Day 709

Saturday.  Sunday.

(Submitted draft---to be post on arXiv if/after accepted)
The timescale of low-mass proto-helium white dwarf evolution
Istrate, Tauris, Langer, Antoniadis

A large number of low-mass (<0.20 Msun) He WDs have recently been discovered.  The majority of these are orbiting another WD or a millisecond pulsar (MSP) in a close binary system; a few examples are found to show pulsations or to have a main sequence star companion.  There appears to be discrepancies between current theoretical modeling of such low-mass He WDs and a number of key observed cases, indicating that their formation scenario remains to be fully understood.  Investigate the formation of detached proto-He WDs in close-orbit low-mass X-ray binaries (LMXBs).  The prime focus is to examine the thermal evolution and the contraction phase towards the WD cooling track and investigate how this evolution depends on the WD mass.  Calculations are then compared to the most recent observational data.  Numerical calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in a large number of close-orbit LMXBs with different initial values of donor star mass, neutron star mass, orbital period and strength of magnetic braking.  Subsequently, follow the evolution of the detached low-mass proto-He WDs, including stages with residual shell hydrogen burning and vigorous flashes caused by unstable CNO burning.  Find that the time between Roche-lobe detachment until the low-mass proto-He WD reaches the WD cooling track is typically Delta t_proto = 0.5-2 Gyr, depending systematically on the WD mass and therefore on its luminosity.  The minimum WD mass for developing shell flashes is ~0.21 Msun for progenitor stars of mass M_2<1.5 Msun (and ~0.18 Msun for M_2=1.6 Msun) [why is shell flash important?].  The long timescale of low-mass proto-He WD evolution can explain a number of recent observations, including some MSP systems hosting He WD companions with very small surface gravities and high effective temperatures.  Find no evidence for Delta t_proto to depend on the occurrence of flashes and thus question the suggested dichotomy in thermal evolution of proto-WDs.

(Submitted draft---to be post on arXiv if/after accepted)
The formation of low-mass helium white dwarfs orbiting pulsars: Evolution of low-mass X-ray binaries below the bifurcation period
Istrate, Tauris, Langer

Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs) which have been spun up to high rotation rates via accretion of matter from a companion star in a LMXB.  This scenario has been strongly supported by various pieces of observational evidence.  However, many details of this recycling scenario remain to be understood.  Investigate binary evolution in close LMXBs to sturdy the formation of radio MSPs with low-mass He WD companions in tight binaries with orbital periods P_orb=2-9 hr.  In particular, examine i) if such observed systems can be reproduced from theoretical modeling using standard prescriptions of orbital angular momentum losses (i.e. with respect to the nature and the strength of magnetic breaking), ii) if the computations of the Roche-lobe detachments can match the observed orbital periods, and iii) if the correlation between WD mass and orbital period (M_WD, P_orb) is valid for systems with P_orb<2days.  Numercial calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in ~400 close LMXB systems with different initial values of donor star mass, NS mass, orbital period and the so-called gamma-index of magnetic breaking.  Subsequently, follow the orbital and the interior evolution of the detached low-mass (proto) HE WDs, including stages with residual shell H burning.  Find that a severe fine-tuning is necessary to reproduce the observed MSPs in tight binaries with He WD companions of mass <0.20 Msun, which suggests that something needs to be modified or is missing in the standard input physics of LMXB modeling.  Results from previous independent studies support this conclusion.  Demonstrate that the theoretically calculated (M_WD, P_orb)-relation is in general also valid for systems with P_orb<2days, although with a large scatter in He WD masses between 0.15-0.20 Msun.  The results of the thermal evolution of the (proto) He WDs are reported in a follow-up paper (Paper II).