Gravity drives the evolution of infrared dark hubs: JVLA observations of SDC13

Context. Converging networks of interstellar filaments, that is hubs, have been recently linked to the formation of stellar clusters and massive stars. Understanding the relationship between the evolution of these systems and the formation of cores and stars inside them is at the heart of current star formation research. Aims. The goal is to study the kinematic and density structure of the SDC13 prototypical hub at high angular resolution to determine what drives its evolution and fragmentation. Methods. We have mapped SDC13, a ~1000 M⊙ infrared dark hub, in NH3(1,1) and NH3(2,2) emission lines, with both the Jansky Very Large Array and Green Bank Telescope. The high angular resolution achieved in the combined dataset allowed us to probe scales down to 0.07 pc. After fitting the ammonia lines, we computed the integrated intensities, centroid velocities and line widths, along with gas temperatures and H2 column densities. Results. The mass-per-unit-lengths of all four hub filaments are thermally super-critical, consistent with the presence of tens of gravitationally bound cores identified along them. These cores exhibit a regular separation of ~0.37 ± 0.16 pc suggesting gravitational instabilities running along these super-critical filaments are responsible for their fragmentation. The observed local increase of the dense gas velocity dispersion towards starless cores is believed to be a consequence of such fragmentation process. Using energy conservation arguments, we estimate that the gravitational to kinetic energy conversion efficiency in the SDC13 cores is ~35%. We see velocity gradient peaks towards ~63% of cores as expected during the early stages of filament fragmentation. Another clear observational signature is the presence of the most massive cores at the filaments’ junction, where the velocity dispersion is largest. We interpret this as the result of the hub morphology generating the largest acceleration gradients near the hub centre. Conclusions. We propose a scenario for the evolution of the SDC13 hub in which filaments first form as post-shock structures in a supersonic turbulent flow. As a result of the turbulent energy dissipation in the shock, the dense gas within the filaments is initially mostly sub-sonic. Then gravity takes over and starts shaping the evolution of the hub, both fragmenting filaments and pulling the gas towards the centre of the gravitational well. By doing so, gravitational energy is converted into kinetic energy in both local (cores) and global (hub centre) potential well minima. Furthermore, the generation of larger gravitational acceleration gradients at the filament junctions promotes the formation of more massive cores.

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Hunting for intermediate-mass black holes in globular clusters: an astrometric study of NGC 6441

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab474 ◽

2021 ◽

Vol 503 (1) ◽

pp. 1490-1506

Author(s):

Maximilian Häberle ◽

Mattia Libralato ◽

Andrea Bellini ◽

Laura L Watkins ◽

Jörg-Uwe Pott ◽

...

Keyword(s):

Globular Clusters ◽

Velocity Dispersion ◽

Angular Resolution ◽

Hubble Space Telescope ◽

High Mass ◽

High Angular Resolution ◽

Stellar Kinematics ◽

Intermediate Mass ◽

The Core ◽

Large Telescopes

ABSTRACT We present an astrometric study of the proper motions (PMs) in the core of the globular cluster NGC 6441. The core of this cluster has a high density and observations with current instrumentation are very challenging. We combine ground-based, high-angular-resolution NACO@VLT images with Hubble Space Telescope ACS/HRC data and measure PMs with a temporal baseline of 15 yr for about 1400 stars in the centremost 15 arcsec of the cluster. We reach a PM precision of ∼30 µas yr−1 for bright, well-measured stars. Our results for the velocity dispersion are in good agreement with other studies and extend already existing analyses of the stellar kinematics of NGC 6441 to its centremost region never probed before. In the innermost arcsecond of the cluster, we measure a velocity dispersion of (19.1 ± 2.0) km s−1 for evolved stars. Because of its high mass, NGC 6441 is a promising candidate for harbouring an intermediate-mass black hole (IMBH). We combine our measurements with additional data from the literature and compute dynamical models of the cluster. We find an upper limit of $M_{\rm IMBH} \lt 1.32 \times 10^4\, \textrm{M}_\odot$ but we can neither confirm nor rule out its presence. We also refine the dynamical distance of the cluster to $12.74^{+0.16}_{-0.15}$ kpc. Although the hunt for an IMBH in NGC 6441 is not yet concluded, our results show how future observations with extremely large telescopes will benefit from the long temporal baseline offered by existing high-angular-resolution data.

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Ammonia observations towards the Aquila Rift cloud complex

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037659 ◽

2020 ◽

Vol 643 ◽

pp. A178

Author(s):

Kadirya Tursun ◽

Jarken Esimbek ◽

Christian Henkel ◽

Xindi Tang ◽

Gang Wu ◽

...

Keyword(s):

Star Formation ◽

Dust Emission ◽

Turbulent Energy ◽

Gravitational Energy ◽

High Mass ◽

Kinetic Temperature ◽

Mass Star ◽

Dense Gas ◽

Star Formation Region ◽

Formation Region

We surveyed the Aquila Rift complex including the Serpens South and W 40 regions in the NH3 (1,1) and (2,2) transitions making use of the Nanshan 26-m telescope. Our observations cover an area of ~ 1.5° × 2.2° (11.4 pc × 16.7 pc). The kinetic temperatures of the dense gas in the Aquila Rift complex obtained from NH3 (2,2)/(1,1) ratios range from 8.9 to 35.0 K with an average of 15.3 ± 6.1 K (errors are standard deviations of the mean). Low gas temperatures are associated with Serpens South ranging from 8.9 to 16.8 K with an average of 12.3 ± 1.7 K, while dense gas in the W 40 region shows higher temperatures ranging from 17.7 to 35.0 K with an average of 25.1 ± 4.9 K. A comparison of kinetic temperatures derived from para-NH3 (2,2)/(1,1) against HiGal dust temperatures indicates that the gas and dust temperatures are in agreement in the low-mass-star formation region of Serpens South. In the high-mass-star formation region W 40, the measured gas kinetic temperatures are higher than those of the dust. The turbulent component of the velocity dispersion of NH3 (1,1) is found to be positively correlated with the gas kinetic temperature, which indicates that the dense gas may be heated by dissipation of turbulent energy. For the fractional total-NH3 (para+ortho) abundance obtained by a comparison with Herschel infrared continuum data representing dust emission, we find values from 0.1 ×10−8 to 2.1 ×10−7 with an average of 6.9 (±4.5) × 10−8. Serpens South also shows a fractional total-NH3 (para+ortho) abundance ranging from 0.2 ×10−8 to 2.1 ×10−7 with an average of 8.6 (±3.8) × 10−8. In W 40, values are lower, between 0.1 and 4.3 ×10−8 with an average of 1.6 (±1.4) × 10−8. Weak velocity gradients demonstrate that the rotational energy is a negligible fraction of the gravitational energy. In W 40, gas and dust temperatures are not strongly dependent on the projected distance to the recently formed massive stars. Overall, the morphology of the mapped region is ring-like, with strong emission at lower and weak emission at higher Galactic longitudes. However, the presence of a physical connection between the two parts remains questionable.

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The Structure of the Neutral Interstellar Medium: A Theory of Interstellar Turbulence

Symposium - International Astronomical Union ◽

10.1017/s0074180900230374 ◽

1996 ◽

Vol 169 ◽

pp. 591-596

Author(s):

Anthony Whitworth

Keyword(s):

Kinetic Energy ◽

Interstellar Medium ◽

Velocity Dispersion ◽

Second Law Of Thermodynamics ◽

Turbulent Energy ◽

Density Structure ◽

Interstellar Turbulence ◽

Kolmogorov Turbulence ◽

Effective Specific Heat ◽

Mass Size

We present a model for the development of density structure in the neutral interstellar medium. In this model, bulk kinetic energy is injected mainly at small scales, by jets and expanding nebulae generated by young and/or massive stars. Subsequently, this bulk kinetic energy propagates to, and is dissipated on, larger scales.This is in contrast to standard (incompressible) Kolmogorov turbulence, where kinetic energy is injected at larger scales, then propagates to, and is dissipated on, smaller scales. The sense in which the Second Law of Thermodynamics drives the propagation of turbulent energy is reversed in the interstellar medium because virialized self-gravitating gas clumps and ensembles of clumps (clouds) have negative effective specific heat.The model is able to explain Larson's relations (between the mass, size, and velocity-dispersion of molecular clouds and clumps; Larson 1981), the maximum masses of giant molecular cloud complexes, the velocity dispersions observed (at low resolution) in face-on spirals like the Milky Way, and the apparent scaling of the interstellar magnetic field with density.

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Extreme fragmentation and complex kinematics at the center of the L1287 cloud

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834173 ◽

2019 ◽

Vol 621 ◽

pp. A140 ◽

Cited By ~ 1

Author(s):

Carmen Juárez ◽

Hauyu Baobab Liu ◽

Josep M. Girart ◽

Aina Palau ◽

Gemma Busquet ◽

...

Keyword(s):

Velocity Gradient ◽

Angular Resolution ◽

Spatial Scales ◽

Young Stellar Objects ◽

Continuum Emission ◽

High Angular Resolution ◽

Dense Gas ◽

Continuum Image ◽

Velocity Gradients ◽

Low Mass

Aims. The filamentary ~10-pc-scale infrared dark cloud L1287 located at a parallax distance of ~929 pc is actively forming a dense cluster of low-mass young stellar objects (YSOs) at its inner ~0.1 pc region. To help understand the origin of this low-mass YSO cluster, the present work aims at resolving the gas structures and kinematics with high angular resolution. Methods. We performed ~1′′ angular resolution (~930 AU) observations at ~1.3 mm wavelengths using the Submillimeter Array (SMA), which simultaneously cover the dust continuum emission and various molecular line tracers for dense gas, warm gas, shocks, and outflows. Results. From a 1.3-mm continuum image with a resolution of ~2′′ we identified six dense cores, namely SMA1-6. Their gas masses are in the range of ~0.4–4 M⊙. From a 1.3-mm continuum image with a resolution of ~1′′, we find a high fragmentation level, with 14 compact millimeter sources within 0.1 pc: SMA3 contains at least nine internal condensations; SMA5 and SMA6 are also resolved with two internal condensations. Intriguingly, one condensation in SMA3 and another in SMA5 appear associated with the known accretion outburst YSOs RNO 1C and RNO 1B. The dense gas tracer DCN (3–2) well traces the dust continuum emission and shows a clear velocity gradient along the NW-SE direction centered at SMA3. There is another velocity gradient with opposite direction around the most luminous YSO, IRAS 00338 + 6312. Conclusions. The fragmentation within 0.1 pc in L1287 is very high compared to other regions at the same spatial scales. The incoherent motions of dense gas flows are sometimes interpreted by being influenced by (proto)stellar feedback (e.g., outflows), which is not yet ruled out in this particular target source. On the other hand, the velocities (with respect to the systemic velocity) traced by DCN are small, and the directions of the velocity gradients traced by DCN are approximately perpendicular to those of the dominant CO outflow(s). Therefore, we alternatively hypothesize that the velocity gradients revealed by DCN trace the convergence from the ≳0.1 pc scales infalling motion towards the rotational motions around the more compact (~0.02 pc) sources. This global molecular gas converging flow may feed the formation of the dense low-mass YSO cluster. Finally, we also found that IRAS 00338 + 6312 is the most likely powering source of the dominant CO outflow. A compact blue-shifted outflow from RNO 1C is also identified.

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Small-Angle Electron Scattering (SAES) of Polymers

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100117467 ◽

1985 ◽

Vol 43 ◽

pp. 78-79

Author(s):

Ralph Oralor ◽

Pamela Lloyd ◽

Satish Kumar ◽

W. W. Adams

Keyword(s):

Electron Scattering ◽

Small Angle ◽

Angular Resolution ◽

Structural Features ◽

Objective Lens ◽

High Angular Resolution ◽

Push Button ◽

First Case ◽

Diffraction Mode ◽

Very High

Small angle electron scattering (SAES) has been used to study structural features of up to several thousand angstroms in polymers, as well as in metals. SAES may be done either in (a) long camera mode by switching off the objective lens current or in (b) selected area diffraction mode. In the first case very high camera lengths (up to 7Ø meters on JEOL 1Ø ØCX) and high angular resolution can be obtained, while in the second case smaller camera lengths (approximately up to 3.6 meters on JEOL 1Ø ØCX) and lower angular resolution is obtainable. We conducted our SAES studies on JEOL 1ØØCX which can be switched to either mode with a push button as a standard feature.

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Large-angle convergent-beam electron-diffraction study of coherent precipitates and decorated dislocations

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100167470 ◽

1996 ◽

Vol 54 ◽

pp. 1002-1004

Author(s):

J.M.K. Wiezorek ◽

H.L. Fraser

Keyword(s):

Electron Diffraction ◽

Angular Resolution ◽

Large Angle ◽

Electron Diffraction Study ◽

Convergent Beam Electron Diffraction ◽

Crystal Defects ◽

High Angular Resolution ◽

Beam Electron ◽

Diffraction Plane ◽

Convergent Beam

Conventional methods of convergent beam electron diffraction (CBED) use a fully converged probe focused on the specimen in the object plane resulting in the formation of a CBED pattern in the diffraction plane. Large angle CBED (LACBED) uses a converged but defocused probe resulting in the formation of ‘shadow images’ of the illuminated sample area in the diffraction plane. Hence, low-spatial resolution image information and high-angular resolution diffraction information are superimposed in LACBED patterns which enables the simultaneous observation of crystal defects and their effect on the diffraction pattern. In recent years LACBED has been used successfully for the investigation of a variety of crystal defects, such as stacking faults, interfaces and dislocations. In this paper the contrast from coherent precipitates and decorated dislocations in LACBED patterns has been investigated. Computer simulated LACBED contrast from decorated dislocations and coherent precipitates is compared with experimental observations.

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