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

`First Light' in the Universe; What Ended the "Dark Age"?

The universe would have been completely dark between the epoch of recombination and the development of the first non-linear structure. But at redshifts beyond 5 -- perhaps even beyond 20 -- stars formed within `subgalaxies' and created the first heavy elements; these same systems (together perhaps with `miniquasars') generated the UV radiation that ionized the IGM, and maybe also the first significant magnetic fields. Although we can already probe back to $z \simeq 5$, these very first objects may be so faint that their detection must await next-generation optical and infrared telescopes. Observations in other wavebands may offer indirect clues to when reionization occurred. Despite the rapid improvements in numerical simulations, the processes of star formation and feedback are likely to remain a challenge for the next decade.Comment: For ``Physics Reports'' special issue in memory of D.N. Schram

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

Soft X-ray Absorption by High-Redshift Intergalactic Helium

The Lyman alpha absorption from intergalactic, once-ionized helium (HeII) has been measured with HST in four quasars over the last few years, over the redshift range 2.4 < z < 3.2. These observations have indicated that the HeII reionization may not have been completed until z\simeq 2.8, and that large fluctuations in the intensity of the HeII-ionizing background were present before this epoch. The detailed history of HeII reionization at higher redshifts is, however, model-dependent and difficult to determine from these observations, because the IGM can be completely optically thick to Lya photons when only a small fraction of the helium remains as HeII. In addition, finding quasars in which the HeII Lya absorption can be observed becomes increasingly difficult at higher redshift, owing to the large abundance of hydrogen Lyman limit systems. It is pointed out here that HeII in the IGM should also cause detectable continuum absorption in the soft X-rays. The spectrum of a high-redshift source seen behind the IGM when most of the helium was HeII should recover from the HeII Lyman continuum absorption at an observed energy \sim 0.1 keV. Galactic absorption will generally be stronger, but not by a large factor; the intergalactic HeII absorption can be detected as an excess over the expected Galactic absorption from the 21cm HI column density. In principle, this method allows a direct determination of the fraction of helium that was singly ionized as a function of redshift, if the measurement is done on a large sample of high-redshift sources over a range of redshift.Comment: accepted to The Astrophysical Journal Letter

The star capture model for fueling quasar accretion disks

Although the powering mechanism for quasars is now widely recognized to be the accretion of matter in a geometrically thin disk, the transport of matter to the inner region of the disk where luminosity is emitted remains an unsolved question. Miralda-Escud\'e & Kollmeier (2005) proposed a model whereby quasars are fuelled when stars are captured by the accretion disk as they plunge through the gas. Such plunging stars can then be destroyed and deliver their mass to the accretion disk. Here we present the first detailed calculations for the capture of stars originating far from the accretion disk near the zone of influence of the central black hole. In particular we examine the effect of adding a perturbing mass to a fixed stellar cusp potential on bringing stars into the accretion disk where they can be captured. The work presented here will be discussed in detail in an upcoming publication Kennedy et al. (2010).Comment: 2 pages, 1 figure, to be published in Proceedings of IAU Symp. 271, Astrophysical Dynamics: from Stars to Galaxies, ed. N. Brummell &amp; A.S. Brun, Cambrige Univ Pres

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