252 research outputs found
A Photometric Study of the Outer Halo Globular Cluster NGC 5824
Multi-wavelength CCD photometry over 21 years has been used to produce deep
color-magnitude diagrams together with light curves for the variables in the
Galactic globular cluster NGC 5824. Twenty-one new cluster RR Lyrae stars are
identified, bringing the total to 47, of which 42 have reliable periods
determined for the first time. The color-magnitude diagram is matched using
BaSTI isochrones with age of ~Gyr. and reddening is found to be ; using the period-Wesenheit relation in two colors the distance
modulus is corresponding to a distance of 30.9 Kpc.
The observations show no signs of populations that are significantly younger
than the ~Gyr stars. The width of the red giant branch does not allow for a
spread in [Fe/H] greater than dex, and there is no photometric
evidence for widened or parallel sequences. The pseudo-color
magnitude diagram shows a bifurcation of the red giant branch that by analogy
with other clusters is interpreted as being due to differing spectral
signatures of the first (75\%) and second (25\%) generations of stars whose age
difference is close enough that main sequence turnoffs in the color-magnitude
diagram are unresolved. The cluster main sequence is visible against the
background out to a radial distance of arcmin. We conclude that NGC
5824 appears to be a classical Oosterhoff Type II globular cluster, without
overt signs of being a remnant of a now-disrupted dwarf galaxy.Comment: 26 pages, 15 figures, 4 tables, accepted for publication in
Astronomical Journa
Constraints on the Formation of the Globular Cluster IC 4499 from Multi-Wavelength Photometry
We present new multiband photometry for the Galactic globular cluster IC 4499
extending well past the main sequence turn-off in the U, B, V, R, I, and DDO51
bands. This photometry is used to determine that IC4499 has an age of 12 pm 1
Gyr and a cluster reddening of E(B-V) = 0.22 pm 0.02. Hence, IC 4499 is coeval
with the majority of Galactic GCs, in contrast to suggestions of a younger age.
The density profile of the cluster is observed to not flatten out to at least
r~800 arcsec, implying that either the tidal radius of this cluster is larger
than previously estimated, or that IC 4499 is surrounded by a halo. Unlike the
situation in some other, more massive, globular clusters, no anomalous color
spreads in the UV are detected among the red giant branch stars. The small
uncertainties in our photometry should allow the detection of such signatures
apparently associated with variations of light elements within the cluster,
suggesting that IC 4499 consists of a single stellar population.Comment: accepted to MNRA
On the density profile of the globular cluster M92
We present new number density and surface brightness profiles for the
globular cluster M92 (NGC 6341). These profiles are calculated from optical
images collected with the CCD mosaic camera MegaCam at the
Canada-France-Hawaii-Telescope and with the Advanced Camera for Surveys on the
Hubble Space Telescope. The ground-based data were supplemented with the Sloan
Digital Sky Survey photometric catalog. Special care was taken to discriminate
candidate cluster stars from field stars and to subtract the background
contamination from both profiles. By examining the contour levels of the number
density, we found that the stellar distribution becomes clumpy at radial
distances larger than about 13 arcminutes, and there is no preferred
orientation of contours in space. We performed detailed fits of King and Wilson
models to the observed profiles. The best-fit models underestimate the number
density inside the core radius. Wilson models better represent the
observations, in particular in the outermost cluster regions: the good global
agreement of these models with the observations suggests that there is no need
to introduce an extra-tidal halo to explain the radial distribution of stars at
large radial distances. The best-fit models for the number density and the
surface brightness profiles are different, even though they are based on the
same observations. Additional tests support the evidence that this fact
reflects the difference in the radial distribution of the stellar tracers that
determine the observed profiles (main sequence stars for the number density,
bright evolved stars for the surface brightness).Comment: 18 pages, 10 figures, Accepted by A
Impact of distance determinations on Galactic structure. II. Old tracers
Here we review the efforts of a number of recent results that use old tracers
to understand the build up of the Galaxy. Details that lead directly to using
these old tracers to measure distances are discussed. We concentrate on the
following: (1) the structure and evolution of the Galactic bulge and inner
Galaxy constrained from the dynamics of individual stars residing therein; (2)
the spatial structure of the old Galactic bulge through photometric
observations of RR Lyrae-type stars; (3) the three\--dimensional structure,
stellar density, mass, chemical composition, and age of the Milky Way bulge as
traced by its old stellar populations; (4) an overview of RR Lyrae stars known
in the ultra-faint dwarfs and their relation to the Galactic halo; and (5)
different approaches for estimating absolute and relative cluster ages.Comment: Review article, 80 pages (25 figures); Space Science Reviews, in
press (chapter of a special collection resulting from the May 2016 ISSI-BJ
workshop on Astronomical Distance Determination in the Space Age
Florix, an index to assess plant species in floodplains for nature conservation â Developed and tested along the river Danube
Natural floodplains are ecosystems with a diverse mosaic of habitats and site conditions, but also highly threatened due to anthropogenic pressures. Plant species occur in all habitat types and can indicate their value for nature conservation. To improve sustainable management of rivers and floodplains, several indices such as the River Ecosystem Service Index (RESI) have been developed. However, there are so far no assessment schemes for the entire range of floodplain plants. The common assessment approaches like biological integrity, achievement rates or threatened species (Red list), applying to other species groups or other ecosystems, are not appropriate in floodplains. Legal obligations and the need to restore floodplains clearly call for an index assessing the ecological value in a reference area which can be combined with a 5-scale assessment in accordance to established assessments like RESI or the Water Framework Directive.
Five typical characteristics describing vascular plantsâ adaptation to floodplain habitats were identified. These can be derived from published data sets available for all species in Germany. We checked these indicators for multicollinearity and selected three of them: species number, hydrodynamic indicators, nature conservation indicators. Species number highly correlate with habitat indicators and geographic occurrence. For the selected three indicators we determined thresholds to group habitats and their indicator rate to five classes (very low to very high value for nature conservation). These thresholds are valid for the river Danube and for the habitat types scrutinized in this study.
The Florix approach was sensitive in data sets testing active against former floodplains and protected against unprotected areas: For the entire reference region âDanube floodplainâ, Florix values were higher in the active floodplain and in the protected areas. Only the habitat type âwater bodiesâ showed better scores for habitats in the former floodplain, for âsoftwood forestsâ the status of being part of a protected area had no effect. Florix results were validated in two case studies differing in land use intensity. The region with dominant agricultural use showed significantly lower values than that with a higher portion of forests and grasslands.
Florix can be used for a floristic conservation status assessment at single habitat level or for the entity of a study region in comparison to a reference region. It allows to identify main pressures and to complement a habitat-type based evaluation. To achieve higher comparability, we should strive for a generalized monitoring in Europe like it is common in aquatic ecosystem monitoring
Alpha Enhancement and the Metallicity Distribution Function of Plaut's Window
We present Fe, Si, and Ca abundances for 61 giants in Plaut's Window
(l=-1,b=-8.5) and Fe abundances for an additional 31 giants in a second, nearby
field (l=0,b=-8) derived from high resolution (R~25,000) spectra obtained with
the Blanco 4m telescope and Hydra multifiber spectrograph. The median
metallicity of red giant branch (RGB) stars in the Plaut field is ~0.4 dex
lower than those in Baade's Window, and confirms the presence of an iron
abundance gradient along the bulge minor axis. The full metallicity range of
our (biased) RGB sample spans -1.5<[Fe/H]<+0.3, which is similar to that found
in other bulge fields. We also derive a photometric metallicity distribution
function for RGB stars in the (l=-1,b=-8.5) field and find very good agreement
with the spectroscopic metallicity distribution. The radial velocity and
dispersion data for the bulge RGB stars are in agreement with previous results
of the BRAVA survey, and we find evidence for a decreasing velocity dispersion
with increasing [Fe/H]. The [alpha/Fe] enhancement in Plaut field stars is
nearly identical to that observed in Baade's window, and suggests that an
[alpha/Fe] gradient does not exist between b=-4 and -8. Additionally, a subset
of our sample (23 stars) appear to be foreground red clump stars that are very
metal--rich, exhibit small metallicity and radial velocity dispersions, and are
enhanced in alpha elements. While these stars likely belong to the Galactic
inner disk population, they exhibit [alpha/Fe] ratios that are enhanced above
the thin and thick disk.Comment: Accepted for publication in ApJ. 38 pages, 11 figures, and 2 tables.
Requests for higher resolution figures and electronic versions of tables 1
and/or 2 in advance of publication may be sent to cijohnson[at]astro.ucla.ed
A colour-excess extinction map of the southern Galactic disc from the VVV and GLIMPSE surveys
An improved high-resolution and deep A Ks foreground dust extinction map is presented for the Galactic disc area within 295⊠⟠l ⟠350âŠ, â1.0⊠⟠b ⟠+1.0âŠ. At some longitudes the map reaches up to |b| ~ 2.25âŠ, for a total of ~148 deg 2. The map was constructed via the RayleighâJeans colour excess (RJCE) technique based on deep near-infrared (NIR) and mid-infrared (MIR) photometry. The new extinction map features a maximum bin size of 1 arcmin, and relies on NIR observations from the Two Micron All-Sky Survey (2MASS) and new data from ESOâs Vista Variables in the VĂa LĂĄctea (VVV) survey, in concert with MIR observations from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire. The VVV photometry penetrates ~4 mag fainter than 2MASS, and provides enhanced sampling of the underlying stellar populations in this heavily obscured region. Consequently, the new results supersede existing RJCE maps tied solely to brighter photometry, revealing a systematic underestimation of extinction in prior work that was based on shallower data. The new high-resolution and large-scale extinction map presented here is readily available to the community through a web query interface.Peer reviewe
Identification of globular cluster stars in RAVE data - I. Application to stellar parameter calibration
We present the identification of potential members of nearby Galactic globular clusters using radial velocities from the RAdial Velocity Experiment Data Release 4 (RAVE-DR4) survey data base. Our identifications are based on three globular clusters â NGC 3201, NGC 5139 (Ï Cen) and NGC 362 â all of which are shown to have âŁRV⣠> 100 km sâ»Âč. The high radial velocity of cluster members compared to the bulk of surrounding disc stars enables us to identify members using their measured radial velocities, supplemented by proper motion information and location relative to the tidal radius of each cluster. The identification of globular cluster stars in RAVE DR4 data offers a unique opportunity to test the precision and accuracy of the stellar parameters determined with the currently available Stellar Parameter Pipelines used in the survey, as globular clusters are ideal test-beds for the validation of stellar atmospheric parameters, abundances, distances and ages. For both NGC 3201 and Ï Cen, there is compelling evidence for numerous members (>10) in the RAVE data base; in the case of NGC 362 the evidence is more ambiguous, and there may be significant foreground and/or background contamination in our kinematically selected sample. A comparison of the RAVE-derived stellar parameters and abundances with published values for each cluster and with BASTI isochrones for ages and metallicities from the literature reveals overall good agreement, with the exception of the apparent underestimation of surface gravities for giants, in particular for the most metal-poor stars. Moreover, if the selected members are part of the main body of each cluster our results would also suggest that the distances from Binney et al., where only isochrones more metal rich than â0.9 dex were used, are typically underestimated by âŒ40âperâcent with respect to the published distances for the clusters, while the distances from Zwitter et al. show stars ranging from 1 to âŒ6.5 kpc â with indications of a trend towards higher distances at lower metallicities â for the three clusters analysed in this study
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