The Araucaria Project: Improving the cosmic distance scale

The book consists of a number of short articles that present achievements of the Araucaria members, collaborators, and friends, in various aspects of distance determinations and related topics. It celebrates the 20-year anniversary of the Araucaria Project, acknowledges the people who worked for its success, and popularises our methods and results among broader readership. This book is a part of a project that has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 695099.Comment: 114 pages, book published in 2021 on behalf of the Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences, to celebrate 20 years of the Arauria Projec

Mid-infrared circumstellar emission of the long-period Cepheid l Carinae resolved with VLTI/MATISSE

Stars and planetary system

Extended envelopes around Galactic Cepheids. V. Multi-wavelength and time-dependent analysis of IR excess

International audienceAims. We aim to investigate the infrared excess of 45 Milky Way (MW) Cepheids combining different observables in order to constrain the presence of circumstellar envelopes (CSEs). Methods. We used the SpectroPhoto-Interferometry of Pulsating Stars (SPIPS) algorithm, a robust implementation of the parallaxof-pulsation method that combines photometry, angular diameter, stellar effective temperature, and radial velocity measurements in a global modelling of the pulsation of the Cepheid. We obtained new photometric measurements at mid-infrared (mid-IR) with the VISIR instrument at the Very Large Telescope complemented with data gathered from the literature. We then compared the mean magnitude of the Cepheids from 0.5 µm to 70 µm with stellar atmosphere models to infer the IR excess, which we attribute to the presence of a circumstellar envelope. Results. We report that at least 29% of the Cepheids of our sample have a detected IR excess (>3σ). We estimated a mean excess of 0.08 ± 0.04 mag at 2.2 µm and 0.13 ± 0.06 mag at 10 µm. Other Cepheids possibly also have IR excess, but they were rejected due to their low detection level compared to a single-star model. We do not see any correlation between the IR excess and the pulsation period as previously suspected for MW Cepheids, but a rather constant trend at a given wavelength. We also do not find any correlation between the CO absorption and the presence of a CSE, but rather with the stellar effective temperature, which confirms that the CO features previously reported are mostly photospheric. No bias caused by the presence of the circumstellar material is detected on the average distance estimates from a SPIPS analysis with a fitted colour excess. We also do not find correlation between the presence of IR excess and the evolution stage of the Cepheids. Conclusions. We report a fraction of 29% of Cepheids with an IR excess likely produced by the circumstellar envelope surrounding the stars. Longer period Cepheids do not exhibit greater excess than short periods as previously suspected from observations and theoretical dusty-wind models. Other mechanisms such as free-free emission, among others, may be at the origin of the formation of the CSEs. We also show that not fitting the colour excess leads to a bias on the distance estimates in our Galaxy

Pulsating chromosphere of classical Cepheids

Context. It has recently been shown that the infrared (IR) emission of Cepheids, constant over the pulsation cycle, might be due to a pulsating shell of ionized gas with a radius of about 15% of that of the star radius, which could be attributed to the chromospheric activity of Cepheids. Aims. The aim of this paper is to investigate the dynamical structure of the chromosphere of Cepheids along the pulsation cycle and to quantify its size. Methods. We present Hα and calcium near-infrared triplet (Ca IR) profile variations using high-resolution spectroscopy with the UVES spectrograph of a sample of 24 Cepheids with a good period coverage from ≈3 to 60 days. After a qualitative analysis of the spectral line profiles, we quantified the Van Hoof effect (velocity gradient between the Hα and Ca IR) as a function of the period of the Cepheids. We then used the Schwarzschild mechanism (a line doubling due to a shock wave) to quantify the size of the chromosphere. Results. We find a significant Van Hoof effect for Cepheids with a period larger than P = 10 days. In particular, Hα lines are delayed with a velocity gradient up to Δv ≈ 30 km s−1 compared to Ca IR. By studying the shocks, we find that the size of the chromosphere of long-period Cepheids is of at least ≈50% of the stellar radius, which is consistent at first order with the size of the shell made of ionized gas previously found from the analysis of IR excess. Last, for most of the long-period Cepheids in the sample, we report a motionless absorption feature in the Hα line that we attribute to a circumstellar envelope that surrounds the chromosphere. Conclusions. Analyzing the Ca IR lines of Cepheids is of importance to potentially unbias the period–luminosity relation from their IR excess, particularly in the context of forthcoming observations of radial velocity measurements from the Radial Velocity Spectrometer on board Gaia, which could be sensitive to their chromosphere

Consistent radial velocities of classical Cepheids from the cross-correlation technique

International audienc

VizieR Online Data Catalog: Classical Cepheids consistent radial velocities (Borgniet+, 2019)

VizieR On-line Data Catalog: J/A+A/631/A37. Originally published in: 2019A&A...631A..37BOur sample is made up of 64 Classical Galactic Cepheids with pulsation periods in the range 2 to 68 days. We provide the full details of our sample in Table A.1. We present both the six correlation templates built to derive tailored cross-correlation functions (CCFs), the CCFs themselves, and the derived radial velocity and line profile observable time series. All data are provided with the corresponding Cepheid name, the spectrograph used to observe the target, the Modified Julian Day (MJD) of the observation and the observation program identifier. The data are provided over three different wavelength ranges: "blue" from 390 to 498nm; "green" from 450 to 680nm; and "red" from 570 to 880nm, corresponding to the letters "b", "g", and "r" (first identifier within the file names). Depending on this range, the data are computed based on one (for the "blue" and "red" ranges) or four (for the "green" range) different correlation templates, corresponding to different sets of spectral lines selected based on their relative depth: weak, medium, deep and all lines. The corresponding identifiers are the letters "w", "m", "d", and "a" (second identifier within the file names). Finally, the radial velocities and other line profile observables are computed in three different ways for each star, each spectrograph, each wavelength range, and each template: first based on the CCF first moment or centroid, second based on a Gaussian fit of the CCF, and third based on a BiGaussian fit of the CCF. The corresponding identifiers are the letters "c", "g" and "b" (third identifier within the rv file names). (28 data files)

A thin shell of ionized gas as the explanation for infrared excess among classical Cepheids

International audienceContext. The infrared (IR) excess of classical Cepheids is seldom studied and poorly understood despite observational evidence and the potential for its contribution to induce systematics on the period-luminosity (PL) relation used in the calibration of the extragalactic distance scale. Aims: This study aims to understand the physical origin of the IR excess found in the spectral energy distribution (SED) of 5 Cepheids: RS Pup (P = 41.46d), ζ Gem (P = 10.15d), η Aql (P = 7.18d), V Cen (P = 5.49d) and SU Cyg (P = 3.85d). Methods: A time series of atmospheric models along the pulsation cycle were fitted to a compilation of data, including optical and near-IR photometry, Spitzer spectra (secured at a specific phase), interferometric angular diameters, effective temperature estimates, and radial velocity measurements. Herschel images in two bands were also analyzed qualitatively. In this fitting process, based on the SPIPS algorithm, a residual was found in the SED, whatever the pulsation phase, and for wavelengths larger than about 1.2 μm, which corresponds to the so-determined infrared excess of Cepheids. This IR excess was then corrected from interstellar medium absorption in order to infer the presence (or absence) of dust shells and was, ultimately, used in order to fit a model for a shell of ionized gas. Results: For all Cepheids, we find a continuum IR excess increasing up to approximately -0.1 magnitudes at 30 μm, which cannot be explained by a hot or cold dust model of CircumStellar Environment (CSE). However, a weak but significant dust emission at 9.7 μm is found for ζ Gem, η Aql and RS Pup, while clear interstellar clouds are seen in the Herschel images for V Cen and RS Pup. We show, for the first time, that the IR excess of Cepheids can be explained by free-free emission from a thin shell of ionized gas, with a thickness of ≃15% of the star radius, a mass of 10-9-10-7M⊙ and a temperature ranging between 3500 and 4500 K. Conclusions: The presence of a thin shell of ionized gas around Cepheids must be tested with interferometers operating in the visible or mid-IR, or using radio telescopes. The impact of such CSEs of ionized gas on the PL relation of Cepheids also calls for further investigation

VizieR Online Data Catalog: Detached eclipsing binaries with Gaia parallaxes (Graczyk+, 2019)

VizieR On-line Data Catalog: J/ApJ/872/85. Originally published in: 2019ApJ...872...85GWe extended the sample of 35 eclipsing binaries compiled by Graczyk+ (2017ApJ...837....7G) by searching for detached systems in the literature suitable for a precise distance determination. Our sample contains 81 systems (51 on the northern hemisphere and 30 on the southern one). Their basic parameters are presented in Table 1. We used Tycho-2 BT and VT photometry (Hog+ 2000, I/259) downloaded from Vizier. The Tycho photometry was subsequently transformed onto the Johnson system using the method outlined by Bessell (2000PASP..112..961B). Whenever possible we used Johnson B, V photometry from the compilation of Mermilliod (1997, II/168) and also absolute optical photometry from literature sources. (4 data files)