arXiv subjects

N. Przybilla

Publications and source records attributed to N. Przybilla.

At least 37 records · Page 2Linked to original sources

arXiv2018

Detection of magnetic fields in He-rich early B-type stars using HARPSpol

We focus on early-B type stars with helium overabundance, for which the presence of a magnetic field has not previously been reported. The measurements were carried out using high-spectral-resolution spectropolarimetric observations obtained with the High Accuracy Radial velocity Planet Searcher (HARPS) in polarimetric mode, installed at the ESO La Silla 3.6m telescope. For five He-rich stars, the longitudinal magnetic field was detected for the first time. For one target, HD58260, the presence of a longitudinal magnetic field of the order of 1.8 kG has already been reported in the literature, but the magnetic field has remained constant over tens of years. Our measurement carried out using the polarimetric spectra obtained in 2015 March indicates a slight decrease of the longitudinal magnetic field strength compared to measurements reported in previous works. A search for periodic modulation in available photometric data allowed us to confidently establish a period of 2.64119$\pm$0.00420 d in archival ASAS3 data for CPD -27$^{\circ}$ 1791. No period could be determined for the other five stars. The obtained results support the scenario that all He-rich stars are detectably magnetic and form an extension of the Ap star phenomenon to higher temperatures.

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arXiv2018

Discovery of extended main sequence turn offs in Galactic open clusters

By far, the color-magnitude diagrams (CMDs) of Galactic open clusters have been considered proto-types of single stellar populations. By using photometry in ultraviolet and optical bands we discovered that the nearby young cluster NGC6705 (M11) exhibits an extended main-sequence turn off (eMSTO) and a broadened main-sequence (MS). This is the first evidence of multiple stellar populations in a Galactic open cluster. By using high-resolution VLT spectra we provide direct evidence that the multiple sequences along the CMD correspond to stellar populations with different rotation rates. Specifically, the blue MS is formed of slow-rotating stars, while red-MS stars are fast rotators. Moreover, we exploit photometry from Gaia DR2 to show that three Galactic open clusters, namely NGC2099, NGC2360, and NGC2818, exhibit the eMSTO, thus suggesting that it is a common feature among these objects. Our previous work on the Large Magellanic Cloud star cluster NGC1818 shows that slowly and fastly-rotating stars populate the blue and red MS observed in its CMD. The similarities between M11 and the young clusters of the Magellanic Clouds suggest that rotation is responsible for the appearance of multiple populations in the CMDs of both Milky Way open clusters and Magellanic Clouds young clusters.

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arXiv2018

Different stellar rotation in the two main sequences of the young globular cluster NGC1818: first direct spectroscopic evidence

We present a spectroscopic analysis of main sequence (MS) stars in the young globular cluster NGC1818 (age~40 Myrs) in the Large Magellanic Cloud. Our photometric survey on Magellanic Clouds clusters has revealed that NGC1818, similarly to the other young objects with age 600 Myrs, displays not only an extended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age~1-2 Gyrs), but also a split MS. The most straightforward interpretation of the double MS is the presence of two stellar populations: a sequence of slowly-rotating stars lying on the blue-MS and a sequence of fast rotators, with rotation close to the breaking speed, defining a red-MS. We report the first direct spectroscopic measurements of projected rotational velocities vsini for the double MS, eMSTO and Be stars of a young cluster. The analysis of line profiles includes non-LTE effects, required for correctly deriving v sini values. Our results suggest that: (i) the mean rotation for blue- and red-MS stars is vsini=71\pm10 km/s (sigma=37 km/s) and vsini=202\pm23 km/s (sigma=91 km/s), respectively; (ii) eMSTO stars have different vsini, which are generally lower than those inferred for red-MS stars, and (iii) as expected, Be stars display the highest vsini values. This analyis supports the idea that distinct rotational velocities play an important role in the appearence of multiple stellar populations in the color-magnitude diagrams of young clusters, and poses new constraints to the current scenarios.

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arXiv2018

On a new and homogeneous metallicity scale for Galactic classical Cepheids - I. Physical parameters

We gathered more than 1130 high-resolution optical spectra for more than 250 Galactic classical Cepheids. The spectra were collected with different optical spectrographs: UVES at VLT, HARPS at 3.6m, FEROS at 2.2m MPG/ESO, and STELLA. To improve the effective temperature estimates, we present more than 150 new line depth ratio (LDR) calibrations that together with similar calibrations already available in the literature allowed us to cover a broad range in wavelength (between 5348 and 8427 angstrom) and in effective temperatures (between 3500 and 7700 K). This means the unique opportunity to cover both the hottest and coolest phases along the Cepheid pulsation cycle and to limit the intrinsic error on individual measurements at the level of ~100 K. Thanks to the high signal-to-noise ratio of individual spectra we identified and measured hundreds of neutral and ionized lines of heavy elements, and in turn, have the opportunity to trace the variation of both surface gravity and microturbulent velocity along the pulsation cycle. The accuracy of the physical parameters and the number of Fe I (more than one hundred) and Fe II (more than ten) lines measured allowed us to estimate mean iron abundances with a precision better than 0.1 dex. Here we focus on 14 calibrating Cepheids for which the current spectra cover either the entire or a significant portion of the pulsation cycle. The current estimates of the variation of the physical parameters along the pulsation cycle and of the iron abundances agree quite well with similar estimates available in the literature. Independent homogeneous estimates of both physical parameters and metal abundances based on different approaches that can constrain possible systematics are highly encouraged.

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arXiv2017

Magnetic field geometry and chemical abundance distribution of the He-strong star CPD -57 3509

The magnetic field of CPD -57 3509 was recently detected in the framework of the BOB (B fields in OB stars) collaboration. We acquired low-resolution spectropolarimetric observations of CPD -57 3509 with FORS2 and high-resolution UVES observations randomly distributed over a few months to search for periodicity, to study the magnetic field geometry, and to determine the surface distribution of silicon and helium. We also obtained supplementary photometric observations at a timeline similar to the spectroscopic and spectropolarimetric observations. A period of 6.36d was detected in the measurements of the mean longitudinal magnetic field. A sinusoidal fit to our measurements allowed us to constrain the magnetic field geometry and estimate the dipole strength in the range of 3.9-4.5kG. Our application of the Doppler imaging technique revealed the presence of He I spots located around the magnetic poles, with a strong concentration at the positive pole and a weaker one around the negative pole. In contrast, high concentration Si III spots are located close to the magnetic equator. Further, our analysis of the spectral variability of CPD -57 3509 on short time scales indicates distinct changes in shape and position of line profiles possibly caused by the presence of beta Cep-like pulsations. A small periodic variability in line with the changes of the magnetic field strength is clearly seen in the photometric data.

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arXiv2017

Candidate exoplanet host HD131399A: a nascent Am star

Direct imaging suggests that there is a Jovian exoplanet around the primary A-star in the triple-star system HD131399. We investigate a high-quality spectrum of the primary component HD131399A obtained with FEROS on the ESO/MPG 2.2m telescope, aiming to characterise the star's atmospheric and fundamental parameters, and to determine elemental abundances at high precision and accuracy. The aim is to constrain the chemical composition of the birth cloud of the system and therefore the bulk composition of the putative planet. A hybrid non-local thermal equilibrium (non-LTE) model atmosphere technique is adopted for the quantitative spectral analysis. Comparison with the most recent stellar evolution models yields the fundamental parameters. The atmospheric and fundamental stellar parameters of HD131399A are constrained to Teff=9200+-100 K, log g=4.37+-0.10, M=1.95+0.08-0.06 Msun, R=1.51+0.13-0.10 Rsun, and log L/Lsun=1.17+-0.07, locating the star on the zero-age main sequence. Non-LTE effects on the derived metal abundances are often smaller than 0.1dex, but can reach up to ~0.8dex for individual lines. The observed lighter elements up to calcium are overall consistent with present-day cosmic abundances, with a C/O ratio of 0.45$\pm$0.07 by number, while the heavier elements show mild overabundances. We conclude that the birth cloud of the system had a standard chemical composition, but we witness the onset of the Am phenomenon in the slowly rotating star. We furthermore show that non-LTE analyses have the potential to solve the remaining discrepancies between observed abundances and predictions by diffusion models for Am stars. Moreover, the present case allows mass loss, not turbulent mixing, to be identified as the main transport process competing with diffusion in very young Am stars.

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arXiv2017

LMC Blue Supergiant Stars and the Calibration of the Flux-weighted Gravity--Luminosity Relationship

High quality spectra of 90 blue supergiant stars in the Large Magellanic Cloud are analyzed with respect to effective temperature, gravity, metallicity, reddening, extinction and extinction law. An average metallicity, based on Fe and Mg abundances, relative to the Sun of [Z] = -0.35 +/- 0.09 dex is obtained. The reddening distribution peaks at E(B-V) = 0.08 mag, but significantly larger values are also encountered. A wide distribution of the ratio of extinction to reddening is found ranging from Rv = 2 to 6. The results are used to investigate the blue supergiant relationship between flux-weighted gravity, and absolute bolometric magnitude. The existence of a tight relationship, the FGLR, is confirmed. However, in contrast to previous work the observations reveal that the FGLR is divided into two parts with a different slope. For flux-weighted gravities larger than 1.30 dex the slope is similar as found in previous work, but the relationship becomes significantly steeper for smaller values of the flux-weighted gravity. A new calibration of the FGLR for extragalactic distance determinations is provided.

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arXiv2017

A comprehensive study of young B stars in NGC 2264: I. Space photometry and asteroseismology

Space photometric time series of the most massive members of the young open cluster NGC 2264 allow us to study their different sources of variability down to the millimagnitude level and permits a search for Slowly Pulsating B (SPB) type pulsation among objects that are only a few million years old. Our goal is to conduct a homogeneous study of young B type stars in the cluster NGC 2264 using photometric time series from space in combination with high-resolution spectroscopy and spectropolarimetry obtained from the ground. The latter will be presented in a separate follow-up article. We performed frequency analyses for eleven B stars in the field of the young cluster NGC 2264 using photometric time series from the MOST, CoRoT and Spitzer space telescopes and the routines Period04 and SigSpec. We employ the MESA stellar evolution code in combination with the oscillation code GYRE to identify the pulsation modes for two SPB stars which exhibit short period spacing series. From our analysis we identify four objects that show SPB pulsations, five stars that show rotational modulation of their light curves caused by spots, one star that is identified to be a binary, and one object in the field of the cluster that is found to be a non-member Be star. In two SPB stars we detect a number of regularly spaced pulsation modes that are compatible with being members of a g mode period series. Despite NGC 2264's young age, our analysis illustrates that its B type members have already arrived on the zero-age main sequence (ZAMS). Our asteroseismic analysis yields masses between 4 and 6 Msun and ages between 1 and 6 million years, which agree well to the overall cluster age.

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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

B fields in OB stars (BOB): Concluding the FORS2 observing campaign

The "B fields in OB stars" (BOB) collaboration is based on an ESO Large Programme, to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. In the framework of this programme, we carried out low-resolution spectropolarimetric observations of a large sample of massive stars using FORS2 installed at the ESO VLT 8-m telescope. We determined the magnetic field values with two completely independent reduction and analysis pipelines. Our in-depth study of the magnetic field measurements shows that differences between our two pipelines are usually well within 3sigma errors. From the 32 observations of 28 OB stars, we were able to monitor the magnetic fields in CPD-57 3509 and HD164492C, confirm the magnetic field in HD54879, and detect a magnetic field in CPD-62 2124. We obtain a magnetic field detection rate of 6+-3% for the full sample of 69 OB stars observed with FORS2 within the BOB programme. For the pre-selected objects with a v sin i below 60 km/s, we obtain a magnetic field detection rate of 5+-5%. We also discuss X-ray properties and multiplicity of the objects in our FORS2 sample with respect to the magnetic field detections.

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arXiv2016

CRIRES-POP: a library of high resolution spectra from 1 to 5 microns II. Data reduction and the spectrum of the K giant 10 Leo

Context. High resolution stellar spectral atlases are valuable resources to astronomy. They are rare in the $1 - 5\,μ$m region for historical reasons, but once available, high resolution atlases in this part of the spectrum will aid the study of a wide range of astrophysical phenomena. Aims. The aim of the CRIRES-POP project is to produce a high resolution near-infrared spectral library of stars across the H-R diagram. The aim of this paper is to present the fully reduced spectrum of the K giant 10 Leo that will form the basis of the first atlas within the CRIRES-POP library, to provide a full description of the data reduction processes involved, and to provide an update on the CRIRES-POP project. Methods. All CRIRES-POP targets were observed with almost 200 different observational settings of CRIRES on the ESO Very Large Telescope, resulting in a basically complete coverage of its spectral range as accessible from the ground. We reduced the spectra of 10 Leo with the CRIRES pipeline, corrected the wavelength solution and removed telluric absorption with Molecfit, then resampled the spectra to a common wavelength scale, shifted them to rest wavelengths, flux normalised, and median combined them into one final data product. Results. We present the fully reduced, high resolution, near-infrared spectrum of 10 Leo. This is also the first complete spectrum from the CRIRES instrument. The spectrum is available online. Conclusions. The first CRIRES-POP spectrum has exceeded our quality expectations and will form the centre of a state-of-the-art stellar atlas. This first CRIRES-POP atlas will soon be available, and further atlases will follow. All CRIRES-POP data products will be freely and publicly available online.

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arXiv2016

MICADO: first light imager for the E-ELT

MICADO will equip the E-ELT with a first light capability for diffraction limited imaging at near-infrared wavelengths. The instrument's observing modes focus on various flavours of imaging, including astrometric, high contrast, and time resolved. There is also a single object spectroscopic mode optimised for wavelength coverage at moderately high resolution. This contribution provides an overview of the key functionality of the instrument, outlining the scientific rationale for its observing modes. The interface between MICADO and the adaptive optics system MAORY that feeds it is summarised. The design of the instrument is discussed, focussing on the optics and mechanisms inside the cryostat, together with a brief overview of the other key sub-systems.

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arXiv2016

B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD-57° 3509

We report the detection of a magnetic field in the helium-strong star CPD-57 3509 (B2 IV), a member of the Galactic open cluster NGC3293, and characterise the star's atmospheric and fundamental parameters. Spectropolarimetric observations with FORS2 and HARPSpol are analysed using two independent approaches to quantify the magnetic field strength. A high-S/N FLAMES/GIRAFFE spectrum is analysed using a hybrid non-LTE model atmosphere technique. Comparison with stellar evolution models constrains the fundamental parameters of the star. We obtain a firm detection of a surface averaged longitudinal magnetic field with a maximum amplitude of about 1 kG. Assuming a dipolar configuration of the magnetic field, this implies a dipolar field strength larger than 3.3 kG. Moreover, the large amplitude and fast variation (within about 1 day) of the longitudinal magnetic field implies that CPD-57 3509 is spinning very fast despite its apparently slow projected rotational velocity. The star should be able to support a centrifugal magnetosphere, yet the spectrum shows no sign of magnetically confined material; in particular, emission in Hα is not observed. Apparently, the wind is either not strong enough for enough material to accumulate in the magnetosphere to become observable or, alternatively, some leakage process leads to loss of material from the magnetosphere. The quantitative spectroscopic analysis of the star yields an effective temperature and a logarithmic surface gravity of 23750+-250 K and 4.05+-0.10, respectively, and a surface helium fraction of 0.28+-0.02 by number. The surface abundances of C, N, O, Ne, S, and Ar are compatible with the cosmic abundance standard, whereas Mg, Al, Si, and Fe are depleted by about a factor of 2. This abundance pattern can be understood as the consequence of a fractionated stellar wind. CPD-57 3509 is one of the most evolved He-strong stars known.

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arXiv2015

On the neutron-capture elements across the Galactic thin disk using Cepheids

We present new accurate abundances for five neutron-capture (Y, La, Ce, Nd, Eu) elements in 73 classical Cepheids located across the Galactic thin disk. Individual abundances are based on high spectral resolution (R ~ 38,000) and high signal-to-noise ratio (S/N ~ 50-300) spectra collected with UVES at ESO VLT for the DIONYSOS project. Taking account for similar Cepheid abundances provided either by our group (111 stars) or available in the literature, we end up with a sample of 435 Cepheids covering a broad range in iron abundances (-1.6 < [Fe/H] < 0.6). We found, using homogeneous individual distances and abundance scales, well defined gradients for the above elements. However, the slope of the light s-process element (Y) is at least a factor of two steeper than the slopes of heavy s- (La, Ce, Nd) and r- (Eu) process elements. The s to r abundance ratio ([La/Eu]) of Cepheids shows a well defined anticorrelation with of both Eu and Fe. On the other hand, Galactic field stars attain an almost constant value and only when they approach solar iron abundance display a mild enhancement in La. The [Y/Eu] ratio shows a mild evidence of a correlation with Eu and, in particular, with iron abundance for field Galactic stars. We also investigated the s-process index - [hs/ls] - and we found a well defined anticorrelation, as expected, between [La/Y] and iron abundance. Moreover, we found a strong correlation between [La/Y] and [La/Fe] and, in particular, a clear separation between Galactic and Sagittarius red giants. Finally, the comparison between predictions for low-mass asymptotic giant branch stars and the observed [La/Y] ratio indicate a very good agreement over the entire metallicity range covered by Cepheids. However, the observed spread, at fixed iron content, is larger than predicted by current models.

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arXiv2015

B fields in OB stars (BOB): low-resolution FORS2 spectropolarimetry of the first sample of 50 massive stars

Within the context of the collaboration "B fields in OB stars (BOB)", we used the FORS2 low-resolution spectropolarimeter to search for a magnetic field in 50 massive stars, including two reference magnetic massive stars. Because of the many controversies of magnetic field detections obtained with the FORS instruments, we derived the magnetic field values with two completely independent reduction and analysis pipelines. We compare and discuss the results obtained from the two pipelines. We obtained a general good agreement, indicating that most of the discrepancies on magnetic field detections reported in the literature are caused by the interpretation of the significance of the results (i.e., 3-4 sigma detections considered as genuine, or not), instead of by significant differences in the derived magnetic field values. By combining our results with past FORS1 measurements of HD46328, we improve the estimate of the stellar rotation period, obtaining P = 2.17950+/-0.00009 days. For HD125823, our FORS2 measurements do not fit the available magnetic field model, based on magnetic field values obtained 30 years ago. We repeatedly detect a magnetic field for the O9.7V star HD54879, the HD164492C massive binary, and the He-rich star CPD -57 3509. We obtain a magnetic field detection rate of 6+/-4%, while by considering only the apparently slow rotators we derive a detection rate of 8+/-5%, both comparable with what was previously reported by other similar surveys. We are left with the intriguing result that, although the large majority of magnetic massive stars is rotating slowly, our detection rate is not a strong function of the stellar rotational velocity.

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arXiv2015

B fields in OB stars (BOB): Detection of a strong magnetic field in the O9.7 V star HD54879

The number of magnetic stars detected among massive stars is small; nevertheless, the role played by the magnetic field in stellar evolution cannot be disregarded. Links between line profile variability, enhancements/depletions of surface chemical abundances, and magnetic fields have been identified for low-mass B-stars, but for the O-type domain this is almost unexplored. Based on FORS2 and HARPS spectropolarimetric data, we present the first detection of a magnetic field in HD54879, a single slowly rotating O9.7 V star. Using two independent and different techniques we obtained the firm detection of a surface average longitudinal magnetic field with a maximum amplitude of about 600 G, in modulus. A quantitative spectroscopic analysis of the star with the stellar atmosphere code FASTWIND results in an effective temperature and a surface gravity of 33000$\pm1000$ K and 4.0$\pm0.1$ dex. The abundances of carbon, nitrogen, oxygen, silicon, and magnesium are found to be slightly lower than solar, but compatible within the errors. We investigate line-profile variability in HD54879 by complementing our spectra with spectroscopic data from other recent OB-star surveys. The photospheric lines remain constant in shape between 2009 and 2014, although H$α$ shows a variable emission. The H$α$ emission is too strong for a standard O9.7 V and is probably linked to the magnetic field and the presence of circumstellar material. Its normal chemical composition and the absence of photospheric line profile variations make HD54879 the most strongly magnetic, non-variable single O-star detected to date.

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arXiv2015

B fields in OB stars (BOB): FORS2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD23478 and HD345439

Massive B-type stars with strong magnetic fields and fast rotation are very rare and provide a mystery for theories of both star formation and magnetic field evolution. Only two such stars, called sigma Ori E analogs, were previously known. Recently, a team involved in APOGEE, one of the Sloan Digital Sky Survey III programs, announced the discovery of two additional rigidly rotating magnetosphere stars, HD23478 and HD345439. The presence of magnetic fields in these newly discovered sigma Ori E analogs was not investigated in the past. In the framework of our ESO Large Programme, and one normal ESO programme, we carried out low-resolution FORS2 spectropolarimetric observations of HD23478 and HD345439. From the measurements using hydrogen lines, we discover a rather strong longitudinal magnetic field of the order of up to 1.5kG in HD23478, and up to 1.3kG using the entire spectrum. The analysis of HD345439 using four subsequent spectropolarimetric subexposures does not reveal the presence of a magnetic field at a significance level of 3sigma. On the other hand, the inspection of individual subexposures indicates that HD345439 may host a strong magnetic field, rapidly varying over 88 minutes. A hint at the fast rotation of HD345439 is also given by the behaviour of several metallic and He I lines in the low-resolution FORS2 spectra, showing profile variations already on such a short time scale.

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