R. E. Luck

Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood

A number of studies of abundance gradients in the galactic disk have been performed in recent years. The results obtained are rather disparate: from no detectable gradient to a rather signicant slope of about 0: 1d ex kpc 1 . The present study concerns the abundance gradient based on the spectroscopic analysis of a sample of classical Cepheids. These stars enable one to obtain reliable abundances of a variety of chemical elements. Additionally, they have well determined distances which allow an accurate determination of abundance distributions in the galactic disc. Using 236 high resolution spectra of 77 galactic Cepheids, the radial elemental distribution in the galactic disc between galactocentric distances in the range 6{11 kpc has been investigated. Gradients for 25 chemical elements (from carbon to gadolinium) are derived. The following results were obtained in this study. Almost all investigated elements show rather flat abundance distributions in the middle part of galactic disc. Typical values for iron-group elements lie within an interval from 0:02 to 0:04 dex kpc 1 (in particular, for iron we obtained d(Fe/H)/dRG = 0:029 dex kpc 1 ). Similar gradients were also obtained for O, Mg, Al, Si, and Ca. For sulphur we have found a steeper gradient ( 0:05 dex kpc 1 ). For elements from Zr to Gd we obtained (within the error bars) a near to zero gradient value. This result is reported for the rst time. Those elements whose abundance is not expected to be altered during the early stellar evolution (e.g. the iron-group elements) show at the solar galactocentric distance (El/H) values which are essentially solar. Therefore, there is no apparent reason to consider our Sun as a metal-rich star. The gradient values obtained in the present study indicate that the radial abundance distribution within 6{11 kpc is quite homogeneous, and this result favors a galactic model including a bar structure which may induce radial flows in the disc, and thus may be responsible for abundance homogenization.

Using Cepheids to determine the galactic abundance gradient II. Towards the galactic center

Based on spectra obtained at the Anglo-Australian Observatory, we present a discussion of the metallic- ity of the galactic disc derived using Cepheids at galactocentric distances of 4{6 kpc. Our new results together with previous gradient determination (Paper I) show that the overall abundance distribution within the galactocentric distances 4{11 kpc cannot be represented by a single gradient value. The distribution is more likely bimodal: it is flatter in the solar neighbourhood with a small gradient, and steepens towards the galactic center. The steepening begins at a distance of about 6.6 kpc.

The Distribution of the Elements in the Galactic Disk

This paper reports on the spectroscopic investigation of 54 Cepheids, deriving parameters and abundances. These Cepheids extend previous samples by about 35% in number and increase the amount of the Galactic disk coverage, especially in the direction of l ≈ 120°. We find that there exists in the Galactic disk at that longitude and at a solar distance of about 3-4 kpc a region that has enhanced abundances, Fe/H] ≈ +0.2, with respect to the local region. A simple linear fit to all Cepheid data now extant yields a gradient d[Fe/H]/dRG = -0.068 ± 0.003 dex kpc-1. After consideration of the spatial abundance inhomogeneities in the sample, we conclude that the best current estimate of the overall gradient is d[Fe/H]/dRG = -0.06 dex kpc-1.

THE DISTRIBUTION OF THE ELEMENTS IN THE GALACTIC DISK. II. AZIMUTHAL AND RADIAL VARIATION IN ABUNDANCES FROM CEPHEIDS

This paper reports on the spectroscopic investigation of 101 Cepheids in the Carina region. These Cepheids extend previous samples by about 35% in number and increase the amount of the galactic disk coverage especially in the direction of l \approx 270{\deg}. The new Cepheids do not add much information to the radial gradient, but provide a substantial increase in azimuthal coverage. We find no azimuthal dependence in abundance over an 80{\deg} angle from the galactic center in an annulus of 1 kpc depth centered on the Sun. A simple linear fit to the Cepheid data yields a gradient d[Fe/H]/dRG = -0.055 \pm 0.003 dex/kpc which is somewhat shallower than found from our previous, smaller Cepheid sample.

Phase-dependent Variation of the Fundamental Parameters of Cepheids. III. Periods between 3 and 6 Days

We present the results of a detailed multiphase spectroscopic analysis of six classical Cepheids with pulsation periods between 3 and 6 days. For each star we have derived phased values of effective temperature, surface gravity, microturbulent velocity, and elemental abundances. We show that the elemental abundance results for these Cepheids are consistent for all pulsational phases.

Phase-dependent Variation of the Fundamental Parameters of Cepheids. II. Periods Longer than 10 Days

We present the results of a detailed multiphase spectroscopic analysis of 14 classical Cepheids with pulsation periods longer than 10 days. For each star, we have derived phased values of effective temperature, surface gravity, microturbulent velocity, and elemental abundances. We show that the elemental abundance results for these Cepheids are consistent for all pulsational phases.

PHASE-DEPENDENT VARIATION OF THE FUNDAMENTAL PARAMETERS OF CEPHEIDS. IV. s-CEPHEIDS

In this fourth of a series of papers presenting the results of a detailed multiphase spectroscopic analysis of classical Cepheids, we report on fundamental parameter variation in a group of so-called s-Cepheids. For each star, we have derived phased values of effective temperature, surface gravity, microturbulent velocity, and elemental abundances. Dependences between these parameters, amplitudes of their variation, and pulsational period are discussed in a framework of such dependences obtained in our previous works for 30 classical Cepheids. Our temperature data reveal that five of the s-Cepheids have mean temperatures that deviate substantially from the remainder of the Cepheid data. The temperature deviations place these s-Cepheids near the edges of the fundamental instability strip. While these deviations cannot differentiate between fundamental and overtone pulsation modes, the deviations are undoubtedly related to the root cause of the small amplitudes found in these stars.

Oxygen abundance distribution in the Galactic disc

Weperformedanon-localthermodynamicequilibrium(NLTE)analysisoftheinfraredoxygen tripletforalargenumberofCepheidspectraobtainedwiththeHobby–EberlyTelescope.These data were combined with our previous NLTE results for stars observed with the Max Planck Gesellschaft Telescope with the aim of investigating the oxygen abundance distribution in the Galactic thin disc. We found the slope of the radial (O/H) distribution to be equal −0.058 dexkpc −1 . However, we found some evidence that the distribution might become flatter in the outer parts of the disc. This is supported by the results of other authors who have studied open clusters, planetary nebulae and H II regions. Some mechanisms of flattening are discussed.

Barium abundances in Cepheids

We derived the barium atmospheric abundances for a large sample of Cepheids, comprising 270 stars. The sample covers a large range of Galactocentric distances, from about 4 to 15 kpc, so it is appropriate to investigate the existence of radial barium abundance gradients in the Galactic disc. In fact, this is the first time that such a comprehensive analysis of the distribution of barium abundances in the Galactic disc has been carried out. As a result, we conclude that the Ba abundance distribution can be characterized by a zero gradient. This result is compared with derived gradients for other elements, and some reasons are briefly discussed for the independence of the barium abundances from Galactocentric distances.

Line profile variations in classical Cepheids Evidence for non-radial pulsations? ?

We have investigated line profiles in a large sample of Cepheid spectra, and found four stars that show unusual (for Cepheids) line profile structure (bumps or/and asymmetries). The profiles can be phase dependent but the behavior persists over many cycles. The asymmetries are unlikely to be due to the spectroscopic binarity of these stars or the specific velocity field in their atmospheres caused by shock waves. As a preliminary hypothesis, we suggest that the observed features on the line profiles in the spectra of X Sgr, V1334 Cyg, EV Sct and BG Cru can be caused by the non-radial oscillations. It is possible that these non-radial oscillations are connected to resonances between the radial modes (3: d 2, 7: 5o r 10: 0).