343 research outputs found
New cluster members and halo stars of the Galactic globular cluster NGC 1851
NGC 1851 is an intriguing Galactic globular cluster, with multiple stellar
evolutionary sequences, light and heavy element abundance variations and
indications of a surrounding stellar halo. We present the first results of a
spectroscopic study of red giant stars within and outside of the tidal radius
of this cluster. Our results identify nine probable new cluster members (inside
the tidal radius) with heliocentric radial velocities consistent with that of
NGC 1851. We also identify, based on their radial velocities, four probable
extratidal cluster halo stars at distances up to ~3.1 times the tidal radius,
which are supportive of previous findings that NGC 1851 is surrounded by an
extended stellar halo. Proper motions were available for 12 of these 13 stars
and all are consistent with that of NGC 1851. Apart from the cluster members
and cluster halo stars, our observed radial velocity distribution agrees with
the expected distribution from a Besancon disk/N-body stellar halo Milky Way
model generated by the Galaxia code, suggesting that no other structures at
different radial velocities are present in our field. The metallicities of
these stars are estimated using equivalent width measurements of the near
infrared calcium triplet absorption lines and are found, within the limitations
of this method, to be consistent with that of NGC 1851. In addition we recover
110 red giant cluster members from previous studies based on their radial
velocities and identify three stars with unusually high radial velocities.Comment: 10 pages, 8 figures. Accepted for publication in MNRA
New halo stars of the Galactic globular clusters M3 and M13 in the LAMOST DR1 Catalog
M3 and M13 are Galactic globular clusters with previous reports of
surrounding stellar halos. We present the results of a search for members and
extratidal cluster halo stars within and outside of the tidal radius of these
clusters in the LAMOST Data Release 1. We find seven candidate cluster members
(inside the tidal radius) of both M3 and M13 respectively. In M3 we also
identify eight candidate extratidal cluster halo stars at distances up to ~9.8
times the tidal radius, and in M13 we identify 12 candidate extratidal cluster
halo stars at distances up to ~13.8 times the tidal radius. These results
support previous indications that both M3 and M13 are surrounded by extended
stellar halos, and we find that the GC destruction rates corresponding to the
observed mass loss are generally significantly higher than theoretical studies
predict.Comment: 21 pages, 10 figures. Accepted for publication in Ap
ESO452-SC11: The lowest mass globular cluster with a potential chemical inhomogeneity
We present the largest spectroscopic investigation of one of the faintest and
least studied stellar clusters of the Milky Way, ESO452-SC11. Using the
Anglo-Australian Telescope AAOmega and Keck HIRES spectrographs we have
identified 11 members of the cluster and found indications of star-to-star
light element abundance variation, primarily using the blue cyanogen (CN)
absorption features. From a stellar density profile, we estimate a total
cluster mass of solar masses. This would make
ESO452-SC11 the lowest mass cluster with evidence for multiple populations.
These data were also used to measure the radial velocity of the cluster
( km s) and confirm that ESO452-SC11 is relatively
metal-rich for a globular cluster ([Fe/H]). All known massive
clusters studied in detail show multiple populations of stars each with a
different chemical composition, but many low-mass globular clusters appear to
be chemically homogeneous. ESO452-SC11 sets a lower mass limit for the multiple
stellar population phenomenon.Comment: 13 pages, 11 figures. Accepted for publication in MNRA
The Highly Unusual Chemical Composition of the Hercules Dwarf Spheroidal Galaxy
We report on the abundance analysis of two red giants in the faint Hercules
dwarf spheroidal (dSph) galaxy. These stars show a remarkable deficiency in the
neutron-capture elements, while the hydrostatic alpha-elements (O, Mg) are
strongly enhanced. Our data indicate [Ba/Fe] and [Mg/Fe] abundance ratios of
<-2 dex and ~+0.8 dex, respectively, with essentially no detection of other
n-capture elements. In contrast to the only other dSph star with similar
abundance patterns, Dra 119, which has a very low metallicity at [Fe/H]=-2.95
dex, our objects, at [Fe/H]~-2.0 dex, are only moderately metal poor. The
measured ratio of hydrostatic/explosive alpha-elements indicates that high-mass
(~35 M_sun) Type II supernovae progenitors are the main, if not only,
contributors to the enrichment of this galaxy. This suggests that star
formation and chemical enrichment in the ultrafaint dSphs proceeds
stochastically and inhomogeneously on small scales, or that the IMF was
strongly skewed to high mass stars. The neutron capture deficiencies and the
[Co/Fe] and [Cr/Fe] abundance ratios in our stars are similar to those in the
extremely low metallicity Galactic halo. This suggests that either our stars
are composed mainly of the ejecta from the first, massive, population III stars
(but at moderately high [Fe/H]), or that SN ejecta in the Hercules galaxy were
diluted with ~30 times less hydrogen than typical for extreme metal-poor stars.Comment: 5 pages, 3 figures, accepted by Astrophysical Journal Letter
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The SPLASH Survey: A Spectroscopic Analysis of the Metal-Poor, Low-Luminosity M31 dSph Satellite Andromeda X
Andromeda X (And X) is a newly discovered low-luminosity M31 dwarf spheroidal
galaxy (dSph) found by Zucker et al. (2007) in the Sloan Digital Sky Survey
(SDSS - York et al. 2000). In this paper, we present the first spectroscopic
study of individual red giant branch stars in And X, as a part of the SPLASH
Survey (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo).
Using the Keck II telescope and multiobject DEIMOS spectrograph, we target two
spectroscopic masks over the face of the galaxy and measure radial velocities
for ~100 stars with a median accuracy of sigma_v ~ 3 km/s. The velocity
histogram for this field confirms three populations of stars along the sight
line: foreground Milky Way dwarfs at small negative velocities, M31 halo red
giants over a broad range of velocities, and a very cold velocity ``spike''
consisting of 22 stars belonging to And X with v_rad = -163.8 +/- 1.2 km/s. By
carefully considering both the random and systematic velocity errors of these
stars (e.g., through duplicate star measurements), we derive an intrinsic
velocity dispersion of just sigma_v = 3.9 +/- 1.2 km/s for And X, which for its
size, implies a minimum mass-to-light ratio of M/L =37^{+26}_{-19} assuming the
mass traces the light. Based on the clean sample of member stars, we measure
the median metallicity of And X to be [Fe/H] = -1.93 +/- 0.11, with a slight
radial metallicity gradient. The dispersion in metallicity is large,
sigma([Fe/H]) = 0.48, possibly hinting that the galaxy retained much of its
chemical enrichment products. We discuss the potential for better understanding
the formation and evolution mechanisms for M31's system of dSphs through
(current) kinematic and chemical abundance studies, especially in relation to
the Milky Way sample. (abridged version)Comment: Accepted for Publication in Astrophys. J. 14 pages including 7
figures and 2 tables (journal format
Chemical Doppelgangers in GALAH DR3: the Distinguishing Power of Neutron-Capture Elements Among Milky Way Disk Stars
The observed chemical diversity of Milky Way stars places important
constraints on Galactic chemical evolution and the mixing processes that
operate within the interstellar medium. Recent works have found that the
chemical diversity of disk stars is low. For example, the APOGEE "chemical
doppelganger rate," or the rate at which random pairs of field stars appear as
chemically similar as stars born together, is high, and the chemical
distributions of APOGEE stars in some Galactic populations are well-described
by two-dimensional models. However, limited attention has been paid to the
heavy elements (Z > 30) in this context. In this work, we probe the potential
for neutron-capture elements to enhance the chemical diversity of stars by
determining their effect on the chemical doppelganger rate. We measure the
doppelganger rate in GALAH DR3, with abundances rederived using The Cannon, and
find that considering the neutron-capture elements decreases the doppelganger
rate from 2.2% to 0.4%, nearly a factor of 6, for stars with -0.1 < [Fe/H] <
0.1. While chemical similarity correlates with similarity in age and dynamics,
including neutron-capture elements does not appear to select stars that are
more similar in these characteristics. Our results highlight that the
neutron-capture elements contain information that is distinct from that of the
lighter elements and thus add at least one dimension to Milky Way abundance
space. This work illustrates the importance of considering the neutron-capture
elements when chemically characterizing stars and motivates ongoing work to
improve their atomic data and measurements in spectroscopic surveys.Comment: 23 pages, 16 figures, 1 table. Submitted to AAS Journals, comments
welcome. Associated catalog of high precision, Cannon-rederived abundances
for GALAH giants to be made publicly available upon acceptance and available
now upon request. See Walsen et al. 2023 for a complementary, high precision,
Cannon-rederived abundance catalog for GALAH solar twin
GHOSTS I: A New Faint very Isolated Dwarf Galaxy at D = 12 +/- 2 Mpc
We report the discovery of a new faint dwarf galaxy, GHOSTS I, using HST/ACS
data from one of our GHOSTS (Galaxy Halos, Outer disks, Substructure, Thick
disk, and Star clusters) fields. Its detected individual stars populate an
approximately one magnitude range of its luminosity function (LF). Using
synthetic color-magnitude diagrams (CMDs) to compare with the galaxy's CMD, we
find that the colors and magnitudes of GHOSTS I's individual stars are most
consistent with being young helium-burning and asymptotic giant branch stars at
a distance of 12 +/- 2 Mpc. Morphologically, GHOSTS I appears to be actively
forming stars, so we tentatively classify it as a dwarf irregular (dIrr)
galaxy, although future HST observations deep enough to resolve a larger
magnitude range in its LF are required to make a more secure classification.
GHOSTS I's absolute magnitude is , making it one
of the least luminous dIrr galaxies known, and its metallicity is lower than
[Fe/H] =-1.5 dex. The half-light radius of GHOSTS I is 226 +/- 38 pc and its
ellipticity is 0.47 +/- 0.07, similar to Milky Way and M31 dwarf satellites at
comparable luminosity. There are no luminous massive galaxies or galaxy
clusters within ~ 4 Mpc from GHOSTS I that could be considered as its host,
making it a very isolated dwarf galaxy in the Local Universe.Comment: 8 pages, 7 figures. Accepted for publication in Ap
The Abundance Spread in the Bootes I Dwarf Spheroidal Galaxy
We present medium-resolution spectra of 16 radial velocity red-giant members of the low-luminosity Bootes I dwarf spheroidal (dSph) galaxy that have sufficient S/N for abundance determination, based on the strength of the Ca ii K line. Assuming [Ca/Fe]-0.3, the abundance range in the sample is Δ[Fe/H]-1.7 dex, with one star having [Fe/H]=-3.4. The dispersion is σ([Fe/H])=0.45 ± 0.08-similar to those of the Galaxy's more luminous dSph systems and ω Centauri. This suggests that the large mass (⊙≥10 7 M&o) normally assumed to foster self-enrichment and the production of chemical abundance spreads was provided by the nonbaryonic material in Boötes I
Transit Timing Observations from Kepler. VIII Catalog of Transit Timing Measurements of the First Twelve Quarters
Following Ford et al. (2011, 2012) and Steffen et al. (2012) we derived the
transit timing of 1960 Kepler KOIs using the pre-search data conditioning (PDC)
light curves of the first twelve quarters of the Kepler data. For 721 KOIs with
large enough SNRs, we obtained also the duration and depth of each transit. The
results are presented as a catalog for the community to use. We derived a few
statistics of our results that could be used to indicate significant
variations. Including systems found by previous works, we have found 130 KOIs
that showed highly significant TTVs, and 13 that had short-period TTV
modulations with small amplitudes. We consider two effects that could cause
apparent periodic TTV - the finite sampling of the observations and the
interference with the stellar activity, stellar spots in particular. We briefly
discuss some statistical aspects of our detected TTVs. We show that the TTV
period is correlated with the orbital period of the planet and with the TTV
amplitude.Comment: Accepted for publication to ApJ. 57 pages, 23 Figures. Machine
readable catalogs are available at ftp://wise-ftp.tau.ac.il/pub/tauttv/TT
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