J/MNRAS/495/663 Gaia DR2 OB associations (Ward+, 2020)
Not all stars form in clusters. Gaia-DR2 uncovers the origin of OB associations.
Ward J.L., Kruijssen J.M.D., Rix H.-W.
<Mon. Not. R. Astron. Soc. 495, 663-685 (2020)>
=2020MNRAS.495..663W 2020MNRAS.495..663W (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, open ; Stars, OB
Keywords: proper motions - stars: formation - stars: kinematics and dynamics -
open clusters and associations: general -
Abstract:
Historically, it has often been asserted that most stars form in
compact clusters. In this scenario, present-day
gravitationally-unbound OB associations are the result of the
expansion of initially gravitationally-bound star clusters. However,
this paradigm is inconsistent with recent results, both theoretical
and observational, that instead favour a hierarchical picture of star
formation in which stars are formed across a continuous distribution
of gas densities and most OB associations never were bound clusters.
Instead they are formed in-situ as the low-density side of this
distribution, rather than as the remnants of expanding clusters. We
utilise the second Gaia data release to quantify the degree to which
OB associations are undergoing expansion and, therefore, whether OB
associations are the product of expanding clusters, or whether they
were born in-situ, as the large-scale, globally-unbound associations
that we see today. We find that the observed kinematic properties of
associations are consistent with highly substructured velocity fields
and additionally require some degree of localised expansion from
sub-clusters within the association. While most present-day OB
associations do exhibit low levels of expansion, there is no
significant correlation between radial velocity and radius. Therefore,
the large-scale structure of associations is not set by the expansion
of clusters, rather it is a relic of the molecular gas cloud from
which the association was formed. This finding is inconsistent with a
monolithic model of association formation and instead favours a
hierarchical model, in which OB associations form in-situ, following
the fractal structure of the gas from which they form.
Description:
We investigate the kinematic properties of a large number
of independently selected OB associations. Using the OPTICS clustering
algorithm, we select 109 likely OB associations from a large sample of
OB stars within the Milky Way.
List of Gaia DR2 designations for all stars in the sample and
corresponding association numbers in allstars.dat.
Association positions and sizes in tablea1.dat.
Association derived kinematic properties in tablea2.dat.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 141 109 Association positions and sizes
tablea2.dat 234 109 Derived kinematic properties
allstars.dat 32 1775465 Gaia DR2 designations for all stars in sample
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See also:
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Assoc Association ID number (G1)
5- 10 I6 --- Nstar [40/177657] Number of stars
12- 14 I3 --- NOB Number of OB stars
16- 30 F15.11 deg RAdeg Mean right ascension of star
association (J2000)
32- 46 F15.11 deg DEdeg Mean declination of star association (J2000)
48- 61 F14.9 pc Dist mean line-of-sight distance to stars
63- 77 F15.11 pc s_Dist standard deviation of distance
79- 93 F15.11 pc stdX standard deviation in plane-of-sky position
95-109 F15.11 pc stdY standard deviation in plane-of-sky position
111-125 F15.12 km/s stdPMX standard deviation in plane-of-sky velocity
127-141 F15.12 km/s stdPMY standard deviation in plane-of-sky velocity
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Byte-by-byte Description of file: tablea2.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Assoc Association ID number (G1)
5- 18 F14.12 --- Nrat Number ratio, NvR/Nvt
20- 35 F16.13 km/s RVmed Median radial velocity
37- 51 F15.12 km/s e_RVmed rms uncertainty in VRmed
53- 69 F17.14 --- RV/Vtmed Median (vR/|vt|) velocity ratio
71- 83 F13.11 --- e_RV/Vtmed rms uncertainty in VT/Vtmed
85-100 F16.13 --- beta Radial-anisotropy parameter
102-118 E17.14 --- e_beta rms uncertainty in beta
120-133 F14.12 --- NratOBs Number ratio using OB star centre
135-150 F16.13 km/s VROBsmed Median (vR) using OB star centre
152-166 F15.12 km/s e_VROBsmed rms uncertainty in VROBsmed
168-184 F17.14 --- VT/VtOBsmed Median (vR/|vt|) using OB star centre
186-198 F13.11 --- e_VT/VtOBsmed rms uncertainty in VT/VtOBsmed
200-216 E17.14 --- betaOBs Radial anisotropy using OB star centre
218-234 E17.14 --- e_betaOBs rms uncertainty in betaOBs
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Byte-by-byte Description of file: allstars.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 designation
30- 32 I3 --- Assoc [1/110] Association ID number (G1)
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Global notes:
Note (G1): Associations numbers: 1-89, 91-110.
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Acknowledgements:
Jake Ward, jakelward1(at)gmail.com
(End) Patricia Vannier [CDS] 20-Apr-2020