OLED - Gaia_nss

OBJETIVO

PROCEDIMIENTOS

ANÁLISIS

TÉCNICAS

PREDICCIONES

OBSERVADORES

REPORTAR

RESULTADOS

DOCUMENTOS

ENLACES

CAMPAÑAS

BOLETINES

Project: Analysis of Gaia DR3 non-single star catalogue with lunar occultations

Abstract: In this project we aim to analyse a subset of the non-single star tables of the Gaia DR3 catalogue, searching for confirmation of duplicities. Also, we will obtain the astrometry of some stars, using the high angular resolution of the lunar occultation (LO) technique.

INTRODUCTION

Analysis of Gaia data has revealed a significant number of unresolved stars that do not fit into the standard single-star model [1, 2]. These deviations are attributed to duplicity. Up to now, more than 800000 stars have been identified in this category (stars that could only be partially resolved during the Gaia satellite’s operational period were excluded from the list; no doubt these sources will undergo further examination by the Gaia team and the scientific community in the coming years, significantly increasing the count of binary stars).

Consequently, stars in the Gaia DR3 non-single star catalogue are considered unresolved binary stars. Yet their binary nature is inferred from the temporal changes in the position of the photocenter of the source. These changes are sometimes periodic and rapid enough to allow for the determination of the orbital parameters of the binary star. In other cases only an accelerated motion can be detected, from which duplicity can be inferred indirectly.

However, there is a need for an independent confirmation of the binary nature of these stars, and the LO technique is ideally suited for this purpose down to apparent separations of a few tens a milliarcseconds (mas).

Stars in the non-single category can be classified into three groups: astrometric binaries, spectroscopic binaries and eclipsing binaries. The first includes unresolved sources for which an astrometric motion can be detected. The second corresponds to sources exhibiting Doppler shifted splitted spectral lines, while in the third group stars show variations in their light curves compatible with eclipses. This information can give rise to essentially two types of solutions: orbital solutions, for which orbital elements can be calculated, and accelerated solutions, i.e. solutions that do not cover a complete orbit but unveil nonlinear motion in three-dimensional space, presumably caused by a star or planetary companion.

Since the non-single stars of the Gaia DR3 catalogue are not resolved pairs, they correspond to new binary stars, not catalogued in the USNO WDS catalogue of double stars. The published parameters, orbital elements for orbital solutions, and accelerations and derivatives of accelerations (jerks) for accelerated solutions, were obtained with data from the first 34 months of operation of the satellite (2014-2017). More advanced versions of the catalogue, DR4 and DR5, and therefore more precise and extensive sets of non-single stars, are expected for the upcoming years. DR5 will be delivered not long after the end of operations of the Gaia satellite (2025). Therefore, not only the number of double stars will increase in the near future, but also the accuracy in their measured motions.

Nevertheless, LO observations of these stars are important as they represent a direct method to directly unveil duplicity of stars that cannot be resolved by Gaia. These observations may lead in many cases to accurate astrometry that can confirm or discard many of the solutions obtained by the Gaia analysts.

Scientists involved in the Gaia project have expressed great interest in the LO technique and strongly support the activity of the OLED observers to analyse these subset of Gaia stars.

A SUBSET OF THE GAIA CATALOGUE FOR THE OLED PROJECT

A first step is the construction of a catalogue with interesting sources, to be used as targets for the OLED observations. A subset of the Gaia DR3 catalogue was first extracted from the Gaia Archive, using the following conditions for the stars:

stars are within the Zodiacal band;
stars have visual magnitude m ≤ 12.0;
the non_single_star parameter of the stars is > 0.

The latter condition encompasses stars with a RUWE (Renormalised Unit Weight Error) value larger than 1.4 (this is considered as an upper tolerance for the fit to the single-star model of Gaia to be considered as bad). The resulting list was further cross-matched with the XZ catalogue of Zodiacal stars. The total number of stars, collected in a file called Merge_Gaia-NSS_XZ, was 11228. Stars with a nonzero non_single_star flag fall into one of the following four tables of the Gaia DR3 catalogue:

nss_two_body_orbit: unresolved stars with calculated orbit (using astrometric and spectroscopic data). Stars in the Merge_Gaia-NSS_XZ file were cross-matched with this table, resulting in 1828 stars. Since the orbital elements of these stars have been derived, it is possible to obtain the apparent semimajor axis a of the orbits (physical semimajor axis multiplied by the parallax).

The figure above shows the distribution of a in the sample. It is strongly peaked at roughly a=1 mas. The mean value is 2.2 mas. The vertical line is an approximate lower limit for the resolution of the lunar-occultation (LO) technique, set to σ=10 mas [3]. The number of stars with a > 10 mas is only 17. Therefore, only very few stars with calculated orbit can be analysed in the context of the OLED project. Ultimately this is due to the short periods of the pairs, which allow for monitoring of the binary stars for more than one orbit, leading to an accurate calculation of their orbit; but short periods are usually associated to very close angular separations between the components, rendering the LO technique useless.

nss_acceleration_astro: stars showing accelerated motion, possibly distant and relatively long-period binary stars (too long for the limited time span of the Gaia satellite), but not necessarily so. Cross-matching of Merge_Gaia-NSS_XZ with this table gives 4166 stars. For these stars, radial velocity, motion in right ascension, motion in declination, and projected accelerations and, in some cases, projected jerks (time derivatives of the accelerations), are provided in the tables. These stars are worth studying by LO observations.
nss_vim_fl: stars which are compatible with a variability-induced mover solution, i.e. are photometrically variable. The table provides values for the coordinates of the variable component of the flux relative to the position of the photocentre. No solutions of this type are found in the Merge_Gaia-NSS_XZ.
nss_non_linear_spectros: stars which show a nonlinear trend in the radial velocity. This set has not been considered in our project.

The total number of non-single stars contained in our catalogue is 4182, collected in a file called nss_Zodiac_mag12.param, henceforth called OLED_Gaia. The file includes identification of the stars in the Gaia DR3 and XZ catalogues, positions and velocities, and type of solution in the non-single star catalogue of Gaia DR3. A sample of the text file is shown below. The OLED webpage will contain a link to the complete database, with more star properties. Predictions for occultations, based on the OLED_Gaia catalogue, will be published in the OLED webpage from December 2023 onwards. Important occultations will be announced by email in the previous days.

ACTIONS

The aim of the project is to record lunar occultations of stars in the OLED_Gaia catalogue. The emphasis is on observations of specific occultations by multiple observers, so that, if possible, positions can be derived at a particular epoch. Only occultations with favourable observational conditions will be selected. These include: stars brighter than visual magnitude 10.0, occultations at the dark limb, preferably with impact factors b < 0.8 and with a fraction of illuminated apparent area for the Moon below 80% (so that the lunar glow is not high). Needless to say these conditions severely limit the number of possible observations by OLED observers. However, at least in a first stage of the project, we should target easy occultations to maximise the chances of having "clean" light curves from which accurate contact times can be estimated.

The observational technique is otherwise similar to the standard one applied in the OLED project. If possible, sampling times should be pushed down a bit, with 10 − 30 ms (i.e. 33 − 100 fps) being a reasonable interval. The choice of the brightest stars possible in the list of proposed targets should facilitate the use of shorter exposures. In case this interval cannot be reached, choose the shortest possible sampling time for your equipment and conditions, maximising S/N to ensure a "clean" light curve.

POSSIBLE SCENARIOS
Obviously most stars of the OLED_Gaia catalogue will be difficult targets, and stepped light curves will be obtained only occasionally. These cases will provide direct evidence that a star is indeed double, and even a few positive observations will constitute a successful outcome of the project, greatly augmented if an accurate astrometric measurement can be extracted from our data.

We should stress that the binary nature of most stars of the OLED_Gaia catalogue (i.e. accelerated solutions) is only hinted from fits of observational data of the Gaia satellite to nonlinear paths in the celestial sphere [4]; some of these results may be inaccurate so the status of a source as a double star is uncertain. Our results could set limits to the accuracy of Gaia results and therefore be useful for future processing by Gaia analysts of sources as binary objects.

It is not possible to anticipate the fraction of successful observations: only time will tell. But it is true that even nonstepped light curves may be useful if a sufficient number of high-quality light curves can be collected for a given star: an accurate position for the star could be obtained. This is very important, since this position may be contrasted with nonlinear trends in the apparent motion of the star as derived by Gaia, thus drawing relevant conclusions from this comparison.

In many cases the accuracy of the OLED measurement will not be enough to reach any conclusion: Either no duplicity is found, or the difference between the astrometric position derived from the observations and the Gaia position is below the uncertainty σ. In addition, it might the case that duplicity is detected, but that data are not of sufficient quality for a reliable quantitative astrometricdetermination. Even in this case detection of duplicity for an unresolved star of the Gaia catalogue would be a great success.

References

[1] Gaia Data Release 3: Stellar multiplicity, a teaser for the hidden treasure, F. Arenou et al. (Gaia Collaboration), A&A 674, A34 (2023).

[2] Gaia Data Release 3: Astrometric binary star processing, J.-L. Halbwachs et al. A&A 674, A9 (2023).

[3] There is a 5 mas contribution from the lunar-limb calculation in the OLED reduction procedure, plus uncertanties related to finite sampling rates and uncertainties in contact-time estimates from light curves. Higher rates are counterproductive for double-star amateur-rank work, due to diffraction effects that cannot be disentangled from signal noise.

[4] Use of the nonlinear constants for extrapolating the positions at the Gaia epoch to the future should be avoided as they are not meant to be coefficients of a Taylor time expansion about the Gaia epoch; only positions and velocities should be used. These nonlinear parameters (accelerations and jerks) only give an idea of how different from a linear path the trajectory is expected to be. For a discussion of this point, see The Hipparcos and Tycho Catalogues. Astrometric and Photometric Star Catalogues derived from the ESA Hipparcos Space Astrometry Mission, ESA, 1997.

Page updated

Report abuse