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AM Canum Venaticorum Stars
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Title: AM Canum Venaticorum Progenitors with Helium Star Donors and the resultant Explosions
Author: Jared Brooks, Lars Bildsten, Pablo Marchant, Bill Paxton

We explore the outcome of mass transfer via Roche lobe overflow (RLOF) of M_He \lesssim 0.51 solar masses pure helium burning stars in close binaries with white dwarfs (WDs). The evolution is driven by the loss of angular momentum through gravitational wave radiation (GWR), and both stars are modelled using Modules for Experiments in Stellar Astrophysics (MESA). The donors have masses of M_He=0.35,0.4,& 0.51 solar masses and accrete onto WDs of mass M_WD from 0.6 solar masses to 1.26 solar masses. The initial orbital periods (Porb) span 20 to 80 minutes. For all cases, the accretion rate onto the WD is below the stable helium burning range, leading to accumulation of helium followed by unstable ignition. The mass of the convective core in the donors is small enough so that the WD accretes enough helium-rich matter to undergo a thermonuclear runaway in the helium shell before any carbon-oxygen enriched matter is transferred. The mass of the accumulated helium shell depends on M_WD and the accretion rate. We show that for M_He \gtrsim 0.4 solar masses and M_WD \gtrsim 0.8 solar masses, the first flash is likely vigorous enough to trigger a detonation in the helium layer. These thermonuclear runaways may be observed as either faint and fast .Ia SNe, or, if the carbon in the core is also detonated, Type Ia SNe. Those that survive the first flash and eject mass will have a temporary increase in orbital separation, but GWR drives the donor back into contact, resuming mass transfer and triggering several subsequent weaker flashes.

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Posts: 131433
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AM Canum Venaticorum binaries
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Title: A search for the hidden population of AM CVn binaries in the Sloan Digital Sky Survey
Authors: P. J. Carter, T. R. Marsh, D. Steeghs, P. J. Groot, G. Nelemans, D. Levitan, A. Rau, C. M. Copperwheat, T.Kupfer, G. H. A. Roelofs

We present the latest results from a spectroscopic survey designed to uncover the hidden population of AM Canum Venaticorum (AM CVn) binaries in the photometric database of the Sloan Digital Sky Survey (SDSS). We selected ~2000 candidates based on their photometric colours, a relatively small sample which is expected to contain the majority of all AM CVn binaries in the SDSS (expected to be ~50). We present two new candidate AM CVn binaries discovered using this strategy: SDSS J104325.08+563258.1 and SDSS J173047.59+554518.5. We also present spectra of 29 new cataclysmic variables, 23 DQ white dwarfs and 21 DZ white dwarfs discovered in this survey.
The survey is now approximately 70 per cent complete, and the discovery of seven new AM CVn binaries indicates a lower space density than previously predicted. From the essentially complete g The sample has been cross-matched with the GALEX All-Sky Imaging Survey database, and with Data Release 9 of the UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey (UKIDSS). The addition of UV photometry allows new colour cuts to be applied, reducing the size of our sample to ~1100 objects. Optimising our followup should allow us to uncover the remaining AM CVn binaries present in the SDSS, providing the larger homogeneous sample required to more reliably estimate their space density.

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Posts: 131433
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AM CVn binaries
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Title: The long term optical behaviour of helium accreting AM CVn binaries
Authors: Gavin Ramsay (1), Thomas Barclay (1,2), Danny Steeghs (3), Peter J. Wheatley (3), Pasi Hakala (4), Iwona Kotko (5), Simon Rosen (6), ((1) Armagh Observatory, (2) MSSL/UCL, (3) Univ Warwick, (4) FINCA, (5) Jagiellonian University, (6) Univ Leicester)

We present the results of a two and a half year optical photometric monitoring programme covering 16 AM CVn binaries using the Liverpool Telescope on La Palma. We detected outbursts in seven systems, one of which (SDSS J0129) was seen in outburst for the first time. Our study coupled with existing data shows that ~1/3 of these helium-rich accreting compact binaries show outbursts. The orbital period of the outbursting systems lie in the range 24-44 mins and is remarkably consistent with disk-instability predictions. The characteristics of the outbursts seem to be broadly correlated with their orbital period (and hence mass transfer rate). Systems which have short periods (<30 min) tend to exhibit outbursts lasting 1--2 weeks and often show a distinct 'dip' in flux shortly after the on-set of the burst. We explore the nature of these dips which are also seen in the near-UV. The longer period bursters show higher amplitude events (5 mag) that can last several months. We have made simulations to estimate how many outbursts we are likely to have missed.

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Binary White Dwarfs
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Title: The Discovery of Binary White Dwarfs that will Merge within 500 Myr
Authors: Mukremin Kilic, Warren R. Brown, Carlos Allende Prieto, S. J. Kenyon

We present radial velocity observations of four extremely low-mass (0.2 Msol) white dwarfs. All four stars show peak-to-peak radial velocity variations of 540 - 710 km/s with 1.0 - 5.9 hr periods. The optical photometry rules out main-sequence companions. In addition, no milli-second pulsar companions are detected in radio observations. Thus the invisible companions are most likely white dwarfs. Due to the loss of angular momentum through gravitational radiation, three of the systems will merge within 500 Myr. The remaining system will merge within a Hubble time. The mass functions for three of the systems imply companions more massive than 0.44 Msol; thus those are carbon/oxygen core white dwarfs. However, the chance of a supernova Ia event is only 1% to 5%. These systems will most likely form single R Coronae Borealis stars, providing evidence for a white dwarf + white dwarf merger mechanism for these unusual objects. One of the systems, SDSS J105353.89+520031.0 has a 70% chance of having a low-mass white dwarf companion. This system will probably form a single helium-enriched subdwarf O star. All four white dwarf systems have unusual mass ratios of < 0.2-0.8 that may also lead to the formation of AM CVn systems. The unknown inclination angles prohibit a definitive conclusion about the future of these systems.

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AM CVn stars
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Title: A census of AM CVn stars: three new candidates and one confirmed 48.3-minute binary
Authors: Arne Rau (1 and 2), Gijs H.A. Roelofs (3), Paul J. Groot (4), Tom R. Marsh (5), Gijs Nelemans (4), Danny Steeghs (3 and 5), Mansi M. Kasliwal (2) ((1) MPE Garching, (2) Caltech, (3) CfA Harvard, (4) Radboud University Nijmegen, (5) University of Warwick)

We present three new candidate AM CVn binaries, plus one confirmed new system, from a spectroscopic survey of colour-selected objects from the Sloan Digital Sky Survey. All four systems were found from their helium emission lines in low-resolution spectra taken on the Hale telescope at Palomar, and the Nordic Optical Telescope and the William Herschel Telescope on La Palma.
The ultra-compact binary nature of SDSS J090221.35+381941.9 was confirmed using phase-resolved spectroscopy at the Keck-I telescope. From the characteristic radial velocity `S-wave' observed in the helium emission lines we measure an orbital period of 48.31 0.08 min. The continuum emission can be described with a blackbody or a helium white dwarf atmosphere of T_eff ~ 15,000K, in agreement with theoretical cooling models for relatively massive accretors and/or donors. The absence in the spectrum of broad helium absorption lines from the accreting white dwarf suggests that the accreting white dwarf cannot be much hotter than 15,000K, or that an additional component such as the accretion disk contributes substantially to the optical flux.
Two of the candidate systems, SDSS J152509.57+360054.5 and SDSS J172102.48+273301.2, do show helium absorption in the blue part of their spectra in addition to the characteristic helium emission lines. This, in combination with the high effective temperatures of ~18,000K and ~16,000K suggests both two be at orbital periods below ~40min. The third candidate, SDSS J164228.06+193410.0, exhibits remarkably strong helium emission on top of a relatively cool (T_eff~12,000K) continuum, indicating an orbital period above ~50min.

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Posts: 131433
Date:
AM_CVn Stars
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Astronomers have reported the first detection of direct radiation from the surface of a white dwarf star in a pre-supernova binary star system using the Hubble Space Telescope. This is a major step forward in identifying the type of star that will become a Type Ia supernova, the type of supernova that is being used to show that the expansion of the universe is accelerating.
These binary objects, called AM Cvn stars, have virtually pure helium in their outer layers, and are considered among the strongest progenitor candidates for the occurrence of Type Ia supernovae, the kind of supernova which are used as "standard candles" to measure the size of our Universe, and being used to show that the expansion of the Universe is speeding up instead of slowing down. The Hubble team is led by Dr. Edward Sion of Villanova University and includes Dr. Paula Szkody of the University of Washington, Seattle, Dr.Jan-Erik Solheim of the University of Oslo, Norway, Dr. Boris Gaensicke, the University of Warwick, England and Dr. Steve Howell of the National Optical Astronomical Observatories, Tucson.

The team is presenting the results of their computer modelling of the Hubble data at the 207th meeting of the American Astronomical Society in Washington, DC. Their work has been accepted for publication in an upcoming issue of the Astrophysical Journal Letters.
The extremely dense, planet-sized white dwarf star is normally surrounded by a swirling disk of helium gas from a very close helium-rich lighter weight donor star. The Hubble Team observed the object during a brief time when the helium disk hiding the white dwarf, temporarily goes away.
A mere teaspoonful of the white dwarf's matter weighs over 100 tons. This heavy weight white dwarf and the lighter weight helium-rich donor star whirl around each other in a stellar dance every 28 minutes. In order for Type Ia supernovae to be used as proper reliable standard candles, we must understand what kind of star exploded. This white dwarf detection helps provide this information.

The Team demonstrates that the pre-supernova object is much cooler and more slowly spinning than predicted by theory. The white dwarf's surface chemistry is laden with heavy metals. In the pre-supernova binary, helium is being transferred from the lightest, largest star to the heaviest, smallest star as they orbit each other every 28 minutes. In most cases the accretion of mass by the heaviest star proceeds via a nearly pure helium accretion disk, At present, only about 10 such double nearly pure helium white dwarf systems are known. But recent estimates predict enough to account for the observed rate of Type Ia supernovae.
This first spectroscopic detection of the white dwarf in a AM CVn system allows us to directly find for the first time the chemical makeup, spin rate, and mass of the white dwarf as well as estimating how fast helium is accumulating onto the primary white dwarf.

These objects can undergo a Type Ia supernova explosions without requiring that the white dwarf star first reach its maximum possible mass, the so-called Chandrasekhar limit. If the incoming helium accumulates slowly enough onto the heavier white dwarf, it will gradually compress or crush the matter below, triggering a helium thermonuclear explosion which will cause the carbon in the core of the white dwarf to detonate 10,000,000 times more violently as a Type Ia supernova. This process called edge-lit detonation (ELD) will occur in an AM CVn system at low enough accretion rates and slow enough rotational velocities.

The AM CVn binary systems are also the only known source of low frequency gravitational waves predicted by Einstein's theory of general relativity.

When the two compact stars in an AM CV binary system revolve around each other, they lose energy and angular momentum through the emission of gravitational waves, at the expense of their own orbital energy. This causes their orbits to shrink ever further. The orbit shrinkage has been observed in binary radiopulsars, e.g. in the famous Hulse-Taylor pulsar PSR B1913+16 (Nobel Prize in Physics 1993) but there has never yet been a direct detection of gravitational waves. AM CVn systems binary white dwarfs are expected to be the dominant sources of gravitational waves to be detected by LISA, the laser interferometer in space which is due for launch be launch early in the next decade.

Source

The AM CVn systems are a select group of 5 stars (AM CVn(HZ 29 ), V803 Cen, CR Boo, CP Eri and GP Com) which show no hydrogen in their spectra along with signs of periodic variability, normally taken to indicate that they are binaries.

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