J/A+A/638/A122 High-speed stars. Galactic hitchhikers (Caffau+, 2020)
High-speed stars: Galactic hitchhikers.
Caffau E., Monaco L., Bonifacio P., Sbordone L., Haywood M., Spite M.,
Di Matteo P., Spite F., Mucciarelli A., Francois P., Matas Pinto A.M.
<Astron. Astrophys. 638, A122 (2020)>
=2020A&A...638A.122C 2020A&A...638A.122C (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Stars, high-velocity ; Abundances ; Space velocities
Keywords: stars: abundances - Galaxy: abundances - Galaxy: evolution -
Galaxy: formation - Galaxy: kinematics and dynamics - Galaxy: halo
Abstract:
The search for stars born in the very early stages of the Milky Way
star formation history is of paramount importance in the study of the
early Universe since their chemistry carries irreplaceable information
on the conditions in which early star formation and galaxy buildup
took place. The search for these objects has generally taken the form
of expensive surveys for faint extremely metal-poor stars, the most
obvious but not the only candidates to a very early formation.
Thanks to Gaia DR2 radial velocities and proper motions, we identified
72 bright cool stars displaying heliocentric transverse velocities in
excess of 500km/s. These objects are most likely members of
extreme outer-halo populations, either formed in the early Milky Way
build-up or accreted from since-destroyed self-gravitating stellar
systems.
We analysed low-resolution FORS spectra of the 72 stars in the sample
and derived the abundances of a few elements. Despite the large
uncertainties on the radial velocity determination, we derived
reliable orbital parameters for these objects.
The stars analysed are mainly slightly metal poor, with a few very
metal-poor stars. Their chemical composition is much more homogeneous
than expected. All the stars have very eccentric halo orbits, some
extending well beyond the expected dimension of the Milky Way.
These stars can be the result of a disrupted small galaxy or they
could have been globular cluster members. Age estimates suggest that
some of them are evolved blue stragglers, now on the subgiant or
asymptotic giant branches.
Description:
From the Gaia DR2 catalogue, we selected stars with transverse
velocity higher than 500km/s, in the G magnitude range 14-14.5.
Further constraints were put on right ascension to ensure
observability in European Southern Observatory (ESO) period 104
(0h≤RA≤16h or RA≥20) and on declination (DE≤-25) to privilege a
south pointing. The latter constraint was set in order to ensure that
the VLT could observe our targets even in the event of fairly strong
northern wind. In this way we were able to ensure observations of
stars that were not too far away, and as a consequence with relatively
small uncertainties on parallaxes and proper motions, and of bright
objects for an 8 m class telescope, allowing good quality observations
even in bad weather conditions.
All 72 stars were observed during ESO period 104.
In the kinematical and chemical investigations, we assumed that all
stars are single.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
sources.dat 75 72 Positions of GHS sources
kinemat.dat 87 72 Kinematic parameters
abund.dat 132 71 Abundances
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Byte-by-byte Description of file: sources.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 5 A5 --- Star Stellar name (GHSNN)
7- 14 A8 --- --- [Gaia DR2]
16- 34 I19 --- GaiaDR2 Gaia DR2 identification number
36- 55 F20.16 deg RAdeg Gaia DR2 right ascension (ICRS) at Ep=2015.5
57- 75 F19.15 deg DEdeg Gaia DR2 declination (ICRS) at Ep=2015.5
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Byte-by-byte Description of file: kinemat.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 5 A5 --- Star Stellar name (GHSNN)
7- 10 I4 km/s RV Radial velocity
12- 13 I2 km/s e_RV Uncertainty on the radial velocity
15- 24 F10.8 kpc rrmin ? Minimum distance from the Galactic centre
in cylindrical coordinates
26- 34 F9.4 kpc rrmax ? Maximum distance from the Galactic centre
in cylindrical coordinates
36- 44 F9.5 kpc zmax ? Maximum distance from the Galactic plane
46- 55 F10.7 kpc rmin ? Minimum distance from the Galactic centre
in spherical coordinates
57- 65 F9.4 kpc rmax ? Maximum distance from the Galactic centre
in spherical coordinates
67- 75 F9.1 km2/s4 Energy ?=0 Energy per mass [km/s2]2
77- 87 F11.5 kpc.km/s Lz ? Vertical component of the angular momentum
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Byte-by-byte Description of file: abund.dat
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Bytes Format Units Label Explanations
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1- 5 A5 --- Star Name of the star
7- 10 I4 K Teff Effective temperature
12- 15 F4.2 [cm/s2] logg Surface gravity
17- 19 F3.1 km/s vt Micro-turbulence
21- 25 F5.2 --- [Fe/H] Metallicity [Fe/H] with A(Fe)sun=7.75
27 A1 --- l_A(C) Upper limit flag on A(C)
from the G-band
28- 31 F4.2 --- A(C) C abundance from the G-band
32 A1 --- u_A(C) Uncertainty flag on A(C)
33 A1 --- l_A(N) Upper limit foag on A(N)
34- 37 F4.2 --- A(N) ? N abundance from CN lines
38 A1 --- u_A(N) Uncertainty flag on A(N)
40- 43 F4.2 --- A(Mg) Mg abundance
45 I1 --- o_MG Number of Mg lines used
47- 50 F4.2 --- s_Mg line-to-line scatter for Mg
52- 55 F4.2 --- A(CaI) ? Ca abundance from CaI lines
57 I1 --- o_CaI ? Number of CaI lines used
59- 62 F4.2 --- s_CaI ? Line-to-line scatter for CaI lines
64- 67 F4.2 --- A(TiI) ? Ti abundance from TiI lines
69 I1 --- o_TiI ? Number of TiI lines used
71- 74 F4.2 --- s_TiI ? Line-to-line scatter for TiI lines
76- 79 F4.2 --- A(TiII) ? Ti abundance from TiII lines
81 I1 --- o_TiII ? Number of TiII lines used
83- 86 F4.2 --- s_TiII ? Line-to-line scatter for TiII lines
88- 91 F4.2 --- A(FeI) Fe abundance from FeI lines
93- 94 I2 --- o_FeI Number of FeI lines used
96- 99 F4.2 --- s_FeI Line-to-line scatter for FeI lines
101-104 F4.2 --- A(FeII) ? Fe abundance from FeII lines
106-109 F4.2 --- A(Ni) ? Ni abundance
111-114 F4.2 --- A(Sr) ? Sr abundance
116-120 F5.2 --- A(BaII) ? Ba abundance from BaII lines
122 I1 --- o_BaII ? number of BaII lines used
124-127 F4.2 --- e_BaII ? line-to-line scatter for BaII lines
129-132 F4.2 --- A(CaII-K) ? Ca abundance from the CaII-K line
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Acknowledgements:
Elisabetta Caffau, Elisabetta.Caffau(at)obspm.fr
History:
23-Jun-2020: on-line version
04-Aug-2020: Gaia DR2 identification numbers and positions added
(End) Patricia Vannier [CDS] 27-Apr-2020