463 research outputs found
Evidence of tidal distortions and mass loss from the old open cluster NGC 6791
We present the first evidence of clear signatures of tidal distortions in the
density distribution of the fascinating open cluster NGC 6791. We used deep and
wide-field data obtained with the Canada-France-Hawaii-Telescope covering a 2x2
square degrees area around the cluster. The two-dimensional density map
obtained with the optimal matched filter technique shows a clear elongation and
an irregular distribution starting from ~300" from the cluster center. At
larger distances, two tails extending in opposite directions beyond the tidal
radius are also visible. These features are aligned to both the absolute proper
motion and to the Galactic center directions. Moreover, other overdensities
appear to be stretched in a direction perpendicular to the Galactic plane.
Accordingly to the behaviour observed in the density map, we find that both the
surface brightness and the star count density profiles reveal a departure from
a King model starting from ~600" from the center. These observational evidence
suggest that NGC 6791 is currently experiencing mass loss likely due to
gravitational shocking and interactions with the tidal field. We use this
evidence to argue that NGC 6791 should have lost a significant fraction of its
original mass. A larger initial mass would in fact explain why the cluster
survived so long. Using available recipes based on analytic studies and N-body
simulations, we derived the expected mass loss due to stellar evolution and
tidal interactions and estimated the initial cluster mass to be M_ini=(1.5-4) x
10^5 M_sun.Comment: Accepted for publication in the MNRAS (9 pages, 8 Figures
FLAMES and XSHOOTER spectroscopy along the two BSS sequences of M30
We present spectroscopic observations acquired with FLAMES and XSHOOTER at
the Very Large Telescope for a sample of 15 Blue Straggler Stars (BSSs) in the
globular cluster (GC) M30. The targets have been selected to sample the two BSS
sequences discovered, with 7 BSSs along the blue sequence and 8 along the red
one. No difference in the kinematical properties of the two groups of BSSs has
been found. In particular, almost all the observed BSSs have projected
rotational velocity lower than ~30 km/s, with only one (blue) fast rotating BSS
(>90 km/s), identified as a W UMa binary. This rotational velocity distribution
is similar to those obtained in 47 Tucanae and NGC 6397, while M4 remains the
only GC studied so far harboring a large fraction of fast rotating BSSs. All
stars hotter than ~7800 K (regardless of the parent BSS sequence) show iron
abundances larger than those measured from normal cluster stars, with a
clearcut trend with the effective temperature. This behaviour suggests that
particle trasport mechanisms driven by radiative levitation occur in the
photosphere of these stars, as already observed for the BSSs in NGC 6397.
Finally, 4 BSSs belonging to the red sequence (not affected by radiative
levitation) show a strong depletion of [O/Fe], with respect to the abundance
measured in Red Giant Branch and Horizontal Branch stars. This O-depletion is
compatible with the chemical signature expected in BSSs formed by mass transfer
processes in binary systems, in agreement with the mechanism proposed for the
formation of BSSs in the red sequence.Comment: Accepted for publication in Ap
Another brick in understanding chemical and kinematical properties of BSSs: NGC 6752
We used high-resolution spectra acquired with the multifiber facility FLAMES
at the Very Large Telescope of the European Southern Observatory to investigate
the chemical and kinematical properties of a sample of 22 Blue Straggler Stars
(BSSs) and 26 red giant branch stars in the nearby globular cluster NGC 6752.
We measured radial and rotational velocities and Fe, O and C abundances.
According to radial velocities, metallicity and proper motions we identified 18
BSSs as likely cluster members. We found that all the BSSs rotate slowly (less
than 40 km/s), similar to the findings in 47 Tucanae, NGC 6397 and M30. The Fe
abundance analysis reveals the presence of 3 BSSs affected by radiative
levitation (showing [Fe/H] significantly higher than that measured in "normal"
cluster stars), confirming that element transport mechanisms occur in the
photosphere of BSSs hotter than 8000 K. Finally, BSS C and O abundances are
consistent with those measured in dwarf stars. No C and O depletion ascribable
to mass transfer processes has been found on the atmospheres of the studied
BSSs (at odds with previous results for 47 Tucanae and M30), suggesting the
collisional origin for BSSs in NGC 6752 or that the CO-depletion is a transient
phenomenon.Comment: ApJ accepte
No evidence of chemical anomalies in the bimodal turnoff cluster NGC 1806 in the LMC
We have studied the chemical composition of NGC 1806, a massive,
intermediate-age globular cluster that shows a double main sequence turnoff. We
analyzed a sample of high-resolution spectra (secured with FLAMES at the Very
Large Telescope) for 8 giant stars, members of the cluster, finding an average
iron content of [Fe/H]=--0.60 +- 0.01 dex and no evidence of intrinsic
star-to-star variations in the abundances of light elements (Na, O, Mg, Al).
Also, the (m_(F814W); m_(F336W)-m_(F814W)) color-magnitude diagram obtained by
combining optical and near-UV Hubble Space Telescope photometry exhibits a
narrow red giant branch, thus ruling out intrinsic variations of C and N
abundances in the cluster. These findings demonstrate that NGC 1806 does not
harbor chemically distinct sub-populations, at variance with what was found in
old globular clusters. In turn, this indicates that the double main sequence
turnoff phenomenon cannot be explained in the context of the self-enrichment
processes usually invoked to explain the chemical anomalies observed in old
globulars. Other solutions (i.e., stellar rotation, merging between clusters or
collisions with giant molecular clouds) should be envisaged to explain this
class of globulars.Comment: Accepted for publication by ApJ Letters; 15 pages, 4 figures, 1 tabl
Multiple populations in the old and massive Small Magellanic Cloud globular cluster NGC121
We used a combination of optical and near-UV Hubble Space Telescope
photometry and FLAMES/ESO-VLT high-resolution spectroscopy to characterize the
stellar content of the old and massive globular cluster (GC) NGC121 in the
Small Magellanic Cloud (SMC). We report on the detection of multiple stellar
populations, the first case in the SMC stellar cluster system. This result
enforces the emerging scenario in which the presence of multiple stellar
populations is a distinctive-feature of old and massive GCs regardless of the
environment, as far as the light element distribution is concerned. We find
that second population (SG) stars are more centrally concentrated than first
(FG) ones. More interestingly, at odds with what typically observed in Galactic
GCs, we find that NGC121 is the only cluster so far to be dominated by FG stars
that account for more than 65% of the total cluster mass. In the framework
where GCs were born with a 90-95% of FG stars, this observational finding would
suggest that either NGC121 experienced a milder stellar mass-loss with respect
to Galactic GCs or it formed a smaller fraction of SG stars.Comment: 25 pages, 7 figures, 3 tables; accepted for publication in Ap
The binary mass transfer origin of the red blue straggler sequence in M30
Two separated sequences of blue straggler stars (BSSs) have been revealed by
Ferraro et al. (2009) in the color-magnitude diagram (CMD) of the Milky Way
globular cluster M30. Their presence has been suggested to be related to the
two BSS formation channels (namely, collisions and mass-transfer in close
binaries) operating within the same stellar system. The blue sequence was
indeed found to be well reproduced by collisional BSS models. In contrast, no
specific models for mass transfer BSSs were available for an old stellar system
like M30. Here we present binary evolution models, including case-B mass
transfer and binary merging, specifically calculated for this cluster. We
discuss in detail the evolutionary track of a binary, which
spends approximately 4 Gyr in the BSS region of the CMD of a 13 Gyr old
cluster. We also run Monte-Carlo simulations to study the distribution of mass
transfer BSSs in the CMD and to compare it with the observational data. Our
results show that: (1) the color and magnitude distribution of synthetic mass
transfer BSSs defines a strip in the CMD that nicely matches the observed red
BSS sequence, thus providing strong support to the mass transfer origin for
these stars; (2) the CMD distribution of synthetic BSSs never attains the
observed location of the blue BSS sequence, thus reinforcing the hypothesis
that the latter formed through a different channel (likely collisions); (3)
most () of the synthetic BSSs are produced by mass-transfer models,
while the remaining requires the contribution from merger models.Comment: 8 pages, 5 figures, accepted to Ap
The Terzan 5 puzzle: discovery of a third, metal-poor component
We report on the discovery of 3 metal-poor giant stars in Terzan 5, a complex
stellar system in the the Galactic bulge, known to have two populations at
[Fe/H]=-0.25 and +0.3. For these 3 stars we present new echelle spectra
obtained with NIRSPEC at Keck II, which confirm their radial velocity
membership and provide average [Fe/H]=-0.79 dex iron abundance and
[alpha/Fe]=+0.36 dex enhancement. This new population extends the metallicity
range of Terzan~5 0.5 dex more metal poor, and it has properties consistent
with having formed from a gas polluted by core collapse supernovae.Comment: Accepted for publication on ApJ Lette
An empirical mass-loss law for Population II giants from the Spitzer-IRAC survey of Galactic globular clusters
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. We used
the Spitzer InfraRed Array Camera (IRAC) photometry obtained in the 3.6-8
micron 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. 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 Msun every dex in [Fe/H]. For Population II asymptotic giant branch stars,
we estimate a total ML of <0.1 Msun, nearly constant with varying metallicity.Comment: 17 pages, 9 figures, in press on A&
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