The formation of low-mass helium white dwarfs orbiting pulsars: Evolution of low-mass X-ray binaries below the bifurcation period

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 low-mass X-ray binary (LMXB). However, many details of this recycling scenario remain to be understood. Here we investigate binary evolution in close LMXBs to study the formation of radio MSPs with low-mass helium white dwarf companions (He WDs) in tight binaries with orbital periods P_orb = 2-9 hr. In particular, we examine: i) if such observed systems can be reproduced from theoretical modelling using standard prescriptions of orbital angular momentum losses (i.e. with respect to the nature and the strength of magnetic braking), ii) if our 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 < 2 days. Numerical 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 braking. Subsequently, we followed the orbital and the interior evolution of the detached low-mass (proto) He WDs, including stages with residual shell hydrogen burning. We find that a severe fine-tuning is necessary to reproduce the observed MSPs in tight binaries with He WD companions of mass < 0.20 M_sun, which suggests that something needs to be modified or is missing in the standard input physics of LMXB modelling. We demonstrate that the theoretically calculated (M_WD, P_orb)-relation is in general also valid for systems with P_orb < 2 days, although with a large scatter in He WD masses between 0.15-0.20 M_sun. The results of the thermal evolution of the (proto) He WDs are reported in a follow-up paper (Paper II).Comment: 14 pages, 13 figures, 1 table, A&A, accepte

The timescale of low-mass proto-helium white dwarf evolution

A large number of low-mass (< 0.20 M_sun) helium white dwarfs (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 appear to be discrepancies between the current theoretical modelling of such low-mass He WDs and a number of key observed cases, indicating that their formation scenario remains to be fully understood. Here we investigate the formation of detached proto-He WDs in close-orbit low-mass X-ray binaries (LMXBs). Our 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. Our 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. Subsequently, we followed 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. We 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 lowest WD mass for developing shell flashes is ~0.21 M_sun for progenitor stars of mass M2 <= 1.5 M_sun (and ~0.18 M_sun for M2 = 1.6 M_sun). 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 low surface gravities and high effective temperatures. We 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.Comment: 6 pages, 6 figures, 2 tables, A&A Letters, accepte

Formation of low-mass helium white dwarfs orbiting binary millisecond pulsars

Millisecond pulsars (MSPs) belong to a class of radio pulsars characterized by high rotational spin rates and low magnetic fields. These neutron stars are believed to be the end-product of binary evolution, in which an old neutron star accretes matter and angular momentum from a close stellar companion for an extended period of time, while being observable as an X-ray binary. During this evolutionary phase, they acquire millisecond spin periods and, after the accretion episode ceases, they are reactivated as radio emitting pulsars. The majority of MSP companions are low-mass helium-core white dwarfs (He WDs). However, MSP systems harbouring carbon-oxygen white dwarfs (CO WD), neutron stars, or ultra light companions have also been found, suggesting a diversity in the nature of their progenitors. A handful of MSPs are observed in very compact orbits, Porb ≃ 2 - 9 hr, orbiting around low-mass He WDs with masses below 0.25 M⊙ and surface gravities 5 Today we know of the existence of more than 80 ELM WDs. The increasing number of discovered ELM WDs reveals that they are formed in different environments, from the Galactic disk to open and globular clusters. ELM WDs are most likely the result of binary evolution as they cannot be formed from single stars within a Hubble time. Indeed, over 80% of the observed ELM WDs are found in binary systems, most commonly around a more massive CO WD. The new wealth of data raises questions regarding the puzzling presence of metals in the atmospheres of young bloated ELM proto-WDs and the newly discovered pulsations in three ELM proto-WDs. In this context, improved stellar evolutionary models in which binary evolution is fully accounted for are needed in order to explain the formation and the observed properties of these objects. In this thesis, we investigate the formation of MSPs found in compact orbits with ELM WD companions through numerical calculations using state-of-the-art stellar evolutionary codes. In particular, we examine if the observed systems can be reproduced by theoretical modelling using standard prescriptions of angular momentum loss with contributions from gravitational wave radiation, magnetic braking and mass loss. We find that a severe fine-tuning in the initial orbital period is necessary to reproduce the observed number of MSPs with ELM WD companions suggesting that something needs to be modified or is missing in the standard input physics of their modelling. Moreover, we explore the formation and cooling of ELM WDs through a large grid of computed models suited for environments with different metallicities, with emphasis on the proto-WD phase. Specifically, we analyse in detail the evolutionary times of these objects which are of great importance in providing an accurate independent age estimate for MSP systems. For the first time, we study the combined effects of element diffusion, i.e. gravitational settling, thermal and chemical diffusion, and rotational mixing on the evolution of these WDs. Our results show that rotational mixing plays a key role in determining the chemical composition of the surface layers of ELM proto-WDs, but it does not affect their internal structure. This finding has important implications for the asteroseismology studies of ELM proto-WDs which are currently pursued. Furthermore, we suggest that the spin frequencies of the resulting WDs are well above the orbital frequencies, a result which needs to be confirmed by further dedicated observations

False angioedema, adverse reaction to mesotherapy – clinical case

Universitatea de Stat de Medicină şi Farmacie „Nicolae Testemiţanu”, Chişinău, Republica MoldovaIntroducere. Angioedemul este o patologie relativ frecventă în practica medicală, poate fi atât alergic cât și non alergic, diagnosticul prezentând o provocare continuă atunci când edemul subcutanat mimează un angioedem. Scopul lucrării. Prin prisma unui caz clinic, ilustrarea particularităților de prezentare a dermatitei de contact care mimează un angioedem localizat al scalpului și feței, ca și reacție adversă a mezoterapiei pentru tratamentul alopeciei. Material și metode. Mezoterapia este o procedură nechirurgicală, care constă în multiple injecții intradermice de substanțe farmacologice. Prezentăm cazul unei paciente de 32 ani, diagnosticată cu alopecie, căreia i-a fost administrat tratament intradermic la nivelul scalpului cu o soluție care conținea emulgatori, solvenți, conservanți, acizi, extracte din plante și benzoați. Rezultate. Intradermoterapia poate avea efecte adverse ușoare: arsuri, hiperemie sau cefalee, dar și efecte adverse severe ca necroza subcutanată, abcese ale scalpului sau angioedem. La 12 ore de la injectare, s-a dezvoltat edem dureros a tegumentelor scalpului, care a migrat în următoarele ore spre frunte, partea superioară a feței și pleoapelor. Edem însoțit de senzație de durere, arsură și godeu la palpare. Remisiunea a fost obținută în 7 zile cu doze mari de corticosteroizi sistemici, fiind urmat de senzație de pareză la nivelul tegumentelor scalpului. Instrucțiunile de utilizare ale substanțelor administrate menționează administrarea topică, în timp ce pacientei aceste substanțe i-au fost injectate intradermic în tegumentele scalpului. Concluzii. Cazul ilustrează o formă rară de reacție adversă la o substanță folosită pentru mezoterapia scalpului. Angioedemul este mimat de dermatita de contact la intradermoterapie, având în vedere debutul tardiv, răspunsul rezervat la tratamentul și absența urticariei.Background. Angioedema is a relatively common pathology in medical practice; it can be allergic and non-allergic, the diagnosis presenting a continuous challenge when subcutaneous edema mimics angioedema. Objective of the study. Through the lens of a clinical case, illustrating the particularities of presentation of contact dermatitis mimicking a localized angioedema of the scalp and face, as an adverse reaction of mesotherapy for the treatment of alopecia. Material and methods. Mesotherapy is a non-surgical procedure, which consists of multiple intradermal injections of pharmacological substances. We present the case of a 32-year-old female patient, diagnosed with alopecia, who was administered intradermal scalp treatment with a solution containing emulsifiers, solvents, preservatives, acids, plant extracts and benzoates. Results. Mesotherapy, although it is a minimally invasive procedure, can have mild side effects: burns, hyperemia or headache, but also severe side effects such as subcutaneous necrosis, scalp abscesses, or angioedema. 12 hours after the injection, the patient developed painful angioedema of the scalp skin, which migrated over the following hours to the forehead, upper face, and eyelids. Edema accompanied by a feeling of pain, burning and pitting on palpation. Remission was obtained in 7 days with high doses of systemic corticosteroids, followed by paresis of the scalp. The instructions for use of the administered substances mention topical administration, while the presented patient had these substances injected intradermally into the skin of the scalp. Conclusions. The case illustrates a rare form of adverse reaction to a substance used for scalp mesotherapy. Angioedema is mimicked by contact dermatitis caused by intradermotherapy, given its late onset, reserved response to treatment, and absence of urticaria

Novel modelling of ultra-compact X-ray binary evolution - stable mass transfer from white dwarfs to neutron stars

Tight binaries of helium white dwarfs (He WDs) orbiting millisecond pulsars (MSPs) will eventually "merge" due to gravitational damping of the orbit. The outcome has been predicted to be the production of long-lived ultra-compact X-ray binaries (UCXBs), in which the WD transfers material to the accreting neutron star (NS). Here we present complete numerical computations, for the first time, of such stable mass transfer from a He WD to a NS. We have calculated a number of complete binary stellar evolution tracks, starting from pre-LMXB systems, and evolved these to detached MSP+WD systems and further on to UCXBs. The minimum orbital period is found to be as short as 5.6 minutes. We followed the subsequent widening of the systems until the donor stars become planets with a mass of ~0.005 Msun after roughly a Hubble time. Our models are able to explain the properties of observed UCXBs with high helium abundances and we can identify these sources on the ascending or descending branch in a diagram displaying mass-transfer rate vs. orbital period.Comment: 6 pages, 4 figures, MNRAS Letters, in pres

Massive donors in interacting binaries: effect of metallicity

Metallicity is known to significantly affect the radial expansion of a massive star: the lower the metallicity, the more compact the star, especially during its post-MS evolution. We study this effect in the context of binary evolution. Using the stellar-evolution code MESA, we computed evolutionary tracks of stars at different metallicities, exploring variations of factors known to affect the radial expansion (e.g. semiconvection, overshooting, rotation). We find observational support for an evolution in which already at metallicity $0.2Z_{\odot}$ massive stars remain relatively compact during the Hertzprung-Gap (HG) phase and most of their expansion occurs during core-helium burning (CHeB). Consequently, we show that metallicity has a strong influence on the type of mass transfer evolution in binary systems. At solar metallicity, a case-B mass transfer is initiated shortly after the end of MS, and a giant donor is almost always a rapidly expanding HG star. At lower metallicity, the parameter space for mass transfer from a more evolved CHeB star increases dramatically. This means that envelope stripping and formation of helium stars in low-metallicity environments occurs later in the evolution of the donor, implying a much shorter duration of the Wolf-Rayet phase (even by an order of magnitude) and higher final core masses. This metallicity effect is independent of the impact of metallicity-dependent stellar winds. At very low metallicities, a significant fraction of massive stars in binaries engages in the first episode of mass transfer very late into their evolution, when they already have a well-developed CO core. The remaining lifetime ($< 10^4$ yr) is unlikely to be enough to strip the entire H-rich envelope. We also briefly discuss the extremely small parameter space for mass transfer from massive convective-envelope donors in the context of binary black hole merger formation.Comment: 15 pages, 8 figures (+ 4 pages, 4 fig. appendix), to appear in A&