247 research outputs found
Massive stellar systems: observational challenges and perspectives in the E-ELT era
We introduce the empirical framework concerning optical and near-infrared (NIR) photometry of crowded stellar fields. In particular, we address the impact that linear detectors and analytical PSF played in improving the accuracy and the precision of multi-band color-magnitude diagrams (CMDs). We focus our attention on recent findings based on deep NIR images collected with Adaptive Optics (AO) systems at the 8-10m class telescopes and discuss pros and cons of the different approaches. We also discuss the estimate of the absolute age of globular clusters using a well defined knee along the lower main sequence. We mention the role which the current AO-assisted instruments will have in addressing longstanding astrophysical problems of the Galactic center. Finally, we outline the role of first generation of E-ELT instruments upon photometry and spectroscopy of crowded stellar fields
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
Variable Stars in Local Group Galaxies. I: Tracing the Early Chemical Enrichment and Radial Gradients in the Sculptor dSph with RR Lyrae Stars
We identified and characterized the largest (536) RR Lyrae (RRL) sample in a
Milky Way dSph satellite (Sculptor) based on optical photometry data collected
over 24 years.
The RRLs display a spread in V-magnitude (0.35 mag) which appears
larger than photometric errors and the horizontal branch (HB) luminosity
evolution of a mono-metallic population. Using several calibrations of two
different reddening free and metal independent Period-Wesenheit relations we
provide a new distance estimate =19.62 mag (=0.04 mag) that
agrees well with literature estimates. We constrained the metallicity
distribution of the old population, using the Period-Luminosity relation,
and we found that it ranges from -2.3 to -1.5 dex. The current estimate is
narrower than suggested by low and intermediate spectroscopy of RGBs
([Fe/H] 1.5).
We also investigated the HB morphology as a function of the galactocentric
distance. The HB in the innermost regions is dominated by red HB stars and by
RRLs, consistent with a more metal-rich population, while in the outermost
regions it is dominated by blue HB stars and RRLs typical of a metal-poor
population. Our results suggest that fast chemical evolution occurred in
Sculptor, and that the radial gradients were in place at an early epoch.Comment: 10 pages, 5 figures, MNRAS accepte
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
Search for the elusive optical counterpart of PSR J0537-6910 with the HST Advanced Camera for Surveys
We present the results of deep, high-resolution, multi-band optical
observations of the field of the young (~ 5,000 yrs) 16 ms X-ray pulsar PSR
J05376910 performed with the Advanced Camera for Surveys (ACS) aboard the
Hubble Space Telescope (HST). Although a few new potential counterparts have
been detected within or close to the revised Chandra X-ray error circle (~1
arcsec) of the pulsar, only two of them (with magnitudes m_{814W} ~ 23.9 and
m_{814W} ~ 24.2) show indications of a peculiar spectrum which could be related
to optical emission from the pulsar. This might be true also for a third,
fainter, candidate detected only in one filter (with magnitude m_{814W} ~
26.7). If either of the two brighter candidates is indeed the actual
counterpart, the optical output of PSR J0537-6910 would make it similar to
young Crab-like pulsars. If not, it would mean that PSR J0537-6910 is
significantly underluminous with respect to all pulsars detected in the opticalComment: 7 pages, 5 figures, Astronomy & Astrophysics, in pres
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
Variable stars in Local Group Galaxies - II. Sculptor dSph
We present the identification of 634 variable stars in the Milky Way dSph
satellite Sculptor based on archival ground-based optical observations spanning
24 years and covering 2.5 deg. We employed the same
methodologies as the "Homogeneous Photometry" series published by Stetson. In
particular, we have identified and characterized one of the largest (536) RR
Lyrae samples so far in a Milky Way dSph satellite. We have also detected four
Anomalous Cepheids, 23 SX Phoenicis stars, five eclipsing binaries, three field
variable stars, three peculiar variable stars located above the horizontal
branch - near to the locus of BL Herculis - that we are unable to classify
properly. Additionally we identify 37 Long Period Variables plus 23 probable
variable stars, for which the current data do not allow us to determine the
period. We report positions and finding charts for all the variable stars, and
basic properties (period, amplitude, mean magnitude) and light curves for 574
of them. We discuss the properties of the RR Lyrae stars in the Bailey diagram,
which supports the coexistence of subpopulations with different chemical
compositions. We estimate the mean mass of Anomalous Cepheids
(1.5M) and SX Phoenicis stars (1M). We discuss
in detail the nature of the former. The connections between the properties of
the different families of variable stars are discussed in the context of the
star formation history of the Sculptor dSph galaxy.Comment: 22 pages, 17 figures, 13 tables. Accepted for publication on MNRA
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