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  • 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.
  • Predicting the heaviest black holes below the pair instability gap

    Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB, UK; School of Maths and Physics, Queen's University Belfast, Northern Ireland, University Road, BT7 1NN, UK; Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB, UK; Winch, Ethan R. J.; Vink, Jorick S.; Higgins, Erin R.; Sabhahitf, Gautham N. (Monthly Notices of the Royal Astronomical Society, 2024-04-01)
    Traditionally, the pair instability (PI) mass gap is located between 50 and 130 M<SUB>⊙</SUB>, with stellar mass black holes (BHs) expected to 'pile up' towards the lower PI edge. However, this lower PI boundary is based on the assumption that the star has already lost its hydrogen (H) envelope. With the announcement of an 'impossibly' heavy BH of 85 M<SUB>⊙</SUB> as part of GW 190521 located inside the traditional PI gap, we realized that blue supergiant (BSG) progenitors with small cores but large hydrogen envelopes at low metallicity (Z) could directly collapse to heavier BHs than had hitherto been assumed. The question of whether a single star can produce such a heavy BH is important, independent of gravitational wave events. Here, we systematically investigate the masses of stars inside the traditional PI gap by way of a grid of 336 detailed MESA stellar evolution models calculated across a wide parameter space, varying stellar mass, overshooting, rotation, semiconvection, and Z. We evolve low Z stars in the range 10<SUP>-3</SUP> &lt; Z/Z<SUB>⊙</SUB> &lt; Z<SUB>SMC</SUB>, making no prior assumption regarding the mass of an envelope, but instead employing a wind mass-loss recipe to calculate it. We compute critical carbon-oxygen and helium core masses to determine our lower limit to PI physics, and we provide two equations for M<SUB>core</SUB> and M<SUB>final</SUB> that can also be of use for binary population synthesis. Assuming the H envelope falls into the BH, we confirm the maximum BH mass below PI is M<SUB>BH</SUB> ≃ 93.3 M<SUB>⊙</SUB>. Our grid allows us to populate the traditional PI gap, and we conclude that the distribution of BHs above the traditional boundary is not solely due to the shape of the initial mass function, but also to the same stellar interior physics (i.e. mixing) that which sets the BH maximum.
  • LISA Galactic Binaries with Astrometry from Gaia DR3

    Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany; Department of Physics and Astronomy, Texas Tech University, P.O. Box 41051, Lubbock, TX 79409, USA; Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany; Institute for Gravitational Wave Astronomy, School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK; NASA Marshall Space Flight Center, Huntsville, AL 35811, USA; Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Callinstrasse 38, 30167 Hannover, Germany; Leibniz Universität Hannover, Institut für Gravitationsphysik, Callinstrasse 38, 30167 Hannover, Germany; Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France; IRFU, CEA, Université Paris-Saclay, F-91191, Gif-sur-Yvette, France; Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands; South African Astronomical Observatory, P.O. Box 9, Observatory, 7935, Cape Town, South Africa; Department of Astronomy &amp; Inter-University Institute for Data Intensive Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France; Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands; SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands; Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium; European Space Agency, European Space Astronomy Centre, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain; et al. (The Astrophysical Journal, 2024-03-01)
    Galactic compact binaries with orbital periods shorter than a few hours emit detectable gravitational waves (GWs) at low frequencies. Their GW signals can be detected with the future Laser Interferometer Space Antenna (LISA). Crucially, they may be useful in the early months of the mission operation in helping to validate LISA's performance in comparison to prelaunch expectations. We present an updated list of 55 candidate LISA-detectable binaries with measured properties, for which we derive distances based on Gaia Data Release 3 astrometry. Based on the known properties from electromagnetic observations, we predict the LISA detectability after 1, 3, 6, and 48 months using Bayesian analysis methods. We distinguish between verification and detectable binaries as being detectable after 3 and 48 months, respectively. We find 18 verification binaries and 22 detectable sources, which triples the number of known LISA binaries over the last few years. These include detached double white dwarfs, AM CVn binaries, one ultracompact X-ray binary, and two hot subdwarf binaries. We find that across this sample the GW amplitude is expected to be measured to ≈10% on average, while the inclination is expected to be determined with ≈15° precision. For detectable binaries, these average errors increase to ≈50% and ≈40°, respectively.
  • Predicting the Heaviest Black Holes below the Pair Instability Gap

    Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB; School of Maths and Physics, Queen's University Belfast, Northern Ireland, University Road, BT7 1NN; Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB; Winch, Ethan R. J.; Vink, Jorick S.; Higgins, Erin R.; Sabhahit, Gautham N. (Monthly Notices of the Royal Astronomical Society, 2024-02-01)
    Traditionally, the pair instability (PI) mass gap is located between 50 and 130 M<SUB>⊙</SUB>, with stellar mass black holes (BHs) expected to pile up towards the lower PI edge. However, this lower PI boundary is based on the assumption that the star has already lost its hydrogen (H) envelope. With the announcement of an impossibly heavy BH of 85 M<SUB>⊙</SUB> as part of GW 190521 located inside the traditional PI gap, we realised that blue supergiant (BSG) progenitors with small cores but large Hydrogen envelopes at low metallicity (Z) could directly collapse to heavier BHs than had hitherto been assumed. The question of whether a single star can produce such a heavy BH is important, independent of gravitational wave events. Here, we systematically investigate the masses of stars inside the traditional PI gap by way of a grid of 336 detailed MESA stellar evolution models calculated across a wide parameter space, varying stellar mass, overshooting, rotation, semi-convection, and Z. We evolve low Z stars in the range 10<SUP>-3</SUP> &lt; Z/Z<SUB>⊙</SUB> &lt; Z<SUB>SMC</SUB>, making no prior assumption regarding the mass of an envelope, but instead employing a wind mass loss recipe to calculate it. We compute critical Carbon-Oxygen and Helium core masses to determine our lower limit to PI physics, and we provide two equations for M<SUB>core</SUB> and M<SUB>final</SUB> that can also be of use for binary population synthesis. Assuming the H envelope falls into the BH, we confirm the maximum BH mass below PI is M<SUB>BH</SUB> ≃ 93.3 M<SUB>⊙</SUB>. Our grid allows us to populate the traditional PI gap, and we conclude that the distribution of BHs above the gap is not solely due to the shape of the initial mass function (IMF), but also to the same stellar interior physics (i.e. mixing) that which sets the BH maximum.
  • 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.
  • Exceptional outburst of the blazar CTA 102 in 2012: the GASP-WEBT campaign and its extension

    Astronomical Institute, St.-Petersburg State University, 198504 St.-Petersburg, Russia; Pulkovo Observatory, 196140 St.-Petersburg, Russia; INAF, Osservatorio Astrofisico di Torino, via Osservatorio 20, I-10025 Pino Torinese, Italy; Astronomical Institute, St.-Petersburg State University, 198504 St.-Petersburg, Russia; Institute for Astrophysical Research, Boston University, Boston, MA, 22015 USA; Institute for Astrophysical Research, Boston University, Boston, MA, 22015 USA; Instituto de Astrofisíca de Andalucía, CSIC, E-18080 Granada, Spain; Steward Observatory, University of Arizona, Tucson, AZ 85721, USA; Instituto de Astrofisica de Canarias (IAC), La Laguna, E-38200 Tenerife, Spain; Departamento de Astrofisica, Universidad de La Laguna, La Laguna, Tenerife, Spain; Pulkovo Observatory, 196140 St.-Petersburg, Russia; Institute of Astronomy, Bulgarian Academy of Sciences, BG-1784 Sofia, Bulgaria; Astronomical Institute, St.-Petersburg State University, 198504 St.-Petersburg, Russia; Department of Physics and Institute for Plasma Physics, University of Crete, GR-71003 Heraklion, Greece; Foundation for Research and Technology - Hellas, IESL, Voutes, GR-7110 Heraklion, Greece; et al. (Monthly Notices of the Royal Astronomical Society, 2016-09-01)
    After several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from γ-ray to near-infrared (NIR) frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive Glast-Agile support programme of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in 2004-2005. We find remarkable similarity between the optical and γ-ray behaviour of CTA 102 during the outburst, with a time lag between the two light curves of ≈1 h, indicative of cospatiality of the optical and γ-ray emission regions. The relation between the γ-ray and optical fluxes is consistent with the synchrotron self-Compton (SSC) mechanism, with a quadratic dependence of the SSC γ-ray flux on the synchrotron optical flux evident in the post-outburst stage. However, the γ-ray/optical relationship is linear during the outburst; we attribute this to changes in the Doppler factor. A strong harder-when-brighter spectral dependence is seen both the in γ-ray and optical non-thermal emission. This hardening can be explained by convexity of the UV-NIR spectrum that moves to higher frequencies owing to an increased Doppler shift as the viewing angle decreases during the outburst stage. The overall pattern of Stokes parameter variations agrees with a model of a radiating blob or shock wave that moves along a helical path down the jet.
  • 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.
  • Natal molecular cloud of SNR Kes 41. Complete characterisation

    CONICET-Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina ; Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina; CONICET-Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina; Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; School of Computing Enginnering and Mathematics, Western Sydney University, Locked Bay 1797, Penrith, NSW 2751, Australia; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; Supan, L.; Castelletti, G.; Supanitsky, A. D.; Burton, M. G.; Wong, G. F.; et al. (Astronomy and Astrophysics, 2018-11-01)
    Using high-resolution data of the <SUP>12</SUP>CO and <SUP>13</SUP>CO (J = 1-0) line emission from the Mopra Southern Galactic Plane CO Survey in conjunction with neutral hydrogen observations from the Southern Galactic Plane Survey (SGPS) and mid-infrared Spitzer data, we have explored the large-scale environment of the supernova remnant Kes 41. On the basis of these data, we identified for the first time the parent cloud of Kes 41 in its whole extension and surveyed the HII regions, masers, and the population of massive young stellar objects in the cloud. The whole unveiled giant cloud, located at the kinematic distance of 12.0 ± 3.6 kpc, whose average total mass and size are 10-30 × 10<SUP>5</SUP> M<SUB>⊙</SUB> and 26', also shines in γ-rays, as revealed by the Large Area Telescope on board the Fermi satellite. We determined a high average proton density 500-1000 cm<SUP>-3</SUP> in the large molecular complex, of which protons from the neutral atomic and ionised gases comprise only 15%.
  • 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.
  • Unidentified γ-ray emission towards the SNR Kes 41 revisited

    CONICET - Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina ; Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina; CONICET - Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina; Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina; School of Physics, University of New South Wales, Sydney, NSW, 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, UK; Supan, L.; Castelletti, G.; Supanitsky, A. D.; Burton, M. G. (Astronomy and Astrophysics, 2018-11-01)
    Kes 41 is one of the Galactic supernova remnants (SNRs) that are proposed to be physically linked to γ-ray emission at GeV energies. The nature of the γ-ray photons has been explained, but inconclusively, as hadronic collisions of particles accelerated at the SNR blast wave with target protons in an adjacent molecular clump. We performed an analysis of Fermi-Large Area Telescope (LAT) data of about nine years to assess the origin of the γ-ray emission. To investigate this matter, we also used spectral modelling constraints from the physical properties of the interstellar medium towards the γ-ray emitting region along with a revised radio continuum spectrum of Kes 41 (α = -0.54 ± 0.10, S ∝ ν<SUP>α</SUP>). We demonstrate that the γ-ray fluxes in the GeV range can be explained through bremsstrahlung emission from electrons interacting with the surrounding medium. We also considered a model in which the emission is produced by pion decay after hadronic collisions, and confirm that this mechanism cannot be excluded.
  • 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.
  • A Study of the Interstellar Medium Towards the Unidentified Dark TeV γ-Ray Sources HESS J1614-518 and HESS J1616-508

    School of Physical Sciences, University of Adelaide, Adelaide, SA 5005, Australia; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, Northern Ireland, UK; Department of Physics, University of Nagoya, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan; Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia; International Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6845, Australia; Lau, J. C.; Rowell, G.; Voisin, F.; Braiding, C.; et al. (Publications of the Astronomical Society of Australia, 2017-12-01)
    HESS J1614-518 and HESS J1616-508 are two tera-electron volt γ-ray sources that are not firmly associated with any known counterparts at other wavelengths. We investigate the distribution of interstellar medium towards the tera-electron volt γ-ray sources using results from a 7-mm-wavelength Mopra study, the Mopra Southern Galactic Plane CO Survey, the Millimetre Astronomer's Legacy Team-45 GHz survey and [C i] data from the HEAT telescope. Data in the CO(1-0) transition lines reveal diffuse gas overlapping the two tera-electron volt sources at several velocities along the line of sight, while observations in the CS(1-0) transition line reveal several interesting dense gas features. To account for the diffuse atomic gas, archival H i data was taken from the Southern Galactic Plane Survey. The observations reveal gas components with masses 10<SUP>3</SUP> to 10<SUP>5</SUP> M<SUB>⊙</SUB> and with densities 10<SUP>2</SUP> to 10<SUP>3</SUP> cm<SUP>-3</SUP> overlapping the two tera-electron volt sources. Several origin scenarios potentially associated with the tera-electron volt γ-ray sources are discussed in light of the distribution of the local interstellar medium. We find no strong convincing evidence linking any counterpart with HESS J1614-518 or HESS J1616-508.
  • Ammonia excitation imaging of shocked gas towards the W28 gamma-ray source HESS J1801-233

    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, UK; 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 I.; de Wilt, Phoebe; Rowell, Gavin P.; Nicholas, Brent P.; et al. (Monthly Notices of the Royal Astronomical Society, 2016-10-01)
    We present 12 mm Mopra observations of the dense (&gt;10<SUP>3</SUP> cm<SUP>-3</SUP>) molecular gas towards the north-east of the W28 supernova remnant (SNR). This cloud is spatially well matched to the TeV gamma-ray source HESS J1801-233 and is known to be an SNR-molecular cloud interaction region. Shock-disruption is evident from broad NH<SUB>3</SUB> (1,1) spectral linewidths in regions towards the W28 SNR, while strong detections of spatially extended NH<SUB>3</SUB> (3,3), NH<SUB>3</SUB>(4,4) and NH<SUB>3</SUB>(6,6) inversion emission towards the cloud strengthen the case for the existence of high temperatures within the cloud. Velocity dispersion measurements and NH<SUB>3</SUB>(n,n)/(1,1) ratio maps, where n = 2, 3, 4 and 6, indicate that the source of disruption is from the side of the cloud nearest to the W28 SNR, suggesting that it is the source of cloud-disruption. Towards part of the cloud, the ratio of ortho to para-NH<SUB>3</SUB> is observed to exceed 2, suggesting gas-phase NH<SUB>3</SUB> enrichment due to NH<SUB>3</SUB> liberation from dust-grain mantles. The measured NH<SUB>3</SUB> abundance with respect to H<SUB>2</SUB> is ∼(1.2 ± 0.5) × 10<SUP>-9</SUP>, which is not high, as might be expected for a hot, dense molecular cloud enriched by sublimated grain-surface molecules. The results are suggestive of NH<SUB>3</SUB> sublimation and destruction in this molecular cloud, which is likely to be interacting with the W28 SNR shock.
  • Detection of a 23.6 min periodic modulation in the optical counterpart of 3XMMJ051034.6-670353

    Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, UK; Department of Physics, University of Warwick, Coventry, CV4 7AL, UK; Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA, 91125, USA; Department of Physics &amp; Astronomy, University of Sheffield, Sheffield, S3 7RH, UK; Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain; Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa; Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL, Nijmegen, Netherlands; Ramsay, G.; Marsh, T. R.; Kupfer, T.; Dhillon, V. S.; et al. (Astronomy and Astrophysics, 2018-09-01)
    We present high speed optical photometric observations made using the NTT and ULTRACAM of the optical counterpart of 3XMMJ051034.6-670353, which was recently identified as an X-ray source showing a modulation on a period of 23.6 min. Although the optical counterpart is faint (g = 21.4), we find that the u'g'r' light curves show a periodic modulation on a period which is consistent with the X-ray period. We also obtained three low resolution spectra of 3XMMJ051034.6-670353 using the Gemini South Telescope and GMOS. There is no evidence for strong emission lines in the optical spectrum of 3XMMJ051034.6-670353. We compare and contrast the optical and X-ray observations of 3XMMJ051034.6-670353 with the ultra compact binaries HM Cnc and V407 Vul. We find we can identify a distribution of binary masses in which stable direct impact accretion can occur.
  • Swift observations of the 2015 outburst of AG Peg - from slow nova to classical symbiotic outburst

    Armagh Observatory, College Hill, Armagh BT61 9DG, UK; Columbia Astrophysics Lab, 550 W120th St., 1027 Pupin Hall, MC 5247 Columbia University, NY 10027, USA; Instituto de Astronomía y Física del Espacio (IAFE, CONICET-UBA), Av. Inte. Güiraldes 2620, C1428ZAA, Buenos Aires, Argentina; Instituto de Ciencias Astronómicas de la Tierra y el Espacio (ICATE-UNSJ), Av. Espanã (sur) 1512, 5400 San Juan, Argentina; Ramsay, Gavin; Sokoloski, J. L.; Luna, G. J. M.; Nuñez, N. E. (Monthly Notices of the Royal Astronomical Society, 2016-10-01)
    Symbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. However, in most symbiotics, the origin of this burning is unclear. In symbiotic slow novae, however, it is linked to a past thermonuclear runaway. In 2015 June, the symbiotic slow nova AG Peg was seen in only its second optical outburst since 1850. This recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks - like outbursts in classical symbiotic stars such as Z And. We report Swift X-ray and UV observations of AG Peg made between 2015 June and 2016 January. The X-ray flux was markedly variable on a time-scale of days, particularly during four days near optical maximum, when the X-rays became bright and soft. This strong X-ray variability continued for another month, after which the X-rays hardened as the optical flux declined. The UV flux was high throughout the outburst, consistent with quasi-steady shell burning on the white dwarf. Given that accretion discs around white dwarfs with shell burning do not generally produce detectable X-rays (due to Compton-cooling of the boundary layer), the X-rays probably originated via shocks in the ejecta. As the X-ray photoelectric absorption did not vary significantly, the X-ray variability may directly link to the properties of the shocked material. AG Peg's transition from a slow symbiotic nova (which drove the 1850 outburst) to a classical symbiotic star suggests that shell burning in at least some symbiotic stars is residual burning from prior novae.
  • V729 Sgr: a long period dwarf nova showing negative superhumps during quiescence

    Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK; Department of Physics and Astronomy, Texas A&amp;M University-Commerce, Commerce, TX 75429, USA; CRESST and Astroparticle Physics Laboratory NASA/GSFC, Greenbelt, MD 20771, USA; Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA; NASA Ames Research Center, Moffett Field, CA 94095, USA; NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; Ramsay, Gavin; Wood, Matt A.; Cannizzo, John K.; Howell, Steve B.; Smale, Alan (Monthly Notices of the Royal Astronomical Society, 2017-07-01)
    We report K2 observations of the eclipsing cataclysmic variable V729 Sgr which covered nearly 80 d in duration. We find five short outbursts and two long outbursts, one of which shows a clear plateau phase in the rise to maximum brightness. The mean time between successive short outbursts is ∼10 d while the time between the two long outbursts is ∼38 d. The frequency of these outbursts is unprecedented for a cataclysmic variable (CV) above the orbital period gap. We find evidence that the mid-point of the eclipse occurs systematically earlier in outburst than in quiescence. During five of the six quiescent epochs we find evidence for a second photometric period which is roughly 5 per cent shorter than the 4.16 h orbital period which we attribute to negative superhumps. V729 Sgr is therefore one of the longest period CVs to show negative superhumps during quiescence.
  • Interstellar gas towards the TeV γ-ray sources HESS J1640-465 and HESS J1641-463

    School of Physical Sciences, University of Adelaide, Adelaide, SA 5005, Australia; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, Northern Ireland, UK; Department of Physics, University of Nagoya, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan; Max-Planck-Institut für Kernphysik, PO Box 103980, D-69029 Heidelberg, Germany; Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland; National Research Nuclear University (MEPHI), 115409, Moscow, Russia; DESY, D-15738 Zeuthen, Germany; GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, the Netherlands; Max-Planck-Institut für Kernphysik, PO Box 103980, D-69029 Heidelberg, Germany; Instytut Fizyki Ja¸drowej PAN, ul. Radzikowskiego 152, PL-31-342 Krakow, Poland; Lau, J. C.; Rowell, G.; Burton, M. G.; et al. (Monthly Notices of the Royal Astronomical Society, 2017-01-01)
    We present a detailed analysis of the interstellar medium towards the tera electron volt (TeV) γ-ray sources HESS J1640-465 and HESS J1641-463 using results from the Mopra Southern Galactic Plane CO Survey and from a Mopra 7 mm-wavelength study. The γ-ray sources are positionally coincident with two supernova remnants (SNRs) G338.3-0.0 and G338.5+0.1, respectively. A bright complex of H II regions connect the two SNRs and TeV objects. Observations in the CO(1-0) transition lines reveal substantial amounts of diffuse gas positionally coincident with the γ-ray sources at multiple velocities along the line of sight, while 7 mm observations in CS, SiO, HC<SUB>3</SUB>N and CH<SUB>3</SUB>OH transition lines reveal regions of dense, shocked gas. Archival H I data from the Southern Galactic Plane Survey was used to account for the diffuse atomic gas. Physical parameters of the gas towards the TeV sources were calculated from the data. We find that for a hadronic origin for the γ-ray emission, the cosmic ray enhancement rates are ∼10<SUP>3</SUP> and 10<SUP>2</SUP> times the local solar value for HESS J1640-465 and HESS J1641-463, respectively.
  • The Superoutburst Duration versus Orbital Period Relation for AM CVn Stars

    CRESST/Joint Center for Astrophysics, University of Maryland, Baltimore County, Baltimore, MD 21250, USA ; Astroparticle Physics Laboratory, NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA ;;; Armagh Observatory, College Hill, Armagh BT61 9DG, UK; Cannizzo, J. K.; Ramsay, G. (The Astronomical Journal, 2019-03-01)
    We examine the relationship between superoutburst duration t <SUB>dur</SUB> and orbital period P <SUB>orb</SUB> in AM CVn ultra-compact binary systems. We show that the previously determined steep relation derived by Levitan et al. was strongly influenced by the inclusion of upper limits for systems with a relatively long orbital period in their fit. Excluding the upper limit values and including t <SUB>dur</SUB> values for three systems at long P <SUB>orb</SUB> that were not considered previously, then d{log}({t}<SUB>dur</SUB>})/d{log}({P}<SUB>orb</SUB>}) is flat as predicted by Cannizzo &amp; Nelemans.
  • 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.
  • Mass loss and stellar superwinds

    Armagh Observatory, College Hill, Armagh BT61 9DG, UK,; Vink, Jorick S. (Philosophical Transactions of the Royal Society of London Series A, 2017-09-01)
    Mass loss bridges the gap between massive stars and supernovae (SNe) in two major ways: (i) theoretically, it is the amount of mass lost that determines the mass of the star prior to explosion and (ii) observations of the circumstellar material around SNe may teach us the type of progenitor that made the SN. Here, I present the latest models and observations of mass loss from massive stars, both for canonical massive O stars, as well as very massive stars that show Wolf-Rayet type features. <P />This article is part of the themed issue 'Bridging the gap: from massive stars to supernovae'.

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