769 research outputs found
On the black hole content and initial mass function of 47 Tuc
The globular cluster (GC) 47 Tuc has recently been proposed to host an
intermediate-mass black hole (IMBH) or a population of stellar-mass black holes
(BHs). To shed light on its dark content, we present an application of
self-consistent multimass models with a varying mass function and content of
stellar remnants, which we fit to various observational constraints. Our
best-fitting model successfully matches the observables and correctly predicts
the radial distribution of millisecond pulsars and their gravitational
accelerations inferred from long-term timing observations. The data favours a
population of BHs with a total mass of , but the
most likely model has very few BHs. Since our models do not include a central
IMBH and accurately reproduce the observations, we conclude that there is
currently no need to invoke the presence of an IMBH in 47 Tuc. The global
present-day mass function inferred is significantly depleted in low-mass stars
(power-law slope ). Given the orbit and predicted
mass-loss history of this massive GC, the dearth of low-mass stars is difficult
to explain with a standard initial mass function (IMF) followed by long-term
preferential escape of low-mass stars driven by two-body relaxation, and
instead suggests that 47 Tuc may have formed with a bottom-light IMF. We
discuss alternative evolutionary origins for the flat mass function and ways to
reconcile this with the low BH retention fraction. Finally, by capturing the
effect of dark remnants, our method offers a new way to probe the IMF in a GC
above the current main-sequence turn-off mass, for which we find a slope of
.Comment: 16 pages, 7 figures, accepted to MNRAS after minor revisio
An Exploratory Study Of Student Satisfaction With University Web Page Design
This exploratory study evaluates the satisfaction of students with a web-based information system at a medium-sized regional university. The analysis provides a process for simplifying data interpretation in captured student user feedback. Findings indicate that student classifications, as measured by demographic and other factors, determine satisfaction levels towards various web sources of information. Differences in satisfaction levels across student groups based on gender, age, minority status, employment, and current course load were found. Implications for university web designers and university administrators are considered and discussed
Identifying Very Metal-Rich Stars with Low-Resolution Spectra: Finding Planet-Search Targets
We present empirical calibrations that estimate stellar metallicity,
effective temperature and surface gravity as a function of Lick/IDS indices.
These calibrations have been derived from a training set of 261 stars for which
(1) high-precision measurements of [Fe/H], T_eff and log g have been made using
spectral-synthesis analysis of HIRES spectra, and (2) Lick indices have also
been measured. Our [Fe/H] calibration, which has precision 0.07 dex, has
identified a number of bright (V < 9) metal-rich stars which are now being
screened for hot Jupiter-type planets. Using the Yonsei-Yale stellar models, we
show that the calibrations provide distance estimates accurate to 20% for
nearby stars.
This paper outlines the second tier of the screening of planet-search targets
by the N2K Consortium, a project designed to identify the stars most likely to
harbor extrasolar planets. Discoveries by the N2K Consortium include the
transiting hot Saturn HD 149026 b (Sato et al. 2005, astro-ph/0507009) and HD
88133 b (Fischer et al. 2005). See Ammons et al. (2005, In Press) for a
description of the first tier of N2K metallicity screening, calibrations using
broadband photometry.Comment: Accepted for publication in the Astrophysical Journa
An Exploratory Study Of Student Satisfaction With University Web Page Design
This exploratory study evaluates the satisfaction of students with a web-based information system at a medium-sized regional university. The analysis provides a process for simplifying data interpretation in captured student user feedback. Findings indicate that student classifications, as measured by demographic and other factors, determine satisfaction levels towards various web sources of information. Differences in satisfaction levels across student groups based on gender, age, minority status, employment, and current course load were found. Implications for university web designers and university administrators are considered and discussed
An HST/WFPC Survey of Bright Young Clusters in M31. II. Photometry of Less Luminous Clusters in the Fields
We report on the properties of 89 low mass star clusters located in the
vicinity of luminous young clusters (blue globulars) in the disk of M31. 82 of
the clusters are newly detected. We have determined their integrated magnitudes
and colors, based on a series of Hubble Space Telescope Wide Field/Planetary
Camera 2 exposures in blue and red (HST filters F450W and F814W). The
integrated apparent magnitudes range from F450W = 17.5 to 22.5, and the colors
indicate a wide range of ages. Stellar color-magnitude diagrams for all
clusters were obtained and those with bright enough stars were fit to
theoretical isochrones to provide age estimates. The ages range from 12 Myr to
>500 Myr. Reddenings, which average E(F450 - F814) = 0.59 with a dispersion of
0.21 magnitudes, were derived from the main sequence fitting for those
clusters. Comparison of these ages and integrated colors with single population
theoretical models with solar abundances suggests a color offset of 0.085
magnitudes at the ages tested. Estimated ages for the remaining clusters are
based on their measured colors. The age-frequency diagram shows a steep decline
of number with age, with a large decrease in number per age interval between
the youngest and the oldest clusters detected.Comment: 20 pages, 9 figure
Partly burnt runaway stellar remnants from peculiar thermonuclear supernovae
We report the discovery of three stars that, along with the prototype
LP40-365, form a distinct class of chemically peculiar runaway stars that are
the survivors of thermonuclear explosions. Spectroscopy of the four confirmed
LP 40-365 stars finds ONe-dominated atmospheres enriched with remarkably
similar amounts of nuclear ashes of partial O- and Si-burning. Kinematic
evidence is consistent with ejection from a binary supernova progenitor; at
least two stars have rest-frame velocities indicating they are unbound to the
Galaxy. With masses and radii ranging between 0.20-0.28 Msun and 0.16-0.60
Rsun, respectively, we speculate these inflated white dwarfs are the partly
burnt remnants of either peculiar Type Iax or electron-capture supernovae.
Adopting supernova rates from the literature, we estimate that ~20 LP40-365
stars brighter than 19 mag should be detectable within 2 kpc from the Sun at
the end of the Gaia mission. We suggest that as they cool, these stars will
evolve in their spectroscopic appearance, and eventually become peculiar O-rich
white dwarfs. Finally, we stress that the discovery of new LP40-365 stars will
be useful to further constrain their evolution, supplying key boundary
conditions to the modelling of explosion mechanisms, supernova rates, and
nucleosynthetic yields of peculiar thermonuclear explosions.Comment: 22 pages, 14 figures, 6 tables. Accepted for publication on MNRA
Identifying Ligand Binding Conformations of the Ξ²2-Adrenergic Receptor by Using Its Agonists as Computational Probes
Recently available G-protein coupled receptor (GPCR) structures and biophysical studies suggest that the difference between the effects of various agonists and antagonists cannot be explained by single structures alone, but rather that the conformational ensembles of the proteins need to be considered. Here we use an elastic network model-guided molecular dynamics simulation protocol to generate an ensemble of conformers of a prototypical GPCR, Ξ²2-adrenergic receptor (Ξ²2AR). The resulting conformers are clustered into groups based on the conformations of the ligand binding site, and distinct conformers from each group are assessed for their binding to known agonists of Ξ²2AR. We show that the select ligands bind preferentially to different predicted conformers of Ξ²2AR, and identify a role of Ξ²2AR extracellular region as an allosteric binding site for larger drugs such as salmeterol. Thus, drugs and ligands can be used as "computational probes" to systematically identify protein conformers with likely biological significance. Β© 2012 Isin et al
Discovery and Early Evolution of ASASSN-19bt, the First TDE Detected by TESS
We present the discovery and early evolution of ASASSN-19bt, a tidal
disruption event (TDE) discovered by the All-Sky Automated Survey for
Supernovae (ASAS-SN) at a distance of Mpc and the first TDE to be
detected by TESS. As the TDE is located in the TESS Continuous Viewing Zone,
our dataset includes 30-minute cadence observations starting on 2018 July 25,
and we precisely measure that the TDE begins to brighten days before
its discovery. Our dataset also includes 18 epochs of Swift UVOT and XRT
observations, 2 epochs of XMM-Newton observations, 13 spectroscopic
observations, and ground data from the Las Cumbres Observatory telescope
network, spanning from 32 days before peak through 37 days after peak.
ASASSN-19bt thus has the most detailed pre-peak dataset for any TDE. The TESS
light curve indicates that the transient began to brighten on 2019 January 21.6
and that for the first 15 days its rise was consistent with a flux power-law model. The optical/UV emission is well-fit by a blackbody SED,
and ASASSN-19bt exhibits an early spike in its luminosity and temperature
roughly 32 rest-frame days before peak and spanning up to 14 days that has not
been seen in other TDEs, possibly because UV observations were not triggered
early enough to detect it. It peaked on 2019 March 04.9 at a luminosity of
ergs s and radiated
ergs during the 41-day rise to peak. X-ray observations after peak indicate a
softening of the hard X-ray emission prior to peak, reminiscent of the
hard/soft states in X-ray binaries.Comment: 23 pages, 14 figures, 5 tables. A machine-readable table containing
the host-subtracted photometry presented in this manuscript is included as an
ancillary fil
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