Steve B. Howell

AN EXPLORATION OF THE PARADIGM FOR THE 2¨3 HOUR PERIOD GAP IN CATACLYSMIC VARIABLES

We critically examine the basic paradigm for the origin of the 2¨3 hr period gap in cataclysmic variables (CVs), i.e., binary systems in which a white dwarf accretes from a relatively unevolved, low-mass donor star. The observed orbital period distribution for D300 CVs shows that these systems typically have orbital periods, in the range of D80 minutes to D8 hr but a distinct dearth of systems with P orb , This latter feature of the period distribution is often referred to as the ii period gap.ˇˇ 2 [ P orb (hr) [ 3. The conventional explanation for the period gap involves a thermal bloating of the donor star for hr due to mass transfer rates that are enhanced over those that could be driven by gravitational P orb Z 3 radiation (GR) losses alone (e.g., magnetic braking). If for some reason the supplemental angular momentum losses become substantially reduced when decreases below D3 hr, the donor star will relax P orb thermally and shrink inside of its Roche lobe. This leads to a cessation of mass transfer until GR losses can bring the system into Roche lobe contact again at hr. We carry out an extensive population P orb D 2 synthesis study of CVs, starting from D3 ] 106 primordial binaries and evolving some D2 ] 104 surviving systems through their CV phase. In particular we study current-epoch distributions of CVs in the and planes, where is the mass transfer rate, q is the M5 P orb , R 2 -P orb , M 2 -P orb , q-P orb , T eff -P orb , L 2 -P orb M0 mass ratio and and are the donor star mass, radius, eUective temperature, and M 2 /M 1 , M 2 , R 2 , T eff , L 2 luminosity, respectively. This work presents a new perspective on theoretical studies of the long-term evolution of CVs. In particular, we show that if the current paradigm is correct, the secondary masses in CVs just above the period gap should be as much as D50% lower than would be inferred if one assumes a main-sequence radius-mass relation for the donor star. We quantify the relations expected M 2 -P orb from models wherein the donor stars are thermally bloated. Finally, we propose speci—c observations, involving the determination of secondary masses in CVs, that would allow for a de—nitive test of the currently accepted model (i.e., interrupted thermal bloating) for the period gap in CVs.

On the Near-Infrared Size of Vega

Near-infrared (2.2 μm) long baseline interferometric observations of Vega are presented. The stellar disk of the star has been resolved, and the data have been fitted with a limb-darkened stellar disk of diameter ΘLD = 3.28 ± 0.01 mas. The derived effective temperature is Teff = 9553 ± 111 K. However, the residuals resulting from the stellar disk model appear to be significant and display organized structure. Instrumental artifacts, stellar surface structure, stellar atmosphere structure, and extended emission/scattering from the debris disk are discussed as possible sources of the residuals. While the current data set cannot uniquely determine the origin of the residuals, the debris disk is found to be the most likely source. A simple debris disk model, with 3%-6% of Vegas flux emanating from the disk at r 4 AU, can explain the residuals.

A Technique for Ultrahigh‐Precision CCD Photometry

We present techniques for wide-field ultrahigh-precision time-sampled CCD photometry. Representing a survey of field stars, our data consist of UBVRI photometry and a V-band time series of exposures on 5 consecutive nights covering 11,500 stars in a ∼1 deg 2 field. The resulting light curves reach precisions of 0.0020 mag per exposure in the brightest stars ( ). Light curves binned and averaged over 4.5 hr reach V ∼ 14 precisions of 0.00019 mag for the brightest stars. We present example light curves and discuss one application for surveys of this type: the search for transits by extrasolar planets.

The soft X-ray eclipses of HU Aqr

We present the results of extended monitoring observations in soft X-rays of the bright eclipsing polar (AM Herculis star) HU Aqr. It was observed between 1990 and 1998 by ROSAT for a total of 230 ksec using the PSPC and the HRI detectors and by EUVE with the Deep Survey Imager and the Spectrometer for a total of 580 ksec. The overall X-ray brightness of HU Aqr varied due to changes of the mass accretion rate by a factor of 40 over that period of time. At all occasions the X-ray light curve was characterized by a marked on-o behavior due to the self-eclipse of the accreting pole. The X-ray light curve showed eclipses by the companion star, the accretion stream and by an accretion curtain raised between the two stars in the binary. Narrow dips prior to the stellar eclipse are caused by the transit of the outer accretion stream. These dips display marked phase shifts, thus indicating a large movement of the threading region, where the stream couples to the magnetic eld. These shifts are shown to be related to changes of the mass accretion rate. Correspondingly, the spot longitude varied between 34 and 50. The X-ray light curves display clear evidence for the presence of an accretion curtain, which is raised all along the ballistic accretion stream down to the region where the bulk of matter couples onto magnetic eld lines. A lower limit to the mass accretion rate in the curtain is 6 10 12 M/yr, which is of order 10% of the total mass accretion rate. A linear t to all available eclipse egress times yields an updated orbital ephemeris of the system: BJED(T0) = 2449217:345872(35) +E 0:086820416195(47) with T0 the time of eclipse of the white dwarf centre of mass (BJED: barycentric Julian ephemeris time). The inclusion of a quadratic term gives a better t to the data but is not regarded as indication of a period change or asynchronous rotation but by a migration of the accretion spot over the surface of the white dwarf. For one particular data set obtained in a high accretion state, detailed light curve modeling was possible. The egress from eclipse lasted 1.3 s, which constrained the azimuthal extent of the accretion spot to less than 4 or 450 km. The spot extended vertically by0.015 Rwd. A comparison of the width of the stream dip and the extent of the accretion spot shows, that only the inner 60{80% of the stream are dense enough to re the soft X-ray engine. During the eclipse, HU Aqr was detected at a flux level of 6 10 14 erg cm 1 s 1 . The implied X-ray luminosity is LX =2 :2 10 29 erg s 1 , comparable with X-ray emission

Investigating the structure of the accretion disc in WZ Sge from multiwaveband time‐resolved spectroscopic observations – II

We present our second paper describing multiwaveband time-resolved spectroscopy of WZ Sge. We analyse the evolution of both optical and IR emission lines throughout the orbital period and find evidence, in the Balmer lines, for an optically thin accretion disc and an optically thick hotspot. Optical and IR emission lines are used to compute radial velocity curves. Fits to our radial velocity measurements give an internally inconsistent set of values for K1, γ and the phase of red-to-blue crossing. We present a probable explanation for these discrepancies, and provide evidence for similar behaviour in other short orbital period dwarf novae. Selected optical and IR spectra are measured to determine the accretion disc radii. Values for the disc radii are found to be strongly dependent on the assumed WD mass and binary orbital inclination. However, the separation of the peaks in the optical emission line (i.e., an indication of the outer disc radius) has been found to be constant during all phases of the supercycle period over the last 40 years.

Hubble Space Telescope Spectra of GW Librae: A Hot Pulsating White Dwarf in a Cataclysmic Variable

We have obtained Hubble Space Telescope UV spectra of the white dwarf in GW Lib, the only known nonradially pulsating white dwarf in a cataclysmic variable and the first known DAZQ variable. The UV light curve reveals large-amplitude (10%) pulsations in the UV with the same periods (646, 376, and 237 s) as those seen at optical wavelengths, but the mean spectrum fits with an average white dwarf temperature (14,700 K for a 0.6 M☉ white dwarf) that is too hot to be in the normal instability strip for ZZ Ceti stars. A better fit is achieved with a dual-temperature model (with 63% of the white dwarf surface at a temperature of 13,300 K and 37% at 17,100 K) and a higher mass (0.8 M☉) white dwarf with 0.1 solar metal abundance. Since the blue edge of the instability strip moves to higher temperature with increasing mass, the lower temperature of this model is within the instability strip. However, the presence of accretion likely causes abundance and atmospheric temperature differences in GW Lib compared to all known single white dwarf pulsators, and the current models that have been capable of explaining ZZ Ceti stars may not apply.

Periodic photometric variability of the black hole binary V404 Cygni

Periodic variations of V404 Cyg having a period of 6.474 +/- 0.005 d with a full amplitude of 0.2 mag were discovered with CCD photometry, in the I band, of the compact star. The light curve consists of two maxima and two minima per cycle, and the shape suggests that it is due to ellipsoidal variations of the secondary star which is tidally distorted by the presence of the massive compact object. It is proposed that the secondary is a 1 solar mass K0 IV star with MV of about 2.5 mag at a distance of about 3.5 kpc. It is suggested that if the secondary star is nearly filling its Roche lobe, then the orbital inclination must be about 60 deg. If the inclination is as high as 80 deg, then the secondary star fills about 90 percent of the Roche lobe. These limits imply a compact star mass of 8-12 solar masses and thus strengthen the evidence that the compact star is extremely massive and possibly a black hole.

Infrared Spectroscopy of the Secondary Star in ST Leonis Minoris: Implications for Evolution and High-/Low-State Behavior in Cataclysmic Variables

We present time-resolved K-band spectra of the polar ST LMi obtained during a time of zero mass transfer. Analysis of these first ever phase-resolved observations of an uncontaminated cataclysmic variable (CV) secondary star show no evidence for ellipsoidal variations, affects due to gravity darkening, or irradiation of the L1 point. The data indicate instead that the secondary star has a phase-dependent temperature including the existence of a localized cooler region centered near orbital phase 0.8. We examine a few possible explanations for such phenomena and conclude that the most likely cause is due to starspot activity. The location of the critical Roche surface calculated with respect to the physical stellar surface of the secondary star is shown to change during the lifetime of a cataclysmic variable and thereby effect its evolution. Previous suggestions of low-state behavior being caused by a starspot placed at L1 are shown not to work. However, an active chromosphere on the secondary star is both consistent with the current observations and may naturally explain high-/low-state behavior in CVs.

A realistic model for point-sources imaged on array detectors: The model and initial results

We have constructed a computer model for simulation of point-sources imaged on two-dimensional detectors. An attempt has been made to ensure that the model produces “data” that mimic real data taken with 2-D detectors. To be realistic, such simulations must include randomly generated noise of the appropriate type from all sources (e.g. source, background, and detector). The model is generic and accepts input values for parameters such as pixel size, read noise, source magnitude, and sky brightness. Point-source profiles are then generated with noise and detector characteristics added via our model. The synthetic data are output as simple integrations (onedimensional), as radial slices (two-dimensional), and as intensity-contour plots (three-dimensional). Each noise source can be turned on or off so that they can be studied separately as well as in combination to yield a realistic view of an image. This paper presents the basic properties of the model and some examples of how it can be used to simulate the effects of changing image position, image scale, signal strength, noise characteristics, and data reduction procedures.Use of the model has allowed us to confirm and quantify three points: 1) The use of traditionalsize apertures for photometry of faint point-sources adds substantial noise to the measurement which can significantly degrade the quality of the observation; 2) The number of pixels used to estimate the background is important and must be considered when estimating errors; and 3) The “CCD equation” normally used by the astronomical community consistently overestimates the signal-to-noise obtainable by a measurement while a revised equation, discussed here, provides a better estimator.

X-ray and ultraviolet observations of the dwarf nova VW Hyi in quiescence

We present an analysis of X-ray and ultraviolet (UV) data of the dwarf nova VW Hyi that were obtained with XMM-Newton during the quiescent state. The X-ray spectrum indicates the presence of an optically thin plasma in the boundary layer that cools as it settles on to the white dwarf. The plasma has a continuous temperature distribution that is well described by a power law or a cooling flow model with a maximum temperature of 6-8 keV. We estimate from the X-ray spectrum a boundary layer luminosity of 8 x 10 3 0 erg s - 1 , which is only 20 per cent of the disc luminosity. The rate of accretion on to the white dwarf is 5 x 10 - 1 2 M O . yr - 1 , about half of the rate in the disc. From the high-resolution X-ray spectra, we estimate that the X-ray emitting part of the boundary layer is rotating with a velocity of 540 km s - 1 , which is close to the rotation velocity of the white dwarf but is significantly smaller than the Keplerian velocity. We detect a 60-s quasi-periodic oscillation of the X-ray flux, which is likely to be due to the rotation of the boundary layer. The X-ray and the UV flux show strong variability on a time-scale of ∼1500 s. We find that the variability in the two bands is correlated and that the X-ray fluctuations are delayed by ∼100 s. The correlation indicates that the variable UV flux is emitted near the transition region between the disc and the boundary layer and that accretion rate fluctuations in this region are propagated to the X-ray emitting part of the boundary layer within ∼100 s. An orbital modulation of the X-ray flux suggests that the inner accretion disc is tilted with respect to the orbital plane. The elemental abundances in the boundary layer are close to their solar values.