arXiv subjects

Silvia Toonen

Publications and source records attributed to Silvia Toonen.

At least 73 records · Page 4Linked to original sources

arXiv2019

Multimessenger science opportunities with mHz gravitational waves

LISA will open the mHz band of gravitational waves (GWs) to the astronomy community. The strong gravity which powers the variety of GW sources in this band is also crucial in a number of important astrophysical processes at the current frontiers of astronomy. These range from the beginning of structure formation in the early universe, through the origin and cosmic evolution of massive black holes in concert with their galactic environments, to the evolution of stellar remnant binaries in the Milky Way and in nearby galaxies. These processes and their associated populations also drive current and future observations across the electromagnetic (EM) spectrum. We review opportunities for science breakthroughs, involving either direct coincident EM+GW observations, or indirect multimessenger studies. We argue that for the US community to fully capitalize on the opportunities from the LISA mission, the US efforts should be accompanied by a coordinated and sustained program of multi-disciplinary science investment, following the GW data through to its impact on broad areas of astrophysics. Support for LISA-related multimessenger observers and theorists should be sized appropriately for a flagship observatory and may be coordinated through a dedicated mHz GW research center.

astro-ph.HE↗

arXiv2019

Stellar Astrophysics and Exoplanet Science with the Maunakea Spectroscopic Explorer (MSE)

The Maunakea Spectroscopic Explorer (MSE) is a planned 11.25-m aperture facility with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. A rebirth of the 3.6m Canada-France-Hawaii Telescope on Maunakea, MSE will use 4332 fibers operating at three different resolving powers (R ~ 2500, 6000, 40000) across a wavelength range of 0.36-1.8mum, with dynamical fiber positioning that allows fibers to match the exposure times of individual objects. MSE will enable spectroscopic surveys with unprecedented scale and sensitivity by collecting millions of spectra per year down to limiting magnitudes of g ~ 20-24 mag, with a nominal velocity precision of ~100 m/s in high-resolution mode. This white paper describes science cases for stellar astrophysics and exoplanet science using MSE, including the discovery and atmospheric characterization of exoplanets and substellar objects, stellar physics with star clusters, asteroseismology of solar-like oscillators and opacity-driven pulsators, studies of stellar rotation, activity, and multiplicity, as well as the chemical characterization of AGB and extremely metal-poor stars.

astro-ph.SR↗

arXiv2019

Neutron star - white dwarf mergers: Early evolution, physical properties, and outcomes

Neutron-star (NS) - white-dwarf (WD) mergers may give rise to observable explosive transients, but have been little explored. We use 2D coupled hydrodynamical-thermonuclear FLASH-code simulations to study the evolution of WD debris-disks formed following WD-disruptions by NSs. We use a 19-elements nuclear-network and a detailed equation-of-state to follow the evolution, complemented by a post-process analysis using a larger 125-isotopes nuclear-network. We consider a wide range of initial conditions and study the dependence of the results on the NS/WD masses ($1.4-2{\rm M_{\odot}}$;$\,{\rm 0.375-0.7\,M_{\odot}}$, respectively), WD-composition (CO/He/hybrid-He-CO) and the accretion-disk structure. We find that viscous inflow in the disk gives rise to continuous wind-outflow of mostly C/O material mixed with nuclear-burning products arising from a weak detonation occurring in the inner-region of the disk. We find that such transients are energetically weak ($10^{48}-10^{49}$ergs) compared with thermonuclear-supernovae (SNe), and are dominated by the (gravitational) accretion-energy. Although thermonuclear-detonations occur robustly in all of our simulations (besides the He-WD) they produce only little energy $(1-10\%$ of the kinetic energy) and $^{56}{\rm Ni}$ ejecta (few$\times10^{-4}-10^{-3}{\rm M_{\odot}})$, with overall low ejecta masses of $\sim0.01-0.1{\rm M_{\odot}}$. Such explosions may produce rapidly-evolving transients, much shorter and fainter than regular type-Ia SNe. The composition and demographics of such SNe appear to be inconsistent with those of Ca-rich type Ib SNe. Though they might be related to the various classes of rapidly evolving SNe observed in recent years, they are likely to be fainter than the typical ones, and may therefore give rise a different class of potentially observable transients.

astro-ph.HE↗

arXiv2018

White dwarfs in the building blocks of the Galactic spheroid

The galactic halo likely grew over time in part by assembling smaller galaxies, the so-called building blocks. We investigate if the properties of these building blocks are reflected in the halo white dwarf (WD) population in the Solar neighborhood. Furthermore, we compute the halo WD luminosity functions (WDLFs) for four major building blocks of five cosmologically motivated stellar haloes. We couple the SeBa binary population synthesis model to the Munich-Groningen semi-analytic galaxy formation model, applied to the high-resolution Aquarius dark matter simulations. Although the semi-analytic model assumes an instantaneous recycling approximation, we model the evolution of zero-age main sequence stars to WDs, taking age and metallicity variations of the population into account. Although the majority of halo stars is old and metal-poor and therefore the WDs in the different building blocks have similar properties (including present-day luminosity), we find in our models that the WDs originating from building blocks that have young and/or metal-rich stars can be distinguished from WDs that were born in other building blocks. In practice however, it will be hard to prove that these WDs really originate from different building blocks, as the variations in the halo WD population due to binary WD mergers result in similar effects. The five joined stellar halo WD populations that we modelled result in WDLFs that are very similar to each other. We find that simple models with a Kroupa or Salpeter initial mass function (IMF) fit the observed luminosity function slightly better, since the Chabrier IMF is more top-heavy, although this result is dependent on our choice of the stellar halo mass density in the Solar neighborhood.

astro-ph.GA↗

arXiv2018

A $Gaia$ Data Release 2 catalogue of white dwarfs and a comparison with SDSS

We present a catalogue of white dwarf candidates selected from the second data release of $Gaia$ (DR2). We used a sample of spectroscopically confirmed white dwarfs from the Sloan Digital Sky Survey (SDSS) to map the entire space spanned by these objects in the $Gaia$ Hertzsprung-Russell diagram. We then defined a set of cuts in absolute magnitude, colour, and a number of $Gaia$ quality flags to remove the majority of contaminating objects. Finally, we adopt a method analogous to the one presented in our earlier SDSS photometric catalogues to calculate a probability of being a white dwarf ($P_{\mathrm{WD}}$) for all $Gaia$ sources which passed the initial selection. The final catalogue is composed of $486\,641$ stars with calculated $P_{\mathrm{WD}}$ from which it is possible to select a sample of $\simeq 260\,000$ high-confidence white dwarf candidates in the magnitude range $8 7000$ K, at high Galactic latitudes ($|b|>20^{\circ}$). However, the completeness drops at low Galactic latitudes, and the magnitude limit of the catalogue varies significantly across the sky as a function of $Gaia$'s scanning law. We also provide the list of objects within our sample with available SDSS spectroscopy. We use this spectroscopic sample to characterise the observed structure of the white dwarf distribution in the H-R diagram.

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arXiv2018

Three Hypervelocity White Dwarfs in Gaia DR2: Evidence for Dynamically Driven Double-Degenerate Double-Detonation Type Ia Supernovae

Double detonations in double white dwarf (WD) binaries undergoing unstable mass transfer have emerged in recent years as one of the most promising Type Ia supernova (SN Ia) progenitor scenarios. One potential outcome of this "dynamically driven double-degenerate double-detonation" (D^6) scenario is that the companion WD survives the explosion and is flung away with a velocity equal to its > 1000 km/s pre-SN orbital velocity. We perform a search for these hypervelocity runaway WDs using Gaia's second data release. In this paper, we discuss seven candidates followed up with ground-based instruments. Three sources are likely to be some of the fastest known stars in the Milky Way, with total Galactocentric velocities between 1000 and 3000 km/s, and are consistent with having previously been companion WDs in pre-SN Ia systems. However, although the radial velocity of one of the stars is > 1000 km/s, the radial velocities of the other two stars are puzzlingly consistent with 0. The combined five-parameter astrometric solutions from Gaia and radial velocities from follow-up spectra yield tentative 6D confirmation of the D^6 scenario. The past position of one of these stars places it within a faint, old SN remnant, further strengthening the interpretation of these candidates as hypervelocity runaways from binary systems that underwent SNe Ia.

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arXiv2018

Detecting hierarchical stellar systems with LISA

A significant fraction of stars are members of gravitationally bound hierarchies containing three or more components. Almost all low mass stars in binaries with periods shorter three days are part of a hierarchical system. We therefore anticipate that a large fraction of compact galactic binaries detected by the Laser Interferometer Space Antenna (LISA) will be members of hierarchical triple or quadruple system. The acceleration imparted by the hierarchical companions can be detected in the gravitational wave signal for outer periods as large as 100 years. For systems with periods that are shorter than, or comparable to, the mission lifetime, it will be possible to measure the period and eccentricity of the outer orbit. LISA observations of hierarchical stellar systems will provide insight into stellar evolution, including the role that Kozai-Lidov oscillations play in driving systems towards merger.

gr-qc↗

arXiv2018

Formation and evolution of hybrid He-CO white dwarfs and their properties

White dwarfs (WDs) are the stellar core remnants of low mass stars. They are typically divided into three main composition groups: Oxygen Neon (ONe), Carbon Oxygen (CO) and Helium (He) WDs. The evolution of binary systems can significantly change the evolution of the binary stellar components. In particular, striping the envelope of an evolved star can give rise to a core remnant, which can later evolve into a WD with significantly different composition. Here we focus on the formation and evolution of hybrid HeCO WDs. We follow the formation and stellar evolution of such WDs for a range of initial conditions and provide their detailed structure, mass-radius relation and luminosity temperature evolution. We find that both low mass WDs (< 0.45M , typically thought to be He WDs) and intermediate-mass WDs (0.45 < MWD < 0.7, typically thought to be CO WDs) could in fact be hybrid HeCO WDs, with 5-25 (75 -95)% of their mass in He (CO). We use population synthesis calculations to infer the birth rate and properties of such WDs. We find that hybrid HeCO WD comprise the majority of young (< 2Gyr) WDs in binaries, but are more rare among older WDs in binaries. The high frequency and large He content of such WDs could have an important role in WD WD mergers, and may give rise to sub Chandrasekhar thermonuclear supernova explosions.

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arXiv2018

Astronomical Distance Determination in the Space Age. Secondary distance indicators

The formal division of the distance indicators into primary and secondary leads to difficulties in description of methods which can actually be used in two ways: with, and without the support of the other methods for scaling. Thus instead of concentrating on the scaling requirement we concentrate on all methods of distance determination to extragalactic sources which are designated, at least formally, to use for individual sources. Among those, the Supernovae Ia is clearly the leader due to its enormous success in determination of the expansion rate of the Universe. However, new methods are rapidly developing, and there is also a progress in more traditional methods. We give a general overview of the methods but we mostly concentrate on the most recent developments in each field, and future expectations.

astro-ph.GA↗

arXiv2017

The Evolution of the Type Ia Supernova Luminosity Function

Type Ia supernovae (SNe Ia) exhibit a wide diversity of peak luminosities and light curve shapes: the faintest SNe Ia are 10 times less luminous and evolve more rapidly than the brightest SNe Ia. Their differing characteristics also extend to their stellar age distributions, with fainter SNe Ia preferentially occurring in old stellar populations and vice versa. In this Letter, we quantify this SN Ia luminosity - stellar age connection using data from the Lick Observatory Supernova Search (LOSS). Our binary population synthesis calculations agree qualitatively with the observed trend in the >1 Gyr-old populations probed by LOSS if the majority of SNe Ia arise from prompt detonations of sub-Chandrasekhar mass white dwarfs (WDs) in double WD systems. Under appropriate assumptions, we show that double WD systems with less massive primaries, which yield fainter SNe Ia, interact and explode at older ages than those with more massive primaries. We find that prompt detonations in double WD systems are capable of reproducing the observed evolution of the SN Ia luminosity function, a constraint that any SN Ia progenitor scenario must confront.

astro-ph.HE↗

arXiv2017

Short Gamma-Ray Bursts following mergers of an ONeMg with a CO white dwarf

We discuss a scenario of short Gamma-Ray Bursts (GRBs) following a merger of a massive ONeMg white dwarf (WD) with a CO WD, and an ensuing accretion induced collapse (AIC). An initial system with the primary mass $M_1 \sim 6-10 \, M_\odot$ and the secondary mass $M_2 \sim 3-6\, M_\odot$ forms, via two distinct evolutionary channels, a double degenerate CO-ONeMg WD system. For sufficiently large mass ratio $q \equiv M_2/M_1 > q_{crit} \sim 0.25$ the ensuing gravitational wave-driven mass transfer is unstable, whereby the less massive CO WD is disrupted and transfers its mass to the primary ONeMg WD on a few orbital time scales. The merger product ignites shell CO burning, adding mass to the degenerate core; at the same time mass and angular momentum is lost due to powerful winds. For an ONeMg WD sufficiently close to the Chandrasekhar mass an electron-capture accretion induced collapse (AIC) follows $\sim 10^4$ years afterwards. We associate the prompt short GRB emission with a direct collapse of an ONeMg WD to a neutron star, without formation of an accretion disk. After the collapse the accretion of the unburnt part of the shell onto the newly formed NS powers the extended emission (EE). During the collapse the neutron star is spun to millisecond periods and produce long lasting relativistic winds that shock against the material lost during the shell-burning stage, and produce afterglow emission from the wind termination shock.

astro-ph.HE↗

arXiv2017

Binary black hole mergers from field triples: properties, rates and the impact of stellar evolution

We consider the formation of binary black hole mergers through the evolution of field massive triple stars. In this scenario, favorable conditions for the inspiral of a black hole binary are initiated by its gravitational interaction with a distant companion, rather than by a common-envelope phase invoked in standard binary evolution models. We use a code that follows self-consistently the evolution of massive triple stars, combining the secular triple dynamics (Lidov-Kozai cycles) with stellar evolution. After a black hole triple is formed, its dynamical evolution is computed using either the orbit-averaged equations of motion, or a high-precision direct integrator for triples with weaker hierarchies for which the secular perturbation theory breaks down. Most black hole mergers in our models are produced in the latter non-secular dynamical regime. We derive the properties of the merging binaries and compute a black hole merger rate in the range (0.3- 1.3) Gpc^{-3}yr^{-1}, or up to ~2.5Gpc^{-3}yr^{-1} if the black hole orbital planes have initially random orientation. Finally, we show that black hole mergers from the triple channel have significantly higher eccentricities than those formed through the evolution of massive binaries or in dense star clusters. Measured eccentricities could therefore be used to uniquely identify binary mergers formed through the evolution of triple stars. While our results suggest up to ~10 detections per year with Advanced-LIGO, the high eccentricities could render the merging binaries harder to detect with planned space based interferometers such as LISA.

astro-ph.GA↗

arXiv2017

Prospects for detection of detached double white dwarf binaries with Gaia, LSST and LISA

Double white dwarf (DWD) binaries are expected to be very common in the Milky Way, but their intrinsic faintness challenges the detection of these systems. Currently, only a few tens of detached DWDs are know. Such systems offer the best chance of extracting the physical properties that would allow us to address a wealth of outstanding questions ranging from the nature of white dwarfs, over stellar and binary evolution to mapping the Galaxy. In this paper we explore the prospects for detections of ultra-compact (with binary separations of a few solar radii or less) detached DWDs in: 1) optical radiation with Gaia and the LSST and 2) gravitational wave radiation with LISA. We show that Gaia, LSST and LISA have the potential to detect respectively around a few hundreds, a thousand, and 25 thousand DWD systems. Moreover, Gaia and LSST data will extend by respectively a factor of two and seven the guaranteed sample of binaries detected in electromagnetic and gravitational wave radiation, opening the era of multi-messenger astronomy for these sources.

astro-ph.HE↗

arXiv2016

Interrupted Binary Mass Transfer in Star Clusters

Binary mass transfer is at the forefront of some of the most exciting puzzles of modern astrophysics, including Type Ia supernovae, gamma-ray bursts, and the formation of most observed exotic stellar populations. Typically, the evolution is assumed to proceed in isolation, even in dense stellar environments such as star clusters. In this paper, we test the validity of this assumption via the analysis of a large grid of binary evolution models simulated with the SeBa code. For every binary, we calculate analytically the mean time until another single or binary star comes within the mean separation of the mass-transferring binary, and compare this time-scale to the mean time for stable mass transfer to occur. We then derive the probability for each respective binary to experience a direct dynamical interruption. The resulting probability distribution can be integrated to give an estimate for the fraction of binaries undergoing mass transfer that are expected to be disrupted as a function of the host cluster properties. We find that for lower-mass clusters ($\lesssim 10^4$ M$_{\odot}$), on the order of a few to a few tens of percent of binaries undergoing mass-transfer are expected to be interrupted by an interloping single, or more often binary, star, over the course of the cluster lifetime, whereas in more massive globular clusters we expect $\ll$ 1% to be interrupted. Furthermore, using numerical scattering experiments performed with the FEWBODY code, we show that the probability of interruption increases if perturbative fly-bys are considered as well, by a factor $\sim 2$.

astro-ph.SR↗

arXiv2014

Binary white dwarfs in the halo of the Milky Way

Aims: We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction. Methods: We simulate the evolution of low-metallicity halo stars at distances up to ~ 3 kpc using the binary population synthesis code SeBa. We use two different white dwarf cooling models to predict the present-day luminosities of halo white dwarfs. We determine the white dwarf luminosity functions (WDLFs) for eight different halo models and compare these with the observed halo WDLF of white dwarfs in the SuperCOSMOS Sky Survey. Furthermore, we predict the properties of binary white dwarfs in the halo and determine the number of halo white dwarfs that is expected to be observed with the Gaia satellite. Results: By comparing the WDLFs, we find that a standard IMF matches the observations more accurately than a top-heavy one, but the difference with a bottom-heavy IMF is small. A burst of star formation 13 Gyr ago fits slightly better than a star formation burst 10 Gyr ago and also slightly better than continuous star formation $10-13$ Gyrs ago. Gaia will be the first instrument to constrain the bright end of the field halo WDLF, where contributions from binary WDs are considerable. Many of these will have He cores, of which a handful have atypical surface gravities ($\log g < 6$) and reach luminosities $\log(L/L_\odot) > 0$ in our standard model for WD cooling. These so called pre-WDs, if observed, can help us to constrain white dwarf cooling models and might teach us something about the fraction of halo stars that reside in binaries.

astro-ph.GA↗

arXiv2014

The influence of mass-transfer variability on the growth of white dwarfs, and the implications for supernova type Ia rates

White dwarfs (WDs) can increase their mass by accretion from companion stars, provided the mass-accretion rate is high enough to avoid nova eruptions. The accretion regimes that allow growth of the WDs are usually calculated assuming constant mass-transfer rates. However, it is possible that these systems are influenced by effects that cause the rate to fluctuate on various timescales. We investigate how long-term mass-transfer variability affects accreting WDs systems. We show that, if such variability is present, it expands the parameter space of binaries where the WD can effectively increase its mass. Furthermore, we find that the supernova type Ia (SNIa) rate is enhanced by a factor 2-2.5 to a rate that is comparable with the lower limit of the observed rates. The changes in the delay-time distribution allow for more SNIae in stellar populations with ages of a few Gyr. Thus, mass-transfer variability gives rise to a new formation channel of SNIa events that can significantly contribute to the SNIa rate. Mass-transfer variability is also likely to affect other binary populations through enhanced WD growth. For example, it may explain why WDs in cataclysmic variables are observed to be more massive than single WDs, on average.

astro-ph.HE↗

arXiv2013

On double-degenerate type Ia supernova progenitors as supersoft X-ray sources - A population synthesis analysis using SeBa

Context: The nature of the progenitors of type Ia supernova (SN) progenitors remains unclear. While it is usually agreed that single-degenerate progenitor systems would be luminous supersoft X-ray sources (SSSs), it was recently suggested that double-degenerate progenitors might also go through a SSS phase. Aims: We aim to examine the possibility of double-degenerate progenitor systems being SSSs, and place stringent upper limits on the maximally possible durations of any SSS phases and expected number of these systems in a galactic population. Method: We employ the binary population synthesis code SeBa to examine the mass-transfer characteristics of a possible SSS phase of double-degenerate type Ia SN progenitor systems for 1) the standard SeBa assumptions, and 2) an optimistic best-case scenario. The latter case establishes firm upper limits on the possible population of supersoft source double-degenerate type Ia SN progenitor systems. Results: Our results indicate that unlike what is expected for single-degenerate progenitor systems, the vast majority of the material accreted by either pure wind mass transfer or a combination of wind and RLOF mass transfer is helium rather than hydrogen. Even with extremely optimistic assumptions concerning the mass-transfer and retention efficiencies, the average mass accreted by systems that eventually become double-degenerate type Ia SNe is small. Consequently, the lengths of time that these systems may be SSSs are short, even under optimal conditions, and the expected number of such systems in a galactic population is negligible. Conclusions: The population of double-degenerate type Ia SN progenitors that are SSSs is at least an order of magnitude smaller than the population of single-degenerate progenitors expected to be SSSs, and the supersoft X-ray behaviour of double-degenerate systems typically ceases long before the SN explosions.

astro-ph.HE↗

arXiv2013

The effect of common-envelope evolution on the visible population of post-common-envelope binaries

Context. An important ingredient in binary evolution is the common-envelope (CE) phase. Although this phase is believed to be responsible for the formation of many close binaries, the process is not well understood. Aims. We investigate the characteristics of the population of post-common-envelope binaries (PCEB). As the evolution of these binaries and their stellar components are relatively simple, this population can be directly used to constraint CE evolution. Methods. We use the binary population synthesis code SeBa to simulate the current-day population of PCEBs in the Galaxy. We incorporate the selection effects in our model that are inherent to the general PCEB population and that are specific to the SDSS survey, which enables a direct comparison for the first time between the synthetic and observed population of visible PCEBs. Results. We find that selection effects do not play a significant role on the period distribution of visible PCEBs. To explain the observed dearth of long-period systems, the α-CE efficiency of the main evolutionary channel must be low. In the main channel, the CE is initiated by a red giant as it fills its Roche lobe in a dynamically unstable way. Other evolutionary paths cannot be constrained more. Additionally our model reproduces well the observed space density, the fraction of visible PCEBs amongst white dwarf (WD)- main sequence (MS) binaries, and the WD mass versus MS mass distribution, but overestimates the fraction of PCEBs with helium WD companions.

astro-ph.SR↗