Examinando por Autor "Hajdu G."

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    Circumstellar emission of Cepheids across the instability strip: Mid-infrared observations with VLTI/MATISSE
    (EDP Sciences, 0025) Hocdé V.; Matter A.; Nardetto N.; Gallenne A.; Kervella P.; Mérand A; Pietrzyń Ski G; Gieren W.; Leftley J; Robbe-Dubois S; Lopez B.; Bailleul M.C.; Bras G; Smolec R; Wielgórski P.; Hajdu G.; Afanasiev A
    Context. The circumstellar envelopes (CSE) of Cepheids are still only poorly characterized despite their potential impact on the distance determination via both the period-luminosity relation and the parallax-of-pulsation method. Aims. This paper aims to investigate Galactic Cepheids across the instability strip in the mid-infrared with MATISSE/VLTI in order to constrain the geometry and physical nature (gas and/or dust) of their CSEs. Methods. We secured observations of eight Galactic Cepheids with short- and up to long-period pulsations with MATISSE/VLTI in the L, M, and N bands. For each star, we calibrated the flux measurements to potentially detect the spectral dust signature in the spectral energy distribution (SED). We then analyzed the closure phase and the visibilities in L, M, and N bands. The parallax-of-pulsation code SPIPS was used in order to derive the infrared excess and the expected angular stellar diameter at the date of the MATISSE observations. We also computed test cases of a radiative transfer model of dusty envelopes with DUSTY to compare them with the visibilities in the N band. Results. The SED analysis in the mid-IR confirmed the absence of a spectral dust signature for the entire star sample. For each star, we observed closure phases in the L, M, and N bands that are consistent with a centro-symmetric geometry for the different targets. Finally, the visibilities in the L, M, and N bands agree with the expected angular stellar diameter. Although we did not resolve any circumstellar emission, the observations are compatible with the presence of compact CSEs within the uncertainties. We provide 2 σ upper limits on the CSE flux contribution based on model residuals for several CSE radii, which exclude models that are simultaneously large and bright (RCSE ≈ 10 R∗ and fCSE ≈ 10%) for all the stars of the sample. Last, the visibilities in the N band rule out CSE models with optical depth τV ≳ 0.001 for different types of dust. Conclusions. The MATISSE observations of eight Cepheids with different pulsation periods (from 7 to 38 days) and evolution stages provide a comprehensive picture of Cepheids from mid-infrared interferometry for the first time. We present additional evidence that circumstellar dust emission is negligible or absent around Cepheids for a wide range of stellar parameters in the instability strip. Further interferometric observations in the visible and near-infrared are required to separate the star and CSE, which is crucial for constraining the CSE contribution and its possible gaseous nature. © The Authors 2025.
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    Establishing the Galactic Centre distance using VVV Bulge RR Lyrae variables
    (Springer Netherlands, 2018-06) Majaess D.; Dékány I.; Minniti D.; Turner D.; Gieren W.; Hajdu G.
    This study’s objective was to exploit infrared VVV (VISTA Variables in the Via Lactea) photometry for high latitude RRab stars to establish an accurate Galactic Centre distance. RRab candidates were discovered and reaffirmed (n= 4194) by matching Ks photometry with templates via χ2 minimization, and contaminants were reduced by ensuring targets adhered to a strict period-amplitude (Δ Ks) trend and passed the Elorietta et al. classifier. The distance to the Galactic Centre was determined from a high latitude Bulge subsample (| b| > 4 ∘, RGC= 8.30 ± 0.36 kpc, random uncertainty is relatively negligible), and importantly, the comparatively low color-excess and uncrowded location mitigated uncertainties tied to the extinction law, the magnitude-limited nature of the analysis, and photometric contamination. Circumventing those problems resulted in a key uncertainty being the MKs relation, which was derived using LMC RRab stars (MKs=−(2.66±0.06)logP−(1.03±0.06), (J−Ks)0=(0.31±0.04)logP+(0.35±0.02), assuming μ0 , LMC= 18.43). The Galactic Centre distance was not corrected for the cone-effect. Lastly, a new distance indicator emerged as brighter overdensities in the period-magnitude-amplitude diagrams analyzed, which arise from blended RRab and red clump stars. Blending may thrust faint extragalactic variables into the range of detectability. © 2018, Springer Science+Business Media B.V., part of Springer Nature.
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    Surface brightness-colour relations of dwarf stars from detached eclipsing binaries: II. Extension of the calibrating sample
    (EDP Sciences, 0025-12) Graczyk D.; Pietrzyński G.; Galan C; Miller N; Gieren W.; Zgirski B; Hajdu G.; Kałuszyński M.; Chini R.; Gallenne A.; Górski M; Karczmarek P.
    Aims. Surface brightness-colour relations (SBCRs) are useful tools for predicting the angular diameters of stars. They offer the possibility to calculate precise spectrophotometric distances based on the eclipsing binary method or the Baade- Wesselink method. Double-lined detached eclipsing binary stars (SB2 DEBs), with precisely known trigonometric parallaxes, allow us to calibrate SBCRs with a high level of precision. To improve such calibrations, it is important to supplement the sample of suitable eclipsing binaries with precisely determined physical parameters. Methods. We selected ten SB2 DEBs within 0.8 kpc of the Sun, which feature components of spectral types ranging from B9 to K3. We analysed their TESS and Kepler K2 space-based photometry simultaneously with the radial velocities derived from HARPS spectra using the Wilson- Devinney code. The disentangled spectra of DEBs were used to derive atmospheric parameters of their components by applying the GSSP code. The direct effective temperatures were also calculated using spectral energy distribution analysis. The O-C diagrams of the minima times were investigated to detect long-term period changes or apsidal motions. Results. Most of the systems are composed of significantly unequal components, with mass ratios as low as ∼0.5. We derived precise masses, radii, and surface temperatures for them, along with their metallicities. The average precision of mass and radii determinations is 0.3% and 1.4%, respectively, for the surface temperature. The spectroscopic and photometric temperatures of the components are usually consistent to within 100 K, but in some systems, the difference is much larger. The components of HD 149946 show the highest difference (up to 400 K), while the atmospheric models favour different surface metallicities. We also provide an updated calibration of the equivalent width of the interstellar sodium D1 line and the reddening E(B-V). © The Authors 2025.