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Register a new userThe Nature of Low-Density Star Formation
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David Thilker
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How do stars manage to form within low-density, HI-dominated gas? Such environments provide a laboratory for studying star formation with physical conditions distinct from starbursts and the metal-rich disks of spiral galaxies where most effort has been invested. Here we outline fundamental open questions about the nature of star formation at low-density. We describe the wide-field, high-resolution UV-optical-IR-radio observations of stars, star clusters and gas clouds in nearby galaxies needed in the 2020s to provide definitive answers, essential for development of a complete theory of star formation.
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Context: Star formation takes place in giant molecular clouds, resulting in mass-segregated young stellar clusters composed of Sun-like stars, brown dwarves, and massive O-type(50-100msun) stars. Aims: To identify candidate hub-filament systems (HFS) in the Milky-Way and examine their role in the formation of the highest mass stars and star clusters. Methods: Filaments around ~35000 HiGAL clumps that are detected using the DisPerSE algorithm. Hub is defined as a junction of three or more filaments. Column density maps were masked by the filament skeletons and averaged for HFS and non-HFS samples to compute the radial profile along the filaments into the clumps. Results: ~3700~(11%) are candidate HFS of which, ~2150~(60%) are pre-stellar, ~1400~(40%) are proto-stellar. All clumps with L>10^4 Lsun and L>10^5 Lsun at distances respectively within 2kpc and 5kpc are located in the hubs of HFS. The column-densities of hubs are found to be enhanced by a factor of ~2 (pre-stellar sources) up to ~10 (proto-stellar sources). Conclusions: All high-mass stars preferentially form in the density enhanced hubs of HFS. This amplification can drive the observed longitudinal flows along filaments providing further mass accretion. Radiation pressure and feedback can escape into the inter-filamentary voids. We propose a filaments to clusters unified paradigm for star formation, with the following salient features: a) low-intermediate mass stars form in the filaments slowly (10^6yr) and massive stars quickly (10^5yr) in the hub, b) the initial mass function is the sum of stars continuously created in the HFS with all massive stars formed in the hub, c) Feedback dissiption and mass segregation arise naturally due to HFS properties, and c) explain age spreads within bound clusters and formation of isolated OB associations.
We have used deep near-infrared observations with adaptive optics to discover a distributed population of low-mass protostars within the filamentary Infrared Dark Cloud G34.43+00.24. We use maps of dust emission at multiple wavelengths to determine the column density structure of the cloud. In combination with an empirically-verified model of the magnitude distribution of background stars, this column density map allows us to reliably determine overdensities of red sources that are due to embedded protostars in the cloud. We also identify protostars through their extended emission in K-band which comes from excited H2 in protostellar outflows or reflection nebulosity. We find a population of distributed low-mass protostars, suggesting that low-mass protostars may form earlier than, or contemporaneously with, high-mass protostars in such a filament. The low-mass protostellar population may also produce the narrow linewidth SiO emission observed in some clouds without high-mass protostars. Finally, we use a molecular line map of the cloud to determine the virial parameter per unit length along the filament and find that the highest mass protostars form in the most bound portion of the filament, as suggested by theoretical models.
Background: low-mass stars are the dominant product of the star formation process, and they trace star formation over the full range of environments, from isolated globules to clusters in the central molecular zone. In the past two decades, our understanding of the spatial distribution and properties of young low-mass stars and protostars has been revolutionized by sensitive space-based observations at X-ray and IR wavelengths. By surveying spatial scales from clusters to molecular clouds, these data provide robust measurements of key star formation properties. Goal: with their large numbers and their presence in diverse environments, censuses of low mass stars and protostars can be used to measure the dependence of star formation on environmental properties, such as the density and temperature of the natal gas, strengths of the magnetic and radiation fields, and the density of stars. Here we summarize how such censuses can answer three basic questions: i.) how is the star formation rate influenced by environment, ii.) does the IMF vary with environment, and iii.) how does the environment shape the formation of bound clusters? Answering these questions is an important step toward understanding star and cluster formation across the extreme range of environments found in the Universe. Requirements: sensitivity and angular resolution improvements will allow us to study the full range of environments found in the Milky Way. High spatial dynamic range (< 1arcsec to > 1degree scales) imaging with space-based telescopes at X-ray, mid-IR, and far-IR and ground-based facilities at near-IR and sub-mm wavelengths are needed to identify and characterize young stars.
We investigate the global properties of the straight and isolated filamentary cloud G350.54+0.69 using Herschel continuum and APEX molecular line data. The overall straight morphology is similar to two other well studied nearby filaments (Musca and Taurus-B211/3) while the isolated nature of G350.54+0.69 appears similar to Musca. G350.54+0.69 is composed of two distinct filaments with a length ~5.9 pc for G350.5-N (~2.3 pc for G350.5-S), a total mass of ~810 $M_{odot}$ (~ 110 $M_{odot}$), and a mean temperature of ~ 18.2 K (~17.7 K). We identify 9 dense and gravitationally bound cores in the whole cloud G350.54+0.69. The separations between cores and the line mass of the whole cloud appear to follow the predictions of the sausage instability theory, which suggests that G350.54+0.69 could have undergone radial collapse and fragmentation. The presence of young protostars is consistent with this hypothesis. The line masses of the two filaments (~120 $M_{odot}$ pc$^{-1}$ for G350.5-N, and ~45 $M_{odot}$ pc$^{-1}$ for G350.5-S), mass-size distributions of the dense cores, and low-mass protostars collectively suggest that G350.54+0.69 is a site of ongoing low-mass star formation. Based on the above evidence, we place G350.54+0.69 in an intermediate evolutionary state between Musca and Taurus-B211/3. We suggest that investigations into straight (and isolated) versus those distributed inside molecular clouds may provide important clues into filament formation and evolution.
Three bright molecular line sources in G333 have recently been shown to exhibit signatures of infall. We describe a molecular line radiative transfer modelling process which is required to extract the infall signature from Mopra and Nanten2 data. The observed line profiles differ greatly between individual sources but are reproduced well by variations upon a common unified model where the outflow viewing angle is the most significant difference between the sources. The models and data together suggest that the observed properties of the high-mass star-forming regions such as infall, turbulence, and mass are consistent with scaled-
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