41 research outputs found
Accretion of Small Satellites and Gas Inflows in a Disc Galaxy
Galaxy interactions can have an important effect in a galaxy's evolution.
Cosmological models predict a large number of small satellites around galaxies.
It is important to study the effect that these small satellites can have on the
host. The present work explores the effect of small N-body spherical satellites
with total mass ratios in the range approx 1:1000-1:100 in inducing gas flows
to the central regions of a disc galaxy with late-type morphology resembling
the Milky Way. Two model galaxies are considered: barred and non-barred models;
the latter one is motivated in order to isolate and understand better the
effects of the satellite. Several circular and non-circular orbits are
explored, considering both prograde and retrogade orientations. We show that
satellites with such small mass ratios can still produce observable distortions
in the gas and stellar components of the galaxy. In terms of gas flows, the
prograde circular orbits are more favourable for producing gas flows, where in
some cases up to $60% of the gas of the galaxy is driven to the central region.
We find, hence, that small satellites can induce significant gas flows to the
central regions of a disc galaxy, which is relevant in the context of fuelling
active galactic nuclei.Comment: Accepted for publication in MNRA
Large-scale gas flows and streaming motions in simulated spiral galaxies
FGR-F and IAB gratefully acknowledge support from the ERC ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC2011-ADG. FGR-F also acknowledges a St. Leonards Scholarship from the University of St Andrews and support from the Hyperstars project (funded by Région Paris Île-de-France DIMACAV+) at the final stages of this project. This equipment is funded by BIS National EInfrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1.From a galactic perspective, star formation occurs on the smallest scales within molecular clouds, but it is likely initiated from the large scale flows driven by galactic dynamics. To understand the conditions for star formation, it is important to first discern the mechanisms that drive gas from large-scales into dense structures on the smallest scales of a galaxy. We present high-resolution smoothed particle hydrodynamics simulations of two model spiral galaxies: one with a live stellar disc (N-body) and one with a spiral potential. We investigate the large-scale flows and streaming motions driven by the simulated spiral structure. We find that the strength of the motions in the radial direction tends to be higher than in the azimuthal component. In the N-body model, the amplitude of these motions decreases with galactocentric radius whereas for the spiral potential, it decreases to a minimum at the corotation radius, and increases again after this point. The results show that in both simulations, the arms induce local shocks, an increase in kinetic energy that can drive turbulence and a means of compressing and expanding the gas. These are all crucial elements in forming molecular clouds and driving the necessary conditions for star formation.PostprintPeer reviewe
Multi-filter transit observations of WASP-39b and WASP-43b with three San Pedro M\'artir telescopes
Three optical telescopes located at the San Pedro M\'artir National
Observatory were used for the first time to obtain multi-filter defocused
photometry of the transiting extrasolar planets WASP-39b and WASP-43b. We
observed WASP-39b with the 2.12m telescope in the U filter for the first time,
and additional observations were carried out in the R and I filters using the
0.84m telescope. WASP-43b was observed in VRI with the same instrument, and in
the i filter with the robotic 1.50m telescope. We reduced the data using
different pipelines and performed aperture photometry with the help of custom
routines, in order to obtain the light curves. The fit of the light curves
(1.5--2.5mmag rms), and of the period analysis, allowed a revision of the
orbital and physical parameters, revealing for WASP-39b a period ( days) which is seconds larger than
previously reported. Moreover, we find for WASP-43b a planet/star radius
() which is larger in the i filter
with respect to previous works, and that should be confirmed with additional
observations. Finally, we confirm no evidence of constant period variations in
WASP-43b.Comment: 13 pages, 7 figures, accepted in PASP, scheduled for the February 1,
2015 issu
Engulfing a radio pulsar: the case of PSR J1023+0038
The binary millisecond radio pulsar PSR J1023+0038 has been recently the
subject of multiwavelength monitoring campaigns which revealed that an
accretion disc has formed around the neutron star (since 2013 June). We present
here the results of X-ray and UV observations carried out by the Swift
satellite between 2013 October and 2014 May, and of optical and NIR
observations performed with the REM telescope, the Liverpool Telescope, the
2.1-m telescope at the San Pedro M\'artir Observatory and the 1.52-m telescope
at the Loiano observing station. The X-ray spectrum is well described by an
absorbed power law, which is softer than the previous quiescent epoch (up to
2013 June). The strong correlation between the X-ray and the UV emissions
indicates that the same mechanism should be responsible for part of the
emission in these bands. Optical and infrared photometric observations show
that the companion star is strongly irradiated. Double-peaked emission lines in
the optical spectra provide compelling evidence for the presence of an outer
accretion disc too. The spectral energy distribution from IR to X-rays is well
modelled if the contributions from the companion, the disc and the intra-binary
shock emission are all considered. Our extensive data set can be interpreted in
terms of an engulfed radio pulsar: the radio pulsar is still active, but
undetectable in the radio band due to a large amount of ionized material
surrounding the compact object. X-rays and gamma-rays are produced in an
intra-binary shock front between the relativistic pulsar wind and matter from
the companion and an outer accretion disc. The intense spin-down power
irradiates the disc and the companion star, accounting for the UV and optical
emissions.Comment: 11 pages, 8 figures, 5 tables; accepted for publication on MNRA
TESS Cycle 2 observations of roAp stars with 2-min cadence data
We present the results of a systematic search of the Transiting Exoplanet Survey Satellite (TESS) 2-min cadence data for new rapidly oscillating Ap (roAp) stars observed during the Cycle 2 phase of its mission. We find seven new roAp stars previously unreported as such and present the analysis of a further 25 roAp stars that are already known. Three of the new stars show multiperiodic pulsations, while all new members are rotationally variable stars, leading to almost 70 per cent (22) of the roAp stars presented being α2 CVn-type variable stars. We show that targeted observations of known chemically peculiar stars are likely to overlook many new roAp stars, and demonstrate that multi-epoch observations are necessary to see pulsational behaviour changes. We find a lack of roAp stars close to the blue edge of the theoretical roAp instability strip, and reaffirm that mode instability is observed more frequently with precise, space-based observations. In addition to the Cycle 2 observations, we analyse TESS data for all known roAp stars. This amounts to 18 further roAp stars observed by TESS. Finally, we list six known roAp stars that TESS is yet to observe. We deduce that the incidence of roAp stars amongst the Ap star population is just 5.5 per cent, raising fundamental questions about the conditions required to excite pulsations in Ap stars. This work, coupled with our previous work on roAp stars in Cycle 1 observations, presents the most comprehensive, homogeneous study of the roAp stars in the TESS nominal mission, with a collection of 112 confirmed roAp stars in total