1,479 research outputs found
Arcfinder: An algorithm for the automatic detection of gravitational arcs
We present an efficient algorithm designed for and capable of detecting
elongated, thin features such as lines and curves in astronomical images, and
its application to the automatic detection of gravitational arcs. The algorithm
is sufficiently robust to detect such features even if their surface brightness
is near the pixel noise in the image, yet the amount of spurious detections is
low. The algorithm subdivides the image into a grid of overlapping cells which
are iteratively shifted towards a local centre of brightness in their immediate
neighbourhood. It then computes the ellipticity for each cell, and combines
cells with correlated ellipticities into objects. These are combined to graphs
in a next step, which are then further processed to determine properties of the
detected objects. We demonstrate the operation and the efficiency of the
algorithm applying it to HST images of galaxy clusters known to contain
gravitational arcs. The algorithm completes the analysis of an image with
3000x3000 pixels in about 4 seconds on an ordinary desktop PC. We discuss
further applications, the method's remaining problems and possible approaches
to their solution.Comment: 12 pages, 12 figure
On the central helium-burning variable stars of the LeoI dwarf spheroidal galaxy
We present a study of short period, central helium-burning variable stars in
the Local Group dwarf spheroidal galaxy LeoI, including 106 RR Lyrae stars and
51 Cepheids. So far, this is the largest sample of Cepheids and the largest
Cepheids to RR Lyrae ratio found in such a kind of galaxy. The comparison with
other Local Group dwarf spheroidals, Carina and Fornax, shows that the period
distribution of RR Lyrae stars is quite similar, suggesting similar properties
of the parent populations, whereas the Cepheid period distribution in LeoI
peaks at longer periods (P \sim 1.26d instead of ~0.5d) and spans over a
broader range, from 0.5 to 1.78d.
Evolutionary and pulsation predictions indicate, assuming a mean metallicity
peaked within -1.5<= [Fe/H]<=-1.3, that the current sample of LeoI Cepheids
traces a unique mix of Anomalous Cepheids (blue extent of the red--clump,
partially electron degenerate central helium-burning stars) and short-period
classical Cepheids (blue-loop, quiescent central helium-burning stars). Current
evolutionary prescriptions also indicate that the transition mass between the
two different groups of stars is MHeF \sim 2.1 Mo, and it is constant for stars
metal-poorer than [Fe/H]\sim-0.7. Finally, we briefly outline the different
implications of the current findings on the star formation history of LeoI.Comment: 5 Pages, 4 Figures, ApJ letter, accepte
High Frame-rate Imaging Based Photometry, Photometric Reduction of Data from Electron-multiplying Charge Coupled Devices (EMCCDs)
The EMCCD is a type of CCD that delivers fast readout times and negligible
readout noise, making it an ideal detector for high frame rate applications
which improve resolution, like lucky imaging or shift-and-add. This improvement
in resolution can potentially improve the photometry of faint stars in
extremely crowded fields significantly by alleviating crowding. Alleviating
crowding is a prerequisite for observing gravitational microlensing in main
sequence stars towards the galactic bulge. However, the photometric stability
of this device has not been assessed. The EMCCD has sources of noise not found
in conventional CCDs, and new methods for handling these must be developed.
We aim to investigate how the normal photometric reduction steps from
conventional CCDs should be adjusted to be applicable to EMCCD data. One
complication is that a bias frame cannot be obtained conventionally, as the
output from an EMCCD is not normally distributed. Also, the readout process
generates spurious charges in any CCD, but in EMCCD data, these charges are
visible as opposed to the conventional CCD. Furthermore we aim to eliminate the
photon waste associated with lucky imaging by combining this method with
shift-and-add.
A simple probabilistic model for the dark output of an EMCCD is developed.
Fitting this model with the expectation-maximization algorithm allows us to
estimate the bias, readout noise, amplification, and spurious charge rate per
pixel and thus correct for these phenomena. To investigate the stability of the
photometry, corrected frames of a crowded field are reduced with a PSF fitting
photometry package, where a lucky image is used as a reference.
We find that it is possible to develop an algorithm that elegantly reduces
EMCCD data and produces stable photometry at the 1% level in an extremely
crowded field.Comment: Submitted to Astronomy and Astrophysic
Optical and Near-Infrared UBVRIJHK Photometry for the RR Lyrae stars in the Nearby Globular Cluster M4 (NGC 6121)
We present optical and near-infrared UBVRIJHK photometry of stars in the
Galactic globular cluster M4 (NGC 6121) based upon a large corpus of
observations obtained mainly from public astronomical archives. We concentrate
on the RR Lyrae variable stars in the cluster, and make a particular effort to
accurately reidentify the previously discovered variables. We have also
discovered two new probable RR Lyrae variables in the M4 field: one of them by
its position on the sky and its photometric properties is a probable member of
the cluster, and the second is a probable background (bulge?) object. We
provide accurate equatorial coordinates for all 47 stars identified as RR
Lyraes, new photometric measurements for 46 of them, and new period estimates
for 45. We have also derived accurate positions and mean photometry for 34 more
stars previously identified as variable stars of other types, and for an
additional five non-RR Lyrae variable stars identified for the first time here.
We present optical and near-infrared color-magnitude diagrams for the cluster
and show the locations of the variable stars in them. We present the Bailey
(period-amplitude) diagrams and the period-frequency histogram for the RR Lyrae
stars in M4 and compare them to the corresponding diagrams for M5 (NGC 5904).
We conclude that the RR Lyrae populations in the two clusters are quite similar
in all the relevant properties that we have considered. The mean periods,
pulsation-mode ratios, and Bailey diagrams of these two clusters show support
for the recently proposed "Oosterhoff-neutral" classification.Comment: 33 pages, 16 figures, 7 table
The Chemical Inhomogeneity of Faint M13 Stars: Carbon and Nitrogen Abundances
Building upon earlier observations that demonstrate substantial star-to-star differences in the carbon abundances of M13 subgiants, we present new Keck LRIS spectra reaching more that 1.5 mag below the M13 main-sequence turnoff (to V ≈ 20). Our analysis reveals a distribution of C abundances similar to that found among the subgiants, implying little change in the compositions of the M13 stars at least through the main-sequence turnoff. We presume these differences to be the result of some process operating early in the cluster history. Additional spectra of previously studied bright M13 giants have been obtained with the 5 m Hale Telescope. A comparison of C abundances derived using the present methods and those from the literature yield a mean difference of 0.03 ± 0.14 dex for four stars in common with the 1996 study by Smith et al. and 0.14 ± 0.07 dex for stars also observed in Suntzeff's 1981 survey (if one extreme case is removed). We conclude that the lower surface C abundances of these luminous giants as compared with the subgiants and main-sequence stars are likely the result of mixing rather than a difference in our abundance scales. NH band strengths have also been measured for a handful of the most luminous M13 turnoff stars. While molecular band formation in such stars is weak, significant star-to-star NH band strength differences are present. Moreover, for the stars with both C and N measurements, differences between stars in these two elements appear to be anticorrelated. Finally, the most recent C and N abundances for main-sequence, main-sequence turnoff, and subgiant stars in 47 Tuc, M71, M5, and the present M13 data are compared
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
Probing the early chemical evolution of the Sculptor dSph with purely old stellar tracers
We present the metallicity distribution of a sample of 471 RR Lyrae (RRL)
stars in the Sculptor dSph, obtained from the -band Period-Luminosity
relation. It is the first time that the early chemical evolution of a dwarf
galaxy is characterized in such a detailed and quantitative way, using
photometric data alone. We find a broad metallicity distribution (FWHM=0.8 dex)
that is peaked at [Fe/H]-1.90 dex, in excellent agreement with
literature values obtained from spectroscopic data. Moreover, we are able to
directly trace the metallicity gradient out to a radius of 55 arcmin. We
find that in the outer regions (r32 arcmin) the slope of the metallicity
gradient from the RRLs (-0.025 dex arcmin) is comparable to the
literature values based on red giant (RG) stars. However, in the central part
of Sculptor we do not observe the latter gradients. This suggests that there is
a more metal-rich and/or younger population in Sculptor that does not produce
RRLs. This scenario is strengthened by the observation of a metal-rich peak in
the metallicity distribution of RG stars by other authors, which is not present
in the metallicity distribution of the RRLs within the same central area.Comment: 5 pages, 4 figures. Accepted for publication on MNRAS Letter
The Carina Project. VI. The helium burning variable stars
We present new optical (BVI) time-series data for the evolved variable stars
in the Carina dwarf spheroidal galaxy. The quality of the data and the
observing strategy allowed us to identify 14 new variable stars. Eight out of
the 14 are RR Lyrae (RRL) stars, four are Anomalous Cepheids (ACs) and two are
geometrical variables. Comparison of the period distribution for the entire
sample of RRLs with similar distributions in nearby dSphs and in the Large
Magellanic Cloud indicates that the old stellar populations in these systems
share similar properties. This finding is also supported by the RRL
distribution in the Bailey diagram. On the other hand, the period distribution
and the Bailey diagram of ACs display significant differences among the above
stellar systems. This evidence suggests that the properties of intermediate-age
stellar populations might be affected both by environmental effects and
structural parameters. We use the BV Period--Wesenheit (PW) relation of RRLs
together with evolutionary prescriptions and find a true distance modulus of
20.09+/-0.07(intrinsic)+/-0.1(statistical) mag that agrees quite well with
similar estimates available in the literature. We identified four peculiar
variables. Taking into account their position in the Bailey diagram and in the
BV PW relation, two of them (V14, V149) appear to be candidate ACs, while two
(V158, V182) might be peculiar RRLs. In particular, the variable V158 has a
period and a V-band amplitude very similar to the low-mass RRL
---RRLR-02792---recently identified by Pietrzynski at al. (2012) in the
Galactic bulge.Comment: Accepted for publication in ApJ, publication date September 20, 2013.
31 pages, 7 figure, 3 tables. Figs. 3 and 4 corrected, references added,
minor correction
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