Population boundaries and gravitational-wave templates for evolving white dwarf binaries

We present results from our analysis of double white dwarf (DWD) binary star systems in the inspiraling and mass-transfer stages of their evolution. Theoretical constraints on the properties of the white dwarf stars allow us to map out the DWD trajectories in the gravitational-wave amplitude-frequency domain and to identify population boundaries that define distinct sub-domains where inspiraling and/or mass-transferring systems will and will not be found. We identify for what subset of these populations it should be possible to measure frequency changes and, hence, directly follow orbit evolutions given the anticipated operational time of the proposed space-based gravitational-wave detector, LISA. We show how such measurements should permit the determination of binary system parameters, such as luminosity distances and chirp masses, for mass-transferring as well as inspiraling systems. We also present results from our efforts to generate gravitational-wave templates for a subset of mass-transferring DWD systems that fall into one of the above mentioned sub-domains. Realizing that the templates from a point-mass approximation prove to be inadequate when the radii of the stars are comparable to the binary separation, we build an evolutionary model that includes finite-size effects such as the spin of the stars and tidal and rotational distortions. In two cases, we compare our model evolution with three-dimensional hydrodynamical models of mass-transferring binaries to demonstrate the accuracy of our results. We conclude that the match is good, except during the final phase of the evolution when the mass transfer rate is rapidly increasing and the mass donating star is severely distorted

On the Frequency of Potential Venus Analogs from Kepler Data

The field of exoplanetary science has seen a dramatic improvement in sensitivity to terrestrial planets over recent years. Such discoveries have been a key feature of results from the {\it Kepler} mission which utilizes the transit method to determine the size of the planet. These discoveries have resulted in a corresponding interest in the topic of the Habitable Zone (HZ) and the search for potential Earth analogs. Within the Solar System, there is a clear dichotomy between Venus and Earth in terms of atmospheric evolution, likely the result of the large difference ($\sim$ factor of two) in incident flux from the Sun. Since Venus is 95\% of the Earth's radius in size, it is impossible to distinguish between these two planets based only on size. In this paper we discuss planetary insolation in the context of atmospheric erosion and runaway greenhouse limits for planets similar to Venus. We define a ``Venus Zone'' (VZ) in which the planet is more likely to be a Venus analog rather than an Earth analog. We identify 43 potential Venus analogs with an occurrence rate (\eta_{\venus}) of $0.32^{+0.05}_{-0.07}$ and $0.45^{+0.06}_{-0.09}$ for M dwarfs and GK dwarfs respectively.Comment: 6 pages, 3 figures, 2 tables. Accepted for publication in the Astrophysical Journal Letters. More information and graphics can be found at the Habitable Zone Gallery (http://hzgallery.org

Observational Constraints on the Great Filter

The search for spectroscopic biosignatures with the next-generation of space telescopes could provide observational constraints on the abundance of exoplanets with signs of life. An extension of this spectroscopic characterization of exoplanets is the search for observational evidence of technology, known as technosignatures. Current mission concepts that would observe biosignatures from ultraviolet to near-infrared wavelengths could place upper limits on the fraction of planets in the galaxy that host life, although such missions tend to have relatively limited capabilities of constraining the prevalence of technosignatures at mid-infrared wavelengths. Yet searching for technosignatures alongside biosignatures would provide important knowledge about the future of our civilization. If planets with technosignatures are abundant, then we can increase our confidence that the hardest step in planetary evolution--the Great Filter--is probably in our past. But if we find that life is commonplace while technosignatures are absent, then this would increase the likelihood that the Great Filter awaits to challenge us in the future.Comment: 13 pages, 2 figure

Population boundaries for compact white-dwarf binaries in LISA's amplitude-frequency domain

In an earlier investigation, we proposed population boundaries for both inspiralling and mass-transferring double white dwarf (DWD) systems in the distance independent "absolute" amplitude-frequency domain of the proposed space-based gravitational-wave (GW) detector, {\it LISA}. The degenerate zero temperature mass-radius (M-R) relationship of individual white dwarf stars that we assumed, in combination with the constraints imposed by Roche geometries, permits us to identify five key population boundaries for DWD systems in various phases of evolution. Here we use the non-zero entropy donor M-R relations of \cite{DB2003} to modify these boundaries for both DWD and neutron star-white dwarf (NSWD) binary systems. We find that the mass-transferring systems occupy a larger fraction of space in ``absolute'' amplitude-frequency domain compared to the simpler T=0 donor model. We also discuss how these boundaries are modified with the new evolutionary phases found by \cite{Deloyeetal2007}. In the initial contact phase, we find that the contact boundaries, which are the result of end of inspiral evolution, would have some width, as opposed to an abrupt cut-off described in our earlier T=0 model. This will cause an overlap between a DWDs & NSWDs evolutionary trajectories, making them indistinguishable with only LISA observations within this region. In the cooling phase of the donor, which follows after the adiabatic donor evolution, the radius contracts, mass-transfer rate drops and slows down the orbital period evolution. Depending upon the entropy of the donor, these systems may then lie inside the fully degenerate T=0 boundaries, but LISA may be unable to detect these systems as they might be below the sensitivity limit or within the unresolved DWD background noise.Comment: 16 pages, 3 figures, 1 table, accepted to Astrophysical Journal; manuscript has been significantly improved from previous version as per referees comments, to include the effects of non-zero entropy WD donors on population boundarie