arXiv subjects

F. R. N. Schneider

Publications and source records attributed to F. R. N. Schneider.

At least 55 records · Page 3Linked to original sources

arXiv2019

The Tarantula Massive Binary Monitoring. IV. Double-lined photometric binaries

A high fraction of massive stars are found to be binaries but only a few of them are reported as photometrically variable. By studying the populations of SB2 in the 30 Doradus region, we found a subset of them that have photometry from the OGLE project and that display variations in their light curves related to orbital motions. The goal of this study is to determine the dynamical masses and radii of the 26 binary components to investigate the mass-discrepancy problem and to provide an empirical mass-luminosity relation for the LMC. We use the PHOEBE programme to perform a systematic analysis of the OGLE V and I light curves obtained for 13 binary systems in 30 Dor. We adopt Teff, and orbital parameters derived previously to obtain the inclinations of the systems and the parameters of the individual components. Three systems display eclipses in their light curves, while the others only display ellipsoidal variations. We classify two systems as over-contact, five as semi-detached, and four as detached. The two remaining systems have uncertain configurations due to large uncertainties on their inclinations. The fact that systems display ellipsoidal variations has a significant impact on the inclination errors. From the dynamical masses, luminosities, and radii, we provide LMC-based empirical mass-luminosity and mass-radius relations, and we compare them to other relations given for the Galaxy, the LMC, and the SMC. These relations differ for different mass ranges, but do not seem to depend on the metallicity regimes. We also compare the dynamical, spectroscopic, and evolutionary masses of the stars in our sample. While the dynamical and spectroscopic masses agree with each other, the evolutionary masses are systematically higher, at least for stars in semi-detached systems. This suggests that the mass discrepancy can be partly explained by past or ongoing interactions between the stars.

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arXiv2019

The young massive SMC cluster NGC 330 seen by MUSE. I. Observations and stellar content

A majority of massive stars are part of binary systems, a large fraction of which will inevitably interact during their lives. Binary-interaction products (BiPs), i.e. stars affected by such interaction, are expected to be commonly present in stellar populations. BiPs are thus a crucial ingredient in the understanding of stellar evolution. We aim to identify and characterize a statistically significant sample of BiPs by studying clusters of 10-40 Myr, an age at which binary population models predict the abundance of BiPs to be highest. One example of such a cluster is NGC 330 in the Small Magellanic Cloud. Using MUSE WFM-AO observations of NGC 330, we resolve the dense cluster core for the first time and are able to extract spectra of its entire massive star population. We develop an automated spectral classification scheme based on the equivalent widths of spectral lines in the red part of the spectrum. We characterize the massive star content of the core of NGC 330 which contains more than 200 B stars, 2 O stars, 6 A-type supergiants and 11 red supergiants. We find a lower limit on the Be star fraction of $32 \pm 3\%$ in the whole sample. It increases to at least $46 \pm 10\%$ when only considering stars brighter than V=17 mag. We estimate an age of the cluster core between 35 and 40 Myr and a total cluster mass of $88^{+17}_{-18} \times 10^3 M_{\odot}$. We find that the population in the cluster core is different than the population in the outskirts: while the stellar content in the core appears to be older than the stars in the outskirts, the Be star fraction and the observed binary fraction are significantly higher. Furthermore, we detect several BiP candidates that will be subject of future studies.

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arXiv2019

The MiMeS survey of Magnetism in Massive Stars: Magnetic properties of the O-type star population

In this paper, we describe an analysis of the MiMeS Survey of O-type stars to explore the range of dipolar field strengths permitted by the polarisation spectra that do not yield a magnetic detection. We directly model the Stokes V profiles with a dipolar topology model using Bayesian inference. The noise statistics of the Stokes V profiles are in excellent agreement with those of the null profiles. Using a Monte-Carlo approach we conclude that a model in which all the stars in our sample were to host a 100\,G, dipolar magnetic field can be ruled out by the MiMeS data. Furthermore, if all the stars with no detection were to host a magnetic field just below their detection limit, the inferred distribution in strength of these undetected fields would be distinct from the known distribution in strength of the known magnetic O-type stars. This indicates that the Initial magnetic (B-)field Function (IBF) is likely bimodal -- young O-type stars are expected to either have weak/absent magnetic fields, or strong magnetic fields. We also find that better upper limits, by at least a factor of 10, would have been necessary to rule out a detection bias as an explanation for the apparent lack of evolved main-sequence magnetic O-type stars reported in the literature, and we conclude that the MiMeS survey cannot confirm or refute a magnetic flux decay in O-type stars.

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arXiv2019

A census of massive stars in NGC 346. Stellar parameters and rotational velocities

Spectroscopy for 247 stars towards the young cluster NGC 346 in the Small Magellanic Cloud has been combined with that for 116 targets from the VLT-FLAMES Survey of Massive Stars. Spectral classification yields a sample of 47 O-type and 287 B-type spectra, while radial-velocity variations and/or spectral multiplicity have been used to identify 45 candidate single-lined systems, 17 double-lined systems, and one triple-lined system. Atmospheric parameters (T$_eff$ and log$g$) and projected rotational velocities ($v_e$sin$i$) have been estimated using TLUSTY model atmospheres; independent estimates of $v_e$sin$i$ were also obtained using a Fourier Transform method. Luminosities have been inferred from stellar apparent magnitudes and used in conjunction with the T$_eff$ and $v_e$sin$i$ estimates to constrain stellar masses and ages using the BONNSAI package. We find that targets towards the inner region of NGC 346 have higher median masses and projected rotational velocities, together with smaller median ages than the rest of the sample. There appears to be a population of very young targets with ages of less than 2 Myr, which have presumably all formed within the cluster. The more massive targets are found to have lower $v_e$sin$i$ consistent with previous studies. No significant evidence is found for differences with metallicity in the stellar rotational velocities of early-type stars, although the targets in the SMC may rotate faster than those in young Galactic clusters. The rotational velocity distribution for single non-supergiant B-type stars is inferred and implies that a significant number have low rotational velocity ($\simeq$10\% with $v_e$<40 km/s), together with a peak in the probability distribution at $v_e \simeq$300 km/s. Larger projected rotational velocity estimates have been found for our Be-type sample and imply that most have rotational velocities between 200-450 km/s.

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arXiv2019

The VLT-FLAMES Tarantula Survey: XXXI. Radial velocities and multiplicity constraints of red supergiant stars in 30 Doradus

The incidence of multiplicity in cool, luminous massive stars is relatively unknown compared to their hotter counterparts. Here we present radial velocity (RV) measurements and investigate the multiplicity properties of red supergiants (RSGs) in the 30~Doradus region of the Large Magellanic Cloud. We provide absolute RV measurements for our sample and estimate line-of-sight velocities for the Hodge 301 and SL 639 clusters, which agree well with those of hot stars in the same clusters. By combining results for the RSGs with those for nearby B-type stars, we estimate systemic velocities and velocity dispersions for the two clusters, obtaining estimates for their dynamical masses of $\log (M_{\rm dyn}/M_{\odot})=$3.8$\pm$0.3 for Hodge 301, and an upper limit of $\log (M_{\rm dyn}/M_{\odot})$<3.1$\pm$0.8 for SL 639, assuming Virial equilibrium. Analysis of the multi-epoch data reveals one RV-variable, potential binary candidate (VFTS744), which is likely a semi-regular variable asymptotic giant branch star. We estimate an upper limit on the observed binary fraction for our sample of 0.3, where we are sensitive to maximum periods for individual objects in the range of 1 to 10 000 days and mass-ratios above 0.3 depending on the data quality. From simulations of the RV measurements from binary systems given the current data we conclude that systems within the parameter range q>0.3, $\log$P[days]<3.5, would be detected by our variability criteria, at the 90% confidence level. The intrinsic binary fraction, accounting for observational biases, is estimated using simulations of binary systems with an empirically defined distribution of parameters where orbital periods are uniformly distributed in the 3.3<$\log$P[days]<4.3 range. A range of intrinsic binary fractions are considered; a binary fraction of 0.3 is found to best reproduce the observed data. [Abridged]

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arXiv2019

The VLT-FLAMES Tarantula Survey. XXX. Red stragglers in the clusters Hodge 301 and SL 639

We estimate physical parameters for the late-type massive stars observed as part of the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Large Magellanic Cloud (LMC). The observational sample comprises 20 candidate red supergiants (RSGs) which are the reddest (($B-V$) $>$ 1 mag) and brightest ($V$ $<$ 16 mag) objects in the VFTS. We use optical and near-IR photometry to estimate their temperatures and luminosities, and introduce the luminosity-age diagram to estimate their ages. We derive physical parameters for our targets, including temperatures from a new calibration of $(J-K_{\rm s})_{0}$ colour for luminous cool stars in the LMC, luminosities from their $J$-band magnitudes (thence radii), and ages from comparisons with state-of-the-art evolutionary models. We show that interstellar extinction is a significant factor for our targets, highlighting the need to take it into account in analysis of the physical parameters of RSGs. We find that some of the candidate RSGs could be massive AGB stars. The apparent ages of the RSGs in the Hodge 301 and SL 639 clusters show a significant spread (12-24 Myr). We also apply our approach to the RSG population of the relatively nearby NGC 2100 cluster, finding a similarly large spread. We argue that the effects of mass-transfer in binaries may lead to more massive and luminous RSGs (which we call `red stragglers') than expected from single-star evolution, and that the true cluster ages correspond to the upper limit of the estimated RSG ages. In this way, the RSGs can serve as a new and potentially reliable age tracer in young star clusters. The corresponding analysis yields ages of 24$^{+5}_{-3}$ Myr for Hodge 301, 22$^{+6}_{-5}$ Myr for SL 639, and 23$^{+4}_{-2}$ Myr for NGC 2100.

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arXiv2018

Gaia and HST astrometry of the very massive $\sim$150 $M_\odot$ candidate runaway star VFTS682

How very massive stars form is still an open question in astrophysics. VFTS682 is among the most massive stars known, with an inferred initial mass of $\sim$150 $M_\odot$ . It is located in 30 Doradus at a projected distance of 29 pc from the central cluster R136. Its apparent isolation led to two hypotheses: either it formed in relative isolation or it was ejected dynamically from the cluster. We investigate the kinematics of VFTS682 as obtained by Gaia and Hubble Space Telescope astrometry. We derive a projected velocity relative to the cluster of $38 \pm 17 \mathrm{km \ s^{-1}}$ (1$σ$ confidence interval). Although the error bars are substantial, two independent measures suggest that VFTS682 is a runaway ejected from the central cluster. This hypothesis is further supported by a variety of circumstantial clues. The central cluster is known to harbor other stars more massive than 150 $M_\odot$ of similar spectral type and recent astrometric studies on VFTS16 and VFTS72 provide direct evidence that the cluster can eject some of its most massive members, in agreement with theoretical predictions. If future data confirm the runaway nature, this would make VFTS682 the most massive runaway star known to date.

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arXiv2018

Response to comment on "An excess of massive stars in the local 30 Doradus starburst"

Farr and Mandel reanalyse our data, finding initial-mass-function slopes for high mass stars in 30 Doradus that agree with our results. However, their reanalysis appears to underpredict the observed number of massive stars. Their technique results in more precise slopes than in our work, strengthening our conclusion that there is an excess of massive stars above $30\,\mathrm{M}_\odot$ in 30 Doradus.

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arXiv2018

Gaia DR2 reveals a very massive runaway star ejected from R136

A previous spectroscopic study identified the very massive O2 III star VFTS 16 in the Tarantula Nebula as a runaway star based on its peculiar line-of-sight velocity. We use the Gaia DR2 catalog to measure the relative proper motion of VFTS 16 and nearby bright stars to test if this star might have been ejected from the central cluster, R136, via dynamical ejection. We find that the position angle and magnitude of the relative proper motion (0.338 +/- 0.046 mas/yr, or approximately 80 +\- 11 km/s) of VFTS 16 are consistent with ejection from R136 approximately 1.5 +/- 0.2 Myr ago, very soon after the cluster was formed. There is some tension with the presumed age of VFTS 16 that, from published stellar parameters, cannot be greater than 0.9 +0.3/-0.2 Myr. Older ages for this star would appear to be prohibited due to the absence of He I lines in its optical spectrum, since this sets a firm lower limit on its effective temperature. The dynamical constraints may imply an unusual evolutionary history for this object, perhaps indicating it is a merger product. Gaia DR2 also confirms that another very massive star in the Tarantula Nebula, VFTS 72 (alias BI253; O2 III-V(n)((f*)), is also a runaway on the basis of its proper motion as measured by Gaia. While its tangential proper motion (0.392 +/-0.062 mas/yr or 93 +/-15 km/s) would be consistent with dynamical ejection from R136 approximately 1 Myr ago, its position angle is discrepant with this direction at the 2$σ$ level. From their Gaia DR2 proper motions we conclude that the two ~100 solar mass O2 stars, VFTS 16 and VFTS72, are fast runaway stars, with space velocities of around 100 km/s relative to R136 and the local massive star population. The dynamics of VFTS16 are consistent with it having been ejected from R136, and this star therefore sets a robust lower limit on the age of the central cluster of ~1.3 Myr.

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arXiv2018

The VLT-FLAMES Tarantula Survey. XXIX. Massive star formation in the local 30 Doradus starburst

The 30 Doradus (30 Dor) nebula in the Large Magellanic Cloud (LMC) is the brightest HII region in the Local Group and a prototype starburst similar to those found in high redshift galaxies. It is thus a stepping stone to understand the complex formation processes of stars in starburst regions across the Universe. Here, we have studied the formation history of massive stars in 30 Dor using masses and ages derived for 452 mainly OB stars from the spectroscopic VLT-FLAMES Tarantula Survey (VFTS). We find that stars of all ages and masses are scattered throughout 30 Dor. This is remarkable because it implies that massive stars either moved large distances or formed independently over the whole field of view in relative isolation. We find that both channels contribute to the 30 Dor massive star population. Massive star formation rapidly accelerated about 8 Myr ago, first forming stars in the field before giving birth to the stellar populations in NGC 2060 and NGC 2070. The R136 star cluster in NGC 2070 formed last and, since then, about 1 Myr ago, star formation seems to be diminished with some continuing in the surroundings of R136. Massive stars within a projected distance of 8 pc of R136 are not coeval but show an age range of up to 6 Myr. Our mass distributions are well populated up to $200\,\mathrm{M}_\odot$. The inferred IMF is shallower than a Salpeter-like IMF and appears to be the same across 30 Dor. By comparing our sample of stars to stellar models in the Hertzsprung-Russell diagram, we find evidence for missing physics in the models above $\log L/\mathrm{L}_\odot=6$ that is likely connected to enhanced wind mass loss for stars approaching the Eddington limit. [abridged]

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arXiv2018

The VLT-FLAMES Tarantula Survey: XXVIII. Nitrogen abundances for apparently single dwarf and giant B-type stars with small projected rotational velocities

Previous analyses of the spectra of OB-type stars in the Magellanic Clouds have identified targets with low projected rotational velocities and relatively high nitrogen abundances. The VFTS obtained spectroscopy for B-type 434 stars. We have estimated atmospheric parameters and nitrogen abundances using TLUSTY model atmospheres for 54 B-type targets that appear to be single, have projected rotational velocities <80 \kms and were not classified as supergiants. In addition, nitrogen abundances for 34 similar stars from a previous survey have been re-evaluated. Approximately 75-80% of the targets have nitrogen enhancements of less than 0.3 dex, consistent with them having experienced only small amounts of mixing. However, stars with low projected rotational velocities <40 \kms and significant nitrogen enrichments are found in both our samples and simulations imply that these cannot all be rapidly rotating objects observed near pole-on. Adopting an enhancement threshold of 0.6 dex, we observed five stars, yet stellar evolution models with rotation predict only 1.25$\pm$1.11 for random stellar viewing inclinations. The excess of such objects is estimated to be 20-30% of all stars with current rotational velocities of less than 40 \kms and 2-4% of the total single B-type sample. These estimates constitute lower limits for stars that appear inconsistent with current grids of stellar evolutionary models. Including targets with smaller nitrogen enhancements of greater than 0.2 dex implies larger percentages of targets that are inconsistent with current evolutionary models, viz. 70% of the stars with rotational velocities less than 40 \kms and 6-8% of the total single stellar population. We consider possible explanations of which the most promising would appear to be breaking due to magnetic fields or stellar mergers with subsequent magnetic braking.

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arXiv2018

The VLT-FLAMES Tarantula Survey XXIV. Stellar properties of the O-type giants and supergiants in 30 Doradus

The Tarantula region in the Large Magellanic Cloud contains the richest population of spatially resolved massive O-type stars known so far. This unmatched sample offers an opportunity to test models describing their main-sequence evolution and mass-loss properties. Using ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to determine stellar, photospheric and wind properties of 72 presumably single O-type giants, bright giants and supergiants and to confront them with predictions of stellar evolution and of line-driven mass-loss theories. We apply an automated method for quantitative spectroscopic analysis of O stars combining the non-LTE stellar atmosphere model {\sc fastwind} with the genetic fitting algorithm {\sc pikaia} to determine the following stellar properties: effective temperature, surface gravity, mass-loss rate, helium abundance, and projected rotational velocity. We present empirical effective temperature versus spectral subtype calibrations at LMC-metallicity for giants and supergiants. In the spectroscopic and classical Hertzsprung-Russell diagrams, our sample O stars are found to occupy the region predicted to be the core hydrogen-burning phase by Brott et al. (2011) and Köhler et al. (2015). Except for five stars, the helium abundance of our sample stars is in agreement with the initial LMC composition. The aforementioned five stars present moderate projected rotational velocities (i.e., $v_{\mathrm{e}}\,\sin\,i\,<\,200\,\mathrm{km\,s^{-1}}$) and hence do not agree with current predictions of rotational mixing in main-sequence stars. Adopting theoretical results for the wind velocity law, we find modified wind momenta for LMC stars that are $\sim$0.3 dex higher than earlier results. [Due to the limitation of characters, the abstract appearing here is slightly shorter than that in the PDF file.]

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arXiv2018

An excess of massive stars in the local 30 Doradus starburst

The 30 Doradus star-forming region in the Large Magellanic Cloud is a nearby analogue of large star-formation events in the distant Universe. We determine the recent formation history and the initial mass function (IMF) of massive stars in 30 Doradus based on spectroscopic observations of 247 stars more massive than 15 solar masses ($\mathrm{M}_\odot$). The main episode of massive star formation started about $8\,\mathrm{Myr}$ ago and the star-formation rate seems to have declined in the last $1\,\mathrm{Myr}$. The IMF is densely sampled up to $200\,\mathrm{M}_\odot$ and contains $32\pm12\%$ more stars above $30\,\mathrm{M}_\odot$ than predicted by a standard Salpeter IMF. In the mass range $15-200\,\mathrm{M}_\odot$, the IMF power-law exponent is $1.90^{+0.37}_{-0.26}$, shallower than the Salpeter value of 2.35.

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arXiv2017

The VLT-FLAMES Tarantula Survey XXVII. Physical parameters of B-type main-sequence binary systems in the Tarantula nebula

A spectroscopic analysis has been undertaken for the B-type multiple systems (excluding those with supergiant primaries) in the VLT-FLAMES Tarantula Survey (VFTS). Projected rotational velocities, $v$sin$i$, for the primaries have been estimated using a Fourier Transform technique and confirmed by fitting rotationally broadened profiles. A subset of 33 systems with $v$sin$i$$\leq$ 80 km/s have been analysed using a TLUSTY grid of model atmospheres to estimate stellar parameters and surface abundances for the primaries. The effects of a potential flux contribution from an unseen secondary have also been considered. For 20 targets it was possible to reliably estimate their effective temperatures (T$_{eff}$) but for the other 13 objects it was only possible to provide a constraint of 20,000$\leq$T$_{eff}$$\leq$26,000 K -- the other parameters estimated for these targets will be consequently less reliable. The estimated stellar properties are compared with evolutionary models and are generally consistent with their membership of 30 Doradus, while the nature of the secondaries of 3 SB2 system is discussed. A comparison with a sample of single stars with $v$sin$i$$\leq$ 80 km/s obtained from the VFTS and analysed with the same techniques implies that the atmospheric parameters and nitrogen abundances of the two samples are similar. However, the binary sample may have a lack of primaries with significant nitrogen enhancements, which would be consistent with them having low rotational velocities and having effectively evolved as single stars without significant rotational mixing. This result, which may be actually a consequence of the limitations of the pathfinder investigation presented in this paper, should be considered as a motivation for spectroscopic abundance analysis of large samples of binary stars, with high quality observational data.

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arXiv2017

The VLT-FLAMES Tarantula Survey XXVI: Properties of the O-dwarf population in 30 Doradus

The VLT-FLAMES Tarantula Survey has observed hundreds of O-type stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). We study the properties of 105 apparently single O-type dwarfs. To determine stellar and wind parameters, we used the IACOB-GBAT package, an automatic procedure based on a large grid of atmospheric models calculated with the FASTWIND code. In addition to classical techniques, we applied the Bayesian BONNSAI tool to estimate evolutionary masses. We provide a new calibration of effective temperature vs. spectral type for O-type dwarfs in the LMC, based on our homogeneous analysis of the largest sample of such objects to date and including all spectral subtypes. Good agreement with previous results is found, although the sampling at the earliest subtypes could be improved. Rotation rates and helium abundances are studied in an evolutionary context. We find that most of the rapid rotators (vsini higher than 300 km/s ) in our sample have masses below 25 MSun and intermediate rotation-corrected gravities (log gc between 3.9 and 4.1). Such rapid rotators are scarce at higher gravities (i.e. younger ages) and absent at lower gravities (larger ages). This is not expected from theoretical evolutionary models, and does not appear to be due to a selection bias in our sample. We compare the estimated evolutionary and spectroscopic masses, finding a trend that the former is higher for masses below 20 MSun. This can be explained as a consequence of limiting our sample to the O-type stars, and we see no compelling evidence for a systematic mass discrepancy. For most of the stars in the sample we were unable to estimate the wind-strength parameter (hence mass-loss rates) reliably, particularly for objects with luminosity lower than logL/LSun about 5.1. Ultraviolet spectroscopy is needed to undertake a detailed investigation of the wind properties of these dwarfs.

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arXiv2017

B fields in OB stars (BOB): The magnetic triple stellar system HD 164492C in the Trifid nebula

HD 164492C is a spectroscopic triple stellar system that has been recently detected to possess a strong magnetic field. We have obtained high-resolution spectroscopic and spectropolarimetric observations over a timespan of two years and derived physical, chemical, and magnetic properties for this object. The system is formed by an eccentric close spectroscopic binary (Ca1-Ca2) with a period of 12.5 days, and a massive tertiary Cb. We calculated the orbital parameters of the close pair, reconstructed the spectra of the three components, and determined atmospheric parameters and chemical abundances by spectral synthesis. From spectropolarimetric observations, multi-epoch measurements of the longitudinal magnetic fields were obtained. The magnetic field is strongly variable on timescales of a few days, with a most probable period in the range of 1.4-1.6 days. Star Cb with Teff~25000 K is the apparently fastest rotator and the most massive star of this triple system and has anomalous chemical abundances with a marked overabundance of helium, 0.35+/-0.04 by number. We identified this star as being responsible for the observed magnetic field, although the presence of magnetic fields in the components of the Ca pair cannot be excluded. Star Ca1 with a temperature of about 24000 K presents a normal chemical pattern, while the least massive star Ca2 is a mid-B type star (Teff~15000 K) with an overabundance of silicon. The obtained stellar parameters of the system components suggest a distance of 1.5 kpc and an age of 10-15 Myr.

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arXiv2016

B field in OB stars (BOB): The outstandingly strong magnetic field in the evolved He-strong star CPD-62 2124

The origin and evolution of magnetism in OB stars is far from being well understood. With approximately 70 magnetic OB stars known, any new object with unusual characteristics may turn out to be a key piece of the puzzle. We report the detection of an exceptionally strong magnetic field in the He-strong B2IV star CPD-62 2124. Spectropolarimetric FORS2 and HARPSpol observations were analysed by two independent teams and procedures, concluding on a strong longitudinal magnetic field of approximately 5.2 kG. The quantitative characterisation of the stellar atmosphere yields an effective temperature of 23650$\pm$250 K, a surface gravity of 3.95$\pm$0.10 dex and a surface helium fraction of 0.35$\pm$0.02 by number. The metal composition is in agreement with the cosmic abundance standard, except for Mg, Si and S, which are slightly non-solar. The strong and broad ($\sim$300 km/s) disc-like emission displayed by the H$α$ line suggests a centrifugal magnetosphere supported by the strong magnetic field. Our results imply that CPD-62 2124 is an early B-type star hosting one of the strongest magnetic fields discovered to date, and one of the most evolved He-strong stars known, with a fractional main-sequence lifetime of approximately 0.6.

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arXiv2016

BONNSAI: correlated stellar observables in Bayesian methods

In an era of large spectroscopic surveys of stars and big data, sophisticated statistical methods become more and more important in order to infer fundamental stellar parameters such as mass and age. Bayesian techniques are powerful methods because they can match all available observables simultaneously to stellar models while taking prior knowledge properly into account. However, in most cases it is assumed that observables are uncorrelated which is generally not the case. Here, we include correlations in the Bayesian code BONNSAI by incorporating the covariance matrix in the likelihood function. We derive a parametrisation of the covariance matrix that, in addition to classical uncertainties, only requires the specification of a correlation parameter that describes how observables co-vary. Our correlation parameter depends purely on the method with which observables have been determined and can be analytically derived in some cases. This approach therefore has the advantage that correlations can be accounted for even if information for them are not available in specific cases but are known in general. Because the new likelihood model is a better approximation of the data, the reliability and robustness of the inferred parameters are improved. We find that neglecting correlations biases the most likely values of inferred stellar parameters and affects the precision with which these parameters can be determined. For example, we apply our technique to massive OB stars, but emphasise that it is valid for any type of stars. For effective temperatures and surface gravities determined from atmosphere modelling, we find that masses can be underestimated on average by $0.5σ$ and mass uncertainties overestimated by a factor of about 2 when neglecting correlations. At the same time, the age precisions are underestimated over a wide range of stellar parameters. [abridged]

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