950 research outputs found
Towards the absolute planes: a new calibration of the Bolometric Corrections and Temperature scales for Population II Giants
We present new determinations of bolometric corrections and effective
temperature scales as a function of infrared and optical colors, using a large
database of photometric observations of about 6500 Population II giants in
Galactic Globular Clusters (GGCs), covering a wide range in metallicity
(-2.0<[Fe/H]<0.0). New relations for BC_K vs (V-K), (J-K) and BC_V vs (B-V),
(V-I), (V-J), and new calibrations for T_eff, using both an empirical relation
and model atmospheres, are provided. Moreover, an empirical relation to derive
the R parameter of the Infrared Flux Method as a function of the stellar
temperature is also presented.Comment: 10 pages, 12 .ps figures, MN Latex, accepted by MNRA
RR Lyrae variables in the globular cluster M3 (NGC5272). I. BVI CCD photometry
New BVI CCD photometry is presented for 60 RR Lyrae variables in the globular
cluster M3. Light curves have been constructed and ephemerides have been
(re)-derived for all of them. Four stars (i.e. V29, V136, V155 and V209),
although recognized as variables, had no previous period determinations. Also,
the period derived for V129 is significantly different from the one published
by Sawyer-Hogg (1973). Light curve parameters, i.e. mean magnitudes, amplitudes
and rise-times, have been derived. The discussion of these results in the
framework of the stellar evolution and pulsation theories will be presented in
a forthcoming paper.Comment: 19 pages, latex, uses mn.sty, 12 encapsulated figures, to be
published in MNRAS, text and figures also available at
http://www.bo.astro.it/bap/BAPhome.html or via anonymous ftp at
ftp://boas3.bo.astro.it/bap/files (bap98-12-textfig.ps
The First Empirical Mass Loss Law for Population II Giants
Using the Spitzer IRAC camera we have obtained mid-IR photometry of the red
giant branch stars in the Galactic globular cluster 47 Tuc. About 100 stars
show an excess of mid-infrared light above that expected from their
photospheric emission. This is plausibly due to dust formation in mass flowing
from these stars. This mass loss extends down to the level of the horizontal
branch and increases with luminosity. The mass loss is episodic, occurring in
only a fraction of stars at a given luminosity. Using a simple model and our
observations we derive mass loss rates for these stars. Finally, we obtain the
first empirical mass loss formula calibrated with observations of Population II
stars. The dependence on luminosity of our mass loss rate is considerably
shallower than the widely used Reimers Law. The results presented here are the
first from our Spitzer survey of a carefully chosen sample of 17 Galactic
Globular Clusters, spanning the entire metallicity range from about one
hundredth up to almost solar
BVI Photometry and the Luminosity Function of the Globular Cluster M92
We present new BVI ground-based photometry and VI space-based photometry for
the globular cluster M92 (NGC 6341) and examine luminosity functions in B, V,
and I containing over 50,000 stars ranging from the tip of the red giant branch
to several magnitudes below the main sequence turn off. Once corrected for
completeness, the observed luminosity functions agree very well with
theoretical models and do not show stellar excesses in any region of the
luminosity function. Using reduced chi squared fitting, the new M92 luminosity
function is shown to be an excellent match to the previously published
luminosity function for M30. These points combine to establish that the
"subgiant excess" found in previously published luminosity functions of
Galactic globular clusters are due to deficiencies in the stellar models used
at that time. Using up to date stellar models results in good agreement between
observations and theory.
Several statistical methods are presented to best determine the age of M92.
These methods prove to be insensitive to the exact choice of metallicity within
the published range. Using [Fe/H]=-2.17 to match recent studies we find an age
of 14.2 plus or minus 1.2 Gyr for the cluster.Comment: 22 pages, 13 figures, 3 tables, accepted for publication in A
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
Globular Clusters in the Magellanic Clouds.I:BV CCD-Photometry for 11 Clusters
We present BV CCD-data for 11 intermediate-age LMC clusters; the main
conclusions are: 1. in the (V_to, V_cl,m) and (V-to, (V_to-V_cl,m)) planes the
models yield a good overall description of the data; 2. with the current
sample, it is impossible to firmly choose between "classical" and
"overshooting" models; 3. the separation in colour between the MS band and the
Red He-burning Clump is smaller than predicted by theoretical tracks; 4. the
existence of the so-called "RGB phase-transition (Renzini and Buzzoni 1986)
seems to be confirmed.Comment: 62 pages, 37 figures and tables 6 to 16 available on request,
uuencoded compressed postscript file with tables 1-5 and 17-18 included, BAP
08-1994-020-OA
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
Massive young clusters in the disc of M31
We have studied the properties of a sample of 67 very blue and likely young
massive clusters in M31 extracted from the Bologna Revised Catalog of globular
clusters, selected according to their color [(B-V) < 0.45] and/or to the
strength of their Hbeta spectral index (Hbeta > 3.5 A). Their existence in M31
has been noted by several authors in the past; we show here that these Blue
Luminous Compact Clusters (BLCCs) are a significant fraction (>~ 15%) of the
whole globular cluster system of M31. Compared to the global properties of the
M31 globular cluster system, they appear to be intrinsically fainter,
morphologically less concentrated, and with a shallower Balmer jump and
enhanced absorption in their spectra.
Empirical comparison with integrated properties of clusters with known age as
well as with theoretical SSP models consistently indicate that their typical
age is less than ~2 Gyr, while they probably are not so metal-poor as deduced
if considered to be old. Either selecting BLCCs by their (B-V) colors or by the
strength of their Hbeta index the cluster sample turns out to be distributed
onto the outskirts of M31 disc, sharing the kinematical properties of the thin,
rapidly rotating disc component.
If confirmed to be young and not metal-poor, these clusters indicate the
occurrence of a significant recent star formation in the thin disc of M31,
although they do not set constraints on the epoch of its early formation.Comment: Submitted for publication in the Astronomical Journal. Aastex Latex
file of 22 pages, 12 figures and 3 table
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