2,048 research outputs found
Differences between surface and column atmospheric CO_2 and implications for carbon cycle research
We used a threeâdimensional atmospheric transport model to investigate several aspects of column CO_2 that are important for the design of new satelliteâbased observation systems and for the interpretation of observations collected by Sunâviewing spectrometers. These aspects included the amplitude of the diurnal cycle and how it is related to surface fluxes, the amplitude and phase of the seasonal cycle, and the magnitude of the northâsouth hemispheric gradient. In our simulation, we found that column CO_2 had less variability than surface CO_2 on all scales. The annual mean column CO_2 northâsouth gradient and seasonal cycle amplitude were approximately one half of their surface counterparts and the column CO_2 diurnal amplitude rarely exceeded 1 ppm. A 1 Gt C yr^(â1) Northern Hemisphere carbon sink decreased the northâsouth column CO_2 gradient by âŒ0.4 ppm
The Contribution of TP-AGB and RHeB Stars to the Near-IR Luminosity of Local Galaxies: Implications for Stellar Mass Measurements of High Redshift Galaxies
Using high spatial resolution HST WFC3 and ACS imaging of resolved stellar
populations, we constrain the contribution of thermally-pulsing asymptotic
giant branch (TP-AGB) stars and red helium burning (RHeB) stars to the 1.6 um
near-infrared (NIR) luminosities of 23 nearby galaxies. The TP-AGB phase
contributes as much as 17% of the integrated F160W flux, even when the red
giant branch is well populated. The RHeB population contribution can match or
even exceed the TP-AGB contribution, providing as much as 21% of the integrated
F160W light. The NIR mass-to-light (M/L) ratio should therefore be expected to
vary significantly due to fluctuations in the star formation rate over
timescales from 25 Myr to several Gyr. We compare our observational results to
predictions based on optically derived star formation histories and stellar
population synthesis (SPS) models, including models based on the Padova
isochrones (used in popular SPS programs). The SPS models generally reproduce
the expected numbers of TP-AGB stars in the sample. The same SPS models,
however, give a larger discrepancy in the F160W flux contribution from the
TP-AGB stars, over-predicting the flux by a weighted mean factor of 2.3 +/-0.8.
This larger offset is driven by the prediction of modest numbers of high
luminosity TP-AGB stars at young (<300 Myrs) ages. The best-fit SPS models
simultaneously tend to under-predict the numbers and fluxes of stars on the
RHeB sequence, typically by a factor of 2.0+/-0.6 for galaxies with significant
numbers of RHeBs. Coincidentally, over-prediction of the TP-AGB and
under-prediction of the RHeBs result in a NIR M/L ratio largely unchanged for a
rapid star formation rate. However, the NIR-to-optical flux ratio of galaxies
could be significantly smaller than AGB-rich models would predict, an outcome
that has been observed in some intermediate redshift post-starburst galaxies.
(Abridged)Comment: 28 Pages, 12 Figures, 5 Tables, Accepted for Publication in the
Astrophysical Journa
Resolved Near-infrared Stellar Populations in Nearby Galaxies
We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the resolved stellar populations within 26 fields of 23 nearby galaxies (âČ 4 Mpc), based on images in the F110W and F160W filters taken with the Wide-Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). The CMDs are measured in regions spanning a wide range of star formation histories, including both old dormant and young star-forming populations. We match key NIR CMD features with their counterparts in more familiar optical CMDs, and identify the red core helium-burning (RHeB) sequence as a significant contributor to the NIR flux in stellar populations younger than a few 100 Myr old. The strength of this feature suggests that the NIR mass-to-light ratio can vary significantly on short timescales in star-forming systems. The NIR luminosity of star-forming galaxies is therefore not necessarily proportional to the stellar mass. We note that these individual RHeB stars may also be misidentified as old stellar clusters in images of nearby galaxies. For older stellar populations, we discuss the CMD location of asymptotic giant branch (AGB) stars in the HST filter set and explore the separation of AGB subpopulations using a combination of optical and NIR colors. We empirically calibrate the magnitude of the NIR tip of the red giant branch in F160W as a function of color, allowing future observations in this widely adopted filter set to be used for distance measurements. We also analyze the properties of the NIR red giant branch (RGB) as a function of metallicity, showing a clear trend between NIR RGB color and metallicity. However, based on the current study, it appears unlikely that the slope of the NIR RGB can be used as an effective metallicity indicator in extragalactic systems with comparable data. Finally, we highlight issues with scattered light in the WFC3, which becomes significant for exposures taken close to a bright Earth limb
Measurement of Free Tropospheric Aerosols in the North Atlantic at the Pico Mountain Observatory.
AAAR 31st Annual Conference. Minneapolis, Minnesota, October 8-12, 2012.The Pico Mountain Observatory is located at 2225 m amsl on an inactive volcano at Pico Island in the Azores archipelago in the North Atlantic ~3900 km east and downwind of North America (38Âș28'15''N; 28Âș24â'14''W). The unique location of the Observatory enables sampling of free tropospheric air transported over long, intercontinental distances and is rarely affected by local emissions. The Observatory is affected mainly by North American outflow after its trans-Atlantic transport. Therefore, its location is ideal for observations of long-range transported pollutants emitted from anthropogenic and biogenic continental sources. The composition of continental pollution outflow is altered during transport by mixing, chemical reactions, phase changes, and removal processes. Thus, the properties of aerosol and trace gases in downwind regions are impacted by the outflow of pollutants, their chemical transformation, and sinks. In previous work, the sampled air-mass measurements (including CO, O3, NOx, NOy, NMHC, black carbon and aerosol optical size) and the simulations of their dispersion indicated outflow of North American tropospheric ozone and its precursors. Although the measurements have been crucial in explaining the evolution of North American gaseous pollution, little is known regarding the nature of the aged aerosol. New work is currently underway at the Observatory to provide chemical characterization of the intercepted free tropospheric aerosols. Here, we show the preliminary results of the free tropospheric aerosol composition and its physical properties. Samples were collected using high-volume filter samplers with quartz filters and analyzed for organic and elemental carbon (OC and EC, respectively). We compare the observed OC and EC values to the collocated measurements of gas- and particle-phase species, meteorological parameters and to the values found in current literature. We highlight the future work in which we will select filter samples based on the arrival of highly polluted air masses from anthropological or biomass burning emissions for further detailed analysis
The ACS Nearby Galaxy Survey Treasury
The ACS Nearby Galaxy Survey Treasury (ANGST) is a systematic survey to
establish a legacy of uniform multi-color photometry of resolved stars for a
volume-limited sample of nearby galaxies (D<4 Mpc). The survey volume
encompasses 69 galaxies in diverse environments, including close pairs, small &
large groups, filaments, and truly isolated regions. The galaxies include a
nearly complete range of morphological types spanning a factor of ~10^4 in
luminosity and star formation rate. The survey data consists of images taken
with ACS on HST, supplemented with archival data and new WFPC2 imaging taken
after the failure of ACS. Survey images include wide field tilings covering the
full radial extent of each galaxy, and single deep pointings in uncrowded
regions of the most massive galaxies in the volume. The new wide field imaging
in ANGST reaches median 50% completenesses of m_F475W=28.0 mag, m_F606W=27.3
mag, and m_F814W=27.3 mag, several magnitudes below the tip of the red giant
branch (TRGB). The deep fields reach magnitudes sufficient to fully resolve the
structure in the red clump. The resulting photometric catalogs are publicly
accessible and contain over 34 million photometric measurements of >14 million
stars. In this paper we present the details of the sample selection, imaging,
data reduction, and the resulting photometric catalogs, along with an analysis
of the photometric uncertainties (systematic and random), for both the ACS and
WFPC2 imaging. We also present uniformly derived relative distances measured
from the apparent magnitude of the TRGB.Comment: 54 pages, including 24 pages of figures and 16 pages of tables.
Project website and data available at http://www.nearbygalaxies.org/ . Data
is also available through MAST. Scheduled to appear in the Astrophysical
Journal Supplements. (Replaced to fix several figures that were damaged
during compression
Ten Years of Black Carbon Measurements in the North Atlantic at the Pico Mountain Observatory, Azores (2225m asl).
45th annual Fall Meeting, AGU. San Francisco, California, 3-7 December.The Pico Mountain Observatory is located in the summit caldera of the Pico mountain, an inactive volcano on the Pico Island in the Azores, Portugal (38.47°N, 28.40°W, Altitude 2225m asl). The Azores are often impacted by polluted outflows from the North American continent and local sources have been shown to have a negligible influence at the observatory. The value of the station stems from the fact that this is the only permanent mountaintop monitoring station in the North Atlantic that is typically located above the marine boundary layer (average MBL heights are below 1200 m and rarely exceed 1300 m) and often receives air characteristic of the lower free troposphere. Measurements of black carbon (BC) mass have been carried out at the station since 2001, mostly in the summer seasons. Here we discuss the BC decadal dataset (2001-2011) collected at the site by using a seven-wavelength AE31 Magee Aethalometer. Measured BC mass and computed Angstrom exponent (AE) values were analysed to study seasonal and diurnal variations. There was a large day-to-day variability in the BC values due to varied meteorological conditions that resulted in different diurnal patterns for different months. The daily mean BC at this location ranged between 0 and ~430 ngm-3, with the most frequently occurring value in the range 0-100 ngm-3. The overall mean for the 10 year period is ~24 ngm-3, with a coefficient of variation of 150%. The BC values exhibited a consistent annual trend being low in winter months and high in summer months, barring year to year variations. To differentiate between BC and other absorbing particles, we analyzed the wavelength dependence of aerosol absorption coefficient and determined a best-fit exponent i.e., the Ă
ngström exponent, for the whole dataset. Visible Ă
ngström exponent (AE: 470-520-590-660 nm) values ranged between 0 and 3.5, with most frequently occurring values in the range 0.85 to 1.25. By making use of the aethalometer light attenuation measurements at different wavelengths and Hysplit back trajectories, we divided the data into two categories. One for periods characterized by AE values close to 1; these periods are typically correlated with back trajectories originating from Canada, North America or northern Europe, indicating the dominance of BC on the light attenuation. Another characterized by AE values substantially different from 1; these periods correlated with back trajectories originating from dust-prone regions (e.g., the Sahara desert).The above measurements, with the aid of ancillary satellite and ground-based measurements will be employed in estimating the radiaitve effects of BC in the North Atlantic
PHAT Stellar Cluster Survey I. Year 1 Catalog and Integrated Photometry
The Panchromatic Hubble Andromeda Treasury (PHAT) survey is an on-going
Hubble Space Telescope (HST) multi-cycle program to obtain high spatial
resolution imaging of one-third of the M31 disk at ultraviolet through
near-infrared wavelengths. In this paper, we present the first installment of
the PHAT stellar cluster catalog. When completed, the PHAT cluster catalog will
be among the largest and most comprehensive surveys of resolved star clusters
in any galaxy. The exquisite spatial resolution achieved with HST has allowed
us to identify hundreds of new clusters that were previously inaccessible with
existing ground-based surveys. We identify 601 clusters in the Year 1 sample,
representing more than a factor of four increase over previous catalogs within
the current survey area (390 arcmin^2). This work presents results derived from
the first \sim25% of the survey data; we estimate that the final sample will
include \sim2500 clusters. For the Year 1 objects, we present a catalog with
positions, radii, and six-band integrated photometry. Along with a general
characterization of the cluster luminosities and colors, we discuss the cluster
luminosity function, the cluster size distributions, and highlight a number of
individually interesting clusters found in the Year 1 search.Comment: 26 pages, 22 figures, Accepted by Ap
Evaluating ozone depletion from very short-lived halocarbons
A new approach is needed for calculating the ozone depletion potential (ODP) of short-lived gases with mean lifetimes less than 100 days. Clearly, the ozone loss from such gases depends strongly on the location and season of emissions. It is proposed that delivery to the tropical tropopause is a surrogate for the amount of halogen entering the stratosphere. Thus 3-D global models, with accurate simulation of tropospheric chemistry and transport, can calculate the ODP specific to tropical sources (high) and high-latitude sources (low). The ODP of CH2BrCH2CH3
 from this analysis ranges from 0.0002 to 0.06 depending on the location and season of emissions
The Panchromatic Hubble Andromeda Treasury I: Bright UV Stars in the Bulge of M31
As part of the Panchromatic Hubble Andromeda Treasury (PHAT) multi-cycle
program, we observed a 12' \times 6.5' area of the bulge of M31 with the
WFC3/UVIS filters F275W and F336W. From these data we have assembled a sample
of \sim4000 UV-bright, old stars, vastly larger than previously available. We
use updated Padova stellar evolutionary tracks to classify these hot stars into
three classes: Post-AGB stars (P-AGB), Post-Early AGB (PE-AGB) stars and
AGB-manqu\'e stars. P-AGB stars are the end result of the asymptotic giant
branch (AGB) phase and are expected in a wide range of stellar populations,
whereas PE-AGB and AGB-manqu\'e (together referred to as the hot
post-horizontal branch; HP-HB) stars are the result of insufficient envelope
masses to allow a full AGB phase, and are expected to be particularly prominent
at high helium or {\alpha} abundances when the mass loss on the RGB is high.
Our data support previous claims that most UV-bright sources in the bulge are
likely hot (extreme) horizontal branch stars (EHB) and their progeny. We
construct the first radial profiles of these stellar populations, and show that
they are highly centrally concentrated, even more so than the integrated UV or
optical light. However, we find that this UV-bright population does not
dominate the total UV luminosity at any radius, as we are detecting only the
progeny of the EHB stars that are the likely source of the UVX. We calculate
that only a few percent of MS stars in the central bulge can have gone through
the HP-HB phase and that this percentage decreases strongly with distance from
the center. We also find that the surface density of hot UV-bright stars has
the same radial variation as that of low-mass X-ray binaries. We discuss age,
metallicity, and abundance variations as possible explanations for the observed
radial variation in the UV-bright population.Comment: Accepted for publication in Ap
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