1,710 research outputs found
The angular sizes of dwarf stars and subgiants - Non-linear surface brightness relations in BVRcIc from interferometry
Context: The prediction of stellar angular diameters from broadband
photometry plays an important role for different applications. In particular,
long-baseline interferometry, gravitational microlensing, extrasolar planet
transits, and many other observing techniques require accurate predictions of
the angular size of stars. These predictions are based on the surface
brightness-colour (SBC) relations. Aims: Our goal is to calibrate
general-purpose SBC relations using visible colours, the most commonly
available data for most stars. Methods: We compiled the existing long-baseline
interferometric observations of nearby dwarf and subgiant stars and the
corresponding broadband photometry in the Johnson B V and Cousins Rc Ic bands.
We then adjusted polynomial SBC models to these data. Results: Due to the
presence of spectral features that depend on the effective temperature, the SBC
relations are usually not linear for visible colours. We present polynomial
fits that can be employed with BVRcIc based colours to predict the
limb-darkened angular diameters (i.e. photospheric) of dwarf and subgiant stars
with a typical accuracy of 5%. Conclusions: The derived polynomial relations
provide a satisfactory approximation to the observed surface brightness of
nearby dwarfs and subgiants. For distant stars, the interstellar reddening
should be taken into account, and will usually introduce an additional
uncertainty to the predicted angular diameters.Comment: 5 pages, accepted for publication in Astronomy & Astrophysics.
Astronomy and Astrophysics (2008) Accepte
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
Interferometry, spectroscopy and astrometry of the bright eclipsing system Delta Velorum
The bright southern star Delta Vel is a multiple system comprising at least
three stars. Its brightest component, Delta Vel A, was identified in 2000 as
one of the brightest eclipsing system in the sky. Its eclipses are easily
observable with the unaided eye, a remarkable property shared only by Algol,
Beta Aur, Alpha CrB and Psi Cen. We determined dynamical masses from a
combination of spectroscopy, high-precision astrometry of the orbits of Aab-B
and Aa-Ab using adaptive optics (VLT/NACO) and optical interferometry
(VLTI/AMBER). The main eclipsing component is a pair of A-type stars in rapid
rotation. We modeled the photometric and radial velocity measurements of the
eclipsing pair Aa-Ab using a self consistent method based on physical
parameters (mass, radius, luminosity, rotational velocity). From this modeling,
we derive the fundamental parameters of the eclipsing stars with a typical
accuracy of 1%. We find that they have similar masses, respectively 2.43 +/-
0.02 and 2.27 +/- 0.02 Msun. The physical parameters of the tertiary component
(Delta Vel B) are also derived, although to a lower accuracy, as well as the
parallax of the system, 39.8 +/- 0.4 mas. This value is in satisfactory
agreement (-1.2 sigma) with the Hipparcos parallax of the system (pi_Hip =40.5
+/- 0.4 mas).Comment: 4 pages, 5 figures, in GREAT-ESF Workshop 'Orbital Couples: "Pas de
Deux" in the Solar System and the Milky Way', Paris, IMCCE proceedings, in
pres
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
The nearby AGB star L2 Puppis: the birth of a planetary nebula ?
Adaptive optics observations in the infrared (VLT/NACO, Kervella et al. 2014)
and visible (VLT/SPHERE, Kervella et al. 2015) domains revealed that the nearby
AGB star L2 Pup (d=64 pc) is surrounded by a dust disk seen almost edge-on.
Thermal emission from a large dust "loop" is detected at 4 microns up to more
than 10 AU from the star. We also detect a secondary source at a separation of
32 mas, whose nature is uncertain. L2 Pup is currently a relatively "young" AGB
star, so we may witness the formation of a planetary nebula. The mechanism that
breaks the spherical symmetry of mass loss is currently uncertain, but we
propose that the dust disk and companion are key elements in the shaping of the
bipolar structure. L2 Pup emerges as an important system to test this
hypothesis.Comment: 6 pages, 3 figures, Proceedings of the Physics of Evolved Stars
conference, 8-12 June 2015, Nice, Franc
The environment of the fast rotating star Achernar - Thermal infrared interferometry with VLTI/MIDI and SIMECA modeling
Context: As is the case of several other Be stars, Achernar is surrounded by
an envelope, recently detected by near-IR interferometry.
Aims: We search for the signature of circumstellar emission at distances of a
few stellar radii from Achernar, in the thermal IR domain.
Methods: We obtained interferometric observations on three VLTI baselines in
the N band (8-13 mic), using the MIDI instrument.
Results: From the measured visibilities, we derive the angular extension and
flux contribution of the N band circumstellar emission in the polar direction
of Achernar. The interferometrically resolved polar envelope contributes 13.4
+/- 2.5 % of the photospheric flux in the N band, with a full width at half
maximum of 9.9 +/- 2.3 mas (~ 6 Rstar). This flux contribution is in good
agreement with the photometric IR excess of 10-20% measured by fitting the
spectral energy distribution. Due to our limited azimuth coverage, we can only
establish an upper limit of 5-10% for the equatorial envelope. We compare the
observed properties of the envelope with an existing model of this star
computed with the SIMECA code.
Conclusions: The observed extended emission in the thermal IR along the polar
direction of Achernar is well reproduced by the existing SIMECA model. Already
detected at 2.2mic, this polar envelope is most probably an observational
signature of the fast wind ejected by the hot polar caps of the star.Comment: A&A Letter, in pres
Interferometric observations of eta Carinae with VINCI/VLTI
Context: The bright star eta Carinae is the most massive and luminous star in
our region of the Milky Way. Though it has been extensively studied using many
different techniques, its physical nature and the mechanism that led to the
creation of the Homunculus nebula are still debated. Aims: We aimed at
resolving the central engine of the eta Carinae complex in the near-infrared on
angular scales of a few milliarcseconds. Methods: We used the VINCI instrument
of the VLTI to recombine coherently the light from two telescopes in the K
band. Results: We report a total of 142 visibility measurements of eta Car,
part of which were analyzed by Van Boekel et al. (2003). These observations
were carried out on projected baselines ranging from 8 to 112 meters in length,
using either two 0.35 m siderostats or two 8-meter Unit Telescopes. These
observations cover the November 2001 - January 2004 period. Conclusions: The
reported visibility data are in satisfactory agreement with the recent results
obtained with AMBER/VLTI by Weigelt et al. (2006), asuming that the flux of eta
Car encircled within 70 mas reaches 56% of the total flux within 1400 mas, in
the K band. We also confirm that the squared visibility curve of eta Car as a
function of spatial frequency follows closely an exponential model.Comment: Accepted for publication in A&A as a Research not
Deep imaging survey of the environment of Alpha Centauri - II. CCD imaging with the NTT-SUSI2 camera
Context: The nearby pair of solar-type stars Alpha Centauri is a favorable
target for an imaging search for extrasolar planets. Indications exist that the
gravitational mass of Alpha Cen B could be higher than its modeled mass, the
difference being consistent with a substellar companion of a few tens of
Jupiter masses. However, Alpha Centauri usually appears in star catalogues
surrounded by a large void area, due to the strong diffused light. Aims: We
searched for faint comoving companions to Alpha Cen located at angular
distances of the order of a few tens of arcseconds, up to 2-3 arcmin. As a
secondary objective, we built a catalogue of the detected background sources.
Methods: In order to complement our adaptive optics search at small angular
distances (Paper I), we used atmosphere limited CCD imaging from the NTT-SUSI2
instrument in the Bessel V, R, I, and Z bands. Results: We present the results
of our search in the form of a catalogue of the detected objects inside a 5.5
arcmin box around this star. A total of 4313 sources down to mV~24 and mI~22
were detected from this wide-field survey. We extracted the infrared photometry
of part of the detected sources from archive images of the 2MASS survey (JHK
bands). We investigate briefly the nature of the detected sources, many of them
presenting extremely red color indices (V-K > 14). Conclusions: We did not
detect any companion to Alpha Centauri between 100 and 300 AU, down to a
maximum mass of ~15 times Jupiter. We also mostly exclude the presence of a
companion more massive than 30 MJup between 50 and 100 AU.Comment: Accepted for publication as a Research Note in A&
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
Experimental evidence and theoretical modeling of two-photon absorption dynamics in the reduction of intensity noise of solid-state Er:Yb lasers
A theoretical and experimental investigation of the intensity noise reduction
induced by two-photon absorption in a Er,Yb:Glass laser is reported. The time
response of the two-photon absorption mechanism is shown to play an important
role on the behavior of the intensity noise spectrum of the laser. A model
including an additional rate equation for the two-photon-absorption losses is
developed and allows the experimental observations to be predicted
- …