5,978 research outputs found
Approximate roots of a valuation and the Pierce-Birkhoff Conjecture
This paper is a step in our program for proving the Piece-Birkhoff Conjecture
for regular rings of any dimension (this would contain, in particular, the
classical Pierce-Birkhoff conjecture which deals with polynomial rings over a
real closed field). We first recall the Connectedness and the Definable
Connectedness conjectures, both of which imply the Pierce - Birkhoff
conjecture. Then we introduce the notion of a system of approximate roots of a
valuation v on a ring A (that is, a collection Q of elements of A such that
every v-ideal is generated by products of elements of Q). We use approximate
roots to give explicit formulae for sets in the real spectrum of A which we
strongly believe to satisfy the conclusion of the Definable Connectedness
conjecture. We prove this claim in the special case of dimension 2. This proves
the Pierce-Birkhoff conjecture for arbitrary regular 2-dimensional rings
A new view on the ISM of galaxies: far-infrared and submillimetre spectroscopy with Herschel
The FIR/submm window is amongst the least explored spectral regions of the
electromagnetic spectrum. It is, however, a key to study the general properties
of the interstellar medium of galaxies, as it contains important spectral line
diagnostics from the neutral, ionized and molecular ISM. The Herschel Space
Observatory, successfully launched on 14 May 2009, is the first observatory to
cover the entire FIR/submm range between 57 and 672 mum. We discuss the main
results from the ISO era on FIR spectroscopy of galaxies and the enormous
science potential of the Herschel mission through a presentation of its
spectroscopic extragalactic key programs.Comment: 10 pages, 4 figures, accepted for publication in New Astronomy
Review
Changes in body composition in women using long-acting reversible contraception
OBJECTIVE:
Users of hormonal long-acting reversible contraception (LARC) report weight gain as a side effect, but few studies have assessed body composition change among LARC users. We evaluated weight and body composition of healthy women using the levonorgestrel intrauterine system (LNG-IUS), copper intrauterine device (copper IUD) or etonogestrel implant (ENG implant). We hypothesized that weight gain and body composition over 12 months would not differ between copper IUD, LNG-IUS and ENG implant users.
STUDY DESIGN:
We performed a prospective cohort study of a subgroup of women enrolled in the Contraceptive CHOICE Project who initiated the LNG-IUS, copper IUD or ENG implant. Inclusion criteria included lack of metabolic and eating disorders or change in body weight of more than 5% in the 6 months before enrollment. We measured changes in weight and body composition (body fat percentage, total body fat mass, total lean mass and total body mass) in women who continued their method for 12 months.
RESULTS:
We analyzed data from 149 participants: 85 LNG-IUS users, 31 copper IUD users and 33 ENG implant users. The mean age was 25.9 years, 56.4% were White, 82.5% had some college education and 67.6% were nulliparous. Although lean body mass increased over 12 months in LNG-IUS and copper IUD users but not in ENG implant users, changes in body weight and body composition did not differ between the groups. In the adjusted model, Black race was associated with change in total body mass (p<.05).
CONCLUSIONS:
Among those who continued the method for 12 months, changes in body weight and composition did not differ between copper IUD, LNG-IUS and ENG implant users.
IMPLICATIONS STATEMENT:
Changes in body weight and composition over 12 months did not differ between copper IUD users and LNG-IUS and ENG implant users among those with 12 months of continuous use
The bolometric and UV attenuation in normal spiral galaxies of the Herschel Reference Survey
The dust in nearby galaxies absorbs a fraction of the
UV-optical-near-infrared radiation produced by stars. This energy is
consequently re-emitted in the infrared. We investigate the portion of the
stellar radiation absorbed by spiral galaxies from the HRS by modelling their
UV-to-submillimetre spectral energy distributions. Our models provide an
attenuated and intrinsic SED from which we find that on average 32 % of all
starlight is absorbed by dust. We define the UV heating fraction as the
percentage of dust luminosity that comes from absorbed UV photons and find that
this is 56 %, on average. This percentage varies with morphological type, with
later types having significantly higher UV heating fractions. We find a strong
correlation between the UV heating fraction and specific star formation rate
and provide a power-law fit. Our models allow us to revisit the IRX-AFUV
relations, and derive these quantities directly within a self-consistent
framework. We calibrate this relation for different bins of NUV-r colour and
provide simple relations to relate these parameters. We investigated the
robustness of our method and we conclude that the derived parameters are
reliable within the uncertainties which are inherent to the adopted SED model.
This calls for a deeper investigation on how well extinction and attenuation
can be determined through panchromatic SED modelling.Comment: 14 pages, 7 figures. Accepted for publication in Astronomy &
Astrophysic
Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust
We present CO(1-0) and CO(2-1) observations of a sample of 20
star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with
oxygen abundances ranging from 12 + log(O/H) ~ 8.1 to 8.8. CO emission is
observed in ten galaxies and marginally detected in another one. CO fluxes
correlate with the FIR 250 m emission, and the dwarfs follow the same
linear relation that holds for more massive spiral galaxies extended to a wider
dynamical range. We compare different methods to estimate H2 molecular masses,
namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on
H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant
at stellar masses <~ 10 M, contrary to the atomic hydrogen
fraction, M/M, which increases inversely with M. The flattening
of the M/M ratio at low stellar masses does not seem to be related
to the effects of the cluster environment because it occurs for both
HI-deficient and HI-normal dwarfs. The molecular-to-atomic ratio is more
tightly correlated with stellar surface density than metallicity, confirming
that the interstellar gas pressure plays a key role in determining the balance
between the two gaseous components of the interstellar medium. Virgo dwarfs
follow the same linear trend between molecular gas mass and star formation rate
as more massive spirals, but gas depletion timescales, , are not
constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo
cluster environment is removing the atomic gas and dust components of the
dwarfs, but the molecular gas appears to be less affected at the current stage
of evolution within the cluster. However, the correlation between HI deficiency
and the molecular gas depletion time suggests that the lack of gas
replenishment from the outer regions of the disc is lowering the star formation
activity.Comment: 19 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
The effects of star formation on the low-metallicity ISM: NGC4214 mapped with Herschel/PACS spectroscopy
We present Herschel/PACS spectroscopic maps of the dwarf galaxy NC4214
observed in 6 far infrared fine-structure lines: [C II] 158mu, [O III] 88mu, [O
I] 63mu, [O I] 146mu, [N II] 122mu, and [N II] 205mu. The maps are sampled to
the full telescope spatial resolution and reveal unprecedented detail on ~ 150
pc size scales. We detect [C II] emission over the whole mapped area, [O III]
being the most luminous FIR line. The ratio of [O III]/[C II] peaks at about 2
toward the sites of massive star formation, higher than ratios seen in dusty
starburst galaxies. The [C II]/CO ratios are 20 000 to 70 000 toward the 2
massive clusters, which are at least an order of magnitude larger than spiral
or dusty starbursts, and cannot be reconciled with single-slab PDR models.
Toward the 2 massive star-forming regions, we find that L[CII] is 0.5 to 0.8%
of the LTIR . All of the lines together contribute up to 2% of LTIR . These
extreme findings are a consequence of the lower metallicity and young,
massive-star formation commonly found in dwarf galaxies. These conditions
promote large-scale photodissociation into the molecular reservoir, which is
evident in the FIR line ratios. This illustrates the necessity to move to
multiphase models applicable to star-forming clusters or galaxies as a whole.Comment: Accepted for publication in the A&A Herschel Special Issu
Process plant safety information repository and support for safety applications
The consideration of safety of processing plants from design to operation involves many tasks. Different techniques and tools have been developed to assist in carry out those tasks. With the advances in computing technologies, both in hardware and software, there is the opportunity to bring together the vast amount of engineering, hazard and risk data and handle them in an efficient way to manage and improve safety throughout the life time of a plant. This paper proposes keeping such data in a process plant safety information repository which can be easily accessed by different safety-related tools to enhance the efficiency of safety identification and analysis tasks
The first maps of κd - the dust mass absorption coefficient - in nearby galaxies, with DustPedia
The dust mass absorption coefficient, κd is the conversion function used to infer physical dust masses from observations of dust emission. However, it is notoriously poorly constrained, and it is highly uncertain how it varies, either between or within galaxies. Here we present the results of a proof-of-concept study, using the DustPedia data for two nearby face-on spiral galaxies M 74 (NGC 628) and M 83 (NGC 5236), to create the first ever maps of κd in galaxies. We determine κd using an empirical method that exploits the fact that the dust-to-metals ratio of the interstellar medium is constrained by direct measurements of the depletion of gas-phase metals. We apply this method pixel-by-pixel within M 74 and M 83, to create maps of κd. We also demonstrate a novel method of producing metallicity maps for galaxies with irregularly sampled measurements, using the machine learning technique of Gaussian process regression. We find strong evidence for significant variation in κd. We find values of κd at 500 μm spanning the range 0.11-0.25 m^{2 kg^{-1}} in M 74, and 0.15-0.80 m^{2 kg^{-1}} in M 83. Surprisingly, we find that κd shows a distinct inverse correlation with the local density of the interstellar medium. This inverse correlation is the opposite of what is predicted by standard dust models. However, we find this relationship to be robust against a large range of changes to our method - only the adoption of unphysical or highly unusual assumptions would be able to suppress it
Enhanced [CII] emission in a z=4.76 submillimetre galaxy
We present the detection of bright [CII] emission in the z=4.76 submillimetre
galaxy LESS J033229.4-275619 using the Atacama Pathfinder EXperiment. This
represents the highest redshift [CII] detection in a submm selected,
star-formation dominated system. The AGN contributions to the [CII] and
far-infrared (FIR) luminosities are small. We find an atomic mass derived from
[CII] comparable to the molecular mass derived from CO. The ratio of the [CII],
CO and FIR luminosities imply a radiation field strength G_0~10^3 and a density
~10^4 cm^-3 in a kpc-scale starburst, as seen in local and high redshift
starbursts. The high L_[CII]/L_FIR=2.4x10^-3 and the very high
L_[CII]/L_CO(1-0) ~ 10^4 are reminiscent of low metallicity dwarf galaxies,
suggesting that the highest redshift star-forming galaxies may also be
characterised by lower metallicities. We discuss the implications of a reduced
metallicity on studies of the gas reservoirs, and conclude that especially at
very high redshift, [CII] may be a more powerful and reliable tracer of the
interstellar matter than CO.Comment: 5 pages, 2 figures; accepted for publication in Astronomy &
Astrophysics Letter
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