Management of a capital stock by Strotz's naive planner

http://dx.doi.org/10.1016/j.jedc.2007.09.01

The Quiescent Spectrum of the AM CVn star CP Eri

We used the 6.5m MMT to obtain a spectrum of the AM CVn star CP Eri in quiescence. The spectrum is dominated by He I emission lines, which are clearly double peaked with a peak-to-peak separation of ~1900 km/s. The spectrum is similar to that of the longer period AM CVn systems GP Com and CE 315, linking the short and the long period AM CVn systems. In contrast with GP Com and CE 315, the spectrum of CP Eri does not show a central 'spike' in the line profiles, but it does show lines of SiII in emission. The presence of these lines indicates that the material being transferred is of higher metallicity than in GP Com and CE 315, which, combined with the low proper motion of the system, probably excludes a halo origin of the progenitor of CP Eri. We constrain the primary mass to M_1>0.27 M_sun and the orbital inclination to 33 degr < i < 80 degr. The presence of the He I lines in emission opens up the possibility for phase resolved spectroscopic studies which allows a determination of the system parameters and a detailed study of helium accretion disks under highly varying circumstances.Comment: 12 pages, 2 figures, accepted for publication in ApJ Letter

The Helium-Rich Cataclysmic Variable ES Ceti

We report photometry of the helium-rich cataclysmic variable ES Ceti during 2001-2004. The star is roughly stable at V ~ 17.0 and has a light curve dominated by a single period of 620 s, which remains measurably constant over the 3 year baseline. The weight of evidence suggests that this is the true orbital period of the underlying binary, not a "superhump" as initially assumed. We report GALEX ultraviolet magnitudes, which establish a very blue flux distribution (F_nu ~ nu^1.3), and therefore a large bolometric correction. Other evidence (the very strong He II 4686 emission, and a ROSAT detection in soft X-rays) also indicates a strong EUV source, and comparison to helium-atmosphere models suggests a temperature of 130+-10 kK. For a distance of 350 pc, we estimate a luminosity of (0.8-1.7)x10^34 erg/s, yielding a mass accretion rate of (2-4)x10^-9 M_sol/yr onto an assumed 0.7 M_sol white dwarf. This appears to be about as expected for white dwarfs orbiting each other in a 10 minute binary, assuming that mass transfer is powered by gravitational radiation losses. We estimate mean accretion rates for other helium-rich cataclysmic variables, and find that they also follow the expected M-dot ~ P_o^-5 relation. There is some evidence (the lack of superhumps, and the small apparent size of the luminous region) that the mass transfer stream in ES Cet directly strikes the white dwarf, rather than circularizing to form an accretion disk.Comment: PDF, 26 pages, 3 tables, 9 figures; accepted, in press, to appear February 2005, PASP; more info at http://cba.phys.columbia.edu

On Echo Outbursts and ER UMa Supercycles in SU UMa-type Cataclysmic Variables

I present a variation on Osaki's tidal-thermal-instability model for SU UMa behavior. I suggest that in systems with the lowest mass ratios, the angular-momentum dissipation in an eccentric disk is unable to sustain the disk on the hot side of the thermal instability. This decoupling of the tidal and thermal instabilities in systems with q < 0.07 allows a better explanation of the `echo' outbursts of EG Cnc and the short supercycles of RZ LMi and DI UMa. The idea might also apply to the soft X-ray transients.Comment: To appear in PASP, April 2001 (6 pages, 4 figs

Characteristics of Pro-c Analogies and Blends between Research Publications

Dr Inventor is a tool that aims to enhance the professional (Pro-c) creativity of researchers by suggesting novel hypotheses, arising from analogies between publications. Dr Inventor processes original research documents using a combination of lexical analysis and cognitive computation to identify novel comparisons that suggest new research hypotheses, with the objective of supporting a novel research publication. Research on analogical reasoning strongly suggests that the value of analogy-based comparisons depends primarily on the strength of the mapping (or counterpart projection) between the two analogs. An evaluation study of a number of computer generated comparisons attracted creativity ratings from a group of practising researchers. This paper explores a variety of theoretically motivated metrics operating on different conceptual spaces, identifying some weak associations with user's creativity ratings. Surprisingly, our results show that metrics focused on the mapping appear to have less relevance to creativity than metrics assessing the inferences (blended space). This paper includes a brief description of a research project currently exploring the best research hypothesis generated during this evaluation. Finally, we explore PCA as a means of specifying a combined multiple metrics from several blending spaces as a basis for detecting comparisons to enhance researchers’ creativity

Mode Identification from Combination Frequency Amplitudes in ZZ Ceti Stars

The lightcurves of variable DA stars are usually multi-periodic and non-sinusoidal, so that their Fourier transforms show peaks at eigenfrequencies of the pulsation modes and at sums and differences of these frequencies. These combination frequencies provide extra information about the pulsations, both physical and geometrical, that is lost unless they are analyzed. Several theories provide a context for this analysis by predicting combination frequency amplitudes. In these theories, the combination frequencies arise from nonlinear mixing of oscillation modes in the outer layers of the white dwarf, so their analysis cannot yield direct information on the global structure of the star as eigenmodes provide. However, their sensitivity to mode geometry does make them a useful tool for identifying the spherical degree of the modes that mix to produce them. In this paper, we analyze data from eight hot, low-amplitude DAV white dwarfs and measure the amplitudes of combination frequencies present. By comparing these amplitudes to the predictions of the theory of Goldreich & Wu, we have verified that the theory is crudely consistent with the measurements. We have also investigated to what extent the combination frequencies can be used to measure the spherical degree (ell) of the modes that produce them. We find that modes with ell > 2 are easily identifiable as high ell based on their combination frequencies alone. Distinguishing between ell=1 and 2 is also possible using harmonics. These results will be useful for conducting seismological analysis of large ensembles of ZZ Ceti stars, such as those being discovered using the Sloan Digital Sky Survey. Because this method relies only on photometry at optical wavelengths, it can be applied to faint stars using 4 m class telescopes.Comment: 73 pages, 22 figures, accepted in the Ap

Persistence in Cluster--Cluster Aggregation

Persistence is considered in diffusion--limited cluster--cluster aggregation, in one dimension and when the diffusion coefficient of a cluster depends on its size $s$ as $D(s) \sim s^\gamma$. The empty and filled site persistences are defined as the probabilities, that a site has been either empty or covered by a cluster all the time whereas the cluster persistence gives the probability of a cluster to remain intact. The filled site one is nonuniversal. The empty site and cluster persistences are found to be universal, as supported by analytical arguments and simulations. The empty site case decays algebraically with the exponent $\theta_E = 2/(2 - \gamma)$. The cluster persistence is related to the small $s$ behavior of the cluster size distribution and behaves also algebraically for $0 \le \gamma < 2$ while for $\gamma < 0$ the behavior is stretched exponential. In the scaling limit $t \to \infty$ and $K(t) \to \infty$ with $t/K(t)$ fixed the distribution of intervals of size $k$ between persistent regions scales as $n(k;t) = K^{-2} f(k/K)$, where $K(t) \sim t^\theta$ is the average interval size and $f(y) = e^{-y}$. For finite $t$ the scaling is poor for $k \ll t^z$, due to the insufficient separation of the two length scales: the distances between clusters, $t^z$, and that between persistent regions, $t^\theta$. For the size distribution of persistent regions the time and size dependences separate, the latter being independent of the diffusion exponent $\gamma$ but depending on the initial cluster size distribution.Comment: 14 pages, 12 figures, RevTeX, submitted to Phys. Rev.

Coaxial Jets and Sheaths in Wide-Angle-Tail Radio Galaxies

We add 20, 6 and 3.6 cm wavelength VLA observations of two WATs, 1231+674 and 1433+553, to existing VLA data at 6 and 20 cm, in order to study the variations of spectral index as a function of position. We apply the spectral tomography process that we introduced in our analysis of 3C67, 3C190 and 3C449. Both spectral tomography and polarization maps indicate that there are two distinct extended components in each source. As in the case of 3C449, we find that each source has a flat spectrum jet surrounded by a steeper spectrum sheath. The steep components tend to be more highly polarized than the flat components. We discuss a number of possibilities for the dynamics of the jet/sheath systems, and the evolution of their relativistic electron populations. While the exact nature of these two coaxial components is still uncertain, their existence requires new models of jets in FR I sources and may also have implications for the dichotomy between FR Is and FR IIs.Comment: 29 text pages plus 13 figures. Scheduled for publication in May 10, 1999 Ap

High Resolution Spectroscopy of the Pulsating White Dwarf G29-38

We present the analysis of time-resolved, high resolution spectra of the cool white dwarf pulsator, G29-38. From measuring the Doppler shifts of the H-alpha core, we detect velocity changes as large as 16.5 km/s and conclude that they are due to the horizontal motions associated with the g-mode pulsations on the star. We detect seven pulsation modes from the velocity time-series and identify the same modes in the flux variations. We discuss the properties of these modes and use the advantage of having both velocity and flux measurements of the pulsations to test the convective driving theory proposed for DAV stars. Our data show limited agreement with the expected relationships between the amplitude and phases of the velocity and flux modes. Unexpectedly, the velocity curve shows evidence for harmonic distortion, in the form of a peak in the Fourier transform whose frequency is the exact sum of the two largest frequencies. Combination frequencies are a characteristic feature of the Fourier transforms of light curves of G29-38, but before now have not been detected in the velocities, nor does published theory predict that they should exist. We compare our velocity combination frequency to combination frequencies found in the analysis of light curves of G29-38, and discuss what might account for the existence of velocity combinations with the properties we observe. We also use our high-resolution spectra to determine if either rotation or pulsation can explain the truncated shape observed for the DAV star's line core. We are able to eliminate both mechanisms: the average spectrum does not fit the rotationally broadened model and the time-series of spectra provides proof that the pulsations do not significantly truncate the line.Comment: 24 pages, 9 figures, Accepted for publication in ApJ (June