The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda I

Images have been obtained with the Hubble Space Telescope WFPC2 camera of Andromeda I, a dwarf spheroidal (dSph) galaxy that lies in the outer halo of M31. The resulting color-magnitude diagrams reveal for the first time the morphology of the horizontal branch in this system. We find that, in a similar fashion to many of the galactic dSph companions, the horizontal branch (HB) of And~I is predominantly red. Combined with the metal abundance of this dSph, this red HB morphology indicates that And I can be classified as a ``second parameter'' system in the outer halo of M31. This result then supports the hypothesis that the outer halo of M31 formed in the same extended chaotic manner as is postulated for the outer halo of the Galaxy.Comment: 26 pages using aas2pp4.sty, including 2 tables and 7 figures, to be published in AJ. Figure 1 is in gif form. To include in main ps file, use xv to create a ps file called Da_Costa.fig1.ps and uncomment appropriate lines in .tex fil

Electron Neutrino Mass Measurement by Supernova Neutrino Bursts and Implications on Hot Dark Matter

We present a new strategy for measuring the electron neutrino mass (\mnue) by future detection of a Galactic supernova in large underground detectors such as the Super-Kamiokande (SK). This method is nearly model-independent and one can get a mass constraint in a straightforward way from experimental data without specifying any model parameters for profiles of supernova neutrinos. We have tested this method using virtual data generated from a numerical model of supernova neutrino emission by realistic Monte-Carlo simulations of the SK detection. It is shown that this method is sensitive to \mnue of $\sim$ 3 eV for a Galactic supernova, and this range is as low as the prediction of the cold+hot dark matter scenario with a nearly degenerate mass hierarchy of neutrinos, which is consistent with the current observations of solar and atmospheric neutrino anomalies and density fluctuations in the universe.Comment: 4 pages including 1 figure, accepted by Phys. Rev. Let

Comments on scalar-tensor representation of nonlocally corrected gravity

The scalar-tensor representation of nonlocally corrected gravity is considered. Some special solutions of the vacuum background equations were obtained that indicate to the nonequivalence of the initial theory and its scalar-tensor representation.Comment: 6 pages, refs adde

On the width of the last scattering surface

We discuss the physical effects of some accelerated world models on the width of the last scattering surface (LSS) of the cosmic microwave background radiation (CMBR). The models considered in our analysis are X-matter (XCDM) and a Chaplygin type gas. The redshift of the LSS does not depend on the kind of dark energy (if XCDM of Chaplygin). Further, for a Chaplygin gas, the width of the LSS is also only weakly dependent on the kind of scenario (if we have dark energy plus cold dark matter or the unified picture).Comment: 10 pages, 1 figure, 2 tables, accepted to IJMP

Collapse of a Circular Loop of Cosmic String

We study the collapse of a circular loop of cosmic string. The gravitational field of the string is treated using the weak field approximation. The gravitational radiation from the loop is evaluated numerically. The memtric of the loop near the point of collapse is found analytically.Comment: 15 page

The Circumstellar Extinction of Planetary Nebulae

We analyze the dependence of circumstellar extinction on core mass for the brightest planetary nebulae (PNe) in the Magellanic Clouds and M31. We show that in all three galaxies, a statistically significant correlation exists between the two quantities, such that high core mass objects have greater extinction. We model this behavior, and show that the relation is a simple consequence of the greater mass loss and faster evolution times of high mass stars. The relation is important because it provides a natural explanation for the invariance of the [O III] 5007 planetary nebula luminosity function (PNLF) with population age: bright Population I PNe are extinguished below the cutoff of the PNLF. It also explains the counter-intuitive observation that intrinsically luminous Population I PNe often appear fainter than PNe from older, low-mass progenitors.Comment: 12 pages, 2 figures, accepted for ApJ, April 10, 199

Towards quantum gravity measurement by cold atoms

Peer reviewedPreprin

Observational constraints on the atmospheres of Uranus and Neptune from new measurements near 10 micron

Uranus was detected at 10.3, 11.6 and 12.5 micrometers approximately 1 micrometer spectral bandpasses, with respective brightness temperatures of 74.0 + 0.9 or -1.1, 67.6 + 0.5 or -0.7, and 65.5 + 0.6 or -0.7 K and the first detection of Neptune at 10.3 micrometers with a brightness temperature of 77.5 + 0.7 or -0.9 K. We also detected Neptune at 11.36 micrometers with 2% spectral resolution at 81.0 + 0.8 or -0.9 K. The 10 micrometers continuous of both Uranus and Neptune may in part be due to reflected solar radiation as well as thermal emission. If all of the observed flux is reflected light, then the maximum geometric albedo of Uranus is 0.115 + or - 0.020, and that of Neptune is 0.229 + or - 0.043. In the context of previous observations in this region, the maximum stratospheric C2H6 mixing ratio is found to be 3 x 10 to the -8 power for Uranus and 3 x 10 to the -6 power for Neptune. A value for the maximum mixing ratio in the stratosphere of Neptune on the order of 1 - 0.004 appears to be consistent with the available data

Large Angular Scale CMB Anisotropy Induced by Cosmic Strings

We simulate the anisotropy in the cosmic microwave background (CMB) induced by cosmic strings. By numerically evolving a network of cosmic strings we generate full-sky CMB temperature anisotropy maps. Based on $192$ maps, we compute the anisotropy power spectrum for multipole moments $\ell \le 20$. By comparing with the observed temperature anisotropy, we set the normalization for the cosmic string mass-per-unit-length $\mu$, obtaining $G\mu/c^2=1.05 {}^{+0.35}_{-0.20} \times10^{-6}$, which is consistent with all other observational constraints on cosmic strings. We demonstrate that the anisotropy pattern is consistent with a Gaussian random field on large angular scales.Comment: 4 pages, RevTeX, two postscript files, also available at http://www.damtp.cam.ac.uk/user/defects/ to appear in Physical Review Letters, 23 September 199