816 research outputs found
The extended structure of the remote cluster B514 in M31. Detection of extra-tidal stars
We present a study of the density profile of the remote M31 globular cluster
B514, obtained from HST/ACS observations. Coupling the analysis of the
distribution of the integrated light with star counts we can reliably follow
the profile of the cluster out to r~35", corresponding to ~130pc. The profile
is well fitted, out to ~15 core radii, by a King Model having C=1.65. With an
estimated core radius r_c=0.38", this corresponds to a tidal radius of r_t~17"
(~65pc). We find that both the light and the star counts profiles show a
departure from the best fit King model for r>~8" - as a surface brightness
excess at large radii, and the star counts profile shows a clear break in
correspondence of the estimated tidal radius. Both features are interpreted as
the signature of the presence of extratidal stars around the cluster. We also
show that B514 has a half-light radius significantly larger than ordinary
globular clusters of the same luminosity. In the M_V vs. log r_h plane, B514
lies in a region inhabited by peculiar clusters, like Omega Cen, G1, NGC2419
and others, as well as by the nuclei of dwarf elliptical galaxies.Comment: 9 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysic
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&
Environmental effects on the structure of the Dwarf Spheroidal Galaxies
The central surface brightness () of the dwarf spheroidal
satellites of our own Galaxy is found to correlate with their distance from the
Galactic Center (). This observational evidence suggests that
environmental effects could strongly influence their structure. This suggestion
is further supported by a clear-cut bivariate correlation linking to
a linear combination of , the absolute integrated magnitude, and ,
which seems to be shared also by the M31 spheroidals. Possible implications for
the Fundamental Plane of elliptical galaxies and Dark Matter content of dwarf
spheroidals are also briefly discussed.Comment: 14 pages, plain TEX, 4 figures soon available via
ftp://boas3.bo.astro .it bap/files/ , get bap95-34fig*.ps or
http://www.bo.astro.it/ or upon request at [email protected]
--accepted by MNRAS-Letter
Different Characteristics of the Bright Branches of the Globular Clusters M3 and M13
We carried out wide-field BVI CCD photometric observations of the GCs M3 and
M13 using the BOAO 1.8 m telescope equipped with a 2K CCD. We present CMDs of
M3 and M13. We have found AGB bumps at V = 14.85 for M3 at V = 14.25 for M13.
It is found that AGB stars in M3 are more concentrated near the bump, while
those in M13 are scattered along the AGB sequence. We identified the RGB bump
of M3 at V = 15.50 and that of M13 at V = 14.80. We have estimated the ratios R
and R2 for M3 and M13 and found that of R for M3 is larger than that for M13
while R2's for M3 and M13 are similar when only normal HB stars are used in R
and R2 for M13. However, we found that R's for M3 and M13 are similar while R2
for M3 is larger than that for M13 when all the HB stars are included in R and
R2 for M13. We have compared the observed RGB LFs of M3 and M13 with the
theoretical RGB LF of Bergbusch & VandenBerg at the same radial distances from
the cluster centers as used in R and R2 for M3 and M13. We found "extra stars"
belonging to M13 in the comparison of the observed RGB LF of M13 and the
theoretical RGB LF of Bergbusch & VandenBerg. In the original definition of R
of Buzzoni et al., N(HB) corresponds to the lifetime of HB stars in the RR
Lyrae instability strip at log T_eff = 3.85. So, the smaller R value resulting
for M13 compared with that for M3 in the case where only normal HB stars are
included in R and R2 for M13 may be partially caused by "extra stars", and the
similar R's for M3 and M13 in the case where the all HB stars are included in R
and R2 for M13 may be caused by "extra stars" in the upper RGB of M13. If
"extra stars" in the upper RGB of M13 are caused by an effective "deep mixing"
these facts support the contention that an effective "deep mixing" could lead
to different HB morphologies between M3 and M13 and subsequent sequences.Comment: 24 pages, 7 figures, to be published in the A
Correlations of Globular Cluster Properties: Their Interpretations and Uses
Correlations among the independently measured physical properties of globular
clusters (GCs) can provide powerful tests for theoretical models and new
insights into their dynamics, formation, and evolution. We review briefly some
of the previous work, and present preliminary results from a comparative study
of GC correlations in the Local Group galaxies. The results so far indicate
that these diverse GC systems follow the same fundamental correlations,
suggesting a commonality of formative and evolutionary processes which produce
them.Comment: An invited review, to appear in "New Horizons in Globular Cluster
Astronomy", eds. G. Piotto, G. Meylan, S.G. Djorgovski, and M. Riello, ASPCS,
in press (2003). Latex file, 8 pages, 5 eps figures, style files include
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