379 research outputs found
Optimized trajectories to the nearest stars using lightweight high-velocity photon sails
New means of interstellar travel are now being considered by various research
teams, assuming lightweight spaceships to be accelerated via either laser or
solar radiation to a significant fraction of the speed of light (c). We
recently showed that gravitational assists can be combined with the stellar
photon pressure to decelerate an incoming lightsail from Earth and fling it
around a star or bring it to rest. Here, we demonstrate that photogravitational
assists are more effective when the star is used as a bumper (i.e. the sail
passes "in front of" the star) rather than as a catapult (i.e. the sail passes
"behind" or "around" the star). This increases the maximum deceleration at
Cen A and B and reduces the travel time of a nominal graphene-class
sail (mass-to-surface ratio 8.6e-4 gram m) from 95 to 75 yr. The maximum
possible velocity reduction upon arrival depends on the required deflection
angle from Cen A to B and therefore on the binary's orbital phase.
Here, we calculate the variation of the minimum travel times from Earth into a
bound orbit around Proxima for the next 300 yr and then extend our calculations
to roughly 22,000 stars within about 300 ly. Although Cen is the most
nearby star system, we find that Sirius A offers the shortest possible travel
times into a bound orbit: 69 yr assuming 12.5% c can be obtained at departure
from the solar system. Sirius A thus offers the opportunity of flyby
exploration plus deceleration into a bound orbit of the companion white dwarf
after relatively short times of interstellar travel.Comment: 14 pages, 7 figures (5 col, 2 b/w), 2 table
The environment of the fast rotating star Achernar - High-resolution thermal infrared imaging with VISIR in BURST mode
Context: The geometry of the circumstellar envelopes (CSE) surrounding Be
stars is still uncertain, although it is often assumed that they are formed by
a disk around the stellar equator and a hot polar wind. Achernar (Alpha Eri) is
the nearest Be star, and we recently detected a CSE using near-IR
interferometry. Aims: Our initial goal was to constrain the geometry and flux
contribution of the CSE of Achernar at distances of a few tens of AU from the
star, in the thermal IR domain to complement our near-IR interferometric
observations. Methods: We obtained diffraction-limited images of Achernar in
the thermal infrared using VISIR at the VLT. In order to freeze the turbulence,
we used the BURST mode of this instrument. Results: The images obtained in the
PAH1 band show a point-like source located 0.280" north-west of Achernar
(projected linear separation of 12.3 AU). Its emission is 1.8 % of the flux of
Achernar in this band, but is not detected in the PAH2, SiC and NeII bands. We
also provide new thermal IR photometry of Achernar in four bands. Conclusions:
Being aligned with the expected azimuth of the equatorial plane of Achernar,
the detected source could be a main sequence stellar companion. In this case,
it apparent brightness would correspond to an A7V spectral type.Comment: 4 pages, 4 figure
Exploring the water and carbon monoxide shell around Betelgeuse with VLTI/AMBER
We present the results of the analysis of our recent interferometric
observations of Betelgeuse, using the AMBER instrument of the VLTI. Using the
medium spectral resolution mode () we detected the presence of the
water vapour and carbon monoxide (CO) molecules in the H and K bands. We also
derived the photospheric angular diameter in the continuum. By analysing the
depth of the molecular lines and the interferometric visibilities, we derived
the column densities of the molecules, as well as the temperature and the size
of the corresponding regions in the atmosphere of Betelgeuse (the MOLsphere)
using a single shell model around the photosphere. Our results confirm the
findings by Perrin \et al\ (\cite{Perrin2004}) and Ohnaka \et al\
(\cite{Ohnaka2011}) that the HO and CO molecules are distributed around
Betelgeuse in a MOLsphere extending to approximately 1.3 times the star's
photospheric radius.Comment: Betelgeuse Workshop, November 2012, Paris. To be published in the
European Astronomical Society Publications Series, 2013, Editors: Pierre
Kervella, Thibaut Le Bertre \& Guy Perri
Preparing an ESO proposal
13 pages, to be published in the Proceedings of the 2006 Euro Summer School "Observation and Data Reduction with the VLTI", 4-16 June 2006, Goutelas (France), http://vltischool.obs.ujf-grenoble.fr/Preparing observing proposals is a classical exercise in an observational astronomer's life. In this paper, we summarize the general requirements to write a good proposal for the ESO telescopes, and eventually obtain telescope time. We detail in particular the different proposal types, how they are evaluated by ESO, what they should contain and what to avoid. The advice given in the present paper are applicable to observing proposals for the AMBER and MIDI instruments of the VLTI, but also to other ESO instruments, and to some extent to other observatories
The convection of close red supergiant stars observed with near-infrared interferometry
Our team has obtained observations of the photosphere of the two closest red
supergiant stars Betelgeuse ( Ori) and Antares ( Sco) using
near infrared interferometry. We have been monitoring the photosphere of
Betelgeuse with the VLTI/PIONIER instrument for three years. On Antares, we
obtained an unprecedented sampling of the visibility function. These data allow
us to probe the convective photosphere of massive evolved stars.Comment: 5 pages, 3 figures. Published in the proceedings of the Physics Of
Evolved Stars conference, dedicated to the memory of Olivier Chesneau (Nice,
France, 2015
On-chip spectro-detection for fully integrated coherent beam combiners
This paper presents how photonics associated with new arising detection
technologies is able to provide fully integrated instrument for coherent beam
combination applied to astrophysical interferometry. The feasibility and
operation of on-chip coherent beam combiners has been already demonstrated
using various interferometric combination schemes. More recently we proposed a
new detection principle aimed at directly sampling and extracting the spectral
information of an input signal together with its flux level measurement. The
so-called SWIFTS demonstrated concept that stands for Stationary-Wave
Integrated Fourier Transform Spectrometer, provides full spectral and spatial
information recorded simultaneously thanks to a motionless detecting device.
Due to some newly available detection principles considered for the
implementation of the SWIFTS concept, some technologies can even provide
photo-counting operation that brought a significant extension of the
interferometry domain of investigation in astrophysics . The proposed concept
is applicable to most of the interferometric instrumental modes including
fringe tracking, fast and sensitive detection, Fourier spectral reconstruction
and also to manage a large number of incoming beams. The paper presents three
practical implementations, two dealing with pair-wise integrated optics beam
combinations and the third one with an all-in-one 8 beam combination. In all
cases the principles turned into a pair wise baseline coding after proper data
processing.Comment: 12 pages, 6 figures, part of the Optics Express special issue
dedicated to Astrophotonic
The circumstellar envelopes of the Cepheids L Car and RS Pup - Comparative study in the infrared with Spitzer, VLT/VISIR and VLTI/MIDI
19 pages, 16 figuresInternational audience[Abridged] - Context: Circumstellar envelopes (CSEs) around Cepheids are particularly interesting as they could impact the Cepheid distance scale, and imply stellar mass loss. Aims: Our goal is to establish the spatial and spectral properties of the CSEs of L Car and RS Pup. This is done through a parametrization of the envelopes in terms of fractional flux (with respect to the star) and angular size. - Methods: We retrieved archival Spitzer images of the two stars (3.5-70 mic), and obtained new imaging with the VLT/VISIR camera in BURST mode (8.6-11.9 mic), as well as interferometry with VLTI/MIDI (8-13 mic). This combination allows us to probe the envelopes over arcminute to milliarcsecond scales. - Results: The CSE of RS Pup is resolved at 24 and 70 mic by Spitzer, and around 10 mic by MIDI and VISIR. The compact envelope of L Car is resolved only in the VISIR and MIDI observations. However, the properties of the IR excesses differ considerably : a warm component is present around both stars at a spatial scale of a few 100 to a few 1 000 AU, but RS Pup presents in addition a large (several 100 000 AU) and cold (~40 K) dusty envelope. - Conclusions: We propose that the reflection nebula surrounding RS Pup has an interstellar origin, while the warm CSEs of both stars were created by ongoing stellar mass loss. Such warm circumstellar envelopes are probably common around Cepheids
The close-in companion of the fast rotating Be star Achernar
Accepted for publication as an A&A LetterContext: Be stars are massive dwarf or subgiant stars that present temporary emission lines in their spectrum, and particularly in the Halpha line. The mechanism triggering these Be episodes is currently unknown, but binarity could play an important role. Aims: Previous observations with the VLT/VISIR instrument (Kervella & Domiciano de Souza 2007) revealed a faint companion to Achernar, the brightest Be star in the sky. The present observations are intended to characterize the physical nature of this object. Methods: We obtained near-IR images and an H-band spectrum of Achernar B using the VLT/NACO adaptive optics systems. Results: Our images clearly show the displacement of Achernar B over a portion of its orbit around Achernar A. Although these data are insuficient to derive the orbital parameters, they indicate a period of the order of 15 yr. The projected angular separation of the two objects in December 2007 was smaller than 0.15 arcsec, or 6.7 AU at the distance of Achernar. Conclusions: From its flux distribution in the near- and thermal-IR, Achernar B is most likely an A1V-A3V star. Its orbital period appears similar to the observed pseudo-periodicity of the Be phenomenon of Achernar. This indicates that an interaction between A and B at periastron could be the trigger of the Be episodes
Properties of the CO and HO MOLsphere of the red supergiant Betelgeuse from VLTI/AMBER observations
Context. Betelgeuse is the closest red supergiant (RSG); therefore, it is
well suited for studying the complex processes in its atmosphere that lead to
the chemical enrichment of the interstellar medium. Aims. We intend to
investigate the shape and composition of the close molecular layer (also known
as the MOLsphere) that surrounds the star. This analysis is part of a wider
program that aims at understanding the dynamics of the circumstellar envelope
of Betelgeuse. Methods. On January and February 2011, Betelgeuse was observed
using the Astronomical Multi-BEam combineR (AMBER) instrument of the Very Large
Telescope Interferometer (VLTI) in the H and K bands. Using the medium spectral
resolution of the instrument (R 1500), we were able to investigate the
carbon monoxide band heads and the water-vapor bands. We used two different
approaches to analyse our data: a model fit in both the continuum and
absorption lines and then a fit with a Radiative HydroDynamics (RHD)
simulation. Results. Using the continuum data, we derive a uniform disk
diameter of ~mas, a power law type limb-darkened disk diameter
of ~mas and a limb-darkening exponent of .
Within the absorption lines, using a single layer model, we obtain parameters
of the MOLsphere. Using a RHD simulation, we unveil the convection pattern in
the visibilities. Conclusions. We derived a new value of the angular diameter
of Betelgeuse in the K band continuum. Our observations in the absorption lines
are well reproduced by a molecular layer at 1.2 stellar radii containing both
CO and HO. The visibilities at higher spatial frequencies are matching a
convection pattern in a RHD simulation.Comment: 13 pages, 11 figures, accepted for publication in Astronomy &
Astrophysics; Language editin
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