3,593 research outputs found
Cool White Dwarfs Identified in the Second Data Release of the UKIRT Infrared Deep Sky Survey
We have paired the Second Data Release of the Large Area Survey of the UKIRT
Infrared Deep Sky Survey with the Fifth Data Release of the Sloan Digital Sky
Survey to identify ten cool white dwarf candidates, from their photometry and
astrometry. Of these ten, one was previously known to be a very cool white
dwarf. We have obtained optical spectroscopy for seven of the candidates using
the GMOS-N spectrograph on Gemini North, and have confirmed all seven as white
dwarfs. Our photometry and astrometry indicates that the remaining two objects
are also white dwarfs. Model analysis of the photometry and available
spectroscopy shows that the seven confirmed new white dwarfs, and the two new
likely white dwarfs, have effective temperatures in the range Teff = 5400-6600
K. Our analysis of the previously known white dwarf confirms that it is cool,
with Teff = 3800 K. The cooling age for this dwarf is 8.7 Gyr, while that of
the nine ~6000 K white dwarfs is 1.8-3.6 Gyr. We are unable to determine the
masses of the white dwarfs from the existing data, and therefore we cannot
constrain the total ages of the white dwarfs. The large cooling age for the
coolest white dwarf in the sample, combined with its low estimated tangential
velocity, suggests that it is an old member of the thin disk, or a member of
the thick disk of the Galaxy, with an age 10-11 Gyr. The warmer white dwarfs
appear to have velocities typical of the thick disk or even halo; these may be
very old remnants of low-mass stars, or they may be relatively young thin disk
objects with unusually high space motion.Comment: 37 pages (referee format), 4 tables, 7 figures, accepted to Ap
Spectroscopic analysis of DA white dwarfs from the McCook & Sion catalog
For some years now, we have been gathering optical spectra of DA white dwarfs
in an effort to study and define the empirical ZZ Ceti instability strip.
However, we have recently expanded this survey to include all the DA white
dwarfs in the McCook & Sion catalog down to a limiting visual magnitude of
V=17.5. We present here a spectroscopic analysis of over 1000 DA white dwarfs
from this ongoing survey. We have several specific areas of interest most
notably the hot DAO white dwarfs, the ZZ Ceti instability strip, and the DA+dM
binary systems. Furthermore, we present a comparison of the ensemble properties
of our sample with those of other large surveys of DA white dwarfs, paying
particular attention to the distribution of mass as a function of effective
temperature.Comment: 8 pages, 7 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
The Origin of C IV Absorption Systems at Redshifts z<1---Discovery of Extended C IV Envelopes Around Galaxies
(Abridged) We report the discovery of extended CIV gaseous envelopes around
galaxies of a wide range of luminosity and morphological type. First, we show
that CIV absorption systems are strongly clustered around galaxies on velocity
scales of v < 250 km/s and impact parameter scales of rho < 100 h^{-1} kpc but
not on larger velocity or impact parameter scales. Next, adopting measurements
of galaxy properties presented in previous papers, we examine how properties of
the CIV absorption systems depend on properties of the galaxies. On the basis
of 14 galaxy and absorber pairs and 36 galaxies that do not produce
corresponding CIV absorption lines to within sensitive upper limits, we find
that: (1) Galaxies of a range of morphological type and luminosity appear to
possess extended CIV gaseous envelopes of radius R ~ 100 h^{-1} kpc, with
abrupt boundaries between the CIV absorbing and non-absorbing regions. (2) The
extent of CIV-absorbing gas around galaxies scales with galaxy B-band
luminosity as R \propto L_B^{0.5 +/- 0.1} but does not depend strongly on
galaxy surface brightness, redshift, or morphological type. And (3) the
covering factor of CIV clouds within ~ 100 h^{-1} kpc of galaxies is nearly
unity, but there is a large scatter in the mean number of clouds encountered
along the line of sight. The most significant implication of the study is that
galaxies of a wide range of luminosity and morphological type are surrounded by
chemically enriched gas that extends for at least ~ 100 h^{-1} kpc. We consider
various scenarios that may have produced metals at large galactic distance and
conclude that accreting satellites are most likely to be responsible for
chemically enriched gas at large galactic distances to regular looking
galaxies.Comment: 19 pages, 3 figures, to appear in ApJ, July 20 200
Prepontine non-giant neurons drive flexible escape behavior in zebrafish
Many species execute ballistic escape reactions to avoid imminent danger. Despite fast reaction times, responses are often highly regulated, reflecting a trade-off between costly motor actions and perceived threat level. However, how sensory cues are integrated within premotor escape circuits remains poorly understood. Here, we show that in zebrafish, less precipitous threats elicit a delayed escape, characterized by flexible trajectories, which are driven by a cluster of 38 prepontine neurons that are completely separate from the fast escape pathway. Whereas neurons that initiate rapid escapes receive direct auditory input and drive motor neurons, input and output pathways for delayed escapes are indirect, facilitating integration of cross-modal sensory information. These results show that rapid decision-making in the escape system is enabled by parallel pathways for ballistic responses and flexible delayed actions and defines a neuronal substrate for hierarchical choice in the vertebrate nervous system
Cool White Dwarfs Found in the UKIRT Infrared Deep Sky Survey
We present the results of a search for cool white dwarfs in the United
Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area
Survey (LAS). The UKIDSS LAS photometry was paired with the Sloan Digital Sky
Survey (SDSS) to identify cool hydrogen-rich white dwarf candidates by their
neutral optical colors and blue near-infrared colors, as well as faint Reduced
Proper Motion magnitudes. Optical spectroscopy was obtained at Gemini
Observatory, and showed the majority of the candidates to be newly identified
cool degenerates, with a small number of G- to K-type (sub)dwarf contaminants.
Our initial search of 280 deg2 of sky resulted in seven new white dwarfs with
effective temperature T_eff ~ 6000 K. The current followup of 1400 deg2 of sky
has produced thirteen new white dwarfs. Model fits to the photometry show that
seven of the newly identified white dwarfs have 4120 K <= T_eff <= 4480 K, and
cooling ages between 7.3 Gyr and 8.7 Gyr; they have 40 km/s <= v_tan <= 85 km/s
and are likely to be thick disk 10-11 Gyr-old objects. The other half of the
sample has 4610 K <= T_eff <= 5260 K, cooling ages between 4.3 Gyr and 6.9 Gyr,
and 60 km/s <= v_tan <= 100 km/s. These are either thin disk remnants with
unusually high velocities, or lower-mass remnants of thick disk or halo late-F
or G stars.Comment: To appear in ApJ, accepted April 18 2011. 34 pages include 11 Figures
and 5 Table
XMM observations of the narrow-line QSO PHL 1092: Detection of a high and variable soft component
We present results based on an XMM-Newton observation of the high luminosity
narrow-line QSO PHL 1092 performed in 2003 January. The 0.3 - 10 keV spectrum
is well described by a model which includes a power-law (Gamma ~ 2.1) and two
blackbody components (kT ~ 130 eV and kT ~ 50 eV). The soft X-ray excess
emission is featureless and contributes ~ 80% to the total X-ray emission in
the 0.3 - 10 keV band. The most remarkable feature of the present observation
is the detection of X-ray variability at very short time scale: the X-ray
emission varied by 35% in about 5000 s. We find that this variability can be
explained by assuming that only the overall normalization varied during the
observation. There was no evidence for any short term spectral variability and
the spectral shape was similar even during the ASCA observation carried out in
1997. Considering the high intrinsic luminosity (~ 2x10^45 erg/s) and the large
inferred mass of the putative black hole (~ 1.6x10^8 M_sun), the observed time
scale of variability indicates emission at close to Eddington luminosity
arising from very close to the black hole. We suggest that PHL 1092 in
particular (and narrow line Seyfert galaxies in general) is a fast rotating
black hole emitting close to its Eddington luminosity and the X-ray emission
corresponds to the high-soft state seen in Galactic black hole sources.Comment: 7 figures, 8 pages, emulateapj style, ApJ in pres
Ethanol and Biodiesel in Canada: Can They Help Meet Canada's Kyoto Commitment?
The Canadian government ratified the Kyoto Protocol in December 2002. The protocol calls for greenhouse gas emissions to be reduced by 6 percent below the 1990 level (approximately 35 percent below what they are expected to be in a business-as-usual situation) by 2008â2012. To help meet this target, both federal and provincial governments in Canada have developed programs (in some cases mandates) for ethanol and biodiesel production and use in fuel blends. Various subsidy schemes have been designed to stimulate development of a biofuels industry in Canada. While the programs have been promoted on the basis of environmental concerns, a major driver has been the search for new markets for Canadian agricultural commodities that face chronically depressed prices. The purposes of this article are to review the current state of policies, programs and production of ethanol and biodiesel in Canada and to assess the prospect of these biofuels to significantly reduce production of greenhouse gases in Canada.Agricultural and Food Policy,
Coloured peak algebras and Hopf algebras
For a finite abelian group, we study the properties of general
equivalence relations on G_n=G^n\rtimes \SG_n, the wreath product of with
the symmetric group \SG_n, also known as the -coloured symmetric group. We
show that under certain conditions, some equivalence relations give rise to
subalgebras of \k G_n as well as graded connected Hopf subalgebras of
\bigoplus_{n\ge o} \k G_n. In particular we construct a -coloured peak
subalgebra of the Mantaci-Reutenauer algebra (or -coloured descent algebra).
We show that the direct sum of the -coloured peak algebras is a Hopf
algebra. We also have similar results for a -colouring of the Loday-Ronco
Hopf algebras of planar binary trees. For many of the equivalence relations
under study, we obtain a functor from the category of finite abelian groups to
the category of graded connected Hopf algebras. We end our investigation by
describing a Hopf endomorphism of the -coloured descent Hopf algebra whose
image is the -coloured peak Hopf algebra. We outline a theory of
combinatorial -coloured Hopf algebra for which the -coloured
quasi-symmetric Hopf algebra and the graded dual to the -coloured peak Hopf
algebra are central objects.Comment: 26 pages latex2
Analysis of a Very Massive DA White Dwarf via the Trigonometric Parallax and Spectroscopic Methods
By two different methods, we show that LHS 4033 is an extremely massive white
dwarf near its likely upper mass limit for destruction by unstable electron
captures. From the accurate trigonometric parallax reported herein, the
effective temperature (T=10,900 K) and the stellar radius (R=0.00368 R_sun) are
directly determined from the broad-band spectral energy distribution -- the
parallax method. The effective temperature and surface gravity are also
estimated independently from the simultaneous fitting of the observed Balmer
line profiles with those predicted from pure-hydrogen model atmospheres -- the
spectroscopic method (T=10,760 K, log g=9.46). The mass of LHS 4033 is then
inferred from theoretical mass-radius relations appropriate for white dwarfs.
The parallax method yields a mass estimate of 1.310--1.330 M_sun, for interior
compositions ranging from pure magnesium to pure carbon, respectively, while
the spectroscopic method yields an estimate of 1.318--1.335 M_sun for the same
core compositions. This star is the most massive white dwarf for which a robust
comparison of the two techniques has been made.Comment: 17 pages, including 4 figures, Accepted for Ap.
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