ABSTRACT
Mid-infrared photometry of the Wolf-Rayet star HD 38030 in the Large Magellanic Cloud from the NEOWISE-R mission show it to have undergone a dust-formation episode in 2018 and the dust to ...have cooled in 2019–20. New spectroscopy with the MagE spectrograph on the Magellan I Baade Telescope in 2019 and 2020 show absorption lines attributable to a companion of type near O9.7III-IV. We found a significant shift in the radial velocity of the C iv λλ5801–12 blend compared with the RVs measured in 1984 and 1993. The results combine to suggest that HD 38030 is a colliding-wind binary having short-lived dust formation episodes, like the Galactic systems WR 140 and WR 19, but at intervals in excess of 20 yr.
Abstract
We summarize the results of our 4 yr survey searching for Wolf-Rayet (WR) stars in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud. Over the course of this survey we have ...discovered 15 new WR stars and 12 Of-type stars. In this last year we discovered two rare Of-type stars: an O6.5f?p and an O6nfp, in addition to the two new Of?p stars discovered in our first year and the three Onfp stars discovered in our second and third years. However, even more exciting was our discovery of a new type of WR star, ones we are calling WN3/O3s owing to their spectroscopic signatures. We describe the completeness limits of our survey and demonstrate that we are sensitive to weak-lined WRs several magnitudes fainter than any we have discovered, arguing that there is not a population of fainter WRs waiting to be discovered. We discuss the nature of the WN3/O3s, summarizing the results of our extensive spectroscopy and modeling. We also examine the important claim made by others that the WN3/O3s are isolated compared to other massive stars. We find that if we use a more complete sample of reference massive stars, the WN3/O3s show the same spatial distribution as other early WNs, consistent with a common origin. Finally, we use this opportunity to present the “Fifth Catalog of LMC Wolf-Rayet Stars,” which includes revised coordinates and updated spectral types for all 154 known LMC WRs.
Abstract
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
λλ
...3811,34 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
12
C+
4
He →
16
O 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.
The binary fraction of unevolved massive stars is thought to be 70%-100% but there are few observational constraints on the binary fraction of the evolved version of a subset of these stars, the red ...supergiants (RSGs). Here we identify a complete sample of RSGs in the Large Magellanic Cloud (LMC) using new spectroscopic observations and archival UV, IR, and broadband optical photometry. We find 4090 RSGs with , with 1820 of them having , which we believe is our completeness limit. We additionally spectroscopically confirmed 38 new RSG + B-star binaries in the LMC, bringing the total known up to 55. We then estimated the binary fraction using a k-nearest neighbors algorithm that classifies stars as single or binary based on photometry with a spectroscopic sample as a training set. We take into account observational biases such as line-of-sight stars and binaries in eclipse while also calculating model-dependent corrections for RSGs with companions that our observations were not designed to detect. Based on our data, we find an initial result of for RSGs with O- or B-type companions. Using the Binary Population and Spectral Synthesis models to correct for unobserved systems, this corresponds to a total RSG binary fraction of . This number is in broad agreement with what we would expect given an initial OB binary distribution of 70%, a predicted merger fraction of 20%-30%, and a binary interaction fraction of 40%-50%.
Abstract
The WN3/O3 Wolf–Rayet (WR) stars were discovered as part of our survey for WRs in the Magellanic Clouds. The WN3/O3s show the emission lines of a high-excitation WN star and the absorption ...lines of a hot O-type star, but our prior work has shown that the absorption spectrum is intrinsic to the WR star. Their place in the evolution of massive stars remains unclear. Here we investigate the possibility that they are the products of binary evolution. Although these are not WN3+O3 V binaries, they could still harbor unseen companions. To address this possibility, we have conducted a multiyear radial velocity study of six of the nine known WN3/O3s. Our study finds no evidence of statistically significant radial velocity variations, and allows us to set stringent upper limits on the mass of any hypothetical companion star: for probable orbital inclinations, any companion with a period less than 100 days must have a mass <2
M
⊙
. For periods less than 10 days, any companion would have to have a mass <1
M
⊙
. We argue that scenarios where any such companion is a compact object are unlikely. The absorption lines indicate a normal projected rotational velocity, making it unlikely that these stars evolved with the aid of a companion star that has since merged. The modest rotation also suggests that these stars are not the result of homogenous evolution. Thus it is likely that these stars are a normal but short-lived stage in the evolution of massive stars.
The percentage of massive main-sequence OB stars in binary systems is thought to be as high as 100%. However, very few Galactic binary red supergiants (RSGs) have been identified, despite the fact ...that these stars are the evolved descendants of OB stars. As shown in our recent paper, binary RSGs will likely have B-type companions, as dictated by stellar evolution considerations. Such a system will have a very unique photometric signature due to the shape of the spectral energy distribution. Using photometric cutoffs, it should therefore be possible to detect candidate RSG+B star binary systems. Here we present our spectroscopic follow-up observations of such candidates. Out of our initial list of 280 candidates in M31 and M33, we observed 149 and confirmed 63 as newly discovered RSG+B star binary systems. Additional spectra of four candidate systems in the Small Magellanic Cloud confirmed all of them as new RSG+B star binaries including the first known RSG+Be star system. By fitting BSTAR06 and MARCS model atmospheres to the newly obtained spectra, we place estimates on the temperatures and subtypes of both the B stars and RSGs. Overall, we have found 87 new RSG+B star binary systems in M31, M33 and the Small and Large Magellanic Clouds. Our future studies are aimed at determining the binary fraction of RSGs.
We present reverberation mapping (RM) results for 17 high-redshift, high-luminosity quasars with good-quality R-band and emission-line light curves. We are able to measure statistically significant ...lags for Ly (11 objects), Si iv (5 objects), C iv (11 objects), and C iii (2 objects). Using our results and previous lag determinations taken from the literature, we present an updated C iv radius-luminosity relation and provide for the first time radius-luminosity relations for Ly , Si iv, and C iii. While in all cases the slopes of the correlations are statistically significant, the zero points are poorly constrained because of the lack of data at the low-luminosity end. We find that the emissivity-weighted distances from the central source of the Ly , Si iv, and C iii line-emitting regions are all similar, which corresponds to about half that of the Hβ region. We also find that 3/17 of our sources show an unexpected behavior in some emission lines, two in the Ly light curve and one in the Si iv light curve, in that they do not seem to follow the variability of the UV continuum. Finally, we compute RM black hole (BH) masses for those quasars with highly significant lag measurements and compare them with C iv single-epoch (SE) mass determinations. We find that the RM-based BH mass determinations seem smaller than those found using SE calibrations.
Since 2016 October, the active galaxy PKS 2247-131 has undergone a γ-ray outburst, which we studied using data obtained with the Fermi Gamma-ray Space Telescope. The emission arises from a ...relativistic jet in PKS 2247-131, as an optical spectrum only shows a few weak absorption lines, typical of the BL Lacertae sub-class of the blazar class of active galactic nuclei. Here we report a ≃34.5 day quasi-periodic oscillation (QPO) in the emission after the initial flux peak of the outburst. Compared to one-year time-scale QPOs, previously identified in blazars in Fermi energies, PKS 2247-131 exhibits the first clear case of a relatively short, month-like oscillation. We show that this QPO can be explained in terms of a helical structure in the jet, where the viewing angle to the dominant emission region in the jet undergoes periodic changes. The time scale of the QPO suggests the presence of binary supermassive black holes in PKS 2247-131.
Abstract
The diversity of Type II supernovae (SNe II) is thought to be driven mainly by differences in their progenitor’s hydrogen-rich (H-rich) envelope mass, with SNe IIP having long plateaus (∼100 ...days) and the most massive H-rich envelopes. However, it is an ongoing mystery why SNe II with short plateaus (tens of days) are rarely seen. Here, we present optical/near-infrared photometric and spectroscopic observations of luminous Type II short-plateau SNe 2006Y, 2006ai, and 2016egz. Their plateaus of about 50–70 days and luminous optical peaks (≲−18.4 mag) indicate significant pre-explosion mass loss resulting in partially stripped H-rich envelopes and early circumstellar material (CSM) interaction. We compute a large grid of
MESA
+
STELLA
single-star progenitor and light-curve models with various progenitor zero-age main-sequence (ZAMS) masses, mass-loss efficiencies, explosion energies,
56
Ni masses, and CSM densities. Our model grid shows a continuous population of SNe IIP–IIL–IIb-like light-curve morphology in descending order of H-rich envelope mass. With large
56
Ni masses (≳0.05
M
⊙
), short-plateau SNe II lie in a confined parameter space as a transitional class between SNe IIL and IIb. For SNe 2006Y, 2006ai, and 2016egz, our findings suggest high-mass red supergiant (RSG) progenitors (
M
ZAMS
≃ 18–22
M
⊙
) with small H-rich envelope masses (
) that have experienced enhanced mass loss (
) for the last few decades before the explosion. If high-mass RSGs result in rare short-plateau SNe II, then these events might ease some of the apparent underrepresentation of higher-luminosity RSGs in observed SN II progenitor samples.
ABSTRACT
WR 21a was known as a massive spectroscopic binary composed of an O2.5 If*/WN6ha primary and an O3 V((f*))z secondary. Although a minimum value, the mass estimated for the primary placed it ...as one of the most massive stars found in our Galaxy. We report the discovery of photometric variations in the time series observations carried out by the Transiting Exoplanet Survey Satellite (TESS). These light variations are interpreted as formed by two main components: a sharp partial eclipse of the O3 secondary by the O2.5/WN6 star, and tidally excited oscillations. Based on the light minima, a new ephemeris for the system is calculated. The system configuration is detached and the observed eclipse corresponds to the periastron passage. During the eclipse, the light curve shape suggests the presence of the heartbeat effect. The frequencies derived for the tidally excited oscillations are harmonics of the orbital period. Combining new and previously published radial velocity measurements, a new spectroscopic orbital solution is also obtained. Using the phoebe code we model the TESS light curve and determine stellar radii of RO2.5/WN6 = 23.4 R⊙ and RO3 = 14.3 R⊙ and an orbital inclination i = 62${_{.}^{\circ}}$2 ± 0${_{.}^{\circ}}$9. The latter combined with the spectroscopic minimum masses lead to absolute masses of MO2.5/WN6 = 93.2 M⊙ and MO3 = 52.9 M⊙, which establishes WR 21a as belonging to the rare group of the very massive stars.
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