30 research outputs found
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 () 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. 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 (Z=0.003), 0.198 (Z=0.01),
0.192 (Z=0.02) or 0.183 (Z=0.03). The duration of the
flashes (independent of chemical composition) is between few
years to few 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 (depending on the heavy element value).Comment: 15 pages, 11 figures, 6 tables, submitted to MNRA
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: = 1 and = 1.4 and
= 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
Magnetic activity and evolution of Algol-type stars - II
We examine the possibility of probing dynamo action in mass-losing stars, components of Algol-type binaries, Our analysis is based on the calculation of nonconservative evolution of these systems, We model the systems U Sge and beta Per where the more massive companion fills its Roche lobe at the main sequence (case AB) and where it has a small helium core (early case B) respectively, We show that to maintain evolution of these systems at the late stages which are presumably driven by stellar 'magnetic braking', an efficient mechanism for producing large-scale surface magnetic fields in the donor star is needed. We discuss the relevance of dynamo operation in the donor star to the accelerated mass transfer during the late stages of evolution of Algol-type binaries. We suggest that the observed X-ray activity in Algol-type systems may be a good indicator of their evolutionary status and internal structure of the mass-losing stellar components
X-ray Evidence of the Common Envelope Phase of V471 Tauri
Chandra Low Energy Transmission Grating Spectrograph observations of the
pre-cataclysmic binary V471 Tau have been used to estimate the C/N abundance
ratio of the K dwarf component for the first time. While the white dwarf
component dominates the spectrum longward of 50 AA, at shorter wavelengths the
observed X-ray emission is entirely due to coronal emission from the K dwarf.
The H-like resonance lines of C and N yield an estimate of their logarithmic
abundance ratio relative to the Sun of [C/N]=-0.38+/-0.15 - half of the
currently accepted solar value. We interpret this result as the first clear
observational evidence for the presumed common envelope phase of this system,
during which the surface of the K dwarf was contaminated by CN-cycle processed
material dredged up into the red giant envelope. We use the measured C/N ratio
to deduce that 0.015-0.04 Msun was accreted by the K dwarf while engulfed, and
show that this is consistent with a recent tentative detection of 13C in the K
dwarf photosphere, and with the measured Li abundance in the scenario where the
red giant companion was Li-rich during the common envelope phase.Comment: 6 pages, 2 figures, ApJL accepte
Validating post-AGB candidates in the LMC and SMC using SALT spectra
We selected a sample of post-AGB candidates in the Magellanic Clouds on the
basis of their near- and mid-infrared colour characteristics. Fifteen of the
most optically bright post-AGB candidates were observed with the South African
Large Telescope in order to determine their stellar parameters and thus to
validate or discriminate their nature as post-AGB objects in the Magellanic
Clouds. The spectral types of absorption-line objects were estimated according
to the MK classification, and effective temperatures were obtained by means of
stellar atmosphere modelling. Emission-line objects were classified on the
basis of the fluxes of the emission lines and the presence of the continuum.
Out of 15 observed objects, only 4 appear to be genuine post-AGB stars (27\%).
In the SMC, 1 out of 4 is post-AGB, and in the LMC, 3 out 11 are post-AGB
objects. Thus, we can conclude that the selected region in the colour-colour
diagram, while selecting the genuine post-AGB objects, overlaps severely with
other types of objects, in particular young stellar objects and planetary
nebulae. Additional classification criteria are required to distinguish between
post-AGB stars and other types of objects. In particular, photometry at far-IR
wavelengths would greatly assist in distinguishing young stellar objects from
evolved ones. On the other hand, we showed that the low-resolution optical
spectra appear to be sufficient to determine whether the candidates are
post-AGB objects.Comment: 19 pages, 19 figures, A&A in pres
Time-delay measurement of MgII broad line response for the highly-accreting quasar HE 0413-4031: Implications for the MgII-based radius-luminosity relation
We present the monitoring of the AGN continuum and MgII broad line emission
for the quasar HE 0413-4031 () based on the six-year monitoring by the
South African Large Telescope (SALT). We managed to estimate a time-delay of
days in the rest frame of the source using seven
different methods: interpolated cross-correlation function (ICCF), discrete
correlation function (DCF), -transformed DCF, JAVELIN, two estimators of
data regularity (Von Neumann, Bartels), and method. This time-delay is
below the value expected from the standard radius-luminosity relation. However,
based on the monochromatic luminosity of the source and the SED modelling, we
interpret this departure as the shortening of the time-delay due to the higher
accretion rate of the source, with the inferred Eddington ratio of .
The MgII line luminosity of HE 0413-4031 responds to the continuum variability
as , which is consistent with
the light-travel distance of the location of MgII emission at . Using the data of 10 other quasars, we confirm the
radius-luminosity relation for broad MgII line, which was previously determined
for broad H line for lower-redshift sources. In addition, we detect a
general departure of higher-accreting quasars from this relation in analogy to
H sample. After the accretion-rate correction of the light-travel
distance, the MgII-based radius-luminosity relation has a small scatter of only
dex.Comment: 39 pages (23 pages - Main text, 16 pages - Appendix), 21 figures, 14
tables; accepted for publication in the Astrophysical Journa