Search

Over the past decades, libraries of stellar spectra have been used in a large variety of science cases, including as sources of reference spectra for a given object or a given spectral type. Despite the existence of large libraries and the increasing number of projects of large-scale spectral surveys, there is to date only one very high-resolution spectral library offering spectra from a few hundred objects from the southern hemisphere (UVES-POP) . We aim to extend the sample, offering a finer coverage of effective temperatures and surface gravity with a uniform collection of spectra obtained in the northern hemisphere. Between 2010 and 2020, we acquired several thousand echelle spectra of bright stars with the Mercator-HERMES spectrograph located in the Roque de Los Muchachos Observatory in La Palma, whose pipeline offers high-quality data reduction products. We have also developed methods to correct for the instrumental response in order to approach the true shape of the spectral continuum. Additionally, we have devised a normalisation process to provide a homogeneous normalisation of the full spectral range for most of the objects. We present a new spectral library consisting of 3256 spectra covering 2043 stars. It combines high signal-to-noise and high spectral resolution over the entire range of effective temperatures and luminosity classes. The spectra are presented in four versions: raw, corrected from the instrumental response, with and without correction from the atmospheric molecular absorption, and normalised (including the telluric correction)., Comment: 17 pages, 18 figures Preview and access to the library: https://www.royer.se/melchiors.html

The temporal and spectral evolution of gamma-ray burst (GRB) afterglows can be used to infer the density and density profile of the medium through which the shock is propagating. In long-duration (core-collapse) GRBs, the circumstellar medium (CSM) is expected to resemble a wind-blown bubble, with a termination shock separating the stellar wind and the interstellar medium (ISM). A long standing problem is that flat density profiles, indicative of the ISM, are often found at lower radii than expected for a massive star progenitor. Furthermore, the presence of both wind-like environments at high radii and ISM-like environments at low radii remains a mystery. In this paper, we perform a 'CSM population synthesis' with long GRB progenitor stellar evolution models. Analytic results for the evolution of wind blown bubbles are adjusted through comparison with a grid of 2D hydrodynamical simulations. Predictions for the emission radii, ratio of ISM to wind-like environments, wind and ISM densities are compared with the largest sample of afterglow-derived parameters yet compiled, which we make available for the community. We find that high ISM densities around 1000/cm3 best reproduce observations. If long GRBs instead occur in typical ISM densities of approximately 1/cm3, then the discrepancy between theory and observations is shown to persist at a population level. We discuss possible explanations for the origin of variety in long GRB afterglows, and for the overall trend of CSM modelling to over-predict the termination shock radius., Comment: 22 pages, 15 figures, 4 tables. Accepted for publication in MNRAS

Massive stars that have been ejected from their parent cluster and supersonically sailing away through the interstellar medium (ISM) are classified as exiled. They generate circumstellar bow shock nebulae that can be observed. We present two-dimensional, axisymmetric hydrodynamical simulations of a representative sample of stellar wind bow shocks from Galactic OB stars in an ambient medium of densities ranging from n_ISM=0.01 up to 10.0/cm3. Independently of their location in the Galaxy, we confirm that the infrared is the most appropriated waveband to search for bow shocks from massive stars. Their spectral energy distribution is the convenient tool to analyze them since their emission does not depend on the temporary effects which could affect unstable, thin-shelled bow shocks. Our numerical models of Galactic bow shocks generated by high-mass (~40 Mo) runaway stars yield H$\alpha$ fluxes which could be observed by facilities such as the SuperCOSMOS H-Alpha Survey. The brightest bow shock nebulae are produced in the denser regions of the ISM. We predict that bow shocks in the field observed at Ha by means of Rayleigh-sensitive facilities are formed around stars of initial mass larger than about 20 Mo. Our models of bow shocks from OB stars have the emission maximum in the wavelength range 3 <= lambda <= 50 micrometer which can be up to several orders of magnitude brighter than the runaway stars themselves, particularly for stars of initial mass larger than 20 Mo., Comment: 13 pages, 12 figures. Accepted to MNRAS (2016)

We aim to examine the role of the interstellar magnetic field in shaping the extended morphologies of slow dusty winds of Asymptotic Giant-branch (AGB) stars in an effort to pin-point the origin of so-called eye shaped CSE of three carbon-rich AGB stars. In addition, we seek to understand if this pre-planetary nebula (PN) shaping can be responsible for asymmetries observed in PNe. Hydrodynamical simulations are used to study the effect of typical interstellar magnetic fields on the free-expanding spherical stellar winds as they sweep up the local interstellar medium (ISM). The simulations show that typical Galactic interstellar magnetic fields of 5 to 10 muG, are sufficient to alter the spherical expanding shells of AGB stars to appear as the characteristic eye shape revealed by far-infrared observations. The typical sizes of the simulated eyes are in accordance with the observed physical sizes. However, the eye shapes are of transient nature. Depending on the stellar and interstellar conditions they develop after 20,000 to 200,000yrs and last for about 50,000 to 500,000 yrs, assuming that the star is at rest relative to the local interstellar medium. Once formed the eye shape will develop lateral outflows parallel to the magnetic field. The "explosion" of a PN in the center of the eye-shaped dust shell gives rise to an asymmetrical nebula with prominent inward pointing Rayleigh-Taylor instabilities. Interstellar magnetic fields can clearly affect the shaping of wind-ISM interaction shells. The occurrence of the eyes is most strongly influenced by stellar space motion and ISM density. Observability of this transient phase is favoured for lines-of-sight perpendicular to the interstellar magnetic field direction. The simulations indicate that shaping of the pre-PN envelope can strongly affect the shape and size of PNe., Comment: Accepted for publication in A&A. Final version will contain animated results

The unparalleled photometric data obtained by NASA's Kepler space telescope led to an improved understanding of red giant stars and binary stars. Seismology allows us to constrain the properties of red giants. In addition to eclipsing binaries, eccentric non-eclipsing binaries, exhibiting ellipsoidal modulations, have been detected with Kepler. We aim to study the properties of eccentric binary systems containing a red giant star and derive the parameters of the primary giant component. We apply asteroseismic techniques to determine masses and radii of the primary component of each system. For a selected target, light and radial velocity curve modelling techniques are applied to extract the parameters of the system. The effects of stellar on the binary system are studied. The paper presents the asteroseismic analysis of 18 pulsating red giants in eccentric binary systems, for which masses and radii were constrained. The orbital periods of these systems range from 20 to 440days. From radial velocity measurements we find eccentricities between e=0.2 to 0.76. As a case study we present a detailed analysis of KIC5006817. From seismology we constrain the rotational period of the envelope to be at least 165 d, roughly twice the orbital period. The stellar core rotates 13 times faster than the surface. From the spectrum and radial velocities we expect that the Doppler beaming signal should have a maximum amplitude of 300ppm in the light curve. Through binary modelling, we determine the mass of the secondary component to be 0.29$\pm$0.03\,$M_\odot$. For KIC5006817 we exclude pseudo-synchronous rotation of the red giant with the orbit. The comparison of the results from seismology and modelling of the light curve shows a possible alignment of the rotational and orbital axis at the 2$\sigma$ level. Red giant eccentric systems could be progenitors of cataclysmic variables and hot subdwarf B stars., Comment: 18 pages, 19 figures, accepted for publication in Astronomy & Astrophysics

Context. The interaction between stellar winds and the interstellar medium (ISM) can create complex bow shocks. The photometers on board the Herschel Space Observatory are ideally suited to studying the morphologies of these bow shocks. Aims. We aim to study the circumstellar environment and wind-ISM interaction of the nearest red supergiant, Betelgeuse. Methods. Herschel PACS images at 70, 100, and 160 micron and SPIRE images at 250, 350, and 500 micron were obtained by scanning the region around Betelgeuse. These data were complemented with ultraviolet GALEX data, near-infrared WISE data, and radio 21 cm GALFA-HI data. The observational properties of the bow shock structure were deduced from the data and compared with hydrodynamical simulations. Results. The infrared Herschel images of the environment around Betelgeuse are spectacular, showing the occurrence of multiple arcs at 6-7 arcmin from the central target and the presence of a linear bar at 9 arcmin. Remarkably, no large-scale instabilities are seen in the outer arcs and linear bar. The dust temperature in the outer arcs varies between 40 and 140 K, with the linear bar having the same colour temperature as the arcs. The inner envelope shows clear evidence of a non-homogeneous clumpy structure (beyond 15 arcsec), probably related to the giant convection cells of the outer atmosphere. The non-homogeneous distribution of the material even persists until the collision with the ISM. A strong variation in brightness of the inner clumps at a radius of 2 arcmin suggests a drastic change in mean gas and dust density some 32 000 yr ago. Using hydrodynamical simulations, we try to explain the observed morphology of the bow shock around Betelgeuse. Conclusions: [abbreviated], Comment: 26 pages

Far-infrared Herschel/PACS images at 70 and 160 micron of a sample of 78 Galactic evolved stars are used to study the (dust) emission structures, originating from stellar wind-ISM interaction. In addition, two-fluid hydrodynamical simulations of the coupled gas and dust in wind-ISM interactions are used to compare with the observations. Four distinct classes of wind-ISM interaction (i.e. "fermata", "eyes", "irregular", and "rings") are identified and basic parameters affecting the morphology are discussed. We detect bow shocks for ~40% of the sample and detached rings for ~20%. De-projected stand-off distances (R_0) -- defined as the distance between the central star and the nearest point of the interaction region -- of the detected bow shocks ("fermata" and "eyes") are derived from the PACS images and compared to previous results, model predictions and the simulations. All observed bow shocks have stand-off distances smaller than 1 pc. Observed and theoretical stand-off distances are used together to independently derive the local ISM density. Both theoretical (analytical) models and hydrodynamical simulations give stand-off distances for adopted stellar properties that are in good agreement with the measured de-projected stand-off distance of wind-ISM bow shocks. The possible detection of a bow shock -- for the distance limited sample -- appears to be governed by its physical size as set roughly by the stand-off distance. In particular the star's peculiar space velocity and the density of the ISM appear decisive in detecting emission from bow shocks or detached rings. Tentatively, the "eyes" class objects are associated to (visual) binaries, while the "rings" generally appear not to occur for M-type stars, only for C or S-type objects that have experienced a thermal pulse., Comment: 29 pages, 15 figures. Accepted for publication in A&A

It has been established that cold plasma condensations can form in a magnetic loop subject to localized heating of the footpoints. In this paper, we use grid-adaptive numerical simulations of the radiative hydrodynamic equations to parametrically investigate the filament formation process in a pre-shaped loop with both steady and finite-time chromospheric heating. Compared to previous works, we consider low-lying loops with shallow dips, and use a more realistic description for the radiative losses. We demonstrate for the first time that the onset of thermal instability satisfies the linear instability criterion. The onset time of the condensation is roughly \sim 2 hr or more after the localized heating at the footpoint is effective, and the growth rate of the thread length varies from 800 km hr-1 to 4000 km hr-1, depending on the amplitude and the decay length scale characterizing this localized chromospheric heating. We show how single or multiple condensation segments may form in the coronal portion. In the asymmetric heating case, when two segments form, they approach and coalesce, and the coalesced condensation later drains down into the chromosphere. With a steady heating, this process repeats with a periodicity of several hours. While our parametric survey confirms and augments earlier findings, we also point out that steady heating is not necessary to sustain the condensation. Once the condensation is formed, it can keep growing also when the localized heating ceases. Finally, we show that the condensation can survive continuous buffeting by perturbations resulting from the photospheric p-mode waves., Comment: 43 pages, 18 figures

The properties of a massive star prior to its final explosion are imprinted in the circumstellar medium (CSM) created by its wind and termination shock. We perform a detailed, comprehensive calculation of the time-variable and angle-dependent transmission spectra of an average-luminosity Gamma-Ray Burst (GRB) which explodes in the CSM structure produced by the collapse of a 20 Msun, rapidly rotating, Z=0.001 progenitor star. We study both the case in which metals are initially in the gaseous phase, as well as the situation in which they are heavily depleted into dust. We find that high-velocity lines from low-ionization states of silicon, carbon, and iron are initially present in the spectrum only if the metals are heavily depleted into dust prior to the GRB explosion. However, such lines disappear on timescales of a fraction of a second for a burst observed on-axis, and of a few seconds for a burst seen at high-latitude, making their observation virtually impossible. Rest-frame lines produced in the termination shock are instead clearly visible in all conditions. We conclude that time-resolved, early-time spectroscopy is not a promising way in which the properties of the GRB progenitor wind can be routinely studied. Previous detections of high velocity features in GRB UV spectra must have been due either due to a superposition of a physically unrelated absorber or to a progenitor star with very unusual properties., Comment: Published in MNRAS; higher resolution figures in published version.

There are large differences in the proposed progenitor models for the Cas A SNR. One of these differences is the presence or absence of a Wolf-Rayet (WR) phase of the progenitor star. The mass loss history of the progenitor star strongly affects the shape of the Supernova remnant (SNR). In this paper we investigate whether the progenitor star of Cas A had a WR phase or not and how long it may have lasted. We performed two-dimensional multi-species hydrodynamical simulations of the CSM around the progenitor star for several WR life times, each followed by the interaction of the supernova ejecta with the CSM. We then looked at the influence of the length of the WR phase and compared the results of the simulations with the observations of Cas A. The difference in the structure of the CSM, for models with different WR life times, has a strong impact on the resulting SNR. With an increasing WR life time the reverse shock velocity of the SNR decreases and the range of observed velocities in the shocked material increases. Furthermore, if a WR phase occurs, the remainders of the WR shell will be visible in the resulting SNR. Comparing our results with the observations suggests that the progenitor star of Cas A did not have a WR phase. We also find that the quasi-stationary flocculi (QSF) in Cas A are not consistent with the clumps from a WR shell that have been shocked and accelerated by the interaction with the SN ejecta. We can also conclude that for a SN explosion taking place in a CSM that is shaped by the wind during a short < 15000 yr WR phase, the clumps from the WR shell will be visible inside the SNR., Comment: 11 figures, 11 pages, accepted for publication in A&A

In the course of the life of a massive star, wind-wind interaction can give rise to the formation of circumstellar nebulae which are both predicted and observed in the nature. We present generic model calculations to predict the properties of such nebulae for blue supergiants. From stellar evolution calculations including rotation, we obtain the time dependence of the stellar wind properties and of the stellar radiation field. These are used as input for hydro-calculations of the circumstellar medium throughout the star's life. Here, we present the results for a rapidly rotating 12 solar masses single star. This star undergoes a blue loop during its post main sequence evolution, at the onset of which its contraction spins it up close to critical rotation. Due to the consequent anisotropic mass loss, the blue supergiant wind sweeps up the preceding slow wind into an hour glass structure. Its collision with the previously formed spherical red supergiant wind shell forms a short-lived luminous nebula consisting of two polar caps and a central inner ring. With time, the polar caps evolve into mid-latitude rings which gradually move toward the equatorial plane while the central ring is fading. These structures are reminiscent to the observed nebulae around the blue supergiant Sher 25 and the progenitor of SN 1987A. The simple model of an hour glass colliding with a spherical shell retrieves most of the intriguing nebula geometries discovered around blue supergiants, and suggests them to form an evolutionary sequence. Our results indicate that binarity is not required to obtain them., Comment: Accepted for publication in Astronomy and Astrophysics Letters; Associated movie available at http://www.astro.uu.nl/~chita

We present the results of numerical simulations of continuum-driven winds of stars that exceed the Eddington limit and compare these against predictions from earlier analytical solutions. Our models are based on the assumption that the stellar atmosphere consists of clumped matter, where the individual clumps have a much larger optical thickness than the matter between the clumps. This `porosity' of the stellar atmosphere reduces the coupling between radiation and matter, since photons tend to escape through the more tenuous gas between the clumps. This allows a star that formally exceeds the Eddington limit to remain stable, yet produce a steady outflow from the region where the clumps become optically thin. We have made a parameter study of wind models for a variety of input conditions in order to explore the properties of continuum-driven winds. The results show that the numerical simulations reproduce quite closely the analytical scalings. The mass loss rates produced in our models are much larger than can be achieved by line driving. This makes continuum driving a good mechanism to explain the large mass loss and flow speeds of giant outbursts, as observed in eta Carinae and other luminous blue variable (LBV) stars. Continuum driving may also be important in population III stars, since line driving becomes ineffective at low metalicities. We also explore the effect of photon tiring and the limits it places on the wind parameters., Comment: Accepted for publication by MNRAS

Continuum driving is an effective method to drive a strong stellar wind. It is governed by two limits: the Eddington limit and the photon-tiring limit. A star must exceed the effective Eddington limit for continuum driving to overcome the stellar gravity. The photon-tiring limit places an upper limit on the mass loss rate that can be driven to infinity, given the energy available in the radiation field of the star. Since continuum driving does not require the presence of metals in the stellar atmosphere it is particularly suited to removing mass from low- and zero-metallicity stars and can play a crucial part in their evolution. Using a porosity length formalism we compute numerical simulations of super-Eddington, continuum driven winds to explore their behaviour for stars both below and above the photon-tiring limit. We find that below the photon tiring limit, continuum driving can produce a large, steady mass loss rate at velocities on the order of the escape velocity. If the star exceeds the photon-tiring limit, a steady solution is no longer possible. While the effective mass loss rate is still very large, the wind velocity is much smaller, Comment: to be published in the conference proceedings of: First Stars III, Santa Fe, 2007

Van Marle et al. (2005) showed that circumstellar absorption lines in early Type Ib/c supernova and gamma-ray burst afterglow spectra may reveal the progenitor evolution of the exploding Wolf-Rayet star. While the quoted paper deals with Wolf-Rayet stars which evolved through a red supergiant stage, we investigate here the initially more massive Wolf-Rayet stars which are thought to evolve through a Luminous Blue Variable (LBV) stage. We perform hydrodynamic simulations of the evolution of the circumstellar medium around a 60 Msol star, from the main sequence through the LBV and Wolf-Rayet stages, up to core collapse. We then compute the column density of the circumstellar matter as a function of radial velocity, time and angle. This allows a comparison with the number and blue-shifts, of absorption components in the spectra of LBVs, Wolf-Rayet stars, Type Ib/c supernovae and gamma-ray burst afterglows. Our simulation for the post-LBV stage shows the formation of various absorption components, which are, however, rather short lived; they dissipate on time scales shorter than 50,000yr. As the LBV stage is thought to occur at the beginning of core helium burning, the remaining Wolf-Rayet life time is expected to be one order of magnitude larger. When interpreting the absorption components in the afterglow spectrum of GRB-021004 as circumstellar, it can be concluded that the progenitor of this source did most likely not evolve through an LBV stage. However, a close binary with late common-envelope phase (Case C) may produce a circumstellar medium that closely resembles the LBV to Wolf-Rayet evolution, but with a much shorter Wolf-Rayet period., Comment: accepted for publication by A&A

We present hydrodynamical models of circumstellar medium (CSM) of long gamma-ray burst (GRB) progenitor candidates. These are massive stars that have lost a large amount of mass in the form of stellar wind during their evolution. There are two possible ways to probe the CSM of long GRB progenitors. Firstly, the GRB afterglow consists of synchrotron radiation, emitted when the GRB jet sweeps up the surrounding medium. Therefore, the lightcurve is directly related to the density profile of the CSM. The density can either decrease with the radius squared (as is the case for a freely expanding stellar wind) or be constant (as we would expect for shocked wind or the interstellar medium). Secondly, material between the GRB and the observer will absorb part of the afterglow radiation, causing absorption lines in the afterglow spectrum. In some cases, such absorption lines are blue-shifted relative to the source indicating that the material is moving away from the progenitor star. This can be explained in terms of wind interactions in the CSM. We can use the CSM of these stars to investigate their prior evolutionary stage., Comment: To appear in the proceedings of: "Circumstellar Media and Late Stages of Massive Stellar Evolution", September 4-8, 2006, Ensenada, Baja California, Mexico

The progenitor stars of long Gamma-Ray Bursts (GRBs) are thought to be Wolf-Rayet stars, which generate a massive and energetic wind. Nevertheless, about 25 percent of all GRB afterglows light curves indicate a constant density medium close to the exploding star. We explore various ways to produce this, by creating situations where the wind termination shock arrives very close to the star, as the shocked wind material has a nearly constant density. Typically, the distance between a Wolf-Rayet star and the wind termination shock is too large to allow afterglow formation in the shocked wind material. Here, we investigate possible causes allowing for a smaller distance: A high density or a high pressure in the surrounding interstellar medium (ISM), a weak Wolf-Rayet star wind, the presence of a binary companion, and fast motion of the Wolf-Rayet star relative to the ISM. We find that all four scenarios are possible in a limited parameter space, but that none of them is by itself likely to explain the large fraction of constant density afterglows. A low GRB progenitor metallicity, and a high GRB energy make the occurrence of a GRB afterglow in a constant density medium more likely. This may be consistent with constant densities beingpreferentially found for energetic, high redshift GRBs., Comment: 13 pages, 13 figures, new version: as accepted by Astronomy & Astrophysics

In the southern Amazon relationships have been established among drought, human activities that cause forest loss, fire, and smoke emissions. We explore the impacts of recent drought on fire, forest loss, and atmospheric visibility in lowland Bolivia. To assess human influence on fire, we consider climate, fire, and vegetation dynamics in an area largely excluded from human activities since 1979, Noel Kempff Mercado National Park (NK) in northeastern Bolivia. We use data from five sources: the Moderate Resolution Imaging Spectroradiometer Collection 6 active fire product (2001-2015) (MODIS C6), Global Fire Weather Database (GFWED) data (1982-2015), MODIS land cover data (2001-2010), MODIS forest loss data (2000-2012), and the regional extinction coefficient for the southwestern Amazon (i.e., B(sub ext)), which is derived from horizontal visibility data from surface stations at the World Meteorological Organization (WMO) level (1973-2015). The B(sub ext) is affected by smoke and acts as a proxy for visibility and regional fire emissions. In lowland Bolivia from 2001 to 2015, interannual Drought Code (DC) variability was linked to fire activity, while from 1982 to 2015, interannual DC variability was linked to B(sub ext). From 2001 to 2015, the B(sub ext) and MODIS C6 active fire data for lowland Bolivia captured fire seasonality, and covaried between low- and high-fire years. Consistent with previous studies, our results suggest B(sub ext)t can be used as a longer-term proxy of regional fire emissions in southwestern Amazonia. Overall, our study found drought conditions were the dominant control on interannual fire variability in lowland Bolivia, and fires within NK were limited to the Cerrado and seasonally inundated wetland biomes. Our results suggest lowland Bolivian tropical forests were susceptible to human activities that may have amplified fire during drought. Human activities and drought need to be considered in future projections of southern Amazonian fire, in regard to carbon emissions and global climate.

Fires have influenced atmospheric composition and climate since the rise of vascular plants, and satellite data has shown the overall global extent of fires. Our knowledge of historic fire emissions has progressively improved over the past decades due mostly to the development of new proxies and the improvement of fire models. Currently there is a suite of proxies including sedimentary charcoal records, measurements of fire-emitted trace gases and black carbon stored in ice and firn, and visibility observations. These proxies provide opportunities to extrapolate emissions estimates based on satellite data starting in 1997, but each proxy has strengths and weaknesses regarding, for example, the spatial and temporal extents over which they are representative. We developed a new historic biomass burning emissions dataset starting in 1750 that merges the satellite record with several existing proxies, and uses the average of six models from the Fire Model Intercomparison Project (FireMIP) protocol to estimate emissions when the available proxies had limited coverage. According to our approach, global biomass burning emissions were relatively constant with 10-year averages varying between 1.8 and 2.3 petagrams of carbon per year. Carbon emissions increased only slightly over the full time period and peaked during the 1990's after which they decreased gradually. There is substantial uncertainty in these estimates and patterns varied depending on choices regarding data representation, especially on regional scales. The observed pattern in fire carbon emissions is for a large part driven by African fires, which accounted for 58 percent of global fire carbon emissions. African fire emissions declined since about 1950 due to conversion of savanna to cropland, and this decrease is partially compensated for by increasing emissions in deforestation zones of South America and Asia. These global fire emissions estimates are mostly suited for global analyses and will be used in the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations.