9,431 research outputs found
Superbubble dynamics in globular cluster infancy. II. Consequences for secondary star formation in the context of self-enrichment via fast-rotating massive stars
Context. The self-enrichment scenario for globular clusters (GC) requires large amounts of residual gas after the initial formation of the first stellar generation. Recently, we found that supernovae may not be able to expel that gas, as required to explain their present-day gas-free state, and suggested that a sudden accretion onto the dark remnants at a stage when type II supernovae have ceased may plausibly lead to fast gas expulsion. Aims. Here, we explore the consequences of these results for the self-enrichment scenario via fast-rotating massive stars (FRMS). Methods. We analysed the interaction of FRMS with the intra-cluster medium (ICM), in particular where, when, and how the second generation of stars may form. From the results, we developed a timeline of the first ≈ 40 Myr of GC evolution. Results. Our previous results imply three phases during which the ICM is in a fundamentally different state, namely the wind bubble phase (lasting 3.5 to 8.8 Myr), the supernova phase (lasting 26.2 to 31.5 Myr), and the dark remnant accretion phase (lasting 0.1 to 4 Myr): (i) Quickly after the first-generation massive stars have formed, stellar wind bubbles compress the ICM into thin filaments. No stars may form in the normal way during this phase because of the high Lyman-Werner flux density. If the first-generation massive stars have equatorial ejections however, as we proposed in the FRMS scenario, accretion may resume in the shadow of the equatorial ejecta. The second-generation stars may then form due to gravitational instability in these disc, which are fed by both the FRMS ejecta and pristine gas. (ii) In the supernova phase the ICM develops strong turbulence, with characteristic velocities below the escape velocity. The gas does not accrete either onto the stars or onto the dark remnants in this phase because of the high gas velocities. The strong mass loss associated with the transformation of the FRMS into dark remnants then leads to the removal of the second-generation stars from the immediate vicinity of the dark remnants. (iii) When the supernovae have ceased, turbulence quickly decays, and the gas can once more accrete, now onto the dark remnants. As discussed previously, this may release sufficient energy to unbind the gas, and may happen fast enough so that a large fraction of less tightly bound first-generation stars are lost. Conclusions. Studying the FRMS scenario for the self-enrichment of GCs in detail reveals the important role of the physics of the ICM for our understanding of the formation and early evolution of GCs. Depending on the level of mass segregation, this sets constraints on the orbital properties of the stars, in particular high orbital eccentricities, which likely has implications on the GC formation scenario.Peer reviewe
The relation between magnetic and material arms in models for spiral galaxies
Context. Observations of polarized radio emission show that large-scale
(regular) magnetic fields in spiral galaxies are not axisymmetric, but
generally stronger in interarm regions. In some nearby galaxies such as NGC
6946 they are organized in narrow magnetic arms situated between the material
spiral arms. Aims. The phenomenon of magnetic arms and their relation to the
optical spiral arms (the material arms) call for an explanation in the
framework of galactic dynamo theory. Several possibilities have been suggested
but are not completely satisfactory; here we attempt a consistent
investigation. Methods. We use a 2D mean-field dynamo model in the no-z
approximation and add injections of small-scale magnetic field, taken to result
from supernova explosions, to represent the effects of dynamo action on smaller
scales. This injection of small scale field is situated along the spiral arms,
where star-formation mostly occurs. Results. A straightforward explanation of
magnetic arms as a result of modulation of the dynamo mechanism by material
arms struggles to produce pronounced magnetic arms, at least with realistic
parameters, without introducing new effects such as a time lag between Coriolis
force and {\alpha}-effect. In contrast, by taking into account explicitly the
small-scale magnetic field that is injected into the arms by the action of the
star forming regions that are concentrated there, we can obtain dynamo models
with magnetic structures of various forms that can be compared with magnetic
arms. (abbrev). Conclusions. We conclude that magnetic arms can be considered
as coherent magnetic structures generated by large-scale dynamo action, and
associated with spatially modulated small-scale magnetic fluctuations, caused
by enhanced star formation rates within the material arms.Comment: 13 pages, 8 figures, accepted for publication to A&
Breakdown of Angular Momentum Selection Rules in High Pressure Optical Pumping Experiments
We present measurements, using two complementary methods, of the breakdown of
atomic angular momentum selection rules in He-broadened Rb vapor. Atomic dark
states are rendered weakly absorbing due to fine-structure mixing during Rb-He
collisions. The effect substantially increases the photon demand for optical
pumping of dense vapors
Triaxial orbit-based modelling of the Milky Way Nuclear Star Cluster
We construct triaxial dynamical models for the Milky Way nuclear star cluster
using Schwarzschild's orbit superposition technique. We fit the stellar
kinematic maps presented in Feldmeier et al. (2014). The models are used to
constrain the supermassive black hole mass M_BH, dynamical mass-to-light ratio
M/L, and the intrinsic shape of the cluster. Our best-fitting model has M_BH =
(3.0 +1.1 -1.3)x10^6 M_sun, M/L = (0.90 +0.76 -0.08) M_sun/L_{sun,4.5micron},
and a compression of the cluster along the line-of-sight. Our results are in
agreement with the direct measurement of the supermassive black hole mass using
the motion of stars on Keplerian orbits. The mass-to-light ratio is consistent
with stellar population studies of other galaxies in the mid-infrared. It is
possible that we underestimate M_BH and overestimate the cluster's triaxiality
due to observational effects. The spatially semi-resolved kinematic data and
extinction within the nuclear star cluster bias the observations to the near
side of the cluster, and may appear as a compression of the nuclear star
cluster along the line-of-sight. We derive a total dynamical mass for the Milky
Way nuclear star cluster of M_MWNSC = (2.1 +-0.7)x10^7 M_sun within a sphere
with radius r = 2 x r_eff = 8.4 pc. The best-fitting model is tangentially
anisotropic in the central r = 0.5-2 pc of the nuclear star cluster, but close
to isotropic at larger radii. Our triaxial models are able to recover complex
kinematic substructures in the velocity map.Comment: 14 pages, 10 figures. Accepted for publication in MNRA
On the Bekenstein-Hawking Entropy, Non-Commutative Branes and Logarithmic Corrections
We extend earlier work on the origin of the Bekenstein-Hawking entropy to
higher-dimensional spacetimes. The mechanism of counting states is shown to
work for all spacetimes associated with a Euclidean doublet
of electric-magnetic dual brane pairs of type II
string-theory or M-theory wrapping the spacetime's event horizon plus the
complete internal compactification space. Non-Commutativity on the brane
worldvolume enters the derivation of the Bekenstein-Hawking entropy in a
natural way. Moreover, a logarithmic entropy correction with prefactor 1/2 is
derived.Comment: 17 pages, 2 figures; refs. adde
- …