Showing papers by "M. S. Oey published in 2023"
06 Mar 2023
TL;DR: In this paper , an analysis of the massive O star AzV 14 was conducted to determine its fundamental and stellar wind parameters, which were then used to investigate evolutionary paths from the O-type to the Wolf-Rayet stage with stellar evolutionary models.
Abstract: The origin of the observed population of Wolf-Rayet (WR) stars in low-metallicity (low-Z) galaxies, such as the Small Magellanic Cloud (SMC), is not yet understood. Standard, single-star evolutionary models predict that WR stars should stem from very massive O-type star progenitors, but these are very rare. On the other hand, binary evolutionary models predict that WR stars could originate from primary stars in close binaries. We conduct an analysis of the massive O star, AzV 14, to spectroscopically determine its fundamental and stellar wind parameters, which are then used to investigate evolutionary paths from the O-type to the WR stage with stellar evolutionary models. Multi-epoch UV and optical spectra of AzV 14 are analyzed using the non-LTE stellar atmosphere code PoWR. An optical TESS light curve was extracted and analyzed using the PHOEBE code. The obtained parameters are put into an evolutionary context, using the MESA code. AzV 14 is a close binary system consisting of two similar main sequence stars with masses of 32 Msol. Both stars have weak stellar winds with mass-loss rates of log $\dot{M}$ = -7.7. Binary evolutionary models can explain the empirically derived stellar and orbital parameters. The model predicts that the primary will evolve into a WR star with T = 100 kK, while the secondary, which will accrete significant amounts of mass during the first mass transfer phase, will become a cooler WR star with T = 50 kK and are predicted to have compared to other WR stars increased oxygen abundances. This model prediction is supported by a spectroscopic analysis of a WR star in the SMC. We hypothesize that the populations of WR stars in low-Z galaxies may have bimodal temperature distributions. Hotter WR stars might originate from primary stars, while cooler WR stars are the evolutionary descendants of the secondary stars if they accreted a significant amount of mass.
1 citations
26 Jan 2023
TL;DR: In this paper , the relative contributions of stars accelerated by the dynamical ejection vs binary supernova mechanisms were estimated using Gaia proper motions, together with stellar masses obtained from RIOTS4 data, and the frequencies of classical OBe stars, high-mass X-ray binaries, and noncompact binaries were used to discriminate between the mechanisms.
Abstract: . We use the RIOTS4 sample of SMC field OB stars to determine the origin of massive runaways in this low-metallicity galaxy using Gaia proper motions, together with stellar masses obtained from RIOTS4 data. These data allow us to estimate the relative contributions of stars accelerated by the dynamical ejection vs binary supernova mechanisms, since dynamical ejection favors faster, more massive runaways, while SN ejection favors the opposite trend. In addition, we use the frequencies of classical OBe stars, high-mass X-ray binaries, and non-compact binaries to discriminate between the mechanisms. Our results show that the dynamical mechanism dominates by a factor of 2 – 3. This also implies a significant contribution from two-step acceleration that occurs when dynamically ejected binaries are followed by SN kicks. We update our published quantitative results from Gaia DR2 proper motions with new data from DR3.
TL;DR: The XShootU project as mentioned in this paper provides enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESO's Very Large Telescope.
Abstract: Observations of individual massive stars, super-luminous supernovae, gamma-ray bursts, and gravitational-wave events involving spectacular black-hole mergers, indicate that the low-metallicity Universe is fundamentally different from our own Galaxy. Many transient phenomena will remain enigmatic until we achieve a firm understanding of the physics and evolution of massive stars at low metallicity (Z). The Hubble Space Telescope has devoted 500 orbits to observe 250 massive stars at low Z in the ultraviolet (UV) with the COS and STIS spectrographs under the ULLYSES program. The complementary ``X-Shooting ULLYSES'' (XShootU) project provides enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESO's Very Large Telescope. We present an overview of the XShootU project, showing that combining ULLYSES UV and XShootU optical spectra is critical for the uniform determination of stellar parameters such as effective temperature, surface gravity, luminosity, and abundances, as well as wind properties such as mass-loss rates in function of Z. As uncertainties in stellar and wind parameters percolate into many adjacent areas of Astrophysics, the data and modelling of the XShootU project is expected to be a game-changer for our physical understanding of massive stars at low Z. To be able to confidently interpret James Webb Space Telescope (JWST) spectra of the first stellar generations, the individual spectra of low Z stars need to be understood, which is exactly where XShootU can deliver.
TL;DR: In this paper , the early Oe star AzV 493 shows strong photometric (ΔI < 1.2 mag) and spectroscopic variability with a dominant, 14.6 yr pattern and ∼40 day oscillations.
Abstract: We present 18 yr of OGLE photometry together with spectra obtained over 12 yr revealing that the early Oe star AzV 493 shows strong photometric (ΔI < 1.2 mag) and spectroscopic variability with a dominant, 14.6 yr pattern and ∼40 day oscillations. We estimate the stellar parameters T eff = 42,000 K, logL/L⊙=5.83±0.15 , M/M ⊙ = 50 ± 9, and v sin i = 370 ± 40 km s−1. Direct spectroscopic evidence shows episodes of both gas ejection and infall. There is no X-ray detection, and it is likely a runaway star. The star AzV 493 may have an unseen companion on a highly eccentric (e > 0.93) orbit. We propose that close interaction at periastron excites ejection of the decretion disk, whose variable emission-line spectrum suggests separate inner and outer components, with an optically thick outer component obscuring both the stellar photosphere and the emission-line spectrum of the inner disk at early phases in the photometric cycle. It is plausible that AzV 493’s mass and rotation have been enhanced by binary interaction followed by the core-collapse supernova explosion of the companion, which now could be either a black hole or a neutron star. This system in the Small Magellanic Cloud can potentially shed light on OBe decretion disk formation and evolution, massive binary evolution, and compact binary progenitors.