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RE: HD49798 white dwarf companion
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Title: Is the X-ray pulsating companion of HD 49798 a possible type Ia supernova progenitor?
Author: Dong-Dong Liu, Wei-Hong Zhou, Cheng-Yuan Wu, Bo Wang

HD 49798 (a hydrogen depleted subdwarf O6 star) with its massive white dwarf (WD) companion has been suggested to be a progenitor candidate of type Ia supernovae (SNe Ia). However, it is still uncertain whether the companion of HD 49798 is a carbon-oxygen (CO) WD or an oxygen-neon (ONe) WD. A CO WD will explode as an SN Ia when its mass grows approach to Chandrasekhar mass, while the outcome of an accreting ONe WD is likely to be a neutron star. We followed a series of Monte Carlo binary population synthesis approach to simulate the formation of ONe WD + He star systems. We found that there is almost no orbital period as large as HD 49798 with its WD companion in these ONe WD + He star systems based on our simulations, which means that the companion of HD 49798 might not be an ONe WD. We suggest that the companion of HD 49798 is most likely a CO WD, which can be expected to increase its mass to the Chandrasekhar mass limit by accreting He-rich material from HD 49798. Thus, HD 49798 with its companion may produce an SN Ia in its future evolution.

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RX J0648.0-4418
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Title: X-ray emission from the luminous O-type subdwarf HD 49798 and its compact companion
Authors: S. Mereghetti, N. La Palombara, A. Tiengo, N. Sartore, P. Esposito, G.L. Israel, L.Stella

The X-ray source RX J0648.0-4418 is the only confirmed binary system in which a compact object, most likely a massive white dwarf, accretes from a hot subdwarf companion, the bright sdO star HD 49798. The X-ray emission from this system is characterised by two periodic modulations caused by an eclipse, at the orbital period of 1.55 d, and by the rotation of the compact object with a spin period of 13.2 s. In 2011 we obtained six short XMM-Newton observations centered at orbital phase 0.75, in order to study the system during the eclipse, and spaced at increasingly long time intervals in order to obtain an accurate measure of the spin-period evolution through phase-connected timing. The duration of the eclipse ingress and egress, 500 s, indicates the presence of an X-ray emitting region with dimensions of the order of a few 10^4 km, surrounding the pulsar and probably due to scattering in the companion's wind. We derived an upper limit on the spin-period derivative |Pdot|<6x10^-15 s/s, more than two orders of magnitude smaller than the previously available value. Significant X-ray emission is detected also during the 1.2 hours-long eclipse, with a luminosity of about 3x10^30 erg/s. The eclipse spectrum shows prominent emission lines of H- and He-like nitrogen, an overabundant element in HD 49798. These findings support the suggestion that the X-ray emission observed during the eclipse originates in HD 49798 and that the processes responsible for X-ray emission in the stellar winds of massive O stars are also at work in the much weaker winds of hot subdwarfs.

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RE: HD49798 white dwarf companion
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Dizzy dwarf star will spin itself to death

Dizzy the white dwarf star is under a powerful curse, doomed to spin faster than any of its peers. No one knows why poor Dizzy was handed such a fate, but its wild gyrations mean that the star could be headed for a violent death, perhaps unlike any stellar explosion we've seen before.
White dwarfs are the dense cores left over when stars like the sun die. Dizzy, officially catalogued as RX J0648.0-4418, is probably the densest known white dwarf, packing more than the mass of our sun into a ball about the size of Mars.
 
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Title: X-ray and optical observations of the unique binary system HD49798/RXJ0648.0-4418
Authors: S.Mereghetti (1), N.La Palombara (1), A.Tiengo (1), F.Pizzolato (1), P.Esposito (2), P.A.Woudt (3), G.L.Israel (4), L.Stella (4) ((1) INAF-IASF MILANO, Italy, (2) INAF-OA CAGLIARI, ITALY, (3) University of Cape Town, South Africa, (4) INAF-OA ROMA, Italy)

We report the results of XMM-Newton observations of HD49798/RXJ0648.0-4418, the only known X-ray binary consisting of a hot sub-dwarf and a white dwarf. The white dwarf rotates very rapidly (P=13.2 s) and has a dynamically measured mass of 1.28±0.05 M_sun. Its X-ray emission consists of a strongly pulsed, soft component, well fit by a blackbody with kT~40 eV, accounting for most of the luminosity, and a fainter hard power-law component (photon index ~1.6). A luminosity of ~10^{32} erg/s is produced by accretion onto the white dwarf of the helium-rich matter from the wind of the companion, which is one of the few hot sub-dwarfs showing evidence of mass-loss. A search for optical pulsations at the South African Astronomical Observatory 1.9-m telescope gave negative results. X-rays were detected also during the white dwarf eclipse. This emission, with luminosity 2x10^{30} erg/s, can be attributed to HD 49798 and represents the first detection of a hot sub-dwarf star in the X-ray band. HD49798/RXJ0648.0-4418 is a post-common envelope binary which most likely originated from a pair of stars with masses ~8-10 M_sun. After the current He-burning phase, HD 49798 will expand and reach the Roche-lobe, causing a higher accretion rate onto the white dwarf which can reach the Chandrasekhar limit. Considering the fast spin of the white dwarf, this could lead to the formation of a millisecond pulsar. Alternatively, this system could be a Type Ia supernova progenitor with the appealing characteristic of a short time delay, being the descendent of relatively massive stars.

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HD 49798
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Title: A likely candidate of type Ia supernova progenitors: the X-ray pulsating companion of the hot subdwarf HD 49798
Authors: Bo Wang, Zhanwen Han
(Version v2)

HD 49798 is a hydrogen depleted subdwarf O6 star and has an X-ray pulsating companion (RX J0648.0-4418). The X-ray pulsating companion is a massive white dwarf. Employing Eggleton's stellar evolution code with the optically thick wind assumption, we find that the hot subdwarf HD 49798 and its X-ray pulsating companion could produce a type Ia supernova (SN Ia) in future evolution. This implies that the binary system is a likely candidate of SN Ia progenitors. We also discussed the possibilities of some other WD + He star systems (e.g. V445 Pup and KPD 1930+2752) for producing SNe Ia.

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RX J0648.0-4418
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Title: An ultra-massive fast-spinning white dwarf in a peculiar binary system
Authors: S. Mereghetti, A. Tiengo, P. Esposito, N. La Palombara (1), G.L. Israel, L. Stella (2) ((1) INAF-IASF MILANO, Italy, (2) INAF-Osservatorio Astronomico di Roma, Italy)

White dwarfs typically have masses in a narrow range centred at about 0.6 solar masses (Msun). Only a few ultra-massive white dwarfs (M>1.2 Msun) are known. Those in binary systems are of particular interest because a small amount of accreted mass could drive them above the Chandrasekhar limit, beyond which they become gravitationally unstable. Using data from the XMM-Newton satellite, we show that the X-ray pulsator RX J0648.0-4418 is a white dwarf with mass > 1.2 Msun, based only on dynamical measurements. This ultra-massive white dwarf in a post-common envelope binary with a hot subdwarf can reach the Chandrasekhar limit, and possibly explode as a Type Ia supernova, when its helium-rich companion will transfer mass at an increased rate through Roche lobe overflow.

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HD 49798
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Title: The discovery of a massive white dwarf in the peculiar binary system HD 49798/RX J0648.0-4418
Authors: S.Mereghetti, A.Tiengo, P.Esposito, N.La Palombara (1), G.L.Israel, L.Stella (2) ((1) INAF-IASF Milano, (2) INAF, Oss. Astron. Roma)

An XMM-Newton observation performed in May 2008 has confirmed that the 13 seconds pulsations in the X-ray binary HD 49798/RX J0648.0-4418 are due to a rapidly rotating white dwarf. From the pulse time delays induced by the 1.55 days orbital motion, and the system's inclination, constrained by the duration of the X-ray eclipse discovered in this observation, we could derive a mass of 1.28±0.05 solar masses for the white dwarf. The future evolution of this post common envelope binary system will likely involve a new phase of mass accretion through Roche-lobe overflow that could drive the already massive white dwarf above the Chandrasekhar limit and produce a Type Ia supernova.

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RX J0648.0-4418
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HD 49798
alpha(2000) = 06h 48m 04.64s , delta(2000) = -44d 18' 59.3''
V = 8.30, B-V = -0.29, Spectral type: O6


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HD 49798
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Title: The Discovery of 13 second X-Ray Pulsations from the Hydrogen-depleted Subdwarf O6 Star Binary HD 49798
Authors: G. L. Israel, L. Stella, L. Angelini, N. E. White, T. R. Kallman, P. Giommi and A. Treves

We have discovered strong ~13 s X-ray pulsations in the ROSAT Position Sensitive Proportional Counter light curve of HD 49798, a 1.55 day single-component spectroscopic binary containing a hydrogen-depleted subdwarf O6 star. We find no evidence for period changes during the ~4 hr ROSAT pointing. The source X-ray spectrum is extremely soft, with an unabsorbed 0.1-2 keV luminosity of a few times 10³² ergs s^-1 (distance of 650 pc). A higher luminosity might be hidden in the EUV. Our results imply that the unseen companion is an accreting degenerate star, a white dwarf, or, more likely, a neutron star. In any case, HD 49798 corresponds to a previously unobserved evolutionary stage of a massive binary system after the common-envelope phase and spiralling in.

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HD 49798

Position(2000): RA  = 06h 48m 04.64s, Dec = -44d 18' 59.3''
V = 8.30

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