arXiv subjects

F. Martins

Publications and source records attributed to F. Martins.

At least 37 records · Page 2Linked to original sources

arXiv2021

The effects of surface fossil magnetic fields on massive star evolution: III. The case of $τ$ Sco

$τ$ Sco, a well-studied magnetic B-type star in the Upper Sco association, has a number of surprising characteristics. It rotates very slowly and shows nitrogen excess. Its surface magnetic field is much more complex than a purely dipolar configuration which is unusual for a magnetic massive star. We employ the CMFGEN radiative transfer code to determine the fundamental parameters and surface CNO and helium abundances. Then, we employ MESA and GENEC stellar evolution models accounting for the effects of surface magnetic fields. To reconcile $τ$ Sco's properties with single-star models, an increase is necessary in the efficiency of rotational mixing by a factor of 3 to 10 and in the efficiency of magnetic braking by a factor of 10. The spin down could be explained by assuming a magnetic field decay scenario. However, the simultaneous chemical enrichment challenges the single-star scenario. Previous works indeed suggested a stellar merger origin for $τ$ Sco. However, the merger scenario also faces similar challenges as our magnetic single-star models to explain $τ$ Sco's simultaneous slow rotation and nitrogen excess. In conclusion, the single-star channel seems less likely and versatile to explain these discrepancies, while the merger scenario and other potential binary-evolution channels still require further assessment as to whether they may self-consistently explain the observables of $τ$ Sco.

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arXiv2020

Spectroscopic evolution of massive stars near the main sequence at low metallicity

We present synthetic spectra and SEDs computed along evolutionary tracks at Z=1/5 Zsun and Z=1/30 Zsun, for masses between 15 and 150 Msun. We predict that the most massive stars all start their evolution as O2 dwarfs at sub-solar metallicities. The fraction of lifetime spent in the O2V phase increases at lower metallicity. The distribution of dwarfs and giants we predict in the SMC accurately reproduces the observations. Supergiants appear at slightly higher effective temperatures than we predict. More massive stars enter the giant and supergiant phases closer to the ZAMS, but not as close as for solar metallicity. This is due to the reduced stellar winds at lower metallicity. Our models with masses higher than ~60 Msun should appear as O and B stars, whereas these objects are not observed, confirming a trend reported in the recent literature. At Z=1/30 Zsun, dwarfs cover a wider fraction of the MS and giants and supergiants appear at lower effective temperatures than at Z=1/5 Zsun. The UV spectra of these low-metallicity stars have only weak P-Cygni profiles. HeII 1640 sometimes shows a net emission in the most massive models, with an equivalent width reaching ~1.2 A. For both sets of metallicities, we provide synthetic spectroscopy in the wavelength range 4500-8000 A. This range will be covered by the instruments HARMONI and MOSAICS on the ELT and will be relevant to identify hot massive stars in Local Group galaxies with low extinction. We suggest the use of the ratio of HeI 7065 to HeII 5412 as a diagnostic for spectral type. We show that this ratio does not depend on metallicity. Finally, we discuss the ionizing fluxes of our models. The relation between the hydrogen ionizing flux per unit area versus effective temperature depends only weakly on metallicity. The ratios of HeI and HeII to H ionizing fluxes both depend on metallicity, although in a slightly different way.

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arXiv2019

Spectral properties and detectability of supermassive stars in protoglobular clusters at high redshift

Globular clusters contain multiple stellar populations with peculiar chemical compositions. Pollution of the intracluster gas by an early population of fast-evolving stars is the most common scenario for explaining the observations. Stars with masses in excess of 1000 Msun have recently been suggested as potential polluters. We investigate the spectral properties of proto-GCs that would host a supermassive star. Our main goal is to quantify how such a star would affect the integrated light of the cluster, and to study the detectability of such objects. We computed non-LTE atmosphere models for SMS with various combinations of stellar parameters (luminosity, effective temperature, and mass) and metallicities appropriate for GCs, and we predict their emergent spectra. Using these spectra, we calculated the total emission of young proto-GCs with SMS as predicted in a previously reported scenario, and we computed synthetic photometry in UV, optical, and near-IR bands, in particular for the JWST. At an effective temperature of 10000 K, the spectrum of SMSs shows a Balmer break in emission. This feature is due to strong non-LTE effects (implied by the high luminosity) and is not observed in normal stars. The hydrogen lines also show a peculiar behavior, with Balmer lines in emission while higher series lines are in absorption. At 7000 K, the Balmer break shows a strong absorption. At high effective temperatures, the Lyman break is found in emission. Cool and luminous SMSs are found to dominate the integrated spectrum of the cluster, except for the UV range. The predicted magnitudes of these proto-GCs are m_AB~28-30 between 0.7 and 8 um and for redshifts z~4-10, which is detectable with the JWST. The peculiar observational features of cool SMSs imply that they might in principle be detected in color-color diagrams that probe the spectral energy distribution below and above the Balmer break.

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arXiv2019

Massive stars in extremely metal-poor galaxies: A window into the past

Cosmic History has witnessed the lives and deaths of multiple generations of massive stars, all of them invigorating their host galaxies with ionizing photons, kinetic energy, fresh material and stellar-mass black holes. Ubiquitous engines as they are, Astrophysics needs a good understanding of their formation, evolution, properties and yields throughout the history of the Universe, and with decreasing metal content mimicking the environment at the earliest epochs. Ultimately, a physical model that could be extrapolated to zero metallicity would enable tackling long-standing questions such as "What did the First, very massive stars of the Universe look like?" or "What was their role in the re-ionization of the Universe?". Yet, most our knowledge of metal-poor massive stars is drawn from one single point in metallicity. Massive stars in the Small Magellanic Cloud (SMC, $\sim 1/5 Z_{\odot}$) currently serve as templates for low-metallicity objects in the early Universe, even though significant differences with respect to massive stars with poorer metal content have been reported. This White Paper summarizes the current knowledge on extremely (sub-SMC) metal poor massive stars, highlighting the most outstanding open questions and the need to supersede the SMC as standard. A new paradigm can be built from nearby extremely metal-poor galaxies that make a new metallicity ladder, but massive stars in these galaxies are out of reach to current observational facilities. Such task would require an L-size mission, consisting of a 10m-class space telescope operating in the optical and the ultraviolet ranges. Alternatively, we propose that ESA unites efforts with NASA to make the LUVOIR mission concept a reality, thus continuing the successful partnership that made Hubble Space Telescope one of the greatest observatories of all time.

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arXiv2019

Massive stars in the young cluster VVV CL074

We obtained K-band spectroscopy of the brightest members of the cluster VVV CL074 in order to identify the massive star population. We also determined the stellar properties of the cluster's massive stars to better quantify the evolutionary sequences linking different types of massive stars. We collected integral field spectroscopy of selected fields in the cluster VVV CL074 with SINFONI on the ESO/VLT. We performed a spectral classification based on the K-band spectra and comparison to infrared spectral atlases. We determined the stellar parameters of the massive stars from analysis with atmosphere models computed with the code CMFGEN. We uncover a population of 25 early-type (OB and Wolf-Rayet) stars, 19 being newly discovered by our observations out of which 15 are likely cluster members. The cluster's spectrophotometric distance is 10.2+/-1.6 kpc, placing it close to the intersection of the galactic bar and the Norma arm, beyond the galactic center. This makes VVV CL074 one the farthest young massive clusters identified so far. Among the massive stars population, three objects are Wolf-Rayet stars, the remaining are O and B stars. From the Hertzsprung-Russell diagram we find that most stars have an age between 3 and 6 Myr according to the Geneva evolutionary tracks. WN8 and WC8-9 stars are the descendants of stars with initial masses between 40 and 60 Msun. The massive star population of VVV CL074 is very similar to that of the cluster DBS2003-179 and to a lesser extent to that of the Quintuplet cluster, indicating the same age. The central cluster of the Galaxy is ~3 Myr older. From the comparison of the massive stars populations in these four clusters, one concludes that galactic stars with an initial mass in the range 40 to 60 Msun likely go through a WN8-9 phase.

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arXiv2019

Thermoelectric Scanning Gate Interferometry on a Quantum Point Contact

We introduce a new scanning probe technique derived from scanning gate microscopy (SGM) in order to investigate thermoelectric transport in two-dimensional semiconductor devices. The thermoelectric scanning gate Microscopy (TSGM) consists in measuring the thermoelectric voltage induced by a temperature difference across a device, while scanning a polarized tip that locally changes the potential landscape. We apply this technique to perform interferometry of the thermoelectric transport in a quantum point contact (QPC). We observe an interference pattern both in SGM and TSGM images, and evidence large differences between the two signals in the low density regime of the QPC. In particular, a large phase jump appears in the interference fringes recorded by TSGM, which is not visible in SGM. We discuss this difference of sensitivity using a microscopic model of the experiment, based on the contribution from a resonant level inside or close to the QPC. This work demonstrates that combining scanning gate microscopy with thermoelectric measurements offers new information as compared to SGM, and provides a direct access to the derivative of the device transmission with respect to energy, both in amplitude and in phase.

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arXiv2018

Extreme resonance line profile variations in the ultraviolet spectra of NGC 1624-2: probing the giant magnetosphere of the most strongly magnetized known O-type star

In this paper, we present high-resolution HST/COS observations of the extreme magnetic O star NGC 1624-2. These represent the first ultraviolet spectra of this archetypal object. We examine the variability of its wind-sensitive resonance lines, comparing it to that of other known magnetic O stars. In particular, the observed variations in the profiles of the CIV and SiIV doublets between low state and high state are the largest observed in any magnetic O-type star, consistent with the expected properties of NGC 1624-2's magnetosphere. We also observe a redshifted absorption component in the low state, a feature not seen in most stars. We present preliminary modelling efforts based on the Analytic Dynamical Magnetosphere (ADM) formalism, demonstrating the necessity of using non-spherically symmetric models to determine wind/magnetospheric properties of magnetic O stars.

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arXiv2018

Synthetic photometry of globular clusters: Uncertainties on synthetic colors

Synthetic photometry is a great tool for studying globular clusters, especially for understanding the nature of their multiple populations. Our goal is to quantify the errors on synthetic photometry that are caused by uncertainties on stellar and observational/calibration parameters. These errors can be taken into account when building synthetic color-magnitude diagrams (CMDs) that are to be compared to observed CMDs. We have computed atmosphere models and synthetic spectra for two stars, Pollux and Procyon, that have stellar parameters typical of turn-off and bottom red giant branch stars in globular clusters. We then varied the effective temperature, surface gravity, microturbulence, the carbon, nitrogen, and oxygen abundances, and [Fe/H]. We quantified the effect on synthetic photometry in the following filters: Johnson UBVRI and HST F275W, F336W, F410M, F438W, F555W, F606W, and F814W. We estimated the effects of extinction, atmospheric correction, and of the Vega reference spectrum on the resulting photometry. We tested the ability of our models to reproduce the observed spectral energy distribution and observed photometry of the two stars. We show that variations are generally stronger in blue filters. Dispersions on synthetic colors due to uncertainties on stellar parameters vary between less than 0.01 and to 0.04 magnitude, depending on the choice of filters. Uncertainties on the zero points, the extinction law, or the atmospheric correction affect the resulting colors at a level of a few 0.01 magnitudes in a systematic way. The models reproduce the flux-calibrated spectral energy distribution of both stars well. Comparison between synthetic and observed UBVRI photometry shows a variable degree of (dis)agreement. The observed differences likely indicate that different calibration processes are performed to obtain respectively observed and synthetic photometry.

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arXiv2018

Quantitative spectral classification of Galactic O stars

Our goal is to provide a quantification of several spectral classification criteria for O stars. We collected high-spectral resolution spectra of 105 Galactic O-type stars from various archives. We measured equivalent widths of classification lines. We defined average values of classification criteria for given spectral types and luminosity classes. We find that the ratio HeI 4471 to HeII 4542 well matches the published ratios for spectral types. We have quantified equivalent width ratios of helium and silicon lines among O8-O9.7 stars to refine spectral class typing in this spectral range. We present quantitative criteria to separate between luminosity class V, IV-III-II (grouped), and I among O3-O8.5 stars, mainly based on the strength of HeII 4686. We find that these criteria also define very well the f, (f), and ((f)) classes for O3-O7.5 stars. Among O9-O9.7 stars we quantify the ratios of HeII 4686 to HeI 4713 and SiIV 4089 to HeI 4026 for all luminosity classes. The tabulated values of the classification criteria should help classify any new O-type stars. The final step of the classification process should rely on a direct comparison to standard stars of the assigned spectral type and luminosity class.

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arXiv2018

The Arches cluster revisited: I. Data presentation and stellar census

Located within the central region of the Galaxy, the Arches cluster appears to be one of the youngest, densest and most massive stellar aggregates within the Milky Way. As such it has the potential to be a uniquely instructive laboratory for the study of star formation in extreme environments and the physics of very massive stars. In order to determine the fundamental physical properties of both cluster and constituent stars, we provide and analyse new HST+VLT near-IR datasets. Stacking multiple epochs of spectroscopy results in the deepest view of the cluster ever obtained, allowing us to to identify candidate giant and main sequence stars for the first time. All cluster members are found to be WNLh or O stars, with the smooth and continuous progression in spectral morphologies from O super-/hypergiants through to the WNLh cohort implying a direct evolutionary connection. Importantly no H-free Wolf-Rayets are found, while no products of binary interaction/mass-transfer may be unambiguously identified, despite the presence of massive binaries within the Arches. We infer a main sequence turn-off around O4-5V, corresponding to ~30-38Msun, while the eclipsing binary F2 implies current masses of ~80Msun and ~60Msun for the WNLh and O hypergiant cohorts, respectively. A cluster age of ~2-3Myr is suggested by the location of the main-sequence turn-off. While the absence of H-free Wolf-Rayets argues against the prior occurrence of SNe, such an age does accommodate such events for exceptionally massive stars. Future progress requires quantitative analysis of cluster members combined with additional spectroscopic observations to better constrain the binary population; nevertheless it is already abundantly clear that the Arches offers an unprecedented insight into the formation, evolution and death of the most massive stars Nature allows to form in the local universe (Abridged).

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arXiv2017

Twelve years of spectroscopic monitoring in the Galactic Center: the closest look at S-stars near the black hole

We study the young S-stars within a distance of 0.04 pc from the supermassive black hole in the center of our Galaxy. Given how inhospitable the region is for star formation, their presence is more puzzling the younger we estimate their ages. In this study, we analyse the result of 12 years of high resolution spectroscopy within the central arcsecond of the Galactic Center (GC). By co-adding between 55 and 105 hours of spectra we have obtained high signal to noise H- and K-band spectra of eight stars orbiting the central supermassive black hole. Using deep H-band spectra, we show that these stars must be high surface gravity (dwarf) stars. We compare these deep spectra to detailed model atmospheres and stellar evolution models to infer the stellar parameters. Our analysis reveals an effective temperature of 21000-28500 K, a rotational velocity of 60-170 km/s, and a surface gravity of 4.1-4.2. These parameters imply a spectral type of B0-B3V for these stars. The inferred masses lie within 8-14 Msun. We derive an age of 6.6^{+3.4}{-4.7} Myr for the star S2, which is compatible with the age of the clockwise rotating young stellar disk in the GC. We estimate the age of all other studied S-stars to be less than 15 Myr, which are compatible with the age of S2 within the uncertainties. The relatively low ages for these S-stars favor a scenario in which the stars formed in a local disk rather than the field-binary-disruption scenario throughout a longer period of time.

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arXiv2017

Notch filtering the nuclear environment of a spin qubit

Electron spins in gate-defined quantum dots provide a promising platform for quantum computation. In particular, spin-based quantum computing in gallium arsenide takes advantage of the high quality of semiconducting materials, reliability in fabricating arrays of quantum dots, and accurate qubit operations. However, the effective magnetic noise arising from the hyperfine interaction with uncontrolled nuclear spins in the host lattice constitutes a major source of decoherence. Low frequency nuclear noise, responsible for fast (10 ns) inhomogeneous dephasing, can be removed by echo techniques. High frequency nuclear noise, recently studied via echo revivals, occurs in narrow frequency bands related to differences in Larmor precession of the three isotopes $\mathbf{^{69}Ga}$, $\mathbf{^{71}Ga}$, and $\mathbf{^{75}As}$. Here we show that both low and high frequency nuclear noise can be filtered by appropriate dynamical decoupling sequences, resulting in a substantial enhancement of spin qubit coherence times. Using nuclear notch filtering, we demonstrate a spin coherence time ($\mathbf{T_{2}}$) of 0.87 ms, five orders of magnitude longer than typical exchange gate times, and exceeding the longest coherence times reported to date in Si/SiGe gate-defined quantum dots.

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arXiv2016

Early-type massive stars in Carina Nebula within the Gaia-ESO Survey

The Gaia-ESO Survey (GES) is obtaining high quality spectra of $\sim$10$^{5}$ stars in our Galaxy, providing an homogeneous and unique overview of all the main components of the Milky Way, its formation history and the evolution of young, mature and ancient Galactic populations. Our group is in charge of the early-type massive stars that define the youngest population in the survey. In this contribution, we present the results of the quantitative spectroscopic analysis of O-type stars in the Carina Nebula within the Gaia-ESO Survey. For this aim, we have used FASTWIND and CMFGEN stellar atmosphere codes, providing stellar parameters for the current sample (GES data release iDR4).

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arXiv2016

Spectroscopic evolution of massive stars on the main sequence

We provide an observational view of evolutionary models in the Hertzsprung--Russell diagram, on the main sequence. For that we computed evolutionary models with the code STAREVOL for 15 < M/Msun < 100. We subsequently calculated atmosphere models at specific points along the evolutionary tracks, using the code CMFGEN. Synthetic spectra obtained in this way were classified as if they were observational data. We tested our spectral classification by comparison to observed spectra of various stars. We also compared our results with empirical data of a large number of OB stars. We obtain spectroscopic sequences along evolutionary tracks. In our computations, the earliest O stars (O2-3.5) appear only above ~50 Msun. For later spectral types, a similar mass limit exists, but is lower. A luminosity class V does not correspond to the entire main sequence. This only holds for the 15 Msun track. As mass increases, a larger portion of the main sequence is spent in luminosity class III. Above 50 Msun, supergiants appear before the end of core-hydrogen burning. Dwarf stars do not occur on the zero-age main sequence above 80 Msun. Consequently, the distribution of luminosity class V in the HR diagram cannot be used to constrain the size of the convective core. The distribution of dwarfs and giants in the HR diagram agrees well with the location of stars analyzed by means of quantitative spectroscopy. For supergiants, there is a slight discrepancy in the sense that luminosity class I is observed slightly earlier than our predictions. This is mainly due to wind densities that affect the luminosity class diagnostic lines. We predict an upper mass limit for dwarf stars (~60 Msun) that is found consistent with the rarity of O2V stars in the Galaxy. Stars with WNh spectral type are not predicted by our models. Stronger winds are required to produce the characteristic emission lines of these objects.

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arXiv2016

A study of the effect of rotational mixing on massive stars evolution: surface abundances of Galactic O7-8 giant stars

Massive star evolution remains only partly constrained. In particular, the exact role of rotation has been questioned by puzzling properties of OB stars in the Magellanic Clouds. Our goal is to study the relation between surface chemical composition and rotational velocity, and to test predictions of evolutionary models including rotation. We have performed a spectroscopic analysis of a sample of fifteen Galactic O7-8 giant stars. This sample is homogeneous in terms of mass, metallicity and evolutionary state. It is made of stars with a wide range of projected rotational velocities. We show that the sample stars are located on the second half of the main sequence, in a relatively narrow mass range (25-40 Msun). Almost all stars with projected rotational velocities above 100 km/s have N/C ratios about ten times the initial value. Below 100 km/s a wide range of N/C values is observed. The relation between N/C and surface gravity is well reproduced by various sets of models. Some evolutionary models including rotation are also able to consistently explain slowly rotating, highly enriched stars. This is due to differential rotation which efficiently transports nucleosynthesis products and allows the surface to rotate slower than the core. In addition, angular momentum removal by winds amplifies surface braking on the main sequence. Comparison of the surface composition of O7-8 giant stars with a sample of B stars with initial masses about four times smaller reveal that chemical enrichment scales with initial mass, as expected from theory. Although evolutionary models that include rotation face difficulties in explaining the chemical properties of O- and B-type stars at low metallicity, some of them can consistently account for the properties of main-sequence Galactic O stars in the mass range 25-40 Msun

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arXiv2016

Electron phase shift at the zero-bias anomaly of quantum point contacts

The Kondo effect is the many-body screening of a local spin by a cloud of electrons at very low temperature. It has been proposed as an explanation of the zero-bias anomaly in quantum point contacts where interactions drive a spontaneous charge localization. However, the Kondo origin of this anomaly remains under debate, and additional experimental evidence is necessary. Here we report on the first phase-sensitive measurement of the zero-bias anomaly in quantum point contacts using a scanning gate microscope to create an electronic interferometer. We observe an abrupt shift of the interference fringes by half a period in the bias range of the zero-bias anomaly, a behavior which cannot be reproduced by single-particle models. We instead relate it to the phase shift experienced by electrons scattering off a Kondo system. Our experiment therefore provides new evidence of this many-body effect in quantum point contacts.

cond-mat.mes-hall↗

arXiv2016

Surface abundances of OC supergiants

Some O and B stars show unusually strong or weak lines of carbon and/or nitrogen. These objects are classified as OBN or OBC stars. It has recently been shown that nitrogen enrichment and carbon depletion are the most likely explanations for the existence of the ON class. We investigate OC stars (all being supergiants) to check that surface abundances are responsible for the observed anomalous line strengths. We perform a spectroscopic analysis of three OC supergiants using atmosphere models. A fourth star was previously studied by us. Our sample thus comprises all OC stars known to date in the Galaxy. We determine the stellar parameters and He, C, N, and O surface abundances. We show that all stars have effective temperatures and surface gravities fully consistent with morphologically normal O supergiants. However, OC stars show little, if any, nitrogen enrichment and carbon surface abundances consistent with the initial composition. OC supergiants are thus barely chemically evolved, unlike morphologically normal O supergiants.

astro-ph.SR↗

arXiv2016

Observational signatures of past mass-exchange episodes in massive binaries: The case of HD 149 404

Mass and momentum exchanges in close massive binaries play an important role in their evolution, and produce several observational signatures such as asynchronous rotation and altered chemical compositions, that remain after the stars detach again. We investigated these effects for the detached massive O-star binary HD 149404 (O7.5If + ON9.7I, P = 9.81 days), which is thought to have experienced a past episode of case A Roche-lobe overflow (RLOF). Using phase-resolved spectroscopy, we performed the disentangling of the optical spectra of the two stars. The reconstructed primary and secondary spectra were then analysed with the CMFGEN model atmosphere code to determine stellar parameters, such as the effective temperatures and surface gravities, and to constrain the chemical composition of the components. We complemented the optical study with the study of IUE spectra, which we compare to the synthetic binary spectra. The properties of the stars were compared to evolutionary models. We confirmed a strong overabundance in nitrogen ($\left[\rm N/C\right]\sim150\left[\rm N/C\right]_{\odot}$) for the secondary and a slight nitrogen overabundance ($\left[\rm N/C\right]\sim5\left[\rm N/C\right]_{\odot}$) for the primary star. Comparing the two stars, we found evidence for asynchronous rotation, with a rotational period ratio of $0.50 \pm 0.11$. The hypothesis of a past case A RLOF interaction in HD 149404 is most plausible to explain its chemical abundances and rotational asynchronicity. Some of the observed properties, such as the abundance pattern, are clearly a challenge for current case A binary evolution models, however.

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