A New Way to Detect Massive Black Holes in Galaxies: The Stellar Remnants of Tidal Disruption

We point out that the tidal disruption of a giant may leave a luminous (10^35-10^39 ergs/s), hot (10-100 eV) stellar core. The ``supersoft'' source detected by Chandra at the center of M31 may be such a core; whether or not it is, the observations have shown that such a core is detectable, even in the center of a galaxy. We therefore explore the range of expected observational signatures and how they may be used to (1) test the hypothesis that the M31 source is a remnant of tidal stripping and (2) discover evidence of black holes and disruption events in other galaxies.Comment: Four pages with 1 figure. Appeared in ApJL (2001, 551, L37

V605 Aql: The Older Twin of Sakurai's Object

New optical spectra have been obtained with VLT/FORS2 of the final helium shell flash (FF) star, V605 Aql, which peaked in brightness in 1919. New models suggest that this star is experiencing a very late thermal pulse. The evolution to a cool luminous giant and then back to a compact hot star takes place in only a few years. V605 Aql, the central star of the Planetary Nebula (PN), A58, has evolved from T$_{eff}\sim$5000 K in 1921 to $\sim$95,000 K today. There are indications that the new FF star, Sakurai's Object (V4334 Sgr), which appeared in 1996, is evolving along a similar path. The abundances of Sakurai's Object today and V605 Aql 80 years ago mimic the hydrogen deficient R Coronae Borealis (RCB) stars with 98% He and 1% C. The new spectra show that V605 Aql has stellar abundances similar to those seen in Wolf-Rayet [WC] central stars of PNe with ~55% He, and ~40% C. The stellar spectrum of V605 Aql can be seen even though the star is not directly detected. Therefore, we may be seeing the spectrum in light scattered around the edge of a thick torus of dust seen edge-on. In the present state of evolution of V605 Aql, we may be seeing the not too distant future of Sakurai's Object.Comment: 12 pages, 1 figure, ApJ Letters in pres

High-Field Quasiparticle Tunneling in Bi_2Sr_2CaCu_2O_8+delta: Negative Magnetoresistance in the Superconducting State

We report on the c-axis resistivity rho_c(H) in Bi_2Sr_2CaCu_2O_{8+\delta} that peaks in quasi-static magnetic fields up to 60 T. By suppressing the Josephson part of the two-channel (Cooper pair/quasiparticle) conductivity \sigma_c (H), we find that the negative slope of \rho_c(H) above the peak is due to quasiparticle tunneling conductivity \sigma_q(H) across the CuO_2 layers below H_{c2}. At high fields (a) \sigma_q(H) grows linearly with H, and (b) \rho_c(T) tends to saturate (sigma_c \neq 0) as T->0, consistent with the scattering at the nodes of the d-gap. A superlinear sigma_q(H) marks the normal state above T_c.Comment: 4p., 5 fig. (.eps), will be published in Phys. Rev. Let

Evidence for coexistence of the superconducting gap and the pseudo - gap in Bi-2212 from intrinsic tunneling spectroscopy

We present intrinsic tunneling spectroscopy measurements on small Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ mesas. The tunnel conductance curves show both sharp peaks at the superconducting gap voltage and broad humps representing the $c$-axis pseudo-gap. The superconducting gap vanishes at $T_c$, while the pseudo-gap exists both above and below $T_c$. Our observation implies that the superconducting and pseudo-gaps represent different coexisting phenomena.Comment: 5 pages, 4 figure

Discrimination between the superconducting gap and the pseudo-gap in Bi2212 from intrinsic tunneling spectroscopy in magnetic field

Intrinsic tunneling spectroscopy in high magnetic field ($H$) is used for a direct test of superconducting features in a quasiparticle density of states of high-$T_c$ superconductors. We were able to distinguish with a great clarity two co-existing gaps: (i) the superconducting gap, which closes as $H \to H_{c2}(T)$ and $T\to T_c(H)$, and (ii) the $c$-axis pseudo-gap, which does not change neither with $H$, nor $T$. Strikingly different magnetic field dependencies, together with previously observed different temperature dependencies of the two gaps ~\cite{Krasnov}, speak against the superconducting origin of the pseudo-gap.Comment: 4 pages, 4 eps figure

Influence of spin fluctuations on the superconducting transition temperature and resistivity in the t-J model at large N

Spin fluctuations enter the calculation of the superconducting transition temperature T$_c$ only in the next-to-leading order (i.e., in O(1/N$^2$) of the 1/N expansion of the t-J model. We have calculated these terms and show that they have only little influence on the value of T$_c$ obtained in the leading order O(1/N) in the optimal and overdoped region, i.e., for dopings larger than the instability towards a flux phase. This result disagrees with recent spin-fluctuation mediated pairing theories. The discrepancies can be traced back to the fact that in our case the coupling between electrons and spins is determined by the t-J model and not adjusted and that the spin susceptibility is rather broad and structureless and not strongly peaked at low energies as in spin-fluctuation models. Relating T$_c$ and transport we show that the effective interactions in the particle-particle and particle-hole channels are not simply related within the 1/N expansion by different Fermi surface averages of the same interactin as in the case of phonons or spin fluctuations. As a result, we find that large values for T$_c$ and rather small scattering rates in the normal state as found in the experiments can easily be reconciled with each other. We also show that correlation effects heavily suppress transport relaxation rates relative to quasiparticle relaxation rates in the case of phonons but not in the case of spin fluctuations.Comment: 16 pages, 10 figures, will appear in Phys. Rev.

The physical parameters, excitation and chemistry of the rim, jets and knots of the planetary nebula NGC 7009

We present long-slit optical spectra along the major axis of the planetary nebula NGC 7009. These data allow us to discuss the physical, excitation and chemical properties of all the morphological components of the nebula, including its remarkable systems of knots and jets. The main results of this analysis are the following: i) the electron temperature throughout the nebula is remarkably constant, T_e[OIII] = 10200K; ii) the bright inner rim and inner pair of knots have similar densities of N_e = 6000cm^{-3}, whereas a much lower density of N_e = 1500cm^{-3} is derived for the outer knots as well as for the jets; iii) all the regions (rim, inner knots, jets and outer knots) are mainly radiatively excited; and iv) there are no clear abundance changes across the nebula for He, O, Ne, or S. There is a marginal evidence for an overabundance of nitrogen in the outer knots (ansae), but the inner ones (caps) and the rim have similar N/H values that are at variance with previous results. Our data are compared to the predictions of theoretical models, from which we conclude that the knots at the head of the jets are not matter accumulated during the jet expansion through the circumstellar medium, neither can their origin be explained by the proposed HD or MHD interacting-wind models for the formation of jets/ansae, since the densities as well as the main excitation mechanisms of the knots, disagree with model predictions.Comment: Figure 1 was changed because features were misidentified in the previous version. 17 pages including 5 figures and 3 tables. ApJ in press. Also available at http://www.iac.es/galeria/denise

A New Look At Carbon Abundances In Planetary Nebulae. IV. Implications For Stellar Nucleosynthesis

This paper is the fourth and final report on a project designed to study carbon abundances in a sample of planetary nebulae representing a broad range in progenitor mass and metallicity. We present newly acquired optical spectrophotometric data for three Galactic planetary nebulae IC 418, NGC 2392, and NGC 3242 and combine them with UV data from the IUE Final Archive for identical positions in each nebula to determine accurate abundances of He, C, N, O, and Ne at one or more locations in each object. We then collect abundances of these elements for the entire sample and compare them with theoretical predictions of planetary nebula abundances from a grid of intermediate mass star models. We find some consistency between observations and theory, lending modest support to our current understanding of nucleosynthesis in stars below 8 M_o in birth mass. Overall, we believe that observed abundances agree with theoretical predictions to well within an order of magnitude but probably not better than within a factor of 2 or 3. But even this level of consistency between observation and theory enhances the validity of published intermediate-mass stellar yields of carbon and nitrogen in the study of the abundance evolution of these elements.Comment: 41 pages, 11 figures. Accepted for publication in the Astrophysical Journa

Theory of vortex excitation imaging via an NMR relaxation measurement

The temperature dependence of the site-dependent nuclear spin relaxation time T_1 around vortices is studied in s-wave and d-wave superconductors.Reflecting low energy electronic excitations associated with the vortex core, temperature dependences deviate from those of the zero-field case, and T_1 becomes faster with approaching the vortex core. In the core region, T_1^{-1} has a new peak below T_c. The NMR study by the resonance field dependence may be a new method to prove the spatial resolved vortex core structure in various superconductors.Comment: 5 pages, 3 figure

Superconducting Fluctuations and the Pseudogap in the Slightly-overdoped High-Tc Superconductor TlSr2CaCu2O6.8: High Magnetic Field NMR Studies

From measurements of the ^{63}Cu Knight shift (K) and the nuclear spin-lattice relaxation rate (1/T_{1}) under magnetic fields from zero up to 28 T in the slightly overdoped superconductor TlSr_{2}CaCu_{2}O_{6.8} (T_{c}=68 K), we find that the pseudogap behavior, {\em i.e.}, the reductions of 1/T_{1}T and K above T_{c} from the values expected from the normal state at high T, is strongly field dependent and follows a scaling relation. We show that this scaling is consistent with the effects of the Cooper pair density fluctuations. The present finding contrasts sharply with the pseudogap property reported previously in the underdoped regime where no field effect was seen up to 23.2 T. The implications are discussed.Comment: 10 pages, 4 GIF figures, to be published in Phys. Rev. Let