Epsilon Aurigae in an evolutionary context

Basic observational data of Epsilon Aurigae are summarized and used as the basis of a discussion of possible evolutionary states of the system. Constraints posed by the presence of a cold disk surrounding the secondary star are also outlined. Possible evolutionary models of the F0 Ia supergiant range from pre-main sequence contraction through shell hydrogen burning, core helium burning, to shell helium burning, depending on the absolute luminosity of the system, for models in which no mass transfer has taken place. Models invoking binary interaction include core and shell helium burning stars, and pre-white dwarfs, again depending on the absolute luminosity of the system. A massive shell helium burning star or a pre-white dwarf mass transfer remnant would appear the most likely of these models at present. Observational tests of these models are briefly outlined

Formation of low-mass x-ray binaries; 1, constraints on hydrogen-rich donors at the onset of the x-ray phase

We identify and quantify the set of constraints that neutron star-normal star binaries must satisfy in order to become observable LMXBs. These constraints are related to (i) the thermal and hydrostatic equilibrium of the donors, (ii) the degree to which the mass transfer process is conservative, and (iii) the age of the systems. They divide the parameter space of potential LMXBs in several distinct parts, of which those that actually become LMXBs at the onset of mass transfer occupy only a small part. Of the remainder, many become unstable to dynamical time scale mass transfer either at the onset or later in the course of mass transfer, and enter common envelope evolution. Others experience super-Eddington mass transfer but may eventually survive to become LMXBs. These survivors arguably include binary millisecond pulsars with orbital periods in excess of 100\,d ultrashort-period LMXBs with hydrogen-deficient donors, and long-period LMXBs with giant donors

LISA Measurement of Gravitational Wave Background Anisotropy: Hexadecapole Moment via a Correlation Analysis

We discuss spatial fluctuations in the gravitational wave background arising from unresolved Galactic binary sources, such as close white dwarf binaries, due to the fact the galactic binary source distribution is anisotropic. We introduce a correlation analysis of the two data streams of the Laser Interferometer Space Antenna (LISA) to extract spherical harmonic coefficients, in an independent manner, of the hexadecapole moment ($l=4$) related to the projected two-dimensional density distribution of the binary source population. The proposed technique complements and improves over previous suggestions in the literature to measure the gravitational wave background anisotropy based on the time modulation of data as LISA orbits around the Sun. Such techniques, however, are restricted only to certain combinations of spherical harmonic coefficients of the galaxy with no ability to separate them individually. With LISA, $m=2,3$ and 4 coefficients of the hexadecapole ($l=4$) can be measured with signal-to-noise ratios at the level of 10 and above in a certain coordinate system. In addition to the hexadecapole coefficients, when combined with the time modulation analysis, the correlation study can also be used, in principle, to measure quadrupole coefficients of the binary distribution.Comment: 8 pages, 2 figure

Cataclysmic Variables and a New Class of Faint UV Stars in the Globular Cluster NGC 6397

We present evidence that the globular cluster NGC 6397 contains two distinct classes of centrally-concentrated UV-bright stars. Color-magnitude diagrams constructed from U, B, V, and I data obtained with the HST/WFPC2 reveal seven UV-bright stars fainter than the main-sequence turnoff, three of which had previously been identified as cataclysmic variables (CVs). Lightcurves of these stars show the characteristic ``flicker'' of CVs, as well as longer-term variability. A fourth star is identified as a CV candidate on the basis of its variability and UV excess. Three additional UV-bright stars show no photometric variability and have broad-band colors characteristic of B stars. These non-flickering UV stars are too faint to be extended horizontal branch stars. We suggest that they could be low-mass helium white dwarfs, formed when the evolution of a red giant is interrupted, due either to Roche-lobe overflow onto a binary companion, or to envelope ejection following a common-envelope phase in a tidal-capture binary. Alternatively, they could be very-low-mass core-He-burning stars. Both the CVs and the new class of faint UV stars are strongly concentrated toward the cluster center, to the extent that mass segregation from 2-body relaxation alone may be unable to explain their distribution.Comment: 11 pages plus 3 eps figures; LaTeX using aaspp4.sty; to appear in The Astrophysical Journal Letter

Consequences of gravitational radiation recoil

Coalescing binary black holes experience an impulsive kick due to anisotropic emission of gravitational waves. We discuss the dynamical consequences of the recoil accompanying massive black hole mergers. Recoil velocities are sufficient to eject most coalescing black holes from dwarf galaxies and globular clusters, which may explain the apparent absence of massive black holes in these systems. Ejection from giant elliptical galaxies would be rare, but coalescing black holes are displaced from the center and fall back on a time scale of order the half-mass crossing time. Displacement of the black holes transfers energy to the stars in the nucleus and can convert a steep density cusp into a core. Radiation recoil calls into question models that grow supermassive black holes from hierarchical mergers of stellar-mass precursors.Comment: 5 pages, 4 figures, emulateapj style; minor changes made; accepted to ApJ Letter

Angular Resolution of the LISA Gravitational Wave Detector

We calculate the angular resolution of the planned LISA detector, a space-based laser interferometer for measuring low-frequency gravitational waves from galactic and extragalactic sources. LISA is not a pointed instrument; it is an all-sky monitor with a quadrupolar beam pattern. LISA will measure simultaneously both polarization components of incoming gravitational waves, so the data will consist of two time series. All physical properties of the source, including its position, must be extracted from these time series. LISA's angular resolution is therefore not a fixed quantity, but rather depends on the type of signal and on how much other information must be extracted. Information about the source position will be encoded in the measured signal in three ways: 1) through the relative amplitudes and phases of the two polarization components, 2) through the periodic Doppler shift imposed on the signal by the detector's motion around the Sun, and 3) through the further modulation of the signal caused by the detector's time-varying orientation. We derive the basic formulae required to calculate the LISA's angular resolution $\Delta \Omega_S$ for a given source. We then evaluate $\Delta \Omega_S$ for two sources of particular interest: monchromatic sources and mergers of supermassive black holes. For these two types of sources, we calculate (in the high signal-to-noise approximation) the full variance-covariance matrix, which gives the accuracy to which all source parameters can be measured. Since our results on LISA's angular resolution depend mainly on gross features of the detector geometry, orbit, and noise curve, we expect these results to be fairly insensitive to modest changes in detector design that may occur between now and launch. We also expect that our calculations could be easily modified to apply to a modified design.Comment: 15 pages, 5 figures, RevTex 3.0 fil

On the fraction of intermediate-mass close binaries that explode as type-Ia supernovae

Type-Ia supernovae (SNe-Ia) are thought to result from a thermonuclear runaway in white dwarfs (WDs) that approach the Chandrasekhar limit, either through accretion from a companion or a merger with another WD. I compile observational estimates of the fraction eta of intermediate-mass stars that eventually explode as SNe-Ia, supplement them with several new estimates, and compare them self-consistently. The estimates are based on five different methods, each utilising some observable related to the SN-Ia rate, combined with assumptions regarding the IMF: the ratio of SN-Ia to core-collapse rates in star-forming galaxies; the SN-Ia rate per unit star-formation rate; the SN-Ia rate per unit stellar mass; the iron to stellar mass ratio in galaxy clusters; and the abundance ratios in galaxy clusters. The five methods indicate that a fraction in the range eta~2-40% of all stars with initial masses of 3-8 M_sun (the generally assumed SN-Ia progenitors) explode as SNe-Ia. A fraction of eta~15% is consistent with all five methods for a range of plausible IMFs. Considering also the binarity fraction among such stars, the mass ratio distribution, the separation distribution, and duplicity (every binary can produce only one SN-Ia explosion), this implies that nearly every intermediate mass close binary ends up as a SN-Ia, or possibly more SNe-Ia than progenitor systems. Theoretically expected fractions are generally one to two orders of magnitude lower. The problem could be solved: if all the observational estimates are in error; or with a ``middle-heavy'' IMF; or by some mechanism that strongly enhances the efficiency of binary evolution toward SN-Ia explosion; or by a non-binary origin for SNe-Ia.Comment: MNRAS, accepted versio