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Register a new userStar Formation in Massive Protoclusters in the Monoceros OB1 Dark Cloud
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Grace A. Wolf-Chase
Publication date
2003
fields
Physics
and research's language is
English
Authors
G. Wolf-Chase
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We present far-infrared, submillimetre, and millimetre observations of bright IRAS sources and outflows that are associated with massive CS clumps in the Monoceros OB1 Dark Cloud. Individual star-forming cores are identified within each clump. We show that combining submillimetre maps, obtained with SCUBA on the JCMT, with HIRES-processed and modelled IRAS data is a powerful technique that can be used to place better limits on individual source contributions to the far-infrared flux in clustered regions. Three previously categorized Class I objects are shown to consist of multiple sources in different evolutionary stages. In each case, the IRAS point source dominates the flux at 12 and 25 microns. In two cases, the IRAS point source is not evident at submillimetre wavelengths. The submillimetre sources contribute significantly to the 60 and 100 micron fluxes, dominating the flux in the 100 micron waveband. Using fluxes derived from our technique, we present the spectral energy distribution and physical parameters for an intermediate-mass Class 0 object in one of the regions. Our new CO J=2-1 outflow maps of the three regions studied indicate complex morphology suggestive of multiple driving sources. We discuss the possible implications of our results for published correlations between outflow momentum deposition rates and source luminosities, and for using these derived properties to estimate the ratio of mass ejection rates to mass accretion rates onto protostars.
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We started a multi-scale analysis of G202.3+2.5, an intertwined filamentary region of Monoceros OB1. In Paper I, we examined the distributions of dense cores and protostars and found enhanced star formation (SF) activity in the junction region of the filaments. In this second paper, we aim to unveil the connections between the core and filament evolutions, and between the filament dynamics and the global evolution of the cloud. We characterise the gas dynamics and energy balance using Herschel and WISE observations and molecular tracers observed with the IRAM 30m and TRAO 14m telescopes. The velocity field of the cloud is examined and velocity-coherent structures are put in perspective with the cloud environment. Two main velocity components (VCs) are revealed, well separated in the north and merged around the location of intense N2H+ emission where Paper I found the peak of SF activity. The relative position of the two VCs along the sightline, and the velocity gradient in N2H+ emission imply that the VCs have been undergoing collision for ~10^5 yrs. The dense gas where N2H+ is detected is interpreted as the compressed region between the two filaments, which corresponds to a high mass inflow rate of ~1e-3 Msun/yr and possibly leads to an increase in its SF efficiency. We identify a protostar in the junction region that possibly powers two crossed intermittent outflows. We show that the HII region around the nearby cluster NCG 2264 is still expanding and its role in the collision is examined. However, we cannot rule out the idea that the collision arises mostly from the global collapse of the cloud. The (sub-)filament-scale observables examined in this paper reveal a collision between G202.3+2.5 sub-structures and its probable role in feeding the cores in the junction region. One must now characterise the cloud morphology, its fragmentation, and magnetic field, all at high resolution.
G0.253+0.016 is a remarkable massive infrared dark cloud located within $sim$100 pc of the galactic center. With a high mass of $1.3 times 10^5 M_odot$, a compact average radius of $sim$2.8 pc and a low dust temperature of 23 K, it has been believed to be a yet starless precursor to a massive Arches-like stellar cluster. We present sensitive JVLA 1.3 and 5.6 cm radio continuum observations that reveal the presence on three compact thermal radio sources projected against this cloud. These radio sources are interpreted as HII regions powered by $sim$B0.5 ZAMS stars. We conclude that although G0.253+0.016 does not show evidence of O-type star formation, there are certainly early B-type stars embedded in it. We detect three more sources in the periphery of G0.253+0.016 with non-thermal spectral indices. We suggest that these sources may be related to the galactic center region and deserve further study.
W51A is one of the most active star-forming region in our Galaxy, which contains giant molecular clouds with a total mass of 10^6 Msun. The molecular clouds have multiple velocity components over ~20 km/s, and interactions between these components have been discussed as the mechanism which triggered the massive star formation in W51A. In this paper, we report an observational study of the molecular clouds in W51A using the new 12CO, 13CO, and C18O (J=1-0) data covering a 1.4x1.0 degree region of W51A obtained with the Nobeyama 45-m telescope at 20 resolution. Our CO data resolved the four discrete velocity clouds at 50, 56, 60, and 68 km/s with sizes and masses of ~30 pc and 1.0-1.9x10^5 Msun. Toward the central part of the HII region complex G49.5-0.4, we identified four C18O clumps having sizes of ~1 pc and column densities of higher than 10^23 cm^-3, which are each embedded within the four velocity clouds. These four clumps are distributed close to each others within a small distance of 5 pc, showing a complementary distribution on the sky. In the position-velocity diagram, these clumps are connected with each others by bridge features with intermediate intensities. The high intensity ratios of 13CO (J=3-2/J=1-0) also indicates that these four clouds are associated with the HII regions. We also found these features in other HII regions in W51A. The timescales of the collisions are estimated to be several 0.1 Myrs as a crossing time of the clouds, which are consistent with the ages of the HII regions measured from the size of the HII regions in the 21 cm continuum emissions. We discuss the cloud-cloud collision scenario and massive star formation in W51A by comparing with the recent observational and theoretical studies of cloud-cloud collision.
As progenitors of the most massive objects, protoclusters are key to tracing the evolution and star-formation history of the Universe, and are responsible for ${gtrsim},20$ per cent of the cosmic star formation at $z,{>},2$. Using a combination of state-of-the-art hydrodynamical simulations and empirical models, we show that current galaxy-formation models do not produce enough star formation in protoclusters to match observations. We find that the star-formation rates (SFRs) predicted from the models are an order of magnitude lower than what is seen in observations, despite the relatively good agreement found for their mass-accretion histories, specifically that they lie on an evolutionary path to become Coma-like clusters at $z,{simeq}, 0$. Using a well-studied protocluster core at $z,{=},4.3$ as a test case, we find that star-formation efficiency of protocluster galaxies is higher than predicted by the models. We show that a large part of the discrepancy can be attributed to a dependence of SFR on the numerical resolution of the simulations, with a roughly factor of 3 drop in SFR when the spatial resolution decreases by a factor of 4. We also present predictions up to $z,{simeq},7$. Compared to lower redshifts, we find that centrals (the most massive member galaxies) are more distinct from the other galaxies, while protocluster galaxies are less distinct from field galaxies. All these results suggest that, as a rare and extreme population at high-$z$, protoclusters can help constrain galaxy formation models tuned to match the average population at $z,{simeq},0$.
A sample of 1.3 mm continuum cores in the Dragon infrared dark cloud (also known as G28.37+0.07 or G28.34+0.06) is analyzed statistically. Based on their association with molecular outflows, the sample is divided into protostellar and starless cores. Statistical tests suggest that the protostellar cores are more massive than the starless cores, even after temperature and opacity biases are accounted for. We suggest that the mass difference indicates core mass growth since their formation. The mass growth implies that massive star formation may not have to start with massive prestellar cores, depending on the core mass growth rate. Its impact on the relation between core mass function and stellar initial mass function is to be further explored.
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