Category Archives: Publications

Extremely slowly rotating magnetic O9.7V star HD54879

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 Hα line.

Generally, magnetic massive O- and B-type stars exhibit a smooth, single-wave variation of the longitudinal magnetic field during the stellar rotation cycle. The approximately sinusoidal variation of ⟨Bz⟩ and the ratio of the values of the ⟨Bz⟩ extrema in previously studied stars suggest that there is an important component of the field that is dipolar. Assuming that the magnetic field of HD 54879 has a pure dipolar configuration and that the negative field extremum is indeed around −570 G and not at a lower value, we fitted a cosine curve to the observed distribution of data points obtained from the high-resolution spectropolarimetric observations and determined a stellar rotation period of 7.2 yr. Certainly, further monitoring of the magnetic field variability is needed to determine the rotation period with more confidence. We note, that magnetic studies of several O-type stars indicate that only one magnetic pole is well visible while the star rotates, implying that the magnetic field structure over the fraction of their invisible surface remains unconstrained.

The four Stokes parameters and their null profiles for two individual lines, He I at 5875.6Å and S III at 5739.7Å, in PEPSI observations obtained on 2020 December 6. In the spectra recorded in circular polarized light, clear Zeeman signatures are detected for both lines whereas the spectra recorded in linear polarized light appear flat.

Read more: Järvinen et al. 2022, MNRAS, 501, 4405. arXiv

The rapidly oscillating Ap star γ Equ: linear polarization as an enhanced pulsation diagnostic?

We present the first short time scale observations of the roAp star γ Equ in linear polarized light obtained with the PEPSI polarimeter installed at the LBT. These observations are used to search for pulsation variability in Stokes Q and U line profiles belonging to different elements. The atmospheres of roAp stars are significantly stratified with spectral lines of different elements probing different atmospheric depths. roAp stars with strong magnetic fields, such as γ Equ with a magnetic field modulus of 4 kG and a pulsation period of 12.21 min, are of special interest because the effect of the magnetic field on the structure of their atmospheres can be studied with greatest detail and accuracy. Our results show that we may detect changes in the transversal field component in Fe I and rare-earth lines possessing large second-order Landé factors. Such variability can be due to the impact of pulsation on the transverse magnetic field, causing changes in the obliquity angles of the magnetic force lines. Further studies of roAp stars in linear polarized light and subsequent detailed modelling are necessary to improve our understanding of the involved physics.

Variabi;ity detected in linear polarization profiles.
Zeeman signatures of γ Equ in the linear polarization line profiles of different lines recorded with PEPSI on two different pulsation phases in 2017 September 11. Individual and overplotted Stokes I profiles for single and LSD profiles are shown in the bottom panels followed by individual and overplotted Stokes Q and U profiles in the middle panels. The upper panels present the differences between the Stokes Q and U profiles with the associated error bars. Since the spectral resolution of R ∼ 130 000 offered by the PEPSI observations is sampled by 4.2 CCD pixels, to achieve a higher S/N, the Stokes Q and U spectra have been smoothed using Gaussians.


Read more: Hubrig et al. 2021, MNRAS 508 L17. arXiv:2108.11272

Searching For Transiting Planets Around Halo Stars. I. Sample Selection and Validation

By measuring the elemental abundances of a star, we can gain insight into the composition of its initial gas cloud — the formation site of the star and its planets. Planet formation requires metals, the availability of which is determined by the elemental abundance. In the case where metals are extremely deficient, planet formation can be stifled. To investigate such a scenario requires a large sample of metal-poor stars and a search for planets therein. This paper focuses on the selection and validation of a halo star sample. We select ~17,000 metal-poor halo stars based on their Galactic kinematics, and confirm their low metallicities ([Fe/H] < -0.5), using spectroscopy from the literature. Furthermore, we perform high-resolution spectroscopic observations using LBT/PEPSI and conduct detailed metallicity ([Fe/H]) analyses on a sample of 13 previously known halo stars that also have hot kinematics. We can use the halo star sample presented here to measure the frequency of planets and to test planet formation in extremely metal-poor environments.

A range of the PEPSI B-band spectrum of HD 160693, with iron line features annotated with their corresponding equivalent width.

Read more: Kolecki et al. 2021, AJ, 162, 125. arXiv:2106.13251

Time-resolved rotational velocities in the upper atmosphere of WASP-33 b

While steady empirical progress has been made in understanding the structure and composition of hot planet atmospheres, direct measurements of velocity signatures, including winds, rotation, and jets, have lagged behind. Quantifying atmospheric dynamics of hot planets is critical to a complete understanding of their atmospheres and such measurements may even illuminate other planetary properties, such as magnetic field strengths. In this manuscript, we present the first detection of the Balmer lines H-alpha and H-beta in the atmosphere of the ultra-hot Jupiter WASP-33b. Using atmospheric models which include the effects of atmospheric dynamics, we show that the shape of the average Balmer line transmission spectrum is consistent with rotational velocities in the planet’s thermosphere of vrot = 10.1 (+0.8 -1.0) km/s. We also measure a low-significance day-to-night side velocity shift of -4.6 +/-3.4 km/s in the transmission spectrum which is naturally explained by a global wind across the planet’s terminator. In a separate analysis the time-resolved velocity centroids of individual transmission spectra show unambiguous evidence of rotation, with a best-fit velocity of 10.0 (+2.4 -2.0) km/s, consistent with the value of vrot derived from the shape of the average Balmer line transmission spectrum. Our observations and analysis confirm the power of high signal-to-noise, time resolved transmission spectra to measure the velocity structures in exoplanet atmospheres. The large rotational and wind velocities we measure highlight the need for more detailed 3D global climate simulations of the rareed upper-atmospheres of ultra-hot gas giants.

Spectral map of the H-alpha and H beta transmission spectra in the stellar rest frame for the entire night. The spectra have been interpolated onto an evenly spaced time vector for display purposes which produces some of the smearing near the beginning and end of the night when exposures were longer on average. The transit contact points T1 and T4 are shown with horizontal purple lines. The star’s  vsini value is marked with the vertical green lines. The planet’s line-of-sight velocity is shown with the blue line. There is a clear H-alpha signature which moves along the planet’s velocity for the duration of the transit. The H-beta absorption is weaker but still present at the expected velocities. Note the pulsation stripes visible in the pre transit data in both lines.

Read more: Cauley et al. 2021, AJ, 161, 152

A Unicorn in Monoceros: the 3M⊙ dark companion to the bright, nearby red giant V723 Mon is a non-interacting, mass-gap black hole candidate

We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby (d=460 pc), bright evolved red giant in a high mass function nearly circular binary (𝑃 = 59•.9 d, e approx. 0). Analyses of the stellar spectra and spectral energy distribution (SED) give 𝑇eff =– 4440 K, 𝐿 = 173 𝐿s and 𝑅 = 22 𝑅. Matching these parameters to MIST evolutionary models indicates a mass of the visible giant of 𝑀giant = 1.•07 +/- 0.•24 𝑀. V723 Mon is a known variable star, previously classified as an eclipsing binary, but its All-Sky Automated Survey (ASAS), Kilodegree Extremely Little Telescope (KELT), and Transiting Exoplanet Survey Satellite (TESS) light curves are those of a nearly edge-on ellipsoidal variable. Detailed models of the light curves constrained by the period, radial velocities and stellar temperature give an inclination of 𝑖 = 87• deg, a mass ratio of 0•.30 +/- 0•.02, and a  companion mass of 𝑀comp = 2.•91 +/- 0•.08 𝑀, a stellar radius of the giant of 𝑅giant = 23.•6 +/-1.•0 𝑅, and a giant mass of 𝑀giant = 0.•87 +/-0.•08 𝑀 , consistent with our other estimates. We identify a likely non-stellar, diffuse veiling component with contributions in the 𝐵 and 𝑉-band of ~64% and ~23%, respectively, and a luminosity of ~20 𝐿. The SED and the absence of continuum eclipses imply that the companion mass must be dominated by a compact object even if the companion is a binary. We do observe eclipses of the Balmer lines when the dark companion passes behind the giant, but their velocity spreads are low compared to observed accretion disks. The X-ray luminosity of the system is 𝐿X = 1• x 10^30 erg/s, corresponding to 𝐿/𝐿edd ~10^-9. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the “mass gap”, although a double neutron star binary is possible.

LBT/PEPSI line profiles for the Balmer H𝛼, H𝛽, Ca I 𝜆6439 and Ca I 𝜆6463 lines (black). A model spectrum using the atmospheric parameters is shown in red. The blue lines show the velocity offset of the Balmer absorption lines (12 km/s) from the rest frame of the giant. PEPSI was used in its R=250,000 resolution mode.

Read more: Jayasinghe et al. 2021, MNRAS, 504, 2577

Strongly magnetic Of?p star NGC 1624-2

NGC 1624-2 is an O7f?p star with a reported probable polar magnetic field strength ≥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 <Bz> and the mean field modulus to obtain constraints on its field geometry. Only one magnetic pole is observable over the rotation cycle. The approximately sinusoidal variation of <Bz> and the ratio of the values of the extrema of indicate that there is an important component of the field that is dipolar. The <Bz> values measured over the rotation cycle are in the range from -0.2 to 4.5 kG, whereas the values for vary between 9 and 12 kG. The <Bz> values obtained using the O III λ7455 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 <Bz> minimum coincides with the phase of the 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 magnetohydrodynamical numerical simulations of the gas flow in the magnetosphere of this star.

The split components of the magnetically resolved line C IV λ5812.

Read more: Järvinen et al., 2021, MNRAS, 501, 4534, in arXiv

Mono-enriched stars and Galactic chemical evolution — Possible biases in observations and theory

A long sought after goal using chemical abundance patterns derived from metal-poor stars is to understand the chemical evolution of the Galaxy and to pin down the nature of the first stars (Pop III). Metal-poor, old, unevolved stars are excellent tracers as they preserve the abundance pattern of the gas from which they were born, and hence they are frequently targeted in chemical tagging studies. Here, we use a sample of 14 metal-poor stars observed with the high-resolution spectrograph called the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT) to derive abundances of 32 elements (34 including upper limits). We present well-sampled abundance patterns for all stars obtained using local thermodynamic equilibrium (LTE) radiative transfer codes and one-dimensional (1D) hydrostatic model atmospheres. However, it is currently well-known that the assumptions of 1D and LTE may hide several issues, thereby introducing biases in our interpretation as to the nature of the first stars and the chemical evolution of the Galaxy. Hence, we use non-LTE (NLTE) and correct the abundances using three-dimensional (3D) model atmospheres to present a physically more reliable pattern. In order to infer the nature of the first stars, we compare unevolved, cool stars, which have been enriched by a single event (‘mono-enriched’), with a set of yield predictions to pin down the mass and energy of the Pop III progenitor. To date, only few bona fide second generation stars that are mono-enriched are known. A simple χ 2 -fit may bias our inferred mass and energy just as much as the simple 1D LTE abundance pattern, and we therefore carried out our study with an improved fitting technique considering dilution and mixing. Our sample presents Carbon Enhanced Metal-Poor (CEMP) stars, some of which are promising bona fide second generation (mono-enriched) stars. The unevolved, dwarf BD+09_2190 shows a mono-enriched signature which, combined with kinematical data, indicates that it moves in the outer halo and likely has been accreted onto the Milky Way early on. The Pop III progenitor was likely of 25.5 M and 0.6 foe (0.6 1051 erg) in LTE and 19.2 M and 1.5 foe in NLTE, respectively. Finally, we explore the predominant donor and formation site of the rapid and slow neutron-capture elements.

Spectrum synthesis of C, Sm, (Nd), Dy, and Rb in various sample stars. Specifically, molecular and atomic C in TYC5481. We note that [C i/Fe] = 0.6±0.1; [CH/Fe] = 0.1 ± 0.1; in BD-0.1_2439, [Sm/Fe] = 0.3 ± 0.1 and the shown Nd line was not used in our average Nd value due to blends and a poor fit – synthesis shows [Nd/Fe] = 0.3, 0.8, 0.9; [Dy/Fe] = 0.5 ± 0.1; and in HD136343, [Rb/Fe] = 0.4 ± 0.1. In all cases, the green dashed line indicates [X/Fe] = −5.


Read more:  Hansen et al. 2020, A&A, 643A, 49, in arXiv

On the Chemical Abundance of HR 8799 and the Planet c

Comparing chemical abundances of a planet and the host star reveals the origin and formation pathway of the planet. Stellar abundance is measured with high-resolution spectroscopy. Planet abundance, on the other hand, is usually inferred from low-resolution data. For directly imaged exoplanets, the data are available from a slew of high-contrast imaging/spectroscopy instruments. Here, we study the chemical abundance of HR 8799 and its planet c. We measure stellar abundance using LBT/PEPSI (R=120,000) and archival HARPS data: stellar [C/H], [O/H], and C/O are 0.11±0.12, 0.12±0.14, and 0.54+0.09-0.12, all consistent with solar values. We conduct atmospheric retrieval using newly obtained Subaru/CHARIS data together with archival Gemini/GPI and Keck/OSIRIS data. We model the planet spectrum with petitRADTRANS and conduct retrieval using PyMultiNest. Retrieved planetary abundance can vary by ∼0.5 dex, from sub-stellar to stellar C and O abundances. The variation depends on whether strong priors are chosen to ensure a reasonable planet mass. Moreover, comparison with previous works also reveals inconsistency in abundance measurements. We discuss potential issues that can cause the inconsistency, e.g., systematics in individual data sets and different assumptions in the physics and chemistry in retrieval. We conclude that no robust retrieval can be obtained unless the issues are fully resolved.


Read more: Wang et al. 2020, AJ, 160, 150

Benchmark stars, benchmark spectrographs: Detailed spectroscopic comparison of ESPRESSO, PEPSI, and HARPS data for Gaia benchmark stars

Gaia benchmark stars are selected to be calibration stars for different spectroscopic surveys. Very high-quality and homogeneous spectroscopic data for these stars are therefore required. We collected ultrahigh-resolution ESPRESSO spectra for 30 of the 34 Gaia benchmark stars and made them public. We quantify the consistency of the results that are obtained with different high-, and ultrahigh-resolution spectrographs. We also comprehensively studied the effect of using different spectral reduction products of ESPRESSO on the final spectroscopic results. We used ultrahigh- and high-resolution spectra obtained with the ESPRESSO, PEPSI, and HARPS spectrographs to measure spectral line characteristics (line depth; line width; and EW) and determined stellar parameters and abundances for a subset of 11 Gaia benchmark stars. The EW spectral line measurements based on the ESPRESSO, PEPSI, and HARPS spectra agree to within a few percent. However, we note that the lines appear deeper in the ESPRESSO spectra than in PEPSI and HARPS. The stellar parameters derived from each spectrograph by combining the several available spectra agree well overall. We conclude that the ESPRESSO, PEPSI, and HARPS spectrographs can deliver spectroscopic results that are sufficiently consistent for most of the science cases in stellar spectroscopy.


Read more: Adibekyan et al. 2020, A&A, 642A, 182, in arXiv

Probing the atmosphere of HD 189733b with the Na I and K I spectral lines

A team around Engin Keles (AIP) compared previously observed high resolution Na I and K I absorption in the atmosphere of HD189733b with synthetic transmission spectra modeled for a variety of temperature and abundance values. The comparison showed that the observed Na I-D-line widths are much larger than the modeled ones. The Na I-D-lines had to be broadened by velocities in the order of 10 km/s to match the observations if only rotational broadening is taken into account. The K I line profile on the other hand showed only a few km/s broadening comparable with the synthetic line profiles. This hints that either the atmosphere of HD 189733b lacks a significant amount of K I or the alkali lines probe different atmospheric regions with different temperature, which could explain the differences in the resolved absorption lines.
Transmission spectrum of the KI-line at 7699 A in [%] subtracted from unity. The black solid line shows the Gaussian fit. The dashed lines show the expected planetary absorption from the synthetic spectra without any broadening (red) and with the best-matching broadening solution of 3.8 km/s (green). The residual spectrum is shifted for clarity.
Read more: Keles et al., 2020, MNRAS, 498, 1023, in arXiv