WC-type Wolf-Rayet Stars Research Articles

The Physical Parameters of Four WC-type Wolf–Rayet Stars in the Large Magellanic Cloud: Evidence of Evolution*

Abstract 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.

Variable dust emission by WC type Wolf–Rayet stars observed in the NEOWISE-R survey

ABSTRACT Photometry at 3.4 and 4.6 ${\mu m}$ of 128 Population I WC type Wolf–Rayet stars in the Galaxy and 12 in the Large Magellanic Cloud (LMC) observed in the WISE NEOWISE-R survey was searched for evidence of circumstellar dust emission and its variation. Infrared spectral energy distributions (SEDs) were assembled, making use of archival r, i, Z, and Y photometry to determine reddening and stellar wind levels for the WC stars found in recent IR surveys and lacking optical photometry. From their SEDs, 10 apparently non-variable stars were newly identified as dust makers, including three, WR 102-22, WR 110-10, and WR 124-10, having subtype earlier than WC8–9, the first such stars to show this phenomenon. The 11 stars found to show variable dust emission include six new episodic dust makers, WR 47c, WR 75-11, WR 91-1, WR 122-14, and WR 125-1 in the Galaxy and HD 38030 in the LMC. Of previously known dust makers, NEOWISE-R photometry of WR 19 captured its rise to maximum in 2018 confirming the 10.1-yr period, that of WR 125 the beginning of a new episode of dust formation suggesting a period near 28.3 yr. while that of HD 36402 covered almost a whole period and forced revision of it to 5.1 yr.

THE EVOLUTION OF LATE-TIME OPTICAL EMISSION FROM SN 1979C

Optical spectra of the bright Type II-L supernova SN 1979C obtained in April 2008 with the 6.5 m Multiple Mirror Telescope are compared with archival late-time spectra to follow the evolution of its optical emission over the age range of 11–29 years. We estimate an Hα flux decrease of around 35% from 1993 to 2008 but noticeable increases in the strength of blueshifted emission of forbidden oxygen lines. While the maximum expansion of the broad ∼6700 km s−1 Hα emission appears largely unchanged from 1993, we find a significant narrowing of the double-peaked emission profiles in the [O i] λλ6300, 6364 and [O ii] λλ7319, 7330 lines. A comparison of late-time optical spectra of a few other Type II SNe which, like SN 1979C, exhibit bright late-time X-ray, optical, and radio emissions, suggests that blueshifted double-peaked oxygen emission profiles may be a common phenomenon. Finally, detection of a faint, broad emission bump centered around 5800 Å suggests the presence of WC-type Wolf–Rayet stars in the SN's host star cluster.

Pair creation supernovae at low and high redshift

Aims. Pair creation supernovae (PCSN) are thought to be produced from very massive low metallicity stars. The spectacularly bright SN 2006gy does show several signatures expected from PCSNe. Here, we investigate the metallicity threshold below which PCSN can form and estimate their occurrence rate. Methods. We perform stellar evolution calculations for stars of 150 Mand 250 Mof low metallicity (Z� / 5a ndZ� /20), and analyze their mass loss rates. Results. We find that the bifurcation between quasi-chemically homogeneous evolution for fast rotation and conventional evolu- tion for slower rotation, which has been found earlier for massive low metallicity stars, persists in the mass range considered here. Consequently, there are two separate PCSN progenitor types: (I) fast rotators produce PCSNe from very massive Wolf-Rayet stars, and (II) slower rotators that generate PCSNe in hydrogen-rich massive yellow hypergiants. Conclusions. We find that hydrogen-rich PCSNe could occur at metallicities as high as Z� /3, which - assuming standard IMFs are still valid to estimate their birth rates - results in a rate of about one PCSN per 1000 supernovae in the local universe, and one PCSN per 100 supernovae at a redshift of z = 5. PCSNe from WC-type Wolf-Rayet stars are restricted to much lower metallicity.

Dust formation around WC stars

The presence of heated circumstellar dust around WC type Wolf-Rayet stars is reviewed in terms of the episodic or persistent condensation of carbon grains in their stellar winds. At present, 19 WC stars are known to form dust persistently and 7 episodically. Infrared spectroscopy suggests that the grains are amorphous rather than graphitic carbon. The properties of the stars which make dust episodically are reviewed in terms of processes in colliding winds in WR binary systems and the possible applicability of these processes to the stars which make dust persistently in examined.

Dust formation in hot stellar winds

WC-type Wolf-Rayet stars show episodic or persistent infrared emission attributed to amorphous carbon grains forming in their winds. The process of dust formation in the hydrogen-poor environments characteristic of WC winds is reviewed and compared to the chemical pathway to soot particle production in AGB stars. Emphasis is put on the formation of the dust precursors, and the physical conditions necessary to nucleate them in WC winds. In particular, it is concluded that dust formation around WC stars occurs in a dense, largely neutral, circumstellar disk.