20 research outputs found
UV radiation below an Arctic vortex with severe ozone depletion
The erythemally weighted (UV) irradiance below the severely depleted Arctic vortices in spring 1996 and 1997 were substantially elevated. On average the UV increased 36 and 33% relative to the 1979-1981 mean assuming clear skies from day 80-100 in 1996 and 1997, respectively. On clear sky days large regions of the Arctic experienced maximum UV increases exceeding 70 and 50% on single days in 1996 and 1997, respectively. A minor fraction of these increases are not anthropogenic and have a dynamical origin as seen by comparison to 1982, when hardly any ozone depletion is expected
The age of the Galaxy's thick disk
We discuss the age of the stellar disks in the solar neighbourhood. After
reviewing the various methods for age dating we discuss current estimates of
the age of both the thin and the thick disk. We present preliminary results for
kinematically-selected stars that belong to the thin as well as the thick disk.
All of these dwarf and sub-giant stars have been studied spectroscopically and
we have derived both elemental abundances as well as ages for them. A general
conclusion is that in the solar neighbourhood, on average, the thick disk is
older than the thin disk. However, we caution that the exclusion of stars with
effective temperatures around 6500 K might result in a biased view on the full
age distribution for the stars in the thick disk.Comment: 6 pages, contribution to IAU Symposium 258: The Ages of Stars, held
in Baltimore, USA, October 13-17, 200
Tracing the vertical composition of disc galaxies through colour gradients
(Abbreviated) Optical observations of a statistically complete sample of
edge-on disc galaxies are used to study the intrinsic vertical colour gradients
in the galactic discs, to constrain the effects of population gradients,
residual dust extinction and gradients in the galaxies' metal abundance. It
appears that the intrinsic vertical colour gradients are either non-existent,
or small and relatively constant as a function of position along the galaxies'
major axes. Our results are consistent with the absence of any vertical colour
gradient in the discs of the early-type sample galaxies. In most galaxies
small-scale variations in the magnitude and even the direction of the vertical
gradient are observed: at larger galactocentric distances they generally
display redder colours with increasing z height, whereas the opposite is often
observed in and near the galactic centres. For a significant fraction of our
sample galaxies another mechanism in addition to the effects of stellar
population gradients is required to explain the magnitude of the observed
gradients. The non-zero colour gradients in a significant fraction of our
sample galaxies are likely (at least) partially due to residual dust extinction
at these z heights, as is also evidenced from the sometimes significant
differences between the vertical colour gradients measured on either side of
the galactic planes. We suggest that initial vertical metallicity gradients, if
any, have likely not been accentuated by accretion or merging events over the
lifetimes of our sample galaxies. On the other hand, they may have weakened any
existing vertical metallicity gradients, although they also may have left the
existing correlations unchanged.Comment: 17 pages LaTeX, incl. 5 embedded postscript figures, resubmitted to
MNRAS (referee's comments taken into account
The ability of intermediate-band Stromgren photometry to correctly identify dwarf, subgiant, and giant stars and provide stellar metallicities and surface gravities
[Abridged] Several large scale photometric and spectroscopic surveys are
being undertaken to provide a more detailed picture of the Milky Way. Given the
necessity of generalisation in the determination of, e.g., stellar parameters
when tens and hundred of thousands of stars are considered it remains important
to provide independent, detailed studies to verify the methods used in the
surveys. We evaluate available calibrations for deriving [M/H] from Stromgren
photometry and develop the standard sequences for dwarf stars to reflect their
metallicity dependence and test how well metallicities derived from ugriz
photometry reproduce metallicities derived from the well-tested system of
Stromgren photometry. We use a catalogue of dwarf stars with both Stromgren
uvby photometry and spectroscopically determined iron abundances (in total 451
dwarf stars with 0.3<(b-y)_0<1.0). We also evaluate available calibrations that
determine log g. A larger catalogue, in which metallicity is determined
directly from uvby photometry, is used to trace metallicity-dependent standard
sequences for dwarf stars. We derive new standard sequences in the c_1,0 versus
(b-y)_0 plane and in the c_1,0 versus (v-y)_0 plane for dwarf stars with 0.40 <
(b-y)_0 < 0.95 and 1.10 < (v-y)_0 < 2.38. We recommend the calibrations by
Ramirez & Me'endez (2005) for deriving metallicities from Stromgren photometry
and find that intermediate band photometry, such as Stromgren photometry, more
accurately than broad band photometry reproduces spectroscopically determined
[Fe/H]. Stromgren photometry is also better at differentiating between dwarf
and giant stars. We conclude that additional investigations of the differences
between metallicities derived from ugriz photometry and intermediate-band
photometry, such as Stromgren photometry, are required.Comment: Accepted for publication in A&A, 34 pages, including on-line materia
The poor cluster of galaxies S639
The relation known as the Fundamental Plane (FP) may be used for determination of relative distances to E and S0 galaxies (e.g., Dressler et al. 1987; Jørgensen, Franx &