J/A+A/609/A8Close encounters to the Sun in Gaia DR1 (Bailer-Jones, 2018)

The completeness-corrected rate of stellar encounters with the Sun from the first Gaia data release. Bailer-Jones C.A.L. <Astron. Astrophys. 609, A8 (2018)> =2018A&A...609A...8B (SIMBAD/NED BibCode)ADC_Keywords: Stars, nearby ; Positional data ; Solar systemKeywords: Oort cloud - solar neighbourhood - stars: kinematics and dynamics - surveys - methods: statistical - methods: analyticalAbstract: I report on close encounters of stars to the Sun found in the first Gaia data release (GDR1). Combining Gaia astrometry with radial velocities of around 320 000 stars drawn from various catalogues, I integrate orbits in a Galactic potential to identify those stars which pass within a few parsecs. Such encounters could influence the solar system, for example through gravitational perturbations of the Oort cloud. 16 stars are found to come within 2pc (although a few of these have dubious data). This is fewer than were found in a similar study based on Hipparcos data, even though the present study has many more candidates. This is partly because I reject stars with large radial velocity uncertainties (>10km/s), and partly because of missing stars in GDR1 (especially at the bright end). The closest encounter found is Gl 710, a K dwarf long-known to come close to the Sun in about 1.3Myr. The Gaia astrometry predict a much closer passage than pre-Gaia estimates, however: just 16000AU (90% confidence interval: 10000-21000AU), which will bring this star well within the Oort cloud. Using a simple model for the spatial, velocity, and luminosity distributions of stars, together with an approximation of the observational selection function, I model the incompleteness of this Gaia-based search as a function of the time and distance of closest approach. Applying this to a subset of the observed encounters (excluding duplicates and stars with implausibly large velocities), I estimate the rate of stellar encounters within 5pc averaged over the past and future 5Myr to be 545±59Myr^{-1}. Assuming a quadratic scaling of the rate within some encounter distance (which my model predicts), this corresponds to 87±9Myr-1 within 2pc. A more accurate analysis and assessment will be possible with future Gaia data releases.Description: The table gives the perihelion (closest approach) parameters of stars in the Gaia-DR1 TGAS catalogue which are found by numerical integration through a Galactic potential to approach within 10pc of the Sun. These parameters are the time (relative to the Gaia measurement epoch), heliocentric distance, and heliocentric speed of the star at perihelion. Uncertainties in these have been calculated by a Monte Carlo sampling of the data to give the posterior probability density function (PDF) over the parameters. For each parameter three summary values of this PDF are reported: the median, the 5% lower bound, the 95% upper bound. The latter two give a 90% confidence interval. The table also reports the probability that each star approaches the Sun within 0.5, 1.0, and 2.0pc, as well as the measured parallax, proper motion, and radial velocity (plus uncertainties) of the stars. Table 3 in the article lists the first 20 lines of this data table (stars with median perihelion distances below 2pc). Some stars are duplicated in this table, i.e. there are rows with the same ID, but different data. Stars with problematic data have not been removed, so some encounters are not reliable. Most IDs are Tycho, but in a few cases they are Hipparcos.File Summary:

FileName Lrecl Records Explanations

ReadMe 80 . This file encount.dat 166 646 Perihelion parameters for all objects with a median perihelion distance below 10pc, in order of increasing median perihelion distance (table3)

See also: I/239 : The Hipparcos and Tycho Catalogues (ESA 1997) I/259 : The Tycho-2 Catalogue (Hog+ 2000) I/337 : Gaia DR1 (Gaia Collaboration, 2016)Byte-by-byte Description of file: encount.dat

Bytes Format Units Label Explanations

1- 12 A12 --- ID Tycho or Hipparcos identifier 14- 15 I2 --- Cat [1/10] Radial velocity catalogue identifier (1) 17- 25 F9.1 kyr tphmed Median perihelion time 27- 35 F9.1 kyr tphlo 5% bound on perihelion time distribution 37- 45 F9.1 kyr tphup 95% bound on perihelion time distribution 47- 53 F7.3 pc dphmed Median perihelion distance 55- 61 F7.3 pc dphlo 5% bound on perihelion distance distribution 63- 69 F7.3 pc dphup 95% bound on perihelion distance distribution 71- 78 F8.2 km/s vphmed Median perihelion speed 80- 87 F8.2 km/s vphlo 5% bound on perihelion speed distribution 89- 96 F8.2 km/s vphup 95% bound on perihelion speed distribution 98-100 I3 --- f0.5 Probability star approaches within 0.5pc 102-104 I3 --- f1.0 Probability star approaches within 1.0pc 106-108 I3 --- f2.0 Probability star approaches within 2.0pc 110-118 F9.3 mas plx Parallax 120-128 F9.3 mas e_plx Parallax uncertainty (1 sigma) 130-138 F9.3 mas/yr pm Proper motion 140-148 F9.3 mas/yr e_pm Proper motion uncertainty (1 sigma) 150-157 F8.2 km/s RV Radial velocity 159-166 F8.2 km/s e_RV Radial velocity uncertainty (1 sigma)

Note (1): Catalogs as follows: 1 = RAVE-DR5, Kunder et al. (2017, Cat. III/279) 2 = GCS2001, Casagrande et al. (2011,Cat. J/A+A/530/A138) 3 = Pulkovo, Gontcharov, 2006, Cat. III/252 4 = Famey et al., 2005, Cat. J/A+A/430/165 5 = Duflot et al., 1995, Cat. III/190 6 = Barbier-Brossat & Figon, 2000, Cat. III/213 7 = Maladora et al., 2000, Cat. III/249 8 = Maldonado et al., 2010, Cat. J/A+A/521/A12 9 = Fehrenbach et al., 1997, Cat. J/A+AS/124/255 10 = APOGEE2, SDSS Collaboration (2016, in prep.) 11 = Gaia-ESO-DR2, https://www.gaia-eso.eu 12 = GALAH, Martell et al., 2010, Cat. J/MNRAS/465/3203

Acknowledgements: Coryn Bailer-Jones, calj(at)mpia.de(End)Coryn Bailer-Jones [MPIA, Heidelberg], Patricia Vannier [CDS] 17-Aug-2017

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