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SEARCH FOR B[E] STARS IN YOUNG OPEN CLUSTERS
D. DE WINTER
Dpto. F'isica Te'orica, C--XI, Universidad Aut'onoma de Madrid,
Cantoblanco, 28049 Madrid, Spain
M.E. VAN DEN ANCKER
Astron. Inst. ``Anton Pannekoek'', University of Amsterdam,
Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
AND
M.R. P '
EREZ
Space Applications Corp., Suite 250,
9315 Largo Drive West, Largo, MD 20774, USA
Abstract. The properties and the stellar content of five young open clusters
have been investigated to identify new B[e] stars. After a detailed analysis
we conclude that none of the ¸ 260 relatively young early type stars can
be identified as such. We propose that either the B[e] characteristic is not
present in young stars or that due to the close proximity of other hot stars
the circumstellar material can not hold sufficient amounts of material to
show this spectroscopic property. After investigating the direct environment
of well known galactic B[e] stars we find support for our hypothesis that
the B[e] phenomenon is mostly seen in rather isolated sources. As many
massive stars are located in clusters it explains the paucity of this spectral
characteristic.
1. Introduction
The stellar population of five well­known galactic young open clusters has
been studied: NGC 2244, 2264, 6383, 6530, 6611. The first goal was to find
Herbig Ae/Be (HAeBe) type stars among the early type members of these
young clusters. However, only relatively few were detected (¸ 21).
For the majority of the bright visible stars (more than 250 objects) their
observable properties (optical to near­IR photometry and low resolution

2 D. DE WINTER ET AL.
spectroscopy), as well as other astrophysical parameters were studied in
some detail. As the sample is obviously biased toward O­ and B­type stars,
it is also perfectly suited for the detection and identification of B[e] stars
among these very young objects of unknown type.
The results of this study will be shown and discussed by presenting the
characteristics of individual objects, as well as the general cluster proper­
ties. Finally, the results will be discussed in relation to known B[e] stars.
2. Description of the sample
An advantages of the use of young open clusters in these kind of studies is
that no confusion exists about the distance and age of the cluster­members.
This has been a major confusion for the classification of known B[e] stars,
see several papers in these proceedings. Most of cluster­members are also
bright enough for a wide range of observational work. Besides these advan­
tages, there is a difficulty to discriminate between real cluster­members and
field stars. For each of the clusters these problems are widely discussed and
sample selections are carefully made in the studies mentioned in Table 1.
In general the selection of B[e] stars is made on the obvious spectroscopic
properties, being spectral type B and the presence of emission lines of [Fe ii]
and [O i], and an indication of a near­infrared (NIR) or far­IR excess.
3. The observational results
We have obtained 4000--8000 š A low resolution spectroscopy (LRS) and
UBV RI \Gamma JHK(LM) photometry of probable cluster members. IRAS data
for cluster­members can not be derived, because of the low spatial resolution
of the instrument and the strong background radiation. Spectral corrections
for emission lines of bright nebulae in these clusters is not always easy as
the emission can be intracluster rather than intrinsic.
The final spectra were studied and the spectral energy distributions
(SEDs) were made and investigated. The observations and all results can
be found in the original studies, see Table 1. In this table we have also sum­
marized the properties for each cluster­sample and of each of the clusters
its properties as derived in these studies.
The results as presented in Table 1 show that in none of the clusters,
all very young, not even a single B[e] star was detected. Why?
4. The circumstellar environment of B[e] stars
We have seen that no B[e] stars were found in large samples of young early
type stars in several very young (Ü10 7 yrs) clusters. It is not an exceptional
property of one of these clusters. We propose, therefore, that B[e] stars are

SEARCH FOR B[E] STARS IN YOUNG OPEN CLUSTERS 3
TABLE 1. Properties of early type stars in several YCOs. The abbreviations in the
heading refer to the original studies: dW = de Winter et al. (1997); Hil = Hillenbrand
et al. (1993); MP = P'erez et al. (1987, 1991); PT2 = Th'e et al. (1985); PT2 = Th'e et
al. (1990); vdA = van den Ancker et al. (1997). For some cluster properties estimates
are given with the error between parentheses.
NGC: \Gamma \Gamma \Gamma\Gamma ? 2244 2264 6383 6530 6611 6611 Total
Characteristic MP MP PT1 vdA dW+PT2 Hil
# Objects studied 104 80 14 132 87 332 749
LR spectra 21 22 14 80 57 67 261
Strong emission 1 1 1 7 18 20 48
Weak/possible emission 0 0 0 12 9 5 26
NIR data 21 22 14 26 49 28 160
NIR excess 1 9 5 9 12 7 43
HAeBes candidates 2 6 1 5 4 3 21
O & B stars with LRS: 21 22 5 54 37 57 196
Emission 1 1 0 13 22 19 56
NIR data 21 22 5 30 19 27 124
NIR excess 1 9 1 9 8 7 35
NIR excess & emission 0 1 0 3 6 3 13
B[e] Stars 0 0 0 0 0 0 0
Cluster properties:
d [kpc] 1.67(1) 0.95(7) 1.5(2) 1.8(2) 2.6(3) 2.1(1) --
interstellar E(B \Gamma V ) 0.48(2) 0.06(1) 0.31(2) 0.30(2) 0.50(2) 0.4 --
tmax [Myr] ! 4 ! 5 ! 6 ! 4 ! 6 ! 6 --
tave [Myr] 1.7(4)* 2(1)** 1--2 2(1) --
t min [Myr] ! 0:3 0 0 0 --
Notes:
*) Taken from Luiken­Miller (1982). **) Taken from Sagar & Joshi (1978).
not extremely young (AE10 7 yrs), or that their CS material (disk?) survival
time is Ü10 7 yrs.
The latter is supported by the detection of Hester et al. (1996) that
dusty regions in NGC 6611 suffer from erosion due to the intense UV ra­
diation fields of nearby O & B stars. This might work also as disk erosion
of young stars in such clusters (P'erez et al. 1998). In fact their calculations
show that in these crowded regions the disk survival time is less than the
pre­main sequence time of a few Myrs for the studied stars, which explains
the lack of detected HAeBe stars, see Table 1.
Note that most well known HAeBes are found in relatively isolated

4 D. DE WINTER ET AL.
positions. What about the known B[e] stars? We have studied this and
found 41 early type (ET) stars (O, B and A) within a circle of 10 0 around 28
known galactic B[e] stars (de Winter & P'erez, these proceedings). However
19 of these nearby ET stars are around only 4 B[e] objects, these are:
CD\Gamma42 ffi 11721, HD 87643, He 2--14 and MWC 17. As 24 B[e] stars have
only 22 ET neighbours within 10 0 we conclude that the known galactic
B[e] stars are isolated objects, a suggestion already proposed by Zickgraf
& Schulte­Ladbeck (1989) for a few B[e] stars.
5. Conclusions
From the non­detection of B[e] stars among more than 250 well studied early
type stars in 5 young open clusters and the investigation of the surroundings
of 28 known galactic B[e] stars we summarize the main possibilities for the
nature of B[e] stars:
\Gamma B[e] stars are young and isolated. In this case the disk material is
young and the B[e] star will be clearly located in or near a SF region,
possible examples are R Mon and V380 Ori (de Winter & P'erez, these
proceedings).
\Gamma B[e] stars are evolved objects with ``young disks''. The ``disk''
can be relatively young when it originates from mass loss. The star
might be isolated or not. Examples of such objects are massive stars
such as LBVs and B[e] supergiants in the MCs, and stars with recent
mass­loss like post­AGB stars.
\Gamma B[e] stars are evolved objects with ``old disks''. The disk sur­
vived a significant time of the stellar evolution for some reason. The
B[e] star will be isolated and does not belong to the previously men­
tioned groups. Examples could be HD 45677 and HD 50138.
The final conclusion is that, as for HAeBes, the environments of B[e]
stars might be very important to understand the presence and the rarity
of the B[e] phenomenon, which is not detected in young open clusters.
Acknowledgements
D. de Winter is supported in part by Spanish grant DGICYT PB94­0165.
M. van den Ancker acknowledges financial support through an NWO Pio­
nier grant to L.B.F.M. Waters.
References
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M.R. 1997, A&AS 123, 63

SEARCH FOR B[E] STARS IN YOUNG OPEN CLUSTERS 5
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