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J. F. Gonzalez

Publications and source records attributed to J. F. Gonzalez.

At least 37 records · Page 2Linked to original sources
arXiv2012

Line identification in high-resolution, near-infrared CRIRES spectra of chemically peculiar and Herbig Ae stars

The recent availability of ESO's high-resolution spectrograph CRIRES offers now the opportunity to study numerous spectral features in the near-IR in intermediate-mass main-sequence and pre-main-sequence stars. High-resolution CRIRES spectra were obtained in three spectral regions, two regions around 1mu and one region around 1.57mu containing magnetically sensitive Fe I lines. The largest number of near-IR spectral features was detected and identified in the well-studied magnetic Ap star gamma Equ. Nearly 30% of the spectral lines in the Ap star HD154708, with one of the strongest magnetic fields known among the Ap stars of the order of 25kG, remain unidentified due to a lack of atomic data. Only very few lines belonging to the rare earth element group have been identified in both Ap stars. A number of spectral lines including the Ce III and Dy II lines appear magnetically split due to the presence of a strong magnetic field in their atmospheres. Variable behaviour of lines of the elements He, N, Mg, Si, and Fe over the rotation period in the spectra of HD101412 confirm our previous finding of variability in the optical region. Due to the very fast rotation of 51Oph, only a few spectral lines have been identified with certainty.

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arXiv2011

Spectroscopic variability and magnetic fields of HgMn stars

The discovery of exotic abundances, chemical inhomogeneities, and weak magnetic fields on the surface of late B-type primaries in spectroscopic binaries has important implications not only for our understanding of the formation mechanisms of stars with Hg and Mn peculiarities themselves, but also for the general understanding of B-type star formation in binary systems. The origin of the abundance anomalies observed in late B-type stars with HgMn peculiarity is still poorly understood. The connection between HgMn peculiarity and membership in binary and multiple systems is supported by our observations during the last decade. The important result achieved in our studies of a large sample of HgMn stars is the finding that most HgMn stars exhibit spectral variability of various chemical elements, proving that the presence of an inhomogeneous distribution on the surface of these stars is a rather common characteristic and not a rare phenomenon. Further, in the studied systems, we found that all components are chemically peculiar with different abundance patterns. Generally, He and Si variable Bp stars possess large-scale organised magnetic fields that in many cases appear to occur essentially in the form of a single large dipole located close to the centre of the star. The presence of magnetic fields in the atmospheres of HgMn stars has been demonstrated in several studies. In addition to the measurements of longitudinal and quadratic magnetic fields, this work also showed evidence for a relative magnetic intensification of Fe II lines produced by different magnetic desaturations induced by different Zeeman-split components.

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arXiv2011

The strong magnetic field of the large-amplitude beta Cephei pulsator V1449 Aql

Only for very few beta Cephei stars has the behaviour of the magnetic field been studied over the rotation cycle. During the past two years we have obtained multi-epoch polarimetric spectra of the beta Cephei star V1449 Aql with SOFIN at the Nordic Optical Telescope to search for a rotation period and to constrain the geometry of the magnetic field. The mean longitudinal magnetic field is measured at 13 different epochs. The new measurements, together with the previous FORS1 measurements, have been used for the frequency analysis and the characterization of the magnetic field. V1449 Aql so far possesses the strongest longitudinal magnetic field of up to 700G among the beta Cephei stars. The resulting periodogram displays three dominant peaks with the highest peak at f=0.0720d^-1 corresponding to a period P=13.893d. The magnetic field geometry can likely be described by a centred dipole with a polar magnetic field strength B_d around 3kG and an inclination angle beta of the magnetic axis to the rotation axis of 76+-4deg. As of today, the strongest longitudinal magnetic fields are detected in the beta Cephei stars V1449 Aql and xi^1 CMa with large radial velocity amplitudes. Their peak-to-peak amplitudes reach ~90km/s and ~33km/s, respectively. Concluding, we briefly discuss the position of the currently known eight magnetic beta Cephei and candidate beta Cephei stars in the Hertzsprung-Russell (H-R) diagram.

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arXiv2010

Rotationally modulated variations and the mean longitudinal magnetic field of the Herbig Ae star HD 101412

Despite of the importance of magnetic fields for the full understanding of the properties of accreting Herbig Ae/Be stars, these fields have scarcely been studied over the rotation cycle until now. One reason for the paucity of such observations is the lack of knowledge of their rotation periods. The sharp-lined young Herbig Ae star HD101412 with a strong surface magnetic field became in the last years one of the most studied targets among the Herbig Ae/Be stars. A few months ago we obtained multi-epoch polarimetric spectra of this star with FORS2 to search for a rotation period and to constrain the geometry of the magnetic field. We measured longitudinal magnetic fields on 13 different epochs distributed over 62 days. These new measurements together with our previous measurements of the magnetic field in this star were combined with available photometric observations to determine the rotation period. The search of the rotation period resulted in P=42.076+-0.01d. According to near-infrared imaging studies the star is observed nearly edge-on. The star exhibits a single-wave variation of the longitudinal magnetic field during the stellar rotation cycle. These observations are usually considered as evidence for a dominant dipolar contribution to the magnetic field topology.

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arXiv2010

Lanthanides and other spectral oddities in a Centauri

Context: There is considerable interest in the helium variable a Cen as a bridge between helium-weak and helium-strong CP stars. Aims: We investigate Ce III and other possible lanthanides in the spectrum the of hottest chemically peculiar (CP) star in which these elements have been found. A {Kr II line appears within a broad absorption which we suggest may be due to a high-level transition in C II. Methods: Wavelengths and equivalent widths are measured on high-resolution UVES spectra, analyzed, and their phase-variations investigated. Results: New, robust identifications of Ce III and Kr II are demonstrated. Nd III is likely present. A broad absorption near 4619[A] is present at all phases of a Cen, and in some other early B stars. Conclusions: The presence of lanthanides in a Cen strengthens the view that this star is a significant link between the cooler CP stars and the hotter helium-peculiar stars. Broad absorptions in a Cen are not well explained.

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arXiv2010

Dynamical evolution of titanium, strontium, and yttrium spots on the surface of the HgMn star HD 11753

Aims. We gathered about 100 high-resolution spectra of three typical HgMn (mercury-manganese) stars, HD 11753, HD 53244, and HD 221507, to search for slowly pulsating B-like pulsations and surface inhomogeneous distribution of various chemical elements. Methods. Classical frequency analysis methods were used to detect line profile variability and to determine the variation period. Doppler imaging reconstruction was performed to obtain abundance maps of chemical elements on the stellar surface. Results. For HD 11753, which is the star with the most pronounced variability, distinct spectral line profile changes were detected for Ti, Sr, Y, Zr, and Hg, whereas for HD 53244 and HD 221507 the most variable line profiles belong to the elements Hg and Y, respectively. We derived rotation periods for all three stars from the variations of radial velocities and equivalent widths of spectral lines belonging to inhomogeneously distributed elements: P_rot (HD 11753)=9.54 d, P_rot (HD 53244)=6.16 d, and P_rot (HD 221507)=1.93 d. For HD 11753 the Doppler imaging technique was applied to derive the distribution of the most variable elements Ti, Sr, and Y using two datasets separated by ~65 days. Results of Doppler imaging reconstruction revealed noticeable changes in the surface distributions of TiII, SrII, and YII between the datasets, indicating the hitherto not well understood physical processes in stars with radiative envelopes that cause a rather fast dynamical chemical spot evolution.

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arXiv2010

The exceptional Herbig Ae star HD101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star

We obtained high-resolution, high signal-to-noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period P_rot=13.86d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non-statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current-driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate-mass stars could be an alternative to a frozen-in fossil field.

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arXiv2009

A high resolution study of isotopic composition and chemical abundances of blue horizontal branch stars in the globular clusters NGC6397 and NGC6752

Large abundance anomalies have been previously detected in Horizontal Branch B-type stars. We present the first high resolution study of isotopic anomalies and chemical abundances in six Horizontal Branch B-type stars in globular clusters NGC6397 and NGC6752 and compare them to those observed in HgMn stars. We obtained high-resolution (up to R~115000) UVES spectra of a representative sample of six B-type stars (T183, T191, T193, B652, B2151, B2206). It is the first time an abundance analysis is performed for all elements for which spectral lines were detected in UVES spectra of Horizontal Branch B-type stars. Our study of these stars revealed no signs of He isotopic anomalies which would produce a ^3He/^4He ratio different from the solar one. The isotopic anomaly of Ca is detected in all six studied stars. The chemical abundance analysis reveals an overabundance over the solar values of P, Ti, Mn, Fe, and Y and an overabundance over the cluster metallicity of Mg, Ca, and Cr. This behaviour is very similar in all six studied stars of both clusters with a few exceptions: The Na abundance is by more than 1.4dex larger than the cluster metallicity in B652, and by more than 0.8dex larger than the cluster metallicity in B2206; the Co abundance is 1.0dex over the solar abundance for T191, while Zr is overabundant over the solar abundance by 0.4dex in B2206. No lines of Hg or other heavy elements were observed in the spectra. Weak emission lines of Ti II, similar to those frequently observed in HgMn stars were discovered in one Horizontal Branch B-type star T191. Further, we detected a radial velocity change of 0.9km s^-1 from one night to the next for T183 and 0.4km s^-1 for B2151.

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arXiv2009

Magnetic fields in O-type stars measured with FORS1 at the VLT

The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of such fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, Halpha emission variations, chemical peculiarity, narrow X-ray emission lines and non-thermal radio/X-ray emission. Here we present the results of our studies of magnetic fields in O-type stars, carried out over the last years.

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arXiv2008

Magnetic field measurements of O stars with FORS1 at the VLT

The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of such fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, H alpha emission variations, chemical peculiarity, narrow X-ray emission lines and non-thermal radio/X-ray emission. To investigate the incidence of magnetic fields in O stars, we acquired 38 new spectropolarimetric observations with FORS1 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Kueyen telescope of the VLT. Spectropolarimetric observations have been obtained at different phases for a sample of 13 O stars. 10 stars were observed in the spectral range 348-589nm, HD36879 and HD148937 were observed in the spectral region 325-621nm, and HD155806 was observed in both settings. To prove the feasibility of the FORS1 spectropolarimetric mode for the measurements of magnetic fields in hot stars, we present in addition 12 FORS1 observations of the mean longitudinal magnetic field in theta^1 Ori C and compare them with measurements obtained with the MuSiCoS, ESPaDOnS and Narval spectropolarimeters. Most stars in our sample which have been observed on different nights show a change of the magnetic field polarity, but a field at a significance level of 3sigma has been detected only in four stars, HD36879, HD148937, HD152408, and HD164794. The largest longitudinal magnetic field, = -276+-88G, was detected in the Of?p star HD148937. We conclude that large-scale organised magnetic fields with polar field strengths larger than 1kG are not widespread among O-type stars.

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arXiv2008

New insights into the nature of the peculiar star theta Carinae

We acquired high resolution spectroscopic and low resolution spectropolarimetric observations to achieve the following goals: a) to improve the orbital parameters to allow a more in-depth discussion on the possibility of mass transfer in the binary system, b) to carry out a non-local thermodynamic equilibrium (NLTE) abundance analysis, and c) to search for the presence of a magnetic field. The study of the radial velocities using CORALIE spectra allowed us to significantly improve the orbital parameters. A comparative NLTE abundance analysis was undertaken for theta Car and two other early B-type stars with recently detected magnetic fields, tau Sco and xi^1 CMa. The analysis revealed significantly different abundance patterns: a one-order-of-magnitude nitrogen overabundance and carbon depletion was found in theta Car, while the oxygen abundance is roughly solar. For the stars xi^1 CMa and tau Sco the carbon abundance is solar and, while an N excess is also detected, it is of much smaller amplitude (0.4-0.6dex). Such an N overabundance is typical of the values already found for other slowly-rotating (magnetic) B-type dwarfs. For theta Car, we attribute instead the chemical peculiarities to a past episode of mass transfer between the two binary components. The results of the search for a magnetic field using FORS1 at the VLT consisting of 26 measurements over a time span of ~1.2h are rather inconclusive: only few measurements have a significance level of 3sigma. Although we detect a periodicity of the order of ~8.8min in the dataset involving the measurements on all hydrogen Balmer lines with the exception of the Halpha and Hbeta lines, these results have to be confirmed by additional time-resolved magnetic field observations.

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arXiv2007

A search for pulsational line profile variations in the delta Scuti star HD21190 and the Ap Sr star HD218994

We present the results of our recent search for pulsational line profile variations in high time resolution UVES spectra of the most evolved Ap star known, the delta Scuti star HD21190, and of the Ap Sr star HD218994. We found that HD218994 is an roAp star with a pulsation period of 5.1 min, which makes it the 36th star known to be a roAp star. No rapid pulsations have been found in the spectra of the delta Scuti star HD21190. However, we detect moving peaks in the cores of spectral lines, which indicate the presence of non-radial pulsations in this star.

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arXiv2007

Spots on the surface of HgMn stars: Clues to the origin of Hg and Mn peculiarities

The important result achieved in our recent study of a large sample of HgMn stars using UVES at the VLT and FEROS at the ESO 2.2m telescope is the finding that most HgMn stars exhibit spectral variability of various chemical elements, proving that the presence of an inhomogeneous distribution on the surface of these stars is a rather common characteristics and not a rare phenomenon.

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arXiv2007

Magnetic fields in massive stars

We review the recent discoveries of magnetic fields in different types of massive stars and briefly discuss strategies for spectropolarimetric observations to be carried out in the future.

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arXiv2007

Discovery of pulsational line profile variations in the delta Scuti star HD21190 and in the Ap Sr star HD218994

Asteroseismology has the potential to provide new insights into the physics of stellar interiors. We have obtained UVES high time resolution observations of the delta Scuti star HD21190 and of the Ap Sr star HD218994 to search for pulsational line profile variations. We report the discovery of a new roAp star, HD218994, with a pulsation period of 14.2 min. This is one of the most evolved roAp stars. No rapid pulsations have been found in the spectra of the cool Ap star - delta Scuti star HD21190. However, we detect with unprecedented clarity for a delta Sct star moving peaks in the cores of spectral lines that indicate the presence of high degree non-radial pulsations in this star.

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arXiv2007

Detection of variable Si II, Mn II and Fe II emission lines in the magnetic Bp star a Centauri

The nature of non-variable high-excitation emission lines detected in the optical spectra of normal late-B type and chemically peculiar HgMn and PGa stars is still poorly understood. To better understand the origin of the weak emission lines in B type stars it is especially important to investigate the spectra of a variety of stars to search for correlations between the emergence of these lines and fundamental stellar parameters. We have acquired high resolution UVES spectra for the sharp-lined magnetic helium-variable star a Cen over the rotation period of 8.82 d to search for the presence of weak emission lines. For the first time we present observational evidence for the appearance of variable high-excitation Si II, Mn II and Fe II emission lines in a magnetic Bp star. Si II emissions are the strongest at the phase corresponding to the maximum strength of He I lines. Mn II and Fe II emissions vary in antiphase to the He I lines. A correlation is found between the probable location of Mn and Fe surface spots and the strength of the emission lines. On the basis of the currently available data it seems possible that the same kind of selective excitation process is working in the atmospheres of objects within a broad parameter space which could be defined by age, effective temperature, chemical composition, rotational velocity, and magnetic field. Neutral iron lines previously reported to appear broad and shallow at certain phases are not detected in our spectra, although two of them are identified as He I forbidden lines, showing maximum strength at the phase of the passage of the He rich region across the visible disk.

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arXiv2007

Heavy Calcium in CP Stars

Large wavelength shifts of infrared triplet lines of Ca II have been observed in the spectra of HgMn and magnetic Ap stars. They have been attributed to the heavy calcium isotopes, including Ca-48. One member of the triplet, $λ$8542, had been either unavailable, or of poor quality in earlier spectra. The present material shows conclusively that the stellar $λ$8542 shifts are consistent with an interpretation in terms of Ca-48. We find no relation between isotopic shifts of the Ca II triplet lines, and those of Hg II $λ$3984. There is a marginal indication that the shifts are {\it anticorrelated} with the surface field strengths of the magnetic stars. We see sparse evidence for Ca-48 in other chemically peculiar stars, e.g. Am's, metal-poor stars, or chemically peculiar red giants. However, the sample is still very small, and the wavelengths of all three triplet lines, including those in the Sun, show slight positive shifts with respect to terrestrial positions. Some profiles of the Ca II infrared triplet in the magnetic stars show extensive wings beyond a well-defined core. We can obtain reasonable fits to these profiles using a stratified calcium abundance similar to that used by previous workers. There is no indication that either the stratification or the Zeeman effect significantly disturbs the measurement of isotope shifts.

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