1,721,041 research outputs found
What we would like to know about extreme horizontal branch stars in globular clusters
No full textIn this contribution, we explore some open questions about Extreme Horizontal Branch (EHB) stars in globular clusters. In particular, we present the current status of the search for rapid pulsators, He-depleted stars, and close binaries, three kind of objects very common among field EHB stars, but which first surveys failed to detect in clusters. We also analyze how the lack of cluster EHB binaries can reconcile with theoretical expectations. We give special attention to the first close EHB binary discovered in a globular cluster, whose characteristics point to a very rare or even unique object. Finally, we analyze some recent puzzling results about spectroscopically derived masses, that could point to the presence of two distinct families of EHB’s in clusters. For all these topics, we present the recent advancement in knowledge, the results requiring more investigation, and what has still to be done to fix the unsolved problems, showing the main points of our studies, the aims of our works, and what we expect to obtain from them
Ruprecht 106: The First Single Population Globular Cluster?
Full text linkAll old Galactic globular clusters (GCs) studied in detail to date host at least two generations of stars, where the second is formed from gas polluted by processed material produced by massive stars of the first. This process can happen if the initial mass of the cluster exceeds a threshold above which ejecta are retained and a second generation is formed. A determination of this mass threshold is mandatory in order to understand how GCs form. We analyzed nine red giant branch stars belonging to the cluster Ruprecht 106. Targets were observed with the UVES@VLT2 spectrograph. Spectra cover a wide range and allowed us to measure abundances for light (O, Na, Mg, Al), α (Si, Ca, Ti), iron-peak (Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), and neutron-capture (Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Dy, Pb) elements. Based on these abundances, we show that Ruprecht 106 is the first convincing example of a single-population GC (i.e., a true simple stellar population), although the sample is relatively small. This result is supported also by an independent photometric test and by the horizontal branch morphology and the dynamical state. It is old (~12 Gyr) and, at odds with other GCs, has no α-enhancement. The material it formed from was contaminated by both s- and r-process elements. The abundance pattern points toward an extragalactic origin. Its present-day mass (M = 104.83 M ⊙) can be assumed as a strong lower limit for the initial mass threshold below which no second generation is formed. Clearly, its initial mass must have been significantly greater, but we have no current constraints on the amount of mass loss during its evolution
A study of the Thick disk of the Galaxy in the direction of the South Galactic Pole in the framework of the Southern Proper Motion Program
No full textThe VVV-SkZ_pipeline: an automatic PSF-fitting photometric pipeline for the VVV survey
Full text linkWe present the VVV-SkZ_pipeline, a DAOPHOT-based photometric pipeline, created to perform PSF-fitting photometry of ``VISTA Variables in the Vía Láctea'' (VVV) ESO Public Survey data. The pipeline replaces the user avoiding repetitive interaction in all the operations, retaining all of the benefits of the power and accuracy of the DAOPHOT suite. The pipeline provides an astrometrized photometric catalog reliable up to more than 2 magnitudes brighter than the saturation limit, where other techniques fail. It also produces deeper and more accurate photometry. These achievements allow the VVV-SkZ_pipeline to produce data well anchored to the selected standard photometric system and analyze important phenomena (i.e. TRGB, RGB slope, HB morphology, RR Lyrae), that other methods are not able to manage
Chemical abundance analysis of red giant branch stars in the globular cluster E3
Full text linkContext. Globular clusters are known to host multiple stellar populations, which are a signature of their formation process. The globular cluster E3 is one of the few low-mass globulars that is thought not to host multiple populations. Aims. We investigate red giant branch stars in E3 with the aim of providing a first detailed chemical inventory for this cluster, we determine its radial velocity, and we provide additional insights into the possible presence of multiple populations in this cluster. Methods. We obtained high-resolution FLAMES-UVES/VLT spectra of four red giant branch stars likely members of E3. We performed a local thermodynamic equilibrium abundance analysis based on one-dimensional plane parallel ATLAS9 model atmospheres. Abundances were derived from line equivalent widths or spectrum synthesis. Results. We measured abundances of Na and of iron peak (Fe, V, Cr, Ni, Mn), α(Mg, Si, Ca, Ti), and neutron capture elements (Y, Ba, Eu). The mean cluster heliocentric radial velocity, metallicity, and sodium abundance ratio are νhelio = 12.6 ± 0.4 km s-1(σ = 0.6 ± 0.2 km s-1), [Fe/H] = -0.89 ± 0.08 dex, and [Na/Fe] = 0.18 ± 0.07 dex, respectively. The low Na abundance with no appreciable spread is suggestive of a cluster dominated by first-generation stars in agreement with results based on lower resolution spectroscopy. The low number of stars observed does not allow us to rule out a minor population of second-generation stars. The observed chemical abundances are compatible with the trends observed in Milky Way stars
A HOT HORIZONTAL BRANCH STAR WITH A CLOSE K-TYPE MAIN-SEQUENCE COMPANION
Full text linkC.M.B., S.V., and M.C. acknowledge support from proyectos FONDECYT regular 1150060, 1130721, and 1141141. M.C. acknowledges support by the Ministry of Economy, Development, and Tourism’s Programa Iniciativa Cienti ́fica Milenio, through grant IC210009, awarded to the Millennium Institute of Astrophysics (MAS) by Proyecto Basal PFB-06/2007. M.M. acknowledges support from FCT through the grant and SFRH/BDP/71230/2010.Dynamical interactions in binary systems are thought to play a major role in the formation of extreme horizontal branch stars (EHBs) in the Galactic field. However, it is still unclear if the same mechanisms are at work in globular clusters (GCs), where EHBs are predominantly single stars. Here, we report on the discovery of a unique close binary system (period ̃1.61 days) in the GC NGC 6752, comprising an EHB and a main-sequence companion of 0.63 ± 0.05 {M}☉ . Such a system has no counterpart among nearly 200 known EHB binaries in the Galactic field. Its discovery suggests that either field studies are incomplete, missing this type of system possibly because of selection effects, or that a particular EHB formation mechanism is active in clusters but not in the field. Based on observations with the ESO Very Large Telescope at Paranal Observatory, Chile (program IDs 69.D-0682, 075.D-0492, and 079.D-0674), and with the 6.5 m Magellan Telescopes at Las Campanas Observatory, Chile (program IDs 2006B-LC-7 and CHILE-2007B-007)
NGC 1252: A high altitude, metal poor open cluster remnant
No full textIf stars form in clusters but most stars belong to the field, understanding the details of the transition from the former to the latter is imperative to explain the observational properties of the field. Aging open clusters are one of the sources of field stars. The disruption rate of open clusters slows down with age but, as an object gets older, the distinction between the remaining cluster or open cluster remnant (OCR) and the surrounding field becomes less and less obvious. As a result, finding good OCR candidates or confirming the OCR nature of some of the best candidates still remain elusive. One of these objects is NGC 1252, a scattered group of about 20 stars in Horologium. Here we use new wide-field photometry in the UBVI passbands, proper motions from the Yale/San Juan SPM 4.0 catalogue and high-resolution spectroscopy concurrently with results from N-body simulations to decipher NGC 1252's enigmatic character. Spectroscopy shows that most of the brightest stars in the studied area are chemically, kinematically and spatially unrelated to each other. However, after analysing proper motions, we find one relevant kinematic group. This sparse object is relatively close (~1 kpc), metal poor and is probably not only one of the oldest clusters (3 Gyr) within 1.5 kpc from the Sun but also one of the clusters located farthest from the disc, at an altitude of nearly -900 pc. That makes NGC 1252 the first open cluster that can be truly considered a high Galactic altitude OCR: an unusual object that may hint at a star formation event induced on a high Galactic altitude gas cloud. We also conclude that the variable TW Horologii and the blue straggler candidate HD 20286 are unlikely to be part of NGC 1252. NGC 1252 17 is identified as an unrelated, Population II cannonball star moving at about 400 km s-1.© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
NGC 1252: a high altitude, metal poor open cluster remnant
Full text linkArtículo de publicación ISIIf stars form in clusters but most stars belong to the field, understanding the details of the
transition from the former to the latter is imperative to explain the observational properties
of the field. Aging open clusters are one of the sources of field stars. The disruption rate of
open clusters slows down with age but, as an object gets older, the distinction between the
remaining cluster or open cluster remnant (OCR) and the surrounding field becomes less and
less obvious. As a result, finding good OCR candidates or confirming the OCR nature of some
of the best candidates still remain elusive. One of these objects is NGC 1252, a scattered
group of about 20 stars in Horologium. Here we use new wide-field photometry in the UBVI
passbands, proper motions from the Yale/San Juan SPM 4.0 catalogue and high-resolution
spectroscopy concurrently with results from N-body simulations to decipher NGC 1252’s
enigmatic character. Spectroscopy shows that most of the brightest stars in the studied area
are chemically, kinematically and spatially unrelated to each other. However, after analysing
proper motions, we find one relevant kinematic group. This sparse object is relatively close
(∼1 kpc), metal poor and is probably not only one of the oldest clusters (3 Gyr) within 1.5 kpc
from the Sun but also one of the clusters located farthest from the disc, at an altitude of nearly
−900 pc. That makes NGC 1252 the first open cluster that can be truly considered a high
Galactic altitude OCR: an unusual object that may hint at a star formation event induced on
a high Galactic altitude gas cloud. We also conclude that the variable TW Horologii and the
blue straggler candidate HD 20286 are unlikely to be part of NGC 1252. NGC 1252 17 is
identified as an unrelated, Population II cannonball star moving at about 400 km s−1
Ghosts of Milky Way's past: The globular cluster ESO 37-1 (E3)
Full text linkContext. In the Milky Way, most globular clusters are highly conspicuous objects that were found centuries ago. However, a few dozen of them are faint, sparsely populated systems that were identified largely during the second half of the past century. One of the faintest is ESO 37-1 (E 3) and as such it remains poorly studied, with no spectroscopic observations published so far although it was discovered in 1976.
Aims: We investigate the globular cluster E 3 in an attempt to better constrain its fundamental parameters. Spectroscopy of stars in the field of E 3 is shown here for the first time.
Methods: Deep, precise VI CCD photometry of E 3 down to V ~ 26 mag is presented and analysed. Low-resolution, medium signal-to-noise ratio spectra of nine candidate members are studied to derive radial velocity and metallicity. Proper motions from the UCAC4 catalogue are used to explore the kinematics of the bright members of E 3.
Results: Isochrone fitting indicates that E 3 is probably very old, with an age of about 13 Gyr; its distance from the Sun is nearly 10 kpc. It is also somewhat metal rich with [Fe/H] = -0.7. Regarding its kinematics, our tentative estimate for the proper motions is (μα cosδ,μδ) = (-7.0 ± 0.8, 3.5 ± 0.3) mas yr-1 (or a tangential velocity of 382 ± 79 km s-1) and for the radial velocity 45 ± 5 km s-1 in the solar rest frame.
Conclusions: E 3 is one of the most intriguing globular clusters in the Galaxy. Having an old age and being metal rich is clearly a peculiar combination, only seen in a handful of objects like the far more conspicuous NGC 104 (47 Tucanae). In addition, its low luminosity and sparse population make it a unique template for the study of the final evolutionary phases in the life of a star cluster. Unfortunately, E 3 is among the most elusive and challenging known globular clusters because field contamination severely hampers spectroscopic studies
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