22 research outputs found
New high-sensitivity, milliarcsecond resolution results from routine observations of lunar occultations at the ESO VLT
(Abridged) Lunar occultations (LO) are a very efficient and powerful
technique, that achieves the best combination of high angular resolution and
sensitivity possible today at near-infrared wavelengths. Given that the events
are fixed in time, that the sources are occulted randomly, and that the
telescope use is minimal, the technique is very well suited for service mode
observations. We have established a program of routine LO observations at the
VLT observatory, especially designed to take advantage of short breaks
available in-between other programs. We have used the ISAAC instrument in burst
mode, capable of producing continuous read-outs at millisecond rates on a
suitable subwindow. Given the random nature of the source selection, our aim
has been primarily the investigation of a large number of stellar sources at
the highest angular resolution in order to detect new binaries. Serendipitous
results such as resolved sources and detection of circumstellar components were
also anticipated. We have recorded the signal from background stars for a few
seconds, around the predicted time of occultation by the Moon's dark limb. At
millisecond time resolution, a characteristic diffraction pattern can be
observed. Patterns for two or more sources superimpose linearly, and this
property is used for the detection of binary stars. The detailed analysis of
the diffraction fringes can be used to measure specific properties such as the
stellar angular size and the presence of extended light sources such as a
circumstellar shell. We present a list of 191 stars for which LO data could be
recorded and analyzed. Results include the detection of 16 binary and 2 triple
stars, all but one of which were previously unknown. The projected angular
separations are as small as 4 milliarcseconds and magnitude differences as high
as ?K=5.8 mag...Comment: 10 pages, 3 figures, to be published in A&
HST-NICMOS Observations of Terzan 5: Stellar Content and Structure of the Core
We report results from HST-NICMOS imaging of the extremely dense core of the
globular cluster Terzan 5. This highly obscured bulge cluster has been
estimated to have one of the highest collision rates of any galactic globular
cluster, making its core a particularly conducive environment for the
production of interacting binary systems. We have reconstructed high-resolution
images of the central 19"x19" region of Terzan 5 by application of the drizzle
algorithm to dithered NIC2 images in the F110W, F187W, and F187N near-infrared
filters. We have used a DAOPHOT/ALLSTAR analysis of these images to produce the
deepest color-magnitude diagram (CMD) yet obtained for the core of Terzan 5. We
have also analyzed the parallel 11"X11" NIC1 field, centered 30" from the
cluster center and imaged in F110W and F160W, and an additional NIC2 field that
is immediately adjacent to the central field. This imaging results in a clean
detection of the red-giant branch and horizontal branch in the central NIC2
field, and the detection of these plus the main-sequence turnoff and the upper
main sequence in the NIC1 field. We have constructed an H versus J-H CMD for
the NIC1 field. We obtain a new distance estimate of 8.7 kpc, which places
Terzan 5 within less than 1 kpc of the galactic center. We have also determined
a central surface-density profile which results in a maximum likelihood
estimate of 7.9" +/- 0.6" for the cluster core radius. We discuss the
implications of these results for the dynamical state of Terzan 5.Comment: 17 pages, 9 figures, accepted for publication in ApJ, for May 20,
200
Deep Radio Imaging of Globular Clusters and the Cluster Pulsar Population
We have obtained deep multifrequency radio observations of seven globular
clusters using the Very Large Array and the Australia Telescope Compact Array.
Five of these, NGC 6440, NGC 6539, NGC 6544, NGC 6624 and Terzan 5 had
previously been detected in a shallower survey for steep spectrum radio sources
in globular clusters (Fruchter and Goss 1990). The sixth, the rich globular
cluster, Liller 1, had heretofore been undetected in the radio, and the
seventh, 47 Tucanae, was not included in our original survey. High resolution 6
and 20 cm images of three of the clusters, NGC 6440, NGC 6539, NGC 6624 reveal
only point sources coincident with pulsars which have been discovered
subsequent to our first imaging survey. 21 and 18 cm images reveal several
point sources within a few core-radii of the center of 47 Tuc. Two of these are
identified pulsars, and a third, which is both variable and has a steep
spectrum, is also most likely a pulsar previously identified by a pulsed
survey. However, the 6, 20 and 90 cm images of NGC 6544, Liller 1 and Terzan 5
display strong steep-spectrum emission which cannot be associated with known
pulsars. The image of the rich cluster Terzan 5 displays numerous point sources
within , or 4 core radii of the cluster center. The density of these
objects rises rapidly toward the core, where an elongated region of emission is
found. The brightest individual sources, as well as the extended emission,
possess the steep spectra expected of pulsars. Furthermore, the flux
distribution of the sources agrees well with the standard pulsar luminosity
function. The total luminosity and number of objects observed suggest that
Terzan 5 contains more pulsars than any other Galactic globular cluster.Comment: 33 pages, 6 Postscript figures; Accepted for publication in the
Astrophysical Journal; abstract abridged. PDF version also available at
http://nemesis.stsci.edu/~fruchter/fg99/fg99.pd
VLT-FLAMES Analysis of 8 giants in the Bulge Metal-poor Globular Cluster NGC 6522: Oldest Cluster in the Galaxy?
NGC 6522 has been the first metal-poor globular cluster identified in the
bulge by W. Baade. Despite its importance, very few high resolution abundance
analyses of stars in this cluster are available in the literature. The bulge
metal-poor clusters may be important tracers of the early chemical enrichment
of the Galaxy. The main purpose of this study is the determination of
metallicity and elemental ratios in individual stars of NGC 6522. High
resolution spectra of 8 giants of the bulge globular cluster NGC 6522 were
obtained at the 8m VLT UT2-Kueyen telescope with the FLAMES+GIRAFFE
spectrograph. Multiband V,I,J,Ks} photometry was used to derive effective
temperatures as reference values. Spectroscopic parameters are derived from FeI
and FeII lines, and adopted for the derivation of abundance ratios. The present
analysis provides a metallicity [Fe/H] = -1.0+-0.2. The alpha-elements Oxygen,
Magnesium and Silicon show [O/Fe]=+0.4, [Mg/Fe]=[Si/Fe]= +0.25, whereas Calcium
and Titanium show shallower ratios of [Ca/Fe]=[Ti/Fe]=+0.15. The
neutron-capture r-process element Europium appears to be overabundant by
[Eu/Fe]=+0.4. The neutron-capture s-elements La and Ba are enhanced by
[La/Fe]=+0.35 and [Ba/Fe]=+0.5. The large internal errors, indicating the large
star-to-star variationin the Ba and Eu abundances, are also discussed. The
moderate metallicity combined to a blue Horizontal Branch (BHB), are
characteristics similar to those of HP~1 and NGC 6558, pointing to a population
of very old globular clusters in the Galactic bulge. Also, the abundance ratios
in NGC 6522 resemble those in HP 1 and NGC 6558. The ultimate conclusion is
that the bulge is old, and went through an early prompt chemical enrichment.Comment: 18 pages, 16 figures. Astronomy & Astrophysics, accepte
R Coronae Borealis stars in the Galactic Bulge discovered by EROS-2
Rare types of variable star may give unique insight into short-lived stages
of stellar evolution. The systematic monitoring of millions of stars and
advanced light curve analysis techniques of microlensing surveys make them
ideal for discovering also such rare variable stars. One example is the R
Coronae Borealis (RCB) stars, a rare type of evolved carbon-rich supergiant. We
have conducted a systematic search of the EROS-2 database for the Galactic
catalogue Bulge and spiral arms to find Galactic RCB stars. The light curves of
100 million stars, monitored for 6.7 years (from July 1996 to February
2003), have been analysed to search for the main signature of RCB stars, large
and rapid drops in luminosity. Follow-up spectroscopy has been used to confirm
the photometric candidates. We have discovered 14 new RCB stars, all in the
direction of the Galactic Bulge, bringing the total number of confirmed
Galactic RCB stars to about 51. After reddening correction, the colours and
absolute magnitudes of at least 9 of the stars are similar to those of
Magellanic RCB stars. This suggests that these stars are in fact located in the
Galactic Bulge, making them the first RCB stars discovered in the Bulge. The
localisation of the 5 remaining RCBs is more uncertain: 4 are either located
behind the Bulge at an estimated maximum distance of 14 kpc or have an unusual
thick circumstellar shell; the other is a DY Per RCB which may be located in
the Bulge, even if it is fainter than the known Magellanic DY Per. From the
small scale height found using the 9 new Bulge RCBs,
pc (95% C.L.), we conclude that the RCB stars follow a disk-like distribution
inside the Bulge.Comment: 20 pages, 26 figures, Accepted in A&
Mira Variables in the OGLE Bulge fields
(abridged) The 222000 I-band light curves of variable stars detected by the
OGLE-II survey in the direction of the Galactic Bulge have been fitted and have
also been correlated with the DENIS and 2MASS databases. Results are presented
for 2691 objects with I-band semi-amplitude larger than 0.45 magnitude,
corresponding to classical Mira variables. The Mira period distribution of 6
fields at similar longitude but spanning latitudes from -1.2 to -5.8 degrees
are statistically indistinguisable indicating similar populations with initial
masses of 1.5-2 \msol (corresponding to ages of 1-3 Gyr). A field at similar
longitude at b = -0.05 from Glass et al. (2001) does show a significantly
different period distribution, indicating the presence of a younger population
of 2.5-3 \msol and ages below 1 Gyr. The K-band period-luminosity relation is
presented for the whole sample, and for sub-fields. The zero point depends on
Galactic longitude. Simulations are carried out to show that the observed
dependence of the zero point with , and the number of stars per field are
naturally explained using the model of disk and bulge stars of Binney et al.
(1997), for a viewing angle of 43 +/- 17 degrees. A comparison is made with
similar objects in the Magellanic Clouds, studied in a previous paper. The
slope of the PL-relation in the Bulge and the MCs agree within the errorbars. A
distance to the Galactic Centre between 8.6 and 9.0 kpc is derived (for an
assumed DM of 18.50 to the LMC).Comment: accepted for publication in Astronomy and Astrophysic
Identification of Five Interacting Binaries in the Galactic Bulge Survey
We present optical light curves, spectroscopy, and classification of five X-ray sources in the Chandra Galactic Bulge Survey (CXOGBS J174009.1â284725 (CX5), CXOGBS J173935.7â272935 (CX18), CXOGBS J173946.9â271809 (CX28), CXOGBS J173729.1â292804 (CX37), CXOGBS J174607.6â261547 (CX561)). These objects were selected based on bright optical counterparts which were quickly found to have emission lines in their optical spectra. This paper presents an illustration of GBS optical follow-up, targeting emission line objects. Of these five objects, four exhibit photometric variability in the Sloan r' band. CX5 shows a tentative period of 2.1 hr and is clearly an intermediate polar (IP). CX28 and CX37 both exhibit flickering with no clear period. Both are also suggested to be IPs. CX18 was observed to undergo two dwarf nova outbursts. Finally, CX561 shows no detectable variability, although its characteristics would be consistent with either a quiescent low-mass X-ray binary or cataclysmic variable
Estimating the parameters of globular cluster M 30 (NGC 7099) from time-series photometry
Aims: We present the analysis of 26 nights of V and I time-series observations from 2011 and 2012 of the globular cluster M 30 (NGC 7099). We used our data to search for variable stars in this cluster and refine the periods of known variables; we then used our variable star light curves to derive values for the cluster's parameters. Methods: We used difference image analysis to reduce our data to obtain high-precision light curves of variable stars. We then estimated the cluster parameters by performing a Fourier decomposition of the light curves of RR Lyrae stars for which a good period estimate was possible. We also derived an estimate for the age of the cluster by fitting theoretical isochrones to our colour-magnitude diagram (CMD). Results: Out of 13 stars previously catalogued as variables, we find that only 4 are bona fide variables. We detect two new RR Lyrae variables, and confirm two additional RR Lyrae candidates from the literature. We also detect four other new variables, including an eclipsing blue straggler system, and an SX Phoenicis star. This amounts to a total number of confirmed variable stars in M 30 of 12. We perform Fourier decomposition of the light curves of the RR Lyrae stars to derive cluster parameters using empirical relations. We find a cluster metallicity [Fe/H][SUB]ZW[/SUB] = -2.01 ± 0.04, or [Fe/H][SUB]UVES[/SUB] = -2.11 ± 0.06, and a distance of 8.32 ± 0.20 kpc (using RR0 variables), 8.10 kpc (using one RR1 variable), and 8.35 ± 0.42 kpc (using our SX Phoenicis star detection in M 30). Fitting isochrones to the CMD, we estimate an age of 13.0 ± 1.0 Gyr for M 30. This work is based on data collected by MiNDSTEp with the Danish 1.54 m telescope at the ESO La Silla Observatory.The full light curves, an extract of which is shown in Table 2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/555/A36Tables 8-10, and Figs. 6 and 9 are available in electronic form at http://www.aanda.org</A
Discovery of new companions to high proper motion stars from the VVV Survey
Accepted for publication in A&A; 14 pages, 3 figures, 6 tablesWe acknowledge support by the FONDAP Center for
Astrophysics 15010003; BASAL CATA Center for Astrophysics and Associated
Technologies PFB-06; the Ministry for the Economy, Development, and
Tourismâs Programa Iniciativa CientĂfica Milenio through grant P07-021-
F, awarded to The Milky Way Millennium Nucleus; FONDECYT grants
No. 1090213 and 1110326 from CONICYT, and the European Southern
Observatory. J.C.B. acknowledge support from a Ph.D. Fellowship from
CONICYT. M.G. is financed by the GEMINI-CONICYT Fund, allocated to
the project 32110014. R.K. acknowledges partial support from FONDECYT
through grant 1130140. E.L.M. acknowledges support from grant AyA2011-
30147-C03-03; J.B. acknowledge support from FONDECYT No. 1120601;
A.N.C. acknowledges support from GEMINI-CONICYT No. 32110005 and
from Comitee Mixto ESO-GOBIERNO DE CHILE. J.A.G. acknowledges support
from Proyecto Fondecyt Postdoctoral 3130552, Fondecyt Regular 1110326,
and Anillos ACT-86
Variable Stars in Galactic Globular Clusters
Based on a search of the literature up to May 2001, the number of known
variable stars in Galactic globular clusters is approximately 3000. Of these,
more than 2200 have known periods and the majority (approximately 1800) are of
the RR Lyrae type. In addition to the RR Lyrae population, there are
approximately 100 eclipsing binaries, 120 SX Phe variables, 60 Cepheids
(including population II Cepheids, anomalous Cepheids and RV Tauri) and 120
SR/red variables. The mean period of the fundamental mode RR Lyrae variables is
0.585, for the overtone variables it is 0.342 (0.349 for the first-overtone
pulsators and 0.296 for the second-overtone pulsators) and approximately 30%
are overtone pulsators. These numbers indicate that about 65% of RR Lyrae
variables in Galactic globular clusters belong to Oosterhoff type I systems.
The mean period of the RR Lyrae variables in the Oosterhoff type I clusters
seems to be correlated with metal abundance in the sense that the periods are
longer in the more metal poor clusters. Such a correlation does not exist for
the Oosterhoff type II clusters. Most of the Cepheids are in clusters with blue
horizontal branches.Comment: 45 pages, 10 figures, to be published in AJ November 200