arXiv subjects

Gilberto C. Gomez

Publications and source records attributed to Gilberto C. Gomez.

16 recordsLinked to original sources

arXiv2023

Direct observational evidence of the multi-scale, dynamical mass accretion toward a high-mass star forming hub-filament system

There is growing evidence that high-mass star formation and hub-filament systems (HFS) are intricately linked. The gas kinematics along the filaments and the forming high-mass star(s) in the central hub are in excellent agreement with the new generation of global hierarchical high-mass star formation models. In this paper, we present an observational investigation of a typical HFS cloud, G310.142+0.758 (G310 hereafter) which reveals unambiguous evidence of mass inflow from the cloud scale via the filaments onto the forming protostar(s) at the hub conforming with the model predictions. Continuum and molecular line data from the ATOMS and MALT90 surveys are used that cover different spatial scales. Three filaments (with total mass $5.7\pm1.1\times 10^3~M_{\odot}$) are identified converging toward the central hub region where several signposts of high-mass star formation have been observed. The hub region contains a massive clump ($1280\pm260~M_{\odot}$) harbouring a central massive core. Additionally, five outflow lobes are associated with the central massive core implying a forming cluster. The observed large-scale, smooth and coherent velocity gradients from the cloud down to the core scale, and the signatures of infall motion seen in the central massive clump and core, clearly unveil a nearly-continuous, multi-scale mass accretion/transfer process at a similar mass infall rate of $\sim 10^{-3}~M_{\odot}~yr^{-1}$ over all scales, feeding the central forming high-mass protostar(s) in the G310 HFS cloud.

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arXiv2022

ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions -- XI. From inflow to infall in hub-filament systems

We investigate the presence of hub-filament systems in a large sample of 146 active proto-clusters, using H$^{13}$CO$^{+}$ J=1-0 molecular line data obtained from the ATOMS survey. We find that filaments are ubiquitous in proto-clusters, and hub-filament systems are very common from dense core scales ($\sim$0.1 pc) to clump/cloud scales ($\sim$1-10 pc). The proportion of proto-clusters containing hub-filament systems decreases with increasing dust temperature ($T_d$) and luminosity-to-mass ratios ($L/M$) of clumps, indicating that stellar feedback from H{\sc ii} regions gradually destroys the hub-filament systems as proto-clusters evolve. Clear velocity gradients are seen along the longest filaments with a mean velocity gradient of 8.71 km s$^{-1}$pc$^{-1}$ and a median velocity gradient of 5.54 km s$^{-1}$pc$^{-1}$. We find that velocity gradients are small for filament lengths larger than $\sim$1~pc, probably hinting at the existence of inertial inflows, although we cannot determine whether the latter are driven by large-scale turbulence or large-scale gravitational contraction. In contrast, velocity gradients below $\sim$1~pc dramatically increase as filament lengths decrease, indicating that the gravity of the hubs or cores starts to dominate gas infall at small scales. We suggest that self-similar hub-filament systems and filamentary accretion at all scales may play a key role in high-mass star formation.

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arXiv2018

The magnetic field structure in molecular cloud filaments

We explore the structure of magnetic field lines in and around filaments in simulations of molecular clouds undergoing global, multi-scale gravitational collapse. In these simulations, filaments are not in a static equilibrium, but are long-lived flow structures that accrete gas from their environment and direct it toward clumps embedded in the filament or at the nodes at the conjunction with other filaments. In this context, the magnetic field is dragged by the collapsing gas, so its structure must reflect the flow that generates the filament. Around the filament, the gas is accreted onto it, and the magnetic lines must then be perpendicular to the filament. As the gas density increases, the gas flow changes direction, becoming almost parallel to the filament, and magnetic lines also tend to align with it. At the spine of the filament, however, magnetic lines become perpendicular again since they must connect to lines on the opposite side of the filament, resulting in "U"-shaped magnetic structures, which tend to be stretched by the longitudinal flow along the filament. Magnetic diffusive processes, however, allow the gas to continue to flow. Assuming a stationary state in which the ram pressure of the flow balances the magnetic tension, the curvature of the field lines is determined by the diffusion rate. We derive an expression relating the curvature of the field lines to the diffusive coefficient, which may be used to observationally determine the nature of the diffusive process.

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arXiv2014

Filaments in Simulations of Molecular Cloud Formation

We report on the filaments that develop self-consistently in a new numerical simulation of cloud formation by colliding flows. As in previous studies, the forming cloud begins to undergo gravitational collapse because it rapidly acquires a mass much larger than the average Jeans mass. Thus, the collapse soon becomes nearly pressureless, proceeding along its shortest dimension first. This naturally produces filaments in the cloud, and clumps within the filaments. The filaments are not in equilibrium at any time, but instead are long-lived flow features, through which the gas flows from the cloud to the clumps. The filaments are long-lived because they accrete from their environment while simultaneously accreting onto the clumps within them; they are essentially the locus where the flow changes from accreting in two dimensions to accreting in one dimension. Moreover, the clumps also exhibit a hierarchical nature: the gas in a filament flows onto a main, central clump, but other, smaller-scale clumps form along the infalling gas. Correspondingly, the velocity along the filament exhibits a hierarchy of jumps at the locations of the clumps. Two prominent filaments in the simulation have lengths ~15 pc, and masses ~600 Msun above density n ~ 10^3 cm-3 (~2x10^3 Msun at n > 50 cm-3). The density profile exhibits a central flattened core of size ~0.3 pc and an envelope that decays as r^-2.5, in reasonable agreement with observations. Accretion onto the filament reaches a maximum linear density rate of ~30 Msun Myr^-1 pc^-1.

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arXiv2013

Molecular Cloud Evolution V. Cloud Destruction by Stellar Feedback

We present a numerical study of the evolution of molecular clouds, from their formation by converging flows in the warm ISM, to their destruction by the ionizing feedback of the massive stars they form. We improve with respect to our previous simulations by including a different stellar-particle formation algorithm, which allows them to have masses corresponding to single stars rather than to small clusters, and with a mass distribution following a near-Salpeter stellar IMF. We also employ a simplified radiative-transfer algorithm that allows the stellar particles to feed back on the medium at a rate that depends on their mass and the local density. Our results are as follows: a) Contrary to the results from our previous study, where all stellar particles injected energy at a rate corresponding to a star of ~ 10 Msun, the dense gas is now completely evacuated from 10-pc regions around the stars within 10-20 Myr, suggesting that this feat is accomplished essentially by the most massive stars. b) At the scale of the whole numerical simulations, the dense gas mass is reduced by up to an order of magnitude, although star formation (SF) never shuts off completely, indicating that the feedback terminates SF locally, but new SF events continue to occur elesewhere in the clouds. c) The SF efficiency (SFE) is maintained globally at the ~ 10% level, although locally, the cloud with largest degree of focusing of its accretion flow reaches SFE ~ 30%. d) The virial parameter of the clouds approaches unity before the stellar feedback begins to dominate the dynamics, becoming much larger once feedback dominates, suggesting that clouds become unbound as a consequence of the stellar feedback. e) The erosion of the filaments that feed the star-forming clumps produces chains of isolated dense blobs reminiscent of those observed in the vicinity of the dark globule B68.

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arXiv2010

Molecular Cloud Evolution III. Accretion vs. stellar feedback

We numerically investigate the effect of feedback from the ionizing radiation heating from massive stars on the evolution of giant molecular clouds (GMCs) and their star formation efficiency (SFE). We find that the star-forming regions within the GMCs are invariably formed by gravitational contraction. 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. The competition of accretion against dense gas consumption by star formation (SF) and evaporation by the feedback, regulates the clouds' mass and energy balance, as well as their SFE. We find that, 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 SF rates (SFRs). However, we observe that the dense gas mass is larger in general in the presence of feedback, while the total (dense gas + stars) is nearly insensitive to the presence of feedback, suggesting that the total mass is determined by the accretion, while the feedback inhibits mainly the conversion of dense gas to stars. 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 ~ 10 pc. This naturally explains the larger observed SFEs of massive-star forming regions. We also find that 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 are the center of a large-scale collapse, continuously accreting mass, rather than being equilibrium entities.

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arXiv2009

Dependence of the Star Formation Efficiency on the Parameters of Molecular Cloud Formation Simulations

We investigate the response of the star formation efficiency (SFE) to the main parameters of simulations of molecular cloud formation by the collision of warm diffuse medium (WNM) cylindrical streams, neglecting stellar feedback and magnetic fields. The parameters we vary are the Mach number of the inflow velocity of the streams, Msinf, the rms Mach number of the initial background turbulence in the WNM, and the total mass contained in the colliding gas streams, Minf. Because the SFE is a function of time, we define two estimators for it, the "absolute" SFE, measured at t = 25 Myr into the simulation's evolution (sfeabs), and the "relative" SFE, measured 5 Myr after the onset of star formation in each simulation (sferel). The latter is close to the "star formation rate per free-fall time" for gas at n = 100 cm^-3. We find that both estimators decrease with increasing Minf, although by no more than a factor of 2 as Msinf increases from 1.25 to 3.5. Increasing levels of background turbulence similarly reduce the SFE, because the turbulence disrupts the coherence of the colliding streams, fragmenting the cloud, and producing small-scale clumps scattered through the numerical box, which have low SFEs. Finally, the SFE is very sensitive to the mass of the inflows, with sferel decreasing from ~0.4 to ~0.04 as the the virial parameter in the colliding streams increases from ~0.15 to ~1.5. This trend is in partial agreement with the prediction by Krumholz & McKee (2005), since the latter lies within the same range as the observed efficiencies, but with a significantly shallower slope. We conclude that the observed variability of the SFE is a highly sensitive function of the parameters of the cloud formation process, and may be the cause of significant scatter in observational determinations.

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arXiv2009

Effects of Non-Circular Motions on Azimuthal Color Gradients

Assuming that density waves trigger star formation, and that young stars preserve the velocity components of the molecular gas where they are born, we analyze the effects that non-circular gas orbits have on color gradients across spiral arms. We try two approaches, one involving semi-analytical solutions for spiral shocks, and another with magnetohydrodynamic (MHD) numerical simulation data. We find that, if non-circular motions are ignored, the comparison between observed color gradients and stellar population synthesis models would in principle yield pattern speed values that are systematically too high for regions inside corotation, with the difference between the real and the measured pattern speeds increasing with decreasing radius. On the other hand, image processing and pixel averaging result in systematically lower measured spiral pattern speed values, regardless of the kinematics of stellar orbits. The net effect is that roughly the correct pattern speeds are recovered, although the trend of higher measured $Ω_p$ at lower radii (as expected when non-circular motions exist but are neglected) should still be observed. We examine the Martinez-Garcia et al. (2009) photometric data and confirm that this is indeed the case. The comparison of the size of the systematic pattern speed offset in the data with the predictions of the semi-analytical and MHD models corroborates that spirals are more likely to end at Outer Lindblad Resonance, as these authors had already found.

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arXiv2009

High- and Low-Mass Star Forming Regions from Hierarchical Gravitational Fragmentation. High local Star Formation Rates with Low Global Efficiencies

We investigate the properties of "star forming regions" in a previously published numerical simulation of molecular cloud formation out of compressive motions in the warm neutral atomic interstellar medium, neglecting magnetic fields and stellar feedback. In this simulation, the velocity dispersions at all scales are caused primarily by infall motions rather than by random turbulence. We study the properties (density, total gas+stars mass, stellar mass, velocity dispersion, and star formation rate) of the cloud hosting the first local, isolated "star formation" event in the simulation and compare them with those of the cloud formed by a later central, global collapse event. We suggest that the small-scale, isolated collapse may be representative of low- to intermediate-mass star-forming regions, while the large-scale, massive one may be representative of massive star forming regions. We also find that the statistical distributions of physical properties of the dense cores in the region of massive collapse compare very well with those from a recent survey of the massive star forming region in the Cygnus X molecular cloud. The star formation efficiency per free-fall time (SFE_ff) of the high-mass SF clump is low, ~0.04. This occurs because the clump is accreting mass at a high rate, not because its specific SFR (SSFR) is low. This implies that a low value of the SFE_ff does not necessarily imply a low SSFR, but may rather indicate a large gas accretion rate. We suggest that a globally low SSFR at the GMC level can be attained even if local star forming sites have much larger values of the SSFR if star formation is a spatially intermittent process, so that most of the mass in a GMC is not participating of the SF process at any given time.

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arXiv2009

Tidal foces as a regulator of star formation in Taurus

Only a few molecular clouds in the Solar Neighborhood exhibit the formation of only low-mass stars. Traditionally, these clouds have been assumed to be supported against more vigorous collapse by magnetic fields. The existence of strong magnetic fields in molecular clouds, however, poses serious problems for the formation of stars and of the clouds themselves. In this {\em Letter}, we review the three-dimensional structure and kinematics of Taurus --the archetype of a region forming only low-mass stars-- as well as its orientation within the Milky way. We conclude that the particularly low star-formation efficiency in Taurus may naturally be explained by tidal forces from the Galaxy, with no need for magnetic regulation or stellar feedback.

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arXiv2006

Errors in kinematic distances and our image of the Milky Way Galaxy

Errors in the kinematic distances, under the assumption of circular gas orbits, were estimated by performing synthetic observations of a model disk galaxy. It was found that the error is < 0.5 kpc for most of the disk when the measured rotation curve was used, but larger if the real rotation curve is applied. In both cases, the error is significantly larger at the positions of the spiral arms. The error structure is such that, when kinematic distances are used to develope a picture of the large scale density distribution, the most significant features of the numerical model are significantly distorted or absent, while spurious structure appears. By considering the full velocity field in the calculation of the kinematic distances, most of the original density structures can be recovered.

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arXiv2004

3D MHD Modeling of the Gaseous Structure of the Galaxy: Description of the Simulations

The extra stiffness that the magnetic field adds to the ISM changes the way it reacts to the presence of a spiral perturbation. At intermediate to high z, the gas shoots up before the arm, flows over, and falls behind it, as it approaches the next arm. This generates a multicell circulation pattern, within each of which the net radial mass flux is positive near the midplane and negative at higher z. The flow distorts the magnetic field lines. In the arm region, the gas flows nearly parallel to the arm, and therefore, the magnetic field adopts a similar pitch angle. Between the arms, the gas flows out in radius, generating a negative pitch angle in the magnetic field. The intensity and direction of the field yield synthetic synchrotron maps that reproduce some features of the synchrotron maps of external galaxies, like the islands of emission and the displacement between the gaseous and synchrotron arms. When comparing the magnitude of the field with the local gas density, two distinctive relations appear, depending on whether the magnetic pressure is dominant. Above the plane, the density structure develops a shape resembling a breaking wave. This structure collapses and rises again with a period of about 60 Myr, similar to that of a vertical oscillation mode. The falling gas plays an important part in the overall hydrostatics, since its deceleration compresses the low z gas, raising the average midplane pressure in the interarm region above that provided by the weight of the material above.

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arXiv2004

3D MHD Modeling of the Gaseous Structure of the Galaxy: Synthetic Observations

We generated synthetic observations from the four-arm model presented in Gomez & Cox (2004) for the Galactic ISM in the presence of a spiral gravitational perturbation. We found that velocity crowding and diffusion have a strong effect in the l-v diagram. The v-b diagram presents structures at the expected spiral arm velocities, that can be explained by the off-the-plane structure of the arms presented in previous papers of this series. Such structures are observed in the Leiden/Dwingeloo HI survey. The rotation curve, as measured from the inside of the modeled galaxy, shows similarities with the observed one for the Milky Way Galaxy, although it has large deviations from the smooth circular rotation corresponding to the background potential. The magnetic field inferred from a synthetic synchrotron map shows a largely circular structure, but with interesting deviations in the midplane due to distortion of the field from circularity in the interarm regions.

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arXiv2002

Analytical Expressions for Spiral Arm Gravitational Potential and Density

When modeling the three-dimensional hydrodynamics of interstellar material rotating in a galactic gravitational potential, it is useful to have an analytic expression for gravitational perturbations due to stellar spiral arms. We present such an expression for which changes in the assumed characteristics of the arms can be made easily and the sensitivity of the hydrodynamics to those characteristics examined. This analytic expression also makes it easy to rotate the force field at the pattern angular velocity with little overhead on the calculations.

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arXiv2002

3D MHD Modeling of the Gaseous Structure of the Galaxy: Setup and Initial Results

We show the initial results of our 3D MHD simulations of the flow of the Galactic atmosphere as it responds to a spiral perturbation in the potential. In our standard case, as the gas approaches the arm, there is a downward converging flow that terminates in a complex of shocks just ahead of the midplane density peak. The density maximum slants forward at high z, preceeded by a similarly leaning shock. The latter diverts the flow upward and over the arm, as in a hydraulic jump. Behind the gaseous arm, the flow falls again, generating further secondary shocks as it approaches the lower z material. Structures similar to the high z part of the gaseous arms are found in the interarm region of our two-armed case, while broken arms and low column density bridges are present in the four-armed case. We present three examples of what can be learned from these models.

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arXiv2001

A Re-examination of the Distribution of Galactic Free Electrons

We present a list of 109 pulsars with independent distance information compiled from the literature. Since the compilation of Frail & Weisberg, there are 35 pulsars with new distance estimate and 25 pulsars for which the distance or distance uncertainty have been revised. We used this data to fit a smooth, axisymmetric, two disk model of the distribution of galactic electrons. The two exponential model components have mean local midplane densities at the solar circle of 2.03e-2 cm^-3 and 0.71e-2 cm^-3, and scale heights of 1.07 and 0.053 kpc. The thick component shows very little radial variation, while the second has a radial scale length of only a few kiloparsecs. We also examined a model which varies as sech^2(x), rather than exp(-x), in both the radial and vertical direction. We prefer this model with no midplane cusp, but find that the fit parameters essentially describe the same electron distribution. The distances predicted by this distribution have a similar scatter as the more complex model of Taylor & Cordes. We examine the pulsars that deviate strongly from this model. There are two regions of enhanced dispersion measure, one of which correlates well with the Sagittarius-Carina spiral arm. We find that the scatter of the observed dispersion measure from the model is not fit well by either a normal or log-normal distribution of lump sizes, but may be caused instead by the uncertainties in the distances.

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