J/A+A/601/A14 Radial velocities of magnetic Ap stars (Mathys, 2017)
Ap stars with magnetically resolved lines:
Magnetic field determinations from Stokes I and V spectra.
Mathys G.
<Astron. Astrophys. 601, A14 (2017)>
=2017A&A...601A..14M 2017A&A...601A..14M (SIMBAD/NED BibCode)
ADC_Keywords: Stars, Ap ; Radial velocities
Keywords: stars: chemically peculiar - stars: magnetic field - stars: rotation -
binaries: general - stars: oscillations
Abstract:
Some Ap stars that have a strong enough magnetic field and a
sufficiently low v sin i show spectral lines resolved into their
magnetically split components.
We present the results of a systematic study of the magnetic fields
and other properties of those stars.
Methods. This study is based on 271 new measurements of the mean
magnetic field modulus of 43 stars, 231 determinations of the mean
longitudinal magnetic field <Bz> and of the crossover <Xz> of 34
stars, and 229 determinations of the mean quadratic magnetic field
<Bq> of 33 stars. Those data were used to derive new values or
meaningful lower limits of the rotation periods Prot of 21 stars.
Variation curves of the mean field modulus were characterised for 25
stars, the variations of the longitudinal field were characterised for
16 stars, and the variations of the crossover and of the quadratic
field were characterised for 8 stars. Our data are complemented by
magnetic measurements from the literature for 41 additional stars with
magnetically resolved lines. Phase coverage is sufficient to define
the curve of variation of Hm for 2 of these stars. Published data
were also used to characterise the Hz curves of variation for 10
more stars. Furthermore, we present 1297 radial velocity measurements
of the 43 Ap stars in our sample that have magnetically resolved
lines. Nine of these stars are spectroscopic binaries for which new
orbital elements were derived.
The existence of a cut-off at the low end of the distribution of the
phase-averaged mean magnetic field moduli av of the Ap stars with
resolved magnetically split lines, at about 2.8kG, is confirmed. This
reflects the probable existence of a gap in the distribution of the
magnetic field strengths in slowly rotating Ap stars, below which
there is a separate population of stars with fields weaker than ∼2kG.
In more than half of the stars with magnetically resolved lines that
have a rotation period shorter than 150 days, av>7.5kG, while those
stars with a longer period all have av<7.5kG. The difference
between the two groups is significant at the 100.0% confidence level.
The relative amplitudes of variation of the mean field modulus may
tend to be greater in stars with Prot>100d than in shorter period
stars. The root-mean-square longitudinal fields of all the studied
stars but one is less than one-third of their phase-averaged mean
field moduli, which is consistent with the expected behaviour for
fields whose geometrical structure resembles a centred dipole.
However, moderate but significant departures from the latter are
frequent. Crossover resulting from the correlation between the Zeeman
effect and the rotation-induced Doppler effect across the stellar
surface is definitely detected in stars with rotation periods of up to
130 days and possibly even up to 500 days. Weak, but formally
significant crossover of constant sign, has also been observed in a
number of longer period stars, which could potentially be caused by
pulsation velocity gradients across the depth of the photosphere. The
quadratic field is in average ∼1.3 times greater than the mean field
modulus and both of those moments vary with similar relative
amplitudes and almost in phase in most stars. Rare exceptions almost
certainly have unusual field structures. The distribution of the known
values and lower limits of the rotation periods of the Ap stars with
magnetically resolved lines indicates that for some of them, Prot
must almost certainly reach 300 years or possibly even much higher
values. Of the 43 Ap stars that we studied in detail, 22 are in binary
systems. The shortest orbital period P_orb of those systems is 27
days. For those non-synchronised Ap binaries for which both the
rotation period and the orbital period, or meaningful lower limits
thereof, are reliably determined, the distribution of the orbital
periods of the systems in which the Ap star has a rotation period that
is shorter than 50 days is different from its distribution for those
systems in which the rotation period of the Ap star is longer, at a
confidence level of 99.6%. The shortest rotation and orbital periods
are mutually exclusive: all but one of the non-synchronised systems
that contain an Ap component with Prot<50d, have Porb>1000d.
Stars with resolved magnetically split lines represent a significant
fraction, of the order of several percent, of the whole population of
Ap stars. Most of these stars are genuine slow rotators, whose
consideration provides new insight into the long-period tail of the
distribution of the periods of Ap stars. Emerging correlations between
rotation periods and magnetic properties provide important clues for
the understanding of the braking mechanisms that have been at play in
the early stages of stellar evolution. The geometrical structures of
the magnetic fields of Ap stars with magnetically resolved lines
appear in general to depart slightly, but not extremely, from centred
dipoles. However, there are a few remarkable exceptions, which deserve
further consideration. Confirmation that pulsational crossover is
indeed occurring at a detectable level would open the door to the
study of non-radial pulsation modes of degree l, which is too high for
photometric or spectroscopic observations. How the lack of short
orbital periods among binaries containing an Ap component with
magnetically resolved lines is related to their (extremely) slow
rotation remains to be fully understood, but the very existence of a
correlation between the two periods lends support to the merger
scenario for the origin of Ap stars.
Description:
Radial velocities measured at multiple epochs for 43 Ap stars with
resolved magnetically split lines, from observations of Mathys
(1991A&AS...89..121M 1991A&AS...89..121M), Mathys et al. (1997, Cat. J/A+AS/123/353),
Mathys & Hubrig (1997A&AS..124..475M 1997A&AS..124..475M), and the present paper. As
described in the latter, the adopted value for the measurements
uncertainties is 2km/s for those based on spectra recorded in
circular polarisation with the ESO CASPEC spectrograph, and 1km/s for
all other determinations, from high-resolution spectra recorded in
natural light with various combinations of telescopes and instruments.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 107 43 Ap stars with resolved magnetically split lines:
stars for which new measurements of the mean
magnetic field modulus are presented in this paper
apres_rv.dat 28 1297 *Radial velocity measured at multiple epochs
for 42 Ap stars (table 7)
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Note on apres_rv.dat: Radial velocities published in Wade et al., 1999,
Cat. J/A+A/347/164).
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See also:
J/A+AS/123/353 : Mean magnetic field modulus of Ap stars (Mathys, 1997)
J/A+A/347/164 : HD 59435 Geneva photometry (Wade+, 1999)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
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1- 6 I6 --- HD/HDE HD/HDE star number
8- 20 A13 --- OName Other id.
22- 27 F6.3 mag Vmag V magnitude
29- 38 A10 --- SpType MK spectral type
41- 42 A2 --- l_Per [> ] Limit flag on Per
43- 52 F10.5 d Per ? Period
53 A1 --- u_Per [:?] Uncertaitny flag on Per
54 A1 --- x_Per [dy] Unit for Period
56- 61 F6.4 --- Per2 ? Second possible period for HD 70331
62 A1 --- x_Per2 [d] Unit for Period2
70- 71 I2 --- r_Per ? Period reference (1)
73- 83 F11.3 d HJD0 ? Phase origin (Julian Date)
85-104 A20 --- n_HJD0 Phase origin information
106-107 I2 --- r_HJD0 ? Reference for HJD0
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Note (1): References as follows:
1 = Romanyuk et al. (2014AstBu..69..427R 2014AstBu..69..427R)
2 = Mathys et al. (1997A&AS..124..475M 1997A&AS..124..475M)
3 = Metlova et al. (2014AstBu..69..315M 2014AstBu..69..315M)
4 = Wade et al. (2000A&A...355.1080W 2000A&A...355.1080W)
5 = Mathys et al. (2016, Cat. J/A+A/586/A85)
6 = Wade et al. (1999, Cat. J/A+A/347/164)
7 = Hill et al. (1998MNRAS.297..236H 1998MNRAS.297..236H)
8 = Adelman (1997, Cat. J/PASP/109/9)
9 = Adelman (1981A&AS...44..265A 1981A&AS...44..265A, Cat. III/89)
10 = Preston (1970ApJ...160.1059P 1970ApJ...160.1059P)
11 = Kurtz (1989MNRAS.238..261K 1989MNRAS.238..261K)
12 = Bailey et al. (2011A&A...535A..25B 2011A&A...535A..25B)
13 = Mathys et al. (2007, Cat. J/A+AS/123/353)
14 = Mathys (1991A&AS...89..121M 1991A&AS...89..121M)
15 = Landstreet (unpublished cited by Mathys 1991A&AS...89..121M 1991A&AS...89..121M)
16 = Wolff (1969ApJ...158.1231W 1969ApJ...158.1231W)
17 = Adelman (2006PASP..118...77A 2006PASP..118...77A)
18 = Wade et al. (1996A&A...313..209W 1996A&A...313..209W)
19 = Wade et al. (1997MNRAS.292..748W 1997MNRAS.292..748W)
20 = Bychkov et al. (2016MNRAS.455.2567B 2016MNRAS.455.2567B)
21 = Manfroid & Mathys (1997A&A...320..497M 1997A&A...320..497M)
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Byte-by-byte Description of file: apres_rv.dat
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Bytes Format Units Label Explanations
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1- 6 I6 --- HD/HDE HD/HDE star number
8- 18 F11.3 --- HJD Heliocentric Julian Date of the observation
20- 24 F5.1 km/s HRV Heliocentric radial velocity
26- 28 F3.1 km/s s_HRV Uncertainty of the measured radial velocity
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Acknowledgements:
Gautier Mathys, gmathys(at)eso.org
(End) Gautier Mathys [JAO/ESO, Chile], Patricia Vannier [CDS] 27-Feb-2017