153 research outputs found
News from the Swampland -- Constraining string theory with astrophysics and cosmology
Our current best guess for a unified theory of gravitation and quantum field
theory (string theory) generically predicts a set of requirements for a
consistently quantized theory, the Swampland criteria. Refined versions of
these criteria have recently been shown to be in mild tension with cosmological
observations. We summarize the status of the current impact of and constraints
on the Swampland conjectures from cosmology, and subject a variety of dark
energy quintessence models to recently released cosmological datasets. We find
that instead of tightening the tension, the new data allows for slightly more
freedom in the Swampland criteria. We further demonstrate that if there is no
theoretical argument made to prevent interactions of the moduli fields with the
electromagnetic sector, a novel fine-tuning argument arises from the extremely
tight current constraints on such interactions. Finally, we conclude with a
cautionary tale on model-independent reconstructions of the Swampland criteria
from expansion rate data.Comment: 35 pages, 20 figures, 4 tables. All comments are welcome! [v2: Added
citations, corrected eq. 2.6
Chemical Cartography with APOGEE: Large-scale Mean Metallicity Maps of the Milky Way
We present Galactic mean metallicity maps derived from the first year of the
SDSS-III APOGEE experiment. Mean abundances in different zones of
Galactocentric radius (0 < R < 15 kpc) at a range of heights above the plane (0
< |z| < 3 kpc), are derived from a sample of nearly 20,000 stars with
unprecedented coverage, including stars in the Galactic mid-plane at large
distances. We also split the sample into subsamples of stars with low and
high-[{\alpha}/M] abundance ratios. We assess possible biases in deriving the
mean abundances, and find they are likely to be small except in the inner
regions of the Galaxy. A negative radial gradient exists over much of the
Galaxy; however, the gradient appears to flatten for R < 6 kpc, in particular
near the Galactic mid-plane and for low-[{\alpha}/M] stars. At R > 6 kpc, the
gradient flattens as one moves off of the plane, and is flatter at all heights
for high-[{\alpha}/M] stars than for low-[{\alpha}/M] stars. Alternatively,
these gradients can be described as vertical gradients that flatten at larger
Galactocentric radius; these vertical gradients are similar for both low and
high-[{\alpha}/M] populations. Stars with higher [{\alpha}/M] appear to have a
flatter radial gradient than stars with lower [{\alpha}/M]. This could suggest
that the metallicity gradient has grown steeper with time or, alternatively,
that gradients are washed out over time by migration of stars.Comment: 16 pages, 12 figures, submitted to A
Very Metal-poor Stars in the Outer Galactic Bulge Found by the Apogee Survey
Despite its importance for understanding the nature of early stellar
generations and for constraining Galactic bulge formation models, at present
little is known about the metal-poor stellar content of the central Milky Way.
This is a consequence of the great distances involved and intervening dust
obscuration, which challenge optical studies. However, the Apache Point
Observatory Galactic Evolution Experiment (APOGEE), a wide-area, multifiber,
high-resolution spectroscopic survey within Sloan Digital Sky Survey III
(SDSS-III), is exploring the chemistry of all Galactic stellar populations at
infrared wavelengths, with particular emphasis on the disk and the bulge. An
automated spectral analysis of data on 2,403 giant stars in twelve fields in
the bulge obtained during APOGEE commissioning yielded five stars with low
metallicity([Fe/H]), including two that are very metal-poor
[Fe/H] by bulge standards.
Luminosity-based distance estimates place the five stars within the outer
bulge, where other 1,246 of the analyzed stars may reside. A manual reanalysis
of the spectra verifies the low metallicities, and finds these stars to be
enhanced in the -elements O, Mg, and Si without significant
-pattern differences with other local halo or metal-weak thick-disk
stars of similar metallicity, or even with other more metal-rich bulge stars.
While neither the kinematics nor chemistry of these stars can yet definitively
determine which, if any, are truly bulge members, rather than denizens of other
populations co-located with the bulge, the newly-identified stars reveal that
the chemistry of metal-poor stars in the central Galaxy resembles that of
metal-weak thick-disk stars at similar metallicity.Comment: 6 pages, 3 figures, 2 table
The VMC Survey -- XXXIV. Morphology of Stellar Populations in the Magellanic Clouds
The Magellanic Clouds are nearby dwarf irregular galaxies whose morphologies
show different properties when traced by different stellar populations, making
them an important laboratory for studying galaxy morphologies. We study the
morphology of the Magellanic Clouds using data from the VISTA survey of the
Magellanic Clouds system (VMC). We used about and million sources
across an area of deg and deg towards the Large and
Small Magellanic Cloud (LMC and SMC), respectively. We estimated median ages of
stellar populations occupying different regions of the near-infrared
() colour-magnitude diagram. Morphological maps
were produced and detailed features in the central regions were characterised
for the first time with bins corresponding to a spatial resolution of
kpc (LMC) and kpc (SMC). In the LMC, we find that main sequence stars
show coherent structures that grow with age and trace the multiple spiral arms
of the galaxy, star forming regions become dimmer as we progress in age, while
supergiant stars are centrally concentrated. Intermediate-age stars, despite
tracing a regular and symmetrical morphology, show central clumps and hints of
spiral arms. In the SMC, young main sequence stars depict a broken bar.
Intermediate-age populations show signatures of elongation towards the
Magellanic Bridge that can be attributed to the LMC-SMC interaction
Myr ago. They also show irregular central features suggesting that the inner
SMC has also been influenced by tidal interactions.Comment: Accepted for publication in MNRAS, 20 pages, 12 figures and 2 table
The APOKASC Catalog: An Asteroseismic and Spectroscopic Joint Survey of Targets in the Kepler Fields
We present the first APOKASC catalog of spectroscopic and asteroseismic
properties of 1916 red giants observed in the Kepler fields. The spectroscopic
parameters provided from the Apache Point Observatory Galactic Evolution
Experiment project are complemented with asteroseismic surface gravities,
masses, radii, and mean densities determined by members of the Kepler
Asteroseismology Science Consortium. We assess both random and systematic
sources of error and include a discussion of sample selection for giants in the
Kepler fields. Total uncertainties in the main catalog properties are of order
80 K in Teff , 0.06 dex in [M/H], 0.014 dex in log g, and 12% and 5% in mass
and radius, respectively; these reflect a combination of systematic and random
errors. Asteroseismic surface gravities are substantially more precise and
accurate than spectroscopic ones, and we find good agreement between their mean
values and the calibrated spectroscopic surface gravities. There are, however,
systematic underlying trends with Teff and log g. Our effective temperature
scale is between 0-200 K cooler than that expected from the Infrared Flux
Method, depending on the adopted extinction map, which provides evidence for a
lower value on average than that inferred for the Kepler Input Catalog (KIC).
We find a reasonable correspondence between the photometric KIC and
spectroscopic APOKASC metallicity scales, with increased dispersion in KIC
metallicities as the absolute metal abundance decreases, and offsets in Teff
and log g consistent with those derived in the literature. We present mean
fitting relations between APOKASC and KIC observables and discuss future
prospects, strengths, and limitations of the catalog data.Comment: 49 pages. ApJSupp, in press. Full machine-readable ascii files
available under ancillary data. Categories: Kepler targets, asteroseismology,
large spectroscopic survey
The Milky Way's circular velocity curve between 4 and 14 kpc from APOGEE data
We measure the Milky Way's rotation curve over the Galactocentric range 4 kpc
<~ R <~ 14 kpc from the first year of data from the Apache Point Observatory
Galactic Evolution Experiment (APOGEE). We model the line-of-sight velocities
of 3,365 stars in fourteen fields with b = 0 deg between 30 deg < l < 210 deg
out to distances of 10 kpc using an axisymmetric kinematical model that
includes a correction for the asymmetric drift of the warm tracer population
(\sigma_R ~ 35 km/s). We determine the local value of the circular velocity to
be V_c(R_0) = 218 +/- 6 km/s and find that the rotation curve is approximately
flat with a local derivative between -3.0 km/s/kpc and 0.4 km/s/kpc. We also
measure the Sun's position and velocity in the Galactocentric rest frame,
finding the distance to the Galactic center to be 8 kpc < R_0 < 9 kpc, radial
velocity V_{R,sun} = -10 +/- 1 km/s, and rotational velocity V_{\phi,sun} =
242^{+10}_{-3} km/s, in good agreement with local measurements of the Sun's
radial velocity and with the observed proper motion of Sgr A*. We investigate
various systematic uncertainties and find that these are limited to offsets at
the percent level, ~2 km/s in V_c. Marginalizing over all the systematics that
we consider, we find that V_c(R_0) 99% confidence. We find an
offset between the Sun's rotational velocity and the local circular velocity of
26 +/- 3 km/s, which is larger than the locally-measured solar motion of 12
km/s. This larger offset reconciles our value for V_c with recent claims that
V_c >~ 240 km/s. Combining our results with other data, we find that the Milky
Way's dark-halo mass within the virial radius is ~8x10^{11} M_sun.Comment: submitted to Ap
A new generation of Parsec-Colibri stellar isochrones including the TP-AGB phase
We introduce a new generation of PARSEC-COLIBRI stellar isochrones that includes a detailed treatment of the thermally pulsing asymptotic giant branch (TP-AGB) phase, covering a wide range of initial metallicities (0.0001. < Z(i) < 0.06). Compared to previous releases, the main novelties and improvements are use of new TP-AGB tracks and related atmosphere models and spectra for M and C-type stars; inclusion of the surface H+He +CNO abundances in the isochrone tables, accounting for the effects of diffusion, dredge-up episodes and hot-bottom burning; inclusion of complete thermal pulse cycles, with a complete description of the in-cycle changes in the stellar parameters; new pulsation models to describe the long-period variability in the fundamental and firstovertone modes; and new dust models that follow the growth of the grains during the AGB evolution, in combination with radiative transfer calculations for the reprocessing of the photospheric emission. Overall, these improvements are expected to lead to a more consistent and detailed description of properties of TP-AGB stars expected in resolved stellar populations, especially in regard to their mean photometric properties from optical to mid-infrared wavelengths. We illustrate the expected numbers of TP-AGB stars of different types in stellar populations covering a wide range of ages and initial metallicities, providing further details on the "C-star island" that appears at intermediate values of age and metallicity, and about the AGB-boosting effect that occurs at ages close to 1.6-Gyr for populations of all metallicities. The isochrones are available through a new dedicated web server
The Ninth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Baryon Oscillation Spectroscopic Survey
The Sloan Digital Sky Survey III (SDSS-III) presents the first spectroscopic
data from the Baryon Oscillation Spectroscopic Survey (BOSS). This ninth data
release (DR9) of the SDSS project includes 535,995 new galaxy spectra (median
z=0.52), 102,100 new quasar spectra (median z=2.32), and 90,897 new stellar
spectra, along with the data presented in previous data releases. These spectra
were obtained with the new BOSS spectrograph and were taken between 2009
December and 2011 July. In addition, the stellar parameters pipeline, which
determines radial velocities, surface temperatures, surface gravities, and
metallicities of stars, has been updated and refined with improvements in
temperature estimates for stars with T_eff<5000 K and in metallicity estimates
for stars with [Fe/H]>-0.5. DR9 includes new stellar parameters for all stars
presented in DR8, including stars from SDSS-I and II, as well as those observed
as part of the SDSS-III Sloan Extension for Galactic Understanding and
Exploration-2 (SEGUE-2).
The astrometry error introduced in the DR8 imaging catalogs has been
corrected in the DR9 data products. The next data release for SDSS-III will be
in Summer 2013, which will present the first data from the Apache Point
Observatory Galactic Evolution Experiment (APOGEE) along with another year of
data from BOSS, followed by the final SDSS-III data release in December 2014.Comment: 9 figures; 2 tables. Submitted to ApJS. DR9 is available at
http://www.sdss3.org/dr
Evolution of Thermally Pulsing Asymptotic Giant Branch Stars V: Constraining the Mass Loss and Lifetimes of Intermediate Mass, Low Metallicity AGB Stars
Thermally pulsing asymptotic giant branch (TP-AGB) stars are relatively short lived (less than a few Myr), yet their cool effective temperatures, high luminosities, efficient mass loss, and dust production can dramatically affect the chemical enrichment histories and the spectral energy distributions of their host galaxies. The ability to accurately model TP-AGB stars is critical to the interpretation of the integrated light of distant galaxies, especially in redder wavelengths. We continue previous efforts to constrain the evolution and lifetimes of TP-AGB stars by modeling their underlying stellar populations. Using Hubble Space Telescope (HST) optical and near-infrared photometry taken of 12 fields of 10 nearby galaxies imaged via the Advanced Camera for Surveys Nearby Galaxy Survey Treasury and the near-infrared HST/SNAP follow-up campaign, we compare the model and observed TP-AGB luminosity functions as well as the ratio of TP-AGB to red giant branch stars. We confirm the best-fitting mass-loss prescription, introduced by Rosenfield et al., in which two different wind regimes are active during the TP-AGB, significantly improves models of many galaxies that show evidence of recent star formation. This study extends previous efforts to constrain TP-AGB lifetimes to metallicities ranging -1.59 less than or similar to [Fe/H] <= -0.56 and initial TP-AGB masses up to similar to 4 M-circle dot, which include TP-AGB stars that undergo hot-bottom burning. \ua9 2016. The American Astronomical Society. All rights reserved
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