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.
  • Recent Advances in the Theoretical Modeling of Pulsating Low-mass He-core White Dwarfs

    Facultad de Ciencias Astronómicas y Geofísicas (UNLP), La Plata, Argentina;; Facultad de Ciencias Astronómicas y Geofísicas (UNLP), La Plata, Argentina; Institute of Space Sciences (IEEC-CSIC), Barcelona, Spain; Instituto de Física (UFRGS), Porto-Alegre, Brazil; Armagh Observatory, College Hill, UK; Córsico, A. H.; Althaus, L. G.; Calcaferro, L. M.; Serenelli, A. M.; Kepler, S. O.; et al. (20th European White Dwarf Workshop, 2017-03-01)
    Many extremely low-mass (ELM) white-dwarf (WD) stars are currently being found in the field of the Milky Way. Some of these stars exhibit long-period nonradial <P />g-mode pulsations, and constitute the class of ELMV pulsating WDs. In addition, several low-mass pre-WDs, which could be precursors of ELM WDs, have <P />been observed to show short-period photometric variations likely due to nonradial p modes and radial modes. They could constitute a new class of pulsating <P />low-mass pre-WD stars, the pre-ELMV stars. Here, we present the recent results of a thorough theoretical study of the nonadiabatic pulsation properties of <P />low-mass He-core WDs and pre-WDs on the basis of fully evolutionary models representative of these stars.
  • 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.
  • Polarimetry as a Tool to Study Multi-Dimensional Winds and Disks

    Armagh Observatory, College Hill, Armagh, Armagh, BT61 9DG, Norn Iron; Vink, J. S. (The B[e] Phenomenon: Forty Years of Studies, 2017-02-01)
    I start with a discussion of spherical winds and small-scale clumping, before continuing with various theories that have been proposed to predict how mass loss depends on stellar rotation - both in terms of wind strength, as well as the latitudinal dependence of the wind. This very issue is crucial for our general understanding of angular momentum evolution in massive stars, and the B[e] phenomenon in particular. I then discuss the tool of linear polarimetry that allows us to probe the difference between polar and equatorial mass loss, allowing us to test B[e] and related disk formation theories.
  • Transition region bright dots in active regions observed by the interface region imaging spectrograph

    Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Physics, Shandong University (Weihai), 264209 Weihai, Shandong, China; Armagh Observatory, College Hill, Armagh BT61 9DG, UK; Hou, Zhenyong; Huang, Zhenghua; Xia, Lidong; Li, Bo; Madjarska, Maria S.; Fu, Hui (Solar Wind 14, 2016-03-01)
    The Interface Region Imaging Spectrograph (IRIS) reveals numerous small-scale (sub-arcsecond) brightenings that appear as bright dots sparkling the solar transition region in active regions. Here, we report a statistical study on these transition-region bright dots. We use an automatic approach to identify 2742 dots in a Si IV raster image. We find that the average spatial size of the dots is 0.8 arcsec<SUP>2</SUP> and most of them are located in the faculae area. Their Doppler velocities obtained from the Si IV 1394 Å line range from -20 to 20 km s<SUP>-1</SUP>. Among these 2742 dots, 1224 are predominantly blue-shifted and 1518 are red-shifted. Their non-thermal velocities range from 4 to 50 km s<SUP>-1</SUP> with an average of 24 km s<SUP>-1</SUP>. We speculate that the bright dots studied here are small-scale impulsive energetic events that can heat the active region corona.
  • Reflections on the Discovery of the First Magnetic White Dwarf

    Department of Physics &amp; Astronomy, University of Western Ontario, London, ON, Canada N6G 1P7; Armagh Observatory &amp; Planetarium, College Hill, Armagh, BT61 9DG, Northern Ireland; Landstreet, J. D. (Stellar Magnetism: A Workshop in Honour of the Career and Contributions of John D. Landstreet, 2020-11-01)
    I was one of the six people most closely involved in the discovery of the first magnetic white dwarf in 1970, now 50 years ago. Thinking back on this event, I have realised that the discovery occurred when and how it did because of a series of lucky coincidences along a strange and winding path. In this paper I recount the events as I recall them, and reflect on how those unlikely coincidences helped us to succeed.
  • Searching for the Origin of Flares in M dwarfs

    Armagh Observatory and Planetarium; Doyle, Lauren; Ramsay, Gavin; Doyle, John G. (20th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, 2018-07-01)
    We present an overview of K2 short cadence observations for 34 M dwarfs observed in Campaigns 1 - 9 which have spectral types between M0 - L1. All of the stars in our sample showed flares with the most energetic reaching 3×10(34) ergs. As previous studies have found, we find rapidly rotating stars tend to show more flares, with evidence for a decline in activity in stars with rotation periods longer than approximately 10 days. We determined the rotational phase of each flare and performed a simple statistical test on our sample to determine whether the phase distribution of the flares is random or if there is a preference for phase. We find, with the exception of one star which is in a known binary system, that none show a preference for the rotational phase of the flares. This is unexpected and all stars in our sample show flares at all rotational phases, suggesting these flares are not all originating from one dominant starspot on the surface of the stars. We outline three scenarios which could explain the lack of a correlation between the number of flares and the stellar rotation phase. In addition we also highlight preliminary observations of DP Cnc, observed in campaigns 16 and 18, and is one of the stars in our extended sample from K2Campaigns 10 -18 which are still to be examined.
  • 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.
  • The VLT-FLAMES Tarantula Survey

    Armagh Observatory, College Hill, BT61 9DG, Armagh, Northern Ireland; ATC, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK; ARC, School of Mathematics and Physics, QUB, Belfast BT7 1NN, UK; Institute of Astrophysics, KU Leuven, Celestijnenlaan 200D, 3001, Leuven, Belgium; Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK; -; Vink, Jorick S.; Evans, C. J.; Bestenlehner, J.; McEvoy, C.; et al. (The Lives and Death-Throes of Massive Stars, 2017-11-01)
    We present a number of notable results from the VLT-FLAMES Tarantula Survey (VFTS), an ESO Large Program during which we obtained multi-epoch medium-resolution optical spectroscopy of a very large sample of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). This unprecedented data-set has enabled us to address some key questions regarding atmospheres and winds, as well as the evolution of (very) massive stars. Here we focus on O-type runaways, the width of the main sequence, and the mass-loss rates for (very) massive stars. We also provide indications for the presence of a top-heavy initial mass function (IMF) in 30 Dor.
  • Massive star evolution revealed in the Mass-Luminosity plane

    Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, N. Ireland; Queen's University of Belfast, Belfast BT7 1NN, N. Ireland; Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin, Ireland; Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA ,; Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, N. Ireland; Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA ,; Higgins, Erin R.; Vink, Jorick S. (High-mass X-ray Binaries: Illuminating the Passage from Massive Binaries to Merging Compact Objects, 2019-12-01)
    Massive star evolution is dominated by key physical processes such as mass loss, convection and rotation, yet these effects are poorly constrained, even on the main sequence. We utilise a detached, eclipsing binary HD166734 as a testbed for single star evolution to calibrate new MESA stellar evolution grids. We introduce a novel method of comparing theoretical models with observations in the `Mass-Luminosity Plane', as an equivalent to the HRD (see Higgins &amp; Vink 2018). We reproduce stellar parameters and abundances of HD166734 with enhanced overshooting (α<SUB>ov</SUB>=0.5), mass loss and rotational mixing. When comparing the constraints of our testbed to the systematic grid of models we find that a higher value of α<SUB>ov</SUB>=0.5 (rather than α<SUB>ov</SUB>=0.1) results in a solution which is more likely to evolve to a neutron star than a black hole, due to a lower value of the compactness parameter.
  • 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.
  • 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.