Extended populations around star forming regions and the field population of the Gould Belt

Fernando COMERÓN

Most of what we know about the Gould Belt is based on studies of its O and B stars. However, other stellar components have been investigated in recent years providing various additional insights on it. The local population of hot, isolated neutron stars derives from massive stars that exploded as supernovae recently (less than 10,000 years ago), and their available proper motions point to an origin in the known associations. Recent indications are that the local population of white dwarfs displays an excess of massive, hot objects that is best explained as the result of a burst of star formation in the solar neighborhood no more than 100 million years ago. Within the limitations set by the small number of known massive, hot white dwarfs, their spatial distribution seems to outline the Gould Belt, perhaps tracing the primeval massive population. Finally, X-ray-selected low-mass stars have been claimed to trace a background population of the Gould Belt unrelated to the known current star formation sites. While this is likely to be the case in some directions, a critical examination of recent studies of the low-mass extended population in the general direction of current star forming sites like Orion shows little evidence for the existence of a distinctly older population. An alternative explanation based on spatially extended formation beyond the most outstanding star forming clouds appears to be more likely. It is concluded that the question of whether or not there is a sharp distinction between the extended populations of present-day star forming regions and the general Gould Belt population still awaits a clear answer.

How long it may be expanding?

Astronomical Institute
Academy of Sciences of the Czech Republic

The local velocity patterns of star forming regions, young OB stars, nearby OB associations, atomic and molecular gas are confronted with models of an expanding region. Free expansion from a point or from a ring, expanding 2D shell, and expanding 3D belt with abrupt or gradual energy injection snow-plowing the ambient medium with or without the drag forces including fragmentation and porosity of the medium are tested. There is no agreement on the expansion time, which varies from 30 - 100 Myr. The inclination of the Gould Belt is not explained by the above models of expansion. An oblique impact of a high velocity cloud may explain it, but the observed velocity pattern is difficult to reproduce. The Gould Belt may be just a meeting place of stellar streams formed in the local spiral arm. Better identification of stellar streams and their formation places will be possible with coming GAIA very precise astrometric data.

Gould's Belt as a laboratory: Herschel results on filaments and a new paradigm for star formation

Philippe ANDRÉ
Laboratoire AIM
CEA Saclay

An overview of the results from the Herschel Gould Belt survey is presented. The Herschel images of nearby interstellar clouds all reveal a rich network of filamentary structure and suggest an intimate connection between these filaments and the formation process of prestellar cores. Remarkably, filaments are omnipresent even in unbound, non-star-forming complexes and seem to be characterized by a narrow distribution of widths around ~ 0.1 pc. This characteristic width approximately corresponds to the sonic scale below which interstellar turbulence becomes subsonic in diffuse gas, supporting the view that the filaments may form as a result of the dissipation of large-scale turbulence. In active star-forming regions, most of the prestellar cores identified with Herschel are located within gravitationally unstable filaments above a critical threshold ~ 16 Msun/pc in mass per unit length or ~ 160 Msun/pc^2 in gas surface density. Altogether, the Herschel results favor a scenario in which interstellar filaments and prestellar cores represent two key steps in the star formation process: first large-scale magneto-hydrodynamic turbulence stirs up the gas, giving rise to a universal web-like structure in the interstellar medium, then gravity takes over and controls the further fragmentation of filaments into prestellar cores and ultimately protostars. This scenario provides new insight into the inefficiency of star formation, the origin of the initial mass function, and the global rate of star formation in galaxies. Despite an apparent complexity, global star formation may be governed by relatively simple universal laws from filament to galactic scales.

The formation and destruction of filaments in GMCs

James E. DALE
Excellence Cluster `Universe'
Boltzmannstraße 2
D-85748 Garching, Germany

I will present simulations of star formation in turbulent GMCs, in some of which feedback from photoionization and/or stellar winds is included. The interaction of the turbulent velocity fields and gas self-gravity naturally leads the clouds to develop complex filamentary structure. I will briefly describe techniques we are using to try to characterize this structure and show some preliminary results regarding filament density profiles and widths, and gas flows along filaments. The largest stellar clusters in these calculations tend to form at filament junctions because the filaments channel gas and stars towards common focal points. This means that massive stars are usually born at filament junctions and one of the main effects of feedback is to destroy the filament networks. I will show that photoionization seems to be a more important feedback mechanism than winds but that its effect in disrupting clouds is limited by the cloud escape velocities. I will also show that feedback is able to trigger star formation, but that this is a local and second-order effect, and that it is very hard to distinguish t

HI shells and other structures, most notably HI belts

Astronomical Institute
Academy of Sciences of the Czech Republic

We applied the algorithm for an automatic identification of HI shells in datacubes on the LAB whole-sky HI survey. We identified 333 shells. Their size distribution can be described as a power-law with the slope of 2.6. The radial distribution has a scale of 2.8 kpc. An extrapolation of these relations gives an estimate of the surface density of HI shells in the Solar neighbourhood for shells with radii between 100 pc and 1 kpc to be apprx. 4 per kpc^2. An analysis of clumps around shells indicates, that these clumps may be sometimes concentrated to a belt which inclination does not coincide with the plane of the maximum brightness temperature.

Gravitational Instability of a Thick Layer

František DINNBIER
Astronomical Institute
Academy of Sciences of the Czech Republic

We study gravitational instability of a thick isothermal self-gravitating gaseous layer by means of 3D hydrodynamic simulations. The layer is confined at both sides by the external pressure P_ext. In order to accurately evaluate the gravitational field, we modified the Ewald method for a planar system (i.e. system with periodic boundary conditions in two directions and isolated ones in the third direction) and implemented it into the hydrodynamic code Flash. We compared our simulations to two dispersion relations: Wunsch et al. (2010, PAGI) and Kim et al. (2012). We found that hydrodynamic simulations are in a good agreement with both dispersion relations in the case of low P_ext (where the two dispersion relations also agree with each other). However, if P_ext is high, our simulations result in perturbation growth rates close to predictions by Kim et al. (2012) differing significantly from predictions of PAGI.

How Significant is Radiation pressure on the dynamics of dusty wind-blown shells?


The impact of radiation pressure on the dynamics of the gas around massive star clusters and recently proposed modifications to the classic wind-blown bubble model are discussed. The re-distribution of the swept-up matter and the amplification of thermal pressure caused by ionizing and non-ionizing photons absorbed by gas and dust in the wind-driven shell are explicitely calculated. The results are then compared to the standard bubble model predictions and are putted on to a diagnostic diagram which allows one to discriminate between the wind-dominated and radiation-dominated regimes. It is show that radiation pressure has only a narrow window of opportunity to affect the dynamics of the wind-driven shell unless a significant fraction of the deposited mechanical energy is lost inside the star cluster volume. The model predicted values of the ionization parameter U are calculated and silich@inaoep.mxcompared to those detected in the local starbursts.

Low-mass Star Formation in Turbulent Molecular Clouds

University Carfiff

First, I review the "star formation in a crossing time" scenario where turbulent flows create shock-compressed layers, which in turn fragment into filaments, and then into prestellar cores. I stress the implausibly high ram pressures required to produce isolated BD(brown dwarf)-mass cores (as a prelude to arguing that BDs may be formed efficiently in protostellar discs, where they can lose entropy at leisure). I suggest that prestellar cores may need a turbulent ram pressure that delivers their gas to sufficiently high densities that it can couple thermally with the dust (and thereby switch from line-cooling to more efficient continuum-cooling, and hence collapse); this corresponds to a critical extinction of A_V~6, which is close to what is inferred observationally.

Second I rehearse the statistical properties of stars which theory must explain, in particular the IMF (Initial Mass Function), and the binary statistics, as a function of primary mass.

Third, I explore the mapping from the CMF (Core Mass Function) to the IMF, and show that if this mapping is simple, a typical core has to spawn 4 or 5 stars, with rather high efficiency and a modest range of masses (factor 2.7 between the 25% and 75% percentiles); just two of these stars should end up in a LONG-LIVED binary, and the probability of a star being part of this binary is proportional to its mass (weak "dynamical biasing").

Fourth, I explain why the SIS (Singular Isothermal Sphere) is not, and never has been, a viable model for low-mass star formation, and has no credible observational backing, despite claims to the contrary).

Fifth, I describe strategies for solving the Inverse Problems that are involved -- and always will be involved -- inconverting detailed observations of prestellar cores (like those in Ophiuchus) into initial conditions for simulations of core collapse, with the minimum of model assumptions.

Sixth, I discuss the phenomenology of core collapse, and the processes that control fragmentation to produce stars.

Finally I present simulations of the collapse and fragmentation of cores, concentrating on the role of disc fragmentation, and the consequences of radiative feedback; I show (a) that disc fragmentation is effective if radiative feedback is episodic (due to episodic accretion, e.g. FUOri bursts) AND the downtime between bursts is long enough (as is suggested by some observations and theoretical models); (b) that the statistical properties of BDs and low-mass H-burning stars produced by this mechanism fit the observations very well, viz. distribution of masses, location, binary properties and attendant disc masses. Hence, I would propose that, as one moves to lower masses, an increasing fraction of stars is formed by disc fragmentation, but there is not an abrupt change in the formation mechanism, just a gradual shift.

Star-Forming Clouds

Thomas G. BISBAS
University College London

We discuss the importance of modelling photodissociation regions (PDRs) in understanding the properties of the interstellar medium as it interacts with the interstellar radiation field. PDRs define the transition zone between an ionized and a dark molecular region. They consist of neutral gas which interacts with far-ultraviolet radiation and are characterized by strong infrared line emission. We have implemented a new three-dimensional PDR code ("3D-PDR") which can calculate the total heating and cooling functions at any point using a given complicated chemical network and applying a three dimensional escape probability method. We show applications in turbulent star-forming clouds as well as in fractal clouds of different fractal dimension. We use the results of 3D-PDR to estimate the X_C*-factor, for both CO(1-0) transition and CI609um transition. We find that for PDRs (which have an average density range of 1e2 - 1e5 cm^-3), CI is a better tracer for the detection of molecular hydrogen (H2) thus having an impact in the current observational techniques that different groups use. In addition, we find evidence of how the column density of H2 correlates with the X_C*-factor as well as the fractal dimension of clumpy HII regions.

Gould's Belt Type Objects Down-stream of Spiral Arms

IBM T.J. Watson Research Center
Yorktown Heights NY 10598 USA

Observations of other galaxies using HST show features of star formation and dust structures that resemble Gould's Belt and the Lindblad Expanding Ring with respect to size, age, and spiral arm position. The origins of these structures may be more easily determined from the extragalactic observations. Star complexes the size of Gould’s Belt and the rings around them can be seen to form commonly inside and down stream of the arms, and to become increasingly dark and stagnant during their long journey to the next arm. The features are so common that Gould’s Belt does not seem to be peculiar at all: many locations in the Milky Way would be likely to have a Gould’s Belt type object nearby. New theoretical work and simulations of spiral arm flows show spurs and pseudo-rings developing as a result of gaseous self-gravity and variable shear. Such development is independent of pressure from star formation, and suggests that a high fraction of the kinetic energy in expanding rings and associations could be galactic potential. The appearance of rings larger than the disk thickness also suggests that spiral arm flows are involved because centralized pressures would only vent to the halo on these scales. One big uncertainty for the local structure is the position of the solar neighborhood relative to corotation. Present observations suggest we are just inside corotation of the local spirals, in which case Gould’s Belt is downstream from the Sgr-Car arm and will eventually enter the inward extension of the Perseus arm.

Dusty supernovae running the thermodynamics of the matter reinserted by young stellar clusters.


Following the observational and theoretical evidence that points at core collapse supernovae as major producers of dust, here we calculate the hydrodynamics of the matter reinserted within young and massive super stellar clusters under the assumption of gas and dust radiative cooling. The large supernova rate expected in massive clusters allows for a continuous replenishment of dust immersed in the high temperature thermalized reinserted matter and warrants a stationary presence of dust within the cluster volume during the type II supernova era (~ 3 Myr - 40 Myr). We first show that such a balance determines the range of dust to gas mass ratio and this the dust cooling law. We then search for the critical line in the cluster mechanical luminosity (or cluster mass) {\it vs} cluster size, that separates quasi- adiabatic and strongly radiative cluster wind solutions from the bimodal cases. In the latter, strong radiative cooling reduces considerably the cluster wind mechanical energy output and affects particularly the cluster central regions, leading to frequent thermal instabilities that diminish the pressure and inhibit the exit of the reinserted matter. Instead matter accumulates there and is expected to eventually lead to gravitational instabilities and to further stellar formation with the matter reinserted by former massive stars. The main outcome of the calculations is that the critical line is almost two orders of magnitude or more, depending on the assumed value of V\infty, lower than when only gas radiative cooling is applied. And thus, massive clusters (M_sc \geq 10^5 Msun) are predicted to enter the bimodal regime.

Delayed massive star formation in clusters

Richard WÜNSCH
Astronomical Institute
Academy of Sciences of the Czech Republic

We develop a simple semi-analytical model of a cluster formation. It is assumed that new gas is accreted onto a sphere with a uniform pressure. The sphere radius is given by the virial equilibrium. The gas in the sphere is turbulent with a constant Mach number and it results in a log-normal probability density distribution. The sphere pressure is calculated from the energy balance between gas cooling (calculated over all density bins) and heating due to accretion f new gas and stellar radiation, winds and outflows. Stars are formed at rate inversely proportional to the free-fall time in a given density bin, with masses corresponding to the Jeans mass. Assuming star formation efficiency 0.01-0.1, this extremely simple model provides approximately correct IMF (with close to Salpeter slope at high mass end) and formation of massive stars delayed by several Myr with respect to the beginning of star formation.

Formation of super star clusters triggered by cloud-cloud collisions

Nagoya University

High-mass stars are major members in starbursts and are influential in galactic evolution by injection of ultraviolet photons and kinetic energy in stellar winds and SN shock waves. The most remarkable objects where starburst takes place are the super star clusters in the Milky Way. NGC3603 is one of the most remarkable starburst clusters, which harbors 30 O stars whose mass ranges up to 100Mo. I present molecular observations with NANTEN2 toward NGC 3603 and show that two giant molecular clouds having 20-km s-1 velocity separation are associated with the cluster as evidenced by temperature rise toward the cluster in the both clouds (Fukui et al. 2013 ApJ in press, arXiv:1306.2090). It is not likely that the two clouds are gravitationally bound with each other since the cloud mass is ten times less than what is required to bind the system. The clouds therefore encountered by chance 1-2Myrs ago. I argue that this encounter led to a supersonic collision which triggered the starburst. The timescale of the star burst can be as small as 10^5yrs as derived by optical age determination of the NGC3603 cluster. Three other young super star clusters including Westerlund 2 in the Milky Way are also associated with two clouds having 15 -20 km relative velocities and the same argument supporting cloud-cloud collisions are applicable. These observations suggest supersonic cloud-cloud collisions are an efficient mechanism to trigger high-mass star formation. It is known that older and lower-mass stars are also members of these clusters and are possibly formed prior to the collisions. The collisions play a crucial role in forming the highest-mass member stars. This view is supported by recent MHD numerical simulations of colliding dense gas flows (Inoue and Fukui 2013 ApJL 774, 31).

Evidence for a top-heavy IMF in starbursts

Helmholtz-Institut fuer Strahlen und Kernphysik (HISKP)
University of Bonn
Auf dem Huegel 71, D-53121 Bonn, Germany

Globular clusters with low concentration have a damaged stellar mass function in the sense of a deficit of low-mass stars. This is not expected from energy-eqipartition-driven evaporation, but can be shown to be consistent with the star clusters having been born significantly more compact, mass segregated and with a top-heavy IMF. The dependence of the top-heaviness on density explains the expansion of the very young clusters after residual gas expulsion. Virtually the same dependence of the IMF on initial density also explains the overabundance of X-ray luminous ultra compact dwarf (UCD) galaxies, and, at the same time, the elevated M/L values of UCDs. The galaxy-wide IMF, the IGIMF, then also becomes top-heavy in galaxy-wide star bursts. Finally, I explain the difference between interpreting the IMF as a probability density distribution function or an optimally sampled distribution function.

Science with the Arecibo Galaxy Environmental Survey

Arecibo Observatory

I present an overview of the Arecibo Galaxy Environment Survey, AGES. AGES is an extragalactic neutral hydrogen survey targeting a range of different environments, from the Local Void to rich clusters. When complete the survey will cover a total of 200 square degrees to an rms sensitivity of 0.7 mJy, equivalent to an HI mass of ~10^7 Msolar at the distance of the Virgo Cluster. I describe some of the results of the survey so far : 1) We have completed the observations for three isolated galaxies and find they have at most one companion each, far less than expected based on the HI mass function from the larger ALFALFA survey; 2) We found 8 HI detections within the Virgo cluster without obvious optical counterparts, some of which have velocity widths too large to fit the Tully-Fisher relation observed in the field (perhaps indicating they are non-primordial debris, but we do not rule out the prospect that they are so-called "dark galaxies"); 3) Behind the galaxy group associated with NGC 7448, we find a dense filamentary structure of galaxies rich in HI streams, some of which are in excess of 800 kpc in length.

Summary and Questions to be Addressed

Jan Palouš & all participants of the wokshop

Gould Belt

  • How much mass there is in gas and stars?
  • How much energy is needed to form it?
  • The tilt: is it just an accident easy to explain, a chance alignment of its main star formation sites, or a distinctive property that is telling us something about its origin?
  • Curling flows, like downstream in the M51: could they be inclined to the galaxy plane?
  • What is the connection to the HI expanding ring?
  • Its radial size versus its thickness?

The Solar Neighborhood

  • The Local Bubble and its chimney to the halo: is it related to the Gould belt?
  • Where is the cold, warm and hot gas?
  • Relation to the Local Spiral Arm and to the grand-design spirals of the MilkyWay?
  • Origins: where is the Solar neigborhood coming from? Formation places of moving groups, superstar clusters, open star clusters, and of the field stars?

Star Formation: Filaments

  • Converging flows and the origin of filaments: have we found the smoking gun?
  • Formation of filaments?
  • Does triggered star formation also happen in filaments, if it happens at all?
  • What is the triggered star formation?
  • Do filaments help with the ultracompact HII region lifetime problem?
  • Are the high-z filaments the same as local?
  • Is 0.1 pc thickness universal?

Star Formation: Disks

  • How do we look for the observational evidence that shows if the very low mass stars and brown dwarfs are born in disks?
  • What show simulations: what is the mass function of stars formed out of disks?

Star Formation

  • Inhomogeneities and turbulence in the ISM?
  • Does most SF happens in Gould’s Belts?

Star Clusters

  • Are there any clusters with the thermally unstable winds in the central parts?
  • What do we need to observe? How to confirm or reject the concept of bimodal clusters?
  • What is the morphology of ionized gas around young stellar clusters? Stromgren spheres, expanding shells, a collection of ionized clumps?
  • Are there observations indicating formation of super-star clusters by the supersonic collision between a giant molecular cloud and the interstellar medium during galaxy versus galaxy encounters? Can it be an alternative mechanism for SSC formation?