Cooling curves and initial models for low-mass white dwarfs (<0.25 Msun) with helium core

We present a detailed calculation of the evolution of low-mass ($< 0.25~M_\odot$) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 to 0.06$~M_\odot$. Due to mixing between the core and outer envelope, the surface hydrogen content is 0.5 to 0.35, depending on the initial value of the heavy element (Z) and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.213$~M_\odot$ (Z=0.003), 0.198$~M_\odot$ (Z=0.01), 0.192$~M_\odot$ (Z=0.02) or 0.183$~M_\odot$ (Z=0.03). The duration of the flashes (independent of chemical composition) is between few $\times 10^6$ years to few $\times 10^7$ years. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporary stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche-lobe overflow) occurs between 0.183-0.213$~M_\odot$ (depending on the heavy element value).Comment: 15 pages, 11 figures, 6 tables, submitted to MNRA

Limits to the Mass and the Radius of the Compact Star in SAX J1808.4--3658 and Their Implications

We show that a survey of equations of state and observations of X-ray pulsations from SAX J1808.4-3658 give 2.27 solar mass as the upper limit of the compact star mass. The corresponding upper limit of the radius comes out to be 9.73 km. We also do a probabilistic study to estimate the lower limit of the mass of the compact star. Such a limit puts useful constraints on equations of state. We also discuss the implications of the upper mass limit for the the evolutionary history of the source, as well as the detection of it in radio frequencies. We envisage that the possible observation of radio-eclipse may be able to rule out several soft equation of state models, by setting a moderately high value for the lower limit of inclination angle.Comment: 7 pages, 1 table, 2 figures, accepted for publication in ApJ Letter

The eclipsing binary millisecond pulsar PSR B1744-24A - possible test for a magnetic braking mechanism

As presented by Nice et al. (2000), long-term timing of the eclipsing binary PSR B1744-24A shows that the orbital period of this system decreases with a time-scale of only ~ 200 Myr. To explain the much faster orbital period decay than that predicted by only emission of the gravitational waves ~ 1000 Myr) we propose that the orbital evolution of this system is also driven by magnetic braking . If magnetic braking is to explain the rapid decay of the orbit, then \lambda characterizing the effectiveness of the dynamo action in the stellar convection zone in the magnetic stellar wind formula must be equal to 1.Comment: 4 pages, 2 figures, uses l-aa.sty and psfig.tex, abstract, accepted for publication in A&

Eclipsing binary millisecond pulsar PSR J1740-5340 -- evolutionary considerations and observational test

We perform evolutionary calculations for a binary system with initial parameters: $M_{sg,i}$= 1$M\odot$ and $M_{ns,i}$= 1.4 $M\odot$ and $P_{orb,i}(RLOF)$= 1.27 d to produce observed binary parameters for the PSR J1740--5340. Our calculations support model proposed by D'Amico et al. (2001) in which this binary may be progenitor of a millisecond pulsar + helium white dwarf system. We propose observational test to verify this hypothesis. If the optical companion lack of carbon lines in its spectrum shows but the oxygen and nitrogen lines are present then our model correctly describes the evolutionary stage of PSR J1740--5340.Comment: 5 pages, 3 figures, uses aa.cls and graphicx, abstract, submitted to A&A at March 2

Is KPD 1930+2752 a good SN Ia progenitor?

We investigate the evolution of a binary system which initially has an orbital period of 2^h 17^m and contains a 0.5 M_sun helium star with a white dwarf companion of 0.97 M_sun, similarly to suggested SN Ia candidate progenitor KPD 1930+2752. We show that the helium star completes core helium burning and becomes a white dwarf before components merge. The most probable outcome of the merger of components is formation of a massive white dwarf, despite initially the total mass of the system is above the Chandrasekhar mass

Transient radio emisison from SAX J1808.4-3658

We report on the detection of radio emission from the accretion-powered X-ray millisecond pulsar SAX J1808.4-3658, using the Australia Telescope Compact Array. We detected a ~0.8 mJy source at the position of SAX J1808.4-3658 on 1998 April 27, approximately one day after the onset of a rapid decline in the X-ray flux; no such source was seen on the previous day. We consider this emission to be related to the radio emission from other X-ray binaries, and is most likely associated with an ejection of material from the system. No radio emission was detected at later epochs, indicating that if SAX J1808.4-3658 is a radio pulsar during X-ray quiescence then its monochromatic luminosity must be less than L(1.4 GHz) ~6 mJy/kpc^2.Comment: 6 pages, uses emulateapj.sty, one embedded PS figure. Accepted to ApJ Letter