1,078 research outputs found
Pre-main sequence stars with disks in the Eagle Nebula observed in scattered light
NGC6611 and its parental cloud, the Eagle Nebula (M16), are well-studied
star-forming regions, thanks to their large content of both OB stars and stars
with disks and the observed ongoing star formation. We identified 834
disk-bearing stars associated with the cloud, after detecting their excesses in
NIR bands from J band to 8.0 micron. In this paper, we study in detail the
nature of a subsample of disk-bearing stars that show peculiar characteristics.
They appear older than the other members in the V vs. V-I diagram, and/or they
have one or more IRAC colors at pure photospheric values, despite showing NIR
excesses, when optical and infrared colors are compared. We confirm the
membership of these stars to M16 by a spectroscopic analysis. The physical
properties of these stars with disks are studied by comparing their spectral
energy distributions (SEDs) with the SEDs predicted by models of T-Tauri stars
with disks and envelopes. We show that the age of these stars estimated from
the V vs. V-I diagram is unreliable since their V-I colors are altered by the
light scattered by the disk into the line of sight. Only in a few cases their
SEDs are compatible with models with excesses in V band caused by optical
veiling. Candidate members with disks and photospheric IRAC colors are selected
by the used NIR disk diagnostic, which is sensitive to moderate excesses, such
as those produced by disks with low masses. In 1/3 of these cases, scattering
of stellar flux by the disks can also be invoked. The photospheric light
scattered by the disk grains into the line of sight can affect the derivation
of physical parameters of ClassII stars from photometric optical and NIR data.
Besides, the disks diagnostic we defined are useful for selecting stars with
disks, even those with moderate excesses or whose optical colors are altered by
veiling or photospheric scattered light.Comment: Accepted for publication in A&
Correlation between the spatial distribution of circumstellar disks and massive stars in the young open cluster NGC 6611. II: Cluster members selected with Spitzer/IRAC
Context: the observations of the proplyds in the Orion Nebula Cluster,
showing clear evidence of ongoing photoevaporation, have provided a clear proof
about the role of the externally induced photoevaporation in the evolution of
circumstellar disks. NGC 6611 is an open cluster suitable to study disk
photoevaporation, thanks to its large population of massive members and of
stars with disk. In a previous work, we obtained evidence of the influence of
the strong UV field generated by the massive cluster members on the evolution
of disks around low-mass Pre-Main Sequence members. That work was based on a
multi-band BVIJHK and X-ray catalog purposely compiled to select the cluster
members with and without disk. Aims: in this paper we complete the list of
candidate cluster members, using data at longer wavelengths obtained with
Spitzer/IRAC, and we revisit the issue of the effects of UV radiation on the
evolution of disks in NGC 6611. Methods: we select the candidate members with
disks of NGC 6611, in a field of view of 33'x34' centered on the cluster, using
IRAC color-color diagrams and suitable reddening-free color indices. Besides,
using the X-ray data to select Class III cluster members, we estimate the disks
frequency vs. the intensity of the incident radiation emitted by massive
members. Results: we identify 458 candidate members with circumstellar disks,
among which 146 had not been revealed in our previous work. Comparing of the
various color indices we used to select the cluster members with disk, we claim
that they detect the excesses due to the emission of the same physical region
of the disk: the inner rim at the dust sublimation radius. Our new results
confirm that UV radiation from massive stars affects the evolution of nearby
circumstellar disks.Comment: Accepted for publication at Astronomy & Astrophysic
The circumstellar disc around the Herbig AeBe star HD169142
We present 7 mm and 3.5 cm wavelength continuum observations toward the
Herbig AeBe star HD169142 performed with the Very Large Array (VLA) with an
angular resolution of ~1". We find that this object exhibits strong (~4.4 mJy),
unresolved (~1") 7 mm continuum emission, being one of the brightest isolated
Herbig AeBe stars ever detected with the VLA at this wavelength. No emission is
detected at 3.5 cm continuum, with a 3 sigma upper limit of ~0.08 mJy. From
these values, we obtain a spectral index of ~2.5 in the 3.5 cm to 7 mm
wavelength range, indicating that the observed flux density at 7mm is most
likely dominated by thermal dust emission coming from a circumstellar disc. We
use available photometric data from the literature to model the spectral energy
distribution (SED) of this object from radio to near-ultraviolet frequencies.
The observed SED can be understood in terms of an irradiated accretion disc
with low mass accretion rate, 10^{-8} solar masses per year, surrounding a star
with an age of ~10 Myr. We infer that the mass of the disc is ~0.04 solar
masses, and is populated by dust grains that have grown to a maximum size of 1
mm everywhere, consistent with the lack of silicate emission at 10 microns.
These features, as well as indications of settling in the wall at the dust
destruction radius, led us to speculate the disc of HD169142 is in an advanced
stage of dust evolution, particularly in its inner regions.Comment: 13 pages, 2 figures. Accepted by MNRA
Vertical structure models of T Tauri and Herbig Ae/Be disks
In this paper we present detailed models of the vertical structure
(temperature and density) of passive irradiated circumstellar disks around T
Tauri and Herbig Ae/Be stars. In contrast to earlier work, we use full
frequency- and angle-dependent radiative transfer instead of the usual moment
equations. We find that this improvement of the radiative transfer has strong
influence on the resulting vertical structure of the disk, with differences in
temperature as large as 70 %. However, the spectral energy distribution (SED)
is only mildly affected by this change. In fact, the SED compares reasonably
well with that of improved versions of the Chiang & Goldreich (CG) model. This
shows that the latter is a reasonable model for the SED, in spite of its
simplicity. It also shows that from the SED alone, little can be learned about
the vertical structure of a passive circumstellar disk. The molecular line
emission from these disks is more sensitive to the vertical temperature and
density structure, and we show as an example how the intensity and profiles of
various CO lines depend on the adopted disk model. The models presented in this
paper can also serve as the basis of theoretical studies of e.g. dust
coagulation and settling in disks.Comment: 12 pages, 15 figures, accepted for publication in A&
Warm Molecular Layers in Protoplanetary Disks
We have investigated molecular distributions in protoplanetary disks,
adopting a disk model with a temperature gradient in the vertical direction.
The model produces sufficiently high abundances of gaseous CO and HCO+ to
account for line observations of T Tauri stars using a sticking probability of
unity and without assuming any non-thermal desorption. In regions of radius R >
10 AU, with which we are concerned, the temperature increases with increasing
height from the midplane. In a warm intermediate layer, there are significant
amounts of gaseous molecules owing to thermal desorption and efficient
shielding of ultraviolet radiation by the flared disk. The column densities of
HCN, CN, CS, H2CO, HNC and HCO+ obtained from our model are in good agreement
with the observations of DM Tau, but are smaller than those of LkCa15.
Molecular line profiles from our disk models are calculated using a
2-dimensional non-local-thermal-equilibrium (NLTE) molecular-line radiative
transfer code for a direct comparison with observations. Deuterated species are
included in our chemical model. The molecular D/H ratios in the model are in
reasonable agreement with those observed in protoplanetary disks.Comment: 11 pages, Latex (aa.cls), to be published in Astronomy and
Astrophysic
Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk
Using the Hubble Space Telescope, the 4 m Blanco telescope at the Cerro
Tololo Inter-American Observatory, and the Spitzer Space Telescope, we have
performed deep imaging from 0.8 to 8 um of the southern subcluster in the
Chamaeleon I star-forming region. In these data, we have discovered an object,
Cha 110913-773444, whose colors and magnitudes are indicative of a very
low-mass brown dwarf with a circumstellar disk. In a near-infrared spectrum of
this source obtained with the Gemini Near-Infrared Spectrograph, the presence
of strong steam absorption confirms its late-type nature (>=M9.5) while the
shapes of the H- and K-band continua and the strengths of the Na I and K I
lines demonstrate that it is a young, pre-main-sequence object rather than a
field dwarf. A comparison of the bolometric luminosity of Cha 110913-773444 to
the luminosities predicted by the evolutionary models of Chabrier and Baraffe
and Burrows and coworkers indicates a mass of 8+7/-3 M_Jup, placing it fully
within the mass range observed for extrasolar planetary companions (M<=15
M_Jup). The spectral energy distribution of this object exhibits mid-infrared
excess emission at >5 um, which we have successfully modeled in terms of an
irradiated viscous accretion disk with M'<=10e-12 M_sun/year. Cha 110913-773444
is now the least massive brown dwarf observed to have a circumstellar disk, and
indeed is one of the least massive free-floating objects found to date. These
results demonstrate that the raw materials for planet formation exist around
free-floating planetary-mass bodies.Comment: 5 pages, accepted to Astrophysical Journal Letter
Canine parvovirus and pseudorabies virus coinfection as a cause of death in a wolf (Canis lupus) from southern Italy
Pseudorabies virus (PRV) or suid herpesvirus 1 (SHV-1) is the causative agent of
Aujeszky's disease, a highly contagious viral infection which causes neurological fatal
illness in mammals other than suids. Here we report a case of a young wolf (Canis
lupus) of around 2 years found dead by a hunter in the province of Avellino, Campania
Region. Necropsy showed pathological findings consistent with encephalitis and gastroenteritis.
Organs were analysed by microbiological and molecular investigations
following standard procedures to ascertain the possible cause of death. Real-time
PCR revealed the presence of PRV in the brain and of canine parvovirus 2b in organs
like intestine, liver, brain, kidney and pancreas. Death probably occurred very shortly
after SHV-1 infection in an animal already weakened by parvovirosis
Testing Zimbardo’s Stanford Time Perspective Inventory (STPI)-Short Form: An Italian study
ABSTRACT. In the present study the psychometric properties of the
Stanford Time Perspective Inventory (STPI – short version) are
assessed in an Italian sample. Factorial analysis of STPI items was
performed on 1507 respondents (965 women and 542 men). Results
showed a clear correspondence between factorial components and a
priori hypothesized dimensions by evidencing three factors (Future,
Hedonistic Present, Fatalistic Present). Italian translation of the STPI
indicates a fair degree of internal consistency and good metrological
characteristics. The present results parallel those previously reported
for an American sample. The present study, however, failed in
evidencing the ‘Past’ as a factor in the factor structure. The results
are discussed in terms of individual differences with respect to
the relationships between demographic variables and temporal
dimensions
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