1,824 research outputs found
Are Swift Long-Lag Gamma-Ray Bursts in the Local Supercluster?
A sample of 18 long-lag (tau_{lag} > 1 s) Gamma-Ray Bursts (GRBs) has been
drawn from our catalog of all Swift long GRBs. Four different tests are done on
this sample to test the prediction that a large fraction of long-lag GRBs are
from our Local Supercluster. The results of these four tests come out that: (1)
the distribution of these GRBs shows no tendency towards the Supergalactic
plane; (2) the distribution shows no tendency towards the Virgo or Coma
Cluster; (3) no associated bright host galaxies (m <=15) in the Local
Supercluster are found for any of the 18 GRBs; (4) 17 of these 18 GRBs have
redshifts of z>0.5, which are too far to be in the Local Supercluster. All
these results disproved the hypothesis that any significant fraction of
long-lag GRBs are from Local Supercluster. Hence these long-lag GRBs can not be
counted in the calculation of LIGO detection rates. An explanation of why we
can detect long-lag GRBs at high redshift is presented.Comment: 14 pages, 1 figure, ApJ accepte
The CH fraction of Carbon stars at high Galactic latitudes
CH stars form a distinct class of objects with characteristic properties like
iron deficiency, enrichment of carbon and overabundance in heavy elements.
These properties can provide strong observational constraints for theoretical
computation of nucleosynthesis at low-metallicity. An important question is the
relative surface density of CH stars which can provide valuable inputs to our
understanding on the role of low to intermediate-mass stars in the early
Galactic chemical evolution. Spectroscopic characterization provides an
effective way of identifying CH stars. The present analysis is aimed at a
quantitative assessment of the fraction of CH stars in a sample of stars using
a set of spectral classification criteria. The sample consists of 92 objects
selected from a collection of candidate Faint High Latitude Carbon stars from
the Hamburg/ESO survey. Medium resolution (R ~ 1300) spectra for these objects
were obtained using OMR at VBO, Kavalur and HFOSC at HCT, IAO, Hanle, during
2007 - 2009 spanning a wavelength range 3800 - 6800 A. Spectral analysis shows
36 of the 92 objects to be potential CH stars; combined with our earlier
studies (Goswami 2005, Goswami et al. 2007) this implies ~ 37% (of 243) objects
as the CH fraction. We present spectral descriptions of the newly identified CH
star candidates. Estimated effective temperatures, 12C/13C isotopic ratios and
their locations on the two colour J-H vs H-K plot are used to support their
identification.Comment: 18 pages, 8 figures,Accepted for publication in MNRA
Affordable Rotating Fluid Demonstrations for Geoscience Education: The DIYnamics Project
Demonstrations using rotating tanks of fluid can help demystify otherwise counterintuitive behaviors of atmospheric, oceanic, and planetary interior fluid motions. But the expense and complicated assembly of existing rotating table platforms limit their appeal for many schools, especially those below the university level. Here, we introduce Do-It-Yourself Dynamics (DIYnamics), a project developing extremely low-cost rotating tank platforms and accompanying teaching materials. The devices can be assembled in a few minutes from household items, all available for purchase online. Ordering, assembly, and operation instructions are available on the DIYnamics website. Videos using these and other rotating tables to teach specific concepts such as baroclinic instability are available on the DIYnamics YouTube channel—including some in Spanish. The devices, lesson plans, and demonstrations have been successfully piloted at multiple middle schools, in a university course, and at public science outreach events. These uses to date convince us of the DIYnamics materials’ pedagogical value for instructors from well-versed university professors to K–12 science teachers with little background in fluid dynamics
Stellar Archaeology -- Exploring the Universe with Metal-Poor Stars
The abundance patterns of the most metal-poor stars in the Galactic halo and
small dwarf galaxies provide us with a wealth of information about the early
Universe. In particular, these old survivors allow us to study the nature of
the first stars and supernovae, the relevant nucleosynthesis processes
responsible for the formation and evolution of the elements, early star- and
galaxy formation processes, as well as the assembly process of the stellar halo
from dwarf galaxies a long time ago. This review presents the current state of
the field of "stellar archaeology" -- the diverse use of metal-poor stars to
explore the high-redshift Universe and its constituents. In particular, the
conditions for early star formation are discussed, how these ultimately led to
a chemical evolution, and what the role of the most iron-poor stars is for
learning about Population III supernovae yields. Rapid neutron-capture
signatures found in metal-poor stars can be used to obtain stellar ages, but
also to constrain this complex nucleosynthesis process with observational
measurements. Moreover, chemical abundances of extremely metal-poor stars in
different types of dwarf galaxies can be used to infer details on the formation
scenario of the halo. and the role of dwarf galaxies as Galactic building
blocks. I conclude with an outlook as to where this field may be heading within
the next decade. A table of ~1000 metal-poor stars and their abundances as
collected from the literature is provided in electronic format.Comment: Ludwig Biermann Award Lecture 2009; data table can be found at
http://www.cfa.harvard.edu/~afrebel/abundances/abund.html; published in AN
331, 47
School Counselor Use of Narrative Therapy to Support Students of Color Transitioning from an Alternative School Setting
The authors propose a collaborative application of narrative therapy utilizing the school counselor, teachers, and parents to support students of color transitioning from an alternative school setting. Research indicates that students of color transitioning from alternative settings often face stigma and marginalization by teachers and peers. The authors contend that school counselors can use this collaborative narrative approach to support students to create new narratives within a supportive environment. Included is an illustration demonstrating the application of the collaborative narrative therapy process
Investigation for the enrichment pattern of the element abundances in r+s star HE 0338-3945: a special r-II star?
The very metal-poor star HE 0338-3945 shows a double-enhanced pattern of the
neutron-capture elements. The study to this sample could make people gain a
better understanding of s- and r-process nucleosynthesis at low metallicity.
Using a parametric model,we find that the abundance pattern of the
neutron-capture elements could be best explained by a binary system formed in a
molecular cloud, which had been polluted by r-process material. The observed
abundance pattern of C and N can be explained by an AGB model(Karakas &
Lattanzio 2007), . Combing with the parameters obtained from Cui & Zhang
(2006), we suggest that the initial mass of the AGB companion is most likely to
be about 2.5Msun, which excludes the possibility of forming a type-1.5
supernova. By comparing with the observational abundance pattern of CS
22892-052, we find that the dominating production of O should accompany with
the production of the heavy r-process elements of r+s stars. Similar to r-II
stars, the heavy r-process elements are not produced in conjunction with all
the light elements from Na to Fe group. The abundance pattern of the light and
r-process elements for HE 0338-3945 is very close to the pattern of the r-II
star CS 22892-052. So, we suggest that this star HE 0338-3945 should be a
special r-II star.Comment: 18 pages, 6 figures, published by Ap
CS 30322-023: an ultra metal-poor TP-AGB star?
With [Fe/H] = -3.5, CS 30322-023 is the most metal-poor star to exhibit a
clear s-process signature and the most metal-poor ``lead star'' known. CS
30322-023 is also remarkable in having the lowest surface gravity (log g <=
-0.3) among the metal-poor stars studied to date. The available evidence
indicates that this star is presently a thermally-pulsing asymptotic giant
branch (TP-AGB) star, with no strong indication of binarity thus far (although
a signal of period 192 d is clearly present in the radial-velocity data, this
is likely due to pulsation of the stellar envelope). We show that low-mass
TP-AGB stars are not expected to be exceedingly rare in a magnitude-limited
sample such as the HK survey, because their high luminosities make it possible
to sample them over a very large volume. The strong N overabundance and the low
12C/13C ratio (4) in this star is typical of the operation of the CN cycle.
Coupled with a Na overabundance and the absence of a strong C overabundance,
this pattern seems to imply that hot-bottom burning operated in this star,
which should then have a mass of at least 2 Msun. However, the luminosity
associated with this mass would put the star at a distance of about 50 kpc, in
the outskirts of the galactic halo. We explore alternative scenarios in which
the observed abundance pattern results from some mixing mechanism yet to be
identified occurring in a single low-metallicity 0.8 Msun AGB star, or from
pollution by matter from an intermediate-mass AGB companion which has undergone
hot-bottom burning. We stress, however, that our abundances may be subject to
uncertainties due to NLTE or 3D granulation effects which were not taken into
consideration.Comment: 17 pages, Astronomy & Astrophysics, in press; also available at
http://www.astro.ulb.ac.be/Html/ps.html#PR
Pollen Nightmare: Elevated Airborne Pollen Levels at Night
High airborne pollen concentrations are generally associated with daylight hours when it is sunny and warm and plants release pollen into the air (Alcázar et al. 1999; Dahl et al. 2013). In contrast, cooler night-time periods are usually considered to be the time of low-allergy risk. This opinion is often reflected in pollen allergy avoidance strategies presented by the media, where the most commonly repeated recommendation is to stay indoors during the day and plan outdoor activities for the evening. However, there is evidence to suggest that elevated concentrations of airborne pollen might also occur during the evening (e.g. Norris-Hill and Emberlin 1991). So, is the night really a time of low-allergy risk? We present the results of the comparative analysis of pollen concentrations during daytime and night-time hours for five allergenic pollen types (Burbach et al. 2009), i.e. alder (Alnus sp.), birch (Betula sp.), grasses (Poaceae), mugwort (Artemisia sp.) and ragweed (Ambrosia sp.)
Unveiling the origin of X-ray flares in Gamma-Ray Bursts
We present an updated catalog of 113 X-ray flares detected by Swift in the
~33% of the X-ray afterglows of Gamma-Ray Bursts (GRB). 43 flares have a
measured redshift. For the first time the analysis is performed in 4 different
X-ray energy bands, allowing us to constrain the evolution of the flare
temporal properties with energy. We find that flares are narrower at higher
energies: their width follows a power-law relation w~E^{-0.5} reminiscent of
the prompt emission. Flares are asymmetric structures, with a decay time which
is twice the rise time on average. Both time scales linearly evolve with time,
giving rise to a constant rise-to-decay ratio: this implies that both time
scales are stretched by the same factor. As a consequence, the flare width
linearly evolves with time to larger values: this is a key point that clearly
distinguishes the flare from the GRB prompt emission. The flare 0.3-10 keV peak
luminosity decreases with time, following a power-law behaviour with large
scatter: L_{pk}~ t_{pk}^{-2.7}. When multiple flares are present, a global
softening trend is established: each flare is on average softer than the
previous one. The 0.3-10 keV isotropic energy distribution is a log-normal
peaked at 10^{51} erg, with a possible excess at low energies. The flare
average spectral energy distribution (SED) is found to be a power-law with
spectral energy index beta~1.1. These results confirmed that the flares are
tightly linked to the prompt emission. However, after considering various
models we conclude that no model is currently able to account for the entire
set of observations.Comment: MNRAS submitte
The Chemistry of the Trailing arm of the Sagittarius Dwarf Galaxy
We present abundances of C, O, Ti, and Fe for eleven M-giant stars in the
trailing tidal arm of the Sagittarius dwarf (Sgr). The abundances were derived
by comparing synthetic spectra with high-resolution infrared spectra obtained
with the Phoenix spectrograph on the Gemini South telescope. The targeted stars
are drawn from two regions of the Sgr trailing arm separated by 66 degrees (5
stars) and 132 degrees (6 stars) from the main body of Sgr. The trailing arm
provides a more direct diagnostic of the chemical evolution of Sgr compared to
the extensively phase-mixed leading arm.
Within our restricted sample of ~2-3 Gyr old stars, we find that the stream
material exhibits a significant metallicity gradient of -(2.4\pm0.3)x10^{-3}
dex / degree (-(9.4\pm1.1)x10^{-4} dex / kpc) away from the main body of Sgr.
The tidal disruption of Sgr is a relatively recently event. We therefore
interpret the presence of a metallicity gradient in the debris as indicative of
a similar gradient in the progenitor. The fact that such a metallicity gradient
survived for almost a Hubble time indicates that the efficiency of radial
mixing was very low in the Sgr progenitor.
No significant gradient is seen to exist in the [alpha/Fe] abundance ratio
along the trailing arm. Our results may be accounted for by a radial decrease
in star formation efficiency and/or radial increase in the efficiency of
galactic wind-driven metal loss in the chemical evolution of the Sgr
progenitor. The [Ti/Fe] and [O/Fe] abundance ratios observed within the stream
are distinct from those of the Galactic halo. We conclude that the fraction of
the intermediate to metal-rich halo population contributed by the recent
dissolution (<3 Gyr) of Sgr-like dwarf galaxies can not be substantial.Comment: 22 pages, 7 figures, ApJ accepte
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