167 research outputs found
Parallax and masses of alpha Centauri revisited
Context. Despite the thorough work of van Leeuwen (2007), the parallax of
alpha Centauri is still far from being carved in stone. Any derivation of the
individual masses is therefore uncertain, if not questionable. And yet, that
does not prevent this system from being used for calibration purpose in several
studies. Aims. Obtaining more accurate model-free parallax and individual
masses of this system. Methods. With HARPS, the radial velocities are not only
precise but also accurate. Ten years of HARPS data are enough to derive the
complement of the visual orbit for a full 3D orbit of alpha Cen. Results. We
locate alpha Cen (743 mas) right where Hipparcos (ESA 1997) had put it, i.e.
slightly further away than derived by Soderhjelm (1999). The components are
thus a bit more massive than previously thought (1.13 and 0.97 Msun for A and B
respectively). These values are now in excellent agreement with the latest
asteroseismologic results.Comment: 4 pages, 3 figures, accepted in Astronomy & Astrophysic
The multiplicity of \phi\ Phe revisited
The chemically peculiar B star Phe was, until very recently,
considered a triple system, even though the data were not conclusive and the
orbits rather uncertain. Very recent results by Korhonen et al. (2013) provided
a revised orbit, different from the then available astrometric Hipparcos orbit.
Additional spectroscopic data, obtained with the BESO spectrograph at Cerro
Armazones, confirm the newly found orbit, even though the resulting radial
velocities do not allow to improve on the recent orbit. We combine the latter
with the Hipparcos measurements to secure the astrometric orbit, and derive the
inclination of the system. Using evolutionary tracks, we can finally constrain
all the parameters of the two components in this system.
We confirm the mass of the primary, 3 M, and find that the companion
has a mass of 0.9 M. The inclination of the system is , and is potentially eclipsing; we predict the time of the next
conjunction. Given that the eccentricity of the orbit and the exact value of
the semi-amplitude of the radial velocity relies on just one set of points, we
also urge observers to measure radial velocities at the next periastron passage
in April 2015.Comment: 5 papes, accepted as Research Note in Astronomy and Astrophysic
HD 112914 : A nearby one solar mass binary system
In paper 167 of his serie published in The Observatory, Griffin presented the
spectroscopic orbit of HD 112914, a late main sequence star. He also noticed
that this star, also known as HIP 63406, was one of the few for which the
DMSA/O Annex of the Hipparcos Catalogue derived an orbit prior to any
spectroscopic one. Albeit in agreement with each others, the astrometric orbit
was however determined with rather large uncertainties. Here, we have
reanalysed the Hipparcos Intermediate Astrometric Data (IAD) using
Griffin'spectroscopic orbit to obtain a much more precise astrometric orbit.
Several parameters of the HD 112914 system are now well constrained.Comment: 4 pages, 1 figure; accepted for publication in The Observator
Limits in astrometric accuracy induced by surface brightness asymmetries in red supergiant stars
Surface brightness asymmetries are a very common feature of stars. Among
other effects they cause a difference between the projected barycentre and
photocentre. The evolution of those surface features makes this difference
time-dependent. In some cases, e. g. for supergiant stars, the displacement can
be a non-negligible fraction of the star radius R, and if R>1 AU, of the
parallax. We investigate the impact of surface brightness asymmetries on both
the Gaia astrometric solution and the data processing flow with a theoretical
approach. We show that when the amplitude of the displacement is comparable to
the epoch astrometric precision, the resulting astrometric solution of a
genuine single star may be, in some cases, of low quality (with some parameters
up to 10 sigma off). In this case, we provide an analytical prediction of the
impact of the photocentre motion on both chi squared and the uncertainty in the
astrometric parameters. Non-single star solutions are found, if allowed for the
closest stars. A closer look at the parameters of the orbital solutions reveals
however that they are spurious (since the semi-major axis is smaller than
either its error or the stellar radius). It is thus possible to filter out
those spurious orbital solutions. Interestingly, for the stocastic solutions,
the stochastic noise appears to be a good estimate of the photocentric noise.Comment: Accepted for publication on Astronomy and Astrophysic
On the derivation of radial velocities of SB2 components: a "CCF vs TODCOR" comparison
The radial velocity (RV) of a single star is easily obtained from
cross-correlation of the spectrum with a template, but the treatment of
double-lined spectroscopic binaries (SB2s) is more difficult. Two different
approaches were applied to a set of SB2s: the fit of the cross-correlation
function with two normal distributions, and the cross-correlation with two
templates, derived with the TODCOR code. It appears that the minimum masses
obtained through the two methods are sometimes rather different, although their
estimated uncertainties are roughly equal. Moreover, both methods induce a
shift in the zero point of the secondary RVs, but it is less pronounced for
TODCOR. All-in-all the comparison between the two methods is in favour of
TODCOR.Comment: 5 pages, 4 figures, SF2A Conference 201
Spectroscopic Orbits for 15 Late-Type Stars
Spectroscopic orbital elements are determined for 15 stars with periods from 8 to 6528 days with six orbits computed for the first time. Improved astrometric orbits are computed for two stars and one new orbit is derived. Visual orbits were previously determined for four stars, four stars are members of multiple systems, and five stars have Hipparcos G designations or have been resolved by speckle interferometry. For the nine binaries with previous spectroscopic orbits, we determine improved or comparable elements. For HD 28271 and HD 200790, our spectroscopic results support the conclusions of previous authors that the large values of their mass functions and lack of detectable secondary spectrum argue for the secondary in each case being a pair of low-mass dwarfs. The orbits given here may be useful in combination with future interferometric and Gaia satellite observations
Spectroscopic Orbits for 15 Late-Type Stars
Spectroscopic orbital elements are determined for 15 stars with periods from 8 to 6528 days with six orbits computed for the first time. Improved astrometric orbits are computed for two stars and one new orbit is derived. Visual orbits were previously determined for four stars, four stars are members of multiple systems, and five stars have Hipparcos G designations or have been resolved by speckle interferometry. For the nine binaries with previous spectroscopic orbits, we determine improved or comparable elements. For HD 28271 and HD 200790, our spectroscopic results support the conclusions of previous authors that the large values of their mass functions and lack of detectable secondary spectrum argue for the secondary in each case being a pair of low-mass dwarfs. The orbits given here may be useful in combination with future interferometric and Gaia satellite observations
Spectroscopic Orbits for Late-type Stars. II
We have determined spectroscopic orbital elements for 13 systems—10 single-lined binaries and three double-lined binaries. For the three binaries with previously published spectroscopic orbits, we have computed improved or comparable elements. While two systems have relatively short periods between 10 and 19 days, the remaining systems have much longer periods ranging from 604 to 9669 days. One of the single-lined systems, HD 142640, shows both short-period and long-period velocity variations and so is triple. For three systems—HD 59380, HD 160933, and HD 161163—we have combined our spectroscopic results with Hipparcos astrometric observations to obtain astrometric orbits. For HD 14802 we have determined a joint orbital solution from spectroscopic velocities and interferometric observations. The orbits given here will be useful in combination with future interferometric and Gaia satellite observations
The LBV HR Car has a partner: Discovery of a companion with the VLTI
Luminous Blue Variables (LBVs) are massive stars caught in a post-main
sequence phase, during which they are losing a significant amount of mass. As,
on one hand, it is thought that the majority of massive stars are close
binaries that will interact during their lifetime, and on the other, the most
dramatic example of an LBV, Eta Car, is a binary, it would be useful to find
other binary LBVs. We present here interferometric observations of the LBV HR
Car done with the AMBER and PIONIER instruments attached to ESO's Very Large
Telescope Interferometer (VLTI). Our observations, spanning two years, clearly
reveal that HR Car is a binary star. It is not yet possible to constrain fully
the orbit, and the orbital period may lie between a few years and several
hundred years. We derive a radius for the primary in the system and possibly
resolve as well the companion. The luminosity ratio in the H-band between the
two components is changing with time, going from about 6 to 9. We also
tentatively detect the presence of some background flux which remained at the
2% level until January 2016, but then increased to 6% in April 2016. Our AMBER
results show that the emission line forming region of Br gamma is more extended
than the continuum emitting region as seen by PIONIER and may indicate some
wind-wind interaction. Most importantly, we constrain the total masses of both
components, with the most likely range being 33.6 and 45 solar masses. Our
results show that the LBV HR Car is possibly an Eta Car analog binary system
with smaller masses, with variable components, and further monitoring of this
object is definitively called for.Comment: A&A, in pres
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