181 research outputs found
The Distribution of the Elements in the Galactic Disk III. A Reconsideration of Cepheids from l = 30 to 250 Degrees
This paper reports on the spectroscopic investigation of 238 Cepheids in the
northern sky. Of these stars, about 150 are new to the study of the galactic
abundance gradient. These new Cepheids bring the total number of Cepheids
involved in abundance distribution studies to over 400. In this work we also
consider systematics between various studies and also those which result from
the choice of models. We find systematic variations exist at the 0.06 dex level
both between studies and model atmospheres. In order to control the systematic
effects our final gradients depend only on abundances derived herein. A simple
linear fit to the Cepheid data from 398 stars yields a gradient d[Fe/H]/dRG =
-0.062 \pm 0.002 dex/kpc which is in good agreement with previously determined
values. We have also reexamined the region of the "metallicity island" of Luck
et al. (2006). With the doubling of the sample in that region and our
internally consistent abundances, we find there is scant evidence for a
distinct island. We also find in our sample the first reported Cepheid (V1033
Cyg) with a pronounced Li feature. The Li abundance is consistent with the star
being on its red-ward pass towards the first giant branch.Comment: 66 pages including tables, 12 figures, Accepted Astronomical Journa
Accurate Fundamental Parameters or A, F, and G-type Supergiants in the Solar Neighbourhood
The following parameters are determined for 63 Galactic supergiants in the
solar neighbourhood: effective temperature Teff, surface gravity log g, iron
abundance log e(Fe), microturbulent parameter Vt, mass M/Msun, age t and
distance d. A significant improvement in the accuracy of the determination of
log g and, all parameters dependent on it, is obtained through application of
van Leeuwens (2007) re-reduction of the Hipparcos parallaxes. The typical error
in the log g values is now +-0.06 dex for supergiants with distances d < 300 pc
and +-0.12 dex for supergiants with d between 300 and 700 pc; the mean error in
Teff for these stars is +-120 K. For supergiants with d > 700 pc parallaxes are
uncertain or unmeasurable, so typical errors in their log g values are 0.2-0.3
dex.
A new Teff scale for A5-G5 stars of luminosity classes Ib-II is presented.
Spectral subtypes and luminosity classes of several stars are corrected.
Combining the Teff and log g with evolutionary tracks, stellar masses and ages
are determined; a majority of the sample has masses between 4 Msun and 15 Msun
and, hence, their progenitors were early to middle B-type main sequence stars.
Using Fe ii lines, which are insensitive to departures from LTE, the
microturbulent parameter Vt and the iron abundance log e(Fe) are determined
from high-resolution spectra. The parameter Vt is correlated with gravity: Vt
increases with decreasing log g. The mean iron abundance for the 48 supergiants
with distances d < 700 pc is log e(Fe)=7.48+-0.09, a value close to the solar
value of 7.45+-0.05, and thus the local supergiants and the Sun have the same
metallicity.Comment: 12 pages, 9 figures. Will be published at MNRA
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