On the origin of variable structures in the winds of hot luminous stars

Examination of the temporal variability properties of several strong optical recombination lines in a large sample of Galactic Wolf-Rayet (WR) stars reveals possible trends, especially in the more homogeneous WC than the diverse WN subtypes, of increasing wind variability with cooler subtypes. This could imply that a serious contender for the driver of the variations is stochastic, magnetic subsurface convection associated with the 170 kK partial-ionization zone of iron, which should occupy a deeper and larger zone of greater mass in cooler WR subtypes. This empirical evidence suggests that the heretofore proposed ubiquitous driver of wind variability, radiative instabilities, may not be the only mechanism playing a role in the stochastic multiple small-scaled structures seen in the winds of hot luminous stars. In addition to small-scale stochastic behaviour, subsurface convection guided by a global magnetic field with localized emerging loops may also be at the origin of the large-scale corotating interaction regions as seen frequently in O stars and occasionally in the winds of their descendant WR stars.Comment: 8 pages, 2 figures and 2 tables. Monthly Notices of the Royal Astronomical Society 201

First detection of phase-dependent colliding wind X-ray emission outside the Milky Way

After having reported the detection of X-rays emitted by the peculiar system HD5980, we assess here the origin of this high-energy emission from additional X-ray observations obtained with XMM-Newton. This research provides the first detection of apparently periodic X-ray emission from hot gas produced by the collision of winds in an evolved massive binary outside the Milky Way. It also provides the first X-ray monitoring of a Luminous Blue Variable only years after its eruption and shows that the dominant source of the X-rays is not associated with the ejecta.Comment: 13 pages, 3 figures and 1 table, accepted for publication in ApJ (letters

Slow expansion of the shell of the recurrent nova T Pyxidis and detection of a faint extended envelope

Deep CCD images of the recurrent nova T Pyx have revealed a faint, extended Hα + [N ii] halo twice as large as the previously detected shell. An [O iii] image of T Pyx shows a smooth, small shell. Comparison of 1980 and 1985 images of the Hα + [N ii] shell show an expansion of less than 10%. If the bright, inner shell is due to the 1966 eruption, it should have expanded ~36% from 1980 to 1985 (assuming uniform shell expansion). We rule out the possibility of the T Pyx shell being associated with a planetary nebula-type ejection for two reasons: the shell mass is less than 10^(-4) M_⊙, and the shell expansion velocity is ~350 km s^(-1). This expansion velocity is much slower than the 850 km s^(-1) and 2000 km s^(-1) velocities reported by Catchpole (1969) during the 1966 outburst. If the 10" diameter shell is from the 1966 outburst, then the ejecta have given up most of their bulk kinetic energy by interaction with circumstellar matter or significant amounts of (now visible) low-velocity material were ejected during the last outburst, or both. The lack of strong [O i] λ6300 and [S ii] λλ6717,34 emission lines argues against much shock interaction at the present era, and, indirectly, for the 1944 identification of the 10" shell, while thermonuclear runaway nova models support the multiple-velocity idea. A point-spread function subtracted from an Hα + [N ii] image of T Pyx has revealed a 2" radius ring around the central star. This may be the ejecta from the 1966 eruption. The photoionized shell gas implies that the central star should be UV-bright. High resolution Hubble Space Telescope imaging observations of the next T Pyx eruption might yield early detection of light echoes

Slow expansion of the shell of the recurrent nova T Pyxidis and detection of a faint extended envelope

Deep CCD images of the recurrent nova T Pyx have revealed a faint, extended Hα + [N ii] halo twice as large as the previously detected shell. An [O iii] image of T Pyx shows a smooth, small shell. Comparison of 1980 and 1985 images of the Hα + [N ii] shell show an expansion of less than 10%. If the bright, inner shell is due to the 1966 eruption, it should have expanded ~36% from 1980 to 1985 (assuming uniform shell expansion). We rule out the possibility of the T Pyx shell being associated with a planetary nebula-type ejection for two reasons: the shell mass is less than 10^(-4) M_⊙, and the shell expansion velocity is ~350 km s^(-1). This expansion velocity is much slower than the 850 km s^(-1) and 2000 km s^(-1) velocities reported by Catchpole (1969) during the 1966 outburst. If the 10" diameter shell is from the 1966 outburst, then the ejecta have given up most of their bulk kinetic energy by interaction with circumstellar matter or significant amounts of (now visible) low-velocity material were ejected during the last outburst, or both. The lack of strong [O i] λ6300 and [S ii] λλ6717,34 emission lines argues against much shock interaction at the present era, and, indirectly, for the 1944 identification of the 10" shell, while thermonuclear runaway nova models support the multiple-velocity idea. A point-spread function subtracted from an Hα + [N ii] image of T Pyx has revealed a 2" radius ring around the central star. This may be the ejecta from the 1966 eruption. The photoionized shell gas implies that the central star should be UV-bright. High resolution Hubble Space Telescope imaging observations of the next T Pyx eruption might yield early detection of light echoes

Resolving Decades of Periodic Spirals from the Wolf-Rayet Dust Factory WR 112

WR 112 is a dust-forming carbon-rich Wolf-Rayet (WC) binary with a dusty circumstellar nebula that exhibits a complex asymmetric morphology, which traces the orbital motion and dust formation in the colliding winds of the central binary. Unraveling the complicated circumstellar dust emission around WR 112 therefore provides an opportunity to understand the dust formation process in colliding-wind WC binaries. In this work, we present a multi-epoch analysis of the circumstellar dust around WR 112 using seven high spatial resolution (FWHM $\sim0.3-0.4''$) N-band ($\lambda \sim12$ $\mu$m) imaging observations spanning almost 20 years and includes newly obtained images from Subaru/COMICS in Oct 2019. In contrast to previous interpretations of a face-on spiral morphology, we observe clear evidence of proper motion of the circumstellar dust around WR 112 consistent with a nearly edge-on spiral with a $\theta_s=55^\circ$ half-opening angle and a $\sim20$-yr period. The revised near edge-on geometry of WR 112 reconciles previous observations of highly variable non-thermal radio emission that was inconsistent with a face-on geometry. We estimate a revised distance to WR 112 of $d = 3.39^{+0.89}_{-0.84}$ kpc based on the observed dust expansion rate and a spectroscopically derived WC terminal wind velocity of $v_\infty= 1230\pm260$ km s$^{-1}$. With the newly derived WR 112 parameters we fit optically-thin dust spectral energy distribution models and determine a dust production rate of $\dot{M}_d=2.7^{+1.0}_{-1.3}\times10^{-6}$ M$_\odot$ yr$^{-1}$, which demonstrates that WR 112 is one of the most prolific dust-making WC systems known.Comment: 17 pages, 9 figures, 1 animated gif, accepted for publication in Ap