2,108 research outputs found
Spectrum in the broken phase of a theory
We derive the spectrum in the broken phase of a theory, in
the limit , showing that this goes as even integers of a
renormalized mass in agreement with recent lattice computations.Comment: 4 pages, 1 figure. Accepted for publication in International Journal
of Modern Physics
Green functions and nonlinear systems: Short time expansion
We show that Green function methods can be straightforwardly applied to
nonlinear equations appearing as the leading order of a short time expansion.
Higher order corrections can be then computed giving a satisfactory agreement
with numerical results. The relevance of these results relies on the
possibility of fully exploiting a gradient expansion in both classical and
quantum field theory granting the existence of a strong coupling expansion.
Having a Green function in this regime in quantum field theory amounts to
obtain the corresponding spectrum of the theory.Comment: 7 pages, 3 figures. Version accepted for publication in International
Journal of Modern Physics
Evidence from stellar rotation of enhanced disc dispersal: (I) The case of the triple visual system BD-21 1074 in the Pictoris association
The early stage of stellar evolution is characterized by a star-disc locking
mechanism. The disc-locking prevents the star to spin its rotation up, and its
timescale depends on the disc lifetime. Some mechanisms can significantly
shorten this lifetime, allowing a few stars to start spinning up much earlier
than other stars. In the present study, we aim to investigate how the
properties of the circumstellar environment can shorten the disc lifetime. We
have identified a few multiple stellar systems, composed of stars with similar
masses, which belong to associations with a known age. Since all parameters
that are responsible for the rotational evolution, with the exception of
environment properties and initial stellar rotation, are similar for all
components, we expect that significant differences among the rotation periods
can only arise from differences in the disc lifetimes. A photometric timeseries
allowed us to measure the rotation periods of each component, while
high-resolution spectra provided us with the fundamental parameters,
and chromospheric line fluxes. The rotation periods of the components differ
significantly, and the component B, which has a closer companion C, rotates
faster than the more distant and isolated component A. We can ascribe the
rotation period difference to either different initial rotation periods or
different disc-locking phases arising from the presence of the close companion
C. In the specific case of BD21 1074, the second scenario seems to be more
favored. In our hypothesis of different disc-locking phase, any planet orbiting
this star is likely formed very rapidly owing to a gravitational instability
mechanism, rather than core accretion. Only a large difference of initial
rotation periods alone could account for the observed period difference,
leaving comparable disc lifetimes.Comment: Accepted by Astronomy & Astrophysics on July 31, 2014; Pages 12,
Figs.
X-Shooter spectroscopy of young stellar objects in Lupus. Atmospheric parameters, membership and activity diagnostics
A homogeneous determination of basic stellar parameters of young stellar
object (YSO) candidates is needed to confirm their evolutionary stage,
membership to star forming regions (SFRs), and to get reliable values of the
quantities related to chromospheric activity and accretion. We used the code
ROTFIT and synthetic BT-Settl spectra for the determination of the atmospheric
parameters (Teff and logg), the veiling, the radial (RV) and projected
rotational velocity (vsini), from X-Shooter spectra of 102 YSO candidates in
the Lupus SFR. We have shown that 13 candidates can be rejected as Lupus
members based on their discrepant RV with respect to Lupus and/or the very low
logg values. At least 11 of them are background giants. The spectral
subtraction of inactive templates enabled us to measure the line fluxes for
several diagnostics of both chromospheric activity and accretion. We found that
all Class-III sources have H fluxes compatible with a pure
chromospheric activity, while objects with disks lie mostly above the boundary
between chromospheres and accretion. YSOs with transitional disks displays both
high and low H fluxes. We found that the line fluxes per unit surface
are tightly correlated with the accretion luminosity () derived
from the Balmer continuum excess. This rules out that the relationships between
and line luminosities found in previous works are simply due to
calibration effects. We also found that the CaII-IRT flux ratio,
, is always small, indicating an optically thick emission
source. The latter can be identified with the accretion shock near the stellar
photosphere. The Balmer decrement reaches instead, for several accretors, high
values typical of optically thin emission, suggesting that the Balmer emission
originates in different parts of the accretion funnels with a smaller optical
depth.Comment: 28 pages, 26 figures, accepted by A&
A veto for continuous gravitational wave searches
vetoes are commonly used in searching for gravitational waves, in
particular for broad-band signals, but they can also be applied to narrow-band
continuous wave signals, such as those expected from rapidly rotating neutron
stars. In this paper we present a veto adapted to the Hough transform
searches for continuous gravitational wave signals; we characterize the
-significance plane for different frequency bands; and discuss the
expected performance of this veto in LIGO analysis.Comment: Final version, 12th Gravitational Waves Data Analysis Worksho
X-shooter spectroscopy of young stellar objects in Lupus: Lithium, iron, and barium elemental abundances
With the purpose of performing a homogeneous determination of elemental
abundances for members of the Lupus T association, we analyzed three chemical
elements: lithium, iron, and barium. The aims were: to derive the Li abundance
for ~90% of known class II stars in the Lupus I, II, III, IV clouds; to perform
chemical tagging of a region where few Fe abundance measurements have been
obtained in the past, and no determination of the Ba content has been done up
to now. We also investigated possible Ba enhancement, as this element has
become increasingly interesting in the last years following the evidence of Ba
over-abundance in young clusters, the origin of which is still unknown. Using
X-shooter@VLT, we analyzed the spectra of 89 cluster members, both class II and
III stars. We measured the strength of the Li line and derived the abundance of
this element through equivalent width measurements and curves of growth. For
six class II stars we also measured the Fe and Ba abundances using the spectral
synthesis and the code MOOG. The veiling contribution was taken into account
for all three elements. We find a dispersion in the strength of the Li line at
low Teff and identify three targets with severe Li depletion. The nuclear age
inferred for these highly Li-depleted stars is around 15 Myr, which exceeds the
isochronal one. As in other star-forming regions, no metal-rich members are
found in Lupus, giving support to a recent hypothesis that the Fe abundance
distribution of most of the nearby young regions could be the result of a
common and widespread star formation episode involving the Galactic thin disk.
We find that Ba is over-abundant by ~0.7 dex with respect to the Sun. Since
current theoretical models cannot reproduce this Ba abundance pattern, we
investigated whether this unusually large Ba content might be related to
effects due to stellar parameters, stellar activity, and accretion.Comment: 15 pages, 14 figures, 3 tables; accepted for publication in A&A;
abstract shortene
X-Shooter spectroscopy of young stellar objects in Lupus: Accretion properties of class II and transitional objects
We present the results of a study of the stellar and accretion properties of
the (almost) complete sample of class II and transitional YSOs in the Lupus I,
II, III and IV clouds, based on spectroscopic data acquired with the
VLT/X-Shooter spectrograph. Our study combines the dataset from our previous
work with new observations of 55 additional objects. We have investigated 92
YSO candidates in total, 11 of which have been definitely identified with giant
stars unrelated to Lupus. The stellar and accretion properties of the 81 bona
fide YSOs, which represent more than 90% of the whole class~II and transition
disc YSO population in the aforementioned Lupus clouds, have been homogeneously
and self-consistently derived, allowing for an unbiased study of accretion and
its relationship with stellar parameters. The accretion luminosity, Lacc,
increases with the stellar luminosity, Lstar, with an overall slope of ~1.6,
similar but with a smaller scatter than in previous studies. There is a
significant lack of strong accretors below Lstar~0.1Lsun, where Lacc is always
lower than 0.01Lstar. We argue that the Lacc-Lstar slope is not due to
observational biases, but is a true property of the Lupus YSOs. The
logMacc-logMstar correlation shows a statistically significant evidence of a
break, with a steeper relation for Mstar<0.2Msun and a flatter slope for higher
masses. The bimodality of the Macc-Mstar relation is confirmed with four
different evolutionary models used to derive the stellar mass. The bimodal
behaviour of the observed relationship supports the importance of modelling
self-gravity in the early evolution of the more massive discs, but other
processes, such as photo evaporation and planet formation during the YSO's
lifetime, may also lead to disc dispersal on different timescales depending on
the stellar mass. We also refined the empirical Lacc vs. Lline relationships.Comment: 43 pages, 22 figure
X-Shooter spectroscopy of young stellar objects: II. Impact of chromospheric emission on accretion rate estimates
Context. The lack of knowledge of photospheric parameters and the level of
chromospheric activity in young low-mass pre-main sequence stars introduces
uncertainties when measuring mass accretion rates in accreting (Class II) Young
Stellar Objects. A detailed investigation of the effect of chromospheric
emission on the estimates of mass accretion rate in young low-mass stars is
still missing. This can be undertaken using samples of young diskless (Class
III) K and M-type stars. Aims. Our goal is to measure the chromospheric
activity of Class III pre main sequence stars to determine its effect on the
estimates of accretion luminosity (Lacc) and mass accretion rate (Macc) in
young stellar objects with disks. Methods. Using VLT/X-Shooter spectra we have
analyzed a sample of 24 non-accreting young stellar objects of spectral type
between K5 and M9.5. We identify the main emission lines normally used as
tracers of accretion in Class II objects, and we determine their fluxes in
order to estimate the contribution of the chromospheric activity to the line
luminosity. Results. We have used the relationships between line luminosity and
accretion luminosity derived in the literature for Class II objects to evaluate
the impact of chromospheric activity on the accretion rate measurements. We
find that the typical chromospheric activity would bias the derived accretion
luminosity by Lacc,noise< 10-3Lsun, with a strong dependence with the Teff of
the objects. The noise on Macc depends on stellar mass and age, and the typical
values of log(Macc,noise) range between -9.2 to -11.6Msun/yr. Conclusions.
Values of Lacc< 10-3Lsun obtained in accreting low-mass pre main sequence stars
through line luminosity should be treated with caution as the line emission may
be dominated by the contribution of chromospheric activity.Comment: accepted for publication in Astronomy & Astrophysic
X-Shooter spectroscopy of young stellar objects: IV -- Accretion in low-mass stars and sub-stellar objects in Lupus
We present X-Shooter/VLT observations of a sample of 36 accreting low-mass
stellar and sub-stellar objects (YSOs) in the Lupus star forming region,
spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun <
Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between
excess-continuum and line emission accretion diagnostics, and, secondly, to
investigate the accretion properties in terms of the physical properties of the
central object. The accretion luminosity (Lacc), and from it the accretion rate
(Macc), is derived by modelling the excess emission, from the UV to the
near-IR, as the continuum emission of a slab of hydrogen. The flux and
luminosity (Ll) of a large number of emission lines of H, He, CaII, etc.,
observed simultaneously in the range from ~330nm to 2500nm, were computed. The
luminosity of all the lines is well correlated with Lacc. We provide empirical
relationships between Lacc and the luminosity of 39 emission lines, which have
a lower dispersion as compared to previous relationships in the literature. Our
measurements extend the Pab and Brg relationships to Lacc values about two
orders of magnitude lower than those reported in previous studies. We confirm
that different methodologies to measure Lacc and Macc yield significantly
different results: Ha line profile modelling may underestimate Macc by 0.6 to
0.8dex with respect to Macc derived from continuum-excess measures. Such
differences may explain the likely spurious bi-modal relationships between Macc
and other YSOs properties reported in the literature. We derive Macc in the
range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to
Mstar^1.8(+/-0.2), with a dispersion lower by a factor of about 2 than in
previous studies. A number of properties indicate that the physical conditions
of the accreting gas are similar over more than 5 orders of magnitude in Macc
Low-Mass Pre-Main Sequence Stars in the Large Magellanic Cloud - III: Accretion Rates from HST-WFPC2 Observations
We have measured the present accretion rate of roughly 800 low-mass (~1-1.4
Mo) pre-Main Sequence stars in the field of Supernova 1987A in the Large
Magellanic Cloud (LMC, Z~0.3 Zo). It is the first time that this fundamental
parameter for star formation is determined for low-mass stars outside our
Galaxy. The Balmer continuum emission used to derive the accretion rate
positively correlates with the Halpha excess. Both these phenomena are believed
to originate from accretion from a circumstellar disk so that their
simultaneous detection provides an important confirmation of the pre-Main
Sequence nature of the Halpha and UV excess objects, which are likely to be the
LMC equivalent of Galactic Classical TTauri stars. The stars with statistically
significant excesses are measured to have accretion rates larger than
1.5x10^{-8}Mo/yr at an age of 12-16 Myrs. For comparison, the time scale for
disk dissipation observed in the Galaxy is of the order of 6 Myrs. Moreover,
the oldest Classical TTauri star known in the Milky Way (TW Hydrae, with 10
Myrs of age) has a measured accretion rate of only 5x10^{-10} Mo/yr, ie 30
times less than what we measure for stars at a comparable age in the LMC. Our
findings indicate that metallicity plays a major role in regulating the
formation of low-mass stars.Comment: Accepted for publication in the Astrophysical Journal (10 June 2004),
28 pages, 9 figures. Typo corrected in the abstract on 21 February 200
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