982 research outputs found
A Superwind from Early Post-Red Giant Stars?
We suggest that the gap observed at 20,000 K in the horizontal branches of
several Galactic globular clusters is caused by a small amount of extra mass
loss which occurs when stars start to "peel off" the red giant branch (RGB),
i.e., when their effective temperature starts to increase, even though they may
still be on the RGB. We show that the envelope structure of RGB stars which
start to peel off is similar to that of late asymptotic giant branch stars
known to have a super-wind phase. An analogous super-wind in the RGB peel-off
stars could easily lead to the observed gap in the distribution of the hottest
HB stars.Comment: 9 pages; Accepted by ApJ Letters; Available also at
http://www.astro.puc.cl/~mcatelan
The Red Giant Branch in Near-Infrared Colour-Magnitude Diagrams. II: The luminosity of the Bump and the Tip
We present new empirical calibrations of the Red Giant Branch (RGB) Bump and
Tip based on a homogeneous near-Infrared database of 24 Galactic Globular
Clusters. The luminosities of the RGB Bump and Tip in the J, H and K bands and
their dependence on the cluster metallicity have been studied, yielding
empirical relationships. By using recent transformations between the
observational and theoretical planes, we also derived similar calibrations in
terms of bolometric luminosity. Direct comparison between updated theoretical
models and observations show an excellent agreement. The empirical calibration
of the RGB Tip luminosity in the near-Infrared passbands presented here is a
fundamental tool to derive distances to far galaxies beyond the Local Group, in
view of using the new ground-based adaptive optics facilities and, in the next
future, the James Webb Space Telescope.Comment: 7 pages, 7 figures, accepted for publication in MNRA
Discovery of > 200 RR Lyrae Variables in M62: An Oosterhoff I Globular Cluster with a Predominantly Blue HB
We report on the discovery of a large number of RR Lyrae variable stars in
the moderately metal-rich Galactic globular cluster M62 (NGC 6266), which
places it among the top three most RR Lyrae-rich globular clusters known.
Likely members of the cluster in our studied field, from our preliminary number
counts, include about 130 fundamental-mode (RRab) pulsators, with =
0.548 d, and about 75 first-overtone (RRc) pulsators, with = 0.300 d.
The average periods and the position of the RRab variables with well-defined
light curves in the Bailey diagram both suggest that the cluster is of
Oosterhoff type I. However, the morphology of the cluster's horizontal branch
(HB) is strikingly similar to that of the Oosterhoff type II globular cluster
M15 (NGC 7078), with a dominant blue HB component and a very extended blue
tail. Since M15 and M62 differ in metallicity by about one dex, we conclude
that metallicity, at a fixed HB type, is a key parameter determining the
Oosterhoff status of a globular cluster and the position of its variables in
the Bailey diagram.Comment: 5 pages, 4 figures. ApJ Letters, in pres
The early evolution of Globular Clusters: the case of NGC 2808
Enhancement and spread of helium among globular cluster stars have been
recently suggested as a way to explain the horizontal branch blue tails, in
those clusters which show a primordial spread in the abundances of CNO and
other elements involved in advanced CNO burning (D'Antona et al. 2002). In this
paper we examine the implications of the hypothesis that, in many globular
clusters, stars were born in two separate events: an initial burst (first
generation), which gives origin to probably all high and intermediate mass
stars and to a fraction of the cluster stars observed today, and a second,
prolonged star formation phase (second generation) in which stars form directly
from the ejecta of the intermediate mass stars of the first generation. In
particular, we consider in detail the morphology of the horizontal branch in
NGC 2808 and argue that it unveils the early cluster evolution, from the birth
of the first star generation to the end of the second phase of star formation.
This framework provides a feasible interpretation for the still unexplained
dichotomy of NGC 2808 horizontal branch, attributing the lack of stars in the
RR Lyr region to the gap in the helium content between the red clump, whose
stars are considered to belong to the first stellar generation and have
primordial helium, and the blue side of the horizontal branch, whose minimum
helium content reflects the helium abundance in the smallest mass
(~4Msun)contributing to the second stellar generation. This scenario provides
constraints on the required Initial Mass Function, in a way that a great deal
of remnant neutron stars and stellar mass black holes might have been produced.Comment: 23 pages, 7 figures, in press on The Astrophysical Journa
Dynamical Mass Constraints on Low-Mass Pre-Main-Sequence Stellar Evolutionary Tracks: An Eclipsing Binary in Orion with a 1.0 Msun Primary and an 0.7 Msun Secondary
We report the discovery of a double-lined, spectroscopic, eclipsing binary in
the Orion star-forming region. We analyze the system spectroscopically and
photometrically to empirically determine precise, distance-independent masses,
radii, effective temperatures, and luminosities for both components. The
measured masses for the primary and secondary, accurate to ~1%, are 1.01 Msun
and 0.73 Msun, respectively; thus the primary is a definitive pre-main-sequence
solar analog, and the secondary is the lowest-mass star yet discovered among
pre-main-sequence eclipsing binary systems. We use these fundamental
measurements to test the predictions of pre-main-sequence stellar evolutionary
tracks. None of the models we examined correctly predict the masses of the two
components simultaneously, and we implicate differences between the theoretical
and empirical effective temperature scales for this failing. All of the models
predict the observed slope of the mass-radius relationship reasonably well,
though the observations tend to favor models with low convection efficiencies.
Indeed, considering our newly determined mass measurements together with other
dynamical mass measurements of pre-main-sequence stars in the literature, as
well as measurements of Li abundances in these stars, we show that the data
strongly favor evolutionary models with inefficient convection in the stellar
interior, even though such models cannot reproduce the properties of the
present-day Sun.Comment: Accepted by Ap
Evolved Stars in the Core of the Massive Globular Cluster NGC 2419
We present an analysis of optical and ultraviolet Hubble Space Telescope
photometry for evolved stars in the core of the distant massive globular
cluster NGC 2419. We characterize the horizontal branch (HB) population in
detail including corrections for incompleteness on the long blue tail. We
present a method for removing (to first order) lifetime effects from the
distribution of HB stars to facilitate more accurate measurements of helium
abundance for clusters with blue HBs and to clarify the distribution of stars
reaching the zero-age HB. The population ratio R = N_HB / N_RGB implies there
may be slight helium enrichment among the EHB stars in the cluster, but that it
is likely to be small (dY < 0.05). An examination of the upper main sequence
does not reveal any sign of multiple populations. Through comparisons of
optical CMDs, we present evidence that the EHB clump in NGC 2419 contains the
end of the canonical horizontal branch, and that the boundary between the
normal HB stars and blue hook stars shows up as a change in the density of
stars in the CMD. This corresponds to a spectroscopically-verified gap in NGC
2808 and an "edge" in omega Cen. The more clearly visible HB gap at V = 23.5
appears to be too bright.(Abridged)Comment: 27 pages, 25 figures (some bitmapped), uses emulateapj, accepted to
Astronomical Journa
Discovery of Two Very Low-Mass Binaries: Final Results of an Adaptive Optics Survey of Nearby M6.0-M7.5 Stars
We present updated results of a high-resolution, magnitude limited (Ks<12
mag) imaging survey of nearby low-mass M6.0-M7.5 field stars. The observations
were carried out using adaptive optics at the Gemini North, VLT, Keck II, and
Subaru telescopes. Our sample of 36 stars consists predominantly of nearby (<30
pc) field stars, 5 of which we have resolved as binaries. Two of the binary
systems, 2MASSI J0429184-312356 and 2MASSI J1847034+552243, are presented here
for the first time. All 5 discovered binary systems have separations between
0.08"-0.53" (2-9 AU) with similar mass ratios (q>0.8, delta Ks < 1 mag). This
result supports the hypothesis that wide (a>20 AU) very low-mass (M_tot<0.19
M_sun) binary systems are rare. The projected semimajor axis distribution of
these systems peak at ~ 5 AU and we report a sensitivity-corrected binary
fraction of 9% (-3%, +4%) for stars with primaries of spectral type M6.0-M7.5
with separations >3 AU and mass ratios q>0.6. Within these instrumental
sensitivities, these results support the overall trend that both the semimajor
axis distribution and binary fraction are a function of the mass of the primary
star and decrease with decreasing primary mass. These observations provide
important constraints for low-mass binary star formation theories.Comment: 20 pages, 8 figures, accepted to Ap
Star Counts in the Globular Cluster Omega Centauri. I. Bright Stellar Components
We present an extensive photometry on HB, RGB, and MSTO stars in Omega Cen.
The central regions of the cluster were covered with a mosaic of F435W, F625W,
and F658N-band data collected with ACS/HST. The outer reaches were covered with
a large set of U,B,V,I-band data collected with the [email protected] ESO/MPI telescope.
The final catalogue includes ~1.7 million stars. We identified ~3,200 likely HB
stars and ~12,500 stars brighter than the subgiant branch and fainter than the
RGB bumps. The HB morphology changes with the radial distance. The relative
number of extreme HB stars decreases from ~30% to ~21% when moving from the
center toward the outer regions of the cluster, while the fraction of less hot
HB stars increases from ~62% to ~72%. We performed a detailed comparison
between observed ratios of different stellar tracers and predictions based on
canonical evolutionary models with a primordial helium (Y=0.23) content and
metal abundances (Z=0.0002,0.001) that bracket the observed spread in
metallicity of Omega Cen stars. We found that the empirical star counts of HB
stars are on average larger (30%-40%) than predicted. Moreover, the rate of HB
stars is 43% larger than the MSTO rate. The discrepancy between the rate of HB
compared with the rate of RG and MSTO stars supports the evidence that we are
facing a true excess of HB stars. The same comparison was performed by assuming
a mix of stellar populations made with 70% of canonical stars and 30% of
He-enhanced stars. The discrepancy between theory and observations decreases by
a factor of two when compared with rates predicted by canonical He content
models, but still 15%-25% (Y=0.42) and 15%-20% (Y=0.33) higher than observed.
Furthermore, the ratio between HB and MSTO star counts are ~24% (Y=0.42) and
30% (Y=0.33) larger than predicted lifetime ratios.Comment: 54 pages, 17 figures,to be published in ApJ, see link at
http://stellari.wiki.zoho.co
Near-Infrared photometry of four metal-rich Bulge globular clusters: NGC6304, NGC6569, NGC6637, NGC6638
We present high-quality near-Infrared photometry of four Bulge metal-rich
globular clusters, namely: NGC 6304, NGC 6569, NGC 6637 and NGC 6638. By using
the observed Colour-Magnitude Diagrams we derived a photometric estimates of
the cluster reddening and distance. We performed a detailed analysis of the Red
Giant Branch, presenting a complete description of morphologic parameters and
evolutionary features (Bump and Tip). Photometric estimates of the cluster
metallicity have been obtained by using the updated set of relations (published
by our group) linking the metal abundance to a variety of near-Infrared indices
measured along the Red Giant Branch. The detection of the Red Giant Branch Bump
and the Tip is also presented and briefly discussed.Comment: 12 pages, 15 figures, accepted for publication in MNRA
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