Image of the Week

Plausible home stars for interstellar object 'Oumuamua

 

Encounter parameters for the stars from Gaia DR2 that encounter 'Oumuamua within 2 parsec. The point shows the median of the estimated encounter time (horizontal axis) and median of the estimated encounter distance (vertical axis). The colour shows the velocity of 'Oumuamua relative to the star at encounter. The error bars show the extent of the 5th and 95th percentiles (the error bars are smaller than the points in some cases). The smallest encounter velocity plotted is 10.7 km/s (this is HD 292249). White circles have median encounter velocities above 50 km/s. (Image credit: Coryn A.L. Bailer-Jones, Davide Farnocchia, Karen J. Meech, Ramon Brasser, Marco Micheli, Sukanya Chakrabarti, Marc W. Buie, Olivier R. Hainaut)

The first interstellar object to pass through our Solar System was detected by the Pan-STARRS survey on 19 October 2017 (Meech et al. 2017). Having already passed the Sun at a distance of 0.25au five weeks earlier, and moving out of the Solar System at nearly 90 km/s, there was a frenzy of observational activity to characterize it before it was too faint to observe (which was the case by January 2018). It turned out to be a small elongated reddish object, possibly cigar-shaped with a length of 800m and width of around 80m, and tumbling around none of its axes of symmetry.

Based on its inferred hyperbolic orbit (i.e. it is unbound to the Sun), various groups traced its motion back in time through the Galaxy to see if they could associate it with a particular star.  The movements of these stars over time must of course also be accommodated in such an analysis, and this was done using astrometry - parallaxes and proper motions - for stars from the TGAS component of Gaia DR1. Combined with radial velocities from other surveys, this provided a list of up to 300 000 stars which could be examined. Unfortunately, none of these studies found any convincing close encounters.

Now, the publication of Gaia DR2 has allowed astronomers to look again into the question of 'Oumuamua's origin. This data release provides much more precise astrometry than DR1, and crucially includes radial velocities for over 7 million stars, thereby increasing the number of stars which can be examined by a factor of 20 over the TGAS-based studies. Furthermore, a team of astronomers led by Marco Micheli at the ESA SSA-NEO Coordination Centre has, in the mean time, used ground-based and HST observations to model 'Oumuamua's orbit more precisely (finding also evidence for a non-gravitational force, which is not uncommon among comets and asteroids).

The new origins study, led by Coryn Bailer-Jones at Max Planck Institute for Astronomy in Heidelberg, has identified four plausible home stars for 'Oumuamua. Assuming that 'Oumuamua was ejected from a stellar system, either through interactions with a giant planet or a second star in that system, then a plausible origin is revealed via a close and slow encounter in the past (which would then indicate the time and speed of ejection). Although none of the encounters are very slow (the slowest is of order 10 km/s), the residual uncertainties in the analysis mean that these are good candidates, and the best found so far. None of the stars has published planets or is known to be a binary, but further investigation will be required to confirm or refute this.

 

The animation above traces the orbit of one of the home candidates ("star") and 'Oumuamua ("iso") back in time to their mutual encounter. The coordinate system is Cartesian with the Galactic centre at (0,0,0) and is inertial with respect to that. Left is the view from the north Galactic pole (the z-axis is perpendicular to the Galactic plane); right is the view from l=90 (the direction of Galactic rotation). As explained in the article, the star and 'Oumuamua are represented not by their nominal measurements but by a set of "surrogates" (just 20 shown here) obtained by sampling the uncertainties in their position and velocity measurements. The uncertainties in 'Oumuamua are so small on this scale that they are barely visible. GIF credit: Coryn A.L. Bailer-Jones, Davide Farnocchia, Karen J. Meech, Ramon Brasser, Marco Micheli, Sukanya Chakrabarti, Marc W. Buie, Olivier R. Hainaut

 

There are of course a number of caveats to this study. All of the home candidate stars are low mass (of order a solar mass or less), relatively nearby, and encountered 'Oumuamua within the past 7 Myr. These are to some extent selection effects of the study, because to have a measurable radial velocity in Gaia DR2 the star must be relatively bright. Indeed, given that there are of order 1011 stars in the Galaxy, and we have no good indications of 'Oumuamua's age, then it is not unlikely that it was ejected from one of the many stars which could not be studied using Gaia Data Release 2. On the other hand, if 'Oumuamua turns out to have been ejected within the past few million years, then there is a reasonable chance this study has identified its home system. Further investigation will have to await the publication of Gaia Data Release 3, which should provide radial velocities for many more stars.

References:
Meech et al. 2017

Micheli et al. 2018

Bailer-Jones et al. 2018 (the paper on plausible home stars for 'Oumuamua)
 

Read the story on ESA Science & Technology here.

Credits: ESA/Gaia/DPAC, Coryn A.L. Bailer-Jones, Davide Farnocchia, Karen J. Meech, Ramon Brasser, Marco Micheli, Sukanya Chakrabarti, Marc W. Buie, Olivier R. Hainaut

[Published: 25/09/2018]

 

Image of the Week Archive

2018
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
2017
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
04/10: 1:1 Gaia model arrives at ESAC
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
27/08: Quest for the Sun's siblings
 
Please note: Entries from the period 2003-2010 are available in this PDF document.