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Neptune-sized planets exhibit a wide range of compositions and densities, depending on factors related to their formation and evolution history, such as the distance from their host stars and atmospheric escape processes. They can vary from relatively low-density planets with thick hydrogen–helium atmospheres1,2to higher-density planets with a substantial amount of water or a rocky interior with a thinner atmosphere, such as HD 95338 b (ref. 3), TOI-849 b (ref. 4) and TOI-2196 b (ref. 5). The discovery of exoplanets in the hot-Neptune desert6, a region close to the host stars with a deficit of Neptune-sized planets, provides insights into the formation and evolution of planetary systems, including the existence of this region itself. Here we show observations of the transiting planet TOI-1853 b, which has a radius of 3.46 ± 0.08 Earth radii and orbits a dwarf star every 1.24 days. This planet has a mass of 73.2 ± 2.7 Earth masses, almost twice that of any other Neptune-sized planet known so far, and a density of 9.7 ± 0.8 grams per cubic centimetre. These values place TOI-1853 b in the middle of the Neptunian desert and imply that heavy elements dominate its mass. The properties of TOI-1853 b present a puzzle for conventional theories of planetary formation and evolution, and could be the result of several proto-planet collisions or the final state of an initially high-eccentricity planet that migrated closer to its parent star.

The interiors of giant planets remain poorly understood. Even for the planets in the Solar System, difficulties in observation lead to large uncertainties in the properties of planetary cores. Exoplanets that have undergone rare evolutionary processes provide a route to understanding planetary interiors. Planets found in and near the typically barren hot-Neptune ‘desert’1,2(a region in mass–radius space that contains few planets) have proved to be particularly valuable in this regard. These planets include HD149026b3, which is thought to have an unusually massive core, and recent discoveries such as LTT9779b4and NGTS-4b5, on which photoevaporation has removed a substantial part of their outer atmospheres. Here we report observations of the planet TOI-849b, which has a radius smaller than Neptune’s but an anomalously large mass of 39.1−2.6+2.7Earth masses and a density of 5.2−0.8+0.7grams per cubic centimetre, similar to Earth’s. Interior-structure models suggest that any gaseous envelope of pure hydrogen and helium consists of no more than 3.9−0.9+0.8per cent of the total planetary mass. The planet could have been a gas giant before undergoing extreme mass loss via thermal self-disruption or giant planet collisions, or it could have avoided substantial gas accretion, perhaps through gap opening or late formation6. Although photoevaporation rates cannot account for the mass loss required to reduce a Jupiter-like gas giant, they can remove a small (a few Earth masses) hydrogen and helium envelope on timescales of several billion years, implying that any remaining atmosphere on TOI-849b is likely to be enriched by water or other volatiles from the planetary interior. We conclude that TOI-849b is the remnant core of a giant planet.

The K2 mission will make use of the Keplerspacecraft and its assets to expand upon Kepler'sgroundbreaking discoveries in the fields of exoplanets and astrophysics through new and exciting observations. K2 will use an innovative way of operating the spacecraft to observe target fields along the ecliptic for the next 2-3 years. Early science commissioning observations have shown an estimated photometric precision near 400 ppm in a single 30 minute observation, and a 6-hr photometric precision of 80 ppm (both at V = 12). The K2 mission offers long-term, simultaneous optical observation of thousands of objects at a precision far better than is achievable from ground-based telescopes. Ecliptic fields will be observed for approximately 75 days enabling a unique exoplanet survey which fills the gaps in duration and sensitivity between the Keplerand TESS missions, and offers pre-launch exoplanet target identification for JWST transit spectroscopy. Astrophysics observations with K2 will include studies of young open clusters, bright stars, galaxies, supernovae, and asteroseismology.

The Z Camelopardalis stars IW And and V513 Cas are unusual in having outbursts following their standstills, in contrast to the usual Z Cam behavior of quiescence following standstills. In order to gain further understanding of these little-studied systems, we obtained spectra correlated with photometry from the AAVSO throughout a 3-4 month interval in 2011. In addition, time-resolved spectra were obtained in 2012 that provided orbital periods of 3.7 hr for IW And and 5.2 hr for V513 Cas. The photometry of V513 Cas revealed a regular pattern of standstills and outbursts with little time at quiescence, while IW And underwent many excursions from quiescence to outburst to short standstills. The spectra of IW And are similar to normal dwarf novae, with strong Balmer emission at quiescence and absorption at outburst. In contrast, V513 Cas shows a much flatter/redder spectrum near outburst with strong He IIemission and prominent emission cores in the Balmer lines. Part of this continuum difference may be due to reddening effects. While our attempts to model the outburst and standstill states of IW And indicate a mass accretion rate near 3 × 10-9M?yr-1, we could find no obvious reason why these systems behave differently following standstill, compared to normal Z Cam stars.

We present optical spectroscopy late in a deep minimum for the quintessential hydrogen-deficient carbon star R Coronae Borealis. Starting 3.5 years into the current deep and long minimum, we have secured observations that reveal some of the oddest optical spectra ever obtained for any astronomical object. Helium emission lines from triplet transitions, strong Ca II H and K emission, and forbidden lines of oxygen and nitrogen are the only spectral features observed. The spectra can be interpreted as coming from a chromospheric-like region lying above a carbon shell ejection front combined with a large-scale nebular-like region surrounding the star.

We have obtained optical wavelength (692 nm and 880 nm) speckle imaging of the planet Pluto and its largest moon Charon. Using our DSSI speckle camera attached to the Gemini North 8 m telescope, we collected high resolution imaging with an angular resolution of ?20 mas, a value at the Gemini-N telescope diffraction limit. We have produced for this binary system the first speckle reconstructed images, from which we can measure not only the orbital separation and position angle for Charon, but also the diameters of the two bodies. Our measurements of these parameters agree, within the uncertainties, with the current best values for Pluto and Charon. The Gemini-N speckle observations of Pluto are presented to illustrate the capabilities of our instrument and the robust production of high accuracy, high spatial resolution reconstructed images. We hope our results will suggest additional applications of high resolution speckle imaging for other objects within our solar system and beyond.

The Keplerspacecraft has collected data of high photometric precision and cadence almost continuously since operations began on 2009 May 2. Primarily designed to detect planetary transits and asteroseismological signals from solar-like stars, Keplerhas provided high-quality data for many areas of investigation. Unconditioned simple aperture time-series photometry is, however, affected by systematic structure. Examples of these systematics include differential velocity aberration, thermal gradients across the spacecraft, and pointing variations. While exhibiting some impact on Kepler'sprimary science, these systematics can critically handicap potentially ground-breaking scientific gains in other astrophysical areas, especially over long timescales greater than 10 days. As the data archive grows to provide light curves for 105stars of many years in length, Keplerwill only fulfill its broad potential for stellar astrophysics if these systematics are understood and mitigated. Post-launch developments in the Keplerarchive, data reduction pipeline and open source data analysis software have helped to remove or reduce systematic artifacts. This paper provides a conceptual primer to help users of the Keplerdata archive understand and recognize systematic artifacts within light curves and some methods for their removal. Specific examples of artifact mitigation are provided using data available within the archive. Through the methods defined here, the Keplercommunity will find a road map to maximizing the quality and employment of the Keplerlegacy archive.

We present the motivations for and methods we used to create a new ground-based photometric survey of the field targeted by the NASA Kepler Mission. The survey contains magnitudes for 4,416,007 sources in one or more of the UBV filters, including 1,861,126 sources detected in all three filters. The typical completeness limit is U [?] 18.7, B [?] 19.3, and V [?] 19.1 mag, but varies by location. The area covered is 191 deg2 and includes the areas on and between the 42 Kepler CCDs, as well as additional areas around the perimeter of the Kepler field. The major significance of this survey is our addition of U to the optical bandpass coverage available in the Kepler Input Catalog, which was primarily limited to the redder SDSS griz and D51 filters. The U coverage reveals a sample of the hottest sources in the field, many of which are not currently targeted by Kepler, but may be objects of astrophysical interest.

We present the motivations for and methods we used to create a new ground-based photometric survey of the field targeted by the NASA KeplerMission. The survey contains magnitudes for 4,416,007 sources in one or more of the UBVfilters, including 1,861,126 sources detected in all three filters. The typical completeness limit is U? 18.7, B? 19.3, and V? 19.1 mag, but varies by location. The area covered is 191 deg2and includes the areas on and between the 42 KeplerCCDs, as well as additional areas around the perimeter of the Keplerfield. The major significance of this survey is our addition of Uto the optical bandpass coverage available in the Kepler Input Catalog, which was primarily limited to the redder SDSS grizand D51 filters. The Ucoverage reveals a sample of the hottest sources in the field, many of which are not currently targeted by Kepler, but may be objects of astrophysical interest.

Fringing in CCD images is troublesome from the aspect of photometric quality and image flatness in the final reduced product. Additionally, defringing during calibration requires the inefficient use of time during the night to collect and produce a "supersky" fringe frame. The fringe pattern observed in a CCD image for a given near-IR filter is dominated by small thickness variations across the detector, with a second-order effect caused by the wavelength extent of the emission lines within the bandpass that produce the interference pattern. We show that essentially any set of emission lines that generally match the wavelength coverage of the night-sky emission lines within a bandpass will produce an identical fringe pattern. We present an easy, inexpensive, and efficient method that uses a neon lamp as a flat-field source and produces high-S/N fringe frames to use for defringing an image during the calibration process.

The recent plethora of sky surveys, especially the Sloan Digital Sky Survey, have discovered many low-mass (M< 0.45 M?) white dwarfs that should have cores made of nearly pure helium. These WDs come in two varieties: those with masses 0.2 < M< 0.45 M?and H envelopes so thin that they rapidly cool and those with M< 0.2 M?(often called extremely low mass [ELM] WDs) that have thick enough H envelopes to sustain 109yr of H burning. In both cases, these WDs evolve through the ZZ Ceti instability strip, Teff? 9000-12,000 K, where g-mode pulsations always occur in carbon/oxygen WDs. This expectation, plus theoretical work on the contrasts between C/O and He-core WDs, motivated our search for pulsations in 12 well-characterized helium WDs. We report here on our failure to find any pulsators among our sample. Though we have varying amplitude limits, it appears likely that the theoretical expectations regarding the onset of pulsations in these objects require closer consideration. We close by encouraging additional observations as new He WD samples become available, and we speculate on where theoretical work may be needed.

Three transiting exoplanet candidate stars were discovered in a ground-based photometric survey prior to the launch of NASA's Kepler mission. Kepler observations of them were obtained during Quarter 1 of the Kepler mission. All three stars are faint by radial velocity follow-up standards, so we have examined these candidates with regard to eliminating false positives and providing high confidence exoplanet selection. We present a first attempt to exclude false positives for this set of faint stars without high-resolution radial velocity analysis. This method of exoplanet confirmation will form a large part of the Kepler mission follow-up for Jupiter-sized exoplanet candidates orbiting faint stars. Using the Kepler light curves and pixel data, as well as medium-resolution reconnaissance spectroscopy and speckle imaging, we find that two of our candidates are binary stars. One consists of a late-F star with an early M companion, while the other is a K0 star plus a late M-dwarf/brown dwarf in a 19 day elliptical orbit. The third candidate (BOKS-1) is an r = 15 G8V star hosting a newly discovered exoplanet with a radius of 1.12 R Jupiter in a 3.9 day orbit.

We present and discuss five candidate exoplanetary systems identified with the Kepler spacecraft. These five systems show transits from multiple exoplanet candidates. Should these objects prove to be planetary in nature, then these five systems open new opportunities for the field of exoplanets and provide new insights into the formation and dynamical evolution of planetary systems. We discuss the methods used to identify multiple transiting objects from the Kepler photometry as well as the false-positive rejection methods that have been applied to these data. One system shows transits from three distinct objects while the remaining four systems show transits from two objects. Three systems have planet candidates that are near mean motion commensurabilities--two near 2:1 and one just outside 5:2. We discuss the implications that multi-transiting systems have on the distribution of orbital inclinations in planetary systems, and hence their dynamical histories, as well as their likely masses and chemical compositions. A Monte Carlo study indicates that, with additional data, most of these systems should exhibit detectable transit timing variations (TTVs) due to gravitational interactions, though none are apparent in these data. We also discuss new challenges that arise in TTV analyses due to the presence of more than two planets in a system.

We report on the discovery and the Rossiter-McLaughlin (R-M) effect of Kepler-8b, a transiting planet identified by the NASA Kepler Mission. Kepler photometry and Keck-HIRES radial velocities yield the radius and mass of the planet around this F8IV subgiant host star. The planet has a radius R P = 1.419 R J and a mass M P = 0.60 M J, yielding a density of 0.26 g cm-3, one of the lowest planetary densities known. The orbital period is P = 3.523 days and the orbital semimajor axis is 0.0483+0.0006 -0.0012 AU. The star has a large rotational vsin i of 10.5 +- 0.7 km s-1 and is relatively faint (V [?] 13.89 mag); both properties are deleterious to precise Doppler measurements. The velocities are indeed noisy, with scatter of 30 m s-1, but exhibit a period and phase that are consistent with those implied by transit photometry. We securely detect the R-M effect, confirming the planet's existence and establishing its orbit as prograde. We measure an inclination between the projected planetary orbital axis and the projected stellar rotation axis of l = -26.deg4 +- 10.deg1, indicating a significant inclination of the planetary orbit. R-M measurements of a large sample of transiting planets from Kepler will provide a statistically robust measure of the true distribution of spin-orbit orientations for hot Jupiters around F and early G stars.

Interacting binaries (IBs) consist of a secondary star that fills or is very close to filling its Roche lobe, resulting in accretion onto the primary star, which is often, but not always, a compact object. In many cases, the primary star, secondary star, and the accretion disk can all be significant sources of luminosity. SIM Lite will only measure the photocenter of an astrometric target, and thus determining the true astrometric orbits of such systems will be difficult. We have modified the Eclipsing Light Curve code to allow us to model the flux-weighted reflex motions of IBs, in a code we call REFLUX. This code gives us sufficient flexibility to investigate nearly every configuration of IB. We find that SIM Lite will be able to determine astrometric orbits for all sufficiently bright IBs where the primary or secondary star dominates the luminosity. For systems where there are multiple components that comprise the spectrum in the optical bandpass accessible to SIM Lite, we find it is possible to obtain absolute masses for both components, although multi-wavelength photometry will be required to disentangle the multiple components. In all cases, SIM Lite will at least yield accurate inclinations and provide valuable information that will allow us to begin to understand the complex evolution of mass-transferring binaries. It is critical that SIM Lite maintains a multi-wavelength capability to allow for the proper deconvolution of the astrometric orbits in multi-component systems.

Time-resolved spectra throughout the orbit of EF Eri during its low accretion state were obtained with the solar blind channel on the Advanced Camera for Surveys on board the Hubble Space Telescope. The overall spectral distribution exhibits peaks at 1500 and 1700 A, while the UV light curves display a quasi-sinusoidal modulation over the binary orbit. Models of white dwarfs (WDs) with a hot spot and cyclotron emission were attempted to fit the spectral variations throughout the orbit. A non-magnetic WD with a temperature of [?]10,000 K and a hot spot with a central temperature of 15,000 K generally match the broad absorptions at 1400 and 1600 A with those expected for the quasi-molecular H features H2 and H+ 2. However, the flux in the core of the Lya absorption does not go to zero, implying an additional component, and the flux variations throughout the orbit are not well matched at long wavelengths. Alternatively, a 9500 K WD with a 100 MG cyclotron component can fit the lowest (phase 0.0) fluxes, but the highest fluxes (phase 0.5) require an additional source of magnetic field or temperature. The 100 MG field required for the UV fit is much higher than that which fits the optical/IR wavelengths, which would support previous suggestions of a complex field structure in polars.

GALEX near-ultraviolet (NUV) and far-ultraviolet (FUV) light curves of three extremely low accretion rate polars show distinct modulations in their UV light curves. While these three systems have a range of magnetic fields from 13 to 70 MG, and of late-type secondaries (including a likely brown dwarf in SDSSJ121209.31+013627.7), the accretion rates are similar, and the UV observations imply some mechanism is operating to create enhanced emission zones on the white dwarf. The UV variations match in phase to the two magnetic poles viewed in the optical in WX LMi and to the single poles evident in the optical in SDSSJ1212109.31+013627.7 and SDSSJ103100.55+202832.2. Simple spot models of the UV light curves show that if hot spots are responsible for the UV variations, the temperatures are on the order of 10,000-14,000 K. For the single pole systems, the size of the FUV spot must be smaller than the NUV and in all cases the geometry is likely more complicated than a simple circular spot.

Do white dwarfs host asteroid systems? Although several lines of argument suggest that white dwarfs may be orbited by large populations of asteroids, transits would provide the most direct evidence. We demonstrate that the Kepler mission has the capability to detect transits of white dwarfs by asteroids. Because white-dwarf asteroid systems, if they exist, are likely to contain many asteroids orbiting in a spatially extended distribution, discoveries of asteroid transits can be made by monitoring only a small number of white dwarfs, compatible with Kepler's primary mission, which is to monitor stars with potentially habitable planets. Possible future missions that survey 10 times as many stars with similar sensitivity and minute-cadence monitoring can establish the characteristics of asteroid systems around white dwarfs, such as the distribution of asteroid sizes and semimajor axes. Transits by planets would be more dramatic, but the probability that they will occur is lower. Ensembles of planetary moons and/or the presence of rings around planets can also produce transits detectable by Kepler. The presence of moons and rings can significantly increase the probability that Kepler will discover planets orbiting white dwarfs, even while monitoring only a small number of them.

We present new Spitzer Infrared Spectrograph (IRS) observations of three intermediate polars: V1223 Sgr, EX Hya, and V603 Aql. We detected a strong, fading flare event from V1223 Sgr. During this event, the flux declined by a factor of 13 in 30 minutes. Given the similarity in the slope of its mid-infrared spectrum during this event to that of AE Aqr, we suggest that this event was caused by transient synchrotron emission. Thus, V1223 Sgr becomes the third cataclysmic variable known to be a synchrotron source. We were unable to confirm the mid-infrared excess noted by Harrison et al. (Paper I) for EX Hya, suggesting that this object is either not a synchrotron source, or is slightly variable. Due to a very high background, V603 Aql was not detected in the long-wavelength regions accessible to the IRS. Given the recent detection of SS Cygni at radio wavelengths during outburst, we extract archival Spitzer IRS spectra for this source obtained during two successive maxima. These spectra do not show a strong excess, but without simultaneous data at shorter wavelengths, it is not possible to determine whether there is any contribution to the mid-infrared fluxes from a synchrotron jet.

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.

We present the results of a search for periodic variables within 4078 time-series light curves and an analysis of the period-color plane for stars in the field of the open cluster NGC 2301. One hundred thirty-eight periodic variables were discovered, of which five are eclipsing binary candidates with unequal minima. The remaining 133 periodic variables appear to consist mainly of late-type stars whose variation is due to rotation modulated by star spot activity. The determined periods range from less than a day to over 14 days and have nearly unreddened B- Rcolors in the range of 0.8 to 2.8. The Barnes (2003) interpretation of the period-color plane of late type stars is tested with our data. Our data did not show distinct I and C sequences, likely due to nonmember field stars contaminating in the background, as we estimate the total contamination to be 43%. Using different assumptions, the gyrochronological age of the cluster is calculated to be 210 ± 25 Myr, which falls in the range of age values (164-250 Myr) determined by previous studies. Finally, we present evidence which nullifies the earlier suggestion that two of the variable stars in NGC 2301 might be white dwarfs.

We present the ultraviolet-optical-infrared spectral energy distribution of the low inclination novalike cataclysmic variable (CV) V592 Cassiopeiae, including new mid-infrared observations from 3.5 to 24 mm obtained with the Spitzer Space Telescope. At wavelengths shortward of 8 mm, the spectral energy distribution of V592 Cas is dominated by the steady state accretion disk, but there is flux density in excess of the summed stellar components and accretion disk at longer wavelengths. Reproducing the observed spectral energy distribution from ultraviolet to mid-infrared wavelengths can be accomplished by including a circumbinary disk composed of cool dust, with a maximum inner edge temperature of [?]500 K. The total mass of circumbinary dust in V592 Cas ([?]1021 g) is similar to that found from recent studies of infrared excess in magnetic CVs, and is too small to have a significant effect on the long-term secular evolution of the cataclysmic variable. The existence of circumbinary dust in V592 Cas is possibly linked to the presence of a wind outflow in this system, which can provide the necessary raw materials to replenish the circumbinary disk on relatively short timescales, and/or could be a remnant from the common envelope phase early in the formation history of the system.

We present new photometric and spectroscopic observations of the Iron stars AS 325 and XX Oph. Both stars are binaries composed of a Be star primary plus a luminous red star. Long-term optical photometry of AS 325 reveals it to be a 513-day period eclipsing binary with highly variable eclipses. The primary eclipse is that of the Be star with no secondary eclipse observed. This is not surprising given the small comparative size of the more massive companion. The red star in AS 325 is observed to be a variable star ranging from K2.5 to a M5 in spectral type and shown to be a supergiant of luminosity class II. The red secondary star is the major cause of the variable eclipses. A simple model is advanced to explain the complex behavior observed for Balmer hydrogen emission and Ca IIH and K absorption lines as a function of eclipse phase. Spatially resolved near-IR spectroscopy of XX Oph shows the binary to be sitting in a large cavity-like region of space surrounded by reflective dust. AS 325 and XX Oph are nearly identical binary systems.

Using Spitzer Space Telescope photometric observations of the eclipsing, interacting binary WZ Sge, we have discovered that the accretion disk is far more complex than previously believed. Our 4.5 and 8 mm time series observations reveal that the well-known gaseous accretion disk is surrounded by an asymmetric disk of dusty material with a radius approximately 15 times larger than the gaseous disk. This dust ring contains only a small amount of mass and is completely invisible at optical and near-IR wavelengths, hence consisting of "dark matter." We have produced a model dust ring using 1 mm spherical particles with a density of 3 g cm[?]3 and with a temperature profile ranging from 700 to 1500 K. Our discovery about the accretion disk structure and the presence of a larger, outer dust ring have great relevance for accretion disks in general, including those in other interacting binary systems, pre-main-sequence stars, and active galaxies.

Ultraviolet light curves constructed from near-ultraviolet (NUV) and far-ultraviolet (FUV) detectors on GALEX reveal large-amplitude variations during the orbital period of the low accretion rate polar MQ Dra (SDSS J1553+55). This unexpected variation from a UV source is similar to that seen and discussed in the polar EF Eri during its low state of accretion, even though the accretion rate in MQ Dra is an order of magnitude lower than even the low state of EF Eri. The similarity in phasing of the UV and optical light curves in MQ Dra imply a similar location for the source of light. We explore the possibilities of hot spots and cyclotron emission with simple models fit to the UV, optical, and IR light curves of MQ Dra. To match the GALEX light curves with a single-temperature circular hot spot requires different sizes of spots for the NUV and FUV, while a cyclotron model that can produce the optical harmonics with a magnetic field near 60 MG requires multipoles with fields >200 MG to match the UV fluxes.

We present phase-resolved low-resolution infrared spectra of the polar EF Eridani obtained over a period of 2 yr with SpeX on the IRTF. The spectra, covering the wavelength range 0.8 m m [?] l [?] 2.4 m m , are dominated by cyclotron emission at all phases. We use a "constant lambda" prescription to attempt to model the changing cyclotron features seen in the spectra. A single cyclotron emission component with B[?] 12.6 MG and a plasma temperature of kT[?] 5.0 keV does a reasonable job in matching the features seen in the H and K bands, but fails to completely reproduce the morphology shortward of 1.6 mm. We find that a two-component model, where both components have similar properties but their contributions differ with viewing geometry, provides an excellent fit to the data. We discuss the implications of our models and compare them with previously published results. In addition, we show that a cyclotron model with similar properties to those used for modeling the infrared spectra, but with a field strength of B = 115 MG, can explain the GALEX observations of EF Eri.

We present the first mid-infrared (5.5-14.5 mm) spectrum of a highly magnetic cataclysmic variable, EF Eridani, obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The spectrum displays a relatively flat, featureless continuum. A spectral energy distribution model consisting of a 9500 K white dwarf, an L5 secondary star, cyclotron emission corresponding to a B [?] 13 MG white dwarf magnetic field, and an optically thin circumbinary dust disk is in reasonable agreement with the extant Two Micron All Sky Survey (2MASS), Infrared Array Camera (IRAC), and IRS observations of EF Eri. Cyclotron emission is ruled out as a dominant contributor to the infrared flux density at wavelengths [?]3 mm. The spectral energy distribution longward of ~5 mm is dominated by dust emission. Even longer wavelength observations would test the model's prediction of a continuing gradual decline in the circumbinary disk-dominated region of the spectral energy distribution.

We present phase-resolved spectroscopy of the short-period cataclysmic variable WZ Sge obtained with the Hubble Space Telescope. We were able to resolve the orbital motion of a number of absorption lines that likely probe the environment near the accreting white dwarf. The radial velocities derived from simultaneous fits to 13 absorption lines indicate an orbital velocity semi-amplitude of KUV = img1.gif km s-1. However, we find that the phase zero is offset from the white dwarf ephemeris by +0.1. Our offset and velocity amplitude are very similar to constraints derived from optical emission lines from the quiescent accretion disk, despite the fact that we are probing material much closer to the primary. If we associate the UV amplitude with K1, our dynamical constraints together with the published K2 estimates and the known binary inclination of i = 77 +- 2 imply 0.88 M < M1 < 1.53 M, 0.078 M < M2 < 0.13 M, and 0.075 < q = M2/M1 < 0.101. If we interpret the mean velocity of the UV lines [-img2.gif km s-1] as being due to the gravitational redshift caused in the high-g environment near the white dwarf, we find vgrav = img3.gif km s-1, which provides an independent estimate on the mass of the primary of M1 = img4.gif M when coupled with a mass-radius relation. Our primary mass estimates are in excellent agreement and are also self-consistent with spectrophotometric fits to the UV fluxes despite the observed phase offset. It is at this point unclear what causes the observed phase offset in the UV spectra and by how much it distorts the radial velocity signature from the underlying white dwarf.

We report the discovery of two new accreting pulsating white dwarf stars among the cataclysmic variables of the Sloan Digital Sky Survey: SDSS J074531.91+453829.5 and SDSS J091945.10+085710.0. We observe high-amplitude nonsinusoidal variations of 4.5%-7% at a period close to 1230 s in the optical light curves of SDSS J074531.91+453829.5 and a low-amplitude variation of 0.7%-1.6% near 260 s in the light curves of SDSS J091945.10+085710.0. We infer that these optical variations are a consequence of nonradial g-mode pulsations in the accreting primary white dwarfs of these cataclysmic variables. However, we cannot rule out the remote possibility that the 260 s period could be the spin period of the accreting white dwarf SDSS J091945.10+085710.0. We also uncovered a nonvariable, SDSS J171145.08+301320.0, during our search; our two observing runs exclude any pulsation-related periodicities in the range 85-1400 s with an amplitude [?]0.5%. This discovery paper brings the total number of known accreting white dwarf pulsators to 11.

We present Spitzer IRS observations of 11 intermediate polars (IPs). Spectra covering the wavelength range from 5.2 to 14 mm are presented for all 11 objects, and longer wavelength spectra are presented for three objects (AE Aqr, EX Hya, and V1223 Sgr). We also present new, moderate-resolution (R ~ 2000) near-infrared spectra for five of the program objects. We find that, in general, the mid-infrared spectra are consistent with simple power laws that extend from the optical into the mid-infrared. There is no evidence for discrete cyclotron emission features in the near- or mid-infrared spectra for any of the IPs investigated, nor for infrared excesses at l [?] 12 mm. However, AE Aqr, and possibly EX Hya and V1223 Sgr, do show longer wavelength excesses. We have used a cyclotron modeling code to put limits on the amount of such emission for magnetic field strengths of 1 [?] B [?] 7 MG. If cyclotron emission is occurring in the 5.2-14.0 mm bandpass, it constitutes less than 1% of the bolometric luminosity of any of the IPs. We were able to model the long-wavelength excess of V1223 Sgr and EX Hya with cyclotron emission from a 1 MG field, but the S/N of those data is very poor, and the reality of those excesses is not established. We attempted to model the long-wavelength excess of AE Aqr with cyclotron emission, but none of our models fit nearly as well as a simple, cool (TBB = 140 K) blackbody. Given the apparent variability of this excess, however, synchrotron radiation remains a better explanation. We discuss our results in the context of the standard model for IPs.

EF Eri is a magnetic cataclysmic variable that has been in a low accretion state for the past 9 yr. Low-state optical spectra reveal the underlying Zeeman-split white dwarf absorption lines. These features are used to determine a value of 13-14 MG as the white dwarf field strength. Recently, 5-7 yr into the low state, Balmer and other emission lines have appeared in the optical. An analysis of the Ha emission line yields the first radial velocity solution for EF Eri, leading to a spectroscopic ephemeris for the binary and, using the best available white dwarf mass of 0.6 M, a mass estimate for the secondary of 0.055 M. For a white dwarf mass of 0.95 M, the average for magnetic white dwarfs, the secondary mass increases to 0.087 M. At EF Eri's orbital period of 81 minutes, this higher mass secondary could not be a normal star and still fit within the Roche lobe. The source of the Balmer and other emission lines is confirmed to be from the substellar secondary, and we argue that it is due to stellar activity. We compare EF Eri's emission-line spectrum and activity behavior to that recently observed in AM Her and VV Pup and attributed to stellar activity. We explore observations and models originally developed for V471 Tau, for the RS CVn binaries, and for extrasolar planets. We conclude that irradiation of the secondary in EF Eri and similar systems is unlikely and, in polars, the magnetic field interaction between the two stars (with a possible tidal component) is a probable mechanism that would concentrate chromospheric activity on the secondary near the substellar point of the white dwarf.

Long-term optical and near-infrared photometric and blue spectroscopic observations were obtained for QS Telescopii, V834 Centauri, and BL Hydri. The optical light curves of all three polars displayed large magnitude changes during our observations. These same high-state and low-state transitions were also apparent in near-infrared JHKphotometry, although with decreased amplitude. The color of the polar with respect to its state was examined and found not to be a good indicator of the instantaneous state. During transitions from low to high state, a nearly constant magnitude difference was observed in all three polars. This ?m value was found to be consistent with the level expected to occur if accretion onto the white dwarf reached the Eddington luminosity during the high state. The high-state Balmer decrement was measured for each star and used to estimate that the temperature of the emission-line-forming region was ?12,000 K, with NHnear 12.8 dex. No relationship between the Balmer emission-line strength and the white dwarf magnetic field strength was seen, in contrast to a good correlation between these two parameters observed for UV emission lines.

We obtained R-band time-series observations of the young, metal-rich open cluster NGC 2301 for 12 nights in 2004 February. B-band images were also obtained, and color-magnitude diagrams with limits of R = 19.5 and B = 21.5 were produced. Only a small effort was made to determine cluster membership, as our magnitude limits are far deeper than previously published values. Our photometric precision for the brightest 5 mag of sources is 1-2 mmag. We determine that for the ?4000 stars that have time-series data, 56% are variable, and of these, approximately 13% are observed to exhibit periodic light curves ranging from tens of minutes to days. We present some examples of the light curves obtained, and produce cuts in variability space based on parameters such as color and amplitude. The percentage of variability is approximately equal across all colors, with the majority of variables having amplitudes of 0.15 mag or less. In general, redder stars show larger variability amplitudes. We find a smooth decline in the number of periodic variables toward longer period. This decline is probably due to a transition from intrinsic to extrinsic variability, and also due in part to our limited observing period of 12 nights. Essentially all the A and F main-sequence stars in our sample are variable (?2 mmag and larger), and most present complex light curves containing multiple periods that are suggestive of their inclusion in the ? Scuti and ? Doradus classes. A variable non-cluster member giant and two variable white dwarf candidates are discussed. Our equational description of variability is shown to be an excellent predictive tool for determining the cumulative fraction of variables that will be observed in a photometric survey. Our entire data set is available electronically.

We have observed the short-period dwarf novae BW Scl, BC UMa, and SW UMa using the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS). In all three systems, the white dwarf is the dominant source of far-ultraviolet flux, even though in BC UMa and SW UMa an additional continuum component contributes ~10% and ~20% of the 1400 A flux, respectively. Fitting the data with detailed white dwarf model spectra, we determine the effective temperatures to be 14,800 +- 900 K (BW Scl), 15,200 +- 1000 K (BC UMa), and 13,900 +- 900 K (SW UMa). The additional continuum component in BC UMa and SW UMa is equally well described by either a blackbody or a power law, which could be associated with emission from the hot spot or from an optically thin accretion disk (or an optically thin layer on top of a colder optically thick disk), respectively. Modeling the narrow metal lines detected in the STIS spectra results in subsolar abundances of carbon, oxygen, and silicon for all three systems and also suggests substantial suprasolar abundances of aluminum. The narrow absorption line profiles imply low white dwarf rotation rates, v sin i [?] 300 km s-1 for the three white dwarfs. SW UMa is the only system that shows significant short-term variability in the far-ultraviolet range, which is primarily associated with the observed emission lines.

We present new optical, near-infrared, and archival ultraviolet observations of XX Ophiuchi and AS 325, two proposed "iron" stars. These unusual stars have optical spectra dominated by emission lines arising from hydrogen, as well as ionized metals such as iron, chromium, and titanium. Both stars have been classified as "iron" stars, and a number of exotic models have been presented for their origin. Using 2 years of moderately high resolution optical spectroscopy, the first high signal-to-noise ratio K-band spectroscopy of these sources (which reveals stellar photospheric absorption lines), and new near-infrared interferometric observations, we confirm that both systems are composed of two stars, likely binaries, containing a hot Be star with an evolved late-type secondary. The hydrogen emission features arise in the hot wind from the Be star, while the corresponding P-Cygni absorption lines are produced from dense material in the expanding, radiation-driven wind around each system. The optical Fe iiemission lines are pumped by ultraviolet Fe iiabsorption lines through fluorescence. Contrary to some claims in the literature, the spectral features of XX Oph and AS 325 are quite similar, evidence that they are comparable systems. We examine the variability of the spectral morphology and radial velocity motions of both sources. We also study the variability of XX Oph during a major photometric event and find that the spectral nature of the system varies during the event. A comparison of the velocity of the absorption-line components in our new spectra with those in the literature show that the structure of the stellar wind from XX Oph has changed since the system was observed in 1951.

We observed the young open cluster NGC 2301 for 14 nights in 2004 February using the orthogonal transfer CCD camera OPTIC. We used point-spread function shaping techniques ("square stars") during the observations, allowing us to obtain a larger dynamic range (4.5 mag) of high photometric precision results (?2 mmag). These results are better than similar observing campaigns using standard CCD imagers. This paper discusses our observational techniques and presents initial results for the variability statistics found in NGC 2301. Details of the variability statistics as functions of color, variability type, stellar type, and cluster location will appear in Paper II.

We present Far Ultraviolet Spectroscopic Explorer(FUSE) observations of the magnetic cataclysmic variable V405 Aurigae. Together with four other DQ Her-type binaries, V405 Aur forms a small subclass of intermediate polars that are likely to evolve into low magnetic field strength polars, similar to AM Her. Our co-added FUSEspectrum exhibits broad O viand C iiiemission lines, in addition to a narrow O viemission-line component that likely forms near the white dwarf surface. The O vi?1032/?1038 narrow emission-line flux ratio is near 2.0 throughout the orbit, which is indicative of its formation in an optically thin gas. Radial velocity measurements of the narrow O viemission lines restrict any orbital modulation to a very low amplitude (KWD= 2.5 ± 0.5 km s?1), indicating, along with other single-peaked broad disk emission lines in the UV and optical, that the binary system is at low inclination.

We present new phase-resolved H- and K-band spectroscopy of the ultrashort-period magnetic cataclysmic variable EF Eri in its current prolonged "low" state obtained using NIRI on Gemini North and NIRSPEC on Keck II. These new data show that the H-band spectrum of EF Eri appears to be dominated by cyclotron emission during the entire orbital cycle. The K-band spectrum of EF Eri is likewise dominated by cyclotron emission during most of an orbital period, but near binary phase 0.0 the secondary star spectrum may be visible. The lack of strong CO or CH4 absorption features and the weakness of the water vapor features in this spectrum, however, suggests the possibility of peculiar abundances for carbon and/or oxygen. We have used the PHOENIX stellar atmosphere code to produce model brown dwarf spectra with nonsolar abundances of carbon, nitrogen, and oxygen, and achieved limited success in fitting the observed spectra. We conclude that strong, highly variable cyclotron emission is responsible for the photometric variation previously reported for EF Eri. The nature of this cyclotron emission is complex: the H-band spectra show that the dominant cyclotron harmonic at phase 0.5 peaks at 1.65 mm, but at phase 0.0 the harmonic peaks near 1.72 mm. At phase 0.5, there is another cyclotron feature present that peaks in between the H and K bands (near 1.93 mm), but at phase 0.0 no such feature is present. These data suggest that cyclotron emission from both poles is occurring. In the high state, the cyclotron emission has been modeled as coming from the pole that is oriented toward the secondary star. One interpretation for the phase 0.5 cyclotron emission is that it originates from the opposite pole. In its current ultralow state, EF Eri reveals no outward signs of accretion (such as H I emission) but continues to have a few, strong cyclotron features. Thus, EF Eri joins the small group of magnetic cataclysmic variables whose accretion rate is so low that they are in the "bombardment scenario" regime.

We present the results of U-band and multicolor photometry during the 2001 superoutburst of WZ Sge. Our 10 nights of U-band photometry span the time interval from HJD 2,452,118 to 2,452,197, while our multicolor observations range from HJD 2,452,115 to 2,452,197. The U-band light curves are generally in agreement with other data sets obtained during the superoutburst, showing highly modulated light early on, rebrightenings, and superhumps of similar shape and period (except during the rebrightening peak). One of our multicolor data sets fortuitously covers the first rebrightening and allows determination of the accretion disk color temperature before, during, and after the event. It is seen that the rebrightening is a change from a neutral disk (T?7000 K) to an ionized disk (T?10,000 K) and back again. We develop a simple limit cycle model for this behavior that approximately predicts the semiperiodic timescale observed for the rebrightenings. We discuss our results in relation to accretion disk structure during superoutburst.

Orthogonal transfer CCDs (OTCCDs) were developed to compensate for real-time image motion, essentially providing tip/tilt corrections without additional optics or moving parts. Because of the complex gate structures of OTCCDs, their ability to provide high-precision photometric observations and their use as high-speed photometers were unclear. We detail new observations obtained with the OPTIC camera at the University of Hawaii's 2.2 m telescope on Mauna Kea that explore both of these areas. We find that OTCCDs provide equally good results in terms of typical time-series differential photometry (compared with typical CCDs) and that their high-speed photometric application is superb. Using point-spread function-shaping techniques, we obtained time-series photometric observations with precisions of ?660 ?mag per 180 s integration. Extending this technique to very bright stars, the potential to reach differential precisions near 10-5per exposure is investigated.

We present Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph E140M spectra of the dwarf nova WZ Sge, following the early superoutburst of 2001 July. Our four far-ultraviolet (FUV) spectra, obtained over a time span of 4 months, monitor changes in the hot component of the system during the decline phase. The spectra cover the wavelength interval 1150-1708 A. They reveal Stark-broadened Lya and He II (1640) absorption and absorption lines due to metals (Si, C, N, Al) from a range of ionization states. Single-temperature white dwarf models provide reasonable qualitative agreement with the HST spectra. We find that the white dwarf appears to dominate the spectra from October through December. However, it is not clear that the absorption lines of metals form in the white dwarf photosphere. Therefore, the derived abundances and rotational velocity must be viewed with caution. Only a modest improvement in the fits to the data results when an accretion belt component is included. If the FUV spectra arise from the white dwarf alone, then we measure a cooling in response to the outburst of at least 11,000 K (29,000-18,000 K). The absence of broad underlying absorption features due to metals at this stage suggests slow rotation (~200 km s-1). It is possible that the white dwarf envelope has expanded due to the heating by the outburst or that the relatively narrow absorption features we observe are forming in an inflated disk atmosphere or curtain associated with the outburst.

We present the results of our analysis of six orbits of Hubble Space Telescope Space Telescope Imaging Spectrograph (HST/STIS) time-tagged spectroscopy of the intermediate polar, EX Hydrae. The high time and wavelength resolution of the HST/STIS spectra provided an excellent opportunity to study the UV properties of EX Hya. Measurements of the continuum and emission-line fluxes corroborate earlier studies that show that the emission-line fluxes are modulated more strongly than the continuum flux and originate from the accretion curtains, while the continuum flux originates from the white dwarf photosphere. The measured K amplitude of the narrow emission-line radial velocity curve is used to calculate a white dwarf mass of 0.91 +- 0.05 M. Synthetic white dwarf photosphere and accretion disk spectral models are used to further refine the white dwarf and accretion disk properties. Using the spectral models, it is determined that EX Hya has a white dwarf of mass 0.9 M, T = 23,000 K, and an accretion disk truncated at 2.5RWD and is at a distance of 60 pc.

We present the results of a multicomponent synthetic spectral analysis of Hubble Space Telescope (HST) STIS spectra of the ultrashort-period dwarf novae VY Aqr and WX Ceti during their deep quiescence following their last superoutburst. The white dwarf in these extremely low accretion rate systems dominates the far-UV light. We find that the accreting white dwarfs in VY Aqr and WX Ceti are remarkably similar. Both systems contain white dwarfs with Teff = 13,000-13,500 K, a rotation velocity below 800-1200 km s-1, and subsolar metallicity. Both white dwarfs are better fitted with a two-temperature white dwarf plus accretion belt model in which part of the white dwarf is cooler and "slowly" rotating and part is hotter, smaller, and spinning at the Keplerian speed. We discuss the implications of the surface temperatures we have derived for the white dwarfs in VY Aqr, WX Ceti, and the nine other WZ Sge-like dwarf novae below the period gap which have been observed with HST. Their surface temperatures cluster closely around 15,000 K and their orbital periods are between 1.3 and 1.5 hr. We show that long-term compressional heating due to time-averaged accretion is the mechanism responsible for the clustering around 15,000 K. The time-averaged accretion rate corresponding to this cluster of observed temperatures is almost precisely what is predicted if gravitational wave emission is driving mass transfer below the period gap.

On 2000 May 5, we began a large, multiwavelength campaign to study the intermediate polar (IP) EX Hydrae. The simultaneous observations from six satellites and four telescopes were centered around a 1 million second observation with Extreme Ultraviolet Explorer (EUVE). Although EX Hya has been studied previously with EUVE, our higher signal-to-noise ratio observations present new results and challenge the current IP models. Previously unseen dips in the light curve are reminiscent of the stream dips seen in polar light curves. Also of interest is the temporal extent of the bulge dip, approximately 0.5 in phase, implying that the bulge extends over half of the accretion disk. We propose that the magnetic field in EX Hya is strong enough (a few MG) to begin pulling material directly from the outer edge of the disk, thereby forming a large accretion curtain, which would produce a very broad bulge dip. This would also result in magnetically controlled accretion streams originating from the outer edge of the disk. We also present a period analysis of the photometric data, which shows numerous beat frequencies with strong power and also intermittent and wandering frequencies, an indication that physical conditions within EX Hya changed over the course of the observation. Iron spectral line ratios give a temperature of log T = 6.5-6.9 K for all spin phases and a poorly constrained density of ne = 1010-1011 cm-3 for the emitting plasma. This paper is the first in a series detailing our results from this multiwavelength observational campaign.