J/A+A/642/A180 Hot subdwarf stars binarity (Pelisoli+, 2020)
Alone but not lonely: Observational evidence that binary interaction is always
required to form hot subdwarf stars.
Pelisoli I., Vos J., Geier S., Schaffenroth V., Baran A.S.
<Astron. Astrophys., 642, A180 (2020)>
=2020A&A...642A.180P 2020A&A...642A.180P (SIMBAD/NED BibCode)
ADC_Keywords: Stars, double and multiple ; Stars, subdwarf
Keywords: subdwarfs - binaries: general - stars: variables: general
Abstract:
Hot subdwarfs are core-helium burning stars that show lower masses and
higher temperatures than canonical horizontal branch stars. They are
believed to be formed when a red giant suffers an extreme mass-loss
episode. Binary interaction is suggested to be the main formation
channel, but the high fraction of apparently single hot subdwarfs (up
to 30%) has prompted single star formation scenarios to be proposed.
We investigate the possibility that hot subdwarfs could form without
interaction by studying wide binary systems. If single formation
scenarios were possible, there should be hot subdwarfs in wide
binaries that have undergone no interaction.
Angular momentum accretion during interaction is predicted to
cause the hot subdwarf companion to spin up to the critical velocity.
The effect of this should still be observable given the timescales of
the hot subdwarf phase. To study the rotation rates of companions, we
have analysed light curves from the Transiting Exoplanet Survey
Satellite for all known hot subdwarfs showing composite spectral
energy distributions indicating the presence of a main sequence wide
binary companion. If formation without interaction were possible, that
would also imply the existence of hot subdwarfs in very wide binaries
that are not predicted to interact. To identify such systems, we have
searched for common proper motion companions with projected orbital
distances of up to 0.1pc to all known spectroscopically confirmed hot
subdwarfs using Gaia DR2 astrometry.
We find that the companions in composite hot subdwarfs show short
rotation periods when compared to field main sequence stars. They
display a triangular-shaped distribution with a peak around 2.5 days,
similar to what is observed for young open clusters. We also report a
shortage of hot subdwarfs with candidate common proper motion
companions. We identify only 16 candidates after probing 2938 hot
subdwarfs with good astrometry. Out of those, at least six seem to be
hierarchical triple systems, in which the hot subdwarf is part of an
inner binary.
The observed distribution of rotation rates for the companions in
known wide hot subdwarf binaries provides evidence of previous
interaction causing spin-up. Additionally, there is a shortage of hot
subdwarfs in common proper motion pairs, considering the frequency of
such systems among progenitors. These results suggest that binary
interaction is always required for the formation of hot subdwarfs.
Description:
The catalogue of spectroscopically confirmed hot subdwarfs from Geier
et al. (2017, Cat. J/A+A/600/A50) has recently been updated taking
into account input from the Gaia data release 2 (DR2, Gaia
Collaboration 2018, Cat. I/345), as well as the latest releases of the
Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST,
Zhao et al., 2012RAA....12..723Z 2012RAA....12..723Z). We have crossmatched the updated
catalogue (Geier 2020, Cat. J/A+A/635/A193) with the list of observed
stars from the TASC WG8. We identified 156 stars classified as
composite hot subdwarfs with main sequence companions (sdO/B + MS, +
A, + F, + G, + K) observed up to Sector 20.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 90 61 61 variables in our sample whose variability was
attributed to rotation of the main sequence
companion, identified by their TESS Input
Catalogue (TIC) numbers
table2.dat 90 27 27 objects whose cause for variability is
undetermined
table3.dat 47 33 Systems for which no periodic variability has
been detected
table4.dat 107 16 Identified common proper motion candidates
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See also:
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
J/A+A/635/A193 : Catalogue of hot subdwarf stars DR2 (Geier, 2020)
Byte-by-byte Description of file: table1.dat table2.dat
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Bytes Format Units Label Explanations
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1- 9 I9 --- TIC TIC identification number
11- 15 F5.2 mag Gmag Gaia DR2 G magnitude
17- 20 F4.2 ---- CROWDSAP Optimal aperture (CROWDSAP) (G1)
22- 31 F10.7 d Per Period (1)
33- 41 F9.7 d e_Per rms uncertainty on Per (1)
44- 50 F7.4 % Amp Amplitude of variation (1)
52- 58 F7.4 % e_Amp rms uncertainty on Amp (1)
60- 67 E8.3 --- FAP False Alarm Probability (2)
69- 72 I4 K Teff Main sequence companion effective
temperature (G2)
74- 76 I3 K e_Teff rms uncertainty on Teff (G2)
78- 83 F6.4 Rsun R Main sequence companion radius (G2)
85- 90 F6.4 Rsun e_R rms uncertainty on R (G2)
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Note (1): Period and amplitude uncertainties were determined with a hundred
Monte-Carlo runs of the multi-component fit to the light curve, re-drawing
the fluxes from a normal distribution taking the quoted uncertainties
into account.
Note (2): The FAP was calculated using the astropy LombScargle function.
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 I9 --- TIC TIC identification number
11- 15 F5.2 mag Gmag Gaia DR2 G magnitude
17- 20 F4.2 ---- CROWDSAP Optimal aperture (CROWDSAP) (G1)
22- 26 F5.3 % DetLim Detection limit (1)
28- 31 I4 K Teff Main sequence companion effective
temperature (G2)
33- 35 I3 K e_Teff rms uncertainty on Teff (G2)
37- 41 F5.3 Rsun R Main sequence companion radius (G2)
43- 47 F5.3 Rsun e_R rms uncertainty on R (G2)
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Note (1): The detection limit has been calculated as five times the average
amplitude in a Fourier transform of the light curve.
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 19 I19 --- Name1 Hot subdwarf Gaia DR2 identification number
20 A1 --- n_Name1 [*] Note on Name1 (1)
22- 25 F4.1 mag Gmag1 Hot subdwarf Gaia DR2 G magnitude
27- 30 F4.2 mag Plx1 Hot subdwarf Gaia DR2 parallax
32- 37 F6.2 mas/yr pmRA1 Hot subdwarf Gaia DR2 proper motion along RA,
pmRA*cosDE
39- 44 F6.2 mas/yr pmDE1 Hot subdwarf Gaia DR2 proper motion along DE
46- 64 I19 --- Name2 Candidate companion Gaia DR2 identification
number
66- 69 F4.1 mag Gmag2 Candidate companion Gaia DR2 G magnitude
71- 74 F4.2 mag Plx2 Candidate companion Gaia DR2 parallax
76- 81 F6.2 mas/yr pmRA2 Candidate companion Gaia DR2 proper motion
along RA, pmRA*cosDE
83- 88 F6.2 mas/yr pmDE2 Candidate companion Gaia DR2 proper motion
along DE
90- 93 F4.1 arcsec Sep Angular separation of the pair
95- 99 I5 AU a Projected orbital separation of the pair,
assuming the distance of the hot subdwarf
101-103 F3.1 --- DPlx Difference in parallax, in units of
standard deviation
105-107 F3.1 --- Dpm Difference in proper motion, in units of
standard deviation
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Note (1): objects marked with asterisks have identified close companions, being
therefore candidate triple systems (see Appendix C).
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Global notes:
Note (G1): the value of CROWDSAP is the one calculated by the SPOC pipeline.
For objects with data from more than one sector, this is the averaged value.
Note (G2): Teff and radius for the main sequence companion were obtained
from a SED fit, as described in the text.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 19-Jan-2021