Matthew R. Templeton

A transient radio jet in an erupting dwarf nova

Astrophysical jets seem to occur in nearly all types of accreting objects, from supermassive black holes to young stellar objects. On the basis of x-ray binaries, a unified scenario describing the disc/jet coupling has evolved and been extended to many accreting objects. The only major exceptions are thought to be cataclysmic variables: Dwarf novae, weakly accreting white dwarfs, show similar outburst behavior to x-ray binaries, but no jet has yet been detected. Here we present radio observations of a dwarf nova in outburst showing variable flat-spectrum radio emission that is best explained as synchrotron emission originating in a transient jet. Both the inferred jet power and the relation to the outburst cycle are analogous to those seen in x-ray binaries, suggesting that the disc/jet coupling mechanism is ubiquitous.

An Accurate Geometric Distance to the Compact Binary SS Cygni Vindicates Accretion Disc Theory

For Good Measure SS Cygni is a well-studied binary star system in the northern constellation Cygnus, consisting of a white dwarf that accretes matter from its companion star. Miller-Jones et al. (p. 950; see the Perspective by Schreiber) used radio observations to derive a model-independent distance to this prototypical accreting white dwarf system. The measurement places the system significantly closer than previously determined, reconciling the observed properties of SS Cygni with our current understanding of accretion theory. SS Cygni is much closer than previously thought, removing a major challenge to our understanding of accretion theory. [Also see Perspective by Schreiber] Dwarf novae are white dwarfs accreting matter from a nearby red dwarf companion. Their regular outbursts are explained by a thermal-viscous instability in the accretion disc, described by the disc instability model that has since been successfully extended to other accreting systems. However, the prototypical dwarf nova, SS Cygni, presents a major challenge to our understanding of accretion disc theory. At the distance of 159 ± 12 parsecs measured by the Hubble Space Telescope, it is too luminous to be undergoing the observed regular outbursts. Using very long baseline interferometric radio observations, we report an accurate, model-independent distance to SS Cygni that places the source substantially closer at 114 ± 2 parsecs. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects.

Finding The Instability Strip For Accreting Pulsating White Dwarfs From Hubble Space Telescope And Optical Observations

Time-resolved low resolution Hubble Space Telescope ultraviolet spectra together with ground-based optical photometry and spectra are used to constrain the temperatures and pulsation properties of six cataclysmic variables containing pulsating white dwarfs (WDs). Combining our temperature determinations for the five pulsating WDs that are several years past outburst with past results on six other systems shows that the instability strip for accreting pulsating WDs ranges from 10,500 to 15,000 K, a wider range than evident for ZZ Ceti pulsators. Analysis of the UV/optical pulsation properties reveals some puzzling aspects. While half the systems show high pulsation amplitudes in the UV compared to their optical counterparts, others show UV/optical amplitude ratios that are less than one or no pulsations at either wavelength region.

Secular Evolution in Mira Variable Pulsations

Stellar evolution theory predicts that asymptotic giant branch (AGB) stars undergo a series of short thermal pulses that significantly change their luminosity and mass on timescales of hundreds to thousands of years. These pulses are confirmed observationally by the existence of the short-lived radioisotope technetium in the spectra of some of these stars, but other observational consequences of thermal pulses are subtle and may only be detected over many years of observations. Secular changes in these stars resulting from thermal pulses can be detected as measurable changes in period if the star is undergoing Mira pulsations. It is known that a small fraction of Mira variables exhibit large secular period changes, and the detection of these changes among a larger sample of stars could therefore be useful in evolutionary studies of these stars. The American Association of Variable Star Observers (AAVSO) International Database currently contains visual data for over 1500 Mira variables. Light curves for these stars span nearly a century in some cases, making it possible to study the secular evolution of the pulsation behavior on these timescales. In this paper we present the results of our study of period change in 547 Mira variables using data from the AAVSO. We use wavelet analysis to measure the period changes in individual Mira stars over the span of available data. By making linear fits to the period versus time measurements, we determine the average rates of period change, d ln P/dt, for each of these stars. We find nonzero d ln P/dt at the 2 σ significance level in 57 of the 547 stars, at the 3 σ level in 21 stars, and at the level of 6 σ or greater in eight stars. The latter eight stars have been previously noted in the literature, and our derived rates of period change largely agree with published values. The largest and most statistically significant d ln P/dt are consistent with the rates of period change expected during thermal pulses on the AGB. A number of other stars exhibit nonmonotonic period change on decades-long timescales, the cause of which is not yet known. In the majority of stars, the period variations are smaller than our detection threshold, meaning the available data are not sufficient to unambiguously measure slow evolutionary changes in the pulsation period. It is unlikely that more stars with large period changes will be found among heretofore well-observed Mira stars in the short term, but continued monitoring of these and other Mira stars may reveal new and serendipitous candidates in the future.

Eclipses during the 2010 Eruption of the Recurrent Nova U Scorpii

The eruption of the recurrent nova U Scorpii on 2010 January 28 is now the all-time best observed nova event. We report 36,776 magnitudes throughout its 67 day eruption, for an average of one measure every 2.6 minutes. This unique and unprecedented coverage is the first time that a nova has had any substantial amount of fast photometry. With this, two new phenomena have been discovered: the fast flares in the early light curve seen from days 9-15 (which have no proposed explanation) and the optical dips seen out of eclipse from days 41-61 (likely caused by raised rims of the accretion disk occulting the bright inner regions of the disk as seen over specific orbital phases). The expanding shell and wind cleared enough from days 12-15 so that the inner binary system became visible, resulting in the sudden onset of eclipses and the turn-on of the supersoft X-ray source. On day 15, a strong asymmetry in the out-of-eclipse light points to the existence of the accretion stream. The normal optical flickering restarts on day 24.5. For days 15-26, eclipse mapping shows that the optical source is spherically symmetric with a radius of 4.1 R ☉. For days 26-41, the optical light is coming from a rim-bright disk of radius 3.4 R ☉. For days 41-67, the optical source is a center-bright disk of radius 2.2 R ☉. Throughout the eruption, the colors remain essentially constant. We present 12 eclipse times during eruption plus five just after the eruption.

GALEX AND OPTICAL OBSERVATIONS OF GW LIBRAE DURING THE LONG DECLINE FROM SUPEROUTBURST

The prototype of accreting, pulsating white dwarfs (GW Lib) underwent a large amplitude dwarf nova outburst in 2007. We used ultraviolet data from Galaxy Evolution Explorer and ground-based optical photometry and spectroscopy to follow GW Lib for three years following this outburst. Several variations are apparent during this interval. The optical shows a superhump modulation in the months following outburst, while a 19 minute quasi-periodic modulation lasting for several months is apparent in the year after outburst. A long timescale (about 4 hr) modulation first appears in the UV a year after outburst and increases in amplitude in the following years. This variation also appears in the optical two years after outburst but is not in phase with the UV. The pre-outburst pulsations are not yet visible after three years, likely indicating the white dwarf has not returned to its quiescent state.

Long-Period Variability in o Ceti

We carried out a new and sensitive search for long-period variability in the prototype of the Mira class of long-period pulsating variables, o Ceti (Mira A), the closest and brightest Mira variable. We conducted this search using an unbroken light curve from 1902 to the present, assembled from the visual data archives of five major variable star observing organizations from around the world. We applied several time-series analysis techniques to search for two specific kinds of variability: long secondary periods (LSPs) longer than the dominant pulsation period of ~333 days, and long-term period variation in the dominant pulsation period itself. The data quality is sufficient to detect coherent periodic variations with photometric amplitudes of 0.05 mag or less. We do not find evidence for coherent LSPs in o Ceti to a limit of 0.1 mag, where the amplitude limit is set by intrinsic, stochastic, low-frequency variability of approximately 0.1 mag. We marginally detect a slight modulation of the pulsation period similar in timescale to that observed in the Miras with meandering periods, but with a much lower period amplitude of ?2 days. However, we do find clear evidence of a low-frequency power-law component in the Fourier spectrum of o Cetis long-term light curve. The amplitude of this stochastic variability is approximately 0.1 mag at a period of 1000 days, and it exhibits a turnover for periods longer than this. This spectrum is similar to the red noise spectra observed in red supergiants.

The Spatially Resolved H(alpha)-Emitting Wind Structure of P Cygni

High spatial resolution observations of the Hα-emitting wind structure associated with the luminous blue variable star P Cygni were obtained with the Navy Prototype Optical Interferometer. These observations represent the most comprehensive interferometric data set on P Cyg to date. We demonstrate how the apparent size of the Hα-emitting region of the wind structure of P Cyg compares between the 2005, 2007, and 2008 observing seasons and how this relates to the Hα line spectroscopy. Using the data sets from 2005, 2007, and 2008 observing seasons, we fit a circularly symmetric Gaussian model to the interferometric signature from the Hα-emitting wind structure of P Cyg. Based on our results, we conclude that the radial extent of the Hα-emitting wind structure around P Cyg is stable at the 10% level. We also show how the radial distribution of the Hα flux from the wind structure deviates from a Gaussian shape, whereas a two-component Gaussian model is sufficient to fully describe the Hα-emitting region around P Cyg.

Multicolor Photometry of the Type II Cepheid Prototype W Virginis

We present the results of recent long-term BVRCIC photometric monitoring of the type II Cepheid prototype W Virginis. These new observations, made during the 2006 and 2007 observing season, represent the longest homogeneous, multicolor light curve of W Vir to date. The BVRCIC light and color curves show conclusively that W Vir exhibits modest but detectable cycle-to-cycle variations, the cause of which appears to be multiperiodicity rather than nonlinearity. We combined our V-band data with the five available years of ASAS-3 V-band photometry to obtain a 6.5 yr light curve that we then analyzed to obtain the pulsation spectrum of W Vir. We find a best-fit period P0 = 17.27134 days; along with this period and the integer-ratio harmonics P0/2 through P0/5 inclusive, we clearly detect two additional periods, P1 and Plow, that are close to but not exactly 2P0/3 and 2P0, respectively. The former, P1 = 11.52562 days, we interpret to be the first overtone mode; the latter, Plow = 34.59760 days, is close to the beat period of (P - P)-1, as well as to the value of 2P0. We interpret the previously reported but thus far unconfirmed descriptions of alternating minima as manifestations of this multiperiodicity. Finally, we use the period derived from the V-band light curve to define a new ephemeris: HJDmax = 2,452,758.172 + 17.27134E. We compiled an (O - C) diagram spanning 75 yr from 1932 to 2007 using a variety of published photometric data and visual observations from the American Association of Variable Star Observers and derived a period-change term for the ephemeris equal to -9.9 ? 10-7E2, indicating a period decrease.

Neutral and ionized emission lines in the type II Cepheid W Virginis

Aims. The aim of this work was to perform a multiphase spectroscopic study of W Vir which represents stars of the class of population II Cepheids, in order to trace the behaviour of emission features in different lines, and to use the data to describe the dynamical processes in the atmosphere of this star associated with the shock wave propagation. Methods. Our spectroscopic study of W Vir involved 18 high-resolution spectra obtained with the help of 3.5-m telescope of Apache Point Observatory. These spectra cover W Vir’s pulsational cycle with good phase resolution that enabled us to precisely fix the intervals of appearance, existence and disappearance of many anomalous spectral features, as well as to construct their radial velocity curves. Results. We detected and investigated the behaviour of emission and line doubling in many metallic lines (Na I, Fe I, Fe II, Ba II etc), as well as in hydrogen and helium lines. Analysis of the temporal characteristics of those emission features allowed us to make, in particular, the following conclusion. Conclusions. W Vir consists of two parts: the inner part, which is, in fact, a pulsating star itself with periodic shocks penetrating into the upper atmosphere, an outer one – a circumstellar envelope. The interaction of the main shock wave with the infalling envelope layers can explain the observed peculiarities of the spectral line variability.