1,749 research outputs found
On the abundance of Lithium in T Coronae Borealis
We have obtained high resolution echelle spectroscopy of the recurrent nova T
CrB. We find that the surface lithium abundance in T CrB is signifcantly
enhanced compared to field M giants, where it is not detectable. We offer
possible explanations for this in terms of either a delay in the onset of
convection in the giant star, enhanced coronal activity due to star-spots or
the enhancement of Li resulting from the nova explosion(s).Comment: 3 pages, 1 figure (a and b), accepted by MNRA
UV excess galaxies: Wolf-Rayet galaxies
We discuss V and R band photometry for 67% of the Sullivan et al. 2000 SA57
ultraviolet-selected galaxy sample. In a sample of 176 UV-selected galaxies,
Sullivan et al. 2000 find that 24% have (UV-B) colors too blue for consistency
with starburst spectral synthesis models. We propose that these extreme blue,
UV excess galaxies are Wolf-Rayet (WR) galaxies, starburst galaxies with strong
UV emission from WR stars. We measure a median (V-R)=0.38+-0.06 for the
UV-selected sample, bluer than a sample optically selected at R but consistent
with starburst and WR galaxy colors. We demonstrate that redshifted WR emission
lines can double or triple the flux through the UV bandpass at high redshifts.
Thus the (UV-B) color of a WR galaxy can be up to 1.3 mag bluer at high
redshift, and the expected selection function is skewed to larger redshifts.
The redshift distribution of the extreme blue, UV excess galaxies matches the
selection function we predict from the properties of WR galaxies.Comment: 4 pages, including 4 figures. Uses AASTeX and emulateapj5.sty.
Includes referee change
Surveying the Inner Halo of the Galaxy with 2MASS-Selected Horizontal Branch Candidates
We use 2MASS photometry to select blue horizontal branch (BHB) candidates
covering the sky |b|>15 deg. A 12.5<J<15.5 sample of BHB stars traces the thick
disk and inner halo to d<9 kpc, with a density comparable to that of M giant
stars. We base our sample selection strategy on the Century Survey Galactic
Halo Project, a survey that provides a complete, spectroscopically-identified
sample of blue stars to a similar depth as the 2MASS catalog. We show that a
-0.20<(J-H)_0<0.10, -0.10<(H-K)_0<0.10 color-selected sample of stars is 65%
complete for BHB stars, and is composed of 47% BHB stars. We apply this
photometric selection to the full 2MASS catalog, and see no spatial
overdensities of BHB candidates at high Galactic latitude |b|>50 deg. We insert
simulated star streams into the data and conclude that the high Galactic
latitude BHB candidates are consistent with having no ~5 deg wide star stream
with density greater than 0.33 objects deg^-2 at the 95% confidence level. The
absence of structure suggests there have been no major accretion events in the
inner halo in the last few Gyr. However, at low Galactic latitudes a two-point
angular correlation analysis reveals structure on angular scales <1 deg. This
structure is apparently associated with stars in the thick disk, and has a
physical scale of 10-100 pc. Interestingly, such structures are expected by
cosmological simulations that predict the majority of the thick disk may arise
from accretion and disruption of satellite mergers.Comment: 11 pages, including figures. Accepted by AJ with minor revision
First Results From The Taiwanese-American Occultation Survey (TAOS)
Results from the first two years of data from the Taiwanese-American
Occultation Survey (TAOS) are presented. Stars have been monitored
photometrically at 4 Hz or 5 Hz to search for occultations by small (~3 km)
Kuiper Belt Objects (KBOs). No statistically significant events were found,
allowing us to present an upper bound to the size distribution of KBOs with
diameters 0.5 km < D < 28 km.Comment: 5 pages, 5 figure, accepted in Ap
Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center
We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars
traveling with such extreme velocities that dynamical ejection from a massive
black hole (MBH) is their only suggested origin. We also detect a population of
possibly bound HVSs. The significant asymmetry we observe in the velocity
distribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf <
-275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun main
sequence stars. Any population of hypervelocity post-main sequence stars should
contain stars falling back onto the Galaxy, contrary to the observations. The
spatial distribution of HVSs also supports the main sequence interpretation:
longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs are
missing at faint magnitudes. We infer that there are 96 +- 10 HVSs of mass 3 -
4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejection
mechanisms and potential models. Depending on the mass function of HVSs, we
predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs
favor a continuous ejection process, although a ~120 Myr-old burst of HVSs is
also allowed.Comment: 10 pages, 8 figures, accepted to ApJ, minor revision
Contamination and exclusion in the sigma Orionis young group
We present radial velocities for 38 low-mass candidate members of the sigma
Orionis young group. We have measured their radial velocities by
cross-correlation of high resolution (R~6000) AF2/WYFFOS spectra of the gravity
sensitive NaI doublet at 8183, 8195Angstroms. The total sample contained 117
objects of which 54 have sufficient signal-to-noise to detect NaI at an
equivalent width of 3Angstroms, however we only detect NaI in 38 of these. This
implies that very low-mass members of this young group display weaker NaI
absorption than similarly aged objects in the Upper Scorpius OB association. We
develop a technique to assess membership using radial velocities with a range
of uncertainties that does not bias the selection when large uncertainties are
present. The resulting membership probabilities are used to assess the issue of
exclusion in photometric selections, and we find that very few members are
likely to be excluded by such techniques.
We also assess the level of contamination in the expected pre-main sequence
region of colour-magnitude space brighter than I = 17. We find that
contamination by non-members in the expected PMS region of the colour-magnitude
diagram is small. We conclude that although radial velocity alone is
insufficient to confirm membership, high signal-to-noise observations of the
NaI doublet provide the opportunity to use the strength of NaI absorption in
concert with radial velocities to asses membership down to the lowest masses,
where Lithium absorption no longer distinguishes youth.Comment: 11 pages, MNRAS accepted. Online data available from:
http://www.astro.ex.ac.uk/people/timn/Catalogues/service.htm
A near-infrared interferometric survey of debris disc stars. II. CHARA/FLUOR observations of six early-type dwarfs
High-precision interferometric observations of six early-type main sequence
stars known to harbour cold debris discs have been obtained in the
near-infrared K band with the FLUOR instrument at the CHARA Array. The measured
squared visibilities are compared to the expected visibility of the stellar
photospheres based on theoretical photospheric models taking into account
rotational distortion, searching for potential visibility reduction at short
baselines due to circumstellar emission. Our observations bring to light the
presence of resolved circumstellar emission around one of the six target stars
(zeta Aql) at the 5 sigma level. The morphology of the emission source cannot
be directly constrained because of the sparse spatial frequency sampling of our
interferometric data. Using complementary adaptive optics observations and
radial velocity measurements, we find that the presence of a low-mass companion
is a likely origin for the excess emission. The potential companion has a
K-band contrast of four magnitudes, a most probable mass of about 0.6 Msun, and
is expected to orbit between about 5.5 AU and 8 AU from its host star assuming
a purely circular orbit. Nevertheless, by adjusting a physical debris disc
model to the observed Spectral Energy Distribution of the zeta Aql system, we
also show that the presence of hot dust within 10 AU from zeta Aql, producing a
total thermal emission equal to 1.69 +- 0.31% of the photospheric flux in the K
band, is another viable explanation for the observed near-infrared excess. Our
re-interpretation of archival near- to far-infrared photometric measurements
shows however that cold dust is not present around zeta Aql at the sensitivity
limit of the IRS and MIPS instruments onboard Spitzer, and urges us to remove
zeta Aql from the category of bona fide debris disc stars.Comment: 14 pages, accepted for publication in A&
Optical Spectroscopy of Embedded Young Stars in the Taurus-Auriga Molecular Cloud
This paper describes the first optical spectroscopic survey of class I
sources (also known as embedded sources and protostars) in the Taurus-Auriga
dark cloud. We detect 10 of the 24 known class I sources in the cloud at
5500-9000 A. All detected class I sources have strong H-alpha emission; most
also have strong [O I] and [S II] emission. These data - together with high
quality optical spectra of T Tauri stars in the Taurus-Auriga cloud -
demonstrate that forbidden emission lines are stronger and more common in class
I sources than in T Tauri stars. Our results also provide a clear discriminant
in the frequency of forbidden line emission between weak-emission and classical
T Tauri stars. In addition to strong emission lines, three class I sources have
prominent TiO absorption bands. The M-type central stars of these sources
mingle with optically visible T Tauri stars in the HR diagram and lie somewhat
below both the birthline for spherical accretion and the deuterium burning
sequence for disc accretion.Comment: 19 pages of text, 4 pages of tables, and 10 pages of figure
Engaging with History after Macpherson
The Race Relations Amendment Act (2000) identifies a key role for education, and more specifically history, in promoting ârace equalityâ in Britain. In this article Ian Grosvenor and Kevin Myers consider the extent of young peopleâs current engagement with the history of âdiversity, change and immigrationâ which underpins the commitment to ârace equalityâ. Finding that in many of Britainâs schools and universities a singular and exclusionary version of history continues to dominate the curriculum, they go on to consider the reasons for the neglect of multiculturalism. The authors identify the development of an aggressive national identity that depends on the past for its legitimacy and argue that this sense of the past is an important obstacle to future progress
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