Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.arcetri.astro.it/~lt/preprints/tegami/tegami.ps.gz Äàòà èçìåíåíèÿ: Tue Sep 11 16:56:55 2007 Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 07:29:13 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: massive stars

Astronomy & Astrophysics manuscript no.
(will be inserted by hand later)
G24.78+0.08: a cluster of high­mass (proto)stars
R.S. Furuya 1 , R. Cesaroni 1 , C. Codella 2 , L. Testi 1 , R. Bachiller 3 , and M. Tafalla 3
1 Osservatorio Astrofisico di Arcetri, INAF, Largo E. Fermi 5, I­50125 Firenze, Italy
2 Istituto di Radioastronomia, CNR, Sezione di Firenze, Largo E. Fermi 5, I­50125 Firenze, Italy
3 Observatorio Astron'omico Nacional (IGN), Apartado 1143, 28800 Alcal'a de Henares (Madrid), Spain
Received date; accepted date
Abstract. We present the results of high angular resolution observations at millimeter wavelengths of the high­
mass star forming region G24.78+0.08, where a cluster of four young stellar objects is detected. We discuss
evidence for these to be high­mass (proto)stars in different evolutionary phases. One of the sources is detected
only in the continuum at 2 and 2.6 mm and we suggest it may represent a good candidate of a high­mass protostar.
Key words. Stars: formation -- Radio lines: ISM -- ISM: jets and outflows -- ISM: individual objects: G24.78+0.08
1. Introduction
High­mass stars are usually defined as stars with mass
above ¸8 M fi : for zero­age main sequence (ZAMS) ob­
jects, this corresponds to a luminosity –5 10 3 L fi and a
spectral type earlier than B3. Such a definition is based
on a basic theoretical result (Palla & Stahler 1993): unlike
their low­mass equivalents, protostars with masses above
8 M fi are expected to evolve on timescales much shorter
than those relevant to accretion. As a consequence, they
reach the ZAMS still deeply embedded in their parental
clouds. Consequently, the dusty parental cocoon makes
difficult to observe the newly born stars which can be
studied only in the IR or at longer wavelengths. Moreover,
massive stars form in clusters, which not only complicates
the observations of each single young stellar object (YSO),
but also profoundly affects the surrounding environment:
copious production of Lyman continuum photons eventu­
ally leads to the destruction of the parental cloud thus
making impossible to trace back the formation process.
Notwithstanding these difficulties, in recent years
much progress has been done to identify earlier and earlier
stages in the evolution of massive stars (see e.g. Kurtz et
al. 2000). This eventually resulted in a scenario according
to which high­mass star formation would proceed in dense,
massive cores ! ¸ 0.1 pc in size: as time goes on, the temper­
ature of such cores would increase and the original spher­
ical symmetry would change into a more axisymmetric
structure; the embedded stars would develop circumstel­
lar disks and bipolar outflows along the disk axes, which
would allow expansion of the ultracompact (UC) Hii re­
Send offprint requests to: R. Cesaroni, e­mail:
cesa@arcetri.astro.it
gions created by the Lyman continuum of the stars. Given
the relatively large number of hot cores and UC Hii re­
gions revealed to date, the latest phases of such a scenario
are quite well assessed; as opposite, we still have a lim­
ited understanding of the very earliest stages, prior to the
arrival onto the ZAMS. Massive YSOs of this type may
be considered the equivalent of class 0 low­mass YSOs,
namely protostars still in the main accretion phase. No
iron clad evidence for the existence of high­mass proto­
stars has been presented so far, but a few candidates have
been detected (Hunter et al. 1998; Molinari et al. 1998).
Here, we present the results of a study of the massive
star forming region G24.78+0.08. This region has been
selected from a sample of OH/H 2 O maser sources iden­
tified by Forster & Caswell (1989) and it was observed
by Codella et al. (1997, hereafter CTC) in the NH 3 (2,2)
and (3,3) inversion transitions. One interesting feature
of G24.78+0.08 is the presence of two groups of maser
spots: one, close to an UC Hii region, consists of both
OH and H 2 O masers, whereas the other, offset by ¸8 00
to the NE, contains only H 2 O masers. Both groups are
embedded in a ¸0.5 pc clump traced by the ammonia
emission. This situation resembles very closely that of the
W3(H 2 O)/W3(OH) system, where a high­mass YSO has
been found in association with W3(H 2 O) (Turner & Welch
1984), whereas W3(OH) coincides with an UC Hii region
created by an early­type star. We have hence decided to
search for a possible source associated with the isolated
H 2 O group in G24.78+0.08. To this purpose, we have stud­
ied the continuum and line emission from this region at
various wavelengths. The basic results are illustrated in
the following, while a more detailed description is post­
poned to a subsequent paper.

2 Furuya et al.: G24.78+0.08: a cluster of high­mass (proto)stars
Fig. 1. Overlay of the 1.3 cm continuum image of CTC (grey scale) with the maps (contours) of the 7 mm (top left panel),
2.6 mm (top right), and 2 mm continuum (bottom left), and CH3CN(8--7) line (bottom right) emission. Circles, squares, and
triangles represent respectively CH3OH (Walsh et al. 1998), OH and H2O (Forster & Caswell 1999) maser spots. Contour levels
are as follows: 0.5, 0.9, 1.5, 6, and 15 to 90 in steps of 15 mJy/beam for the 7 mm map; 15 and 30 to 150 in steps of 30 mJy/beam
for 2.6 mm map; 7 and 17 to 122 in step of 15 mJy/beam for the 2 mm map; 70, 120, and 220 to 1420 in steps of 200 mJy/beam
for the CH3CN map. The 3oe RMS sensitivities are 0.45, 11, 7.6, and 82 mJy/beam for the 7, 2.6, and 2 mm continuum and the
CH3CN line maps, respectively. The letters mark the four objects detected in the region (see text)
2. Observations
In 1998­2002, we carried out continuum emission imaging
using the Nobeyama Millimeter Array (NMA) at 2 mm,
the Plateau de Bure interferometer (PdBI) at 2.6 mm, and
Very Large Array (VLA) in the D­array configuration at
7 mm. The NMA observations were performed together
with the enhanced Rainbow mode using the 45­m tele­
scope as one of the array elements. At 2 and 2.6 mm,
we observed lines of CH 3 CN(8--7) and 12 CO(1--0), respec­
tively. The resulting synthesised beam sizes were 2: 00 3\Theta1: 00 6
at 2 mm, 5: 00 3\Theta4: 00 1 at 2.6 mm, and 2: 00 3\Theta1: 00 7 at 7 mm. For
all datasets the inner hole in the (u; v) plane has a ra­
dius in the range 4.5 to 6.5 k–. Attained sensitivities in
individual maps are described in the caption of Fig. 1.
3. Results and discussion
Our main findings are illustrated in Figs. 1 and 2. The for­
mer shows the maps of the continuum emission from 2 to
7 mm and the integrated intensity map of the CH 3 CN(8--
7) K=0 and 1 line emission overlaied to the 1.3 cm con­
tinuum map of CTC. Also shown are the positions of the
OH and H 2 O maser spots from Forster & Caswell (1999)
and those of the CH 3 OH masers from Walsh et al. (1998).
The most important result is the detection of four sepa­
rate sources, which are best seen in the 2 mm continuum
map: these have been identified with letters A to D. Of
these, A and B were already detected by CTC and are
associated with two compact Hii regions (see also Forster
& Caswell 2000). Source C is seen in the mm continuum
and CH 3 CN line maps: this confirms our expectation that

Furuya et al.: G24.78+0.08: a cluster of high­mass (proto)stars 3
the H 2 O masers to the NE were associated with a com­
pact molecular core. A surprising result is the detection
at 2 and 2.6 mm of a continuum peak, D, to the NW of
the UC Hii region in A. This must trace a compact dusty
core, which is not detected in any of the molecular lines
observed: such a lack of line emission may be indicative
of molecular depletion and hence high density and low
temperature.
Figure 2 presents maps of the blue­ and red­shifted
emission in the wings of the 12 CO(1--0) line: these reveal
two bipolar outflows centred on A and C. It is thus likely
that one of the flows originates from the early­type star
ionising the UC Hii region in A, while the other may be
powered by a deeply embedded YSO in C. The parame­
ters of the outflow can be derived as usual by integrating
the emission under the line wings and assuming an age
equal to the kinematical time scale (2 10 4 yr) given by
the ratio between the size of the lobes (0.45 pc) and the
maximum velocity reached in the flow (20 km s \Gamma1 ): we
find very similar values for both outflows, corresponding
to masses of ¸10 M fi , mechanical luminosities of ¸10 L fi ,
and mass loss rates of ¸5 10 \Gamma4 M fi yr \Gamma1 . Such values are
to be taken as lower limits, as the 12 CO(1--0) line may be
optically thick and the lobes might extend over a larger
region than that imaged by the interferometer. Also, cor­
recting for the (unknown) inclination of the outflow axis
would increase the real velocity and hence the mass loss
rate and mechanical luminosity. We conclude that the val­
ues quoted above are typical of high­mass stars, as one can
see e.g. from Table 1 of Churchwell (1997).
Finally, it is worth noting that the two outflow axes
are parallel to the direction outlined by the maser spots:
this result leads support to the belief that H 2 O masers
could be strongly associated with outflows (see Felli et al.
1992), as already pointed out by CTC.
Three cores (A, C, D) and two compact Hii regions
(A and B) are seen towards G24.78. Now, we try to
shed light on the nature of these objects. In Fig. 3,
we plot their continuum spectra obtained by adding the
6 cm and 3.6 cm measurements of Becker et al. (1994)
and Forster & Caswell (2000) to our data. The four
spectra have been fitted with a simple model consisting
of an Hii region surrounded by a dusty core: spherical
symmetry and constant density and temperature have
been assumed. We adopted a dust absorption coefficient
Ÿ(š)=0.005 cm 2 g \Gamma1 (š=231GHz) 2 (following Preibisch et
al. 1993 and Molinari et al. 2000). The flux depends on
the radius (R HII ) and emission measure (EM) of the Hii
region, and only on the mass (M core ) and temperature
(T dust ) of the surrounding core because at – – 2 mm the
thermal emission from dust is optically thin. In order to
minimise the number of free parameters, we have used the
results of the present work and of CTC to fix the diameters
of the Hii regions in A and B and the temperatures of the
cores in A and C. Under these assumptions, we estimated
the emission measure of the Hii regions and the mass of
the cores in A and C: in the other cases only limits can be
set. The fit parameters are given in Table 1. For core D
Fig. 2. Overlay of the 2 mm continuum image (grey scale)
with the outflow maps (contours) obtained by integrating
the 12 CO(1--0) line emission under the wings, from 90 to
105 km s \Gamma1 (full contours) and from 116 to 131 km s \Gamma1 (dashed
contours). The cloud LSR velocity is ¸111 km s \Gamma1 . Contour
levels correspond to 1.1, 1.4, and 1.7 Jy/beam (dashed con­
tours) and 0.4, 0.6, 0.8, 1.2, and 1.6 Jy/beam (full contours).
Symbols and letters have the same meaning as in Fig. 1
Table 1. Parameters for the fits in Fig. 3. An electron temper­
ature of 10 4 K and a distance of 7.7 kpc have been assumed
Source RHII EM T dust Mcore
(pc) (pc cm \Gamma6 ) (K) (M fi )
A 0.005( a ) 2 10 9 90( a ) 550
B 0.05( a ) 6.5 10 6 90( b ) Ÿ40
C !0.005( c ) ?9 10 6 30( a ) 250
D !0.005( c ) ?7.5 10 6 Ÿ30( d ) –100
( a ) derived by CTC
( b ) no T dust estimate: same temperature as in A assumed
( c ) upper limit assumed equal to diameter of Hii region in A
( d ) assumed equal or less than the temperature of core C
we have assumed a maximum temperature equal to that
of core C. Although such an assumption is arbitrary, it
seems unlikely that the gas is hotter, otherwise one would
expect to detect line emission from molecules evaporated
from grain mantles: as discussed above, such emission is
not seen at the same level as in A and C.
4. Nature of the sources and evolutionary
sequence
In the light of the previous results, we can now discuss the
nature of the four sources, whose properties are schemat­
ically summarised in Table 2.
B The negligible molecular line emission seen towards this
object indicates that the Hii region has already ex­
panded destroying the densest portion of the molecu­
lar surroundings. The ionising star is hence relatively
old, although less than ¸10 5 yr, the expected life time
of UC Hii regions (Wood & Churchwell 1989).

4 Furuya et al.: G24.78+0.08: a cluster of high­mass (proto)stars
Fig. 3. Spectrum of the continuum emission of the four ob­
jects detected towards G24.78+0.08. The letters identify each
spectrum according to the notation in Fig. 1. The lines are fits
to the data obtained with the model described in the text and
for the parameters listed in Table 1
A The UC Hii region is unresolved at 1.3 cm (CTC) and
deeply embedded in a dense molecular core, as wit­
nessed by the absorption detected in the NH 3 (2,2) and
(3,3) lines. The compactness of the ionised region, the
temperature and mass of the surrounding core, the ex­
istence of a bipolar outflow, the strong emission in rare
molecules such as CH 3 CN, the presence of maser emis­
sion in various species all indicate that we are dealing
with a young early­type ZAMS star strongly interact­
ing with the surrounding environment and in an earlier
evolutionary phase than the star in B.
C The basic difference with respect to A which suggests
a younger age for C is represented by the absence of
an UC Hii region toward C even though the large
mass of the molecular core, the outflow parameters,
and the detection of hot core species such as CH 3 CN
are all typical of high­mass YSOs. This conclusion is
also supported by the fact that C, unlike A, harbours
only H 2 O maser emission: the common belief is that
H 2 O masers should appear at a very early stage of the
evolution of a high­mass star, which also suggests that
C is younger than A. Further evidence in favour of C
being associated with massive YSOs is represented by
the large mass of the core, ¸250 M fi . In fact, such a
mass is comparable to that of cores hosting high­mass
YSOs (see Table 1 of Kurtz et al. 2000); moreover, it
is contained in 0.07 pc and is ¸6 times greater than
that (40 M fi over 0.2 pc) of the low­mass star forming
clumps observed by Testi et al. (1998) in the Serpens
region. Therefore, it is unlikely that we are observing
a low­mass star forming core.
Table 2. Characteristics of the four objects observed
YSO D C A B
Dusty Core (mm cont.) Y Y Y N
Molecular Core (NH3 , CH3CN) N Y Y N
Bipolar Outflow ( 12 CO) N Y Y N
H2O masers N Y Y N
CH3OH masers N N Y N
OH masers N N Y N
UC Hii region (cm free­free) N N Y Y
D This is the most intriguing object, as it is traced only
by the millimeter continuum emission and is not de­
tected in any molecular species (CO, CH 3 CN, NH 3 ).
One cannot rule out the possibility that this is a quies­
cent core without star formation. However, this is hard
to believe given the large value of the mass, ?100 M fi :
as already discussed for C, values such large are not
appropriate for low­mass star forming cores.
In conclusion, we have detected a cluster containing at
least 4 high­mass YSOs, in different evolutionary phases.
More precisely, the ages of these YSOs are likely to be in
the order t B ? t A ? t C ? t D . We suggest that core D rep­
resents an excellent candidate for a high­mass protostar.
Acknowledgements. It is a pleasure to thank the staff of IRAM,
NRAO, and NRO for their help during the observations.
Special thanks are due to Prof. Sachiko Okumura for taking
care of our observations with the Nobeyama Millimeter Array.
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