168 research outputs found
Planet forming disks, debris disks and the Solar System
VLT instruments and ALMA with their high spatial resolution have revolutionized in the past five years our view and understanding of how disks turn into planetary systems. This talk will briefly outline our current understanding of the physical processes occurring and chemical composition evolving as these disks turn into debris disks and eventually planetary systems like our own solar system. I will especially focus on the synergy between disk structure/evolution modeling and astrochemical laboratory/theoretical work to highlight the most recent advances, and open questions such as (1) how much of the chemical composition in disks is inherited from molecular clouds, (2) the relevance of snowlines for planet formation, and (3) what is the origin of the gas in debris disks and what can we learn from it. For each of the three, I will outline briefly how the combination of theory/lab astrochemistry, astrophysical models and observations are required to advance our understanding
The status of Galactic field lambda Bootis stars in the post-Hipparcos era
The lambda Bootis stars are Population I, late B to early F-type stars, with
moderate to extreme (up to a factor 100) surface underabundances of most
Fe-peak elements and solar abundances of lighter elements (C, N, O, and S). To
put constraints on the various existing theories that try to explain these
peculiar stars, we investigate the observational properties of lambda Bootis
stars compared to a reference sample of normal stars. Using various photometric
systems and Hipparcos data, we analyze the validity of standard photometric
calibrations, elemental abundances, and Galactic space motions. There
crystallizes a clear picture of a homogeneous group of Population I objects
found at all stages of their main-sequence evolution, with a peak at about 1
Gyr. No correlation of astrophysical parameters such as the projected
rotational velocities or elemental abundances with age is found, suggesting
that the a-priori unknown mechanism, which creates lambda Bootis stars, works
continuously for late B to early F-type stars in all stages of main-sequence
evolution. Surprisingly, the sodium abundances seem to indicate an interaction
between the stars and their local environment.Comment: 14 pages, 9 figures, accepted by MNRA
Multi-wavelength observations of planet forming disks: Constraints on planet formation processes
Our understanding of protoplanetary disks has greatly improved over the last
decade due to a wealth of data from new facilities. Unbiased dust surveys with
Spitzer leave us with good constraints on the dust dispersal timescale of small
grains in the terrestrial planet forming region. In the ALMA era, this can be
confronted for the first time also with evolutionary timescales of mm grains in
the outer disk. Gas surveys in the context of the existing multi-wavelength
dust surveys will be a key in large statistical studies of disk gas evolution.
Unbiased gas surveys are limited to ALMA CO submm surveys, where the
quantitative interpretation is still debated. Herschel gas surveys have been
largely biased, but [OI] 63 mic surveys and also accretion tracers agree
qualitatively with the evolutionary timescale of small grains in the inner
disk. Recent advances achieved by means of consistent multi-wavelength studies
of gas AND dust in planet forming disks reveal the subtleties of the
quantitative interpretation of gas surveys. Observational methods to determine
disk masses e.g. from CO submm lines require the knowledge of the dust
properties in the disk. Understanding not only the gas evolution, but also its
chemical composition will provide crucial input for planet formation models.
Kinetic chemical results give profoundly different answers than thermodynamic
equilibrium in terms of the C/O ratios as well as the water ice/rock ratios.
Again, dust has a key impact on the chemical evolution and composition of the
gas. Grain growth for example affects freeze-out processes and strongly
increases the cosmic ray induced UV field.Comment: appears in the proceedings of the conference "The Cosmic Wheel and
the Legacy of the AKARI archive: from galaxies and stars to planets and
life", October 17-20, 2017, Tokyo, Japa
Diagnostic value of far-IR water ice features in T Tauri disks
This paper investigates how the far-IR water ice features can be used to
infer properties of disks around T Tauri stars and the water ice thermal
history. We explore the power of future observations with SOFIA/HIRMES and
SPICA's proposed far-IR instrument SAFARI. A series of detailed radiative
transfer disk models around a representative T Tauri star are used to
investigate how the far-IR water ice features at 45 and 63 micron change with
key disk properties: disk size, grain sizes, disk dust mass, dust settling, and
ice thickness. In addition, a series of models is devised to calculate the
water ice emission features from warmup, direct deposit and cooldown scenarios
of the water ice in disks. Photodesorption from icy grains in disk surfaces
weakens the mid-IR water ice features by factors 4-5. The far-IR water ice
emission features originate from small grains at the surface snow line in disks
at distance of 10-100 au. Unless this reservoir is missing in disks (e.g.
transitional disks with large cavities), the feature strength is not changing.
Grains larger than 10 micron do not contribute to the features. Grain settling
(using turbulent description) is affecting the strength of the ice features by
at most 15%. The strength of the ice feature scales with the disk dust mass and
water ice fraction on the grains, but saturates for dust masses larger than
1.e-4 Msun and for ice mantles that increase the dust mass by more than 50%.
The various thermal histories of water ice leave an imprint on the shape of the
features (crystalline/amorphous) as well as on the peak strength and position
of the 45 micron feature. SOFIA/HIRMES can only detect crystalline ice features
much stronger than simulated in our standard T Tauri disk model in deep
exposures (1 hr). SPICA/SAFARI can detect the typical ice features in our
standard T Tauri disk model in short exposures (10 min). (abbreviated)Comment: accepted for publication in A&
Volatile-Rich Circumstellar Gas in the Unusual 49 Ceti Debris Disk
We present Hubble Space Telescope STIS far-UV spectra of the edge-on disk
around 49 Ceti, one of the very few debris disks showing sub-mm CO emission.
Many atomic absorption lines are present in the spectra, most of which arise
from circumstellar gas lying along the line-of-sight to the central star. We
determined the line-of-sight CI column density, estimated the total carbon
column density, and set limits on the OI column density. Surprisingly, no
line-of-sight CO absorption was seen. We discuss possible explanations for this
non-detection, and present preliminary estimates of the carbon abundances in
the line-of-sight gas. The C/Fe ratio is much greater than the solar value,
suggesting that 49 Cet harbors a volatile-rich gas disk similar to that of Beta
Pictoris.Comment: Accepted for publication in ApJ Letters. 5 pages, 4 figure
Tracing the Physical Conditions in Active Galactic Nuclei with Time-Dependent Chemistry
We present an extension of the code ProDiMo that allows for a modeling of
processes pertinent to active galactic nuclei and to an ambient chemistry that
is time dependent. We present a proof-of-concept and focus on a few
astrophysically relevant species, e.g., H+, H2+ and H3+; C+ and N+; C and O; CO
and H2O; OH+, H2O+ and H3O+; HCN and HCO+. We find that the freeze-out of water
is strongly suppressed and that this affects the bulk of the oxygen and carbon
chemistry occurring in AGN. The commonly used AGN tracer HCN/HCO+ is strongly
time-dependent, with ratios that vary over orders of magnitude for times longer
than 10^4 years. Through ALMA observations this ratio can be used to probe how
the narrow-line region evolves under large fluctuations in the SMBH accretion
rate. Strong evolutionary trends, on time scales of 10^4-10^8 years, are also
found in species such as H3O+, CO, and H2O. These reflect, respectively, time
dependent effects in the ionization balance, the transient nature of the
production of molecular gas, and the freeze-out/sublimation of water.Comment: Accepted for publication at the Journal of Physical Chemistry A "Oka
Festschrift: Celebrating 45 Years of Astrochemistry
PAH emission in the proplyd HST10: what is the mechanism behind photoevaporation?
Proplyds are photodissociation region (PDR)-like cometary cocoons around
young stars which are thought to originate through photo-evaporation of the
central protoplanetary disk by external UV radiation from the nearby OB stars.
This letter presents spatially resolved mid-infrared imaging and spectroscopy
of the proplyd HST10 obtained with the VLT/VISIR instrument. These observations
allow us to detect Polycyclic Aromatic Hydrocarbons (PAH) emission in the
proplyd photodissociation region and to study the general properties of PAHs in
proplyds for the first time. We find that PAHs in HST10 are mostly neutral and
at least 50 times less abundant than typical values found for the diffuse ISM
or the nearby Orion Bar. With such a low PAH abundance, photoelectric heating
is significantly reduced. If this low abundance pertains also to the original
disk material, gas heating rates could be too low to efficiently drive
photoevaporation unless other processes can be identified. Alternatively, the
model behind the formation of proplyds as evaporating disks may have to be
revised.Comment: 5 pages, 3 figures, 1 tabl
Continuum and line modelling of discs around young stars. I. 300000 disc models for Herschel/GASPS
We have combined the thermo-chemical disc code ProDiMo with the Monte Carlo
radiative transfer code MCFOST to calculate a grid of ~300000 circumstellar
disc models, systematically varying 11 stellar, disc and dust parameters
including the total disc mass, several disc shape parameters and the
dust-to-gas ratio. For each model, dust continuum and line radiative transfer
calculations are carried out for 29 far IR, sub-mm and mm lines of [OI], [CII],
12CO and o/p-H2O under 5 inclinations. The grid allows to study the influence
of the input parameters on the observables, to make statistical predictions for
different types of circumstellar discs, and to find systematic trends and
correlations between the parameters, the continuum fluxes, and the line fluxes.
The model grid, comprising the calculated disc temperatures and chemical
structures, the computed SEDs, line fluxes and profiles, will be used in
particular for the data interpretation of the Herschel open time key programme
GASPS. The calculated line fluxes show a strong dependence on the assumed UV
excess of the central star, and on the disc flaring. The fraction of models
predicting [OI] and [CII] fine-structure lines fluxes above Herschel/PACS and
Spica/SAFARI detection limits are calculated as function of disc mass. The
possibility of deriving the disc gas mass from line observations is discussed.Comment: accepted by MNRAS. 5 pages, 4 figures, 3 table
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