اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدConstraining GRB progenitor models by probing Wolf-Rayet wind geometries in the Large Magellanic Cloud
109
0
0.0
(
0
)
تأليف
Jorick S. Vink
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
The favoured progenitors of long-duration gamma-ray bursts (GRBs) are rapidly rotating Wolf-Rayet (WR) stars. However, most Galactic WR stars are slow rotators, as stellar winds are thought to remove angular momentum. This poses a challenge to the collapsar model. Recent observations indicate that GRBs occur predominately in low metallicity (Z) environments, which may resolve the problem: lower Z leads to less mass loss, which may inhibit angular momentum removal, allowing WR stars to remain rotating rapidly until collapse. We wish to determine whether low Z WR stars rotate on average more rapidly than Galactic WR stars, and perform a Very Large Telescope (VLT) linear spectropolarimetry survey of WR stars in the low Z environment of the Large Magellanic Cloud (LMC) and compare our results with the Galactic sample. We find that only 2 out of 13 (i.e. 15%) of LMC WR stars show line polarization effects, compared to a similarly low fraction of ~15-20% for Galactic WR stars. The low incidence of line polarization effects in LMC WR stars suggests that the threshold metallicity where significant differences in WR rotational properties occur is below that of the LMC (Z ~ 0.5 Zsun), possibly constraining GRB progenitor channels to this upper metallicity.
قيم البحث
اقرأ أيضاً
Infrared photometry of the probable triple WC4(+O?)+O8I: Wolf-Rayet system HD 36402 (= BAT99-38) in the Large Magellanic Cloud (LMC) shows emission characteristic of heated dust. The dust emission is variable on a time-scale of years, with a period n
ear 4.7 yr, possibly associated with orbital motion of the O8 supergiant and the inner P ~ 3.03-d WC4+O binary. The phase of maximum dust emission is close to that of the X-ray minimum, consistent with both processes being tied to colliding wind effects in an elliptical binary orbit. It is evident that Wolf-Rayet dust formation occurs also in metal-poor environments.
Surveys of Wolf-Rayet (WR) stars in the Large Magellanic Cloud (LMC) have yielded a fairly complete catalog of 154 known stars. We have conducted a comprehensive, multiwavelength study of the interstellar/circumstellar environments of WR stars, using
the Magellanic Cloud Emission Line Survey (MCELS) images in the H$alpha$, [O III], and [S II] lines; Spitzer Space Telescope 8 and 24 $mu$m images; Blanco 4m Telescope H$alpha$ CCD images; and Australian Telescope Compact Array (ATCA) + Parkes Telescope H I data cube of the LMC. We have also examined whether the WR stars are in OB associations, classified the H II environments of WR stars, and used this information to qualitatively assess the WR stars evolutionary stages. The 30 Dor giant H II region has active star formation and hosts young massive clusters, thus we have made statistical analyses for 30 Dor and the rest of the LMC both separately and altogether. Due to the presence of massive young clusters, the WR population in 30 Dor is quite different from that from elsewhere in the LMC. We find small bubbles ($<$50 pc diameter) around $sim$12% of WR stars in the LMC, most of which are WN stars and not in OB associations. The scarcity of small WR bubbles is discussed. Spectroscopic analyses of abundances are needed to determine whether the small WR bubbles contain interstellar medium or circumstellar medium. Implications of the statistics of interstellar environments and OB associations around WR stars are discussed. Multiwavelength images of each LMC WR star are presented.
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 spectro
graph 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 5801-12 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.
Aims: Following our comprehensive studies of the WR stars in the Milky Way, we now present spectroscopic analyses of almost all known WN stars in the LMC. Methods: For the quantitative analysis of the wind-dominated emission-line spectra, we employ t
he Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to the observed spectral energy distribution and the available spectra (ultraviolet and optical), we obtain the physical properties of 107 stars. Results: We present the fundamental stellar and wind parameters for an almost complete sample of WN stars in the LMC. Among those stars that are putatively single, two different groups can be clearly distinguished. While 12% of our sample are more luminous than 10^6 Lsun and contain a significant amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate the luminosity range between log (L/Lsun) = 5.3...5.8. Conclusions: While the few extremely luminous stars (log (L/Lsun) > 6), if indeed single stars, descended directly from the main sequence at very high initial masses, the bulk of WN stars have gone through the red-supergiant phase. According to their luminosities in the range of log (L/Lsun) = 5.3...5.8, these stars originate from initial masses between 20 and 40 Msun. This mass range is similar to the one found in the Galaxy, i.e. the expected metallicity dependence of the evolution is not seen. Current stellar evolution tracks, even when accounting for rotationally induced mixing, still partly fail to reproduce the observed ranges of luminosities and initial masses. Moreover, stellar radii are generally larger and effective temperatures correspondingly lower than predicted from stellar evolution models, probably due to subphotospheric inflation.
Vigorous mass loss in the classical Wolf-Rayet (WR) phase is important for the late evolution and final fate of massive stars. We develop spherically symmetric time-dependent and steady-state hydrodynamical models of the radiation-driven wind outflow
s and associated mass loss from classical WR stars. The simulations are based on combining the opacities typically used in static stellar structure and evolution models with a simple parametrised form for the enhanced line-opacity expected within a supersonic outflow. Our simulations reveal high mass-loss rates initiated in deep and hot optically thick layers around Tapprox 200kK. The resulting velocity structure is non-monotonic and can be separated into three phases: i) an initial acceleration to supersonic speeds ii) stagnation and even deceleration, and iii) an outer region of rapid re-acceleration. The characteristic structures seen in converged steady-state simulations agree well with the outflow properties of our time-dependent models. By directly comparing our dynamic simulations to corresponding hydrostatic models, we demonstrate explicitly that the need to invoke extra energy transport in convectively inefficient regions of stellar structure and evolution models is merely an artefact of enforcing a hydrostatic outer boundary. Moreover, the dynamically inflated inner regions of our simulations provide a natural explanation for the often-found mismatch between predicted hydrostatic WR radii and those inferred from spectroscopy. Finally, we contrast our simulations with alternative recent WR wind models based on co-moving frame radiative transfer for computing the radiation force. Since CMF transfer currently cannot handle non-monotonic velocity fields, the characteristic deceleration regions found here are avoided in such simulations by invoking an ad-hoc very high degree of clumping.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
