A star disrupted by a stellar black hole as the origin of the cloud falling toward the Galactic center

We propose that the cloud moving on a highly eccentric orbit near the central black hole in our Galaxy, reported by Gillessen et al., is formed by a photoevaporation wind originating in a disk around a star that is tidally perturbed and shocked at every peribothron passage. The disk is proposed to have formed when a stellar black hole flew by the star, tidally disrupted its envelope, and placed the star on its present orbit with some of the tidal debris forming a disk. A disrupting encounter at the location of the observed cloud is most likely to be caused by a stellar black hole because of the expected dynamical mass segregation; the rate of these disk-forming encounters may be as high as $\sim 10^{-6}$ per year. The star should also be spun up by the encounter, so the disk may subsequently expand by absorbing angular momentum from the star. Once the disk expands up to the tidal truncation radius, the tidal perturbation of the outer disk edge at every peribothron may place gas streams on larger orbits which can give rise to a photoevaporation wind that forms the cloud at every orbit. This model predicts that, after the cloud is disrupted at the next peribothron passage in 2013, a smaller unresolved cloud will gradually grow around the star on the same present orbit. An increased infrared luminosity from the disk may also be detectable when the peribothron is reached. We also note that this model revives the encounter theory for planet formation.Comment: To be published in Ap

Microlensing of Extremely Magnified Stars near Caustics of Galaxy Clusters

Recent observations of lensed galaxies at cosmological distances have detected individual stars that are extremely magnified when crossing the caustics of lensing clusters. In idealized cluster lenses with smooth mass distributions, two images of a star of radius $R$ approaching a caustic brighten as $t^{-1/2}$ and reach a peak magnification $\sim 10^{6}\, (10\, R_{\odot}/R)^{1/2}$ before merging on the critical curve. We show that a mass fraction ($\kappa_\star \gtrsim \, 10^{-4.5}$) in microlenses inevitably disrupts the smooth caustic into a network of corrugated microcaustics, and produces light curves with numerous peaks. Using analytical calculations and numerical simulations, we derive the characteristic width of the network, caustic-crossing frequencies, and peak magnifications. For the lens parameters of a recent detection and a population of intracluster stars with $\kappa_\star \sim 0.01$, we find a source-plane width of $\sim 20 \, {\rm pc}$ for the caustic network, which spans $0.2 \, {\rm arcsec}$ on the image plane. A source star takes $\sim 2\times 10^4$ years to cross this width, with a total of $\sim 6 \times 10^4$ crossings, each one lasting for $\sim 5\,{\rm hr}\,(R/10\,R_\odot)$ with typical peak magnifications of $\sim 10^{4} \left( R/ 10\,R_\odot \right)^{-1/2}$. The exquisite sensitivity of caustic-crossing events to the granularity of the lens-mass distribution makes them ideal probes of dark matter components, such as compact halo objects and ultralight axion dark matter.Comment: 29 pages, 11 figures, 2 tables; changed to match published versio

Gravitational Lensing Signatures of Axion Dark Matter Minihalos in Highly Magnified Stars

Axions are a viable candidate for Cold Dark Matter (CDM) which should generically form minihalos of sub-planetary masses from white-noise isocurvature density fluctuations if the Peccei-Quinn phase transition occurs after inflation. Despite being denser than the larger halos formed out of adiabatic fluctuations from inflation, axion minihalos have surface densities much smaller than the critical value required for gravitational lensing to produce multiple images or high magnification, and hence are practically undetectable as lenses in isolation. However, their lensing effect can be enhanced when superposed near critical curves of other lenses. We propose a method to detect them through photometric monitoring of recently discovered caustic transiting stars behind cluster lenses, under extreme magnification factors $\mu \gtrsim 10^3$--$10^4$ as the lensed stars cross microlensing caustics induced by intracluster stars. For masses of the first gravitationally collapsed minihalos in the range $\sim 10^{-15}$--$10^{-8}\,h^{-1}\,M_\odot$, we show that axion minihalos in galaxy clusters should collectively produce subtle surface density fluctuations of amplitude $\sim 10^{-4}$--$10^{-3}$ on projected length scales of $\sim 10$--$10^4\,$AU, which imprint irregularities in the microlensing light curves of caustic transiting stars. We estimate that, inside a cluster halo and over the age of the Universe, most of these minihalos are likely to avoid dynamic disruption by encounters with stars or other minihalos.Comment: Accepted for publication in the Astronomical Journal; Figure 6 updated with time axis corrected; no major changes in conclusion and tex

On the Importance of Local Sources of Radiation in Cosmological Absorption Systems

An upper limit to the importance of local sources of radiation compared to the cosmic background in cosmological absorption systems is derived, as a simple consequence of the conservation of surface brightness. The limit depends only on the rate of incidence of the absorbers and the mean free path of the radiation. It is found that, on average, the ionizing radiation intensity from local sources in Lyman limit systems at z>2 must be less than half of the intensity of the cosmic background. In absorbers with column densities much lower than Lyman limit systems, the local source contribution must be negligible. The limit on the ratio of local source to background intensities is then applied to the class of damped Lya absorption systems with detectable excited CII lines. A cooling rate of the gas in these systems has been measured by Wolfe et al., who assumed that the balancing heating source is photoelectric heating on dust by light at ~ 1500 A . The intensity from local star formation at this wavelength in this class of damped Lya systems is found to be at most ~ 3 times the background intensity. If the heating source is indeed photoelectric heating of dust, the background created by sources associated with damped Lya systems can then be estimated from the average cooling rates measured in the absorbers. Current results yield a background intensity higher than previous estimates based on observed galaxy and quasar luminosity functions, although with a large uncertainty. The possibility of other sources of heating, such as shock-heating in a turbulent medium, should be explored.Comment: submitted to ApJ

The Distribution of Mass and Gas in the Center of Clusters of Galaxies Implied by X-Ray and Lensing Observations

Observations of gravitational lensing indicate that the mass distribution in clusters of galaxies (where most of the mass is dark matter) is highly peaked towards the center, while X-ray observations imply that the gas is more extended. The additional fact that the gas is cooling in the center has often led one to expect that the gas temperature should be lower near the center, and therefore the gas should be more concentrated than the dark matter. We show that such expectation is not correct, and that the gas temperature must remain approximately constant within the cooling region in order to have consistency with the observed X-ray profiles and lenses. A multiphase cooling flow naturally produces an approximately constant temperature profile, and a more extended distribution for the gas compared to the mass. Cool phases are deposited at relatively large radius, while hot phases are adiabatically heated as they flow inwards and can keep the average temperature constant. Thus, cooling flows result in an {\it increase} of the central temperature, relative to a case where there is no cooling and the gas follows the mass distribution. The increased central temperatures caused by cooling flows give a characteristic core radius to the gas profiles, which is of order the cooling radius. This provides a natural explanation for the typical cores observed in X-ray clusters. It also brings into better agreement with observations the rate of cluster evolution expected in self-similar hierarchical models. We propose that clusters having core radii much larger than their cooling radii are in the process of merging and are not in dynamical equilibrium.Comment: 30 pages, PostScrip

Gravitational lensing effects on the baryonic acoustic oscillation signature in the redshift-space correlation function

Measurements of the baryonic acoustic oscillation (BAO) peak in the redshift-space correlation function yield the angular diameter distance D A ( z ) and the Hubble parameter H ( z ) as a function of redshift, constraining the properties of dark energy and space curvature. We discuss the perturbations introduced in the galaxy correlation function by gravitational lensing through the effect of magnification bias and its cross correlation with the galaxy density. At the BAO scale, gravitational lensing adds a small and slowly varying component to the galaxy correlation function and does not change its shape significantly, through which the BAO peak is measured. The relative shift in the position of the BAO peak caused by gravitational lensing in the angle-averaged correlation function is 10 − 4 at z = 1 , rising to 10 − 3 at z = 2.5 . Lensing effects are stronger near the line of sight; however, the relative peak shift increases only to 10 − 3.3 and 10 − 2.4 at z = 1 and z = 2.5 , when the galaxy correlation is averaged within 5 degrees of the line of sight (containing only 0.4% of the galaxy pairs in a survey). Furthermore, the lensing contribution can be measured separately and subtracted from the observed correlation at the BAO scale