5,241 research outputs found
Star formation rates and efficiencies in the Galactic Centre
The inner few hundred parsecs of the Milky Way harbours gas densities, pressures, velocity dispersions, an interstellar radiation field and a cosmic ray ionisation rate orders of magnitude higher than the disc; akin to the environment found in star-forming galaxies at high-redshift. Previous studies have shown that this region is forming stars at a rate per unit mass of dense gas which is at least an order of magnitude lower than in the disc, potentially violating theoretical predictions. We show that all observational star formation rate diagnostics - both direct counting of young stellar objects and integrated light measurements - are in agreement within a factor two, hence the low star formation rate is not the result of the systematic uncertainties that affect any one method. As these methods trace the star formation over different timescales, from Myr, we conclude that the star formation rate has been constant to within a factor of a few within this time period. We investigate the progression of star formation within gravitationally bound clouds on parsec scales and find per cent of the cloud masses are converted into stars per free-fall time, consistent with a subset of the considered "volumetric" star formation models. However, discriminating between these models is obstructed by the current uncertainties on the input observables and, most importantly and urgently, by their dependence on ill-constrained free parameters. The lack of empirical constraints on these parameters therefore represents a key challenge in the further verification or falsification of current star formation theories
Cytisine versus varenicline for smoking cessation for Māori (the indigenous people of New Zealand) and their extended family:Protocol for a randomized non‐inferiority trial
Background and aims Cytisine, a nicotinic acetylcholine receptor partial agonist (like varenicline) found in some plants, is a low-cost, effective smoking cessation medication that may appeal to Māori [the indigenous people of New Zealand (NZ)]. The RAUORA trial aims to determine the effectiveness, safety and cost-effectiveness of cytisine (Tabex®) versus varenicline (Champix®) for smoking cessation in Māori and the whānau (extended family) of Māori. Design Pragmatic, community-based, open-label randomized non-inferiority trial. Setting Lakes District Health Board region, NZ. Participants Daily smokers (n = 2140) who self-identify as Māori or whānau of Māori, and are: aged ≥ 18 years, motivated to quit smoking in the next 2 weeks, eligible for subsidized varenicline, able to provide verbal consent and have daily access to a mobile phone/internet. Recruitment uses multi-media advertising. Intervention and comparator Participants are randomized (1 : 1 ratio) to receive a prescription for 12 weeks of cytisine tablets [following the manufacturer’s dosing regimen for 25 days, then one 1.5-mg tablet every 6 hours (two per day) until 12 weeks] or varenicline tablets (following the manufacturer’s dosing regimen). Both groups receive brief stop-smoking advice from the prescribing doctor and withdrawal-orientated behavioural support via community-based stop-smoking counselling services (frequency, duration and mode of delivery tailored for participants) or a research assistant (six weekly 10–15- minute calls). Participants are advised to reduce their smoking over the first 4 days of treatment, with day 5 as their designated quit-date. Measurements The primary outcome is carbon monoxide-verified continuous abstinence at 6 months post-quit date. Secondary outcomes at 1, 3, 6 and 12 months post-quit date include: self-reported continuous abstinence, 7-day point prevalence abstinence, cigarettes per day, time to (re)lapse, adverse events, treatment adherence/compliance, treatment acceptability, nicotine withdrawal/urge to smoke and health-care utilization/health- related quality of life. Comments This trial compares cytisine and varenicline when used by the indigenous people of NZ and their extended family for smoking cessation
WASP-33: The first delta Scuti exoplanet host star
We report the discovery of photometric oscillations in the host star of the
exoplanet WASP-33 b (HD 15082). The data were obtained in the R band in both
transit and out-of-transit phases from the 0.3-m telescope and the Montcabrer
Observatory and the 0.8-m telescope at the Montsec Astronomical Observatory.
Proper fitting and subsequent removal of the transit signal reveals stellar
photometric variations with a semi-amplitude of about 1 mmag. The detailed
analysis of the periodogram yields a structure of significant signals around a
frequency of 21 cyc per day, which is typical of delta Scuti-type variable
stars. An accurate study of the power spectrum reveals a possible
commensurability with the planet orbital motion with a factor of 26, but this
remains to be confirmed with additional time-series data that will permit the
identification of the significant frequencies. These findings make WASP-33 the
first transiting exoplanet host star with delta Sct variability and a very
interesting candidate to search for star-planet interactions.Comment: 5 pages, 6 figures. Revised version accepted for publication in A&A
Letter
'The Brick' is not a brick : A comprehensive study of the structure and dynamics of the Central Molecular Zone cloud G0.253+0.016
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.In this paper we provide a comprehensive description of the internal dynamics of G0.253+0.016 (a.k.a. 'the Brick'); one of the most massive and dense molecular clouds in the Galaxy to lack signatures of widespread star formation. As a potential host to a future generation of high-mass stars, understanding largely quiescent molecular clouds like G0.253+0.016 is of critical importance. In this paper, we reanalyse Atacama Large Millimeter Array cycle 0 HNCO data at 3 mm, using two new pieces of software which we make available to the community. First, scousepy, a Python implementation of the spectral line fitting algorithm scouse. Secondly, acorns (Agglomerative Clustering for ORganising Nested Structures), a hierarchical n-dimensional clustering algorithm designed for use with discrete spectroscopic data. Together, these tools provide an unbiased measurement of the line of sight velocity dispersion in this cloud, kms, which is somewhat larger than predicted by velocity dispersion-size relations for the Central Molecular Zone (CMZ). The dispersion of centroid velocities in the plane of the sky are comparable, yielding . This isotropy may indicate that the line-of-sight extent of the cloud is approximately equivalent to that in the plane of the sky. Combining our kinematic decomposition with radiative transfer modelling we conclude that G0.253+0.016 is not a single, coherent, and centrally-condensed molecular cloud; 'the Brick' is not a \emph{brick}. Instead, G0.253+0.016 is a dynamically complex and hierarchically-structured molecular cloud whose morphology is consistent with the influence of the orbital dynamics and shear in the CMZ.Peer reviewedFinal Accepted Versio
Unequal-mass galaxy merger remnants: spiral-like morphology but elliptical-like kinematics
It is generally believed that major galaxy mergers with mass ratios in the
range 1:1-3:1 result in remnants that have properties similar to elliptical
galaxies, and minor mergers below 10:1 result in disturbed spiral galaxies. The
intermediate range of mass ratios 4:1-10:1 has not been studied so far. Using
N-body simulations, we show that such mergers can result in very peculiar
systems, that have the morphology of a disk galaxy with an exponential profile,
but whose kinematics is closer to that of elliptical systems. These objects are
similar to those recently observed by Jog & Chitre (2002). We present two cases
with mass ratios 4.5:1 and 7:1, and show that the merging causes major heating
and results in the appearance of elliptical-type kinematics, while surprisingly
the initial spiral-like mass profile is conserved.Comment: 4 pages, 3 figures, Accepted for publication in A&A Letter
Using young massive star clusters to understand star formation and feedback in high-redshift-like environments
The formation environment of stars in massive stellar clusters is similar to the environment of stars forming in galaxies at a redshift of 1 - 3, at the peak star formation rate density of the Universe. As massive clusters are still forming at the present day at a fraction of the distance to high-redshift galaxies they offer an opportunity to understand the processes controlling star formation and feedback in conditions similar to those in which most stars in the Universe formed. Here we describe a system of massive clusters and their progenitor gas clouds in the centre of the Milky Way, and outline how detailed observations of this system may be able to: (i) help answer some of the fundamental open questions in star formation and (ii) quantify how stellar feedback couples to the surrounding interstellar medium in this high-pressure, high-redshift analogue environment
Cepheid variables in the LMC cluster NGC 1866. I. New BVRI CCD photometry
We report BV(RI)c CCD photometric data for a group of seven Cepheid variables
in the young, rich cluster NGC 1866 in the Large Magellanic Cloud. The
photometry was obtained as part of a program to determine accurate distances to
these Cepheids by means of the infrared surface brightness technique, and to
improve the LMC Cepheid database for constructing Cepheid PL and PLC relations.
Using the new data together with data from the literature, we have determined
improved periods for all variables. For five fundamental mode pulsators, the
light curves are now of excellent quality and will lead to accurate distance
and radius determinations once complete infrared light curves and radial
velocity curves for these variables become available.Comment: To appear in ApJ Supp., AASTeX, 24 pages, 8 tables, 8 figure
Rotation profiles of solar-like stars with magnetic fields
The aim of this work is to investigate rotation profile of solar-like stars
with magnetic fields. A diffusion coefficient of magnetic angular momentum
transport is deduced. Rotating stellar models with different mass are computed
under the effect of the coefficient. Then rotation profiles are obtained from
the theoretical stellar models. The total angular momentum of solar model with
only hydrodynamic instabilities is about 13 times larger than that of the Sun
at the age of the Sun, and this model can not reproduce quasi-solid rotation in
the radiative region. However, not only can the solar model with magnetic
fields reproduce an almost uniform rotation in the radiative region, but its
total angular momentum is consistent with helioseismic result at the level of 3
at the age of the Sun. The rotation of solar-like stars with magnetic
fields is almost uniform in the radiative region. But there is an obvious
transition region of angular velocity between the convective core and the
radiative region of models with 1.2 - 1.5 , where angular velocity
has a sharp radial change, which is different from the rotation profile of the
Sun and massive stars with magnetic fields. Moreover the changes of the angular
velocity in the transition region increase with the increasing in the age and
mass.Comment: Accepted for publication in ChjA
Luminosity profiles of advanced mergers of galaxies using 2MASS data
A sample of 27 disturbed galaxies that show signs of interaction but have a
single nucleus were selected from the Arp and the Arp-Madore catalogues. For
these, the K_s band images from the Two Micron All Sky Survey (2MASS) are
analysed to obtain their radial luminosity profiles and other structural
parameters. We find that in spite of their similar optical appearance, the
sample galaxies vary in their dynamical properties, and fall into two distinct
classes. The first class consists of galaxies which can be described by a
single r^{1/4} law and the second class consists of galaxies that show an outer
exponential disk. A few galaxies that have disturbed profiles cannot be fit
into either of the above classes. However, all the galaxies are similar in all
other parameters such as the far-infrared colours, the molecular hydrogen
content and the central velocity dispersion. Thus, the dynamical parameters of
these sets seem to be determined by the ratio of the initial masses of the
colliding galaxies. We propose that the galaxies in the first class result from
a merger of spiral galaxies of equal masses whereas the second class of
galaxies results from a merger of unequal mass galaxies. The few objects that
do not fall into either category show a disturbed luminosity profile and a
wandering centre, which is indicative of these being unrelaxed mergers. Of the
27 galaxies in our sample, 9 show elliptical-like profiles and 13 show an outer
exponential. Interestingly, Arp 224, the second oldest merger remnant of the
Toomre sequence shows an exponential disk in the outer parts.Comment: 18 pages, 8 figures, uses aastex, to appear in A&
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