53 research outputs found
Diffusion in supersonic, turbulent, compressible flows
We investigate diffusion in supersonic, turbulent, compressible flows.
Supersonic turbulence can be characterized as network of interacting shocks. We
consider flows with different rms Mach numbers and where energy necessary to
maintain dynamical equilibrium is inserted at different spatial scales. We find
that turbulent transport exhibits super-diffusive behavior due to induced bulk
motions. In a comoving reference frame, however, diffusion behaves normal and
can be described by mixing length theory extended into the supersonic regime.Comment: 11 pages, incl. 5 figures, accepted for publication in Physical
Review E (a high-resolution version is available at
http://www.aip.de./~ralf/Publications/p21.abstract.html
The effect of the dynamical state of clusters on gas expulsion and infant mortality
The star formation efficiency (SFE) of a star cluster is thought to be the
critical factor in determining if the cluster can survive for a significant
(>50 Myr) time. There is an often quoted critical SFE of ~30 per cent for a
cluster to survive gas expulsion. I reiterate that the SFE is not the critical
factor, rather it is the dynamical state of the stars (as measured by their
virial ratio) immediately before gas expulsion that is the critical factor. If
the stars in a star cluster are born in an even slightly cold dynamical state
then the survivability of a cluster can be greatly increased.Comment: 6 pages, 2 figures. Review talk given at the meeting on "Young
massive star clusters - Initial conditions and environments", E. Perez, R. de
Grijs, R. M. Gonzalez Delgado, eds., Granada (Spain), September 2007,
Springer: Dordrecht. Replacement to correct mistake in a referenc
Simulating Turbulence Using the Astrophysical Discontinuous Galerkin Code TENET
In astrophysics, the two main methods traditionally in use for solving the
Euler equations of ideal fluid dynamics are smoothed particle hydrodynamics and
finite volume discretization on a stationary mesh. However, the goal to
efficiently make use of future exascale machines with their ever higher degree
of parallel concurrency motivates the search for more efficient and more
accurate techniques for computing hydrodynamics. Discontinuous Galerkin (DG)
methods represent a promising class of methods in this regard, as they can be
straightforwardly extended to arbitrarily high order while requiring only small
stencils. Especially for applications involving comparatively smooth problems,
higher-order approaches promise significant gains in computational speed for
reaching a desired target accuracy. Here, we introduce our new astrophysical DG
code TENET designed for applications in cosmology, and discuss our first
results for 3D simulations of subsonic turbulence. We show that our new DG
implementation provides accurate results for subsonic turbulence, at
considerably reduced computational cost compared with traditional finite volume
methods. In particular, we find that DG needs about 1.8 times fewer degrees of
freedom to achieve the same accuracy and at the same time is more than 1.5
times faster, confirming its substantial promise for astrophysical
applications.Comment: 21 pages, 7 figures, to appear in Proceedings of the SPPEXA
symposium, Lecture Notes in Computational Science and Engineering (LNCSE),
Springe
The Cloud Factory II: gravoturbulent kinematics of resolved molecular clouds in a galactic potential
Interstellar matter and star formatio
The HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and Hi overview
Context. The Galactic plane has been observed extensively by a large number of Galactic plane surveys from infrared to radio wavelengths at an angular resolution below 40". However, a 21 cm line and continuum survey with comparable spatial resolution is still missing. Aims. The first half of THOR data (l = 14.0 37.9, and l = 47.1 51.2, |b| < 1.25) has been published in our data release 1 paper (Beuther et al. 2016). With this data release 2 paper, we publish all the remaining spectral line data and Stokes I continuum data with high angular resolution (1000–4000) including a new H i dataset for the whole THOR survey region (l = 14.0 67.4 and |b| < 1.25). As we have published the results of OH lines and continuum emission elsewhere, we concentrate on the H i analysis in this paper. Methods. With the Karl G. Jansky Very Large Array (VLA) in C-configuration, we observed a large portion of the first Galactic quadrant achieving an angular resolution of < 40. At L Band, the WIDAR correlator at the VLA was set to cover the 21 cm H i line, four OH transitions, a series of Hn↵ radio recombination lines (RRLs; n = 151 to 186), and eight 128 MHz wide continuum spectral windows (SPWs) simultaneously. Results. We publish all OH and RRL data from the C-configuration observations, and a new H i dataset combining VLA C+D+GBT (VLA D-configuration and GBT data are from the VLA Galactic Plane Survey, Stil et al. 2006) for the whole survey. The H i emission shows clear filamentary substructures at negative velocities with low velocity crowding. The emission at positive velocities is more smeared-out likely due to higher spatial and velocity crowding of structures at the positive velocities. Comparing to the spiral arm model of the Milky Way, the atomic gas follows the Sagittarius and Perseus Arm well but with significant material in the inter-arm regions. With the C-configuration-only H i+continuum data, we produced a H i optical depth map of the THOR areal coverage from 228 absorption spectra with the nearest-neighbor method. With this ⌧ map, we corrected the H i emission for optical depth and the derived column density is 38% higher than the column density with optically thin assumption. The total H i mass with optical depth correction in the survey region is 4.7⇥108 M, 31% more than the mass derived assuming the emission is optically thin. If we apply this 31% correction to the whole Milky Way, the total atomic gas mass would be 9.4–10.5⇥109 M. Comparing the H i with existing CO data, we find a significant increase in the atomic-to-molecular gas ration from the spiral arms to the inter-arm regions. Conclusions. The high sensitivity and resolution THOR H i dataset provides an important new window on the physical and kinematic properties of gas in the inner Galaxy. Although the optical depth we derive is a lower limit, our study shows that the optical depth correction is significant for H i column density and mass estimation. Together with the OH, RRL and continuum emission from the THOR survey, these new H i data provide the basis for high angular-resolution studies of the interstellar medium (ISM) in different phases
Star Formation in Molecular Clouds
Star formation is one of the least understood processes in cosmic evolution.
It is difficult to formulate a general theory for star formation in part
because of the wide range of physical processes involved. The interstellar gas
out of which stars form is a supersonically turbulent plasma governed by
magnetohydrodynamics. This is hard enough by itself, since we do not understand
even subsonic hydrodynamic turbulence very well, let alone supersonic non-ideal
MHD turbulence. However, the behavior of star-forming clouds in the ISM is also
obviously influenced by gravity, which adds complexity, and by both continuum
and line radiative processes. Finally, the behavior of star-forming clouds is
influenced by a wide variety of chemical processes, including formation and
destruction of molecules and dust grains (which changes the thermodynamic
behavior of the gas) and changes in ionization state (which alter how strongly
the gas couples to magnetic fields). As a result of these complexities, there
is nothing like a generally agreed-upon theory of star formation, as there is
for stellar structure. Instead, we are forced to take a much more
phenomenological approach. These notes provide an introduction to our current
thinking about how star formation works.Comment: To appear in the XVth Special Courses of the National Observatory of
Rio de Janeiro, 49 pages, 11 figures, AIP conference format. This is a
pedagogic introduction to star formation science, intended for beginning grad
students or advanced undergraduate
Interstellar MHD Turbulence and Star Formation
This chapter reviews the nature of turbulence in the Galactic interstellar
medium (ISM) and its connections to the star formation (SF) process. The ISM is
turbulent, magnetized, self-gravitating, and is subject to heating and cooling
processes that control its thermodynamic behavior. The turbulence in the warm
and hot ionized components of the ISM appears to be trans- or subsonic, and
thus to behave nearly incompressibly. However, the neutral warm and cold
components are highly compressible, as a consequence of both thermal
instability in the atomic gas and of moderately-to-strongly supersonic motions
in the roughly isothermal cold atomic and molecular components. Within this
context, we discuss: i) the production and statistical distribution of
turbulent density fluctuations in both isothermal and polytropic media; ii) the
nature of the clumps produced by thermal instability, noting that, contrary to
classical ideas, they in general accrete mass from their environment; iii) the
density-magnetic field correlation (or lack thereof) in turbulent density
fluctuations, as a consequence of the superposition of the different wave modes
in the turbulent flow; iv) the evolution of the mass-to-magnetic flux ratio
(MFR) in density fluctuations as they are built up by dynamic compressions; v)
the formation of cold, dense clouds aided by thermal instability; vi) the
expectation that star-forming molecular clouds are likely to be undergoing
global gravitational contraction, rather than being near equilibrium, and vii)
the regulation of the star formation rate (SFR) in such gravitationally
contracting clouds by stellar feedback which, rather than keeping the clouds
from collapsing, evaporates and diperses them while they collapse.Comment: 43 pages. Invited chapter for the book "Magnetic Fields in Diffuse
Media", edited by Elisabete de Gouveia dal Pino and Alex Lazarian. Revised as
per referee's recommendation
TOPoS: III. An ultra iron-poor CEMP multiple system
Aims. One of the primary objectives of the TOPoS survey is to search for the most metal-poor stars. Our search has led to the discovery of one of the most iron-poor objects known, SDSS J092912.32+023817.0. This object is a multiple system, in which two components are clearly detected in the spectrum. Methods. We have analysed 16 high-resolution spectra obtained using the UVES spectrograph at the ESO 8.2 m VLT telescope to measure radial velocities and determine the chemical composition of the system. Results. Cross correlation of the spectra with a synthetic template yields a double-peaked cross-correlation function (CCF) for eight spectra, and in one case there is evidence for the presence of a third peak. Chemical analysis of the spectrum obtained by averaging all the spectra for which the CCF showed a single peak found that the iron abundance is [Fe/H]=–4.97. The system is also carbon enhanced with [C/Fe]=+3.91 (A(C) = 7.44). From the permitted oxygen triplet we determined an upper limit for oxygen of [O/Fe] 1.3. We are also able to provide more stringent upper limits on the Sr and Ba abundances ([Sr/Fe] < +0.70, and [Ba/Fe] < +1.46, respectively)
Feedback in W94A diagnosed with Radio Recombination Lines and Models
We present images of radio recombination lines (RRLs) at wavelengths around 18 cm from the star-forming region W49A to determine the kinematics of ionized gas in the THOR survey (The Hi/OH/Recombination line survey of the inner Milky Way) at an angular resolution of 16:08 X 13:08. The distribution of ionized gas appears to be affected by feedback processes from the star clusters inW49A. The velocity structure of the RRLs shows a complex behaviour with respect to the molecular gas. We find a shell-like distribution of ionized gas as traced by RRL emission surrounding the central cluster of OB stars in W49A. We describe the evolution of the shell with the recent feedback model code warpfield that includes the important physical processes and has previously been applied to the 30 Doradus region in the Large Magellanic Cloud. The cloud structure and dynamics of W49A are in agreement with a feedbackdriven shell that is re-collapsing. The shell may have triggered star formation in other parts of W49A. We suggest that W49A is a potential candidate for star formation regulated by feedback-driven and re-collapsing shells
The VLT-FLAMES Tarantula Survey. XXIX. Massive star formation in the local 30 Doradus starburst
The 30 Doradus (30 Dor) nebula in the Large Magellanic Cloud (LMC) is the brightest HII region in the Local Group and a prototype starburst similar to those found in high redshift galaxies. It is thus a stepping stone to understand the complex formation processes of stars in starburst regions across the Universe. Here, we have studied the formation history of massive stars in 30 Dor using masses and ages derived for 452 mainly OB stars from the spectroscopic VLT-FLAMES Tarantula Survey (VFTS). We find that stars of all ages and masses are scattered throughout 30 Dor. This is remarkable because it implies that massive stars either moved large distances or formed independently over the whole field of view in relative isolation. We find that both channels contribute to the 30 Dor massive star population. Massive star formation rapidly accelerated about 8 Myr ago, first forming stars in the field before giving birth to the stellar populations in NGC 2060 and NGC 2070. The R136 star cluster in NGC 2070 formed last and, since then, about 1 Myr ago, star formation seems to be diminished with some continuing in the surroundings of R136. Massive stars within a projected distance of 8 pc of R136 are not coeval but show an age range of up to 6 Myr. Our mass distributions are well populated up to 200 M⊙. The inferred IMF is shallower than a Salpeter-like IMF and appears to be the same across 30 Dor. By comparing our sample of stars to stellar models in the Hertzsprung–Russell diagram, we find evidence for missing physics in the models above log L/L⊙ = 6 that is likely connected to enhanced wind mass loss for stars approaching the Eddington limit. Our work highlights the key information about the formation, evolution and final fates of massive stars encapsulated in the stellar content of 30 Dor, and sets a new benchmark for theories of massive star formation in giant molecular clouds
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