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5. Section Online only : Analysis of the SED fits

We now discuss the results of the SED fits for the programme stars for which we altered the originally derived astrophysical parameters or which show peculiarities.

Table 7 (click here) lists all astrophysical parameters of the programme stars. Parameters obtained from the SED fits were used and superseded previous determinations.

5.1. Remarks on individual fits

For most of the SED fits special remarks are given in Sect. 5.1. of Koulis (1993). The SEDs for which we could not obtain satisfactory fits, encountered some problems, or for which several fits were possible, are discussed now:
W213: we consider two options, denoted by W213(1) and W213(2), below. In both cases the photometry used is identical except for the UBV set. The subscript 2 in tex2html_wrap_inline5644 indicates the reference of the data-set used in the last column of Tables 2 and 3, and in Sect. 2.1.
W213(1): we take the tex2html_wrap_inline5646 data and the spectral type A7. Because of the IR-excess, we have corrected with tex2html_wrap_inline5648 = 3.4. Except for the Balmer discontinuity, the obtained fit is reasonable (except that the J datapoint lies below the model).
W213(2): we use the averaged tex2html_wrap_inline5652 data and fit a spectral type F9. The obtained fit becomes acceptable after we have applied an extinction law with tex2html_wrap_inline5654 = 3.4. However, the tex2html_wrap_inline5656 and J datapoints still lie above and below the Kurucz model respectively. We keep both options for further consideration.
W232: we use spectral type F8. The fit shows an UBV as well as a near-IR excess, while the observed tex2html_wrap_inline5662 datapoint lies below the fit. We eliminate the IR-excess by correction with tex2html_wrap_inline5664 = 4.0. However, the fit remains unsatisfactory for the UBV datapoints.
W273: this proves to be an interesting case. We have started using a spectral type B9, but have to modify this to A0, with E(B-V) = 0tex2html_wrap5780 64. The obtained fit is not good, because most of the photometric blue and visual datapoints are just covered by the fit, whereas the tex2html_wrap_inline5672 datapoint is too high. Furthermore, the star shows an IR-excess for which we have corrected by considering two possibilities:
(a) Fitting an anomalous extinction law with tex2html_wrap_inline5674 = 4.8, we found that the blue, visual and red part of the fit are improved. However, the IR-excess remains, and the J and K datapoints are asymmetrically above and below the fit.
(b) W273sh: we have fitted the IR-excess by a Planck curve (see Sect. 5.3) with tex2html_wrap_inline5680200 K. The combination of such a near-IR explanation and a fit with tex2html_wrap_inline5682 = 3.1 gives reasonably good results at all pass-bands. The addition sh in W273sh indicates that the IR-excess is assumed to be due to a dust shell.
W276: like W213 here we also consider two options, in which the JHK sets are different:
W276(1): with the tex2html_wrap_inline5686 set the fit becomes satisfactory if an anomalous extinction with tex2html_wrap_inline5688 = 3.4 is applied, although the U datapoint does not fit in the Balmer discontinuity and the K datapoint is located slightly below the fit.
W276(2): we use the same procedure for tex2html_wrap_inline5694, but with tex2html_wrap_inline5696 = 4.0 instead. Although the K datapoint fits well, the J stays below the fit. Furthermore, the blue part of the spectrum appears to fit less well compared to the SED fit of W276(1). Option W276(1) provides a relatively better fit, which we adopt.
W299: here again we have two options:
W299(1): we use the averaged tex2html_wrap_inline5702 data.
W299(2): we adopt the tex2html_wrap_inline5704 data.
For both options we have used a spectral type B6 and the same sets of Walraven and near-IR data. If the Walraven data set proved not to be in mutual agreement, we discarded them. Both fits show approximately the same characteristics. Because of the presence of an IR-excess we have fitted with tex2html_wrap_inline5706 = 3.8. The J and K datapoints fall asymmetrically below and above the fit. This motivates us to consider fitting a Planck curve, see Sect. 5.3.
W336: we have W336(1), where we have fitted the tex2html_wrap_inline5712 data, and W336(2) the tex2html_wrap_inline5714 data. W336(1) shows a JHK depletion and W336(2) an IR-excess. We chose to consider W336(2) and rejected W336(1). We then derived tex2html_wrap_inline5718 = 3.5 for a B3 spectral type with E(B-V) = 0tex2html_wrap5782 69. This is a satisfactory fit.
W339: we considered 3 different options:
W339(1): based on tex2html_wrap_inline5724 data. The spectral type is adopted to be B3 and E(B-V) = 1tex2html_wrap5784 08 is taken. This SED fit shows no IR-excess.
W339(2): tex2html_wrap_inline5730 has been used and we obtained an extinction law with tex2html_wrap_inline5732 = 3.9, the fit was very satisfactory.
W339(2)sh: we have fitted the IR-excess with a Planck curve, see Sect. 5.3.
We notice that the SED fit of W339(2) is the best one, that of W339(2)sh being of somewhat poorer quality. We therefore discarded option W339(1). Note that the CCD spectrum of W339 seems to have characteristics of a symbiotic system.
W374: with spectral type F2 we have obtained a satisfactory SED fit after correcting the presence of a large IR-excess with tex2html_wrap_inline5734 = 4.4. Since the IR-excess is strong, we decided to make a second fit with a CS dust shell (see Sect. 5.3).
W396: two possible spectral types were considered, because it lies low in the TCDs.
W396(a): spectral type ranging from A8 to F5 (photometry) and F9 from the CCD spectrum. Several weak emission lines are visible.
W396(b): spectral type ranging from G2 to G6 (photometry), in which G2 is deduced from an IDS spectrum.
The SED fits for both W396(a) (spectral type F9, E(B-V) = 0tex2html_wrap5786 64) and W396(b) (spectral type G2, E(B-V) = 0tex2html_wrap5788 56) are both very satisfactory. However, we have corrected the SED with tex2html_wrap_inline5744 = 3.5 for W396(a) and with tex2html_wrap_inline5746 = 3.7 for W396(b). Probably W396(a) and W396(b) are two different neighbouring stars, lying almost along the same line of sight. This would explain the two different spectra as well as the relative discrepancy in their tex2html_wrap_inline5748 value; they are not resolved in the photometric observations.
W440(1): Originally we have also considered W440(2). From tex2html_wrap_inline5750 and a very poor CCD spectrum (not shown here) the spectral classifications (type B9) seems to be of another star, as the SED fit is also poor. This option is therefore discarded.
W455: the SED fit shows a near-IR depletion. Changing the spectral type of the star from B8 to B7 and fitting for different luminosity classes gave no satisfactory result. Finally we have decided to keep the original fit.
W489: we have two somewhat different sets of JHK data. For both an SED fit is made with the same Walraven and tex2html_wrap_inline5754 data. Although both fits are of poor quality in the near-IR (the J and K data lying asymmetrically below and above the fit) we have obtained the same results: spectral type B7 (note that initially we have taken B9) and an anomalous extinction law with tex2html_wrap_inline5760 = 3.3 are found.
W536: initially four different SED fits, using 2 different sets of Walraven data and two different sets of JHK data, are made. We have discarded the two SED fits with the tex2html_wrap_inline5764 data, because the tex2html_wrap_inline5766 data yield better fits. In addition to this, we have averaged the 2 Walraven data sets as they are in agreement with each other. The spectral type (B1) remains the same, but we have to correct for an anomalous extinction with tex2html_wrap_inline5768 = 3.5.
W611: we have made numerous SED fits by varying the temperature and luminosity class, and staying within the spectral range determined from our photometric and spectroscopic results. The tex2html_wrap_inline5770 data fitted well and the extinction appeared to be normal. Because of a remaining depletion at the JHK data, we finally have classified W611 as G8 V. Initially we used K0 V.
W617: we have two very different UBV data sets. The tex2html_wrap_inline5776 data do not match with the other photometric data. Furthermore, we have to discard tex2html_wrap_inline5778 as these data lie far below the fit. Although the blue data still lie somewhat lower than the fit, the resulting fit with spectral type K5 V is satisfactory. No indication for an anomalous extinction law is found.

5.2. Adapting the luminosity class; the near-IR depletion

After we have selected self-consistent data and removed erroneous data points at the SED-fits, several objects still show a remarkable depletion in the near-IR: W103, W349, W402, W406, W411, W504, W525 and W534.

The IR-depletion was explained in Thé et al. (1990) by tex2html_wrap_inline5790 values lower than 3.1. However, the depletion remained after trying to vary the extinction law and the Kurucz model. If the photometry and spectroscopy are reliable and the stellar astrophysical parameters derived from them are correct, the only adjustable parameter left (besides the presence of strong absorption bands in the near-IR) is the luminosity class. Indeed, by fitting at the red, optical and blue data we are able to change the log g in such a way that reasonable fits in the near-IR for several late type objects can be made.

We have obtained satisfactory fits for W349: G8 I, W406: K0 III, W525: K5 III and W534: K2 III.

For the other two objects, W402 and W411, the depletion at the JHK(LM) pass-bands can not easily be compensated using this procedure. The best fits obtained are spectral type G9 III for W402 and K5 III for W411.

As we have mentioned before, a PMS characteristic is brightness variability. Normally this will not alter the stellar spectral type. However, for several PMS objects the colours can change rapidly (see Bibo & Thé 1991). This happens especially in the optical and the blue pass-bands. We must, therefore, keep in mind that the inability to obtain a satisfactory SED fit in cases where there is a near-IR depletion, is probably because the photometric data were taken at different brightness stages. Furthermore, strong molecular bands can influence the near-IR photometry significantly. This could be the origin of the near-IR depletion in the SEDs of W103: B9 and W504: B5.

5.3. Circumstellar dust shells

In Sect. 5.1. we encountered some objects for which a fit of a CS dust shell to their IR-properties was proposed. Additionally, we have some remaining cases for which the IR-excess could not be fitted by anomalous extinction: W245, W262, W266, W494 and W605. Sometimes it is possible to obtain a reasonable fit by using an anomalous extinction law with extreme tex2html_wrap_inline5796 values, far above 4.0, but then the obtained fit shows a poor agreement in the blue.

A strong IR-excess, other than one due to anomalous extinction or free-free emission, is a well known PMS characteristic. Such excesses are normally explained as due to thermal re-radiation of circumstellar dust grains. Also an M-type ``companion'' (in a star forming region could be a T Tauri- or a proto-star) to explain the IR-excess, show approximately, a Planck spectrum. This latter explanation is presented as the explanation for the variable IR-excess of NGC 6611-W409 (Thé et al. 1985). To study these two options we calculate the amount of near-IR excess in each passband and try to fit the flux differences to a black body. We thus obtain a temperature of the black body by using Wien's law (tex2html_wrap_inline5798 cm K, where tex2html_wrap_inline5800 is the wavelength corresponding to the maximum flux of the used Planck curve). This temperature is used to approximate the temperature of the radiating dust.

The stars showing one of these two properties are: W245sh, W273sh, W299sh, W339(2)sh, W374sh, W494sh and W605sh, where the abbreviation sh stands for shell (we will not discuss here the correct morphology of the circumstellar material distribution: disk or spherical shell).
W245sh: its SED fits a spectral type B6 well, with a strong IR excess. An IR-excess for this star was also found by Hillenbrand et al. (1993), but a photometric spectral type of B9.5 was used. We have tried to fit tex2html_wrap_inline5802 3.4, but it was soon clear that the SED at the UBV passbands became tilted off the Kurucz model significantly, while the JHKL points could not be fitted appropriately. However, we obtained a satisfactory fit for a Planck curve of 1880tex2html_wrap_inline5808150 K. We assume therefore that this radiation is caused by a CS dust shell.
W262sh: the IR-excess has been fitted with a Planck curve of 2550tex2html_wrap_inline5810200 K. The SED fit is very satisfactory. However the temperature is much too high for CS dust. We therefore suggest the presence of an M-type companion.
W266sh: has an SED with a black body of 1970tex2html_wrap_inline5812150 K. This temperature is somewhat high for a CS shell. However, considering the errors in the temperature estimate, the presence of a CS dust shell is possible.
W273sh: has been considered in sect. 4.1. We obtained a temperature of 2440tex2html_wrap_inline5814200 K for the Planck curve, which indicates the presence of an M-type companion. Judging the two SED fits, the Planck curve fit is relatively better.
W299sh: an SED fit with tex2html_wrap_inline5816 = 3.8 gives a reasonable result. However, we also tried a 2070tex2html_wrap_inline5818200 K black body. The best fit is the one for the anomalous extinction law.
W339: the different possibilities of the SEDs are already discussed. For W339sh we have also tried a Planck curve of 2550tex2html_wrap_inline5820200 K to fit the near-IR-excess. However, this fit is of poorer quality compared to the one of W339(2), especially in the near-IR. We are thus compelled to assume that this object suffers from anomalous extinction with tex2html_wrap_inline5822 = 3.9. This option must compete with the possibility that the IR-excess is due to an M-type companion, as is noticed from the spectroscopic observations, Fig. 1 (click here).
W374sh: is another star for which we considered different options. We compared the SED fit for tex2html_wrap_inline5824 = 4.4 with the one for a Planck curve of 2550tex2html_wrap_inline5826200 K. The quality of the SED fit of the former is the best.
W494sh: a Planck curve of 1760tex2html_wrap_inline5828100 K fits very well to the IR-excess in the SED. The radiating body can thus be a CS dust shell. For this object an IR-excess was also found by Hillenbrand et al. (1993).
W605sh: a CS dust shell with 1360tex2html_wrap_inline583080 K is found to be the best fit for the near-IR-excess.
In conclusion, we find that W245sh, W266sh, W494sh and W605sh clearly present dust properties as known in HAeBe candidates. On the other hand W262sh, W273sh and W339(2) appear to have an M-type companion, whereas W299sh and W374sh show a strong near-IR-excess due to highly anomalous extinction (tex2html_wrap_inline5832 = 3.8-4.4).

In the cases above, free-free emission might also be considered as young objects have a significant amount of gaseous circumstellar material.

Note that for those objects showing a strong near-IR-excess we have adopted a normal extinction law (tex2html_wrap_inline5834 = 3.1). Although we suspect that the cluster is mostly influenced by anomalous extinction, we are not able to make appropriate conclusions about the extinction of the previously discussed shell-type stars. If their extinctions are indeed anomalous, they could change the amount of near-IR-excess, thereby lowering the estimated temperatures. Consequently, the temperatures derived above should be considered as upper limits. However, the overall conclusions will not change significantly.

  figure798 figure804 figure808 figure812 figure816

figure820 figure824 figure828 figure832 figure836

figure840 figure844 figure848 figure852 figure856

figure860 figure864 figure868 figure872 figure876

figure880 figure884 figure888 figure892 figure896

Figure 1: Low resolution spectra of the PMS programme stars of NGC 6611

Figure 2: The U-B, B-V diagram for our stars in NGC 6611. The intrinsic luminosity class V relation (solid line) and the one for class III (dashed line) of Schmidt-Kaler (1982) are drawn

figure906 figure910 figure914 figure918 figure922

figure926 figure930 figure934   figure938 figure944

figure948 figure952 figure956 figure960 figure964

figure968 figure972 figure976 figure980 figure984

figure988 figure992 figure996 figure1000 figure1004

figure1008 figure1012 figure1016

Figure 3: SEDs of our sample stars. In some cases the different options are also given, see text and Table 7. Note that a correction with a high tex2html_wrap_inline5840 value causes a small UV-depletion when the IR-excess is probably not due to anomalous extinction

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