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Recent surveys monitoring millions of light curves of resolved stars in the LMC have discovered several microlensing events. Unresolved stars could however significantly contribute to the microlensing rate towards the LMC. Monitoring pixels, as opposed to individual stars, should be able to detect stellar variability as a variation of the pixel flux. We present a first application of this new type of analysis (Pixel Method) to the LMC Bar. We describe the complete procedure applied to the EROS 91-92 data (one tenth of the existing CCD data set) in order to monitor pixel fluxes. First, geometric and photometric alignments are applied to each image. Averaging the images of each night reduces significantly the noise level. Second, one light curve for each of the $2.1~10^6$pixels is built and pixels are lumped into 3.6″ × 3.6″ super-pixels, one for each elementary pixel. An empirical correction is then applied to account for seeing variations. We find that the final super-pixel light curves fluctuate at a level of 1.8% of the flux in blue and 1.3% in red. We show that this noise level corresponds to about twice the expected photon noise and confirms previous assumptions used for the estimation of the contribution of unresolved stars. We also demonstrate our ability to correct very efficiently for seeing variations affecting each pixel flux. The technical results emphasised here show the efficacy of the Pixel Method and allow us to study luminosity variations due to possible microlensing events and variable stars in two companion papers.

The first stars in the history of the universe are likely to form in the dense central regions of [?]105-106 M cold dark matter halos at z [?] 10-50. The annihilation of dark matter particles in these environments may lead to the formation of so-called dark stars, which are predicted to be cooler, larger, more massive, and potentially more long-lived than conventional population III stars. Here, we investigate the prospects of detecting high-redshift dark stars with the upcoming James Webb Space Telescope (JWST). We find that all dark stars with masses up to 103 M are intrinsically too faint to be detected by JWST at z > 6. However, by exploiting foreground galaxy clusters as gravitational telescopes do, certain varieties of cool (T eff [?] 30, 000 K) dark stars should be within reach at redshifts up to z [?] 10. If the lifetimes of dark stars are sufficiently long, many such objects may also congregate inside the first galaxies. We demonstrate that this could give rise to peculiar features in the integrated spectra of galaxies at high redshifts, provided that dark stars make up at least [?]1% of the total stellar mass in such objects.

We have proposed that the first phase of stellar evolution in the history of the universe may be dark (matter powered) stars (DSs), luminous objects powered by dark matter (DM) heating rather than by nuclear fusion, and in this paper we examine the history of these DSs. The power source is annihilation of weakly interacting massive particles (WIMPs) which are their own antiparticles. These WIMPs are the best motivated DM candidates and may be discovered by ongoing direct or indirect detection searches (e.g., Fermi/GLAST) or at the Large Hadron Collider at CERN. A new stellar phase results, powered by DM annihilation as long as there is a DM fuel, from millions to billions of years. We build up the DSs from the time DM heating becomes the dominant power source, accreting more and more matter onto them. We have included many new effects in the current study, including a variety of particle masses and accretion rates, nuclear burning, feedback mechanisms, and possible repopulation of DM density due to capture. Remarkably, we find that in all these cases, we obtain the same result: the first stars are very large, 500-1000 times as massive as the Sun; as well as puffy (radii 1-10 AU), bright (106-107 L ), and cool (T surf < 10, 000 K) during the accretion. These results differ markedly from the standard picture in the absence of DM heating, in which the maximum mass is about 140 M and the temperatures are much hotter (T surf > 50,000 K). Hence DSs should be observationally distinct from standard Pop III stars. In addition, DSs avoid the (unobserved) element enrichment produced by the standard first stars. Once the DM fuel is exhausted, the DS becomes a heavy main-sequence star; these stars eventually collapse to form massive black holes that may provide seeds for the supermassive black holes observed at early times as well as explanations for recent ARCADE data and for intermediate-mass black holes.

The first stars in the universe form inside [?]106 M dark matter (DM) halos whose initial density profiles are laid down by gravitational collapse in hierarchical structure-formation scenarios. During the formation of the first stars in the universe, the baryonic infall compresses the DM further. The resultant DM density is presented here, using an algorithm originally developed by Young to calculate changes to the profile as a result of adiabatic infall in a spherical halo model; the Young prescription takes into account the noncircular motions of halo particles. The density profiles obtained in this way are found to be within a factor of 2 of those obtained using the simple adiabatic contraction prescription of Blumenthal and colleagues. Our results hold regardless of the nature of the DM or its interactions and rely merely on gravity. If the DM consists of weakly interacting massive particles, which are their own antiparticles, their densities are high enough that their annihilation in the first protostars can indeed provide an important heat source and prevent the collapse all the way to fusion. In short, a "Dark Star" phase of stellar evolution, powered by DM annihilation, may indeed describe the first stars in the universe.

The position of the giant elliptical galaxy M87 at the center of the Virgo Cluster means that the inferred column density of dark matter associated with both the cluster halo and the galaxy halo is quite large. This system is thus an important laboratory for studying massive dark objects in elliptical galaxies and galaxy clusters by gravitational microlensing, strongly complementing the studies of spiral galaxy halos performed in the Local Group. We have performed a microlensing survey of M87 with the WFPC2 instrument on the Hubble Space Telescope. Over a period of 30 days, with images taken once daily, we discover seven variable sources. Four are variable stars of some sort, two are consistent with classical novae, and one exhibits an excellent microlensing light curve, although with a very blue color, implying the somewhat disfavored possibility of a horizontal-branch source being lensed. On the basis of sensitivity calculations from artificial stars and from artificial light curves, we estimate the expected microlensing rate. We find that the detection of one event is consistent with a dark halo with a 20% contribution of microlensing objects for both M87 and the Virgo Cluster, similar to the value found from observations in the Local Group. Further work is required to test the hypothesized microlensing component to the cluster.

We present the AGAPE astrometric and photometric catalogue of 1579 variable stars in a $14'$ $\times$ $10'$ field centred on M 31. This work is the first survey devoted to variable stars in the bulge of M 31. The $R$ magnitudes of the objects and the $B-R$ colours suggest that our sample is dominated by red long-period variable stars (LPV), with a possible overlap with Cepheid-like type II stars. Fits of the light curves with sinusoids suggest that a large fraction of the stars correspond to periodic or semi-periodic objects with periods longer than 100 days. Twelve nova candidates are identified. Correlations with other catalogues suggest that 2 novae could be recurrent novae and provide possible optical counterparts for 2 supersoft X-ray sources candidates observed with Chandra.

In the Cardassian model, dark energy density arises from modifications to the Friedmann equation, which becomes H2 = g(rM), where g(rM) is a new function of the energy density. The universe is flat, matter dominated, and accelerating. The distance-redshift relation predictions of generalized Cardassian models can be very different from generic quintessence models, and can be differentiated with data from upcoming pencil beam surveys of Type Ia supernovae such as Supernova/Acceleration Probe (SNAP). We have found the interesting result that, once Om is known to 10% accuracy, SNAP will be able to determine the sign of the time dependence of the dark energy density. Knowledge of this sign (which is related to the weak energy condition) will provide a first discrimination between various cosmological models that fit the current observational data (cosmological constant, quintessence, Cardassian expansion). Further, we have performed Monte Carlo simulations to illustrate how well one can reproduce the form of the dark energy density with SNAP. To be concrete we study a class of two-parameter (n, q) generalized Cardassian models that includes the original Cardassian model (parameterized by n only) as a special case. Examples are given of modified polytropic (MP) Cardassian models that fit current supernova and cosmic microwave background data, and prospects for differentiating between MP Cardassian and other models in future data are discussed. We also note that some Cardassian models can satisfy the weak energy condition w > -1 even with a dark energy component that has an effective equation of state wX < -1.

Pixel microlensing, i.e., gravitational microlensing of unresolved stars, can be used to explore distant stellar systems and, as a bonus, may be able to detect extragalactic planets. In these studies, binary lens events with multiple high-magnification peaks are crucial. Considering only those events that exhibit caustic crossings, we estimate the fractions of binary events in several pixel microlensing example surveys and compare them to the fraction of binary events in a classical survey with resolved stars. We find a considerable enhancement of the relative rate of binary events in pixel microlensing surveys relative to surveys with resolved sources. We calculate the rate distribution of binary events with respect to the time between caustic crossings. We consider possible surveys of M31 with ground-based telescopes and surveys of M87 with the Hubble Space Telescope and the Next Generation Space Telescope. For the latter, a pixel microlensing survey taking one image a day may observe on the order of 12 binary events per month.

We consider the lightest neutralino, a general combination of photino, zino and higgsinos in the minimal supersymmetric extension of the standard model, as a component of the dark matter in the halo of our galaxy. We compute the neutralino cosmological relic abundance to estimate its contribution to the dark halo density. Due to its spin-independent coherent interactions with nuclei, mediated mainly by Higgs bosons, neutralinos may be efficiently captured by the earth and the sun or interact in detectors. They could thus be detected in indirect and direct searches. Accelerator bounds have recently severely restricted the parameter space. We study neutralinos lighter than the W-boson and find that present dark matter searches impose practically no bounds in the regions allowed by accelerators. We show that part of those regions will be accessible to improved searches in the near future (particularly, looking for upgoing muons in underground detectors).

The first stars to form in the universe may have been dark stars, powered by dark matter annihilation instead of nuclear fusion. The initial amount of dark matter gathered by the star gravitationally can sustain it only for a limited period of time. It has been suggested that capture of additional dark matter from the environment can prolong the dark star phase even to the present day. Here we show that this capture process is ineffective to prolong the life of the first generation of dark stars. We construct a Monte Carlo simulation that follows each weakly interacting massive particle (WIMP) in the dark matter halo as its orbit responds to the formation and evolution of the dark star, as it scatters off the star's nuclei, and as it annihilates inside the star. A rapid depletion of the WIMPs on orbits that cross the star causes the demise of the first generation of dark stars. We suggest that a second generation of dark stars may in principle survive much longer through capture. We comment on the effect of relaxing our assumptions.

The article presents the poem "Spring and Autumn," by Elisabeth Gondolo. First Line: Spring things are beautiful: Last Line: Stormy night polishing the ancient sun.