Mass Transfer In Binary Systems Research Articles

Carbon Stars in the Hamburg/ESO Survey: Abundances

We have carried out a detailed abundance analysis using high-dispersion spectra from HIRES at Keck for a sample of 16 carbon stars found among candidate extremely metal-poor (EMP) stars from the Hamburg/ESO Survey (HES). We find that the Fe metallicities for the cooler C stars (Teff ~ 5100 K) have been underestimated by a factor of ~10 by the standard HES tools. The results presented here provided crucial supporting data used recently by Cohen et al. to derive the frequency of C stars among EMP stars. C enhancement in these EMP C stars appears to be independent of Fe metallicity and approximately constant at ~ the solar (C). The C enhancement shows some evidence of decreasing with decreasing Teff (increasing luminosity), presumably due to mixing and dredge-up of C-depleted material. The mostly low 12C/13C ratios (~4) and the high N abundances in many of these stars suggest that material that has been through proton burning via the CN cycle comprises most of the stellar envelope. C enhancement in this sample is associated with strong enrichment of heavy nuclei beyond the Fe peak for 12 of the 16 stars. The remaining C stars from the HES, which tend to be the most Fe-poor, show no evidence for enhancement of the heavy elements. Very high enhancements of lead are detected in some of the C stars with highly enhanced Ba. The strong lead lines, the high Ba/Eu ratios, and the high ratios of abundances of the diagnostic elements in the first and second s-process peaks demonstrate that the s-process is responsible for the enhancement of the heavy elements for the majority of the C stars in our sample. The low 12C/13C ratios and large C and N enhancements of the EMP C stars are more extreme than those of intrinsic asymptotic giant branch C stars of near-solar Fe metallicity, but closer to the composition of CH stars. Our subsample of EMP C stars without s-process enhancement is reminiscent of the R-type C stars in the solar neighborhood; thus, we expect that they are formed by similar mechanisms. We suggest that both the s-process-enhanced and Ba-normal C stars result from phenomena associated with mass transfer in binary systems. This leads directly to the progression from C stars to CH stars and then to Ba stars as the Fe metallicity increases.

Modelling of direct-contact evaporation using a simultaneous heat and multicomponent mass-transfer model for superheated bubbles

A recently developed model for coupled heat and mass transfer in binary systems during the formation and ascension of superheated bubbles was extended to a multicomponent system comprising N volatile species. The model allows variable properties and bubble radius changes, assuming diffusive mass fluxes to be properly described by Fick's law. Experimental direct-contact evaporation tests were conducted with ethyl acetate aqueous solutions to provide data for assessing the developed model. In addition, the model was tested against available literature data for an air-stripper. In both cases, a good agreement between simulation and experimental results was verified.

Abundance Variations within Globular Clusters

▪ Abstract Abundance variations within globular clusters (GCs), and of GC stars with respect to field stars, are important diagnostics of a variety of physical phenomena, related to the evolution of individual stars, mass transfer in binary systems, and chemical evolution in high density environments. The broad astrophysical implications of GCs as building blocks of our knowledge of the Universe make a full understanding of their history and evolution basic in a variety of astrophysical fields. We review the current status of the research in this field, comparing the abundances in GCs with those obtained for field stars, discussing in depth the evidence for H-burning at high temperatures in GC stars, describing the process of self-enrichment in GCs with particular reference to the case of the most massive Galactic GC (ω Cen), and discussing various classes of cluster stars with abundance anomalies. Whereas the overall pattern might appear very complex at first sight, exciting new scenarios are opening where the interplay between GC dynamical and chemical properties are closely linked with each other.

A New Interpretation of the Low Radial Velocity Determined from the HeII Emission Line in the Galactic Supersoft X-Ray Source RX J0019.8+2156

We propose a new interpretation for the low radial velocity of He II emission line in the Galactic supersoft X-ray source RX J0019.8+2156 based on a 2-D numerical simulation of dynamical mass transfer in binary systems. A more massive main-sequence (MS) companion of ~ 2.0 M⊙ is derived for a ~ 0.6 M⊙ white dwarf (WD) primary.The Galactic supersoft X-ray source RX J0019.8+2156 (hereafter, RX J0019) with a 12.2 mag optical counterpart was discovered by ROSAT. The source is a binary with the period of 0.66 days and its X-ray luminosity is fairly constant, at least, for 7 yrs. Therefore RX J0019 seems to be a typical luminous supersoft X-ray source (SSS) with steady hydrogen shell-burning on a mass-accreting white dwarf (WD).

S‐Process Nucleosynthesis in Carbon Stars

We present the first detailed and homogeneous analysis of the s-element content in Galactic carbon stars of N type. Abundances of Sr, Y, Zr (low-mass s-elements, or ls), Ba, La, Nd, Sm, and Ce (high-mass s-elements, or hs) are derived using the spectral synthesis technique from high-resolution spectra. The N stars analyzed are of nearly solar metallicity and show moderate s-element enhancements, similar to those found in S stars, but smaller than those found in the only previous similar study (Utsumi 1985), and also smaller than those found in supergiant post-asymptotic giant branch (post-AGB) stars. This is in agreement with the present understanding of the envelope s-element enrichment in giant stars, which is increasing along the spectral sequence M → MS → S → SC → C during the AGB phase. We compare the observational data with recent s-process nucleosynthesis models for different metallicities and stellar masses. Good agreement is obtained between low-mass AGB star models (M 3 M☉) and s-element observations. In low-mass AGB stars, the 13C(α, n)16O reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of 13C that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colors, and the theoretical relations between stellar mass, metallicity, and the final C/O ratio, we conclude that most (or maybe all) of the N stars studied in this work are intrinsic, thermally pulsing AGB stars; their abundances are the consequence of the operation of third dredge-up and are not to be ascribed to mass transfer in binary systems.

Nonconservative evolution of cataclysmic binaries

We present a mechanism to take into account angular-momentum loss in binary systems with non-conservative mass transfer. In a number of cases, mass loss in the system can increase the orbital angular momentum of the stars. Including this mechanism in evolutionary models substantially expands the domain of stable mass transfer in binary systems. All observed cataclysmic binaries with known component masses fall within the calculated area for stable mass transfer.

The Formation of Carbon Stars in the Magellanic Clouds from Mass Transfer in Close Binaries

The presence of the carbon stars in the MC with luminosities higher or lower than predicted for thermally-pulsing (TP) AGB stars, can be explained by processes that happen during the early AGB (E–AGB) stage. I examine this assumption by means of a population simulation technique. I find that there must be TP–AGB C and S stars in the MC that formed as a result of mass transfer in binary systems. Their presence may influence the age determinations of MC clusters.

Gradients in giant branch morphology in the core of 47 Tucanae

I describe an algorithm which uses the high spatial resolution of the Hubble Space Telescope to complement the high spatial-to-noise, approximately symmetric point response function, relatively large spatial coverage, and standard filters available from ground based images of crowded fields. Applying this technique to the central regions of the globular cluster 47 Tucanae, I find that the morphology of the giant branch in the core is significantly different from that in more distant regions (r approximately equals 5 to 10 core radii) of the cluster. In particular, there appear to be fewer bright giants in the core, along with an enhanced `asymptotic giant branch' (AGB) sequence. Depletion of giants has been observed in the cores of other dense clusters, and may be due to `stripping' of large stars by stellar encounters and/or mass transfer in binary systems. Central concentrations of true asymptotic giant branch stars are not expected to result from dynamical processes; possibly some of these stars may be evolved blue stragglers.

RAPID MASS TRANSFER IN BINARY SYSTEMS

This thesis investigates the conditions for rapid mass transfer in binary stars. Previous theoretical calculations and observations of binaries imply the existence of several different time scales for mass transfer: nuclear, thermal, or dynamical. Since the mass transfer rates differ by several orders of magnitude, it is important to know which time scales are relevant to different systems. Dynamical time-scale mass transfer is thought to cause substantial decreases in the orbital period through mass and angular momentum losses. Thermal time-scale mass transfer is thought to transform the appearance ofthe binary as mass exchange occurs in a short time. Binaries currently transferring mass are doing so on the longest, nuclear time scale. The characteristics of binaries which divide the three time scales are estimated by calculating the response of potential mass donors in two idealized limits: an adiabatic response, where the entropy profile does not change with mass loss, and a thermal response, where thermal relaxation is allowed but nuclear burning is not. A comparison ofthe changing surface radius, £ = d'nR/d'n3)l to the Roche-lobe radius implies the critical mass ratios for dynamically and thermally unstable mass transfer. These calculations are performed here for donors between 0.25 and 20 TtQ which would fill their Roche lobes before helium burning. The adiabatic mass-loss calculations have provided a clear relation between £ad and the donor's convective envelope mass fraction (fce), with a smaller dependence on the state of the interior. Low-mass ZAMS donors and models near the base ofthe giant branch have £ad ^> 1 change to £ad ~

Thresholds for rapid mass transfer in binary systems. I - Polytropic models

The adiabatic responses to mass loss of simple and compound polytropic star models are explored. The results show that stars in thermal equilibrium with isentropic surface layers are linearly unstable if the mass ratio exceeds a finite critical threshold q(0). Unless the mass fraction in these surface layers is quite small, q(0) is of the order of unity. On the other hand, stars in thermal equilibrium with subadiabatic surface layers are linearly stable for any value of the mass ratio. A finite-amplitude instability exists to which radiative stars are subject. If the initial phases of mass transfer are sufficiently rapid, such a star may undergo a delayed transition to dynamical time scale mass loss. Red giants with small core mass fractions behave very much like fully convective stars, expanding adiabatically as they lose mass. 32 references.

Infrared spectroscopy of symbiotic stars and the nature of their cool components

We present low-resolution 2--4 ..mu..m spectroscopy of a small sample of symbiotic stars, in an effort to determine if the giant components of these systems fill their Roche Lobes. A (2.35)-(2.2) color index measures the strength of the CO absorption band and provides a useful discriminant of luminosity class among single M-type giants which separates normal giants from supergiants at the same spectral type. Although interpretation of symbiotic spectra is complicated somewhat by their binary nature, our results suggest the late-type components in these systems range from normal red giants to bright asymptotic giants. The possible presence of non-Roche Lobe filling, low-luminosity giants in some symbiotic stars cannot be understood within the framework of existing theories for these interesting objects, and thus may provide important information for understanding mass transfer in binary systems.

X-ray and optical observations of HD 155638 - A remarkably active cool star

X-ray and optical observations of the 8.5-mag star HD 155638 are reported which reveal a remarkably active chromosphere and corona for the star. The object was first observed in soft X-rays with the HEAO 1 A2 low-energy detectors, and later reobserved with the HEAO 2 IPC. X-ray fluxes of about 2 x 10 to the -11th and 3 x 10 to the -12th ergs/sq cm per sec were obtained on the two occasions, indicative of source variability over the 2.5-year period between observations. SIT spectrographs taken in July-August 1980 show evidence of Ca II H and K core emission reversals and filled-in H alpha, while spectra taken in March and July 1981 show evidence of asymmetry and variability in the Ca II reversals. The spectral data are consistent with a G8 IV-V classification, from which is inferred a distance of 35-120 pc, and an X-ray luminosity from 4 x 10 to the 29th to 4 x 10 to the 31st ergs/sec. An absence of emission lines characteristic of mass transfer in binary systems indicates the X-rays to be coronal in origin. The properties of the star are noted to be similar to those of the RS CVn binaries.

Pre-Main-Sequence Mass Transfer in Binary Systems

Pre-Main-Sequence Mass Transfer in Binary Systems

Diffusion in binary systems in a critical state

The phenomenological theory is proposed of mass transfer in binary systems near the critical point.