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GEOS GEOS CIRCULAR ON CEPHEID VARIABLES GEOS
CEP 2 Parc de Levesville, 23
July 2014 F-28300 BAILLEAU L'EV ^EQUE
GEOS RR Lyr Survey: FM DEL IS INDEED A CEPHEID
Jean-Fran cois Le Borgne1;2;3, Alain Klotz1;2;3.
1GEOS (Groupe Europ een d'Observations Stellaires), 23 Parc de Levesville, 28300 Bailleau l'Ev^ eque, France
2Universit e de Toulouse; UPS-OMP; IRAP; Toulouse, France
3CNRS; IRAP; 14, avenue Edouard Belin, F-31400 Toulouse, France
ABSTRACT
Though FM Del has been considered as a RR Lyr star by Preston et al. in 1959 (following discovery by
Huth, 1957), Huth (1960) eventually changed his mind by showing that it is in fact a cepheid of W Vir
type of period of 3.95452 days. Various authors since then have considered it as a cepheid indeed, with
the exception of Wils et al. (2006) who list this star in their RR Lyr catalog with a period of 0.79688
days. On this basis, FM Del was added to Tarot RR Lyr program. We present here these observations
which con rm the cepheid type.
RESUM E
Quoique l' etoile FM Del ait et e consid er ee comme une RR Lyr par Preston et al. en 1959 suivant la
d ecouverte par Huth (1957), Huth (1960) changea d'id ee en montrant qu'il s'agit en fait d'une c eph eide
de type W Vir de p eriode 3.95452 jours. Plusieurs auteurs ont consid er e cette etoile comme une c eph eide
depuis, a l'exception de Wils et al. (2006) qui la listent dans leur catalogue de RR Lyr avec une p eriode
de 0.79688 jour. Sur cette base, FM Del a et e ajout ee au programme RR Lyr de Tarot. Nous pr esentons
ici ces observations qui con rment le type c eph eide.
RIASSUNTO
Scoperta da Huth (1959), FM Del e stata classi cata come una variabile di tipo RR Lyr da Preston
et al. (1959). Successivamente, Huth (1960) ha mostrato che si tratta di una Cefeide tipo W Vir con
periodo 3.95452 d. Questa classi cazione e stata adottata da molti autori ad eccezione di Wils et al.
(2006), i quali la riportano nel loro catalogo di variabili RR Lyr con periodo 0.79688 d. Sulla base di
questa nuova indicazione, FM Del e stata aggiunta al programma RR Lyr svolto con Tarot. L'analisi
delle nuove osservazioni conferma de nitivamente che si tratta di una cefeide.
RESUMEN
La estrella FM Del fue clasi cada como de tipo RR Lyr por Preston et al. en 1959, tras su descubrimiento
por Huth (1957). Posteriormente, Huth (1960) demostr o que se trata de una Cefeida de tipo W Vir,
con periodo de 3.95452 d as. Desde entonces numerosos autores la han considerado como Cefeida, con
la excepci on de Wils et al. (2006), que la incluyeron en su cat alogo de variables de tipo RR Lyr, con
un periodo de 0.79688 das. En base a esto, FM Del ha sido a~ nadida al programa de RR Lyr de Tarot.
Presentamos aqu estas observaciones, que con rman que es una Cefeida.arXiv:1407.4961v1 [astro-ph.SR] 18 Jul 2014
GEOS CIRCULAR CEP 2 FM Del 2
1 Introduction
As recalled by Diethelm (1986), FM Del was discovered by Huth (1957) as an RR Lyr of period 0.79739
days and was studied as such by Preston (1959). Preston's study concerned metallicity which was found
to be about solar. This pushed Huth to reconsider the type determination and eventually revised (Huth,
1960) its type to cepheid with period 3.95542 days. GCVS (Samus et al., 2007-2012) gives CWB type
(population II cepheid) and variation from 12.3 to 13.3 (p) with reference to Huth (1960).
Since then, several authors used FM Del as a cepheid. It was listed cepheid general studies by Petit
(1960a, 1960b) and Harris (1985). New observations only appear in Diethelm (1986, 1990) who made a
single photometric measurement in Walraven VBLUW system determining a metallicity index [Fe/H]=-
0.9. Having only one measurement, Diethelm supposed that it is a cepheid, but with caution. Schmidt
et al. (2003) eventually published electronic measurements in V and R lters, followed by spectroscopic
observations (2005). Though Schmidt et al. assume the same period as determined by Huth (1960),
their light curve leaves no doubt on the cepheid type.
Strangely enough, FM Del appears in Wils et al. (2006) catalog of RR Lyr stars from Rotse measure-
ments with period of 0.79688 days (NSVS 11462023), a period close to Huth's. We may guess that this
is an artifact from the time sampling of one measurement per night. It is worth to note that the ratio
between the two periods is close to 5 (4.96). A consequence of FM Del being in Wils et al. catalog have
been to schedule FM Del in Tarot RR Lyr program.
2 TAROT observations
A description of TAROT telescopes may be found in Klotz et al. (2008) and GEOS RR Lyr survey,
including TAROT RR Lyr program in Le Borgne et al. (2007, 2012). To summarize, let us say that
Figure 1: Folded light curve of FM Del (TAROT)
using elements (1). Figure 2: Star eld around FM Del.
TAROT telescopes are robotic 25cm telescopes aimed to the observation of optical counterparts of
events triggered by
-ray satellite alerts or astro-particles detectors (neutrinos, gravitational waves).
One is located in France (Calern Observatory) and the other in Chile (La Silla Observatory). Between
alerts, the telescopes are used for several programs, one of them being to contribute to GEOS RR Lyr
survey.
FM Del has been observed by the northern TAROT telescope at Calern Observatory from JD 2455401.414
GEOS CIRCULAR CEP 2 FM Del 3
(23 July 2010) to 2456162.582 (23 August 2012). 872 measurements spread over 29 nights have been
obtained. As for the other stars of TAROT RR Lyr program, data reduction, from bias subtraction and
at elding to photometry using SExtractor (Bertin and Arnouts, 1996), is performed automatically.
In TAROT RR Lyr program, observations are scheduled in order to obtain times of maximum dur-
ing selected nights. For FM Del, the nights where selected according the elements given in Wils et
al. (2006). It appeared that no maximum was observed during the selected nights, and furthermore,
the star varied very few during all of them, and at di erent mean brightness. This is not typical of
a RR Lyr star. We then plotted a folded light curve with the period given by Huth and later used
by Schmidt et al. (Figure 1). We rst used the elements given in GCVS (Samus et al., 2011) but the
maximum of the light curve did not correspond to phase 0. We then adjusted the origin of the elements:
3 Discussion
One fact is to be noted: there is no reference to Wils et al. (2006) in CDS/SIMBAD entry for FM Del,
nor NSVS 11462023 is given as cross identi cation. The question is then to investigate if these are 2
di erent stars. The coordinates of FM Del given in GCVS are the same as those given in SIMBAD.
The coordinates of NSVS 11462023 from ROTSE are 0.2 arc minutes from GCVS FM Del coordinates.
Figure 2 shows the star eld around FM Del. The circle shows the position of NSVS 11462023 from
Wils et al. (203343.42+161619.2). There is no star at ROTSE position and the closest 12th magnitude
star is FM Del. Then it is most probable that FM Del and NSVS 11462023 are the same star.
HJD 2456111:65 + 3:95452E: (1)
Next step is to check by our self the frequencies present in Rotse measurements which are available at
Figure 3: Periodograms of Rotse measurements. left: Schwarzenberg's method. Right: Vani^ cek's method.
the web site http://skydot.lanl.gov/ (Wozniak, 2004). Rotse database contains 103 measurements for
NSVS 11462023 between JD 2451420.746 (30 August 1999) and 2451511.631 (29 November 1999). A
periodogram of these data (Figure 3, left), using multiharmonic Fourier series method (Schwarzenberg-
Czerny, 1996), gives a main frequency fat 0.25307 d 1, that is 3.95147 days. We also see aliases at
1 fand 1 +f, as well as their half frequencies f=2, (1 f)=2 and (1 +f)=2. Other aliases appear at
1 f=2, 1 +f=2 and 1 + (1 + f)=2. Note that because fis close to 0.25, 5 fis close to 1 + f. As noted
above, 5 is the ratio between the GCVS cepheid period and the period given by Wils et al.. Obviously,
they used the alias 1 + fas the main frequency. In principle, since the time sampling of Rotse is about
one measurement per day, they should have considered such a frequency only with caution.
GEOS CIRCULAR CEP 2 FM Del 4
We also use a second method, the iterative sine{wave least{squares method (Vani^ cek, 1971) to build a
second periodogram (Figure 3, right) the frequency ffound is 0.25274 d 1, corresponding to a period
of 3.95663 days. As with the former method, we see aliases at 1 fand 1 +fwhich is expected with
the time sampling of the measurements. However, none of the other aliases appears.
The folded light curve of Rotse measurements are plotted with the same elements as TAROT mea-
Figure 4: Folded light curve of Rotse measurements using elements (1).
surements in gure 4.
As a comparison, periodograms of Tarot data obtained with the two methods are given in gures 5.
Thanks to TAROT time sampling and duration, only fandf=2 appear in periodogram obtained with
Schwarzenberg-Czerny's method while only fis found with Vani^ cek's method. However, Vani^ cek's
periodogram is noisier. The remaining lines are aliases of 1 d 1frequency.
All these periodograms, on Rotse and Tarot data, give an uncertainty on period of about 0.003 day.
This does not allow to improve the period given in GCVS. Note that FM Del is not in Catalina Survey
Figure 5: Periodograms of Tarot measurements. left: Schwarzenberg's method. Right: Vani^ cek's method.
database.
GEOS CIRCULAR CEP 2 FM Del 5
4 Conclusion
We have con rmed that FM Del is a cepheid as it was supposed to be since 1960 although it was rst
identi ed as an RR Lyr at its discovery in 1957. It was erroneously added to Wils et al. (2006) RR
Lyr catalog with a period which is 5 times less than true one. Included in RR Lyr TAROT program on
this basis, it clearly appeared not to be a RR Lyr star. TAROT data t nicely Huth's 1960 period, as
do ROTSE measurements which were at origin of Wils et al. paper.
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