Grzegorz Pietrzynski

Context . Galactic Cepheid variable stars are fundamental calibrators of the cosmic distance scale through their period-luminosity (PL) relation. The high-precision parallaxes expected from Gaia Data Release 4 (DR4) will enable a calibration of the Milky Way PL relation with unprecedented precision. This will place stringent requirements on interstellar extinction corrections, which remain a dominant source of uncertainty. Aims . We strive to derive highly accurate extinction values for Galactic Cepheids. We present an innovative methodology relying on the analysis of neighboring field stars and apply it to a pilot sample of 16 Cepheids. Methods . We identified physically near field stars using Gaia DR3 astrometry. For these stars, we determined atmospheric parameters from medium- to high-resolution VLT/FLAMES spectroscopy. Fixing these parameters, we derived individual extinctions by fitting spectral energy distributions constructed from Gaia XP spectra and infrared photometry. We then reconstructed the three-dimensional distribution of extinction and interpolate extinction to the Cepheid position. Results . We report the extinction, A _V , total-to-selective extinction ratio, R _V , and color excess, E _{B– V} , of all 16 Cepheids. Our method achieves a mean precision of approximately 5%, improving upon typical literature values, with no significant systematic offset detected. Conclusions . We demonstrate that using field stars bypasses uncertainties associated with Cepheid variability and can provide robust extinction estimates for Galactic Cepheids.

Context. The Baade–Wesselink method, while theoretically applicable for calibrating the distance scale via Cepheids, is not currently employed in the cosmic distance ladder. There are various versions of this method based mainly on interferometry and/or surface–brightness color relations. In all these approaches, the projection factor used to convert the radial into the pulsation velocity remains the limiting quantity. Aims. By comparing for the first time simultaneous cross-correlated optical and H-band radial velocities of five Cepheids, we aim to better understand the geometric part of the projection factor that is directly linked to the limb darkening of the stars. The five Cepheids were observed with the HARPS-N and GIANO-B instruments mounted at the Telescopio Nazionale Galileo in the GIARPS configuration. Methods. A two-parameter linear fit was applied to the simultaneous radial velocity measurements of each star in order to extract the amplitude ratio of both curves ( f _VH ) as well as the difference in mean velocities. Results. We find a mean value of f _VH of 1.056 with a root mean square over the five stars of 0.016, which means that the V-band radial velocity amplitudes are around 5% larger than the ones obtained in the H band. In addition, we find that the H-band radial velocity curves have an average (i.e., a γ velocity) of about 1 km s ⁻¹ blueshifted compared to the visible ones, except for one star, X Cyg, which has a blueshift of 0.4 km s ⁻¹ . At first order, the f _VH values are directly linked to the ratio of limb darkening coefficients in both bands (or equivalently the ratio of geometrical projection factors). The f _VH values are found to be consistent with expectations from static stellar atmosphere models. The difference in the γ velocity is attributed to the interplay of convection and the deeper location of H-band line-forming regions. Conclusions. This work shows that the differential limb darkening of Cepheids in visible and H bands, and consequently the geometrical part of the projection factor, follows what is expected from stellar atmosphere models. The current dispersion of about 8% in the projection factors (for a given pulsation period) is most likely due to poorly understood dynamical effects in the atmosphere of Cepheids.

Context . The Baade–Wesselink (BW) method, also called the parallax of pulsation method, compares the linear and angular variations of Cepheids to derive their distance. This method is limited, however, by the projection factor, which relates the observed radial velocity to the true pulsation velocity of the star. In this context, interferometry is crucial. Firstly, to apply the BW method to bright Cepheids, and secondly, to calibrate the surface brightness-colour relations (SBCRs). These relations can then be used to derive the angular diameter curve as a function of the pulsation phase of faint Cepheids. Aims . Using simultaneous observations from the CHARA Array interferometer in the K, H, and R bands with the MYSTIC, MIRC-X, and SPICA combiners, respectively, we aim to understand the physics of Cepheid atmospheres better. Methods . Applying a specific method to multi-chromatic simultaneous interferometric observations of Cepheids, we derived robust limb-darkened angular diameters that were then used to calibrate the SBCR and study the projection factor. We also developed a strategy to measure the limb-darkening of Cepheids in R, H, and K bands. These measurements were then used to constrain the geometrical component of the projection factor. Results . From the limb-darkened angular diameter curves of three Cepheids, we decreased the scatter of the SBCR in V − K colour to 0.0011 magnitude and to 0.0040 for the SBCR in G _BP − G _RP . These SBCRs are particularly robust because they are based on multi-chromatic diameters and are homogeneous, which previous calibrations for Cepheids were not. For the very first time, we derived the limb-darkening of Cepheids in R, H, and K bands. For δ Cep, we derived an R-based projection factor of 1.275 ± 0.051, and the geometrical part obtained from the measured limb-darkening coefficient in R band was estimated to 1.420 ± 0.016 (on average), as expected from stellar static and hydrodynamical atmosphere models. The limb-darkening coefficients obtained in H and K band are consistent with models. Conclusions . These results demonstrate that multi-chromatic interferometry can improve the accuracy of the BW method. It is therefore essential to continue the CHARA survey of Cepheids in the coming years.

Context. The instability strip (IS) is a region on the Hertzsprung-Russell diagram occupied by various types of pulsating stars. However, among them there are also stars that appear photometrically stable at a certain level of detectability. Although several scenarios have been proposed to explain this phenomenon, their nature remains unknown. Aims. We analyzed photometric and spectroscopic data for 11 candidates for non-pulsating stars located in the Cepheid IS of the Large Magellanic Cloud (LMC) in order to investigate the reasons for the lack of pulsations. Methods. We used available temperature calibrations based on photometric colors to estimate the effective temperatures of the candidates, which served as initial parameters for the spectroscopic analysis. We also applied surface brightness–color relations calibrated for Cepheid variables, giants, and supergiants to estimate stellar radii. The spectral analysis was performed using the spectral synthesis method to determine the atmospheric parameters; namely, the effective temperature, metallicity, surface gravity, microturbulent velocity, projected rotational velocity, and chemical abundances for up to ∼30 elements. Results. For most objects, only a single spectrum was available. However, no significant variations in radial velocities were detected among the stars with repeat observations; therefore, all stars were treated as single object in the analysis. Two stars exhibit broad spectral lines, which may indicate high rotational velocities or possible binarity; however, additional spectra are required to confirm this interpretation. One of these objects also shows asymmetric line profiles, which might be related to the presence of non-radial pulsation modes causing line-shape variations. A common feature among all analyzed candidates is an enhancement of barium-peak s -process elements compared to solar values. This may indicate past mass transfer from a companion during its post-asymptotic giant branch phase. Conclusions. This study provides an insight into the physical parameters of candidates for non-pulsating stars residing within the Cepheid IS in the LMC. Most of the stars have parameters very similar to Cepheids and the lack of pulsations remains a mystery and challenges pulsation theory. Additional spectroscopic observations are required to confirm and further investigate the possible origins of the observed properties of these objects.

Aims . Despite their key role in astrophysics, the binary properties of RR Lyrae stars (RRLs) are almost completely unknown since only a single RRL is confirmed as belonging to a binary system. Finding companions to RRLs is difficult since most of them will be at wider orbits, given that close orbits will likely cause mass transfer, thus disrupting the conditions for developing stellar pulsations. These wide orbits open the possibility that RRL companions may be more easily found by high-resolution imaging. Methods . We observed 81 RRLs with the speckle interferometers Zorro and ’Alopeke at the Gemini telescopes, reaching the diffraction limit of ∼20 mas of these 8m-class telescopes, and therefore exploring a new parameter space around RRLs. Results . We have detected ten newly identified companions around these 81 RRLs, with projected separations between 20 AU to 220 AU. An analysis of the field contamination shows that all of these detected companions are most likely gravitationally bound binaries. From these observations we can estimate an RRL binary fraction higher than 12%, ruling out a binary fraction higher than 25% at the 99% confidence level. These numbers are significantly more elevated than previous estimations which were close to a binary fraction of only 1%, although derived with methods exploring a different parameter space. For RRLs with thin disk kinematics, we find that the binary fraction is significantly lower, around 6%, with a single thin disk RRLs having a companion out of the 16 observed. The nature of the companions, found to be stars in the lower red giant branch and upper main sequence, is also studied via the measurement of the minimum light colors of the RRL, which is a useful method for the search and analysis of RRLs in binary systems.

Context. Anomalous Cepheids (ACEPs) are radially pulsating stars observed in dwarf galaxies, the Galactic bulge and halo, and globular clusters. Similarly to other radially pulsating stars, they can be used as distance indicators through their period-luminosity relations (PLRs) and the geometrical Baade-Wesselink (BW) method. Aims. We aim to calibrate the zero point of the distance scale of ACEPs using nearby representatives of this class of pulsating stars. Methods. We collected optical/near-infrared photometry and spectra for a sample of nearby ACEPs with two telescopes located at the RolfChini Cerro Murphy Observatory (formerly Observatory Cerro Armazones) and optical telescopes offered by the Las Cumbres Observatory, along with instruments hosted and operated by the European Southern Observatory. Using parallaxes measured by the Gaia space mission and mean magnitudes from our new photometry, we calibrated the zero point of the period-luminosity relations (PLRs) in Johnson B , V , 2MASS J , H , K _S , and Pan-STARRS g , r , i passbands, as well as select Wesenheit indices. The slopes were adopted from the PLRs of the LMC ACEPs from the literature. Using the surface brightness-color relation version of the BW technique, we were also able to determine the projection factors (p-factors) and mean radii of three nearby ACEPs. Results. The precision of the measured zero points is at the level of 0.04–0.05 mag and their systematic uncertainty was estimated to be about 0.1 mag. We used our zero point values and literature photometry related to LMC ACEPs to measure the distance modulus of this galaxy. We obtained a value of 18.454 ± 0.045 (statistical) mag, which is in very good agreement with the most accurate value from eclipsing binaries. The p-factors were 1.38 ± 0.13, 1.59 ± 0.21, and 1.35 ± 0.14 for V716 Oph, XX Vir, and UY Eri, respectively. We suspect that the very high p-factor of XX Vir results from the underestimated parallax of this star in the Gaia catalog, as its radius and absolute brightness are also higher than expected for ACEPs at this period. Finally, the radii measured for V716 Oph and UY Eri are in agreement with the period-radius relation obtained from LMC ACEPs.

Context . Radial velocity (RV) curves of Cepheids are essential for studying their pulsation properties and also for measuring their distance via the parallax-of-pulsation method. Although precise RV curves are available for hundreds of Cepheids, it is still not possible to predict the complete RV curve from other observables such as their pulsation period or metallicity. Aims . This paper aims to develop the first method of reconstructing the shape of the RV curves of short-period fundamental-mode Cepheids, based exclusively on their pulsation period and the morphology of their V -band light curves (LCs). Methods . We compiled a dataset of high-quality spectroscopic and photometric measurements from the literature for 81 short-period fundamental-mode Galactic Cepheids up to a pulsation period of 8 days, enabling the precise determination of the Fourier parameters and their uncertainties. By performing a detailed comparative analysis of their shapes, we investigated correlations between LC and RV Fourier parameters and used these relations to reconstruct the RV curves. We further assessed the accuracy of these reconstructions by examining potential metallicity effects with an additional dataset of 24 metal-poor Cepheids. Results . For pulsation periods between 3.5 and 7.0 days, we found tight correlations between different combinations of LC and RV Fourier parameters up to order 7. In particular, we discovered that the ratios R ₂₁ ( RV ) /R ₂₁ ( LC ) and R ₃₁ ( RV ) /R ₃₁ ( LC ) are tightly correlated with the pulsation period. These relationships enable the reconstruction of RV curves of Cepheids with their LC. The reconstructed curve has an uncertainty of about 0.60 km s ⁻¹ relative to the Fourier fit of true spectroscopic RV measurements. For individual Cepheids, the reconstructed RV curves integrated along the pulsation cycle (i.e. the linear radius variations) are accurate to less than 1% and precise to within 4.16% in comparison to the integrated true spectroscopic RV curves. Last, our sample of metal-poor Cepheids shows a good agreement with the empirical relations calibrated on Galactic pulsators, indicating that the method of reconstruction is weakly dependent on metallicity. Conclusions . For the first time, we present a method of reconstructing the shape of the RV curves of Cepheids using solely the V -band LC and the pulsation period. This approach provides a valuable tool for the reconstruction of RV curves for extragalactic Cepheids through photometric data alone. It opens the road to a purely photometric parallax-of-pulsation method in the context of photometric surveys, such as the Vera Rubin Telescope. Further calibration in different photometric bands and for a larger metallicity baseline will be useful to improve the method.

To measure precise distances beyond the Magellanic Clouds and determine an accurate value of the Hubble constant, eclipsing binary systems composed of early-type stars can play a crucial role. However, it is fundamental to first obtain a reliable empirical surface brightness–color relation (SBCR) for the hottest possible stars. Based on our previous study of six detached eclipsing binaries composed of O- and B-type stars in the Large Magellanic Cloud, we calibrated the SBCR using 12 stars with V − K _s < –0.6 mag. We found a significant difference between O-type and B-type stars in SBCRs, which are clearly separated in mass. The relation based on B-type stars is consistent with the relation for redder stars from the literature. This allowed us to provide a combined relation valid for stars less massive than ∼16 M _⊙ in the wide color range −0.9 < V − K _s < 2.1 mag, with σ = 0.025 mag. Such a relation can provide extragalactic distances precise to as high as ∼1.2% given the sufficient quality and number of target objects. The relation for O-type stars ( σ = 0.055 mag) remains uncertain due to its strong dependence on the method used to determine reddening and requires further study. However, we tested it on the only known eclipsing system in M33 and obtained distance modulus DM = 24.90 ± 0.17 mag, which perfectly agrees with the published distance to the system.

Context . Surface brightness–colour relations (SBCRs) are widely used to estimate the angular diameters of stars. In particular, they are employed in the Baade–Wesselink distance determination method, which relies on the comparison between the linear and angular amplitudes of Cepheids. The SBCR can be calibrated by combining different photometric systems. An SBCR was recently calibrated based on Gaia DR3 magnitudes alone for fundamental-mode classical Cepheids with solar metallicity. This relation appears to be strongly affected by metallicity, however. Aims . We derive SBCRs for classical Cepheids in the Milky Way and in the Magellanic Clouds using the photometric data available in the Gaia database, and we quantify the metallicity effect. Methods . We first selected the data on the basis of a number of quality criteria and chose the best photometric data and the best parallaxes available in Gaia for Milky Way classical Cepheids. Secondly, we compiled an extensive list of period–radius (PR) relations available in the literature, and we also provide a new PR relation based on interferometric data in our previous work. Thirdly, combining the radius of classical Cepheids with distance estimates (based on Gaia parallaxes for the Milky Way and on eclipsing binaries for the Magellanic Clouds), we derived the surface brightness and colour of about 1700 classical Cepheids. Results . We first derived a new PR relation based on interferometric data and distances from the literature of seven classical Cepheids: log(R/R _⊙ ) = 1.133 _±0.019 + 0.688 _±0.016 log(P). The metallicity does not affect the PR relations. Secondly, we calculated three different SBCRs for the Milky Way and Large and Small Magellanic Cloud classical Cepheids based on this new PR relation that clearly show the dependence of the metallicity on the SBCR based on Gaia magnitudes alone. Finally, we derived relations between the slopes, the zero points (ZP), and the metallicity ([Fe/H]) of these three SBCRs: Slope _SBCR = −0.0663 _±0.0121 [Fe/H] − 0.3010 _±0.0030 and ZP _SBCR = −0.1016 _±0.0091 [Fe/H] + 3.9988 _±0.0029 . Conclusions . These new SBCRs, dedicated to classical Cepheids in the Milky Way and Magellanic Clouds, are of particular importance to apply the inverse Baade–Wesselink method to classical Cepheids observed by Gaia in a forthcoming study.

We present precise J - and K -band photometric measurements for 128 near-infrared (NIR) secondary standard stars, located in the 19 United Kingdom Infrared Telescope/Maunakea Observatories (UKIRT/MKO) primary faint standard fields. The data were collected over more than 50 nights, covering a decade of observations between 2008 and 2018 at the ESO La Silla Observatory, using the New Technology Telescope equipped with the SOFI NIR camera. Presented magnitudes are calibrated onto the MKO photometric system. The J - and K -band magnitudes range from 10 to 15.8 mag, with median values of J ˜ = 13.5 mag and K ˜ = 13 mag. The selection process ensured high photometric quality, with a precision better than 0.01 mag for all stars. The catalog excludes stars with close neighbors, high proper motion, or variable stars. Using these fields for standardization can improve the precision and accuracy of photometric calibrations without increasing the observational time cost.