1,231 research outputs found
Gamma-GQM Time Headway Model: Endogenous Effects in Rural Two-lane Two-way Roads
AbstractStudy of vehicle time headway distributions is essential in many traffic engineering applications, such as capacity and level of service analysis and, in recent years, in the fields of vehicle generation in traffic micro-simulation models and driving simulation applications. This paper presents results from an experimental analysis of vehicle time headway distributions on two-lane two-way rural roads. Analysis focused on estimating a well-known model, the gamma-generalized queuing model (gamma-GQM). A trendless analysis of observed time headways was also carried out. The endogenous traffic parameters considered as affecting time headway distributions were flow rate and flow composition (percentage of heavy vehicles). Exogenous conditions, such as weather and geometric futures, were common to all time periods and cross-sections analysed. Gamma-GQM pdf appears to be very suitable for representing real headway distributions in all the analysed situations; it fits real-time headway distributions well, despite flow rate range and traffic composition (range of percentage of heavy vehicles)
Uncertainties in water chemistry in disks: An application to TW Hya
Context. This paper discusses the sensitivity of water lines to chemical
processes and radiative transfer for the protoplanetary disk around TW Hya. The
study focuses on the Herschel spectral range in the context of new line
detections with the PACS instrument from the Gas in Protoplanetary Systems
project (GASPS). Aims. The paper presents an overview of the chemistry in the
main water reservoirs in the disk around TW Hya. It discusses the limitations
in the interpretation of observed water line fluxes. Methods. ... (abbreviated)
Results. We report new line detections of p-H2O (3_22-2_11) at 89.99 micron and
CO J=18-17 at 144.78 micron for the disk around TW Hya. Disk modeling shows
that the far-IR fine structure lines ([OI], [CII]) and molecular submm lines
are very robust to uncertainties in the chemistry, while the water line fluxes
can change by factors of a few. The water lines are optically thick,
sub-thermally excited and can couple to the background continuum radiation
field. The low-excitation water lines are also sensitive to uncertainties in
the collision rates, e.g. with neutral hydrogen. The gas temperature plays an
important role for the [OI] fine structure line fluxes, the water line fluxes
originating from the inner disk as well as the high excitation CO, CH+ and OH
lines. Conclusions. Due to their sensitivity on chemical input data and
radiative transfer, water lines have to be used cautiously for understanding
details of the disk structure. Water lines covering a wide range of excitation
energies provide access to the various gas phase water reservoirs (inside and
outside the snow line) in protoplanetary disks and thus provide important
information on where gas-phase water is potentially located. Experimental
and/or theoretical collision rates for H2O with atomic hydrogen are needed to
diminish uncertainties from water line radiative transfer.Comment: accepted for publication in A&
Relations between morphological settings and vegetation covers in a medium relief landscape of Central Italy
Morphometric units and vegetation classes were determined by applying two classification methods to the Soratte Mount area, a medium relief structure within the Italian Latium region. The study aims at defining the relationships between vegetation and landform types and highlighting the main morphological characteristics within examined land cover classes. These were the result of the application of a supervised classification method to the first 28 (VIS-NIR) bands of the airborne MIVIS data collected within an extensive survey campaign over Rome Province. The analysis was supported by photo-interpretation of peculiar MIVIS band combinations and by data acquired during field surveys and from a pre-existing vegetation map. The morphometric data were obtained by processing a raster DEM created from topographic maps. These data were processed by means of a new morphometric classification method based on the statistical multivariate investigation of local topographic gradients, calculated along the 8 azimuth directions of each pixel neighbourhood. Such approach quickly estimates the spatial distribution of different types of homogeneous terrain units, emphasizing the impact of erosional and tectonic processes on the overall relief. Mutual relations between morphometric units and vegetation types were assessed by performing a correspondence analysis between the results of the two classifications
The Photoevaporative Wind from the Disk of TW Hya
Photoevaporation driven by the central star is expected to be a ubiquitous
and important mechanism to disperse the circumstellar dust and gas from which
planets form. Here, we present a detailed study of the circumstellar disk
surrounding the nearby star TW Hya and provide observational constraints to its
photoevaporative wind. Our new high-resolution (R ~ 30,000) mid-infrared
spectroscopy in the [Ne II] 12.81 {\mu}m line confirms that this gas diagnostic
traces the unbound wind component within 10AU from the star. From the blueshift
and asymmetry in the line profile, we estimate that most (>80%) of the [Ne II]
emission arises from disk radii where the midplane is optically thick to the
redshifted outflowing gas, meaning beyond the 1 or 4AU dust rim inferred from
other observations. We re-analyze high-resolution (R ~ 48, 000) archival
optical spectra searching for additional transitions that may trace the
photoevaporative flow. Unlike the [Ne II] line, optical forbidden lines from
OI, SII, and MgI are centered at the stellar velocity and have symmetric
profiles. The only way these lines could trace the photoevaporative flow is if
they arise from a disk region physically distinct from that traced by the [Ne
II] line, specifically from within the optically thin dust gap. However, the
small (~10 km/s) FWHM of these lines suggest that most of the emitting gas
traced at optical wavelengths is bound to the system rather than unbound. We
discuss the implications of our results for a planet-induced versus a
photoevaporation-induced gap.Comment: Accepted for publication in Ap
Anachronistic Grain Growth and Global Structure of the Protoplanetary Disk Associated with the Mature Classical T Tauri Star, PDS 66
We present ATCA interferometric observations of the old (13 Myr), nearby
(86pc) classical T Tauri star, PDS 66. Unresolved 3 and 12 mm continuum
emission is detected towards PDS 66, and upper limits are derived for the 3 and
6 cm flux densities. The mm-wave data show a spectral slope flatter than that
expected for ISM-sized dust particles, which is evidence of grain growth. We
also present HST/NICMOS 1.1 micron PSF-subtracted coronagraphic imaging of PDS
66. The HST observations reveal a bilaterally symmetric circumstellar region of
dust scattering about 0.32% of the central starlight, declining radially in
surface brightness. The light-scattering disk of material is inclined 32
degrees from face-on, and extends to a radius of 170 AU. These data are
combined with published optical and longer wavelength observations to make
qualitative comparisons between the median Taurus and PDS 66 spectral energy
distributions (SEDs). By comparing the near-infrared emission to a simple
model, we determine that the location of the inner disk radius is consistent
with the dust sublimation radius (1400 K at 0.1 AU). We place constraints on
the total disk mass using a flat-disk model and find that it is probably too
low to form gas giant planets according to current models. Despite the fact
that PDS 66 is much older than a typical classical T Tauri star (< 5 Myr), its
physical properties are not much different.Comment: 31 pages, 7 figure
Discovery of molecular gas around HD 131835 in an APEX molecular line survey of bright debris disks
Debris disks are considered to be gas-poor, but recent observations revealed
molecular or atomic gas in several 10-40 Myr old systems. We used the APEX and
IRAM 30m radiotelescopes to search for CO gas in 20 bright debris disks. In one
case, around the 16 Myr old A-type star HD 131835, we discovered a new
gas-bearing debris disk, where the CO 3-2 transition was successfully detected.
No other individual system exhibited a measurable CO signal. Our Herschel Space
Observatory far-infrared images of HD 131835 marginally resolved the disk both
at 70 and 100m, with a characteristic radius of ~170 au. While in stellar
properties HD 131835 resembles Pic, its dust disk properties are
similar to those of the most massive young debris disks. With the detection of
gas in HD 131835 the number of known debris disks with CO content has increased
to four, all of them encircling young (40 Myr) A-type stars. Based on
statistics within 125 pc, we suggest that the presence of detectable amount of
gas in the most massive debris disks around young A-type stars is a common
phenomenon. Our current data cannot conclude on the origin of gas in HD 131835.
If the gas is secondary, arising from the disruption of planetesimals, then HD
131835 is a comparably young and in terms of its disk more massive analogue of
the Pic system. However, it is also possible that this system similarly
to HD 21997 possesses a hybrid disk, where the gas material is predominantly
primordial, while the dust grains are mostly derived from planetesimals.Comment: Accepted for publication in ApJ, 18 pages, 9 figures, 5 table
Relations between morphological settings and vegetation covers in a medium relief landscape of Central Italy
Mount area, a medium relief structure within the Italian Latium region. The study aims at defining the relationships
between vegetation and landform types and highlighting the main morphological characteristics within examined
land cover classes. These were the result of the application of a supervised classification method to the first 28 (VISNIR)
bands of the airborne MIVIS data collected within an extensive survey campaign over Rome Province. The
analysis was supported by photo-interpretation of peculiar MIVIS band combinations and by data acquired during
field surveys and from a pre-existing vegetation map. The morphometric data were obtained by processing a raster
DEM created from topographic maps. These data were processed by means of a new morphometric classification
method based on the statistical multivariate investigation of local topographic gradients, calculated along the 8 azimuth
directions of each pixel neighbourhood. Such approach quickly estimates the spatial distribution of different
types of homogeneous terrain units, emphasizing the impact of erosional and tectonic processes on the overall relief.
Mutual relations between morphometric units and vegetation types were assessed by performing a correspondence
analysis between the results of the two classifications
Mid-infrared interferometry of massive young stellar objects. I. VLTI and Subaru observations of the enigmatic object M8E-IR
[abridged] Our knowledge of the inner structure of embedded massive young
stellar objects is still quite limited. We attempt here to overcome the spatial
resolution limitations of conventional thermal infrared imaging. We employed
mid-infrared interferometry using the MIDI instrument on the ESO/VLTI facility
to investigate M8E-IR, a well-known massive young stellar object suspected of
containing a circumstellar disk. Spectrally dispersed visibilities in the 8-13
micron range were obtained at seven interferometric baselines. We resolve the
mid-infrared emission of M8E-IR and find typical sizes of the emission regions
of the order of 30 milli-arcseconds (~45 AU). Radiative transfer simulations
have been performed to interpret the data. The fitting of the spectral energy
distribution, in combination with the measured visibilities, does not provide
evidence for an extended circumstellar disk with sizes > 100 AU but requires
the presence of an extended envelope. The data are not able to constrain the
presence of a small-scale disk in addition to an envelope. In either case, the
interferometry measurements indicate the existence of a strongly bloated,
relatively cool central object, possibly tracing the recent accretion history
of M8E-IR. In addition, we present 24.5 micron images that clearly distinguish
between M8E-IR and the neighbouring ultracompact HII region and which show the
cometary-shaped infrared morphology of the latter source. Our results show that
IR interferometry, combined with radiative transfer modelling, can be a viable
tool to reveal crucial structure information on embedded massive young stellar
objects and to resolve ambiguities arising from fitting the SED.Comment: 7 pages, 5 figures, accepted for publication in A&A, new version
after language editing, one important reference added, conclusions unchange
Probing stellar accretion with mid-infrared hydrogen lines
In this paper we investigate the origin of the mid-infrared (IR) hydrogen
recombination lines for a sample of 114 disks in different evolutionary stages
(full, transitional and debris disks) collected from the {\it Spitzer} archive.
We focus on the two brighter {H~{\sc i}} lines observed in the {\it Spitzer}
spectra, the {H~{\sc i}}(7-6) at 12.37m and the {H~{\sc i}}(9-7) at
11.32m. We detect the {H~{\sc i}}(7-6) line in 46 objects, and the {H~{\sc
i}}(9-7) in 11. We compare these lines with the other most common gas line
detected in {\it Spitzer} spectra, the {[Ne~{\sc iii}]} at 12.81m. We
argue that it is unlikely that the {H~{\sc i}} emission originates from the
photoevaporating upper surface layers of the disk, as has been found for the
{[Ne~{\sc iii}]} lines toward low-accreting stars. Using the {H~{\sc
i}}(9-7)/{H~{\sc i}}(7-6) line ratios we find these gas lines are likely
probing gas with hydrogen column densities of 10-10~cm.
The subsample of objects surrounded by full and transitional disks show a
positive correlation between the accretion luminosity and the {H~{\sc i}} line
luminosity. These two results suggest that the observed mid-IR {H~{\sc i}}
lines trace gas accreting onto the star in the same way as other hydrogen
recombination lines at shorter wavelengths. A pure chromospheric origin of
these lines can be excluded for the vast majority of full and transitional
disks.We report for the first time the detection of the {H~{\sc i}}(7-6) line
in eight young (< 20~Myr) debris disks. A pure chromospheric origin cannot be
ruled out in these objects. If the {H~{\sc i}}(7-6) line traces accretion in
these older systems, as in the case of full and transitional disks, the
strength of the emission implies accretion rates lower than
10M/yr. We discuss some advantages of extending accretion
indicators to longer wavelengths
First Detection of Millimeter Dust Emission from Brown Dwarf Disks
We report results from the first deep millimeter continuum survey targeting
Brown Dwarfs (BDs). The survey led to the first detection of cold dust in the
disks around two young BDs (CFHT-BD-Tau 4 and IC348 613), with deep JCMT and
IRAM observations reaching flux levels of a few mJy. The dust masses are
estimated to be a few Earth masses assuming the same dust opacities as usually
applied to TTauri stars.Comment: 5 pages, accepted for ApJ
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