7,352 research outputs found
An empirical mass-loss law for Population II giants from the Spitzer-IRAC survey of Galactic globular clusters
The main aim of the present work is to derive an empirical mass-loss (ML) law
for Population II stars in first and second ascent red giant branches. We used
the Spitzer InfraRed Array Camera (IRAC) photometry obtained in the 3.6-8
micron range of a carefully chosen sample of 15 Galactic globular clusters
spanning the entire metallicity range and sampling the vast zoology of
horizontal branch (HB) morphologies. We complemented the IRAC photometry with
near-infrared data to build suitable color-magnitude and color-color diagrams
and identify mass-losing giant stars. We find that while the majority of stars
show colors typical of cool giants, some stars show an excess of mid-infrared
light that is larger than expected from their photospheric emission and that is
plausibly due to dust formation in mass flowing from them. For these stars, we
estimate dust and total (gas + dust) ML rates and timescales. We finally
calibrate an empirical ML law for Population II red and asymptotic giant branch
stars with varying metallicity. We find that at a given red giant branch
luminosity only a fraction of the stars are losing mass. From this, we conclude
that ML is episodic and is active only a fraction of the time, which we define
as the duty cycle. The fraction of mass-losing stars increases by increasing
the stellar luminosity and metallicity. The ML rate, as estimated from
reasonable assumptions for the gas-to-dust ratio and expansion velocity,
depends on metallicity and slowly increases with decreasing metallicity. In
contrast, the duty cycle increases with increasing metallicity, with the net
result that total ML increases moderately with increasing metallicity, about
0.1 Msun every dex in [Fe/H]. For Population II asymptotic giant branch stars,
we estimate a total ML of <0.1 Msun, nearly constant with varying metallicity.Comment: 17 pages, 9 figures, in press on A&
Detections of massive stars in the cluster MCM2005b77, in the star-forming regions GRS G331.3400.36 (S62) and GRS G337.9200.48 (S36)
Large infrared and millimeter wavelength surveys of the Galactic plane have
unveiled more than 600 new bubble HII regions and more than 3000 candidate star
clusters. We present a study of the candidate clusters MCM2005b72, DBS2003-157,
DBS2003-172, and MCM2005b77, based on near-infrared spectroscopy taken with
SofI on the NTT and infrared photometry from the 2MASS, VVV, and GLIMPSE
surveys. We find that (1) MCM2005b72 and DBS2003-157 are subregions of the same
star-forming region, HII GRS G331.34-00.36 (bubble S62). MCM2005b72 coincides
with the central part of this HII region, while DBS2003-157 is a bright
mid-infrared knot of the S62 shell. We detected two O-type stars at extinction
\Aks=1.0-1.3 mag. Their spectrophotometric properties are consistent with the
near-kinematic distance to GRS G331.34-00.36 of 3.9pm0.3 kpc. (2) DBS2003-172
coincides with a bright mid-infrared knot in the S36 shell (GRS G337.92-00.48),
where we detected a pair of candidate He I stars embedded in a small cometary
nebula. (3) The stellar cluster MCM2005b77 is rich in B-type stars, has an
average Aks of 0.91 mag, and is adjacent to the HII region IRAS 16137-5025. The
average spectrophotometric distance of kpc matches the
near-kinematic distance to IRAS 16137-5025 of 5.2pm0.1 kpc.Comment: 22 pages, 11 Figures, ApJ accepte
The Terzan 5 puzzle: discovery of a third, metal-poor component
We report on the discovery of 3 metal-poor giant stars in Terzan 5, a complex
stellar system in the the Galactic bulge, known to have two populations at
[Fe/H]=-0.25 and +0.3. For these 3 stars we present new echelle spectra
obtained with NIRSPEC at Keck II, which confirm their radial velocity
membership and provide average [Fe/H]=-0.79 dex iron abundance and
[alpha/Fe]=+0.36 dex enhancement. This new population extends the metallicity
range of Terzan~5 0.5 dex more metal poor, and it has properties consistent
with having formed from a gas polluted by core collapse supernovae.Comment: Accepted for publication on ApJ Lette
Evidence of a Metal Rich Galactic Bar from the Vertex Deviation of the Velocity Ellipsoid
We combine radial velocities, proper motions, and low resolution abundances
for a sample of 315 K and M giants in the Baade's Window (l,b)=(0.9,-4)
Galactic bulge field. The velocity ellipsoid of stars with [Fe/H]>-0.5 dex
shows a vertex deviation in the plot of radial versus transverse velocity,
consistent with that expected from a population with orbits supporting a bar.
We demonstrate that the significance of this vertex deviation using
non-parametric rank correlation statistic is >99%. The velocity ellipsoid for
the metal poor ([FeH]<-0.5) part of the population shows no vertex deviation
and is consistent with an isotropic, oblate rotating population. We find no
evidence for kinematic subgroups, but there is a mild tendency for the vertical
velocity dispersion sigma_b to decrease with increasing metallicity.Comment: 4 pages, ApJ Letters, submitte
Condition-based maintenance for major airport baggage systems
Purpose: The aim of this paper is to develop a contribution to knowledge that adds to theempirical evidence of predictive condition-based maintenance by demonstrating how theavailability and reliability of current assets can be improved without costly capital investment,resulting in overall system performance improvements.Methodology: The empirical, experimental approach, technical action research (TAR), wasdesigned to study a major Middle-Eastern airport baggage handling operation. A predictivecondition-based maintenance prototype station was installed to monitor the condition of ahighly complex system of static and moving assets.Findings. The research provides evidence that the performance frontier for airport baggagehandling systems can be improved using automated dynamic monitoring of the vibration anddigital image data on baggage trays as they pass a service station. The introduction of low-endinnovation, which combines advanced technology and low-cost hardware, reduced assetfailures in this complex, high speed operating environment.Originality/Value: The originality derives from the application of existing hardware with thecombination of Edge and Cloud computing software through architectural innovation resultingin adaptations to an existing baggage handling system within the context of a time-criticallogistics system.Keywords: IoT, Condition-based maintenance, Predictive maintenance, Edge computing, IoT,Technical Action Research, Theory of Performance Frontiers,Case Stud
Deep near-IR observations of the Globular Cluster M4: Hunting for Brown Dwarfs
We present an analysis of deep HST/WFC3 near-IR (NIR) imaging data of the
globular cluster M4. The best-photometry NIR colour-magnitude diagram (CMD)
clearly shows the main sequence extending towards the expected end of the
Hydrogen-burning limit and going beyond this point towards fainter sources. The
white dwarf sequence can be identified. As such, this is the deepest NIR CMD of
a globular cluster to date. Archival HST optical data were used for
proper-motion cleaning of the CMD and for distinguishing the white dwarfs (WDs)
from brown dwarf (BD) candidates. Detection limits in the NIR are around F110W
approx 26.5 mag and F160W approx27 mag, and in the optical around F775W approx
28 mag. Comparing our observed CMDs with theoretical models, we conclude that
we have reached beyond the H-burning limit in our NIR CMD and are probably just
above or around this limit in our optical-NIR CMDs. Thus, any faint NIR sources
that have no optical counterpart are potential BD candidates, since the optical
data are not deep enough to detect them. We visually inspected the positions of
NIR sources which are fainter than the H-burning limit in F110W and for which
the optical photometry did not return a counterpart. We found in total five
sources for which we did not get an optical measurement. For four of these five
sources, a faint optical counterpart could be visually identified, and an upper
optical magnitude was estimated. Based on these upper optical magnitude limits,
we conclude that one source is likely a WD, one source could either be a WD or
BD candidate, and the remaining two sources agree with being BD candidates. For
only one source no optical counterpart could be detected, which makes this
source a good BD candidate. We conclude that we found in total four good BD
candidates.Comment: ApJ accepted, 28 pages including 16 figure
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