J/A+A/566/L10 Cepheid radial velocity amplitude modulations (Anderson, 2014)
Tuning in on Cepheids: radial velocity amplitude modulations.
A source of systematic uncertainty for Baade-Wesselink distances.
Anderson R.I.
<Astron. Astrophys. 566, L10 (2014)>
=2014A&A...566L..10A 2014A&A...566L..10A
ADC_Keywords: Stars, variable ; Stars, giant ; Stars, supergiant ;
Radial velocities
Keywords: methods: observational - techniques: radial velocities -
stars: variables: Cepheids - stars: distances - distance scale -
Abstract:
Classical Cepheids are crucial calibrators of the extragalactic
distance scale. The Baade-Wesselink technique can be used to calibrate
Cepheid distances using Cepheids in the Galaxy and the Magellanic
Clouds.
To report the discovery of modulations in radial velocity (RV) curves
of four Galactic classical Cepheids and to investigate their impact as
a systematic uncertainty for Baade-Wesselink distances.
Highly precise Doppler measurements were obtained using the Coralie
high-resolution spectrograph since 2011. Particular care was taken to
sample all phase points in order to very accurately trace the RV curve
during multiple epochs and to search for differences in linear radius
variations derived from observations obtained at different epochs.
Different timescales are sampled, ranging from cycle-to-cycle to
months and years.
The unprecedented combination of excellent phase coverage obtained
during multiple epochs and high precision enabled the discovery of
significant modulation in the RV curves of the short-period s-Cepheids
QZ Normae and V335 Puppis, as well as the long-period fundamental mode
Cepheids l Carinae and RS Puppis. The modulations manifest as shape
and amplitude variations that vary smoothly over timescales of years
for short-period Cepheids, and from one pulsation cycle to the next in
the long-period Cepheids. The order of magnitude of the effect ranges
from several hundred m/s to a few km/s. The resulting difference among
linear radius variations derived using data from different epochs can
lead to systematic errors of up to 15% for Baade-Wesselink-type
distances, if the employed angular and linear radius variations are
not determined contemporaneously.
The different nature of the Cepheids exhibiting modulation in their RV
curves suggests that this phenomenon is common. The observational
baseline is not yet sufficient to conclude whether these modulations
are periodic. In order to ensure the accuracy of Baade-Wesselink
distances, angular and linear radius variations should always be
determined contemporaneously.
Description:
A total of 983 radial velocity measurements of the four Cepheids
QZ Nor (125), V335 Pup (95), l Car (324), and RS Pup (439) are
provided. The measurements are based on observations carried out
between April 2011 and February 2014 that were obtained using the
Coralie spectrograph, mounted to the Swiss 1.2m Euler telescope
located at La Silla Observatory, Chile.
For each Cepheid, a table with the barycentric Julian date of
observation, radial velocity, and the measurement uncertainty are
provided. In addition, a table containing the identifiers,
coordinates, and pulsation periods used to phase-fold the data (see
the figures in the article) is provided.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
cepheids.dat 57 4 Identifiers, coordinates, pulsation periods
qznor.dat 26 125 Coralie radial velocity data for QZ Normae
v335pup.dat 26 95 Coralie radial velocity data for V335 Puppis
lcar.dat 26 324 Coralie radial velocity data for l Carinae
rspup.dat 26 439 Coralie radial velocity data for RS Puppis
--------------------------------------------------------------------------------
See also:
III/229 : Radial Velocities of Cepheids (Gorynya+ 1992-98)
J/A+A/318/416 : Classical Cepheids RV + photometry (Pont+ 1997)
J/A+A/323/775 : Cepheids radial velocities and coordinates (Mishurov+ 1997)
J/AJ/115/635 : Cepheid radial velocities (Metzger+ 1998)
J/A+A/341/818 : s-Cepheid velocity curves (Kienzle+, 1999)
J/A+A/415/531 : Radial velocities of 10 Galactic Cepheids (Storm+, 2004)
J/MNRAS/362/1167 : Radial velocities of 14 southern Cepheids (Petterson+, 2005)
J/A+A/453/309 : HARPS observations of 9 galactic Cepheids (Nardetto+, 2006)
J/A+A/489/1255 : HARPS observations of 8 galactic Cepheids (Nardetto+, 2008)
J/A+A/502/951 : HARPS observations of 9 galactic Cepheids (Nardetto+, 2009)
J/A+A/504/959 : Amplitudes of single-mode Cepheids (Klagyivik+, 2009)
J/A+A/534/A94 : Milky Way Cepheids radial velocities (Storm+, 2011)
Byte-by-byte Description of file: cepheids.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Name Cepheid name according to the GCVS
10- 15 I6 --- HD HD number
17- 18 I2 h RAh Right ascension (J2000.0)
20- 21 I2 min RAm Right ascension (J2000.0)
23- 26 F4.1 s RAs Right ascension (J2000.0)
28 A1 --- DE- Declination sign (J2000.0)
29- 30 I2 deg DEd Declination (J2000.0)
32- 33 I2 arcmin DEm Declination (J2000.0)
35- 36 I2 arcsec DEs Declination (J2000.0)
38- 45 F8.5 d Per Pulsation period
47- 57 A11 --- FileName Name of the file with RV data
--------------------------------------------------------------------------------
Byte-by-byte Description of file: qznor.dat v335pup.dat lcar.dat rspup.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 11 F11.5 --- BJD Barycentric Julian Date (BJD-2400000)
13- 20 F8.3 km/s RV Barycentric radial velocity
22- 26 F5.3 km/s e_RV Radial velocity uncertainty
--------------------------------------------------------------------------------
Acknowledgements:
Richard I. Anderson, richard.anderson(at)unige.ch
(End) R.I. Anderson [Geneva Obs., Switzerland], P. Vannier [CDS] 05-Jun-2014