376 research outputs found
HIP10680/HIP10679: a visual binary in the Pictoris Association with the fastest rotating member
We present the results of a multi-filter photometric monitoring of the wide
binary HIP10680/HIP10679. We found both component to be variable with amplitude
up to V = 0.03 mag in the case of HIP10680 and V = 0.07 mag in
the case of HIP10679. We could measure the rotation periods P = 0.2396d of the
hotter F5V component HIP10680 and P = 0.777d of the cooler G5V component
HIP10679. We found that the rotation axes of both components are aligned with
an inclination = 10. Although the two components have a
mass difference not larger than 15%, they exhibit a significant difference
between their rotation periods. Such difference may arise either from different
initial rotation periods or to different disc life times. For instance, the
slower rotating component HIP 10679 hosts a well know debris disc.Comment: In press by IBVS in Information Bulletin on Variable Stars, 201
Photometric study of HD 155555 C in the Pictoris Association
We are carrying out a series of photometric monitoring to measure the
rotation periods of members in the young Pictoris Association, as part
of the RACE-OC project (Rotation and ACtivity Evolution in Open Clusters). In
this paper, we present the results for HD 155555C which is believed to be
physically associated to the spectroscopic binary V824 Ara (HD155555) and thus
constituting a triple system. We collected B, V, and R-band photometric data
timeseries and discovered from periodogram analysis the rotation period P =
4.43d. Combined with stellar radius and projected rotational velocity, we find
this star almost equator-on with an inclination 90. The
rotational properties of HD155555C fit well into the period distribution of
other Pic members, giving further support to the suggested membership
to the association and to its physical association to V824 Ara. A comparison
with Pre-Main-Sequence isochrones from various models allows us to estimate an
age of 2015 Myr for this triple system.Comment: 16 pages, 7 figure
Impact of photometric variability on age and mass determination of Young Stellar Objects: A case study on Orion Nebula Cluster
In case of pre-main sequence objects, the only way to determine age and mass
is by fitting theoretical isochrones on color-magnitude (alternatively
luminosity-temperature) diagrams. Since young stellar objects exhibit
photometric variability over wide range in magnitude and colors, the age and
mass determined by fitting isochrones is expected to be inaccurate, if not
erroneous. These in turn will badly affect any study carried out on age spread
and process of star formation. Since we have carried out very extensive
photometric observations of the Orion Nebula Cluster (ONC), we decided to use
our multi-band data to explore the influence of variability in determining mass
and age of cluster members. In this study, we get the amplitudes of the
photometric variability in V, R, and I optical bands of a sample of 346 ONC
members and use it to investigate how the variability affects the inferred
masses and ages and if it alone can take account for the age spread among the
ONC members reported by earlier studies. We find that members that show
periodic and smooth photometric rotational modulation have their masses and
ages unaffected by variability. On other hand, we found that members with
periodic but very scattered photometric rotational modulation and members with
irregular variability have their masses and ages significantly affected.
Moreover, using Hertzsprung-Russell (HR) diagrams we find that the observed I
band photometric variability can take account of only a fraction (about 50%) of
the inferred age spread, whereas the V band photometric variability is large
enough to mask any age spread.Comment: Accepted by MNRAS; 17 pages, 4 Tables, 15 Figure
Evidence from stellar rotation for early disc dispersal owing to close companions
Young low-mass stars of equal-mass exhibit a distribution of rotation
periods. At the very early phases of stellar evolution, this distribution is
set by the star-disc locking mechanism. The primordial disc lifetime and,
consequently, the duration of the disc-locking mechanism, can be significantly
shortened by the presence of a close companion, making the rotation period
distribution of close binaries different from that of either single stars or
wide binaries. We use new data to investigate and better constrain the range of
ages, the components separation and the mass ratio dependence at which the
rotation period distribution has been significantly affected by the disc
dispersal that is enhanced by close companions. We select a sample of close
binaries in the Upper Scorpius association (age 8 Myr) whose components
have measured the separation and the rotation periods and compare their period
distribution with that of coeval stars that are single stars. We find that
components of close binaries have on average rotation periods shorter than
single stars. More precisely, binaries with about equal-mass components (0.9
M2/M1 1.0) have rotation periods on average by 0.4 d shorter
than single stars; binaries with smaller mass ratios (0.8 M2/M1 0.9)
have rotation periods on average by 1.9 d the primary components, and by
1.0 d the secondary components shorter than single stars. A comparison
with the older 25-Myr Pictoris association shows that, whereas in the
latter all close binaries with projected separation 80 AU all
rotate faster than single stars, in the Upper Scorpius that has happened for
about 70% stars, yet. We interpret the enhanced rotation in close binaries with
respect to single stars as the consequence of an early disc dispersal induced
by the presence of close companions.Comment: 7 pages, 5 figures, 1 table, Accepted by Astronomy & Astrophysic
On the rotation periods of the components of the triple system TYC9300-0891-1AB/TYC9300-0529-1 in the Octans Association
Stellar rotation depends on different parameters. The range of values of
these parameters causes the dispersion in the rotation period distributions
observed in young stellar clusters/associations. We focus our investigation on
the effects of different circumstellar environments on stellar rotation. More
specifically, we are searching in stellar Associations for visual triple
systems where all stellar parameters are similar, with the only exceptions of
the unknown initial rotation period, and of the circum-stellar environment, in
the sense that one of the two about equal-mass components has a close-by third
'perturber' component. In the present study we analyse the 35-Myr old visual
triple system TYC 9300-0891-1AB + TYC 9300-0529-1 in the young Octans stellar
association consisting of three equal-mass K0V components. We collected from
the literature all information that allowed us to infer that the three
components are actually physically bound forming a triple system and are
members of the Octans Association. We collected broad-band photometric
timeseries in two observation seasons. We discovered that all the components
are variable, magnetically active, and from periodogram analysis we found the
unresolved components TYC 9300-0891-1AB to have a rotation period P = 1.383d
and TYC 9300-0529-1 a rotation period P = 1.634d. TYC 9300-0891-1A, TYC
9300-0891-1B, and TYC 9300-0529-1 have same masses, ages, and initial chemical
compositions. The relatively small 16% rotation period difference measured by
us indicates that all components had similar initial rotation periods and disc
lifetimes, and the separation of 157AU between the component A and the
'perturber' component B (or vice-versa) has been sufficiently large to prevent
any significant perturbation/shortening of the accretion-disc lifetime.Comment: Accepted by New Astronomy 201
Il concetto di 'democrazia ecologica' nel dibattito filosofico e giuspolitico contemporaneo. Tra autorità e libertà, comunità e natura
2013 - 2014My thesis is based on the philosophical-scientific notion of "epistemic complexity" and how that has been "incorporated" in the contemporary environmental law that has already an "interdisciplinary" nature, since on the border between law, science, ethics and economics. It constitutes the "scene" where the need of reweaving the idea of development, unlimited growth and progress manifests both at national and international level through a reorientation of the objectives and the economical political strategies of the last forty years . A key element of this "legal branch" is represented by the precautionary principle (expressly recognized as "mandatory" by the international community since 1992 in the Rio de Janeiro Declaration), which provides a general "duty to act" for the protection of the environment, even if there is no absolute certainty of the risk of serious and irreparable damage. Therefore, several responsibilities arise from both the political decision makers and in the scientific and economic world relatively to the possible risks that a particular "community" could face, accepting the relative consequences. For this reason even not "sectorial" mind-sets appear gradually at substantial legal level of the natural environment, as well as a conception of "strong" environmental sustainability (and "ecological modernization"), not limited to a mere "technical adjustment", nor to precautionary action on an economic cost-benefit analysis of environmental protection; procedurally new "forms" and "spaces" of deliberative "reflexive" democracy (i.e. not simply "pragmatic" ones) are created where rights to information, participation, access to the courts can be practiced more broadly beyond individual preferences of people, not only towards an "individualistic" or "communitarian" perception, but also towards an "ecological" one (inter-relationships between parts and the whole, where the whole has not to be interpreted as the mere sum of the parts). Forms and spaces of an "ecological democracy" which find again in the state, and particularly in a not "interventionist" and not "competitive" state, not enslaved to the logic of competition, market and "financial thinking", their starting point and contemporaneously their "development". At the same time they should and could turn even the state itself into a continuous "process", within "deliberative systems" (on a "transnational" scale), focused on "anthropo-decentralized" ethics. The experiences of Andes neo-constitutionalism (Ecuador and Bolivia cases particularly) finally mark the transition to a new "state based" paradigm oriented to the concept of "multinationality" where "Western" and "indigenous" ideas of “Nature” are harmonized, encouraging to enrich in this way the "systemic" framework for a possible transnational "ecological democracy" towards new ways of considering law, economy, welfare and happiness, different from the material accumulation or simply from the mere "personal life plan"; in other terms towards a new "common sense" and a new "right" to an alternative form of individual and collective life. [edited by Author]XIII n.s
The visual binary AG Tri in Pictoris Association: can a debris disc cause very different rotation periods of its components?
We measure the photometric rotation periods of the components of multiple
systems in young stellar associations to investigate the causes of the observed
rotation period dispersion. We present the case of the wide binary AG Tri in
the 23-Myr young beta Pictoris Association consisting of K4 + M1 dwarfs. Our
multi-band, multi-season photometric monitoring allowed us to measure the
rotation periods of both components P_A = 12.4d and P_B = 4.66d, to detect a
prominent magnetic activity in the photosphere, likely responsible for the
measured radial velocity variations, and for the first time, a flare event on
the M1 component AG Tri B. We investigate either the possibility that the
faster rotating component may have suffered an enhanced primordial disc
dispersal, starting its PMS spin-up earlier than the slower rotating component,
or the possibility that the formation of a debris disc may have prevented AG
Tri A from gaining part of the angular momentum from the accreting disc.Comment: 28 pages, 7 figures, accepted for publication in Astrophysics and
Space Science 2015, (ASTR-D-15-00445R2
International Student Mobility: An Identity Development Task?
Based on the review of literature on internationalization of education and on identity formation pro
cesses in
young adults, this cross
-
sectional study aims to investigate to which extent self
-
perceived dimensions of identity
are associated to the main moti
vations to study abroad.
The
participants in this study were 429
international
university students of different nationalities. Findings revealed that the motivation to study abroa
d for personal
growth is strongly associated to the commitment and in
-
depth e
xploration
identity
processes, whereas
the
motivation to study abroad with the aim
of changing
life style and enlarging
job opportunities is positively
associated with reconsideration of commitment and in
-
depth exploration. Furthermore,
identity achieved s
tudents
showed the highest motivation to s
tudy abroad for personal growth,
while
the motivation to study abroad to
positively change
life
-
styles and work conditions
is strongly associated with the positive facet of identity crisis,
which is otherwise calle
d searching
-
moratorium status.
Based on these results, the present
survey provides useful
questions and hypothesis for future researc
Tracking active nests in solar-type pulsators: Ensemble starspot modelling of Kepler asteroseismic targets
The satellite Planetary Transits and Oscillations of stars (PLATO), due to be
launched late 2026, will provide us with an unprecedented sample of light
curves of solar-type stars that will exhibit both solar-type oscillations and
signatures of activity-induced brightness modulations. Solar-type pulsators
only have moderate levels of activity because high levels of activity inhibit
oscillations. This means that these targets represent a specific challenge for
starspot modelling. In order to assess the possibilities that PLATO will soon
open, we wish to characterise the morphology of active regions at the surface
of stars for which we also have a detection of solar-like acoustic
oscillations. In this context, we report the results of an ensemble starspot
modelling analysis of the Sun and ten solar-type pulsators observed by the
Kepler satellite. We implement a Bayesian starspot modelling approach based on
a continuous-grid model, accounting for the combined starspot and facular
contribution to activity-induced brightness modulations. From our analysis, we
find that several stars of our sample exhibit clear signatures of stable
longitudinal active nests while sharing activity levels and convection versus
rotation regimes similar to the solar regime. By searching for modulations in
the reconstructed starspot coverage, we found significant periodicities that we
identify as possible signatures of cyclic modulations similar to the
quasi-biennal oscillation or the Rieger cycle. We can infer the corresponding
intensity of the magnetic field at the bottom of the convective envelope based
on the hypothesis that internal magneto-Rossby waves acting on the tachocline
cause these modulations.Comment: 18 pages, 23 figures, accepted for publication in A&
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