121 research outputs found
GAIA: AGB stars as tracers of star formation histories in the Galaxy and beyond
We discuss the tracing of star formation histories with ESA's space
astrometry mission GAIA, emphasizing the advantages of AGB stars for this
purpose. GAIA's microarcsecond-level astrometry, multi-band photometry and
spectroscopy will provide individual distances, motions, effective
temperatures, gravities and metallicities for vast numbers of AGB stars in the
Galaxy and beyond. Reliable ages of AGB stars can be determined to distances of
\~200 kpc in a wide range of ages and metallicities, allowing star formation
histories to be studied in a diversity of astrophysical environments.Comment: 4 pages, 1 figure, to be appear in 'Mass-Losing Pulsating Stars and
their Circumstellar Matter', eds. Y. Nakada, M. Honma & M. Seki, Kluwer ASSL
series, vol. 28
Dynamical age differences among coeval star clusters as revealed by blue stragglers
Globular star clusters that formed at the same cosmic time may have evolved
rather differently from a dynamical point of view (because that evolution
depends on the internal environment) through a variety of processes that tend
progressively to segregate stars more massive than the average towards the
cluster centre. Therefore clusters with the same chronological age may have
reached quite different stages of their dynamical history (that is, they may
have different dynamical ages). Blue straggler stars have masses greater than
those at the turn-off point on the main sequence and therefore must be the
result of either a collision or a mass-transfer event. Because they are among
the most massive and luminous objects in old clusters, they can be used as test
particles with which to probe dynamical evolution. Here we report that globular
clusters can be grouped into a few distinct families on the basis of the radial
distribution of blue stragglers. This grouping corresponds well to an effective
ranking of the dynamical stage reached by stellar systems, thereby permitting a
direct measure of the cluster dynamical age purely from observed properties.Comment: Published on the 20 December 2012 issue of Natur
Two distinct sequences of blue straggler stars in the globular cluster M30
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
M30. 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 arise from
the 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.Comment: Published on the 24th December 2009 issue of Natur
The double blue-straggler sequence in NGC 2173: an artifact of field contamination
Here we discuss the case of the double blue straggler star (BSS) sequence recently detected in the young stellar cluster NGC2173 in the Large Magellanic Cloud (LMC) by Li et al. (2018, ApJ, 856, 25). In order to investigate this feature we made use of two Hubble Space Telescope (HST) sets of observations, one (the same one used by Li et al.) probing the cluster central regions, and the other sampling the surrounding field. We demonstrate that when field star decontamination is applied, _40% of the BSS population selected by Li et al. turns out to be composed of field stars interlopers. This contamination mainly affects one of the two sequences, which therefore disappears in the decontaminated color-magnitude diagram. We analyze the result of tens different decontamination realizations: we find no evidence of a double BSS sequence in any of them. We therefore conclude that NGC2173 harbors a normal single (poorly populated) BSS sequence and that particular care needs to be devoted to the field decontamination process in any study aimed at probing stellar population features or star counts in the LMC clusters
HST - WFPC2 photometry of the globular cluster ngc 288: binary systems, blue stragglers and very blue stars
We report on new WFPC2 observations of the globular cluster NGC 288, focusing
our attention on peculiar stars. A very pronounced binary sequence, paralleling
the ordinary Main Sequence (MS) is clearly observed in the Color Magnitude
Diagram (CMD) and a huge relative fraction of Blue Straggler Stars is measured.
The dataset offers the opportunity of studying the evolution of a large
population of binaries (and binary evolution by-products) in a low density
environment, where the evolution of such systems is not dominated by collisions
and encounters. Three (very) Extreme Horizontal Branch Stars have been found,
all lying outside of the cluster core.Comment: 6 pages, 3 figures, in press in the chemical evolution of the Milky
Way: stars versus clusters, F. Matteucci and F. Giovannelli eds, Kluwe
Digging for Relics of the Past: The Ancient and Obscured Bulge Globular Cluster NGC 6256
We used a set of moderately deep and high-resolution optical observations obtained with the Hubble Space Telescope to investigate the properties of the stellar population in the heavily obscured bulge globular cluster (GC) NGC 6256. The analysis of the colorâmagnitude diagram (CMD) revealed a stellar population with an extended blue horizontal branch severely affected by differential reddening, which was corrected, taking into account color excess variations up to ÎŽE(B â V) ~ 0.51. We implemented a Monte Carlo Markov Chain technique to perform the isochrone fitting of the observed CMD in order to derive the stellar age, the cluster distance, and the average color excess in the cluster direction. Using three different sets of isochrones we found that NGC 6256 is characterized by a very old stellar age around 13.0 Gyr, with a typical uncertainty of ~0.5 Gyr. We also found an average color excess of E(B â V) = 1.19 and a distance from the Sun of 6.8 kpc. We then derived the cluster gravitational center and measured its absolute proper motion using the Gaia-DR2 catalog. All this was used to back-integrate the cluster orbit in a Galaxy-like potential and measure its integrals of motion. It turned out that NGC 6256 is currently in a low-eccentricity orbit entirely confined within the bulge and its integrals of motion are fully compatible with a cluster purely belonging to the Galaxy native GC population. All these pieces of evidence suggest that NGC 6256 is an extremely old relic of the past history of the Galaxy, formed during the very first stages of its assembly
A Chemical Trompe-L'ceil: No Iron Spread In The Globular Cluster M22
We present the analysis of high-resolution spectra obtained with UVES and UVES-FLAMES at the Very Large Telescope of 17 giants in the globular cluster M22, a stellar system suspected to have an intrinsic spread in the iron abundance. We find that when surface gravities are derived spectroscopically (by imposing to obtain the same iron abundance from FeI and FeII lines) the [Fe/H] distribution spans _0.5 dex, according to previous analyses. However, the gravities obtained in this way correspond to unrealistic low stellar masses (0.1-0.5 Mâ) for most of the surveyed giants. Instead, when photometric gravities are adopted, the [FeII/H] distribution shows no evidence of spread at variance with the [FeI/H] distribution. This difference has been recently observed in other clusters and could be due to non-local thermodynamical equilibrium effects driven by over- ionization mechanisms, that mainly affect the neutral species (thus providing lower [FeI/H]) but leave [FeII/H] unaltered. We confirm that the s-process elements show significant star-to-star variations and their abundances appear to be correlated with the difference between [FeI/H] and [FeII/H]. This puzzling finding suggests that the peculiar chemical composition of some cluster stars may be related to effects able to spuriously decrease [FeI/H]. We conclude that M22 is a globular cluster with no evidence of intrinsic iron spread, ruling out that it has retained the supernovae ejecta in its gravitational potential well
On the Globular Cluster IMF below 1 Solar Mass
(Abridged) Accurate luminosity functions (LF) for a dozen globular clusters
have now been measured at or just beyond their half-light radius using HST.
They span almost the entire cluster main sequence below ~ 0.75 Msolar. All
these clusters exhibit LF that rise continuously from an absolute I magnitude
M_I ~ 6 to a peak at M_I ~ 8.5-9 and then drop with increasing M_I.
Transformation of the LF into mass functions (MF) by means of the most recent
mass luminosity relations that are consistent with all presently available data
on the physical properties of low mass, low metallicity stars shows that all
the LF observed so far can be obtained from MF having the shape of a log-normal
distribution with characteristic mass m_c=0.33 +/- 0.03 Msolar and standard
deviation sigma = 1.81 +/- 0.19. After correction for the effects of mass
segregation, the variation of the ratio of the number of higher to lower mass
stars with cluster mass or any simple orbital parameter or the expected time to
disruption recently computed for these clusters shows no statistically
significant trend over a range of this last parameter of more than a factor of
100. We conclude that the global MF of these clusters have not been measurably
modified by evaporation and tidal interactions with the Galaxy and, thus,
should reflect the initial distribution of stellar masses. Since the log-normal
function that we find is also very similar to the one obtained independently
for much younger clusters and to the form expected theoretically, the
implication seems to be unavoidable that it represents the true stellar IMF for
this type of stars in this mass range.Comment: Accepted for publication in The Astrophysical Journal. Contains 28
pages with 6 figure
Slowly cooling white dwarfs in M13 from stable hydrogen burning
White dwarfs (WDs) are the final evolutionary product of the vast majority of stars in the Universe. They are electron-degenerate structures characterized by no stable thermonuclear activity, and their evolution is generally described as a pure cooling process. Their cooling rate is adopted as cosmic chronometer to constrain the age of several Galactic populations, including the disk, globular and open clusters. By analysing high-resolution photometric data of two very similar Galactic globular clusters (M3 and M13), we find a clear-cut and unexpected overabundance of bright WDs in M13. Theoretical models suggest that, consistent with the horizontal branch morphology, this overabundance is due to a slowing down of the cooling process in ~70% of the WDs in M13, caused by stable thermonuclear burning in their residual hydrogen-rich envelope. The presented observational evidence of quiescent thermonuclear activity occurring in cooling WDs brings new attention on the use of the WD cooling rate as cosmic chronometer for low-metallicity environments
Slowly Cooling White Dwarfs in NGC 6752
Recently, a new class of white dwarfs (âslowly cooling WDsâ) has been identified in the globular cluster M13. The cooling time of these stars is increased by stable thermonuclear hydrogen burning in their residual envelope. These WDs are thought to be originated by horizontal branch (HB) stars populating the HB blue tail that skipped the asymptotic giant branch phase. To further explore this phenomenon, we took advantage of deep photometric data acquired with the Hubble Space Telescope in the near-ultraviolet and investigate the bright portion of the WD cooling sequence in NGC 6752, another Galactic globular cluster with a metallicity, age, and HB morphology similar to M13. The normalized WD luminosity function derived in NGC 6752 turns out to be impressively similar to that observed in M13, in agreement with the fact that the stellar mass distribution along the HB of these two systems is almost identical. As in the case of M13, the comparison with theoretical predictions is consistent with âŒ70% of the investigated WDs evolving at slower rates than standard, purely cooling WDs. Thanks to its relatively short distance from Earth, NGC 6752 photometry reaches a luminosity 1 order of a magnitude fainter than the case of M13, allowing us to sample a regime where the cooling time delay, with respect to standard WD models, reaches âŒ300 Myr. The results presented in this paper provide new evidence for the existence of slowly cooling WDs and further support to the scenario proposing a direct causal connection between this phenomenon and the HB morphology of the host stellar cluster
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