616 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
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 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
No evidence of mass segregation in the low mass Galactic globular cluster NGC 6101
We used a combination of Hubble Space Telescope and ground based data to
probe the dynamical state of the low mass Galactic globular cluster NGC 6101.
We have re-derived the structural parameters of the cluster by using star
counts and we find that it is about three times more extended than thought
before. By using three different indicators, namely the radial distribution of
Blue Straggler Stars, that of Main Sequence binaries and the luminosity (mass)
function, we demonstrated that NGC 6101 shows no evidence of mass segregation,
even in the innermost regions. Indeed, both the BSS and the binary radial
distributions fully resemble that of any other cluster population. In addition
the slope of the luminosity (mass) functions does not change with the distance,
as expected for non relaxed stellar systems. NGC 6101 is one of the few
globulars where the absence of mass segregation has been observed so far. This
result provides additional support to the use of the "dynamical clock"
calibrated on the radial distribution of the Blue Stragglers as a powerful
indicator of the cluster dynamical age.Comment: Accepted for publication by ApJ; 33 pages, 13 figure
A Family Picture: Tracing the Dynamical Path of the Structural Properties of Multiple Populations in Globular Clusters
We studied the spatial distributions of multiple stellar populations (MPs) in a sample of 20 globular clusters (GCs) spanning a broad range of dynamical ages. The differences between first-population (FP) and second-population (SP) stars were measured by means of the parameter A +, defined as the area enclosed between their cumulative radial distributions. We provide the first purely observational evidence of the dynamical path followed by MPs from initial conditions toward a complete FP–SP spatial mixing. Less dynamically evolved clusters have SP stars more centrally concentrated than FPs, while in more dynamically evolved systems the spatial differences between FP and SP stars decrease and eventually disappear. By means of an appropriate comparison with a set of numerical simulations, we show that these observational results are consistent with the evolutionary sequence expected by the long-term dynamical evolution of clusters forming with an initially more centrally concentrated SP subsystem. This result is further supported by the evidence of a trend between A + and the stage of GC dynamical evolution inferred by the ratio between the present-day and the initial mass of the cluster
Blue straggler masses from pulsation properties. II. Topology of the Instability Strip
We present a new set of nonlinear, convective radial pulsation models for
main sequence stars computed assuming three metallicities: Z=0.0001, 0.001 and
0.008. These chemical compositions bracket the metallicity of stellar systems
hosting SX Phoenicis stars (SXPs or pulsating Blue Stragglers), namely Galactic
globular clusters and nearby dwarf spheroidals. Stellar masses and luminosities
of the pulsation models are based on alpha--enhanced evolutionary tracks from
the BASTI website. We are able to define the topology of the instability strip
(IS), and in turn the pulsation relations for the first four pulsation modes.
We found that third overtones approach a stable nonlinear limit cycle.
Predicted and empirical IS agree quite well in the case of 49 SXPs belonging to
omega Cen. We used theoretical Period-Luminosity relations in B,V bands to
identify their pulsation mode. We assumed Z=0.001 and Z=0.008 as mean
metallicities of SXPs in omega Cen. We found respectively 13-15 fundamental,
22-6 first and 9-4 second overtone modes. Five are unstable in the third
overtone mode only for Z=0.001. Using the above mode identification and
applying the proper mass-dependent Period-Luminosity relations we found masses
ranging from ~1.0 to 1.2 Mo (=1.12, sigma=0.04 Mo) and from ~1.2 to 1.5 Mo
(=1.33, sigma=0.03 Mo) for Z=0.001 and 0.008 respectively. Our investigation
supports the use of evolutionary tracks to estimate of SXP masses. We will
extend our analysis to higher Helium content that may have an impact in our
understanding of the BSS formation scenario.Comment: 14 pages, 5 figures, 7 tables, ApJ accepte
UV observations of the globular cluster M10 from HST and GALEX. The BSS population
We present a combination of high-resolution Hubble Space Telescope and
wide-field ground-based and Galaxy Evolution Explorer data of the Galactic
Globular Cluster M10 (NGC6254). By using this large data-set we determined the
center of gravity of the cluster and we built its density profile from star
counts over its entire radial extension. We find that the density profile is
well reproduced by a single-mass King model with structural parameters c=1.41
and r_c=41". We also studied the Blue Straggler Star population and its radial
distribution. We count a total number of 120 BSS within the tidal radius. Their
radial distribution is bimodal: highly peaked in the cluster center, decreasing
at intermediate distances and rising again outwards. We discuss these results
in the context of the dynamical clock scheme presented by Ferraro et al. (2012)
and of recent results about the radial distribution of binary systems in this
cluster.Comment: Accepted for publication by ApJ; 26 pages, 11 figures, 1 tabl
Deep multi-telescope photometry of NGC 5466. I. Blue Stragglers and binary systems
We present a detailed investigation of the radial distribution of blue
straggler star and binary populations in the Galactic globular cluster NGC
5466, over the entire extension of the system. We used a combination of data
acquired with the ACS on board the Hubble Space Telescope, the LBC-blue mounted
on the Large Binocular Telescope, and MEGACAM on the Canadian-France-Hawaii
Telescope. Blue straggler stars show a bimodal distribution with a mild central
peak and a quite internal minimum. This feature is interpreted in terms of a
relatively young dynamical age in the framework of the "dynamical clock"
concept proposed by Ferraro et al. (2012). The estimated fraction of binaries
is 6-7% in the central region (r<90") and slightly lower (5.5%) in the
outskirts, at r>200". Quite interestingly, the comparison with the results of
Milone et al. (2012) suggests that also binary systems may display a bimodal
radial distribution, with the position of the minimum consistent with that of
blue straggler stars. If confirmed, this feature would give additional support
to the scenario where the radial distribution of objects more massive than the
average cluster stars is primarily shaped by the effect of dynamical friction.
Moreover, this would also be consistent with the idea that the unperturbed
evolution of primordial binaries could be the dominant BSS formation process in
low-density environments.Comment: Accepted for publication on 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
Constraining the true nature of an exotic binary in the core of NGC 6624
We report on the identification of the optical counterpart to Star1, the
exotic object serendipitously discovered by Deutsch et al. in the core of the
Galactic globular cluster NGC 6624. Star1 has been classified by Deutsch et al.
as either a quiescent Cataclysmic Variable or a low-mass X-ray binary. Deutsch
et al. proposed StarA as possible optical counterpart to this object. We used
high-resolution images obtained with the Hubble Space Telescope to perform a
variability analysis of the stars close to the nominal position of Star1. While
no variability was detected for StarA, we found another star, here named
COM_Star1, showing a clear sinusoidal light modulation with amplitude \Delta
m_F435W~0.7 mag and orbital period of P_orb~98 min. The shape of the light
curve is likely caused by strong irradiation by the primary heating one
hemisphere of the companion, thus suggesting a quite hot primary.Comment: Accepted for publication by ApJ Letters; 6 pages, 5 figure
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