I. A. Usenko

Spectroscopic studies of southern-hemisphere Cepheids: XX Sgr, AP Sgr, RV Sco, RY Sco, V482 Sco, and V636 Sco

We present the results obtained by analyzing 20 spectra of six bright Cepheids taken with the 1.9-m telescope at the South African Astronomical Observatory. The chemical composition and atmospheric parameters have been determined for XX Sgr, AP Sgr, RV Sco, RY Sco, V482 Sco, and V636 Sco.

Spectroscopic studies of Cepheids (S Cru, AP Pup, AX Cir, S TrA, T Cru, R Mus, S Mus, U Car) and semiregular bright supergiants (V382 Car, HD 75276, R Pup) in the southern hemisphere

We present the second part of the results of our work to investigate the atmospheric parameters and chemical composition of southern-hemisphere Cepheids obtained by analyzing 24 spectra for eight bright Cepheids and three semiregular bright supergiants taken with the 1.9-m telescope of the South African Astronomical Observatory. The chemical composition and atmospheric parameters have been determined for S Cru, AP Pup, AX Cir, S TrA, T Cru, R Mus, S Mus, U Car, V382 Car, HD 75276, and R Pup.

Spectroscopic studies of southern-hemisphere Cepheids: Six objects in Centaurus (V Cen, V737 Cen) and Sagittarius (BB Sgr, W Sgr, X Sgr, Y Sgr)

This paper is the next one in the series of our works aimed at determining the atmospheric parameters and chemical composition of southern-hemisphere Cepheids. We present the results of our study for six bright Cepheids: V Cen, V737 Cen, BB Sgr, W Sgr, X Sgr, and Y Sgr. We have analyzed 14 high-resolution spectra taken with the 1.9-m telescope at the South African Astronomical Observatory. In addition to determining the chemical composition and atmospheric parameters, we point out and discuss several features in the spectra of individual Cepheids. In particular,we have detected emission in the cores of the Hβ and Hα lines forWSgr near its maximum light, while X Sgr shows the splitting of metal absorption lines into individual components without any change of the hydrogen lines. These peculiarities can be explained by different manifestations of shock waves in the Cepheid atmospheres and by the presence of circumstellar envelopes around X Sgr and W Sgr. The chemical composition has been estimated for V737 Cen, BB Sgr, and X Sgr for the first time. On the whole, our abundance estimates for α-elements, iron-peak elements, and r- and s-process elements are close to the solar ones for all objects, carbon is underabundant, the oxygen abundance is nearly solar, the “odd” elements, Na and Al, are overabundant (except X Sgr), magnesium is underabundant for V Cen and X Sgr and overabundant for the remaining objects. Such a chemical composition is typical of yellow supergiants after the first dredge-up. Keywords: Cepheids, spectra, atmospheric parameters, chemical composition.

Spectroscopic studies of three Cepheids with high positive pulsation period increments: SZ Cas, BY Cas, and RU Sct

Three high-resolution spectra have been taken at different times with the 6-m SAO RAS telescope (LYNX and PFES spectrographs) for three Cepheids exhibiting high positive period increments: the small-amplitude (DCEPS) SZ Cas and BY Cas and the classical (DCEP) RU Sct. SZ Cas and RU Sct are members of the Galactic open clusters χ and h Per and Trump 35, respectively. Analysis of the spectra has shown that the interstellar Na I D1 and D2 lines in all objects are considerably stronger than the atmospheric ones and are redshifted in SZ Cas and BY Cas and blushifted in RU Sct. The core of the Hα absorption line in BY Cas has an asymmetric knifelike shape, while RU Sct exhibits an intense emission in the blue wing of this line. Such phenomena are observed in long-period Cepheids and bright hypergiants with an extended envelope. In this case, the strong Mg Ib 5183.62 Å and Ba II 5853.67, 6141.713, and 6496.90 Å lines with low χlow in SZ Cas and RU Sct also show characteristic knifelike profiles with an asymmetry in the red region, while the Ba II 4934.095 Å line shows similar profiles in the blue one. The absorption lines of neutral atoms and singly ionized metals with different lowerlevel excitation potentials exhibit different degrees of asymmetry: from a pronounced one with secondary components in BY Cas (similar to those in the small-amplitude Cepheid BG Cru pulsating in the first overtone and having an envelope) to its insignificance or virtual absence in SZ Cas and RU Sct. Analysis of the secular changes in mean Teff determined from photometric color indices and spectra over the last 55 years for these stars has revealed periodic fluctuations of 200 K for SZ Cas and BY Cas and 500 K for RU Sct. For SZ Cas and RU Sct, Teff determined in some years from some color indices show much lower values, which together with the temperature fluctuations can be associated with mass loss and dust formation. Based on these facts, we hypothesize the existence of circumstellar envelopes around all three Cepheids. We have determined the atmospheric parameters and chemical composition of the program Cepheids. An appreciable carbon underabundance, a nitrogen overabundance (the result was obtained only for BY Cas), a nearly solar oxygen abundance, a sodium overabundance, and solar magnesium and aluminum abundances have been revealed in all stars, suggesting that these yellow supergiants has already passed the first dredge-up. The abundances of the Fe-peak elements, a-elements, and r- and s-process elements are nearly solar. [Fe/H] = −0.05 dex for SZ Cas and [Fe/H]= +0.05 dex for RU Sct can be used to estimate the metallicities of the open clusters χ and h Per and Trump 35, respectively.

Spectroscopic studies of southern-hemisphere Cepheids: Three Cepheids in Crux (BG Cru, R Cru, and T Cru)

This paper is devoted to spectroscopic studies of three bright Cepheids (BG Cru, R Cru, and T Cru) and continues the series of our works aimed at determining the atmospheric parameters and chemical composition of southern-hemisphere Cepheids. We have studied 12 high-resolution spectra taken with the 1.9-m telescope of the South African Astronomical Observatory and the 8-m VLT telescope of the European Southern Observatory in Chile. The atmospheric parameters and chemical composition have been determined for these stars. The averaged atmospheric parameters are: Teff = 6253 ± 30 K, log g = 2.15, Vt = 4.30 km s−1 for BG Cru; Teff = 5812 ± 22 K, log g = 1.65, Vt = 3.80 km s−1 for R Cru; and Teff = 5588 ± 21 K, log g = 1.70, Vt = 4.30 km s−1 for T Cru. All these Cepheids exhibit a nearly solar metallicity ([Fe/H] = +0.04 dex for BG Cru, +0.06 dex for R Cru, and +0.08 dex for T Cru); the carbon, oxygen, sodium, magnesium, and aluminum abundances suggest that the objects have already passed the first dredge-up. The abundances of other elements are nearly solar. An anomalous behavior of the absorption lines of metals (neutral atoms and ions) in the atmosphere of the small-amplitude Cepheid BG Cru is pointed out. The main components in these lines split up into additional blue and red analogs that are smaller in depth and equivalent width and vary with pulsation phase. Such splitting of the absorption lines of metals (with the hydrogen lines being invariable) is known for the classical Cepheid X Sgr. The calculated nonlinear pulsation model of BG Cru with the parameters L = 2000 L⊙, Teff = 6180 K, and M = 4.3M⊙ shows that this small-amplitude Cepheid pulsates in the first overtone and is close to the blue boundary of the Cepheid instability strip. According to the model, the extent of the Cepheid’s atmosphere is relatively small. Therefore, no spectroscopic manifestations of shock waves through variability are possible in this Cepheid and the observed blue and red components in metal absorption lines can be explained solely by the presence of an extended circumstellar envelope around BG Cru.

Spectroscopic studies of Cepheids in Circinus (AV Cir, BP Cir) and Triangulum Australe (R TrA, S TrA, U TrA, LR TrA)

Based on high-resolution spectra taken with the 1.9-m telescope of the South African Astronomical Observatory, we have determined the atmospheric parameters and chemical composition for three small-amplitude (AV Cir, BP Cir, and LR TrA), two classical (R TrA and S TrA), and one double-mode (U TrA) Cepheids. The averaged atmospheric parameters have been estimated for three Cepheids (AV Cir, BP Cir, and U TrA) observed at various pulsation phases. In all Cepheids, except U TrA, the metallicity has turned out to be higher than the solar one by 0.1–0.2 dex. The abundances of the key elements of the evolution of yellow supergiants (C, O, Na, Mg, Al) show that these objects have already passed the first dredge-up, while those of the remaining elements are nearly solar. Comparison of our results on the Cepheids from the list (except U TrA) with those of other authors shows significant differences in C and O abundance estimates for AV Cir, R TrA, S TrA, and LR TrA. For AV Cir and BP Cir, the Hα line profiles are symmetric but with a slight asymmetry in the core at approximately the same phase near 0·P 7: on the “blue” side for AV Cir and on the “red” one for BP Cir. BP Cir exhibits a distinct asymmetry in the absorption lines of neutral atoms and ions at various pulsation phases, which can be explained by nonradial first-overtone pulsations. The constancy of the Hα absorption line profiles with pulsation phase for AV Cir and BP Cir may suggest the presence of a hydrogen envelope around them. For the double-mode Cepheid U TrA, an asymmetry is observed in the cores of the Hα line and the absorption lines of neutral atoms and ions at various pulsation phases, which can be explained by nonradial pulsations in the Cepheid’s atmosphere. The absorption lines of neutral atoms and ions of metals in LR TrA closely resemble those in the well-known Cepheid BG Cru: secondary “blue” and “red” components whose line depths vary with pulsation phase are noticeable. This Cepheid can also pulsate in the first overtone and have an extended hydrogen envelope. Careful multiphase spectroscopic observations with a sufficiently high resolution are needed to test this assumption.

Spectroscopic studies of the small-amplitude Cepheid SU Cas

A new set of 16 high-resolution spectra for the small-amplitude Cepheid SU Cas obtained in 2007–2009 has allowed us to determine its atmospheric parameters (Teff = 6345 ± 30 K, log g = 2.40, Vt = 3.25 km s−1) and to measure its radial velocities. The latter were added to the general list of radial velocities (375 estimates) obtained in the last 90 years. Using a frequency analysis, we have refined the pulsation and orbital periods of the Cepheid. Apart from the well-known fundamental pulsation period , we have detected a possible secondary period of . Their ratio of 0.96 suggests the existence of nonradial pulsations in the Cepheid’s atmosphere. Based on photoelectric photometry in the last 60 years, we have shown that the effective temperature undergoes cyclic secular changes of ±200 K with an unknown period. The mean effective temperature Teff = 6395 ± 52 K estimated from photometric data agrees well with our estimate from spectroscopic data. The variations of the mean color index, effective temperature, and γ-velocity (in 90 years of observations) point to a possible orbital motion of the well-known hot companion with the most probable periods of , , and . The elemental abundances in the atmosphere of SU Cas confirm the conclusion that this Cepheid is a typical yellow supergiant after the first dredge-up. Our Teff estimate gives a radius of 32R⊙ and a distance of 455 pc for it, which is inconsistent with its membership in the open cluster Alessi 95. The question about the pulsation mode of SU Cas still remains open.

Spectroscopic studies of southern-hemisphere cepheids: WW Car, SX Car, UZ Car, UY Car, GX Car, HW Car, YZ Car

For seven faint southern Cepheids (WW Car, SX Car, UZ Car, UY Car, GX Car, HW Car, YZ Car), we have determined their atmospheric parameters and chemical composition for the first time based on ten high-resolution (R = 50 000) spectra taken with the 1.5-m Hexapod telescope at the Joint Observatory of the Northern Catholic University (Antofagasta, Chile) and the Ruhr University (Bochum, Germany). Six objects from the list demonstrate atmospheric parameters and chemical composition typical of Cepheids that have passed through the first dredge-up phase, while WW Car is probably an anomalous Cepheid. According to our preliminary estimates, it has an overabundance of CNO, a deficit of sodium and aluminum, and a slight deficit of magnesium, with iron and other elements being underabundant relative to the Sun.

Spectroscopic studies of yellow supergiants in the Cepheid instability strip

High-resolution spectra of nine yellow nonvariable supergiants (NVSs) located within the canonical Cepheid instability strip from Sandage and Tammann (1969) (α Aqr, ϵ Leo, μ Per, ω Gem, BD+60 2532, HD 172365, HD 187299, HD 190113, and HD 200102) were taken with the 1-m Zeiss and 6-m BTA telescopes at the Special Astrophysical Observatory of the Russian Academy of Sciences in the 1990s. These have been used to determine the atmospheric parameters, chemical composition, radial velocities, reddenings, luminosities, distances, and radii. The spectroscopic estimates of Teff and the luminosities determined from the Hipparcos parallaxes have shown eight of the nine program NVSs on the Teff−log(L/L⊙) diagram to be outside the canonical Cepheid instability strip. When the edges of the Cepheid instability strip from Bono et al. (2000) are used, out of the NVSs from the list on the diagram one is within the Cepheid instability strip but closer to the red edge, two are at the red edge, three are beyond the red edge, two are at the blue edge, and one is beyond the blue edge. The evolutionary masses of the objects have been estimated. The abundances of α-elements, r- and s-process elements for all program objects have turned out to be nearly solar. The СNO, Na, Mg, and Al abundance estimates have shown that eight of the nine NVSs from the list have already passed the first dredge-up. Judging by the abundances of the key elements and its position on the Teff−log(L/L⊙) diagram, the lithium-rich supergiant HD 172365 is at the post-main-sequence evolutionary stage of gravitational helium core contraction and moves toward the first crossing of the Cepheid instability strip. The star ϵ Leo should be assigned to bright supergiants, while HD 187299 and HD 190113 may have already passed the second dredge-up and move to the asymptotic branch.

Spectroscopic studies of the classical Cepheid ζ Gem: Analysis of the velocity field in the atmosphere and manifestation of the presence of a circumstellar envelope

Based on five high-resolution spectra in the range 5625–7525 ˚A taken in 1995 and covering the ascending branch of the light curve from minimum to maximum, we have performed spectroscopic studies of the classical Cepheid ζ Gem. The atmospheric parameters and chemical composition of the Cepheid have been refined. The abundances of the key elements of the evolution of yellow supergiants are typical for an object that has passed the first dredge-up: a C underabundance, N, Na, and Al overabundances, and nearly solar O and Mg abundances. We have estimated [Fe/H] = +0.01 dex; the abundances of the remaining elements are also nearly solar. The metal absorption lines in all spectra show a clear asymmetry and the formation of secondary blue (B1 and B2) and red (R1 and R2) components, just as for the Cepheid X Sgr. The Hα absorption line is also split into blue (B) and red (R) components with different depths changing with pulsation phase. To analyze the velocity field in the atmosphere of ζ Gem, we have estimated the radial velocities from specially selected (with clear signatures of the B1, B2, R1, and R2 components) absorption lines (neutral atoms and ions) of metals (38 lines) and the B and R components of the Hα line. Analysis of these estimates has shown that their scatter is from −22 to 36 km s−1 for all pulsation phases but does not exceed 35–40 km s−1 for each individual phase, while it does not exceed 22 km s−1 for the Hα line components. The radial velocity estimates for the metal lines and their B1 and B2 components have been found to depend on the depths, suggesting the presence of a velocity gradient in the atmosphere. No significant difference in velocities between the atoms and ions of the metal lines is observed, i.e., there is no significant inhomogeneity in the upper atmospheric layers of the Cepheid. Since the averaged radial velocity estimates for the cores of the metal lines and their B1 and B2 components change with pulsation phase and coincide with those for the B component of the Hα line, they are all formed in the Cepheid’s atmosphere. The formation and passage of a shock wave due to the κ-mechanism at work can be responsible for the stronger scatter of the B1 and B2 components in their velocities at phases after the Cepheid’s minimum radius. The averaged velocities of the R1 components also change with pulsation phase and differ only slightly from the remaining ones. On the other hand, the mean velocity estimate for the R component of the Hα line at all phases is +32.72 ± 2.50 km s−1 and differs significantly from the bulk of the velocities, suggesting the formation of this component in the envelope around the Cepheid. The unusual behavior of the mean velocities for the R2 components of the metal absorption lines can also point to their formation in the envelope and can be yet another indicator of its presence around ζ Gem.