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High-Mass Protostellar Candidates. I. The Sample and Initial Results
We describe a systematic program aimed at identifying and characterizingcandidate high-mass protostellar objects (HMPOs). Our candidate sampleconsists of 69 objects selected by criteria based on those establishedby Ramesh & Sridharan using far-infrared, radio continuum, andmolecular line data. IRAS and Midcourse Space Experiment data were usedto study the larger scale environments of the candidate sources anddetermine their total luminosities and dust temperatures. To derive thephysical and chemical properties of our target regions, we observedcontinuum and spectral line radiation at millimeter and radiowavelengths. We imaged the free-free and dust continuum emission atwavelengths of 3.6 cm and 1.2 mm, respectively, searched forH2O and CH3OH maser emission, and observed the COJ=2-->1 line and several NH3 lines toward all sources inour sample. Other molecular tracers were observed in a subsample. Whiledust continuum emission was detected in all sources, most of them showonly weak or no emission at 3.6 cm. Where detected, the centimeteremission is frequently found to be offset from the millimeter emission,indicating that the free-free and dust emissions arise from differentsubsources possibly belonging to the same (proto)cluster. A comparisonof the luminosities derived from the centimeter emission with bolometricluminosities calculated from the IRAS far-infrared fluxes shows that thecentimeter emission very likely traces the most massive source, whereasthe whole cluster contributes to the far-infrared luminosity. Estimatesof the accretion luminosity indicate that a significant fraction of thebolometric luminosity is still due to accretion processes. The earlieststages of HMPO evolution we seek to identify are represented by dustcores without radio emission. Line wings due to outflow activity arenearly omnipresent in the CO observations, and the molecular line dataindicate the presence of hot cores for several sources, where theabundances of various molecular species are elevated because ofevaporation of icy grain mantles. Kinetic gas temperatures of 40 sourcesare derived from NH3 (1,1) and (2, 2) data, and we comparethe results with the dust temperatures obtained from the IRAS data.Comparing the amount of dust, and hence the gas, associated with theHMPOs and with ultracompact H II (UCH II) regions, we find that the twotypes of sources are clearly separated in mass-luminosity diagrams: forthe same dust masses, the UCH II regions have higher bolometricluminosities than HMPOs. We suggest that this is an evolutionary trend,with the HMPOs being younger and reprocessing less (stellar) radiationin the IR than the more evolved UCH II regions. These results indicatethat a substantial fraction of our sample harbors HMPOs in a pre-UCH IIregion phase, the earliest known stage in the high-mass star formationprocess.
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