Hairy Stars Research Articles

Instability of compact stars with a nonminimal scalar-derivative coupling

For a theory in which a scalar field ϕ has a nonminimal derivative coupling to the Einstein tensor Gμ ν of the form ϕ Gμ ν∇μ∇ν ϕ, it is known that there exists a branch of static and spherically-symmetric relativistic stars endowed with a scalar hair in their interiors. We study the stability of such hairy solutions with a radial field dependence ϕ(r) against odd- and even-parity perturbations. We show that, for the star compactness \U0001d49e smaller than 1/3, they are prone to Laplacian instabilities of the even-parity perturbation associated with the scalar-field propagation along an angular direction. Even for \U0001d49e>1/3, the hairy star solutions are subject to ghost instabilities. We also find that even the other branch with a vanishing background field derivative is unstable for a positive perfect-fluid pressure, due to nonstandard propagation of the field perturbation δ ϕ inside the star. Thus, there are no stable star configurations in derivative coupling theory without a standard kinetic term, including both relativistic and nonrelativistic compact objects.

Stationary scalar hairy configurations supported by Neumann compact stars

We study stationary scalar field hairy configurations supported by asymptotically flat horizonless compact stars. At the star surface, we impose Neumann boundary conditions for the scalar field. With analytical methods, we obtain bounds on the frequency of scalar fields. For certain discrete frequency satisfying the bounds, we numerically get solutions of scalar hairy stars. We also disclose effects of model parameters on the discrete frequency of scalar fields.

On instabilities of stationary scalar field configurations supported by reflecting compact stars

We study instabilities of the system composed of stationary scalar fields and asymptotically flat horizonless reflecting compact stars. In the probe limit, we obtain bounds on the scalar field frequency. Below this bound, stationary hairy stars are expected to suffer from nonlinear instabilities under massless field perturbations. In other words, we prove that stationary scalar hairy stars are unstable for scalar fields with small frequency.

On instabilities of scalar hairy regular compact reflecting stars

We study the system constructed by charged scalar fields linearly coupled to asymptotically flat horizonless compact reflecting stars. We obtain bounds on the charge of the scalar field, below which the scalar hairy star is expected to suffer from nonlinear instabilities. It means that scalar hairy regular configurations are unstable for scalar fields of small charge. For the highly-charged star, there are also bounds on radii of regular compact reflecting stars. When the star radius is below the bound, the hairy star is always unstable.

Static scalar field condensation in regular asymptotically AdS reflecting star backgrounds

We study condensation behaviors of static scalar fields in the regular asymptotically AdS reflecting star spacetime. With analytical methods, we provide upper bounds for the radii of the scalar hairy reflecting stars. Above the bound, there is no scalar hair theorem for the star. Below the bound, we numerically obtain charged scalar hairy reflecting star solutions and in particular, the radii of the hairy stars are discrete, which is similar to known results in other reflecting object backgrounds. For every set of parameters, we search for the largest AdS hairy star radius, study effects of parameters on the largest hairy star radius and also find difference between properties in this AdS reflecting star background and those in the flat reflecting star spacetime. Moreover, we show that scalar fields cannot condense around regular AdS reflecting stars when the star charge is small or the cosmological constant is negative enough.

Scalar field configurations supported by charged compact reflecting stars in a curved spacetime

We study the system of static scalar fields coupled to charged compact reflecting stars through both analytical and numerical methods. We enclose the star in a box and our solutions are related to cases without box boundaries when putting the box far away from the star. We provide bottom and upper bounds for the radius of the scalar hairy compact reflecting star. We obtain numerical scalar hairy star solutions satisfying boundary conditions and find that the radius of the hairy star in a box is continuous in a range, which is very different from cases without box boundaries where the radius is discrete in the range. We also examine effects of the star charge and mass on the largest radius.

Polystyrene/Poly(2-vinylpyridine) Heteroarm Star Copolymer Micelles in Aqueous Media and Onion Type Micelles Stabilized by Diblock Copolymers,

Micellization of polystyrene/poly(2-vinylpyridine) heteroarm star copolymers, PSnPVPn, in aqueous media has been studied by a combination of light scattering and fluorescence techniques. Three copolymer samples differing in numbers of arms and their length form micelles with compact hydrophobic cores and polyelectrolyte poly(2-vinylpyridine) shells. The association number decreases with increasing number of water-soluble arms and their length. Micellization equilibrium is kinetically frozen in aqueous media. The micellar cores are frozen and the behavior of the micellar solutions is controlled by the polyelectrolyte behavior of the polyelectrolyte shell. In the case of the present heteroarm stars with ca. 101 arms, the soluble and insoluble arms are strongly segregated and the micellar cores are formed of pure polystyrene as with micelles formed by linear diblock copolymers. As concerns the sample with several tens of arms, a significant steric hindrance, together with a low association number (ca. 4), does not allow for a complete segregation of polystyrene and the poly(2-vinylpyridine) arms. The micellar cores formed by the very hairy star copolymer with several tens of arms contain a fraction of poly(2-vinylpyridine) arms as shown by fluorescence measurements with solubilized pyrene. Alkalimetric titration of an acidic mixture of PSnPVPn micelles with a linear poly(2-vinylpyridine)-block-poly(ethylene oxide) copolymer, PVP−PEO, yields fairly monodisperse spherical onion-skin micelles. The onion-skin micelle has a compact spherical PS core deriving from the parent PSnPVPn micelle, a fairly thin and compact middle layer formed by PVP both from the parent micelle and PVP−PEO copolymer, and a protective PEO shell. The formation of onion-skin micelles is fully reversible and occurs suddenly and very rapidly at pH values higher than 4.8. The “onions” dissociate immediately into PSnPVPn micelles and PVP−PEO copolymer below pH 4.8. Evaluation of the alkalimetric titration data shows that only ca. 40% of the PVP units in the micellar shells are protonized (even in strongly acidic solutions that contain a large surplus of a strong acid, e.g., in 0.1 M HCl). This observation is in agreement with indirect fluorometric data which indicate that in the case of a weak polyelectrolyte, such as poly(2-vinylpyridine), the inner layer of the polyelectrolyte shell close to the core is not ionized.

From comets to asteroids: When hairy stars go bald

From comets to asteroids: When hairy stars go bald