213 research outputs found
The BLAST Survey of the Vela Molecular Cloud: Dynamical Properties of the Dense Cores in Vela-D
The Vela-D region, according to the nomenclature given by Murphy & May
(1991), of the star forming complex known as the Vela Molecular Ridge (VMR),
has been recently analyzed in details by Olmi et al. (2009), who studied the
physical properties of 141 pre- and proto-stellar cold dust cores, detected by
the ``Balloon-borne Large-Aperture Submillimeter Telescope'' (BLAST) during a
much larger (55 sq. degree) Galactic Plane survey encompassing the whole VMR.
This survey's primary goal was to identify the coldest, dense dust cores
possibly associated with the earliest phases of star formation. In this work,
the dynamical state of the Vela-D cores is analyzed. Comparison to dynamical
masses of a sub-sample of the Vela-D cores estimated from the 13CO survey of
Elia et al. (2007), is complicated by the fact that the 13CO linewidths are
likely to trace the lower density intercore material, in addition to the dense
gas associated with the compact cores observed by BLAST. In fact, the total
internal pressure of these cores, if estimated using the 13CO linewidths,
appears to be higher than the cloud ambient pressure. If this were the case,
then self-gravity and surface pressure would be insufficient to bind these
cores and an additional source of external confinement (e.g., magnetic field
pressure) would be required. However, if one attempts to scale down the 13CO
linewidths, according to the observations of high-density tracers in a small
sample of sources, then most proto-stellar cores would result effectively
gravitationally bound.Comment: This paper has 12 pages and 6 figures. Accepted for publication by
the Astrophysical Journal on July 19, 201
Pre-Main Sequence variables in the VMR-D : identification of T Tauri-like accreting protostars through Spitzer-IRAC variability
We present a study of the infrared variability of young stellar objects by
means of two Spitzer-IRAC images of the Vela Molecular Cloud D (VMR-D) obtained
in observations separated in time by about six months. By using the same
space-born IR instrumentation, this study eliminates all the unwanted effects
usually unavoidable when comparing catalogs obtained from different
instruments. The VMR-D map covers about 1.5 square deg. of a site where star
formation is actively ongoing. We are interested in accreting pre-main sequence
variables whose luminosity variations are due to intermittent events of disk
accretion (i.e. active T Tauri stars and EXor type objects). The variable
objects have been selected from a catalog of more than 170,000 sources detected
at a S/N ratio > 5. We searched the sample of variables for ones whose
photometric properties are close to those of known EXor's. These latter are
monitored in a more systematic way than T Tauri stars and the mechanisms that
regulate the observed phenomenology are exactly the same. Hence the modalities
of the EXor behavior is adopted as driving criterium for selecting variables in
general. We selected 19 bona fide candidates that constitute a well-defined
sample of new variable targets for further investigation. Out of these, 10
sources present a Spitzer MIPS 24 micron counterpart, and have been classified
as 3 Class I, 5 flat spectrum and 2 Class II objects, while the other 9 sources
have spectral energy distribution compatible with phases older than Class I.
This is consistent with what is known about the small sample of known EXor's,
and suggests that the accretion flaring or EXor stage might come as a Class
I/II transition. We present also new prescriptions that can be useful in future
searches for accretion variables in large IR databases.Comment: 35 pages, 12 figures To appear in Ap
Spitzer-MIPS survey of the young stellar content in the Vela Molecular Cloud-D
A new, unbiased Spitzer-MIPS imaging survey (~1.8 square degs) of the young
stellar content of the Vela Molecular Cloud-D is presented. The survey is
complete down to 5mJy and 250mJy at 24micron (mu) and 70mu, respectively. 849
sources are detected at 24mu and 52 of them also have a 70mu counterpart. The
VMR-D region is one that we have already partially mapped in dust and gas
millimeter emission, and we discuss the correlation between the Spitzer compact
sources and the mm contours. About half of the 24mu sources are located inside
the region delimited by the 12CO(1-0) contours (corresponding to only one third
of the full area mapped with MIPS) with a consequent density increase of about
100% of the 24mu sources [four times for 70mu ones] moving from outside to
inside the CO contours. About 400 sources have a 2MASS counterpart. So we have
constructed a Ks vs. Ks-[24] diagram and identified the protostellar
population. We find an excess of Class I sources in VMR-D in comparison with
other star forming regions. This result is reasonably biased by the sensitivity
limits, or, alternatively, may reflect a very short lifetime (<=10^6yr) of the
protostellar content in this cloud. The MIPS images have identified embedded
cool objects in most of the previously identified starless cores; in addition,
there are 6 very young, possibly Class 0 objects identified. Finally we report
finding of the driving sources for a set of five out of six very compact
protostellar jets previously discovered in near-infrared images.Comment: 29 pages, 14 figures. To appear in Ap.
Near- and Far-Infrared Counterparts of Millimeter Dust Cores in the Vela Molecular Ridge Cloud D
The aim of this paper is to identify the young protostellar counterparts
associated to dust millimeter cores of the Vela Molecular Ridge Cloud D through
new IR observations (H_2 narrow-band at 2.12 micron and N broad band at 10.4
micron) along with an investigation performed on the existing IR catalogues.
The association of mm continuum emission with infrared sources from catalogues
(IRAS, MSX, 2MASS), JHK data from the literature and new observations, has been
established according to spatial coincidence, infrared colours and spectral
energy distributions. Only 7 out of 29 resolved mm cores (and 16 out of the 26
unresolved ones) do not exhibit signposts of star formation activity. The other
ones are clearly associated with: far-IR sources, H_2 jets or near-IR objects
showing a high intrinsic colour excess. The distribution of the spectral
indices pertaining to the associated sources is peaked at values typical of
Class I objects, while three objects are signalled as candidates Class 0
sources. We remark the high detection rate (30%) of H_2 jets driven by sources
located inside the mm-cores. They appear not driven by the most luminous
objects in the field, but rather by less luminous objects in young clusters,
testifying the co-existence of both low- and intermediate-mass star formation.
The presented results reliably describe the young population of VMR-D. However,
the statistical evaluation of activity vs inactivity of the investigated cores,
even in good agreement with results found for other star forming regions, seems
to reflect the limiting sensitivity of the available facilities rather than any
property intrinsic to the mm-condensations.Comment: 38 pages. To be published to Astronomy & Astrophysic
Postpartum Ovarian Vein Thrombosis
Postpartum ovarian vein thrombosis carries a significant risk of morbidity and mortality if not recognized early and managed appropriately
Spitzer-IRAC survey of molecular jets in Vela-D
We present a survey of H2 jets from young protostars in the Vela-D molecular
cloud (VMR-D), based on Spitzer -IRAC data between 3.6 and 8.0 micron. Our
search has led to the identification of 15 jets and about 70 well aligned knots
within 1.2 squared degree. We compare the IRAC maps with observations of the H2
1-0 S(1) line at 2.12 micron, with a Spitzer-MIPS map at 24 and 70 micron, and
with a map of the dust continuum emission at 1.2 mm. We find a association
between molecular jets and dust peaks. The jet candidate exciting sources have
been searched for in the published catalog of the Young Stellar Objects of
VMR-D. We selected all the sources of Class II or earlier which are located
close to the jet center and aligned with it.The association between jet and
exciting source was validated by estimating the differential extinction between
the jet opposite lobes. We are able to find a best-candidate exciting source in
all but two jets. Four exciting sources are not (or very barely) observed at
wavelengths shorter than 24 micron, suggesting they are very young protostars.
Three of them are also associated with the most compact jets. The exciting
source Spectral Energy Distributions have been modeled by means of the
photometric data between 1.2 micron and 1.2 mm. From SEDs fits we derive the
main source parameters, which indicate that most of them are low-mass
protostars. A significant correlation is found between the projected jet length
and the [24] - [70] color, which is consistent with an evolutionary scenario
according to which shorter jets are associated with younger sources. A rough
correlation is found between IRAC line cooling and exciting source bolometric
luminosity, in agreement with the previous literature. The emerging trend
suggests that mass loss and mass accretion are tightly related phenomena and
that both decrease with time.Comment: Accepted by The Astrophysical Journa
The Spitzer-IRAC Point Source Catalog of the Vela-D Cloud
This paper presents the observations of the Cloud D in the Vela Molecular
Ridge, obtained with the IRAC camera onboard the Spitzer Space Telescope at the
wavelengths \lambda = 3.6, 4.5, 5.8, 8.0 {\mu}m. A photometric catalog of point
sources, covering a field of approximately 1.2 square degrees, has been
extracted and complemented with additional available observational data in the
millimeter region. Previous observations of the same region, obtained with the
Spitzer MIPS camera in the photometric bands at 24 {\mu}m and 70 {\mu}m, have
also been reconsidered to allow an estimate of the spectral slope of the
sources in a wider spectral range. A total of 170,299 point sources, detected
at the 5-sigma sensitivity level in at least one of the IRAC bands, have been
reported in the catalog. There were 8796 sources for which good quality
photometry was obtained in all four IRAC bands. For this sample, a preliminary
characterization of the young stellar population based on the determination of
spectral slope is discussed; combining this with diagnostics in the
color-magnitude and color-color diagrams, the relative population of young
stellar objects in the different evolutionary classes has been estimated and a
total of 637 candidate YSOs have been selected. The main differences in their
relative abundances have been highlighted and a brief account for their spatial
distribution is given. The star formation rate has been also estimated and
compared with the values derived for other star forming regions. Finally, an
analysis of the spatial distribution of the sources by means of the two-point
correlation function shows that the younger population, constituted by the
Class I and flat-spectrum sources, is significantly more clustered than the
Class II and III sources.Comment: Accepted by Ap
Ship-Induced Depression Wakes and Shoreline Erosion
Shoreline retreat as an effect of ship wakes was studied in a navigation channel of the industrial port of Venice, Italy: the Malamocco -Marghera Channel. The investigation revealed unprecedented erosion rates, up to 4 m y , that determined a total loss of about 1.2 million of m of soil in the period 1970 -2015. This interaction between navigation and the channel margins must be considered in order to understand the past evolution of the central Venice Lagoon and for a sustainable management of the port traffic in the future sea -level rise scenario
The Effects of Ship Wakes in the Venice Lagoon and Implications for the Sustainability of Shipping in Coastal Waters
We analyse the impact of ship traffic in the vicinity of navigation channels in a wide shallow waterbody. The crucial hydrodynamic driver in this situation is the depression (Bernoulli) wake that may be transferred into a long-living solitary wave of depression over the shoals. The analysis considers navigation channels in the Venice Lagoon using a new large dataset of approximately 600 measured wake events associated to specific ships whose data are provided by the AIS system. Since the development of the modern industrial port and the opening of the Malamocco–Marghera channel in the late 1960s, growing pressure on the lagoon caused by ship traffic has raised concerns about its physical integrity and habitat survival. The transit of large vessels has been shown to have serious impacts on the shallow water areas adjacent to waterways. Depression wakes created by such vessels can reach significant dimensions (water level dropdown of up to 2.45 m at the channel margin), causing unusually large retreat rates of several sections of the shoreline and which may adversely affect the lagoon morphology. The wakes are analysed in relation to ship and morphological parameters. A formulation is proposed to predict wake amplitude on the basis of ship characteristics and motion
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