J/A+A/682/A92 Distinguishing exoplanet comp. from field stars (Herz+, 2024)
Distinguishing exoplanet companions from field stars in direct imaging using
Gaia astrometry.
Herz P., Samland M., Bailer-Jones C.A.L.
<Astron. Astrophys. 682, A92 (2024)>
=2024A&A...682A..92H 2024A&A...682A..92H (SIMBAD/NED BibCode)
ADC_Keywords: Stars, double and multiple ; Exoplanets ; Photometry, infrared
Keywords: methods: statistical - techniques: high angular resolution -
planets and satellites: detection - stars: early-type
Abstract:
Direct imaging searches for exoplanets around stars detect many
spurious candidates that are in fact background field stars. To help
distinguish these from genuine companions, multi-epoch astrometry can
be used to identify a common proper motion with the host star.
Although this is frequently done, many approaches lack an appropriate
model for the motions of the background population, or do not use a
statistical framework to properly quantify the results. Here we use
Gaia astrometry combined with 2MASS photometry to model the parallax
and proper motion distributions of field stars around exoplanet host
stars as a function of candidate magnitude. We develop a
likelihood-based method that compares the positions of a candidate at
multiple epochs with the positions expected under both this field star
model and a co-moving companion model. Our method propagates the
covariances in the Gaia astrometry and the candidate positions. True
companions are assumed to have long periods compared to the
observational baseline, so we currently neglect orbital motion. We
apply our method to a sample of 23 host stars with 263 candidates
identified in the B-Star Exoplanet Abundance Study (BEAST) survey on
VLT/SPHERE. We identify seven candidates in which the odds ratio
favours the co-moving companion model by a factor of 100 or more. Most
of these detections are based on only two or three epochs separated by
less than three years, so further epochs should be obtained to
reassess the companion probabilities. Our method is publicly available
as an open-source python package from
https://github.com/herzphi/compass to use with any data.
Description:
This work has introduced a statistical method that uses multi
epoch astrometry of an imaged exoplanet candidate to compare a
co-moving companion model with a chance-aligned field star model. It
puts what is commonly referred to as the 'common proper motion
test' on a probabilistic footing.
We applied our method to a sample of 263 candidates around 23 stars
from the B-Star Exoplanet Abundance Study. The results are shown in
Table 1.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 176 267 Candidates with logarithmic odds ratio greater
than zero under the parallax and
proper motion model
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- Seq Sequential number
5- 13 A9 --- Target Identifier of the target star
17- 25 A9 --- UUID Unique identifier for the candidate
27- 36 A10 "date" Date1 Date 1 of observation of the candidate
38- 47 A10 "date" Date2 Date 2 of observation of the candidate
49- 58 A10 "date" Date3 Date 3 of observation of the candidate
60- 78 F19.16 mag Ksmag Ks magnitude of the candidate
80- 97 F18.16 mag e_Ksmag Ks magnitude error of the candidate
99-119 F21.16 --- 2Dnmodel Likelihood ratio of the covariance method
121-141 F21.16 --- pmmodel Likelihood ratio of the pm only method
143-155 A13 --- catalogue [gaiacalctmass] Catalogues used to determine
the ratios
157-162 F6.1 yr refepoch1 Epoch of observation 1
164-169 F6.1 yr refepoch2 Epoch of observation 2
171-176 F6.1 yr refepoch3 ? Epoch of observation 3
--------------------------------------------------------------------------------
History:
From Philipp Herz, herzphilipp(at)outlook.com
Acknowledgements:
We are grateful for the helpful comments during the refereeing process
which improve the paper. The observations this study is based on
were acquired at the ESO VLT telescope (program 1101.C-025). This
publication ma kes use of data from the European Space Agency (ESA)
mission Gaia (http://www.cosmos.esa.int/gaia), processed by the Gaia
Data Processing and Anal ysis Consortium (DPAC,
http://www.cosmos.esa.int/web/ gaia/dpac/consortium). Funding for the
DPAC has been provided by national institutions, in particular the
institutions participating in the Gaia Multilateral Agreement. This
publication also makes use of data from the Two Micron All Sky Survey,
which is a joint project of the University of Massachusetts and the
Infrared Processing and Analysis Center/California Institute of
Technology, funded by the National Aeronautics and Space
Administration and the National Science Foundation.
(End) Patricia Vannier [CDS] 13-Dec-2023