On the Evolution of Ultracompact H<scp>ii</scp>Regions

Keto, Eric

doi:10.1086/343794

Cited by 122 publications

(137 citation statements)

References 11 publications

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“…Additional observations of very high angular resolution are needed to test the nature of this source as a hypercompact H ii region. Hypercompact H ii regions are thought to mark the earliest stages of evolution of regions of ionized gas, being formed in the accretion phase of hot molecular cores (Keto 2002(Keto , 2003González-Avilés et al 2005;Avalos et al 2006). We conclude that IRAS 23151+5912 is indeed associated with a massive-star-forming region in a very early stage of evolution.…”

Section: Iras 23151+5912mentioning

confidence: 73%

“…If massive O stars are formed by accretion, we expect that disks and jets will be present in their earliest stages of evolution, as in the case of low-mass stars. Due to their large luminosities, hypercompact H ii regions are also expected to be formed while the protostar is still undergoing accretion (Keto 2002(Keto , 2003. On the other hand, if formed by random coalescence of lower mass stars, neither disks, jets, or hypercompact H ii regions are expected.…”

Section: Introductionmentioning

confidence: 99%

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Very Large Array Observations of Candidate High-Mass Protostellar Objects at 7 Millimeters

2007

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We present radio-continuum observations at 7 mm made using the Very Large Array toward three massive starforming regions thought to be in very early stages of evolution, selected from the 2002 sample of Sridharan et al. Emission was detected toward all three sources (IRAS 18470À0044, IRAS 19217+1651, and IRAS 23151+5912). We find that in all cases the 7 mm emission corresponds to thermal emission from ionized gas. The regions of ionized gas associated with IRAS 19217+1651 and IRAS 23151+5912 are hypercompact, with diameters of 0.009 and 0.0006 pc and emission measures of 7:0 ; 10 8 and 2:3 ; 10 9 pc cm À6 , respectively.

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Section: Iras 23151+5912mentioning

confidence: 73%

Section: Introductionmentioning

confidence: 99%

Very Large Array Observations of Candidate High-Mass Protostellar Objects at 7 Millimeters

2007

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“…However, for a non-uniform density source with high temperature and density, a more significant contribution from free-free emission is possible (e.g. Beuther et al 2004;Keto 2002Keto , 2003. This may lead to an overestimate of the mass.…”

Section: Temperature Estimatesmentioning

confidence: 99%

The ATLASGAL survey: The sample of young massive cluster progenitors

Csengeri

Bontemps

Wyrowski

et al. 2017

A&A

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Context. The progenitors of high-mass stars and clusters are still challenging to recognise. Only unbiased surveys, sensitive to compact regions of high dust column density, can unambiguously reveal such a small population of particularly massive and cold clumps. Aims. Here we use the ATLASGAL survey to identify a sample of candidate progenitors of massive clusters in the inner Galaxy. Methods. We characterise a flux limited sample of compact sources selected from the ATLASGAL survey. Sensitive mid-infrared data at 21−24 µm from the WISE and MIPSGAL surveys were explored to search for embedded objects, and complementary spectroscopic data were used to investigate their stability and their star formation activity. Results. We identify an unbiased sample of infrared-quiet massive clumps in the Galaxy that potentially represent the earliest stages of massive cluster formation. An important fraction of this sample consists of sources that have not been studied in detail before. We first find that clumps hosting more evolved embedded objects and infrared-quiet clumps exhibit similar physical properties in terms of mass and size, suggesting that the sources are not only capable of forming high-mass stars, but likely also follow a single evolutionary track leading to the formation of massive clusters. The majority of the clumps are likely not in virial-equilibrium, suggesting collapse on the clump scale. Conclusions. We identify the precursors of the most massive clusters in the Galaxy within our completeness limit, and argue that these objects are undergoing large-scale collapse. This is in line with the low number of infrared-quiet massive clumps and earlier findings that star formation, in particular for high-mass objects is a fast, dynamic process. We propose a scenario in which massive clumps start to fragment and collapse before their final mass is accumulated indicating that strong self-gravity and global collapse is needed to build up rich clusters and the most massive stars.

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“…These include accretion driven formation through massive disks (e.g. Krumholz et al 2009;McKee & Tan 2003;Yorke & Sonnhalter 2002;Keto et al 2002), competitive accretion in dense clusters (Bonnell et al 2004), and merging of low-mass protostars (Bonnell & Bate 2002).…”

Section: Introductionmentioning

confidence: 99%

A comparative study of high-mass cluster forming clumps

López-Sepulcre

Cesaroni

Walmsley

2010

A&A

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Aims. We have searched for star formation activity (mainly infall and outflow signatures) in a sample of high-mass molecular clumps (M > 100 M ) in different evolutionary stages and with a wide range of surface densities, with the aim of looking for evolutionary trends and testing observationally recent theoretical models which predict the need for a minimum surface density to form high-mass stars. Methods. Our sample has been selected from single-dish 1.2 mm continuum surveys and is composed of 48 massive molecular clumps, of which 29 are IR-loud and 19 are IR-dark. Each of these has been mapped in the HCO + (1-0), HCN(1-0) and C 18 O(2-1) transitions with the IRAM-30 m telescope on Pico Veleta (Spain). We derive basic parameters (mass, momentum, kinetic energy) for the clumps and their associated outflows and examine the HCO + (1-0) line profiles for evidence of infall or expansion. Results. Molecular outflows have been detected in 75% of our targets from the presence of high-velocity wings in the HCO + (1-0) spectra. These are equally frequent and massive (between ∼1 and ∼100 M ) in IR-dark and IR-loud clumps, implying similar levels of star formation activity in both kinds of objects. A surface density threshold at Σ = 0.3 g cm −2 has been found above which the outflow detection rate increases significantly and the outflows are on average more massive. The infall detection rate in our sample is low, but significantly higher in the IR-dark sub-sample. Our clump mass estimates using the mm dust emission and C 18 O(2-1) are sensitive to the temperature, but assuming a value of 15 K for the IR-dark sub-sample, we find evidence that C 18 O is depleted by a factor ∼4.5. The HCO + (1-0) to HCN(1-0) integrated intensity ratios measured reveal a greater dispersion about the mean value in the IR-dark sub-sample than in the IR-loud by a factor of about 5. We find that a considerable number of IR-dark sources are self-absorbed in HCN(1-0) suggesting that radiative transport effects in the ground state transitions have an important influence on the integrated intensity ratio. Conclusions. Our results indicate that, in terms of outflow frequency and energetics, both IR-dark and IR-loud molecular clumps present equivalent signatures of star formation activity, and that the formation of high-mass stars requires sufficiently high clump surface densities. The higher infall detection rate measured for the IR-dark subsample suggests that these objects could be associated with the onset of star formation.

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On the Evolution of Ultracompact HiiRegions

Cited by 122 publications

References 11 publications

Very Large Array Observations of Candidate High-Mass Protostellar Objects at 7 Millimeters

Very Large Array Observations of Candidate High-Mass Protostellar Objects at 7 Millimeters

The ATLASGAL survey: The sample of young massive cluster progenitors

A comparative study of high-mass cluster forming clumps

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