Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.arcetri.astro.it/~lt/preprints/herbig/herbig.ps.gz Äàòà èçìåíåíèÿ: Tue Sep 11 16:56:43 2007 Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 07:28:46 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: massive stars

A&A manuscript no.
(will be inserted by hand later)
Your thesaurus codes are:
08(08.06.2; 08.16.5; 13.09.6)
ASTRONOMY
AND
ASTROPHYSICS
17.9.1996
A search for clustering around Herbig Ae/Be stars
Leonardo Testi 1 , Francesco Palla 2 , Timo Prusti 3 , Antonella Natta 2 and Silvia Maltagliati 1
1 Dipartimento di Astronomia e Scienza dello Spazio, Universit`a degli Studi di Firenze, Largo E. Fermi 5, I­50125 Firenze, Italy
2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I­50125 Firenze, Italy
3 Astrophysics Division, Space Science Department, ESTEC, Postbus 299, NL­2200 AG Noordwijk, The Netherlands
Received xxxx; accepted xxxx
Abstract. We present the results of new near­infrared
observations of the fields around a sample of 19 Her­
big Ae/Be stars. The observations reveal the popula­
tion of young stars that accompanies the formation of
intermediate­mass stars.
The richness of the detected star clusters is investi­
gated. We find a clear dependence of the richness of the
cluster on the spectral type of the Herbig Ae/Be star, con­
firming that the mode of formation of intermediate­mass
stars represents the transition between the high­mass and
the low­mass modes. In particular, we find that the clus­
ter nature of star formation appears at a significant (i.e.
detectable) level for stars of B7 spectral type or earlier.
Key words: stars: formation -- stars: pre­main sequence
-- infrared: stars
1. Introduction
It is a well established result that many stars do not form
in isolation; young stars are usually found to be members
of clusters or associations (see e.g. Lada et al. 1993; Zin­
necker et al. 1993). Similarly, even apparently isolated low
mass pre­main­sequence stars form small groups with den­
sities of 10­100 stars per cubic parsec (Gomez et al. 1993).
At the other extreme, high mass stars are associated with
rich clusters with densities up to 10 4 objects per cubic par­
sec, as in the Trapezium cluster (McCaughrean and Stauf­
fer 1994). Here and in the following we will call ``in loose
groups'' or ``in aggregates'' the low­mass star formation
mode, in analogy with the study of Gomez et al. (1993).
Typical scales of this mode of formation are: a mean pro­
jected separation between the young stars of 0:3 pc and
groups of ¸ 15 stars within 0:5 \Gamma 1 pc. On the opposite
side, the term ``cluster mode'' does not imply a gravita­
tionally bound system, but a group of stars from several
Send offprint requests to: L. Testi
tens to a few hundreds distributed over a scale !
¸ 0:5 pc,
as observed in high­mass star forming regions like Orion
(McCaughrean and Stauffer 1994).
Herbig Ae/Be stars should represent the transition be­
tween the high­mass star formation mode in clusters and
the low mass­mode in loose groups. In recent years, the
evidence that the clustering effect begins at intermedi­
ate mass level has been found in several studies at opti­
cal and infrared wavelength (Barsony et al. 1991; Hillen­
brand 1994; Palla et al. 1995). In spite of the fact that
the near infrared wavelengths are the ideal choice for the
study of the embedded population of young stars around
Herbig Ae/Be stars, the literature on the subject is still
scant (see however Hillenbrand 1995).
Li et al. (1994) performed near infrared imaging of 16
Herbig Ae/Be stars. Their primary goal was to investi­
gate the possibility that diffuse circumstellar emission and
nearby companions would have affected the large beam
photometry of Herbig stars. For this reason the study was
limited to a small area around the Herbig star itself (few
tens of arcsec), and to the brigthest and closest compan­
ions.
In this paper, we report the results of wide­field and
deep images in the broad­band near­infrared (NIR) filters
J,H, and K of large (4--7 arcmin) fields around 19 Herbig
Ae/Be stars. The NIR data (especially the K­band) are ex­
pected to reveal stars undetectable at optical wavelenghts,
enabling a detailed study of the embedded population of
young stellar objects. As we will show below, although the
present sample is still limited, the data clearly indicate the
existence of a correlation between the stellar density of the
fields and the spectral type of the Herbig star.
2. Observations
The nineteen Herbig stars of our sample have been se­
lected with two criteria: (i) they should not be members
of extended star forming complexes and (ii) they should
cover a wide range of spectral types. These two require­
ments were set in order to study regions not affected by

2 L. Testi et al.: A search for clustering around Herbig Ae/Be stars
Table 1. Properties of the Herbig stars considered.
# Star R.A. Dec. Sp. Dist
(1950) (1950) type (pc)
1 LkHff 198 00 : 08 : 47:5 +58 : 33 : 05 A5 600
2 BD+61 ffi 154 00 : 40 : 21:9 +61 : 38 : 15 B8 650
3 RNO 6 02 : 13 : 03:0 +55 : 09 : 03 B1 1600
4 XY Per 03 : 46 : 17:4 +38 : 49 : 50 B6 160
5 AB Aur 04 : 52 : 34:2 +30 : 28 : 22 A0 160
6 UX Ori 05 : 02 : 00:6 \Gamma03 : 51 : 20 A2 450
7 HK Ori 05 : 28 : 40:1 +12 : 07 : 00 A4 450
8 T Ori 05 : 33 : 23:1 \Gamma05 : 30 : 26 A2 450
9 BF Ori 05 : 34 : 47:2 \Gamma06 : 36 : 45 A7 450
10 HD 37490 05 : 36 : 33:0 +04 : 05 : 00 B3 360
11 HD 250550 05 : 59 : 06:4 +16 : 30 : 59 B7 700
12 MWC 137 06 : 15 : 53:5 +15 : 18 : 09 B0 1300
13 LkHff 215 06 : 29 : 56:2 +10 : 11 : 51 B7 800
14 HD 259431 06 : 30 : 19:4 +10 : 21 : 38 B5 800
15 R Mon 06 : 36 : 26:1 +08 : 46 : 55 B0 800
16 LkHff 25 06 : 37 : 59:5 +09 : 50 : 53 B7 800
17 HD 52721 06 : 59 : 28:6 \Gamma11 : 13 : 41 B1 1150
18 LkHff 218 07 : 00 : 21:9 \Gamma11 : 21 : 46 B9 1150
19 BD+40 ffi 4124 20 : 18 : 42:7 +41 : 12 : 18 B2 1000
Table 2. Parameters of the Observations.
Star Date Field Limiting Magnitude Acc. Abs. com. a
( 0 ) (pc) J H K MK
LkHff 198 15 Jan 1996 7 \Theta 7 1:2 \Theta 1:2 17:7 16:9 16:6 8% 6.7
BD+61 ffi 154 9 Jan 1993 4 \Theta 4 0:76 \Theta 0:76 16:4 16:0 15:7 20% 5.6
RNO 6 20 Jan 1996 7 \Theta 7 3:3 \Theta 3:3 17:6 16:8 16:3 5% 4.3
XY Per 20 Jan 1996 7 \Theta 7 0:33 \Theta 0:33 17:8 16:8 16:8 5% 9.8
AB Aur 8 Jan 1993 4 \Theta 4 0:19 \Theta 0:19 16:5 15:5 15:5 20% 8.5
UX Ori 15 Jan 1996 7 \Theta 7 0:92 \Theta 0:92 17:8 16:8 16:8 8% 7.5
HK Ori 8 Jan 1993 4 \Theta 4 0:52 \Theta 0:52 16:6 16:0 15:7 20% 6.4
T Ori 15 Jan 1996 7 \Theta 7 0:92 \Theta 0:92 17:4 16:2 16:0 8% 6.7
BF Ori 20 Jan 1996 7 \Theta 7 0:92 \Theta 0:92 17:8 16:7 16:6 5% 7.3
HD 37490 15 Jan 1996 7 \Theta 7 0:73 \Theta 0:73 17:7 16:6 16:6 8% 7.8
HD 250550 9 Jan 1993 4 \Theta 4 0:81 \Theta 0:81 16:5 15:7 15:7 20% 5.5
MWC 137 18 Jan 1996 7 \Theta 7 2:6 \Theta 2:6 17:4 16:5 16:5 7% 4.9
LkHff 215 11 Feb 1994 7 \Theta 7 1:6 \Theta 1:6 17:5 16:0 16:0 8% 5.5
HD 259431 12 Feb 1994 7 \Theta 7 1:6 \Theta 1:6 16:8 15:6 15:7 8% 5.2
R Mon 15 Jan 1996 7 \Theta 7 1:6 \Theta 1:6 17:9 16:8 16:6 8% 6.1
LkHff 25 11 Feb 1994 7 \Theta 7 1:6 \Theta 1:6 17:9 16:0 16:0 8% 5.5
HD 52721 20 Jan 1996 7 \Theta 7 2:3 \Theta 2:3 17:7 16:8 16:8 5% 5.5
LkHff 218 20 Jan 1996 7 \Theta 7 2:3 \Theta 2:3 17:7 16:4 16:5 5% 5.2
BD+40 ffi 4124 30 Sep 1993 7 \Theta 7 2:0 \Theta 2:0 17:3 16:1 16:0 10% 5.0
a ) Completeness absolute magnitudes in K have been calculated from the K limiting magnitudes assuming the distance quoted
in Table 1 (see text 2.1).

L. Testi et al.: A search for clustering around Herbig Ae/Be stars 3
large scale star formation processes and to probe the de­
pendence of the population of young stars on the spectral
type of the target star.
In Table 1 we report the relevant parameters of
the observed stars as obtained from the literature
(Berrilli et al. 1992; Finkenzeller & Mundt 1984; Herbig
& Bell 1988; Hillenbrand et al. 1992; Scarrot et al. 1986;
Th`e et al. 1994). The target stars span a range of spec­
tral types from A7 to B0, ensuring a good coverage of the
intermediate mass range.
The observations were carried out using the Arcetri
NIR camera ARNICA mounted at the 1:5 meter tele­
scope TIRGO 1 . ARNICA is equipped with a NIC­
MOS3 256\Theta256 pixels array, the scale on the detector is
0:96 arcsecond per pixel. For a complete description of
the instrument and of its performances at TIRGO see
Lisi et al. (1996), Hunt et al. (1996a). The sources were
observed during several observing runs in 1993, 1994, and
1996. The size of the observed field was in some cases
¸ 4\Theta4 square arcmin with constant signal to noise ratio
over the field, in other cases ¸ 7 \Theta 7 square arcmin were
imaged with poorer signal to noise ratio at the edges than
at the center. All the sources have been observed in the
three J, H and K broad bands. In most cases the Herbig
star itself is saturated in the images.
Table 2 summarizes the parameters of the observation
of each field: in the first column is the name of the Her­
big star, in the second the date of the observation, in the
third and the fourth the field imaged (in arcminutes and in
parsecs), in the fifth, sixth and seventh, the limiting mag­
nitudes achieved (3 oe in 4 arcsec aperture) in each band,
and in the last two the photometric calibration accuracy
and the completeness absolute magnitude in K (see be­
low). For the large fields the limiting magnitudes quoted
are those measured at the edges of the mosaic, while in
the central region they are about one magnitude fainter.
All the data reduction and analysis were performed
using the IRAF 2 and the ARNICA software packages
(Hunt et al. 1994). The raw images were median averaged
in order to obtain the flat field images. After flat field­
ing the images were registered and combined to form the
large mosaics. Aperture photometry was performed using
the DAOPHOT package. A four arcsecond aperture was
used in all fields.
The photometric calibration was achieved observing
during each night a set of UKIRT faint standards (Casali
et al. 1992) or of TIRGO standards (Hunt et al. 1996b).
The standard stars were observed at similar airmasses as
the sources of interest; hence, no correction for airmass
1 The TIRGO telescope is operated by the C.A.I.S.M.I.--
C.N.R., Firenze, Italy.
2 IRAF is the Image Reduction and Analysis Facility made
available to the astronomical community by the National
Optical Astronomy Observatories, which are operated by
AURA Inc., under contract with the U.S. National Science
Foundation.
was applied. As explained in Hunt et al. (1996a), the pho­
tometry of point sources in the first runs of the camera
were position­dependent on the array; for this reason the
photometric accuracy of the data taken during the Jan­
uary 93 run is of the order of ¸ 20%.
2.1. Source detection and completeness
In detecting all the point sources in the images, the
DAOFIND algorithm has proven unreliable for the faintest
stars. For this reason the lists of sources in each field were
compiled ``by hand'' inspecting the images at different
contrast levels. Accurate centering of each source was ob­
tained with a gaussian fitting algorithm, before performing
the photometry.
To be conservative we assumed our data to be complete
down to one magnitude brighter than the limiting magni­
tude of each field (quoted in Table 2). For the large field
mosaics the ``edge'' limiting magnitudes have been consid­
ered. In the last column of Table 2 the absolute K­band
completeness magnitudes are reported, these have been
calculated assuming the distances reported in Table 1.
3. Results
Even a quick look at the images shows that the fields
present large variations. As an example, Figure 1 shows
that the fields around MWC 137 and HD 52721 are quite
crowded, while those around UX Ori and BF Ori contain
just a few stars. Whether this reflects the fact that some
of the Herbig stars do have a large number of companions
that formed together in a cluster/group whereas others
formed almost alone, is the main subject of this paper
and will be investigated in the following.
Due to the reduced amount of extinction, the K­band
images are the the most suitable to reveal the embedded
objects around the Herbig stars. For this reason the ef­
fects of variable extinction toward different fields, which
may introduce significant errors at optical wavelengths,
are expexted to affect only marginally the results of our
study.
The simple approach of counting the stars in each field
cannot be used, since the target stars are located at dif­
ferent distances from the Sun and the sensitivity, as well
as the total field of view of the observations, change from
star to star. Moreover, the fields are located towards differ­
ent regions of the galaxy, hence a different contamination
from background objects is expected. In order to compen­
sate for these effects, we will extract from the data a set
of indicators of the clustering of stars around each Herbig
star, and we will investigate the properties of such indica­
tors.
3.1. Distance and sensitivity correction
Since our program stars are at different distances and the
observations reach different sensitivity limits, the absolute

4 L. Testi et al.: A search for clustering around Herbig Ae/Be stars
45 s
50 s
55 s
6 16 00
h m s
+15 16'
o
17'
18'
19'
20'
22 s
24 s
26 s
28 s
30 s
32 s
34 s
6 59 36
h m s
­11 15'
o
14'
13'
12'
54 s
56 s
58 s
5 02 00
h m s
02 s
04 s
06 s
08 s
­3 53'00"
o
30"
­3 52'00"
o
30"
­3 51'00"
o
30"
­3 50'00"
o
30"
40 s
42 s
44 s
46 s
48 s
50 s
52 s
54 s
5 34 56
h m s
30"
­6 38'00"
o
30"
­6 37'00"
o
30"
­6 36'00"
o
30"
­6 35'00"
o
Fig. 1. K­band images of four Herbig stars: MWC 137 (upper left), HD 52721 (upper right), UX Ori (lower left) and BF Ori
(lower right). On the axes are reported right ascension and declination for the 1950:0 epoch. Rich clusters are evident in
MWC 137 and HD 52721, while the other two stars appear isolated.
K­band completeness magnitudes (assuming null extinc­
tion) are different. Thus, we have calculated one indica­
tor (NK ) corrected for this effect: NK is the number of
K­band sources detected with MK ! 5:2 within 0:21 pc
from the Herbig star. The physical radius of 0:21 pc was
chosen because it is large enough to contain the clusters
observed in BD+40 ffi 4124 and MWC 137, and at the same
time is sufficiently small that only two of the nearest stars
(AB Aur and XY Per) have been observed with a smaller
field of view. Such a choice of the radius is also based on
the results of Hillenbrand (1995) who fixed the effective
size of the cluster at 0.17 pc. Due to the larger extent of
our fields, we can be more conservative and take a larger
value of the cluster size. The threshold at MK = 5:2 in­
cludes most of the fields (cf. Table 2, last column), but is
high enough to be able to count some stars in most fields.
With this choice five fields have only lower lim­
its of NK . These fields are: MWC 137, RNO 6 and
BD+40 ffi 4124, whose absolute completeness magnitude is
lower than 5:2, and AB Aur and XY Per, which have been
imaged with a field smaller than 0:21 pc. The values of NK
for each star are reported in Table 3 (column 3).
3.2. Infrared excess sources
Since we are trying to find a way to separate the
young stars around the Herbig stars from the fore­

L. Testi et al.: A search for clustering around Herbig Ae/Be stars 5
ground/background objects, the colour­colour diagram
(J--H,H--K) could be a very useful tool. In fact main­
sequence (MS) stars and reddened MS stars tend to lie
in a different region of the (J--H,H--K) plane than young
stars (Lada & Adams 1992). The latter usually show a
marked NIR excess and are located to the right of the MS
and reddened MS loci.
A problem in using this method to discriminate be­
tween young stars (members of the cluster) and field stars,
is that some kind of young stars (the Weak Line T­Tauri
stars or in general the infrared Class III sources) do not
show infrared excess at all. For example, about 50% of
the known PMS stars in Taurus­Auriga fall within or very
close the reddening band (Kenyon & Hartmann 1995).
Also Class I sources have been found, in some cases,
to fall inside the ``reddening belt'' (see e.g. Greene &
Meyer 1995), and thus a fraction of them may not be eas­
ily detected in the colour--colour diagram. This means that
using this method we may strongly underestimate the ac­
tual number of source members of the cluster around the
Herbig star.
Notwithstanding these limitations, we define a rich­
ness indicator, NEX , based on the colour properties of the
sources: this quantity represents the number of NIR ex­
cess sources in each field with the same constraints as
NK , plus the requirement that each source should have
been detected in all the three bands. A star is con­
sidered to have NIR excess if it satisfies the condition
(J \Gamma H) ! 1:75(H \Gamma K) \Gamma 0:35. This relation takes into
account 10% error in the color determination. To give an
estimate of the fraction of the sources detected in all the
three bands that show infrared excess we have calculated
FEX as the ratio of NEX to the number of sources de­
tected in all the three bands (with the same constraints
as for NK ). These two quantities are listed in column 4
and 5 of Table 3.
3.3. K­band sources density profiles
The indexes NK , NEX and FEX are still not corrected
for the contamination from background/foreground stars,
which may affect fields at different galactic latitude and
longitude in different ways. We have tried to account for
this effect using the predictions of analytic star counts
models. In particular, we have used the model of Ortiz &
L'epine (1993), but with little success. The relatively small
fields surveyed and the ``local'' small­scale extinction en­
hancement (not considered in the models) lead to great
uncertainties in the predicted background/foreground star
counts and we have decided that the results of such anal­
ysis were not meaningful.
A more straightforward way of estimating the amount
of contamination is by plotting the stellar surface density
in the K­band for each field as a function of the distance
from the Herbig star. In practice, the local source density
at radius r i has been measured from the position of the
Herbig star (see Table 1) in annulii of 12 00 . As a result,
it is found that there are some fields in which the Her­
big star is located near the center of a marked local star
density enhancement, while in other cases the density is
almost constant across the whole field. In Figure 2 two ex­
treme cases are shown: the richest cluster associated with
MWC 137 and an empty field around UX Ori.
More formally, we define another richness indicator of
the cluster around each star as the number of sources in
the density enhancement, I C = 2ú
R 1
0 r(n(r) \Gamma n1 )dr,
where n(r) is the density of stars at radius r and
n1 = lim r!1 n(r). Practically, we will define n1 as
the mean density in the outer parts of the plot, and
I C = ú
P imax
i=1 (r 2
i \Gamma r 2
i\Gamma1 )(n(r i ) \Gamma n1 ) (r 0 = 0, and r imax
is chosen in such a way to contain all the cluster members
in the field). Table 3 lists the values of I C in each field
(column 6).
In principle, I C is a very good indicator, since the con­
tribution from field stars is evaluated close to the cluster
area and subtracted before the integration. On the other
hand, the correct determination of n1 is critical for the
quantitative result, as shown by the error on I C quoted in
Table 3 and reported in Figure 4, which has been deter­
mined by propagating the uncertainty in the determina­
tion of n1 on the sum defined above.
Two possible sources of systematic errors in the com­
parison of the values of I C in different fields are: (i) the
fact that all the sources detected within the complete­
ness of each observations have been considered and (ii)
the extinction toward each of the fields may differ by sev­
eral magnitudes. Unfortunately the low number of stars
around the sources does not allow a meaningful determi­
nation of the source densities maintaining the constraint
of the same absolute limiting magnitude in all the fields.
Nevertheless, we do not find a correlation between the ab­
solute completeness magnitudes and the presence of a star
cluster. For instance, HD 37490 and HD 52721 have abso­
lute completeness magnitudes that differ by more than
two magnitudes, but the cluster appearence and the value
of I C are similar.
The effect of extinction is more difficult to estimate,
because of the uncertainties in the determination of this
quantity from the NIR data alone. On the other hand,
since n1 is calculated around the cluster, only the ``local''
extinction variations on the same scale of the clusters may
affect the calculation of I C . Hillenbrand et al. (1995) used
the measured column densities of molecular gas to get an
estimate of the extinction effect toward different fields. She
found variations in extinction as large as ten magnitudes
in the visual, but she also noted that the amount of ex­
tinction is not correlated with the spectral type. Thus, we
can conclude that while this effect may affect the direct
comparison of some fields, it should not alter the general
trend in I C that we find and that will be discussed below.
Finally, we like to stress that in all the cases in which a
well defined cluster has been found, the bulk of the cluster

6 L. Testi et al.: A search for clustering around Herbig Ae/Be stars
Fig. 2. Two examples of sources density profiles, on the left a rich cluster field, on the right an empty field. The error bars
represent the statistical uncertainties.
is within 0:21 pc from the Herbig star. This result con­
firms the goodness of our choice of the physical size in the
calculation of NK . In two cases, LkHff 198 and R Mon,
the value of I C is highly negative, indicating that n1 is
much greater than the density around the Herbig star.
There are two possible explanations for this result: one
is that when the Herbig star is very bright and with an
extended nebulosity, as in the case of LkHff 198, nearby
faint stars may be hidden, and be completely missed in
our counts. Another possibility is the presence of a com­
pact molecular clump around the star that enhances the
value of the extinction along the line of sight close to the
star and that obscures background objects. Clearly, fur­
ther observations of these two regions are needed in order
to understand what is the actual situation in these special
cases.
4. Correlation between the richness indicators and
the spectral type
Having defined suitable richness indicators, we can now
study their dependence on the spectral type of the Herbig
stars. We have decided to use the spectral type and not the
mass of the star (which would be the physical quantity of
relevance) for several reasons. The most important is that
the determination of the mass is always indirect and relies
on the use of HR diagrams. Although the knowledge of
the PMS evolution has improved lately (Palla & Stahler
1993), the uncertainty with which both the distance and
the luminosity of the Herbig stars are known is still too
large to allow any reliable mass estimate using classical
methods of stellar evolution. Unlike the mass, the spec­
tral types of Herbig stars are known sufficiently well: the
typical error found in the literature is of only one or two
subclasses.
Table 3 reports the values of the richness indicators in
each of the observed fields. The fields have been sorted by
the spectral type of the Herbig star. In Figures 3 and 4 the
richness indicators have been plotted against the spectral
type of the central star. We do not show the error bar in
the spectral type since it is the same for all the stars of
the sample.
A first result from Figure 3 is that NK , NEX and
FEX show a rough correlation with the spectral type,
with earlier types having higher values of the indicators.
MWC 137, which is the Herbig star with the earlier spec­
tral type (B0) in our sample shows the highest values of
the richness indicators. The only early type B star for
which all the indicators give negative results is R Mon.
From Table 3, low­mass Herbig stars have NK !
¸ 4,
which corresponds to a density of about 100 stars per cu­
bic parsec. This density is similar to that found by Gomez
et al. (1993) around known TTauri stars in Taurus ( !
¸ 60
stars per cubic parsec). B­type Herbig stars have mean
density values of about 250 \Xi 500 stars per cubic parsec,
but in the case of MWC 137 the density is 1:5 \Theta 10 3 stars
per cubic parsec, a value typical of massive star forming
regions.
Our results are very similar to those obtained by Hil­
lenbrand (1995) for seventeen fields around Herbig Ae/Be
stars. In particular, she finds that the local density of K­
band sources spans from a few tens per square parsecs
for the fields around low­mass stars to several hundreds
for those around high­mass stars. There are eight Herbig

L. Testi et al.: A search for clustering around Herbig Ae/Be stars 7
Fig. 3. Richness indicators versus spectral type of the Herbig star. On the left NK : sources detected in K band with MK – 5:2
and within 0:21 pc from the Herbig star; center NEX : sources detected in all the three bands, with infrared excess, with MK – 5:2
and within 0:21 pc from the Herbig star; on the right FEX : ratio between NEX and the sources detected in all the three bands
with MK – 5:2 and within 0:21 pc from the Herbig star. The fields for which NEX = 0 have been represented with dashes in
the central panel and have not been reported in the right panel. Lower limits are indicated with an arrow.
Table 3. Values of the richness indicators in each field.
Star type NK Nex FEX IC
MWC 137 B0 59 a 26 a 0:55 a 76:0 \Sigma 9
R Mon B0 0 -- -- \Gamma12:8 \Sigma 3
HD 52721 B1 10 -- -- 20:5 \Sigma 4
RNO 6 B1 11 a 3 a 0:5 a 11:0 \Sigma 1
BD+40 ffi 4124 B2 19 a 12 a 0:75 a 11:0 \Sigma 3
HD 37490 B3 9 6 0.75 9:9 \Sigma 3
HD 259431 B5 2 -- -- 0:9 \Sigma 2
XY Per B6 3 a -- a -- a 11:3 \Sigma 3
LkHff 25 B7 11 2 0.18 14:5 \Sigma 5
LkHff 215 B7 7 6 1.0 3:9 \Sigma 1
HD 250550 B7 4 -- -- 2:2 \Sigma 2
BD+61 ffi 154 B8 8 1 0.13 \Gamma1:4 \Sigma 3
LkHff 218 B9 8 1 0.14 2:0 \Sigma 5
AB Aur A0 0 a -- a -- a 1:2 \Sigma 1
UX Ori A2 0 -- -- \Gamma0:3 \Sigma 1
T Ori A2 5 -- -- 1:0 \Sigma 1
HK Ori A4 7 -- -- 2:2 \Sigma 1
LkHff 198 A5 6 -- -- \Gamma10:6 \Sigma 11
BF Ori A7 4 -- -- 1:1 \Sigma 1
a ) The values of the indicators for these sources are only lower
limits.
stars that are common to the two surveys. Considering the
differences in the completeness limit and region surveyed
of the two studies, the agreement in the numerical values
of NK for the fields in common is quite satisfactory.
The best evidence for the existence of a trend in the
clustering properties with the spectral type of the Her­
big star is shown in Figure 4 where we plot Ic vs. spec­
tral type. Early type stars have values of I C above ten,
while late type stars are characterized by values of or­
der unity. MWC 137 has the highest value of I C . In terms
of I C , the clustering around the Herbig Be stars is re­
vealed at a level of 5­10 ``effective stars'' above the back­
ground (note that I C takes into account the different galac­
tic background/foreground contamination suffered by the
different fields). Considering the results of Hillenbrand
(1995) and of Barsony et al. (1991), the sample of Her­
big stars with rich clusters includes MWC 137, MWC 297,
MWC 1080, BD+40 ffi 4124, and LkHff 101.
From our results, the variation of I C with spectral type
is not smooth. As shown in fig. 2, there is an indication
of the possible presence of a threshold (or sudden break)
effect in I C around a spectral type B5­B7. It is too early
to say whether this effect is real or an artifact of our small
sample of stars. Hillenbrand (1995) finds a linear relation­
ship of the star density with the stellar mass. However,
we have already warned about the large uncertainty in
the mass assignment of the Herbig stars and the derived
relation may suffer from this drawback. Judging from the
behaviour of the indicator NK , a quantity more similar to
that used by Hillenbrand than I C , there is no evidence
in our data for such a linear correlation (cf. Figure 3). On
the other hand, the existence of a minimum mass for the
presence of clustering effects (also related to environmen­
tal conditions) would provide a strong constraint on star
formation theories.
5. Conclusions
Although our data are limited to a subsample of the in­
termediate mass pre­main sequence Herbig Ae/Be stars,
a few conclusions can be drawn from this study.
Our NIR images reveal the population of low­mass
stars around Herbig Ae/Be stars, undetectable at shorter
wavelengths (Goodrich 1993; Aspin et al. 1994; Hillen­
brand et al. 1995). We have investigated the clustering of
young stars around the target stars by means of several
richness indicators. The main results of this analysis can

8 L. Testi et al.: A search for clustering around Herbig Ae/Be stars
Fig. 4. The richness indicator IC versus the spectral type of
the Herbig star.
be summarized in two points: 1) there is a clear depen­
dence between the spectral type of the Herbig star and
the richness of the embedded cluster around it: the earlier
the spectral type, the richer the cluster; 2) the presence of
clusters appears at a detectable level only for stars earlier
than B5­B7. While the former result is supported by the
study of Hillenbrand (1995) on a similar set of stars, the
existence of a threshold spectral type (or, more physically,
of a minimum mass) has to be verified on a larger sample,
using a homogeneous richness indicator, such as I C .
These initial results confirm the idea that the class
of Herbig Be stars represents the transition between the
isolated star formation mode, typical of low­mass stars,
which includes Ae­type stars, and the rich cluster mode
typical of high­mass O stars. These statements can be,
and will be, made more precise as soon as observations
of other stars of this class will become available. Ideally,
Herbig Be stars are the most interesting candidates to
study the appearance of associated clusters.
Acknowledgements. We thank the TIRGO and ARNICA staff,
especially Filippo Mannucci and Ruggero Stanga, for nice
scheduling and for service observing. LT acknowledges many
interesting discussions with Thierry Montmerle and Paolo
Saraceno, and would like to thank Jaques L'epine for useful sug­
gestions and for providing the fortran code of the star counts
model. We also thank the referee, Hans Zinnecker, whose sug­
gestions greatly improved this work. This search has made use
of the Simbad database, operated at CDS, Strasbourg, France.
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