322 research outputs found
CN Bimodality at Low Metallicity: The Globular Cluster M53
We present low resolution UV-blue spectroscopic observations of red giant
stars in the globular cluster M53 ([Fe/H]=-1.84), obtained to study primordial
abundance variations and deep mixing via the CN and CH absorption bands. The
metallicity of M53 makes it an attractive target: a bimodal distribution of
3883 angstrom CN bandstrength is common in moderate- and high-metallicity
globular clusters ([Fe/H] > -1.6) but unusual in those of lower metallicity
([Fe/H] < -2.0). We find that M53 is an intermediate case, and has a broad but
not strongly bimodal distribution of CN bandstrength, with CN and CH
bandstrengths anticorrelated in the less-evolved stars. Like many other
globular clusters, M53 also exhibits a general decline in CH bandstrength and
[C/Fe] abundance with rising luminosity on the red giant branch.Comment: 8 pages including 11 figures and 1 table, accepted by PAS
Clear evidence for the presence of second-generation asymptotic giant branch stars in metal-poor Galactic globular clusters
Galactic globular clusters (GCs) are known to host multiple stellar
populations: a first generation with a chemical pattern typical of halo field
stars and a second generation (SG) enriched in Na and Al and depleted in O and
Mg. Both stellar generations are found at different evolutionary stages (e.g.,
the main-sequence turnoff, the subgiant branch, and the red giant branch). The
non detection of SG asymptotic giant branch (AGB) stars in several metal-poor
([Fe/H] < -1) GCs suggests that not all SG stars ascend the AGB phase, and that
failed AGB stars may be very common in metal-poor GCs. This observation
represents a serious problem for stellar evolution and GC formation/evolution
theories. We report fourteen SG-AGB stars in four metal-poor GCs (M 13, M 5, M
3, and M 2) with different observational properties: horizontal branch (HB)
morphology, metallicity, and age. By combining the H-band Al abundances
obtained by the APOGEE survey with ground-based optical photometry, we identify
SG Al-rich AGB stars in these four GCs and show that Al-rich RGB/AGB GC stars
should be Na-rich. Our observations provide strong support for present,
standard stellar models, i.e., without including a strong mass-loss efficiency,
for low-mass HB stars. In fact, current empirical evidence is in agreement with
the predicted distribution of FG and and SG stars during the He-burning stages
based on these standard stellar models.Comment: Accepted for publication in The Astrophysical Journal Letters (16
pages, 4 figures, and 1 table
Photometry and spectroscopy of multiple populations along the AGB of NGC2808 and NGC6121 (M4)
We present a photometric and spectroscopic study of multiple populations
along the asymptotic-giant branch (AGB) of the intermediate-metallicity
globular clusters (GCs) NGC2808 and NGC6121 (M4). Chemical abundances of O, Na,
Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Co, Ni, Zn, Y, and Ce in AGB stars from
high-resolution FLAMES+UVES@VLT spectra are reported for both clusters. Our
spectroscopic results have been combined with multi-wavelength photometry from
the HST UV survey of Galactic GCs and ground-based photometry, plus proper
motions derived by combining stellar positions from ground-based images and
Gaia DR1. Our analysis reveals that the AGBs of both clusters host multiple
populations with different chemical composition.
In M4 we have identified two main populations of stars with different Na/O
content, lying on distinct AGBs in the mF438W vs. C_F275W,F336W,F438W and the V
vs.C_U,B,I pseudo-CMDs. In the more massive and complex GC NGC2808 three groups
of stars with different chemical abundances occupy different locations on the
so-called "chromosome map" photometric diagram. The spectroscopic+photometric
comparison of stellar populations along the AGB and the red giants of this GC
suggests that the AGB hosts stellar populations with a range in helium
abundances spanning from primordial up to high contents of Y~0.32. On the other
hand, from our dataset, there is no evidence for stars with extreme helium
abundance (Y~0.38) on the AGB, suggesting that the most He-rich stars of
NGC2808 do not reach this phase.Comment: 33 pages, 8 figures, accepted for publication in Ap
Bright Variable Stars in NGC 6819 - An Open Cluster in the Kepler Field
We describe a variability study of the moderately old open cluster NGC 6819.
We have detected 4 new detached eclipsing binaries near the cluster turnoff
(one of which may be in a triple system). Several of these systems should be
able to provide mass and radius information, and can therefore constrain the
age of the cluster. We have also newly detected one possible detached binary
member about 3.5 magnitudes below the turnoff. One EW-type binary (probably not
a cluster member) shows unusually strong night-to-night light curve variations
in sets of observations separated by 8 years. According to the best current
information, the three brightest variables we detected (2 of them new) are
cluster members, making them blue stragglers. One is a delta Scu pulsating
variable, one is a close but detached binary, and the third contains a detached
short period binary that shows total eclipses. In each case, however, there is
evidence hinting that the system may have been produced through the interaction
of more than two stars.Comment: 33 pages, 15 figures, accepted to A
The Nature of the Red Giant Branches in the Ursa Minor and Draco Dwarf Spheroidal Galaxies
Spectra for stars located redward of the fiducial red giant branches of the
Ursa Minor and Draco dwarf spheroidal galaxies have been obtained with the
Hobby-Eberly telescope and the Marcario Low Resolution Spectrometer. From a
comparison of our radial velocities with those reported in previous
medium-resolution studies, we find an average difference of 10 km/s with a
standard deviation of 11 km/s. On the basis of these radial velocities, we
confirm the membership of five stars in Ursa Minor, and find two others to be
nonmembers. One of the confirmed members is a known carbon star which lies
redward of RGB; three others are previously unidentified carbon stars. The
fifth star is a red giant which was found previously by Shetrone et al. (2001)
to have [Fe/H] =-1.68+/-0.11 dex. In Draco, we find eight nonmembers, confirm
the membership of one known carbon star, and find two new members. One of these
stars is a carbon star, while the other shows no evidence for C2 bands or
strong atomic bands, although the signal-to-noise ratio of the spectrum is low.
Thus, we find no evidence for a population of stars more metal-rich than [Fe/H]
\~ -1.45 dex in either of these galaxies. Indeed, our spectroscopic survey
suggests that every candidate suspected of having a metallicity in excess of
this value based on its position in the color-magnitude diagram is, in
actuality, a carbon star. Based on the census of 13 known carbon stars in these
two galaxies, we estimate of the carbon star specific frequency to be e(dSph) ~
2.4E-5/Lsolarv, 25-100 times higher than that of Galactic globular clusters.Comment: 8 pages including 3 figures accepted in the Publications of the
Astronomical Society of the Pacific. This work is based on observations
obtained with the Hobby-Eberly Telescop
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