4,583 research outputs found
High Spectral Resolution Observations of the Massive Stars in the Galactic Center
We present high-resolution near-infrared spectra, obtained with the NIRSPEC
spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive
stars within the central 25 arcseconds of the Galactic center. We have
identified a total of twenty-one emission-line stars, seven of which are new
radial velocity detections with five of those being classified as He I
emission-line stars for the first time. These stars fall into two categories
based on their spectral properties: 1) those with narrow 2.112, 2.113 micron He
I doublet absorption lines, and 2) those with broad 2.058 micron He I emission
lines. These data have the highest spectral resolution ever obtained for these
sources and, as a result, both components of the absorption doublet are
separately resolved for the first time. We use these spectral features to
measure radial velocities. The majority of the measured radial velocities have
relative errors of 20 kms, smaller than those previously obtained with
proper-motion or radial velocity measurements for similar stellar samples in
the Galactic center. The radial velocities estimated from the He I absorption
doublet are more robust than those previously estimated from the 2.058 micron
emission line, since they do not suffer from confusion due to emission from the
surrounding ISM. Using this velocity information, we agree that the stars are
orbiting in a somewhat coherent manner but are not as defined into a disk or
disks as previously thought. Finally, multi-epoch radial velocity measurements
for IRS 16NE show a change in its velocity presumably due to an unseen stellar
companion.Comment: ApJ accepted, 42 pages, 16 figure
Graduate students\u27 teaching experiences improve their methodological research skills
Science, technology, engineering, and mathematics (STEM) graduate students are often encouraged to maximize their engagement with supervised research and minimize teaching obligations. However, the process of teaching students engaged in inquiry provides practice in the application of important research skills. Using a performance rubric, we compared the quality of methodological skills demonstrated in written research proposals for two groups of early career graduate students (those with both teaching and research responsibilities and those with only research responsibilities) at the beginning and end of an academic year. After statistically controlling for preexisting differences between groups, students who both taught and conducted research demonstrate significantly greater improvement in their abilities to generate testable hypotheses and design valid experiments. These results indicate that teaching experience can contribute substantially to the improvement of essential research skills
New distances to RAVE stars
Probability density functions are determined from new stellar parameters for
the distance moduli of stars for which the RAdial Velocity Experiment (RAVE)
has obtained spectra with S/N>=10. Single-Gaussian fits to the pdf in distance
modulus suffice for roughly half the stars, with most of the other half having
satisfactory two-Gaussian representations. As expected, early-type stars rarely
require more than one Gaussian. The expectation value of distance is larger
than the distance implied by the expectation of distance modulus; the latter is
itself larger than the distance implied by the expectation value of the
parallax. Our parallaxes of Hipparcos stars agree well with the values measured
by Hipparcos, so the expectation of parallax is the most reliable distance
indicator. The latter are improved by taking extinction into account. The
effective temperature absolute-magnitude diagram of our stars is significantly
improved when these pdfs are used to make the diagram. We use the method of
kinematic corrections devised by Schoenrich, Binney & Asplund to check for
systematic errors for general stars and confirm that the most reliable distance
indicator is the expectation of parallax. For cool dwarfs and low-gravity
giants tends to be larger than the true distance by up to 30 percent. The
most satisfactory distances are for dwarfs hotter than 5500 K. We compare our
distances to stars in 13 open clusters with cluster distances from the
literature and find excellent agreement for the dwarfs and indications that we
are over-estimating distances to giants, especially in young clusters.Comment: 20 pages accepted by MNRAS. Minor changes to the submitted versio
Gendered endings: Narratives of male and female suicides in the South African Lowveld
This is the author's accepted manuscript. The final publication is available at Springer via http://dx.doi.org/10.1007/s11013-012-9258-y. Copyright @ Springer Science+Business Media, LLC 2012.Durkheimâs classical theory of suicide rates being a negative index of social solidarity downplays the salience of gendered concerns in suicide. But gendered inequalities have had a negative impact: worldwide significantly more men than women perpetrate fatal suicides. Drawing on narratives of 52 fatal suicides in Bushbuckridge, South Africa, this article suggests that Bourdieuâs concepts of âsymbolic violenceâ and âmasculine dominationâ provide a more appropriate framework for understanding this paradox. I show that the thwarting of investments in dominant masculine positions have been the major precursor to suicides by men. Men tended to take their own lives as a means of escape. By contrast, women perpetrated suicide to protest against the miserable consequences of being dominated by men. However, contra the assumption of Bourdieuâs concept of âhabitusâ, the narrators of suicide stories did reflect critically upon gender constructs
Group Theory and Quasiprobability Integrals of Wigner Functions
The integral of the Wigner function of a quantum mechanical system over a
region or its boundary in the classical phase plane, is called a
quasiprobability integral. Unlike a true probability integral, its value may
lie outside the interval [0,1]. It is characterized by a corresponding
selfadjoint operator, to be called a region or contour operator as appropriate,
which is determined by the characteristic function of that region or contour.
The spectral problem is studied for commuting families of region and contour
operators associated with concentric disks and circles of given radius a. Their
respective eigenvalues are determined as functions of a, in terms of the
Gauss-Laguerre polynomials. These polynomials provide a basis of vectors in
Hilbert space carrying the positive discrete series representations of the
algebra su(1,1)or so(2,1). The explicit relation between the spectra of
operators associated with disks and circles with proportional radii, is given
in terms of the dicrete variable Meixner polynomials.Comment: 11 pages, latex fil
Extracting science from surveys of our Galaxy
Our knowledge of the Galaxy is being revolutionised by a series of
photometric, spectroscopic and astrometric surveys. Already an enormous body of
data is available from completed surveys, and data of ever increasing quality
and richness will accrue at least until the end of this decade. To extract
science from these surveys we need a class of models that can give probability
density functions in the space of the observables of a survey -- we should not
attempt to "invert" the data from the space of observables into the physical
space of the Galaxy. Currently just one class of model has the required
capability, so-called "torus models". A pilot application of torus models to
understanding the structure of the Galaxy's thin and thick discs has already
produced two significant results: a major revision of our best estimate of the
Sun's velocity with respect to the Local Standard of Rest, and a successful
prediction of the way in which the vertical velocity dispersion in the disc
varies with distance from the Galactic plane.Comment: 13 pages. Invited review to appear in Pramana - journal of physics
(Indian Academy of Sciences
High Precision Stellar Radial Velocities in the Galactic Center
We present radial velocities for 85 cool stars projected onto the central
parsec of the Galaxy. The majority of these velocities have relative errors of
1 km/s, or a factor of 30-100 smaller than those previously
obtained with proper motion or other radial velocity measurements for a similar
stellar sample. The error in a typical individual stellar velocity, including
all sources of uncertainty, is 1.7 km/s. Two similar data sets were obtained
one month apart, and the total error in the relative velocities is 0.80 km/s\
in the case where an object is common to both data sets. The data are used to
characterize the velocity distribution of the old population in the Galctic
Center. We find that the stars have a Gaussian velocity distribution with a
mean heliocentric velocity of 11.0 km/s (blueshifted) and a standard
deviation of 100.9 km/s; the mean velocity of the sample is consistent
with no bulk line-of-sight motion with respect to the Local Standard of Rest.
At the 1 sigma level, the data are consistent with a symmetric velocity
distribution about any arbitrary axis in the plane of the sky. We find evidence
for a flattening in the distribution of late-type stars within a radius of
0.4 \pc, and infer a volume density distribution of r in this
region. Finally, we establish a first epoch of radial velocity measurements
which can be compared to subsequent epochs to measure small accelerations (1
km/s/yr), corresponding to the magnitude expected over a timespan of several
years for stars nearest to Sgr A*.Comment: retrieve full version at
http://www-int.stsci.edu/figer/papers/nirspec/vel
Detection of a Thick Disk in the edge-on Low Surface Brightness Galaxy ESO 342-G017: I. VLT Photometry in V and R Bands
We report the detection of a thick disk in the edge-on, low surface
brightness (LSB), late-type spiral ESO 342-G017, based on ultra-deep images in
the V and R bands obtained with the VLT Test Camera during Science Verification
on UT1. All steps in the reduction procedure are fully described, which,
together with an extensive analysis of systematic and statistic uncertainties,
has resulted in surface brightness photometry that is reliable for the
detection of faint extended structure to a level of V = 27.5 and R = 28.5
mag/square arcsec. The faint light apparent in these deep images is
well-modeled by a thick exponential disk with an intrinsic scale height about
2.5 times that of the thin disk, and a comparable or somewhat larger scale
length. Deprojection including the effects of inclination and convolution with
the PSF allow us to estimate that the thick disk contributes 20-40% of the
total (old) stellar disk luminosity of ESO 342-G017. To our knowledge, this is
the first detection of a thick disk in an LSB galaxy, which are generally
thought to be rather unevolved compared to higher surface brightness galaxies.Comment: Accepted for publication in Astronomy & Astrophysics; 18 pages, 12
figure
Simulations of energetic beam deposition: from picoseconds to seconds
We present a new method for simulating crystal growth by energetic beam
deposition. The method combines a Kinetic Monte-Carlo simulation for the
thermal surface diffusion with a small scale molecular dynamics simulation of
every single deposition event. We have implemented the method using the
effective medium theory as a model potential for the atomic interactions, and
present simulations for Ag/Ag(111) and Pt/Pt(111) for incoming energies up to
35 eV. The method is capable of following the growth of several monolayers at
realistic growth rates of 1 monolayer per second, correctly accounting for both
energy-induced atomic mobility and thermal surface diffusion. We find that the
energy influences island and step densities and can induce layer-by-layer
growth. We find an optimal energy for layer-by-layer growth (25 eV for Ag),
which correlates with where the net impact-induced downward interlayer
transport is at a maximum. A high step density is needed for energy induced
layer-by-layer growth, hence the effect dies away at increased temperatures,
where thermal surface diffusion reduces the step density. As part of the
development of the method, we present molecular dynamics simulations of single
atom-surface collisions on flat parts of the surface and near straight steps,
we identify microscopic mechanisms by which the energy influences the growth,
and we discuss the nature of the energy-induced atomic mobility
Improved distances and ages for stars common to TGAS and RAVE
ABSTRACT
We combine parallaxes from the first Gaia data release with the spectrophotometric distance
estimation framework for stars in the fifth RAVE survey data release. The combined distance
estimates are more accurate than either determination in isolation â uncertainties are on average
two times smaller than for RAVE-only distances (three times smaller for dwarfs), and 1.4 times
smaller than TGAS parallax uncertainties (two times smaller for giants). We are also able to
compare the estimates from spectrophotometry to those from Gaia, and use this to assess the
reliability of both catalogues and improve our distance estimates.We find that the distances to
the lowest log g stars are, on average, overestimated and caution that they may not be reliable.
We also find that it is likely that the Gaia random uncertainties are smaller than the reported
values. As a byproduct we derive ages for the RAVE stars, many with relative uncertainties less
than 20 percent. These results for 219 566 RAVE sources have been made publicly available,
and we encourage their use for studies that combine the radial velocities provided by RAVE
with the proper motions provided by Gaia. A sample that we believe to be reliable can be
found by taking only the stars with the flag notification âflag_any=0â.
Key words: Galaxy: fundamental parameters â methods: statistical âFunding for the research in this study came from the Swedish National Space Board, the Royal Physiographic Society in Lund, and some of the computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at Lunarc under project SNIC 2016/4-17. Funding for RAVE has been provided by: the Australian Astronomical Observatory; the Leibniz-Institut fuer Astrophysik Potsdam (AIP); the Australian National University; the Australian Research Council; the French National Research Agency; the German Research Foundation (SPP 1177 and SFB 881); the European Research Council (ERC-StG 240271 Galactica); the Istituto Nazionale di Astrofisica at Padova; The Johns Hopkins University; the National Science Foundation of the USA (AST-0908326); the W. M. Keck foundation; the Macquarie University; the Netherlands Research School for Astronomy; the Natural Sciences and Engineering Research Council of Canada; the Slovenian Research Agency (research core funding No. P1-0188); the Swiss National Science Foundation; the Science & Technology Facilities Council of the UK; Opticon; Strasbourg Observatory; and the Universities of Groningen, Heidelberg and Sydney. The RAVE web site is https://www.rave-survey.org. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement
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