35 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
The distance to the LMC cluster Reticulum from the K-band Period-Luminosity-Metallicity relation of RR Lyrae stars
We present new and accurate Near-Infrared J and Ks-band data of the Large
Magellanic Cloud cluster Reticulum. Data were collected with SOFI available at
NTT and covering an area of approximately (5 x 5) arcmin^2 around the center of
the cluster. Current data allowed us to derive accurate mean K-band magnitudes
for 21 fundamental and 9 first overtone RR Lyrae stars. On the basis of the
semi-empirical K-band Period-Luminosity-Metallicity relation we have recently
derived, we find that the absolute distance to this cluster is 18.52 +- 0.005
(random) +- 0.117 (systematic). Note that the current error budget is dominated
by systematic uncertainty affecting the absolute zero-point calibration and the
metallicity scale.Comment: 14 pages, 2 figures, ApJ accepted. Full resolution figure 1 on
request ([email protected]
Variable Stars in Local Group Galaxies. IV. RR Lyrae stars in the central regions of the low-density galaxy Crater II
We present a search and analysis of variable stars in the recently discovered
Crater~II dwarf galaxy. Based on , , data collected with the Isaac
Newton Telescope (FoV0.44 square degrees) we detected 37 variable stars,
of which 34 are bone-fide RR Lyrae stars of Crater~II (28 RRab, 4 RRc, 2 RRd).
We applied the metal-independent (, ) Period--Wesenheit relation and
derived a true distance modulus ( = 20.300.08 mag (=0.16
mag). Individual metallicities for RR Lyrae stars were derived by inversion of
the predicted -band Period-Luminosity relation. We find a mean metallicity
of [Fe/H]=-1.64 and a standard deviation of =0.21 dex,
compatible with either negligible or vanishing intrinsic metallicity
dispersion. The analysis of the Colour-Magnitude Diagram reveals a stark
paucity of blue horizontal branch stars, at odds with other Galactic dwarfs,
and globular clusters with similar metal abundances.Comment: 14 pages, 8 figures, accepted for publications on MNRAS. Time series
photometry is available in the manuscript source ta
The RR Lyrae distance scale from near-infrared photometry: current results
We present new observational results on the RR Lyrae K-band Period-Luminosity
relation (PLK). Data on the Galactic globular clusters NGC 3201 and NGC 4590
(M68), and on the Large Magellanic Cloud cluster Reticulum are shown. We
compare the observed slopes of the PLK relations for these three clusters with
those predicted by pulsational and evolutionary models, finding a fair
agreement. Trusting on this finding we decided to adopt these theoretical
calibrations to estimate the distance to the target clusters,finding a good
agreement with optical-based RR Lyrae distances, but with a smaller formal
scatter.Comment: Proceedings of the Stellar Pulsation and Evolution meeting, Rome,
June 200
The white dwarf cooling sequence in the old open cluster NGC 188
We develop the white dwarf luminosity function (LF) of the old open cluster
NGC 188 in order to determine a lower limit to the age of the cluster by using
the faint end of the cooling sequence.
To produce an extensive sequence of the cooling white dwarfs we imaged four
contiguous HST-WFPC2 fields in the center of the cluster in the F555W and F814W
filters. After imposing selection criteria on the detected objects we found a
white dwarf cooling sequence (down to V ~26.5) including 28 candidate white
dwarfs in the cluster. The exposures are not deep enough to reach the end of
this sequence, but the results of our analysis allow us to establish a lower
limit to the age of the cluster independently of the isochrone fit to the
cluster turnoff.
The most ancient white dwarfs found are ~4 Gyr old, an age that is set solely
by the photometric limit of our data. Classical methods provide an estimate of
\~7 Gyr (Sarajedini et al., 1999).Comment: 6 pages, 5 figures, accepted on Astronomy and Astrophysic
A DECam View of the Diffuse Dwarf Galaxy Crater II: Variable Stars
Time series observations of a single dithered field centered on the diffuse
dwarf satellite galaxy Crater II were obtained with the Dark Energy Camera
(DECam) at the 4m Blanco Telescope at Cerro Tololo Inter-American Observatory,
Chile, uniformly covering up to two half-light radii. Analysis of the and
time series results in the identification and characterization of 130
periodic variable stars, including 98 RR Lyrae stars, 7 anomalous Cepheids, and
1 SX Phoenicis star belonging to the Crater II population, and 24 foreground
variables of different types. Using the large number of ab-type RR Lyrae stars
present in the galaxy, we obtained a distance modulus to Crater II of
(stat) (sys). The distribution of the RR
Lyrae stars suggests an elliptical shape for Crater II, with an ellipticity of
0.24 and a position angle of . From the RR Lyrae stars we infer a
small metallicity dispersion for the old population of Crater II of only 0.17
dex. There are hints that the most metal-poor stars in that narrow distribution
have a wider distribution across the galaxy, while the slightly more metal rich
part of the population is more centrally concentrated. Given the features in
the color-magnitude diagram of Crater II, the anomalous Cepheids in this galaxy
must have formed through a binary evolution channel of an old population.Comment: Accepted for publication in MNRA
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The GAPS Programme at TNG XXXVII. A precise density measurement of the young ultra-short period planet TOI-1807 b
Context. Great strides have been made in recent years in the understanding of the mechanisms involved in the formation and evolution of planetary systems. Despite this, many observational findings have not yet been corroborated by astrophysical explanations. A fine contribution to the study of planetary formation processes comes from the study of young, low-mass planets, with short orbital periods (.100 days). In the last three years, the NASA/TESS satellite has identified many planets of this kind and their characterization is clearly necessary in order to understand how they formed and evolved. Aims. Within the framework of the Global Architecture of Planetary System (GAPS) project, we performed a validation and characterization (radius and mass) of the ultra-short period planet TOI-1807 b, which orbits its young host star BD+39 2643 (âŒ300 Myr) in only 13 h. This is the youngest ultra-short period planet discovered so far. Methods. Thanks to a joint modeling of the stellar activity and planetary signals in the TESS light curve and in new HARPS-N radial-velocity measurements, combined with accurate estimation of stellar parameters, we validated the planetary nature of TOI-1807 b and measured its orbital and physical parameters. Results. By using astrometric, photometric, and spectroscopic observations, we found that BD+39 2643 is a young, active K dwarf star and a member of a 300 ± 80 Myr old moving group. Furthermore, it rotates in Prot = 8.8 ± 0.1 days. This star hosts an ultra-short period planet, exhibiting an orbital period of only Pb = 0.54937 ± 0.00001 days. Thanks to the exquisite photometric and spectroscopic series, along with the accurate information on its stellar activity, we measured both the radius and the mass of TOI-1807 b with high precision, obtaining RP,b = 1.37 ± 0.09 Râ and MP,b = 2.57 ± 0.50 Mâ. These planet parameters correspond to a rocky planet with an Earth-like density (Ïb = 1.0 ± 0.3 Ïâ) and no extended H/He envelope. From the analysis of the age-RP distribution for planets with well measured ages, we inferred that TOI-1807 b may have already lost a large part of its atmosphere over the course of its 300 Myr lifetime