J/MNRAS/498/1920 The death throes of the Hyades with Gaia (Oh+, 2020)
Kinematic modelling of clusters with Gaia: the death throes of the Hyades.
Oh S., Evans N.W.
<Mon. Not. R. Astron. Soc., 498, 1920-1938 (2020)>
=2020MNRAS.498.1920O 2020MNRAS.498.1920O (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, open ; Stars, fundamental ; Positional data ;
Parallaxes, trigonometric ; Proper motions ; Radial velocities ;
Models ; Optical
Keywords: methods: data analysis - software: data analysis - astrometry -
stars: distances - stars: fundamental parameters -
open clusters and associations: individual: Hyades
Abstract:
The precision of the Gaia data offers a unique opportunity to study
the internal velocity field of star clusters. We develop and validate
a forward-modelling method for the internal motions of stars in a
cluster. The model allows an anisotropic velocity dispersion matrix
and linear velocity gradient describing rotation and shear, combines
radial velocities available for a subset of stars, and accounts for
contamination from background sources via a mixture model. We apply
the method to Gaia DR2 data of the Hyades cluster and its tidal tails,
dividing and comparing the kinematics of stars within and beyond 10pc,
which is roughly the tidal radius of the cluster. While the velocity
dispersion for the cluster is nearly isotropic, the velocity ellipsoid
for the tails is clearly elongated with the major axis pointing
towards the Galactic centre. We find positive and negative expansions
at ∼2σ significance in Galactic azimuthal and vertical
directions for the cluster but no rotation. The tidal tails are
stretching in a direction tilted from the Galactic centre while
equally contracting as the cluster in Galactic vertical direction. The
tails have a shear (A) of 16.90±0.92m/s/pc and a vorticity (B) of
-6.48±1.15m/s/pc, values distinct from the local Oort constants. By
solving the Jeans equations for flattened models of the Hyades, we
show that the observed velocity dispersions are a factor of ∼2 greater
than required for virial equilibrium due to tidal heating and
disruption. From simple models of the mass loss, we estimate that the
Hyades is close to final dissolution with only a further ~<30Myr left.
Description:
We present a method to model the internal kinematics of stars in a
cluster or association, which builds upon and extends previous works
with Hipparcos and Gaia. Our model allows for anisotropic velocity
dispersions and a linear velocity gradient (equivalently, rotation and
shear). It incorporates RV measurements for a subset of stars with
astrometry, and accounts for contamination by background sources (in
terms of velocity) via a mixture model. We implemented the method in a
modern statistical modelling language and validated the implementation
with mock data generated with similar quality as Gaia DR2.
We applied the method to the Gaia DR2 data of the Hyades cluster and
its tails, which have recently been discovered using the same data
(Meingast & Alves 2019A&A...621L...3M 2019A&A...621L...3M, Cat. J/A+A/621/L3; Roser et al.
2019A&A...621L...2R 2019A&A...621L...2R, Cat. J/A+A/621/L2). We divided the sample into
two, the cluster proper (cl, rc<10pc) and the tidal tails (tails,
rc>10pc).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 134 1103 The merged Hyades sample used for kinematic
modelling
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See also:
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
J/A+A/616/A10 : 46 open clusters GaiaDR2 HR diagrams
(Gaia Collaboration, 2018)
J/A+A/621/L3 : Hyades tidal tails with Gaia DR2 (Meingast+, 2019)
J/A+A/621/L2 : Hyades tidal tails revealed by Gaia DR2 (Roeser+, 2019)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- --- [Gaia DR2]
10- 28 I19 --- GaiaDR2 Gaia DR2 source identifier
30- 39 F10.6 deg RAdeg Right ascension (ICRS) at Ep=2015.5
41- 50 F10.6 deg DEdeg Declination (ICRS) at Ep=2015.5
52- 60 F9.6 mas plx Parallax from Gaia DR2
62- 72 F11.6 mas/yr pmRA Proper motion in right ascension from Gaia
DR2
74- 84 F11.6 mas/yr pmDE Proper motion in declination from Gaia DR2
86- 92 F7.3 km/s RV ? Radial velocity
94- 99 F6.3 mag BP-RP ? BP-RP colour from Gaia DR2
101- 106 F6.3 mag Gmag G-band magnitude from Gaia DR2
108- 111 A4 --- inGaia Indicates if the source was included the
Gaia Collaboration et al.
(2018A&A...616A..10G 2018A&A...616A..10G, Cat. J/A+A/616/A10)
membership list
113- 116 A4 --- inMeingast Indicates if the source was included the
membership list of Meingast & Alves
(2019A&A...621L...3M 2019A&A...621L...3M, Cat. J/A+A/621/L3)
118- 121 A4 --- inRoser Indicates if the source was included the
membership list of Roser et al.
(2019A&A...621L...2R 2019A&A...621L...2R, Cat. J/A+A/621/L2)
123- 128 F6.4 --- Pmem Mean membership probability from kinematic
modelling
130- 134 A5 --- Group Fit group (1)
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Note (1): With the cluster centre determined, we divide the sample into two:
400 stars within 10pc which we call 'cl' and 703 stars beyond 10pc
which we call 'tails'
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History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 03-Aug-2023