1,478 research outputs found
The absence of the 10 um silicate feature in the isolated Herbig Ae star HD 100453
We analyse the optical and IR spectra, as well as the spectral energy
distribution (UV to mm) of the candidate Herbig Ae star HD100453. This star is
particular, as it shows an energy distribution similar to that of other
isolated Herbig Ae/Be stars (HAEBEs), but unlike most of them, it does not have
a silicate emission feature at 10 um, as is shown in Meeus (2001). We confirm
the HAEBE nature of HD100453 through an analysis of its optical spectrum and
derived location in the H-R diagram. The IR spectrum of HD100453 is modelled by
an optically thin radiative transfer code, from which we derive constraints on
the composition, grain-size and temperature distribution of the circumstellar
dust. We show that it is both possible to explain the lack of the silicate
feature as (1) a grain-size effect - lack of small silicate grains, and (2) a
temperature effect - lack of small, hot silicates, as proposed by Dullemond
(2001), and discuss both possibilities.Comment: 9 pages, 7 figures; accepted by A&
ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be systems: towards an understanding of dust processing
We present Infrared Space Observatory (ISO) spectra of fourteen isolated
Herbig Ae/Be (HAEBE) stars, to study the characteristics of their circumstellar
dust. These spectra show large star-to-star differences, in the emission
features of both carbon-rich and oxygen-rich dust grains. The IR spectra were
combined with photometric data ranging from the UV through the optical into the
sub-mm region. We defined two key groups, based upon the spectral shape of the
infrared region. The derived results can be summarized as follows: (1) the
continuum of the IR to sub-mm region of all stars can be reconstructed by the
sum of a power-law and a cool component, which can be represented by a black
body. Possible locations for these components are an optically thick,
geometrically thin disc (power-law component) and an optically thin flared
region (black body); (2) all stars have a substantial amount of cold dust
around them, independent of the amount of mid-IR excess they show; (3) also the
near-IR excess is unrelated to the mid-IR excess, indicating different
composition/location of the emitting material; (4) remarkably, some sources
lack the silicate bands; (5) apart from amorphous silicates, we find evidence
for crystalline silicates in several stars, some of which are new detections;
(6) PAH bands are present in at least 50% of our sample, and their appearance
is slightly different from PAHs in the ISM; (7) PAH bands are, with one
exception, not present in sources which only show a power-law continuum in the
IR; their presence is unrelated to the presence of the silicate bands; (8) the
dust in HAEBE stars shows strong evidence for coagulation; this dust processing
is unrelated to any of the central star properties (such as age, spectral type
and activity).Comment: 15 pages, accepted by A&
Herschel observations of the circumstellar environment of the two Herbig Be stars R Mon and PDS27
We report and analyse FIR observations of two Herbig Be stars, R Mon and PDS
27, obtained with Herschel's instruments PACS and SPIRE. We construct SEDs and
derive the infrared excess. We extract line fluxes from the PACS and SPIRE
spectra and construct rotational diagrams in order to estimate the excitation
temperature of the gas. We derive CO, [OI] and [CI] luminosities to determine
physical conditions of the gas, as well as the dominant cooling mechanism. We
confirm that the Herbig Be stars are surrounded by remnants from their parental
clouds, with an IR excess that mainly originates in a disc. In R Mon we detect
[OI], [CI], [CII], CO (26 transitions), water and OH, while in PDS 27 we only
detect [CI] and CO (8 transitions). We attribute the absence of OH and water in
PDS 27 to UV photo-dissociation and photo-evaporation. From the rotational
diagrams, we find several components for CO: we derive 94990 K,
35820 K & 7712 K for R Mon, 9612 K & 314 K for PDS 27 and
258 K & 276 K for their respective compact neighbours. The forsterite
feature at 69m was not detected in either of the sources, probably due to
the lack of (warm) crystalline dust in a flat disc. We find that cooling by
molecules is dominant in the Herbig Be stars, while this is not the case in
Herbig Ae stars where cooling by [OI] dominates. Moreover, we show that in the
Herbig Be star R Mon, outflow shocks are the dominant gas heating mechanism,
while in Herbig Ae stars this is stellar. The outflow of R Mon contributes to
the observed line emission by heating the gas, both in the central spaxel/beam
covering the disc and the immediate surroundings, as well as in those
spaxels/beams covering the parabolic shell around it. PDS 27, a B2 star, has
dispersed a large part of its gas content and/or destroyed molecules; this is
likely given its intense UV field.Comment: Accepted for publication in Astronomy & Astrophysic
Herschel GASPS spectral observations of T Tauri stars in Taurus: unraveling far-infrared line emission from jets and discs
At early stages of stellar evolution young stars show powerful jets and/or
outflows that interact with protoplanetary discs and their surroundings.
Despite the scarce knowledge about the interaction of jets and/or outflows with
discs, spectroscopic studies based on Herschel and ISO data suggests that gas
shocked by jets and/or outflows can be traced by far-IR (FIR) emission in
certain sources. We want to provide a consistent catalogue of selected atomic
([OI] and [CII]) and molecular (CO, OH, and HO) line fluxes observed in
the FIR, separate and characterize the contribution from the jet and the disc
to the observed line emission, and place the observations in an evolutionary
picture. The atomic and molecular FIR (60-190 ) line emission of
protoplanetary discs around 76 T Tauri stars located in Taurus are analysed.
The observations were carried out within the Herschel key programme Gas in
Protoplanetary Systems (GASPS). The spectra were obtained with the
Photodetector Array Camera and Spectrometer (PACS). The sample is first divided
in outflow and non-outflow sources according to literature tabulations. With
the aid of archival stellar/disc and jet/outflow tracers and model predictions
(PDRs and shocks), correlations are explored to constrain the physical
mechanisms behind the observed line emission. The much higher detection rate of
emission lines in outflow sources and the compatibility of line ratios with
shock model predictions supports the idea of a dominant contribution from the
jet/outflow to the line emission, in particular at earlier stages of the
stellar evolution as the brightness of FIR lines depends in large part on the
specific evolutionary stage. [Abridged Abstract]Comment: 37 pages, 27 figures, accepted for publication in A&
Identities: A developmental social-psychological perspective
In this contribution, we review research that uses a cross-fertilisation approach to integrate developmental and social-psychological perspectives on how identities are formed and changed over time and how identity processes are genuinely social, being embedded in social contexts and fed by social contents. First, we outline the three-factor identity model as a parsimonious approach to understanding the dynamics of identity development. Second, we review empirical studies with longitudinal approaches to shed light on how identity processes are embedded in key contexts such as family, friendships and society at large through behaviours such as civic engagement. Third, we discuss the interplay between personal and social identities. We conclude by highlighting how adopting a cross-fertilisation approach that combines social-psychological and developmental perspective can significantly advance the theoretical understanding of identity dynamics. Finally, we address similarities and differences between personal identity and social identity approaches, and we provide an agenda for future research
Contradictory yet coherent? Inconsistency in performance feedback and R&D investment change
This paper is based upon doctoral research of the first author
conducted while at the Center for Innovation Research, Tilburg University.
The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.In this paper, we study to what extent inconsistent feedback signals about performance affect firm adaptive behavior in terms of changes made to research-and-development (R&D) investments. We argue that inconsistency in performance feedback—based on discrepancies between two distinct performance signals—affects the degree to which such investments will be changed. Our aim is to show that accounting for inconsistent performance feedback is necessary as predictions for the direction of change in R&D investments based on the individual performance feedback signals are contradictory. Furthermore, we contribute by proposing a holistic consideration mechanism as an alternative to the selective attention mechanism previously applied to inconsistent performance feedback. Our findings show that the impact of inconsistency depends on the exact configuration of the underlying performance feedback signal discrepancies. While consistently negative performance feedback signals would amplify their impact in stimulating increased R&D investments, inconsistent performance feedback signals created more nuanced effects. Having lower performance compared to an industry-based peer group—despite doing well compared to the previous year—made firms decrease their R&D investments. For the opposite case of inconsistent performance feedback, we did not find an effect on change in R&D investments. These findings support to a degree our contention that explaining the effects of inconsistent performance feedback requires a holistic consideration theoretical mechanism instead of one involving selective attention. In sum, these findings suggest future research should take into account the differences between distinct instances of inconsistent performance feedback
The Sun's position in the sky
We express the position of the Sun in the sky as a function of time and the
observer's geographic coordinates. Our method is based on applying rotation
matrices to vectors describing points on the celestial sphere. We also derive
direct expressions, as functions of date of the year and geographic latitude,
for the duration of daylight, the maximum and minimum altitudes of the Sun, and
the cardinal directions to sunrise and sunset. We discuss how to account for
the eccentricity of the earth's orbit, the precessions of the equinoxes and the
perihelion, the size of the solar disk, and atmospheric refraction. We
illustrate these results by computing the dates of "Manhattanhenge" (when
sunset aligns with the east-west streets on the main traffic grid for
Manhattan, in New York City), by plotting the altitude of the Sun over
representative cities as a function of time, and by showing plots ("analemmas")
for the position of the Sun in the sky at a given hour of the day.Comment: 19 pages, 16 figures. v3: Replaced to match published version and to
re-package Mathematica notebook as an ancillary fil
Dust evolution in protoplanetary disks around Herbig Ae/Be stars - The Spitzer view
In this paper we present mid-infrared spectra of a comprehensive set of
Herbig Ae/Be stars observed with the Spitzer Space Telescope. The
signal-to-noise ratio of these spectra is very high, ranging between about a
hundred and several hundreds. During the analysis of these data we tested the
validity of standard protoplanetary dust models and studied grain growth and
crystal formation. On the basis of the analyzed spectra, the major constituents
of protoplanetary dust around Herbig Ae/Be stars are amorphous silicates with
olivine and pyroxene stoichiometry, crystalline forsterite and enstatite and
silica. No other solid state features, indicating other abundant dust species,
are present in the Spitzer spectra. Deviations of the synthetic spectra from
the observations are most likely related to grain shape effects and
uncertainties in the iron content of the dust grains. Our analysis revealed
that larger grains are more abundant in the disk atmosphere of flatter disks
than in that of flared disks, indicating that grain growth and sedimentation
decrease the disk flaring. We did not find, however, correlations between the
value of crystallinity and any of the investigated system parameters. Our
analysis shows that enstatite is more concentrated toward the warm inner disk
than forsterite, in contrast to predictions of equilibrium condensation models.
None of the three crystal formation mechanisms proposed so far can alone
explain all our findings. It is very likely that all three play at least some
role in the formation of crystalline silicates.Comment: 56 pages, 21 figures, accepted for publication in Ap
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