Theoretical analysis of surface brightness-colour relations for late-type stars using MARCS model atmospheres

Salsi, A.; Nardetto, N.; Plez, B.; Mourard, D.

doi:10.1051/0004-6361/202142133

Cited by 4 publications

(2 citation statements)

References 72 publications

(48 reference statements)

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“…Applying a homogeneous approach on a large number of interferometric data, these recent studies have shown that the SBCRs depend not only on the temperature of stars, but also on their luminosity class. This result was also confirmed from atmospheric models (Salsi et al 2022). The SBCRs are also of high interest for the study of transiting exoplanet host stars.…”

Section: Introductionsupporting

confidence: 76%

HARPS-N high spectral resolution observations of Cepheids

Nardetto

Gieren

Storm

et al. 2023

A&A

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Context. The Baade-Wesselink (BW) method of distance determination of Cepheids is used to calibrate the distance scale. Various versions of this method are mainly based on interferometry and/or the surface-brightness color relation (SBCR). Aims. We quantify the impact of the SBCR, its slope, and its zero point on the projection factor. This quantity is used to convert the pulsation velocity into the radial velocity in the BW method. We also study the impact of extinction and of a potential circumstellar environment on the projection factor. Methods. We analyzed HARPS-N spectra of η Aql to derive its radial velocity curve using different methods. We then applied the inverse BW method using various SBCRs in the literature in order to derive the BW projection factor. Results. We find that the choice of the SBCR is critical: a scatter of about 8% is found in the projection factor for different SBCRs in the literature. The uncertainty on the coefficients of the SBCR affects the statistical precision of the projection factor only little (1–2%). Confirming previous studies, we find that the method with which the radial velocity curve is derived is also critical, with a potential difference on the projection factor of 9%. An increase of 0.1 in E(B − V) translates into a decrease in the projection factor of 3%. A 0.1 mag effect of a circumstellar envelope (CSE) in the visible domain is rather small on the projection factor, about 1.5%. However, we find that a 0.1 mag infrared excess in the K band due to a CSE can increase the projection factor by about 6%. Conclusions. The impact of the surface-brightness color relation on the BW projection factor is found to be critical. Efforts should be devoted in the future to improve the SBCR of Cepheids empirically, but also theoretically, taking their CSE into account as well.

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Section: Introductionsupporting

confidence: 76%

HARPS-N high spectral resolution observations of Cepheids

Nardetto

Gieren

Storm

et al. 2023

A&A

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“…It is a grid of about 10 4 model atmospheres with nearly 52 000 stellar spectra containing F, G, and K types of stars. This grid of one-dimensional LTE model atmospheres can be combined with atomic and molecular spectral line data and software to generate stellar spectra, which has been widely used in a variety of studies (Roederer et al 2014;Lu et al 2018;Reggiani et al 2019;VandenBerg et al 2021;Salsi et al 2022).…”

Section: Marcs Datasetmentioning

confidence: 99%

Estimating stellar parameters and identifying very metal-poor stars for low-resolution spectra (R ∼ 200)

Wu,

Bu,

Xie

et al. 2023

Publ. Astron. Soc. Aust.

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Very metal-poor (VMP, [Fe/H]<-2.0) stars serve as invaluable repositories of insights into the nature and evolution of the first-generation stars formed in the early galaxy. The upcoming China Space Station Telescope (CSST) will provide us with a large amount of spectral data that may contain plenty of VMP stars, and thus it is crucial to determine the stellar atmospheric parameters (Teff , log g, and [Fe/H]) for low-resolution spectra similar to the CSST spectra (R ∼ 200). This study introduces a novel two-dimensional Convolutional Neural Network (CNN) model, comprised of three convolutional layers and two fully connected layers. The model’s proficiency is assessed in estimating stellar parameters, particularly metallicity, from low-resolution spectra (R ∼ 200), with a specific focus on enhancing the search for VMP stars within the CSST spectral data. We mainly use 10,008 spectra of VMP stars from LAMOST DR3, and 16,638 spectra of non-VMP stars ([Fe/H]>-2.0) from LAMOST DR8 for the experiments and apply random forest and support vector machine methods to make comparisons. The resolution of all spectra is reduced to R ∼ 200 to match the resolution of the CSST, followed by preprocessing and transformation into two-dimensional spectra for input into the CNN model. The validation and practicality of this model are also tested on the MARCS synthetic spectra. The results show that using the CNN model constructed in this paper, we obtain Mean Absolute Error (MAE) values of 99.40 K for Teff , 0.22 dex for log g, 0.14 dex for [Fe/H], and 0.26 dex for [C/Fe] on the test set. Besides, the CNN model can efficiently identify VMP stars with a precision rate of 94.77%, a recall rate of 93.73%, and an accuracy of 95.70%. This paper powerfully demonstrates the effectiveness of the proposed CNN model in estimating stellar parameters for low-resolution spectra (R ∼ 200) and recognizing VMP stars that are of interest for stellar population and galactic evolution work.

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Projection factor and radii of Type II Cepheids

Wielgórski,

Pietrzyński,

Gieren

et al. 2024

A&A

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Type II Cepheids are old pulsating stars that can be used to trace the distribution of an old stellar population and to measure distances to globular clusters and galaxies within several megaparsecs, and by extension, they can improve our understanding of the cosmic distance scale. One method that can be used to measure the distances of Type II Cepheids relies on period-luminosity relations, which are quite widely explored in the literature. The semi-geometrical Baade-Wesselink technique is another method that allows distances of radially pulsating stars, such as Type II Cepheids, to be measured if the so-called projection factor is known. However, the literature concerning this parameter for Type II Cepheids is limited to just a few pioneering works. In determining projection factors for eight nearby short-period Type II Cepheids, also known as BL Her type stars, we aim to calibrate the Baade-Wesselink method for measuring distances for this class of stars. Using the surface brightness-colour relation version of the Baade-Wesselink technique, we determined the projection factors and radii of eight nearby BL Her type stars. We adopted accurate distances of target stars from $Gaia$ Data Release 3. Time series photometry in the $V$ and $K_ S $ bands have been collected with two telescopes located at the $Rolf$ $Chini$ Cerro Murphy Observatory (former Cerro Armazones Observatory), while spectroscopic data have been obtained within dedicated programmes with instruments hosted by the European Southern Observatory. The measured projection factors for the stars with good quality data are in the range between 1.21 and 1.36. The typical uncertainty of projection factors is 0.1. The mean value is 1.330pm 0.058, which gives the uncertainty of sim 4<!PCT!>. The main sources of uncertainty on the $p$-factors are statistical errors of the Baade-Wesselink fit (related to the dispersion and coverage of light and radial velocity curves) and parallax. In the case of radii, the biggest contribution to the error budget comes from the $K_ S $ band photometry's systematic uncertainty and parallax. The determined radii allowed us to construct the period-radius relation for BL Her stars. Our period-radius relation is in good agreement with the previous empirical calibration, while two theoretical calibrations found in the literature agree with our relation within 2sigma . We also confirm that BL Her and RR Lyr stars obey an apparent common period-radius relation.

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Theoretical analysis of surface brightness-colour relations for late-type stars using MARCS model atmospheres

Cited by 4 publications

References 72 publications

HARPS-N high spectral resolution observations of Cepheids

HARPS-N high spectral resolution observations of Cepheids

Estimating stellar parameters and identifying very metal-poor stars for low-resolution spectra (R ∼ 200)

Projection factor and radii of Type II Cepheids

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