We present a spectral analysis of four Large Magellanic Cloud (LMC) WC-type Wolf–Rayet (WR) stars (BAT99-8, BAT99-9, BAT99-11, and BAT99-52) to shed light on two evolutionary questions surrounding massive stars. The first is: are WO-type WR stars more oxygen enriched than WC-type stars, indicating further chemical evolution, or are the strong high-excitation oxygen lines in WO-type stars an indication of higher temperatures. This study will act as a baseline for answering the question of where WO-type stars fall in WR evolution. Each star’s spectrum, extending from 1100 to 25000 Å, was modeled using cmfgen to determine the star’s physical properties such as luminosity, mass-loss rate, and chemical abundances. The oxygen abundance is a key evolutionary diagnostic, and with higher resolution data and an improved stellar atmosphere code, we found the oxygen abundance to be up to a factor of 5 lower than that of previous studies. The second evolutionary question revolves around the formation of WR stars: do they evolve by themselves or is a close companion star necessary for their formation? Using our derived physical parameters, we compared our results to the Geneva single-star evolutionary models and the Binary Population and Spectral Synthesis (BPASS) binary evolutionary models. We found that both the Geneva solar-metallicity models and BPASS LMC-metallicity models are in agreement with the four WC-type stars, while the Geneva LMC-metallicity models are not. Therefore, these four WC4 stars could have been formed either via binary or single-star evolution.

We present the first ever X-ray data taken of an intermediate polar, FO Aqr, when in a low accretion state and during the subsequent recovery. The Swift and Chandra X-ray data taken during the low accretion state in July 2016 both show a softer spectrum when compared to archival data taken when FO Aqr was in a high state. The X-ray spectrum in the low state showed a significant increase in the ratio of the soft X-ray flux to the hard X-ray flux due to a change in the partial covering fraction of the white dwarf from > 85% to 70 +5 −8 % and a change in the hydrogen column density within the disc from 19 +1.2 −0.9 × 10 22 cm −2 to 1.3 +0.6 −0.3 × 10 22 cm −2 . XMM-Newton observations of FO Aqr during the subsequent recovery suggest that the system had not yet returned to its typical high state by November 2016, with the hydrogen column density within the disc found to be 15 +3.0 −2.0 cm −2 . The partial covering fraction varied in the recovery state between 85% and 95%. The spin period of the white dwarf in 2014 and 2015 has also been refined to 1254.3342(8) s. Finally, we find an apparent phase difference between the high state X-ray pulse and recovery X-ray pulse of 0.17, which may be related to a restructuring of the X-ray emitting regions within the system.

Publication date 2016-12-09Original citation Colin, L. et al (2016) ABSTRACT In 2016 May, the intermediate polar FOAqr was detected in a low state for the first time in its observational history. We report time-resolved photometry of the system during its initial recovery from this faint state. Our data, which includes high-speed photometry with cadences of just 2 s, show the existence of very strong periodicities at 22.5 and 11.26 minutes, equivalent to the spin-orbit beat frequency and twice its value, respectively. A pulse at the spin frequency is also present but at a much lower amplitude than is normally observed in the bright state. By comparing our power spectra with theoretical models, we infer that a substantial amount of accretion was streamfed during our observations, in contrast to the disk-fed accretion that dominates the bright state. In addition, we find that FOAqr's rate of recovery has been unusually slow in comparison to rates of recovery seen in other magnetic cataclysmic variables, with an e-folding time of 115±7 days. The recovery also shows irregular variations in the median brightness of as much as 0.2 mag over a 10-day span. Finally, we show that the arrival times of the spin pulses are dependent upon the system's overall brightness.

Are WO-type Wolf–Rayet (WR) stars in the final stage of massive star evolution before core-collapse? Although WC- and WO-type WRs have very similar spectra, WOs show a much stronger O vi λλ3811,34 emission-line feature. This has usually been interpreted to mean that WOs are more oxygen rich than WCs, and thus further evolved. However, previous studies have failed to model this line, leaving the relative abundances uncertain, and the relationship between the two types unresolved. To answer this fundamental question, we modeled six WCs and two WOs in the LMC using UV, optical, and NIR spectra with the radiative transfer code cmfgen in order to determine their physical properties. We find that WOs are not richer in oxygen; rather, the O vi feature is insensitive to the abundance. However, the WOs have a significantly higher carbon and lower helium content than the WCs, and hence are further evolved. A comparison of our results with single-star Geneva and binary BPASS evolutionary models show that, while many properties match, there is more carbon and less oxygen in the WOs than either set of evolutionary model predicts. This discrepancy may be due to the large uncertainty in the 12C+4He → 16O nuclear reaction rate; we show that if the Kunz et al. rate is decreased by a factor of 25%–50%, then there would be a good match with the observations. It would also help explain the LIGO/VIRGO detection of black holes whose masses are in the theoretical upper mass gap.

In the standard view of massive star evolution, luminous blue variables (LBVs) are transitional objects between the most massive O-type stars and Wolf-Rayet (WR) stars. With short lifetimes, these stars should all be found near one another. A recent study of LBVs in the Large Magellanic Cloud (LMC) found instead that LBVs are considerably more isolated than either O-type stars or WRs, with a distribution intermediate between that of the WRs and red supergiants (RSGs). A similar study, using a more restricted sample of LBVs, reached the opposite conclusion. Both studies relied upon the distance to the nearest spectroscopically identified O-type star to define the degree of isolation. However, our knowledge of the spectroscopic content of the LMC is quite spotty. Here we re-examine the issue using carefully defined photometric criteria to select the highest-mass unevolved stars ("bright blue stars," or BBSs), using spatially complete photometric catalogs of the LMC, M31, and M33. Our study finds that the LBVs are no more isolated than BBSs or WRs. This result holds no matter which sample of LBVs we employ. A statistical test shows that we can rule out the LBVs having the same distribution as the RSGs, which are about 2× more isolated. We demonstrate the robustness of our results using the second-closest neighbor. Furthermore, the majority of LBVs in the LMC are found in or near OB associations as are the BBS and WRs; the RSGs are not. We conclude that the spatial distribution of LBVs therefore is consistent with the standard picture of massive star evolution.

An analysis of the Large Magellanic Cloud (LMC) WC4 star BAT99-9 (HD 32125, FD 4, Brey 7, WS 3) shows that the star still contains photospheric nitrogen. Three N emission features (N V λλ1238, 1242, N IV λ1719, N IV λλ3479 − 3485) are unambiguously identified in the spectrum. CMFGEN models of the star yield a N/C ratio of 0.004 ± 0.002 (by number) and a C/He ratio of $0.15_{-0.05}^{+0.10}$. Due to the similarity of BAT99-9 to other WC4 stars, and the good fit achieved by CMFGEN to both the classic WC4 spectrum, and the N lines, we argue that the N lines are intrinsic to BAT99-9. An examination of a limited set of rotating models for single star evolution at LMC and Galactic metallicities shows that a model with a Galactic metallicity (z = 0.014) and a progenitor mass of around 50 M⊙ can have a N/C ratio similar to, or larger than, what we observe for a significant fraction of its lifetime. However, the LMC models (z = 0.006) are inconsistent with the observations. Both the single and binary BPASS models predict that many WC stars can have a N/C ratio similar to, or larger than, what we observe for a significant fraction of their lifetime. While the binary models cover a wider range of luminosities and provide a somewhat better match to BAT99-9, it is not currently possible to rule out BAT99-9 being formed through single star evolution, given the uncertainties in mass-loss rates, and the treatment of convection and mixing processes.

Most asteroids are somewhat elongated and have non-zero lightcurve amplitudes. Such asteroids can be detected in large-scale sky surveys even if their mean magnitudes are fainter than the stated sensitivity limits. We explore the detection of elongated asteroids under a set of idealized but useful approximations. We find that objects up to 1 mag fainter than a survey’s sensitivity limit are likely to be detected, and that the effect is most pronounced for asteroids with lightcurve amplitudes 0.1–0.4 mag. This imposes a bias on the derived size and shape distributions of the population that must be properly accounted for.

Most asteroids are somewhat elongated and have non-zero lightcurve amplitudes. Such asteroids can be detected in large-scale sky surveys even if their mean magnitudes are fainter than the stated sensitivity limits. We explore the detection of elongated asteroids under a set of idealized but useful approximations. We find that objects up to 1 magnitude fainter than a survey's sensitivity limit are likely to be detected, and that the effect is most pronounced for asteroids with lightcurve amplitudes 0.1-0.4 mag. This imposes a bias on the derived size and shape distributions of the population that must be properly accounted for.