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The bright and distant past A dwarf nova is a type of cataclysmic variable containing a collapsed white dwarf star that accretes matter from its close companion in a binary system, a red dwarf. An instability periodically dumps material onto the white dwarf, increasing the luminosity by up to a hundredfold. Classical novae are thousands of times brighter than dwarf novae, and are accompanied by the formation of shells around the system. Theory predicts that dwarf novae will eventually gain sufficient mass to undergo classical nova eruptions. This suspected link between dwarf and classical novae now has an observational basis with the discovery of an ancient nova shell around the dwarf nova Z Camelopardalis. The nature of the shell suggests that a few thousand years ago, Z Cam underwent a classical nova eruption and for some days was one of the brightest stars in the sky. Classical novae are thousands of times brighter than dwarf novae, and are accompanied by the formation of shells around the system. This paper reports the discovery of a shell an order of magnitude more extended than those detected around many other classical novae surrounding the prototypical dwarf nova Z Camelopardalis, thereby observationally linking the objects. Cataclysmic variables (classical novae and dwarf novae) are binary star systems in which a red dwarf transfers hydrogen-rich matter, by way of an accretion disk, to its white dwarf companion 1 . In dwarf novae, an instability 2 is believed to episodically dump much of the accretion disk onto the white dwarf. The liberation of gravitational potential energy then brightens these systems by up to 100-fold every few weeks or months 2 . Thermonuclear-powered eruptions thousands of times more luminous 3 , 4 occur in classical novae 5 , accompanied by significant mass ejection 6 and formation of clearly visible shells 7 , 8 from the ejected material. Theory predicts that the white dwarfs in all dwarf novae must eventually accrete enough mass to undergo classical nova eruptions 9 . Here we report a shell, an order of magnitude more extended than those detected around many classical novae, surrounding the prototypical dwarf nova Z Camelopardalis. The derived shell mass matches that of classical novae, and is inconsistent with the mass expected from a dwarf nova wind or a planetary nebula. The shell observationally links the prototypical dwarf nova Z Camelopardalis with an ancient nova eruption and the classical nova process.

Data obtained in the 1950–1955 Palomar campaign for the discovery of classical novae in M81 are set out in detail. Positions and apparentBmagnitudes are listed for the 23 novae that were found. There is modest evidence that the spatial distribution of the novae does not track theBbrightness distribution of either the total light or the light beyond an isophotal radius that is 70 \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $\\arcsec$\\end{document} from the center of M81. The nova distribution is more extended than the aforementioned light, with a significant fraction of the sample appearing in the outer disk/spiral arm region. We suggest that many (perhaps a majority) of the M81 novae that are observed at any given epoch (compared with, say, 1010years ago) are daughters of Population I interacting binaries. The conclusion that the present‐day novae are drawn from two population groups—one from low‐mass white dwarf secondaries of close binaries identified with the bulge/thick disk population, and the other from massive white dwarf secondaries identified with the outer thin disk/spiral arm population—is discussed. We conclude that the M81 data are consistent with the two population division as argued previously from (1) observational studies on other grounds of nearby galaxies, (2) Monte Carlo simulations of novae in M31 and in the Galaxy, and (3) population synthesis modeling of nova binaries. Two different methods of using M81 novae as distance indicators give a nova distance modulus for M81 as (m− \\documentclass{aastex} \\usepackage{amsbsy} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{bm} \\usepackage{mathrsfs} \\usepackage{pifont} \\usepackage{stmaryrd} \\usepackage{textcomp} \\usepackage{portland,xspace} \\usepackage{amsmath,amsxtra} \\usepackage[OT2,OT1]{fontenc} \\newcommand\\cyr{ \\renewcommand\\rmdefault{wncyr} \\renewcommand\\sfdefault{wncyss} \\renewcommand\\encodingdefault{OT2} \\normalfont \\selectfont} \\DeclareTextFontCommand{\\textcyr}{\\cyr} \\pagestyle{empty} \\DeclareMathSizes{10}{9}{7}{6} \\begin{document} \\landscape $M) _{0}=27.75$ \\end{document} , consistent with the Cepheid modulus that is the same value.

I present a far-ultraviolet study, taken with the Hubble Space Telescope (HST), of the globular clusters NGC 1851 and NGC 6681. My focus is primarily on the time-domain, and I present searches for – and classification of - far-ultraviolet variables and transients in my data. The far-ultraviolet sources have been cross-identified with objects detected at near-ultraviolet and optical wavelengths in archival HST imaging. The combined photometric catalogs were used to identify several core populations, including some that are likely to be binaries containing a hot component. In NGC 1851, I identified 36 variable candidates in the 273 exposures taken over 3 HST visits of 4 orbits each. Twenty five of the variable candidates are distributed along the horizontal branch, 12 are likely RR Lyraes and 13 are on the blue horizontal branch. Five variable candidates are located among the blue straggler populations with one of them a likely SX Phoenicis pulsator. The remaining six variable candidates have photometric properties that point to white dwarf + main sequence binary systems. One of the far-ultraviolet variables in NGC 1851, FUV1, was found to have a period of 18 minutes in our far-ultraviolet time-series photometry. The two possibilities for the nature of this far-ultraviolet variable are: (i) it may be an intermediate polar (i.e. acompact binary containing an accreting magnetic white dwarf), or (ii) it may be an AM CVn (i.e. and interacting double-white dwarf system). The AM CVn interpretation is favoured because of the lack of an X-ray detection in ≅ 65 ksec of Chandra imaging. I also present a search for transients (e.g. dwarf novae) in 20 years of far-ultraviolet imaging of the core of globular cluster NGC 6681. No such transients were found in the 80 epochs of observations. If all of these epochs can be considered independent, the search should be nearly 100% complete for dwarf novae with duty cycles > 5%, and still ≅ 60% complete for systems with duty cycles of ∼ 1%. The detection of of zero outbursts allows me to place a 2 - σ upper limit of 3 on the mean number that might have been detected in similar surveys. This places strongish constraints on models for the cataclysmic variable populations.

Asynchronous polars (APs) are accreting white dwarfs (WDs) that have different WD and orbital angular velocities, unlike the rest of the known polars, which rotate synchronously (i.e., their WD and orbital angular velocities are the same). Past nova eruptions are the predicted cause of the asynchronicity, in part due to the fact that one of the APs, V1500 Cyg, was observed to undergo a nova eruption in 1975. We used the Southern African Large Telescope 10m class telescope and the MDM 2.4m Hiltner telescope to search for nova shells around three of the remaining four APs (V1432 Aql, BY Cam, and CD Ind) as well as one Intermediate Polar with a high asynchronicity (EX Hya). We found no evidence of nova shells in any of our images. We therefore cannot say that any of the systems besides V1500 Cyg had nova eruptions, but because not all post-nova systems have detectable shells, we also cannot exclude the possibility of a nova eruption occurring in any of these systems and knocking the rotation out of sync.

Accurate determination of the rates of nova eruptions in different kinds of galaxies give us strong constraints on those galaxies' underlying white dwarf and binary populations, and those stars' spatial distributions. Until 2016, limitations inherent in ground-based surveys of external galaxies - and dust extinction in the Milky Way - significantly hampered the determination of those rates and how much they differ between different types of galaxies. Infrared Galactic surveys and dense cadence Hubble Space Telescope (HST)-based surveys are overcoming these limitations, leading to sharply increased nova-in-galaxy rates relative to those previously claimed. Here we present 14 nova candidates that were serendipitously observed during a year-long HST survey of the massive spiral galaxy M51 (the \"Whirlpool Galaxy\"). We use simulations based on observed nova light curves to model the incompleteness of the HST survey in unprecedented detail, determining a nova detection efficiency \\(\\epsilon = 20.3\\) percent. The survey's M51 area coverage, combined with \\(\\epsilon\\), indicates a conservative M51 nova rate of \\(172^{+46}_{-37}\\) novae yr\\(^{-1}\\), corresponding to a luminosity-specific nova rate (LSNR) of \\(\\sim10.4^{+2.8}_{-2.2}\\) novae yr\\(^{-1}\\)/\\(10^{10} L_{\\odot,K}\\). Both these rates are approximately an order of magnitude higher than those estimated by ground-based studies, contradicting claims of universal low nova rates in all types of galaxies determined by low cadence, ground-based surveys. They demonstrate that, contrary to theoretical models, the HST-determined LSNR in a giant elliptical galaxy (M87) and a giant spiral galaxy (M51) likely do not differ by an order of magnitude or more, and may in fact be quite similar.

We present a far-ultraviolet (FUV) study of the globular cluster M30 (NGC 7099). The images were obtained using the Advanced Camera for Surveys (ACS/SBC, F150LP, FUV) and the Wide Field Planetary Camera 2 (WFPC2, F300W, UV) on board the Hubble Space Telescope (HST). We compare the catalogue of FUV objects to ten known X-ray sources and find six confident matches of two cataclysmic variables (CVs), one RS CVn, one red giant with strong FUV emission and two sources only detected in the FUV. We also searched for variable sources in our dataset and found a total of seven blue stragglers (BSs), four horizontal branch (HB) stars, five red giant branch stars, 28 main sequence stars and four gap objects that demonstrated variability. One BS star is a known W-UMa contact binary, one of the gap objects is a known CV identified in this work to be a dwarf nova, and the three other gap sources are weak variables. The periods and positions of two of the variable HB stars match them to two previously known RR Lyrae variables of types RRab and RRc.

We present a far-ultraviolet (FUV) study of the globular cluster M30 (NGC 7099). The images were obtained using the Advanced Camera for Surveys (ACS/SBC, F150LP, FUV) and the Wide Field Planetary Camera 2 (WFPC2, F300W, UV) which were both on board the Hubble Space Telescope (HST). The FUV-UV colour-magnitude diagram (CMD) shows a main sequence (MS) turnoff at FUV \\(\\approx\\) 22 mag and FUV-UV \\(\\approx\\) 3 mag. The MS extends 4 mag below the turnoff, and a prominent horizontal branch (HB) and blue straggler (BS) sequence can be seen. A total of 1218 MS stars, 185 red giant branch stars, 47 BS stars and 41 HB stars are identified, along with 78 sources blueward of the MS which consist of white dwarfs (WDs) and objects in the gap between the WDs and the MS that include potential cataclysmic variable (CV) candidates. The radial distribution of the BS population is concentrated towards the cluster centre, indicating that mass segregation has occurred. The blue and red sub-populations of the double BS sequence appear mixed in the ultraviolet CMD, and no significant central concentration of CV candidates is seen in this cluster.

The existence of a vast nova shell surrounding the prototypical dwarf nova Z Camelopardalis (Z Cam) proves that some old novae undergo metamorphosis to appear as dwarf novae thousands of years after a nova eruption. The expansion rates of ancient nova shells offer a way to constrain both the time between nova eruptions and the time for post-nova mass transfer rates to decrease significantly, simultaneously testing nova thermonuclear runaway models and hibernation theory. Previous limits on the expansion rate of part of the Z Cam shell constrain the inter-eruption time between Z Cam nova events to be \\(>\\) 1300 years. Deeper narrow-band imaging of the ejecta of Z Cam with the Condor Array Telescope now reveals very low surface brightness areas of the remainder of the shell. A second, even fainter shell is also detected, concentric with and nearly three times the size of the \"inner\" shell. This is the first observational support of the prediction that concentric shells must surround the frequently-erupting novae of relatively massive white dwarfs. The Condor images extend our Z Cam imaging baseline to 15 years, yielding the inner shell's expansion rate as \\(v = 83 \\pm 37\\) km s\\(^{-1}\\) at 23 degrees South of West, in excellent agreement with our 2012 prediction. This velocity corresponds to an approximate age of \\(t = 2672^{-817}_{+2102}\\) yr. While consistent with the suggestion that the most recent nova eruption of Z Cam was the transient recorded by Chinese Imperial astrologers in the year 77 BCE, the age uncertainty is still too large to support or disprove a connection with Z Cam.

Gravitationally bound companions to stars enable determinations of their masses, and offer clues to their formation, evolution and dynamical histories. So motivated, we have carried out a speckle imaging survey of eight of the nearest and brightest Wolf-Rayet (WR) stars to directly measure the frequency of their resolvable companions, and to search for much fainter companions than hitherto possible. We found one new, close companion to each of WR 113, WR 115 and WR 120 in the separation range 0.2\" - 1.2\". Our results provide more evidence that similar-brightness, close companions to WR stars are common. More remarkably, they also demonstrate that the predicted, but much fainter and thus elusive companions to WR stars are now within reach of modern speckle cameras on 8m class telescopes by finding the first example. The new companion to WR 113 is just 1.16\" distant from it, and is 8 magnitudes fainter than the WR star. The empirical probability of a chance line-of-sight of the faint companion at the position of WR 113 is < 0.5%, though we cannot yet prove or disprove if the two stars are gravitationally bound. If these three new detections are physical companions we suggest, based on their narrowband magnitudes, colors, reddenings and GAIA distances that the companions to WR113, WR 115 and WR 120 are an F-type dwarf, an early B-type dwarf, and a WNE-type WR star, respectively.