We have made a detailed study of the color distribution of the main sequence of the globular cluster (GC) NGC 2808, based on new deep HST WFPC2 photometry of a field in the uncrowded outskirts of the ...cluster. The observed color distribution of main-sequence stars is not Gaussian and is wider than expected for a single stellar population, given our (carefully determined) measurement errors. About 20% of the sample stars are much bluer than expected and are most plausibly explained as a population having a much larger helium abundance than the bulk of the main sequence. Using synthetic color-magnitude diagrams based on new stellar models, we estimate that the helium mass fraction of these stars is Y 6 0.4. The newly found anomaly on the main sequence gives credence to the idea that GCs like NGC 2808 have undergone self-enrichment and that different stellar populations were born from the ejecta of the intermediate-mass asymptotic giant branch (AGB) stars of the first generation. The enhancement and spread of helium among the stars in NGC 2808 have recently been suggested as a simple way to explain the very peculiar morphology of its horizontal branch. We find that if in addition to the Y = 0.40 stars, roughly 30% of the stars have Y distributed between 0.26-0.29, while 50% have primordial Y, this leads to a horizontal-branch morphology similar to that observed. In this framework, three main stages of star formation are identified, the first with primordial helium content Y 0.24, the second born from the winds of the most massive AGBs of the first stellar generation (66-7 M sub( )), with Y 6 0.4, and a third born from the matter ejected from less massive AGBs (63.5-4.5 M sub( )), with Y 6 0.26-0.29. There could have been a long hiatus (several times 10 super(7) yr), between the second and third generation in which no star formed in the protocluster. We suggest that during this period, star formation has been inhibited by the explosion of late Type II supernovae deriving from binary evolution.
We used deep observations collected with Advanced Camera for Surveys (ACS) at Hubble Space Telescope (HST) to derive the fraction of binary systems in a sample of 13 low-density Galactic globular ...clusters. By analysing the colour distribution of main-sequence stars we derived the minimum fraction of binary systems required to reproduce the observed colour–magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6 per cent within the core radius. The estimated global fractions of binary systems range from 10 to 50 per cent depending on the cluster. A dependence of the relative fraction of binary systems on the cluster age has been detected, suggesting that the binary disruption process within the cluster core is active and can significantly reduce the binary content in time.
The peculiar horizontal branch of NGC 2808 Dalessandro, E.; Salaris, M.; Ferraro, F. R. ...
Monthly notices of the Royal Astronomical Society,
01/2011, Volume:
410, Issue:
1
Journal Article
Peer reviewed
Open access
We present an accurate analysis of the peculiar horizontal branch (HB) of the massive Galactic globular cluster NGC 2808, based on high-resolution far-UV and optical images of the central region of ...the cluster obtained with the Hubble Space Telescope. We confirm the multimodal distribution of stars along the HB: four sub-populations separated by gaps are distinguishable. The detailed comparison with suitable theoretical models showed that (i) it is not possible to reproduce the luminosity of the entire HB with a single helium abundance, while an appropriate modelling is possible for three HB groups by assuming different helium abundances in the range 0.24 < ΔY < 0.4 that are consistent with the multiple populations observed in the main sequence; and (ii) canonical HB models are not able to properly match the observational properties of the stars populating the hottest end of the observed HB distribution, the so-called blue hook region. These objects are probably 'hot flashers', stars that peel off the red giant branch before reaching the tip and ignite helium at high T
eff. Both of these conclusions are based on the luminosity of the HB in the optical and ultraviolet bands and do not depend on specific assumptions about mass loss.
Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy's history. 'Blue stragglers' are stars massive enough that they should have evolved into ...white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions and stellar mergers resulting from direct collisions between two stars. Recently the binary explanation was claimed to be dominant. Here we report that there are two distinct parallel sequences of blue stragglers in M 30. This globular cluster is thought to have undergone 'core collapse', during which both the collision rate and the mass transfer activity in binary systems would have been enhanced. We suggest that the two observed sequences are a consequence of cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer.
We have investigated the relevant trend of the bolometric correction (BC) at the cool-temperature regime of red giant stars and its possible dependence on stellar metallicity. Our analysis relies on ...a wide sample of optical–infrared spectroscopic observations, along the 3500 Å⇒ 2.5 μm wavelength range, for a grid of 92 red giant stars in five (three globular + two open) Galactic clusters, along the full metallicity range covered by the bulk of the stars, −2.2 ≤Fe/H≤+0.4. Synthetic BVRCIC JHK photometry from the derived spectral energy distributions allowed us to obtain robust temperature (Teff) estimates for each star, within ±100 K or less. According to the appropriate temperature estimate, blackbody extrapolation of the observed spectral energy distribution allowed us to assess the unsampled flux beyond the wavelength limits of our survey. For the bulk of our red giants, this fraction amounted to 15 per cent of the total bolometric luminosity, a figure that raises up to 30 per cent for the coolest targets (Teff≲ 3500 K). Overall, we obtain stellar Mbol values with an internal accuracy of a few percentages. Even neglecting any correction for lost luminosity etc., we would be overestimating Mbol by ≲0.3 mag, in the worst cases. Making use of our new data base, we provide a set of fitting functions for the V and K BC versus Teff and versus (B−V) and (V−K) broad-band colours, valid over the interval 3300 ≤Teff≤ 5000 K, especially suited for red giants. The analysis of the BCV and BCK estimates along the wide range of metallicity spanned by our stellar sample shows no evident drift with Fe/H. Things may be different for the B-band correction, where the blanketing effects are more and more severe. A drift of Δ(B−V) versus Fe/H is in fact clearly evident from our data, with metal-poor stars displaying a ‘bluer’(B−V) with respect to the metal-rich sample, for fixed Teff. Our empirical bolometric corrections are in good overall agreement with most of the existing theoretical and observational determinations, supporting the conclusion that (a) BCK from the most recent studies are reliable within ≲±0.1 over the whole colour/temperature range considered in this paper, and (b) the same conclusion apply to BCV only for stars warmer than ≃3800 K. At cooler temperatures the agreement is less general, and MARCS models are the only ones providing a satisfactory match to observations, in particular in the BCV versus (B−V) plane.
We have identified in the 2MASS database 693 known and candidate globular clusters in M 31. The 2MASS $J,~H,~K$ magnitudes of these objects have been transformed to the same homogeneous photometric ...system of existing near infrared photometry of M 31 globulars, finally yielding $J,~H,~K$ integrated photometry for 279 confirmed M 31 clusters, 406 unconfirmed candidates and 8 objects with controversial classification. Of these objects 529 lacked any previous estimate of their near infrared magnitudes. The newly assembled near infrared dataset has been implemented into a revised version of the Bologna Catalogue of M 31 globulars, with updated optical (UBVRI) photometry taken, when possible, from the most recent sources of CCD photometry available in the literature and transformed to a common photometric system. The final Revised Bologna Catalogue (available in electronic form) is the most comprehensive list presently available of confirmed and candidate M 31 globular clusters, with a total of 1164 entries. In particular, it includes 337 confirmed GCs, 688 GC candidates, 10 objects with controversial classification, 70 confirmed galaxies, 55 confirmed stars, and 4 H ii regions lying within ~$3\degr$ from the center of the M 31 galaxy. Using the newly assembled database we show that the $V-K$ color provides a powerful tool to discriminate between M 31 clusters and background galaxies, and we identify a sample of 83 globular cluster candidates, which is not likely to be contaminated by misclassified galaxies.
Aims. The main aim of the present work is to derive an empirical mass-loss (ML) law for Population II stars in first and second ascent red giant branches. Methods. We used the Spitzer InfraRed Array ...Camera (IRAC) photometry obtained in the 3.6–8 μm range of a carefully chosen sample of 15 Galactic globular clusters spanning the entire metallicity range and sampling the vast zoology of horizontal branch (HB) morphologies. We complemented the IRAC photometry with near-infrared data to build suitable color-magnitude and color-color diagrams and identify mass-losing giant stars. Results. We find that while the majority of stars show colors typical of cool giants, some stars show an excess of mid-infrared light that is larger than expected from their photospheric emission and that is plausibly due to dust formation in mass flowing from them. For these stars, we estimate dust and total (gas + dust) ML rates and timescales. We finally calibrate an empirical ML law for Population II red and asymptotic giant branch stars with varying metallicity. We find that at a given red giant branch luminosity only a fraction of the stars are losing mass. From this, we conclude that ML is episodic and is active only a fraction of the time, which we define as the duty cycle. The fraction of mass-losing stars increases by increasing the stellar luminosity and metallicity. The ML rate, as estimated from reasonable assumptions for the gas-to-dust ratio and expansion velocity, depends on metallicity and slowly increases with decreasing metallicity. In contrast, the duty cycle increases with increasing metallicity, with the net result that total ML increases moderately with increasing metallicity, about 0.1 M⊙ every dex in Fe/H. For Population II asymptotic giant branch stars, we estimate a total ML of ≤0.1 M⊙, nearly constant with varying metallicity.
Aims. We present the first results of a large spectroscopic survey of candidate globular clusters located in the extreme outskirts of the nearby M 31 galaxy. The survey is aimed at ascertaining the ...nature of the selected candidates to increase the sample of confirmed M 31 clusters lying more that 2away from the center of the galaxy. Methods. We obtained low resolution spectra ( \lambda/\Delta\lambda \simeq 800-1300) of 48 targets selected from the Extended Source Catalogue of 2MASS, as in Galleti et al. (2005, A&A, 436, 535). The observed candidates have been robustly classified according to their radial velocity and by verifying their extended/point- source nature from ground-based optical images. We have also obtained a spectrum and a radial velocity estimate for the remote M 31 globular discovered by Martin et al. (2006b, MNRAS, 371, 1983). Results. Among the 48 observed candidates clusters we found: 35 background galaxies, 8 foreground Galactic stars, and 5 genuine remote globular clusters. One of them has been already identified independently by Mackey et al. (2007, ApJ, 655, L85), their GC1; the other four are completely new discoveries: B516, B517, B518, B519. The newly discovered clusters lie at projected distance 40 kpc \lesssim R_{\rm p}\lesssim 100 kpc from the center of M 31, and have absolute integrated magnitude-9.5 \lesssim M sub( )V\lesssim-7.5. For all the observed clusters we have measured the strongest Lick indices and we have obtained spectroscopic metallicity estimates. Mackey-GC1, Martin-GC1, B517 and B518 have spectra typical of old and metal poor globular clusters (Fe/H \lesssim-1.3) ; B519 appears old but quite metal-rich (Fe/H similar to \simeq-0.5) ; B516 presents very strong Balmer absorption lines: if this is indeed a cluster it should have a relatively young age (likely < 2 Gyr). Conclusions. The present analysis nearly doubles the number of M 31 globulars at R_{\rm p}\ge 40 kpc. At odds with the Milky Way, M 31 appears to have a significant population of very bright globular clusters in its extreme outskirts.
We use high-resolution spectra obtained with the ESO Very Large Telescope to measure surface abundance patterns of 43 blue straggler stars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs ...shows a significant depletion of carbon and oxygen with respect to the dominant population. This evidence would suggest the presence of CNO burning products on the BSS surface coming from a deeply peeled parent star, as expected in the case of a mass transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster.
Abstract
We describe two ground-based observing campaigns aimed at building a grid of approximately 200 spectrophotometric standard stars (SPSS), with an internal ≃1 per cent precision and tied to ...Vega within ≃3 per cent, for the absolute flux calibration of data gathered by Gaia, the European Space Agency (ESA) astrometric mission. The criteria for the selection and a list of candidates are presented, together with a description of the survey strategy and the adopted data analysis methods. We also discuss a short list of notable rejected SPSS candidates and difficult cases, based on identification problems, literature discordant data, visual companions and variability. In fact, all candidates are also monitored for constancy (within ±5 mmag, approximately). In particular, we report on a CALSPEC standard, 1740346, that we found to be a δ Scuti variable during our short-term monitoring (1-2 h) campaign.
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