Issue |
A&A
Volume 370, Number 1, April IV 2001
|
|
---|---|---|
Page(s) | 230 - 264 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20010190 | |
Published online | 15 April 2001 |
A molecular-line study of clumps with embedded high-mass protostar candidates
1
Istituto di Radioastronomia, CNR, Via Gobetti 101, 40129 Bologna, Italy
2
Osservatorio Astrofisica di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
3
Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
Corresponding author: J. Brand, brand@ira.bo.cnr.it
Received:
2
October
2000
Accepted:
29
January
2001
We present molecular line observations made with the IRAM 30-m
telecopes of the immediate surroundings of a
sample of 11 candidate high-mass protostars. These observations are part of an
effort to clarify the evolutionary status of a set of objects which we
consider to be precursors of UC Hii regions.
In a preceding series of papers we have studied a sample of objects, which
on the basis of their IR colours are likely to be associated with compact
molecular
clouds. The original sample of 260 objects was divided approximately evenly
into a High group, with IR colour indices and
, and a Low group with complementary colours.
The FIR luminosity of the Low
sources, their distribution in the IR colour-colour diagram, and their lower
detection rate in H2O maser emission compared to the High sources,
led to the hypothesis that the majority of these objects represent an earlier
stage in the evolution than the members of the
High group, which are mostly identifiable with UC Hii regions.
Subsequent observations led to the selection of 12 Low sources that
have FIR luminosities indicating the presence of B2.5 to O8.5 V0 stars,
are associated with dense gas and dust, have (sub-)mm continuum spectra
indicating temperatures of ~30 K, and have no detectable radio
continuum emission. One of these sources has been proposed by us to be a
good candidate for the high-mass equivalent of a Class 0 object. In the
present paper we present observations of the molecular environment of 11 of
these 12 objects, with the aim to derive the physical parameters of the gas
in which they are embedded, and to find further evidence in support of our
hypothesis that these sources are the precursors to UC Hii regions. We
find that the data are consistent with such an interpretation.
All observed sources are associated with well-defined molecular clumps.
Masses, sizes, and other parameters depend on the tracer used, but typically
the cores have average diameters of ~0.5-1 pc (with a range of
0.2 to 2.2 pc), and masses of a few tens to a few thousand solar masses.
Compared to a similar analysis of High sources, the present sample has
molecular clumps that are more massive, larger, cooler, and less turbulent.
They also tend to have a smaller ratio of virial-to-luminous mass,
indicating they are less dynamically stable than their counterparts in which
the High sources are embedded. The large sizes suggest these clumps
should still undergo substantial contraction (their densities are ~10
times smaller than those of the High sources). The lower
temperatures and small linewidths are also expected in objects in an earlier
evolutionary state. In various sources indications are found for outflowing
gas, though its detection is hampered by the presence of multiple emission
components in the line spectra. There are also signs of self-absorption,
especially in the spectra of 13CO and HCO+.
We find that the masses of the molecular clumps associated with our objects
increase with Lfir (
),
and that there is
a (weak) relation between the clump mass and the mass of the embedded
protostellar object
.
The large amount of observational data is necessarily presented in a
compact, reduced form. Yet we supply enough information to allow further
study. These data alone cannot prove or disprove the hypothesis that among
these objects a high-mass protostar is truly present. More observations, at
different wavelenghts and spatial resolutions are needed to provide enough
constraints on the number of possible interpretations.
Key words: ISM: clouds / molecules, radio lines: ISM
© ESO, 2001
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