1,721,056 research outputs found
RADIAL VELOCITIES AND METALLICITIES OF RED GIANT STARS IN THE OLD OPEN CLUSTER NGC 7762
Full text linkWe present and discuss radial velocity and the very first metallicity measurements for nine evolved stars in the poorly known old open cluster NGC 7762. We isolated eight radial velocity cluster members and one interloper. Radial velocities are in good agreement with previous studies. NGC 7762 turns out to be of solar metallicity within the uncertainties ([Fe/H] = 0.04 ± 0.12). For this metallicity, the cluster age is 2.5 ± 0.2 Gyr, and falls in an age range where only a few old open clusters are known. With respect to previous studies, we find a larger distance, implying the cluster is located {900}-50+70 pc from the Sun. For most of the elements we measure solar-scaled abundance ratios. We searched the literature for open clusters of similar age in the solar vicinity and found that NGC 7762 can be considered a twin of Ruprecht 147, a similar age cluster located only 300 pc from the Sun. In fact, as well as age, metallicity and abundance ratios are very close to Ruprecht 147 values within the observational uncertainties
NGC 1883: a neglected intermediate-age open cluster located in the outskirts of the Galactic disc
Full text linkWe report on BVI CCD photometry of a field centred in the region of the open cluster NGC 1883, down to V= 21. This cluster has never been studied so far; we provide, for the first time, estimates of its fundamental parameters - namely radial extent, age, distance and reddening. We find that the cluster has a radius of about 2.5 arcmin, and shows signatures of dynamical relaxation. NGC 1883 is located in the anticentre direction, and exhibits a reddening in the range E(B-V) = 0.23-0.35, depending on the metal abundance. It turns out to be of intermediate age (1 Gyr old), and is quite distant for an open cluster. In fact, it is located 4.8 kpc from the Sun, and more than 13 kpc from the Galactic centre. This results makes NGC 1883 one of the most peripheral old open clusters, with important consequences for the trend of the metallicity with distance in the outer Galactic disc.Fil: Carraro, Giovanni. Università di Padova; ItaliaFil: Baume, Gustavo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica la Plata; Argentina. Università di Padova; ItaliaFil: Villanova, S.. Università di Padova; Itali
Uncovering Multiple Populations in Globular Clusters with Washington Photometry
No full textGlobular Clusters (GCs), long considered as ideal Simple Stellar Populations, are now known to harbor a wide variety of chemical inhomogeneities. Multiple populations (MP) are being found in a growing number of Galactic globular clusters (GCs) via both photometric and spectroscopic techniques. Indeed, it has been suggested that a GC is an object that possesses MP. A definitive investigation of MP in GCs will undoubtedly provide a profound improvement in our understanding of their formation and evolution.However, most studies employ either high resolution VLT spectroscopy, HST photometry or inefficient filters from the ground. A ground-based photometric system which is both efficient and effective would be especially excellent for uncovering MP. We demonstrate that the Washington system meets these goals. The Washington C filter, in addition to being specifically designed for the purpose of detecting MPs, is both much broader and redder than competing UV filters, making it far more efficient at detecting MPs and much less sensitive to reddening and extinction.Our analysis of the well-studied GC NGC 1851 shows indeed that the C filter is both very efficient and effective at detecting its previously discovered MPs in the RGB and SGB, using relatively little telescope time on only a 1-meter telescope. Remarkably, we have also detected an intrinsically broad MS best characterized by two distinct but heavily overlapping populations that cannot be explained by binaries, field stars, or photometric errors. Detailed analysis shows that the MS distribution is in very good agreement with that seen on the RGB. This is the first time MPs in a MS have been discovered from the ground, and just as strikingly, using only a 1-meter telescope. The Washington system thus proves to be a very powerful tool for investigating MPs, and holds particular promise for extragalactic objects where photons are limited
Optical photometry and spectral classification of stars in the field of the open cluster NGC 6996 in the North America Nebula
No full textThe Helium Content of Globular Clusters: NGC 6121 (M4)
No full textIn the context of the multiple stellar population scenario in globular clusters, helium (He) has been proposed as a key element to interpret the observed multiple main sequences, subgiant branches, and red giant branches, as well as the complex horizontal branch (HB) morphology. In particular, second-generation stars belonging to the bluer part of the HB are thought to be more He-rich (ΔY = 0.03 or more) but also more Na-rich/O-poor than those located in the redder part that should have Y equal to the cosmological value. Up to now this hypothesis was only partially confirmed in NGC 6752, where stars of the redder zero-age HB showed an He content of Y = 0.25 ± 0.01, fully compatible with the primordial He content of the universe, and were all Na-poor/O-rich. Here we study hot blue horizontal branch (BHB) stars in the GC NGC 6121 (M4) to measure their He plus O/Na content. Our goal is to complete the partial results obtained for NGC 6752, focusing our attention on targets located on the bluer part of the HB of M4. We observed six BHB stars using the VLT2/UVES spectroscopic facility. Spectra of signal-to-noise ratio ~ 150 were obtained and the very weak He line at 5875 Å measured for all our targets. We compared this feature with synthetic spectra to obtain He abundances. In addition O, Na, and Fe abundances were estimated. Stars turned out to be all Na-rich and O-poor and to have a homogeneous He content with a mean value of Y = 0.29 ± 0.01(random) ± 0.01(systematic), which is enhanced by ΔY ~ 0.04 with respect to the most recent measurements of the primordial He content of the universe (Y ~ 0.24/0.25). The high He content of blue HB stars in M4 is also confirmed by the fact that they are brighter than red HB stars (RHB). Theoretical models suggest the BHB stars are He-enhanced by Δ(Y) = 0.02/0.03 with respect to the RHB stars. The whole sample of stars has a metallicity of [Fe/H] = -1.06 ± 0.02 (internal error), in agreement with other studies available in the literature. This is a rare direct measurement of the (primordial) He abundance for stars belonging to the Na-rich/O-poor population of GC stars in a temperature regime where the He content is not altered by sedimentation or extreme mixing as suggested for the hottest, late helium flash HB stars. Our results support theoretical predictions that the Na-rich/O-poor population is also more He-rich than the Na-poor/O-rich generation and that a leading contender for the second parameter is the He abundance.
Based on observations made with ESO telescopes at La Silla Paranal Observatory under programme ID 083.B-0083
Detailed Abundances of Red Giants in the Globular Cluster NGC 1851: C+N+O and the Origin of Multiple Populations
No full textWe present chemical abundance analysis of a sample of 15 red giant branch (RGB) stars of the globular cluster NGC 1851 distributed along the two RGBs of the (v, v-y) color-magnitude diagram. We determined abundances for C+N+O, Na, α, iron-peak, and s-elements. We found that the two RGB populations significantly differ in their light (N, O, Na) and s-element content. On the other hand, they do not show any significant difference in their α and iron-peak element content. More importantly, the two RGB populations do not show any significant difference in their total C+N+O content. Our results do not support previous hypotheses suggesting that the origins of the two RGBs and the two subgiant branches of the cluster are related to different content of either α (including Ca) or iron-peak elements, or C+N+O abundance, due to a second generation polluted by Type II supernovae
The helium content of globular clusters: light element abundance correlations and HB morphology I. NGC 6752
No full textContext: In the context of the multiple stellar population scenario in globular clusters (GC), helium (He) has been proposed as the key element to interpret the observed multiple main sequences (MS), subgiant branches (SGB) and red giant branches (RGB), as well as the complex horizontal branch (HB) morphology. However, up to now, He has never been directly measured in GC stars (8500 < T_eff < 11 500 K) to verify this hypothesis.
Aims: We studied hot blue horizontal branch (BHB) stars in the GC NGC 6752 to measure their He content. Our goal is to verify the feasibility of He measurement from high resolution spectra in stars cooler than 11 500 K, where chemical abundances are not altered by sedimentation or levitation.
Methods: We observed 7 BHB stars using the UVES@VLT2 spectroscopic facility. Spectra of S/N~200 were obtained and the very weak He line at 5875 Å measured. We compared this feature with synthetic spectra to obtain He abundances. In addition, iron peak (Fe, Cr), α (Si, Ti), light (O, Na), and s-element (Ba) abundances were measured.
Results: We could measure He abundance only for stars warmer than T_eff = 8500 K. All our targets with measurable He are zero age HB (ZAHB) objects and turned out to have a homogeneous He content with a mean value of Y = 0.245±0.012, compatible with the most recent measurements of the primordial He content of the Universe (Y~0.25). The whole sample of stars has a metallicity of [Fe/H] = -1.56±0.03 and [ α/Fe] = +0.21±0.03, in agreement with other studies available in the literature. Our HB targets show the same Na-O anticorrelation identified among the TO-SGB-RGB stars.
Conclusions: This is the first direct measurement of the He abundance for a significative sample of GC stars in a temperature regime where the He content is not altered by sedimentation or extreme mixing as suggested for the hottest, late helium flasher HB stars
Testing Newtonian gravity with distant globular clusters: NGC 1851 and NGC 1904
Full text linkContext. Globular clusters are useful for testing the validity of Newtonian dynamics in the low acceleration regime typical of galaxies, without the complications of non-baryonic dark matter. In the absence of disturbing effects, such as tidal heating, the velocity dispersion of globular clusters is expected to vanish at large radii. If this is not observed, and in particular if, as observed in elliptical galaxies, the dispersion is found to be constant at large radii below a certain threshold acceleration, this might indicate a breakdown of Newtonian dynamics.
Aims. To minimize the effects of tidal heating that can increase the velocity dispersion at large radii, we study the velocity dispersion profile of two distant globular clusters, NGC 1851 and NGC 1904.
Methods. The velocity dispersion profile is derived from accurate radial velocity measurements, obtained at the ESO 8m VLT telescope with the FLAMES multi-object spectrograph. Reliable data for 184 and 146 bona fide cluster star members were obtained for NGC 1851 and NGC 1904, respectively.
Results. These data allow us to trace the velocity dispersion profile to ~2r0, where r0 is the radius at which the cluster internal acceleration of gravity is a0 ~ 10-8 cm s-2. It is found that in both clusters the velocity dispersion is maximal at the center, decreases moving outward, and then becomes constant beyond ~r0. Since the distance of these clusters from the Milky Way is large, the observed flattening of the velocity dispersion profile cannot be ascribed to tidal heating effects, as proposed in the case of nearer globular clusters.
Conclusions. These results are in full agreement with those found for another five globular clusters previously investigated as part of this project. Taken together, our results for these 7 clusters support the claim that the velocity dispersion is constant beyond r0, irrespectively of the specific physical properties of the clusters: mass, size, dynamical history, and distance from the Milky Way. The strong similarity to the constant velocity dispersion observed in elliptical galaxies beyond r0 is indicative of a common origin for this phenomenon in the two class of objects, and possibly a breakdown of Newtonian dynamics below a0
A study of the two northern open clusters NGC 1582 and NGC 1663
Full text linkWe present CCD UBV(I)C observations obtained in the field of the previously unstudied northern open clusters NGC 1582 and NGC 1663. For the former, we also provide high-resolution spectra of the brightest stars and complement our data with Two-Micron All-Sky-Survey (2MASS) near-infrared photometry and with astrometric data from the Tycho-2 catalog. From the analysis of all these data, we argue that NGC 1582 is a very poor, quite large and heavily contaminated open cluster. It turns out to have a reddening EB-V = 0.35 +/- 0.03, to be situated 1100 +/- 100 pc from the Sun and to have an age of 300 +/- 100 Myr. On the other hand, we were not able to unambiguously clarify the nature of NGC 1663. By assuming it is a real cluster and from the analysis of its photometric diagrams, we found a color excess value EB-V = 0.20, an intermediate age value ( ~ 2000 Myr) and a distance of about 700 pc. The distribution of the stars in the region however suggests we are probably facing an open cluster remnant. As an additional result, we obtained aperture photometry of three previously unclassified galaxies placed in the field of NGC 1663 and performed a preliminary morphological classification of them.Fil: Baume, Gustavo Luis. Università di Padova; ItaliaFil: Villanova, S.. Università di Padova; Italia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica la Plata; ArgentinaFil: Carraro, Giovanni. Università di Padova; Itali
NGC 5385, NGC 2664 and Collinder 21: Three candidate open cluster remnants
No full textWe present CCD UBVI photometric and medium/high resolution
spectroscopic observations obtained in the field
of the previously unstudied dissolving open cluster
candidates NGC 5385, NGC 2664 and Collinder 21. Our
analysis is based on the discussion of star counts, photometry,
radial velocity distribution, and proper
motions available from the Tycho 2 catalogue. All
three aggregates clearly emerge from the mean Galactic
field, but, regrettably, the close scrutiny of proper
motions and radial velocities reveals that we are not
facing any physical group. Instead, what we are looking
at are just chance alignments of a few bright unrelated stars.
Our analysis casts some doubt on the Bica et al. ([CITE], A&A, 366, 827) criterion
to look for Possible Open Cluster Remnants. It seems mandatory to
define a better criterion to adopt for further investigations
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