18,418 research outputs found
Circumstellar environment of the M-type AGB star R Dor. APEX spectral scan at GHz
Our current insights into the circumstellar chemistry of asymptotic giant
branch (AGB) stars are largely based on studies of carbon-rich stars and stars
with high mass-loss rates. In order to expand the current molecular inventory
of evolved stars we present a spectral scan of the nearby, oxygen-rich star R
Dor, a star with a low mass-loss rate (/yr). We
carried out a spectral scan in the frequency ranges 159.0-321.5GHz and
338.5-368.5 GHz (wavelength range 0.8-1.9mm) using the SEPIA/Band-5 and SHeFI
instruments on the APEX telescope and we compare it to previous surveys,
including one of the oxygen-rich AGB star IK Tau, which has a high mass-loss
rate (/yr). The spectrum of R Dor is dominated by
emission lines of SO and the different isotopologues of SiO. We also detect
CO, HO, HCN, CN, PO, PN, SO, and tentatively TiO, AlO, and NaCl.
Sixteen out of approximately 320 spectral features remain unidentified. Among
these is a strong but previously unknown maser at 354.2 GHz, which we suggest
could pertain to HSiO, silanone. With the exception of one, none of these
unidentified lines are found in a similarly sensitive survey of IK Tau
performed with the IRAM 30m telescope. We present radiative transfer models for
five isotopologues of SiO (SiO, SiO, SiO, SiO,
SiO), providing constraints on their fractional abundance and radial
extent. We derive isotopic ratios for C, O, Si, and S and estimate that R Dor
likely had an initial mass in the range 1.3-1.6, in agreement with
earlier findings based on models of HO line emission. From the presence of
spectral features recurring in many of the measured thermal and maser emission
lines we tentatively identify up to five kinematical components in the outflow
of R Dor, indicating deviations from a smooth, spherical wind.Comment: 66 pages, 25 figures, Accepted for publication in Astronomy &
Astrophysics. Fully reduced FITS spectrum made available through CD
An independent distance estimate to the AGB star R Sculptoris
For the carbon AGB star R Sculptoris, the uncertain distance significantly
affects the interpretation of observations regarding the evolution of the
stellar mass loss during and after the most recent thermal pulse. We aim to
provide a new, independent measurement of the distance to R Sculptoris,
reducing the absolute uncertainty of the distance estimate to this source. R
Scl is a semi-regular pulsating star, surrounded by a thin shell of dust and
gas created during a thermal pulse approximately 2000 years ago. The stellar
light is scattered by the dust particles in the shell at a radius of 19 arcsec.
The variation in the stellar light affects the amount of dust-scattered light
with the same period and amplitude ratio, but with a phase lag that depends on
the absolute size of the shell. We measured this phase lag by observing the
star R Scl and the dust-scattered stellar light from the shell at five epochs
between June - December 2017. By observing in polarised light, we imaged the
shell in the plane of the sky, removing any uncertainty due to geometrical
effects. The phase lag gives the absolute size of the shell, and together with
the angular size of the shell directly gives the absolute distance to R
Sculptoris. We measured a phase lag between the stellar variations and the
variation in the shell of 40.0 +/- 4.0 days. The angular size of the shell is
measured to be 19.1 arcsec +/- 0.7 arcsec. Combined, this gives an absolute
distance to R Sculptoris of 361 +/- 44 pc. We independently determined the
absolute distance to R Scl with an uncertainty of 12%. The estimated distance
is consistent with previous estimates, but is one of the most accurate
distances to the source to date. In the future, using the variations in
polarised, dust-scattered stellar light, may offer an independent possibility
to measure reliable distances to AGB stars.Comment: accepted by A&A, 8 pages, 8 figure
Sulphur molecules in the circumstellar envelopes of M-type AGB stars
The sulphur compounds SO and SO have not been widely studied in the
circumstellar envelopes of asymptotic giant branch (AGB) stars. By presenting
and modelling a large number of SO and SO lines in the low mass-loss rate
M-type AGB star R Dor, and modelling the available lines of those molecules in
a further four M-type AGB stars, we aim to determine their circumstellar
abundances and distributions. We use a detailed radiative transfer analysis
based on the accelerated lambda iteration method to model circumstellar SO and
SO line emission and molecular data files for both SO and SO that are
more extensive than those previously available. Using 17 SO lines and 98 SO2
lines to constrain our models for R Dor, we find an SO abundance of
6.7x10 and an SO abundance of 5x10 with both species having
high abundances close to the star. We also modelled SO and found an
abundance of 3.1x10, giving an SO/SO ratio of 21.6. We
derive similar results for the circumstellar SO and SO abundances and their
distributions for the low mass-loss rate object W Hya. For these stars, the
circumstellar SO and SO abundances are much higher than predicted by
chemical models and these two species may account for all available sulphur.
For the higher mass-loss rate stars, we find shell-like SO distributions with
peak abundances that decrease and peak abundance radii that increase with
increasing mass-loss rate. The positions of the peak SO abundance agree very
well with the photodissociation radii of HO. We find evidence that SO is
most likely through the photodissociation of HO and the subsequent reaction
between S and OH. The S-bearing parent molecule appears not to be HS. The
SO models suggest an origin close to the star for this species, also
disagreeing with current chemical models.Comment: 25 page
The surprisingly carbon-rich environment of the S-type star W Aql
W Aql is an asymptotic giant branch (AGB) star with an atmospheric elemental
abundance ratio C/O0.98 and reported circumstellar molecular
abundances intermediate between those of M-type (C/O1)
AGB stars. This intermediate status is considered typical for S-type stars,
although our understanding of the chemical content of their circumstellar
envelopes (CSEs) is currently rather limited. We performed observations in the
frequency range 159-268 GHz with the APEX telescope and make abundance
estimates through comparison to available spectra towards some well-studied AGB
stars and based on rotational diagram analysis in the case of SiC2. We conclude
that W Aql's CSE appears considerably closer to that of a C-type AGB star than
to that of an M-type AGB star. In particular, we detect emission from C2H,
SiC2, SiN, and HC3N, molecules previously only detected towards the CSEs of
C-type stars. This conclusion, based on the chemistry of the gaseous component
of the CSE, is further supported by reports in the literature on the presence
of atmospheric molecular bands and spectral features of dust species typical
for C-type AGB stars. Although our observations mainly trace species in the
outer regions of the CSE, our conclusion matches closely that based on recent
chemical equilibrium models for the inner wind of S-type stars: the atmospheric
and circumstellar chemistry of S-type stars likely resembles that of C-type AGB
stars much more closely than that of M-type AGB stars. Further observational
investigation of the gaseous circumstellar chemistry of S-type stars is
required to characterise its dependence on the atmospheric C/O. Non-equilibrium
chemical models of the CSEs of AGB stars need to address the particular class
of S-type stars and the chemical variety that is induced by the range in
atmospheric C/O.Comment: 12 pages, 8 figures (+26 pages appendix) Accepted for publication in
Astronomy & Astrophysic
Properties of dust in the detached shells around U Ant, DR Ser, and V644 Sco
Understanding the properties of dust produced during the asymptotic giant
branch phase of stellar evolution is important for understanding the evolution
of stars and galaxies. Recent observations of the carbon AGB star R Scl have
shown that observations at far-infrared and submillimetre wavelengths can
effectively constrain the grain sizes in the shell, while the total mass
depends on the structure of the grains (solid vs. hollow or fluffy). We aim to
constrain the properties of the dust observed in the submillimetre in the
detached shells around the three carbon AGB stars U Ant, DR Ser, and V644 Sco,
and to investigate the constraints on the dust masses and grain sizes provided
by far-infrared and submm observations. We observed the carbon AGB stars U Ant,
DR Ser, and V644 Sco at 870 micron using LABOCA on APEX. Combined with
observations from the optical to far-infrared, we produced dust radiative
transfer models of the spectral energy distributions (SEDs) with contributions
from the stars, present-day mass-loss and detached shells. We tested the effect
of different total dust masses and grain sizes on the SED, and attempted to
consistently reproduce the SEDs from the optical to the submm. We derive dust
masses in the shells of a few 10e-5 Msun, assuming spherical, solid grains. The
best-fit grain radii are comparatively large, and indicate the presence of
grains between 0.1 micron-2 micron. The LABOCA observations suffer from
contamination from 12CO(3-2), and hence gives fluxes that are higher than the
predicted dust emission at submm wavelengths. We investigate the effect on the
best-fitting models by assuming different degrees of contamination and show
that far-infrared and submillimetre observations are important to constrain the
dust mass and grain sizes in the shells.Comment: Accepted by A&
Sulphur-bearing molecules in AGB stars I: The occurrence of hydrogen sulfide
Through a survey of (sub-)millimetre emission lines of various
sulphur-bearing molecules, we aim to determine which molecules are the primary
carriers of sulphur in different types of AGB stars. In this paper, the first
in a series, we investigate the occurrence of HS in AGB circumstellar
envelopes and determine its abundance, where possible. We have surveyed 20 AGB
stars with a range of mass-loss rates and of different chemical types using the
APEX telescope to search for rotational transition lines of five key
sulphur-bearing molecules: CS, SiS, SO, SO and HS. Here we present our
results for HS, including detections, non-detections and detailed radiative
transfer modelling of the detected lines. We compare results based on different
descriptions of the molecular excitation of HS and different abundance
distributions, including those derived from chemical modelling results. We
detected HS towards five AGB stars, all of which have high mass-loss rates
of yr and are oxygen-rich. HS
was not detected towards the carbon or S-type stars that fall in a similar
mass-loss range. For the stars in our sample with detections, we find peak
o-HS abundances relative to H between and . Overall, we conclude that HS can play a significant role in
oxygen-rich AGB stars with higher mass-loss rates, but is unlikely to play a
key role in stars of other chemical types or the lower mass-loss rate
oxygen-rich stars. For two sources, V1300 Aql and GX Mon, HS is most likely
the dominant sulphur-bearing molecule in the circumstellar envelope.Comment: 14 pages, 7 figures, accepted in A&
ALMA observations of the vibrationally-excited rotational CO transition towards five AGB stars
We report the serendipitous detection with ALMA of the vibrationally-excited
pure-rotational CO transition towards five asymptotic giant branch
(AGB) stars, Cet, R Aqr, R Scl, W Aql, and Gru. The observed lines
are formed in the poorly-understood region located between the stellar surface
and the region where the wind starts, the so-called warm molecular layer. We
successfully reproduce the observed lines profiles using a simple model. We
constrain the extents, densities, and kinematics of the region where the lines
are produced. R Aqr and R Scl show inverse P-Cygni line profiles which indicate
infall of material onto the stars. The line profiles of Cet and R Scl show
variability. The serendipitous detection towards these five sources shows that
vibrationally-excited rotational lines can be observed towards a large number
of nearby AGB stars using ALMA. This opens a new possibility for the study of
the innermost regions of AGB circumstellar envelopes.Comment: 6 pages, 2 figures, 2 tables, 2016MNRAS.463L..74
CO and HCN isotopologue ratios in the outflows of AGB stars
Isotopologue line intensity ratios of circumstellar molecules have been
widely used to trace the photospheric elemental isotopic ratios of evolved
stars. However, depending on the molecular species and the physical conditions
of the environment, the circumstellar isotopologue ratio may deviate
considerably from the stellar atmospheric value. In this paper, we aim to
examine how the CO and HCN abundance ratios vary radially due to chemical
reactions in the outflows of AGB stars and the effect of excitation and optical
depth on the resulting line intensity ratios. We find that the circumstellar
12CO/13CO can deviate from its atmospheric value by up to 25-94% and 6-60% for
C- and O-type CSEs, respectively. We show that variations of the intensity of
the ISRF and the gas kinetic temperature can significantly influence the CO
isotopologue ratio in the outer CSEs. On the contrary, the H12CN/H13CN ratio is
stable for all tested mass-loss rates. The RT modeling shows that the
integrated line intensity ratio of CO of different rotational transitions
varies significantly for stars with intermediate mass-loss rates due to
combined chemical and excitation effects. In contrast, the excitation
conditions for the both HCN isotopologues are the same. We demonstrate the
importance of using the isotopologue abundance profiles from chemical models as
inputs to RT models in the interpretation of isotopologue observations.
Previous studies of CO isotopologue ratios are based on multi-transition data
for individual sources and it is difficult to estimate the errors in the
reported values due to assumptions that are not entirely correct according to
this study. If anything, previous studies may have overestimated the
circumstellar 12CO/13CO abundance ratio. The use of the HCN as a tracer of C
isotope ratios is affected by fewer complicating problems, provided one
accounts corrections for high optical depths.Comment: 14 pages, 11 figure
How do consumers react differently to a brand owned by a New Digital Influencer compared to a traditional brand, in the high-end beauty segment?
The present thesis aims at studying and comparing reactions consumers have towards two types of brands: a traditional brand, one that has been in the high-end beauty segment for generations, and a brand owned by a New Digital Influencer -NDI - in the same segment.
A New Digital Influencer is someone who gathers a large online following, who has their audience’s trust by posting reliable content. The goal was to understand consumers’ reactions in Purchase Intention, Brand Attitude, Word-of-Mouth, and Engagement. Furthermore, high-end stands for “very good quality” and “expensive”, being this the criterion applied to select the studied brands.
Following the Literature Review, a quantitative research was performed. An online questionnaire targeted at women who are on social media and who don’t reject recommendations from influencers was considered the ideal approach. An experimental in-between groups design was conducted with respondents being randomly allocated either to the traditional brand, or to the brand owned by an NDI.
Research Questions were answered by performing a Kruskal-Wallis test, where both groups were considered statistically different in three of the four dimensions analysed. Furthermore, the traditional brand has higher scores for Purchase Intention and Word-of-Mouth; for Engagement the public engages more with a brand owned by an NDI; and for Brand Attitude the reaction by consumers is equal for both brand types. All these discoveries are justified with previous literature.
The gap in academic literature has been significantly reduced. Also, it is confirmed: influencers are a possible competitor in the retail industry.A presente tese pretende estudar e comparar a reação de consumidores a dois tipos de marcas: uma marca tradicional, marca presente no mercado de beleza de luxo há gerações, e uma marca lançada por um Novo Influenciador Digital – NDI- no mesmo segmento.
Novo Influenciador Digital é alguém que ganha uma rede de seguidores considerável online, que detém a confiança dos mesmos através da postagem de conteúdo confiável. O objetivo traçado foi o de compreender a reação de consumidores em termos de Intenção de Compra, Atitude com a Marca, Boca-a-Boca e Interação. Ainda, o conceito de luxo é “alta qualidade” e “caro” pelo que foi este o critério utilizado na seleção das marcas estudadas.
Posteriormente à Revisão de Literatura, foi efetuada uma análise quantitativa. Um questionário online tendo como alvo mulheres ativas nas redes sociais, que não rejeitam influenciadores foi considerada a abordagem ideal. Foi feita uma experiência entre grupos na qual os respondentes foram aleatoriamente alocados à marca tradicional, ou à marca lançada por um NDI.
As Perguntas de Pesquisa foram respondidas através do teste de Kruskal-Wallis, e os dois grupos foram considerados estatisticamente diferentes em três dimensões. A marca tradicional obteve resultados mais elevados em Intenção de Compra e Boca-a-Boca; Interação com uma marca lançada por um NDI é superior; Atitude com a Marca é igual em ambos os grupos. Os resultados foram justificados de acordo com a literatura.
A lacuna na literatura académica foi significativamente reduzida, e é possível afirmar: influenciadores são possível concorrência no mundo do retalho
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