L. A. Yakovina

Metallicity and effective temperature of the secondary of RS Ophiuchi

Context. The recurrent nova RS Ophiuchi undergoes nova eruptions every � 10 20 years as a result of thermonuclear runaway on the surface of a white dwarf close to the Chandrasekhar limit. Both the progress of the eruption, and its aftermath, depend on the (poorly known) composition of the red giant in the RS Oph system. Aims. Our aim is to understand better the effect of the giant secondary on the recurrent nova eruption. Methods. Synthetic spectra were computed for a grid of M-giant model atmospheres having a range of effective temperatures 3200 < Teff < 4400 K, gravities 0 < log g < 1 and abundances -4 <[Fe/H] < 0.5, and fit to infrared spectra of RS Oph as it returned to quiescence after its 2006 eruption. We have modelled the infrared spectrum in the range 1.4 2.5µm to determine metallicity and effective temperature of the red giant. Results. We find Teff = 4100 ± 100 K, log g = 0.0 ± 0.5, [Fe/H] = 0.0 ± 0.5, [C/H] = 0.8 ± 0.2, [N/H] = +0.6 ± 0.3 in the atmosphere of the secondary, and demonstrate that that inclusion of some dust ‘veiling’ in the spectra cannot improve our fits.Context. The recurrent nova RS Ophiuchi undergoes nova eruptions every ∼ 10− 20 years as a result of thermonuclear runaway on the surface of a white dwarf close to the Chandrasekhar limit. Both the progress of the eruption, and its aftermath, depend on the (poorly known) composition of the red giant in the RS Oph system. Aims. Our aim is to understand better the effect of the giant secondary on the recurrent nova eruption. Methods. Synthetic spectra were computed for a grid of M-giant model atmospheres having a range of effective temperatures 3200 < Teff < 4400 K, gravities 0 < log g < 1 and abundances -4 <[Fe/H] < 0.5, and fit to infrared spectra of RS Oph as it returned to quiescence after its 2006 eruption. We have modelled the infrared spectrum in the range 1.4 − 2.5 μm to determine metallicity and effective temperature of the red giant. Results. We find Teff = 4100 ± 100 K, log g = 0.0 ± 0.5, [Fe/H] = 0.0 ± 0.5, [C/H] = −0.8 ± 0.2, [N/H] = +0.6 ± 0.3 in the atmosphere of the secondary, and demonstrate that that inclusion of some dust ‘veiling’ in the spectra cannot improve our fits.

CO bands in V4334 Sgr (Sakurai's Object): The

We present the results of our analysis of a high resolution (R ~ 30000) infrared spectrum of V4334 Sgr (Sakurais Object) around the first overtone CO bands, obtained in 1998 July. The 12CO and 13CO bands are well-resolved, and we compute synthetic hydrogen-deficient model atmosphere spectra to determine the 12C/13C ratio. We find 12C/13C ~ 4 +/-1, consistent with the interpretation of V4334 Sgr as an object that has undergone a very late thermal pulse.

\mathsf{^{12}}

We calculated 240 model atmospheres of carbon giants with high carbon abundance (2800 K ≤ Teff ≤ 3400 K, 0.06 ≤ log(C/O) ≤ 2.7). This set of models was used for modelling of the energy distribution in the spectrum of an evolved carbon star DY Per demonstrating photometric features of the R CrB type stars. Most of the models are metal-poor (−3.5 ≤ [Fe/H] < 0) and a portion of them was calculated with the hydrogen deficiency (1/9 ≤ H/He < 9/1). The calculations of the models were carried out taking into account specific for carbon stars sources of opacity in the frame of the classic approaches. The opacity sampling method was used to calculate the opacity due to the atomic and molecular line absorption.

C/

We have derived the atmospheric parameters of the R CrB carbon star DY Per. The spectrum of DY Per near its maximum brightness was obtained using the 2.6-m ZTSh telescope of the Crimean Astrophysical Observatory, and has a resolution of about 1.74 Å per pixel. We compare the absolute observed and theoretical spectral energy distributions (SEDs) of DY Per for λλ 430–730 nm for the first time. The model atmospheres were computed using a code written by Ya.V. Pavlenko in the classical approximation, taking into account the main opacity sources in carbon-star atmospheres. The theoretical SEDs were computed using the list of atomic lines from the VALD database and the molecular line lists from CD-ROM No. 18 of Kurucz’s database. The estimated by spectral synthesis effective temperature of DY Per is in the range of 2900–3100 K, if we assume log g = 0. We find a metal deficiency in the atmosphere of DY Per. Quantitative estimates of the overall metallicity, carbon and nitrogen abundances, and the H/He ratio are somewhat uncertain: −2 ≤ [Fe/H] ≤ −0.5, 0.65 ≤ [C/Fe] ≤ 1.35, 0.0 ≤ [N/Fe] ≤ 0.8, 1/9 ≤ H/He ≤ 9/1. These high H/He values do not quite agree with qualitative observational estimates made by other authors. Our results confirm that DY Per is a unique stellar object. This is the coolest and possibly the most metal-poor of all known R CrB stars. We conclude that the large deficiencies of metals and hydrogen lead to [C/Fe] values in the atmosphere of DY Per characteristic of R CrB stars.

\mathsf{^{13}}

Theoretical spectral energy distributions were computed for a grid of hydrogen-deficient and carbon-rich model atmospheres of T(eff) in the range of 5000-6250 K and log g = 1.0 - 0.0 by the technique of opacity sampling, taking into account continuous, molecular band and atomic line absorption. These energy distributions were compared with the spectrum of V4334 Sgr (Sakurais object) of April, 1997 in the wavelength interval 300-1000 nm. We show that (1) the shape of the theoretical spectra depends strongly on T(eff) but only very weakly on the hydrogen abundance; (2) the comparison of the observed and computed spectra permits to estimate T(eff) approximately 5500 K for V4334 Sgr in April, 1997, and its interstellar reddening (plus a possible circumstellar contribution) E(B-V) approximately 0.70.Theoretical spectral energy distributions for Sakurais object at 300–1000 nm are derived. A model-atmosphere grid with Teff=5000–6250 K and logg=0.0–1.0 is computed for the chemical composition of Sakurais object using opacity sampling including molecular and atomic absorption. Opacity due to absorption in 20 band systems of diatomic molecules is computed using the JOLA technique. The theoretical fluxes are compared with the observed energy distribution in a spectrum of Sakurais object taken in April 1997. It is shown that (a) the theoretical energy distributions agree well qualitatively with the observed spectrum and depend strongly on the effective temperature; (b) C2 and CN molecular bands are dominant in the visible and near-infrared spectrum, while atomic absorption is important at UV and blue wavelengths; and (c) comparison of the observed and computed spectra yields an effective temperature for Sakurais object in April 1997 Teff≈5250–5500 K. The dependence of the computed spectra at 300–1000 nm on the input parameters and adopted approximations is also discussed.

C ratio

We determine lithium abundances in atmospheres of three carbon stars from synthetic spectrum fitting in the λλ 668–674 nm range using the Li I λ 670.8 nm resonance doublet. To produce synthetic spectra, we use a modified list of atomic lines from the VALD database and three alternative line lists of CN and C2 molecules which are modifications of line lists from the Jorgensen’s website (http://stella.nbi.dk) and from the Kurucz database (1993, CD-ROM nos. 1–23). The spectral lines from these lists were tested by matching synthetic spectra to observed spectra of the sun, Arcturus, and early R star HD 100764. We perform analysis of the blends involving the Li I λ 670.8 nm doublet in spectra of N stars AW Cyg and UX Dra. The lithium abundances in HD 100764, AW Cyg, and UX Dra are estimated to be lgN(Li) ≈ 2, −1.4, and −0.9, respectively. Discrepancies of lithium abundances lgN(Li) obtained with the help of molecular line lists do not seem to exceed 0.2 dex.

Model atmospheres of carbon giants with high carbon abundance

EU PF7 Marie Curie Initial Training Networks (ITN) RoPACS project 213646 Fondecyt grant 1161218 CATA-Basal (CONICYT) PB06 UK STFC ST/M001008/1 Leverhulme Trust RPG-2014-281 NAS Ukraine under the Main Astronomical Observatory GRAPE/GPU/GRID computing cluster project

DY Persei, the Coolest Metal-Poor R CrB Carbon Star

We present the results of modelling the 0.45-1 μm spectral energy distribution of V838 Mon for 2002 November. Synthetic spectra were calculated using the NextGen model atmospheres of Hauschildt et al. (1999, ApJ, 512, 377), which incorporate line lists for H 2 O, TiO, CrH, FeH, CO, and MgH, as well as the VALD atomic line list. Fits to the observed spectra show that, in 2002 November, the effective temperature of V838 Mon was approximately 2000 ± 100 K. Our theoretical spectra show a comparatively weak dependence on log g. Preliminary analysis of the hot star observed together with V838 Mon shows it to be a normal B3V dwarf.

A model for the spectrum of Sakurai's object (V4334 Sgr)

The TU Gem star has long been known as a peculiar carbon giant of the Galaxy halo, but its classification as a CH star is still debated. We estimated the TU Gem atmosphere parameters through modeling its spectrum and comparision one with the spectra of the star observed in two wide spectral ranges (λλ 400–720 nm and λλ 900–2440 nm). The low-dispersion optical TU Gem spectrum obtained by Barnbaum et al. (2006) (R ∼ 600) and the infrared spectrum presented by Tanaka et al. (2007) (R ∼ 2600) were used in the analysis. The model atmospheres were calculated using the SAM12 software (Pavlenko, 2003). Since the metallicity ([Fe/H]) value could not be determined conclusively based on our spectral data, only the TU Gem effective temperature Teff (that depends weakly on metallicity) was defined with certainty (Teff = 3000 ± 100 K). We determined the C/O, [C/Fe], and [N/Fe] values for the −2.0 ≤ [Fe/H] ≤ 0.0 range with a step of Δ[Fe/H] = 0.5. Our estimate of [C/Fe] (0.63–0.67 at [Fe/H] = −1.0) is higher than the corresponding estimate ([C/Fe] = 0.21 at [Fe/H] = −1.1) given in (Kipper et al., 1996), while the estimates for [N/Fe] at the stated metallicities agree with each other: [N/Fe] = +1.0. This brings TU Gem closer to CH stars, but a detailed analysis of the chemical composition of the TU Gem atmosphere is required to reach a definite conclusion.

Lithium abundances in atmospheres of carbon stars estimated with the help of modern lists of spectral lines

The LTE lithium abundances, logN (Li), have been determined for the atmospheres of the four carbon giants AQ And, HK Lyr, UX Dra, and WZ Cas, applying the synthetic-spectrum technique to the LiI λ670.8 nm resonance doublet. We used estimates of the effective temperatures Teff, metallicities, elemental abundances, and carbon-isotope ratios for these stars from the literature [12C/13C]. The resulting lithium abundances depend significantly on the adopted Teff, so that we can find each star’s range of possible logN(Li) values for a given range of effective temperatures (ΔTeff = 200−300 K). The uncertainty in the derived logN(Li) values is 1.3 for AQ And and UX Dra, 0.5 for HK Lyr, and 0.8 for WZ Cas. Our most probable logN(Li) values (or ranges) are −1.25 for AQ And, −1.2 for HK Lyr, −1.0 ≤ logN(Li) ≤ +0.3 for UX Dra, and 4.3 ≤ logN(Li) ≤ 4.8 for WZ Cas. The derived C/O ratios for the stellar atmospheres also depend on Teff. Possible uncertainties in other atmospheric parameters do not change logN(Li) by more than 0.2.