arXiv subjects

O. R. Pols

Publications and source records attributed to O. R. Pols.

At least 55 records · Page 3Linked to original sources

arXiv2007

Can low metallicity binaries avoid merging?

Rapid mass transfer in a binary system can drive the accreting star out of thermal equilibrium, causing it to expand. This can lead to a contact system, strong mass loss from the system and possibly merging of the two stars. In low metallicity stars the timescale for heat transport is shorter due to the lower opacity. The accreting star can therefore restore thermal equilibrium more quickly and possibly avoid contact. We investigate the effect of accretion onto main sequence stars with radiative envelopes with different metallicities. We find that a low metallicity (Z<0.001), 4 solar mass star can endure a 10 to 30 times higher accretion rate before it reaches a certain radius than a star at solar metallicity. This could imply that up to two times fewer systems come into contact during rapid mass transfer when we compare low metallicity. This factor is uncertain due to the unknown distribution of binary parameters and the dependence of the mass transfer timescale on metallicity. In a forthcoming paper we will present analytic fits to models of accreting stars at various metallicities intended for the use in population synthesis models.

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arXiv2007

Building Blue Stragglers with Stellar Collisions

The evolution of stellar collision products in cluster simulations has usually been modelled using simplified prescriptions. Such prescriptions either replace the collision product with an (evolved) main sequence star, or assume that the collision product was completely mixed during the collision. It is known from hydrodynamical simulations of stellar collisions that collision products are not completely mixed, however. We have calculated the evolution of stellar collision products and find that they are brighter than normal main sequence stars of the same mass, but not as blue as models that assume that the collision product was fully mixed during the collision.

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arXiv2007

When Stars Collide

When two stars collide and merge they form a new star that can stand out against the background population in a starcluster as a blue straggler. In so called collision runaways many stars can merge and may form a very massive star that eventually forms an intermediate mass blackhole. We have performed detailed evolution calculations of merger remnants from collisions between main sequence stars, both for lower mass stars and higher mass stars. These stars can be significantly brighter than ordinary stars of the same mass due to their increased helium abundance. Simplified treatments ignoring this effect give incorrect predictions for the collision product lifetime and evolution in the Hertzsprung-Russell diagram.

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arXiv2007

Binaries at Low Metallicity: ranges for case A, B and C mass transfer

The evolution of single stars at low metallicity has attracted a large interest, while the effect of metallicity on binary evolution remains still relatively unexplored. We study the effect of metallicity on the number of binary systems that undergo different cases of mass transfer. We find that binaries at low metallicity are more likely to start transferring mass after the onset of central helium burning, often referred to as case C mass transfer. In other words, the donor star in a metal poor binary is more likely to have formed a massive CO core before the onset of mass transfer. At solar metallicity the range of initial binary separations that result in case C evolution is very small for massive stars, because they do not expand much after the ignition of helium and because mass loss from the system by stellar winds causes the orbit to widen, preventing the primary star to fill its Roche lobe. This effect is likely to have important consequences for the metallicity dependence of the formation rate of various objects through binary evolution channels, such as long GRBs, double neutron stars and double white dwarfs.

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arXiv2007

Critically rotating stars in binaries - an unsolved problem -

In close binaries mass and angular momentum can be transferred from one star to the other during Roche-lobe overflow. The efficiency of this process is not well understood and constitutes one of the largest uncertainties in binary evolution. One of the problems lies in the transfer of angular momentum, which will spin up the accreting star. In very tight systems tidal friction can prevent reaching critical rotation, by locking the spin period to the orbital period. Accreting stars in systems with orbital periods larger than a few days reach critical rotation after accreting only a fraction of their mass, unless there is an effective mechanism to get rid of angular momentum. In low mass stars magnetic field might help. In more massive stars angular momentum loss will be accompanied by strong mass loss. This would imply that most interacting binaries with initial orbital periods larger than a few days evolve very non-conservatively. In this contribution we wish to draw attention to the unsolved problems related to mass and angular momentum transfer in binary systems. We do this by presenting the first results of an implementation of spin up by accretion into the TWIN version of the Eggleton stellar evolution code.

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arXiv2007

Efficiency of mass transfer in massive close binaries, Tests from double-lined eclipsing binaries in the SMC

One of the major uncertainties in close binary evolution is the efficiency of mass transfer beta: the fraction of transferred mass that is accreted by a secondary star. We attempt to constrain the mass-transfer efficiency for short-period massive binaries undergoing case A mass transfer. We present a grid of about 20,000 detailed binary evolution tracks with primary masses 3.5-35 Msun, orbital periods 1-5 days at a metallicity Z=0.004, assuming both conservative and non-conservative mass transfer. We perform a systematic comparison, using least-squares fitting, of the computed models with a sample of 50 double-lined eclipsing binaries in the Small Magellanic Cloud, for which fundamental stellar parameters have been determined. About 60% of the systems are currently undergoing slow mass transfer. In general we find good agreement between our models and the observed detached systems. However, for many of the semi-detached systems the observed temperature ratio is more extreme than our models predict. For the 17 semi-detached systems that we are able to match, we find a large spread in the best fitting mass-transfer efficiency; no single value of beta can explain all systems. We find a hint that initially wider systems tend to fit better to less conservative models. We show the need for more accurate temperature determinations and we find that determinations of surface abundances of nitrogen and carbon can potentially constrain the mass-transfer efficiency further.

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arXiv2007

The s-process in stellar population synthesis: a new approach to understanding AGB stars

Thermally pulsating asymptotic giant branch (AGB) stars are the main producers of slow neutron capture (s-) process elements, but there are still large uncertainties associated with the formation of the main neutron source, 13C, and with the physics of these stars in general. Observations of s-process element enhancements in stars can be used as constraints on theoretical models. For the first time we apply stellar population synthesis to the problem of s-process nucleosynthesis in AGB stars, in order to derive constraints on free parameters describing the physics behind the third dredge-up and the properties of the neutron source. We utilize a rapid evolution and nucleosynthesis code to synthesize different populations of s-enhanced stars, and compare them to their observational counterparts to find out for which values of the free parameters in the code the synthetic populations fit best to the observed populations. These free parameters are the amount of third dredge-up, the minimum core mass for third dredge-up, the effectiveness of 13C as a source of neutrons and the size in mass of the 13C pocket. We find that galactic disk objects are reproduced by a spread of a factor of two in the effectiveness of the 13C neutron source. Lower metallicity objects can be reproduced only by lowering by at least a factor of 3 the average value of the effectiveness of the 13C neutron source needed for the galactic disk objects. Using observations of s-process elements in post-AGB stars as constraints we find that dredge-up has to start at a lower core mass than predicted by current theoretical models, that it has to be substantial ($λ$ >~ 0.2) in stars with mass M <~ 1.5 M_sun and that the mass of the 13C pocket must be about 1/40 that of the intershell region.

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arXiv2006

Modelling the formation of double white dwarfs

We investigate the formation of the ten double-lined double white dwarfs that have been observed so far. A detailed stellar evolution code is used to calculate grids of single-star and binary models and we use these to reconstruct possible evolutionary scenarios. We apply various criteria to select the acceptable solutions from these scenarios. We confirm the conclusion of Nelemans et al. (2000) that formation via conservative mass transfer and a common envelope with spiral-in based on energy balance or via two such spiralins cannot explain the formation of all observed systems. We investigate three different prescriptions of envelope ejection due to dynamical mass loss with angular-momentum balance and show that they can explain the observed masses and orbital periods well. Next, we demand that the age difference of our model is comparable to the observed cooling-age difference and show that this puts a strong constraint on the model solutions. However, the scenario in which the primary loses its envelope in an isotropic wind and the secondary transfers its envelope, which is then re-emitted isotropically, can explain the observed age differences as well. One of these solutions explains the DB-nature of the oldest white dwarf in PG1115+116 along the evolutionary scenario proposed by Maxted et al. (2002a), in which the helium core of the primary becomes exposed due to envelope ejection, evolves into a giant phase and loses its hydrogen-rich outer layers.

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arXiv2006

Population synthesis of s-process element enhanced stars: Constraining the 13C efficiency

We study s-process element abundance ratios in stars by carrying out stellar population synthesis, using a rapid synthetic stellar evolution code which includes an up-to-date treatment of AGB nucleosynthesis and evolution. In contrast to other studies, we find that a large spread in the 13C efficiency parameter (13C_eff) is not needed to explain the observed spread in the ratios of heavy s-process to light s-process elements ([hs/ls]), but this comes naturally from the range of different initial stellar masses and their time evolution. As a result, the 13C efficiency needed for fitting most stars in the galactic disk is constrained to 1 < 13C_eff < 2.5. In the same fashion we also study the [Pb/Ce] ratios of lead stars and find out that for low metallicities 13C_eff is approximately 0.5.

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arXiv2005

On the possibility of a helium white dwarf donor in the presumed ultracompact binary 2S 0918-549

2S 0918-549 is a low-mass X-ray binary (LMXB) with a low optical to X-ray flux ratio. Probably it is an ultracompact binary with an orbital period shorter than 60 min. Such binaries cannot harbor hydrogen rich donor stars. As with other (sometimes confirmed) ultracompact LMXBs, 2S 0918-549 is observed to have a high neon-to-oxygen abundance ratio (Juett et al. 2001) which has been used to argue that the companion star is a CO or ONe white dwarf. However, type-I X-ray bursts have been observed from several of these systems implying the presence of hydrogen or helium on the neutron star surface. In this paper, we argue that the companion star in 2S 0918-549 is a helium white dwarf We first present a Type I X-ray burst from 2S 0918-549 with a long duration of 40 minutes. We show that this burst is naturally explained by accretion of pure helium at the inferred accretion rate of ~0.01 times the Eddington accretion rate. At higher accretion rates of ~0.1 Eddington, hydrogen is required to explain long duration bursts. However, at low rates the long duration is due to the large amount of helium that accumulates prior to the burst. We show that it is possible to form a helium white dwarf donor in an ultracompact binary if accretion starts during the first ascent of the giant branch, when the core is made of predominantly helium. Furthermore, this scenario naturally explains the high neon-to-oxgen ratio, without requiring a CO or ONe white dwarf companion. The only observational aspect of 2S 0918-549 that we cannot explain is the absence of helium lines in the optical spectrum. Model calculations of optical accretion disk spectra need to be carried out in order to obtain limits on the helium abundance.

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arXiv2005

Reduced magnetic braking and the magnetic capture model for the formation of ultra-compact binaries

A binary in which a slightly evolved star starts mass transfer to a neutron star can evolve towards ultra-short orbital periods under the influence of magnetic braking. This is called magnetic capture. In a previous paper we showed that ultra-short periods are only reached for an extremely small range of initial binary parameters, in particular orbital period and donor mass. Our conclusion was based on one specific choice for the law of magnetic braking, and for the loss of mass and angular momentum during mass transfer. In this paper we show that for less efficient magnetic braking it is impossible to evolve to ultra-short periods, independent of the amount of mass and associated angular momentum lost from the binary.

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arXiv2004

Creating ultra-compact binaries through stable mass transfer

A binary in which a slightly evolved star starts mass transfer to a neutron star can evolve towards ultra-short orbital periods under the influence of magnetic braking. This is called magnetic capture. We investigate in detail for which initial orbital periods and initial donor masses binaries evolve to periods less than 30-40 minutes within the Hubble time. We show that only small ranges of initial periods and masses lead to ultra-short periods, and that for those only a small time interval is spent at ultra-short periods. Consequently, only a very small fraction of any population of X-ray binaries is expected to be observed at ultra-short period at any time. If 2 to 6 of the 13 bright X-ray sources in globular clusters have an ultra-short period, as suggested by recent observations, their formation cannot be explained by the magnetic capture model.

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arXiv2004

Creating ultra-compact binaries in globular clusters through stable mass transfer

A binary in which a slightly evolved star starts mass transfer to a neutron star can evolve towards ultra-short orbital periods under the influence of magnetic braking. This is called magnetic capture. We investigate in detail for which initial orbital periods and initial donor masses binaries evolve to periods less than 30-40 minutes within the Hubble time. We show that only small ranges of initial periods and masses lead to ultra-short periods, and that for those only a small time interval is spent at ultra-short periods. Consequently, only a very small fraction of any population of X-ray binaries is expected to be observed at ultra-short period at any time. If 2 to 6 of the 13 bright X-ray sources in globular clusters have an ultra-short period, as suggested by recent observations, their formation cannot be explained by the magnetic capture model.

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arXiv2004

Deep Dredge-up in Intermediate-Mass TP-AGB Stars

We present results of the evolution of AGB stars of 3M_sol and 5M_sol with solar metallicity calculated with the Eggleton stellar evolution code (STARS), which has a fully implicit and simultaneous method for solving for the stellar structure, convective mixing and nuclear burning. We introduce the concept of a viscous mesh in order to improve the numerical stability of the calculations. For the 5M_sol star, we evolve through 25 thermal pulses and their associated third dredge-up events. We obtain a maximum helium luminosity of 1.7x10^9 L_sol and significantly deep dredge-up after the second pulse. Strong hot-bottom burning is observed after the 5th pulse. The 3M_sol model is evolved through 20 thermal pulse events and we find third dredge-up after the 7th pulse. During the 14th pulse sufficient carbon has been brought to the surface to produce a carbon star. We find that dredge-up and the transformation into a carbon star occur at significantly smaller core masses (0.584M_sol and 0.608M_sol, respectively) than in previous calculations for 3M_sol.

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arXiv2003

The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems

With a view to understanding the formation of double neutron-stars (DNS), we investigate the late stages of evolution of helium stars with masses of 2.8 - 6.4 Msun in binary systems with a 1.4 Msun neutron-star companion. We found that mass transfer from 2.8 - 3.3 Msun helium stars and from 3.3 - 3.8 Msun in very close orbits (P_orb > 0.25d) will end up in a common-envelope (CE) and spiral-in phase due to the development of a convective helium envelope. If the neutron star has sufficient time to complete the spiraling-in process before the core collapses, the system will produce very tight DNSs (P_orb ~ 0.01d) with a merger timescale of the order of 1 Myr or less. These systems would have important consequences for the detection rate of GWR and for the understanding of GRB progenitors. On the other hand, if the time left until the explosion is shorter than the orbital-decay timescale, the system will undergo a SN explosion during the CE phase. Helium stars with masses 3.3 - 3.8 Msun in wider orbits (P_orb > 0.25d) and those more massive than 3.8 Msun do not go through CE evolution. The remnants of these massive helium stars are DNSs with periods in the range of 0.1 - 1 d. This suggests that this range of mass includes the progenitors of the galactic DNSs with close orbits (B1913+16 and B1534+12). A minimum kick velocity of 70 km/s and 0 km/s (for B1913+16 and B1534+12, respectively) must have been imparted at the birth of the pulsar's companion. The DNSs with wider orbits (J1518+4904 and probably J1811-1736) are produced from helium star-neutron star binaries which avoid RLOF, with the helium star more massive than 2.5 Msun. For these systems the minimum kick velocities are 50 km/s and 10 km/s (for J1518+4904 and J1811-1736, respectively).

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arXiv2002

Parameterising the third dredge-up in asymptotic giant branch stars

We present new evolutionary sequences for low and intermediate mass stars for three different metallicities, Z = 0.02,0.008 and 0.004. We evolve the models from the pre-main sequence to the thermally-pulsing asymptotic giant branch phase. We have two sequences of models for each mass, one which includes mass loss and one without mass loss. Typically 20 or more pulses have been followed for each model, allowing us to calculate the third dredge-up parameter for each case. Using the results from this large and homogeneous set of models, we present an approximate fit for the core mass at the first thermal pulse, as well as for the third dredge-up efficiency parameter, and the core mass at the first third dredge-up episode as a function of metallicity and total mass. We also examine the effect of a reduced envelope mass on the value of the third dredge-up efficiency parameter.

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arXiv2002

The evolution of naked helium stars with a neutron-star companion in close binary systems

The evolution of helium stars with masses of 1.5 - 6.7 M_sun in binary systems with a 1.4 M_sun neutron-star companion is presented. Such systems are assumed to be the remnants of Be/X-ray binaries with B-star masses in the range of 8 - 20 M_sun which underwent a case B or case C mass transfer and survived the common-envelope and spiral-in process. The orbital period is chosen such that the helium star fills its Roche lobe before the ignition of carbon in the centre. We distinguish case BA (in which mass transfer is initiated during helium core burning) from case BB (onset of Roche-lobe overflow occurs after helium core burning is terminated, but before the ignition of carbon). We found that the remnants of case BA mass transfer from 1.5 - 2.9 M_sun helium stars are heavy CO white dwarfs. This implies that a star initially as massive as 12 M_sun is able to become a white dwarf. CO white dwarfs are also produced from case BB mass transfer from 1.5 - 1.8 M_sun helium stars, while ONe white dwarfs are formed from 2.1 - 2.5 M_sun helium stars. Case BB mass transfer from more-massive helium stars with a neutron-star companion will produce a double neutron-star binary. We are able to distinguish the progenitors of type Ib supernovae (as the high-mass helium stars or systems in wide orbits) from those of type Ic supernovae (as the lower-mass helium stars or systems in close orbits). Finally, we derive a "zone of avoidance" in the helium star mass vs. initial orbital period diagram for producing neutron stars from helium stars.

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arXiv2000

Comprehensive analytic formulae for stellar evolution as a function of mass and metallicity

We present analytic formulae that approximate the evolution of stars for a wide range of mass and metallicity. Stellar luminosity, radius and core mass are given as a function of age, M and Z, for all phases from the zero-age main-sequence up to, and including, the remnant stages. For the most part we find continuous formulae accurate to within 5% of detailed models. These formulae are useful for purposes such as population synthesis that require very rapid but accurate evaluation of stellar properties, and in particular for use in combination with N-body codes. We describe a mass loss prescription that can be used with these formulae and investigate the resulting stellar remnant distribution.

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