1,039 research outputs found
VLBA images of the precessing jet of LSI+61303
Context: In 2004, changes in the radio morphology of the Be/X-ray binary
system LSI+61303 suggested that it is a precessing microquasar. In 2006, a set
of VLBA observations performed throughout the entire orbit of the system were
not used to study its precession because the changes in radio morphology could
tentatively be explained by the alternative pulsar model. However, a recent
radio spectral index data analysis has confirmed the predictions of the
two-peak microquasar model, which therefore does apply in LSI+61303. Aims: We
revisit the set of VLBA observations performed throughout the orbit to
determine the precession period and improve our understanding of the physical
mechanism behind the precession. Methods: By reanalyzing the VLBA data set, we
improve the dynamic range of images by a factor of four, using
self-calibration. Different fitting techniques are used and compared to
determine the peak positions in phase-referenced maps. Results: The improved
dynamic range shows that in addition to the images with a one-sided structure,
there are several images with a double-sided structure. The astrometry
indicates that the peak in consecutive images for the whole set of observations
describes a well-defined ellipse, 6-7 times larger than the orbit, with a
period of about 28 d. Conclusions: A double-sided structure is not expected to
be formed from the expanding shocked wind predicted in the pulsar scenario. In
contrast, a precessing microquasar model can explain the double- and one-sided
structures in terms of variable Doppler boosting. The ellipse defined by the
astrometry could be the cross-section of the precession cone, at the distance
of the 8.4 GHz-core of the steady jet, and 28d the precession period.Comment: 7 pages, 5 figures, Accepted for publication in Astronomy and
Astrophysics, added references for sect.
The broad-band radio spectrum of LSI+61303 in outburst
Aims: Our aim is to explore the broad-band radio continuum spectrum of
LSI+61303 during its outbursts by employing the available set of secondary
focus receivers of the Effelsberg 100 m telescope. Methods: The clear
periodicity of the system LSI+61303 allowed observations to be scheduled
covering the large radio outburst in March-April 2012. We observed LSI+61303 on
14 consecutive days at 2.6, 4.85, 8.35, 10.45, 14.3, 23, and 32 GHz with a
cadence of about 12 hours followed by two additional observations several days
later. Based on these observations we obtained a total of 24 quasi-simultaneous
broad-band radio spectra. Results: During onset, the main flare shows an almost
flat broad-band spectrum, most prominently seen on March 27, 2012, where - for
the first time - a flat spectrum (alpha=0.00+/-0.07, S nu^alpha) is observed up
to 32 GHz (9 mm wavelength). The flare decay phase shows superimposed
'sub-flares' with the spectral index oscillating between -0.4 and -0.1 in a
quasi-regular fashion. Finally, the spectral index steepens during the decay
phase, showing optically thin emission with values alpha -0.5 to -0.7.
Conclusions: The radio characteristics of LSI+61303 compare well with those of
the microquasars XTE J1752-223 and Cygnus X-3. In these systems the flaring
phase is actually also composed of a sequence of outbursts with clearly
different spectral characteristics: a first outburst with a flat/inverted
spectrum followed by a bursting phase of optically thin emission.Comment: 4 pages, 3 figures, accepted for publication in A&
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
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
Feasibility Study of Lense-Thirring Precession in LS I +61303
Very recent analysis of the radio spectral index and high energy observations
have shown that the two-peak accretion/ejection microquasar model applies for
LSI+61303. The fast variations of the position angle observed with MERLIN and
confirmed by consecutive VLBA images must therefore be explained in the context
of the microquasar scenario. We calculate what could be the precessional period
for the accretion disk in LSI+61303 under tidal forces of the Be star
(P_{tidal-forces}) or under the effect of frame dragging produced by the
rotation of the compact object (P_{Lense-Thirring}). P_{tidal-forces}R_{tr}r_g$, where the Bardeen-Petterson effect
should align the midplane of the disk. For this truncated radius of the
accretion disk P_{Lense-Thirring} for a slow rotator results in a few days.
Therefore, Lense-Thirring precession induced by a slowly rotating compact
object could be compatible with the daily variations of the ejecta angle
observed in LSI+61303.Comment: 6 pages, 5 figures, accepted for publication in Astronomy and
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
Triggered massive-star formation on the borders of Galactic HII regions. III. Star formation at the periphery of Sh2-219
Context. Massive-star formation triggered by the expansion of HII regions.
Aims. To understand if sequential star formation is taking place at the
periphery of the HII region Sh2-219. Methods. We present 12CO(2-1) line
observations of this region, obtained at the IRAM 30-m telescope (Pico Veleta,
Spain). Results. In the optical, Sh2-219 is spherically symmetric around its
exciting star; furthermore it is surrounded along three quarters of its
periphery by a ring of atomic hydrogen. This spherical symmetry breaks down at
infrared and millimetre wavelengths. A molecular cloud of about 2000\msol lies
at the southwestern border of Sh2-219, in the HI gap. Two molecular
condensations, elongated along the ionization front, probably result from the
interaction between the expanding HII region and the molecular cloud. In this
region of interaction there lies a cluster containing many highly reddened
stars, as well as a massive star exciting an ultracompact HII region. More
surprisingly, the brightest parts of the molecular cloud form a `chimney',
perpendicular to the ionization front. This chimney is closed at its south-west
extremity by H-alpha walls, thus forming a cavity. The whole structure is 7.5
pc long. A luminous H-alpha emission-line star, lying at one end of the chimney
near the ionization front, may be responsible for this structure. Confrontation
of the observations with models of HII region evolution shows that Sh2-219 is
probably 10^5 yr old. The age and origin of the near-IR cluster observed on the
border of Sh2-219 remain unknown.Comment: 11 pages, 10 figures. To be published in A&
Implications of the radio spectral index transition in LS I +61{\deg}303 for its INTEGRAL data analysis
The TeV emitting X-ray binary LS I +61{\deg}303 has two radio periodicities
that correspond to a large periodic outburst with the same period as the orbit,
26.5 days (phase \Phi), and a second periodicity of 1667 days (phase \Theta),
which modulates the orbital phase and amplitude of the large outburst. Analyses
of the radio spectral index revealed in LS I +61{\deg}303 the presence of the
critical transition typical for microquasars from optically thick emission
(related to a steady jet) to an optically thin outburst (related to a transient
jet), and found that it occurs at \Phi_{crit}, which is modulated by \Theta:
\Phi_{crit}=f(\Theta). We examine the possible implications of averaging high
energy data over large \Theta and \Phi intervals in the light of puzzling
published INTEGRAL results, which differ for different averaging of the data.
In microquasars, a simultaneous transition between two X-ray states occurs at
the switch from optically thick radio emission to an optically thin radio
outburst, from the low/hard to the steep power-law state. Assuming that the
same transition occurs in LS I +61{\deg}303 at \Phi_{crit}, we can show
qualitatively the effect of averaging high energy data on \Theta, by analysing
the effects of averaging radio spectral index data across the same \Theta
interval. We then model the two X-ray states, low/hard and steep power-law
state, and show quantitatively how their mixing can affect the results. When
folded over too large a \Theta interval, spectral data from INTEGRAL can yield
a false picture of the emission behaviour of the source along the orbit because
it may be mixing two spectral states. Furthermore, averaging the data along the
orbit may result in a dominant low/hard spectral state, which, for
insufficiently extended sampling, might appear without a cut-off.Comment: 5 pages, 3 figures, accepted for publication in A&
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