86 research outputs found
Towards a photometric metallicity scale for open clusters
Open clusters are a useful tool when investigating several topics connected
with stellar evolution; for example the age or distance can be more accurately
determined than for field stars. However, one important parameter, the
metallicity, is only known for a marginal percentage of open clusters. We aim
at a consistent set of parameters for the open clusters investigated in our
photometric Delta-a survey of chemically peculiar stars. Special attention is
paid to expanding our knowledge of cluster metallicities and verifying their
scale. Making use of a previously developed method based on normalised
evolutionary grids and photometric data, the distance, age, reddening, and
metallicity of open clusters were derived. To transform photometric
measurements into effective temperatures to use as input for our method, a set
of temperature calibrations for the most commonly used colour indices and
photometric systems was compiled. We analysed 58 open clusters in total. Our
derived metallicity values were in excellent agreement with about 30
spectroscopically studied targets. The mean value of the absolute deviations
was found to be 0.03 dex, with no noticeable offset or gradient. The method was
also applied using recent evolutionary models based on the currently accepted
lower solar abundance value Z=0.014. No significant differences were found
compared to grids using the former adopted solar value Z=0.02. Furthermore,
some divergent photometric datasets were identified and discussed. The method
provides an accurate way of obtaining properly scaled metallicity values for
open clusters. In light of present and future homogeneous photometric sky
surveys, the sample of stellar clusters can be extended to the outskirts of the
Milky Way, where spectroscopic studies are almost impossible. This will help
for determining galactic metallicity gradients in more detail.Comment: Accepted for publication in A&A (19 pages including online material
A comparative study on the reliability of open cluster parameters
Context. Open clusters are known as excellent tracers of the structure and chemical evolution of the Galactic disk, however, the accuracy and reliability of open cluster parameters is poorly known.
Aims: In recent years, several studies aimed to present homogeneous open cluster parameter compilations, which are based on some different approaches and photometric data. These catalogues are excellent sources to facilitate testing of the actual accuracy of open cluster parameters.
Methods: We compare seven cluster parameter compilations statistically and with an external sample, which comprises the mean results of individual studies. Furthermore, we selected the objects IC 4651, NGC 2158, NGC 2383, NGC 2489, NGC 2627, NGC 6603, and Trumpler 14, with the main aim to highlight differences in the fitting solutions.
Results: We derived correction terms for each cluster parameter, using the external calibration sample. Most results by the compilations are reasonable scaled, but there are trends or constant offsets of different degree. We also identified one data set, which appears too erroneous to allow adjustments. After the correction, the mean intrinsic errors amount to about 0.2 dex for the age, 0.08 mag for the reddening, and 0.35 mag for the distance modulus. However, there is no study that characterises the cluster morphologies of all test cases in a correct and consistent manner. Furthermore, we found that the largest compilations probably include at least 20 percent of problematic objects, for which the parameters differ significantly. These could be among others doubtful or unlikely open clusters that do not facilitate an unambiguous fitting solution
On the metallicity of open clusters. III. Homogenised sample
Open clusters are known as excellent tools for various topics in Galactic
research. For example, they allow accurately tracing the chemical structure of
the Galactic disc. However, the metallicity is known only for a rather low
percentage of the open cluster population, and these values are based on a
variety of methods and data. Therefore, a large and homogeneous sample is
highly desirable. In the third part of our series we compile a large sample of
homogenised open cluster metallicities using a wide variety of different
sources. These data and a sample of Cepheids are used to investigate the radial
metallicity gradient, age effects, and to test current models. We used
photometric and spectroscopic data to derive cluster metallicities. The
different sources were checked and tested for possible offsets and
correlations. In total, metallicities for 172 open cluster were derived. We
used the spectroscopic data of 100 objects for a study of the radial
metallicity distribution and the age-metallicity relation. We found a possible
increase of metallicity with age, which, if confirmed, would provide
observational evidence for radial migration. Although a statistical
significance is given, more studies are certainly needed to exclude selection
effects, for example. The comparison of open clusters and Cepheids with recent
Galactic models agrees well in general. However, the models do not reproduce
the flat gradient of the open clusters in the outer disc. Thus, the effect of
radial migration is either underestimated in the models, or an additional
mechanism is at work. [abridged]Comment: Accepted for publication in Astronomy and Astrophysics: 18 pages, 10
figures, 4 table
The Hvar survey for roAp stars: II. Final results (Research Note)
The 60 known rapidly oscillating Ap (roAp) stars are excellent laboratories
to test pulsation models in the presence of stellar magnetic fields. Our survey
is dedicated to search for new group members in the Northern Hemisphere. We
attempt to increase the number of known chemically peculiar stars that are
known to be pulsationally unstable. About 40 h of new CCD photometric data of
21 roAp candidates, observed at the 1m Austrian-Croatian Telescope (Hvar
Observatory) are presented. We carefully analysed these to search for
pulsations in the frequency range of up to 10mHz. No new roAp star was detected
among the observed targets. The distribution of the upper limits for roAp-like
variations is similar to that of previoius similar efforts using
photomultipliers and comparable telescope sizes. In addition to photometric
observations, we need to consolidate spectroscopic information to select
suitable targets.Comment: 8 pages, 5 figures, accepted by Astronomy & Astrophysic
A probable pre-main sequence chemically peculiar star in the open cluster Stock 16
We used the Ultraviolet and Visual Echelle Spectrograph of the ESO-Very Large
Telescope to obtain a high resolution and high signal-to-noise ratio spectrum
of Stock 16-12, an early-type star which previous Delta-a photometric
observations suggest being a chemically peculiar (CP) star. We used spectral
synthesis to perform a detailed abundance analysis obtaining an effective
temperature of 8400 +/- 400 K, a surface gravity of 4.1 +/- 0.4, a
microturbulence velocity of 3.4 +0.7/-0.3 km/s, and a projected rotational
velocity of 68 +/- 4 km/s. We provide photometric and spectroscopic evidence
showing the star is most likely a member of the young Stock 16 open cluster
(age 3-8 Myr). The probable cluster membership, the star's position in the
Hertzsprung-Russell diagram, and the found infrared excess strongly suggest the
star is still in the pre-main-sequence (PMS) phase. We used PMS evolutionary
tracks to determine the stellar mass, which ranges between 1.95 and 2.3 Msun,
depending upon the adopted spectroscopic or photometric data results.
Similarly, we obtained a stellar age ranging between 4 and 6 Myr, in agreement
with that of the cluster. Because the star's chemical abundance pattern
resembles well that known of main sequence CP metallic line (Am) stars, the
object sets important constraints to the diffusion theory. Additional
spectroscopic and spectropolarimetric data allowed us to conclude that the
object is probably a single non-magnetic star.Comment: Accepted for publication in MNRAS; 8 pages, 5 figures, 1 tabl
CCD photometric search for peculiar stars in open clusters VIII. King 21, NGC 3293, NGC 5999, NGC 6802, NGC 6830, Ruprecht 44, Ruprecht 115, and Ruprecht 120
Context. We continue our survey of magnetic chemically peculiar (CP2) stars in galactic open clusters to shed more light on their origin and evolution. Aims. To study the group of CP2 stars, it is essential to find these objects in different galactic environments and at a wide range of evolutionary stages. The knowledge of open cluster ages and metallicities can help for finding a correlation between these parameters and the (non-)presence of peculiarities, which has to be taken into account in stellar evolution models. Methods. The intermediate band Δa photometric system samples the depth of the 5200 Å flux depression by comparing the flux at the centre with the adjacent regions with bandwidths of 110 A to 230 A. It is capable of detecting magnetic CP2 and CP4 stars with high efficiency, but also the groups of (metal-weak) λBootis and classical Be/shell stars can be successfully investigated. In addition, it allows the age, reddening, and distance modulus to be determined with appropriate accuracy by fitting isochrones. Results. From the 1677 observed members of the eight open clusters, one Ae and twenty-five CP2 stars were identified. Furthermore nineteen deviating stars are designated as questionable for several reasons. The estimated age, reddening, and distance for the programme clusters were compared with published values of the literature and discussed in this context. Conclusions. The current paper shows that CP2 stars are continuously present in very young (7 Myr) to intermediate age (500 Myr) open clusters at distances greater than 2 kpc from the Sun. © ESO 2007.Fil: Netopil, M.. Universidad de Viena; AustriaFil: Paunzen, E.. Universidad de Viena; AustriaFil: Maitzen, H.M.. Universidad de Viena; AustriaFil: Pintado, Olga Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Complejo Astronómico "El Leoncito". Universidad Nacional de Córdoba. Complejo Astronómico "El Leoncito". Universidad Nacional de la Plata. Complejo Astronómico "El Leoncito". Universidad Nacional de San Juan. Complejo Astronómico "El Leoncito"; ArgentinaFil: Claret, A.. Instituto de Astrofísica de Andalucía - Csic;Fil: Miranda, L.F.. Instituto de Astrofísica de Andalucía - Csic;Fil: Hiev, I.Kh.. National Astronomical Observatory Rozhen; BulgariaFil: Casanova, V.. Instituto de Astrofísica de Andalucía - Csic
A search for peculiar stars in the open cluster Hogg 16
The study of chemically peculiar (CP) stars in open clusters provides
valuable information about their evolutionary status. Their detection can be
performed using the photometric system, which maps a characteristic
flux depression at \AA. This paper aims at studying the
occurrence of CP stars in the earliest stages of evolution of a stellar
population by applying this technique to Hogg 16, a very young Galactic open
cluster (about 25 Myr). We identified several peculiar candidates: two B-type
stars with a negative index (CD-60 4701, CPD-60 4706) are likely
emission-line (Be) stars, even though spectral measurements are necessary for a
proper classification of the second one; a third object (CD-60 4703),
identified as a Be candidate in literature, appears to be a background B-type
supergiant with no significant index, which does not rule out the
possibility that it is indeed peculiar as the normality line of for
supergiants has not been studied in detail yet. A fourth object (CD-60 4699)
appears to be a magnetic CP star of 8 M, but obtained spectral data
seem to rule out this hypothesis. Three more magnetic CP star candidates are
found in the domain of early F-type stars. One is a probable nonmember and
close to the border of significance, but the other two are probably pre-main
sequence cluster objects. This is very promising, as it can lead to very strong
constraints to the diffusion theory. Finally, we derived the fundamental
parameters of Hogg 16 and provide for the first time an estimate of its metal
content.Comment: Accepted for publication in New Astronomy. 9 pages, 4 figures, and 4
tables. A complete data table is available via CDS or upon request from the
corresponding autho
Photoelectric search for peculiar stars in open clusters XV. Feinstein 1, NGC 2168, NGC 2323, NGC 2437, NGC 2547, NGC 4103, NGC 6025, NGC 6633, Stock 2, and Trumpler 2
The chemically peculiar (CP) stars of the upper main sequence are mainly characterized by strong overabundances of heavy elements. Two subgroups (CP2 and CP4) have strong local magnetic fields which make them interesting targets for astrophysical studies. This star group, in general, is often used for the analysis of stellar formation and evolution in the context of diffusion as well as meridional circulation. In continuation of a long term study of CP stars (initiated in the 1980s), we present new results based on photoelectric measurements for ten open clusters that are, with one exception, younger than 235 Myr. Observations in star clusters are favourable because they represent samples of stars of constant age and homogeneous chemical composition. The very efficient tool of Δa photometry was applied. It samples the flux depression at 5200 Å typically for CP stars. In addition, it is able to trace emission line Be/Ae and lambda Bootis stars. Virtually all CP2 and CP4 stars can be detected via this tool, and it has been successfully applied even in the Large Magellanic Cloud. For all targets in the cluster areas, we performed a kinematic membership analysis. We obtained new photoelectric Δa photometry of 304 stars from which 207 objects have a membership probability higher than 50%. Our search for chemically peculiar objects results in fifteen detections. The stars have masses between 1.7 Msun and 7.7 Msun and are between the zero- and terminal-age-main-sequence. We discuss the published spectral classifications in the light of our delta-a photometry and identify several misclassified CP stars. We are also able to establish and support the nature of known bona fide CP candidates. It is vital to use kinematic data for the membership determination and also to compare published spectral types with other data, such as Δa photometry. There are no doubts about the accuracy of photoelectric measurements, especially for stars brighter than 10th magnitude. The new and confirmed CP stars are interesting targets for spectroscopic follow-up observations to put constraints on the formation and evolution of CP stars
Search for variability of five central stars of planetary nebulae
The central stars of planetary nebulae are interesting objects
which are important to understand the very late stages of stellar
evolution. They exhibit variability due to binarity, stellar winds,
and pulsation. We searched for variability in five of
such objects and found only upper limits. However, also this null
result is important for the understanding of these stars
CCD photometric search for peculiar stars in open clusters. VII. Berkeley 11, Berkeley 94, Haffner 15, Lynga 1, NGC 6031, NGC 6405, NGC 6834 and Ruprecht 130
The detection of magnetic chemically peculiar (CP2) stars in open clusters of
the Milky Way can be used to study the influence of different galactic
environments on the (non-)presence of peculiarities, which has to be taken into
account in stellar evolution models. Furthermore it is still unknown if the CP2
phenomenon evolves, i.e. does the strength of the peculiarity feature at 5200A,
increase or decrease with age. We have observed eight young to intermediate age
open clusters in the Delta a photometric system. This intermediate band
photometric system samples the depth of the 5200A, flux depression by comparing
the flux at the center with the adjacent regions having bandwidths of 110A, to
230A. The Delta a photometric system is most suitable to detect CP2 stars with
high efficiency, but is also capable of detecting a small percentage of
non-magnetic CP objects. Also, the groups of (metal-weak) lambda Bootis, as
well as classical Be/shell stars, can be successfully investigated. This
photometric system allows one to determine the age, reddening and distance
modulus by fitting isochrones. Among the presented sample of eight galactic
clusters, we have detected twenty three CP2, eight Be/Ae and eight metal-weak
stars. Another six objects show a peculiar behaviour which is most probably due
to a non-membership,variability or duplicity. Fitting isochrones to Delta a
photometry yields estimates of the age, reddening and distance that are in
excellent agreement with published values
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