Recent Submissions

  • X-Shooting ULLYSES: Massive Stars at Low Metallicity

    Armagh Observatory and Planetarium, UK; Department of Physics & Astronomy, University of Sheffield, UK; Space Telescope Science Institute, Baltimore, USA; Centre for Astrobiology (CSIC-INTA), Torrejón de Ardoz, Madrid, Spain; Montpellier Universe and Particles Laboratory, Montpellier University, France; Las Campanas Observatory, Carnegie Observatories, Chile; Institute for Physics and Astronomy, University of Potsdam, Germany; Department of Physics, University of Montreal, Canada; Penn State Scranton, Dunmore, PA, USA; Astronomy Centre, Heidelberg University, Germany; et al. (The Messenger, 2024-03-01)
    The Hubble Space Telescope has devoted 500 orbits to observing 250 massive stars with low metallicity in the ultraviolet (UV) range within the framework of the ULLYSES program. The X-Shooting ULLYSES (XShootU) project enhances the legacy value of this UV dataset by providing high-quality optical and near-infrared spectra, which are acquired using the wide-wavelength- coverage X-shooter spectrograph at ESO's Very Large Telescope. XShootU emphasises the importance of combining UV with optical spectra for the consistent determination of key stellar parameters such as effective temperature, surface gravity, luminosity, abundances, and wind characteristics including mass-loss rates as a function of metallicity. Since uncertainties in these parameters have implications across various branches of astrophysics, the data and modelling generated by the XShootU project are poised to significantly advance our understanding of massive stars at low metallicity. This is particularly crucial for confidently interpreting James Webb Space Telescope (JWST) data of the earliest stellar generations, making XShootU a unique resource for comprehending individual spectra of low-metallicity stars.
  • Multimessenger science opportunities with mHz gravitational waves

    NASA Goddard Space Flight Center; Columbia University; Leiden University; Harvard University; The Pennsylvania State University; University of Cambridge; CITA, University of Toronto; California Institute of Technology; City University of New York/American Museum of Natural History; Princeton University; et al. (Bulletin of the American Astronomical Society, 2019-05-01)
    We review opportunities for multi-messenger science breakthroughs involving mHz gravitational waves with electromagnetic observations.
  • Constraining the progenitor evolution of GW 150914

    Armagh Observatory, and Planetarium, BT61 9DG Armagh, College Hill, Northern Ireland,; Vink, Jorick S. (High-mass X-ray Binaries: Illuminating the Passage from Massive Binaries to Merging Compact Objects, 2019-12-01)
    One of the largest surprises from the LIGO results regarding the first gravitational wave detection (GW 150914) was the fact the black holes (BHs) were heavy, of order 30 - 40 M<SUB>⊙</SUB>. The most promising explanation for this obesity is that the BH-BH merger occurred at low metallicity (Z): when the iron (Fe) contents is lower this is expected to result in weaker mass loss during the Wolf-Rayet (WR) phase. We therefore critically evaluate the claims for the reasons of heavy BHs as a function of Z in the literature. Furthermore, weaker stellar winds might lead to more rapid stellar rotation, allowing WR and BH progenitor evolution in a chemically homogeneous manner. However, there is as yet no empirical evidence for more rapid rotation amongst WR stars in the low Z environment of the Magellanic Clouds. Due to the intrinsic challenge of determining WR rotation rates from emission lines, the most promising avenue to constrain rotation-rate distributions amongst various WR subgroups is through the utilisation of their emission lines in polarised light. We thus provide an overview of linear spectro-polarimetry observations of both single and binary WRs in the Galaxy, as well as the Large and Small Magellanic Clouds, at 50% and 20% of solar Z, respectively. Initial results suggest that the route of chemically homogeneous evolution (CHE) through stellar rotation is challenging, whilst the alternative of a post-LBV or common envelope evolution is more likely.
  • Molecular shocks and the gamma-ray clouds of the W28 supernova remnant

    School of Physics, University of New South Wales, Sydney, 2052, Australia; School of Physical Sciences, Adelaide University, Adelaide, 5005, Australia; School of Physics, University of New South Wales, Sydney, 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, Northern Ireland, United Kingdom; International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, Australia; Department of Astrophysics, Nagoya University, Furocho, Chikusa-ku, Nagoya, Aichi, 464-8602, Japan; National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan; Maxted, Nigel; Rowell, Gavin; de Wilt, Phoebe; Burton, Michael; et al. (6th International Symposium on High Energy Gamma-Ray Astronomy, 2017-01-01)
    Interstellar medium clouds in the W28 region are emitting gamma-rays and it is likely that the W28 supernova remnant is responsible, making W28 a prime candidate for the study of cosmic-ray acceleration and diffusion. Understanding the influence of both supernova remnant shocks and cosmic rays on local molecular clouds can help to identify multi-wavelength signatures of probable cosmic-ray sources. To this goal, transitions of OH, SiO, NH<SUB>3</SUB>, HCO<SUP>+</SUP> and CS have complemented CO in allowing a characterisation of the chemically rich environment surrounding W28. This remnant has been an ideal test-bed for techniques that will complement arcminute-scale studies of cosmic-ray source candidates with future GeV-PeV gamma-ray observations.
  • Massive star winds and HMXB donors

    Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, Northern Ireland, UK, ; Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, D-14476 Potsdam, Germany; Sander, Andreas A. C. (High-mass X-ray Binaries: Illuminating the Passage from Massive Binaries to Merging Compact Objects, 2019-12-01)
    Understanding the complex behavior of High Mass X-ray binaries (HMXBs) is not possible without detailed information about their donor stars. While crucial, this turns out to be a challenge on multiple fronts. First, multi-wavelength spectroscopy is vital. As such systems can be highly absorbed, this is often already hard to accomplish. Secondly, even if the spectroscopic data is available, the determination of reliable stellar parameters requires sophisticated model atmospheres that accurately describe the outermost layers and the wind of the donor star.
  • Vela X-1 as a laboratory for accretion in high-mass X-ray binaries

    European Space Astronomy Centre (ESA/ESAC), Science Operations Department, E-28692, Villanueva de la Cañada, Madrid, Spain,; Instituto de Física de Cantabria (CSIC-Universidad de Cantabria), E-39005, Santander, Spain,; European Space Astronomy Centre (ESA/ESAC), Science Operations Department, E-28692, Villanueva de la Cañada, Madrid, Spain; Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany; European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands; Centre for Mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium; University of Sharjah, University City Rd, Sharjah, United Arab Emirates; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, UK; Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, The Netherlands; Kretschmar, P.; et al. (Memorie della Societa Astronomica Italiana, 2019-01-01)
    Vela X-1 is an eclipsing high mass X-ray binary (HMXB) consisting of a 283s accreting X-ray pulsar in a close orbit of 8.964 days around the B0.5Ib supergiant HD77581 at a distance of just 2.4 kpc. The system is considered a prototype of wind-accreting HMXB and it has been used as a baseline in different theoretical or modelling studies. <P />We discuss the observational properties of the system and the use of the observational data as laboratory to test recent developments in modelling the accretion process in High-Mass X-ray Binaries \citep[e.g.,][]{Sander:2018,El-Mellah:2018}, which range from detailed descriptions of the wind acceleration to modelling of the structure of the flow of matter close to the neutron star and its variations.
  • The origin and impact of Wolf-Rayet-type mass loss

    Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany,; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, N. Ireland; Anton Pannekoek Institute for Astronomy, Science Park 904, 1098 XH, Amsterdam, The Netherlands; Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, D-14476 Potsdam, Germany; Sander, Andreas A. C.; Vink, Jorick S.; Higgins, Erin R.; Shenar, Tomer; Hamann, Wolf-Rainer; Todt, Helge (The Origin of Outflows in Evolved Stars, 2022-01-01)
    Classical Wolf-Rayet (WR) stars mark an important stage in the late evolution of massive stars. As hydrogen-poor massive stars, these objects have lost their outer layers, while still losing further mass through strong winds indicated by their prominent emission line spectra. Wolf-Rayet stars have been detected in a variety of different galaxies. Their strong winds are a major ingredient of stellar evolution and population synthesis models. Yet, a coherent theoretical picture of their strong mass-loss is only starting to emerge. In particular, the occurrence of WR stars as a function of metallicity (Z) is still far from being understood.
  • The Link between Hot and Cool Outflows

    Armagh Observatory and Planetarium, College Hill, BT61 9DG, Armagh, Northern Ireland,; Vink, Jorick S.; Sander, A. A. C.; Higgins, E. R.; Sabhahit, G. N. (The Origin of Outflows in Evolved Stars, 2022-01-01)
    The link between hot and cool stellar outflows is shown to be critical for correctly predicting the masses of the most massive black holes (BHs) below the so-called pair-instability supernova (PISN) mass gap. Gravitational Wave (GW) event 190521 allegedly hosted an "impossibly" heavy BH of 85 M <SUB>⊙</SUB>. Here we show how our increased knowledge of both metallicity Z and temperature dependent mass loss is critical for our evolutionary scenario of a low-Z blue supergiant (BSG) progenitor of an initially approx 100 M <SUB>⊙</SUB> star to work. We show using MESA stellar evolution modelling experiments that as long as we can keep such stars above 8000 K such low-Z BSGs can avoid strong winds, and keep a very large envelope mass intact before core collapse. This naturally leads to the Cosmic Time dependent maximum BH function below the PISN gap.