source 06158+1517 near to MWC 137 was identified as an H II  region at a distance of 9.4 kpc by Rudolph et al. (1996). We found an extended bipolar millimetre continuum source with maxima [FORMULA] north-east and [FORMULA] south-west of the main star of MWC 137. It would be interesting to study these millimetre cores in more detail to find out if there are embedded stellar sources or if the structure is associated with a bipolar molecular outflow. Our NIR imaging does not reveal the presence of a bright object at the positions of the millimetre "lobes".

VY Mon  (IRAS 06283+1028): VY Mon is located [FORMULA] south of the reflection nebula IC 446. Casey & Harper (1990) detected a globule at 160 and 370 µm south of VY Mon. VY Mon itself is an eruptive, highly reddened (AV [FORMULA] 7.4 mag), Algol-like variable HAEBE star. It dominates the NIR and MIR emission in this region. The K-band emission associated with the star appears to be slightly extended to the south. We found a cometary-shaped millimetre source with the core located at the stellar position of VY Mon and the tail extending to the south. In addition, we detected a second component on a very low flux density level (3 [FORMULA]) at the position of the globule. Both components are within the 2 [FORMULA] contour. To separate the components we assigned the emission north of DEC [FORMULA] [FORMULA] to the HAEBE star VY Mon. The emission around the globule covers a region of [FORMULA] and includes a mass of 30 [FORMULA]. For the mass estimation we assumed [FORMULA]=15K (see Casey & Harper 1990) and [FORMULA].

HD 100546  (IRAS 11312-6955): In the K-band the star appears to be point-like and is surrounded by many fainter sources. The NIR object closest to HD 100546 is only 0.5 arcsec away (west of HD 100546). ISO observations showed the presence of crystalline silicate and PAH features in the SWS spectrum (Waelkens et al. 1996).

Despite the large beam size of the presented 1.3 mm map (23"), we see an extension of the dust continuum emission to the south-east. This faint extension is close to the detection limit, therefore, we do not give flux densities and physical parameters in Tables 3 and 5. To explain the spectral energy distribution of this star a 1D-model was successfully used by Henning et al. (1994). However they had problems to fit the high 1.3 mm flux density which was 1.5 times smaller than the total flux density estimated from our new map. IUE data revealed the existence of in-falling gas similar to the [FORMULA] Pic system (Grady et al. 1997).

I12496-7650  (IRAS 12496-7650): This source has strong flux densities in all four IRAS bands as well as high flux densities of 0.68 Jy and 0.53 Jy in pointed 1.3 mm observations performed by Henning et al. (1994) and Reipurth et al. (1993), respectively. From our map, we obtained 0.5 Jy for the total flux density. The optically visible source is variable, shows blue-shifted optical (forbidden) emission lines and a prominent P Cygni profile in H [FORMULA] (Hughes et al. 1991). The object is also associated with a relatively weak molecular outflow (Knee 1992). Altogether it can be classified as a very active HAEBE star. With multi-aperture photometry at MIR wavelengths, Prusti et al. (1994) found no evidence for the object being extended. This object should be a prime candidate for the search of a small-scale disk-like structure with high-resolution techniques.

CoD -42o11721  (IRAS 16555-4237): The HAEBE star CoD -42o11721 was first discovered by Merrill & Burwell (1949) and was considered to be a supergiant with nebulosity by Hutsemékers & Van Drom (1990). It is contained in the catalogue of HAEBE stars by Thé et al. (1994). The nebulosity has a size of [FORMULA] [FORMULA]x 80[FORMULA]. CoD -42o11721 shows PAH features (Jourdain de Muizon et al. 1990, Brooke et al. 1993) in its spectrum. Brooke et al. (1993) mentioned that the visual spectrum contains primarily emission lines which makes the spectral classification difficult. Based on ultraviolet absorption lines Shore et al. (1990) classified this object as of spectral type B0.

The emission around the HAEBE star was found to be extended with a wing to the NW by KAO observations (Natta et al. 1993, also present in our K band image when visualized with another intensity scale). Our millimetre map shows two distinct cores which are clearly shifted in comparison to the optical position.

MWC 297  (IRAS 18250-0351): A visible nebula was found near the HAEBE star MWC 297 extending in north-east direction by Herbig (1960). The 1.3 mm map shows a central core at the position of the optical star together with a more extended envelope. In addition, there is a weak component in the west associated with a NIR source and a second one in the south-east. For these objects, we estimated masses of 0.04 [FORMULA] and 0.01 [FORMULA], respectively, assuming [FORMULA]=50K and [FORMULA].
The brightest NIR source south-west of MWC 297 is the H II region SH 2-62 which is not associated with detectable millimetre emission. MWC 297 is relatively bright in the IRAS 100 µm band and also has high flux densities at 3.6 and 6 cm (Skinner et al. 1993). Cantó et al. (1984) performed CO (1-0) observations and found two velocity components at [FORMULA] km s-1 and [FORMULA] km s-1. However, there was no clear pattern in the spatial distribution of these components, suggesting two independent clouds.

The region around LkH[FORMULA] 225: This region attracted special attention because some HAEBE stars (V 1685 Cyg, V 1686 Cyg and probably LkH[FORMULA] 225 S) and a lot of other NIR sources are located close together in a relatively small region (Aspin et al. 1994, Li et al. 1994, Hillenbrand et al. 1995, Palla et al. 1995). Aspin et al. found LkH[FORMULA] 225 (= V 1318 Cyg) to be a binary object with faint emission between both stars at near-infrared wavelengths. This faint emission is also evident in our NIR image. LkH[FORMULA] 225 S is luminous enough to be a HAEBE candidate (Thé et al. 1994). This star shows a bipolar CO outflow (Palla et al. 1995) and is located in the centre of the point-like continuum emission component (this paper). The CS J=5[FORMULA]4 emission which is tracing high-density molecular gas is highly concentrated at LkH[FORMULA] 225 and V 1686 Cyg. We could not detect significant 1.3 mm emission at the position of V 1686 Cyg as may be expected from the CS observations. The C18O emission measured by Palla et al. (1995) traces the column density of the gas. It shows a ridge structure which is similar to the result of our continuum map.

V 1685 Cyg is an Algol-type star with CO band-head emission, a flat spectrum in the 10 µm region (Aspin et al. 1994), and 3.6 cm continuum emission (Skinner et al. 1993). The HAEBE stars V 1685 Cyg and V 1686 Cyg are examples for sources where the main millimetre emission comes from another object (LkH[FORMULA] 225).

LkH[FORMULA] 234  (IRAS 21418+6552): The HAEBE star LkH[FORMULA] 234, located in the star-forming region NGC 7129, is associated with a large amount of cold dust both in the envelope and the core. In the south-east direction a prominent optical nebula is present which can also be seen in the NIR image. It is remarkable that the cold dust envelope has also an elongated and curved structure.

Weintraub et al. (1994) found evidence for a deeply embedded companion [FORMULA] north-west of LkH[FORMULA]2_34 in their 2 µm polarization maps, which may be responsible for the CO outflow detected by Edwards & Snell (1983) and for the 6 cm emission (1:007 NW of LkH[FORMULA] 234) detected by Skinner et al. (1993). Recently, Cabrit et al. (1997) detected the companion 2:007 north-west of the optical star at MIR wavelengths. Its role in driving the optical jet is not clear. The optical star is found to have unresolved mid-infrared emission. The presence of a dust ring with a radius of 0.15 pc was suggested by Dent et al. (1989) based on 1100 µm mapping. This ring would be identical with the extended emission at 1.3 mm.

MWC 1080  (IRAS 23152+6034): MWC 1080 is a luminous object with L[FORMULA]4 104 [FORMULA] associated with a strong wind which has velocities up to 1100 km/s. The source is an eclipsing binary (Shevchenko et al. 1993) where the observed X-ray emission may origin from colliding wind components (Zinnecker & Preibisch 1994). NIR speckle observations showed the presence of an additional companion [FORMULA] west of MWC 1080 for which a luminosity of about 250 [FORMULA] based on a simple disk model was derived (Leinert et al. 1997). Our K-band image shows extended emission in the NE-SW direction.

An 8.8 µm image obtained by Deutsch et al. (1995) shows extended elliptical emission of about [FORMULA] or 4000 AU in diameter. A bipolar molecular CO outflow is aligned perpendicular to this structure in the N-S direction (Yoshida et al. 1991). At longer wavelengths (100 µm), the dust emission is also extended, however, with a deconvolved size less than the uncertainty of the point source profile (Di Francesco et al. 1994). Our 1.3 mm map shows the presence of two cores separated by an angular distance of [FORMULA], near MWC 1080 together with a cometary-shaped envelope and an extension to the south-west at low flux density level. The south-east core has a mass of 1.6 [FORMULA], the north-west core of 1.0 [FORMULA], respectively. The mass of the extension to the south-west, not included in the mass estimate given in Table 5, amounts to 10.3 [FORMULA]. In addition, three weak separate millimetre sources west, south-west and south of MWC 1080 with masses of 0.3, 0.2, and 0.3 [FORMULA] were detected.

RNO 1B: The object RNO 1B/C is characterized by a flat envelope and a core. The stars RNO 1B/C and RNO 1 are located in the cometary nebula GN 00.33.9 which is part of the L 1287 cloud in the globule filament GF 11. Staude & Neckel (1991) and Kenyon et al. (1993) suggested that RNO 1B/C form a binary FU Orionis system. The distance of a fourth object (IRAS 00338+6312) is [FORMULA] and [FORMULA] relative to RNO 1B and RNO 1C, respectively. A bipolar outflow with its major axis in a north-east to south-west direction found by Snell et al. (1990) and Yang et al. (1991) was assigned to the IRAS source. IRAS 00338+6312 may be a deeply embedded young stellar object, responsible for the 3.6 and 6 cm continuum emission detected by Anglada et al. (1994) and McCutcheon et al. (1991). The millimetre and submm observations performed by McMuldroch et al. (1995) suggest that RNO 1C is the driving source of the outflow. The centre of the 1.3 mm (core) emission is very close to RNO 1B/C.

Z CMa (IRAS 07013-1128): Z CMa was initially classified as a Herbig Ae/Be star (Herbig 1960). Hartmann et al. (1989) considered this object with a system luminosity of [FORMULA] [FORMULA] as of FU Orionis type. The object is very well studied at all wavelengths and different high-resolution speckle observations revealed a close companion ([FORMULA] separation) at optical and NIR wavelengths (Leinert & Haas 1987, Koresko et al. 1991, Barth et al. 1994, Thiébaut et al. 1995). The south-east component is considered as a FU Ori object, the north-west component is similar to NIR companions of several T Tauri stars (Hartmann et al. 1989). Whitney et al. (1993) suggested that the north-west component is a HAEBE star. Persi et al. (1996) found Z CMa unresolved in their MIR measurements, derived an upper limit of [FORMULA] 2000 AU (d=1150pc) for the disk size and mentioned that this size is consistent with constraints derived from 50 µm observations by Natta et al. (1993). The claim of having seen a circumbinary disk at 3-5 µm (Malbet et al. 1993) has been questioned (Tessier et al. 1994).

3.6. Non-detected sources

HK Ori  (IRAS 05286+1207): HK Ori is a star of spectral type A 4ep at a distance of 460 pc with an extended emission in the 100 µm IRAS band (70 Jy). Despite the low rms of 9 mJy/beam we reached, we did not detect any 1.3 mm emission from this source. HK Ori is a binary with a strong wavelength dependence of the brightness ratio of the components in the near infrared (Leinert et al. 1997). This binarity may be responsible for the destruction of a cold dust envelope.

HD 250550  (IRAS 05591+1630): HD 250550 is associated with the Bok globule CB 39 (Clemens & Barvainis 1988). The source shows variations in the Ca II lines with a period of 41h (Catala et al. 1991) as well as in the polarization degree with a time scale of [FORMULA]1 month (Jain & Bhatt 1995). We did not detect any cold dust emission (upper limit 18 mJy). A similar 3 [FORMULA] upper limit (30 mJy) was found by Launhardt & Henning (1997) during On-On measurements. Therefore, the disk/envelope model applied by Hartmann et al. (1993) may not be appropriate in this case. More likely is a close companion for the explanation of the NIR excess. Using the FineGuidanceSensor(FGS) of the Hubble space telescope, Bernacca et al. (1995) restricted the distance of a possible companion to be smaller than 0.05 AU with a mass of less than 2.9 [FORMULA].

LkH[FORMULA] 25 (IRAS 06379+0950): LkH[FORMULA] 25 is located in the Monoceros OB I region and is the third source of our sample not detected at 1.3 mm. Like HK Ori this source has low flux densities in the first three IRAS bands (between 1 and 4 Jy) and extended emission at 100 µm (620 Jy). Extended background emission was also reported at 25 µm by Margulis et al. (1989).

V 1515 Cyg  (IRAS 20220+4202): The eruptive pre-main-sequence star V 1515 Cyg shows a variation of spectral type with wavelength, a large infrared excess, and strong 2.3 µm CO absorption bands consistent with an accretion disk model for FU Orionis stars (Kenyon et al. 1991). They explained the optical photometric minimum of the year 1980 as a dust condensation event in the out-flowing wind. The slow rise time of 20 yr in the light curve of V 1515 Cyg up to the outburst could be matched by a self-regulated outburst (accretion disk) model developed by Bell et al. (1995). We derived a mass limit from our non-detection of 0.18 [FORMULA].

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Online publication: July 20, 1998
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