We present a quantitative study of the variability in ultraviolet resonance lines of N v, Si IV a... more We present a quantitative study of the variability in ultraviolet resonance lines of N v, Si IV and C iv of the Be starr 7 Cas. For this purpose we used IUE spectra obtained over a period of eleven years. Variability occurs in the form of discretee absorption components (DACs), which are formed in the fast-outflowing radiatively driven part of the stellar wind. Wee constructed a template spectrum from spectra containing no or minor extra absorption due to DACs and modelled the isolatedd DACs in the obtained quotient spectra. Besides the frequently observed narrow components (vi typically < 250 km/s)) at high velocity, we found several broad components occurring at low and intermediate wind velocities. Wee confirm the finding of Doazan et al. (1987) who reported that the number of observed DACs is associated with the cyclic V/RR variability of the Balmer-emission lines. We show that when V/R<1 the central optical depth of DACs is significantly lowerr than when V/R>1. In our interpretation this is due to a correlation between the column density associated with the DACss and the phase of the V/R cycle. Wee find that the H/3 observations of Doazan et al. are consistent with a model in which the cyclic V/R variability is due too a global, one-armed oscillation moving through an equatorial disc. We suggest that the higher column density of DACs inn phases of V/R> 1 is the result of the higher density in the region of their origin, namely close to or in the part of the equatoriall disc which is rotating towards the observer.
arXiv: Instrumentation and Methods for Astrophysics, 2020
MOSAIC is the planned multi-object spectrograph for the 39m Extremely Large Telescope (ELT). Conc... more MOSAIC is the planned multi-object spectrograph for the 39m Extremely Large Telescope (ELT). Conceived as a multi-purpose instrument, it offers both high multiplex and multi-IFU capabilities at a range of intermediate to high spectral resolving powers in the visible and the near-infrared. MOSAIC will enable unique spectroscopic surveys of the faintest sources, from the oldest stars in the Galaxy and beyond to the first populations of galaxies that completed the reionisation of the Universe--while simultaneously opening up a wide discovery space. In this contribution we present the status of the instrument ahead of Phase B, showcasing the key science cases as well as introducing the updated set of top level requirements and the adopted architecture. The high readiness level will allow MOSAIC to soon enter the construction phase, with the goal to provide the ELT community with a world-class MOS capability as soon as possible after the telescope first light.
Johan Richard1, Roland Bacon1, Jérémy Blaizot1, Samuel Boissier2, Alessandro Boselli2, Nicolas Bo... more Johan Richard1, Roland Bacon1, Jérémy Blaizot1, Samuel Boissier2, Alessandro Boselli2, Nicolas Bouché1, Jarle Brinchmann3,4, Norberto Castro5, Laure Ciesla2, Paul Crowther6, Emanuele Daddi7, Stefan Dreizler8, Pierre-Alain Duc9, David Elbaz7, Benoit Épinat2, Chris Evans10, Matteo Fossati11, Michele Fumagalli11, Miriam Garcia12, Thibault Garel1,13, Matthew Hayes14, Artemio Herrero15,16, Emmanuel Hugot2, Andrew Humphrey3, Pascale Jablonka17, Sebastian Kamann18, Lex Kaper19, Andreas Kelz5, Jean-Paul Kneib17, Alex de Koter19,20, Davor Krajnović5, Rolf-Peter Kudritzki21, Norbert Langer22, Carmela Lardo17, Floriane Leclercq13, Danny Lennon15, Guillaume Mahler23, Fabrice Martins24, Richard Massey11, Peter Mitchell4, Ana Monreal-Ibero15,16, Paco Najarro12, Cyrielle Opitom25, Polychronis Papaderos3,26, Céline Péroux28,2, Yves Revaz17, Martin M. Roth5, Philippe Rousselot29, Andreas Sander30, Charlotte Simmonds Wagemann13, Ian Smail11, Anthony Mark Swinbank11, Frank Tramper31, Tanya Urrutia5, A...
We report on UBVRIZJsHKs-band photometry of the dark GRB 000210 host galaxy. Fitting a grid of sp... more We report on UBVRIZJsHKs-band photometry of the dark GRB 000210 host galaxy. Fitting a grid of spectral templates to its Spectral Energy Distribution (SED), we derived a photometric redshift (z=0.842\+0.0540.042) which is in excellent agreement with the spectroscopic one (z=0.8463+/-0.0002; Piro et al. 2002). The best fit to the SED is obtained with a blue starburst template with an age of 0.181\+0.0370.026 Gyr. We discuss the implications of the inferred low value of Av and the age of the dominant stellar population for the non detection of the GRB 000210 optical afterglow.
Until a few years ago the common understanding was that neutron stars, the compact remnants of ma... more Until a few years ago the common understanding was that neutron stars, the compact remnants of massive stars, have a canonical mass of about 1.4 MA. Recent observations with VLT/UVES support the view that the neutron stars in high-mass X-ray binaries display a relatively large spread in mass, ranging from the theoretical lower mass limit of 1 MA up to over 2 MA. Such a mass distribution provides important information on the formation mechanism of neutron stars (i.e. the supernovae), and on the (unknown) behaviour of matter at supranuclear densities.
A new time-series analysis of profile changes in the photospheric He I λ4713 spectral line from d... more A new time-series analysis of profile changes in the photospheric He I λ4713 spectral line from data taken during 5 days in 1989 at the Calar Alto and Kitt Peak observatories has provided evidence for the presence of a non-radial prograde p-mode in the O 7.5 giant ξ Per (̀ = 3, P = 3.5) and probably two such modes in the O6 supergiant λ Cep (̀ = 3, P = 12.3 h and̀ = 5, P = 6.6 h). The corotating pulsation periods are in both cases much shorter than the estimated stellar rotation period. Modeling the observed amplitude of the line-profile changes (assuming |m| = `) yields a velocity amplitude of approximately 5 km s −1 of the pulsation inξ Per and 6 km s −1 in λ Cep. Any such a pulsation by itself is unlikely to be the cause of the well-known cyclical wind variability in these stars because the pulsation period is too short, but the cumulative action of multiple modes could cause such an effect. Weak magnetic fields anchored at the surface remain the strongest candidate for the origi...
Context. A pioneering study showed that the fine structure in the luminosity function (LF) of you... more Context. A pioneering study showed that the fine structure in the luminosity function (LF) of young star clusters contains information about the evolutionary stage (age) and composition of the stellar population. The notable features include the H-peak, which is the result of the onset of hydrogen burning turning pre-main sequence stars into main sequence stars. The feature moves toward the faint end of the LF, and eventually disappears as the population evolves. Another detectable feature is the Wielen dip, a dip at MV ≃ 7 mag in the LF first identified in 1974 for stars in the solar environment. Later studies also identified this feature in the LF of star clusters. The Wielen dip is caused by the increased importance of H− opacity in a certain range of low-mass stars. Aims. We studied the detailed structure in the luminosity function using the data from Gaia DR2 and PARSEC stellar evolution models with the aim to further our understanding of young stellar populations. Methods. We ...
Rapid rotation in massive stars imposes a latitudinal variation in the mass loss from radiatively... more Rapid rotation in massive stars imposes a latitudinal variation in the mass loss from radiatively driven winds that can lead to enhanced mass loss at the poles (with little angular momentum loss) and/or equator (with maximal angular momentum loss). Here we present an examination of the stellar wind lines of the two O-type stars with the fastest known equatorial velocities, VFTS 102 (V sin i = 610 ± 30 km s −1 ; O9: Vnnne+) and VFTS 285 (V sin i = 609 ± 29 km s −1 ; O7.5 Vnnn) in the Large Magellanic Cloud. Ultraviolet spectra of both stars were obtained with the Hubble Space Telescope Cosmic Origins Spectrograph. The spectrum of VFTS 285 displays a fast outflow in N V and a much slower wind in Si IV, and we argue that there is a two-wind regime in which mass loss is strong at the poles (fast and tenuous wind) but dominant at the equator (slow and dense winds). These ions and wind lines are not present in the spectrum of the cooler star VFTS 102, but the double-peaked Hα emission in its spectrum implies equatorial mass loss into a circumstellar disk. The results suggest that in the fastest rotating O-stars, most mass is lost as an equatorial outflow, promoting angular momentum loss that contributes to a spin down over time.
MAXI J1621-501 is the first Swift/XRT Deep Galactic Plane Survey transient that was followed up w... more MAXI J1621-501 is the first Swift/XRT Deep Galactic Plane Survey transient that was followed up with a multitude of space missions (NuSTAR, Swift, Chandra, NICER, INTEGRAL, and MAXI) and ground-based observatories (Gemini, IRSF, and ATCA). The source was discovered with MAXI on 2017 October 19 as a new, unidentified transient. Further observations with NuSTAR revealed 2 Type I X-ray bursts, identifying MAXI J1621-501 as a Low Mass X-ray Binary (LMXB) with a neutron star primary. Overall, 24 Type I bursts were detected from the source during a 15 month period. At energies below 10 keV, the source spectrum was best fit with three components: an absorbed blackbody with kT = 2.3 keV, a cutoff power law with index Γ = 0.7, and an emission line centered on 6.3 keV. Timing analysis of the X-ray persistent emission and burst data has not revealed coherent pulsations from the source or an orbital period. We identified, however, a super-orbital period ∼ 78 days in the source X-ray light curve. This period agrees very well with the theoretically predicted radiative precession period of ∼ 82 days. Thus, MAXI J1621-501 joins a small group of sources characterized with super-orbital periods.
Polarimetric studies of light transmitted through interstellar clouds may give constraints on the... more Polarimetric studies of light transmitted through interstellar clouds may give constraints on the properties of the interstellar dust grains. Traditionally, broadband linear polarisation (BBLP) measurements have been considered an important diagnostic tool for the study of the interstellar dust, while comparatively less attention has been paid to spectropolarimetric measurements. However, spectropolarimetry may offer stronger constraints than BBLP, for example by revealing narrowband features, and by allowing us to distinguish the contribution of dust from the contribution of interstellar gas. Therefore, we have decided to carry out a Large Interstellar Polarisation Survey (LIPS) using spectropolarimetric facilities in both hemispheres. Here we present the results obtained in the Southern Hemisphere with the FORS2 instrument of the ESO Very Large Telescope. Our spectra cover the wavelength range 380-950 nm at a spectral resolving power of about 880. We have produced a publicly available catalogue of 127 linear polarisation spectra of 101 targets. We also provide the Serkowski-curve parameters, as well as the wavelength gradient of the polarisation position angle for the interstellar polarisation along 76 different lines of sight. In agreement with previous literature, we found that the best-fit parameters of the Serkowski-curve are not independent of each other. However, the relationships that we obtained are not always consistent with what has been found in previous studies.
The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquito... more The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquitously present in the interstellar medium (ISM). After decades of study, two strong and possibly three weak near-infrared DIBs have recently been attributed to the C + 60 fullerene based on observational and laboratory measurements. There is great promise for the identification of the over 400 other known DIBs, as this result could provide chemical hints towards other possible carriers. In an effort to systematically study the properties of the DIB carriers, we have initiated a new large-scale observational survey: the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES). The main objective is to build on and extend existing DIB surveys to make a major step forward in characterising the physical and chemical conditions for a statistically significant sample of interstellar lines-of-sight, with the goal to reverse-engineer key molecular properties of the DIB carriers. EDIBLES is a filler Large Programme using the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope at Paranal, Chile. It is designed to provide an observationally unbiased view of the presence and behaviour of the DIBs towards early-spectral-type stars whose lines-of-sight probe the diffuse-to-translucent ISM. Such a complete dataset will provide a deep census of the atomic and molecular content, physical conditions, chemical abundances and elemental depletion levels for each sightline. Achieving these goals requires a homogeneous set of high-quality data in terms of resolution (R ∼ 70 000-100 000), sensitivity (S/N up to 1000 per resolution element), and spectral coverage (305-1042 nm), as well as a large sample size (100+ sightlines). In this first paper the goals, objectives and methodology of the EDIBLES programme are described and an initial assessment of the data is provided.
Publications of the Astronomical Society of the Pacific
We provide new observations of the LMC X-1 O star and its extended nebula structure using spectro... more We provide new observations of the LMC X-1 O star and its extended nebula structure using spectroscopic data from VLT/UVES as well as Hα imaging from the Wide Field Imager on the Max Planck Gesellschaft / European Southern Observatory 2.2m telescope and ATCA imaging of the 2.1 GHz radio continuum. This nebula is one of the few known to be energized by an X-ray binary. We use a new spectrum extraction technique that is superior to other methods to obtain both radial velocities and fluxes. This provides an updated spatial velocity of 21.0 ± 4.8 km s −1 for the O star. The slit encompasses both the photo-ionized and shock-ionized regions of the nebula. The imaging shows a clear arc-like structure reminiscent of a wind bow shock in between the ionization cone and shockionized nebula. The observed structure can be fit well by the parabolic shape of a wind bow shock. If an interpretation of a wind bow shock system is valid, we investigate the N159-O1 star cluster as a potential parent of the system, suggesting a progenitor mass of ∼ 60 M for the black hole. We further note that the radio emission could be non-thermal emission from the wind bow shock, or synchrotron emission associated with the jet inflated nebula. For both wind and jet-powered origins, this would represent one of the first radio detections of such a structure.
Proceedings of the International Astronomical Union, 2015
We have obtained optical to near-infrared (300-2500 nm) VLT/X-shooter spectra of six candidate mY... more We have obtained optical to near-infrared (300-2500 nm) VLT/X-shooter spectra of six candidate mYSOs, deeply embedded in the massive star forming region M17. These mYSO candidates have been identified based on their infrared excess and spectral features (double-peaked emission lines, CO band-head emission) indicating the presence of a disk (Hanson et al. 1997). In most cases, we detect a photospheric spectrum allowing us to measure the physical properties of the mYSOs and to confirm their PMS nature.
We present the results of a study focused on the morphology and dynamical evolution of wind bow s... more We present the results of a study focused on the morphology and dynamical evolution of wind bow shocks produced by massive runaway stars in the diffuse interstellar medium. A revision of current analytical models describing the shape and physical parameters of a wind bow shock is proposed, motivated by the recognition that the shocked stellar wind does not need to incorporate into the bow shock as a result of its long cooling time. The semi-analytical expressions we derive, suggest that actual surface densities and velocity patterns in wind bow shocks differ significantly from those previously predicted using the assumption of instantaneous cooling of the shocked gas. To test the general validity of these simplified models and to obtain a more realistic picture of the bow shock characteristics, we have carried out a number of two-dimensional numerical simulations. We have examined the influence of the stellar wind strength, the ambient interstellar density, and the relative velocity of the star on the resulting structures. Particular attention is paid to the role of the physical ingredients used to model the dynamics of the gas, especially the effects of finite cooling times and thermal conduction. The numerical simulations, while confirming in general terms the validity of our simplified, semi-analytical approach, reveal a wealth of details in the structure and evolution of wind bow shocks. By exploring a representative range of the parameter space, we show that the interaction of the stellar wind with the ambient medium can give rise to a remarkable diversity of structures, even with moderate changes in the basic input parameters. Depending on these, a bow shock may be stable or unstable, have a simple or a layered structure with different density and temperature domains, or may not form at all. In this context, we briefly discuss the potential of observations of wind bow shocks as diagnostic tools of the stellar wind of the runaway star and the medium in which it travels.
MOONS is a new conceptual design for a Multi-Object Optical and Near-infrared Spectrograph for th... more MOONS is a new conceptual design for a Multi-Object Optical and Near-infrared Spectrograph for the Very Large Telescope (VLT), selected by ESO for a Phase A study. The baseline design consists of ~1000 fibers deployable over a field of view of ~500 square arcmin, the largest patrol field offered by the Nasmyth focus at the VLT. The total wavelength coverage is 0.8µm-1.8µm and two resolution modes: medium resolution and high resolution. In the medium resolution mode (R~4,000-6,000) the entire wavelength range 0.8µm-1.8µm is observed simultaneously, while the high resolution mode covers simultaneously three selected spectral regions: one around the CaII triplet (at R~8,000) to measure radial velocities, and two regions at R~20,000 one in the J-band and one in the H-band, for detailed measurements of chemical abundances. The grasp of the 8.2m Very Large Telescope (VLT) combined with the large multiplex and wavelength coverage of MOONS-extending into the near-IR-will provide the observational power necessary to study galaxy formation and evolution over the entire history of the Universe, from our Milky Way, through the redshift desert and up to the epoch of re-ionization at z>8-9. At the same time, the high spectral resolution mode will allow astronomers to study chemical abundances of stars in our Galaxy, in particular in the highly obscured regions of the Bulge, and provide the necessary follow-up of the Gaia mission. Such characteristics and versatility make MOONS the long-awaited workhorse near-IR MOS for the VLT, which will perfectly complement optical spectroscopy performed by FLAMES and VIMOS.
The recent finding by Chevalier & Ilovaisky (1998) from Hipparcos observations that OB-supergiant... more The recent finding by Chevalier & Ilovaisky (1998) from Hipparcos observations that OB-supergiant X-ray binaries have relatively large runaway velocities (mean peculiar tangential velocity 1 v tr = 42 ± 14 km s −1), whereas Be/X-ray binaries have low runaway velocities (v tr = 15 ± 6 km s −1), provides confirmation of the current models for the formation of these two types of systems. These predict a difference in runaway velocity of this order of magnitude. This difference basically results from the variation of the fractional helium core mass as a function of stellar mass, in combination with the conservation of orbital angular momentum during the mass transfer phase that preceded the formation of the compact object in the system. This combination results into: (i) Systematically narrower pre-supernova orbits in the OB-supergiant systems than in the Be-systems, and (ii) A larger fractional amount of mass ejected in the supernovae in high-mass systems relative to systems of lower mass. Regardless of possible kick velocities imparted to neutron stars at birth, this combination leads to a considerable difference in average runaway velocity between these two groups. If one includes the possibility for non-conservative mass transfer the predicted difference between the runaway velocity of the two groups becomes even more pronounced. The observed low runaway velocities of the Be/X-ray binaries confirm that in most cases not more than 1 to 2 M was ejected in the supernovae that produced their neutron stars. This, in combination with the-on average-large orbital eccentricities of these systems, indicates that their neutron stars must have received a velocity kick in the range 60-250 km s −1 at birth. The considerable runaway velocity of Cygnus X-1 (v tr = 50± 15 km s −1) shows that also with the formation of a black hole considerable mass ejection takes place.
Iau Colloq 113 Physics of Luminous Blue Variables, 1989
We report rapid, systematic changes in the absorption parts of the P Cygni profiles of the resona... more We report rapid, systematic changes in the absorption parts of the P Cygni profiles of the resonance lines of Si IV, CIV, and N v in four 0 stars. We find two types of variability: (1) in the form of the well-known "discrete absorption components", which appear episodically at low (but supersonic) velocity in the P Cygni profiles and accelerate through the profile. The recurrence time scale varies approximately with the stellar rotation period. The acceleration differs from star to star and, for a given star, from episode to episode; (2) significant (10%) changes in the velocity of the steep edge of the saturated P Cygni profiles, which are correlated with the discrete component behavior in the unsaturated Si IV line. This implies that the steep edge can not be a good measure of the terminal velocity of the wind. We propose an alternative determination of the maximum bulk velocity. On the ground of these (and other) observations we suggest that both types of variability are typical for all early-type stars. The magnitude of the effect varies, however, from star to star as well as for a given star.
The Influence of Binaries on Stellar Population Studies, 2001
High-mass X-ray binaries (HMXBs) represent an important phase in the evolution of massive binary ... more High-mass X-ray binaries (HMXBs) represent an important phase in the evolution of massive binary systems. HMXBs provide unique diagnostics to test massive-star evolution, to probe the physics of radiationdriven winds, to study the process of mass accretion, and to measure fundamental parameters of compact objects. As a consequence of the supernova explosion that produced the neutron star (or black hole) in these systems, HMXBs have high space velocities and thus are runaways. Alternatively, OB-runaway stars can be ejected from a cluster through dynamical interactions. Observations obtained with the Hipparcos satellite indicate that both scenarios are at work. Only for a minority of the OB runaways (and HMXBs) a wind bow shock has been detected. This might be explained by the varying local conditions of the interstellar medium.
We present a quantitative study of the variability in ultraviolet resonance lines of N v, Si IV a... more We present a quantitative study of the variability in ultraviolet resonance lines of N v, Si IV and C iv of the Be starr 7 Cas. For this purpose we used IUE spectra obtained over a period of eleven years. Variability occurs in the form of discretee absorption components (DACs), which are formed in the fast-outflowing radiatively driven part of the stellar wind. Wee constructed a template spectrum from spectra containing no or minor extra absorption due to DACs and modelled the isolatedd DACs in the obtained quotient spectra. Besides the frequently observed narrow components (vi typically < 250 km/s)) at high velocity, we found several broad components occurring at low and intermediate wind velocities. Wee confirm the finding of Doazan et al. (1987) who reported that the number of observed DACs is associated with the cyclic V/RR variability of the Balmer-emission lines. We show that when V/R<1 the central optical depth of DACs is significantly lowerr than when V/R>1. In our interpretation this is due to a correlation between the column density associated with the DACss and the phase of the V/R cycle. Wee find that the H/3 observations of Doazan et al. are consistent with a model in which the cyclic V/R variability is due too a global, one-armed oscillation moving through an equatorial disc. We suggest that the higher column density of DACs inn phases of V/R> 1 is the result of the higher density in the region of their origin, namely close to or in the part of the equatoriall disc which is rotating towards the observer.
arXiv: Instrumentation and Methods for Astrophysics, 2020
MOSAIC is the planned multi-object spectrograph for the 39m Extremely Large Telescope (ELT). Conc... more MOSAIC is the planned multi-object spectrograph for the 39m Extremely Large Telescope (ELT). Conceived as a multi-purpose instrument, it offers both high multiplex and multi-IFU capabilities at a range of intermediate to high spectral resolving powers in the visible and the near-infrared. MOSAIC will enable unique spectroscopic surveys of the faintest sources, from the oldest stars in the Galaxy and beyond to the first populations of galaxies that completed the reionisation of the Universe--while simultaneously opening up a wide discovery space. In this contribution we present the status of the instrument ahead of Phase B, showcasing the key science cases as well as introducing the updated set of top level requirements and the adopted architecture. The high readiness level will allow MOSAIC to soon enter the construction phase, with the goal to provide the ELT community with a world-class MOS capability as soon as possible after the telescope first light.
Johan Richard1, Roland Bacon1, Jérémy Blaizot1, Samuel Boissier2, Alessandro Boselli2, Nicolas Bo... more Johan Richard1, Roland Bacon1, Jérémy Blaizot1, Samuel Boissier2, Alessandro Boselli2, Nicolas Bouché1, Jarle Brinchmann3,4, Norberto Castro5, Laure Ciesla2, Paul Crowther6, Emanuele Daddi7, Stefan Dreizler8, Pierre-Alain Duc9, David Elbaz7, Benoit Épinat2, Chris Evans10, Matteo Fossati11, Michele Fumagalli11, Miriam Garcia12, Thibault Garel1,13, Matthew Hayes14, Artemio Herrero15,16, Emmanuel Hugot2, Andrew Humphrey3, Pascale Jablonka17, Sebastian Kamann18, Lex Kaper19, Andreas Kelz5, Jean-Paul Kneib17, Alex de Koter19,20, Davor Krajnović5, Rolf-Peter Kudritzki21, Norbert Langer22, Carmela Lardo17, Floriane Leclercq13, Danny Lennon15, Guillaume Mahler23, Fabrice Martins24, Richard Massey11, Peter Mitchell4, Ana Monreal-Ibero15,16, Paco Najarro12, Cyrielle Opitom25, Polychronis Papaderos3,26, Céline Péroux28,2, Yves Revaz17, Martin M. Roth5, Philippe Rousselot29, Andreas Sander30, Charlotte Simmonds Wagemann13, Ian Smail11, Anthony Mark Swinbank11, Frank Tramper31, Tanya Urrutia5, A...
We report on UBVRIZJsHKs-band photometry of the dark GRB 000210 host galaxy. Fitting a grid of sp... more We report on UBVRIZJsHKs-band photometry of the dark GRB 000210 host galaxy. Fitting a grid of spectral templates to its Spectral Energy Distribution (SED), we derived a photometric redshift (z=0.842\+0.0540.042) which is in excellent agreement with the spectroscopic one (z=0.8463+/-0.0002; Piro et al. 2002). The best fit to the SED is obtained with a blue starburst template with an age of 0.181\+0.0370.026 Gyr. We discuss the implications of the inferred low value of Av and the age of the dominant stellar population for the non detection of the GRB 000210 optical afterglow.
Until a few years ago the common understanding was that neutron stars, the compact remnants of ma... more Until a few years ago the common understanding was that neutron stars, the compact remnants of massive stars, have a canonical mass of about 1.4 MA. Recent observations with VLT/UVES support the view that the neutron stars in high-mass X-ray binaries display a relatively large spread in mass, ranging from the theoretical lower mass limit of 1 MA up to over 2 MA. Such a mass distribution provides important information on the formation mechanism of neutron stars (i.e. the supernovae), and on the (unknown) behaviour of matter at supranuclear densities.
A new time-series analysis of profile changes in the photospheric He I λ4713 spectral line from d... more A new time-series analysis of profile changes in the photospheric He I λ4713 spectral line from data taken during 5 days in 1989 at the Calar Alto and Kitt Peak observatories has provided evidence for the presence of a non-radial prograde p-mode in the O 7.5 giant ξ Per (̀ = 3, P = 3.5) and probably two such modes in the O6 supergiant λ Cep (̀ = 3, P = 12.3 h and̀ = 5, P = 6.6 h). The corotating pulsation periods are in both cases much shorter than the estimated stellar rotation period. Modeling the observed amplitude of the line-profile changes (assuming |m| = `) yields a velocity amplitude of approximately 5 km s −1 of the pulsation inξ Per and 6 km s −1 in λ Cep. Any such a pulsation by itself is unlikely to be the cause of the well-known cyclical wind variability in these stars because the pulsation period is too short, but the cumulative action of multiple modes could cause such an effect. Weak magnetic fields anchored at the surface remain the strongest candidate for the origi...
Context. A pioneering study showed that the fine structure in the luminosity function (LF) of you... more Context. A pioneering study showed that the fine structure in the luminosity function (LF) of young star clusters contains information about the evolutionary stage (age) and composition of the stellar population. The notable features include the H-peak, which is the result of the onset of hydrogen burning turning pre-main sequence stars into main sequence stars. The feature moves toward the faint end of the LF, and eventually disappears as the population evolves. Another detectable feature is the Wielen dip, a dip at MV ≃ 7 mag in the LF first identified in 1974 for stars in the solar environment. Later studies also identified this feature in the LF of star clusters. The Wielen dip is caused by the increased importance of H− opacity in a certain range of low-mass stars. Aims. We studied the detailed structure in the luminosity function using the data from Gaia DR2 and PARSEC stellar evolution models with the aim to further our understanding of young stellar populations. Methods. We ...
Rapid rotation in massive stars imposes a latitudinal variation in the mass loss from radiatively... more Rapid rotation in massive stars imposes a latitudinal variation in the mass loss from radiatively driven winds that can lead to enhanced mass loss at the poles (with little angular momentum loss) and/or equator (with maximal angular momentum loss). Here we present an examination of the stellar wind lines of the two O-type stars with the fastest known equatorial velocities, VFTS 102 (V sin i = 610 ± 30 km s −1 ; O9: Vnnne+) and VFTS 285 (V sin i = 609 ± 29 km s −1 ; O7.5 Vnnn) in the Large Magellanic Cloud. Ultraviolet spectra of both stars were obtained with the Hubble Space Telescope Cosmic Origins Spectrograph. The spectrum of VFTS 285 displays a fast outflow in N V and a much slower wind in Si IV, and we argue that there is a two-wind regime in which mass loss is strong at the poles (fast and tenuous wind) but dominant at the equator (slow and dense winds). These ions and wind lines are not present in the spectrum of the cooler star VFTS 102, but the double-peaked Hα emission in its spectrum implies equatorial mass loss into a circumstellar disk. The results suggest that in the fastest rotating O-stars, most mass is lost as an equatorial outflow, promoting angular momentum loss that contributes to a spin down over time.
MAXI J1621-501 is the first Swift/XRT Deep Galactic Plane Survey transient that was followed up w... more MAXI J1621-501 is the first Swift/XRT Deep Galactic Plane Survey transient that was followed up with a multitude of space missions (NuSTAR, Swift, Chandra, NICER, INTEGRAL, and MAXI) and ground-based observatories (Gemini, IRSF, and ATCA). The source was discovered with MAXI on 2017 October 19 as a new, unidentified transient. Further observations with NuSTAR revealed 2 Type I X-ray bursts, identifying MAXI J1621-501 as a Low Mass X-ray Binary (LMXB) with a neutron star primary. Overall, 24 Type I bursts were detected from the source during a 15 month period. At energies below 10 keV, the source spectrum was best fit with three components: an absorbed blackbody with kT = 2.3 keV, a cutoff power law with index Γ = 0.7, and an emission line centered on 6.3 keV. Timing analysis of the X-ray persistent emission and burst data has not revealed coherent pulsations from the source or an orbital period. We identified, however, a super-orbital period ∼ 78 days in the source X-ray light curve. This period agrees very well with the theoretically predicted radiative precession period of ∼ 82 days. Thus, MAXI J1621-501 joins a small group of sources characterized with super-orbital periods.
Polarimetric studies of light transmitted through interstellar clouds may give constraints on the... more Polarimetric studies of light transmitted through interstellar clouds may give constraints on the properties of the interstellar dust grains. Traditionally, broadband linear polarisation (BBLP) measurements have been considered an important diagnostic tool for the study of the interstellar dust, while comparatively less attention has been paid to spectropolarimetric measurements. However, spectropolarimetry may offer stronger constraints than BBLP, for example by revealing narrowband features, and by allowing us to distinguish the contribution of dust from the contribution of interstellar gas. Therefore, we have decided to carry out a Large Interstellar Polarisation Survey (LIPS) using spectropolarimetric facilities in both hemispheres. Here we present the results obtained in the Southern Hemisphere with the FORS2 instrument of the ESO Very Large Telescope. Our spectra cover the wavelength range 380-950 nm at a spectral resolving power of about 880. We have produced a publicly available catalogue of 127 linear polarisation spectra of 101 targets. We also provide the Serkowski-curve parameters, as well as the wavelength gradient of the polarisation position angle for the interstellar polarisation along 76 different lines of sight. In agreement with previous literature, we found that the best-fit parameters of the Serkowski-curve are not independent of each other. However, the relationships that we obtained are not always consistent with what has been found in previous studies.
The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquito... more The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquitously present in the interstellar medium (ISM). After decades of study, two strong and possibly three weak near-infrared DIBs have recently been attributed to the C + 60 fullerene based on observational and laboratory measurements. There is great promise for the identification of the over 400 other known DIBs, as this result could provide chemical hints towards other possible carriers. In an effort to systematically study the properties of the DIB carriers, we have initiated a new large-scale observational survey: the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES). The main objective is to build on and extend existing DIB surveys to make a major step forward in characterising the physical and chemical conditions for a statistically significant sample of interstellar lines-of-sight, with the goal to reverse-engineer key molecular properties of the DIB carriers. EDIBLES is a filler Large Programme using the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope at Paranal, Chile. It is designed to provide an observationally unbiased view of the presence and behaviour of the DIBs towards early-spectral-type stars whose lines-of-sight probe the diffuse-to-translucent ISM. Such a complete dataset will provide a deep census of the atomic and molecular content, physical conditions, chemical abundances and elemental depletion levels for each sightline. Achieving these goals requires a homogeneous set of high-quality data in terms of resolution (R ∼ 70 000-100 000), sensitivity (S/N up to 1000 per resolution element), and spectral coverage (305-1042 nm), as well as a large sample size (100+ sightlines). In this first paper the goals, objectives and methodology of the EDIBLES programme are described and an initial assessment of the data is provided.
Publications of the Astronomical Society of the Pacific
We provide new observations of the LMC X-1 O star and its extended nebula structure using spectro... more We provide new observations of the LMC X-1 O star and its extended nebula structure using spectroscopic data from VLT/UVES as well as Hα imaging from the Wide Field Imager on the Max Planck Gesellschaft / European Southern Observatory 2.2m telescope and ATCA imaging of the 2.1 GHz radio continuum. This nebula is one of the few known to be energized by an X-ray binary. We use a new spectrum extraction technique that is superior to other methods to obtain both radial velocities and fluxes. This provides an updated spatial velocity of 21.0 ± 4.8 km s −1 for the O star. The slit encompasses both the photo-ionized and shock-ionized regions of the nebula. The imaging shows a clear arc-like structure reminiscent of a wind bow shock in between the ionization cone and shockionized nebula. The observed structure can be fit well by the parabolic shape of a wind bow shock. If an interpretation of a wind bow shock system is valid, we investigate the N159-O1 star cluster as a potential parent of the system, suggesting a progenitor mass of ∼ 60 M for the black hole. We further note that the radio emission could be non-thermal emission from the wind bow shock, or synchrotron emission associated with the jet inflated nebula. For both wind and jet-powered origins, this would represent one of the first radio detections of such a structure.
Proceedings of the International Astronomical Union, 2015
We have obtained optical to near-infrared (300-2500 nm) VLT/X-shooter spectra of six candidate mY... more We have obtained optical to near-infrared (300-2500 nm) VLT/X-shooter spectra of six candidate mYSOs, deeply embedded in the massive star forming region M17. These mYSO candidates have been identified based on their infrared excess and spectral features (double-peaked emission lines, CO band-head emission) indicating the presence of a disk (Hanson et al. 1997). In most cases, we detect a photospheric spectrum allowing us to measure the physical properties of the mYSOs and to confirm their PMS nature.
We present the results of a study focused on the morphology and dynamical evolution of wind bow s... more We present the results of a study focused on the morphology and dynamical evolution of wind bow shocks produced by massive runaway stars in the diffuse interstellar medium. A revision of current analytical models describing the shape and physical parameters of a wind bow shock is proposed, motivated by the recognition that the shocked stellar wind does not need to incorporate into the bow shock as a result of its long cooling time. The semi-analytical expressions we derive, suggest that actual surface densities and velocity patterns in wind bow shocks differ significantly from those previously predicted using the assumption of instantaneous cooling of the shocked gas. To test the general validity of these simplified models and to obtain a more realistic picture of the bow shock characteristics, we have carried out a number of two-dimensional numerical simulations. We have examined the influence of the stellar wind strength, the ambient interstellar density, and the relative velocity of the star on the resulting structures. Particular attention is paid to the role of the physical ingredients used to model the dynamics of the gas, especially the effects of finite cooling times and thermal conduction. The numerical simulations, while confirming in general terms the validity of our simplified, semi-analytical approach, reveal a wealth of details in the structure and evolution of wind bow shocks. By exploring a representative range of the parameter space, we show that the interaction of the stellar wind with the ambient medium can give rise to a remarkable diversity of structures, even with moderate changes in the basic input parameters. Depending on these, a bow shock may be stable or unstable, have a simple or a layered structure with different density and temperature domains, or may not form at all. In this context, we briefly discuss the potential of observations of wind bow shocks as diagnostic tools of the stellar wind of the runaway star and the medium in which it travels.
MOONS is a new conceptual design for a Multi-Object Optical and Near-infrared Spectrograph for th... more MOONS is a new conceptual design for a Multi-Object Optical and Near-infrared Spectrograph for the Very Large Telescope (VLT), selected by ESO for a Phase A study. The baseline design consists of ~1000 fibers deployable over a field of view of ~500 square arcmin, the largest patrol field offered by the Nasmyth focus at the VLT. The total wavelength coverage is 0.8µm-1.8µm and two resolution modes: medium resolution and high resolution. In the medium resolution mode (R~4,000-6,000) the entire wavelength range 0.8µm-1.8µm is observed simultaneously, while the high resolution mode covers simultaneously three selected spectral regions: one around the CaII triplet (at R~8,000) to measure radial velocities, and two regions at R~20,000 one in the J-band and one in the H-band, for detailed measurements of chemical abundances. The grasp of the 8.2m Very Large Telescope (VLT) combined with the large multiplex and wavelength coverage of MOONS-extending into the near-IR-will provide the observational power necessary to study galaxy formation and evolution over the entire history of the Universe, from our Milky Way, through the redshift desert and up to the epoch of re-ionization at z>8-9. At the same time, the high spectral resolution mode will allow astronomers to study chemical abundances of stars in our Galaxy, in particular in the highly obscured regions of the Bulge, and provide the necessary follow-up of the Gaia mission. Such characteristics and versatility make MOONS the long-awaited workhorse near-IR MOS for the VLT, which will perfectly complement optical spectroscopy performed by FLAMES and VIMOS.
The recent finding by Chevalier & Ilovaisky (1998) from Hipparcos observations that OB-supergiant... more The recent finding by Chevalier & Ilovaisky (1998) from Hipparcos observations that OB-supergiant X-ray binaries have relatively large runaway velocities (mean peculiar tangential velocity 1 v tr = 42 ± 14 km s −1), whereas Be/X-ray binaries have low runaway velocities (v tr = 15 ± 6 km s −1), provides confirmation of the current models for the formation of these two types of systems. These predict a difference in runaway velocity of this order of magnitude. This difference basically results from the variation of the fractional helium core mass as a function of stellar mass, in combination with the conservation of orbital angular momentum during the mass transfer phase that preceded the formation of the compact object in the system. This combination results into: (i) Systematically narrower pre-supernova orbits in the OB-supergiant systems than in the Be-systems, and (ii) A larger fractional amount of mass ejected in the supernovae in high-mass systems relative to systems of lower mass. Regardless of possible kick velocities imparted to neutron stars at birth, this combination leads to a considerable difference in average runaway velocity between these two groups. If one includes the possibility for non-conservative mass transfer the predicted difference between the runaway velocity of the two groups becomes even more pronounced. The observed low runaway velocities of the Be/X-ray binaries confirm that in most cases not more than 1 to 2 M was ejected in the supernovae that produced their neutron stars. This, in combination with the-on average-large orbital eccentricities of these systems, indicates that their neutron stars must have received a velocity kick in the range 60-250 km s −1 at birth. The considerable runaway velocity of Cygnus X-1 (v tr = 50± 15 km s −1) shows that also with the formation of a black hole considerable mass ejection takes place.
Iau Colloq 113 Physics of Luminous Blue Variables, 1989
We report rapid, systematic changes in the absorption parts of the P Cygni profiles of the resona... more We report rapid, systematic changes in the absorption parts of the P Cygni profiles of the resonance lines of Si IV, CIV, and N v in four 0 stars. We find two types of variability: (1) in the form of the well-known "discrete absorption components", which appear episodically at low (but supersonic) velocity in the P Cygni profiles and accelerate through the profile. The recurrence time scale varies approximately with the stellar rotation period. The acceleration differs from star to star and, for a given star, from episode to episode; (2) significant (10%) changes in the velocity of the steep edge of the saturated P Cygni profiles, which are correlated with the discrete component behavior in the unsaturated Si IV line. This implies that the steep edge can not be a good measure of the terminal velocity of the wind. We propose an alternative determination of the maximum bulk velocity. On the ground of these (and other) observations we suggest that both types of variability are typical for all early-type stars. The magnitude of the effect varies, however, from star to star as well as for a given star.
The Influence of Binaries on Stellar Population Studies, 2001
High-mass X-ray binaries (HMXBs) represent an important phase in the evolution of massive binary ... more High-mass X-ray binaries (HMXBs) represent an important phase in the evolution of massive binary systems. HMXBs provide unique diagnostics to test massive-star evolution, to probe the physics of radiationdriven winds, to study the process of mass accretion, and to measure fundamental parameters of compact objects. As a consequence of the supernova explosion that produced the neutron star (or black hole) in these systems, HMXBs have high space velocities and thus are runaways. Alternatively, OB-runaway stars can be ejected from a cluster through dynamical interactions. Observations obtained with the Hipparcos satellite indicate that both scenarios are at work. Only for a minority of the OB runaways (and HMXBs) a wind bow shock has been detected. This might be explained by the varying local conditions of the interstellar medium.
Uploads
Papers by Lex Kaper