Engineering topics

Vazquez-Semadeni, Enrique

Publications and source records attributed to Vazquez-Semadeni, Enrique.

6 records

DOE OSTI2012

FROM DUSTY FILAMENTS TO MASSIVE STARS: THE CASE OF NGC 7538 S

We report on high-sensitivity and high angular resolution archival Submillimeter Array observations of the large ({approx}15,000 AU) putative circumstellar disk associated with the O-type protostar NGC 7538 S. Observations of the continuum resolve this putative circumstellar disk into five compact sources, with sizes {approx}3000 AU and masses {approx}10 M{sub Sun }. This confirms the results of recent millimeter observations made with CARMA/BIMA toward this object. However, we find that most of these compact sources eject collimated bipolar outflows, revealed by our silicon monoxide (SiO J = 5-4) observations, and confirm that these sources have a (proto)stellar nature. All outflows are perpendicular to the large and rotating dusty structure. We propose therefore that, rather than being a single massive circumstellar disk, NGC 7538 S could instead be a large and massive contracting or rotating filament that is fragmenting at scales of 0.1-0.01 pc to form several B-type stars, via the standard process involving outflows and disks. As in recent high spatial resolution studies of dusty filaments, our observations also suggest that thermal pressure does not seem to be sufficient to support the filament, so that either additional support needs to be invoked or else the filament must be in the process of collapsing. A smoothed particle hydrodynamics numerical simulation of the formation of a molecular cloud by converging warm neutral medium flows produces contracting filaments whose dimensions and spacings between the stars forming within them, as well as their column densities, strongly resemble those observed in the filament reported here.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

ASPECT RATIO DEPENDENCE OF THE FREE-FALL TIME FOR NON-SPHERICAL SYMMETRIES

We investigate the collapse of non-spherical substructures, such as sheets and filaments, which are ubiquitous in molecular clouds. Such non-spherical substructures collapse homologously in their interiors but are influenced by an edge effect that causes their edges to be preferentially accelerated. We analytically compute the homologous collapse timescales of the interiors of uniform-density, self-gravitating filaments and find that the homologous collapse timescale scales linearly with the aspect ratio. The characteristic timescale for an edge-driven collapse mode in a filament, however, is shown to have a square-root dependence on the aspect ratio. For both filaments and circular sheets, we find that selective edge acceleration becomes more important with increasing aspect ratio. In general, we find that lower dimensional objects and objects with larger aspect ratios have longer collapse timescales. We show that estimates for star formation rates, based upon gas densities, can be overestimated by an order of magnitude if the geometry of a cloud is not taken into account.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

AN EVOLUTIONARY MODEL FOR COLLAPSING MOLECULAR CLOUDS AND THEIR STAR FORMATION ACTIVITY

We present an idealized, semi-empirical model for the evolution of gravitationally contracting molecular clouds (MCs) and their star formation rate (SFR) and efficiency (SFE). The model assumes that the instantaneous SFR is given by the mass above a certain density threshold divided by its free-fall time. The instantaneous number of massive stars is computed assuming a Kroupa initial mass function. These stars feed back on the cloud through ionizing radiation, eroding it. The main controlling parameter of the evolution turns out to be the maximum cloud mass, M{sub max}. This allows us to compare various properties of the model clouds against their observational counterparts. A giant molecular cloud (GMC) model (M{sub max} {approx} 10{sup 5} M{sub Sun }) adheres very well to the evolutionary scenario recently inferred by Kawamura et al. for GMCs in the Large Magellanic Cloud. A model cloud with M{sub max} Almost-Equal-To 2000 M{sub Sun} evolves in the Kennicutt-Schmidt diagram, first passing through the locus of typical low-to-intermediate-mass star-forming clouds, and then moving toward the locus of high-mass star-forming ones over the course of {approx}10 Myr. Also, the stellar age histograms for this cloud a few Myr before its destruction agree very well with those observed in the {rho}-Oph stellar association, whose parent cloud has a similar mass, and imply that the SFR of the clouds increases with time. Our model thus agrees well with various observed properties of star-forming MCs, suggesting that the scenario of gravitationally collapsing MCs, with their SFR regulated by stellar feedback, is entirely feasible and in agreement with key observed properties of MCs.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

THE FREE-FALL TIME OF FINITE SHEETS AND FILAMENTS

Molecular clouds often exhibit filamentary or sheet-like shapes. We compute the free-fall time ({tau}{sub ff}) for finite, uniform, self-gravitating circular sheets and filamentary clouds of small but finite thickness, so that their volume density {rho} can still be defined. We find that, for thin sheets, the free-fall time is larger than that of a uniform sphere with the same volume density by a factor proportional to {radical}A, where the aspect ratio A is given by A = R/h, R being the sheet's radius and h is its thickness. For filamentary clouds, the aspect ratio is defined as A=L/R, where L is the filament's half-length and R is its (small) radius, and the modification factor is more complicated, although in the limit of large A it again reduces to nearly {radical}A. We propose that our result for filamentary shapes naturally explains the ubiquitous configuration of clumps fed by filaments observed in the densest structures of molecular clouds. Also, the longer free-fall times for non-spherical geometries in general may contribute toward partially alleviating the 'star formation conundrum', namely, the star formation rate in the Galaxy appears to be proceeding in a timescale much larger than the total molecular mass in the Galaxy divided by its typical free-fall time. If molecular clouds are in general formed by thin sheets and long filaments, then their relevant free-fall time may have been systematically underestimated, possibly by factors of up to one order of magnitude.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

FROM THE CONVERGENCE OF FILAMENTS TO DISK-OUTFLOW ACCRETION: MASSIVE STAR FORMATION IN W33A

Interferometric observations of the W33A massive star formation region, performed with the Submillimeter Array and the Very Large Array at resolutions from 5'' (0.1 pc) to 0.''5 (0.01 pc), are presented. Our three main findings are: (1) parsec-scale, filamentary structures of cold molecular gas are detected. Two filaments at different velocities intersect in the zone where the star formation is occurring. This is consistent with triggering of the star formation activity by the convergence of such filaments, as predicted by numerical simulations of star formation initiated by converging flows. (2) The two dusty cores (MM1 and MM2) at the intersection of the filaments are found to be at different evolutionary stages, and each of them is resolved into multiple condensations. MM1 and MM2 have markedly different temperatures, continuum spectral indices, molecular-line spectra, and masses of both stars and gas. (3) The dynamics of the 'hot-core' MM1 indicates the presence of a rotating disk in its center (MM1-Main) around a faint free-free source. The stellar mass is estimated to be {approx}10 M{sub sun}. A massive molecular outflow is observed along the rotation axis of the disk.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

MOLECULAR CLOUD EVOLUTION. III. ACCRETION VERSUS STELLAR FEEDBACK

We numerically investigate the effect of feedback from the ionization heating from massive stars on the evolution of giant molecular clouds (GMCs) and their star formation efficiency (SFE), which we treat as an instantaneous, time-dependent quantity. We follow the GMCs' evolution from their formation to advanced star-forming stages. After an initial period of contraction, the collapsing clouds begin forming stars, whose feedback evaporates part of the clouds' mass, opposing the continuing accretion from the infalling gas. Our results are as follows: (1) in the presence of feedback, the clouds attain levels of the SFE that are consistent at all times with observational determinations for regions of comparable star formation rates. (2) However, the dense gas mass is larger in general in the presence of feedback, while the total mass (dense gas + stars) is nearly insensitive to the presence of feedback, suggesting that it is determined mainly by the accretion, while the feedback inhibits mainly the conversion of dense gas to stars, because it acts directly to reheat and disperse the gas that is directly on its way to forming stars. (3) The factor by which the SFE is reduced upon the inclusion of feedback is a decreasing function of the cloud's mass, for clouds of size {approx}10 pc. This naturally explains the larger observed SFEs of massive-star-forming regions. (4) The clouds may attain a pseudo-virialized state, with a value of the virial mass very similar to the actual cloud mass. However, this state differs from true virialization in that the clouds, rather than being equilibrium entities, are the centers of a larger-scale collapse, in which accretion replenishes the mass consumed by star formation. (5) The higher-density regions within the clouds are in a similar situation, accreting gas infalling from the less-dense, more extended regions of the clouds. (6) The density probability density functions of the regions containing the clouds in general exhibit a shape characteristic of thermally bistable flows, and extend down to densities corresponding to the warm atomic medium, because the warm gas interpenetrates the clouds. The general picture arising from our simulations is that the gas involved in the gravitational contraction leading to star formation extends to large scales and is not confined to the local environment of the final collapse, where the effect of the feedback is active, in agreement with recent proposals of a gravitationally driven mass cascade from large to small scales.

79 ASTRONOMY AND ASTROPHYSICS↗