Nikolai Piskunov

Uppsala University | UU · Department of Physics and Astronomy

Accurately estimating the C/O ratio of hot Jupiter atmospheres is a promising pathway towards understanding planet formation and migration, as well as the formation of clouds and the overall atmospheric composition. The atmosphere of the hot Jupiter WASP-43b has been extensively analysed using low-resolution observations with HST and Spitzer, but t...

Accurately estimating the C/O ratio of hot Jupiter atmospheres is a promising pathway towards understanding planet formation and migration, as well as the formation of clouds and the overall atmospheric composition. The atmosphere of the hot Jupiter WASP-43b has been extensively analysed using low-resolution observations with HST and Spitzer, but t...

Aims. We aim to characterise the small-scale magnetic fields of a sample of 16 Sun-like stars and investigate the capabilities of the newly upgraded near-infrared (NIR) instrument CRIRES ⁺ at the Very Large Telescope in the context of small-scale magnetic field studies. Our targets also had their magnetic fields studied with optical spectra, which...

We aim to characterise the small-scale magnetic fields for a sample of 16 Sun-like stars and investigate the capabilities of the newly upgraded near-infrared (NIR) instrument CRIRES$^+$ at the VLT in the context of small-scale magnetic field studies. Our targets also had their magnetic fields studied in the optical, which allows us to compare magne...

The dayside atmospheres of ultra-hot Jupiters (UHJs) are predicted to possess temperature inversion layers with extremely high temperatures at high altitudes. We observed the dayside thermal emission spectra of WASP-18b and WASP-76b with the new CRIRES ⁺ high-resolution spectrograph at near-infrared wavelengths. Using the cross-correlation techniqu...

The dayside atmospheres of ultra-hot Jupiters (UHJs) are predicted to possess temperature inversion layers with extremely high temperatures at high altitudes. We observed the dayside thermal emission spectra of WASP-18b and WASP-76b with the new CRIRES+ high-resolution spectrograph at near-infrared wavelengths. Using the cross-correlation technique...

The CRyogenic InfraRed Echelle Spectrograph (CRIRES) Upgrade project CRIRES + extended the capabilities of CRIRES. It transformed this VLT instrument into a cross-dispersed spectrograph to increase the wavelength range that is covered simultaneously by up to a factor of ten. In addition, a new detector focal plane array of three Hawaii 2RG detector...

The CRyogenic InfraRed Echelle Spectrograph (CRIRES) Upgrade project CRIRES$^{+}$ extended the capabilities of CRIRES. It transformed this VLT instrument into a cross-dispersed spectrograph to increase the wavelength range that is covered simultaneously by up to a factor of ten. In addition, a new detector focal plane array of three Hawaii 2RG dete...

Context. The characterization of exoplanets requires the reliable determination of the fundamental parameters of their host stars. Spectral fitting plays an important role in this process. For the majority of stellar parameters, matching synthetic spectra to the observations provides a robust and unique solution for the fundamental parameters, such...

Simultaneous analysis of the C_2 and CN molecular bands in the 5100-5200 and 7930-8100A spectral regions is a promising alternative for the accurate determination of the carbon (C) and nitrogen (N) abundance in the atmospheres of the solar-like stars. Practical implementation of this new method became possible after recent improvements of the molec...

The correct interpretation of the large amount of complex data from next-generation (in particular, space-based) observational facilities requires a very strong theoretical underpinning. One can thus predict that in the near future the use of atmospheric models obtained through the use of three-dimensional (3-D) radiation magnetohydrodynamics (RMHD...

We performed an extensive comparative analysis of the recent experimental data on Fe i transition probabilities (TP) based on the observed solar and stellar spectra. This work is part of the Vienna Atomic Line database (VALD) activities. Our motivation is to keep the VALD line list as complete as possible and provide VALD users with substantiated r...

Context. LQ Hya is one of the most frequently studied young solar analogue stars. Recently, it has been observed to show intriguing behaviour when analysing long-term photometry. For instance, from 2003–2009, a coherent spot structure migrating in the rotational frame was reported by various authors. However, ever since, the star has entered a chao...

Context . Studying the spots of late-type stars is crucial for distinguishing between the various proposed dynamo mechanisms believed to be the main cause of starspot activity. For this research it is important to collect observation time series that are long enough to unravel both long- and short-term spot evolution. Doppler imaging is a very effi...

Context . Starspots are important manifestations of stellar magnetic activity. By studying their behaviour in young solar analogues, we can unravel the properties of their magnetic cycles. This gives crucial information of the underlying dynamo process. Comparisons with the solar cycle enable us to infer knowledge about how the solar dynamo has evo...

Starspots are important manifestations of stellar magnetic activity. By studying their behaviour in young solar analogues, we can unravel the properties of their magnetic cycles. This gives crucial information of the underlying dynamo process. Comparisons with the solar cycle enable us to infer knowledge about how the solar dynamo has evolved durin...

Vienna Atomic Line Database (VALD) contains data on atomic and molecular energy levels and parameters of spectral lines required for stellar spectra analysis. Hundreds of millions of lines for fine spectral synthesis and for opacity calculations are collected in the present version of VALD (VALD3). Critical evaluation of the data and the diversity...

The program package SME (Spectroscopy Made Easy), designed to perform an analysis of stellar spectra using spectral fitting techniques, was updated due to adding new functions (isotopic and hyperfine splittins) in VALD and including grids of NLTE calculations for energy levels of few chemical elements. SME allows to derive automatically stellar atm...

Today Vienna Atomic Line Database (VALD) is one of main databases of atomic and molecular parameters required for stellar spectra analysis. We present the new features that recently appeared in the VALD3 release, including the effects of isotopic composition and hyperfine splitting. The latest version of VALD contains parameters for several isotope...

We provide a brief outline of the concepts and facilities of the Vienna Atomic Line DataBase in its new version 2. A summary of contents and recommendations how to use the VALD-2 are given. We conclude by a few applications planned for future releases of VALD.

We describe the main structure of the Vienna Atomic Line Database, the tools provided for the users and the statistics of its use in the last two years. Our plans for future developments of the database are discussed.

The first generation of E-ELT instruments will include an optical-infrared High Resolution Spectrograph, conventionally indicated as EELT-HIRES, which will be capable of providing unique breakthroughs in the fields of exoplanets, star and planet formation, physics and evolution of stars and galaxies, cosmology and fundamental physics. A 2-year long...

The first generation of E-ELT instruments will include an optical-infrared High Resolution Spectrograph, conventionally indicated as EELT-HIRES, which will be capable of providing unique breakthroughs in the fields of exoplanets, star and planet formation, physics and evolution of stars and galaxies, cosmology and fundamental physics. A 2-year long...

The Virtual Atomic and Molecular Data Centre (VAMDC) Consortium is a worldwide consortium which federates atomic and molecular databases through an e-science infrastructure and an organisation to support this activity. About 90% of the inter-connected databases handle data that are used for the interpretation of astronomical spectra and for modelli...

The article presents a review of scientific problems and methods of ultraviolet astronomy, focusing on perspective scientific problems (directions) whose solution requires UV space observatories. These include reionization and the history of star formation in the Universe, searches for dark baryonic matter, physical and chemical processes in the in...

The discovery of almost 2000 exoplanets has revealed an unexpectedly diverse planet population. Observations to date have shown that our Solar System is certainly not representative of the general population of planets in our Milky Way. The key science questions that urgently need addressing are therefore: What are exoplanets made of? Why are plane...

We performed extensive tests of the accuracy of atmospheric parameter determination for FGK stars based on the spectrum fitting procedure Spectroscopy Made Easy (sme). Our stellar sample consists of 13 objects, including the Sun, in the temperature range 5000–6600 K and metallicity range −1.4–+0.4. The analysed stars have the advantage of having pa...

CRIRES is one of the few IR (0.92-5.2 μm) high-resolution spectrographs in operation at the VLT since 2006. Despite good performance it suffers a limitation that significantly hampers its ability: a small spectral coverage per exposure. The CRIRES upgrade (CRIRES+) proposes to transform CRIRES into a cross-dispersed spectrograph while maintaining t...

Vienna atomic line database (VALD) is a collection of critically evaluated laboratory parameters for individual atomic transitions, complemented by theoretical calculations. VALD is actively used by astronomers for stellar spectroscopic studies—model atmosphere calculations, atmospheric parameter determinations, abundance analysis etc. The two firs...

We study the spot distribution on the surface of LQ~Hya during the observing seasons October 1998 -- November 2002. We look for persistent active longitudes, trends in the level of spot activity and compare to photometric data, specifically to the derived time epochs of the lightcurve minima. We apply the Doppler Imaging technique on photospheric s...

We demonstrate a new procedure to derive accurate and precise surface gravities from high resolution spectra without the use of external constraints. Our analysis utilizes Spectroscopy Made Easy (SME) with robust spectral line constraints and uses an iterative process to mitigate degeneracies in the fitting process. We adopt an updated radiative tr...

The Kepler satellite provides a unique window into stellar temporal variability by observing a wide variety of stars with multi-year, near-continuous, high precision, optical photometric time series. While most Kepler targets are faint stars with poorly known physical properties, many unexpected discoveries should result from a long photometric sur...

The detection of exoplanets using any method is prone to confusion due to the intrinsic variability of the host star. We investigate the effect of cool starspots on the detectability of the exoplanets around solar-like stars using the radial velocity method. For investigating this activity-caused ‘jitter’ we calculate synthetic spectra using radiat...

Exoplanet science is booming. In 20 years our knowledge has expanded considerably, from the first discovery of a Hot Jupiter, to the detection of a large population of Neptunes and super-Earths, to the first steps toward the characterization of exoplanet atmospheres. Between today and 2025, the field will evolve at an even faster pace with the adve...

Intermediate-mass, magnetic chemically peculiar (Ap) stars provide a unique opportunity to study the topology of stellar magnetic fields in detail and to investigate magnetically driven processes of spot formation. Here we aim to derive the surface magnetic field geometry and chemical abundance distributions for the extraordinary Ap star HD 75049....

Aims: We present a magnetic Doppler imaging study from all Stokes parameters of the cool, chemically peculiar star HD 24712. This is the very first such analysis performed at a resolving power exceeding 10^5. Methods: The analysis is performed on the basis of phase-resolved observations of line profiles in all four Stokes parameters obtained with t...

CRIRES at the VLT is one of the few adaptive optics enabled instruments that offer a resolving power of 105 from 1 − 5 μm. An instrument upgrade (CRIRES+) is proposed to implement cross-dispersion capabilities, spectro-polarimetry modes, a new detector mosaic, and a new gas absorption cell. CRIRES+ will boost the simultaneous wavelength coverage of...

Accurate spectroscopic measurements of magnetic fields in low mass stars remain challenging because of their cool temperatures, strong line blending, and often fast rotation. This is why previous estimates were based either on the analysis of only a few lines or made use of some indirect techniques. This frequently led to noticeable scatter in obta...

Magnetic Doppler imaging from four Stokes parameter observations has uncovered a new level of complexity of stellar magnetic fields previously not known. This new information is of interest to the evolution and structure of magnetic fields of intermediate and high-mass stars. Recently, we performed magnetic Doppler imaging study of the chemically p...

The CRIRES infrared spectrograph at the European Southern Observatory (ESO) Very Large Telescope (VLT) facility will soon receive an upgrade. This upgrade will include the addition of a module for performing highresolution spectropolarimetry. The polarimetry module will incorporate a novel infrared beamsplitter based on polarization gratings (PGs)....

CRIRES, the ESO high resolution infrared spectrometer, is a unique instrument which allows astronomers to access a parameter space which up to now was largely uncharted. In its current setup, it consists of a single-order spectrograph providing long-slit, single-order spectroscopy with resolving power up to R=100,000 over a quite narrow spectral ra...

High-resolution infrared spectroscopy plays an important role in astrophysics from the search for exoplanets to cosmology. Yet, many existing infrared spectrographs are limited by a rather small simultaneous wavelength coverage. The AO assisted CRIRES instrument, installed at the ESO VLT on Paranal, is one of the few IR (0.92-5.2 μm) highresolution...

The current instrumentation plan for the E-ELT foresees a High Resolution Spectrograph conventionally indicated as HIRES. Shaped on the study of extra-solar planet atmospheres, Pop-III stars and fundamental physical constants, HIRES is intended to embed observing modes at high-resolution (up to R=150000) and large spectral range (from the blue limi...

Aims: () is a Li-rich, rapidly rotating single K giant. We set out to study the spot configuration and activity level by calculating surface temperature maps of the star. Methods: We apply the Doppler imaging method on high-resolution optical spectroscopy obtained during 2004-2006. Results: In July-August 2004, no clear spot structures were visible...

Based on detailed spectral synthesis we carry out quantitative measurements of the strength and complexity of surface magnetic fields in the four well-known M-dwarfs GJ 388, GJ 729, GJ 285, and GJ 406 populating the mass regime around the boundary between partially and fully convective stars. Very high resolution R=100000, high signal-to-noise (up...

We present the results of special tests of the automatic procedure for atmospheric parameters determination based on spectroscopy.

Our current knowledge about the stellar magnetic fields relies almost entirely on the circular polarization observations. Very few objects have been observed so far in all four Stokes parameters. The magnetic Ap star HD24712 (DO Eri, HR1217) was recently observed in the Stokes IQUV parameters with the HARPSpol instrument at the 3.6-m ESO telescope....

High-resolution spectropolarimetric observations provide simultaneous information about stellar magnetic field topologies and three-dimensional distributions of chemical elements. Here we present analysis of a unique full Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap star HD 24712. The goal of our work is to examine...

Several recent spectrophotometric studies failed to detect significant global magnetic fields in late-B HgMn chemically peculiar stars, but some investigations have suggested the presence of strong unstructured or tangled fields in these objects. We used detailed spectrum synthesis analysis to search for evidence of tangled magnetic fields in high-...

In atmospheres of magnetic main-sequence stars, the diffusion of chemical elements leads to a number of observed anomalies, such as abundance spots across the stellar surface. The aim of this study was to derive a detailed picture of the surface abundance distribution of the magnetic chemically peculiar star HD 3980. Based on high-resolution, phase...

Stellar activity signatures such as spots and plage can significantly limit the search for extrasolar planets. Current models of activity-induced radial velocity (RV) signals focused on the impact of temperature contrast in spots predicting the signal to diminish toward longer wavelengths. On the other hand, the relative importance of the Zeeman ef...

We present high spectral resolution ($R\approx108,000$) Stokes $V$ polarimetry of the Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization propertie...

We present high spectral resolution Stokes V polarimetery of the Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emissio...

The Kepler satellite is providing spectacular optical photometric light-curves of unprecedented precision and duration that routinely allow detailed studies of stellar magnetic activity on late-type stars that were difficult previously. Kepler provides multi-year duration light-curves that allow investigation of how activity phenomena -- such as th...

We have observed about 325 stars in our Kepler Guest Observer programs (Cycles 1 through 4). For most of these targets, we are analyzing extremely high-precision light curves that have been continuously sampled every 30 minutes for up to 3 years. Our sample of candidate magnetically-active stars was selected primarily using GALEX colors. Starspots,...

We present a database of well determined orbital parameters of exoplanets, and their host stars' properties. This database comprises spectroscopic orbital elements measured for 427 planets orbiting 363 stars from radial velocity and transit measurements as reported in the literature. We have also compiled fundamental transit parameters, stellar par...

We present two new accurate and efficient method to compute the formal solution of the polarized radiative transfer equation. In this work, the source function and the absorption matrix are approximated using quadratic and cubic Bezier spline interpolants. These schemes provide 2nd and 3rd order approximation respectively and don't suffer from erra...

The cryogenic high resolution IR echelle spectrograph CRIRES is the ESO infrared (0.95&#87225.4 μm) high resolution spectrograph operating at the Nasmyth A focus of VLT-UT1. The instrument provides long-slit (31") spectroscopy with resolving power up to R=100,000 over a quite narrow wavelengths range, about 1/70 of the central wavelength. Observati...

The observations were carried out in service mode with the infrared spectrometer CRIRES at ESO-VLT. In total 14 targets were observed during periods 82 (1st of October 2008 to 31st of March 2009) and 84 (1st of October 2009 to 31st of March 2010). (4 data files).

We present high spectral resolution Stokes V polarimetery of the Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He I emissi...

The purpose of the Working Group on Ap and Related Stars (ApWG) is to promote and facilitate research about stars in the spectral type range from B to early F that exhibit surface chemical peculiarities and related phenomena. This is a very active field of research, in which a wide variety of new developments have taken place since 2009, as illustr...

This Division IV was started on a trial basis at the General Assembly in The Hague 1994 and was formally accepted at the Kyoto General Assembly in 1997. Its broad coverage of ``Stars'' is reflected in its relatively large number of Commissions and so of members (1266 in late 2011). Its kindred Division V, ``Variable Stars'', has the same history of...

Spectroscopy Made Easy (SME) is IDL software and a compiled external library that fits an observed high-resolution stellar spectrum with a synthetic spectrum to determine stellar parameters. The SME external library is available for Mac, Linux, and Windows systems. Atomic and molecular line data formatted for SME may be obtained from VALD. SME can...

Doppler observations from Keck Observatory have revealed a triple-planet system orbiting the nearby K4V star, HIP 57274. The inner planet, HIP 57274b, is a super-Earth with Msin i = 11.6 M ⊕ (0.036 M Jup), an orbital period of 8.135 ± 0.004 days, and slightly eccentric orbit e = 0.19 ± 0.1. We calculate a transit probability of 6.5% for the inner p...

We have studied the long-term spot activity of the RS CVn star II Peg by means of Doppler imaging based on spectroscopy and time series analysis of photometry. We present 28 Doppler imaging temperature maps spanning the years 1994-2010, of which 14 were calculated for the present study. The longitudinal spot distribution, derived from the surface t...

Development of high-resolution spectropolarimetry has stimulated a major progress in our understanding of the magnetism and activity of late-type stars. During the last decade magnetic fields were discovered and mapped for various types of active stars using spectropolarimetric methods. However, these observations and modeling attempts are inherent...

The relativley large spread in the derived metallicities ([Fe/H]) of M dwarfs shows that various approaches have not yet converged to consistency. The presence of strong molecular features, and incomplete line lists for the corresponding molecules have made metallicity determinations of M dwarfs difficult. Furthermore, the faint M dwarfs require lo...

Mercury-manganese (HgMn) stars have been considered as non-magnetic and non-variable chemically peculiar (CP) stars for a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surface distribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known tha...

Chemically peculiar stars of the mercury-manganese (HgMn) type represent a new class of spotted late-B stars, in which evolving surface chemical inhomogeneities are apparently unrelated to the presence of strong magnetic fields but are produced by some hitherto unknown astrophysical mechanism. The goal of this study is to perform a detailed line pr...

Close M-dwarf binaries and higher multiples allow the investigation of rotational evolution and mean magnetic flux unbiased from scatter in inclination angle and age since the orientation of the spin axis of the components is most likely parallel and the individual systems are coeval. Systems composed of an early (M0.0 -- M4.0) and a late (M4.0 --...