435 research outputs found
The time resolved measurement of ultrashort THz-band electric fields without an ultrashort probe
The time-resolved detection of ultrashort pulsed THz-band electric field
temporal profiles without an ultrashort laser probe is demonstrated. A
non-linear interaction between a narrow-bandwidth optical probe and the THz
pulse transposes the THz spectral intensity and phase information to the
optical region, thereby generating an optical pulse whose temporal electric
field envelope replicates the temporal profile of the real THz electric field.
This optical envelope is characterised via an autocorrelation based FROG
measurement, hence revealing the THz temporal profile. The combination of a
narrow-bandwidth, long duration, optical probe and self-referenced FROG makes
the technique inherently immune to timing jitter between the optical probe and
THz pulse, and may find particular application where the THz field is not
initially generated via ultrashort laser methods, such as the measurement of
longitudinal electron bunch profiles in particle accelerators.Comment: 7 pages, 3 figures, submitted to AP
The Snedden-Farnsworth Exchanges of 1917 and 1918 on the Value of Music and Art in Education
In 1917 and 1918, Charles Hubert Farnsworth, a leading music educator from Teachers College, Columbia University, and David Snedden, a critic and educational theorist of national repute, privately exchanged views on the role of art and music in society and in education. Snedden mulled over Herbert Spencer's query “What knowledge is of most worth?” and concluded that music must have practical survival value: it must contribute primarily to the maintenance of social and political order and secondarily to other aims. Farnsworth, on the other hand, thought that music performance or appreciation should be for the immediate joy that it gives the individual, not for some deferred social purpose no matter how important it might be. These divergent positions are explained in light of Farnsworth's interests in philosophy and Snedden's schooling in Spencerian and Darwinian thought.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68979/2/10.2307_3345173.pd
A Search for Planetary Nebulae With the SDSS: the outer regions of M31
We have developed a method to identify planetary nebula (PN) candidates in
imaging data of the Sloan Digital Sky Survey (SDSS). This method exploits the
SDSS' five-band sampling of emission lines in PN spectra, which results in a
color signature distinct from that of other sources. Selection criteria based
on this signature can be applied to nearby galaxies in which PNe appear as
point sources. We applied these criteria to the whole area of M31 as scanned by
the SDSS, selecting 167 PN candidates that are located in the outer regions of
M31. The spectra of 80 selected candidates were then observed with the 2.2m
telescope at Calar Alto Observatory. These observations and cross-checks with
literature data show that our method has a selection rate efficiency of about
90%, but the efficiency is different for the different groups of PNe
candidates.
In the outer regions of M31, PNe trace different well-known morphological
features like the Northern Spur, the NGC205 Loop, the G1 Clump, etc. In
general, the distribution of PNe in the outer region 8<R<20 kpc along the minor
axis shows the "extended disk" - a rotationally supported low surface
brightness structure with an exponential scale length of 3.21+/-0.14 kpc and a
total mass of ~10^10 M_{\sun}, which is equivalent to the mass of M33. We
report the discovery of three PN candidates with projected locations in the
center of Andromeda NE, a very low surface brightness giant stellar structure
in the outer halo of M31. Two of the PNe were spectroscopically confirmed as
genuine PNe. These two PNe are located at projected distances along the major
axis of ~48 Kpc and ~41 Kpc from the center of M31 and are the most distant PNe
in M31 found up to now.Comment: 58 pages, 17 figures, 2 tables, Accepted to Astronomical Journa
Tilted Pulse-Front Phase-matching in Three Dimensions:Overcoming The Cherenkov Angle Restrictions
We consider the non-linear generation of THz with tilted pulse-fronts in three dimensions and show that, contrary to the widely held expectations, coherent phase matching can be obtained for pulse-front tilt angles other the Cherenkov angle
A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
We present a catalog of 9316 spectroscopically confirmed white dwarfs from
the Sloan Digital Sky Survey Data Release 4. We have selected the stars through
photometric cuts and spectroscopic modeling, backed up by a set of visual
inspections. Roughly 6000 of the stars are new discoveries, roughly doubling
the number of spectroscopically confirmed white dwarfs. We analyze the stars by
performing temperature and surface gravity fits to grids of pure hydrogen and
helium atmospheres. Among the rare outliers are a set of presumed helium-core
DA white dwarfs with estimated masses below 0.3 Msun, including two candidates
that may be the lowest masses yet found. We also present a list of 928 hot
subdwarfs.Comment: Accepted by the Astrophysical Journal Supplements, 25 pages, 24
figures, LaTeX. The electronic catalog, as well as diagnostic figures and
links to the spectra, is available at http://das.sdss.org/wdcat/dr4
Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey
We report the discovery of five very cool white dwarfs in the Sloan Digital
Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced
absorption (CIA) from molecular hydrogen and are similar in color to the three
previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402.
The fifth, an ultracool white dwarf candidate, shows milder CIA flux
suppression and has a color and spectral shape similar to WD 0346+246. All five
new white dwarfs are faint (g > 18.9) and have significant proper motions. One
of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system
with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.Comment: 15 pages, 3 figures, submitted to ApJL. Higher resolution versions of
finding charts are available at
http://astro.uchicago.edu/~gates/findingchart
Optically Selected BL Lacertae Candidates from the Sloan Digital Sky Survey Data Release Seven
We present a sample of 723 optically selected BL Lac candidates from the SDSS
DR7 spectroscopic database encompassing 8250 deg^2 of sky; our sample
constitutes one of the largest uniform BL Lac samples yet derived. Each BL Lac
candidate has a high-quality SDSS spectrum from which we determine
spectroscopic redshifts for ~60% of the objects. Redshift lower limits are
estimated for the remaining objects utilizing the lack of host galaxy flux
contamination in their optical spectra; we find that objects lacking
spectroscopic redshifts are likely at systematically higher redshifts.
Approximately 80% of our BL Lac candidates match to a radio source in
FIRST/NVSS, and ~40% match to a ROSAT X-ray source. The homogeneous
multiwavelength coverage allows subdivision of the sample into 637 radio-loud
BL Lac candidates and 86 weak-featured radio-quiet objects. The radio-loud
objects broadly support the standard paradigm unifying BL Lac objects with
beamed radio galaxies. We propose that the majority of the radio-quiet objects
may be lower-redshift (z<2.2) analogs to high-redshift weak line quasars (i.e.,
AGN with unusually anemic broad emission line regions). These would constitute
the largest sample of such objects, being of similar size and complementary in
redshift to the samples of high-redshift weak line quasars previously
discovered by the SDSS. However, some fraction of the weak-featured radio-quiet
objects may instead populate a rare and extreme radio-weak tail of the much
larger radio-loud BL Lac population. Serendipitous discoveries of unusual white
dwarfs, high-redshift weak line quasars, and broad absorption line quasars with
extreme continuum dropoffs blueward of rest-frame 2800 Angstroms are also
briefly described.Comment: 24 pages, 14 figures, 8 tables. Accepted for publication in A
The Hercules-Aquila Cloud
We present evidence for a substantial overdensity of stars in the direction
of the constellations of Hercules and Aquila. The Cloud is centered at a
Galactic longitude of about 40 degrees and extends above and below the Galactic
plane by at least 50 degrees. Given its off-centeredness and height, it is
unlikely that the Hercules-Aquila Cloud is related to the bulge or thick disk.
More likely, this is a new structural component of the Galaxy that passes
through the disk. The Cloud stretches about 80 degrees in longitude. Its
heliocentric distance lies between 10 and 20 kpc so that the extent of the
Cloud in projection is roughly 20 kpc by 15 kpc. It has an absolute magnitude
of -13 and its stellar population appears to be comparable to, but somewhat
more metal-rich than, M92.Comment: ApJ (Letters), in pres
Discovery of an Unusual Dwarf Galaxy in the Outskirts of the Milky Way
In this Letter, we announce the discovery of a new dwarf galaxy, Leo T, in
the Local Group. It was found as a stellar overdensity in the Sloan Digital Sky
Survey Data Release 5 (SDSS DR5). The color-magnitude diagram of Leo T shows
two well-defined features, which we interpret as a red giant branch and a
sequence of young, massive stars. As judged from fits to the color-magnitude
diagram, it lies at a distance of about 420 kpc and has an intermediate-age
stellar population with a metallicity of [Fe/H]= -1.6, together with a young
population of blue stars of age of 200 Myr. There is a compact cloud of neutral
hydrogen with mass roughly 10^5 solar masses and radial velocity 35 km/s
coincident with the object visible in the HIPASS channel maps. Leo T is the
smallest, lowest luminosity galaxy found to date with recent star-formation. It
appears to be a transition object similar to, but much lower luminosity than,
the Phoenix dwarf.Comment: Ap J (Letters) in press, the subject of an SDSS press release toda
- …