J/MNRAS/475/5659 OB association members (Ward+, 2018)
Not all stars form in clusters -
measuring the kinematics of OB associations with Gaia.
Ward J.L., Kruijssen J.M.D.
<Mon. Not. R. Astron. Soc., 475, 5659-5676 (2018)>
=2018MNRAS.475.5659W 2018MNRAS.475.5659W (SIMBAD/NED BibCode)
ADC_Keywords: Associations, stellar ; Stars, OB ; Proper motions ;
Parallaxes, trigonometric ; Optical
Keywords: proper motions - stars: formation - stars: kinematics and dynamics -
open clusters and associations: general
Abstract:
It is often stated that star clusters are the fundamental units of
star formation and that most (if not all) stars form in dense stellar
clusters. In this monolithic formation scenario, low-density OB
associations are formed from the expansion of gravitationally bound
clusters following gas expulsion due to stellar feedback. N-body
simulations of this process show that OB associations formed this way
retain signs of expansion and elevated radial anisotropy over tens of
Myr. However, recent theoretical and observational studies suggest
that star formation is a hierarchical process, following the fractal
nature of natal molecular clouds and allowing the formation of
large-scale associations in situ. We distinguish between these two
scenarios by characterizing the kinematics of OB associations using
the Tycho-Gaia Astrometric Solution catalogue. To this end, we
quantify four key kinematic diagnostics: the number ratio of stars
with positive radial velocities to those with negative radial
velocities, the median radial velocity, the median radial velocity
normalized by the tangential velocity, and the radial anisotropy
parameter. Each quantity presents a useful diagnostic of whether the
association was more compact in the past. We compare these diagnostics
to models representing random motion and the expanding products of
monolithic cluster formation. None of these diagnostics show evidence
of expansion, either from a single cluster or multiple clusters, and
the observed kinematics are better represented by a random velocity
distribution. This result favours the hierarchical star formation
model in which a minority of stars forms in bound clusters and
large-scale, hierarchically structured associations are formed in
situ.
Description:
Accurate and precise distances towards OB association members are
crucial for both membership selection and the analysis of dynamical
properties of those associations. This study is therefore limited to a
strictly enforced 3σ confidence limit on the distances (i.e.
σD≤D/3) derived from the TGAS catalogue parallaxes. We
obtained parallaxes and corresponding uncertainties from the Gaia
archive (https://gea.esac.esa.int/archive/) for the entire TGAS
catalogue (entries for ∼2 million stars). From these parallax
measurements, we calculate distances and distance uncertainties.
We present All OB association members selected in this study.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tableb1.dat 170 3450 All OB association members
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See also:
I/259 : The Tycho-2 Catalogue (Hog+ 2000)
I/337 : Gaia DR1 (Gaia Collaboration, 2016)
Byte-by-byte Description of file: tableb1.dat
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Bytes Format Units Label Explanations
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1- 7 A7 --- OBAss OB association to which the star has been
assigned membership
9- 14 I6 --- HIP ? HIP number
16- 27 A12 --- Tycho2 Tycho-2 designation
29- 30 I2 h RAh Right ascension (J2000)
32- 33 I2 min RAm Right ascension (J2000)
35- 42 F8.5 s RAs Right ascension (J2000)
44 A1 --- DE- Declination sign (J2000)
45- 46 I2 deg DEd Declination (J2000)
48- 49 I2 arcmin DEm Declination (J2000)
51- 57 F7.4 arcsec DEs Declination (J2000)
59- 75 F17.15 mas plx TGAS parallax
77- 93 F17.15 mas e_plx TGAS parallax error
95-112 E18.15 mas/yr pmRA TGAS proper motion along RA
114-130 F17.15 mas/yr e_pmRA TGAS proper motion along RA error
132-150 F19.15 mas/yr pmDE TGAS proper motion along DE
152-168 F17.15 mas/yr e_pmDE TGAS proper motion along DE error
170 I1 --- OB? [0/1] indicates whether the star is listed as
spectral type O or B in the Simbad data base
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 21-Sep-2023