arXiv subjects

Dimitri Pourbaix

Publications and source records attributed to Dimitri Pourbaix.

At least 19 recordsLinked to original sources

arXiv2023

Discovery of astrometric accelerations by dark companions in the globular cluster $\omega$ Centauri

We present results from the search for astrometric accelerations of stars in $\omega$ Centauri using 13 years of regularly-scheduled {\it Hubble Space Telescope} WFC3/UVIS calibration observations in the cluster core. The high-precision astrometry of $\sim$160\,000 sources was searched for significant deviations from linear proper motion. This led to the discovery of four cluster members and one foreground field star with compelling acceleration patterns. We interpret them as the result of the gravitational pull by an invisible companion and determined preliminary Keplerian orbit parameters, including the companion's mass. {For the cluster members} our analysis suggests periods ranging from 8.8 to 19+ years and dark companions in the mass range of $\sim$0.7 to $\sim$1.4$M_\mathrm{sun}$. At least one companion could exceed the upper mass-boundary of white dwarfs and can be classified as a neutron-star candidate.

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arXiv2023

An update of SB9 orbits using HERMES/Mercator radial velocities

The Gaia mission is delivering a large number of astrometric orbits for binary stars. By combining these with spectroscopic orbits for systems with two observable spectra (SB2), it is possible to derive the masses of both components. However, to get masses with a good accuracy requires accurate spectroscopic orbits, which is the major aim of the present paper. A subsidiary aim is to discover SB2 systems hiding among known SB1, and even though this search may often prove unsuccessful, the acquired radial velocities may be used anyway to improve the existing spectroscopic orbits. New radial velocities for 58 binary systems from the Ninth Catalogue of Spectroscopic Binary Orbits (SB9), obtained using the high-resolution HERMES spectrograph installed on the 1.2 m Mercator telescope, were used to possibly identify hitherto undetected SB2 systems. For SB1 systems with inaccurate orbits, these new radial-velocity measurements were used to improve the orbital accuracy. This paper provides 51 orbits (41 SB1 and 10 SB2) that have been improved with respect to the solution listed in the SB9 catalogue, out of the 58 SB9 orbits studied, which belong to 56 stellar systems. Among them, there are five triple and four quadruple systems. Despite the high resolution of HERMES, HIP 115142 A is the only system which we detected as a new SB2 system. The B component of the visual binary HIP 92726 has now been found to be a spectroscopic system as well, which makes HIP 92726 a newly discovered quadruple system (SB1+SB1). The high HERMES resolution allowed us moreover to better isolate the signature of the secondary component of HIP 12390, HIP 73182 and HIP 111170. More accurate masses have thus been derived for them. Among the 30 SB also present in Gaia Data Release 3 (DR3) and with periods shorter than the Gaia DR3 time span (1000 d), only 5 were flagged as binaries by DR3.

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arXiv2022

Gaia Data Release 3. Astrometric binary star processing

Context.The Gaia Early Data Release 3 contained the positions, parallaxes and proper motions of 1.5 billion sources, among which some did not fit well the "single star" model. Binarity is one of the causes of this. Aims. Four million of these stars were selected and various models were tested to detect binary stars and to derive their parameters. Methods. A preliminary treatment was used to discard the partially resolved double stars and to correct the transits for perspective acceleration. It was then investigated whether the measurements fit well with an acceleration model with or without jerk. The orbital model was tried when the fit of any acceleration model was beyond our acceptance criteria. A Variability-Induced Mover (VIM) model was also tried when the star was photometrically variable. A final selection has been made in order to keep only solutions that probably correspond to the real nature of the stars. Results. At the end, 338,215 acceleration solutions, about 165,500 orbital solutions and 869 VIM solutions were retained. In addition, formulae for calculating the uncertainties of the Campbell orbital elements from orbital solutions expressed in Thiele-Innes elements are given in an appendix.

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arXiv2020

Masses of the components of SB2 binaries observed with Gaia. V. Accurate SB2 orbits for 10 binaries and masses of the components of 5 binaries

Double-lined spectroscopic binaries (SB2s) are one of the main sources of stellar masses, as additional observations are only needed to give the inclinations of the orbital planes in order to obtain the individual masses of the components. For this reason, we are observing a selection of SB2s using the SOPHIE spectrograph at the Haute-Provence observatory in order to precisely determine their orbital elements. Our objective is to finally obtain masses with an accuracy of the order of one percent by combining our radial velocity (RV) measurements and the astrometric measurements that will come from the Gaia satellite. We present here the RVs and the re-determined orbits of 10 SB2s. In order to verify the masses we will derive from Gaia, we obtained interferometric measurements of the ESO VLTI for one of these SB2s. Adding the interferometric or speckle measurements already published by us or by others for 4 other stars, we finally obtain the masses of the components of 5 binary stars, with masses ranging from 0.51 to 2.2 solar masses, including main-sequence dwarfs and some more evolved stars whose location in the HR diagram has been estimated.

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arXiv2018

${\delta}$ MUS Revisited:Rectifying an 82 Yr-Old Mistake

The red giant ${\delta}$ Muscae is known since 1919 to be a spectroscopic binary, and the first and only orbit was determined in 1936, claiming the period to be 847 days. This was discrepant with the Hipparcos determined astrometric orbit. Using the latest data available for this object - leading to a 100 yr time-span - we show here that the correct period is 423 d, and are able for the first time to combine the spectroscopic orbit with the Hipparcos orbit. Using all the available information, we find that the $\sim$ 1.2 solar mass red giant must have a $\sim$ 0.3 - 0.4 solar mass M dwarf companion, and that the system will soon evolve towards a He WD binary system. Given its relatively short period, ${\delta}$ Muscae may be an ideal benchmark for testing astrometric orbits obtained by Gaia for very bright stars.

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arXiv2017

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.

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arXiv2017

The programme "accurate masses for SB2 components"

Accurate stellar masses are requested in order to improve our understanding of stellar interiors, but they are still rather rare. Fortunately, the forthcoming Gaia Mission will provide astrometric measurements permitting the derivation of the orbital inclinations of nearby binaries which are also observed as double-lined spectroscopic binaries (SB2s) with ground-based telescopes. A programme of radial velocity (RV) measurements was initiated in 2010 with the Sophie spectrograph of the Haute-Provence observatory in order to derive accurate SB2 orbits for a large set of stars. Therefore, combined SB2+astrometric orbits will be derived thanks to Gaia, and masses with errors around 1~\% are expected for both components. The programme includes 70 SB2s, and the accurate SB2 orbits of 24 of them were already derived. In addition, two complementary programmes devoted to southern stars or to late-type dwarf stars were also initiated with the HERMES and the CARMENES spectrographs, respectively. Interferometric measurements were obained with the VLTI/PIONIER for 7 SB2s, and were taken from other sources for 4 others. Currently, combined "visual binary" (VB) +SB2 solutions were derived for 7 binaries, leading to the masses of the components and to the parallaxes. The parallaxes from the Hipparcos 2 catalogue were corrected for orbital motion and compared to our solution, confirming the high quality of Hipparcos 2.

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arXiv2016

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.

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arXiv2016

Masses of the components of SB2 binaries observed with Gaia. III. Accurate SB2 orbits for 10 binaries and masses of HIP 87895

In anticipation of the Gaia astrometric mission, a large sample of spectroscopic binaries has been observed since 2010 with the SOPHIE spectrograph at the Haute--Provence Observatory. Our aim is to derive the orbital elements of double-lined spectroscopic binaries (SB2s) with an accuracy sufficient to finally obtain the masses of the components with relative errors as small as 1 % when the astrometric measurements of Gaia are taken into account. In this paper we present the results from five years of observations of 10 SB2 systems with periods ranging from 37 to 881 days. Using the TODMOR algorithm we computed radial velocities from the spectra, and then derived the orbital elements of these binary systems. The minimum masses of the components are then obtained with an accuracy better than 1.2 % for the ten binaries. Combining the radial velocities with existing interferometric measurements, we derived the masses of the primary and secondary components of HIP 87895 with an accuracy of 0.98 % and 1.2 % respectively.

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arXiv2016

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.

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arXiv2015

Detecting the Companions and Ellipsoidal Variations of RS CVn Primaries: I. sigma Geminorum

To measure the properties of both components of the RS CVn binary sigma Geminorum (sigma Gem), we directly detect the faint companion, measure the orbit, obtain model-independent masses and evolutionary histories, detect ellipsoidal variations of the primary caused by the gravity of the companion, and measure gravity darkening. We detect the companion with interferometric observations obtained with the Michigan InfraRed Combiner (MIRC) at Georgia State University's Center for High Angular Resolution Astronomy (CHARA) Array with a primary-to-secondary H-band flux ratio of 270+/-70. A radial velocity curve of the companion was obtained with spectra from the Tillinghast Reflector Echelle Spectrograph (TRES) on the 1.5-m Tillinghast Reflector at Fred Lawrence Whipple Observatory (FLWO). We additionally use new observations from the Tennessee State University Automated Spectroscopic and Photometric Telescopes (AST and APT, respectively). From our orbit, we determine model-independent masses of the components (M_1 = 1.28 +/- 0.07 M_Sun, M_2 = 0.73 +/- 0.03 M_Sun), and estimate a system age of 5 -/+ 1 Gyr. An average of the 27-year APT light curve of sigma Gem folded over the orbital period (P = 19.6027 +/- 0.0005 days) reveals a quasi-sinusoidal signature, which has previously been attributed to active longitudes 180 deg apart on the surface of sigma Gem. With the component masses, diameters, and orbit, we find that the predicted light curve for ellipsoidal variations due to the primary star partially filling its Roche lobe potential matches well with the observed average light curve, offering a compelling alternative explanation to the active longitudes hypothesis. Measuring gravity darkening from the light curve gives beta < 0.1, a value slightly lower than that expected from recent theory.

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arXiv2015

The Gaia Mission, Binary Stars and Exoplanets

On the 19th of December 2013, the Gaia spacecraft was successfully launched by a Soyuz rocket from French Guiana and started its amazing journey to map and characterise one billion celestial objects with its one billion pixel camera. In this presentation, we briefly review the general aims of the mission and describe what has happened since launch, including the Ecliptic Pole scanning mode. We also focus especially on binary stars, starting with some basic observational aspects, and then turning to the remarkable harvest that Gaia is expected to yield for these objects.

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arXiv2014

Masses of the components of SB2 binaries observed with Gaia. I. Selection of the sample and mass ratios of 20 new SB2s discovered with Sophie

In anticipation of the Gaia astrometric mission, a large sample of spectroscopic binaries is being observed since 2010 with the Sophie spectrograph at the Haute--Provence Observatory. Our aim is to derive the orbital elements of double-lined spectroscopic binaries (SB2s) with an accuracy sufficient to finally obtain the masses of the components with relative errors as small as 1% when the astrometric measurements of Gaia are taken into account. Simultaneously, the luminosities of the components in the Gaia photometric band G will also be obtained. Our observation program started with 200 SBs, including 152 systems that were only known as single-lined. Thanks to the high efficiency of the Sophie spectrograph, an additional component was found for 25 SBs. After rejection of 5 multiple systems, 20 new SB2s were retained, including 8 binaries with evolved primary, and their mass ratios were derived. Our final sample contains 68 SB2s, including 2 late-type giants and 10 other evolved stars.

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arXiv2013

The multiplicity of ϕ Phe revisited

The chemically peculiar B star $ϕ$ 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$_\odot$, and find that the companion has a mass of 0.9 M$_\odot$. The inclination of the system is $i=93^{\circ} \pm 4.7^{\circ}$, 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.

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arXiv2011

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.

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arXiv2003

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.

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arXiv2002

A Planet at 5 AU Around 55 Cancri

We report precise Doppler shift measurements of 55 Cancri (G8V) obtained from 1989 to 2002 at Lick Observatory. The velocities reveal evidence for an outer planetary companion to 55 Cancri orbiting at 5.5 AU. The velocities also confirm a second, inner planet at 0.11 AU. The outer planet is the first extrasolar planet found that orbits near or beyond the orbit of Jupiter. It was drawn from a sample of ~50 stars observed with sufficient duration and quality to detect a giant planet at 5 AU, implying that such planets are not rare. The properties of this jupiter analog may be compared directly to those of the Jovian planets in our Solar System. Its eccentricity is modest, e=0.16, compared with e=0.05 for both Jupiter and Saturn. Its mass is at least 4.0 jupiter masses (M sin i). The two planets do not perturb each other significantly. Moreover, a third planet of sub-Jupiter mass could easily survive in between these two known planets. Indeed a third periodicity remains in the velocity measurements with P = 44.3 d and a semi-amplitude of 13 m/s. This periodicity is caused either by a third planet at a=0.24 AU or by inhomogeneities on the stellar surface that rotates with period 42 d. The planet interpretation is more likely, as the stellar surface is quiet, exhibiting log(R'_{HK}) = -5.0 and brightness variations less than 1 millimag, and any hypothetical surface inhomogeneity would have to persist in longitude for 14 yr. Even with all three planets, an additional planet of terrestrial--mass could orbit stably at ~1 AU. The star 55 Cancri is apparently a normal, middle-aged main sequence star with a mass of 0.95 solar masses, rich in heavy elements ([Fe/H] = +0.27). This high metallicity raises the issue of the relationship between its age, rotation, and chromosphere.

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arXiv2001

A probable planetary companion to HD 39091 from the Anglo-Australian Planet Search

We report the detection of a extrasolar planet candidate orbiting the G1 V star HD 39091. The orbital period is 2083 d and the eccentricity is 0.62. With a minimum (M sin $i$) mass of 10.3 M$_{\rm JUP}$ this object falls near the high mass end of the observed planet mass function, and may plausibly be a brown dwarf. Other characteristics of this system, including orbital eccentricity and metallicity, are typical of the well populated class of radial velocity planets in eccentric orbits around metal-rich stars.

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