1,431 research outputs found
New advances in photoionisation codes: How and what for?
The study of photoionised gas in planetary nebulae (PNe) has played a major
role in the achievement, over the years, of a better understanding of a number
of physical processes, pertinent to a broader range of fields than that of PNe
studies, spanning from atomic physics to stellar evolution theories. Whilst
empirical techniques are routinely employed for the analysis of the emission
line spectra of these objects, the accurate interpretation of the observational
data often requires the solution of a set of coupled equations, via the
application of a photoionisation/plasma code. A number of large-scale codes
have been developed since the late sixties, using various analytical or
statistical techniques for the transfer of continuum radiation, mainly under
the assumption of spherical symmetry and a few in 3D. These codes have been
proved to be powerful and in many cases essential tools, but a clear idea of
the underlying physical processes and assumptions is necessary in order to
avoid reaching misleading conclusions. A brief review of the field of
photoionisation today is given here, with emphasis on the recent developments,
including the expansion of the models to the 3D domain. Attention is given to
the identification of new available observational constraints and how these can
used to extract useful information from realistic models. (abridged)Comment: 8 pages, 3 figures, conference proceeding
Effects of X-ray irradiation and disk flaring on the [NeII] 12.8 micron emission from young stellar objects
The [Ne II] fine-structure emission line at 12.8 micron has been detected in
several young stellar objects (YSO) spectra. This line is thought to be
produced by X-ray irradiation of the warm protoplanetary disk atmospheres,
however the observational correlation between [Ne II] luminosities and measured
X-ray luminosities shows a large scatter. Such spread limits the utility of
this line as a probe of the gaseous phase of disks, as several authors have
suggested pollution by outflows as a probable cause of the observed scatter. In
this work we explore the possibility that the large variations in the observed
[Ne II] luminosity may be caused instead by different star-disk parameters. In
particular we study the effects that the hardness of the irradiating source and
the structure (flaring) of the disk have on the luminosity and spectral profile
of the [Ne II] 12.8 micron line. We find that varying these parameter can
indeed cause up to an order of magnitude variation in the emission luminosities
which may explain the scatter observed, although our models predict somewhat
smaller luminosities than those recently reported by other authors who observed
the line with the Spitzer Space Telescope. Our models also show that the
hardness of the spectrum has only a limited (undetectable) effect on the line
profiles, while changes in the flaring power of the disk significantly affect
the size of the [Ne II] emission region and, as a consequence, its line
profile. In particular we suggest that broad line profiles centred on the
stellar radial velocity may be indicative of flat disks seen at large
inclination angles.Comment: 9 pages, 8 figures. accepted for publication in MNRA
The imprint of photoevaporation on edge-on discs
We have performed hydrodynamic and radiative transfer calculations of a
photoevaporating disc around a Herbig Ae/Be star to determine the evolution and
observational impact of dust entrained in the wind. We find that the wind
selectively entrains grains of different sizes at different radii resulting in
a dust population that varies spatially and increases with height above the
disc at radii > 10 AU. This variable grain population results in a 'wingnut'
morphology to the dust density distribution. We calculate images of this dust
distribution at NIR wavelengths that also show a wingnut morphology at all
wavelengths considered. We have also considered the contribution that
crystalline dust grains will have in the wind and show that a photoevaporative
wind can result in a significant crystallinity fraction at all radii, when the
disc is edge-on. However, when the disc's photosphere is unobscured, a
photoevaporative wind makes no contribution to the observable crystallinity
fraction in the disc. Finally, we conclude that the analysis of extended
emission around edge-on discs could provide a new and independent method of
testing photoevaporation models.Comment: 8 pages, 6 figures, accepted for publication in MNRA
The clearing of discs around late type T Tauri stars: constraints from the infrared two colour plane
We have undertaken SED modeling of discs around low mass T Tauri stars in
order to delineate regions of the infrared two colour plane (K - [8] versus K -
[24]) that correspond to discs in different evolutionary stages. This provides
a ready tool for classifying the nature of star-disc systems based on infrared
photometry. In particular we demonstrate the distinct loci followed by discs
that undergo `uniform draining' (reduction in surface density by a spatially
uniform factor) from those that clear from the inside out. We draw attention to
the absence of objects on this `draining locus' in those star forming regions
where the 24um sensitivity would permit their detection, as compared with the
~20 objects in these regions with colours suggestive of inner holes. We thus
conclude that discs predominantly clear from the inside out. We also apply our
classification of the infrared two colour plane to stars of spectral type M3-M5
in the IC 348 cluster and show that some of its members (dubbed `weak excess'
sources by Muzerolle et al 2010) that had previously been assumed to be in a
state of clearing are instead likely to be optically thick discs in which the
dust is well settled towards the mid-plane. Nevertheless, there are many discs
in a state of partial clearing in IC 348, with their abundance (relative to the
total population of disc bearing stars) being around four times higher than for
comparable stars in Taurus. However, the number of partially cleared discs
relative to the total number of late type stars is similarly low in both
regions (10 and 20 % respectively). We therefore conclude that IC 348
represents a more evolved version of the Taurus population (with more of its
discs being highly settled or partially cleared) but that the timescale for
clearing is similarly short (a few times 10^5 years) in both cases.Comment: 9 pages, 6 figures, accepted, MNRA
The lifetime of protoplanetary discs: Observations and Theory
The time-scale over which and modality by which young stellar objects (YSOs)
disperse their circumstellar discs dramatically influences the eventual
formation and evolution of planetary systems. By means of extensive radiative
transfer (RT) modelling, we have developed a new set of diagnostic diagrams in
the infrared colour-colour plane (K-[24] vs. K-[8]), to aid with the
classiffication of the evolutionary stage of YSOs from photometric
observations. Our diagrams allow the differentiation of sources with un-evolved
(primordial) discs from those evolving according to different clearing
scenarios (e.g. homologous depletion vs. inside-out dispersal), as well as from
sources that have already lost their disc. Classification of over 1500 sources
in 15 nearby star-forming regions reveals that approximately 39% of the sources
lie in the primordial disc region, whereas between 31% and 32% disperse from
the inside-out and up to 22% of the sources have already lost their disc. Less
than 2% of the objects in our sample lie in the homogeneous draining regime.
Time-scales for the transition phase are estimated to be typically a few 10^5
years independent of stellar mass. Therefore, regardless of spectral type, we
conclude that currently available infrared photometric surveys point to fast
(of order 10% of the global disc lifetime) inside-out clearing as the preferred
mode of disc dispersal.Comment: Conference Proceedings: The Labyrinth of Start Formation - 18-22 June
Crete, Greec
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