386 research outputs found
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
The optical counterpart to the X-ray transient IGR J18245-2452 in the globular cluster M28
We report on the identification of the optical counterpart to the recently
detected INTEGRAL transient IGR J18245-2452 in the Galactic globular cluster
M28. From the analysis of a multi epoch HST dataset we have identified a
strongly variable star positionally coincident with the radio and Chandra X-ray
sources associated to the INTEGRAL transient. The star has been detected during
both a quiescent and an outburst state. In the former case it appears as a
faint, unperturbed main sequence star, while in the latter state it is about
two magnitudes brighter and slightly bluer than main sequence stars. We also
detected Halpha excess during the outburst state, suggestive of active
accretion processes by the neutron star.Comment: Accepted for publication by ApJ; 15 pages, 4 figures, 1 tabl
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
Blue Straggler masses from pulsation properties. I. The case of NGC6541
We used high-spatial resolution images acquired with the WFC3 on board HST to
probe the population of variable blue straggler stars in the central region of
the poorly studied Galactic globular cluster NGC 6541. The time sampling of the
acquired multi wavelength (F390W, F555W and F814W) data allowed us to discover
three WUma stars and nine SX Phoenicis. Periods, mean magnitudes and pulsation
modes have been derived for the nine SX Phoenicis and their masses have been
estimated by using pulsation equations obtained from linear non adiabatic
models. We found masses in the range 1.0-1.1Mo, with an average value of
1.06+-0.09 Mo (sigma = 0.04), significantly in excess of the cluster Main
Sequence Turn Off mass (~ 0.75Mo). A mild trend between mass and luminosity
seems also to be present. The computed pulsation masses turn out to be in very
good agreement with the predictions of evolutionary tracks for single stars,
indicating values in the range ~ 1.0-1.2 Mo for most of the BSS population, in
agreement with what discussed in a number of previous studies.Comment: 8 pages, 9 figures, ApJ, accepte
The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440
We used a combination of deep optical and Halpha images of the Galactic
globular cluster NGC 6440, acquired with the Hubble Space Telescope, to
identify the optical counterpart to the accreting millisecond X-ray pulsar SAX
J1748.9-2021during quiescence. A strong Halpha emission has been detected from
a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15"
from the nominal position of the X-ray source. The position of the star also
agrees with the optical counterpart found by Verbunt et al. (2000) during an
outburst. We propose this star as the most likely optical counterpart to the
binary system. By direct comparison with isochrones, we estimated that COM-SAX
J1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsun
and a superficial temperature of 5250pm80 K. These parameters combined with the
orbital characteristics of the binary suggest that the system is observed at a
very low inclination angle (~8 deg -14 deg) and that the star is filling or
even overflowing its Roche Lobe. This, together with the equivalent width of
the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The
possibile presence of such a on-going mass transfer during a quiescence state
also suggests that the radio pulsar is not active yet and thus this system,
despite its similarity with the class of redback millisecond pulsars, is not a
transitional millisecond pulsar.Comment: 8 pages, 6 figures. Accepted for publication in Ap
Potassium: a new actor on the globular cluster chemical evolution stage. The case of NGC 2808
We derive [K/Fe] abundance ratios for 119 stars in the globular cluster NGC
2808, all of them having O, Na, Mg and Al abundances homogeneously measured in
previous works. We detect an intrinsic star-to-star spread in the Potassium
abundance. Moreover [K/Fe] abundance ratios display statistically significant
correlations with [Na/Fe] and [Al/Fe], and anti-correlations with [O/Fe] and
[Mg/Fe]. All the four Mg deficient stars ([Mg/Fe]<0.0) discovered so far in NGC
2808 are enriched in K by ~0.3 dex with respect to those with normal [Mg/Fe].
NGC 2808 is the second globular cluster, after NGC 2419, where a clear Mg-K
anti-correlation is detected, albeit of weaker amplitude. The simultaneous
correlation/anti-correlation of [K/Fe] with all the light elements usually
involved in the chemical anomalies observed in globular cluster stars, strongly
support the idea that these abundance patterns are due to the same
self-enrichment mechanism that produces Na-O and Mg-Al anti-correlations. This
finding suggests that detectable spreads in K abundances may be typical in the
massive globular clusters where the self-enrichment processes are observed to
produce their most extreme manifestations.Comment: Accepted for publication by ApJ, 5 pages, 3 figure
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