Using Cepheids to determine the galactic abundance gradient

Abstract

Abstract.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 significant slope of about −0.1 dex 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 first 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.

PN and Galactic Chemical Evolution

“…Data on cepheid variables should in principle agree with the B star results, which is confirmed by a recent analysis by Caputo et al (2001), but a flatter gradient and/or a discontinuity around 10 kpc is suggested from the high resolution data by Andrievsky et al (2002).…”

Non-LTE abundances of magnesium, aluminum and sulfur in OB stars near the solar circle

“…3 include the recent cepheid results from Andrievsky et al (2002), shown as open circles, and H II regions from the optical emission lines analyzed by Deharveng et al (2000), shown as six-pointed stars, and H II region far-IR fine-structure lines by Simpson et al (1995) and Afflerbach et al (1997), shown as three-pointed stars.…”

The Oxygen Abundance in Our Galaxy

“…Thus, the value of the oxygen abundance at the solar galactocentric distance (12+logO/H∼8.6) derived by Andrievsky et al (2002) from the study of Cepheids and the value of the oxygen abundance (12+logO/H∼8.5) derived by Daflon et al (1999Daflon et al ( , 2001) from the study of OB stars are in agreement with the oxygen abundance in the solar vicinity region derived from the interstellar absorption towards stars and from the study of H ii regions. At the same time, there is an appreciable disagreement between values of the radial oxygen abundance gradient derived from the study of Cepheids (-0.022dex/kpc, Andrievsky et al 2002), from the study of early Btype main-sequence stars (-0.067dex/kpc, Rolleston et al 2000), and from the study of H ii regions (-0.048dex/kpc, this study).…”

“…At the same time, there is an appreciable disagreement between values of the radial oxygen abundance gradient derived from the study of Cepheids (-0.022dex/kpc, Andrievsky et al 2002), from the study of early Btype main-sequence stars (-0.067dex/kpc, Rolleston et al 2000), and from the study of H ii regions (-0.048dex/kpc, this study). This disagreement can be explained by two reasons.…”

“…the present-day "recommended" solar abundance is less (by more than 0.2 dex) than the value recommended several years ago (Anders & Grevesse 1989;Grevesse, Noels & Sauval 1996;Grevesse & Sauval 1998). With this recent solar oxygen abundance, the data of Andrievsky et al (2002) result in a 12+logO/H value of around 8.60 for the solar vicinity.…”