1,053 research outputs found
Long-term periodicity in LSI+61303 as beat frequency between orbital and precessional rate
Context: In the binary system LSI+61303 the peak flux density of the radio
outburst, which is related to the orbital period of 26.4960 +/- 0.0028d,
exibits a modulation of 1667 +/- 8 d. The radio emission at high spatial
resolution appears structured in a precessing jet with a precessional period of
27-28 d. Aims: How close is the precessional period of the radio jet to the
orbital period? Any periodicity in the radio emission should be revealed by
timing analysis. The aim of this work is to establish the accurate value of the
precessional period. Methods: We analyzed 6.7 years of the Green Bank
Interferometer database at 2.2 GHz and 8.3 GHz with the Lomb-Scargle and phase
dispersion minimization (PDM) methods and performed simulations. Results: The
periodograms show two periodicities, P1 = 26.49 +/- 0.07 d (\nu1=0.03775
d^{-1}) and P2 = 26.92 +/- 0.07 d (\nu2 = 0.03715 d^{-1}). Whereas radio
outbursts have been known to have nearly orbital occurrence P1 with timing
residuals exhibiting a puzzling sawtooth pattern, we probe in this paper that
they are actually periodical outbursts and that their period is Paverage=
(2/(\nu1 + \nu2)= 26.70 +/- 0.05 d. The period Paverage as well as the
long-term modulation Pbeat=1/(\nu1 - \nu2)=1667 +/- 393 d result from the beat
of the two close periods, the orbital P1 and the precessional P2 periods.
Conclusions: The precessional period, indicated by the astrometry to be of
27--28 d, is P2=26.92 d. The system \lsi seems to be one more case in astronomy
of beat, i.e., a phenomenon occurring when two physical processes create stable
variations of nearly equal frequencies. The very small difference in frequency
creates a long-term variation of period 1/(\nu1-\nu2). The long-term modulation
of 1667 d results from the beat of the two close orbital and precessional
rates.Comment: 8 pages, 4 figures, accepted for publication in A&
Long-term OVRO monitoring of LSI+61303: confirmation of the two close periodicities
Context: The gamma-ray binary LSI+61303 shows multiple periodicities. The
timing analysis of 6.7 yr of GBI radio data and of 6 yr of Fermi-LAT GeV
gamma-ray data both have found two close periodicities P1(GBI) = 26.49 \pm 0.07
d, P2(GBI)=26.92 \pm 0.07 d and P1(gamma)=26.48 \pm 0.08 d, P2(gamma) = 26.99
\pm 0.08 d. Aims: The system LSI+61303 is the object of several continuous
monitoring programs at low and high energies. The frequency difference between
f1 and f2 of only 0.0006 d(-1) requires long-term monitoring because the
frequency resolution in timing analysis is related to the inverse of the
overall time interval. The Owens Valley Radio Observatory (OVRO) 40 m telescope
has been monitoring the source at 15 GHz for five years and overlaps with
Fermi-LAT monitoring. The aim of this work is to establish whether the two
frequencies are also resolved in the OVRO monitoring. Methods: We analysed OVRO
data with the Lomb-Scargle method. We also updated the timing analysis of
Fermi-LAT observations. Results: The periodograms of OVRO data confirm the two
periodicities P1(OVRO) = 26.5 \pm 0.1 d and P2(OVRO) = 26.9 \pm 0.1 d.
Conclusions: The three indipendent measurements of P1 and P2 with GBI, OVRO,
and Fermi-LAT observations confirm that the periodicities are permanent
features of the system LSI+61303. The similar behaviours of the emission at
high (GeV) and low (radio) energy when the compact object in LSI+61303 is
toward apastron suggest that the emission is caused by the same periodically
(P1) ejected population of electrons in a precessing (P2) jet.Comment: 4 pages, 7 figures, A&A Letters in pres
Tribologists on the path of tactile perception
Between human senses, tactile perception is nowadays object of several research area, due to the lack of knowledge and to its crucial role on several social and industrial issues. Being the contact between skin and touched surfaces at the origin of the tactile sense, tribologists play a key role in the identification and understanding of the signals at the origin of touch. In particular, the quasi-static deformation of the fingertip skin, due to distributed contact forces, together with its transient deformation, due to the vibrations induced by the sliding of the fingerprints on the surface, activate the mechanoreceptors, who collect information to the brain. In this work, the main contributions dealing with the role of friction-induced vibrations and skin-surface contact on tactile perception are discussed
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
Estimation of the normal contact stiffness for frictional interface in sticking and sliding conditions
Modeling of frictional contact systems with high accuracy needs the knowledge of several contact parameters, which are mainly related to the local phenomena at the contact interfaces and affect the complex dynamics of mechanical systems in a prominent way. This work presents a newer approach for identifying reliable values of the normal contact stiffness between surfaces in contact, in both sliding and sticking conditions. The combination of experimental tests, on a dedicated set-up, with finite element modeling, allowed for an indirect determination of the normal contact stiffness. The stiffness was found to increase with increasing contact pressure and decreasing roughness, while the evolution of surface topography and third-body rheology affected the contact stiffness when sliding
The IMF and Star Formation History of the Stellar Clusters in the Vela D Cloud
We present the results of a Near-Infrared deep photometric survey of a sample
of six embedded star clusters in the Vela-D molecular cloud, all associated
with luminous (~10^3 Lsun) IRAS sources. The clusters are unlikely to be older
than a few 10^6 yrs, since all are still associated with molecular gas. We
employed the fact that all clusters lie at the same distance and were observed
with the same instrumental setting to derive their properties in a consistent
way, being affected by the same instrumental and observational biases. We
extracted the clusters' K Luminosity Functions (KLF) and developed a simple
method to correct them for extinction, based on colour-magnitude diagrams. The
reliability of the method has been tested by constructing synthetic clusters
from theoretical tracks for pre-main sequence stars and a standard Initial Mass
Function (IMF). The clusters' IMFs have been derived from the dereddened KLFs
by adopting a set of pre-main sequence evolutionary tracks and assuming coeval
star formation. All clusters are small (~100 members) and compact (radius
\~0.1-0.2 pc); their most massive stars are intermediate-mass (~2-10 Msun)
ones. The dereddened KLFs are likely to arise from the same distribution,
suggesting that the selected clusters have quite similar IMFs and star
formation histories. The IMFs are consistent with those derived for field stars
and clusters. Adding them together we found that the ``global'' IMF appears
steeper at the high-mass end and exhibits a drop-off at ~10 Msun. In fact, a
standard IMF would predict a star with M>22.5 Msun within one of the clusters,
which is not found. Hence, either high-mass stars need larger clusters to be
formed, or the IMF of the single clusters is steeper at the high-mass end
because of the physical conditions in the parental gas.Comment: 17 pages, 14 figures, to be published in Astronomy & Astrophysic
Triggered massive-star formation on the borders of Galactic HII regions. II. Evidence for the collect and collapse process around RCW 79
We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFI JHK images of
the Galactic HII region RCW 79. The millimetre continuum data reveal the
presence of massive fragments located in a dust emission ring surrounding the
ionized gas. The two most massive fragments are diametrically opposite each
other in the ring. The near-IR data, centred on the compact HII region located
at the south-eastern border of RCW 79, show the presence of an IR-bright
cluster containing massive stars along with young stellar objects with near-IR
excesses. A bright near- and mid-IR source is detected towards maser emissions,
1.2 pc north-east of the compact HII region centre. Additional information,
extracted from the Spitzer GLIMPSE survey, are used to discuss the nature of
the bright IR sources observed towards RCW 79. Twelve luminous Class I sources
are identified towards the most massive millimetre fragments. All these facts
strongly indicate that the massive-star formation observed at the border of the
HII region RCW 79 has been triggered by its expansion, most probably by the
collect and collapse process.Comment: 14 pages, 15 figures. Accepted for publication in A&A. The images
have been highly compressed for astro-ph. A version of this paper with
higher-resolution figures is available at
http://www.oamp.fr/matiere/rcw79.pd
Discovery of a periodical apoastron GeV peak in LS I +61{\deg}303
Aims. The aim of this paper is to analyse the previously discovered
discontinuity of the periodicity of the GeV -ray emission of the
radio-loud X-ray binary LS I +61{\deg}303 and to determine its physical origin.
Methods. We used wavelet analysis to explore the temporal development of
periodic signals. The wavelet analysis was first applied to the whole data set
of available Fermi-LAT data and then to the two subsets of orbital phase
intervals and . We also performed a
Lomb-Scargle timing Analysis. We investigated the similarities between GeV
-ray emission and radio emission by comparing the folded curves of the
Fermi-LAT data and the Green Bank Interferometer radio data. Results. During
the epochs when the timing analysis fails to determine the orbital periodicity,
the periodicity is present in the two orbital phase intervals and . That is, there are two periodical signals, one
towards periastron (i.e., ) and another one towards apoastron
(). The apoastron peak seems to be affected by the same
orbital shift as the radio outbursts and, in addition, reveals the same two
periods and that are present in the radio data. Conclusions. The
-ray emission of the apoastron peak normally just broadens the emission
of the peak around periastron. Only when it appears at , because of the orbital shift, it is enough detached from the first peak
to become recognisable as a second orbital peak, which is the reason why the
timing analysis fails. Two -ray peaks along the orbit are predicted by
the two-peak accretion model for an eccentric orbit, that was proposed by
several authors for LS I +61{\deg}303.Comment: 7 pages, 4 figures, A&A (2014
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.
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