146 research outputs found
Recommended from our members
Croll Revisited: Why Is the Northern Hemisphere Warmer Than the Southern Hemisphere?
The question of why, in the annual-mean, the Northern Hemisphere is warmer than the Southern Hemisphere is addressed, revisiting an 1870 paper by James Croll. We first show that ocean is warmer than land in general which, acting alone, would make the Southern Hemisphere with greater ocean fraction warmer. Croll thought it was caused by greater specific humidity and greenhouse trapping over ocean than over land. However, for any given temperature, greenhouse trapping is actually greater over land. Instead, oceans are warmer than land because of smaller surface albedo. However, inter-hemispheric differences in total albedo are negligible because the impact of differences in land-sea fraction are o set by Southern Hemisphere ocean and land reflecting more than their Northern Hemisphere counterparts. In agreement with Croll, it is shown that northward cross-equatorial ocean heat transport is critical for the warmer Northern Hemisphere. This is examined in a simple box model based on the energy budget of each hemisphere. The inter-hemispheric difference forced by ocean heat transport is enhanced by the positive water vapor-greenhouse feedback, and is partly compensated by the southward atmospheric energy transport. To fully explain the temperature difference in this way, requires a northward ocean heat transport at the extreme of observational estimates. A better fit to data is found when a larger basic state greenhouse trapping in the Northern Hemisphere, conceived as imposed by continental geometry, is imposed. Therefore, despite some modifications to his theory, analysis of modern data confirms Croll's 140 year-old theory that the warmer Northern Hemisphere is partly because of northward cross-equatorial ocean heat transport
A Search for a Sub-Earth Sized Companion to GJ 436 and a Novel Method to Calibrate Warm Spitzer IRAC Observations
We discovered evidence for a possible additional 0.75 R_Earth transiting
planet in the NASA EPOXI observations of the known M dwarf exoplanetary system
GJ 436. Based on an ephemeris determined from the EPOXI data, we predicted a
transit event in an extant Spitzer Space Telescope 8 micron data set of this
star. Our subsequent analysis of those Spitzer data confirmed the signal of the
predicted depth and at the predicted time, but we found that the transit depth
was dependent on the aperture used to perform the photometry. Based on these
suggestive findings, we gathered new Warm Spitzer Observations of GJ 436 at 4.5
microns spanning a time of transit predicted from the EPOXI and Spitzer 8
micron candidate events. The 4.5 micron data permit us to rule out a transit at
high confidence, and we conclude that the earlier candidate transit signals
resulted from correlated noise in the EPOXI and Spitzer 8 micron observations.
In the course of this investigation, we developed a novel method for correcting
the intrapixel sensitivity variations of the 3.6 and 4.5 micron channels of the
Infrared Array Camera (IRAC) instrument. We demonstrate the sensitivity of Warm
Spitzer observations of M dwarfs to confirm sub-Earth sized planets. Our
analysis will inform similar work that will be undertaken to use Warm Spitzer
observations to confirm rocky planets discovered by the Kepler mission.Comment: 22 pages, 8 figures, accepted for publication in PAS
Intersecting Disadvantages for Married Adolescents: Life After Marriage Pre- and Post-COVID-19 in Contexts of Displacement.
PURPOSE: Although there is a growing evidence base on the drivers of child marriage, comparatively little is known about the experiences of married girls in refugee settings and how their development trajectories diverge from those of their nonmarried peers, particularly in the context of the COVID-19 pandemic. Drawing on cross-national panel data from Bangladesh and Jordan, this article explores diversity in child marriage experiences in contexts affected by forced displacement, highlighting how married girls\u27 well-being differs from that of their unmarried peers, and how COVID-19 has reinforced these differences.
METHODS: We analyzed longitudinal survey data-collected pre- and post-COVID-19-from the Gender and Adolescence: Global Evidence study with 293 ever-married and 1,102 never-married adolescent girls. Multivariate regression analysis assessed the well-being of married and unmarried girls across contexts and refugee status, both prior to and during the COVID-19 pandemic. These quantitative data are complemented by in-depth qualitative data from adolescents (n = 112), and key informant interviews with service providers and community leaders (n = 62).
RESULTS: Our findings highlight that married girls in contexts affected by displacement are disadvantaged in multiple ways, but that the patterning of disadvantage varies across contexts, and that marriage can also have protective effects in certain contexts. The COVID-19 pandemic has, however, served to exacerbate existing inequalities in all contexts.
DISCUSSION: Although child marriage prevention efforts remain critical, there is also an urgent need for programming that targets married girls in refugee and host communities to mitigate negative outcomes among this vulnerable group
Low False-Positive Rate of Kepler Candidates Estimated From A Combination Of Spitzer And Follow-Up Observations
(Abridged) NASA's Kepler mission has provided several thousand transiting
planet candidates, yet only a small subset have been confirmed as true planets.
Therefore, the most fundamental question about these candidates is the fraction
of bona fide planets. Estimating the rate of false positives of the overall
Kepler sample is necessary to derive the planet occurrence rate. We present the
results from two large observational campaigns that were conducted with the
Spitzer telescope during the the Kepler mission. These observations are
dedicated to estimating the false positive rate (FPR) amongst the Kepler
candidates. We select a sub-sample of 51 candidates, spanning wide ranges in
stellar, orbital and planetary parameter space, and we observe their transits
with Spitzer at 4.5 microns. We use these observations to measures the
candidate's transit depths and infrared magnitudes. A bandpass-dependent depth
alerts us to the potential presence of a blending star that could be the source
of the observed eclipse: a false-positive scenario. For most of the candidates
(85%), the transit depths measured with Kepler are consistent with the depths
measured with Spitzer as expected for planetary objects, while we find that the
most discrepant measurements are due to the presence of unresolved stars that
dilute the photometry. The Spitzer constraints on their own yield FPRs between
5-40%, depending on the KOIs. By considering the population of the Kepler field
stars, and by combining follow-up observations (imaging) when available, we
find that the overall FPR of our sample is low. The measured upper limit on the
FPR of our sample is 8.8% at a confidence level of 3 sigma. This observational
result, which uses the achromatic property of planetary transit signals that is
not investigated by the Kepler observations, provides an independent indication
that Kepler's false positive rate is low.Comment: 33 pages, 16 figures, 3 tables; accepted for publication in ApJ on
February 7, 201
Spitzer IRAC Secondary Eclipse Photometry of the Transiting Extrasolar Planet HAT-P-1b
We report Spitzer/IRAC photometry of the transiting giant exoplanet HAT-P-1b
during its secondary eclipse. This planet lies near the postulated boundary
between the pM and pL-class of hot Jupiters, and is important as a test of
models for temperature inversions in hot Jupiter atmospheres. We derive eclipse
depths for HAT-P-1b, in units of the stellar flux, that are: 0.080% +/-
0.008%,[3.6um], 0.135% +/- 0.022%,[4.5um],0.203% +/- 0.031%,[5.8um], and
$0.238% +/- 0.040%,[8.0um]. These values are best fit using an atmosphere with
a modest temperature inversion, intermediate between the archetype inverted
atmosphere (HD209458b) and a model without an inversion. The observations also
suggest that this planet is radiating a large fraction of the available stellar
irradiance on its dayside, with little available for redistribution by
circulation. This planet has sometimes been speculated to be inflated by tidal
dissipation, based on its large radius in discovery observations, and on a
non-zero orbital eccentricity allowed by the radial velocity data. The timing
of the secondary eclipse is very sensitive to orbital eccentricity, and we find
that the central phase of the eclipse is 0.4999 +/- 0.0005. The difference
between the expected and observed phase indicates that the orbit is close to
circular, with a 3-sigma limit of |e cosw| < 0.002.Comment: 5 pages, 6 figures, 1 table. Accepted by The Astrophysical Journal,
10 Nov 200
Preliminary Results on HAT-P-4, TrES-3, XO-2, and GJ 436 from the NASA EPOXI Mission
EPOXI (EPOCh + DIXI) is a NASA Discovery Program Mission of Opportunity using
the Deep Impact flyby spacecraft. The EPOCh (Extrasolar Planet Observation and
Characterization) Science Investigation will gather photometric time series of
known transiting exoplanet systems from January through August 2008. Here we
describe the steps in the photometric extraction of the time series and present
preliminary results of the first four EPOCh targets.Comment: 4 pages, 2 figures. To appear in the Proceedings of the 253rd IAU
Symposium: "Transiting Planets", May 2008, Cambridge, M
Studying the atmosphere of the exoplanet HAT-P-7b via secondary eclipse measurements with EPOXI, Spitzer and Kepler
The highly irradiated transiting exoplanet, HAT-P-7b, currently provides one
of the best opportunities for studying planetary emission in the optical and
infrared wavelengths. We observe six near-consecutive secondary eclipses of
HAT-P-7b at optical wavelengths with the EPOXI spacecraft. We place an upper
limit on the relative eclipse depth of 0.055% (95% confidence). We also analyze
Spitzer observations of the same target in the infrared, obtaining secondary
eclipse depths of 0.098+/-0.017%, 0.159+/-0.022%, 0.245+/-0.031% and
0.225+/-0.052% in the 3.6, 4.5, 5.8 and 8.0 micron IRAC bands respectively. We
combine these measurements with the recently published Kepler secondary eclipse
measurement, and generate atmospheric models for the day-side of the planet
that are consistent with both the optical and infrared measurements. The data
are best fit by models with a temperature inversion, as expected from the high
incident flux. The models predict a low optical albedo of ~< 0.13, with
subsolar abundances of Na, K, TiO and VO. We also find that the best fitting
models predict that 10% of the absorbed stellar flux is redistributed to the
night side of the planet, which is qualitatively consistent with the
inefficient day-night redistribution apparent in the Kepler phase curve. Models
without thermal inversions fit the data only at the 1.25 sigma level, and also
require an overabundance of methane, which is not expected in the very hot
atmosphere of HAT-P-7b. We also analyze the eight transits of HAT-P-7b present
in the EPOXI dataset and improve the constraints on the system parameters,
finding a period of P = 2.2047308+/-0.0000025 days, a stellar radius of R* =
1.824+/-0.089Rsun, a planetary radius of Rp = 1.342+/-0.068RJup and an
inclination of i = 85.7+3.5-2.2 deg.Comment: 21 pages, 8 figures, accepted by the Astrophysical Journa
Spitzer transit and secondary eclipse photometry of GJ 436b
We report the results of infrared (8 mu m) transit and secondary eclipse photometry of the hot Neptune exoplanet, GJ 436b using Spitzer. The nearly photon-limited precision of these data allows us to measure an improved radius for the planet and to detect the secondary eclipse. The transit (centered at HJD = 2454280.78149 +/- 0.00016) shows the flat-bottomed shape typical of infrared transits, and it precisely defines the planet-to-star 0.00016 radius ratio (), independent of the stellar properties. However, we obtain the planetary radius, 0.0839 +/- 0.0005 as well as the stellar mass and radius, by fitting to the transit curve simultaneously with an empirical mass-radius relation for M dwarfs (M = R). We find R* = M* 0.47 +/- 0.02 in solar units, and R-p = 27,600 +/- 1170 km 4.33 +/- 0.18 R-circle plus). This radius significantly exceeds the radius of a naked ocean planet and requires a gaseous hydrogen-helium envelope. The secondary eclipse occurs at phase, proving a significant orbital 0.587 +/- 0.005 eccentricity (e = 0.150 +/- 0.012). The amplitude of the eclipse [(5.7 +/- 0.8) x 10(-4)] indicates a brightness tem- perature for the planet of T = 712 +/- 36 K. If this is indicative of the planet\u27s physical temperature, it suggests T = 712 +/- 36 the occurrence of tidal heating in the planet. An uncharacterized second planet likely provides ongoing gravitational perturbations that maintain GJ 436b\u27s orbit eccentricity over long timescales
A TESS Dress Rehearsal: Planetary Candidates and Variables from K2 Campaign 17T
We produce light curves for all ∼34,000 targets observed with K2 in Campaign 17 (C17), identifying 34 planet candidates, 184 eclipsing binaries, and other 222 periodic variables. The forward-facing direction of the C17 field means follow-up can begin immediately now that the campaign has concluded and interesting targets have been identified. The C17 field has a large overlap with C6, so this latest campaign also offers an infrequent opportunity to study a large number of targets already observed in a previous K2 campaign. The timing of the C17 data release, shortly before science operations begin with the Transiting Exoplanet Survey Satellite (TESS), also lets us exercise some of the tools and methods developed for identification and dissemination of planet candidates from TESS. We find excellent agreement between these results and those identified using only K2-based tools. Among our planet candidates are several planet candidates with sizes <4 R[subscript ⊕] and orbiting stars with Kp ≲ 10 (indicating good RV targets of the sort TESS hopes to find) and a Jupiter-sized single-transit event around a star already hosting a 6 day planet candidate. Key words: methods, data analysis, planets and satellites, detection – techniques, photometricUnited States. National Aeronautics and Space Administration (K2GO Grant 80NSSC18K0308
- …