arXiv subjects

John J. Eldridge

Publications and source records attributed to John J. Eldridge.

At least 19 recordsLinked to original sources

arXiv2013

The death of massive stars - II. Observational constraints on the progenitors of type Ibc supernovae

The progenitors of many type II core-collapse supernovae have now been identified directly on pre-discovery imaging. Here we present an extensive search for the progenitors of type Ibc supernovae in all available pre-discovery imaging since 1998. There are 12 type Ibc supernovae with no detections of progenitors in either deep ground-based or Hubble Space Telescope archival imaging. The deepest absolute BVR magnitude limits are between -4 and -5. We compare these limits with the observed Wolf-Rayet population in the Large Magellanic Cloud and estimate a 16 per cent probability we have failed to detect such a progenitor by chance. Alternatively the progenitors evolve significantly before core-collapse or we have underestimated the extinction towards the progenitors. Reviewing the relative rates and ejecta mass estimates from lightcurve modelling of Ibc SNe, we find both incompatible with Wolf-Rayet stars with initial masses >25Msun being the only progenitors. We present binary evolution models that fit these observational constraints. Stars in binaries with initial masses <20Msun lose their hydrogen envelopes in binary interactions to become low mass helium stars. They retain a low mass hydrogen envelope until approximately 10,000 years before core-collapse; hence it is not surprising that galactic analogues have been difficult to identify.

astro-ph.SR↗

arXiv2013

Hα to FUV ratios in resolved star forming region populations of nearby spiral galaxies

We present a new study of Hα/FUV flux ratios of star forming regions within a sample of nearby spiral galaxies. We search for evidence of the existence of a cluster mass dependent truncation in the underlying stellar initial mass function (IMF). We use an automated approach to identification of extended objects based on the SExtractor algorithm to catalogue resolved Hii regions within a set of nearby spiral galaxies. Corrections due to dust attenuation effects are applied to avoid artificially boosted Hα/FUV values. We use the BPASS stellar population synthesis code of Eldridge & Stanway (2009) to create a benchmark population of star forming regions to act as a reference for our observations. Based on those models, we identify a zone of parameter space populated by regions that cannot be obtained with a cluster mass dependent truncation in the stellar IMF imposed. We find that the investigated galaxies display small subpopulations of star forming regions falling within our zone of interest, which appears to be inconsistent with imposing an IMF truncation at maximum stellar mass dependent on the total cluster mass. The presence of those regions is expected in models using both stochastic and sorted sampling of the full extent of the stellar IMF. This result persists after taking dust attenuation effects into account. We highlight the significance of stochastic effects in environments with low star formation activity and in studies describing systems associated with small physical scales.

astro-ph.CO↗

arXiv2012

The structure and evolution of quasi-stars

The existence of bright quasars at high redshifts implies that supermassive black holes were able to form in the early Universe. Though a number of mechanisms to achieve this have been proposed, none yet stands out. A recent suggestion is the formation of quasi-stars, initially stellar-mass black holes accreting from hydrostatic giant-like envelopes of gas, formed from the monolithic collapse of pre-galactic gas clouds. In this work, we modify the Cambridge STARS stellar evolution package to construct detailed models of the evolution of these objects. We find that, in all of our models, the black hole inside the envelope is able to reach slightly more than one-tenth of the total mass of the system before hydrostatic equilibrium breaks down. This breakdown occurs after a few million years of evolution. We show that the mechanism which causes the hydrostatic evolution to end is present in polytropic models. We also show that the solutions are highly sensitive to the size of the inner boundary radius and that no physical solutions exist if the inner boundary is chosen to be less than about 0.3 of the Bondi radius.

astro-ph.HE↗

arXiv2012

Stochasticity, a variable stellar upper-mass limit, binaries and star-formation rate indicators

Using our Binary Population And Spectral Synthesis (BPASS) code we explore the effects on star-formation rate indicators of stochastically sampling the stellar initial mass function, adding a cluster mass dependent stellar upper-mass limit and including binary stars. We create synthetic spectra of young clusters and star-forming galaxies and compare these to observations of H(alpha) emission from isolated clusters and the relation between H(alpha) and FUV emission from nearby galaxies. We find that observations of clusters tend to favour a purely stochastic sampling of the initial mass function for clusters less than 100M(Sun), rather than the maximum stellar mass being dependant on the total cluster mass. It is more difficult to determine whether the same is true for more massive clusters. We also find that binary stars blur some of the observational differences that occur when a cluster-mass dependent stellar upper-mass limit is imposed when filling the IMF. The effect is greatest when modelling the observed H(alpha) and FUV star-formation rate ratios in galaxies. This is because mass transfer and merging of stars owing to binary evolution creates more massive stars and stars that have greater mass than the initial maximum imposed on the stellar population.

astro-ph.SR↗

arXiv2011

Circumstellar dust as a solution to the red supergiant supernova progenitor problem

We investigate the red supergiant problem: the apparent dearth of Type IIP supernova progenitors with masses between 16 and 30 M_sun. Although red supergiants with masses in this range have been observed, none have been identified as progenitors in pre-explosion images. We show that by failing to take into account the additional extinction resulting from the dust produced in the red supergiant winds, we risk underestimating the luminosity of the most massive red supergiants at the end of their lives. We estimate the initial masses of all Type IIP progenitors for which observations exist and analyse the resulting population. We find that the most likely maximum mass for a Type IIP progenitor is 21^{+2}_{-1} M_sun. This is in closer agreement with the limit predicted from single star evolution models.

astro-ph.SR↗

arXiv2011

Close to the Dredge: Precise X-ray C and N Abundances in lambda Andromeda and its Precocious RGB Mixing Problem

Chandra LETG+HRC-S and XMM-Newton RGS spectra of H-like C and N formed in the corona of the lambda And primary star, a mildly metal-poor G8 III-IV first ascent giant that completed dredge-up ~50 Myr ago, have been used to make a precise measurement of its surface C/N ratio. We obtain the formal result [C/N]=0.03+/-0.07, which is typical of old disk giants and in agreement with standard dredge-up theory for stars of about 1 M_sun or lower. In contrast, these stars as a group, including lambda And, have 12C/13C ~< 20, which is much lower than standard model predictions. We show that the abundances of the old disk giants are consistent with models including thermohaline mixing that begins at the red giant branch luminosity function "bump". Instead, lambda And indicates that the 12C/13C anomaly can be present immediately following dredge-up, contrary to current models of extra mixing on the red giant branch.

astro-ph.SR↗

arXiv2011

Towards a unified model of stellar rotation

The effects of rapid rotation on stellar evolution can be profound. We are now beginning to gather enough data to allow a realistic comparison between different physical models. Two key tests for any theory of stellar rotation are first whether it can match observations of the enrichment of nitrogen, and potentially other elements, in clusters containing rapid rotators and secondly whether it can reproduce the observed broadening of the main sequence in the Hertzsprung-Russel diagram. Models of stellar rotation have been steadily increasing in number and complexity over the past two decades but the lack of data makes it difficult to determine whether such additions actually give a closer reflection of reality. One of the most poorly explored features of stellar rotation models is the treatment of angular momentum transport within convective zones. If we treat the core as having uniform specific angular momentum the angular momentum distribution in the star, for a given surface rotation, is dramatically different from what it is when we assume the star rotates as a solid body. The uniform specific angular momentum also generates strong shears which can drive additional transport of chemical elements close to the boundary of a convection zone. A comparison of different models and their reproduction of observable properties with otherwise identical input physics is essential to properly distinguish between them. We compare detailed grids of stellar evolution tracks of intermediate and high-mass stars produced using several models for rotation generated with our new stellar rotation code.

astro-ph.SR↗

arXiv2011

The effect of stellar evolution uncertainties on the rest-frame ultraviolet stellar lines of CIV and HeII in high-redshift Lyman-break galaxies

Young, massive stars dominate the rest-frame ultraviolet spectra of star-forming galaxies. At high redshifts (z>2), these rest-UV features are shifted into the observed-frame optical and a combination of gravitational lensing, deep spectroscopy and spectral stacking analysis allows the stellar population characteristics of these sources to be investigated. We use our stellar population synthesis code BPASS to fit two strong rest-UV spectral features in published Lyman-break galaxy spectra, taking into account the effects of binary evolution on the stellar spectrum. In particular, we consider the effects of quasi-homogeneous evolution (arising from the rotational mixing of rapidly-rotating stars), metallicity and the relative abundance of carbon and oxygen on the observed strengths of HeII (1640 Angstroms) and CIV (1548,1551 Angstroms) spectral lines. We find that Lyman-break galaxy spectra at z=2-3 are best fit with moderately sub-solar metallicities, and with a depleted carbon-to-oxygen ratio. We also find that the spectra of the lowest metallicity sources are best fit with model spectra in which the HeII emission line is boosted by the inclusion of the effect of massive stars being spun-up during binary mass-transfer so these rapidly-rotating stars experiencing quasi-homogeneous evolution.

astro-ph.CO↗

arXiv2011

The Great Escape: How Exoplanets and Smaller Bodies Desert Dying Stars

Mounting discoveries of extrasolar planets orbiting post-main sequence stars motivate studies aimed at understanding the fate of these planets. In the traditional "adiabatic" approximation, a secondary's eccentricity remains constant during stellar mass loss. Here, we remove this approximation, investigate the full two-body point-mass problem with isotropic mass loss, and illustrate the resulting dynamical evolution. The magnitude and duration of a star's mass loss combined with a secondary's initial orbital characteristics might provoke ejection, modest eccentricity pumping, or even circularisation of the orbit. We conclude that Oort clouds and wide-separation planets may be dynamically ejected from 1-7 Solar-mass parent stars during AGB evolution. The vast majority of planetary material which survives a supernova from a 7-20 Solar-mass progenitor will be dynamically ejected from the system, placing limits on the existence of first-generation pulsar planets. Planets around >20 Solar-mass black hole progenitors may easily survive or readily be ejected depending on the core collapse and superwind models applied. Material ejected during stellar evolution might contribute significantly to the free-floating planetary population.

astro-ph.EP↗

arXiv2011

Runaway stars as progenitors of supernovae and gamma-ray bursts

When a core collapse supernova occurs in a binary system, the surviving star as well as the compact remnant emerging from the SN, may reach a substantial space velocity. With binary population synthesis modelling at solar and one fifth of solar metallicity, we predict the velocities of such runaway stars or binaries. We compile predictions for runaway OB stars, red supergiants and Wolf-Rayet stars. For those stars or binaries which undergo a second stellar explosion we compute their further evolution and the distance travelled until a Type II or Type Ibc SN or a long or short gamma-ray burst occurs. We find our predicted population of OB runaway stars broadly matches the observed population of stars but, to match the fastest observed WR runaway stars, we require that black holes receive an asymmetric kick upon formation. We find that at solar metallicity Type Ic SN progenitors travel shorter distances than the progenitors of other SN types because they are typically more massive and thus have shorter lifetimes. Those of Type IIP SN can fly farthest about 48 pc on average at solar metallicity. In considering the consequences of assuming that the progenitors of long GRBs are spun-up secondary stars that experience quasi-homogeneous evolution, we find that such evolution has a dramatic effect on the population of runaway WR stars and that some 30 per cent of GRBs could occur a hundred parsecs or more from their initial positions. We also consider mergers of double compact object binaries consisting of neutron stars and/or black holes. We find the most common type of visible mergers are neutron star--black hole mergers that are roughly ten times more common than neutron star--neutron star mergers. We also find that there may be a population of low-velocity neutron stars that are ejected from a binary rather than by their own natal kick.

astro-ph.SR↗

arXiv2010

The red supergiants & Wolf-Rayet stars of NGC 604

We study the post-main sequence stars in NGC 604, the most luminous HII region in M33. Previously, a number of Wolf-Rayet (WR) stars and one red supergiant (RSG) have been discovered. Based on broadband photometry of the region, we present evidence that is consistent with the presence of this RSG and with three more RSG candidates. Using SED fitting based on HST UVIJHK photometry we estimate the ages of the WR stars and RSGs finding that the two populations are from distinct formation episodes with ages 3.2$\pm%1.0Myrs and 12.4$\pm$2.1Myrs, respectively. The RSGs have greater extinctions towards their line of sight than the WR stars consistent with the RSGs producing large amount of dust. Using the WR and RSG populations and similar SED fits to the most massive O stars we estimate that the total stellar mass is (3.8 $\pm$ 0.6) x 10^5Msun. We find a large discrepancy between the expected Hα flux from such a massive cluster and that one observed. This suggests that 49 (+16,-19) percent of the ionizing photons produced by massive stars in NGC 604 is leaking from this HII region. We also suggest that the implications of an old RSG population mean that if NGC 604 was more distant and only observed in the infrared (IR) it would be difficult to study the youngest burst of star formation due to the contamination of RSGs.

astro-ph.SR↗

arXiv2009

A new age determination Gamma^2 Velorum from binary stellar evolution models

We derive a new age for the Gamma^2 Velorum binary by comparing recent observations to our set of binary models. We find that it is very unlikely the stars have not interacted, which implies that previous estimates of the age from single-star models of 3.5+/-0.4 Myrs are incorrect. We prefer an older age of 5.5+/-1 Myrs that agrees with the age of other lower mass stars in the Vela OB association. We also find that our favoured binary model shows the components of the binary have interacted in a Case B, post main-sequence, mass-transfer event. During the mass-transfer event, the envelopes of both components where radiative and therefore the damping of tidal forces are relatively weak. This explains why the binary is still eccentric after mass-transfer.

astro-ph.SR↗

arXiv2009

Spectral synthesis including massive binaries

We have constructed a new code to produce synthetic spectra of stellar populations that includes massive binaries. We have tested this code against the broadband colours of unresolved young massive stellar clusters in nearby galaxies, the equivalent widths of the Red and Blue Wolf-Rayet bumps in star-forming SDSS galaxies and the UV and optical spectra of the star forming regions Tol-A and B in NGC5398. In each case we find a good agreement between our models and observations. We find that in general binary populations are bluer and have fewer red supergiants, and thus significantly less flux in the I-band and at longer wavelengths, than single star populations. Also we find that Wolf-Rayet stars occur over a wider range of ages up to 10^7 years in a stellar population including binaries, increasing the UV flux and Wolf-Rayet spectral features at later times. In addition we find that nebula emission contributes significantly to these observed properties and must be considered when comparing stellar models with observations of unresolved stellar populations. We conclude that incorporation of massive stellar binaries can improve the agreement between observations and synthetic spectral synthesis codes, particularly for systems with young stellar populations.

astro-ph.CO↗

arXiv2009

Modelling the binary progenitor of supernova 1993J

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 thermohaline mixing.

astro-ph.SR↗

arXiv2007

The birth place of the type Ic Supernova 2007gr

We report our attempts to locate the progenitor of the peculiar type Ic SN 2007gr in HST pre-explosion images of the host galaxy, NGC 1058. Aligning adaptive optics Altair/NIRI imaging of SN 2007gr from the Gemini (North) Telescope with the pre-explosion HST WFPC2 images, we identify the SN position on the HST frames with an accuracy of 20 mas. Although nothing is detected at the SN position we show that it lies on the edge of a bright source, 134+/-23 mas (6.9 pc) from its nominal centre. Based on its luminosity we suggest that this object is possibly an unresolved, compact and coeval cluster and that the SN progenitor was a cluster member, although we note that model profile fitting favours a single bright star. We find two solutions for the age of this assumed cluster; 7-/+0.5 Myrs and 20-30 Myrs, with turn-off masses of 28+/-4 Msun and 12-9 Msun respectively. Pre-explosion ground-based K-band images marginally favour the younger cluster age/higher turn-off mass. Assuming the SN progenitor was a cluster member, the turn-off mass provides the best estimate for its initial mass. More detailed observations, after the SN has faded, should determine if the progenitor was indeed part of a cluster, and if so allow an age estimate to within ~2 Myrs thereby favouring either a high mass single star or lower mass interacting binary progenitor.

astro-ph↗

arXiv2007

The effect of massive binaries on stellar populations and supernova progenitors

We compare our latest single and binary stellar model results from the Cambridge STARS code to several sets of observations. We examine four stellar population ratios, the number of blue to red supergiants, the number of Wolf-Rayet stars to O supergiants, the number of red supergiants to Wolf-Rayet stars and the relative number of Wolf-Rayet subtypes, WC to WN stars. These four ratios provide a quantitative measure of nuclear burning lifetimes and the importance of mass loss during various stages of the stars' lifetimes. In addition we compare our models to the relative rate of type Ib/c to type II supernovae to measure the amount of mass lost over the entire lives of all stars. We find reasonable agreement between the observationally inferred values and our predicted values by mixing single and binary star populations. However there is evidence that extra mass loss is required to improve the agreement further, to reduce the number of red supergiants and increase the number of Wolf-Rayet stars.

astro-ph↗

arXiv2006

Implications of the metallicity dependence of Wolf-Rayet winds

Aims: Recent theoretical predictions for the winds of Wolf-Rayet stars indicate that their mass-loss rates scale with the initial stellar metallicity in the local Universe.We aim to investigate how this predicted dependence affects the models of Wolf-Rayet stars and their progeny in different chemical environments. Methods: We compute models of stellar structure and evolution for Wolf-Rayet stars for different initial metallicities, and investigate how the scaling of the Wolf-Rayet mass-loss rates affects the final masses, the lifetimes of the WN and WC subtypes, and how the ratio of the two populations vary with metallicity. Results: We find significant effects of metallicity dependent mass-loss rates for Wolf-Rayet stars. For models that include the scaling of the mass-loss rate with initial metallicity, all WR stars become neutron stars rather than black holes at twice the solar metallicity; at lower $Z$, black holes have larger masses. We also show that our models that include the mass-loss metallicity scaling closely reproduce the observed decrease of the relative population of WC over WN stars at low metallicities.

astro-ph↗

arXiv2005

Progenitors of Core-Collapse Supernovae

The progenitors of core-collapse supernovae are stars with an initial mass greater than about 8M(sun). Understanding the evolution of these stars is necessary to comprehend the evolution and differences between supernovae. We have constructed new and unique opacity tables to increase model accuracy during the latest stages of stellar evolution. We have investigated how initial mass, initial composition and mass loss affects the progenitors and their populations. There are many prescriptions for mass loss. Different research groups use their preferred rates. We have compared 12 different prescriptions and determined which provides the best fit to observations. We use our preferred mass-loss scheme to make suggestions as to the source of the differences between supernova types from our progenitor models. Binary evolution is considered in order to search for low luminosity SN progenitors and progenitor types not possible from single stars. Removal of the hydrogen envelope is more common and we find quite different hydrogen deficient SN progenitors. We discuss the implications of our binary models for ultra-luminous X-ray sources and gamma-ray bursts. We present an estimation of the mass distribution for black holes at various metallcities showing that massive black holes are not formed until very low metallicities. Finally we combine the single star and binary results to determine their relative populations and compare to observations. However it is not possible to draw many firm conclusions because of the uncertainty in observations to date.

astro-ph↗