2,051 research outputs found
Evolution of brown dwarf disks: A Spitzer survey in Upper Scorpius
We have carried out a Spitzer survey for brown dwarf (BD) disks in the ~5 Myr
old Upper Scorpius (UpSco) star forming region, using IRS spectroscopy from 8
to 12\mu m and MIPS photometry at 24\mu m. Our sample consists of 35 confirmed
very low mass members of UpSco. Thirteen objects in this sample show clear
excess flux at 24\mu m, explained by dust emission from a circum-sub-stellar
disk. Objects without excess emission either have no disks at all or disks with
inner opacity holes of at least ~5 AU radii. Our disk frequency of 37\pm 9% is
higher than what has been derived previously for K0-M5 stars in the same region
(on a 1.8 sigma confidence level), suggesting a mass-dependent disk lifetime in
UpSco. The clear distinction between objects with and without disks as well as
the lack of transition objects shows that disk dissipation inside 5 AU occurs
rapidly, probably on timescales of <~10^5 years. For the objects with disks,
most SEDs are uniformly flat with flux levels of a few mJy, well modeled as
emission from dusty disks affected by dust settling to the midplane, which also
provides indirect evidence for grain growth. The silicate feature around 10\mu
m is either absent or weak in our SEDs, arguing for a lack of hot, small dust
grains. Compared with younger objects in Taurus, BD disks in UpSco show less
flaring. Taken together, these results clearly demonstrate that we see disks in
an advanced evolutionary state: Dust settling and grain growth are ubiquituous
in circum-sub-stellar disks at ages of 5 Myr, arguing for planet forming
processes in BD disks. For almost all our targets, results from high-resolution
spectroscopy and high-spatial resolution imaging have been published before,
thus providing a large sample of BDs for which information about disks,
accretion, and binarity is available. (abridged)Comment: 39 pages, 7 figures, accepted for publication in Ap
Polarized point sources in the LOFAR Two-meter Sky Survey: A preliminary catalog
The polarization properties of radio sources at very low frequencies (h45m–15h30m right ascension, 45°–57° declination, 570 square degrees). We have produced a catalog of 92 polarized radio sources at 150 MHz at 4.′3 resolution and 1 mJy rms sensitivity, which is the largest catalog of polarized sources at such low frequencies. We estimate a lower limit to the polarized source surface density at 150 MHz, with our resolution and sensitivity, of 1 source per 6.2 square degrees. We find that our Faraday depth measurements are in agreement with previous measurements and have significantly smaller errors. Most of our sources show significant depolarization compared to 1.4 GHz, but there is a small population of sources with low depolarization indicating that their polarized emission is highly localized in Faraday depth. We predict that an extension of this work to the full LOTSS data would detect at least 3400 polarized sources using the same methods, and probably considerably more with improved data processing
F-theory and Neutrinos: Kaluza-Klein Dilution of Flavor Hierarchy
We study minimal implementations of Majorana and Dirac neutrino scenarios in
F-theory GUT models. In both cases the mass scale of the neutrinos m_nu ~
(M_weak)^2/M_UV arises from integrating out Kaluza-Klein modes, where M_UV is
close to the GUT scale. The participation of non-holomorphic Kaluza-Klein mode
wave functions dilutes the mass hierarchy in comparison to the quark and
charged lepton sectors, in agreement with experimentally measured mass
splittings. The neutrinos are predicted to exhibit a "normal" mass hierarchy,
with masses m_3,m_2,m_1 ~ .05*(1,(alpha_GUT)^(1/2),alpha_GUT) eV. When the
interactions of the neutrino and charged lepton sectors geometrically unify,
the neutrino mixing matrix exhibits a mild hierarchical structure such that the
mixing angles theta_23 and theta_12 are large and comparable, while theta_13 is
expected to be smaller and close to the Cabibbo angle: theta_13 ~ theta_C ~
(alpha_GUT)^(1/2) ~ 0.2. This suggests that theta_13 should be near the current
experimental upper bound.Comment: v2: 83 pages, 10 figures, references adde
Systematic study of trace radioactive impurities in candidate construction materials for EXO-200
The Enriched Xenon Observatory (EXO) will search for double beta decays of
136Xe. We report the results of a systematic study of trace concentrations of
radioactive impurities in a wide range of raw materials and finished parts
considered for use in the construction of EXO-200, the first stage of the EXO
experimental program. Analysis techniques employed, and described here, include
direct gamma counting, alpha counting, neutron activation analysis, and
high-sensitivity mass spectrometry.Comment: 32 pages, 6 figures. Expanded introduction, added missing table
entry. Accepted for publication in Nucl. Instrum. Meth.
Asexuality: Classification and characterization
This is a post-print version of the article. The official published version can be obtaineed at the link below.The term “asexual” has been defined in many different ways and asexuality has received very little research attention. In a small qualitative study (N = 4), individuals who self-identified as asexual were interviewed to help formulate hypotheses for a larger study. The second larger study was an online survey drawn from a convenience sample designed to better characterize asexuality and to test predictors of asexual identity. A convenience sample of 1,146 individuals (N = 41 self-identified asexual) completed online questionnaires assessing sexual history, sexual inhibition and excitation, sexual desire, and an open-response questionnaire concerning asexual identity. Asexuals reported significantly less desire for sex with a partner, lower sexual arousability, and lower sexual excitation but did not differ consistently from non-asexuals in their sexual inhibition scores or their desire to masturbate. Content analyses supported the idea that low sexual desire is the primary feature predicting asexual identity
Overturning established chemoselectivities : selective reduction of arenes over malonates and cyanoacetates by photoactivated organic electron donors
The prevalence of metal-based reducing reagents, including metals, metal complexes, and metal salts, has produced an empirical order of reactivity that governs our approach to chemical synthesis. However, this reactivity may be influenced by stabilization of transition states, intermediates, and products through substrate-metal bonding. This article reports that in the absence of such stabilizing interactions, established chemoselectivities can be overthrown. Thus, photoactivation of the recently developed neutral organic superelectron donor 5 selectively reduces alkyl-substituted benzene rings in the presence of activated esters and nitriles, in direct contrast to metal-based reductions, opening a new perspective on reactivity. The altered outcomes arising from the organic electron donors are attributed to selective interactions between the neutral organic donors and the arene rings of the substrates
Search for Neutrinoless Double-Beta Decay in Xe with EXO-200
We report on a search for neutrinoless double-beta decay of Xe with
EXO-200. No signal is observed for an exposure of 32.5 kg-yr, with a background
of ~1.5 x 10^{-3} /(kg yr keV) in the region of interest. This
sets a lower limit on the half-life of the neutrinoless double-beta decay
(Xe) > 1.6 x 10 yr (90% CL),
corresponding to effective Majorana masses of less than 140-380 meV, depending
on the matrix element calculation
Social representations and the politics of participation
Recent work has called for the integration of different perspectives into the field of political psychology (Haste, 2012). This chapter suggests that one possible direction that such efforts can take is studying the role that social representations theory (SRT) can play in understanding political participation and social change. Social representations are systems of common-sense knowledge and social practice; they provide the lens through which to view and create social and political realities, mediate people's relations with these sociopolitical worlds and defend cultural and political identities. Social representations are therefore key for conceptualising participation as the activity that locates individuals and social groups in their sociopolitical world. Political participation is generally seen as conditional to membership of sociopolitical groups and therefore is often linked to citizenship. To be a citizen of a society or a member of any social group one has to participate as such. Often political participation is defined as the ability to communicate one's views to the political elite or to the political establishment (Uhlaner, 2001), or simply explicit involvement in politics and electoral processes (Milbrath, 1965). However, following scholars on ideology (Eagleton, 1991; Thompson, 1990) and social knowledge (Jovchelovitch, 2007), we extend our understanding of political participation to all social relations and also develop a more agentic model where individuals and groups construct, develop and resist their own views, ideas and beliefs. We thus adopt a broader approach to participation in comparison to other political-psychological approaches, such as personality approaches (e.g. Mondak and Halperin, 2008) and cognitive approaches or, more recently, neuropsychological approaches (Hatemi and McDermott, 2012). We move away from a focus on the individual's political behaviour and its antecedents and outline an approach that focuses on the interaction between psychological and political phenomena (Deutsch and Kinnvall, 2002) through examining the politics of social knowledge
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