1,203 research outputs found
High resolution elemental abundance analysis of the Hyades Supercluster
The existence of a kinematically defined moving group of stars centred at U =
-40, V = -17 km/s referred to as the Hyades Supercluster, has been suggested as
the debris of an originally large star forming event, with its core being the
present day Hyades open cluster. Using high-resolution UVES spectra, we present
elemental abundances for a range of alpha, Fe-peak and neutron-capture elements
for 26 proposed supercluster stars. Our results show that the sample stars
display a heterogeneous abundance distribution, with a clump around [Fe/H] =
+0.15. We also calculate stellar radial velocities and U,V,W space velocities.
Enforcing a strict chemical and kinematical membership criteria, we find 4
supercluster stars share the Hyades open cluster abundances and kinematics,
while many of the remaining stars fit the disc field kinematics and abundance
range. We discuss our findings in the context of the Hyades supercluster being
a dispersed star-forming remnant, a stellar stream of purely dynamical origin
or a result of several processes.Comment: accepted by MNRA
Systematic motions in the Galactic plane found in the Hipparcos Catalogue using Herschel's Method
Two motions in the galactic plane have been detected and characterized, based
on the determination of a common systematic component in Hipparcos catalogue
proper motions. The procedure is based only on positions, proper motions and
parallaxes, plus a special algorithm which is able to reveal systematic trends.
Our results come from two stellar samples. Sample 1 has 4566 stars and defines
a motion of apex (l,b)=(177.8,3.7)+/-(1.5,1.0) and space velocity V=27+/-1
km/s. Sample 2 has 4083 stars and defines a motion of apex
(l,b)=(5.4,-0.6)+/-(1.9,1.1) and space velocity V=32+/-2 km/s. Both groups are
distributed all over the sky and cover a large variety of spectral types, which
means that they do not belong to a specific stellar population. Herschel's
method is used to define the initial samples of stars and later to compute the
common space velocity. The intermediate process is based on the use of a
special algorithm to determine systematic components in the proper motions. As
an important contribution, this paper sets out a new way to study the
kinematics of the solar neighborhood, in the search for streams, associations,
clusters and any other space motion shared by a large number of stars, without
being restricted by the availability of radial velocities.Comment: 12 pages and 12 eps figures. LaTe
Resolving the pulsations of subdwarf B stars: HS 0039+4302, HS 0444+0458, and an examination of the group properties of resolved pulsators
We continue our program of single-site observations of pulsating subdwarf B
(sdB) stars and present the results of extensive time series photometry of HS
0039+4302 and HS 0444+0458. Both were observed at MDM Observatory during the
fall of 2005. We extend the number of known frequencies for HS 0039+4302 from 4
to 14 and discover one additional frequency for HS 0444+0458, bringing the
total to three. We perform standard tests to search for multiplet structure,
measure amplitude variations, and examine the frequency density to constrain
the mode degree .
Including the two stars in this paper, 23 pulsating sdB stars have received
follow-up observations designed to decipher their pulsation spectra. It is
worth an examination of what has been detected. We compare and contrast the
frequency content in terms of richness and range and the amplitudes with
regards to variability and diversity. We use this information to examine
observational correlations with the proposed pulsation mechanism as
well as alternative theories.Comment: 32 pages, 18 figures, 7 tables. Accepted for publication in MNRA
Climate change effects on human health: projections of temperature-related mortality for the UK during the 2020s, 2050s and 2080s
Background The most direct way in which climate change is expected to affect public health relates to changes in mortality rates associated with exposure to ambient temperature. Many countries worldwide experience annual heat-related and cold-related deaths associated with current weather patterns. Future changes in climate may alter such risks. Estimates of the likely future health impacts of such changes are needed to inform public health policy on climate change in the UK and elsewhere.
Methods Time-series regression analysis was used to characterise current temperature-mortality relationships by region and age group. These were then applied to the local climate and population projections to estimate temperature-related deaths for the UK by the 2020s, 2050s and 2080s. Greater variability in future temperatures as well as changes in mean levels was modelled.
Results A significantly raised risk of heat-related and cold-related mortality was observed in all regions. The elderly were most at risk. In the absence of any adaptation of the population, heat-related deaths would be expected to rise by around 257% by the 2050s from a current annual baseline of around 2000 deaths, and cold-related mortality would decline by 2% from a baseline of around 41 000 deaths. The cold burden remained higher than the heat burden in all periods. The increased number of future temperature-related deaths was partly driven by projected population growth and ageing.
Conclusions Health protection from hot weather will become increasingly necessary, and measures to reduce cold impacts will also remain important in the UK. The demographic changes expected this century mean that the health protection of the elderly will be vital
Chromospheric activity, lithium and radial velocities of single late-type stars possible members of young moving groups
We present here high resolution echelle spectra taken during three observing
runs of 14 single late-type stars identified in our previous studies (Montes et
al. 2001b, hereafter Paper I) as possible members of different young stellar
kinematic groups (Local Association (20 - 150 Myr), Ursa Major group (300 Myr),
Hyades supercluster (600 Myr), and IC 2391 supercluster (35 Myr)). Radial
velocities have been determined by cross correlation with radial velocity
standard stars and used together with precise measurements of proper motions
and parallaxes taken from Hipparcos and Tycho-2 Catalogues, to calculate
Galactic space motions (U, V, W) and to apply Eggen's kinematic criteria. The
chromospheric activity level of these stars have been analysed using the
information provided for several optical spectroscopic features (from the Ca II
H & K to Ca II IRT lines) that are formed at different heights in the
chromosphere. The Li I 6707.8 AA line equivalent width (EW) has been determined
and compared in the EW(Li I) versus spectral type diagram with the EW(Li I) of
stars members of well known young open clusters of different ages, in order to
obtain an age estimation. All these data allow us to analyse in more detail the
membership of these stars in the different young stellar kinematic groups.
Using both kinematic and spectroscopic criteria we have confirmed PW And, V368
Cep, V383 Lac, EP Eri, DX Leo, HD 77407, and EK Dra as members of the Local
Association and V834 Tau, pi^{1} UMa, and GJ 503.2 as members of the Ursa Major
group. A clear rotation-activity dependence has been found in these stars.Comment: Latex file with 19 pages, 7 figures tar'ed gzip'ed. Full postscript
(text, figures and tables) available at
http://www.ucm.es/info/Astrof/p_skg_stars_I_fv.ps.gz Accepted for publication
in: Astronomy & Astrophysics (A&A
The Masses Of The B-Stars In The High Galactic Latitude Eclipsing Binary IT Lib
A number of blue stars which appear to be similar to Population I B-stars in
the star forming regions of the galactic disk are found more than 1 kpc from
the galactic plane. Uncertainties about the true distances and masses of these
high latitude B-stars has fueled a debate as to their origin and evolutionary
status. The eclipsing binary IT Lib is composed of two B-stars, is
approximately one kiloparsec above the galactic plane, and is moving back
toward the plane. Observations of the light and velocity curves presented here
lead to the conclusion that the B-stars in this system are massive young
main-sequence stars. While there are several possible explanations, it appears
most plausible that the IT Lib system formed in the disk about 30 million years
ago and was ejected on a trajectory taking it to its present position.Comment: 26 pages, 3 figures, accepted for publication in the PASP (January
2003
A spectroscopy study of nearby late-type stars, possible members of stellar kinematic groups
Nearby late-type stars are excellent targets for seeking young objects in
stellar associations and moving groups. The origin of these structures is still
misunderstood, and lists of moving group members often change with time and
also from author to author. Most members of these groups have been identified
by means of kinematic criteria, leading to an important contamination of
previous lists by old field stars. We attempt to identify unambiguous moving
group members among a sample of nearby-late type stars by studying their
kinematics, lithium abundance, chromospheric activity, and other age-related
properties. High-resolution echelle spectra () of a sample of
nearby late-type stars are used to derive accurate radial velocities that are
combined with the precise Hipparcos parallaxes and proper motions to compute
galactic-spatial velocity components. Stars are classified as possible members
of the classical moving groups according to their kinematics. The spectra are
also used to study several age-related properties for young late-type stars,
i.e., the equivalent width of the lithium Li~{\sc i} \space 6707.8 \space \AA
\space line or the index. Additional information like X-ray
fluxes from the ROSAT All-Sky Survey or the presence of debris discs is also
taken into account. The different age estimators are compared and the moving
group membership of the kinematically selected candidates are discussed. From a
total list of 405 nearby stars, 102 have been classified as moving group
candidates according to their kinematics. i.e., only 25.2 \% of the
sample. The number reduces when age estimates are considered, and only 26
moving group candidates (25.5\% of the 102 candidates) have ages in agreement
with the star having the same age as an MG memberComment: 39 pages, 11 figures. Accepted for publication in Astronomy \&
Astrophysic
The Age of the Oldest Stars in the Local Galactic Disk From Hipparcos Parallaxes of G and K Subgiants
We review the history of the discovery of field subgiant stars and their
importance in the age dating of the Galactic disk. We use the cataloged data
from the Hipparcos satellite in this latter capacity. Based on Hipparcos
parallaxes accurate to 10% or better, the absolute magnitude of the lower
envelope of the nearly horizontal subgiant sequence for field stars in the H-R
diagram for B-V colors between 0.85 and 1.05 is measured to be M_V = 4.03 +/-
0.06. The age of the field stars in the solar neighborhood is found to be 7.9
+/- 0.7 Gyr by fitting the theoretical isochrones for [Fe/H] = +0.37 to the
lower envelope of the Hipparcos subgiants. The same grid of isochrones yields
ages, in turn, of 4.0 +/- 0.2 Gyr, 6.2 +/- 0.5 Gyr, and 7.5 to 10 Gyr for the
old Galactic clusters M67, NGC188, and NGC6791. The ages of both the Galactic
disk in the solar neighborhood and of NGC6791 are, nevertheless, likely between
3 and 5 Gyr younger than the oldest halo globular clusters, which have ages of
13.5 Gyr. The most significant results are (1) the supermetallicity of the
oldest local disk stars, and (2) the large age difference between the most
metal-poor component of the halo and the thick and thin disk in the solar
neighborhood. These facts are undoubtedly related and pose again the problem of
the proper scenario for the timing of events in the formation of the halo and
the Galactic disk in the solar neighborhood. [Abstract Abridged]Comment: 44 pages, 12 Figures; accepted for publication in PASP; high
resolution versions of Figures 1, 2, 6 and 9 available at
http://bubba.ucdavis.edu/~lubin/Sandage
Multiwavelength optical observations of chromospherically active binary systems V. FF UMa (2RE J0933+624): a system with orbital period variation
This is the fifth paper in a series aimed at studying the chromospheres of
active binary systems using several optical spectroscopic indicators to obtain
or improve orbital solution and fundamental stellar parameters. We present here
the study of FF UMa (2RE J0933+624), a recently discovered, X-ray/EUV selected,
active binary with strong H_alpha emission. The objectives of this work are, to
find orbital solutions and define stellar parameters from precise radial
velocities and carry out an extensive study of the optical indicators of
chromospheric activity. We obtained high resolution echelle spectroscopic
observations during five observing runs from 1998 to 2004. We found radial
velocities by cross correlation with radial velocity standard stars to achieve
the best orbital solution. We also measured rotational velocity by
cross-correlation techniques and have studied the kinematic by galactic space-
velocity components (U, V, W) and Eggen criteria. Finally, we have determined
the chromospheric contribution in optical spectroscopic indicators, from Ca II
H & K to Ca II IRT lines, using the spectral subtraction technique. We have
found that this system presents an orbital period variation, higher than
previously detected in other RS CVn systems. We determined an improved orbital
solution, finding a circular orbit with a period of 3.274 days. We derived the
stellar parameters, confirming the subgiant nature of the primary component and
obtained rotational velocities (vsini), of 33.57 km/s and 32.38 km/s for the
primary and secondary components respectively. From our kinematic study, we can
deduce its membership to the Castor moving group. Finally, the activity study
has given us a better understanding of the possible mechanisms that produce the
orbital period variation.Comment: Latex file with 16 pages, 18 figures. Available at
http://www.ucm.es/info/Astrof/invest/actividad/actividad_pub.html Accepted
for publication in: Astronomy & Astrophysics (A&A
Search for low instability strip variables in the young open cluster NGC 2516
In this paper we revise and complete the photometric survey of the
instability strip of the southern open cluster NGC 2516 published by Antonello
and Mantegazza (1986). No variable stars with amplitudes larger than
were found. However by means of an accurate analysis based on a new statistical
method two groups of small amplitude variables have been disentangled: one with
periods (probably Scuti stars) and one with periods
. The position in the HR diagram and the apparent time-scale may
suggest that the stars of the second group belong to a recently discovered new
class of variables, named Dor variables. They certainly deserve
further study. We also present a comparison between the results of the
photometric survey and the available pointed ROSAT observations of this
cluster.Comment: 7 pages, 2 ps figures. Accepted for P.A.S.
- …