Testing the Larson relations in massive clumps Traficante, A; Duarte-Cabral, A; Elia, D ...
Monthly notices of the Royal Astronomical Society,
06/2018, Letnik:
477, Številka:
2
Journal Article
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Abstract
We tested the validity of the three Larson relations in a sample of 213 massive clumps selected from the Herschel infrared Galactic Plane (Hi-GAL) survey, also using data from the Millimetre ...Astronomy Legacy Team 90 GHz (MALT90) survey of 3-mm emission lines. The clumps are divided into five evolutionary stages so that we can also discuss the Larson relations as a function of evolution. We show that this ensemble does not follow the three Larson relations, regardless of the clump's evolutionary phase. A consequence of this breakdown is that the dependence of the virial parameter αvir on mass (and radius) is only a function of the gravitational energy, independent of the kinetic energy of the system; thus, αvir is not a good descriptor of clump dynamics. Our results suggest that clumps with clear signatures of infall motions are statistically indistinguishable from clumps with no such signatures. The observed non-thermal motions are not necessarily ascribed to turbulence acting to sustain the gravity, but they might be a result of the gravitational collapse at the clump scales. This seems to be particularly true for the most massive (M ≥ 1000 M⊙) clumps in the sample, where exceptionally high magnetic fields might not be enough to stabilize the collapse.
The relative importance of primordial molecular cloud fragmentation versus large-scale accretion still remains to be assessed in the context of massive core/star formation. Studying the kinematics of ...the dense gas surrounding massive-star progenitors can tell us the extent to which large-scale flow of material impacts the growth in mass of star-forming cores. Here we present a comprehensive dataset of the 5500(±800) M⊙ infrared dark cloud SDC335.579-0.272 (hereafter SDC335), which exhibits a network of cold, dense, parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0 observations reveal two massive star-forming cores, MM1 and MM2, sitting at the centre of SDC335 where the filaments intersect. With a gas mass of 545(-385+770) M⊙ contained within a source diameter of 0.05 pc, MM1 is one of the most massive, compact protostellar cores ever observed in the Galaxy. As a whole, SDC335 could potentially form an OB cluster similar to the Trapezium cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense gas furthermore reveal that the kinematics of this hub-filament system are consistent with a global collapse of the cloud. These molecular-line data point towards an infall velocity Vinf = 0.7( ± 0.2) km s-1, and a total mass infall rate Ṁinf ≃ 2.5(±1.0) × 10-3 M⊙ yr-1 towards the central pc-size region of SDC335. This infall rate brings 750(±300) M⊙ of gas to the centre of the cloud per free-fall time (tff = 3 × 105 yr). This is enough to double the mass already present in the central pc-size region in 3.5-1.0+2.2 × tff. These values suggest that the global collapse of SDC335 over the past million year resulted in the formation of an early O-type star progenitor at the centre of the cloud’s gravitational potential well.
ABSTRACT The evolutionary classification of massive clumps that are candidate progenitors of high-mass young stars and clusters relies on a variety of independent diagnostics based on observables ...from the near-infrared to the radio. A promising evolutionary indicator for massive and dense cluster-progenitor clumps is the L/M ratio between the bolometric luminosity and the mass of the clumps. With the aim of providing a quantitative calibration for this indicator, we used SEPIA/APEX to obtain CH3C2H(J = 12-11) observations, which is an excellent thermometer molecule probing densities ≥ 10 5 cm−3, toward 51 dense clumps with M ≥ 1000 M and uniformly spanning −2 Log(L/M) L /M 2.3. We identify three distinct ranges of L/M that can be associated to three distinct phases of star formation in massive clumps. For L/M ≤ 1 no clump is detected in CH3C2H, suggesting an inner envelope temperature below ∼30K. For 1 L/M 10 we detect 58% of the clumps with a temperature between ∼30 and ∼35 K independently from the exact value of L/M; such clumps are building up luminosity due to the formation of stars, but no star is yet able to significantly heat the inner clump regions. For L/M 10 we detect all the clumps with a gas temperature rising with Log(L/M), marking the appearance of a qualitatively different heating source within the clumps; such values are found toward clumps with UCH ii counterparts, suggesting that the quantitative difference in T versus L/M behavior above L/M ∼ 10 is due to the first appearance of ZAMS stars in the clumps.
The conversion of gas into stars is a fundamental process in astrophysics and cosmology. Stars are known to form from the gravitational collapse of dense clumps in interstellar molecular clouds, and ...it has been proposed that the resulting star formation rate is proportional to either the amount of mass above a threshold gas surface density, or the gas volume density. These star formation prescriptions appear to hold in nearby molecular clouds in our Milky Way Galaxy's disc as well as in distant galaxies where the star formation rates are often much larger. The inner 500 pc of our Galaxy, the Central Molecular Zone (CMZ), contains the largest concentration of dense, high-surface density molecular gas in the Milky Way, providing an environment where the validity of star formation prescriptions can be tested. Here, we show that by several measures, the current star formation rate in the CMZ is an order-of-magnitude lower than the rates predicted by the currently accepted prescriptions. In particular, the region 1° < l < 3°.5, |b| < 0°.5 contains ∼107 M of dense (> several 103 cm−3) molecular gas - enough to form 1000 Orion-like clusters - but the present-day star formation rate within this gas is only equivalent to that in Orion. In addition to density, another property of molecular clouds must be included in the star formation prescription to predict the star formation rate in a given mass of molecular gas. We discuss which physical mechanisms might be responsible for suppressing star formation in the CMZ.
The electronic properties of interfaces between two different solids can differ strikingly from those of the constituent materials. For instance, metallic conductivity-and even superconductivity-have ...recently been discovered at interfaces formed by insulating transition-metal oxides. Here, we investigate interfaces between crystals of conjugated organic molecules, which are large-gap undoped semiconductors, that is, essentially insulators. We find that highly conducting interfaces can be realized with resistivity ranging from 1 to 30 kohms per square, and that, for the best samples, the temperature dependence of the conductivity is metallic. The observed electrical conduction originates from a large transfer of charge between the two crystals that takes place at the interface, on a molecular scale. As the interface assembly process is simple and can be applied to crystals of virtually any conjugated molecule, the conducting interfaces described here represent the first examples of a new class of electronic systems.
Abstract
The composition of giant planets is imprinted by their migration history and the compositional structure of their hosting disks. Studies in recent literature have investigated how the ...abundances of C and O can constrain the formation pathways of giant planets forming within few tens of au from a star. New ALMA observations, however, suggest planet-forming regions possibly extending to hundreds of au. We explore the implications of these wider formation environments through
n
-body simulations of growing and migrating giant planets embedded in planetesimal disks, coupled with a compositional model of the protoplanetary disk where volatiles are inherited from the molecular cloud and refractories are calibrated against extrasolar and Solar System data. We find that the C/O ratio provides limited insight on the formation pathways of giant planets that undergo large-scale migration. This limitation can be overcome, however, thanks to nitrogen and sulfur. Jointly using the C/N, N/O, and C/O ratios breaks any degeneracy in the formation and migration tracks of giant planets. The use of elemental ratios normalized to the respective stellar ratios supplies additional information on the nature of giant planets, thanks to the relative volatility of O, C, and N in disks. When the planetary metallicity is dominated by the accretion of solids C/N* > C/O* > N/O* (* denoting this normalized scale), otherwise N/O* > C/O* > C/N*. The S/N ratio provides an additional independent probe into the metallicity of giant planets and their accretion of solids.
Nutritional Programming (NP) has been shown to counteract the negative effects of dietary plant protein (PP) by introducing PP at an early age towards enhancement of PP utilization during later life ...stages. This study explored the effect of NP and its induction time on growth, expression of appetite-stimulating hormones, and any morphological changes in the gut possibly responsible for improved dietary PP utilization. At 3 days post-hatch (dph) zebrafish were distributed into 12 (3 L) tanks, 100 larvae per tank. This study included four groups: 1) The control (NP-FM) group received fishmeal (FM)-based diet from 13-36 dph and was challenged with PP-based diet during 36-66 dph; 2) The NP-PP group received NP with dietary PP in larval stage via live food enrichment during 3-13 dph followed by FM diet during 13-36 dph and PP diet during 36-66 dph; 3) The T-NP group received NP between 13-23 dph through PP diet followed by FM diet during 23-36 dph and PP diet during 36-66 dph; and 4) The PP group received PP diet from 13-66 dph. During the PP challenge the T-NP group achieved the highest weight gain compared to control and PP. Ghrelin expression in the brain was higher in T-NP compared to NP-FM and NP-PP, while in the gut it was reduced in both NP-PP and T-NP groups. Cholecystokinin expression showed an opposite trend to ghrelin. The brain neuropeptide Y expression was lower in NP-PP compared to PP but not different with NP-FM and T-NP groups. The highest villus length to width ratio in the middle intestine was found in T-NP compared to all other groups. The study suggests that NP induced during juvenile stages improves zebrafish growth and affects digestive hormone regulation and morphology of the intestinal lining-possible mechanisms behind the improved PP utilization in pre-adult zebrafish stages.
Super star clusters are the end product of star formation under the most extreme conditions. As such, studying how their final stellar populations are assembled from their natal progenitor gas clouds ...can provide strong constraints on star formation theories. An obvious place to look for the initial conditions of such extreme stellar clusters is gas clouds of comparable mass and density, with no star formation activity. We present a method to identify such progenitor gas clouds and demonstrate the technique for the gas in the inner few hundred pc of our Galaxy. The method highlights three clouds in the region with similar global physical properties to the previously identified extreme cloud, G0.253 + 0.016, as potential young massive cluster (YMC) precursors. The fact that four potential YMC progenitor clouds have been identified in the inner 100 pc of the Galaxy, but no clouds with similar properties have been found in the whole first quadrant despite extensive observational efforts, has implications for cluster formation/destruction rates across the Galaxy. We put forward a scenario to explain how such dense gas clouds can arise in the Galactic Centre environment, in which YMC formation is triggered by gas streams passing close to the minimum of the global Galactic gravitational potential at the location of the central supermassive black hole, Sgr A*. If this triggering mechanism can be verified, we can use the known time interval since closest approach to Sgr A* to study the physics of stellar mass assembly in an extreme environment as a function of absolute time.
Context. The mechanism of formation of massive stars is still a matter of debate. It is not yet clear if it can be considered to be a scaled-up analogue of the low-mass star regime, or if there are ...additional agents like merging of lower-mass forming objects or accretion from initially unbound material. Most of the uncertainties come from the lack of diagnostic tools to evolutionarily classify large samples of candidate massive protostellar objects that can then be studied in more detail. Aims. We want to verify whether diagnostic tools like the SED shape and the relationship between envelope mass and bolometric luminosity can be extended to the study of high-mass star formation. Methods. The 8-1200 μm SED of YSOs in 42 regions of massive star formation has been reconstructed using MSX, IRAS, and submm data partly available from previous works. Apart from IRAS catalogue fluxes, the fluxes in the Mid-IR and sub-mm/mm were derived directly from the images. The SEDs were fitted to an extensive grid of envelope models with embedded ZAMS stars, available from the literature. Sources that could not be fitted with a single model were then fitted with a two-component model composed of an embedded ZAMS for the mid-IR part and a single-temperature optically thin greybody for the longer wavelength emitting component. Sources were classified as “IR” if they were fitted with an embedded ZAMS envelope, and “MM” if they could only be fitted with a greybody with a peak at high λ; further subclassification was based on being the most massive object in the field (“P”, for primary) or not (“S”, for secondary). Results. The different classes of sources identified in our analysis have very different SEDs and occupy distinct areas in the $L_\mathrm{\rm bol}{-}M_\mathrm{env}$ diagram; by analogy with the low-mass regime, we see that MM-P, IR-P and IR-S objects could be interpreted as the high-mass analogue of Class 0-I-II. Evolutionary tracks obtained from a simple model based on the turbulent core prescriptions show that the three classes of sources possibly mark different periods in the formation of a massive YSO. The IR-P objects are consistent with being at the end of the main accretion phase, while MM-P sources are probably in an earlier evolutionary stage. The timescales for the formation decrease from ~ 4$\times$105 to ~ 1$\times$105 years with stellar mass increasing from ~6 to ~40 $M_{\odot}$; these timescales, and the association with young clusters with median stellar age of a few 106 years suggest that the most massive objects are among the last ones to form. Conclusions. Our results are consistent with the high-mass star formation being a scaled-up analogue of the traditional accretion-dominated paradigm valid for the low-mass regime.
The dynamics of massive clumps, the environment where massive stars originate, is still unclear. Many theories predict that these regions are in a state of near-virial equilibrium, or near energy ...equi-partition, while others predict that clumps are in a sub-virial state. Observationally, the majority of the massive clumps are in a sub-virial state with a clear anti-correlation between the virial parameter αvir and the mass of the clumps Mc, which suggests that the more massive objects are also the more gravitationally bound. Although this trend is observed at all scales, from massive clouds down to star-forming cores, theories do not predict it. In this work we show how, starting from virialized clumps, an observational bias is introduced in the specific case where the kinetic and the gravitational energies are estimated in different volumes within clumps and how it can contribute to the spurious αvir-Mc anti-correlation in these data. As a result, the observed effective virial parameter α ∼ eff $ \tilde{\alpha}_{\mathrm{eff}} $ < αvir, and in some circumstances it might not be representative of the virial state of the observed clumps.
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