arXiv subjects

F. Tramper

Publications and source records attributed to F. Tramper.

53 records · Page 3Linked to original sources

arXiv2016

The Tarantula Massive Binary Monitoring project: II. A first SB2 orbital and spectroscopic analysis for the Wolf-Rayet binary R145

We present the first SB2 orbital solution and disentanglement of the massive Wolf-Rayet binary R145 (P = 159d) located in the Large Magellanic Cloud. The primary was claimed to have a stellar mass greater than 300Msun, making it a candidate for the most massive star known. While the primary is a known late type, H-rich Wolf-Rayet star (WN6h), the secondary could not be so far unambiguously detected. Using moderate resolution spectra, we are able to derive accurate radial velocities for both components. By performing simultaneous orbital and polarimetric analyses, we derive the complete set of orbital parameters, including the inclination. The spectra are disentangled and spectroscopically analyzed, and an analysis of the wind-wind collision zone is conducted. The disentangled spectra and our models are consistent with a WN6h type for the primary, and suggest that the secondary is an O3.5 If*/WN7 type star. We derive a high eccentricity of e = 0.78 and minimum masses of M1 sin^3 i ~ M2 sin^3 i ~ 13 +- 2 Msun, with q = M2 / M1 = 1.01 +- 0.07. An analysis of emission excess stemming from a wind-wind collision yields a similar inclination to that obtained from polarimetry (i = 39 +- 6deg). Our analysis thus implies M1 = 53^{+40}_{-20} and M2 = 54^{+40}_{-20} Msun, excluding M1 > 300Msun. A detailed comparison with evolution tracks calculated for single and binary stars, as well as the high eccentricity, suggest that the components of the system underwent quasi-homogeneous evolution and avoided mass-transfer. This scenario would suggest current masses of ~ 80 Msun and initial masses of Mi,1 ~ 105 and Mi,2 ~ 90Msun, consistent with the upper limits of our derived orbital masses, and would imply an age of ~2.2 Myr.

astro-ph.SR↗

arXiv2016

A new prescription for the mass-loss rates of WC and WO stars

We present a new empirical prescription for the mass-loss rates of carbon and oxygen sequence Wolf-Rayet stars as a function of their luminosity, surface chemical composition, and initial metallicity. The new prescription is based on results of detailed spectral analyses of WC and WO stars, and improves the often applied Nugis & Lamers (2000) relation. We find that the mass-loss rates of WC and WO stars (with $X=0$ and $Y < 0.98$) can be expressed as $\log{\dot{M}} = -9.20 + 0.85\log{(L/L_{\odot})} + 0.44\log{Y} + 0.25\log{(Z_{\mathrm{Fe}}/Z_{\mathrm{Fe}, \odot})}$. This relation is based on mass-loss determinations that assume a volume-filling factor of 0.1, but the prescription can easily be scaled to account for other volume-filling factors. The residual of the fit is $σ= 0.06$ dex. We investigated whether the relation can also describe the mass loss of hydrogen-free WN stars and showed that it can when an adjustement of the metallicty dependence ($\log{\dot{M}} \propto 1.3\log{(Z_{\mathrm{Fe}}/Z_{\mathrm{Fe}, \odot})}$) is applied. Compared to Nugis & Lamers (2000), $\dot{M}$ is less sensitive to the luminosity and the surface abundance, implying a stronger mass loss of massive stars in their late stages of evolution. The modest metallicity dependence implies that if WC or WO stars are formed in metal deficient environments, their mass-loss rates are higher than currently anticipated. These effects may result in a larger number of type Ic supernovae and less black holes to be formed, and may favour the production of superluminous type Ic supernovae through interaction with C and O rich circumstellar material or the dense stellar wind.

astro-ph.SR↗

arXiv2016

The VLT-FLAMES Tarantula Survey XXV. Surface nitrogen abundances of O-type giants and supergiants

Theoretically, rotation-induced chemical mixing in massive stars has far reaching evolutionary consequences, affecting the sequence of morphological phases, lifetimes, nucleosynthesis, and supernova characteristics. Using a sample of 72 presumably single O-type giants to supergiants observed in the context of the VLT-FLAMES Tarantula Survey (VFTS), we aim to investigate rotational mixing in evolved core-hydrogen burning stars initially more massive than $15\,M_\odot$ by analysing their surface nitrogen abundances. Using stellar and wind properties derived in a previous VFTS study, we constrained the nitrogen abundance by fitting the equivalent widths of relatively strong lines that are sensitive to changes in the abundance of this element. Given the quality of the data, we constrained the nitrogen abundance in 38 cases; for 34 stars only upper limits could be derived, which includes almost all stars rotating at $v_\mathrm{e}\sin i >200\,\mathrm{km s^{-1}}$. We analysed the nitrogen abundance as a function of projected rotation rate $v_\mathrm{e}\sin i$ and confronted it with predictions of rotational mixing. The upper limits on the nitrogen abundance of the rapidly rotating stars are not in apparent violation with theoretical expectations. However, we found a group of N-enhanced slowly-spinning stars that is not in accordance with predictions of rotational mixing in single stars. Among O-type stars with (rotation-corrected) gravities less than $\log\,g_c = 3.75$ this group constitutes 30$-$40 percent of the population. We found a correlation between nitrogen and helium abundance which is consistent with expectations, suggesting that, whatever the mechanism that brings N to the surface, it displays CNO-processed material.

astro-ph.SR↗

arXiv2015

The mass of the very massive binary WR21a

We present multi-epoch spectroscopic observations of the massive binary system WR21a, which include the January 2011 periastron passage. Our spectra reveal multiple SB2 lines and facilitate an accurate determination of the orbit and the spectral types of the components. We obtain minimum masses of $64.4\pm4.8 \ M_{\odot}$ and $36.3\pm1.7 \ M_{\odot}$ for the two components of WR21a. Using disentangled spectra of the individual components, we derive spectral types of O3/WN5ha and O3Vz~((f*)) for the primary and secondary, respectively. Using the spectral type of the secondary as an indication for its mass, we estimate an orbital inclination of $i=58.8\pm2.5^{\mathrm{o}}$ and absolute masses of $103.6\pm10.2 \ M_{\odot}$ and $58.3\pm3.7 \ M_{\odot}$, in agreement with the luminosity of the system. The spectral types of the WR21a components indicate that the stars are very young (1$-$2 Myr), similar to the age of the nearby Westerlund 2 cluster. We use evolutionary tracks to determine the mass-luminosity relation for the total system mass. We find that for a distance of 8 kpc and an age of 1.5 Myr, the derived absolute masses are in good agreement with those from evolutionary predictions.

astro-ph.SR↗

arXiv2015

Discovery of the massive overcontact binary VFTS 352: Evidence for enhanced internal mixing

The contact phase expected to precede the coalescence of two massive stars is poorly characterized due to the paucity of observational constraints. Here we report on the discovery of VFTS 352, an O-type binary in the 30 Doradus region, as the most massive and earliest spectral type overcontact system known to date. We derived the 3D geometry of the system, its orbital period $P_{\rm orb}=1.1241452(4)$ d, components' effective temperatures -- $T_1=42\,540\pm280$ K and $T_2=41\,120\pm290$ K -- and dynamical masses -- $M_1=28.63\pm0.30 M_{\odot}$ and $M_2 = 28.85\pm0.30 M_{\odot}$. Compared to single-star evolutionary models, the VFTS 352 components are too hot for their dynamical masses by about 2700 and 1100 K, respectively. These results can be explained naturally as a result of enhanced mixing, theoretically predicted to occur in very short-period tidally-locked systems. The VFTS 352 components are two of the best candidates identified so far to undergo this so-called chemically homogeneous evolution. The future of VFTS 352 is uncertain. If the two stars merge, a very rapidly rotating star will be produced. Instead, if the stars continue to evolve homogeneously and keep shrinking within their Roche Lobes, coalescence can be avoided. In this case, tides may counteract the spin down by winds such that the VFTS 352 components may, at the end of their life, fulfill the requirements for long gamma-ray burst (GRB) progenitors in the collapsar scenario. Independently of whether the VFTS 352 components become GRB progenitors, this scenario makes VFTS 352 interesting as a progenitor of a black hole binary, hence as a potential gravitational wave source through black hole-black hole merger.

astro-ph.SR↗

arXiv2015

The VLT-FLAMES Tarantula Survey XXI. Stellar spin rates of O-type spectroscopic binaries

The initial distribution of spin rates of massive stars is a fingerprint of their elusive formation process. It also sets a key initial condition for stellar evolution and is thus an important ingredient in stellar population synthesis. So far, most studies have focused on single stars. Most O stars are however found in multiple systems. By establishing the spin-rate distribution of a sizeable sample of O-type spectroscopic binaries and by comparing the distributions of binary sub-populations with one another as well as with that of presumed single stars in the same region, we aim to constrain the initial spin distribution of O stars in binaries, and to identify signatures of the physical mechanisms that affect the evolution of the massive stars spin rates. We use ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial rotational velocities (\vrot) for components of 114 spectroscopic binaries in 30 Doradus. The \vrot\ values are derived from the full-width at half-maximum (FWHM) of a set of spectral lines, using a FWHM vs. \vrot\ calibration that we derive based on previous line analysis methods applied to single O-type stars in the VFTS sample. The overall \vrot\ distribution of the primary stars resembles that of single O-type stars in the VFTS, featuring a low-velocity peak (at $\vrot < 200$ kms) and a shoulder at intermediate velocities ($200 < \vrot < 300$ kms). The distributions of binaries and single stars however differ in two ways. First, the main peak at $\vrot \sim$100 kms is broader and slightly shifted toward higher spin rates in the binary distribution compared to that of the presumed-single stars. Second, the \vrot distribution of primaries lacks a significant population of stars spinning faster than 300 kms while such a population is clearly present in the single star sample.

astro-ph.SR↗

arXiv2015

Massive stars on the verge of exploding: the properties of oxygen sequence Wolf-Rayet stars

Context. Oxygen sequence Wolf-Rayet (WO) stars represent a very rare stage in the evolution of massive stars. Their spectra show strong emission lines of helium-burning products, in particular highly ionized carbon and oxygen. The properties of WO stars can be used to provide unique constraints on the (post-)helium burning evolution of massive stars, as well as their remaining lifetime and the expected properties of their supernovae. Aims. We aim to homogeneously analyse the currently known presumed-single WO stars to obtain the key stellar and outflow properties and to constrain their evolutionary state. Methods. We use the line-blanketed non-local thermal equilibrium atmosphere code cmfgen to model the X-Shooter spectra of the WO stars and deduce the atmospheric parameters. We calculate dedicated evolutionary models to determine the evolutionary state of the stars. Results. The WO stars have extremely high temperatures that range from 150 kK to 210 kK, and very low surface helium mass fractions that range from 44% down to 14%. Their properties can be reproduced by evolutionary models with helium zero-age main sequence masses of $M_{\mathrm{He, ini}} = 15-25 M_{\odot}$ that exhibit a fairly strong (on the order of a few times $10^{-5} M_{\odot} \mathrm{yr}^{-1}$), homogeneous ($f_\mathrm{c} > 0.3$) stellar wind. Conclusions. WO stars represent the final evolutionary stage of stars with estimated initial masses of $M_{\mathrm{ini}} = 40-60 M_{\odot}$. They are post core-helium burning and predicted to explode as type Ic supernovae within a few thousand years.

astro-ph.SR↗

arXiv2014

The properties of ten O-type stars in the low-metallicity galaxies IC 1613, WLM and NGC 3109

Massive stars likely played an important role in the reionization of the Universe, and the formation of the first black holes. Massive stars in low-metallicity environments in the local Universe are reminiscent of their high redshift counterparts. In a previous paper, we reported on indications that the stellar winds of low-metallicity O stars may be stronger than predicted, which would challenge the current paradigm of massive star evolution. In this paper, we aim to extend our initial sample of six O stars in low-metallicity environments by four. We aim to derive their stellar and wind parameters, and compare these to radiation-driven wind theory and stellar evolution models. We have obtained intermediate-resolution VLT/X-Shooter spectra of our sample of stars. We derive the stellar parameters by fitting synthetic fastwind line profiles to the VLT/X-Shooter spectra using a genetic fitting algoritm. We compare our parameters to evolutionary tracks and obtain evolutionary masses and ages. We also investigate the effective temperature versus spectral type calibration for SMC and lower metallicities. Finally, we reassess the wind momentum versus luminosity diagram. The derived parameters of our target stars indicate stellar masses that reach values of up to 50 $M_{\odot}$. The wind strengths of our stars are, on average, stronger than predicted from radiation-driven wind theory and reminiscent of stars with an LMC metallicity. We discuss indications that the iron content of the host galaxies is higher than originally thought and is instead SMC-like. We find that the discrepancy with theory is lessened, but remains significant for this higher metallicity. This may imply that our current understanding of the wind properties of massive stars, both in the local universe as well as at cosmic distances, remains incomplete.

astro-ph.SR↗

arXiv2014

A resolved, au-scale gas disk around the B[e] star HD 50138

HD 50138 is a B[e] star surrounded by a large amount of circumstellar gas and dust. Its spectrum shows characteristics which may indicate either a pre- or a post-main-sequence system. Mapping the kinematics of the gas in the inner few au of the system contributes to a better understanding of its physical nature. We present the first high spatial and spectral resolution interferometric observations of the Br-gamma line of HD~50138, obtained with VLTI/AMBER. The line emission originates from a region more compact (up to 3 au) than the continuum-emitting region. Blue- and red-shifted emission originates from the two different hemispheres of an elongated structure perpendicular to the polarization angle. The velocity of the emitting medium decreases radially. An overall offset along the NW direction between the line- and continuum-emitting regions is observed. We compare the data with a geometric model of a thin Keplerian disk and a spherical halo on top of a Gaussian continuum. Most of the data are well reproduced by this model, except for the variability, the global offset and the visibility at the systemic velocity. The evolutionary state of the system is discussed; most diagnostics are ambiguous and may point either to a post-main-sequence or a pre-main-sequence nature.

astro-ph.SR↗

arXiv2014

Rotational velocities of single and binary O-type stars in the Tarantula Nebula

Rotation is a key parameter in the evolution of massive stars, affecting their evolution, chemical yields, ionizing photon budget, and final fate. We determined the projected rotational velocity, $v_e\sin i$, of $\sim$330 O-type objects, i.e. $\sim$210 spectroscopic single stars and $\sim$110 primaries in binary systems, in the Tarantula nebula or 30 Doradus (30\,Dor) region. The observations were taken using VLT/FLAMES and constitute the largest homogeneous dataset of multi-epoch spectroscopy of O-type stars currently available. The most distinctive feature of the $v_e\sin i$ distributions of the presumed-single stars and primaries in 30 Dor is a low-velocity peak at around 100\,$\rm{km s^{-1}}$. Stellar winds are not expected to have spun-down the bulk of the stars significantly since their arrival on the main sequence and therefore the peak in the single star sample is likely to represent the outcome of the formation process. Whereas the spin distribution of presumed-single stars shows a well developed tail of stars rotating more rapidly than 300\,$\rm{km s^{-1}}$, the sample of primaries does not feature such a high-velocity tail. The tail of the presumed-single star distribution is attributed for the most part -- and could potentially be completely due -- to spun-up binary products that appear as single stars or that have merged. This would be consistent with the lack of such post-interaction products in the binary sample, that is expected to be dominated by pre-interaction systems. The peak in this distribution is broader and is shifted toward somewhat higher spin rates compared to the distribution of presumed-single stars. Systems displaying large radial velocity variations, typical for short period systems, appear mostly responsible for these differences.

astro-ph.SR↗

arXiv2014

The properties of single WO stars

The enigmatic oxygen sequence Wolf-Rayet (WO) stars represent a very late stage in massive star evolution, although their exact nature is still under debate. The spectra of most of the WO stars have never been analysed through detailed modelling with a non-local thermodynamic equilibrium expanding atmosphere code. Here we present preliminary results of the first homogeneous analysis of the (apparently) single WOs.

astro-ph.SR↗

arXiv2013

On the nature of the WO3 star DR1 in IC 1613

We present the results of a quantitative spectroscopic analysis of the oxygen-sequence Wolf- Rayet star DR1 in the low-metallicity galaxy IC 1613. Our models suggest that the strong oxygen emission lines are the result of the high temperature of this WO3 star and do not necessarily reflect a more advanced evolutionary stage than WC stars.

astro-ph.SR↗

arXiv2013

On the nature of WO stars: a quantitative analysis of the WO3 star DR1 in IC 1613

Context. Oxygen sequence Wolf-Rayet (WO) stars are thought to represent the final evolutionary stage of the most massive stars. The characteristic strong O vi emission possibly originates from an enhanced oxygen abundance in the stellar wind. Alternatively, the O vi emission can be caused by the high temperature of these stars, in which case the WO stars are the high-temperature extension of the more common carbon sequence Wolf-Rayet (WC) stars. Aims. By constraining the physical properties and evolutionary status of DR1, a WO star in the low-metallicity Local Group dwarf galaxy IC 1613 and one of only two objects of its class known in a SMC-like metallicity environment, we aim to investigate the nature of WO stars and their evolutionary connection with WC stars. Methods. We use the non-LTE atmosphere code cmfgen to model the observed spectrum of DR1 and to derive its stellar and wind parameters. We compare our values with other studies of WC and WO stars, as well as with the predictions of evolutionary models. We also model the surrounding nebula using the photo-ionization code cloudy. Results. The oxygen and carbon abundances that we obtain are comparable to values found for WC stars. The temperature and luminosity are, however, higher than those of WC stars. DR1 is embedded in the hottest known H ii region in the Local Group. The nebular properties can be consistently reproduced by cloudy models adopting DR1 as central ionizing source. Conclusions. Comparison of the abundances and temperature of DR1 with core helium-burning models show that DR1 is currently well into the second half of helium burning. If the properties of DR1 are representative for the WO class, it would imply that WO stars are the high-temperature and high-luminosity extension of the WC stars, and do not necessarily represent a later evolutionary stage.

astro-ph.SR↗

arXiv2013

RCW36: characterizing the outcome of massive star formation

Massive stars play a dominant role in the process of clustered star formation, with their feedback into the molecular cloud through ionizing radiation, stellar winds and outflows. The formation process of massive stars is poorly constrained because of their scarcity, the short formation timescale and obscuration. By obtaining a census of the newly formed stellar population, the star formation history of the young cluster and the role of the massive stars within it can be unraveled. We aim to reconstruct the formation history of the young stellar population of the massive star-forming region RCW 36. We study several dozens of individual objects, both photometrically and spectroscopically, look for signs of multiple generations of young stars and investigate the role of the massive stars in this process. We obtain a census of the physical parameters and evolutionary status of the young stellar population. Using a combination of near-infrared photometry and spectroscopy we estimate ages and masses of individual objects. We identify the population of embedded young stellar objects (YSO) by their infrared colors and emission line spectra. RCW 36 harbors a stellar population of massive and intermediate-mass stars located around the center of the cluster. Class 0/I and II sources are found throughout the cluster. The central population has a median age of 1.1 +/- 0.6 Myr. Of the stars which could be classified, the most massive ones are situated in the center of the cluster. The central cluster is surrounded by filamentary cloud structures; within these, some embedded and accreting YSOs are found. Our age determination is consistent with the filamentary structures having been shaped by the ionizing radiation and stellar winds of the central massive stars. The formation of a new generation of stars is ongoing, as demonstrated by the presence of embedded protostellar clumps, and two exposed jets.

astro-ph.SR↗

arXiv2013

R144 revealed as a double-lined spectroscopic binary

R144 is a WN6h star in the 30 Doradus region. It is suspected to be a binary because of its high luminosity and its strong X-ray flux, but no periodicity could be established so far. Here, we present new Xshooter multi-epoch spectroscopy of R144 obtained at the ESO Very Large Telescope (VLT). We detect variability in position and/or shape of all the spectral lines. We measure radial velocity variations with an amplitude larger than 250 km/s in NIV and NV lines. Furthermore, the NIII and NV line Doppler shifts are anti-correlated and the NIV lines show a double-peaked profile on six of our seven epochs. We thus conclude that R144 is a double-lined spectroscopic binary. Possible orbital periods range from 2 to 6 months, although a period up to one year is allowed if the orbit is highly eccentric. We estimate the spectral types of the components to be WN5-6h and WN6-7h, respectively. The high luminosity of the system (log Lbol/Lsun ~ 6.8) suggests a present-day total mass content in the range of about 200 to 300 Msun, depending on the evolutionary stage of the components. This makes R144 the most massive binary identified so far, with a total mass content at birth possibly as large as 400 Msun. We briefly discuss the presence of such a massive object 60 pc away from the R136 cluster core in the context of star formation and stellar dynamics.

astro-ph.SR↗

arXiv2013

Very Massive Stars (VMS) in the Local Universe

Recent studies have claimed the existence of very massive stars (VMS) up to 300 solar masses in the local Universe. As this finding may represent a paradigm shift for the canonical stellar upper-mass limit of 150 Msun, it is timely to discuss the status of the data, as well as the far-reaching implications of such objects. We held a Joint Discussion at the General Assembly in Beijing to discuss (i) the determination of the current masses of the most massive stars, (ii) the formation of VMS, (iii) their mass loss, and (iv) their evolution and final fate. The prime aim was to reach broad consensus between observers and theorists on how to identify and quantify the dominant physical processes.

astro-ph.SR↗

arXiv2011

On the Mass-Loss Rates of Massive Stars in the Low-Metallicity Galaxies IC 1613, WLM and NGC 3109

We present a spectroscopic analysis of VLT/X-Shooter observations of six O-type stars in the low-metallicity (Z ~ 1/7 Z\odot) galaxies IC 1613, WLM and NGC 3109. The stellar and wind parameters of these sources allow us, for the first time, to probe the mass-loss versus metallicity dependence of stellar winds below that of the Small Magellanic Cloud (at Z ~ 1/5Z\odot) by means of a modified wind momentum versus luminosity diagram. The wind strengths that we obtain for the objects in WLM and NGC 3109 are unexpectedly high and do not agree with theoretical predictions. The objects in IC 1613 tend towards a higher than expected mass-loss rate, but remain consistent with predictions within their error bars. We discuss potential systematic uncertainties in the mass-loss determinations to explain our results. However, if further scrutinization of these findings point towards an intrinsic cause for this unexpected sub-SMC mass-loss behavior, implications would include a higher than anticipated number of Wolf-Rayet stars and Ib/Ic supernovae in low-metallicity environments, but a reduced number of long-duration gamma-ray bursts produced through a single-star evolutionary channel.

astro-ph.SR↗