Ashley Ruiter, University of New South Wales Canberra,
Characteristics of close white dwarf binaries from theory (and their fate). Most stars end their nuclear burning stage as white dwarfs. With the majority of stars residing in binary or multiple systems, many double white dwarf binaries exist throughout the Universe, with >10^8 predicted to exist in our Milky Way alone. When binaries containing two white dwarfs interact, a variety of outcomes are possible: Type Ia supernovae, R Coronae Borealis stars, AM CVn stars, accretion-induced collapse neutron stars, etc., depending on the properties of the binary. Confirming the progenitor nature of these events, however, is extremely difficult observationally due to the (electromagnetically) faint nature of double white dwarfs in general. Making connections between transient events observed in sky surveys, with progenitor scenario predictions from theory, will set the groundwork for future searches of electromagnetic counterparts to LISA gravitational wave events. Though the launch of the first space-based gravitational wave observatory (LISA) is still ~15 years away, predictions of birthrates, ages, and physical characteristics of transient progenitors involving white dwarfs is increasingly important in today's productive era of synoptic sky surveys.
Roberto Raddi, Dr.Karl-Remeis-Sternwarte Bamberg - Astronomisches Institut
Partly burnt white dwarfs from peculiar thermonuclear supernovae. LP 40-365 is an hypervelocity star, whose atmosphere is dominated by oxygen, neon, and magnesium, and it is sprinkled with the ashes of partial oxygen and silicon burning. Its composition and remarkable kinematics suggest this star formed via the failed disruption of a near-Chandrasekhar mass white dwarf in a peculiar thermonuclear supernova that unbound the binary progenitor.LP 40-365 now appears as a low-mass, inflated star of 0.2 Msun and 0.16 Rsun.We have identified three new stars, which help us to define LP 40-365 as the prototype of its own class that is characterised by a relatively homogeneous chemical composition, physical and kinematic properties. The "LP 40-365 stars" will help to better constrain their binary progenitors, explosion mechanisms, and nucleosynthetic yields.
Koji Mukai, NASA/GSFC and UMBC, USA
Magnetic cataclysmic variables and related objects. After a brief reminder of major subclasses and unusual objects, I will present a two-part overview of recent developments regarding magnetic cataclysmic variables (CVs) and related compact binaries. First, I will present selected recent observational advances on the physics of accretion in magnetic CVs. This will include comprehensive X-ray spectral models of intermediate polars (IPs) that include X-ray "reflection" and complex absorption, leading to inferences regarding the size of the X-ray emission region. I will also summarize the recent advances in the observations and modeling of the ~2 second quasi-periodic oscillations in polars. I will also discuss the messy details of how ballistic stream is captured by the magnetic field in polars, using the TESS data on the asynchronous polar, CD Ind. In the second part of the talk, I will discuss the collective properties of magnetic CVs. This will include a summary of recent discoveries of IPs through hard X-ray surveys, and our updated understanding of space density and hard and soft X-ray luminosities of polars and IPs thanks to the Gaia DR2 distances. I will provide an updated estimate of the true fraction of magnetic CVs, in comparison with the incidence of magnetism among field white dwarfs and in symbiotic stars. This will lead to some thoughts on the origin of magnetic field in white dwarfs, and on the evolution of magnetic CVs.
David Jones, Instituto de Astrofísica de Canarias, Spain
Post-common-envelope Planetary Nebulae. As the immediate products of the common envelope, close-binary central stars of planetary nebulae offer a unique tool with which to study this rather poorly understood phase of binary evolution. Furthermore, as the nebula itself represents the ionised remnant of the ejected common-envelope, such planetary nebulae can be used to directly probe the mass, morphology and dynamics of the ejecta. In this invited review, I will summarise our current understanding of the importance of binarity in the formation of planetary nebulae as well as what they can tell us about the common envelope phase - including the possible relationships with other post-common-envelope phenomena like novae and type Ia supernovae.
Anna Francesca Pala, European Southern Observatory, Germany
Fundamental properties of cataclysmic variables from Gaia DR2. Cataclysmic variables (CVs) are the best-suited laboratories in which to test the models of compact binary evolution as they are numerous, relatively bright, and both stellar components are structurally simple. A large and well-defined sample of CVs is fundamental to study the global properties of such stars and to critically test our understanding of the evolution of all types of binaries. However, until recently, all available CV samples were affected by strong selection biases. Only the advent of the second data release (DR2) of the ESA mission Gaia has offered the unique opportunity to construct a volume-limited sample of CV that can provide accurate constraints on the properties of the Galactic CV population. In this talk, I present the first population study of the 150 pc CV sample to accurately constrain the space density of CVs, the composition of the intrinsic population as well as the white dwarf mass and effective temperature distributions. Being independent of selection effects, the results from this study represent one of the most accurate and reliable tests currently available for the present models of compact binary evolution.
Simone Scaringi, Texas Tech University, USA
Accretion-induced variability across all scales. Accreting objects are intrinsically variable on a wide range of timescales and wavelengths. This variability is mostly attributed to the accretion disks themselves, which transport material inwards through complex viscous processes. I will give a brief review of aperiodic variability (flickering) in accreting compact objects, with special emphasis on observations of accreting white dwarfs over the last decade, and present new observational results linking accreting systems across a wide range of scales, masses and magnetic field strengths.
Stuart Littlefair, University of Sheffield, UK
The donor stars in compact white dwarf binaries. This invited review will cover what we know to date about the donor stars in accreting compact white dwarf binaries, and what light the donor stars can shed on the evolution of these binaries. Since the donor star properties change in response to long-term changes in mass loss rate, they represent our best opportunity to study the secular evolution of accreting white dwarf binaries, without the complications introduced by short-term changes. In this review I will summarise our current understanding of the donor stars in accreting white dwarf binaries.
Steven Parsons, University of Sheffield, UK
White dwarfs with main-sequence AFGK star companions. The number of white dwarf plus main-sequence star binaries has increased rapidly in the last decade, jumping from only ~30 in 2003 to over 3000. However, in the majority of systems the companion to the white dwarf is a low mass M dwarf, since these are relatively easy to identify from optical colours and spectra. White dwarfs with more massive A, F, G or K type companions have remained elusive due to the large difference in optical brightness between the two stars. In this talk I will present our work to uncover this hidden population of systems. I'll also present our follow up observations of these systems designed to identify the post common envelope systems and characterise the stars in order to determine their past and future evolution. Finally, I will discuss the potential of this sample for testing thermonuclear supernovae formation channels
Matthew Green, University of Warwick, UK
AM CVn binary formation channels. AM CVn stars are ultracompact, accreting binaries in which both stars are either degenerate or semi-degenerate. Several formation channels have been suggested for AM CVn binaries, including a favoured model in which they descend directly from double white dwarf binaries. The double white dwarf channel has uncertainties around the fraction of double degenerate binaries which will reach a state of stable accretion versus those that will merge. Historic observations of the composition of the accreted material have favoured the double white dwarf channel as a prominent source of AM CVn binaries. On the other hand, recent observations seem to challenge current models of the double white dwarf channel. These observations include donor mass measurements (especially from eclipsing systems) and absolute magnitude measurements from Gaia. Both sets of observations suggest that the donors in AM CVn binaries are more inflated than is predicted by current models of the double degenerate channel. I will present an overview of these results and how they challenge current models.
Alberto Rebassa-Mansergas, Universitat Politècnica de Catalunya, Spain
Double degenerate binaries: from the cradle to the grave. In this presentation I will review our current knowledge on the formation, evolution, and final fates of double white dwarfs.
Diogo Belloni, National Institute for Space Research, Brazil
On the Formation and Evolution of White Dwarf Binaries. I will discuss recent progress in modeling the evolution of cataclysmic variables (CVs). I will show that the empirical consequential angular momentum loss can solve several long-standing problems of CV evolution and will discuss a possible mechanism behind this angular momentum loss. I will furthermore present the first population synthesis of magnetic CVs. I conclude that the agreement between observations and theoretical predictions has improved significantly during the last years, but some serious problems remain to be solved. I will finally expand on these findings and briefly discuss binary population models of type Ia supernova progenitors.
Banibrata Mukhopadhyay, Indian Institute of Science, India
Gravitational waves from white dwarf sources. I will explore plausible gravitational wave emission from sources involved with white dwarfs. They are either isolated magnetized, spinning white dwarfs or binary white dwarfs. I will also explore the additional spin-down electromagnetic counter part of energy of magnetized white dwarfs, which will help highly magnetized white dwarfs to distinguish from their non-magnetic counter part. Further, the amplitude of gravitational waves and the underlying frequencies appear to be different.between magnetized white dwarfs emitting continuously and binary white dwarfs, hence their detections in LISA, ET, DECIGO/BBO do not interfere each other. Based on this work, we propose that highly magnetized super-Chandrasekhar white dwarfs, proposed to be exist to explain peculiar type Ia supernovae by us since last half a decade, could be directly detected in the premise of gravitational wave astronomy.
Elme Breedt, Institute of Astronomy, Cambridge University
AM CVn stars in Gaia and Gaia Alerts. AM CVn stars are expected to be among the brightest low frequency gravitational wave sources in the LISA sky. The most compact among them will be used as verification sources for the mission and the their space density is a key ingredient for modelling the background against which new gravitational wave sources will be detected. Yet, despite the increase in the number of known systems in recent years, primarily due to transient surveys, their space density is still very uncertain. The most recent estimate, based on distances from Gaia DR2, indicates that a large population of AM CVn stars still remain to be discovered. In this talk we will give an overview of the known population of AM CVn stars and discuss our ongoing work, based on Gaia Alerts and the Gaia data releases, to identify new examples of these ultracompact binaries.
Juan Echevarria, Instituto de Astronomia, UNAM
New Observations of DW Cnc: where is the 33 min signal? A new spectroscopic study of the intermediate polar DW Cancri is presented. There are, to date, no derivations of the masses or inclination angle of this poorly know dwarf nova. We show that the 84 min signal, associated with the orbital period is evident in our radial velocity analysis as well as in the Tomography. Is the 33 min signal, associated with the spin of the white dwarf evident in out tomograms?
Juan Venancio Hernandez Santisteban, University of St Andrews
The superoutburst of V1838 Aql inside a bow shock nebula. Bow-shocks around cataclysmic variables are rare, with only a a handful of systems discovered to date. In fact, those known are observed in high-mass transfer systems where outflows (winds and/or jets) are expected to blow into the interstellar medium. The recent superoutburst of V1838 Aql, while a normal event in most accounts, provided us with a rare view of the feedback interaction of a low-mass transfer CV into the ISM. Surprisingly, a bow shock nebula was discovered in deep Halpha imageing a few days after the peak of the outburst and persisted for more than 2 weeks. In this talk, I will present the spectroscopic and photometric follow-up of V1838 Aql, from outburst and back into quiescence. I will discuss the characterisation of this new short-orbital period system and the implications of this discovery in the context of outflows from quiescent systems.
Noel Castro Segura, Physics and Astronomy, University of Southampton
V341 Ara - The roseta stone of accretion disk winds. Non-magnetic nova-likes are cataclysmic variables with mass-transfer rates high enough to keep the accretion disk in a permanent high state. These systems are therefore great test beds for our understanding of steady-state accretion disks.Despite the extreme blue color of this 10th magnitude star, it was (mis-)classified as Cepheid for more than 60 years. V341~ Ara is actually one of the brightest CVs known, though it was not identified as such until recently. However, the most important property of V341 Ara is the presence of an impressive parabolic bowshock, thought to due to the interaction of its accretion disk wind with the ISM. This bowshock is seen via nebular [OIII] surrounded by diffuse nebular H alpha emission and makes this source a Rosetta Stone for the study of accretion disk winds. Since the system is virtually unknown, we will first constrain the basic binary parameters and accretion geometry. In a second paper the origin of the nebula will also be discussed. Ultimately, UV spectral modeling will be carried out to estimate the mass, energy and momentum flux, and this will be compared directly to estimates of these parameters obtained from the interaction of the wind with the ISM (i.e. the bow shock).
Nicole Reindl, University of Potsdam
Recent progress on our understanding of Henize 2-428. The nucleus of the planetary nebula Henize 2-428 is a short orbital period (4.2 h) spectroscopic binary, whose status as potential supernovae type Ia progenitor has raised some controversy in the literature. Here we report recent pitfalls and progress in our analysis of this unique system.
Nicole Reindl, University of Potsdam
Hot white dwarfs showing ultra-high excited metals - a new class of close binary systems? A large fraction of the hottest white dwarfs exhibits broad absorption lines in their optical spectra, which were identified as Rydberg lines of ultra-high excited metals in ionization stages V-X. This has never been observed in any other astrophysical object and indicates a line formation in a dense environment with temperatures near 10^6 K. We find that the majority of these objects is photometrically variable with periods ranging from a few hours to days - consistent with either a typical white dwarf rotational period or an orbital period of a post-common envelope system. Here, we present preliminary results of recent investigations on the search for close binaries amongst these objects.
Alexey Bobrick, Lund University
White dwarf-neutron star binary progenitors for ultra-compact X-ray binaries. White dwarf-neutron star binaries are among the main progenitors for ultra-compact X-ray binaries. They spiral in to contact by emitting gravitational waves and depending on the mass of the white dwarf turn into stably transferring ultra-compact X-ray binaries or produce supernova-like events following a tidal disruption of the white dwarf. Nearly all the stably transferring systems evolve through a phase during which mass transfer rates exceed the Eddington rate by orders of magnitude. Standard population synthesis codes assume that during this phase all the excess material is lost from the system through a tightly collimated jet originating at the accretor. This is in contrast to observations of systems accreting at highly super-Eddington rates, such as SS 433, which are characterised by significant outflows from the accretion disc. To assess the role of such disc outflows, we perform hydrodynamic simulations of super-Eddington accretion in white dwarf-neutron star binaries. We measure the specific angular momentum lost through disc winds in our simulations and find that it is significantly larger than the angular momentum carried away through a jet. We use the measured angular momentum to construct a model of the long-term evolution of white dwarf-neutron star binaries and predict the outcomes of mass transfer. We find that more binaries result in unstable mass transfer than previously thought. In particular, all the binaries with CO white dwarfs and He white dwarfs more massive than about 0.2 solar masses, which were assumed to be stable, were found to be unstable. This result leads to a better agreement between the empirical inspiral rates of binary pulsars containing white dwarfs and empirical formation rates and compositions of ultra-compact X-ray binaries in the Galaxy.
Luke Shingles, Queen's University Belfast, N. Ireland
Late-phase radiative transfer of Type Ia supernovae. Type Ia supernovae are well-studied objects, largely because of their usefulness as precise distance indicators for cosmology and their role in the production of Fe-peak elements for chemical evolution. However, while Type Ia supernovae almost certainly involve the thermonuclear explosion and unbinding of a carbon-oxygen white dwarf, understanding the possible role of a binary companion and the mechanism of explosion remain open problems in supernova research. Multi-band observations including radio and X-ray have placed deep upper limits on the density of the surrounding medium and disfavour most scenarios involving a non-degenerate companion. Several other scenarios remain, and thanks to theoretical developments in the last decade, we now have a variety of 3D hydrodynamic explosion models available. These can can be tested with nucleosynthesis and radiative transfer simulations to compare to observational light curves and spectra. I will describe recent work to improve the physics of the ARTIS radiation transport code to extend its validity to late times (>100 days in the nebular phase) for application to existing 3D explosion models. The code developments are initially tested with 1D models, including the well-known W7 fast-deflagration model, and then applied to sub-Chandrasekhar mass detonation models that account for the effect of gravitational settling in the progenitor white dwarf. I will finish by outlining the next step to three-dimensional radiative transfer simulations of 3D hydrodynamic explosion models.
Ingrid Pelisoli, University of Potsdam, Germany
Gaia DR2 Catalogue of Extremely-low Mass White Dwarf Candidates. Extremely-low mass white dwarf stars (ELMs) are white dwarfs with M < 0.3 MSun, born either as a result of a common-envelope phase or after a stable Roche-lobe overflow episode in a multiple system. The Universe is not old enough for ELMs to have formed through single-star evolution channels. As remnants of binary evolution, ELMs can shed light onto the poorly understood yet crucial phase of common-envelope evolution and provide constraints to the physics of mass accretion. Most known ELMs will merge in less than a Hubble time, providing an important contribution to the signal to be detected by upcoming space-based gravitational wave detectors. There are currently less than 150 known ELMs; most were selected by colour, with a bias towards hot objects, in a magnitude-limited survey of the Northern hemisphere only. Recent theoretical models have predicted a much larger spacial density for ELMs than estimated observationally based on this limited sample. In order to perform meaningful comparisons with theoretical models and test their predictions, a larger and unbiased sample is required. In this work, we present a catalogue of ELM candidates selected from the second data release of Gaia (DR2). We have used predictions from theoretical models and analysed the properties of the known sample to map the space spanned by these objects in the Gaia Hertzsprung-Russell diagram. Defining a set of colour cuts and quality flags, we have obtained a final sample of 1720 ELM candidates, out of which 1164 are within 1 kpc.
Axel Schwope, Leibniz-Institute for Astrophysics Potsdam (AIP),Germany
X-ray surveys with eROSITA. I will describe the scope and the promise of X-ray surveys for compact white-dwarf binaries to be performed with SRG/eROSITA. Launch is foreseen for summer 2019 and the 1st light observation to be performed around the time of the conference.
Boris Gaensicke, University of Warwick, UK
Dissecting the pulsations of GW Lib with simultaneous K2 and HST observations. A small number of white dwarfs in cataclysmic variables exhibit non-radial pulsations. As the envelope of these white dwarfs is continuously enriched with helium accreted from the companion star, they are not confined to the canonical DAV and DBV instability strips, and the characteristics of their pulsations remain poorly explored. We present the results of a co-ordinated 88-day campaign on GW Lib, the prototype of this class of pulsators, using K2, HST and Swift. The short-cadence K2 data shows a persistent modulation at ∼ 20 min, plus two harmonics, super-imposed by periods lasting two to ten days during which a larger amplitude variability with a period of ∼ 4 h is present. The simultaneous HST observations confirm that both types of variability are related to temperature variations of the white dwarf. We will discuss the results of this campaign in the context of the current theories of non-radial pulsations in mixed H/He envelopes.
Stella Kafka, AAVSO, USA
The AAVSO's contribution to compact WD binary research. The American Association of Variable Star Observers was formed in 1911 as a group of US-based amateur observers obtaining data in support of professional astronomy projects. Now, it has evolved into an International Association with members and observers from both the professional and non-professional astronomical community, contributing photometry to a public photometric database of more than 25,000 variable objects, and using it for research projects. As such, the AAVSO’s main claim to fame is that it successfully engages backyard Astronomers, educators, students and professional astronomers in astronomical research. I will present the main aspects of the association and how it has evolved with time to become a premium resource for variable star researchers. I will also discuss the various means that the AAVSO is using to support cutting-edge compact WD binary research, and give updates on the tools and resources that are now available to researchers worldwide.
James Court, Texas Tech University, USA
Z Cha as seen by TESS: A New Method to trace the State of the Disk During Outburst. I present the results of a study on the eclipsing CV Z Cha using data from TESS observations of the source during both an outburst and a superoutburst in 2019. Using our data we are able to place better constraints on the postulated third body in this system which causes periodic fluctuations in the observed orbital period over time. We also present a new method for tracing the period associated with a superhump, and show that it tends towards the orbital period on the same timescale that the superoutburst decays. Finally, we show evidence for hysteresis in a plot of eclipse depth against out-of-eclipse flux during an outburst, and show how this hysteresis can be used to estimate the response time between the disk increasing in temperature and the disk increasing in size, breaking this long-standing degeneracy.
Maxim Lyutikov, Purdue University, USA
AT2018cow following electron-capture collapse of merged white dwarfs. We develop a model of AT2018cow resulting from electron-capture collapse following a merger of a massive ONeMg white dwarf (WD) with another WD. Two distinct evolutionary channels lead to the disruption of the less massive WD during the merger and formation of a shell burning non-degenerate star incorporating the ONeMg core. During the electron-capture collapse little of the material is ejected with mildly relativistic velocities. The remnant neutron star is spun up and magnetic field is amplified. The ensuing fast magnetically-dominated relativistic wind from the newly formed neutron star shocks against the ejecta, and later against the wind. The model explains many of the puzzling properties of AT2018cow: host galaxies, fast and light anisotropic ejecta producing bright optical peak, afterglow high energy emission of similar luminosity to optical, late infra-red features.
David Buckley, South African Astronomical Observatory, SA
Q: When is a pulsar not a neutron star? A: when its a white dwarf! Multi-wavelength observations of AR Sco have established that it exhibits pulsed non-thermal emission, from X-ray to radio wavelengths, at a 2 min period, ostensibly powered by the spin-down power of a magnetic white dwarf. This has therefore been suggested to be the first clear white dwarf analogue of a pulsar. Polarimetric observations undertaken at SAAO have strengthened these assertions. I will discuss the nature of this fascinating system and some recent results which bolster the pulsar analogue case through application of the rotating vector model.
Elias Aydi, Michigan State University, USA
Investigating flaring novae. Classical novae are transient events taking place on the surface of accreting white dwarfs in interacting binaries. While a stereotypical nova light curve shows a quick rise to maximum followed by a smooth decline, there have long been poorly-understood observational complications to this picture. Many light curves deviate from the stereotypical trend and show multiple peaks (flares), which might imply multiple ejections of discrete shells rather than a simple ballistic ejection. Therefore, investigating flaring novae is of a great importance to understand the diversity of ejection mechanisms that govern nova eruptions. In this talk I will highlight recent efforts towards a better understanding of flaring novae, focusing particularly on two remarkable examples, novae ASASSN-17pf and ASASSN-18fv.
Tom Marsh, University of Warwick, UK
High time resolution photometry of the white dwarf pulsar AR Sco and the evolution of its magnetic field. AR Sco is a unique white dwarf system which is powered by its rapidly spinning magnetic white dwarf, and which seems not to accrete. I will present high-speed multi-band photometry of AR Sco taken with the 5-band high-speed CCD camera, HiPERCAM. I will use these and other data to discuss the present day spin evolution of the white dwarf. The white dwarf in AR Sco must once have had a much weaker field. I will show that the field might have emerged when the system detached near the period gap at 3 hours, with the transfer of angular momentum from spin to orbit resulting in the present day period above 3.5 hours. It is possible that many present day polars may have had an AR Sco-like phase in their past.
Stephania Hernandez, Universidad de Valparaiso, Chile
Searching for SN Ia progenitors among white dwarf with early type secondary stars. The unique capabilities of Type Ia Supernovae (SNIa) as distance indicators, sufficiently bright to serve as yardsticks on cosmological distance scales, has resulted in them becoming some of the most important objects in the Universe. Although it is well established that SN Ia are related to the thermonuclear ignition of a carbon-oxygen core white dwarf, there is not yet a general consensus on the pathways leading to the explosion. The two main formation channels are thought to be the double-degenerate channel, i.e. the merger of two white dwarfs, and the single-degenerate channel, where a white dwarf accretes from a variety of non- degenerate mass donors at a sufficiently high rate to sustain stable hydrogen burning. In both SN Ia formation channels, the progenitors are the descendants of detached binaries containing a WD with a main-sequence star companion of spectral type AFGK. By combining optical data from Tycho-Gaia Astrometric Solution (TGAS) with ultraviolet data from GALEX, we identified hundreds of main sequence stars of spectral type AFGK with large UV excesses, indicative of the presence of a white dwarf. among these objects, we identified 110 close binaries and measured 12 periods. In this talk, I will discuss in more detail some particularly interesting objects: two systems with very short orbital periods and F secondaries that will potentially evolve into SN Ia, and system with eccentric orbits.
Yossef Zenati, Technion, Israel
Normal type-Ia supernovae from disruptions of hybrid He-CO white-dwarfs by CO white-dwarfs. Type-Ia supernovae (SNe) are thought to originate from the thermonu- clear explosions of carbon-oxygen (CO) white-dwarf (WD) stars. They produce most of the Iron-peak elements in the universe, and bright Ia-SNe serve as important “standard candle” cosmological distance-indicators. The proposed progenitors of standard type Ia-SNe had been studied for decades, and can be generally divided between explosions of CO-WDs ac- creting material from stellar non-degenerate companions (single-degenerate; SD models), and those arising from the mergers of two CO-WDs (double- degenerage; DD models). However, current models for the progentiors of such SNe failed to reproduce the diverse properties of the observed explo- sions, nor did they explain the inferred rates and the characteristics of the observed populations of type Ia-SNe. Here we use detailed thermonuclear- hydrodynamical and radiative-transfer models to show that mergers of CO-WDs with hybrid CO-He WDs can give rise to normal type Ia-SNe. We find that such mergers give rise to explosions that which synthetic light-curves and spectra resemble those of observed type Ia-SNe, and in particular can produce a wide range of peak-luminousities and light-curve width, consistent with those observed for normal Ia-SNe. Moreover, our population synthesis models show that together with the contribution of massive CO-WD mergers they can reproduce the rate and delay-time dis- trbution of type Ia SNe. Mergers of hybrid He-CO WDs can therefore play a key role in explaining the origin of type Ia SNe, serve to study their detailed compostion yields, and potentially probe the systematics involved in Ia-SNe measurements of the cosmological parameters of the universe.
Gagik Tovmassian, Institute of Astronomy, UNAM, Mexico
The donor star of V1082 Sgr as seen from the IR spectroscopy. We will report the results of JHK IR spectroscopic observations obtained at the GTC with EMIR. We will discuss the status of the binary based on these observations.
Irma Fuentes Morales, Universidad de Valparaiso, Chile
The orbital period of old novae. Following with the project to recover old-novae with eruption before 1986, we reported four new orbital periods for the old novae X Cir, V2572 Sgr, XX Tau and DY Pup. Additionally, we present a updated version of the observed orbital period distribution of novae including our last results. This is compared with the predicted period distribution for post-novae using Montecarlo simulations.
Felipe Lagos, Universidad de Valparaiso, Chile
Are all low-mass white dwarfs formed in triple systems? In spite of their importance for modern astronomy, we do not fully understand how close binary stars containing at least one compact object form from main sequence binary stars. This is mostly because our prescriptions for mass transfer interactions and angular momentum loss are highly uncertain. The discovery of EL CVn binaries, extremely low mass pre white dwarfs with close A/F main sequence star companions, offers now the unique possibility to test our models of close compact binary star formation. Binary evolution theories predict that these EL CVn stars descend from very close main sequence binaries (orbital period less than 3 days). If this is correct, nearly all EL CVn stars should be inner binaries of hierarchical triples because more than 95% of the alleged progenitor systems are found to be hierarchical triples. If true, the third component might play crucial role in the close binary formation for example through Kozai-Lidov oscillations. We performed a survey with SPHERE searching for the third component in a sample of EL CVn stars, and preliminary results reveal indeed the presence of tertiary components. We discuss this fascinating result in the context of close white dwarf binary star formation.
Marina Orio, INAF-Padova, Italy and University of Wisconsin, Madison, USA
Timing supersoft X-ray sources with NICER. Intriguing modulation of supersoft X-ray flux on timescales clustering either around half an hour or 1 minute have been discovered in white dwarfs that are burning hydrogen in an accreted shell, either after a nova outburst, or in a semi-steady mode in other supersoft X-ray sources. I will discuss the possible root causes) of these modulations and present new high quality timing data obtained with the NICER (Neutron Stars Interior Composition Explorer Mission) payload on the International Space Station. I will also connect the timing data with the evidence obtained with high resolution X-ray spectra with the XMM-Newton and Chandra grating spectrographs.
Lilit Barkhudaryan, Byurakan Astrophysical Observatory, Armenia
Normal Type Ia and 91bg-like supernovae in elliptical host galaxies. In this talk I will present the results of our recent analysis of the global parameters of elliptical host galaxies of "normal" and peculiar "91bg-like" subclasses of supernovae (SNe) Ia with the aim to find links between the properties of SN Ia progenitors and host stellar populations. Among the studied parameters, only the distributions of u-r colours and ages are inconsistent significantly between the ellipticals of different SN Ia subclasses: the hosts of normal SNe are on average bluer and younger than those of 91bg-like SNe. In the colour-mass diagram, the tail of colour distribution of normal SN hosts stretches into green valley, a transitional state of galaxy evolution, while the same tail of 91bg-like SN hosts barely reaches that region. Therefore, the bluer and younger ellipticals might have more residual star formation that gives rise to younger "prompt" SN Ia progenitors, resulting in normal SNe with shorter delay times. These ellipticals can also produce "delayed" 91bg-like events with lower rate, because of very long delay times. The redder and older ellipticals that already exhausted nearly all star formation budget may produce significantly less normal SNe Ia with shorter delay times, prevailing by 91bg-like SNe with very long delay times. Our results favor SN Ia progenitor models such as He-ignited violent mergers as a unified model for normal and 91bg-like SNe that have the potential to explain their observed properties.
Isabel de Jesus Lima, National Institute of Space Research Brazil and University of Washington, USA
Simultaneous modelling of X-ray emission and optical polarization of Intermediate polars. CYCLOPS, acronym for CYCLOtron emission of PolarS, is a code that calculates the radiative transport along a 3D accretion column in magnetic cataclysmic variables assuming non-homogeneous magnetic field, temperature, and density distributions. CYCLOPS also includes the numerical shock solution from magnetized post-shock plasma. V405 Aurigae is considered the highest magnetic field intermediate polars. Previous studies of V405 Aur were not successful in proposing a geometry that explains both the optical and X-ray data. In this study, we present a model that reproduces the observations: broadband (UBVRI) circular polarization and photometry, as well as the X-ray spectrum and light curve. We compare scenarios using one or two post-shock regions. In particular, we confirm the high magnetic field intensity (B=36MG) proposed in the literature using the one-region model.
Claudia Rodrigues, Instituto Nacional de Pesquisas Espaciais, Brazil
Polarimetry, spectroscopy and modelling of the polar V348 Pav. Many evidences point to significant differences in the mass distributions of isolated white dwarfs (WDs) and of WDs in cataclysmic variables (CVs). However, the differences between the distributions of WD masses in magnetic and in non-magnetic CVs are less clear. Clarifying this would bring important consequences to the understanding of CV evolution. We investigate the physical and geometrical properties of the polar V348 Pav using optical spectroscopic, photometric and polarimetric data. This object has a high level of circular polarization, reaching around 30%, and a high HeII 4686AA to H Beta line ratio, confirming it is a polar. We determined an orbital period of 1.33 h, below the orbital period gap of CVs. Doppler tomography showed that most of the spectral line emission in this system is originated in the X-ray heated surface of the companion star. To model the rotational modulation of flux and polarization in four optical bands, we use a code that solves the shock structure and then calculates the radiative transfer in the post-shock region emitting by the cyclotron mechanism. This modelling combined with the radial-velocity curve of the companion star indicates a high value for the WD mass, placing V348 Pav among the most massive WD in polars.
Kiril Stoyanov, Institute of Astronomy and National Astronomical Observatory, Bulgaria
Optical observations of the recurrent nova RS Oph. We present optical observations of the recurrent nova RS Oph in the recent years. On the base of simultaneous UBVRI photometry, we estimated the physical parameters of the flickering source. We also present high-resolution spectral observations and discuss the emission lines variability.
Elena Pavlenko, Crimean Astrophysical Observatory, Ukraine
Evolution of negative superhumps. A photometric study of the negative superhumps for three SU UMa-type dwarf novae MN Dra, NY Her and V503 Cyg is presented. It is based on intensive observations from 2012 to 2018 obtained at five observatories. These dwarf novae are known as systems with strong variations (two to three times) of the interval between successive normal outbursts (cycle) and superoutbursts (supercycle). The negative superhumps were recorded with an increased cycle. We investigated the evolution of negative superhumps on a long time scale and revealed systematic changes in the periods of negative superhumps during the cycle and supercycle that are identical for the three systems. These variations may indicate changes in the radius of accretion disk between outbursts and superoutbursts. We found that the observed variations fit with a prediction of the thermal-tidal instability model of the superoutburst and supercycle of SU UMa-type dwarf novae.
Tomasz Bulik, University of Warsaw, Poland
How to search for common envelope events? Classic binary evolutionary scenarios predict that a transitional common-envelope (CE) phase could be preceded as well as succeeded by the evolutionary stage when a binary consists of a compact object and a star, that is, an X-ray binary. The observational manifestations of common envelope are poorly constrained. We speculate that its ejection might be observed in some cases as a transient event at mid-infrared (IR) wavelengths. We estimate the rate density of CE events in the local Universe and discuss its observability.
Odette Toloza, University of Warwick, UK
White dwarfs accreting from nuclear evolved companions. Accreting white dwarf can lead to numerous important outcomes from SNIa to low frequency gravitational wave emitters. These outcomes are likely related to the evolution of white dwarfs accreting from companions that have undergone considerable nuclear evolution before the onset of mass transfer. However, while current theoretical models can broadly reproduce observationally the bulk of the population of their analogues i.e. of white dwarfs accreting from low mass (un-evolved) companions (known as cataclysmic variables), these models fail to reproduce the accretion rates observed in white dwarf with evolved companions. Specifically, the standard prescription of magnetic braking which drives the evolution of these systems results in substantial overestimates of the accretion rates. Here, I present simulations with MESA exploring the different prescriptions of magnetic braking, including the most recent braking laws for the spin-down single stars.
Kirill Sokolovsky, Michigan State University
X-ray spectroscopy of the brightest gamma-ray nova ASASSN-18fv. Nova events occur when hydrogen burning ignites in the hydrogen-rich layer near the surface of an accreting white dwarf. The resulting optical flare is accompanied by emission in other bands of the electromagnetic spectrum including GeV gamma rays and X rays. The X-ray emission may be produced in the following stages of the nova outburst: 1) soft X rays emitted by the fireball during the first hours of explosion (never observed); 2) hard X rays from plasma heated by internal shocks in the nova outflow; 3) super-soft thermal X rays from the surface of the nuclear-burning white dwarf become visible when the ejecta clears. Shock waves produced within the expanding nova ejecta are powering gamma rays and a fraction of optical emission, but most of their energy is expected to be dissipated as X rays. We test this scenario with NuSTAR, Swift and XMM-Newton observations of the brightest gamma-ray nova observed so far - Nova Carinae 2018 (= ASASSN-18fv = V906 Car). The observations reveal X-ray emission consistent with being produced by single-temperature thermal plasma that gradually cools. The emission first appears at the NuSTAR 3.5-78 keV band (detected simultaneously with GeV gamma rays observed by Fermi) and becomes visible at the Swift 0.3-10 keV band two months after explosion. The element abundances in the ejecta are clearly non-solar. The observations support the view that shocks play a crucial role in nova events and may provide constraints on the particle acceleration efficiency in novae.
Joanna Mikołajewska, Nicolaus Copernicus Astronomical Center of Polish Academy of Sciences
Symbiotic paths to close white dwarf pairs and SN Ia. Symbiotic stars (SySt) are interacting binaries in which an evolved red giant (RG) transfers material to a hot and luminous white dwarf (WD). This presentation aims at presenting their basic characteristics and discussing their possible link with SN Ia progenitors. SySt are promising progenitors for SN Ia regardless of whether the path to SN Ia explosion is trough the accretion (single degenerate, SD) scenario or through merging a double WD system (double degenerate, DD) scenario. Our recent finding that the WD in the symbiotic recurrent nova RS Oph is clearly made of CO and its mass has grown significantly since its birth makes it a prime candidate to eventually produce SN Ia via the SD scenario. Most SNe Ia seem to result from the DD merger, and SySt are obvious nurseries for double WD systems. Our ongoing study of Galactic nad Magellanic SySt shows that majority of SySt contain WDs with masses between 0.5-1 Msun and 1-3 Msun RG companions with core masses of ~0.45-0.55 Msun. Their orbital period are <~1000 yrs, and many of them after a common envelope phase will become close pairs of WDs. Before this will happen, the present WD masses are likely to more or less steadily burn accreted material and increase their mass for long enough time that the total mass of the final WD pair would become comparable to or higher than the Chandrasekhar limit.
Valeri Hambaryan, Astrophysical Institute, University of Jena, Germany
A pulsar-runaway-pair from a nearby supernova ~1.8 Myr ago that ejected 60Fe found on Earth. The detection of 1.5-3.2 Myr 60-Fe on Earth indicates recent nearby core-collapse supernovae. For supernovae in multiple stars, the primary stars become neutron stars, while former companions can get unbound (runaway stars). By tracing back the space motion of runaway and neutron stars to the nearest stellar associations (Scorpius-Centaurus-Lupus and Tucana-Horologium), we found kinematic evidence that a certain runaway stars and a certain pulsars were released by a supernova in a binary about 1.6-1.8 Myr ago at about 50-10 pc distance; associations age and flight time determine the progenitor mass (16-18 Msun), which can constrain supernova nucleosynthesis yields and 60-Fe uptake on Earth.Our scenario links 60-Fe found on Earth and Moon to an individual supernova in a binary.
Krystian Iłkiewicz, Nicolaus Copernicus Astronomical Center of Polish Academy of Sciences
Hibernation scenario as seen by population synthesis. The apparent lack of classical novae outbursts in dwarf novae has been puzzling astronomers for decades. The solution to this problem, proposed over 30 years ago, is a hibernation scenario in which all cataclysmic variables undergo cycles of low and high states of mass transfer rate. The recent observations seem to support the hypothesis of hibernation scenario, however it was not yet proven. In my talk I will present the results of population synthesis of classical novae. In particular I will present the predicted rate of classical nova outbursts in dwarf novae, which makes challenge to the hibernation scenario.
Nazar Ikhsanov, Pulkovo Observatory, St. Petersburg, Russia
On the origin of pulsar-like white dwarfs in AR Scorpii and AE Aquarii. AR Scorpii is a 3.56h orbital period binary system composed of a M5V red dwarf and a white dwarf which rotates with the period of 117s and manifests itself as a spin-powered radio pulsar. The rapid spin-down of the white dwarf indicates that its surface magnetic field exceeds 150 MG and prevents the surrounding material from penetrating to within its Roche lobe. Since the cooling age of the white dwarf significantly exceeds its spin-down timescale the system could not be formed in its current state. On the other hand, an attempt to model the origin of the system in terms of accretion-driven spin-up scenario also encounters difficulties. Using currently adopted disk accretion scenario one finds that a white dwarf with the surface field of 150 MG could be spun-up to a period of 117s only if the mass accretion rate onto the white dwarf were in excess of the Eddington limit. Otherwise, the centrifugal barrier at the Alfven radius would prevent material from reaching the surface of the white dwarf well before its spin period reaches the currently observed value. This may indicate that either the surface magnetic field of the white dwarf during a previous spin-up epoch was buried by the accreted material or/and the inner radius of the accretion disk was significantly smaller than the conventional Alfven radius. In my talk I discuss both of these possibilities and show that similar corrections to the accretion scenario may help to model the origin of the fast rotating white dwarf in AE Aquarii.
Joannes van Roestel, Caltech, USA
Eclipsing white dwarfs discovered by the Zwicky Transient Factory. The Zwicky Transient Facility is a large field-of-view synoptic survey which has been observing the sky since spring 2018. It has observed the entire northern sky with a median of 100 epochs (g&r) with a limiting magnitude of 20.5. I will present the ZTF survey and the different methods we employ to search the data for both outbursting and periodically variable white dwarfs; CVs, AM CVns, interacting sdB-WD binaries, eclipsing WD+RD/BD binaries and (very) short period eclipsing double WDs. I'll focus on two results in particular; a sample of 200 new eclipsing WD-RD systems, which can be used to test binary evolution models, and on 6 new short period eclipsing double white dwarfs, including a detached system with an orbital period of just 7 minutes.
Todd Hillwig, Valparaiso University, Valparaiso, IN USA
Characterizing the Compact Double White Dwarf Population Using Binary Central Stars of Planetary Nebulae. We now have a large enough sample of close binary central stars of planetary nebulae to begin making statistical descriptions of the sample. I use a Monte Carlo routine with the existing close binary sample to determine properties of the compact double white dwarf population in the Galaxy. The results include the birthrate of compact double white dwarf binaries, the birthrate of double white dwarf type Ia supernova progenitors, the observable fractions of these systems, and merger rates for double white dwarf systems.
Maryam Ghasemnezhad, Shahid Bahonar university of Kerman, Iran
The role of anisotropic thermal conduction in a collisionless magnetized hot accretion flow. We study the importance and the effects of anisotropic thermal conduction in a collisionless magnetized advection dominated accretion flow in the presence of discontinuity of mass, angular momentum and energy between inflow and outflow. In this paper, we have considered that the thermal conduction is a heating mechanism like viscosity and leads to an increase in the temperature of the gas. A set of self similar solutions are used for steady state and axisymmetric structure of such hot accretion disc to solve the MHD equations in our model. Based on these solutions, we have found that increasing the level of two parts of anisotropic thermal conduction (parallel & transverse) results in increasing the mass accretion rate or radial velocity but decreasing the rotational velocity. Also both radial and rotational velocities are sub-Keplerian.
Dave Sahman, University of Sheffield, UK
Searching for nova shells around cataclysmic variables. Our understanding of CV evolution has made great strides in recent years, but one of the major remaining unsolved problems is: how can the different types of CV co-exist at the same orbital period? Theory predicts that all CVs evolve from longer to shorter orbital periods on timescales of gigayears, and as they do so the mass-transfer rate also declines. At periods longer than approximately 5 hours, all CVs should have high mass-transfer rates and appear as novalikes, whereas below this period the lower mass-transfer rate allows the disc-instability mechanism to operate and will hence make the CV appear as a dwarf nova. This theoretical expectation, however, is in stark contrast to observations which show that novalikes are far more common than dwarf novae in the 3-4 hr period range. The most plausible explanation for this is that there must be short-timescale cycles in mass-transfer rate which temporarily make some CVs appear as novalikes. In this cyclical evolution theory (aka Hibernation theory), the cycles in mass-transfer rate are caused by nova eruptions. Some fraction of the energy released in the nova event will heat up the white dwarf leading to irradiation and subsequent bloating of the secondary. This increases the mass-transfer rate and the system appears as a novalike. As the white dwarf cools, the mass-transfer rate reduces (or possibly ceases) and the system becomes a dwarf nova. Hence CVs are expected to cycle between nova, novalike and dwarf nova states, on timescales of 10^4 – 10^5 yrs. One test of this theory is to look for old nova shells around novalikes and dwarf novae, to determine if they have experienced a prior nova event. We have undertaken two campaigns to search for nova shells around CVs. We examined a total of 132 targets, including 56 novalikes. We discovered a shell around the novalike V1315 Aql, the first shell ever found around a novalike. We found no shells around any other targets. Combining these results with those of Schmidtobreick et al. (2015) and Pagnotta & Zurek (2016), we find that the novalike phase for CVs is ~5,600 years. This supports the theory of a nova-induced cycle to explain how the different types of CV can co-exist at the same orbital period.
Alexa Muethel, Michigan State University, USA
Searching for Shocks in Novae from Hard X-ray Emission. With the recent detection of GeV gamma-ray emission from novae, it has been discovered that shocks in novae are energetically important and effective at accelerating relativistic particles. However, the bulk of the shock energy should be thermal, and best traced by hard X-rays. In order to better understand the mechanisms and energetics of nova shocks, we are conducting a survey of hard X-ray emission in novae. We present an analysis of X-ray light curves obtained by the Swift X-Ray Telescope over the last 20 years and an interpretation in the context of other wavelengths. This unprecedentedly large sample size allows us to look for population trends with the goal of understanding why certain novae show hard X-ray emission and the relationship to gamma-ray production.
Deanne Coppejans, Northwestern University, USA
Radio jets in compact white dwarf binaries. Jets are found in a variety of accreting white dwarf classes such as super soft sources, novae and symbiotics. Until recently, cataclysmic variables (CVs) were thought to be the one exception. However, there is now evidence indicating that they likely do launch jets. The key argument for jets in these systems is that the accretion-outflow cycle in X-ray Binaries (XRBs) can be mapped to CVs, and that multi-wavelength observations of the exemplar system SS Cyg have spectacularly met the key predictions of this model. In particular, SS Cyg consistently shows a bright radio flare on the rise to outburst (analogous to that of the transient jet emission from an XRB transitioning into the soft state). Current radio observations of other (fainter) CVs are consistent with this behaviour, but with one clear exception. Conclusively establishing whether CVs launch jets is important to understanding jet-launching in compact accretors: they would provide a non-relativistic environment to study jet physics as well as important constraints on the properties needed to launch and collimate jets. I will review the case for jets in CVs and outline several key areas for further investigation.
Sergey Shugarov, Astronomical Institute of Slovak Academy of Sciences
Photometric study of the X-ray Nova MAXI J1820 + 070 in the optical range. UBVRI observations of the optical counterpart of the X-Ray nova MAXI J1820+070 with relativistic component, probable black hole, were carried out from April 2018 to June 2019 on telescopes located in Slovakia and Crimea. The data covers the stages from the beginning of outburst to quiescence. On separate nights, monitoring was conducted to study flickering in different color bands with a time resolution of 15-45 s. The correlation of the optical and X-ray luminosity is found according to our multicolor photometry and the SWIFT data. The changes in the amplitude and shape of a periodic ~0.7-d wave during the outburst and quiescence are studied. The peculiarities of Nova MAXI J1820 + 070 are compared with those of other black hole candidate binaries. Preliminary conclusions of the physical nature of variability are made.
Gohar Petrosyan, Yerevan State University, Armenia
Investigation of evolution of the late-type binary stars of the catalog KP2001, as potential white dwarf stars. In this poster I present the results of investigation of the optical variability of stars of the catalog KP2001. For this purpose, the Northern Sky Variability-Survey (NSVS) monitoring data were used. From 257 investigated objects, 5 are variables of the type - myrids, 33 - semiregular (SR) and 108 - irregular variables (Ir). The behavior of KP2001 stars on the color diagrams of 2MASS and WISE is considered.
Adam Kawash, Michigan State University
A Systematic Comparison of Dwarf Novae vs. Classical Novae. For decades, there has been an inconsistency between the expected rate of classical novae in the Galaxy and the numbered discovered annually. This was expected to be reconciled with many all-sky surveys starting operations in the last few years. The All-Sky Automated Survey for Supernovae (ASAS-SN) is a project that observes the entire night sky nightly down to g ~ 18 mag, making it an ideal survey to search for galactic classical novae. However, the vastly improved sky coverage that ASAS-SN provides has not yielded an increase in the number of discovered novae. To ensure that classical novae are not hiding in the large sample of CVs, we perform a systematic study of the light curve properties of CVs observed by ASAS-SN.
Nikolaus Vogt, Instituto de Física y Astronomía, Universidad de Valparaiso, Chile
Is the shell evolution of a Classical Nova related to its eruption light curve shape or its speed class? We present a new analysis of the nova shell luminosities based on Hα and O[III] λ5007 emission lines fluxes, given by Downes & Duerbeck, 2001 (JAD 7, 6) for epochs between one week and several decades after the eruption maximum of classical and recurrent novae. For this purpose, we use new distances from the Gaia Collaboration DR2 (2018) and investigate whether there are significant correlations of shell luminosities to nova speed classes and/or to the different nova light curve types defined by Strope et al 2010 (AJ 140, 34). We describe the differences among these parameters and add some conclusions on the long-term nova shell evolution.
Alejandra Romero, Universidade Federal do Rio Grande do Sul
Magnetic breaking and the formation of ELM WD – C/O WD binary systems. Extremely low-mass white dwarf (ELMs) are thought to be the result of binary evolution in which a low-mass donor star is stripped by its companion leaving behind a helium-core white dwarf with a typical mass of ~0.17 solar masses. The binary evolution is dominated at short periods by the magnetic braking mechanism, which is the main uncertainty in modeling such systems. In the case of modeling the formation of the observed ELMs orbiting millisecond pulsars in very compact orbits (2-9 hr) a severe fine-tuning in the initial orbital period is necessary suggesting that probably the standard magnetic braking formulation needs to be revised (Istrate et al. 2014). Recently Van et al. (2018) presented a new prescription for the magnetic breaking law that includes a scaling of the magnetic field strength with the convective turnover time and the wind mass loss rate. This prescription seems to be more adequate to reproduce the observed low-mass X-ray binaries properties. We apply this new magnetic braking formulation to the formation of ELMs in compact binaries with millisecond pulsars as well as CO WD companions. In this work we present our first preliminary results
Georgi Latev, Institute of Astronomy and National Astronomical Observatory, Bulgaria
Rapid variability of the intemediate polar AE Aquarii. We present UBVRI photometric observations of the intemediate polar AE Aquarii and estimate the physical parameters of the blobs. The amplitudes and rise and decline times are discussed.
Evgeny Mikhailov, M.V.Lomonosov Moscow State University, Russia
Dynamo models and structure of the magnetic fields in accretion disks around white dwarfs. The magnetic fields in the accretion disks can be very important for the motions of the medium there, they can describe such processes as angular momentum transition and another effects. So it is important to have the models for the magnetic field which can give the realistic structures of the field. Usually the magnetic field generation in the astrophysical objects is described by dynamo theory, which explains the transition of energy of the turbulent motions to the energy of the magnetic field. In the galaxy dynamo theory there are approaches that allow to model the field in the disks using relatively small computational resources. They are based on using the no-z approximation and axisymmetric model with vertical structure of the field. Such approaches have shown good results while modeling the magnetic fields of galaxy disks and outer rings. So it would be useful to try to use them to describe the field in the accretion disks, which have geometrically similar structure (of course, with another length scales). Here we present the results of using “galaxy” models for studying the magnetic field in the accretion disk. There are strong radial flows, so the maximum values of the field are quite close to the center. The dynamo mechanism is a threshold effect: the field grows only for intensive motions. The critical values of the parameters, when the field can grow, are quite low: it is much simplier for the field to be generates in the accretion discs than in the galaxies. We also discuss the influence of the generated magnetic field on the structure of the motions and give some computations.
Michael Politano, Department of Physics, Marquette University, Milwaukee, USA
On the Final Orbital Separation in Common Envelope Evolution. In most population synthesis calculations of close binary stars, the common envelope (CE) phase is modeled using a standard prescription based upon conservation of energy. In this prescription, the orbital separation of the secondary and giant core at the end of the CE phase is taken to be the orbital separation when the envelope becomes unbound. In this poster, we explore a modified treatment of the CE phase, in which the final orbital separation is dictated additionally by the dynamical constraint that the spiral-in of the secondary will be halted when the frictional torque on the secondary is reduced to approximately zero. We crudely estimate this separation as a function of core mass based upon existing stellar interior models of AGB stars.
Todd Hillwig, National Astronomical Observatories, Chinese Academy of Sciences
Binary Central Stars of Planetary Nebulae: Spectra and Photometry. We present spectra and photometry for two recently identified close binary systems in identified planetary nebulae, which both have likely a white dwarf companion. We employed synthetic spectra calculated with the Tübingen non-LTE model-atmosphere package to derive the effective temperatures, surface gravities, and the H/He of the central stars from optical spectroscopy. Current evolutionary models are used to further determine the luminosity and mass of the stars. Radial velocity curves from time-resolved spectra are used with photometric light curves to further refine our understanding of the binary systems.
Keith Inight, Liverpool John Moores University
Federico Bernardini, INAF Rome, INAF Naples, NYUAD, Italy