69 research outputs found
Evolution of the Small Magellanic Cloud
Based on the results of N-body simulations on the last 2.5 Gyr evolution of
the Large and Small Magellanic Clouds (LMC and SMC, respectively) interacting
with the Galaxy, we firstly show when and where the leading arms (LAs) of the
Magellanic stream (MS) can pass through the Galactic plane after the MS
formation. We secondly show collisions between the outer Galactic HI disk and
the LAs of the MS can create giant HI holes and chimney-like structures in the
disk about 0.2 Gyr ago. We thirdly show that a large amount of metal-poor gas
is stripped from the SMC and transfered to the LMC during the tidal interaction
between the Clouds and the Galaxy about 0.2 and 1.3 Gyr ago. We thus propose
that this metal-poor gas can closely be associated with the origin of LMC's
young and intermediate-age stars and star clusters with distinctively
low-metallicities with [Fe/H] < -0.6.Comment: 4 pages, 3 figures, to appear in the proceedings of ``Galaxies in the
Local Volume'', Sydney, 8 to 13 July, 200
The first chemical abundance analysis of K giants in the inner Galactic disc
The elemental abundance structure of the Galactic disc has been extensively
studied in the solar neighbourhood using long-lived stars such as F and G
dwarfs or K and M giants. These are stars whose atmospheres preserve the
chemical composition of their natal gas clouds, and are hence excellent tracers
of the chemical evolution of the Galaxy. As far as we are aware, there are no
such studies of the inner Galactic disc, which hampers our ability to constrain
and trace the origin and evolution of the Milky Way. Therefore, we aim in this
study to establish the elemental abundance trend(s) of the disc(s) in the inner
regions of the Galaxy. Based on equivalent width measurements in
high-resolution spectra obtained with the MIKE spectrograph on the Magellan II
telescope on Las Campanas in Chile, we determine elemental abundances for 44
K-type red giant stars in the inner Galactic disc, located at Galactocentric
distances of 4-7\,kpc. The analysis method is identical to the one recently
used on red giant stars in the Galactic bulge and in the nearby thin and thick
discs, enabling us to perform a truly differential comparison of the different
stellar populations. We present the first detailed elemental abundance study of
a significant number of red giant stars in the inner Galactic disc. We find
that these inner disc stars show the same type of chemical and kinematical
dichotomy as the thin and thick discs show in the solar neighbourhood. The
abundance trends of the inner disc agree very well with those of the nearby
thick disc, and also to those of the Bulge. The chemical similarities between
the Bulge and the Galactic thick disc stellar populations indicate that they
have similar chemical histories, and any model trying to understand the
formation and evolution of either of the two should preferably incorporate both
of them.Comment: A&A Letters, accepte
Evolutionary Trajectories for the Functional Diversification of Anthracycline Methyltransferases
The puzzling dredge-up pattern in NGC 1978
Low-mass stars are element factories that efficiently release their products
in the final stages of their evolution by means of stellar winds. Since they
are large in number, they contribute significantly to the cosmic matter cycle.
To assess this contribution quantitatively, it is crucial to obtain a detailed
picture of the stellar interior, particularly with regard to nucleosynthesis
and mixing mechanisms. We seek to benchmark stellar evolutionary models of
low-mass stars. In particular, we measure the surface abundance of ^{12}C in
thermally pulsing AGB stars with well-known mass and metallicity, which can be
used to infer information about the onset and efficiency of the third
dredge-up. We recorded high-resolution near-infrared spectra of AGB stars in
the LMC cluster NGC 1978. The sample comprised both oxygen-rich and carbon-rich
stars, and is well-constrained in terms of the stellar mass, metallicity, and
age. We derived the C/O and ^{12}C/^{13}C ratio from the target spectra by a
comparison to synthetic spectra. Then, we compared the outcomes of stellar
evolutionary models with our measurements. The M stars in NGC 1978 show values
of C/O and ^{12}C/^{13}C that can best be explained with moderate extra-mixing
on the RGB coupled to a moderate oxygen enhancement in the chemical
composition. These oxygen-rich stars do not seem to have undergone third
dredge-up episodes (yet). The C stars show carbon-to-oxygen and carbon isotopic
ratios consistent with the occurrence of the third dredge-up. We did not find S
stars in this cluster. None of the theoretical schemes that we considered was
able to reproduce the observations appropriately. Instead, we discuss some
non-standard scenarios to explain the puzzling abundance pattern in NGC 1978.Comment: 16 pages, 9 figures, 4 tables, accepted for publication in A&A,
language revise
Tracing the long bar with red-clump giants
Over the last decade a series of results have lent support to the hypothesis
of the existence of a long thin bar in the Milky Way with a half-length of 4.5
kpc and a position angle of around 45 deg. This is apparently a very different
structure from the triaxial bulge of the Galaxy.
In this paper, we analyse the stellar distribution in the inner 4 kpc of the
Galaxy to see if there is clear evidence for two triaxial or barlike
structures, or whether there is only one. By using the red-clump population as
a tracer of the structure of the inner Galaxy we determine the apparent
morphology of the inner Galaxy. Star counts from 2MASS are used to provide
additional support for this analysis.
We show that there are two very different large-scale triaxial structures
coexisting in the inner Galaxy: a long thin stellar bar constrained to the
Galactic plane (|b|<2 deg) with a position angle of 43.1 +- 1.8 deg, and a
distinct triaxial bulge that extends to at least |b|<7.5 deg with a position
angle of 12.6 +- 3.2 deg. The scale height of the bar source distribution is
around 100 pc, whereas for the bulge the value of this parameter is five times
larger.Comment: 16 pages, 35 figures, accepted for publication in A&
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
MORGOTH: incorporating horizontal branch modelling into star formation history determinations
We present a new method that incorporates the horizontal branch morphology into synthetic colour-magnitude diagram based star formation history determinations. This method, we call MORGOTH, self-consistently takes into account all the stellar evolution phases up to the early asymptothic giant branch, flexibly modelling red giant branch mass loss. We test MORGOTH on a range of synthetic populations, and find that the inclusion of the horizontal branch significantly increases the precision of the resulting star formation histories. When the main sequence turn-off is detected, MORGOTH can fit the star formation history and the red giant branch mass loss at the same time, efficiently breaking this degeneracy. As part of testing MORGOTH, we also model the observed colour-magnitude diagram of the well studied Sculptor dwarf spheroidal galaxy. We recover a new more detailed star formation history for this galaxy. Both the new star formation history and the red giant branch mass loss we determined for Sculptor with MORGOTH are in good agreement with previous analyses, thus demonstrating the power of this new approach
The influence of chemical composition on the properties of Cepheid stars. II-The iron content
The Cepheid period-luminosity (PL) relation is unquestionably one of the most
powerful tools at our disposal for determining the extragalactic distance
scale. While significant progress has been made in the past few years towards
its understanding and characterization both on the observational and
theoretical sides, the debate on the influence that chemical composition may
have on the PL relation is still unsettled. With the aim to assess the
influence of the stellar iron content on the PL relation in the V and K bands,
we have related the V-band and the K-band residuals from the standard PL
relations of Freedman et al. (2001) and Persson et al. (2004), respectively, to
[Fe/H]. We used direct measurements of the iron abundances of 68 Galactic and
Magellanic Cepheids from FEROS and UVES high-resolution and high
signal-to-noise spectra. We find a mean iron abundance ([Fe/H]) about solar
(sigma = 0.10) for our Galactic sample (32 stars), -0.33 dex (sigma = 0.13) for
the Large Magellanic Cloud (LMC) sample (22 stars) and -0.75 dex (sigma = 0.08)
for the Small Magellanic Cloud (SMC) sample (14 stars). Our abundance
measurements of the Magellanic Cepheids double the number of stars studied up
to now at high resolution. The metallicity affects the V-band Cepheid PL
relation and metal-rich Cepheids appear to be systematically fainter than
metal-poor ones. These findings depend neither on the adopted distance scale
for Galactic Cepheids nor on the adopted LMC distance modulus. Current data do
not allow us to reach a firm conclusion concerning the metallicity dependence
of the K-band PL relation. The new Galactic distances indicate a small effect,
whereas the old ones support a marginal effect.Comment: 18 pages, 17 figures, accepted for publication in Astronomy and
Astrophysic
Need for recovery amongst emergency physicians in the UK and Ireland: A cross-sectional survey
OBJECTIVES: To determine the need for recovery (NFR) among emergency physicians and to identify demographic and occupational characteristics associated with higher NFR scores. DESIGN: Cross-sectional electronic survey. SETTING: Emergency departments (EDs) (n=112) in the UK and Ireland. PARTICIPANTS: Emergency physicians, defined as any registered physician working principally within the ED, responding between June and July 2019. MAIN OUTCOME MEASURE: NFR Scale, an 11-item self-administered questionnaire that assesses how work demands affect intershift recovery. RESULTS: The median NFR Score for all 4247 eligible, consented participants with a valid NFR Score was 70.0 (95% CI: 65.5 to 74.5), with an IQR of 45.5-90.0. A linear regression model indicated statistically significant associations between gender, health conditions, type of ED, clinical grade, access to annual and study leave, and time spent working out-of-hours. Groups including male physicians, consultants, general practitioners (GPs) within the ED, those working in paediatric EDs and those with no long-term health condition or disability had a lower NFR Score. After adjusting for these characteristics, the NFR Score increased by 3.7 (95% CI: 0.3 to 7.1) and 6.43 (95% CI: 2.0 to 10.8) for those with difficulty accessing annual and study leave, respectively. Increased percentage of out-of-hours work increased NFR Score almost linearly: 26%-50% out-of-hours work=5.7 (95% CI: 3.1 to 8.4); 51%-75% out-of-hours work=10.3 (95% CI: 7.6 to 13.0); 76%-100% out-of-hours work=14.5 (95% CI: 11.0 to 17.9). CONCLUSION: Higher NFR scores were observed among emergency physicians than reported in any other profession or population to date. While out-of-hours working is unavoidable, the linear relationship observed suggests that any reduction may result in NFR improvement. Evidence-based strategies to improve well-being such as proportional out-of-hours working and improved access to annual and study leave should be carefully considered and implemented where feasible
Strong Interaction Physics at the Luminosity Frontier with 22 GeV Electrons at Jefferson Lab
This document presents the initial scientific case for upgrading the
Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab (JLab)
to 22 GeV. It is the result of a community effort, incorporating insights from
a series of workshops conducted between March 2022 and April 2023. With a track
record of over 25 years in delivering the world's most intense and precise
multi-GeV electron beams, CEBAF's potential for a higher energy upgrade
presents a unique opportunity for an innovative nuclear physics program, which
seamlessly integrates a rich historical background with a promising future. The
proposed physics program encompass a diverse range of investigations centered
around the nonperturbative dynamics inherent in hadron structure and the
exploration of strongly interacting systems. It builds upon the exceptional
capabilities of CEBAF in high-luminosity operations, the availability of
existing or planned Hall equipment, and recent advancements in accelerator
technology. The proposed program cover various scientific topics, including
Hadron Spectroscopy, Partonic Structure and Spin, Hadronization and Transverse
Momentum, Spatial Structure, Mechanical Properties, Form Factors and Emergent
Hadron Mass, Hadron-Quark Transition, and Nuclear Dynamics at Extreme
Conditions, as well as QCD Confinement and Fundamental Symmetries. Each topic
highlights the key measurements achievable at a 22 GeV CEBAF accelerator.
Furthermore, this document outlines the significant physics outcomes and unique
aspects of these programs that distinguish them from other existing or planned
facilities. In summary, this document provides an exciting rationale for the
energy upgrade of CEBAF to 22 GeV, outlining the transformative scientific
potential that lies within reach, and the remarkable opportunities it offers
for advancing our understanding of hadron physics and related fundamental
phenomena.Comment: Updates to the list of authors; Preprint number changed from theory
to experiment; Updates to sections 4 and 6, including additional figure
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