[Show abstract][Hide abstract] ABSTRACT: We present the Kepler photometric light-variation analysis of the late-type double-lined binary system V568 Lyr that is in the field of the high
metallicity old open cluster NGC 6791. The radial velocity and the high-quality short-cadence light curve of the system are
analysed simultaneously. The masses, radii and luminosities of the component stars are M1 = 1.0886 ± 0.0031 M⊙, M2 = 0.8292 ± 0.0026 M⊙, R1 = 1.4203 ± 0.0058 R⊙, R2 = 0.7997 ± 0.0015 R⊙, L1 = 1.85 ± 0.15 L⊙, L2 = 0.292 ± 0.018 L⊙ and their separation is a = 31.060 ± 0.025 R⊙. The distance to NGC 6791 is determined to be 4.260 ± 0.290 kpc by analysis of this binary system. We fit the components
of this well-detached binary system with evolution models made with the Cambridge stars and ev(twin) codes to test low-mass binary star evolution. We find a good fit with a metallicity of Z = 0.04 and an age of 7.704 Gyr. The standard tidal dissipation, included in ev(twin) is insufficient to arrive at the observed circular orbit unless it formed rather circular to begin with.
Monthly Notices of the Royal Astronomical Society 08/2015; 453(3). DOI:10.1093/mnras/stv1773 · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: I describe a series of processes, including hierarchical fragmentation, gravitational scattering, Kozai cycles within triple systems, tidal friction and magnetic braking, that I believe are responsible for producing the modest but significant fraction of stars that are observed as contact binaries. I also discuss further processes, namely heat transport, mass transport, nuclear evolution, thermal relaxation oscillations, and further magnetic braking with tidal friction, that influence the evolution during contact. The endpoint, for contact, is that the two components merge into a single star, as recently was observed in the remarkable system V1309 Sco. The single star probably throws off some mass and rotates rapidly at first, and then slows by magnetic braking to become a rather inconspicuous but normal dwarf or subgiant. If however the contact binary was part of a triple system originally-as I suggested above was rather likely-then the result could be a binary with apparently non-coeval components. There are several such known.
[Show abstract][Hide abstract] ABSTRACT: What is the fate of planetary systems when the star goes
through the red giant phase and eventually becomes a white dwarf? Do we
expect to detect planets around white dwarfs? Are the planets ejected
when perturbed by the high mass-loss episodes? This study aims at
answering all of these questions using our own Solar system as a case
study. The interior planets are either directly engulfed by the Sun in
the red-giant phase or - for Mars - the tidal forces cause the planet to
fall on the swollen Sun. Using an adapted version of the code Mercury,
we integrated the remaining four exterior planets to the white dwarf
phase of the Sun, running hundreds of simulations with different initial
conditions. Our results show that it is very likely that some of the
planets will be ejected during the red giant phase and others will
survive. The fate of these planets depend on their orbits and their
[Show abstract][Hide abstract] ABSTRACT: We have calculated the equilibrium properties of a star in a circular,
equatorial orbit about a Super-Massive Black Hole (SMBH), when the star fills
and overflows its Roche lobe. The mass transfer time scale is anticipated to be
long compared with the dynamical time and short compared with the thermal time
of the star, so that the entropy as a function of the interior mass is
conserved. We have studied how the stellar entropy, pressure, radius, mean
density, and orbital angular momentum vary when the star is evolved
adiabatically, for a representative set of stars. We have shown that the
stellar orbits change with the stellar mean density. Therefore, sun-like stars,
upper main sequence stars and red giants will spiral inward and then outward
with respect to the hole in this stable mass transfer process, while lower main
sequence stars, brown dwarfs and white dwarfs will always spiral outward.
Monthly Notices of the Royal Astronomical Society 10/2011; 434(4). DOI:10.1093/mnras/stt1208 · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an evolutionary study of 28 Tau, a Be star, in connection with its rapid rotation. The photometric data during the absence of its envelope in 1921 have been used to determine the effective temperature and luminosity of the star at the main sequence of the HR diagram. From an evolutionary model, we found that the mass and radius of the star are about 3.2 Msolar and 3.2 Rsolar respectively. The equatorial rotation velocity of the star, nue found to be close to its critical velocity, nucr where nue/nucr ~= 0.87.
Proceedings of the International Astronomical Union 07/2011; 272(S272):274-275. DOI:10.1017/S1743921311010532
[Show abstract][Hide abstract] ABSTRACT: We have developed a detailed stellar evolution code capable of following
the simultaneous evolution of both stars in a binary system, together
with their orbital properties. To demonstrate the capabilities of the
code we investigate potential progenitors for the Type IIb supernova
1993J, which is believed to have been an interacting binary system prior
to its primary exploding. We use our detailed binary stellar evolution
code to model this system to determine the possible range of primary and
secondary masses that could have produced the observed characteristics
of this system, with particular reference to the secondary. Using the
luminosities and temperatures for both stars (as determined by Maund et
al. 2004) and the remaining mass of the hydrogen envelope of the primary
at the time of explosion, we find that if mass transfer is 100 per cent
efficient the observations can be reproduced by a system consisting of a
15 solar mass primary and a 14 solar mass secondary in an orbit with an
initial period of 2100 days. With a mass transfer efficiency of 50 per
cent, a more massive system consisting of a 17 solar mass primary and a
16 solar mass secondary in an initial orbit of 2360 days is needed. We
also investigate some of the uncertainties in the evolution, including
the effects of tidal interaction, convective overshooting and
[Show abstract][Hide abstract] ABSTRACT: For me, and for many astrophysicists of my generation, Chandrasekhar’s book An Introduction to the Study of Stellar Structure was very important. I could not have done my PhD (1962–1965) without it. Much more recently (1998) I realized that I could
not have written my lecture course on thermodynamics and statistical mechanics without much of it, particularly the first
chapter. I shall present anecdotal evidence that the influence of his discussion on the second law of thermodynamics has been
important not just for astrophysics but for a much wider range of physics.
Chandrasekhar’s discussion of polytropes was masterly. Even today polytropes play an important role as an aid for understanding
stellar structure. I believe that to the list of analytic solutions of the polytrope only one more has to be added: a curious
n = 5 model of Srivastava (1962).
Stellar structure is nowadays a very computationally intensive subject. I shall illustrate this with a couple of topics from
my experience with Djehuty, a supercomputer code for modelling stars in 3D. Nevertheless it remains true, I believe, that analytical mathematical entities
like polytropes are fundamental as aids for understanding what the computers churn out.
How close are we to seeing a book with the title ‘The Last Word on the Study of Stellar Structure’? Not very, although much
has been learned in 70 years. I shall discuss a few of the aspects of stellar evolution that are problematic today.
I shall discuss a couple of aspects where I believe analysis of ‘piecewise polytropic’ structures sheds light on the question
‘Why do stars become red giants?’
[Show abstract][Hide abstract] ABSTRACT: Several studies in the last three years indicate that close binaries,
i.e. those with periods of ≲3 d, are very commonly found to have a
third body in attendance. We argue that this proves that the third body
is necessary in order to make the inner period so short, and further
argue that the only reasonable explanation is that the third body causes
shrinkage of the inner period, from perhaps a week or more to the
current short period, by means of the combination of Kozai cycles and
tidal friction (KCTF). In addition, once KCTF has produced a rather
close binary, magnetic braking also combined with tidal friction (MBTF)
can decrease the inner orbit further, to the formation of a contact
binary or even a merged single star.
Some of the the products of KCTF that have been suggested, either by
others or by us, are W UMa binaries, Blue Stragglers, X-ray active BY
Dra stars, and short-period Algols. We also argue that some components
of wide binaries are actually merged remnants of former close inner
pairs. This may include such objects as rapidly rotating dwarfs (AB Dor,
BO Mic) and some (but not all) Be stars.
[Show abstract][Hide abstract] ABSTRACT: We discuss the incidence of multiplity, particularly among the bright and therefore relatively thoroughly examined stars, and note certain types of (a) binary stars that might be expected to merge, and (b) single stars with characteristics that suggest they may be former binaries that merged. Some Be stars and rapidly rotating red giants seem like possible merger products; and perhaps even some magnetic peculiar stars that are rapidly rotating.
[Show abstract][Hide abstract] ABSTRACT: We consider the multiplicity of stellar systems with (combined)
magnitude brighter than 6.00 in Hipparcos magnitudes. We identify 4559
such bright systems (including the Sun), and the frequencies of
multiplicities 1, 2, ... , 7 are found to be 2718, 1437, 285, 86, 20, 11
and 2. We discuss the uncertainties, which are substantial. We also
consider the distributions of periods of orbits and suborbits. We note
that for even more restricted set of 478 systems with
VH<=4.00 , the proportions of higher multiples up to
sextuple are progressively larger (213, 179, 54, 19, 8, 5), suggesting
substantial incompleteness in even the reasonably well studied larger
(4 data files).
[Show abstract][Hide abstract] ABSTRACT: The multiplicities of stars, and some other properties, were collected recently by Eggleton & Tokovinin, for the set of 4559 stars with Hipparcos magnitude brighter than 6.0 (4558 excluding the Sun). In this paper I give a numerical recipe for constructing, by a Monte Carlo technique, a theoretical ensemble of multiple stars that resembles the observed sample. Only multiplicities up to 8 are allowed; the observed set contains only multiplicities up to 7. In addition, recipes are suggested for dealing with the selection effects and observational uncertainties that attend the determination of multiplicity. These recipes imply, for example, that to achieve the observed average multiplicity of 1.53, it would be necessary to suppose that the real population has an average multiplicity slightly over 2.0. This numerical model may be useful for (a) comparison with the results of star and star cluster formation theory, (b) population synthesis that does not ignore multiplicity above 2, and (c) initial conditions for dynamical cluster simulations.
Monthly Notices of the Royal Astronomical Society 08/2009; 399(3). DOI:10.1111/j.1365-2966.2009.15372.x · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Luminous X-ray stars are very often found in visual double or multiple
stars. Binaries with periods of a few days possess the highest degree of
coronal X-ray activity among regular, non-relativistic stars because of
their fast, tidally driven rotation. But the orbital periods in visual
double stars are too large for any direct interaction between the
companions to take place. We suggest that most of the strongest X-ray
components in resolved binaries are yet-undiscovered short-period
binaries, and that a few are merged remnants of such binaries. The
omnipresence of short-period active stars, e.g. of BY-Dra-type binaries,
in multiple systems is explained via the dynamical evolution of triple
stars with large mutual inclinations. The dynamical perturbation on the
inner pair pumps up the eccentricity in a cyclic manner, a phenomenon
known as Kozai cycling. At times of close periapsis, tidal friction
reduces the angular momentum of the binary, causing it to shrink. When
the orbital period of the inner pair drops to a few days, fast surface
rotation of the companions is driven by tidal forces, boosting activity
by a few orders of magnitude. If the period drops still further, a
merger may take place leaving a rapidly-rotating active dwarf with only
a distant companion.
[Show abstract][Hide abstract] ABSTRACT: Luminous X-ray stars are very often found in visual double or multiple stars. Binaries with periods of a few days possess the highest degree of coronal X-ray activity among regular, non-relativistic stars because of their fast, tidally driven rotation. But the orbital periods in visual double stars are too large for any direct interaction between the companions to take place. We suggest that most of the strongest X-ray components in resolved binaries are yet-undiscovered short-period binaries, and that a few are merged remnants of such binaries. The omnipresence of short-period active stars, e.g., of BY-Dra-type binaries, in multiple systems is explained via the dynamical evolution of triple stars with large mutual inclinations. The dynamical perturbation on the inner pair pumps up the eccentricity in a cyclic manner, a phenomenon known as Kozai cycling. At times of close periapsis, tidal friction reduces the angular momentum of the binary, causing it to shrink. When the orbital period of the inner pair drops to a few days, fast surface rotation of the companions is driven by tidal forces, boosting activity by a few orders of magnitude. If the period drops still further, a merger may take place leaving a rapidly rotating active dwarf with only a distant companion.
The Astrophysical Journal 04/2009; 703(2). DOI:10.1088/0004-637X/703/2/1760 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The results of the models studied in the paper are stored in the
"evgrid" subdirectory, which includes for the metallicities/masses
studied the ZAMS parameters and the evolutionary tracks computed with or
without overshooting. A set of Fortran programs is also included which
computes luminosities, radiuses, absolute magnitudes, and colors in the
UBVRI Johnson-Cousins as a function of stellar age for any interpolation
in the grid of models; a README file in this "evgrid" subdirectory gives
details for an extended usage of the models.
(4 data files).
[Show abstract][Hide abstract] ABSTRACT: We undertake a comparison of observed Algol-type binaries with a library of computed Case A binary evolution tracks. The library consists of 5500 binary tracks with various values of initial primary mass M10, mass ratio q0, and period P0, designed to sample the phase-space of Case A binaries in the range -0.10 ≤ log M10 ≤ 1.7. Each binary is evolved using a standard code with the assumption that both total mass and orbital angular momentum are conserved. This code follows the evolution of both stars to the point where contact or reverse mass transfer occurs. The resulting binary tracks show a rich variety of behavior that we sort into several subclasses of case A and case B. We present the results of this classification, the final mass ratio, and the fraction of time spent in Roche Lobe overflow for each binary system. The conservative assumption under which we created this library is expected to hold for a broad range of binaries, where both components have spectra in the range G0 to B1 and luminosity classes III to V. We gather a list of relatively well-determined, observed hot Algol-type binaries meeting this criterion, as well as a list of cooler Algol-type binaries, for which we expect significant dynamo-driven mass loss and angular momentum loss. We fit each observed binary to our library of tracks using a χ2-minimizing procedure. We find that the hot Algols display overall acceptable χ2, confirming the conservative assumption, while the cool Algols show much less acceptable χ2, suggesting the need for more free parameters, such as mass and angular momentum loss.
The Astrophysical Journal 12/2008; 552(2):664. DOI:10.1086/320560 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We collect data on the masses, radii, etc., of three classes of close binary stars: low-temperature contact binaries (LTCBs), near-contact binaries (NCBs), and detached close binaries (DCBs). We restrict ourselves to systems in which (1) both components are, at least arguably, near the main sequence, (2) the periods are less than a day, and (3) there is both spectroscopic and photometric analysis leading to reasonably reliable data. We discuss the possible evolutionary connections between these three classes, emphasizing the roles played by mass loss and angular momentum loss in rapidly rotating cool stars. We describe a new mechanism, differential rotation as observed in the Sun, which can explain the remarkable efficiency of heat transport in the outer envelopes of contact binaries.
The Astrophysical Journal 12/2008; 629(2):1055. DOI:10.1086/431300 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present equations governing the way in which both the orbit and the intrinsic spins of stars in a close binary should evolve subject to a number of perturbing forces, including the effect of a third body in a possibly inclined wider orbit. We illustrate the solutions in some binary star and triple star situations: tidal friction in a wide but eccentric orbit of a radio pulsar about a B star (0045-7319), the Darwin and eccentricity instabilities in a more massive but shorter period massive X-ray binary, and the interaction of tidal friction with Kozai cycles in a triple star, such as β Per at an early stage in that star's life, when all three components were zero-age main sequence stars. We also attempt to model in some detail the interesting triple system SS Lac, which stopped eclipsing in about 1950. We find that our model of SS Lac is quite constrained by the relatively good observational data of this system and leads to a specific inclination (29°) of the outer orbit relative to the inner orbit at epoch zero (1912). We make some predictions about changes to system parameters in the short term (20-40 yr) and also in the medium term (up to ~3000 yr). Although the intrinsic spins of the stars have little effect on the orbit, the converse is not true: the spin axes can vary their orientation relative to the close binary by up to 120° on a timescale of about a century.
The Astrophysical Journal 12/2008; 562(2):1012. DOI:10.1086/323843 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We consider the multiplicity of stellar systems with (combined) magnitude brighter than 6.00 in Hipparcos magnitudes. We identify 4559 such bright systems (including the Sun), and the frequencies of multiplicities 1, 2, … , 7 are found to be 2718, 1437, 285, 86, 20, 11 and 2. We discuss the uncertainties, which are substantial. We also consider the distributions of periods of orbits and suborbits. We note that for even more restricted set of 478 systems with VH≤ 4.00, the proportions of higher multiples up to sextuple are progressively larger (213, 179, 54, 19, 8, 5), suggesting substantial incompleteness in even the reasonably well studied larger sample.
This sample can be seen as relatively thoroughly studied for multiplicity, and reasonably representative of stars more massive than the Sun. But the restriction to VH≤ 6 means that our sample contains hardly any systems where all components are low-mass main-sequence stars (K or M).
Data on multiplicity are important as a constraint on (i) the star formation problem, (ii) the problem of the evolution of the Galactic stellar population and (iii) the interaction of dynamics and evolution through the effect of Kozai cycles. We discuss these topics briefly.
Monthly Notices of the Royal Astronomical Society 09/2008; 389(2):869 - 879. DOI:10.1111/j.1365-2966.2008.13596.x · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: I discuss my stellar evolution code Ev in the context of simulations of large clusters of stars. It has long been able to handle single stars, and also binary stars up to a point. That point is far beyond what other codes are able to do, but well short of what is necessary for believable simulations. A recent version, Ev(Twin), can in principle deal with the contact phase of binary evolution, but it is not yet clear what the physical interaction is that needs to be simulated. An upgrade, which I hope will be only a few lines, should allow it to follow Kozai cycles with tidal friction, a process that strongly influences the orbital period of close pairs that reside within wide, non-coplanar triples. However, there are many substantial gaps in the physics of even single stars, let alone binaries or triples.
Proceedings of the International Astronomical Union 09/2008; 246:228-232. DOI:10.1017/S1743921308015640
[Show abstract][Hide abstract] ABSTRACT: Three-dimensional stellar modeling has enabled us to identify a deep-mixing mechanism that must operate in all low mass giants. This mixing process is not optional, and is driven by a molecular weight inversion created by the 3He(3He,2p)4He reaction. In this paper we characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of 3He overproduction, reconciling stellar and big bang nucleosynthesis with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low mass giants, a problem of more than 3 decades' standing. This mixing mechanism, which we call `$\delta\mu$-mixing', operates rapidly (relative to the nuclear timescale of overall evolution, ~ 10^8 yrs) once the hydrogen burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Pop I stars between 0.8 and 2.0$\Msun$ develop 12C/13C ratios of 14.5 +/- 1.5, while Pop II stars process the carbon to ratios of 4.0 +/- 0.5. In stars less than 1.25$\Msun$, this mechanism also destroys 90% to 95% of the 3He produced on the main sequence.
The Astrophysical Journal 04/2008; 677(1):581. DOI:10.1086/529024 · 5.99 Impact Factor