21 research outputs found
The Return of the Proplyds - Understanding the Dynamics of Ionization Triggered Stars
Proplyds and stars inside HII-regions are a well studied phenomenon. It is
possible that they were triggered by the expansion of the HII-region itself.
Here, we present calculations on the dynamics of HII-regions. We show that the
triggered stars that form in the expanding shell of swept up material around
the HII region rarely return into the HII regions on timescales that are
inferred for the proplyds and observed young stars. However, in very dense
environments like Orion, the triggered stars return in time. Thus, our model
can explain why proplyds are barely observed in other HII regions. We propose
that the properties of young stellar objects in HII regions in general depend
critically on the distance from the massive, ionizing central star cluster.
Closest in, there are proplyds, where the disk of a young star interacts
directly with the feedback of the massive star. Further out are Class II
protostars, where the ionization already removed the envelope. Even further
away, one should find Class I stars, which either have been triggered by the
ionizing radiation or pre-existed and have not lost their envelope yet. This
radial sequence is not necessarily an age sequence but rather a result of the
dwindling importance of stellar winds and ionizing radiation with distance. We
investigate the observational signature of triggered star formation and find
that the stellar distribution for ionization triggered star formation shows a
distinct feature, a peak at the current position of the ionization front.
Therefore, it is generally possible to tell triggered and in situ distributions
of stars apart.Comment: 6 pages, 5 figures, submitted to MNRA
Accretion driven turbulence in filaments I: Non-gravitational accretion
We study accretion driven turbulence for different inflow velocities in star
forming filaments using the code ramses. Filaments are rarely isolated objects
and their gravitational potential will lead to radially dominated accretion. In
the non-gravitational case, accretion by itself can already provoke
non-isotropic, radially dominated turbulent motions responsible for the complex
structure and non-thermal line widths observed in filaments. We find that there
is a direct linear relation between the absolute value of the total density
weighted velocity dispersion and the infall velocity. The turbulent velocity
dispersion in the filaments is independent of sound speed or any net flow along
the filament. We show that the density weighted velocity dispersion acts as an
additional pressure term supporting the filament in hydrostatic equilibrium.
Comparing to observations, we find that the projected non-thermal line width
variation is generally subsonic independent of inflow velocity
Accretion driven turbulence in filaments II: Effects of self-gravity
We extend our previous work on simulations with the code RAMSES on accretion
driven turbulence by including self-gravity and study the effects of core
formation and collapse. We show that radial accretion onto filaments drives
turbulent motions which are not isotropic but radially dominated. In contrast
to filaments without gravity, the velocity dispersion of self-gravitating
filaments does not settle in an equilibrium. Despite showing similar amounts of
driven turbulence, they continually dissipate their velocity dispersion until
the onset of core formation. This difference is connected to the evolution of
the radius as it determines the dissipation rate. In the non-gravitational case
filament growth is not limited and its radius grows linearly with time. In
contrast, there is a maximum extent in the self-gravitational case resulting in
an increased dissipation rate. Furthermore, accretion driven turbulence shows a
radial profile which is anti-correlated with density. This leads to a constant
turbulent pressure throughout the filament. As the additional turbulent
pressure does not have a radial gradient it does not contribute to the
stability of filaments and does not increase the critical line-mass. However,
this radial turbulence does affect the radius of a filament, adding to the
extent and setting its maximum value. Moreover, the radius evolution also
affects the growth timescale of cores which compared to the timescale of
collapse of an accreting filament limits core formation to high line-masses
Explaining the observed velocity dispersion of dwarf galaxies by baryonic mass loss during the first collapse
In the widely adopted LambdaCDM scenario for galaxy formation, dwarf galaxies
are the building blocks of larger galaxies. Since they formed at relatively
early epochs when the background density was relatively high, they are expected
to retain their integrity as satellite galaxies when they merge to form larger
entities. Although many dwarf spheroidal galaxies (dSphs) are found in the
galactic halo around the Milky Way, their phase space density (or velocity
dispersion) appears to be significantly smaller than that expected for
satellite dwarf galaxies in the LambdaCDM scenario. In order to account for
this discrepancy, we consider the possibility that they may have lost a
significant fraction of their baryonic matter content during the first infall
at the Hubble expansion turnaround. Such mass loss arises naturally due to the
feedback by relatively massive stars which formed in their centers briefly
before the maximum contraction. Through a series of N-body simulations, we show
that the timely loss of a significant fraction of the dSphs initial baryonic
matter content can have profound effects on their asymptotic half-mass radius,
velocity dispersion, phase-space density, and the mass fraction between
residual baryonic and dark matter.Comment: 6 pages, 6 figures, accepted for publication in the Ap
Ionisation Feedback in Star and Cluster Formation Simulations
Feedback from photoionisation may dominate on parsec scales in massive
star-forming regions. Such feedback may inhibit or enhance the star formation
efficiency and sustain or even drive turbulence in the parent molecular cloud.
Photoionisation feedback may also provide a mechanism for the rapid expulsion
of gas from young clusters' potentials, often invoked as the main cause of
'infant mortality'. There is currently no agreement, however, with regards to
the efficiency of this process and how environment may affect the direction
(positive or negative) in which it proceeds. The study of the photoionisation
process as part of hydrodynamical simulations is key to understanding these
issues, however, due to the computational demand of the problem, crude
approximations for the radiation transfer are often employed. We will briefly
review some of the most commonly used approximations and discuss their major
drawbacks. We will then present the results of detailed tests carried out using
the detailed photoionisation code MOCASSIN and the SPH+ionisation code iVINE
code, aimed at understanding the error introduced by the simplified
photoionisation algorithms. This is particularly relevant as a number of new
codes have recently been developed along those lines. We will finally propose a
new approach that should allow to efficiently and self-consistently treat the
photoionisation problem for complex radiation and density fields.Comment: Invited review presented at the IAU Symposium 270: Computational Star
Formation held in Barcelona (May 31st- June 4th 2010) - Refereed paper
version; 8 Pages, 4 Figure
Ionization and Triggered Star Formation in Turbulent Molecular Clouds
Einige der spektakulärsten Beobachtungen unserer Milchstrasse zeigen die filamentären Strukturen in der Umgebung von heissen massereichen O-Sternen. Sobald diese Sterne beginnen zu leuchten, ionisiert ihre ultraviolette Strahlung das umgebende Gas und erzeugt eine heisse HII-Region. Das erhitzte Gas expandiert in die umgebende kalte Molekülwolke. Die dabei entstehende Schockwelle komprimiert das kalte Gas in die auffälligen Strukturen. An den Spitzen dieser Strukturen entstehen neue, masseärmere Sterne. Bis heute ist die präzise Entstehung dieser Regionen nicht vollständig verstanden.
Ziel dieser Arbeit ist die Simulation dieser Entwicklung anhand hydrodynamischer Methoden. Dazu wird ionisierende Strahlung in einen Smoothed Particle Hydrodynamics (SPH) Code namens VINE, der vollständig OpenMP-parallelisiert ist, implementiert. Für die Berechnung der Ionisation wird angenommen, dass die betrachtete Region so weit von dem Stern entfernt ist, dass die Strahlung näherungsweise plan-parallel eintrifft. Zunächst wird die Eintrittsfläche in gleich grosse Strahlen unterteilt. Dann wird die Ionisation entlang dieser Strahlen propagiert. Die neue Implementation ist vollständig parallelisiert und trägt den Namen iVINE.
Zuerst wird anhand mehrerer Tests die Übereinstimmung von iVINE mit bekannten analytischen Lösungen gezeigt. Danach wird der durch Ionisation induzierte gravitative Kollaps einer marginal stabilen Sphäre untersucht. In allen drei simulierten Fällen mit unterschiedlichem einfallenden ionisierenden Fluss kollabiert die Sphäre. Zusätzlich kann die beobachtete Tendenz, dass jüngere Sterne weiter entfernt von der Quelle der Ionisation entstehen, bestätigt werden.
Desweiteren werden Simulationen über den Einfluss ionisierender Strahlung auf turbulente Molekülwolken durchgeführt. Hier zeigt sich, dass die beobachteten, komplexen Strukturen durch die Kombination von Ionisation, Hydrodynamik und Gravitation reproduziert werden können. An den Spitzen der Strukturen wird das Gas stark komprimiert und kollabiert unter dem Einfluss seiner Eigengravitation, genau wie beobachtet. Gleichzeitig treibt die ionisierende Strahlung die Turbulenz im kalten Gas weit stärker als bisher angenommen. Anhand von einer Parameterstudie folgt, dass die entstehenden Strukturen kritisch von dem jeweiligen Anfangsstadium der Wolke zur Zeit der Zündung des O-Sterns abhängen. Dies ergibt die einmalige Gelegenheit, zusätzliche Informationen über Molekülwolken, die ansonsten schwierig zu beobachten sind, in den von O-Sternen stark illuminierten Regionen zu erhalten.
Die Implementation ionisierender Strahlung im Rahmen dieser
Doktorarbeit ermöglicht die Untersuchung der Einwirkung
massereicher Sterne auf ihre Umgebung in bislang Unerreichter Genauigkeit. Die durchgeführten Simulationen vertiefen unser Verständnis der Wechselwirkung von Turbulenz und Gravitation im Rahmen der Sternentstehung. Weitere erstrebenswerte Schritte wären die genauere Berücksichtigung der Kühlprozesse innerhalb der Molekülwolke und die Implementation der Winde massereicher O-Sterne
Triggered Star Formation in the Environment of Young Massive Stars
Recent observations with the Spitzer Space Telescope show clear evidence that
star formation takes place in the surrounding of young massive O-type stars,
which are shaping their environment due to their powerful radiation and stellar
winds. In this work we investigate the effect of ionising radiation of massive
stars on the ambient interstellar medium (ISM): In particular we want to
examine whether the UV-radiation of O-type stars can lead to the observed
pillar-like structures and can trigger star formation. We developed a new
implementation, based on a parallel Smooth Particle Hydrodynamics code (called
IVINE), that allows an efficient treatment of the effect of ionising radiation
from massive stars on their turbulent gaseous environment. Here we present
first results at very high resolution. We show that ionising radiation can
trigger the collapse of an otherwise stable molecular cloud. The arising
structures resemble observed structures (e.g. the pillars of creation in the
Eagle Nebula (M16) or the Horsehead Nebula B33). Including the effect of
gravitation we find small regions that can be identified as formation places of
individual stars. We conclude that ionising radiation from massive stars alone
can trigger substantial star formation in molecular clouds.Comment: 4 pages, 2 figures. To appear in: "Triggered Star Formation in a
Turbulent ISM", IAU Symposium 237, Prague, Czech Republic, August 2006; eds.
B.G.Elmegreen & J. Palou
On the IMF in a Triggered Star Formation Context
The origin of the stellar initial mass function (IMF) is a fundamental issue
in the theory of star formation. It is generally fit with a composite power
law. Some clues on the progenitors can be found in dense starless cores that
have a core mass function (CMF) with a similar shape. In the low-mass end,
these mass functions increase with mass, albeit the sample may be somewhat
incomplete; in the high-mass end, the mass functions decrease with mass. There
is an offset in the turn-over mass between the two mass distributions. The
stellar mass for the IMF peak is lower than the corresponding core mass for the
CMF peak in the Pipe Nebula by about a factor of three. Smaller offsets are
found between the IMF and the CMFs in other nebulae. We suggest that the offset
is likely induced during a starburst episode of global star formation which is
triggered by the formation of a few O/B stars in the multi-phase media, which
naturally emerged through the onset of thermal instability in the cloud-core
formation process. We consider the scenario that the ignition of a few massive
stars photoionizes the warm medium between the cores, increases the external
pressure, reduces their Bonnor?Ebert mass, and triggers the collapse of some
previously stable cores. We quantitatively reproduce the IMF in the low-mass
end with the assumption of additional rotational fragmentation.Comment: 3 figure
ON THE IMF IN A TRIGGERED STAR FORMATION CONTEXT
The origin of the stellar initial mass function (IMF) is a fundamental issue in the theory of star formation. It is generally fit with a composite power law. Some clues on the progenitors can be found in dense starless cores that have a core mass function (CMF) with a similar shape. In the low-mass end, these mass functions increase with mass, albeit the sample may be somewhat incomplete; in the high-mass end, the mass functions decrease with mass. There is an offset in the turn-over mass between the two mass distributions. The stellar mass for the IMF peak is lower than the corresponding core mass for the CMF peak in the Pipe Nebula by about a factor of three. Smaller offsets are found between the IMF and the CMFs in other nebulae. We suggest that the offset is likely induced during a starburst episode of global star formation which is triggered by the formation of a few O/B stars in the multi-phase media, which naturally emerged through the onset of thermal instability in the cloud-core formation process. We consider the scenario that the ignition of a few massive stars photoionizes the warm medium between the cores, increases the external pressure, reduces their Bonnor-Ebert mass, and triggers the collapse of some previously stable cores. We quantitatively reproduce the IMF in the low-mass end with the assumption of additional rotational fragmentation
Detailed Numerical Simulations on the Formation of Pillars around HII-regions
We study the structural evolution of turbulent molecular clouds under the
influence of ionizing radiation emitted from a nearby massive star by
performing a high resolution parameter study with the iVINE code. The
temperature is taken to be 10K or 100K, the mean number density is either
100cm^3 or 300cm^3. Besides, the turbulence is varied between Mach 1.5 and Mach
12.5 and the main driving scale between 1pc and 8pc. We vary the ionizing flux
by an order of magnitude. In our simulations the ionizing radiation enhances
the initial turbulent density distribution and thus leads to the formation of
pillar-like structures observed adjacent to HII regions in a natural way.
Gravitational collapse occurs regularly at the tips of the structures. We find
a clear correlation between the initial state of the turbulent cold cloud and
the final morphology and physical properties of the structures formed. The most
favorable regime for the formation of pillars is Mach 4-10. Structures and
therefore stars only form if the initial density contrast between the high
density unionized gas and the gas that is going to be ionized is lower than the
temperature contrast between the hot and the cold gas. The density of the
resulting pillars is determined by a pressure equilibrium between the hot and
the cold gas. A thorough analysis of the simulations shows that the complex
kinematical and geometrical structure of the formed elongated filaments
reflects that of observed pillars to an impressive level of detail. In
addition, we find that the observed line-of sight velocities allow for a
distinct determination of different formation mechanisms. Comparing the current
simulations to previous results and recent observations we conclude that e.g.
the pillars of creation in M16 formed by the mechanism proposed here and not by
the radiation driven implosion of pre-existing clumps.Comment: 15 pages, 12 figures, accepted for publication in Ap