arXiv subjects

S. P. Järvinen

Publications and source records attributed to S. P. Järvinen.

At least 19 recordsLinked to original sources

arXiv2026

Detection of a large-scale organized 2 kG order magnetic field in the Herbig Ae star HD179218

While about two dozen Herbig Ae/Be stars have been reported to be magnetic, only two of them, HD101412 and HD190073, have had their magnetic field geometries studied in the past. The knowledge of the magnetic field structure is important to understanding how magnetospheric accretion works in these stars. We aim to study in detail the spectral and magnetic variability of HD179218, which is necessary to put constraints on its magnetic field geometry. We measured the mean longitudinal magnetic field, $\langle B_{\rm z}\rangle$, from newly acquired and archival high-resolution spectropolarimetric observations of HD179218 using the least-squares deconvolution technique. Additionally, we studied the spectral variability of the hydrogen lines using dynamical spectra. Based on our analysis of the Stokes V spectra of HD179218, we report for the first time the definite detection of a magnetic field. Using a slightly refined rotation period of P$_{rot}$ = 1.34102 d, we constrained its geometry as follows: an estimated magnetic obliquity angle of $β$= 79.9 $\pm$ 0.7 deg and a dipole strength of B$_d$ = 2142 $\pm$ 52 G. The $\langle B_{\rm z}\rangle$ variation is best fitted by the superposition of a sine wave and of its first harmonic, but more spectropolarimetric observations are necessary to test the impact of the limited measurement precision and the uneven coverage of the rotation cycle. The strongest emission in the H$α$ and H$β$ line profiles in the medium-resolution spectra acquired in 2025 was detected close to the phases of the best visibility of the magnetic poles. HD179218 is the second Herbig Ae/Be star after HD190073 for which a first snapshot of a magnetosphere is presented.

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arXiv2026

The magnetic fields in Be stars are stronger than previously suggested

Recent observational studies suggest that Be stars most likely are formed through the process of mass transfer in binary systems. In view of the wide consensus that the origin of the magnetic field in stars with radiative envelopes involves binary interaction processes, searching for magnetic fields in Be stars appears especially promising. As a pilot project, we searched for the presence of magnetic fields in a sample of seven well-known Be stars. We used high-resolution HARPSpol spectra to measure the mean longitudinal magnetic field, employing the least squares deconvolution technique. A dedicated measurement procedure introduced by our group in recent years was applied. Opposite to previous spectropolarimetric studies reporting that magnetic fields in Be stars are weak and usually below 100 G, our study presents the first observational evidence that magnetic fields in Be stars can be as strong as a few hundred gauss. Magnetic fields are detected in all studied Be stars, with the strongest magnetic field being about -460 G for the B0.5 III star HD 184915. Magnetic fields in the range between 338 and 380 G (in absolute values) are detected in three other Be stars, HD 209409, HD 209522, and HD 224686. Due to the fact that magnetic fields in Be stars are stronger than previously believed, we must re-evaluate our understanding of the initial conditions of massive binaries to be able to determine the origin of such systems.

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arXiv2025

Magnetic activity on the young Sun: a case study of EK Draconis

Context. Young, solar analogue stars provide key insights into the early stages of stellar evolution, particularly in terms of magnetic activity and rotation. Their rapid rotation, high flaring rate, and enhanced surface activity make them ideal laboratories for testing stellar models or even the solar dynamo. Aims. Using long-term photometric data, we investigated the cyclic behaviour of EK Dra over the last century. We analyze its short-term activity based on 13 Transiting Exoplanet Survey Satellite (TESS) sectors. Applying Doppler imaging on high-resolution spectral data we investigate short and long-term spot evolution and surface differential rotation. Methods. We use Short-term Fourier-transform on a 120 years long archival photometric data in order to search for activity cycles. The short-term space photometry data is fitted with an analytic three-spot model, and we hand-select flares from it to analyze their phase and frequency distribution. Spectral synthesis is used to determine the astrophysical parameters of EK Dra. Using the iMap multi-line Doppler imaging code, we reconstruct 13 Doppler images. Differential rotation is derived by cross-correlating consecutive Doppler maps. Results. Long-term photometric data reveal a 10.7-12.1 year cycle that was persistently present for 120 years. In the more recent half of the light curve a 7.3-8.2 years-long signal is also visible. The distribution of the 142 flares in the TESS data shows no correlation with the rotational phase or with the spotted longitudes. The reconstructed Doppler images show a surface that varies from rotation to rotation, putting the lower limit of the spot lifetime between 10-15 days. Based on the cross-correlation of the Doppler maps, EK Dra has a solar-type differential rotation with a surface shear parameter of $α_{DR} = 0.030 \pm 0.008$.

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arXiv2025

The incidence of magnetism in blue and yellow straggler stars

Our understanding of the generation of magnetic fields in intermediate-mass and massive OBA stars remains limited. Some theories have proposed that their magnetic fields could be a result of strong binary interactions, including stellar mergers. Blue stragglers, which lie well beyond the main sequence turn-off point on the colour-magnitude diagram of stellar clusters, are widely theorised to be merger products or interacting binaries and therefore can be considered excellent test targets to get insights into the origin of magnetic fields in stars with radiative envelopes. We search for the presence of magnetic fields in a sample of blue and yellow stragglers listed in the Gaia DR2-based catalogue of blue stragglers in open clusters. We measured the mean longitudinal magnetic field from HARPSpol spectra of five blue stragglers and three yellow stragglers using the least-squares deconvolution technique. We present the first observational evidence that blue and yellow stragglers possess magnetic fields of the order of a hundred to a few hundred Gauss. The targets in our sample belong to open clusters of very different ages and metallicities, but we do not detect any relationship between the presence or strength of the detected magnetic field and the cluster characteristics. For the first time, using high-resolution spectropolarimetric observations, a definite detection of a magnetic field is achieved in a Be-shell star, HD61954. The two yellow stragglers, HD62329 and HD65032, appear to be members in binary systems, whereas the blue straggler HD62775 is possibly a triple system. HD62329 and HD65032 exhibit in their spectra weak Nd III 6145 lines, which are usually prominent in magnetic Ap and Bp stars.

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arXiv2025

A magnetic tertiary in the most massive compact triple-star system

The system TIC 470710327 is comprised of three main-sequence OB stars, with an inner compact 1.10 d eclipsing binary and a non-eclipsing tertiary on a 52.04 d orbit. With the tertiary mass of 14.5-16 $M_{\odot}$ and both components in the inner eclipsing binary with individual masses of 6--7 and 5.5-6.3 $M_{\odot}$, it is currently the most massive compact system known. The formation scenario of such a compact triple is uncertain. It has been suggested that `2 + 2' quadruple dynamics can lead to a stellar merger in the initially more massive binary and finally result in a highly magnetised tertiary. Our study confirms the presence of a kG-order magnetic field in the tertiary and the slow rotation typical for massive magnetic stars. We conclude that finding massive merger candidates by studies of dynamics in compact, multiple-star systems is an efficient way to understand the evolution of massive stellar multiplicity and the generation of magnetic fields.

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arXiv2025

A search for Maunder-minimum candidate stars

Stars with very low levels of magnetic activity provide an opportunity for a more quantitative comparison with the Sun during its Maunder minimum. We employ spectra from the RAVE survey in a search for particularly low-activity stars with the goal of identifying candidates for so-called Maunder-minimum stars. Spectra were used to measure the relative flux in the cores of the Ca II infrared-triplet (IRT) lines. Those were converted to absolute emission-line fluxes and were corrected with target fluxes from high-resolution STELLA and ultra-high-resolution PEPSI spectra. Absolute Ca II IRT fluxes for a total of 78111 RAVE dwarf stars are presented and compared with fluxes of the 123 stars from our high-resolution STELLA+PEPSI sample. RAVE fluxes appear higher than the STELLA and PEPSI fluxes by on average 19% for IRT-1, 21% for IRT-2, and 25% for IRT-3 due to their lower spectral resolution. Our sample also spans a metallicity [Fe/H] range relative to the Sun of -1.5 to +0.5 dex. We confirm the strong dependency of IRT fluxes on metallicity and quantify it to be at most $\pm$14% in the B-V range 0.53-0.73. Without a metallicity correction, practically all very-low-activity RAVE dwarfs show a super-solar metallicity. After correcting for spectral resolution and for metallicity, we find 13 RAVE stars out of 13326 (0.1%) that fall well below our empirical lower flux bound from high-resolution versus B-V. For solar B-V, this relates to a photospheric uncorrected radiative loss in the IRT lines of log R_IRT=-4.13 (~20% below the solar-minimum value in late 2016). However, 11 targets turned out to be evolved stars based on their Gaia DR3 parallaxes. Two stars, TIC 352227373 (G2V) and TYC 7560-477-1 (G7V), are our only Maunder-minimum candidates from the present search. Contrary to the initial suggestion from the Mount-Wilson H&K Survey, we conclude that such stars are very rare.

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arXiv2025

First snapshot of a magnetosphere around a Herbig Ae/Be star

The Herbig Ae/Be star HD190073 is one of the very few magnetic Herbig Ae/Be stars for which close low-mass companions have been reported. Previously published magnetic field measurements indicated an annual change in the field configuration. We aim to study in detail the spectral and magnetic variability of this star and characterise its magnetosphere for the first time. Newly acquired and archival spectropolarimetric observations are combined to determine a more precise magnetic period and to constrain the geometry of the magnetic field. The variability of hydrogen line profiles is studied using dynamical spectra. Archival X-shooter observations of the He I 10830 ang triplet are used to characterise its variability over the rotation cycle. Further, we carry out 2D magnetohydrodynamical simulations of the magnetosphere using the Nirvana code. From the spectropolarimetric observations, we determine for HD190073 a magnetic period P=51.70 d. We estimate a magnetic obliquity angle 82.9 degr and a dipole strength 222 G. Our dynamical spectra constructed for the hydrogen line profiles observed during 2011 clearly reveal a ringlike magnetospheric structure appearing at the rotation phase of best visibility of the positive magnetic pole. These spectra present the first snapshot of a magnetosphere around a Herbig Ae/Be star. 2D MHD simulations involving nonisothermal gas show that the magnetosphere is compact, with a radius of about $3\,R_*$, and that the wind flow extends over tens of $R_*$. With a reported radius of the accretion disk of 1.14 au around HD190073, the distance between the star and the disk is about 25 $R_*$. The detection of a magnetosphere around HD190073, and the possible presence of lower-mass companions at different distances, make this system a valuable laboratory for studying the magnetic interaction between the host star, its companions, and the accretion disk.

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arXiv2024

TESS detections of pulsations in Of?p stars in the Magellanic Clouds

We used TESS observations to search for pulsations in six known Of?p stars in the Magellanic Clouds. We find evidence for pulsational variability in three Of?p stars: UCAC4 115-008604, OGLE SMC-SC6 237339, and AzV 220. Two of them, UCAC4 115-008604 and OGLE SMC-SC6 237339, have been reported to possess kG-order magnetic fields. The obtained results are important to constrain and improve stellar evolution models.

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arXiv2024

Testing pulsation diagnostics in the rapidly oscillating magnetic Ap star $γ$ Equ using near-infrared CRIRES+ observations

Pulsations of rapidly oscillating Ap stars and their interaction with the stellar magnetic field have not been studied in the near-IR region despite the benefits these observations offer compared to visual wavelengths. The main advantage of the near-IR is the quadratic dependence of the Zeeman effect on the wavelength, as opposed to the linear dependence of the Doppler effect. To test pulsation diagnostics of roAp stars in the near-IR, we aim to investigate the pulsation behaviour of one of the brightest magnetic roAp stars, $γ$Equ, which possesses a strong surface magnetic field of the order of several kilogauss and exhibits magnetically split spectral lines in its spectra. Two magnetically split spectral lines belonging to different elements, the triplet Fe I at 1563.63nm and the pseudo-doublet Ce III at 1629.2nm, were recorded with CRIRES+ over about one hour in the H band with the aim of understanding the character of the line profile variability and the pulsation behaviour of the magnetic field modulus. The profile shapes of both studied magnetically split spectral lines vary in a rather complex manner probably due to a significant decrease in the strength of the longitudinal field component and an increase in the strength of the transverse field components over the last decade. A mean magnetic field modulus of 3.9kG was determined for the Fe I, whereas for the Ce III we observe only about 2.9kG. For comparison, a mean field modulus of 3.4kG was determined using the Zeeman doublet Fe II at 6249.25 in optical PEPSI spectra recorded just about two weeks before the CRIRES+ observations. Different effects that may lead to the differences in the field modulus values are discussed. Our measurements of the mean magnetic field modulus using the line profiles recorded in different pulsational phase bins suggest a field modulus variability of 32G for the Fe I and 102G for the Ce III.

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arXiv2023

The magnetic, spectroscopic, and photometric variability of the Wolf-Rayet star WR55

Studies of magnetic fields in the most evolved massive stars, the Wolf-Rayet stars, are of special importance because they are progenitors of certain types of supernovae. The first detection of a magnetic field of the order of a few hundred Gauss in the WN7 star WR55, based on a few FORS2 low-resolution spectropolarimetric observations, was reported in 2020. In this work we present new FORS2 observations allowing us to detect magnetic and spectroscopic variability with a period of 11.90 h. No significant frequencies were detected in TESS and ASAS-SN photometric observations. Importantly, magnetic field detections are achieved currently only in two Wolf-Rayet stars, WR6 and WR55, both showing the presence of corotating interacting regions.

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arXiv2022

Magnetic field measurements of sharp-lined Ap stars

Previous observations suggested that Ap and Bp stars exhibit a bimodal distribution of surface magnetic field strengths and that actually only few or no stars exist with magnetic dipole field strengths below 300 G down to a few Gauss. As the number of Ap and Bp stars currently known to possess weak magnetic fields is not large, it is necessary to carry out additional spectropolarimetric studies of Ap and Bp stars to prove whether the assumption of the existence of a critical value for the stability of magnetic fields is realistic. In this study, we present high-resolution HARPSpol magnetic field measurements for a sample of Ap stars with sharp spectral lines with a view to characterize the strengths of their magnetic fields. Out of the studied seven sharp-lined stars, two stars, HD 174779 and HD 203932, exhibit a rather weak longitudinal magnetic field with $\left< B_{\rm z}\right>=-45\pm3$ G and $\left< B_{\rm z}\right>=21\pm4$ G, respectively. Additionally, TESS observations were used to test previous conclusions on the differentiation of rotation periods of Ap and Bp stars. Apart from HD 189832 and HD 203932, all other studied sharp-lined stars have long rotation periods. Since an explanation for the slow rotation of Ap stars is currently missing, additional studies of slowly rotating Ap and Bp stars are necessary to improve our understanding of the formation and evolution of Ap and Bp stars.

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arXiv2021

Detection of anomalous element distribution in the extremely slowly rotating magnetic O9.7 V star HD 54879

The O9.7 V star HD 54879 is currently the only massive magnetic star whose magnetic field geometry and rotation period are not constrained. Over the last three years, we gathered additional observations of this star, obtained using various instruments at several astronomical facilities with, the aim to constrain the rotation period and the magnetic field geometry. The new data include the first full Stokes vector observations with the PEPSI spectropolarimeter, installed at the Large Binocular Telescope. The acquired spectropolarimetric observations show a very slow magnetic field variability related to the extremely slow rotation of HD 54879, which is also indicated in a dynamical spectrum, displaying variability of the H$α$ line. The most intriguing result of our study is the discovery of differences in longitudinal magnetic field strengths measured using different LSD masks containing lines belonging to different elements. It is the first time that such a differential analysis of the field strength in dependence of the used lines is carried out for a magnetic O-type star. Since the LSD Stokes $I$ profiles of the studied O, Si, and He line masks remain stable over all observing epochs, we conclude that the detection of different field strengths using lines belonging to these elements is related to the different formation depths, with the He lines formed much higher in the stellar atmosphere compared to the silicon and the oxygen lines, and NLTE effects. Our numerical magnetospherical model suggests the presence of enhanced gas density that fills the volume inside the field lines close to the star.

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arXiv2021

A detailed understanding of the rotation-activity relationship using the 300 Myr old open cluster NGC 3532

The coeval stars of young open clusters provide insights into the formation of the rotation-activity relationship that elude studies of multi-age field populations. We measure the chromospheric activity of cool stars in the 300 Myr old open cluster NGC 3532 in concert with their rotation periods to study the mass-dependent morphology of activity for this transitional coeval population. Using multi-object spectra of the Ca ii infrared triplet region obtained with the AAOmega spectrograph at the Anglo- Australian Telescope, we measure the chromospheric emission ratios for 454 FGKM cluster members of NGC 3532. The morphology of activity against colour appears to be a near-mirror image of the cluster's rotational behaviour. In particular, we identify a group of 'desaturated transitional rotators' that branches off from the main group of unsaturated FGK slow rotators, and from which it is separated by an 'activity gap'. The few desaturated gap stars are identical to the ones in the rotational gap. Nevertheless, the rotation-activity diagram is completely normal. In fact, the relationship is so tight that it allows us to predict rotation periods for many additional stars. We then precisely determine these periods from our photometric light curves. Our activity measurements show that all fast rotators of near-solar mass have evolved to become slow rotators, demonstrating that the absence of fast rotators in a colour-period diagram is not a detection issue but an astrophysical fact. We also identify a new population of low-activity stars among the early M dwarfs, enabling us to populate the extended slow rotator sequence in the colour-period diagram. The joint analysis of chromospheric activity and photometric time series data thus enables comprehensive insights into the evolution of the rotation and activity of stars during the transitional phase between the Pleiades and Hyades age. (Abridged)

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arXiv2021

Magnetic field geometry and magnetospheric environment of the strongly magnetic Of?p star NGC 1624-2

NGC 1624-2 is an O7f?p star with a reported probable polar magnetic field strength $\ge$ 20 kG, which is the strongest magnetic field ever measured in an O-type star. We study the variability of the mean longitudinal magnetic field $\langle B_z \rangle$ and the mean field modulus $\langle B \rangle$ to obtain constraints on its field geometry. Only one magnetic pole is observable over the rotation cycle. The approximately sinusoidal variation of $\langle B_z \rangle$ and the ratio of the values of the extrema of $\langle B \rangle$ indicate that there is an important component of the field that is dipolar. The $\langle B_z \rangle$ values measured over the rotation cycle are in the range from $-0.2$ to 4.5 kG, whereas the values for $\langle B \rangle$ vary between 9 and 12 kG. The $\langle B_z \rangle$ values obtained using the O III $λ$7455 emission line are in the range from 0.4 to 2.3 kG and show a variability pattern similar to that detected for the absorption lines. The fact that the phase of the $\langle B_z \rangle$ minimum coincides with the phase of the $\langle B \rangle$ maximum, indicates that the field structure must significantly depart from a centred dipole. Further, we discuss the nature of the observed variable Stokes $V$ profiles corresponding to a longitudinal field of negative polarity detected in the emission He I lines and present the first MHD numerical simulations of the gas flow in the magnetosphere of this star.

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arXiv2020

The anomalous atmospheric structure of the strongly magnetic Ap star HD166473

High resolution spectropolarimetric observations of the strongly magnetic, super-slowly rotating rapidly oscillating Ap star HD166473 are used to investigate the implications of the presence of a variable strong magnetic field on the vertical and surface horizontal distribution of various chemical elements. The analysis of the calculated LSD Stokes $I$ and $V$ profiles confirms the previously reported detection of non-uniform horizontal surface distribution of several chemical elements. To test the vertical abundance stratification of iron peak and rare earth elements, magnetic field measurements were carried out using spectral lines of these elements belonging to neutral and ionised stages. We find clear indication of the existence of a relation between the magnetic field strength and its orientation and vertical element stratification: magnetic field values obtained for elements in different stages close to the magnetic equator are rather similar whereas the dispersion in field strengths is remarkably large in the regions close to magnetic field poles. At the phases of negative and positive extrema the mean longitudinal field strength determined from the analysis of the REE lines is usually stronger than when using Fe and Cr. The strongest mean longitudinal magnetic field, up to $-$4160$\pm$226 G, is detected using the La II line list at the negative extremum, followed by the measurements using the Pr III lines with $\langle B_z \rangle$=$-$3740$\pm$343 G and the Ce II lines with $\langle B_z \rangle$=$-$3372$\pm$247 G. The strongest mean longitudinal magnetic field of positive polarity, up to 3584$\pm$354 G is detected using the Pr III lines, followed by the measurement $\langle B_z \rangle$=2517$\pm$249 G using the Ce II lines.

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arXiv2019

New evidence for weak magnetic fields in Herbig Ae/Be stars

In recent years Herbig Ae/Be stars receive considerable attention as their disks are believed to be the sites of on-going planet formation. Confirming the presence of magnetic fields in these stars is critical for understanding the transport of angular momentum during the protostellar phase. Furthermore, magnetic fields set the conditions for strongly anisotropic accretion. In this study we present the results of our recent observing campaigns of a sample of Herbig Ae/Be stars aimed at measurements of their magnetic fields applying the Singular Value Decomposition method to high resolution spectropolarimetric observations. The strongest longitudinal magnetic field of 209 G is detected in the Herbig Be star HD 58647, whereas the weakest field of 17 G is measured in the Herbig Ae star HD 190073. A change of polarity is detected for HD 58647 and in the Herbig Be star HD 98922. The obtained results provide further evidence that Herbig Ae/Be stars possess much weaker magnetic fields than their lower mass counterpart T Tauri stars with magnetic fields of kG order.

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arXiv2019

The two magnetic components in the Herbig Ae SB2 system HD 104237

We present longitudinal magnetic field measurements $\langle B_z \rangle$ for the Herbig Ae primary and the T Tauri secondary in the SB2 system HD 104237. These measurements were carried out using high spectral resolution observations obtained with the High Accuracy Radial velocity Planet Searcher in polarimetric mode, installed at the ESO La Silla 3.6 m telescope. In agreement with previous studies of Herbig Ae stars, the longitudinal magnetic field in the primary is rather weak, ranging from 47 G to 72 G. The secondary component possesses a variable, much stronger magnetic field, up to 600 G, as expected for a magnetically active T Tauri star. We estimated the rotation period of the primary, $P_{\mathrm{rot}}=4.33717\pm0.00316$ d ($=104\pm0.08$ h), from metal line equivalent width variations.

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arXiv2018

Mapping EK Draconis with PEPSI - Possible evidence for starspot penumbrae

We present the first temperature surface map of EK Dra from very-high-resolution spectra obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope. Changes in spectral line profiles are inverted to a stellar surface temperature map using our $i$Map code. The long-term photometric record is employed to compare our map with previously published maps. Four cool spots were reconstructed, but no polar spot was seen. The temperature difference to the photosphere of the spots is between 990 and 280K. Two spots are reconstructed with a typical solar morphology with an umbra and a penumbra. For the one isolated and relatively round spot (A), we determine an umbral temperature of 990K and a penumbral temperature of 180K below photospheric temperature. The umbra to photosphere intensity ratio of EK Dra is approximately only half of that of a comparison sunspot. A test inversion from degraded line profiles showed that the higher spectral resolution of PEPSI reconstructs the surface with a temperature difference that is on average 10% higher than before and with smaller surface areas by 10-20%. PEPSI is therefore better suited to detecting and characterising temperature inhomogeneities. With ten more years of photometry, we also refine the spot cycle period of EK Dra to 8.9$\pm$0.2 years with a continuing long-term fading trend. The temperature morphology of spot A so far appears to show the best evidence for the existence of a solar-like penumbra for a starspot. We emphasise that it is more the non-capture of the true umbral contrast rather than the detection of the weak penumbra that is the limiting factor. The relatively small line broadening of EK Dra, together with the only moderately high spectral resolutions previously available, appear to be the main contributors to the lower-than-expected spot contrasts when comparing to the Sun.

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