Stars which pass close to the Sun can perturb the Oort cloud, injecting comets into the inner solar system where they may collide with the Earth. Using van Leeuwen's re-reduction of the Hipparcos data complemented by the original Hipparcos and Tycho-2 catalogues, along with recent radial velocity surveys, I integrate the orbits of over 50 000 stars through the Galaxy to look for close encounters. The search uses a Monte Carlo simulation over the covariance of the data in order to properly characterize the uncertainties in the times, distances, and speeds of the encounters. I show that modelling stellar encounters by assuming instead a linear relative motion produces, for many encounters, inaccurate and biased results. I find 42, 14, and 4 stars which have encounter distances below 2, 1, and 0.5 pc respectively, although some of these stars have questionable data. Of the 14 stars coming within 1 pc, 5 were found by at least one of three previous studies (which found a total of 7 coming within 1 pc). The closest encounter appears to be Hip 85605, a K or M star, which has a 90% probability of coming between 0.04 and 0.20 pc between 240 and 470 kyr from now (90% Bayesian confidence interval). However, its astrometry may be incorrect, in which case the closest encounter found is the K7 dwarf GL 710, which has a 90% probability of coming within 0.10-0.44 pc in about 1.3 Myr. A larger perturbation may have been caused by gamma Microscopii, a G6 giant with a mass of about 2.5 Msol, which came within 0.35-1.34 pc (90% confidence interval) around 3.8 Myr ago.

- Article: Astronomy & Astrophysics, 575, A35 (2015) (accepted 11 December 2014)

[PDF] [ADS] [arXiv] [journal link] - Full version of Table 3 (1548 rows + header line, ascii file) (at CDS)
- Additional encounter data (1704 rows + header line, ascii file). Data on all stars with dph_nom < 10 pc (see section 4.1 and Table 2).

10 columns (units are kyr, pc, km/s):- 1: Hipparcos ID + i/10 where i indicates source catalogue: 1 GCS; 2 RAVE; 3 Pulkovo; 4 xhip
- 2-4: perihelion time, distance, speed according to the linear motion approximation
- 5-7: perihelion time, distance, speed for the nominal data
- 8-10: mean of distributions over the perihelion time, distance, speed (as reported in Table 3)

- Supplementary information comparing the mean to the median as estimators of the perihelion distributions

- Document and plots
- Data file (1548 rows + header line, ascii file). 4 columns: Hip ID + i/10; median perihelion time (kyr), distance (pc), speed (km/s)

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- A (12 week) BSc thesis on this topic by Victor Ksoll

Coryn Bailer-Jones, calj at mpia-hd.mpg.de

Last modified: 4 September 2015